Chem.Soc.Rev.,2012,41,7938–Defect-related luminescent materials

Cite this:Chem.Soc.Rev .,2012,41,7938–7961Defect-related luminescent materials:synthesis,emission properties and applications

Cuimiao Zhang ab and Jun Lin*a

Received 25th March 2012DOI:10.1039/c2cs35215j

Luminescent materials have found a wide variety of applications,including information displays,lighting,X-ray intensi?cation and scintillation,and so on.Therefore,much e?ort has been devoted to exploring novel luminescent materials so far.In the past decade,defect-related luminescent materials have inspired intensive research e?orts in their own right.This kind of luminescent material can be basically classi?ed into silica-based materials,phosphate systems,metal oxides,BCNO phosphors,and carbon-based materials.These materials combine several favourable attributes of traditional commercially available phosphors,which are stable,e?cient,and less toxic,being free of the burdens of intrinsic toxicity or elemental scarcity and the need for stringent,intricate,tedious,costly,or ine?cient preparation steps.Defect-related luminescent

materials can be produced inexpensively and on a large scale by many approaches,such as sol–gel process,hydro(solvo)thermal reaction,hydrolysis methods,and electrochemical methods.This review article highlights the recent advances in the chemical synthesis and luminescent properties of the defect-related materials,together with their control and tuning,and emission mechanisms (solid state physics).We also speculate on their future and discuss potential developments for their applications in lighting and biomedical ?elds.

1.Introduction

Luminescent materials (or phosphors)are generally characterized by the emission of light in the visible range but can also be in other spectral regions (such as ultraviolet or infrared)with energy beyond thermal equilibrium.1,2The ever-growing demand for advanced luminescent materials has motivated scienti?c and

a

State Key Laboratory of Rare Earth Resources Utilization,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022,P.R.China.E-mail:jlin@https://www.doczj.com/doc/8e13031946.html,;Fax:(+86)431-85698041b

College of Chemistry and Environmental Science,Hebei University,Baoding 071002,P.R.China.E-mail:cuimiaozhang@https://www.doczj.com/doc/8e13031946.html,

Cuimiao Zhang

Cuimiao Zhang was born in Hebei,China,in 1980.She received her BS (2004)in chemistry from Hebei Univer-sity.Then she has been a grad-uate student in Jun Lin’s group at Changchun Institute of Applied Chemistry (CIAC),Chinese Academy of Sciences,and received a PhD degree in 2010.After graduation,she has been working as a research fellow in Nanyang Technolo-gical University (Singapore,2010–2012).Her current research interests include the

synthesis of environmentally-friendly luminescence materials and their bioapplication.She came back to China in 2012,and since then has been working as an assistant professor in Hebei University.

Jun Lin

Jun Lin was born in Chang-chun,China,in 1966.He received BS and MS degrees in inorganic chemistry from Jilin University,China,in 1989and 1992,respectively,and a PhD degree (inorganic chemistry)from the Chang-chun Institute of Applied Chemistry (CIAC),Chinese Academy of Sciences,in 1995.Then he went to CityU (HK,1996),INM (Germany,1997),VCU and UNO (USA,1998–1999)working as a postdoctor.He came back to

China in 2000,and since then has been working as a professor in CIAC.His research interests include luminescent materials and multifunctional composite materials together with their applica-tions in display,lighting and biomedical ?elds.

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technological e?orts dedicated to improving existing phos-phors and to developing new e?ective luminescent materials.3Generally,inorganic luminescent materials can be classi?ed into two groups:metal-activator based and non-activator based luminescent materials.The ?rst case is represented by transitions between energy levels of metal activator ions containing the rare earth and/or transition metal ions (e.g.f–f transitions of Eu 3+in Y 2O 3:Eu 3+)4–7or complex ions (e.g.the charge-transfer transition on [WO 4]2àin CaWO 4).8,9These kinds of luminescent materials are often expensive and contain non-environmentally friendly elements,such as Ag (in ZnS:Ag +)and lanthanides.1–15On the other hand,it is necessary for most metal-activator-based phosphors to be excited by short-wavelength ultraviolet (UV)light for operation,which results in the extensive use of a mercury vapor plasma in ?uorescent lighting products.1,10The excess usage of mercury vapor will give rise to environmental contamination and technical di?culties.The other group of luminescent materials consists of semiconductors and defect-related materials.For most semiconductors,the luminescence normally results from band-to-band excitation between impurity states within the bandgap.However,some serious intrinsic toxicity and potential environmental hazards associated with many of these semiconductors are critically pronounced,which have limited their applications.For the defect-related luminescent materials,although the roots can be traced to 1986,16they have had a remarkable growth since Sailor et al.reported the white phosphors from a silicate-carboxylate sol–gel precursor.17Since then,the emerging defect-related luminescent materials,which do not contain any transition metal or rare earth activators,appear to be promising alternatives to traditional phosphors in many applications because of their advantages of low toxicity,stability,tunable emission color,and low cost.17–30Although the origins of photoluminescence (PL)are not yet entirely understood in defect-related materials,there is mounting evidence that emission arises from the special defects.Typically,the defects in these kind of materials contain vacancies,impurities,radical impurities,donor–acceptor pairs,etc.,22,27,31–33which may arouse charge imbalances at these defect sites.The charge imbalances must be recti?ed by localization of electrons and electron holes or give rise to impurity states within the bandgap.Therefore,the defect sites are apparently the precursors for various centers containing the emission center for defect-related luminescent materials and impart them with excellent photo-luminescent emission.

On the other hand,in modern chemistry and materials science,there have been tremendous developments in the synthetic control of size,shape,and composition,thus allowing the tailoring of their properties.34–40Therefore,the synthesis methods have been inten-sively pursued for their technological and fundamental scienti?c importance.41,42Among them,the luminescence properties of defect-related luminescent materials can also be in?uenced by their phase,surface chemistry,size distribution,morphology and/or porosity.43–45Thus,the rational control over these factors has become an important research issue in recent years.46So far,valuable explorations have been carried out based on soft chemical synthetic strategies [such as Pechini sol–gel (PSG)method,hydro-thermal process,and so on]to luminescent materials as well as controlled morphologies,formation,and luminescence properties.

As we know,the tremendous recent advances in functionalized nanomaterials with distinct ?uorescent properties,47–54mesoporous properties,55and biocompatability 56,57have received considerable attention in the past decades,since nanomaterials provide a new platform for biomedical applications such as drug delivery,bio-separation,?uorescent labeling,58diagnosis and treatment of disease.59–68During the past decades,the use of semiconductor ?uorescent nanocrystals or quantum dots has notably extended the scope of possible biological applications (?uorescent probes,labeling and tracking,etc.)due to their high quantum yield and high photostability.69,70However,their disadvantages of blinking,potential cytotoxicity in vivo ,and complex functionalization strategies have limited their use.Additionally,some serious problems of photobleaching and quenching of ?uorescent organic molecules are critically pronounced,which have seriously limited their applications in biomedical areas.60,71,72In contrast to traditional ?uorescent materials in biological areas such as organic dyes and quantum dots,the novel defect-related luminescent nanomaterials may be promising ?uorescent materials in biological ?elds due to their good optical properties,high chemical stability,and low toxicity.29,30,73–77Especially,multifunctional defect-related materials with excellent luminescent,structural,and/or surface properties seem to be ideal systems for biomedical applications.To date,many recent reports have shown that multifunctional defect-related materials,as the most e?cient ?uorescent materials,can be used for the intracellular imaging,drug delivery,and so on.26,29,30,78

In this review article,we mainly focus on the recent progress in various chemical syntheses and luminescent properties of defect-related luminescent materials,together with their tuning and possible emission mechanisms as well as their applications in lighting,display,and biological ?elds (including bioimaging and tracking in drug delivery systems),based on our recent e?orts and those of others in this area.Moreover,we hope to inspire research into the origins of the unique properties of this emergent class of novel luminescent materials and to encourage their exploration in a multitude of exciting areas from lighting to display and medical diagnostics.

2.Synthetic methods

Generally,as the defect sites are mainly created in the preparing process,the luminescent properties of defect-related materials seem to be dominated by the preparation methods.In recent years,a large number of synthetic methods have been developed for generating defect-related luminescent materials.These methods can be divided into several categories:sol–gel method,hydro-(solvo)thermal reaction,hydrolysis method,electrospinning process,sonochemical synthesis,chemical vapor deposition (CVD),and so on.Typically,the common features of all these methods of synthesis are the need of heat treatments (high or low temperature)and/or organic groups (surfactants,organic additives,templates,etc.).2.1.

Sol–gel method

Undoubtedly,the sol–gel technique is one of the most well-known soft solution processes,which can be generally divided into three types:sol–gel route based on the hydrolysis and

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condensation of molecular precursors,gelation route based on concentration of aqueous solutions containing metal-chelates (called ‘‘chelate gel’’route),and polymerizable complex route (named Pechini-type sol–gel process).79–81Commonly,silicon alkoxides,metal alkoxides,or organic additives [such as citric acid and polyethylene glycol (PEG)]are employed in the sol–gel process,following heat treatment to remove the organic groups produced by alcoholysis of alkoxides or organic additives.Interestingly,it is also possible to utilize some defects (such as carbon impurities and vacancies)as emission centers to get novel defect-related luminescent materials.17,79It has been reported that sol–gel-derived SiO 2-based materials,including silica gels (xerogels or aerogels),28,82porous silica,83multicomponent silica-based oxide materials,84silicate-carboxylate,17and inorganic–organic hybrids,18,85all show strong luminescence from the blue to red spectral region,17–19,27and even ultraviolet photoluminescence.83They can potentially be used as environ-mentally friendly luminescent materials without expensive or toxic metal elements as activators.

Pechini-type sol–gel process (PSG),which uses common metal salts as precursors,citric acid as a chelating ligand of metal ions and polyhydroxy alcohol as a cross-linking agent to form a polymeric resin on the molecular level,can reduce segregation of particles and ensure compositional homogene-ity.These advantages can overcome most of the di?culties and disadvantages that frequently occur in the alkoxide-based sol–gel process (high cost,fast hydrolysis rate,and so on).Pechini-type sol–gel method followed by heat treatment at moderate temperature can remove organic additives and generate a pure phase of multiple components (Scheme 1).79During this annealing process,hydrocarbon residues cannot be removed completely,which may induce carbon impurities and form other kinds of defects in the host lattice (such as the oxygen defects).It is possible to utilize these defects as emission centers to obtain novel phosphors without rare earth or transition metal ions as activators.In the past several years,our group has synthesized various kinds of highly e?cient defect-related phosphors by PSG.We found that the PSG process-derived BPO 4exhibits a weak purple-colored emission under UV irradiation caused by the carbon impurities.Doping it with alkaline earth metal and alkali metal ions 86,87or mixing with SiO 2or Al 2O 388produced a series of BPO 4-based materials that emit an e?cient bluish-white light with a quantum yield more than 31%,which is induced by carbon impurities,peroxyl radicals,and/or oxygen vacancies defects,respectively.Besides BPO 4-based phosphors,the same phenomena were also observed in Al 2O 3,89ZrO 2,43and silica/apatite composite 90prepared by PSG process (Fig.1),which

can also show e?cient,stable,and/or tunable luminescence.In addition,some titanates (CaTiO 3,SrTiO 3,and BaTiO 3)exhibiting an intense photoluminescence were also prepared by PSG method followed by calcination at 300–4001C.91,92For the sol–gel method,the annealing procedure (temperature and annealing time)is the key step in the synthesis process,which can seriously a?ect the types and quantities of the defects in the materials and their luminescent properties.2.2.

Hydro(solvo)thermal reaction

It is well known that the hydrothermal method is a typical solution-based method that exploits water or organic solvents under certain temperatures and pressures,and has received much attention.93Over the past decade,various micro-and/or nano-crystals,which exhibit many interesting shape-and size-dependent phenomena and properties,have been synthesized by the hydrothermal process and extensively investigated for both their scienti?c and technological applications.94–99Especially,the water-based systems with environmentally acceptable and relatively low cost advantages provide a relatively green chemical alternative to the preparation of various inorganic materials.38,100–102To date,di?erent defect-related luminescent

Scheme 1Basic principle of Pechini sol–gel process to synthesize phosphors (reproduced with permission from ref.79,copyright 2007,American Chemical Society).

Fig.1Representative shapes of defect-related luminescent materials obtained by Pechini-type sol–gel process.SEM images of BPO 4:6%Li +(a),Al 2O 3(b),ZrO 2(c)(reproduced with permission from ref.87,89and 43,copyright 2009,2008,and 2007,American Chemical Society),and silica/apatite composite (d)(reproduced with permission from ref.90,copyright 2011,Royal Society of Chemistry).

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materials have been successfully prepared by the simple hydro-thermal method.For instance,based on the hydrothermal treatment at designated reaction conditions (such as reaction temperature and time,chelating ligands,organic additives,etc.),a systematic study has been carried out to investigate the controlled formation of a series of apatite structure micro-/nano-crystals with environmentally friendly defect-related luminescent properties.103–106The general synthetic methodology for the apatite structures is shown in the top of Fig.2.As an example,the hydroxyapatite (HAp)phosphors via hydrothermal process [trisodium citrate as chelating ligands and hexadecyltri-methylammonium bromide (CTAB)as surfactants]with narrow size distribution and novel morphologies and architectures,such as nanorods,nanowires,nanoparticles,microspheres,micro-?owers,microsheets,and microspheres (Fig.2a–f),are of special signi?cance in understanding the growth behavior and in?uence on defect-related luminescent properties.103Moreover,there are several important key factors for ?nal shapes of HAp nano/microcrystals,which contains pH values,organic additive (CTAB),and trisodium citrate (Fig.2).107,108The metal-chelating trisodium citrate ligands can not only a?ect the morphology of the sample,but also the key factor for the defect-related luminescent properties.During the crystal growth of HAp,the trisodium citrate can slow down the nucleation and subsequent crystal growth of HAp particles and change the relative surface energy of di?erent crystal facets,which results in the formation of di?erent HAp structures.109On the other hand,the electron paramagnetic resonance (EPR)results indicates that the lumi-nescent-related paramagnetic defects (CO 2 à)can be induced by trisodium citrate during the hydrothermal process.Under the high pressure and thermal process,bond cleavages can occur in some citrate anions,which result in R–C and CO 2 à.Small amounts of CO 2 àradicals resulting from the bond cleavages are trapped by the already formed hydroxyapatite lattice or interstitial positions.The residual fragmented bonds (CO 2 à)are apparently the precursors for various centers,such as luminescent centers.

Apart from the investigation of the HAp nano/microcrystals,we also utilized a similar method to synthesize other apatite structures with defect-related luminescent properties,such as calcium ?uorapatite (FAP)nanorods,strontium hydroxyapatite (SrHAp)with various morphologies (nanorods and architecture microspheres).Fig.2g–i shows the representative morpho-logies of FAP and SrHAp products.We found that the molar ratio of metal ions and chelating ligands,reaction time,and acetone treatment are also critical to obtain the apatite structures.Especially,multifunctional strontium hydroxy-apatite nanorods with luminescent and mesoporous properties can be easily synthesized at a low temperature,and have good biocompatibility,stability,nontoxicty,and water-soluble properties.106

In addition,the defect-related ?uoride materials prepared by the solvothermal method were also reported.97,110The as-obtained alkaline earth metal ?uoride (CaF 2,SrF 2,BaF 2)nanocrystals based on the solvothermal method exhibit a narrow size distribution,and can be well dispersed in nonpolar solvents.The colloidal solutions of these three ?uoride nano-crystals dispersed in cyclohexane present a strong emission

Fig.2The general synthetic methodology for a series of apatite nano/microstructures by hydrothermal method and the typical methodologies for defect-related luminescent apatite structures by hydrothermal method.Typical TEM and SEM images of calcium hydroxyapatite (a–f)(reproduced with permission from ref.103,copyright 2009,American Chemical Society),SEM images of calcium ?uorapatite (g)(reproduced with permission from ref.105,copyright 2010,Royal Society of Chemistry),and strontium hydroxyapatite (h,i)(reproduced with permission from ref.106and 104,copyright 2010,2009,Elsevier and American Chemical Society)with multiform shapes.

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band around 400nm in the PL spectra.97In addition,a series of well-dispersed KRE 3F 10(RE =rare earth:Sm–Lu,Y)colloidal nanocrystals with a rich variety of morphologies,such as hexagonal nanoplates,nanocubes and nanoparticles have been synthesized by a facile solvothermal route.The as-prepared nanocrystals are highly crystalline and can be well dispersed in nonpolar solvents such as cyclohexane and chloroform to form stable and clear colloids,which all display self-activated luminescence.1102.3.

Hydrolysis method

It is well known that the hydrolysis method is often used to prepare hydroxide and oxide materials using alkoxides as precursors.Over the past decade,monodisperse nano-/microspheres,which exhibit tunable or size-dependent photoluminescent properties,have been synthesized by the hydrolysis method and extensively investigated.111–115Our group has reported the simple synthesis of nearly monodisperse zirconia spheres (ZrO 2)with defect-related tunable luminescent properties (Fig.3a and b)by the controlled hydrolysis of zirconium butoxide in ethanol,followed by heat treatment in air at low temperature from 300to 5001C.106,116

In addition,the well-known Sto ber method,117based on hydro-lysis of tetraethylorthosilicate (TEOS)in an alcohol medium in the presence of water and ammonia,has been extensively employed to prepare the monodisperse silica (SiO 2)spheres with di?erent sizes by a modi?ed procedure (Fig.3).These monodisperse SiO 2spheres could be further annealed at low temperature (generally less than 6001C),and exhibited bright,stable,and tunable defect-related luminescence.111,113Apart from the methodology presented by Sto ber and modi?ed Sto ber methods,other hydrolysis-related strategies have recently been developed,based mainly on reverse microemulsions using a stable and macroscopically isotropic dispersion of a surfactant and water in a hydrocarbon (Fig.3c),114,118,119or on direct micelles as templates.120,1212.4.

Electrospinning process

The electrospinning technique was developed for the synthesis of one-dimensional (1D)nanomaterials.122–128During the synthesis process,the as-synthesized 1D precursor needs to be annealed to remove the organic additives.Moreover,it is also possible to form some carbon-related impurities induced by residual organic additives and other oxygen-related defects as emission centers during the annealing process.Recently,the electrospinning method was employed to prepare multifunctional silica ?bers,which possess irregular porous structure,and display ?ber-like morphology with dimensions of several hundred nanometers in width and several millimeters in length (Fig.4).129Furthermore,the as-obtained silica ?bers exhibit an intense broad bluish emission,which might be attributed to impurities and/or defects in the silica ?bers.

In addition,Qiu and co-workers reported tunable emission of BCNO nanoparticle-embedded polymer electrospun nano-?bers.130Firstly,BCNO nanoparticles were prepared by a low-temperature method.Then,the BCNO nanoparticle-embedded polymer electrospun nano?bers were synthesized by an electrospinning process using synthesized BCNO phosphors as raw materials.Photoluminescence properties of these nano?bers indicate that the emission can be readily tuned by adjusting the C concentration in the BCNO phosphors.

Fig.3Typical SEM images for unannealed ZrO 2(a),heat-treated ZrO 2spheres (b)(reproduced with permission from ref.112,copyright 2009,American Chemical Society),and (c–f)SiO 2spheres with di?erent size (reproduced with permission from ref.114,copyright 2010,Elsevier).

Fig.4Typical SEM images of the as-formed precursor (a)and porous silica ?bers annealed at 6001C (b).The possible formation process of the porous structures in silica ?bers (c)(reproduced with permission from ref.129,copyright 2010,Wiley).

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2.5.Sonochemical synthesis

The sonochemical method has been widely used to prepare a variety of nanostructured materials.131,132During the sonication process,propagation of pressure waves is intense enough to cause the formation,growth and implosive collapse of bubbles in liquid medium.These bubbles generate localized hotspots,which exhibit extreme temperatures,pressure and high cooling rates.133Sonochemical e?ects can primarily drive a variety of chemical reactions such as oxidation,reduction,and so on.During this special process,some defects can also be formed as a result of localized hotspots.Recently,Uchino and co-workers 134synthesized luminescent silica crystals without luminescent activator ions via a sonochemical reduction route.The neutral oxygen vacancy or the F center induced by the sonochemical process might be the emission centers and give a broad visible PL band peaking at B 500nm under UV irradiation.The sonochemical reduction of TEOS by a colloidal solution of sodium performed by Gedanken’s group led to the formation of silicon.135Ultrasound-induced reduction was further applied to a system with silicon alkoxides and colloidal sodium to prepare luminescent silicon nanoparticles.2.6.

Chemical vapor deposition

The chemical vapor deposition (CVD)system is believed as an important synthesis method with several advantages,such as relatively low temperature and low cost,adaptable to a wide variety of structures,rapid growth,and direct growth on a large number of substrates.136Controlling the di?erent reaction conditions (such as substrate temperature,pressure,reactant composition,reactant pre-treatment,supply rates,etc.),a number of forms of CVD are in wide use,including atmospheric pressure CVD (APCVD),aerosol-assisted CVD (AACVD),plasma-enhanced CVD (PECVD),and so on.137–139So far,a large number of compounds,including oxides,sul?des,semiconduc-tors,carbon-based materials,etc.,have been grown by these CVD methods.140Among them,some defect-related luminescent materials can also be obtained by this kind of method.Previously,o?-stoichiometric silicon oxide (SiO x ,x o 2)thin ?lms can often be synthesized via chemical vapor deposition.141,142In addition,Becher’s group reported the ?uorescence and polarization spectroscopy of single silicon vacancy centers in heteroepitaxial nanodiamonds on iridium using microwave plasma chemical vapor deposition.143Yang and co-workers have successfully fabricated the In 2O 3–ZnO one-dimensional nanosized heterostructures constructed by In 2O 3quadrangular columns and ZnO hexagonal disks by thermal chemical vapor transport.144

Besides the above methods,multistep heat treatment,145thermal decomposition,16?ame spray pyrolysis,146microwave irradiation method,147etc.can also be employed to prepare the defect-related luminescent materials.

3.Classi?cation and optical properties

Defect-related luminescent materials can be classi?ed into

silica-based materials,phosphate systems,metal oxides,BCNO phosphors,and carbon-based materials,etc .Here we will elucidate the luminescent properties together with their

control and tuning,and emission mechanisms (solid state physics)of these materials.3.1.

Silica and silica-based materials

Earlier defect-related luminescent materials were mainly based upon silica and silica-based materials,including silicate-carboxylate,17SiO 2glass,18,22SiO 2gels,27,28SiO 2spheres,111,114organic/inorganic hybrid silicones,23–25silica nanotubes,148molecular sieves,31,149etc.150–156As the most promising materials for environmentally friendly phosphors,these kinds of luminescent materials have been the focus of research e?orts since the pioneering work of Sailor and co-workers.17,121It has been reported that a kind of white phosphor with high emissive broadband spectra (Fig.5a)(emission maximum between 450and 600nm)can be synthesized from a sol–gel precursor and a variety of organic carboxylic acid with a low heat treatment.The PL quantum yield of these materials ranged from 20%to 45%under UV (365nm)excitation with PL lifetime less than 10ns.In addition,it is worth pointing out that the bright white photoluminescence was ?rst attributed to carbon defect centers and the detailed scheme of the carbon substitutional defect for Si as the luminescent species in the lattice is shown in Fig.5b.Apart from this,silica gels have been also extensively investigated as defect-related phosphors.Among them,silica aerogels,which are well-known to show photoluminescence generally from the blue to red spectral regions,exhibited several structural defects advantageous to photoluminescent properties,such as a non-bridged oxygen hole center (NBOHC)(described as SiOH -SiO + H;this defect was connected with bands at 1.8and 1.9eV),151–153peroxy linkage (POL)(labeled as ROO ,where R represents an unspeci?ed radical or part of the structure),154and E 0center (R Si )on the basis of the basic emission characteristic band at 2.2eV in the PL spectra.152,156In

addition,Hinic

’s group has reported PL spectra measured Fig.5Optical characterization of SiO 2-based materials.Emission spectra (337nm excitation at 298K)of sol–gel-prepared SiO 2-based phosphors (a)and a scheme of the carbon substitutional defect for Si as the luminescent species in the lattice (b)(reproduced with permission from ref.17,copyright 1997,https://www.doczj.com/doc/8e13031946.html,).Emission spectra of monodisperse SiO 2spheres excited at 300nm (c)and the corresponding photographs (d)under a 365nm lamp (reproduced with permission from ref.111,copyright 2006,American Chemical Society).

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for two series of silica aerogel sintered at 10001C for di?erent time intervals.The photoluminescence band at 1.8eV for silica aerogels originates from non-bridged oxygen hole center defects,and that at 2.0eV originates from silane in the gel network.82

The earlier research of silica/silica-based luminescent materials was mainly focused on the luminescent,physical,and chemistry properties.However,the luminescent properties can also be tailored by the size,shape,and composition.For example,the spherical morphology with narrow size distribution for phosphors is good for high brightness and high resolution because high packing densities and low scattering of light can be obtained by using such phosphors.79,157–161Therefore,morphology-controlled silica-based luminescent materials have appeared over the past decades.The controlled preparation of colloidal silica was proposed for the ?rst time by Kolbe in 1956and then developed by Sto ber et al.in the late 1960s.117Based on modi?ed Sto ber procedures and following a heat treatment,Schmedake’s group has reported a novel approach to synthesize mono-disperse luminescent silica spheres.111The tunable luminescent properties of the silica spheres depend on the quantity of (aminopropyl)triethoxysilane in the precalcined material (Fig.5c).The possible luminescent mechanism was ascribed to carbon/oxygen-related defect centers resulting from calci-nation of the APTES-containing SiO 2spheres.Luminescent and porous silica ?bers prepared by the electrospinning process have also been extensively investigated.129These kinds of multifunctional silica ?bers showed a strong self-activated photoluminescence ranging from 300to 600nm (centered at 405nm),which might be caused by impurities and/or defects in the silica lattice.Moreover,molecular sieves with porous structure (such as MCM-4131and MCM-4832)also show excellent and tunable luminescent properties in the visible region.Most recently,as particularly interesting nanomaterials,luminescent silica nanoparicles were developed by Davies et al.based on the aqueous sol–gel technique.78The as-obtained silica nanomaterials without metal activators or traditional dyes displayed a broad white emission.They also proposed that the source of emission was from the formation of hydrocarbon-based defects during synthesis,which originated from the alkoxysilane used to prepare the nanostructures.

Due to the good optical properties,high chemical stability,and nontoxicity,the applications of these multifunctional SiO 2-based materials have extended in biomedicine ?elds.Therefore,innovative research is ongoing in the silica-based defect-related phosphors.Clearly,we cannot exhaustively cover this ?eld,so our e?orts here are to highlight the recent studies.In the above silica-based materials,a summary description of general properties and luminescent mechanisms for silica-based materials is listed in Table 1.3.2.

Phosphate system

3.2.1.BPO 4-related system.The crystal structure of BPO 4is built by sharing the corner of the BO 4and PO 4tetrahedra,leading to a continuous three-dimensional framework,which is isomorphous with silica oxides.This information told us that BPO 4-based materials might be another kind of defect-related phosphor.Our group ?rstly reported the luminescent properties of BPO 4-based materials prepared by PSG method.86–88This

PSG-derived BPO 4can show a weak purple emission under UV irradiation,which may be caused by the carbon impurities in the host lattice (Fig.6a and e).In further study,when a slight amount of alkali metal or alkaline earth metal ions (Sr 2+,Ca 2+,Mg 2+,Ba 2+Li +,Na +,or K +,)were doped into the BPO 4host lattice,the luminescent intensities can be enhanced (Fig.6a,b and 7A),which are due to the oxygen-related defects (peroxyl radical defects or oxygen vacancy)(Fig.6d).86,87As structural disturbance of BPO 4by dopants can create more defect centers in BPO 4-based systems,especially BPO 4/Ba 2+and BPO 4/Li +,leading to be the most e?cient phosphors (Fig.6e and 7A).The energy band diagram of the possible defects and emission process in sol–gel-derived BPO 4and BPO 4/Ba 2+was also investigated (Fig.6f and 7D).The possible luminescent mechanisms (carbon impurities,peroxyl radical defects,or oxygen vacancy)for these kind of phosphors were ?rst proposed in BPO 4-based materials.In addition,the BPO 4–x SiO 2(x :SiO 2/BPO 4molar ratio,0–70%)and BPO 4–x Al 2O 3(x :Al 2O 3/BPO 4molar ratio,0–20%)hybrid systems prepared by the PSG process are also stable,e?cient,and defect-related luminescent materials.88Further-more,the doping/mixing concentration,annealing tempera-ture,and the amount of additives in the precursors also have a great e?ect on the defect-related luminescent properties of BPO 4-based materials (Fig.7B and C).And the band struc-tures of defective BPO 4,which were employed to further investigate the defect-related luminescent mechanisms,were calculated using CASTEP code based on density-function theory (DFT).In this calculation,the structure models of ideal and defective [interstitial carbon (C i )and oxygen vacancy (V O )]BPO 4are established (Fig.7D).The band gaps for ideal and defective BPO 4are calculated based on the corresponding models (Fig.7D).The calculated results indicated that both the C i and V O can cause great change in the band structure (size of band gap,site of conduction band and valence band,and band structure),which can well explain the luminescence phenomenon and the proposed mechanisms.Therefore,the analysis of the band structure can o?er a valuable alternative to investigate the luminescent mechanisms of defect-related luminescent materials.

3.2.2.Apatite-structured materials.It is well known that apatite-structured materials,including HAp,SrHAp,FAP,etc.,are regarded as a typical class of biological materials due to their super-biological stability,biocompatibility,and a?nity.Obviously,these kinds of nano/micromaterials with excellent ?uorescence may serve as ideal candidates for biomedicine applications,such as cancer diagnosis,drug delivery,and so on.Therefore,they have attracted widespread attention.In earlier research,the luminescence of apatite struc-ture materials is usually realized by depositing optically active rare-earth-doped nanophosphors (i.e.,Y 2O 3:Eu 3+nanoparticles)on the surface or doping the materials with lanthanide/transition metal ions.56,127,162,163However,the lanthanide/transition metal ions are often expensive and non-environmentally friendly ele-ments,which may have toxic e?ects.

In the past few years,much e?ort has been put into the investigation of defect-related luminescent properties in apatite materials.Defect-related apatite materials have been successfully

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synthesized by a facile hydrothermal process.The as-obtained samples showed strong blue emission with controllable morphologies and structures (Fig.8).103–106Here we take the preparation and properties of HAp and SrHAp phosphors as representative examples to illustrate the advantages of the defect-related apatite materials.The hydrothermal-derived HAp samples with trisodium citrate in the precursor can show an intense and bright blue emission under long-wavelength UV light excitation (341nm).This emission spectrum consists of a strong broad band ranging from 350to 570nm with a max-imum at 414–434nm (Fig.8a).103The corresponding excitation

spectrum of the HAp burr-like microspheres includes two broad bands:a weak band from 200to 280nm and a very strong and broadband from 280to 420nm (Fig.8b),which can be excited by a wide range of UV light.It is presumed that CO 2 àradical impurities in the crystal lattice may play a key role for the luminescent properties,based on the results of the analysis and experiments such as Fourier transform infrared (FT-IR)spectra,EPR,and control experiments.These defect centers induce an electron localized in the 2p orbital of the single bonded carbon,which can give rise to photoluminescence through a strong electron-photon coupling.43,88

Table 1Summary comparison of the defect-related silica-based luminescent materials

Materials

Fabrication route Excitation Emission

Possible mechanism

Ref.Silicate-carboxylate

Sol–gel method

365nm

Broad and maximum between 450and 600nm

Carbon substitutional defect 17SiO 2glass

Sol–gel method UV excitation at 337.1nm Broad emission,maxima at 420and 520nm (5001C)Defects of oxygen de?ciencies

(420nm),radical carbonyl

defects (520nm)

18Peaking at 360and 225nm Entire visible spectrum,peaking at 425and 500nm Chemical bond cleavage and

resultant carbon formation

and/or non-stoichiometry

22Vapor phase axial deposition Absorption at 3.8eV (oxygen plasma process) 1.9eV luminescence NBOHC (1.9eV PL)and peroxy linkage (3.8eV)154SiO 2gels

Sol–gel method 355nm Blue luminescence O 2àspecies 27Broad band peaking at 363,236,243nm

Broad band (360-600nm)Carbon impurity mechanism 28SiO 2spheres Similar to Sto ber

method 300nm Maximum red-shifted with increasing APTES concentration

Carbon/oxygen-related defect center

111A water/oil microemulsion method and Sto ber method

Broad band ranging from 280to 400nm Broad emission and maximum between 385and 470nm,Carbon impurities and oxygen defects 114Organic/inorganic hybrid silicones Sol–gel method 330,350,365,395,365,400,and 420nm Emission spectra are excitation wavelength dependent Emission originated in the NH (NH 2)groups of the

urea or urethane bridges with

electron–hole recombinations

23

Broad band,depending on monitored emission wavelengths

Broad band between 350and 680nm NH-related emission 24Broad band between 315and 450nm,recorded at 462nm Broad and red shift with the decrease of excitation energy

Recombination electron–hole mechanisms 25SiO 2nanotubes

Reverse-microemulsion mediated sol–gel method 350nm Three emission bands at 410,435(maximum),460nm Intrinsic diamagnetic centers (O–Si–O,violet emission)and

neutral oxygen vacancy

(R Si–Si R ,blue emission)

148

Molecular sieves Siliceous synthesis gel 350,300.250,and 200nm 420,473,and 620nm Oxygen-related defect sites

31Hydrothermal process and following heat treatment

365nm Blue emission Oxygen-related defects (R Si–O )in MCM-41series 149SiO 2nanowires Heating method Two excitation bands at 260(4.77eV)and 368nm (3.37eV)Broad PL band at 443nm (2.8eV)with a shoulder at 413nm (3.0eV)

Defects

150Silica ?bers Electrospinning process A double broad band (200–370nm)peaking at 255and 355nm PL emission ranging from 300to 600nm and maximum at 405nm

Impurities and/or defects in the

silica lattice 129Silica nanoparticles Sol–gel technique 457nm Broad white emission Hydrocarbon-based defects

78g -Irradiated a -SiO 2Plasma CVD method

Excited by 2–4eV photons Two bands at 1.9eV and 2.2eV Nonbridging oxygen hole centers (1.9eV PL)and

E 0centers (2.2eV PL)

152Silica aerogels Two-step acid-base catalyzed process 300–400nm Emission wavelength ranges from 460–500nm Oxygen defect centers in the

aerogel matrix 155Calcination Excited by a 488nm (2.54eV)Ar ion laser Three PL bands at 1.8,2.0and 2.2eV Nonbridged oxygen hole center

defect (1.8eV)and silane in gel

network (2.0)

82Silicon-rich

oxynitride

(SiO x N y :H)

Plasma-enhanced chemical vapor deposition Excited in the range of 2.4–3.8eV A broad tunable PL emission Silicon suboxide bonding (E 0centers)156

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Upon further investigation of the luminescent properties,it can be found that HAp samples with di?erent morphologies

show di?erent emission intensities (Fig.8a).Among them,HAp microspheres show the strongest blue emission and HAp nanocrystals exhibit the weakest luminescence.The as-obtained HAp microspheres have higher crystallinity,and spherical morphology is good for high brightness and high resolution because of high packing densities and low scattering of light.79Based on the above situation and consideration,other important apatite structures of FAP nanomaterials 105and SrHAp samples with di?erent size and morphologies were also obtained.The luminescent properties of the SrHAp can be tuned by the reaction time and the molar ratio of metal ions and chelating ligands.Normally,the photoluminescence in-tensity ?rst increased with increasing the reaction time,reached a maximum at a certain time,and then nearly kept the maximum with further increase of reaction time (Fig.8c).This might be due to the improvement of the crystallinity.104On the other hand,the photoluminescence intensity varied with the change of the molar ratio of metal ions and trisodium citrate (Fig.8d).At a high molar ratio,the emission intensity decreased greatly,which may be caused by the quenching e?ect of the luminescent centers.This luminescence study can con?rm the key role of trisodium citrate on the luminescence properties.106The comparison of luminescent properties and mechanisms of these defect-related phosphate phosphors are summarized in Table 2.3.3.

Metal oxides

Metal oxides now represent the most mature luminescent materials,which can be classi?ed into two major categories:metal activator-doped metal oxide phosphors (e.g.Y 2O 3:Eu 3+)

Fig.6(a,b)Emission spectra for BPO 4and BPO 4doped with alkaline earth metal ions [SG =sol–gel;SG/CH 3COO à=sol–gel with CH 3COONH 4instead of Ba(CH 3COO)2as dopant;SS =solid-state reaction using H 3BO 3and (NH 4)2HPO 4].(c)Scheme of excita-tion and emission process induced by C impurities in BPO 4.(d)Scheme of the production of peroxyl radicals in BPO 4by Ba 2+.(e)Photographs of the sol–gel-derived BPO 4and BPO 4/6mol%Ba 2+phosphor pellets under a UV (365nm)lamp in the dark.(f)Energy band diagram showing the possible defects and emission process in BPO 4and BPO 4/6mol%Ba 2+(reproduced with permission from ref.86,copyright 2006,American Chemical Society).

Fig.7(A,B,C)Luminescent properties of the BPO 4-based materials a?ected by dopants,concentration,and temperature.(D)Structure models of BPO 4:ideal (a),interstitial carbon (c),oxygen vacancy (e),and the calculated band structures according to models of ideal BPO 4(b),interstitial carbon-defective BPO 4(d),and oxygen vacancy defective BPO 4(f)(reproduced with permission from ref.87and 88,copyright 2009,2008,American Chemical Society).

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and defect-related materials.For the defect-related metal oxide phosphors,the species,synthesis,properties,and lumi-nescent mechanisms are classi?ed and listed in Table 3.

3.3.1.Zirconia.Zirconia (ZrO 2)is an important ceramic material and widely used in high-performance ceramics,catalysts,cosmetics,and new emergent applications of high-temperature fuel cells and bioceramics (such as dental prostheses).Along with the further investigation on phosphors,some attention has been paid to the luminescence properties of ZrO 2.164During the early study,Li and co-workers reported that tetragonal ZrO 2nano-particles synthesized by a microwave irradiation method showed sharp emission peaks at 402,420,and 459nm under 254nm UV excitation and a broad-band emission at 608nm under 412nm UV excitation.147The ZrO 2nanocrystals prepared by a two-phase process presented by Zhao et al.165exhibited a broad-band emission with a maximum at 365nm under UV excitation (250nm).These reports indicated that the luminescence intensity of ZrO 2nanocrystals seemed to depend strongly on the prepara-tion method.Moreover,the luminescent mechanisms need to be further investigated and clari?ed.

In a further study,Lin and co-workers 43have found that PSG-derived nanocrystalline ZrO 2powders annealed at 5001C and crystallized as tetragonal could exhibit white-blue emission centered at 425nm under a wide range of UV light excitation,which is suitably excited by di?erent light sources.By further increasing the annealing temperature,the phase transformation of ZrO 2from tetragonal to monoclinic occurs.The monoclinic ZrO 2samples could give an intense blue-green emission (470nm)after being annealed at 10001C (Fig.9A).This result con?rmed that the phase of the phosphor can a?ect the emission color and/or luminescent intensity.Moreover,the detailed study indicated that the whitish blue and blue-green emission bands can be ascribed to interstitial carbon defects (C i )in the tetragonal ZrO 2and oxygen defects in the mono-clinic ZrO 2,respectively.43The PSG-derived ZrO 2presented

Table 2Summary comparison of the defect-related phosphate-system luminescent materials

Materials Fabrication route Excitation

Emission

Possible mechanism Ref.BPO 4/Ba 2+(Sr 2+,Ca 2+,and Mg 2+)PSG process Broad band (220–450nm)with maximum at 310nm Bluish-white emission band peaking from 416to 451nm Carbon impurities and oxygen-related defects 86Alkali metal ion-doped BPO 4PSG process Broad band (220–400nm)peaking at 311and 375nm

Strong emission (360–570nm,BPO 4:6%Li +maximum at 416nm)

Carbon impurities and

oxygen vacancy (V

O

center)87BPO 4–SiO 2–Al 2O 3mixture

PSG process BPO 4–x SiO 2One broad band with a maximum at 313nm Emission band centered at 428nm,SiO 2concentration

and annealing temperature dependent

Carbon impurities and oxygen vacancy (V

O

center)88

PSG process BPO 4–x Al 2O 3

Two broad bands peaking at 308and 372nm Emission band centered at 413nm,Al 2O 3concentration

and annealing temperature dependent

Carbon impurities and oxygen vacancy (V

O center)88

Hydroxyapatite Ca 5(PO 4)3OH nano/microcrystals Hydrothermal process A weak band (200–280nm)and a strong broadband (280–420nm)

Blue emission (350–570nm)and morphologies dependent Carbon dioxide radical

(CO 2 à)

103Strontium hydroxyapatite Hydrothermal process (microsphere)

Long-wavelength UV light excitation (344nm).Blue emission centered at 427nm

Carbon dioxide radical (CO 2 à)

104Hydrothermal process (nanorods)Two broad bands peaking at 254and 345nm Blue emission and dependent on cumulatively released

amount of drug

Carbon dioxide radical (CO 2 à)

106Mesoporous silica/apatite composite Modi?ed Pechini sol–gel process

Two bands centered at 280and 330nm Blue luminescence (maximum at 410nm)Carbon dioxide radical (CO 2 à)

90Calcium ?uorapatite Hydrothermal process

Broad excitation band (200–400nm)

Blue emission Ca 5(PO 4)3F Carbon dioxide radical (CO 2 à)

105

White emission color for Ce 3+,Mn 2+-codoped Ca 5(PO 4)3F excited by 308nm UV light

Carbon dioxide radical (CO 2 à)and Ce 3+,Mn 2+activators emission

Fig.8(a)PL emission spectra of hydrothermal-derived HAp samples (S1:nanorods and nanoparticles obtained at pH =9.0,S2:nanorods obtained at pH =7.0,S3:burr-like microspheres obtained at pH =5.0,S4:micro?owers obtained at pH =4.5,and S5:microsheets obtained at pH =4.0).(b)PL excitation for HAp sample S3(reproduced with permission from ref.103,copyright 2009,American Chemical Society).(c)Emission spectra of SrHAp samples obtained from di?erent reaction times (reproduced with permission from ref.104,copyright 2009,American Chemical Society).(d)Emission spectra for the SrHAp samples prepared with di?erent molar ratios of Cit 3à/Sr 2+(reproduced with permission from ref.106,copyright 2010,Elsevier).

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strong and bright emission,however the high annealing temperature caused serious aggregation.Therefore,to avoid this situation,other methods to control the synthesis of ZrO 2luminescent materials were explored.Well-dispersed ZrO 2spheres with a narrow size distribution were successfully prepared by the controlled hydrolysis of zirconium butoxide in ethanol.With a subsequent heat treatment in air at low temperature (300–5001C),the annealed ZrO 2spheres exhibited broad,intense,and visible photoluminescence (Fig.9B).112More-over,the emission colors of the ZrO 2spheres can be tuned from blue to nearly white to dark orange by varying the annealing temperature.Based on the spectral analysis,luminescence lifetimes,and EPR results,the luminescent centers might be attributed to the carbon impurities in the ZrO 2lattice.And the concentration of the carbon impurities is closely related with the annealing temperature and the amount of the organic additive.

Based on these researches,defect-related luminescent properties of ZrO 2and ZrO 2-based phosphors can be in?uenced by synthesis method,species of defect,phase,and surface/interface properties.By controlling these factors,many defect-related ZrO 2phosphors with di?erent emission color or intensity could be achieved,which enriches the inorganic luminescent materials and provides novel phosphors as alternatives to traditional phosphors.

3.3.2.Alumina.Cai et al.and Chen et al.found photo-luminescence properties in porous alumina membranes.166,167Zhang et al.reported a blue luminescence from amorphous Al 2O 3nanoparticles suspended in toluene solution through ultrasonic treatment of a porous anodic alumina membrane.168From an earlier study,it was found that the intense blue emission of as-obtained nanoparticles strongly depended on the particle size and chosen solvent.Moreover,a mixed mechanism combining the surface bonding states with widened bandgaps of alumina nanoparticles by the quantum con?nement e?ect is presented.168In a follow-up study,our group also demonstrated the amorphous Al 2O 3powder samples prepared by the PSG process and annealed at 500and 6001C exhibited bright bluish-white emission centered at 430and 407nm,respectively.89The most probable luminescent center was ascribed to radical carbonyl defects such as R Al–O–C Q O on the pore surface.

Besides the defect-related Al 2O 3phosphors,the Al 2O 3–SiO 2porous glasses with high luminescence quantum e?ciencies have also been well investigated by Hayakawa et al.Further investigation of Al 2O 3–SiO 2porous glasses indicated that the Al 2O 3content could a?ect the luminescent properties.18More recently,Raubach and co-workers reported interfacial photo-luminescence emission properties of core/shell Al 2O 3/ZrO 2.In their research work,the general aspect of the spectra is a broad band ranging from 350to 620nm (maximum at 455nm),which covered a large part of the visible light.A possible mechanism was ascribed to the multi-photonic process based on the recombination of excited electrons and holes.Interestingly,PL spectra (l ex =350.7nm)also revealed that the e?ect of the zirconia coating on the alumina was more e?ective for a -Al 2O 3than for g -Al 2O 3,which may be related to the process of coating zirconia on alumina.169

3.3.3.Rare earth oxides.Rare earth oxides have been extensively used in high-performance luminescent devices,magnets,catalysts,and other functional materials because of their electronic,optical,and chemical characteristics resulting from the 4f electronic shells.However,most rare earth oxide phosphors are metal activator-doped materials,for example traditional red phosphor Y 2O 3:Eu 3+.Research on defect-related rare earth oxide phosphors could decrease the amount of rare earth ions,particularly very expensive rare earth activators (such as Eu 3+,Tb 3+,Er 3+,Dy 3+,etc.),which would greatly reduce the cost and increase safety for the environment and people’s health.As a result,defect-related luminescent materials without metal activators have attracted much attention.

Lin and co-workers have successfully synthesized Y 2O 3powders via the sol–gel process using glycerol and polyethyl-eneglycol as additives,followed by low-temperature treatment.170The as-prepared Y 2O 3sample shows intense bluish-white emission (350–600nm)with a quantum yield as high as 64.6%under a wide range of UV light excitation (235–400nm).These luminescent properties were caused by carbon impurities in the Y 2O 3host.In addition,highly uniform and well-dispersed CeO 2nanocrystals (3.5nm in diameter)were prepared by a nonhydrolytic solution route.171This kind of undoped CeO 2can show a weak emission band with a maximum at 501nm.The emission band is not due to the characteristic emission of the lanthanide ions but might arise from the oxygen vacancy which is introduced in the ?uorite lattice of the CeO 2nanocrystals to compensate the e?ective negative charge associated with the trivalent ions.In a further study,Tanner and Wang 172presented the intense and white upcon-version luminescence from several rare earth oxides (Ln 2O 3,Ln =Sm,Tm,Yb,and CeO 2)excited by a 808nm laser diode,which is unlike the upconversion spectra of lanthanide-ion activators.The color purity is excellent for the prepared oxides.Among them,the coordinates for neat Tm 2O 3and Yb 2O 3are close to those for pure white light.Moreover,they speculated that the mechanism involved thermally induced production of free carriers.

3.3.

4.ZnO,SnO 2,and In 2O 3.As a wide-band-gap semi-conductor,zinc oxide (ZnO)has been known as a luminescent material for a long time,and it is of great interest for photonic applications such as UV luminescent devices,low threshold

Fig.9(A)Photographs under UV light from mercury lamp in a dark room and corresponding emission spectra of the ZrO 2crystals annealed at di?erent temperature (ZOSG:ZrO 2via sol–gel process;ZOSS:ZrO 2via the solid-state reaction).(B)Luminescence photo-graphs under a 365nm UV lamp and emission spectra of ZrO 2annealed at di?erent temperature (reproduced with permission from ref.38and 112,copyright 2007and 2009,American Chemical Society).

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lasers,nonlinear optical devices and so forth.173–175ZnO is a famous

defect-related phosphor which has good characteristics such as low outgassing,high electric conductivity (B 10à3O cm à1),low

excitation voltage,and high stability,176and has been exten-sively used as a bright green phosphor.177Besides the green emission,violet,178–180yellow,181,182and orange-red 183–185

Table 3Summary of the defect-related oxide luminescent materials

Representative examples Synthetic method

Excitation

Emission

Possible mechanism Ref.ZrO 2

Nanoparticles Microwave irradiation method

254and 412nm Sharp emission excited by 254nm and broad-band emission excited by 412nm Surface defects and oxygen vacancies 147Colloidal nanocrystals Two-phase process 250nm Broad-band with

a maximum at 365nm

A mid-gap trap state induced by surface passivation.

165Nanocrystals

PSG process

302nm White-blue emission (tetragonal phase)Interstitial carbon defects

43

280nm

Blue-green emission (monoclinic phase)Oxygen defects

Spheres

Hydrolysis and followed by a heat treatment

365nm

Tunable emission (blue-white-dark orange)by varying the annealing temperature

Carbon impurities 112

Al 2O 3–ZrO 2Core–shell Polymeric precursor method

350.7nm Broad band (350–620nm)Recombination of

excited electrons and holes

169Al 2O 3

Porous membrane

Electrochemical method

360nm Blue PL band (400–600nm)Singly ionized oxygen

vacancies (F +centers)

166Two-step anodizing process

200–400nm Blue-emitting bands (420–600nm)peaking at 452nm

Oxygen defects and oxalic impurities 167Nanoparticles Ultrasonic treatment 330nm Broad band and a blue-shift as the decrease of particle sizes Surface bonding energy levels and the oxygen defect levels 168Porous Al 2O 3

PSG process Broad band (235–425nm)Bluish–white emission (350–600nm)Carbon-related defect (radical carbonyl defect)89Al 2O 3–SiO 2Porous glasses Sol–gel method 337.1nm White light emission peaking at 420and 520nm Oxygen defects and radical carbonyl-terminations

18Y 2O 3

PSG process 235–400nm Bluish–white emission (350–600nm)

Carbon impurities 170Calcination

808nm

Upconversion white luminescence Thermally induced production of free carriers 171CeO 2Nanocrystals

Non-hydrolytic solution route Broad band (280–420nm)

Emission band with a maximum at 501nm Oxygen vacancy 172ZnO

Polyol method

followed by annealing,electrodeposition,chemical solution deposition,two-step process,molecular-beam epitaxy,or sol–gel process,etc.

UV light excitation for PL spectra or low-voltage electron beam excitation for cathodoluminescent spectra (CL)

Bright green luminescent Interstitial oxygen ion

(O i à),ionized oxygen vacancies (V O +),or Zn vacancy

176,

177,182Violet luminescence Defects associated with

zinc or oxygen vacancies or vacancy related complexes

178,179,180Yellow emission Oxygen vacancy O i à181,182Orange-red emission Oxygen interstitials 183,184,185SnO 2Nanorods Hydrothermal method 295nm Broad emission (350–550nm)Oxygen vacancies 187Nanowires Vapor-liquid-solid process

Excitation power (0.5–10mW)Ultraviolet

photoluminescence

Defect states

190Nanoparticles Sol–gel method 300nm Emission band centered at 400nm

Oxygen vacancy (V O

)191Hierarchical nanostructures

Multistep thermal vapor deposition

325nm

Blue–violet and yellow bands centered at 432and 539nm

Oxygen vacancies and structural defects 192In 2O 3In 2O 3/SiO 2Soaking and thermal-decomposition 275and 250nm Ultraviolet and visible emissions

Sulfur impurities and quantum size e?ect 194Nanocubes Hydrothermal treatment

380nm Blue-green emission centered at 450nm Oxygen vacancies 195Nano?bers Solvothermal reaction 325nm

Three emission peaks at 378,398,and 420nm Oxygen vacancies 196Nanorod bundles and spheres Hydrothermal process 300–425nm with a maximum at 383nm Blue emission peaking at 416and 439nm

Oxygen vacancies 197Nanocrystals Solvothermal method 234nm

Blue emission centered at 423nm and size dependent

Weak quantum-con?nement-e?ects

198

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emissions have been also observed and investigated.Since these emissions are located in a wide range of the visible spectrum,great e?orts have been made to tailor these visible emissions.186As a representative example,ZnO and ZnO:Zn phosphors have been successfully synthesized by the polyol-method followed by annealing in air and reducing gas,respec-tively,which showed tunable PL and cathodoluminescent (CL)properties.176In this research,the emission spectra of ZnO powders annealed at 9501C in air contained a weak ultraviolet emission band at 390nm and a broad emission band centered at about 527nm,exhibiting yellow emission (Fig.10a).However,when the sample was annealed in redu-cing conditions at temperatures from 500to 10501C,the yellow emission decreased gradually and disappeared completely at 8001C,whereas the ultraviolet emission band became the strongest.Above this temperature,the green emission (centered at 500nm)appeared and increased with increasing reducing temperatures (Fig.10b).Experimental results indicated that a singly ionized oxygen vacancy existed in the ZnO and ZnO:Zn powders (Fig.10c).Based on the analysis,the yellow and green emissions may arise from the transitions of photogenerated electrons close to the conduc-tion band to the deeply trapped hole in the single negatively charged interstitial oxygen ion (O i à)and the single ionized oxygen vacancy (V O )centers,respectively.176A schematic diagram of the radiative transitions responsible for emissions of these kinds of ZnO-based phosphors is shown in Fig.10d.Tin oxide (SnO 2)is another important metal oxide,which has an n-type wide band gap (3.6eV at 300K)but is dipole forbidden due to its special wave function symmetry.187–189Furthermore,the exciton binding energy of SnO 2is very large,which suggests e?cient exciton emission at room temperature and even at higher temperatures.From the viewpoint of fundamental physics,190considerable research has been focused on the exploration of novel properties of SnO 2nano-materials

(nanoparticles,191nanowires,190nanorods,187hierarchical nanostructures,192etc.),especially the photoluminescent properties.Photoluminescence of SnO 2materials,generally observed in the UV and/or visible region (350–550nm),may reveal the presence of crystalline defects (such as oxygen vacancies and tin interstitial or dangling bonds)resulting from various synthesis processes.187,190–192Among them,oxygen vacancies have been assumed to be the most likely candidates for the recombination centers in emission processes for SnO 2samples.Lu et al.presented the oxygen-vacancy-related photolumines-cence of SnO 2nanoparticles and the origin of the emission was assigned to the recombination of electrons in a conduction

band with holes in the V

O center.191Another research group produced defect-related luminescent SnO 2nanowires and proposed a di?erent luminescent mechanism,which was iden-ti?ed:recombination of donor–acceptor pairs,excitons bound to neutral and ionized donor impurities,and optical transi-tions from free electrons to neutral acceptor impurities in the lattice.190In addition,Patra and co-workers have investigated the surface-defect-related luminescent properties of SnO 2nanorods and nanoparticles.187This research work suggests that the luminescent properties can strongly depend on the shape.In conclusion,the emission of SnO 2nanocrystals is due to a transition of an electron from a level close to the conduction

band edge to a deeply trapped hole (V

O )in the bulk of the SnO 2nanocrystal and the surface-related defects are more prominent in smaller nanocrystals than in nanorods.

Similarly,indium oxide (In 2O 3)is also a very important wide-band-gap (direct band gap around 3.6eV)and n-type transparent semiconductor due to its oxygen-e?cient ?uorite structure.193In recent years,fabrication of In 2O 3has received much interest,such as luminescent spectra peaking at 480and 520nm from In 2O 3nanoparticles reported by Zhou et al.,194450nm from In 2O 3nanocubes 195and 378,398,and 420nm from In 2O 3nano?bers reported by Qian et al .196The emission from oxide semiconductors is mainly attributed to the oxygen vacancies.In addition,Yang et al.197have well investigated the synthesis and luminescent properties of the In 2O 3nano/micro-structures with two di?erent morphologies (nanorod bundles and caddice sphere-like agglomerates).In 2O 3nanorod bundles and spheres showed similar blue emission peaking around 416and 439nm under 383nm UV excitation.Moreover,Fang et al.198also reported highly uniform and quasi-monodisperse In 2O 3nanocrystals,which gave a relatively strong emission centered at 423nm and a shoulder located at about 392nm with weak quantum con?nement e?ects at room temperature.All the research work suggests that the indium oxide is also an important alternative defect-related luminescent material.According to the existing investigations,in summary,some low-toxic and wide-band-gap semiconductor oxides can potentially be used as e?cient and defect-related luminescent materials.Detailed information and representative research is listed in Table 3.3.4.

BCNO phosphors

Recently,oxynitride and nitride compounds have emerged as promising environmentally friendly phosphor candidates because of their excellent properties of non-toxicity,outstanding thermal

Fig.10(a)PL spectra (inset:photographs under irradiation of a 365nm UV lamp),(b)CL spectra,and (c)EPR curves of ZnO-based phosphors.(d)Schematic diagram of the energy band of a ZnO-based particle in cross section (Ec:conduction band,Ev:valence band,Eg:band gap)(reproduced with permission from ref.176,copyright 2006,American Chemical Society).

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and chemical stability,and high luminescence e?ciency.26,199In previous work,BCN compounds were expected to behave as semiconductors with bandgap energies that could be tunable by varying the atomic composition.Therefore,this kind of materials was considered to be intermediate between graphite and hexagonal-BN (h-BN).200However,it was very di?cult to improve the light-emitting properties of this BCN phosphor,which had only single emission peaks in the PL and showed low PL intensity and quantum e?ciency.Most recently,Okuyama and co-workers reported a simple and useful synthetic route for BCNO phosphor with high quantum e?ciency (as high as 79%)and wavelength-tunable emission ranging from 387to 571nm.26,199,201–203The experimental results indicated that this phosphor was polycrystalline in nature and composed of several distinct nanocrystals.More-over,higher PL intensity and quantum e?ciency have been achieved by controlling the carbon content,PEG/B ratio in the precursor solution,and reaction time during the preparation of BCNO phosphors (Fig.11A).199This result showed that carbon atom impurities were able to substitute for boron or nitrogen atoms in the t -BN crystallites with increased carbon concentration.As a result,the allowed electronic transitions in the matrix were modi?ed and the states generated by carbon were changed,that is to say the variations of the BCNO band gap results from changes in the chemical composition of the BCNO crystals.Furthermore,the variations of band gap can cause shifts of their emission spectra and di?erent emission colors.199,201–203

Another research group of Qiu et al.also fabricated BCNO-based phosphors and well investigated their photolumines-cence properties.204Some of their research work focused on BCNO-nanoparticle-embedded polymer electrospun nano?-bers prepared by an electrospinning technique.These nano?-bers showed a wide band emission in the visible region,which originated from the defect-related BCNO nanocrystals and can be changed by adjusting the C content during the pre-paration of BCNO phosphors.In addition,they also assumed the paramagnetic defect centre originated from electrons trapped at nitrogen vacancies,and this defect was responsible for the observed luminescence of the BCNO nanocrystals.204In a further study,Qiu and co-workers also reported a BCNO-based long-persistence phosphor,205,206which shows a phosphorescence peak centered at 520nm.Its afterglow decay curve ?tting t à0.93indicated a relatively slow decay rate (Fig.11B).Moreover,they pointed out that this defect-related long-persistence luminescent BCNO phosphor was competitive as a commercial long-persistence phosphor (e.g.,SrAl 2O 4:Eu 2+,Dy 3+)207for appli-cation in many ?elds due to its excellent luminescent properties,low cost,and low toxicity.3.5.

Titanates

ATiO 3(A =Ca,Sr,Ba,Pd,etc.)-type perovskites,which have an energy band gap of about 3.0eV,have recently attracted a good deal of attention due to their ferroelectric and electro-optical properties.208–211Pizani et al.reported a series of disordered (amorphous)BaTiO 3,PbTiO 3,and SrTiO 3materials prepared by a sol–gel method.91These three materials displayed a similar broad emission band in the visible region under UV

excitation.The origin of the photoluminescence was not exactly identi?ed.However,the experimental results indicated that it could be related to the disordered perovskite structure.In addition,they also synthesized amorphous luminescent PbTiO 3thin ?lms.92Lazaro and co-workers produced ordered-disordered CaTiO 3with photoluminescence prepared by a poly-meric precursor method and annealed at 450–6001C for 2h.212The authors proposed that this photoluminescence in CaTiO 3was a?ected not only by disorder in the lattice former but also by structural disorder in the lattice modi?er.Pontes et al.also reported the role of network modi?ers in the creation of photo-luminescence in CaTiO 3.213

Takahashi et al.produced orange luminescent barium tita-nosilicate BaTiSi 2O 7by a conventional solid-state reaction.214This product displayed visible orange luminescence.Corre-spondingly,the broad PL emission with a peak at 580nm was observed by ultraviolet excitation at 321nm.The authors further suggested that this luminescence property was related

Fig.11(A)Photographs and corresponding emission spectra excited at 365nm of BCNO samples prepared under various conditions of PRG/B ratio,temperature,and heating time [(a)2.0?10à3,9001C,30min;(b)2.0?10à3,8001C,30min;(c)4.0?10à3,7001C,60min;(d)4.0?10à3,7001C,45min;(e)4.4?10à3,7001C,30min](reproduced with permission from ref.199,copyright 2008,Wiley).(B)Afterglow decays of BCNO-1to BCNO-6recorded at the respective peaking wavelength after termination of irradiation with an UV lamp for 10min (starting materials with ?xed atomic ratios of B/C =2/3and B/N =1/7,1/6,1/5,1/4,1/3for BCNO-1to BCNO-5,and B/C =1/2,B/N =1/3for BCNO-6,respectively)(reproduced with permission from ref.206,copyright 2009,IOP Science).

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to both the absolutely isolated pyramidal TiO 5unit pair and the oxygen defect in the TiO 5unit in the BaTiSi 2O 7phase.Besides the above-mentioned titanates,a strong blue emission band at 480nm has been observed in nano-sized La 0.5Na 0.5TiO 3,reported by Zhang.215In this report,the emission band is closely related to the electron transitions between the conduction band and the defect levels,between the defect levels and valence band,as well as between the defect levels.Interestingly,a emission pattern very similar to that of ATiO 3was also observed,and the independence of the emission band on the preparation method of the material was revealed.3.6.

Fluoride materials

Fluoride materials have been studied for a very long time because they have a wide range of potential optical applica-tions based on their low energy phonons and high ionicity.216In the past few years,our group has successfully synthesized well-dispersed ?uoride nanocrystals,including alkaline earth metal ?uorides (CaF 2,SrF 2,BaF 2)97and KRE 3F 10(RE =Sm–Lu,Y).110These as-obtained ?uoride nanocrystals can show a strong emission band with maximum at 400nm under UV excitation.97Since we did not introduce any other activated ions in the experiments,the emission has to be caused by some kind of defect or electronic centre.In addition,we also observed that the lifetimes of these three ?uoride nanocrystals are 7.6ns (CaF 2),8.4ns (SrF 2)and 17.4ns (BaF 2),respectively.The short lifetimes further proved that their luminescence originated from defects or electronic centres in the ?uoride nanocrystals.These defect emission centers may be induced by the adsorbed oleic acid molecules onto the surfaces of the nanocrystals in the formation process.Similarly,the as-prepared well-dispersed KRE 3F 10(RE =Sm–Lu,Y)nanocrystals also displayed self-activated luminescence due to the trap states of surface defects.1103.7.

Carbon nanomaterials

3.7.1.Carbon nanodots.Recently,emerging light-emitting quantum-sized (less than 10nm)carbon nanodots (CNDs)have appeared to be a promising alternative to traditional luminescent materials in many applications because of their advantages in size,biocompatibility,low toxicity,and cheaper cost.29The synthesis of the CNDs can be generally classi?ed into two kinds of approaches.Firstly,CNDs can be obtained from a larger structure (Fig.12a),such as graphite,217carbon nanotubes,218and so on.219In the other kind of strategy,CNDs are prepared from some organic molecular precursors.220–223Generally,during these methods,CNDs often undergo oxidation (such as HNO 3and/or H 2SO 4)and further puri?cation processes.Therefore,CNDs typically contain many functional groups (such as carboxylic and hydroxyl groups)on the surface (Fig.12b),thus imparting them with excellent water solubility and suitability for subsequent functionalization with various organic,polymeric,inorganic,or biological species.For the physical and chemical properties of CNDs,Ray et al.224reported that CNDs formed from oxidation of candle soot consisted of a nanocrystalline core of graphitic sp 2carbon atoms functionalized with peripheral carboxylic/carbonyl moieties.Another research group demonstrated that Raman

spectra of CNDs that were produced by electrochemical oxidation of nanotubes showed characteristics of both sp 2and disordered carbon.218Based on further and detailed study,CNDs consist of an amorphous to nanocrystalline core with predominantly sp 2carbon and the lattice spacings are consis-tent with graphitic or turbostratic carbon.223,225,226

Typically,CNDs can show strong optical absorption in the UV region and even the visible range,which depends on the synthesis method and surface modi?cation.217However,the most fascinating features of CNDs are the photoluminescence properties.The emission spectra of CNDs prepared by Chen and co-workers have a broad color range with the maximum at the range of 415–615nm.225Ray et al.reported ?uorescent carbon nanoparticles with a size of 2–6nm which can be prepared by oxidation of soot particles.Surface oxidation and subsequent nitrogen/oxygen doping a?orded a light emission property of the carbon particles.The luminescence of carbon particles depends on the wavelength of excitation light and the particle size,that is,the smaller the size,the higher the luminescence e?ciency.224In general,the emission intensities and the color can be in?uenced by the synthesis methods,di?erent excitation wavelengths (Fig.12c),particle size (Fig.12d),surface properties of the CNDs themselves,219,225,226and other external factors (such as ionic strength and pH values).221,227Since the elemental carbon itself is nonluminous and has no extrinsic activators,the observed luminescence from CNDs must be related to some defects and/or impurities in the system.Due to its complexity and di?culty,the exact mechanism is still a matter of current debate and requires further investiga-tion.However,based on the research results,besides the quan-tum e?ect of the CNDs,the carbon-based photoluminescence has been attributed by many researchers to passivated defects on the carbon particle surface acting as excitation energy traps.217,218

Fig.12(a)Representative TEM micrographs of CNDs (reproduced with permission from ref.225,copyright 2009,American Chemical Society).(b)Depiction of CNDs after surface oxidative treatment (reproduced with permission from ref.29,copyright 2010,Wiley).(c)PL emission spectra of 1.9nm CNDs at di?erent excitation wavelengths of 290–380nm (reproduced with permission from ref.226,copyright 2008,Royal Society of Chemistry).(d)Optical images of polyacrylamide gel electrophoresis (PAGE)-separated CNDs illuminated under white (top)and UV light (312nm;center)(reproduced with permission from ref.227,copyright 2007,Wiley).

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Additionally,the performance in an in vivo study using CD-1mice suggests the nontoxicity of CNDs at exposure levels and times beyond those typically used for optical in vivo imaging studies.228

3.7.2.Graphene oxide.Graphene oxide (GO)is an atomically thin sheet of graphite,and is increasingly attracting chemists for its characteristics.Graphene oxide contains oxygen-containing func-tional groups on the surface,such as carboxylic and hydroxy groups,thus imparting it with excellent water solubility,suitability,and a mixture of sp 2-and sp 3-hybridized carbon atoms,which may provide interesting possibilities for optical applications.29

In 2008,Dai and co-workers 229,230reported intrinsic ?uores-cence in both the visible and the near-infrared (NIR)regions from nanosized GO functionalized with PEG.Owing to its small size,intrinsic optical properties,large speci?c surface area,low cost,and low toxicity,GO as a promising new material has attracted much attention in the following studies.231–234Chhowalla and co-workers reported near-UV-to-blue photoluminescence from solution-processed graphene oxide.Moreover,they presumed that the PL of this kind of GO originated from the recombination of electron–hole (e–h)pairs,localized within small sp 2carbon clusters embedded within the sp 3matrix.235The PL behavior of GO is very similar to that observed in CNDs.29Therefore,both GO and CNDs may become promising platforms that pave the way toward interesting biomedical labeling and optoelectronics applications.3.8.

Inorganic–organic hybrids

Other attractive species of defect-related luminescent materials include inorganic–organic hybrids with advanced optical properties,such as high laser e?ciencies,good photostability,photopattern waveguiding structures for integrated optics,electroluminescent diodes,and so on.23–25,236–238Carlos and co-workers have paid much attention on the hybrid concept to synthesize stable and e?cient photoluminescent materials lacking metal activator ions,such as urea cross-linked inorganic–organic hybrids (so-called ureasils),amine-functionalized inorganic–organic hybrids,and urea/urethane cross-linked nanohybrids prepared by a sol–gel method at room tempera-ture.239,240Generally,the host matrix of this kind of hybrid material is a silica-based network and covalently grafted by means of urea,urethane,or amido-group linkages.Control-ling the experimental conditions and excitation energy,the inorganic–organic hybrids can show tunable photolumines-cence,and even e?cient white-light hybrids with high photo-luminescence yield.Moreover,Carlos et al.proposed a mechanism based on NH 3+/NH à(or NH 2+/N à)donor–acceptor pairs to explain the emission of these materials.23Similarly,Lianos et al.prepared hybrid inorganic–organic nanoclusters.These inorganic–organic hybrids were founded on a silica backbone but they were also facilitated and assisted by forces arising from hydrophilic/hydrophobic balance and hydrogen-bonding interactions.Luminescence was attributed to electron–hole recombination on delocalized states so that emission wavelength can be tuned in the range of almost the entire visible spectrum by choosing the excitation wavelength.19

In a recent study,Guo et al.discovered the white-light-emitting borate-based inorganic–organic hybrid open framework.241

The two three-dimensional (3D)inorganic–organic hybrid isopolyacid borates,B 6O 9(en)and (H 2en)2(Hen)2B 16O 27(en =ethylenediamine)were ?rst prepared under solvothermal conditions.Especially,the (H 2en)2(Hen)2B 16O 27hybrid can show bright luminescence and be modi?ed from blue to white by means of a simple heat-treatment process.Since no metal activator ions exist in this compound,the self-activating photo-luminescence probably originates from structural defects caused by heat treatment.This is supported by the t f value at ns level and the results of the PL spectra,suggesting its ?uorescence character.In addition,they also pointed that the compound of (H 2en)2(Hen)2B 16O 27with large void spaces may ?nd utility in ion exchange in the future.

4.Emission mechanisms

To date,researchers have extensively investigated defect-related phosphors,however the origins of luminescence are not yet entirely understood in defect-related materials.Unlike emission from rare earth activator-doped luminescent materials,it is di?cult to con?rm the kinds of defects that cause emission and how it works in the matrix.A variety of possible mechanisms have been proposed.According to the existing investigations,we grouped the defect-related luminescent mechanisms into four systems,which have some special characters,and can be summarized as follows:(i)

Carbon impurities (defects)

Carbon defects are only luminescently active after heat treatment,because these samples are not luminescent prior to this procedure.These carbon-related defect centers include an electron being localized in a 2p orbital of a single-bonded carbon and single-bonded oxygen.This would give rise to photoluminescence through a strong electron–photon coupling.43,112Generally,the luminescence from carbon defects contains several common features,such as characteristic emission range (350–700nm),maximum of emission spectrum (405–550nm)and shorter PL lifetime (less than 10ns).17,43,86–88(ii)

Oxygen vacancies

Oxygen vacancies are known to be one of the most common defects in oxides and usually act as radiative centers in luminescence processes.191As the annealing process can cause recovery of the structural defects and eliminate e?ciently the non-radiative combination centers,the oxygen vacancy-related photoluminescence is strongly dependent on the calcination temperature and time.176The recombination of a conduction band electron with an oxygen vacancy can yield visible emission.Moreover,each recombination luminescence includes the following processes:ionization,migration,recombination,and emission.87,88,176This recombination process results in the long lifetime (m s or ms).(iii)

E 0-type luminescent centers

An E 0-type luminescent center ( Si R )is an unpaired electron localized in a single Si sp 3orbital (i.e.a silicon dangling bond),where the spin is highly localized on the silicon atom.152This is a prominent paramagnetic defect in conventional a-SiO 2.This

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defect can result in characteristic photoluminescence at about 288nm when excited with high-energy photons.152,156(iv)

Nonbridging oxygen hole centers

Nonbridging oxygen hole centers are intrinsic defect centers in amorphous and porous silica and can be described as SiOH -SiO + H,where dots represent unpaired electrons.In the PL spectra of silica-based systems,this defect was connected with bands at 1.8and 1.9eV.151–153Unfortunately,the silanol groups (SiOH)are unstable when exposed to ambient air and oxidized slowly to form Si dangling bonds,which form non-radiative recombination centers and in?uence PL e?ciency.(v)

Peroxy-radical hole trap

Peroxy-radical hole traps (PRHT, O–O–Si R O3)are also intrinsic defect sites in the silica lattice.This kind of defect consists of an unpaired electron localized over two oxygens,one of which is bonded to Si.31Therefore,they can introduce an electronic state into the band gap,resulting in characteristic photoluminescence at about 460nm (2.7eV)when excited with high energy photons (h n Z 3.5eV or l r 357nm).86(vi)

Carbon dioxide radical

Carbon dioxide radicals (CO 2 à)are a kind of paramagnetic defect resulting from bond cleavages of some organic additives during the synthesis process.90,103–106These radicals are trapped into the matrix lattice or interstitial positions,which are apparently the precursors for various centers.These defect centers induce an electron localized in the 2p orbital of the single-bonded carbon.This would give rise to photoluminescence with a short lifetime through strong electron–photon coupling.(vii)

Donor–acceptor pairs

Donor–acceptor pairs (such as NH 3+/NH àor NH 2+/N à)can usually be found in room temperature-obtained organic/inorganic hybrids.This mechanism was proposed by Carlos et al..23,239,240Photoinduced proton-transfer between defects such as NH 3+and NH àor NH 2+and N àcan give rise to photoluminescence.This process is a shorter-lived electron–hole recombination occurring in organic/inorganic hybrids.Similarly,radiative recombination of electron–hole pairs in carbon nanomaterials (i.e.graphene)can also lead to ?uorescence.30The characteristic luminescent properties are a broad emission band,a short lifetime,and tunable emission color.

5.Potential applications

5.1.

Lighting

Because of the great interest in the generation of white-light emission for a variety of applications (such as solid-state multicolor three-dimensional displays,back lighting,etc.),researchers have paid much attention in this ?eld.242–244At present,the most commonly used white-light source is white light-emitting diodes (LEDs),245,246which is a GaN blue LED encapsulated with a yellow-emitting phosphor coating (commercial YAG:Ce 3+phosphors,which are expensive and require high production temperatures).247Because of the exceptional performance characteristics (e.g.low cost and non-toxicity),a

novel yellow-emitting defect-related BCNO phosphor has been used in a white LED by Okuyama’s group.28,213In their study,defect-related BCNO phosphor particles were coated on the surface of a commercial blue GaN/SiC LED chip.Bright white-light emission was achieved when it was excited by the blue emission from the GaN/SiC chip (Fig.13).Therefore,it can be inferred that the environmentally-friendly and low cost defect-related luminescent materials may ?nd potential applications in the ?eld of white LEDs.

On the other hand,white light-emitting devices with multiple emitting components,which are generally rather complicated and expensive,can be problematic as it is di?cult to control the color balance.For this reason,one current academic interest in white light is pursuing single white light-emitting phosphors to avoid the intrinsic color-balance,device-complication,and high-cost problems.241,248–251Due to the excellent blue emission of defect-related apatite structure phosphors described above,it may be an attractive alternative to commercially blue phos-phors.In further study,our group induced Ce 3+and Mn 2+to defect-related FAP nanorods and obtained the FAP:Ce 3+,Mn 2+samples with multicolor emission,which consists of an ultraviolet emission centered at 357nm attributed to the 5d–4f transition of Ce 3+,a radical-related blue emission band ranging from 360to 500nm,and a yellow emission centered at 561nm from the 4T 1-6A 1transition of Mn 2+.105Moreover,with increase of the excitation wavelength from 295to 342nm,the PL intensities of ultraviolet and yellow emissions decrease,and the blue emission increases (Fig.14).The change in relative intensities of these three color parts can result in tunable luminescence from blue to white to yellow for these samples.Especially,when adjusting the excitation light to 308nm,the blue and yellow emissions are comparable in intensity to give white-light emission.The corresponding chromaticity coordi-nate for this white light excited by 308nm is calculated to be x =0.316and y =0.328,which is very close to the standard equal energy of ideal white-light illumination (x =0.333and y =0.333)according to the 1931CIE coordinate diagram.Similarly,in order to improve the emission in the red-light range,Guo and co-workers introduced Eu 3+ions into a borate-based inorganic–organic hybrid open framework by soaking it in an aqueous solution of EuCl 3,241resulting in a white light-emitting hybrid sample.This novel strategy of combining defect-related luminescence and metal-activator emissions might serve as guidance for the design and fabrication of other

Fig.13An example application of BCNO phosphors in white LEDs.(a)A 5mm reference GaN/SiC LED chip illuminating blue light,(b)the same LED chip coated with a thin layer of the BCNO sample prepared at 7001C after treatment with a polymer solution,and (c)upon illumination,the same LED shows a bright white light (reproduced with permission from ref.26and 203,copyright 2009,Royal Society of Chemistry and ECS The Electrochemical Society).

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materials with white-light emission and tunable luminescent properties.5.2.

Application in biomedical ?elds

Luminescent materials including ?uorescent organic molecules 252and semiconductor nanoparticles 253have been widely investigated in biomedical areas.However,some serious problems of photo-bleaching and quenching of ?uorescent organic molecules and the toxicity of semiconductor quantum dots are critically

pronounced,which have seriously limited their applications in biomedical areas.Because of the excellent characteristics of low cost and non-toxicity,the defect-related luminescent materials have great potential in medical applications.

5.2.1.Drug delivery.An ideal drug-delivery system should possess several important properties,which are biocompatibility,nontoxicity,proper particle size,and the ability to transport the desired drug molecules to the targeted cells or tissues and release in a controlled manner.254–257So far,di?erent types of drug-delivery systems have been developed.254Among them,meso-porous nanomaterials have gained enhanced interest with particular attention as drug storage and release hosts due to their unique surface and textural properties.255,258–261Recently,our group has paid much attention to luminescent nanomaterials for biomedical application as drug delivery systems.We designed and synthesized the multifunctional SrHAp nanorods with mesoporous and defect-related lumi-nescence by a simple hydrothermal process.106The experi-mental process,loading and release of drug molecules are presented in Fig.15A.During the synthesis process,CTAB was selected as the organic template for the mesoporous structure of multifunctional SrHAp and removed through re?uxing in acetone to form a mesoporous structure with mesopore size of 3–5nm.Drug storage/release tests suggest that the multifunctional SrHAp nanorods show high drug loading and a controlled release property for ibuprofen (IBU)(Fig.15B).Although the mesoporous SrHAp sample also exhibited blue emission (Fig.15Cg)even after loading the drug molecules (Fig.15Ca),the IBU–SrHAp sample (Fig.15Ca)has a lower PL intensity than SrHAp (Fig.15Cg).Moreover,the PL emission intensities of the multifunctional drug carriers increase with the amount of IBU released (Fig.15C).106Therefore,this

Fig.14PL emission spectra for as-synthesized FAP:Ce 3+,Mn 2+sample under di?erent excitation wavelengths:(a)295,(b)305,(c)308,(d)310,(e)312,(f)314,(g)320,(h)342nm.The inset shows the corresponding luminescence photograph for the sample under excita-tion at 308nm (reproduced with permission from ref.105,copyright 2010,Royal Society of Chemistry).

Fig.15(A)Schematic for the experimental process for luminescence mesostructure of SrHAp nanorods and subsequent loading and release of IBU.(B)Cumulative ibuprofen release from IBU–SrHAp system as a function of release time in the release media of the simulated body ?uid (SBF)system.(C)PL emission spectra of IBU–SrHAp nanorods at di?erent release time (reproduced with permission from ref.106,copyright 2010,Elsevier).

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relationship between the luminescence properties and drug release has potential as a probe for monitoring and tracking the drug release during the drug-release process.

In a follow-up study,to improve the mesoporous property,the design of multifunctional composite materials combining the meso-porous textures (MCM-48),spherical shape,and defect-related ?uorescence were also prepared.90In this study,defect-related luminescent apatite was deposited on the surface and/or in the pores of the bioactive,mesoporous,and amorphous MCM-48network,resulting in a higher drug loading system (33.7wt%).In addition,defect-related luminescent porous silica ?bers as drug carriers were also prepared and investigated due to many advantages,such as stable porous structure,tunable pore size,high speci?c surface area with abundant Si–OH active bonds on the pore walls,nontoxic nature,and good biocompatibility.129

Shi et al.reported oxygen-de?cient luminescent mesoporous silica nanoparticles with uniform morphology,size,and inte-grated mesoporosity–luminescent properties,which were synthesized by a bottom-up self-assembly route followed by a calcination process.262They also developed the biological applications of these functional nanoparticles for synchronous drug delivery and imaging.

Owing to the nano-size,intrinsic PL properties,and large speci?c surface area,GO is another promising new material for medical applications.The PEG-modi?ed GO was exploited as a vehicle by Dai and co-workers for the antibody-targeted delivery of the widely used chemotherapy drug doxorubicin (DOX),which was loaded onto the GO-PEG surface by simple p -stacking-mediated physisorption.229All these research reports

indicate that defect-related luminescent nanomaterials have good application prospects in the biomedical ?eld of drug delivery.5.2.2.Bioimaging.Fluorescence imaging is a very important technique for biological studies and clinical applications.Quantum dots and related core–shell nanoparticles (such as CdSe,NaGd-F 4:Yb 3+,Er 3+/NaGdF 4core–shell upconversion luminescent nanoparticles,and so on)have been used in various in vitro and in vivo optical imaging experiments.263–267However,since heavy metals and/or rare earth ions are the essential elements of these two kinds of luminescent nanoparticles,they have prompted serious health and environmental concerns.264Therefore,the search for benign alternatives has become increasingly important and urgent.With characteristics of tunable PL properties and low toxicity,defect-related luminescent nanomaterials form an attractive alternative for bioimaging applications.

Sun et al.have studied carbon dots for multiphoton bioima-ging.268First,they prepared nanosized carbon particles (sub-10nm)with surface passivation by poly(propionylethylenimine-co-ethylenimine)(PPEI-EI).These kinds of CNDs can show strong luminescence with two-photon excitation in the near-infrared area,which has been attributed to passivated defects on the carbon particle surface acting as excitation energy traps.217Then,after incubation with CNDs for 2h in an aqueous bu?er at 371C followed by washing to remove any extracellular CNDs,the MCF-7cells exhibited bright luminescence in both the cell membrane and cytoplasm regions when imaged on the ?uores-cence microscope with excitation by 800nm laser pulses (Fig.16a–c).Furthermore,the translocation of the CNDs

Fig.16(a)Schematic for the CND structure prepared by Sun et al.and (b,c)two-photon luminescence image (800nm excitation)of human breast cancer MCF-7cells with internalized CNDs passivated with PPEI-EI (reproduced with permission from ref.268,copyright 2007,American Chemical Society).(d–i)Fluorescent CND-labeled EAC cells.Washed cells were imaged under bright ?eld,UV,and blue light excitation.The bottom row images correspond to the control experiment where no CNDs were used.Cells become bright blue–green under UV excitation and yellow under blue excitation but were colorless in the control sample.The light blue color of the control sample under UV excitation is due to the well-known auto?uorescence of cells (reproduced with permission from ref.224,copyright 2009,American Chemical Society).

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from outside the cell membrane into the cytoplasm is tempera-ture dependent,and no meaningful CND internalization was observed at 41C.These results indicate that the luminescent CNDs can be potentially used for cell imaging with two-photon luminescence microscopy.

Additionally,many other studies on the bioimaging cap-abilities of CNDs have followed Sun et al.’s work.217,228,268,269Among them,Ray and co-workers reported ?uorescent car-bon nanoparticles (2–6nm)with excellent water solubility used for conventional bioimaging.224In this study,a CND solution was mixed with cell culture media along with Ehrlich ascites carcinoma cells (EACs),incubated for 30min,and then the cells were washed.These labeled cells can show a bright blue–green color under UV excitation and yellow under blue excitation,but they are colorless in the control sample without CNDs (Fig.16d–i).This result demonstrates that CNDs can enter into the cells without any further functionalization and the CND-labeled cells show encouraging cell-imaging applica-tions using a conventional ?uorescence microscope.Some other researchers also predict that the biocompatibility of CNDs may be competitive with some currently FDA-approved dyes used as optical imaging agents such as indo-cyanine green (LD 50=60mg kg à1body weight).228Similarly,luminescent GO is also an excellent alternative in bioimaging applications because of its advantages in size,biocompatibil-ity,and low toxicity.229,230

6.Summary and outlook

Defect-related luminescent materials are interesting newco-mers to the world of phosphors.In the past decade,the development of novel defect-related phosphors has greatly accelerated due to the emphasis on environment/health and the combination of nanotechnology and biology.In this review,we have discussed the importance of metal-activator-free phosphors and their unique luminescent properties for technological advances.We have also outlined the common strategies for the synthesis of defect-related luminescent materials using inexpensive raw materials with control over compositions,sizes,and shapes,and their applications in lighting and biology.The novel defect-related phosphors exhibit tunable color emission within the violet-to-red range and even near-infrared with high e?ciency and stability,which can also be excited by a wide range of light,ranging from short UV (254nm)to blue light.In addition,long-duration phos-phorescence was observed in these kinds of phosphors.The defect-related phosphors stand to have a huge e?ect on both health and environmental applications due to their potential to serve as low-/nontoxic replacements to toxic traditional heavy-metal-or rare earth-ion-based luminescent materials.Although substantial and rapid progress has been made in the synthesis,properties,and applications of defect-related phosphors in the recent years,developing better synthetic routes and more detailed fundamental studies of their proper-ties have a level of urgency,as there still remains much room for improvement in the coming years.First,despite the fact that there are many kinds of possible luminescent mechanisms proposed by researchers,further and detailed investigations are still required to explore exactly how the defects work,and

to develop better synthesis routes to these emerging materials.Secondly,as these kinds of phosphors are related to defects/impurities in the matrix,how to precisely control the content and species of defects in the matrix is also a di?cult issue to be addressed.Thirdly,although the photoluminescence has been extensively investigated by many researchers,basic studies of electroluminescence and cathodoluminescence properties of defect-related phosphors should be carried out for various electronics-related applications.Finally,to move forward,researchers with various backgrounds may be needed to develop strategies to further tune the properties and applica-tions of defect-related environmentally-friendly phosphors.One key area will be the integration of defect-related phos-phors with organic molecules or special characteristics (such as magnetization)to open the door to novel multifunctional applications,for example magnetic resonance imaging (MRI).On the other hand,from the point of view of devices,?uorescence from defect-related phosphors over a wide range of wavelengths (emission in the violet-to-red region,and even the near-infrared region)might o?er possibilities for incorporation in solar photovoltaic conversion,sensors (i.e.electrochemilu-minescence sensors,?uorescent sensors,etc.),?eld emission displays,optical information transmission,and display applica-tions on ?exible platforms.

Acknowledgements

This project is ?nancially supported by National Basic Research Program of China (Grant No.2010CB327704),National High Technology Program of China (2011AA03A407),the National Natural Science Foundation of China (Grant Nos.NSFC 50872131,51172228,21101149,51172227,20921002),and Science Foundation for Excellent Youth Scholars of Hebei Education Department (Grant No.Y2012007).

Notes and references

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2G.Blasse and B.C.Grabmaier,Luminescent Materials ,Springer-Verlag,Berlin,1994.

3Y.Liu,D.Tu,H.Zhu,R.Li,W.Luo and X.Y.Chen,Adv.Mater.,2011,22,3266–3271.

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7W.Luo,C.Fu,R.Li,Y.Liu,H.Zhu and X.Y.Chen,Small ,2011,7,3046–3056.

8L.X.Yu and M.Nogami,Mater.Lett.,2010,64,1644–1646.9C.H.Zheng,C.G.Hu,X.Y.Chen,H.Liu,Y.F.Xiong,J.Xu,B.Y.Wan and L.Y.Huang,CrystEngComm ,2010,12,3277–3282.

10R.C.Ropp,Luminescence and the Solid State ,Elsevier,Amsterdam,

Netherlands,1991,vol.12,p.283.

11Q.Y.Zhang and X.Y.Huang,Prog.Mater.Sci.,2010,55,

353–427.

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J.Alloys Compd.,2011,509,2076–2080.

13G.G.Li,D.M.Geng,M.M.Shang,C.Peng,Z.Y.Cheng and

J.Lin,J.Mater.Chem.,2011,21,13334–13344.

14G.G.Li,Z.Y.Hou,C.Peng,W.X.Wang,Z.Y.Cheng,C.X.Li,

H.Z.Lian and J.Lin,Adv.Funct.Mater.,2010,20,3446–3456.

P u b l i s h e d o n 27 S e p t e m b e r 2012. D o w n l o a d e d o n 06/01/2015 10:49:26.

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2012年01月壬辰年【龙年】 一二三四五六日 1 腊八节 2 初九 3 初十 4 十一 5 十二 6 小寒 7 十四 8 十五 9 十六10 十七 11 十八 12 十九 13 二十 14 廿一 15 廿二 16 小年17 廿四 18 廿五 19 廿六 20 大寒 21 廿八 22 除夕 23 春节24 初二 25 初三 26 初四 27 初五 28 初六 29 初七 30 初八31 初九 2012年02月壬辰年【龙年】 一二三四五六日 1 初十 2 十一 3 十二 4 立春 5 十四 6 元宵节 7 十六 8 十七 9 十八 10 十九 11 二十 12 廿一 13 廿二 14 情人节 15 廿四 16 廿五 17 廿六 18 廿七 19 雨水 20 廿九21 三十 22 2月小 23 初二 24 初三 25 初四 26 初五 27 初六28 初七 29 初八

2012年03月壬辰年【龙年】 一二三四五六日 1 初九 2 初十 3 十一 4 十二 5 惊蛰 6 十四 7 十五 8 妇女节 9 十七 10 十八 11 十九 12 植树节13 廿一 14 廿二 15 廿三 16 廿四 17 廿五 18 廿六 19 廿七20 春分 21 廿九 22 3月大 23 初二 24 初三 25 初四 26 初五27 初六 28 初七 29 初八 30 初九 31 初十2012年04月壬辰年【龙年】 一二三四五六日 1 十一 2 十二 3 十三 4 清明节 5 十五 6 十六 7 十七 8 十八 9 十九10 二十 11 廿一 12 廿二 13 廿三 14 廿四 15 廿五 16 廿六17 廿七 18 廿八 19 廿九 20 谷雨 21 4月大 22 初二 23 初三24 初四 25 初五 26 初六 27 初七 28 初八 29 初九 30 初十

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1.男生不能防守女生，撞到女生算犯规。 2.男生不能抢女生手中的球，男生只能在三分线外投篮，每次进攻球必须经过女生，否则进球无效。 3.男女比例：男生2个，女生1个。 4.一场分两节，一节十分钟。 以上规则暂定，如有变动于比赛前通知。 三、实际操作 以实际情况为准，由裁判把握尺度。 四、参赛方式 五、时间地点 流程 一、8：20前所有工作人员到场，并由总负责人安排具体工作。 二、8：50前所有人到场。 三、9：00—9：10由会长简短发言并宣布开始。 四、开始第一项内容：篮球蹦蹦跳。 五、第二项内容：“3+1”男女混合篮球赛。(中间休息时间分别穿插6，7中游戏) 六、第三项内容：“天旋神射手”，时间安排：“男女混合篮球赛”第一次休息时。 七、第四项内容：“二人夹球接力赛”，时间安排：“男女混合篮球赛”第二次休息时。 八、比赛结束，计算最后总分，并由会长颁奖。

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1 系统概述 1.1系统需求 网络考试系统是实现网络教育的一个重要组成部分，通过网络考试系统你可以及时测试自己的学习进度、自己对已学习知识的掌握程度。网络考试与传统考试相比有着明显的优势，考生和教师可以从世界的任何角落，通过浏览器接入因特网来使用考试系统，不受地域的限制，同时也使得用户可以更加灵活地安排考试时间，可以在任何时候使用网络考试系统进行自测，快速查询考试成绩。所以，网络考试系统的开发和应用现在正受到越来越多的大学的重视。在本单位，网络考试模式是教学改革的正式项目之一，它可以运用到多种教学过程中。例如对于本单位公共课程“计算机应用基础”，要求在计算机等级考试前进行一次拟上机考试，以前都是由老师自己命题，然后复制题目到每个学生的机器上，学生把答案写一个文件中，做完题后由教师回收该文件进行判分。这样的考试，过程变得非常复杂，让教师的负担很重;而且考试题目不统一，经常出现很多无法估计的问题，考试效果不能达到预想的目的。基于上述情况，迫切需要一个界面统一、管理使用方便的网络考试系统的出现。 校园网建设的普及和完善以及Web数据库技术的快速发展为网络考试系统开发提供了良好的媒介和基础，使我们对考试过程的实施和管理由书面化和单机化过渡到了网络化。 1.2 国内外研究现状 互联网的考试用途很早就受到了各方面的重视，既然考试能够通过电脑完成，那么网上考试从技术上来说就是可行的。国内著名的工商管理考试GMAT于1999年就开始实行算机考试，2000年后GRE，TOEFL考试也开始采用计算机考试，向着网络化方向发展。近年来出现的各种rI’认证考试以及英语GM八T和GRE考试已经是一种基于计算机网络的考试，完全摆脱了纸和笔。考生使用计算机答卷，通过网络提交答卷，自动阅卷。网络考试的优点是显然的:阅卷更加迅速，考试结束考生即可知道考试结果，还免了纸介质的试卷和答卷的传输，从而降低了保密成本。通过抽取不同的试题，或者是随机变换试题顺序或答案顺序，可以做到一人一卷，极大地降低了作弊的几率。另外，网络考试为自适应考试提供了可能。基于计算机网络的考试需要专门的较高水平的技术支持，考试己经发展为一种产 业。 网络考试系统的实现技术有多种，可以采用传统的客户机/服务器(C/S)型的Mls型架构，即试题内容放在远程的服务器上，在考试机上安装考试应用程序和数据库客户机配置，因此每次考试时要对机器进行安装、配置，这样一来考务工作比较烦琐;而且考试程序放在客户机上，安全性也受到一定影响。另外一种考试系统采用W七b技术实现。节几b技术超越了传统的“客户机/服务器”两层结构，采用了三层体系结构:用户界面层/事务层/数据库层。因此Web结构有着更好的安全性。在用户机上不需要安装任何应用程序，只需要有一个标准的web浏览器就可以。应用程序可以安装在事务层所在的计算机上，试题存放在数据库服务器上(事务层和数据库可以是同一台机器)。采用这种体系结构的计算机网络考试和管理系统适用于一个考场或几十个考场、几十人或几万人同时进行无纸化考试，解决了考试系统对客户机软件过分依赖的问题，减轻了客户机软件维护工作量。比以往的考试系统更具有实用性。而且，考试系统安装和试题装卸的程序简易，进行不同类别试题的考试时仅需要在服务器上进行试题的更换处理，对考场的计算机配置应考专业的相应软件，考生就可以进行考试。目前国内成熟的网络考试系统多为QS模式，比如清华毫太网络考试系统， B/S模式的比较少，其中基于JavaZ技术的网络考试平台则更少。 1.3网站的开发语言和语言功能

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摘要： 2011 年8 月（记住，一定要写和考试时间接近的），我作为项目经理参与了xx 省公安厅 xxxxx项目，该项目投资共500 万元人民币（一定需要写），建设工期为1年，通过该项目的建设，实现了该省公安信息化工作的……..（自己补充完善），该项目于2012 年8 月，通过了业主方的验收，赢得了用户的好评。本文结合作者的实际经验，以该项目为例，讨论了信息系统项目建设过程中的xx 管理（具体根据考试题目来），主要从如下几个方面进行了阐述：（一定要根据题目，如果说是写进度管理的过程，那就是活动定义，活动排序，资源等等----一定要和下面正文里的提纲对应上。）------摘要就是这个格式，可以写350-400 字。 正文： 2011年8月，我作为项目经理参与了xx 省公安厅xxxxx项目，该项目投资共500万元人民币，建设工期为1 年，通过该项目的建设，实现了该省公安信息化工作的……..（自己补充完善），该系统采用java 语言开发，数据库采用oracle 10g，用到了什么中间件、采用什么架构，数据库服务器、应用服务器分别采用什么？？这些大家根据自己的项目去写，写个500字左右，别太多，别太少。由于本项目的顺利上线涉及到业务的考核，因此，在本项目中，xx 管理尤为重要，在本项目中，我作为项目经理特别除了对其余管理领域进行克制恪守的管理外，特别对xx 管理从如下几个方面进行了管理-----这是一个过渡段，非常重要。 结尾：（这2 个字在论文里可以不写，我这里写，是让大家好看） 经过我们团队不懈的努力，历时1 年，本项目终于于2012 年8 月，通过了业主方组织的验收，为用户解决了什么问题，或者是达到了什么目的（根据自己的项目去写）得到了业主的好评。本项目的成功得益于我成功的xx 管理。当然，在本项目中，还有一些不足之处，比如：在项目的实施过程中，由于项目组2 名成员因为自身原因突然离职，导致项目的团队建设出现一些小问题，还有，曾经由于需要购买的服务器由于连日暴雨的不可抗力导致环境搭建进度出现些许异常（自己去想一些小问题，切忌，别出现什么大问题），不过，经过我后期的纠偏，并没有对项目产生什么影响。在后续的学习和工作中，我将不断的充电学习，和同行进行交流，提升自己的业务和管理水平，力争为我国信息化建设做出自己的努力。