Photoluminescence enhancement induced by nanoparticles from La2O3 and CeO2 doped diamond-like carbon

Journal of Alloys and Compounds 476(2009)318–323Contents lists available at ScienceDirectJournal of Alloys andCompoundsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j a l l c omPhotoluminescence enhancement induced by nanoparticles from La 2O 3and CeO 2doped diamond-like carbon filmsZhenyu Zhang a ,∗,Hongxiu Zhou b ,∗,Dongming Guo a ,Dongjiang Wu a ,Yu Tong aa Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education,Dalian University of Technology,Dalian 116024,PR China bInstitute of Internal Combustion Engines,Dalian University of Technology,Dalian 116024,PR Chinaa r t i c l e i n f o Article history:Received 26May 2008Received in revised form 16August 2008Accepted 23August 2008Available online 11October 2008Keywords:Nanoparticles DLC CeO 2La 2O 3Optical propertiesa b s t r a c tLa 2O 3and CeO 2doped composite hydrogen-free diamond-like carbon (DLC)films with thickness of 230–280nm were deposited by unbalanced magnetron sputtering.Nanoparticles are formed on the surface of deposited films.Auger electron spectroscopy shows the C,O,La and Ce elements distribute uniformly along the depth direction,and they diffuse into the Si(100)substrate.X-ray photoelectron spectroscopy confirms the forming of La 2O 3and CeO 2within the DLC films,and the amorphous DLC structure is not changed with the doping of rare earth oxides.G peak positions shift to higher wave num-bers,and relative intensities ratio I D /I G increase of DLC films with the doping of La 2O 3and CeO 2,indicating the coherence between the I D /I G and G peak position.Furthermore,the photoluminescence (PL)obviously increase with the forming of nanoparticles and doping of La 2O 3and CeO 2.Specially,the PL intensity of 4%La 2O 3and 4%CeO 2doped composite DLC films is 18.4times of that of pure DLC films,which is beneficial for the increase of photoconversion efficiency of solar cells.©2008Elsevier B.V.All rights reserved.1.IntroductionDiamond-like carbon (DLC)films have been widely used in the fields of optics,electricity,solid state devices,etc.,due to their unique properties,such as high hardness,high electrical resistivity,low infrared absorption,transparency to visible light and chem-ical inertness [1,2].With the lacking of energy sources in recent years,the research on DLC films as anti-reflection coatings has attracted more and more attention,attributing to that DLC films have wide bandgaps,and adjustable refractive indices [3,4].The DLC coated Si solar cells increase the solar cell efficiency by 20%[1],and the transmission efficiency in the infrared and visible regions is 68.83and 85%,respectively,close to theoretical value for non-absorption carbon material [5,6].For the development of solar cells,the pure DLC anti-reflection coatings can not meet the require-ments of photoconversion,such as low photoconversion efficiency,photoluminescence (PL),etc.,of pure DLC films,thus doping ele-ments into DLC films is attempted to improve the optical properties of DLC films [7].Ge doped DLC films increase the transmission effi-ciency by 14%,compared with widely used ZnS coatings.H doped DLC films have higher transparency and lower refractive index,that those of pure DLC films [8].Si-doped DLC films increase the opti-cal gap [9].N doped DLC films have higher extinction coefficients∗Corresponding authors.Tel.:+8641184709794;fax:+8641184709794.E-mail addresses:zzy@ (Z.Zhang),hxzhou@ (H.Zhou).than those of pure DLC films [10].So the elements doped DLC (E-DLC)films have improved the optical properties of pure DLC films.However,Ge,N,H,Si doped DLC films are not stable,due to the mismatch of atomic radii,weak bonding states between the car-bon and doped elements,and oxidizing of doped elements.The E-DLC films change the carbon bonding state,replacing the carbon atom with doped atom.On the other hand,the photoconversion efficiency,such as PL of E-DLC films has not been reported.To enhance the photoconversion efficiency and PL properties of DLC films,DLC/porous silicon (PS)structure and ZnO dope DLC films have been deposited by PECVD and FCVA methods,respectively [11,12].While,DLC/PL structure is also not stable,and the PS will oxidize and form SiO 2with the time going on,then lose the pho-toconversion function.As ZnO is a kind of soft material,the doping of ZnO will decrease the mechanical and tribological properties of DLC films.Rare earth oxides,such as La 2O 3and CeO 2,are excellent optical 2O 3and CeO 2doped TiO 2glasses can change the opti-cal gap and the width of localized states [13].La 2O 3films deposited by spray pyrolysis method are photoactive,and La 2O 3based borate glasses with various visible emissions are useful for developing new color light sources,fluorescent display devices,UV-sensor and tun-able visible lasers [14,15].La 2O 3doped germanate glasses results in the red shifts of ultraviolet bands [16].Also,5%CeO 2doped Ta 2O 5films possess the highest refractive index value [17],and CeO 2films exhibit excellent thermal stability and irradiation resistance [18].CeO 2doped B 2O 3glasses increase the mechanical strength0925-8388/$–see front matter ©2008Elsevier B.V.All rights reserved.doi:10.1016/j.jallcom.2008.08.050Z.Zhang et al./Journal of Alloys and Compounds 476(2009)318–323319Table 1Compound concentration of DLC and La 2O 3and CeO 2doped DLC films Films Compound concentration within the DLC films (at.%)S0DLCS12%La 2O 3and 2%CeO 2doped DLC films S24%La 2O 3and 4%CeO 2doped DLC films S311%La 2O 3and 15%CeO 2doped DLC films S414%La 2O 3and 11%CeO 2doped DLC films S516%La 2O 3and 11%CeO 2doped DLC films S619%La 2O 3and 12%CeO 2doped DLC films S721%La 2O 3and 13%CeO 2doped DLC filmsand cross-linking of the glass network resulting in the decrease of polaron hopping energy [19].CeO 2/Fe 3O 4coating has very good UV-sheering and heat insulating properties,which can be used as an effective solar control glass [20].CeO 2phosphor films have strong PL properties [21].CeO 2films have high optical transmit-tance up to over 80%in the visible and near-infrared region,which can be used as counter electrode in smart window devices [22].CeO 2/MgF 2double layer anti reflection films reduce the reflectance to lower than 1.87%,which can replace the conventional anti-reflect materials,such as ZnS,TiO 2and Ta 2O 5,and produce low-cost solar cells with high conversion efficiency [23].Furthermore,in our previous study,La 2O 3and CeO 2as dopants exhibit excellent tribological,thermal,and mechanical properties in Cr 3C 2-NiCr [24,25]and CoCrW [26,27]coatings.The doping of La 2O 3and CeO 2into DLC films,respectively,correspondingly shows goodtribological [28,29]and mechanical [39,31]properties,and do not change the amorphous DLC matrix.Various physical and chemical methods have been employed to deposit DLC films,such as pulsed laser deposition,cathodic vacuum arc,plasma deposition,ion deposition,ion assisted sputter-ing,sputtering [32].Among these methods,unbalanced magnetron sputtering (UMS)is widely used to prepare the DLC films.UMS allows the simple and relatively inexpensive low temperature coat-ing of a range of temperature sensitive substrates over a large,and the uniform coating of substrates having different shapes and sizes.Fig.1.AFM surface morphology of S4film.Fig.2.AES elemental depth profile of S4film.Besides these advantages,UMS enables to generate higher rate of energetic ion bombardment on the substrate.It has the char-acteristics of conventional magnetron sputtering and ion-assisted deposition.The additional magnetron field extends an ionized region near the substrate and then makes ion bombardment on the growing film,resulting in the improving of the coatings,and releasing the residual compressive stress [5].To resolve the instable problem of dopants existing in the DLC films as anti-reflect films of solar cells,do not change the amor-phous DLC matrix,and enhance the PL property to increase the photoconversion efficiency,in this study,La 2O 3and CeO 2doped DLC films are deposited by UMS method.2.Experimental 2.1.Deposition methodTwo graphite targets,a pure metal La and Ce targets with purities of 99.99%were selected as deposition sources.Before the targets were fixed on the target holders,they were ultrasonically cleaned in the acetone solution for 10min to eliminate the purities on the surface.A mid-frequency 40kHz alternative current power con-trols the two graphite targets,and a pair of mid-frequency 40kHz direct current power,respectively,operated the La and Ce targets.Si(100)wafers with diameter of 100mm were used as substrates,and they were also ultrasonically cleaned in the acetone solution for 10min,before mounted on the substrate holder.Prior to depo-sition,the vacuum chamber was evacuated to a base pressure of 1×10−4Pa,then Ar gas was fed into the chamber,until to 2Pa.Substrate bias voltage was adjusted to 600V,and the Ar +ion bombardment started and continued for 10min,to eliminate the thin oxide layer on the Si substrate.O 2and Ar complex gas with volume ratio of 1:9was imported to the vacuum chamber to a pressure of 0.55Pa,and the sub-strate bias was adjusted to 50V.The deposition process was started and continued for 15min.Under above mentioned parameters,DLC and different concentration La 2O 3and CeO 2doped DLC films are deposited.The concentration of La 2O 3and CeO 2within the DLC films was accurately controlled by the power ratio among La,Ce and graphite targets,and the actual concentration is measured by Auger electron microscopy.Table 1shows the compound concentration of DLC and La 2O 3and CeO 2doped DLC films.It is noticed that in the following figures the numbers,such as 0,1,2,3,4,5,6,and 7of horizontal axis of coordinates represent S0,S1,S2,S3,S4,S5,S6and S7films,respectively.2.2.Measuring methodsSurface roughness and morphology was characterized by atomic force microscopy (AFM,Nanoscope ®IIIa,DI,Veeco,USA).Film thickness was measured by high-resolution surface profiler (XP-1,Ambios,USA).Elemental depth profile and concentration were obtained by Auger electron microscopy (AES,PHI-700,ULVAC-PHI,Japan).Elemental bonding valence was achieved by X-ray photoelec-tron microscopy (XPS,Quantera ULVAC-PHI,Japan).Raman spectroscopy and PL were measured by micro-confocal Raman spectrometer operated by 514nm Ar +laser (Renishaw 1000,UK).320Z.Zhang et al./Journal of Alloys and Compounds 476(2009)318–323Fig.3.XPS spectra of (a)La3d,(b)Ce3d,and (c)C1s of S4film.3.Results and discussion 3.1.Surface morphologyThe film thickness of DLC is 185nm,and those of La 2O 3and CeO 2doped DLC films are in the range of 230–280nm.Fig.1shows the AFM surface morphology of S4film.Nanoparticles with diameter of around 20–30nm are formed on the surface,and surface is very compact and smooth.The surface roughness is 1.95nm,and those of DLC and La 2O 3and CeO 2doped DLC films are in the range of 1.6–2.0nm.3.2.AES spectraFig.2shows the AES elemental depth profile of S4film.C,O,La and Ce elements distribute uniformly along the depth direction,and they diffuse into the Si substrate at the interface between the film and substrate,suggesting better adhesion strength of film on the substrate.As the diffusion of deposited elements,a widened interface appears between the substrate and film,indicating better ion/atom penetrating into the Si substrate.3.3.XPS spectraFig.3a–c shows the XPS spectra of La 3d,Ce 3d and C 1s of S4film, 3d spectrum exhibits 3d 3/2and 3d 5/2spin–orbit doublets.3d 3/2and 3d 5/2peaks correspondingly appear at 851.1and834.3eV,and the interval between two peaks is 16.8,as shown in Fig.3a,which agrees well with the characteristics of La 2O 3[33,34].Ce 3d spectrum has 3d 5/2and 3d 3/2transitions located at 881.5and 900.0eV,respectively,and the distance between two transi-tions is 18.5,as seen in Fig.3b.The strong satellite peaks appear at around 902.6and 884.5eV.These characteristics are consistent with those of CeO 2[33,35].C 1s is deconvoluted into three Gaus-sian curves to identify the three components,as drawn in Fig.3c.To distinct the high-order components of C 1s and eliminate the effect of background noise,Shirley’s function and fast Fourier transfor-mation (FFT)are employed to treat the experimental curve.The three deconvoluted Gaussian curves centered at 283.4,284.0and 286.9eV,correspond to sp 2,sp 3and C–O bonds.The XPS spec-tra is obtained before and after Ar +ion bombardment,and the results is very similar,which confirms that the La 2O 3and CeO 2form within the DLC amorphous matrix.C,La,Ce elements do not react with each other,and form compounds,thus the DLC amorphous structure is not changed with the doping of La 2O 3and CeO 2.3.4.Raman spectraFig.4shows the Raman spectra of DLC and seven kinds of La 2O 3and CeO 2doped DLC films.Raman spectra display an asymmetric broad shape in the range of 1000–1800cm −1,and D and G peaks are located at around 1350and 1560cm −1,respectively,as shown in Fig.4a.These features agree well with the obvious amorphousZ.Zhang et al./Journal of Alloys and Compounds 476(2009)318–323321Fig.4.Raman spectra of (a)DLC and seven kinds of La 2O 3and CeO 2doped DLC films and (b)S4film deconvoluted with two Gaussian curves.characteristics of DLC films [36,37].With the doping of La 2O 3and CeO 2,the intensities of Raman spectra are higher than that of pure DLC film.Although the La 2O 3and CeO 2are insensitive to the Raman spectrum irradiation,the doping of these two compounds can alter the fraction of sp 2/sp 3,sizes of sp 2and sp 3clusters and concentra-tion of sp 2and sp 3,resulting in the change of Raman spectra [30].To further district the D and G peaks,the Raman spectrum of S4film is deconvoluted into two Gaussian lines assisted with Shirley and FFT functions,as seen in Fig.4b.3.5.PL spectraFig.5shows the PL of DLC and seven kinds of La 2O 3and CeO 2doped DLC films.The sharp peaks situated at 529nm are Raman signal,and the high asymmetric peaks located at approximately 560nm correspond to G peaks.The low broad asymmetric peaks centered at around 606nm are 2G peaks,and lower peaks cen-tered at about 663nm are PL peaks [38,39],as seen in Fig.5a.The PL intensities of La 2O 3and CeO 2doped DLC films are higher than that of pure DLC films.To further decompose the four compo-nents of PL spectra and identify the PL spectrum,S4PL spectrum is deconvoluted into four Gaussian lines also assisted with Shirley and FFT functions,as shown in Fig.5b.The intensity of PL spectrum is obtained through this kind of method.3.6.G peak position,I D /I G and relative PL intensityFig.6a and b shows the G peak position,relative intensity ratio I D /I G ,and relative PL intensity as a function of La 2O 3and CeO 2con-centration within the DLC films,respectively.The G peak positions of La 2O 3and CeO 2doped DLC films are higher than that of pure DLC films,which can be empirically used to identify the concentration of sp 2and sp 3,indicating the increase of sp 2concentration [40,41].Moreover,the relative intensity ratio I D /I D of DLC films increases with the doping of La 2O 3and CeO 2,which also suggests the increase of sp 2concentration,revealing the coherence between the relative intensity ratio I D /I G and G peak position,as seen in Fig.6a.The rel-ative PL intensities calculated from the area between deconvoluted PL lines and X horizontal axis of coordinates,of La 2O 3and CeO 2doped DLC films are obviously higher than that of pure DLC films,as shown in Fig.6b.Specially,the relative PL intensities of S2,S5and S6films are 18.4,12.4,and 14.8times of that of pure DLC films,respectively,which suggests the obvious increase of PL property and photoconversion efficiency.This is beneficial for the increase of efficiency of solar cells as anti-reflect coatings.Rare earth has been considered the most important optical activators for luminescent devices [42].The consequent appearence of deep electronic states,due to the doping of rare earth oxides in the amorphous DLC matrix,provides to this kind of composite DLC films liminescentpropertiesFig.5.PL of (a)DLC and seven kinds of la 2O 3and CeO 2doped DLC films and (b)S4film deconvoluted with four Gaussian lines.322Z.Zhang et al./Journal of Alloys and Compounds 476(2009)318–323Fig.6.(a)G peak position and relative intensity ratio I D /I G and (b)relative PL intensity as a function of La 2O 3and CeO 2concentration within the DLC films.and consequent emissions in the visible range of electromagnetic spectrum (625–700nm),preserving and sometines enhancing the intrinsic properties of DLC films.The fluorescent emission in rare earth oxides doped DLC films commonly comes from 5d to 4f or 4f to 4f electronic transitions in trivalent rare earth ually,the responsible of the emission are the rare earth ions [43,44].The obvious increase of relative PL intensity of La 2O 3and CeO 2doped DLC films is partially attributed to the change of concentration of sp 2and sp 3,fraction of sp 2/sp 3and sizes of sp 2and sp 3clusters [30].Furthermore,the La 2O 3nanoparticles [45,46]and coatings [47]can increase the PL intensity,and CeO 2powers [48,49],nanoparticles [50]and nanostructured CeO 2[51]films and CeO 2films [52,53]can also enhance the PL property.As nanoparticles consisting of La 2O 3,CeO 2and amorphous DLC structure form on the surface of deposited nanothickness films,the united actions of nanoparticles and films of La 2O 3,CeO 2,and amorphous DLC structure make the PL intensity obviously increase.4.ConclusionLa 2O 3and CeO 2doped DLC films have been deposited at the background Ar and O 2gases by UMS.As the La 2O 3and CeO 2are stable compounds,they will not decompose or oxidize with the time going on,which avoid the instable problem existing in the single element doped DLC films.With the doping of La 2O 3and CeO 2,the C element does not react with La and Ce elements,and the amorphous DLC structure is not changed.The PL property of La 2O 3and CeO 2doped DLC films obviously increase compared with pure DLC films,which is beneficial for the increase of photoconversion efficiency.With the forming of nanoparticles consisting of La 2O 3,CeO 2,and amorphous DLC structure,the PL intensities of S2,S5and S6films are one order magnitude higher than that of pure DLC films,thus this kinds of novel rare earth oxides doped DLC films have broad prospect in the fields of photoconversion devices and anti-reflect films.AcknowledgementsThe authors appreciate the financial support from the National Key Basic Research Program of China (Grant No.2009CB724306),the National Natural Science Foundation of China (Grant Nos.50535020and 50805017),the Natural Science Foundation of Liaon-ing Province of China (Grant No.20071083),the Open Foundation of the State Key Laboratory of Digital Manufacturing Equipment andTechnology,Huazhong University of Science and Technology,and the Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education of Dalian University of 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