Spectrally-Resolved Dielectric Functions of Solution-Cast Quantum ...
Spectrally-Resolved Dielectric Functions of Solution-Cast Quantum Dot Thin Films Benjamin T. Diroll, E. Ashley Gaulding, Cherie R. Kagan, and Christopher B. Murray Materials and Methods Materials. CdO (99.99%, Strem), PbO (99.99%, Aldrich), 1-octadecene (ODE, 90%, Acros), selenium powder (99.99%, Aldrich), bis(trimethylsilyl) sulfide (TMS, 95%, Aldrich), calcium carbonate (>99.0%, Aldrich), trifluoroacetic acid (TFA, 99.8%, Fisher), tetrabutylammonium iodide (TBAI, 98%, Acros), mercaptopropyltrimethoxysilane (MPTS, 95%, Aldrich), 1,3benzenedithiol (BDT, 98%, Aldrich), ethanedithiol (>98%, Aldrich), mercaptopropionic acid (99+%, Aldrich) oleic acid (90%, Aldrich), myristic acid (99.5+%, Aldrich), oleylamine (8090%, Acros) purchased from commercial sources. NH4SCN (99+%, Acros) was recrystallized air-free using a 1:1 mixture of anhydrous isopropanol and methanol. Quartz plates (3/16” thick by 3/4” diameter) were purchased from GM associates. Synthesis of CdSe Nanocrystals (NCs). Zinc blende CdSe NCs were synthesized following literature procedures with slight modifications.1,2 Cadmium myristate was synthesized by dissolving 1 eq. CdO in 2.1 equivalents of molten myristic acid at 250 °C, followed by drying, and washing with acetone, ethanol, and methanol to remove excess myristic acid. For secondary injections of precursors, 0.5 M CdO in oleic acid was prepared by dissolving CdO in neat oleic acid at 250 °C and then drying the solution under vacuum at 120 °C for 1 hour. A 0.1 M Se stock solution was prepared by dissolving 1 mmol of Se powder in 100 mL of ODE by heating under nitrogen for 2 hours at 280 °C, then cooling and drying under vacuum at 120 °C for 1 hour. For the synthesis of CdSe QDs, 0.6 mmol cadmium myristate, 0.3 mmol Se powder, 30 mL of ODE were loaded into a reaction flask and dried at 120 °C for 1 hour, then heated to 240 °C. After 3 minutes at 240 °C, a dropwise injection of 1 mL oleylamine, 1 mL oleic acid, and 4 mL ODE was performed at 1 mL/min. After the injection, the reaction was allowed to proceed for 30 minutes at 240 °C. This procedure yields 3.6 nm CdSe QDs. For larger QDs, the temperature was raised to 280 °C and a dropwise injection was performed with a 5:1 by volume mixture of 0.1 M Se and 0.5 M Cd stock solution injected at 0.2 mL/min. To obtain larger particles, this injection proceeded until aliquots of optical spectra confirmed the desired size was reached (between 12 and 36 mL total stock injection). To make smaller QDs, 0.6 mmol cadmium myristate was replaced with 1.2 mmol CdO and 4.5 mmol oleic acid in 60 mL ODE. Upon heating to 240 °C, an injection of 2 mL oleic acid, 2 mL oleylamine, and 8 mL ODE was made immediately. All CdSe QDs were washed by precipitation with a mixture of isopropanol and
ethanol, then four additional washing steps using hexanes/isopropanol mixtures and finally dispersed in hexanes. These reactions were found to scale to at least twice the scale described without changing particle quality and as described this strategy was used to generate the necessary larger quantities for experiments with smaller QDs. Synthesis of PbS NCs. PbS nanocrystals were synthesized using a variation from Hines and Scholes.3 1.88 g (8.4 mmol) PbO, 55.2 mL ODE, and 5.6 mL (17.7 mmol) OA were degassed on a schlenk line at 120 °C under vacuum for 2 hrs, then put under nitrogen. The temperature was reduced to 110 °C, then 20 mL of a TMS/ODE (21uL/1 mL) (2 mmol TMS) solution was rapidly injected. The synthesis was allowed to proceed for 30 seconds. The heating mantle was then removed and the solution allowed to cool to 35 °C before transferring to schlenk tube to undergo purification inside a nitrogen filled glovebox. The wash steps were as follows: The solution was split into six 50 mL centrifuge tubes. 1st: 35 mL of acetone was added to each tube and the solution centrifuged. 2nd: pellet was redispersed in 3mL toluene/tube, then 3 mL acetone and 3 mL methanol were added. Centrifuge. 3rd: pellet was redispersedd in 3 mL toluene/tube, 3 mL of ethanol added. Centrifuge. All centrifugation was done at 8000 rpm for 5 min. The final product was redispersed in 20 mL of hexane total. Synthesis of CaF2 NCs. CaF2 NCs were synthesized by thermal decomposition of calcium trifluoroacetate (synthesized by dissolving CaCO3 in TFA) in a reaction medium of 50 vol% oleic acid and 50 vol% ODE similar to rare earth phosphors.4 2 mmol calcium trifluoroacetate in 15 mL oleic acid and 15 mL ODE was evacuated for 1 hour at 120 °C, then heated under nitrogen to 290 °C using a heating mantle, held for 30 minutes, then cooled to room temperature and isolated by three precipitation steps with ethanol. Thin Film Deposition. Spin-coating of CdSe thin films was performed on quartz plate substrates between 800 rpm and 3000 rpm depending on the desired thickness. The QDs were dispersed in octane (after drying hexanes solutions to obtain mass) and dispensed through a 0.2 µm PTFE filter to remove particulates. Ligand exchange was performed using a 130 mM solution of NH4SCN in anhydrous methanol which was dropped on the film, held 30 s, then spun off at 2500 rpm. Films were then washed three times with clean, filtered methanol by dropping on the film, then spinning at 2500 rpm after 30 s. Annealing of the CdSe film was performed for 10 minutes in a nitrogen glovebox on a hotplate with a surface temperature thermometer at 250 °C. Spin-coating of PbS: All films were deposited on MPTS treated quartz plates. The substrates were immersed in a 5% by volume MPTS in toluene solution overnight. For non-ligand exchanged films, an aliquot of the PbS/hexane stock solution was dried under vacuum then redispersed in a 5:1 ratio of octane:hexane at half the original volume (double the concentration). The NCs were deposited in one layer at 800 rpm. For ligand exchanged films, an aliquot of the stock PbS/hexane solution was dried under vacuum then redispered in a 5:1 ratio of octane:hexane at the original stock concentration. One drop of the NC solution was deposited on the substrate and spun off at 2500 rpm. Then the film was completely coated with the ligand solution and allowed to sit for 30 seconds before being spun off. The film was then washed with the dispersive solvent twice. This process was repeated 5 to 8 times to achieve the desired thickness. The following ligand solutions were used:
10 mg/mL (130 mM) SCN/acetonitrile5 10 mg/mL (27 mM) TBAI/methanol6,7 2 uL/10 mL (2.4 mM) EDT/acetonitrile7,8 2 uL/10 mL (1.7 mM) BDT/acetonitrile8 1% vol (114 mM) MPA/methanol6 Conversions used in this work. Several standard conversions are employed in this work which are repeated here for reference. First, the complex refractive index and complex dielectric functions may be converted as follows: = − and = 2. n and k may be described
as a function of the complex dielectric function as = √′ + ′′ + ′⁄2 and =
√′ + ′′ − ′⁄2. The linear extinction coefficient, the most common measurement of
absorbance in thin films, is frequently used in this work and can be obtained from = 4 ⁄. The molar extinction coefficient (base 10, M-1L-1), which is commonly used in the literature of NCs in solution, can be obtained from the linear extinction coefficient using the volume (V) of a NC and Avogadro’s number (NA): = ⁄1000ln 10 . Electron Microscopy. Routine transmission electron microscopy (TEM) characterization was performed using a JEOL 2100 microscope operating at 200 keV. Sizing estimates in this work are based upon TEM images. Scanning electron microscopy characterization was performed using a JEOL 7500 microscope operating at 5 keV. To prepare a sample for the cross section image in Figure 1b, a 250 nm SiO2/Si wafer was used as the substrate. Atomic Force Microscopy (AFM). AFM characterization was performed using MFP-Bio-3D AFM (Asylum Research) in tapping mode. Scans were typically performed over 50 µm at scan rates of 0.2 Hz. Thermogravimetric analysis (TGA). TGA was conducted using a TA Instruments SDT Q600. Samples were heated at 20 °C/min under air to 550 °C.
Figure S1. (a) Ellipsometric data for quartz plate substrates. Angles are labeled by color with delta values indicated with open circles and psi values indicated with lines. (b) Transmission spectrum of the quartz plate substrate with respect to air. Note increased noise from infrared detector. Table S1. Cauchy fitting parameters for transparent region of CdSe QD solids. CdSe QD size (nm) 3.4 3.6 4.7 5.1 6.6 3.6 (after ligand-exchange) 3.6 (exchange and annealing)
Cauchy A parameter 1.60 1.64 1.70 1.73 1.74 1.78 1.92
Cauchy B parameter 0.02 0.02 0.02 0.04 0.05 0.04 0.06
Figure S2. TEM micrographs of CdSe QDs of (a) 3.4 nm, (b) 3.6 nm, (c) 4.7 nm, (d) 5.1 nm, and (e) 6.6 nm. (f) PbS QDs used in this work prior to ligand exchange processing.
Figure S3. AFM line scan from a 3.6 nm CdSe QD film. An AFM spot-check showed a thickness of ~388 nm whereas the same film was fitted by ellipsometry to a thickness of 403 nm.
Figure S4. Comparison of ellipsometric fitting of 3.6 nm CdSe thin film with (a) and without (b) surface roughness as an added additional variable in the fitting process. Psi is represented by open squares and the fit lines are solid red lines; delta is represented by closed circles fitted with solid black lines. Data points have been removed for clarity. This sample represents the largest relative improvement which was obtained from adding surface roughness as a variable, from a mean square error (MSE) of 4.4 without surface roughness to a MSE of 2.6 with surface roughness of 2.37 nm. Typically, MSE values of fits improved by approximately 2 with the addition of surface roughness as a variable, but it was also found to generate non-physical (e.g. negative) surface roughness and to increase computational fitting time substantially.
Figure S5. Solution absorption spectra of QDs used in this work. (a) CdSe and (b) PbS QDs.
Figure S6. (a) Linear extinction coefficients of CdSe QD film before and after ligand exchange. (b) Ratio of the extinction coefficient after ligand exchange to extinction before ligand exchange.
Figure S7. TGA data for CdSe and PbS nanocrystals capped with oleic acid.
Concentration-dependence of QD extinction and mixed composition films. Following the Beer-Lambert law, the absorption of a material is linearly-dependent on the concentration of that material, but this presupposes that the extinction coefficient of an absorbing material is not a function of concentration. In fact, the Beer-Lambert law can break down at high concentrations of absorber when those absorbers interact strongly. Because previous reports have suggested that interaction between neighboring particles in a self-assembled monolayer generates giant and broad-band increases of the extinction coefficient of QD,9 we tested the concentrationdependence of the absorption from CdSe QDs in a 3D solid-state matrix with a transparent nanocrystal (CaF2) to act as a non-interacting host. A solution of QDs and a solution of CaF2 NCs were prepared at 150 mg/mL using at least 400 mg of material each. Then, mixture solutions were prepared with compositions varying from 5% CdSe by weight to 100% CdSe. These mixed solutions were then spin-coated into thin films at 800 rpm and the absorption spectra measured for each solid thin film sample as well as a diluted aliquot of the solution. For the plot in Figure S4, the absorption value recorded at the first excitonic absorption or 350 nm was normalized to the absorption measured at the relevant position for the sample containing 100 wt% CdSe. The results indicate that the enhanced probability of near-neighbor coupling at higher wt% fractions of CdSe causes no appreciable deviation from expected linearity. This was reflected also in the unchanged position of the first excitonic feature. A similar test was conducted with both wurtzite (polar) and zinc blende (apolar) QDs, but results were unchanged.
Figure S8. Absorbance of mixed hexanes solutions and spin-coated thin films of CdSe and CaF2 NCs prepared by mixing two solutions with the same weight concentration (150 mg/mL). The absorbance of each series was normalized to the value of the relevant absorbance maximum for comparison between series. Solution absorption spectra were taken at the same dilution. Drop lines from each of the points are shown for ease of comparison.
References (1)
Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C. Synthesis of CdSe and CdTe Nanocrystals Without Precursor Injection. Angew. Chem., Int. Ed. Engl. 2005, 44, 6712– 6715.
(2)
Goodwin, E. D.; Diroll, B. T.; Oh, S. J.; Paik, T.; Murray, C. B.; Kagan, C. R. Effects of Post-Synthesis Processing on CdSe Nanocrystals and Their Solids: Correlation between Surface Chemistry and Optoelectronic Properties. J. Phys. Chem. C 2014, 118, 27097– 27105.
(3)
Hines, M. a.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater. 2003, 15, 1844–1849.
(4)
Mai, H.-X.; Zhang, Y.-W.; Si, R.; Yan, Z.-G.; Sun, L.; You, L.-P.; Yan, C.-H. HighQuality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties. J. Am. Chem. Soc. 2006, 128, 6426–6436.
(5)
Fafarman, A. T.; Koh, W.; Diroll, B. T.; Kim, D. K.; Ko, D.-K.; Oh, S. J.; Ye, X.; DoanNguyen, V.; Crump, M. R.; Reifsnyder, D. C.; Murray, C. B.; Kagan, C. R. ThiocyanateCapped Nanocrystal Colloids: Vibrational Reporter of Surface Chemistry and SolutionBased Route to Enhanced Coupling in Nanocrystal Solids. J. Am. Chem. Soc. 2011, 133, 15753–15761.
(6)
Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J.-P.; Minor, J.; Kemp, K. W.; Dong, H.; Rollny, L.; Labelle, A.; Carey, G.; Sutherland, B.; Hill, I.; Amassian, A.; Liu, H.; Tang, J.; Bakr, O. M.; Sargent, E. H. Airstable n-type colloidal quantum dot solids. Nat. Mater. 2014, 13, 822–828.
(7)
Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796–801.
(8)
Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulović, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8, 5863–5872.
(9)
Geiregat, P.; Justo, Y.; Abe, S.; Flamee, S.; Hens, Z. Giant and Broad-Band Absorption Enhancement in Colloidal Quantum Dot Monolayers through Dipolar Coupling. ACS Nano 2013, 7, 987–993.
ethanol, then four additional washing steps using hexanes/isopropanol mixtures and finally dispersed in hexanes. These reactions were found to scale to at least twice the scale described without changing particle quality and as described this strategy was used to generate the necessary larger quantities for experiments with smaller QDs. Synthesis of PbS NCs. PbS nanocrystals were synthesized using a variation from Hines and Scholes.3 1.88 g (8.4 mmol) PbO, 55.2 mL ODE, and 5.6 mL (17.7 mmol) OA were degassed on a schlenk line at 120 °C under vacuum for 2 hrs, then put under nitrogen. The temperature was reduced to 110 °C, then 20 mL of a TMS/ODE (21uL/1 mL) (2 mmol TMS) solution was rapidly injected. The synthesis was allowed to proceed for 30 seconds. The heating mantle was then removed and the solution allowed to cool to 35 °C before transferring to schlenk tube to undergo purification inside a nitrogen filled glovebox. The wash steps were as follows: The solution was split into six 50 mL centrifuge tubes. 1st: 35 mL of acetone was added to each tube and the solution centrifuged. 2nd: pellet was redispersed in 3mL toluene/tube, then 3 mL acetone and 3 mL methanol were added. Centrifuge. 3rd: pellet was redispersedd in 3 mL toluene/tube, 3 mL of ethanol added. Centrifuge. All centrifugation was done at 8000 rpm for 5 min. The final product was redispersed in 20 mL of hexane total. Synthesis of CaF2 NCs. CaF2 NCs were synthesized by thermal decomposition of calcium trifluoroacetate (synthesized by dissolving CaCO3 in TFA) in a reaction medium of 50 vol% oleic acid and 50 vol% ODE similar to rare earth phosphors.4 2 mmol calcium trifluoroacetate in 15 mL oleic acid and 15 mL ODE was evacuated for 1 hour at 120 °C, then heated under nitrogen to 290 °C using a heating mantle, held for 30 minutes, then cooled to room temperature and isolated by three precipitation steps with ethanol. Thin Film Deposition. Spin-coating of CdSe thin films was performed on quartz plate substrates between 800 rpm and 3000 rpm depending on the desired thickness. The QDs were dispersed in octane (after drying hexanes solutions to obtain mass) and dispensed through a 0.2 µm PTFE filter to remove particulates. Ligand exchange was performed using a 130 mM solution of NH4SCN in anhydrous methanol which was dropped on the film, held 30 s, then spun off at 2500 rpm. Films were then washed three times with clean, filtered methanol by dropping on the film, then spinning at 2500 rpm after 30 s. Annealing of the CdSe film was performed for 10 minutes in a nitrogen glovebox on a hotplate with a surface temperature thermometer at 250 °C. Spin-coating of PbS: All films were deposited on MPTS treated quartz plates. The substrates were immersed in a 5% by volume MPTS in toluene solution overnight. For non-ligand exchanged films, an aliquot of the PbS/hexane stock solution was dried under vacuum then redispersed in a 5:1 ratio of octane:hexane at half the original volume (double the concentration). The NCs were deposited in one layer at 800 rpm. For ligand exchanged films, an aliquot of the stock PbS/hexane solution was dried under vacuum then redispered in a 5:1 ratio of octane:hexane at the original stock concentration. One drop of the NC solution was deposited on the substrate and spun off at 2500 rpm. Then the film was completely coated with the ligand solution and allowed to sit for 30 seconds before being spun off. The film was then washed with the dispersive solvent twice. This process was repeated 5 to 8 times to achieve the desired thickness. The following ligand solutions were used:
10 mg/mL (130 mM) SCN/acetonitrile5 10 mg/mL (27 mM) TBAI/methanol6,7 2 uL/10 mL (2.4 mM) EDT/acetonitrile7,8 2 uL/10 mL (1.7 mM) BDT/acetonitrile8 1% vol (114 mM) MPA/methanol6 Conversions used in this work. Several standard conversions are employed in this work which are repeated here for reference. First, the complex refractive index and complex dielectric functions may be converted as follows: = − and = 2. n and k may be described
as a function of the complex dielectric function as = √′ + ′′ + ′⁄2 and =
√′ + ′′ − ′⁄2. The linear extinction coefficient, the most common measurement of
absorbance in thin films, is frequently used in this work and can be obtained from = 4 ⁄. The molar extinction coefficient (base 10, M-1L-1), which is commonly used in the literature of NCs in solution, can be obtained from the linear extinction coefficient using the volume (V) of a NC and Avogadro’s number (NA): = ⁄1000ln 10 . Electron Microscopy. Routine transmission electron microscopy (TEM) characterization was performed using a JEOL 2100 microscope operating at 200 keV. Sizing estimates in this work are based upon TEM images. Scanning electron microscopy characterization was performed using a JEOL 7500 microscope operating at 5 keV. To prepare a sample for the cross section image in Figure 1b, a 250 nm SiO2/Si wafer was used as the substrate. Atomic Force Microscopy (AFM). AFM characterization was performed using MFP-Bio-3D AFM (Asylum Research) in tapping mode. Scans were typically performed over 50 µm at scan rates of 0.2 Hz. Thermogravimetric analysis (TGA). TGA was conducted using a TA Instruments SDT Q600. Samples were heated at 20 °C/min under air to 550 °C.
Figure S1. (a) Ellipsometric data for quartz plate substrates. Angles are labeled by color with delta values indicated with open circles and psi values indicated with lines. (b) Transmission spectrum of the quartz plate substrate with respect to air. Note increased noise from infrared detector. Table S1. Cauchy fitting parameters for transparent region of CdSe QD solids. CdSe QD size (nm) 3.4 3.6 4.7 5.1 6.6 3.6 (after ligand-exchange) 3.6 (exchange and annealing)
Cauchy A parameter 1.60 1.64 1.70 1.73 1.74 1.78 1.92
Cauchy B parameter 0.02 0.02 0.02 0.04 0.05 0.04 0.06
Figure S2. TEM micrographs of CdSe QDs of (a) 3.4 nm, (b) 3.6 nm, (c) 4.7 nm, (d) 5.1 nm, and (e) 6.6 nm. (f) PbS QDs used in this work prior to ligand exchange processing.
Figure S3. AFM line scan from a 3.6 nm CdSe QD film. An AFM spot-check showed a thickness of ~388 nm whereas the same film was fitted by ellipsometry to a thickness of 403 nm.
Figure S4. Comparison of ellipsometric fitting of 3.6 nm CdSe thin film with (a) and without (b) surface roughness as an added additional variable in the fitting process. Psi is represented by open squares and the fit lines are solid red lines; delta is represented by closed circles fitted with solid black lines. Data points have been removed for clarity. This sample represents the largest relative improvement which was obtained from adding surface roughness as a variable, from a mean square error (MSE) of 4.4 without surface roughness to a MSE of 2.6 with surface roughness of 2.37 nm. Typically, MSE values of fits improved by approximately 2 with the addition of surface roughness as a variable, but it was also found to generate non-physical (e.g. negative) surface roughness and to increase computational fitting time substantially.
Figure S5. Solution absorption spectra of QDs used in this work. (a) CdSe and (b) PbS QDs.
Figure S6. (a) Linear extinction coefficients of CdSe QD film before and after ligand exchange. (b) Ratio of the extinction coefficient after ligand exchange to extinction before ligand exchange.
Figure S7. TGA data for CdSe and PbS nanocrystals capped with oleic acid.
Concentration-dependence of QD extinction and mixed composition films. Following the Beer-Lambert law, the absorption of a material is linearly-dependent on the concentration of that material, but this presupposes that the extinction coefficient of an absorbing material is not a function of concentration. In fact, the Beer-Lambert law can break down at high concentrations of absorber when those absorbers interact strongly. Because previous reports have suggested that interaction between neighboring particles in a self-assembled monolayer generates giant and broad-band increases of the extinction coefficient of QD,9 we tested the concentrationdependence of the absorption from CdSe QDs in a 3D solid-state matrix with a transparent nanocrystal (CaF2) to act as a non-interacting host. A solution of QDs and a solution of CaF2 NCs were prepared at 150 mg/mL using at least 400 mg of material each. Then, mixture solutions were prepared with compositions varying from 5% CdSe by weight to 100% CdSe. These mixed solutions were then spin-coated into thin films at 800 rpm and the absorption spectra measured for each solid thin film sample as well as a diluted aliquot of the solution. For the plot in Figure S4, the absorption value recorded at the first excitonic absorption or 350 nm was normalized to the absorption measured at the relevant position for the sample containing 100 wt% CdSe. The results indicate that the enhanced probability of near-neighbor coupling at higher wt% fractions of CdSe causes no appreciable deviation from expected linearity. This was reflected also in the unchanged position of the first excitonic feature. A similar test was conducted with both wurtzite (polar) and zinc blende (apolar) QDs, but results were unchanged.
Figure S8. Absorbance of mixed hexanes solutions and spin-coated thin films of CdSe and CaF2 NCs prepared by mixing two solutions with the same weight concentration (150 mg/mL). The absorbance of each series was normalized to the value of the relevant absorbance maximum for comparison between series. Solution absorption spectra were taken at the same dilution. Drop lines from each of the points are shown for ease of comparison.
References (1)
Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C. Synthesis of CdSe and CdTe Nanocrystals Without Precursor Injection. Angew. Chem., Int. Ed. Engl. 2005, 44, 6712– 6715.
(2)
Goodwin, E. D.; Diroll, B. T.; Oh, S. J.; Paik, T.; Murray, C. B.; Kagan, C. R. Effects of Post-Synthesis Processing on CdSe Nanocrystals and Their Solids: Correlation between Surface Chemistry and Optoelectronic Properties. J. Phys. Chem. C 2014, 118, 27097– 27105.
(3)
Hines, M. a.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater. 2003, 15, 1844–1849.
(4)
Mai, H.-X.; Zhang, Y.-W.; Si, R.; Yan, Z.-G.; Sun, L.; You, L.-P.; Yan, C.-H. HighQuality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties. J. Am. Chem. Soc. 2006, 128, 6426–6436.
(5)
Fafarman, A. T.; Koh, W.; Diroll, B. T.; Kim, D. K.; Ko, D.-K.; Oh, S. J.; Ye, X.; DoanNguyen, V.; Crump, M. R.; Reifsnyder, D. C.; Murray, C. B.; Kagan, C. R. ThiocyanateCapped Nanocrystal Colloids: Vibrational Reporter of Surface Chemistry and SolutionBased Route to Enhanced Coupling in Nanocrystal Solids. J. Am. Chem. Soc. 2011, 133, 15753–15761.
(6)
Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J.-P.; Minor, J.; Kemp, K. W.; Dong, H.; Rollny, L.; Labelle, A.; Carey, G.; Sutherland, B.; Hill, I.; Amassian, A.; Liu, H.; Tang, J.; Bakr, O. M.; Sargent, E. H. Airstable n-type colloidal quantum dot solids. Nat. Mater. 2014, 13, 822–828.
(7)
Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796–801.
(8)
Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulović, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8, 5863–5872.
(9)
Geiregat, P.; Justo, Y.; Abe, S.; Flamee, S.; Hens, Z. Giant and Broad-Band Absorption Enhancement in Colloidal Quantum Dot Monolayers through Dipolar Coupling. ACS Nano 2013, 7, 987–993.
