Quantum Dot Light-Emitting Diodes in the Visible Region: Energy

Nov 19, 2013 - The redox energy level of the dithiols, on the other hand, lies above the ..... nanocrystals under different applied voltages (shown in...
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Quantum Dot Light-Emitting Diodes in the Visible Region: Energy Level of Ligands and Their Role in Controlling Interdot Spacing and Device Performance Saikat Bhaumik and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India ABSTRACT: In fabricating quantum dot light-emitting diodes (QDLEDs) based on manganese-doped zinc sulfide (Mn-doped ZnS) nanocrystals, we use a range of dithiolcontaining ligands in forming thin films of nanocrystals to study the role of interdot spacing on the performance of the QDLEDs. Use of dithiols as ligands to the doped nanocrystals is crucial since the redox energy levels of such ligands lie below both the d-states of Mn ions, and since a transition between the d-states yields photoluminescence (PL) emission in the doped system, the ligands could not quench the PL emission of the nanocrystals. In thin films of nanocrystals, the length of the ligands controls the intensity of the PL emission due to several factors. When the length of the ligands is long, the ligands on one hand increase the PL intensity, and they also reduce the conductivity of the emitting layer due to an increase in the interdot spacing. We observe that the performance of the QDLEDs is optimum at an intermediate length of the ligands. The spectral range of electroluminescence (EL) emission does not depend on the length of the ligands since the PL emission of Mn-doped ZnS nanocrystals, and hence also the EL of the devices, appears to be due to a radiative transition between the d-states of manganese ions.

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dots dispersible for facile film formation, the ligands, in thin films of the nanostructures, also act as insulators for the interdot conduction process. That is, while the long-chain ligands enhance PL emission by reducing aggregation of the nanostructures, the short ones favor electrical conduction through the quantum dots.23,24 The use of dithiols as ligands may become useful since such ligands would reduce the interdot distance and hence enhance conductivity of nanocrystalline thin films. The redox energy level of the dithiols, on the other hand, lies above the valence band-edge of a range of nanocrystals (such as CdSe, CdTe, etc.) and thus acts as a trapping site, thereby quenching the PL emission.25 To circumvent the problem introduced by the trapping sites of the ligands, manganese (Mn) as dopants may become useful. Since the PL emission in Mn-doped systems appears due to a radiative transition between the d-states (4T1 → 6A1) of Mn ions26 and since the redox levels of dithiolcontaining ligands lie below the 6A1 level of Mn ions, the energy level of dithiols could not quench the PL emission of the doped nanostructures. In this manuscript, we report the role of different dithiols as ligands of Mn-doped ZnS nanocrystals on the visible QDLEDs. We have varied the length of the dithiol ligands in this direction.

INTRODUCTION Researchers have incorporated colloidal quantum dots in a range of devices such as light-emitting diodes (LEDs),1,2 solar cells,3−6 photodetectors,7 field-effect transistors,8 sensors,9 etc. The ability to form solution-processed films, apart from the other usual advantages of quantum dots in general, makes the lower-dimensional materials appropriate in fabricating the electronic and opto-electronic devices. A range of materials, such as oxides,10,11 binary (II−VI and IV−VI),1,2,12−14 quaternary (I−II−III−IV2),15 doped,16,17 and alloyed semiconductors,18 in the form of nanostructures have been used in the LEDs. Depending on the choice of the material, the focus of the quantum dot LEDs (QDLEDs) has spread from brightness to the range of wavelength (for near-infrared emission)12−14 or color tuning.15,17,19,20 In the QDLEDs based on these direct bandgap semiconductors, the origin of photoluminescence (PL) emission, and hence also of the electroluminescence (EL), depends on the material itself. Depending on the selection of the semiconductor, the emission may be termed as band-edge emission, 2 defect-state emission, 10 donor−acceptor pair recombination,15,21,22 or emission involving one or both dopant level(s).16,17 In the QDLEDs, the choice of ligands of the quantum dots is crucial since the ligands affect several parameters of the devices, such as PL (and EL) emission, interdot distance for charge transport, etc. The ligands expectedly control dispersion of the nanostructures in film-forming solvents. In this direction, although the long-chain ligands are suitable to make quantum © 2013 American Chemical Society

Received: October 7, 2013 Revised: November 16, 2013 Published: November 19, 2013 25390

dx.doi.org/10.1021/jp409937z | J. Phys. Chem. C 2013, 117, 25390−25396

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on top of the PEDOT:PSS film. (2) The film of the nanocrystals was covered with 2 vol % of a dithiol-containing ligand solution in acetonitrile for 45 s for exchange of the ligands to occur. The excess ligands were physically removed by spinning the substrate. (3) To remove the excess dithiols, six drops of anhydrous acetonitrile were spun. (4) The ligands that were replaced by the dithiols were removed by spinning chloroform at which they were soluble to clean the surface for deposition of the next layer. Each step was carried out with an interval of 30 s to dry the surface. The steps were repeated until a desired number of layers of Mn-doped ZnS nanocrystals were formed. In the present work, we deposited up to six layers of the nanocrystals. To measure the thickness of the film of the nanocrystals, the depth profile of an intentional scratch on a multilayered film deposited separately on a glass substrate was captured by an atomic force microscope (AFM, Nanosurf Easyscan 2). The thickness of the nanocrystalline film, that turned out to be 25 nm per deposited layer, did not depend on the length of the ligands. Scanning Tunnleing Spectroscopy (STS) of the Nancocrystals. The tunneling current of the monolayers of the Mn-doped nanocrystals was measured with a PAN-style Ultra-High Vacuum Scanning Tunneling Spectroscopy (UHVSTS, M/s RHK Technologies, USA). The base pressure of the microcope chamber during the measurement was 1.8 × 10−10 Torr. Platinum/iridium (Pt/Ir) wire was used as the tip of the scanning tunneling microscope (STM) that had a work function of 5.3 eV. To record the tunneling current, ultrathin layers of the nanocrystals were formed on highly doped Si ⟨111⟩ substrates (arsenic-doped having a resistivity of 3−10 mΩ·cm). One of the films was treated with HDT. The treated and untreated films were characterized with the same STM tip with the same tip-approaching condition. The tip voltage was scanned from −2.5 to +2.5 V. Bias was applied with respect to the tip. For each of the monolayers, measurements were carried out on many different points on monolayer films. Device Fabrication. The light-emitting diodes were fabricated on indium tin oxide (ITO) coated glass substrates having a surface resistance of 15 Ω/sq. At first ITO-coated glass substrates were cleaned by sequential sonication in soap solution, methanol, acetone, and hot 2-propanol. The surface of the substrates was treated by oxygen plasma for improved contacts with the electrodes. A layer of PEDOT:PSS was spun on the stripped ITO electrodes at 5000 rpm. The film was annealed at 150 °C in nitrogen environment for 15 min to remove water from the PSS shell of the PEDOT:PSS grains. On top of the hole-transport layer, Mn-doped ZnS nanocrystals were deposited through the layer-by-layer approach followed by treatment with different dithiols (as described in the previous paragraph). The thin films were annealed at 130 °C in a nitrogen environment for 25 min. Finally, a layer of ZnO nanoparticles was spun from 20 mg/mL solution in ethanol at 2000 rpm. The trilayer structures were annealed at 110 °C in the inert environment for 10 min. As the top electrode, 100 nm thick aluminum strips, orthogonal to the ITO electrodes, were thermally evaporated at a pressure of 5 × 10−6 Torr to complete the process of device fabrication. Characterization of the Devices. The QDLEDs were characterized by mounting them in a shielded vacuum chamber fitted with a quartz window. Current−voltage (I−V) characteristics of the devices were measured with a Keithley DualChannel System SourceMeter Instrument model 2636A; luminous output of the devices was measured by a Konica

EXPERIMENTAL METHODS Growth of Nanocrystals. Stearic acid (SA), manganese(II) chloride tetrahydrate (MnCl2·4H2O), octadecylamine (ODA), octadecene (ODE), tetramethylammonium hydroxide (TMAH), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), 1,2-ethanedithiol (EDT), 1,4-butanedithiol (BDT), 1,6-hexanedithiol (HDT), and 1,8-octanedithiol (ODT) were purchased from Sigma-Aldrich Co. Zinc-stearate and sulfur powder were purchased from Loba Chemie Pvt. Ltd. Formation of Manganese Stearate. An amount of 10 mmol of stearic acid was dissolved in methanol at 50 °C under a vigorous stirring condition. Then, 10 mmol of TMAH in methanol was added dropwise. After 30 min, 5 mmol of MnCl2· 4H2O in methanol solution was added dropwise until manganese stearate precipitated in the form of white powder that was separated by adding excess methanol. Formation of Mn-Doped ZnS Nanocrystals. Mn-doped ZnS nanocrystals with a shell layer of ZnS were grown following the reported routes.27 In a typical synthesis, 0.126 g of Zn-stearate (0.2 mM), 5 mg of Mn-stearate (0.008 mM), 28 mg of sulfur (0.875 mM), 1.5 g of ODA (5.5 mM), and 15 mL of ODE were loaded in a three-neck round-bottom flask. The mixed solution was degassed at 100 °C by purging ultrapure nitrogen for 15 min. The temperature of the mixed solution was then increased to 270 °C so that Mn-doped ZnS nanocrystals formed. After a wait of 5 min, the temperature of the flask was cooled to 240 °C. We then grew a shell layer of ZnS on the Mn-doped ZnS nanocrystals. To do so, 1.26 g of Zn-stearate (2 mM), 0.586 g of stearic acid (2 mM), and 10 mL of ODE were mixed together in a glass vial and degassed by purging nitrogen. An amount of 3 mL of this solution was quickly injected into the reaction flask containing Mn-doped ZnS nanocrystals. The reaction was allowed to continue for 10 min. This process was repeated to obtain the desired thickness of the shell layer. The reaction was then stopped by cooling the flask followed by a collection of nanocrystals through repeated centrifuging and washing. Formation of ZnO Nanocrystals. Room-temperature synthesis of ZnO nanoparticles, following a reported route,28 was carried out by reacting 30 mL of 0.101 M zinc acetate dihydrate in DMSO with 10 mL of 0.552 M TMAH in ethanol (added dropwise at approximately 2 mL/min). The reaction was stopped by the addition of 50 mL of ethyl acetate. The reaction mixture was then cooled immediately; ZnO precipitates were centrifuged, collected, and redissolved in ethanol for further use. Film Formation and Ligand Exchange. While forming films of Mn-doped ZnS nanocrystals, we aimed to remove the long-chain ligands with short ones. In this work, we used different dithiol-containing ligands that had an additional two to eight units of −CH2− groups, such as EDT (2), BDT (4), HDT (6), and ODT (8). Thin films of the nanocrystals were formed through a multistep process involving removal of longchain surfactants. Since a surfactant-removal process would make the particles insoluble leading to their precipitation, we introduced the dithiols in each ultrathin layer so that the ligand acted as a linker between the layers of the nanocrystals through the two sulfur atoms of the dithiol. Such a process has earlier been used in other quantum dots.6,13,29−32 The steps were as follows: (1) an ultrathin layer of Mn-doped ZnS nanocrystals was spun from chloroform solution (15 mg/mL) at 2000 rpm 25391

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Figure 1. (a) XRD patterns and (b) TEM image of Mn-doped ZnS nanocrystals. HR-TEM image of a nanocrystal is shown in the inset of (b).

energy transfer to the d-state of Mn ions from the band-edge excitons of ZnS nanocrystals followed by a radiative transition between the d-states (4T1 → 6A1) of Mn ions. Such an origin (of PL) ensures that the wavelength of PL emission is independent of many parameters, such as diameter of the host nanocrystals or concentration of the dopant ions.34 Doping concentration has been chosen to be 1−1.5% to maximize the intensity of PL emission. The strong PL of the quantum dots prompted us to fabricate LEDs based on the nanostructures. Ligand Exchange during Film Formation: Redox Energy Levels of a Dithiol. We formed thin films of the Mn-doped ZnS nanocrystals through a layer-by-layer approach. Formation of thin films of nanocrystals was monitored by recording optical absorption and PL emission of the films (Figure 3). The figures, representing absorbance and PL at a

Minolta Luminance Meter model LS-110. All the instruments were connected to a computer and operated through LabVIEW software. The spectral response of EL emission was recorded with a Spex Fluoromax 4P Emission spectrophotometer after blocking the excitation beam.

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RESULTS AND DISCUSSION Characterization of Nanocrystals. We have recorded XRD patterns and HR-TEM images of the Mn-doped ZnS nanocrystals (Figure 1) which form the active layer of the devices. Diffraction peaks of the nanostrctures, marked in the spectra, readily matched the cubic crystal structure of ZnS (JCPDS file #77-2100) implying formation of the nanocrystals. TEM image of the nanocrystals and a HR-TEM image of a typical particle showed that the diameter of the nanocrystals was about 5.0−5.5 nm. The TEM image further shows that the lattice spacing of the nanocrystals was about 0.285 nm, which matched well with the spacing of ⟨111⟩ planes of ZnS crystals. The images further showed that the nanoparticles were largely single crystalline in nature. Optical absorption and PL spectra of the doped nanostructures are shown in Figure 2. While the optical absorption falls

Figure 3. (a) Absorbance at 310 nm and (b) PL intensity at 584 nm versus number of layers of thin films of Mn-doped ZnS nanocrystals. (c) Energy level diagram of Mn-doped ZnS nanoparticles along with the redox energy level of 1,6-hexanedithiol (HDT) used as ligands.

wavelength versus the number of layers of the doped nanocrystals, show that the film grew linearly with the number of layers. The appearance of PL in the dithiol-containing ligandcapped quantum dots is itself of interest. As such, in many nanocrystals, the redox energy level of a dithiol ligand is situated above the valence band-edge of the nanocrystals.25 Hole trapping hence occurs in such systems resulting in quenching of PL emission. In the Mn-doped ZnS, the two dstates of Mn ions are located above the valence band-edge and below the conduction band-edge. Since the redox energy level of a dithiol ligand falls below the d-states of Mn ions, quenching

Figure 2. Optical absorption and PL spectra of Mn-doped ZnS nanocrystals in dispersed solution.

in the UV region, the PL is in the yellow region. We looked for PL intensity in the low-wavelength region. Upon doping, the emission from the defect states of ZnS arising out of vacancies at sulfur sites, appearing in the 400−500 nm region,33 is greatly suppressed due to the strong PL band observed at about 584 nm. The negligible defect-state emission is visible only in an expanded y-scale. The red-shifted strong PL at 584 nm is due to 25392

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Figure 4. (a) Tunneling current and (b) density of states versus tip voltage characteristics of doped ZnS nanocrystals before and after treatment of the nanocrystals with ligands containing a dithiol. White the valence-band (VB) and conduction-band (CB) edges are shown as black lines, the redox levels of the dithiol ligands are shown as blue broken lines. Inset of (a) shows a typical STM topography (200 nm × 200 nm) of an ultrathin film of the nanocrystals before the treatment with the ligands.

Figure 5. Optical absorption and PL spectra of thin films of Mn-doped ZnS nanocrystals having different dithiols as ligands.

that the redox levels of the dithiol ligands appeared above the valence band-edge of the nanocrystals. The intensity of the DOS spectrum of the HDT-treated ZnS was as such higher than that of the pristine nanocrystals supporting the earlier analysis that the dithiol-capped nanocrystals have a higher conductance as compared to that of the pristine ones. Absorption and PL of Thin Films. Since in the present work we aimed to study the effect of ligands on the LEDs, we formed thin films of the nanocrystals. As stated in the Experimental section, we have used four different ligands during the film formation. Optical absorption and PL emission spectra of the thin films are shown in Figure 5. While the optical absorption spectra did not exhibit an appreciable change upon incorporation of different ligands, the PL intensity responded strongly with the nature of the ligands. With an increase in the length of the ligands, the PL intensity increased followed by a decrease. The intensity of PL of the nanocrystals in the thin film after the ligand-exchange process depends on nucleophilicity of the nucleophilic center and steric hindrance of carbon chains of the ligands. If the number of units of −CH2− groups increases, the PL intensity increases due to an increase in the charge density at nucleophilic centers; ligand exchange was favorable in such cases. When the chain length was further higher, the affinity of ligand exchange reduces due to a steric effect resulting in a decrease in the intensity of PL emission. We hence observed an optimum intensity when the long-chain ligands of the QDs were exchanged by HDT that had six units of −CH2− groups. In other words, when the length of the ligands is small, assemblies of nanocrystals form nanocrystalline solids in which PL quenching is significant. The

of PL emission does not occur. The energy level diagram of Mn-doped ZnS nanoparticles, along with the redox energy level of the ligands, is shown in Figure 3(c). While the values of 4T1 and 6A1 levels of Mn ions were taken from the literature,35 the dithiol levels along with the conduction and valence band-edges have been taken from the STM results (as shown in the next paragraph). To locate the redox energy level of dithiol-containing ligands of the nanocrystals along with the valence and conduction band-edges of the host, we have carried out STS of the nanocrystals. A typical STM topography is shown in the inset of Figure 4(a). Plots of tunneling current versus tip voltage for the nanocrystals before and after HDT treatment are shown in Figure 4(a). The plots show that upon treatment with ligands containing a dithiol conductance of the nanocrystals in the ultrathin film increased. The percentage of increase in current upon treatment with dithiol ligands was substantial (>30%). To know the location of the conduction and valence band-edges of the nanocrystals, we have calculated the density of states (DOS) from the tunneling current. As shown in Figure 4(b), the DOS spectrum of the as-grown nanocrystals evidences the location of conduction and valence band-edges. Here since bias was applied with respect to the tip, the peaks at the positive voltage, at which electrons can be injected (from the tip) to the nanocrystals, denote the location of conduction bands. Similarly, the peaks at the negative voltages denote the valence band; at such voltages, electrons can be withdrawn from the nanocrystals. The energies shown in the plot are with respect to the work function of the tip used for STS characterization. In the DOS spectra of the HDT-treated nanocrystals, we observe 25393

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Figure 6. (a) Current−voltage and (b) luminance−current plots of QDLEDs based on a different number of layers of Mn-doped ZnS nanocrystals. Inset of (a) shows field dependence of current in the three cases.

quenching occurs due to fast exciton dissociation via tunneling to surface states.36,37 When the length of the ligands of the nanocrystals is further increased (for example, ODT), the PL decreased; the trend which is similar to PL of CdSe quantum dots in the presence of different amines as ligands,38 can be explained by considering lesser surface coverage of the nanocrystals by long-chain ligands prompting aggregation of the nanocrystals followed by a decrease in the PL intensity. The wavelength of the PL emission did not depend on the length of the dithiols. This implied that the redox level of the dithiols never went above the 6A1 level of Mn ions. It may be relevant to cite that the energy of the dithiol levels did not show an appreciable change with the increase in the length of the carbon chain.39 Characterization of QDLEDs. We have characterized the QDLEDs based on Mn-doped ZnS nanocrystals. As holetransporting and electron-transporting layers, we have used layers of PEDOT:PSS and ZnO nanoparticles, respectively, that are being used in several QDLEDs. In the ITO/PEDOT:PSS/ Mn-doped ZnS/ZnO/Al structure, we have varied the thickness of the Mn-doped ZnS emitting layer. I−V characteristics of the devices are shown in Figure 6(a). For devices with each thickness of Mn-doped ZnS nanocrystals, we have presented results from three cells to show the degree of reproducibility. The characteristics were nonlinear in nature implying that barriers existed for injection of carriers that decreased upon an increase in the applied voltage. As expected, the thicker devices yielded lower current at a voltage. The I−V characteristics have been replotted as the field dependence of the device current. Such characteristics, as shown in the inset of Figure 6(a), are independent of the thickness of the active layer. This implied that the electric field dictated the device current in the QDLEDs. Luminance−current density plots of the devices are shown in Figure 6(b). The figure plotted in double logarithmic scale shows that there is a turn-on current beyond which the EL was detectable. The turn-on current seemed not to depend on the thickness of the active layer of the nanocrystals strongly. Above the turn-on current, the EL intensity increased linearly with the device current. The EL intensity versus current plots also did not mostly depend on the thickness of the Mn-doped ZnS layer. When the thickness was too thin (two layers), EL intensity was a little lower; with holes and electrons approaching the thin emitting layer from both sides, many of them may have escaped Mn-doped ZnS nanocrystals instead of forming excitons.

We have recorded EL spectra of the devices under different applied voltages. For this measurement, we chose the optimum device that had four layers of Mn-doped ZnS nanocrystals. The spectra for a typical device are shown in Figure 7 under 7.0−9.5

Figure 7. EL spectra of QDLEDs based on HDT-treated Mn-doped ZnS nanocrystals under different applied voltages (shown in legends).

V of device operation in steps of 0.5 V. The EL spectra matched the PL spectrum of the nanocrystals implying that excitons indeed formed in Mn-doped ZnS nanostructures followed by a radiative emission. As stated earlier, the transition between the d-states of Mn ions resulted in the PL and hence also the EL emission in these nanocrystals. Intensity of the EL emission expectedly increased with the applied voltage since the number of excitons formed in the devices through injection of electrons and holes increased with the applied voltage. Effect of (Length of) Ligands on QDLEDs. To study the effect of the length of the ligands (of the nanocrystals) on the performance of QDLEDs, we have characterized such devices. The other parameters of the devices, such as the thickness of hole- and electron-transporting layers, were kept the same. For comparison of I−V characteristics, we present the results of devices based on four layers of Mn-doped ZnS nanocrystals having different lengths of dithiol ligands. The results presented in Figure 8(a) show that while all the I−V’s were nonlinear in nature the device based on nanocrystals having ligands of longer chain lengths yielded a lower current. With thickness of the active layer in these devices being the same or very similar, the I−V characteristics therefore responded to the interdot distance and hence conductivity of nanocrystalline thin films. 25394

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Figure 8. (a) Current−voltage and (b) EL intensity versus current plots of QDLEDs based on an active layer of Mn-doped ZnS nanocrystals having different ligands. In the inset of (b), external quantum efficiency versus current density plots of the four devices are shown.

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CONCLUSIONS In conclusion, we have shown that since the redox energy level of dithiol-containing ligands does not quench PL emission of Mn-doped nanocrystals we could fabricate QDLEDs based on such doped nanocrystalline systems. From STM characteristics, we determined the redox levels of the ligands on the nanocrystals. From the energy level diagram of doped nanocrystals, we inferred that quenching of PL emission did not occur since the energy level of dithiol ligands was lower than the d-state (6A1) of manganese ions so that holes were not trapped to the dithiol states. This allowed a radiative transition between the d-states (4T1 and 6A1) to yield PL emission in thin films of the nanocrystals and hence an EL emission in the QDLEDs. In the present work, we have studied the role of the length of dithiol ligands that controls the interdot spacing and hence conductivity of thin films of the nanocrystals on QDLEDs. From the current−voltage, luminance−current, and EQE%−current characteristics of the QDLEDs, we have observed an optimum length of the ligands on the nanocrystals for improved device performance. The EL response of the devices did not depend on the length of the ligands since the PL and also the EL emission originated due to the radiative transition between d-states of Mn ions without involving the redox level of the dithoiol ligands.

The impact of the length of the ligands on the EL intensity of the QDLEDs was complex. Longer ligands on one hand increase the interdot spacing, thereby reducing the conduction process of electrons and holes through the layer of the nanocrystals, and they (the longer ligands) improve the possibility of radiative emission. The impact of the length of the ligands is reflected in the plots of EL intensity versus current density, as shown in Figure 8(b). Though the turn-on current did not vary in a large manner, the EL intensity at a current was higher for devices based on nanocrystals having moderately long stabilizers. The EL intensity versus current density plots above the turn-on current remained linear in the double-log scale. The effect of the length of the ligands (of the nanocrystals) on the performance of LEDs can also be observed in the external quantum efficiency (EQE%) versus current characteristics. Such plots for all the devices are presented in the inset of Figure 8(b). Though the efficiency of some devices saturated or even decreased at a higher device current, the device based on nanocrystals having HDT as the ligands excelled compared to other devices. The length of the ligands must have been optimum for device operation. The decrease in the EQE% at a higher device current could be due to a nonradiative Auger recombination process. We compared the EL spectra of the devices. The spectra as presented in Figure 9 show that the spectral response of the devices did not change upon the use of different ligands in the Mn-doped ZnS nanocrystals since the PL and also the EL emission originated due to a radiative transition between d-states of the nanocrystals.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-33-24734971. Fax: +91-33-24732805. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work of S.B. was funded by CSIR − Junior Research Fellowship No. 09/080(0692)/2010-EMR-I (Roll No. 507693).

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REFERENCES

(1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Light-Emitting Diodes Made from Cadmium Selenide Nanocrystals and a Semiconducting Polymer. Nature 1994, 370, 354−357. (2) Qian, L.; Zheng, Y.; Xue, J. G.; Holloway, P. H. Stable and Efficient Quantum-Dot Light-Emitting Diodes Based on SolutionProcessed Multilayer Structures. Nat. Photonics 2011, 5, 543−548. (3) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Air-Stable All-Inorganic Nanocrystal Solar Cells Processed from Solution. Science 2005, 310, 462−465.

Figure 9. EL spectra of QDLEDs based on an active layer of Mndoped ZnS nanocrystals having different ligands. 25395

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp409937z | J. Phys. Chem. C 2013, 117, 25390−25396