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2016-Review—Quantum Dots and Their Application in Lighting, display and biology

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2162-8769/2016/5(1)/R3019/13/$33.00?The Electrochemical

Society

JSS F OCUS I SSUE ON N OVEL A PPLICATIONS OF L UMINESCENT O PTICAL M ATERIALS

Review—Quantum Dots and Their Application in Lighting,Displays,and Biology

Talitha Frecker,a,b Danielle Bailey,a,c,d Xochitl Arzeta-Ferrer,a James McBride,a,b,z and Sandra J.Rosenthal a,b,d,e,f,g,z

a Department of Chemistry,Vanderbilt University,Nashville,Tennessee 37235,USA

b Vanderbilt Institute for Nanoscale Science and Engineering,Vanderbilt University,Nashville,Tennessee 37235,USA

c Department of Interdisciplinary Materials Science,Vanderbilt University,Nashville,Tennessee 37235,USA

d Department of Pharmacology,Vanderbilt University,Nashville,Tennesse

e 37235,USA

e Department o

f Physics and Astronomy,Vanderbilt University,Nashville,Tennessee 37235,USA

f Department of Chemical and Biomolecular Engineering,Vanderbilt University,Nashville,Tennessee 37235,USA

g Materials Science and Technology Division,Oak Ridge National Laboratory,Oak Ridge,Tennessee 37831-6071,USA

Quantum dots have attracted considerable interest in the ?elds of solid state lighting,displays,and ?uorescent imaging.Their tunable optical properties by changing the size and solution processability lead to commercial applications.In this review,we focus on the advancement of white light emitting nanocrystals,their usage as the emissive layer in LEDs and display backlights,and examine the increased ef?ciency and longevity of quantum dots based colored LEDs.In addition,we also explore recent discoveries on quantum dots as biological labels,dynamic trackers,and applications in drug delivery.

Manuscript received July 16,2015.Published August 18,2015.This paper is part of the JSS Focus Issue on Novel Applications of Luminescent Optical Materials .

Colloidal quantum dots are one of the most visually compelling examples of how materials can behave differently at the https://www.doczj.com/doc/3b7350869.html,ing solution chemistry,one can grow crystals of a semiconductor in the reaction ?ask.In early growth,when the crystals are below 10nm in diameter,the bandgap of these semiconductor nanocrystals is size dependent,allowing for simple tuning of their absorption and emission spectra.Louis Brus was the ?rst to show that when the radius of the crystal is below the bulk Bohr exciton radius,con?nement en-ergy of the exciton modi?es the bandgap energy.1Murray et al.would later publish a seminal paper describing the synthesis of monodisperse CdSe nanocrystals.2The incredible power to tune a single material’s optical properties simply by size in addition to the added advantages of solution processability indicated early on that colloidal quantum dots could have commercial applications in lighting and display tech-nology.However,as synthesized,the ef?ciency of the light emitted is very low.In this review article,we will discuss emissive quantum dots and their uses in solid state lighting,displays,and biological applications.

Emissive Quantum Dots

Core shells.—The primary roadblock toward immediate commer-cial application was that the as-synthesized nanocrystals were not ef?cient nor photostable enough to begin to compete with contempo-rary lighting and display technologies.The semiconductor industry has long been aware of the technical challenges surfaces can https://www.doczj.com/doc/3b7350869.html,pared to thin ?lm devices,colloidal nanocrystals are dominated by surfaces.Speci?cally,the surface of a nanocrystal is composed of cations predominantly passivated by the surfactants used in the syn-thesis,while the anions remain mostly unpassivated and subject to oxidation.The dangling bonds as a result of under-coordinated sur-face atoms act as charge traps for photogenerated carriers,lowering the ?uorescence ef?ciency.

To eliminate these surface traps,Hines et al.developed a method of growing a shell of a wider bandgap material (Figure 1).3These early core/shell quantum dots demonstrated a dramatic increase in ?uorescence ef?ciency,which was on the order of 30%,and improved photostability.A start-up company (Quantum Dot Corp.)would use these crude core/shell quantum dots as a starting point for what would be the ?rst major commercial product based on colloidal quantum

E-mail:james.r.mcbride@https://www.doczj.com/doc/3b7350869.html, ;sandra.j.rosenthal@https://www.doczj.com/doc/3b7350869.html,

dots.Their goal was to build technology platforms based on stable and bright ?uorescence.This could evolve to the use of QDs as probes for biological https://www.doczj.com/doc/3b7350869.html,pared to conventional ?uorescent dyes,the quantum dots exhibit superior brightness and far narrower emission spectrum.Further,the continuum absorption of the quantum dots could enable multi-color imaging with the need for only one excitation wavelength.

At the time of the initial start-up,little was known about the nature of the inorganic shell and there were very few protocols for transfer-ring quantum dots from non-polar to aqueous,bio-friendly solutions.In order to develop improved shell coverage,advanced electron mi-croscopy in conjunction with Rutherford backscattering spectroscopy was employed to gain unprecedented look into the true shell cover-age.Through collaboration with Stephen Pennycook,atomic number contrast scanning transmission electron microscopy,or Z-STEM,was utilized to obtain the ?rst true images of the shell coverage.4,

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Unlike convention HRTEM,the intensity of the electrons collected by a high angle annular dark ?eld detector (HAADF)is dependent on the atomic number of the scattering atom.6This elegant mode of ‘what you see is what you get’imaging showed in great detail how the shell deviated strongly from the perfectly uniform coating oft depicted in cartoons.As shown in Figure 2,the shell clearly growing off of one surface,leaving the other surfaces of the core with minimal or no passivation.The result of this shell motif is an improved quantum yield but continued susceptibility to quenching and photobleaching.There were two possible reasons why the shell preferred to grow off of one surface.Since the CdSe cores had a wurtzite crystal struc-ture,not all surfaces are chemically equivalent,yielding surfaces that can be cation or anion rich.7,8In particular,the Se-rich surfaces are relatively un-passivated by ligands.This increases their reactivity re-sulting in accelerated shell https://www.doczj.com/doc/3b7350869.html,ter,with the atomic resolution afforded by aberration-correction,the shell growth would unambigu-ously be shown to favor the anion rich surfaces.To a lesser extent,lattice mismatch between CdSe and ZnS may also contribute to the anisotropic growth.After a monolayer of ZnS is grown on a surface,the shell material can relax creating a defect at the interface,allowing for latticed-matched shell growth.

With a clear picture and mechanistic understanding of the anisotropic shell growth,shells of CdS were chosen instead of ZnS.Aberration-corrected Z-STEM images shown in Figure 3indicate

that

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although the shell coverage is still preferentially covering the anion-rich surfaces yielding a bullet-shape particle,enough shell is grown to cover all the surfaces.

With the addition of amphiphilic polymer‘shell’,the quantum yield of this material approached unity in water.Currently,CdS is still the shell material of choice for CdSe and other core materials.Through successive ion layer addition and reaction(SILAR)processes,shells of CdS can be grown upwards to20monolayers.9These‘giant shelled’quantum dots exhibit extreme photostability as the core is believed to be completely isolated from the surface.10

White light-emitting nanocrystals.—The most common sources of lighting today are incandescent and?uorescent bulbs.These sources of light only have ef?ciencies of about5and25%,respectively.In both designs,most of the energy lost is due to the release of heat. Solid state lighting releases very little heat,requiring less energy to produce the same amount of light.A pure white light LED is needed to replace the traditional lighting.11Previous methods of producing white light LEDs have utilized a blue LED with a yellow?uorescent phosphor or combining red,green,and blue emitting nanocrystals. The yellow phosphors require rare earth elements and have a low color quality and a halo effect due to scattering.When using multiple sizes of nanocrystals to create the red,green,and blue emission,the emission intensity is decreased due to self-absorption.

First attempts to synthesize white light emitting nanocrystals in-cluded the combination of band edge emission with conventional deep trap states of ZnSe quantum dots.12In2005,Bowers et al.synthesized ultrasmall CdSe nanocrystals,via a pyrolytic method,that emit a broad spectrum of light from420to710nm.13These nanocrystals do not suffer from self-absorption due to their large Stokes shift of 40–50nm.This broad emission has three unique features.14The?rst peak at440nm is pinned at diameters less than1.7nm because of an energy state mediated by the alkyl phosphonic acid surface ligand. The second peak at480nm has an unknown origin,but it is believed to be due to residual tributylphosphine coordinated to the surface.The third broad peak at560nm is due to conventional deep trap emission believed to be associated with surface Se dangling bonds.With the high surface-to-volume ratio,the intensity of the third peak is equal to that of the blue and green peaks(Figure4).This discovery stimulated a tremendous amount of research to improve and provide alternates for solid state lighting.

Although this leads to a balanced white light with CIE(Com-mission Internationale de L’Eclairage,1931)coordinates of(0.32, 0.37),these nanocrystals suffered from a low quantum yield of10%. Rosson et al.brightened the ultrasmall CdSe nanocrystals by per-forming a post-synthesis treatment with formic acid.15This treatment increased the quantum yield up to45%(Figure5).The formic

acid Figure5.Absorbance(dashed)and emission(solid)of original(blue)and formic acid treated(red)CdSe nanocrystals.Inset:Vials containing concen-trated white-light CdSe nanocrystal solutions before(left)and after(right) a sample formic acid treatment.Reprinted with permission from Ref.15. Copyright2012American Chemical Society.

ligand partially exchanges with the phosphonic acid ligand,while also passivating previously non-radiative trap states.With this increase in quantum yield,the CIE coordinates also change to a bluer emission of(0.24,0.24).

Recently,Dolai et al.reported ultrasmall,white light-emitting CdSe nanocrystals synthesized via a low temperature method.16This is possible due to the highly reactive,phosphine-free Se precursor. This reaction produces a broad emission consisting of a sharp band edge emission and a broad trap state emission from various surface de-fects.The CdSe nanocrystals were synthesized with either oleylamine (CdSe-OLA)or both oleylamine and benzoic acid(CdSe-OLA/BA). The CdSe-OLA displayed CIE coordinates of(0.31,0.32)and a QY of11.2%.The CIE coordinates values shifted to(0.33,0.34)and the quantum yield increased26.9%when CdSe was synthesized with OLA and BA(Figure6).When semiconductor nanocrystals are syn-thesized in the presence of anionic ligand,such as benzoic acid,the surfaces are found to be metal rich.This Cd(O2CPh)2is bound to the surface Se sites,whereas the OLA is bound to the surface Cd sites. This passivation of both the surface Cd and Se prevents the formation of charge trapping sites,leading to the increase in the QY.

Another method of generating white light emitting nanocrystals is by doping.This was?rst done in2007by Nag et al.by combining the surface state emission of CdS nanocrystals(broad peak,～500 nm)with the inner core transition,4T1-6A1,emission of doped Mn2+ (620nm).17The emission is extended to longer wavelengths compared to conventional trap states because of the dopant.The advantage of using a dopant material,such as Mn2+,is the decreased sensitivity of the chromaticity due to particle size and distribution.With a dopant, the emission color is not dependent on the bandgap emission,but on the ratio between the surface state and dopant emission.Since the intensity of the emission at620nm can be tuned by changing the Mn2+concentration,the shade of white light can be altered.When the CdS nanocrystals with an average particle size of1.8nm were doped with0.10%and0.19%Mn2+,the CIE chromaticity coordinates were (0.30,0.40)and(0.35,0.40),respectively(Figure7).As the Mn2+ concentration increased,the peak at620nm increased,generating a more yellow emission.Although the tunability of white light is advantageous,these doped particles only have a measured quantum yield of about2%.

To avoid using toxic materials,such as cadmium,Sharma et al.reported tunable white-light emission from Mn-doped ZnSe nanocrystals.18This white-light emission originates from three emis-sion states.The?rst,at410and435nm,is from the MnSe cores; the second,at520nm,from Zn-vacancy related defect states;and the third,at580nm,from the Mn-dopant.Once again,the emission can be tuned to emit different shades of white,in this case by varying the excitation wavelength from325to400nm(Figure8).Spherical

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Figure 6.(a)UV ?vis absorption (black)and PL (blue)of CdSe(OLA)nanocrystals.(b)CIE coordinates (0.310,0.316)of CdSe(OLA)nanocrystals.(c)UV ?vis absorption (black)and PL (blue)of CdSe(BA/OLA)nanocrystals.(d)CIE coordinates (0.330,0.337)of CdSe(BA/OLA)nanocrystals.Reprinted with permission from Ref.16.Copyright 2015American Chemical Society.

MnSe cores are ?rst synthesized (2and 4nm diameters),followed by three Zn injections.As the number of Zn injections increased,the shaped turned from spherical to branched with a core size of 4.3nm and branch length of 5.2nm.With this deviation from a spherical to a branched shape,the PL intensity decreases.When excited at 300nm,these nanocrystals have luminous ef?cacy up to 401lm/W,and CIE coordinates of (0.34,0.46).As the excitation wavelength is increased,the luminous ef?cacy decreases,but the emission color shifts toward pure white light.

An issue that has been found with previous methods of doping nanocrystals is their stability.Since there is a strong lattice mismatch between the nanocrystal lattice and the doping atoms,the dopants are found mainly on the surface.This leads to low luminescence inten-sity in aqueous environments.By doping with both Mn and Cu into separate layers,Wang,C.et al.was able to generate a stable aqueous white-light generator.19Nanocrystals were synthesized by ?rst prepar-ing a Mn-doped ZnSe core,followed by a ZnS layer.CuCl 2was then injected to obtain a surface doped Mn:ZnSe/Cu:ZnS nanocrystal.To increase the stability of the nanocrystals,they were then enveloped with additional ZnS layers.The Mn:ZnSe emit at 480and 585nm,due to ZnSe trap states and Mn-dopant states,respectively.After the ?rst layer of ZnS was synthesized,the PL intensity increased without a change in the peak wavelengths.The trap emission at 480nm shifted to 477nm after Cu doping.As the outer ZnS layers are added,the PL increases in intensity due to passivation of ZnSe surface defects.Also,as the number of ZnS layers are increased,the CIE coordinates shift from (0.29,0.32)to (0.41,0.38)(Figure 9).Values for quantum yield were not given.Most recently,Pramanik et al.reported forming two different com-plexes with organic ligands on the surface of Mn:ZnS nanocrystals that display white light.20The prepared Mn-ZnS nanocrystals were reacted with a mixture of hydroxyquinoline (HQ)and acetylsalicylic acid (ASA)to form MQ 2and M(ASA)2,where M =Zn and Mn.The M(ASA)2emits at 410nm,ZnQ 2at 500nm,and Mn 2+-dopant at 588nm.The intensity of these peaks can be tuned by changing the concentration of the ligands (Figure 10).When reacted with a 1:1mo-lar ratio of HQ and ASA,the complexed Mn:ZnSe nanocrystals have a 2.2%QY and exhibit CIE coordinates of (0.30,0.33)when excited at 320nm.It was also found that as the excitation was changed,the chromaticity could be tuned.This complex is also photostable in the solid state,which will be useful in future light-emitting diodes.

Another more environmentally friendly method of generating white light from semiconductor nanoparticles is by using Group IV materials.Carbon containing nanoparticles have been used as a light emitting material since 2006.There are many facile methods of synthe-sizing carbon nanoparticles that are capable of controlling shape,size,and physical properties.Wang,F.et al.produced carbogenic (oxygen containing carbon)nanoparticles by thermal oxidation of citric acid in molten LiNO 3,which serves as an oxidizing and basic media.21The surface of these particles are passivated with poly(ethylene gly-col)(PEG 1500),which allows them to be dispersed in various sol-vents.When excited with blue light,the carbon nanocrystals have a broad emission that covers the entire visible spectrum.After passiva-tion with PEG 1500,the quantum yield increases from 3%to 10%.As the excitation wavelength is increased,the emission shifts to longer wavelengths.At an excitation wavelength of 407nm,the carbon

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Figure7.(a)UV-visible absorption and PL spectra of0.10and0.19%Mn2+-doped CdS NCs.(b)CIE diagram showing the chromaticity coordinates of the produced white light of different shades corresponding to the PL spectra of(a). Reprinted with permission from Ref.17.Copyright2007American Chemical Society.

nanocrystals have CIE coordinates of(0.31,0.32),almost pure white(0.33,0.33).Mao et al.reported similar quantum yields (9.0%)with carbon dots synthesized by a one-step pyrolysis of poly(acrylic acid)(PAA)mixed with glycerol(Figure11).22The PAA is the carbon source,while the glycerol is used as the

sol-Figure9.(a)(CIE)color coordinates of the Mn:ZnSe/Cu:ZnS QDs with dif-ferent growth times of the ZnS layers outside the Cu impurities The corre-sponding CIE coordinates are(0.29,0.32),(0.32,0.36),(0.33,0.32),(0.35, 0.36),(0.39,0.37),and(0.41,0.38).(b-d)Images of QDs under the irradiation of a365nm UV lamp(10W).(b)Aqueous Mn:ZnSe/Cu:ZnS QD solution (left)and water(right).(c)Mixture of QDs and PVP aqueous solution.(D) Spin coating of the mixture in(c)on a glass slide.Reproduced from Ref.19 with permission of The Royal Society of Chemistry.

vent and for surface passivation.Unlike the carbon nanocrystals passivated by PEG1500,these carbon nanocrystals have a more monodisperse sample,with an average size of3.4nm.These par-ticles also show chromaticity tunability due to varying the excita-tion wavelength.The9.0%quantum yield was obtained when ex-cited at347nm and had CIE coordinates at(0.24,0.29).After30h of UV irradiation it was found that the PL intensity does not decrease. Another extremely abundant and nontoxic Group IV material that has been used,but not widely developed as a white-light generator are silicon nanocrystals.

Lee et al.synthesized silicon nanocrystals from sodium silicide in organic solvents using ultrasonic energy and adding a SiCl4modi?er.23 After an hour of sonication,the nanocrystals emit from340to700nm (Figure12).Neither quantum yield nor CIE coordinates were given for this experiment.

Applications

Lighting.—Colloidal quantum dots are ideal in the application as the emitting layer in light emitting diodes(LEDs)due to their tunable colors,bright emission,solution processability,and stability.The ear-liest experimental QD-LEDs were fabricated in the early1990s and were comprised of a CdSe core or a core/shell QD–polymer bilayer or blend charge transport layer(CTLs).24External quantum

ef?ciencies Figure8.(a)PL spectra of the modi?ed Mn-doped ZnSe NCs under excitation at various wavelengths.(b)CIE chromaticity diagram for Mn-doped ZnSe NCs. Reprinted with permission from Ref.18.Copyright2014American Chemical Society.

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Figure 10.(a)UV ?vis and (b)emission (λex =320nm)spectra of (i)ligand-free Mn2+-doped ZnS QDs and (ii)ASA-added Mn2+-doped ZnS QDs.(c)Excitation spectra of (i)ligand-free Mn2+-doped ZnS QDs at λem =588nm and (ii,iii)ASA-added Mn2+-doped ZnS QDs at λem =410and 588nm,respectively.(d)Emission (λex =320nm)spectra of QDs following complexation with the mixture of 1.0mM of ASA and HQ with ratio:(i)0:0,(ii)1:0,(iii)4:1,(iv)3:2,(v)1:1(vi)2:3,(vii)1:4,and (viii)0:1.Reprinted with permission from Ref.20.Copyright 2015American Chemical Society.

(EQE)of up to 0.22%were reported.The low EQE is due to parasitic polymer electroluminescence.The next generation of QD-LEDs was introduced in 2002by Coe et al.25In this study,a monolayer of QDs is sandwiched between two organic small molecule thin ?lm CTLs,recording a EQE of 0.5%.Anikeeva et al.fabricated QD-LEDs by microcontact printing the QD layer,which avoids exposing the or-ganic charge transport layer to solvents.26These devices reported

maximum EQE of 2.7%for orange emission.The third generation of QD-LEDs replaced both of the organic with inorganic charge trans-port layers.Inorganic CTLs are more stable to air compared to organic ones.This stability could lead to higher current densities.Examples of inorganic charge transport layers are sputtered metal oxide thin ?lms.The energy band of these oxides can be ?ne-tuned due to the variety of compositions.Inorganic layers are also more conductive than the organic layers.Caruge,et al.applied this method using zinc tin oxide and NiO as the n-and p-type CTLs,respectively.27The re-ported EWE was low at <0.1%,due to damage of the QD layer during the sputtering process.The fourth generation of QD-LEDs consists of hybrid organic and inorganic CTLs.Generally the n-type layer

Figure 12.(a)Room temperature photoluminescence spectra of silicon col-loids.The excitation wavelength used was 325nm He–Cd laser.(b)The color photograph displays white as the sample is irradiated with the commercial low-intensity UV lamp (360nm).Reproduced with permission from Ref.23.Copyright 2004Wiley InterScience.

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Figure13.(a)Normalized electroluminescence spectra and images of blue,green and red QD-LEDs.(b)CIE coordinates of the three QD-LEDs(triangles) compared to the NTSC color standards(stars).(c)Current ef?ciency(ηA)andηEQE as a function of luminance of the best performing red and blue QD-LED based on quantum dots.Inset:normalized electroluminescence spectrum of the red and blue QD-LEDs.Reprinted by permission from Macmillan Publishers Ltd: Nature Photonics Ref.28,copyright2015.

a metal oxide which,in recent research,has been solution processed metal oxide nanoparticles.Since the QD layer is not damaged during fabrication,EQEs have greatly improved.A series of blue,green, and red QD-LEDs with external quantum ef?ciencies over10%,have been reported by Yang et al.that are fabricated with an electron trans-port layer(ETL)of ZnO nanoparticles.28They have achieved these ef?ciencies by tailoring the nanostructure and composition of the quantum dots.A graded alloy intermediate shell is inserted between the CdSe core and the ZnS outer shell to reduce Auger recombina-tion.This recombination occurs when an electron and hole recombine and transfer the energy to an electron in the conduction band,which thermalizes back down to the valence band.ZnSe provides better con-?nement of the electron wavefunction compared to the commonly used outer shell material,CdS,due to the higher conduction band. Since thick shells of ZnSe and ZnS are not necessary to guarantee car-rier con?nement in the QD core,charge injection into the QD-LEDs is improved,leading to long device lifetime with a high ef?ciency. Green QD-LEDs fabricated with the ZnSe rich QDs had an external quantum ef?ciency and current ef?ciency(ηA)of14.5%and63cd A?1,compared to that of7.5%and31cd A?1from CdS rich QDs.In addition,the blue and red QD-LEDs had EQEs of10.7%and12.0% andηA of4.4and15cd A?1,respectively(Figure13).

Shen et al.reported blue QD-LEDs with a comparable EQE of 10.3%and a maximumηA of1.3cd A?1.29This QD-LED is fabricated using ZnCdS/ZnS graded core-shell QDs with1-octanethiol capping ligands.These capping ligands are shorter than the previously used oleic acid,and allow for an increase in charge mobility.Evidence for this is given by the low turn-on voltage of about2.6V and a maximum luminance of7600cd m?2(Figure14).

Displays.—In1953,the National Television System Committee (NTSC)developed one of the?rst color standards that tracked color separately from brightness.30This standard was based on the best cathode-ray-tube materials available at the time,although it was not until the2010s that this standard was able to be fully met.In many smaller devices,OLED displays can now exceed this color standard, although this technology has been slow to move toward larger systems. This is due to the lower cost of production of LCDs.Although LCD displays dominate in the display market,they can only reach about 70%of the color performance of OLEDs.LCDs can reach the sRGB color gamut at best because of the white LEDs that are used as the backlight to illuminate the liquid crystal module(LCM).There are millions of pixels that are divided into three subpixels(red,green, and blue)in the LCM.The color of the subpixel is controlled by the ef?ciency of the?lter and of the spectral energy of the white light from the backlight.To make a high quality color,the?lter either needs to be very narrow,which will let less light through,or the speci?c color component from the white light must be narrow and tuned to the desired wavelength.The LED light source does not meet the requirement for narrow color peaks.Quantum dots can solve this problem owing to their narrow emission and high quantum yield. QD Vision and Nanosys are currently developing products that can easily be placed in existing LCD manufacturing.The quantum dots are packaged into either a tube(QD Vision)which is placed adjacent

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Figure 14.(a)Schematic illustration of energy levels for the multilayer QD-LEDs.(b)Current density (J)and luminance (L)of the devices based on QDs with OA and OT ligands as a function of driving voltage (V).(c)External quantum ef?ciency (ηEQE)and current ef?ciency (ηA)of these devices as a function of L.Reprinted with permission from Ref.29.Copyright 2015American Chemical Society.

to edge-lit LEDs,or in a ?lm (Nanosys)on top of the LED backlight.Recently,quantum dot LEDs are being researched as the backlight in displays.Kim et al.fabricated a full color QD display using a solvent-free transfer printing method to pattern the individual red,green,and blue emitting QDs onto the pixelated display panel.31Previous methods of patterning QDs onto the pixels,such as spin-coating,cause a cross-contamination between the three sizes.Other methods lead to non-uniform ?lms with a rough surface.This results in a decrease in the charge transfer and a decline of quantum ef?ciency.In the transfer printing method,QDs are spin-coated onto a surface modi?ed donor substrate,followed by the application of a polydimethylsiloxane (PDMS)stamp to the QD ?lm.Due to the surface energy of the stamp being less than that of the donor substrate,with suf?cient pressure,the QDs are picked up.The QDs are then transferred to a device stack into an array of narrow https://www.doczj.com/doc/3b7350869.html,pared to a ?lm that was fabricated using a spin-coated method,the density of the printed ?lm has a 20%greater density of QDs.When integrated into an organic/inorganic hybrid LED structure,the printed red,green,and blue QDs had brightness values of 16,380,6,425,and 423cd m ?2and current ef?ciencies of approximately 3.00,0.55,and 0.05cd A ?1,respectively (Figure 15).These values are about 25–52%greater than the values collected with the spin-coated device.The method of QD printing was employed to produce a 4-inch full-color active matrix QD display that showed a near perfect image.Due to the elastomeric PDMS stamp,this printing method can transfer QDs onto ?exible substrates,which is necessary for scaling up to a roll-to-roll system.

As previously noted,Yang et al.fabricated red,green,and blue LEDs with EQEs greater than 10%.28In the report,the red and green QD-LEDs were then fabricated into 4.3inch monochrome active ma-trix displays with a resolution of 480×800on low-temperature

poly-silicon (LTPS)thin-?lm transistor backplanes as a preliminary demonstration toward the application of QD-LEDs in displays.

Biological Applications

QDs as biological labels.—Quantum dots exhibit several charac-teristics that make them appropriate for biological applications.As previously stated,they demonstrate broad excitation,narrow size-tunable emission and a strong resistance to photobleaching,mak-ing them preferable to conventional organic dyes for long term data collection.32

The synthesis of QDs results in surface-bound organic nonpolar ligands that are not biocompatible.Surface modi?cation is required before the QDs are viable.Initially,surface modi?cation was done through ligand exchange,as seen in Rosenthal et al.2002.33TOPO surface ligands of CdSe/ZnS core shell QDs were ?rst exchanged with pyridine in order to allow attachment of serotonin-based linkers for serotonin transporter protein labeling.The QDs were further modi?ed by the addition of mercaptoacetic acid,a short polar ligand.Electro-physiological measurements were taken to show binding of these serotonin-linker arm conjugated nanocrystals (LSNACs).However,the exchange of the surface ligands results in ?uorescence quenching;in this example the QDs went from 38%?uorescent quantum yield to 3%after all modi?cations.

To overcome this ?uorescent quenching,an amphiphilic polymer was added instead to both encapsulate the nonpolar ligands without displacement and have polar reactive groups exposed on the outer surface.34Also of note is the addition of poly-ethylene glycol (PEG)chains to the surface of the QDs.While amphiphilic QDs

increase

Figure 15.(a)Schematic of transfer printing process for patterning of quantum dots.(b)Fluorescence micrograph of the transfer-printed RGB QD stripes onto the glass substrate,excited by 365nm UV radiation.(c)Electroluminescence image of a 4-inch full-color QD display using a HIZO TFT backplane with a 320×240pixel array.Reprinted by permission from Macmillan Publishers Ltd:Nature Photonics Ref.31,copyright 2011.

ECS Journal of Solid State Science and Technology,5(1)R3019-R3031(2016)R3027 Table I.Small molecules used for neurotransmitter transporter labeling.

Name Target Small molecule structure QD surface ligand

Serotonin(LSNACs)33SERT

TOPO

IDT31843,48SERT

Steptavidin

Biotin-GBR1290949DAT

Steptavidin

IDT44450,51DAT

Steptavidin

Muscimol47GABA

Carboxyl-AMP

solubility,Bentzen et al.2005found that surface addition of PEG chains reduces nonspeci?c binding during live cell labeling.35 QDs can be functionalized with terminating groups on surface ligands such as carboxylic acid,azide,amine or streptavidin/avidin protein.Carboxyl dots can be conjugated to amine terminated molecules,while amine dots can be conjugated to either amine or thiol-terminated molecules.One of the most proli?c applications in-volves binding streptavidin/avidin conjugated QDs to biotinylated terminated molecules.CdSe/ZnS core shells are available commer-cially for imaging and can be purchased with surface streptavidin for bioconjugation.36

These well characterized constructs allow for cellular label-ing using antibodies,37peptides/proteins,38–40sugars41or small molecules42–44in?xed and un?xed cell and tissue cultures.Both in-tracellular and extracellular labelling has been accomplished,with extracellular labeling resulting in more speci?c binding due to abil-ity to wash away unbound QDs.45Surface proteins with a revealed extracellular component can be labeled speci?cally through use of antibodies,through either an exposed amino acid sequence,or,in the case of extracellular termini,through use of encoded fusion tags such as GFP.46Additionally,small molecule-functionalized QDs can be anchored to an activation site or binding pocket.33,43The Rosen-thal group specializes in such small molecule functionalization in order to visualize and track neurotransmitter transporters(Table I), starting with the synthesized LSNACs33and then making use of com-mercially available AMP-QDs for muscimol-QD conjugates,47and ?nally taking advantage of streptavidin-coated QDs for easy labeling with biotinylated ligands.48–50

In order to label intracellular structures,the cells need to elicit endocytosis of the QDs.Nonspeci?c uptake has been studied using transferrin-conjugated QDs in an attempt to study cancer markers,52 while speci?c labeling of cytoskeletal actin has been done using lipid-associated phalloidin-conjugated QDs to penetrate living cells.53 However,intracellular quantum dots are susceptible to endosomal trapping and aggregation,and dif?culty in distinguishing bound and unbound probes.45,53

QDs as dynamic trackers for biological events.—In live cells and tissue cultures,QDs can be used to dynamically track different cellular targets and events.The bright emission of QDs allows for estimation of the probe down to a pixel resolution,and easy determination of the position of individual labels which can then be mapped.Unlike organic dyes,the photostability of QDs allows for both longer and more detailed trajectories to be obtained.54The method of obtaining trajectory data is described in Figure16.55

The construction of trajectories allows for statistical analysis of molecule displacement,velocity,and diffusion coef?cient.Mapping mean-square displacement as a function of time,allows for the de-termination of diffusion mode.56This allows for quantitative analysis of the cellular targets,and de?ning the diffusion mode allows for in-sights into the functionality of the target both natively and in response to stimuli.An example of this is con?ned mobility seen in membrane compartments or a directed motion seen in motor proteins.42 In2003,the?rst single protein tracking using QDs studied the 2D diffusion dynamics of glycine receptors in spinal neuronal cells.57 The Rosenthal group has since used QDs to study the membrane

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Figure16.(a)A Gaussian?t of intensity is used to determine location of particles from a timelapse series.(b)the x and y positions derived from the Gaussian?t allows for the construction of a trajectory with positioning related to t,allowing for plotting mean square displacement(MSD)as a function of time.(c)The?tted slope of MSD vs time allows for determining the mode of diffusion,where a linear slope denotes a normal or Brownian diffusion. Reprinted by permission from Macmillan Publishers Ltd:Molecular Therapy Ref.54,copyright2011.

dynamics of the dopamine and serotonin neurotransmitter transporters (DAT,SERT)and their relationship to signaling pathways,such as lipid raft membrane domains and kinase activation pathways.58,48,50 They found that DAT and SERT demonstrate con?ned diffusion,due to localization with lipid raft membrane domains and as such will exhibit increased mobility when the domains are disrupted.48,50In addition,variants of the wild-type proteins can be tracked to correlate functionalization and mobility,50

QDs can also be used to study endocytic traf?cking,such as Li et al.demonstrating the effect from Paclitaxel exposure on the rate of uptake of epidermal growth factor in cancer cells.44

Biermann et al.has demonstrated short trajectory tracking of gly-cosylphosphatidylinositol(GPI-anchor)-GFP mobility in organotypic mouse brain slices.37

Drug delivery applications using QD-FRET-based nanosensors.—Developing more ef?cient and precise disease detection and drug de-livery systems leads to timely and accurate diagnoses and subsequent treatments.Recently,nanomedicine,and speci?cally quantum dots, has emerged as an alternative to traditional drug delivery and sens-ing approaches that exploits QD properties such as brightness and photo-stability,size-tunability,and multi-functionality,incorporating a variety of drugs and ligands on the surface.59One popular QD-based approach includes using QDs in conjunction with F¨o rster resonance energy transfer(FRET)as a nanosensor system.FRET involves the transfer of electronic energy from a donor chromophore to an ac-ceptor chromophore.60This transfer occurs through intermolecular dipole-dipole interactions over distances as small as10?to100?, making it an ideal tool for determining distances between biological molecules,for acting as a sensor when two?uorophores are present and interacting,and for con?rming that drugs have reached their target locations.QDs are typically used as donors in FRET interactions,as emission intensity changes can easily be quantitated in the presence of acceptor molecules.61The narrow emission spectra,the diverse sur-face functionalization capabilities,small size,and brightness of QDs have made them ideal donor molecules for use in FRET-nanomedicine applications.

Zhang et https://www.doczj.com/doc/3b7350869.html,ed single quantum dots conjugated to DNA probes to detect DNA implicated in genetic diseases.62They take advantage of a single-strand DNA“capture probe”labeled with biotin that binds to a streptavidin-coated QD,and the target DNA sequence becomes sandwiched between this sequence and another single-strand DNA reporter probe,which also includes a?uorophore(Figure17).When the DNA target is present,the QD,which acts as a donor,is brought into close proximity with the acceptor,which is the reporter probe. The QD can then non-radiatively transfer energy to the acceptor,and the acceptor emits a photon,allowing for easy con?rmation that the DNA target is present.As expected,in the absence of the acceptor, the FRET ef?ciency is zero,since all of the?uorescence is from the donor.As acceptor/donor ratios increased,so did the FRET ef?ciency, con?rming the presence of the DNA target of interest.In another ap-plication,Bagalkot et https://www.doczj.com/doc/3b7350869.html,e a QD-aptamer with a?uorescent drug loaded into the aptamer.63When the drug is loaded onto the QD,both the QD and drug are in the“off”state due to a FRET interaction.The aptamer serves to speci?cally target the cancer cells,and the particle is then taken up into the cell.Once the drug is released from the QD, both the QD and the drug are?uorescent again,as the FRET interac-tion can no longer take place.This?uorescence con?rms that the drug has reached its target.This design offers the potential to revolution-ize the way speci?c diseases,including cancer,are treated.Prasuhn et al.take advantage of two QD-conjugated dye-labeled peptides to detect caspase3,a protease of interest in cancer research,and cal-cium ions,which are critical in many biological pathways.64When caspase3is present,the peptide is cleaved from the QD,disturbing the FRET interaction,resulting in QD emission.Alternatively,when calcium is present,it increases dye emission of the peptide.These applications utilize straightforward chemistries that can be applied to other molecules and ions of interest,making this a diverse and relevant method.

QDs will continue to be a material of choice for biosensing applica-tions.Their large surface area allows for multivalent functionalization, increasing FRET signal and allowing for widespread applications. Unique QD functionalization and chemistries will continue to evolve, offering widespread applications as nanosensors.

Speci?c targeting for in vivo imaging.—One advantage to having speci?cally binding QDs includes targeted tumor imaging.In vivo targeting is advantageous to in vitro targeting as it provides more complex information about the natural environment of the protein

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Figure 17.Zhang et al.design a QD nanosensor using a ?uorescent reporter probe that has a FRET interaction with the QD when the target DNA is present.Reprinted by permission from Macmillan Publishers Ltd:Nature Materials,copyright 2005.

of interest.Zhang et al.have synthesized speci?c CdTe:Zn 2+QDs functionalized with a phosphorothioate DNA aptamer with a target-ing sequence.65These QDs target lung cancer tumors in vivo for easy and speci?c tumor identi?cation.Non-aptamer QDs show no signal,further con?rming speci?city.These QDs offer exciting possibilities for advancements in disease detection and eventual treatments.Han et al.conjugate tetrazine-modi?ed antibodies to quantum dots passi-vated with a polyimidazole ligand including norbornene,which main-tains an overall neutral charge,decreasing nonspeci?c binding.66Us-ing their design,they target a rare cell population in bone marrow at the single cell level in live animals using multiphoton microscopy.The advantages to their QD design include high stability and quan-tum yield in vivo,low nonspeci?c binding,compact size,and easily adaptable for a variety of targeting antibodies.

Summary and Future Prospects

With the enhancements in stability,brightness,and water solubil-ity,the commercialization of quantum dots has been on the rise in recent years.QD Vision,founded in 2004,is focused on producing nanomaterials for the use in lighting and display technologies.Most recently,they have released their quantum dot-based Color IQ tech-nology.With a color performance that achieves 100%of the NTSC standard compared to 60–70%from most mainstream LCD designs,quantum dots are ideal for use in TV displays.At the 2015Consumer Electronics Show,QD Vision along with TCL Multimedia debuted the ?rst quantum dot based TV ,TCL 55 4K UHD.67Nanosys,founded in 2001,is producing a Quantum Dot Enhancement Film,QDEF,which is compatible with current LCD displays as a replacement backlight ?lm (Figure 18).At a fraction of the cost,this product exceeds the color output and ef?ciency of OLEDs.

Figure 18.A sheet of QDEF (left)can be seen converting some of the blue light emitted by a BLU (right)into white.Reprinted with permission from Ref.30.

In addition to being commercially produced for display and lighting technologies,quantum dots are also utilized as ?uorescent probes in biological assays.Life Technologies produces standardized aqueous QDs with various functional groups depending on biolog-ical labeling need.Standard functionalization includes amino-and carboxyl-terminated ligands,surface streptavidin,surface biotin,or antibodies.36

Precise control of emission.—As could be seen by Rosson et al.,performing a post-synthesis treatment on the ultrasmall white light-emitting CdSe changed the emission intensity of the energy state correlated to the surface ligand.15It has been shown that for ultrasmall sizes of CdSe,the emission is no longer controlled by band edge emission,but is pinned at a speci?c energy.14In 2009,Schreuder et al.reported a tunability of the wavelength of the pinned emission by varying the electronegativity of the alkyl phosphonic acid surface ligand.68As the alkyl chain length was increased,the wavelength of the pinned emission blue shifted from 425to 445nm,due to the de-creasing electron withdrawing nature of the ligands on the adjacent dangling bond of the Cd.Varying the alkyl phosphonic acid chain length also led to a change in the quantum yield of the QDs.As the chain length decreased,the quantum yield also decreased.This may be due to the stronger bond between the Cd and the ligand leading to few ligands being removed during the cleaning process,and prevent-ing trap states from forming.By applying the knowledge of the effect of ligand electronegativity and the increase in quantum yield with a post-synthesis treatment,precisely tuning the emission spectrum of the ultrasmall white-light emitting CdSe QDs is a possibility.Correlating atomic structure with optical properties.—Recently,Or?eld et al.have developed a reliable methodology that enables direct correlation of the optical properties of an individual quantum dot with its atomic structure.69Commercial SiO 2TEM grids with 8nm thick were used as support ?lm for both single particle ?uorescence and high resolution STEM imaging,while polystyrene spheres were used as ?ducial markers.As a proof of concept,commercial QD655quantum dots were imaged under wide ?eld conditions to measure their blinking behavior.Afterwards,the same quantum dots were identi?ed under Z-STEM condition in an electron microscope and their atomic structures were imaged.A total number of 84particles were correlated,allowing for ?rst time,direct evidence that stacking faults negatively impact the ?uorescence of colloidal quantum dots.An example of some of the correlation data is shown in Figure 19.Additionally,shell coverage was also directly shown to impact a quantum dots ‘on time’.Ultimately,this new methodology will accel-erate the development of emergent quantum dot systems by allowing for the identi?cation of the best performing structures enabling tar-geted synthesis strategies.Further,one can speci?cally design struc-tures with pre-determined blinking rates.Quantum dots with dis-tinguishable on times will open a new modality for single particle tracking experiments.

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Figure19.Top:blinking traces and atomic resolution STEM image of the same quantum dots.Bottom:expanded view of the blinking trace in(a)showing the‘on’and‘off’threshold.Reprinted with permission from Ref.70.Copyright 2015American Chemical Society.

Summary

As discussed in this review,the stability,brightness,and process-ability of quantum dots makes them a material of choice for a variety of applications.The quantum dots technologies described above will al-low scientists to target problems pertaining to lighting ef?ciencies and color,display resolutions,and specialized drug delivery and imaging at the molecular level.

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