Low-Temperature Annealing for Highly Conductive Lead

Dec 30, 2010 - Additionally, these materials have potentially lower fabrication costs and simpler processing methods than conventional materials. ...
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J. Phys. Chem. C 2011, 115, 607–612

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Low-Temperature Annealing for Highly Conductive Lead Chalcogenide Quantum Dot Solids Seung Jae Baik,*,† Kyungnam Kim,‡ Koeng Su Lim,† SoMyung Jung,‡ Yun-Chang Park,§ Dong Geon Han,† Sooyeon Lim,† Seunghyup Yoo,† and Sohee Jeong*,‡ Department of Electrical Engineering, Korea AdVanced Institute of Science and Technology (KAIST), 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Korea, Nano Mechanical Systems Research DiVision, Korea Institute of Machinery and Materials, 104 Sinseong-ro, Yuseong-gu, Daejeon 305-343, Korea, and Measurement and Analysis DiVision, National Nanofab Center (NNFC), Daejeon 305-806, Korea ReceiVed: September 5, 2010; ReVised Manuscript ReceiVed: NoVember 18, 2010

Electrical conductivity in quantum dot solids is crucial for application in devices. In addition to the wellknown ligand exchange strategies for enhanced conductivity, the current study examined the optical, structural, and electrical properties of ethanedithiol-treated layer-by-layer (LbL) assembled quantum dot solid (QDS) films following low-temperature annealing (room temperature to 170 °C). As the annealing temperature increased, it was induced that the average separation between nanocrystal quantum dots is decreased, and accordingly, the overall conductivity of the QDS increased exponentially. From a simplified percolation model, the activation energy of temperature-dependent quantum dot attachment was estimated to be around 0.26-0.27 eV both for PbS and PbSe quantum dot solids. Furthermore, the results of this study indicated that device applications requiring higher conductivity, attainable through high-temperature annealing, may also require repassivation after annealing. 1. Introduction Quantum dot solids (QDSs), formed by the assembly of individual, nanocrystalline quantum dots (NQDs), are promising in the context of material design and low-cost processing. The physical properties of QDSs can be controlled by tailoring the size of their constituent NQDs. Additionally, these materials have potentially lower fabrication costs and simpler processing methods than conventional materials. Recent advances in the use of hot carriers in QDSs1 and multiple-exciton generation in NQD solutions2 have demonstrated potential efficiency improvements when used in photovoltaic devices. Despite these advantages, poor transport properties represent a significant technological barrier for QDS device applications. There have been extensive efforts aimed at overcoming the poor conductivity of QDSs, including (1) the creation of QDSs with modified surfaces by attaching short ligands, such as hydrazine,3 carboxyl acids,4 and alkanedithiols,5-8 or by creating a thin inorganic shell,9 (2) the application of physical processes, such as heat-annealing after the initial assembly,10,11 and (3) coupling the NQDs to one-dimensional nanostructures.12 Transport between NQDs can be modeled by direct tunneling through a thickness determined by the inter-NQD separation. The exponential dependence of electron mobility and coupling energy on the ligand length6 (inter-NQD separation) is the signature of direct tunneling transport between NQDs. The field effect mobility of thin-film transistors made with PbS QDSs is in the range of 10-4-0.1 cm2/(V · s) with a controlled interNQD separation using ethanedithiol.6,11 Comparison with the transport properties of amorphous Si (∼0.6 cm2/(V · s)) and polycrystalline Si (10-100 cm2/(V · s))13 revealed that decreasing * Corresponding authors. E-mail: [email protected] and solar100@ kaist.ac.kr. † Korea Advanced Institute of Science and Technology. ‡ Korea Institute of Machinery and Materials. § National Nanofab Center.

the inter-NQD separation is crucial for competitive device performance. A thermal annealing approach has been successfully applied to form various semiconducting thin films with good electrical conductivity. The ability of NQDs to fuse at relatively modest temperatures14 provides a low-temperature route to thin-film growth. Recently, thermal treatment of ethanedithiol-treated PbS QDSs was shown to enhance photovoltaic light conversion efficiency.11 Also, temperature elevation was shown to cause desorption of ligand molecules and subsequent attachment of NQDs in monolayer-thick NQD arrays.14 Both of these results suggest that thermal annealing may enhance the electrical conductivity of QDSs without causing NQD growth or elemental segregation. The presence of large organic ligands after thermal annealing may increase the density of trap sites and prevent the ordered arrangement of QDS, which would cause a decrease in carrier lifetime and mobility.10 Thus, NQDs surface-passivated by small molecules would be more appropriate for the annealing of QDS. On the other hand, weakly bound small molecules with high volatility, such as hydrazine, do not remain after annealing. Thus, the surface-defect density of NQDs increases,1 which is not appropriate for photovoltaic applications, and small molecules with moderate volatility would be the appropriate choice in such an annealing experiment. The present study examined the effects of thermal annealing on lead chalcogenide QDSs assembled via layer-by-layer (LbL) assembly at relatively low temperatures where the electronic structure of each NQD was retained while enhancing the transport properties of the film. More importantly, optical, structural, and electrical characterization and analyses allowed the derivation of a carrier transport mechanism. We believe that QDSs with zero inter-QD separation would be the final goal to optimize device performance, and thermal annealing may be a simple, controllable, and viable method to attain this goal.

10.1021/jp1084668  2011 American Chemical Society Published on Web 12/30/2010

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Figure 1. (a) Absorbance spectra of PbSe and PbS NQDs dispersed in tetrachloroethylene (TCE). (b) The 1S exciton transition wavelength and (c) energy of LbL-coated QDS is shown as a function of the inverse of the annealing temperature.

2. Experimental Section Chemicals. Oleic acid, lead(II) acetate trihydrate, bis(trimethylsilyl)sulfide, and octadecene (ODE) were purchased from Sigma-Aldrich (St. Louis, MO) and used as received with no further purification. 1,2-Hexadecanediol was purchased from Sigma-Aldrich and was further purified by filtration.15 Trioctylphosphine (TOP) was purchased from Sigma-Aldrich and was degassed prior to use. Selenium shot was purchased from Alfa Aesar (Ward Hill, MA). TOP/selenide stock solutions, either with 1.0 or 1.5 M, were prepared in a nitrogen-filled glovebox by stirring TOP with selenium shot overnight. All chemicals were stored in an inert atmosphere glovebox prior to use. NQD Synthesis. PbSe and PbS nanocrystalline quantum dots were prepared by using established colloidal synthetic techniques with a few modifications.15 Briefly, lead(II) acetate trihydrate (0.76 g, 2 mmol) and oleic acid (1.4 mL, 4.4 mmol) were heated under vacuum (100 °C, 3 h) in ODE to complete lead oleate formation (incomplete oleate formation can result in unstable product). TOP-Se (4 mL (1 M), 4 mmol) was introduced into a lead precursor solution at room temperature, and the mixture was injected rapidly into the 1,2-hexadecanediol (2 g) and ODE solution at 160 °C. After 2-8 min, the solution was rapidly cooled with ice-cold toluene. The resulting PbSe NQDs were precipitated five times from a mixture of butanol and methanol. PbS NQDs were prepared by rapidly injecting bis(trimethylsilyl)sulfide into the lead oleate precursor solutions in ODE at 160 °C. The QDS film was subsequently formed by a 10-cycle LbL deposition according to the following sequence. The substrate was dipped in PbS or PbSe dissolved in hexane for a few seconds, removed and dried, then dipped in 0.1 M EDT in acetonitrile for a few seconds and removed and dried. Thermal annealing was performed on a hot plate in a nitrogen-filled glovebox. The surface temperature of the glass substrate was monitored and controlled during the 30 min annealing time. Room-temperature optical absorption spectra were collected by using a UV/vis/NIR spectrophotometer (Shimadzu, UV3600). Device Fabrication and Characterization. A device for measuring thin-film conductivity was fabricated as follows. Al was evaporated onto a clean glass substrate to form coplanar electrodes with a length of 1.5 cm and a width of 100 µm. The gap between the electrodes varied in 50 µm increments from 100 to 500 µm. The QDS film was subsequently formed by a 10-cycle LbL deposition according to the sequence mentioned above. The conductivity of the QDS film resulting from a specific set of experimental conditions was extracted from the slope of the resistance as a function of electrode separation, and conductivities were calculated from the corresponding I-V at 10 V, by assuming the film thickness was constant at 50 nm. Conductivity measurements were performed in ambient air by

using a HP4156A precision semiconductor parameter analyzer and a homemade probe station. XPS Characterization. Samples for XPS characterization were fabricated by using LbL methods for PbSe and PbS QDS. The LbL was performed five times, and the estimated thickness was 25 nm, corresponding to five layers of NQDs. Coating was performed in a nitrogen-filled glovebox that was exposed to air for about 1 h before loading to the vacuum chamber for XPS analysis. A monochromatic Al-KR source and an anode energy of 15 KV were used for the XPS analysis with a filament emission of 10 mA. The obtained spectra were aligned by placing C1s peak at the binding energy of 284.8 eV. Electron Microscopy. The microstructure and crystallographic structure of the Pb chalcogenide arrays were investigated by field emission transmission electron microscopy (FETEM, FEI, Tecnai F30 Super-Twin). TEM sample arrays were prepared by using the LbL method on a carbon-coated copper mesh grid, and cross-sectional TEM samples were prepared by using an ex situ lift-out technique, based on focused ion beam (FIB, Hitachi, FB-2100) milling of specific areas. The FIBmilled samples were placed on a carbon-coated Cu mesh grid for further observation.22 3. Results and Discussion Assembly of NQD Films. PbSe and PbS NQDs were synthesized according to standard literature procedures with slight modifications.15 As prepared, the NQDs were passivated with trioctylphosphine and oleic acid, precipitated at least five times to remove any excess ligands, and in turn dispersed in a nonpolar solvent, such as toluene. Optical measurements showed a first excitonic transition at 1735 nm (0.71 eV) for PbSe and at 1406 nm (0.88 eV) for PbS NQDs dispersed in tetrachloroethylene (TCE). The full-width-at-half-maximum values of the first exciton peak of PbSe and PbS were 39 and 57 meV, respectively, indicating a highly monodisperse building block (Figure 1a). QDSs were formed by an LbL dip coating method.8 Substrates were dipped into a NQD solution to obtain monolayer coverage and subsequently dipped into an ethanedithiol (EDT) solution for ligand exchange. These two steps were repeated to form a continuous film with a smooth surface profile of the desired thickness. All processes were conducted in a nitrogen-filled glovebox to prevent complications from exposure to air or moisture. Typically, this procedure resulted in 50 nm thick QDS films on glass substrates suitable for both electrical and optical characterization. Monolayer QDS films were formed on TEM grids for structural characterization. Quantum dots created from lead chalcogenides offer great advantages over other colloidal quantum dots because of their large exciton Bohr radius (20 nm for PbS, 46 nm for PbSe)

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Figure 2. Cross-sectional TEM micrographs show PbSe QDS processed at (a) room temperature and (b) 55 °C. A TEM micrograph shows a cross section of PbS QDS films annealed at (c) room temperature and (d) 55 °C. A micrograph shows a PbSe NQD on a TEM grid after annealing at (e) room temperature, (f) 55 °C, and (g) 100 °C. The 50 nm scale bar is common to (e-g). A PbS NQD is shown on a TEM grid following annealing at (h) room temperature, (i) 70 °C, (j) 100 °C, and (k) 170 °C.

and high electron and hole mobilities.16 In this regime, their optical properties can be evaluated in the limit of extremely strong quantum confinement even when using relatively large quantum dots. Additionally, advances in synthetic routes have led to the fabrication of lead chalcogenides with very narrow line widths, which provide a higher density of electronic states (DOS) available for charge transport. QDSs using lead chalcogenide have been studied extensively in the hope of obtaining device-applicable quality. Quantum dots, as synthesized, were passivated by surfactants where the hydrocarbon chains of the surfactant acted as a direct tunneling barrier for carrier transport when assembled into arrays. NQD arrays that exhibit enhanced transport properties while preserving the electronic structure of each individual NQD have been achieved by replacing the hydrophobic alkyl tail with small molecules,3-8 conjugated molecules,6 or an inorganic, thinlayer shell.9 Although hydrazine effectively removes the surfactant molecules and is a feasible coating for relatively thick films, hydrazine does not remain after thermal treatment and creates defects in the final film.1 Ethanedithiol-exchanged NQD arrays resulted in robust films with reduced interdot distances. We further examined the effects of thermal annealing on the preformed QDS of lead chalcogenides. Optical and Structural Characteristics of QDS with LowTemperature Annealing. Law et al.10 reported an improvement in transport properties following the sintering of individual nanocrystals into a QDS film at 200-500 °C. They observed a bulk-like absorption spectrum when the film was annealed at temperatures above 200 °C. Neither conductivity enhancement nor the removal of passivating molecules (oleate) was observed at lower annealing temperatures. In the current study, conductive

QDSs were first created by removing the insulating organic ligands. The effects of heat annealing at relatively low temperatures, at which the electronic structures of individual NQDs are retained, were then evaluated as a means of enhancing the transport properties of the QDS film. QDS films were fabricated with a thickness of about 50 nm. Films were heat-annealed between 40 and 170 °C. Figure 1b,c shows that thermal annealing shifts the 1S exciton peak toward longer wavelengths; this peak disappears at sufficiently high temperatures (Figure S1 in the Supporting Information). Volume shrinkage after thermal annealing at 55 °C was assessed by electron microscopy. Figure 2a,b show that NQD attachment caused volume shrinkage after annealing. This reduced film thickness and the surface profile became undulated. However, TEM of PbS QDSs, shown in Figure 2c,d, show a smooth surface profile after thermal annealing, suggesting that NQD attachment may have been less sensitive to temperature. QDS films (50 nm thick) did not provide detailed information on individual NQDs. Thus, submonolayer lead chalcogenide NQD arrays were prepared on TEM grids by a single-step LbL deposition. Thermal annealing was performed on a hot plate for 30 min in a nitrogen-filled glovebox. The TEM in Figure 2e-k show that NQD attachment occurrs with increasing probability as the annealing temperature is increased for both PbSe and PbS NQDs. In situ TEM micrographs showed that PbSe NQD attachment occurred through an oriented attachment mechanism at temperatures far below the melting point of PbSe.14 The PbSe NQD array annealed at 55 °C shows a mixture of attached NQDs and individual NQDs (Figure 2f), whereas only attached NQDs were seen after annealing at 100 °C (Figure 2g). In contrast, the PbS NQD array shown in Figure 2h-k

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Figure 3. (a) S 2s spectra of PbSe QDS and (b) S 2p XPS spectra of PbS QDS for different annealing temperatures.

exhibited only complete attachment at the higher temperature of 170 °C. When considering that the diameter of both the PbS and PbSe NQDs was around 5 nm, the different temperature sensitivities were likely a function of the different desorption rates of thiols on PbS and PbSe NQDs, because attachment was driven by the desorption of ligand molecules; van Huis et al. observed that oleate-passivated PbSe NQDs attached at much higher temperatures than hexylamine-passivated NQDs.14 A recent report demonstrated that air-stable, thiol-passivated PbS QD solar cells were more robust than thiol-passivated PbSe QD solar cells, suggesting tighter binding of ligands on the PbS surface.17 Individual NQD attachment could enhance optical absorption and decrease the observed bandgap of the QDS film because of enhancedelectroniccouplingenergy10 ortheincreaseddipole-dipole interaction between NQDs,18 as indicated by the optical and structural characterizations described here. These phenomena are the result of collective shrinkage of separation between NQDs, although microscopically this shrinkage is highly localized. That is, NQD attachment starts where surface ligands are removed, and the removal of surface ligands occurs preferentially from locations with the highest free energy. For the LbLcoated QDS with a disordered microstructure,8 these locations would be randomly distributed in the QDS film. Thus, the observed absorption spectra were red-shifted, and the width of the absorption peak increased gradually with increasing annealing temperature (Figure S1 in the Supporting Information) as a result of collective absorption from a mixture of ligand-separated and mutually attached NQDs. On the other hand, this broadening behavior might be due to the spreading of NQD subbands with decreased inter-NQD separation.18 It is still under debate whether these red shifts and accompanying broadening are caused by electronic coupling or dipole-dipole interaction. Our experimental results provide additional evidence that decreased interNQD separation leads to red shifts and line broadening of exciton transition in a systematic manner. Furthermore, the disappearance of absorption peak at high temperatures could be attributed to the bulk-like absorption caused by the fusion of NQDs.10 To further investigate ligand desorption and the fate of desorbed EDT molecules in QDS, XPS analysis was performed, as shown in Figure 3a,b. S2s spectra of PbSe QDS show that the bound thiolate at 226 eV diminished as the annealing temperature increased, supporting desorption of bound thiols at the surface of PbSe NQDs. At an annealing temperature of 100 °C, the peak at 226 eV did not completely disappear. S2p spectra of PbS QDS also showed a decrease in the S-S bond at 163.7 eV and an increase in the Pb-S bond at 160.8 eV,

indicating a decrease of bound thiolates after annealing. Moreover, no noticeable change was observed for the S-C bond at 162.0 eV, suggesting that desorbed thiol molecules remained in the QDS film. Consequently, bound thiols in PbSe and PbS QDS desorbed from the surface of NQDs as the annealing temperature increased, and desorbed thiol molecules remained in QDS films. Enhanced Conductivity with Low-Temperature Annealing. To characterize the changes in QDS conductivity with thermal annealing, coplanar electrodes were prepared with a variety of separation distances, from 100 to 500 µm. The use of several distances allowed the removal of electrode contributionstotheQDScontactresistance(Figure4d).Thecurrent-voltage (I-V) characteristics for an electrode separation distance of 350 µm are shown for PbSe and PbS in Figure 4a,b, respectively. I-V characteristics were ohmic for low voltages and turned to follow the dependence of I-Vu, with the exponent u larger than 1. The power law dependence of I-V can be interpreted as a trap-controlled space charge-limited current (SCLC). By adopting the model of SCLC formulated by Lampert, the exponent u represents the trap distribution, and larger values of u correspond to an increased number of traps.19 The plot of u versus the inverse of the annealing temperature shows an increase in trap density with annealing temperature, supporting the XPS analysis regarding ligand desorption. The shape of the curve exactly coincided with that of Figure 1b, suggesting that the increase in electronic coupling energy was driven by ligand desorption that increased NQD surface traps. Figure 5 shows the extracted conductivity as a function of annealing temperature. PbSe QDS exhibited higher conductivity than PbS, which was expected on the basis of the larger exciton Bohr radius16 and bulk conductivity of PbSe.20 Both QDSs exhibited exponentially increasing conductivity as a function of annealing temperature, and the enhancement was saturated at sufficiently high temperatures, which was the same trend as that observed with the 1S exciton transition wavelength. This annealing-induced conductivity enhancement can also be understood in terms of NQD attachment or changes in the proximity of neighboring NQDs. The conductivity level saturated at relatively higher annealing temperatures was much lower than that of the bulk material,20 indicating that NQD fusion occurred at local sites. Thus, the QDS film conductivity did not span uniformly over the area of the film but was instead limited to localized percolation paths. Moreover, as pointed out by the XPS analysis, remaining EDT may have obstructed further shrinkage of the QDS lattice, thereby limiting the decrease in inter-NQD distances.

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Figure 4. Current-voltage characteristics are shown for (a) PbSe QDS and (b) PbS QDS (WQ ) 350 µm). Different annealing temperatures ranging from room temperature to 170 °C are represented by different line colors. (c) The extracted exponent u is shown as a function of the inverse of the annealing temperature. (d) The device used to measure the conductivity of QDS films. Two NQD layers are shown for illustrative purposes; actual measurements were performed on films composed of about ten NQD layers. The separation between coplanar Al electrodes is denoted as W Q.

σ-1 )

N

N-M

i)1

i)1

∑ σi-1 + r0 ) ∑ R exp(-β∆) + r0 ) (N - M)R exp(-β∆) + r0 (1)

Figure 5. Conductivity is shown as a function of the inverse of the annealing temperature. Symbols denote the experimental results extracted from I-V data from the coplanar conductivity device described in Figure 3a. Lines represent the predicted trend from a simplified 1D percolation model.

To gain further insight, a simplified conduction model was developed, based on the percolation approach. The measurement of I-V was performed within an electric field of 1000 V/cm, and this low-field range verified that the current transport followed a variable range hopping.21 Variable range hopping is a percolating conduction mechanism that represents conduction along the lowest activation energy path or the shortest hopping distance. The conducting path must be a multiple linkage of one-dimensional conducting paths. Thus, for simplicity, it was assumed that the conduction path consisted of multiple, isolated 1D percolation paths, and the variation of hopping activation energy was negligible. Additionally, site-to-site conduction along the 1D percolation path was modeled as direct tunneling. The probability of direct tunneling is proportional to exp(a∆Eb1/2), where a is a constant, ∆ is the inter-NQD separation, and Eb is the barrier height. ∆ was modeled as a constant or zero to alternatively represent separated and attached NQDs, respectively, with no intermediate arrangement permitted. The conductivity of the QDS film can then be represented by a series connection of tunnel resistances, shown in eq 1:

where N is the total number of inter-NQD tunneling resistances, M is the number of inter-NQD tunneling resistances with zero value, r0 is the resistance when all of the NQDs in the percolation paths are attached, and R and β are constants. On the basis of the stochastic nature of NQD attachment, the number of attached QDs, M, can be represented as:

j ) M)M

N

∑ Q ) NQ

(2)

i)1

where Q is the probability of NQD attachment. NQD attachment was modeled as an exponential random process, and its mean frequency of occurrence was assumed to be proportional to exp(-Ea/kT), where Ea is the activation energy of NQD attachment. When considering that QD attachment is followed by ligand desorption, Ea may be related to the activation energy of ligand desorption if sufficient QD mobility is maintained. However, in 3D QDSs, each QD is bound to adjacent QDs, and local ligand removal does not necessarily lead to QD attachment. Ea then represents the activation energy of QD attachment in the framework of 3D QDS. Thus, Q can be represented as:

(

( ))

Q ) 1 - exp -γt exp -

Ea kT

(3)

where γ, t, k, and T represent a constant, time, the Boltzmann constant, and temperature, respectively. Equation 4 can then be derived from eqs 1-3 if r0 is neglected for simplicity.

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( )

log σ ) -γt exp -

Ea +C kT C ) -log N - log R + β∆ (4)

Equation 4 does not describe saturation behavior and is, therefore, not applicable at high annealing temperatures. Fitting the experimental data to eq 4 yielded Ea values of 0.26 eV for PbS and 0.27 eV for PbSe. There are two outcomes of desorption of ligand molecules passivating the surface of NQDs: (1) attachment of NQDs, thereby increasing the conductivity, and (2) loss of surface passivation, thereby increasing the surface trap density of NQDs. The first outcome enhances device quality, whereas the second degrades it. Thus, thermal annealing alone cannot lead to deviceapplicable quality for QDS. To attain high conductivity as well as long carrier lifetime (low trap density), the development of a repassivation scheme coupled with low-temperature annealing is key. Air annealing11 may provide a repassivation scheme, but it must be carefully controlled to prevent loss of conductivity from excessive oxidation. In summary, the effects of annealing temperature on PbS and PbSe QDSs were investigated in the low-temperature regime at which highly conductive QDS films were obtained. There is currently a great deal of interest in developing QDS-based electronic devices, including photovoltaics. However, the low intrinsic conductivity of these materials represents a significant obstacle. The present study describes the optical, electrical, and structural characterization of QDS films annealed at relatively low temperatures. With thermal annealing, a red shift of the first exciton transition energy, a monotonic change of SCLC exponent representing trap density, and an exponential increase in conductivity were observed. The film behavior was modeled as a thermally induced QD attachment with an averaged decrease of inter-NQD separation. Low-temperature annealing may be a promising method for obtaining device-quality QDS with a close-packed superlattice and negligible inter-QD separation. Although the current experimental method did not achieve that goal, the results presented here indicate that an ordered QDS film and a repassivation method applied to QD surfaces after thermal annealing would aid in the development of highperformance devices. Acknowledgment. This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant, funded by the Ministry of Education, Science, and Technology (20090083219, 2008-0062241), 21C Frontier Research Program, and Brain Korea 21 Project, the School of Information Technology, KAIST in 2010. Supporting Information Available: Absorption spectra of PbSe and PbS QDS after annealing at various temperatures and electron micrographs of PbS NQDs on a TEM grid, annealed

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