Challenges and Prospects of Electronic Doping of Colloidal Quantum

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Challenges and Prospects of Electronic Doping of Colloidal Quantum Dots: Case Study of CdSe Viktor Chikan Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-3701, United States ABSTRACT: Colloidal quantum dots from cheap chemical synthesis are becoming important building blocks for constructing practical electronic devices. Here, we review some of the challenges and prospects of electronic doping of colloidal CdSe quantum dots. There are fundamental challenges to produce doped quantum dots with uniform size and composition. However, kinetics and thermodynamic argument suggests promising strategies that could be used to overcome the challenges. Doped CdSe quantum dots can be the starting point of other scientific discoveries that could be also applicable to practical devices such as quantum dot photovoltaics.

Q

uantum confined materials have been the center of interest of nanotechnology for the past decades1
3 for several reasons. First of all, because of their size-dependent properties and their ease of preparation,4 quantum dots (QDs) are important materials for constructing the next generation of solar cells5 that can efficiently utilize the entire solar spectrum. A significant amount of research is devoted to control the size6 and surface characteristics of these materials7 to interface them for applications. A somewhat less explored area is the electrical control of the QDs8
10 by chemical doping. In bulk semiconductors, doping helps achieve the desired conductivity and Fermi level by increasing electron or hole concentration, meanwhile preserving the electrical neutrality of the material. In contrast, in QDs, doping may produce a very different outcome.11 For example, there is no infinite lattice within the QD; therefore, it would be more appropriate to consider polarizability12,13 of the “free” carriers from doping than macroscopic conductivity. QDs made of CdSe are one of the most studied QDs prepared by colloidal synthesis.14 Although CdSe is not a sustainable material15,16 to address our solar energy needs on a global scale, it holds many desirable properties. Because of the good overlap with the solar energy spectrum with its band gap, CdSe has been extensively used to experiment with next-generation solar cells. The efficiency of these solar cells can reach a few percent.5,17 Incremental increase in the efficiency will translate into economically viable solar cells.18 Doping was a very critical step in the development of the p
n junction solar cell. Boron and phosphorus dopants allow modification of the Fermi level of bare silicon. When the p- and n-type materials are placed in contact, a built-in electric field develops, resulting in an efficient charge separation from light absorption. The rhetorical question is if the same success can be implemented with quantum-dot-based solar cell technology. Here, CdSe QDs serve as a platform to explore basic questions and the challenges about doping QDs. Although r 2011 American Chemical Society

there are several very important studies on Mn-doped CdSe,19
21 in this paper, we focus on doping that aims at manipulating the electronic conductivity or polarizibility of this material. Colloidal synthesis is perhaps the most viable and economical choice to produce doped QDs on a large scale. As the simplest strategy, a dopant can be introduced into the growth solution of CdSe QDs. This approach brings challenges into the goal of doping QDs.22 The material science community has devoted considerable efforts to reduce he size dispersity23 of colloidal QDs in a colloidal growth solution via controlling the nucleation and growth process. Size focusing of CdSe QDs is achieved by artificially maintaining a CdSe monomer supersaturated solution that leads to size focusing during growth. The colloidal control of the growth solution produces uniform-sized QDs with small variations in the physical properties of the QDs.24 Poissonian Dopant Distribution. Doping introduces additional uncertainty into the colloidal synthesis.25 Dopants are randomly dispersed in the growth solution, which makes the incorporation of the dopant into the QDs highly probabilistic. From the statistical point of view, the probability of a dopant incorporated into the volume of the QDs is driven by Poissonian statistics (eq 1).25 PðkÞ ¼

eVn ðVnÞk k!

ð1Þ

Here, V is the QD volume, n is the dopant concentration, and k is the number of dopant/QD. As an example, the calculation in Figure 1 shows the probability of a dopant atom occupying the host QDs with a radius r. For the calculation, 5 mol % dopant and 95% host material concentration are chosen, which have been Received: September 9, 2011 Accepted: October 17, 2011 Published: October 17, 2011 2783

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Figure 1. (Top) Calculated dopant distribution in CdSe QDs (published work26) as a function of QD size for an arbitrary 5 mol % dopant concentration (n). The calculation is based on eq 1. (Bottom Left) Calculated dopant distribution in CdSe QDs for an arbitrary CdSe QD with 1.5 nm radius. (Bottom Right) Variation of the number of most probable dopants as a function of QD radius. The green curve shows a Gaussian size distribution of QDs with 1.5 nm average radius and 10% size distribution.

shown to be achievable in CdSe QDs.26 As one would expect, larger QDs contain more dopants. However, the variation of the number of dopant/QD is significant for any given size. In addition, for a QD solution with reasonable size variation (5%), the most probable dopant number will also vary considerably over the size distribution. From this prospect, it seems challenging to hope that dopant uniformity can be achieved in a colloidal synthesis. Can chemical control be developed, similar to size control, to reduce variation in dopant distributions in QDs? On the basis of statistics, the answer is no. However, thermodynamic consideration contributing to the growth kinetics of the quantum dots can help overcome this variation.

Can chemical control be developed, similar to size control, to reduce variation in dopant distributions in QDs? On the basis of statistics, the answer is no. However, thermodynamic consideration contributing to the growth kinetics of the quantum dots can help overcome this variation. Thermochemistry and Growth Kinetics of Doped QDs. Differential Growth As a Key Component to Reduce Size Distribution. While we previously used a 5% concentration to model the variation in dopant levels, we now examine what thermochemical factors govern the actual incorporation of dopants into growing

QDs to begin with. Talapin et al. proposed an elegant theoretical model to describe the growth of colloidal QDs and explain the origin of size focusing based on the size-dependent activation energies of the growth and dissolution processes in a onecomponent solution.27 According to this model, the growth of QDs is driven by the chemical potential difference between the QD monomer in the solution and the monomer in the solid QD. The chemical potential of the monomer in the solid is dependent on the curvature of the QD(r) via the Gibbs
Thomson effect, which produces dispersion of the growth and dissolution rates of QDs (see eq 2). 8
 9 ½M 2γVm > > > > > > = < C
exp rRT dr flat
 ð2Þ ¼ Vm DCflat  > D 4γVm > dt > > > > ; :r þ  exp kg rRT Here, γ is the surface energy, Vm is the molar volume, R is the gas constant, T is the temperature, D is the monomer diffusion coefficient, Cflat is the solubility limit of the monomer above a flat surface, and r is the radius of the nanoparticle. The dispersion of growth rates creates faster growth for medium-sized particles than for larger particles at high monomer concentrations at well over the solubility limit of the monomer in the solution when the reaction is under diffusion control. This differential growth, which is a key component to reduce the size distribution in colloidal solutions, can produce self-focusing of QDs.23 At low monomer concentrations, the larger particles will grow at the expense of the small ones, resulting in Ostwald ripening of the colloidal particles.28 Surface Energy and Chemical Potential of the Host. Doping will likely affect the growth of the particles through changing the surface energy and the chemical potential of the host in the nanoparticle. In a two-component growth solution in which the dopant and the host semiconductor are present in both the solution and the solid phase, the growth rate of the particles and the rate of dopant incorporation into the particles depend on the chemical 2784

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Figure 2. Chemical potential of the host in the absence and the presence of dopant in the nanoparticle.

potential difference of the components between the solid phase and the solution. In the solution, the chemical potentials of the components can be considered independent from one another. In other words, host and dopant are not likely to interact in solution; therefore, these components are less likely to affect the growth of the particles. In the solid phase (nanoparticle), strong interactions of the dopant with the host could be expected, which affects the chemical potential of both components and the surface energy of the particles. As shown in Figure 2, the chemical potential of the host (and the dopant) will be influenced significantly in the solid phase by doping (mixing). The two important contributions to the freeenergy change of mixing are the entropy and the enthalpy of mixing. The entropy of mixing will always lower the chemical potential of the host in the nanoparticle compared to that of the host in an undoped nanoparticle. For a typical nanoparticle synthesis, the entropy of mixing is on the order of a kJ and only will be important for larger nanoparticle sizes.29 More significant contribution is expected from the enthalpy of mixing if there are significant differences between the bond energies of the host
host, host
dopant, and dopant
dopant. Generally, it is thought that doping will have positive enthalpy of mixing. This positive enthalpic contribution from the mixing is a basis to explain the difficulty of making doped QDs and the self-purification of QDs.30 Doping can affect the dispersion of growth rate through the change of surface energy as well. Decrease of the surface energy is considered a viable route to make the dopant stick to the nanoparticles.20,31 On the other hand, the increase in surface energy could account for the lack of success in producing doped QDs. Both the decrease in surface energy and the negative enthalpy of mixing in the presence of strong host
dopant interaction can result in conditions that lower the chemical potential of the host. The lowering of the chemical potential of the host will result in important consequences on the growth kinetics and dopant distribution of the QDs.As the chemical potential of the host is lowered, that will have important consequences on the growth kinetics and dopant distribution of the QDs.

As the chemical potential of the host is lowered, that will have important consequences on the growth kinetics and dopant distribution of the QDs.

Figure 3. Effect of indium dopant on the growth of CdSe QDs (published work32).

Effect of Lower Chemical Potential and Surface Energy. The prediction based on lower host chemical potential and the decrease of surface energy is that the solubility of the host decreases. The change in solubility leads to faster growth in the presence of the dopant. In addition, QDs with no dopant atoms will grow slower than their dopant-free counterparts. This could result in a significant increase in the size distribution of the particles during growth and dispersion of the dopant distribution as a function of size. The dopant distribution in the QD will be different from what is predicted based on the statistical distribution imposed by the Poissonian statistics when a narrow size of the particles is considered out of the ensemble. This difference can potentially open a way to produce more uniformly doped QDs if the larger particles can be separated from the smaller particles. Additional strategies may also depend on manipulating the chemical potential of the dopant in the solution, which could result in faster doping rates during the growth of nanoparticles. While Talapin has shown that supersaturation of the host monomer is critical to achieve size focusing, it plausible (Le Chatelier’s principle) that supersaturation of the dopant will be essential to produce a more uniform dopant distribution of the QDs. The opposite is true as well; if the coordinating solvent strongly binds with the dopant, successful doping is less likely. Colloidal Synthesis of Doped QDs. The effect of doping on the growth rates of the QDs has been briefly explored in the literature. The effect of indium doping on the growth of CdSe is investigated via in situ monitoring of the size of CdSe QDs.32 The kinetic measurements show that the indium accelerates the growth of the CdSe QDs as expected from the arguments above (Figure 3). The second prediction that the larger QDs will contain more dopants than the small ones is not verified experimentally yet. In agreement with the above picture, it is predicted that dispersion of growth rate will result in lower-quality size distributions in colloidal growth solutions containing dopants. The larger size distribution in the presence of dopant also has been reported in the literature.32 Calculations on indium-doped CdSe QDs suggest that the best overlap between the QD wave function and the dopant level could be expected if the dopant is located at the center of the QD.33 Peng et al. suggested that perhaps QDs could be doped more uniformly when the particles nucleate.34 Doping is expected to affect the nucleation of the QD during growth. Gamelin et al. have shown that the Mn doping increases the barrier for the nucleation event.35 From the barrier increase, the 2785

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Figure 4. (Top Left) Transient absorption spectrum of indium-doped CdSe QDs. (Top Right) Transient absorption spectrum of undoped CdSe QDs (unpublished data). (Bottom Left) Temperature-dependent Stokes shift of undoped and tin- and indium-doped CdSe QDs (published work26). (Bottom Right) Absorption and photoluminescence spectra of tin- and indium-doped CdSe QDs, indicating the absence of a Stokes shift (published work26).

probability of dopant incorporation could be suppressed. He argues that the increased barrier height also helps stabilize the trapped dopant atom at the early stage of the growth. Effect of Doping on the Electronic Structure of CdSe. Incorporation of a single dopant in an average sized CdSe QD (1019
1020/ cm
3) will result in doping levels significantly higher than those observed in bulk semiconductors (1013
1018/cm
3). According to the Moss
Burstein effect,36,37 heavily doped n-type CdSe particles are expected to show a blue shift in the optical absorption spectrum because of the Pauli Exclusion Principle. At room temperature, the dopant levels in QDs are not expected to produce a significant amount of free electrons to the conduction band as easily as in bulk26 because of the quantum-confined energy shift and the reduced density of states. The temperaturedependent Stokes shift of indium- and tin-doped samples indicates that the expected blue shift becomes prominent above room temperature. When the indium-doped CdSe QD samples are exposed to an ultrafast light source, the transient bleach data clearly show a blue shift of the transient bleach, which remains persistent for a significant time. These data indicate that the electronic structure of the CdSe QDs has been significantly altered by the presence of the indium dopants. Analogous to the temperature-dependent Stokes shift, transient absorption experiments can be thought of as a process to produce hot carriers fairly rapidly (non-Boltzmann in nature). Interestingly, the roomtemperature absorption and emission of doped samples lack a Stokes shift, which suggests relaxation of the selection rules for the states in the presence of the indium and tin dopants (see Figure 4). The chemical doping is expected to produce free electrons in the conduction band or free holes in the valence band of the QD.

Figure 5. Calculation (blue curves) of the temperature-dependent PL quenching based on the Fermi
Dirac statistics of the 1Se level of CdSe QD as a function of increasing dopant level from the conduction band edge (published work26). The size of the QD is 3.2 nm in the experiment and the in the calculation. (b) PL quenching of indium-doped CdSe QDs after removal of PL quenching of undoped CdSe QDs. (4) PL quenching of tin-doped CdSe QDs after removal of PL quenching of undoped CdSe QDs.

Traditionally, the conductivity of semiconductors is measured via electrical measurements. The measurement of available carriers in colloidal QDs presents some challenges due to the difficulty in attaching electrodes to these particles. Guyot-Sionnest et al. observed that the presence of extra electron in the conduction band results in photoluminescence quenching.10 This quenching is the result of an Auger process, where the exciton energy is 2786

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Figure 6. (Left) Calculated surface plasmon resonance of the single electron in CdSe QDs in toluene. (Right) Experimental measurement of the Frolich mode of the CdSe QDs in the absence and presence of a single electron on the 1Se level.13

transferred to the free electron rather than to produce a photon via recombination. The photoluminescence quenching could be an effective tool to probe the occupation of the 1Se electron energy level of the CdSe QDs. The experimental results in Figure 5 on tin- and indium-doped QDs show that the dopant introduces a strong temperature-dependent photoluminescence quenching relative to the undoped QDs. Comparison of predicted dopant levels could provide a good estimate of the dopant energies relative to those of the 1Se quantum confined electronic level. Calculation shows that the tin and indium dopant levels are approximately 100 and 280 meV below the conduction band edge for QDs 3.2 nm in diameter. Similar results are also obtained for gallium-doped CdSe QDs (unpublished data), which shows an activation energy similar to that of tin (∼100 meV). The ionization of the dopant to the quantum-confined level could produce an extra electron. This extra electron could turn the CdSe QDs more electrochemically active (more reducing for n-type CdSe) than their undoped counterpart. This has been noted by Guyot-Sionnest et al. when investigating the effect of the electron in the conduction band of CdSe QDs.10

This extra electron could turn the CdSe QDs more electrochemically active (more reducing for n-type CdSe) than their undoped counterpart. The extra electron in the conduction band of the CdSe QDs could introduce new interesting physical and chemical phenomena. Plasmon resonance of doped QDs could be expected from the few (single) electrons or holes. Figure 6 shows the calculated surface plasmon resonance of singly charged CdSe QDs in toluene. The surface plasmon resonance is similar to the plasmon resonance of metal nanoparticles such as gold, copper, or silver. The key difference between the doped QDs and metal nanoparticles is the significantly lower resonance frequency of the doped QDs. The plasmon resonance of the doped QDs is expected to fall into a terahertz (far-infrared) regime due to the low free carrier density that determines the plasmon resonance frequency. The terahertz regime also overlaps with the phonons present in the material. This is significant because the overlap of the plasmon and phonon

results in resonances that can contribute to the energy dissipation in charged or doped QD systems. Similarly to bulk semiconductors, plasmon
phonon coupling in charged CdSe QDs has been observed experimentally by using terahertz time domain spectroscopy.13 Figure 6 shows the expected and measured plasmon resonance of CdSe QDs. This observation is not unique to CdSe QDs. In a recent report, Alivisatos et al. has also shown the presence of surface plasmon resonance in p-type Cu2
xS QDs.38 Effect of Doping on the Solar Energy Conversion in Solar Cells. Eletronic doping could improve the performance of devices that are constructed of CdSe QDs in many ways. The excess amount of charge carriers could improve the overall conductivity of these devices, as in the case of photovoltaic cells. Doping could provide free carriers for the solar cell to fill up trap states. Recently, Fu et al. have shown that doping has a sigificant impact on the photovoltaic efficiency in PbS QD solar cells.39 The introduced defects could provide scattering centers for charge carrieres, resulting in not improved but reduced overall pefomance. Optimizing the two opposing effects will be just as important in QDbased devices as it was in eletronic components of bulk semiconductors. Zhang et al. have proven that nitrogen doping of CdSe quantum-dot-sensitized nanocrystalline TiO2 film solar cells can impact the photoconversion efficiency.40 In this research, the doping has the role of extending the absorption spectrum of the TiO2 into the visible spectrum, which indirectly impacts the charge separation efficiency. Kamat et al. have been investigating the charge-transfer rates between TiO2 and different size CdSe QDs.41 They have found 3 orders of magnitude variations in the charge-transfer rates when the CdSe QD size is decreased from 7.5 to 2.5 nm. While these results are important, the key advantage of QD size tunability is to match its absorption spectrum to the solar spectrum. Doping could effectively manipulate the Fermi level of the QDs without changing the size, which would allow optimization of charge-transfer rates. Future Challenges. Doping presents us with synethetic challanges that could be potentially circumvented, considering both the thermodynamics and the kinetics of the growth of CdSe QDs. While there are a significant number of studies on doping QDs conducted in the laboratory, virtually no studies are completed about how laboratory synethsis will scale up for industrial production where the limited mass and heat transport results in additional challenges. We note that the new physical properties of electroncially doped QDs are not fully explored. For example, the plasmon
phonon coupling in doped QDs could considerably modify the charge relaxation and heat dissipation dynamics in these particles. Theory needs to predict how dopants could impact 2787

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The Journal of Physical Chemistry Letters these dynamics in QDs. This would also be of great interest to the QD-based solar cell community. In addition, experimental data are needed to show quantitatively the impact of doping in practical quanum dot devices such as photodetectors and solar cells.

’ BIOGRAPHY Prof. Viktor Chikan: MSc (1998) in organic chemistry at University of Szeged, Hungary, in 1998; Analytical chemist (1999), Bend Research Inc.; Ph.D. (2002) in physical Chemistry, Kansas State University; Postdoctoral researcher (2002
2005), UC Berkeley; Assistant professor (2005
2011), Kansas State University; Associate Professor (2011
present), Kansas State University. ’ ACKNOWLEDGMENT The author wish to thank Kansas State University and the Department of Chemistry at Kansas State University for funding. This research has been made possible by Grant ACS PRF # 49320-DNI10 from the American Chemical Society. The author would like to acknowledge Prof. Prashant Kamat and Kevin Tvrdy at the University of Notre Dame for the transient absorption data. Finally, the authors would like to acknowledge Dr. Jacek Jasinski at the University Louisville for taking the high resolution image of the indium doped CdSe QDs and Hyeon Jung Kim for drawing the structure of CdSe for the table of contents image. ’ REFERENCES (1) Ashoori, R. C. Electrons in Artificial Atoms. Nature 1996, 379, 413–419. (2) Makhlin, Y.; Schon, G.; Shnirman, A. Quantum-State Engineering with Josephson-Junction Devices. Rev. Mod. Phys. 2001, 73, 357–400. (3) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Quantum Dot Bioconjugates for Imaging, Labelling and Sensing. Nat. Mater. 2005, 4, 435–446. (4) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies. Annu. Rev. Mater. Sci. 2000, 30, 545–610. (5) Nozik, A. J. Quantum Dot Solar Cells. Phys. E 2002, 14, 115–120. (6) Trindade, T.; O’Brien, P.; Pickett, N. L. Nanocrystalline Semiconductors: Synthesis, Properties, and Perspectives. Chem. Mater. 2001, 13, 3843–3858. (7) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671–679. (8) Tsu, R.; Babic, D. Doping of a Quantum-Dot. Appl. Phys. Lett. 1994, 64, 1806–1808. (9) Yu, D.; Wang, C. J.; Guyot-Sionnest, P. n-Type Conducting CdSe Nanocrystal Solids. Science 2003, 300, 1277–1280. (10) Shim, M.; Guyot-Sionnest, P. N-Type Colloidal Semiconductor Nanocrystals. Nature 2000, 407, 981–983. (11) Mocatta, D.; Cohen, G.; Schattner, J.; Millo, O.; Rabani, E.; Banin, U. Heavily Doped Semiconductor Nanocrystal Quantum Dots. Science 2011, 332, 77–81. (12) Wang, F.; Shan, J.; Islam, M. A.; Herman, I. P.; Bonn, M.; Heinz, T. F. Exciton Polarizability in Semiconductor Nanocrystals. Nat. Mater. 2006, 5, 861–864. (13) Mandal, P. K.; Chikan, V. Plasmon
Phonon Coupling in Charged n-Type CdSe Quantum Dots: A THz Time-Domain Spectroscopic Study. Nano Lett. 2007, 7, 2521–2528. (14) Reiss, P.; Protiere, M.; Li, L. Core/Shell Semiconductor Nanocrystals. Small 2009, 5, 154–168. (15) Fadeel, B.; Garcia-Bennett, A. E. Better Safe than Sorry: Understanding the Toxicological Properties of Inorganic Nanoparticles

PERSPECTIVE

Manufactured for Biomedical Applications. Adv. Drug Delivery Rev. 2010, 62, 362–374. (16) Gupta, N.; Alapatt, G. F.; Podila, R.; Singh, R.; Poole, K. F. Prospects of Nanostructure-Based Solar Cells for Manufacturing Future Generations of Photovoltaic Modules. Int. J. Photoenergy 2009, 154059. (17) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid Nanorod
Polymer Solar Cells. Science 2002, 295, 2425–2427. (18) Gimenez, S.; Mora-Sero, I.; Macor, L.; Guijarro, N.; Lana-Villarreal, T.; Gomez, R.; Diguna, L. J.; Shen, Q.; Toyoda, T.; Bisquert, J. Improving the Performance of Colloidal Quantum-Dot-Sensitized Solar Cells. Nanotechnology 2009, 20, 295204. (19) Mikulec, F. V.; Kuno, M.; Bennati, M.; Hall, D. A.; Griffin, R. G.; Bawendi, M. G. Organometallic Synthesis and Spectroscopic Characterization of Manganese-Doped CdSe Nanocrystals. J. Am. Chem. Soc. 2000, 122, 2532–2540. (20) Erwin, S. C.; Zu, L. J.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Doping Semiconductor Nanocrystals. Nature 2005, 436, 91–94. (21) Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. Magnetic Quantum Dots: Synthesis, Spectroscopy, and Magnetism of Co2+- and Ni2+-Doped ZnO Nanocrystals. J. Am. Chem. Soc. 2003, 125, 13205–13218. (22) Galli, G. Solid-State Physics — Doping the Undopable. Nature 2005, 436, 32–33. (23) Peng, X. G.; Wickham, J.; Alivisatos, A. P. Kinetics of II-VI and III-V Colloidal Semiconductor Nanocrystal Growth: “Focusing” of Size Distributions. J. Am. Chem. Soc. 1998, 120, 5343–5344. (24) Qu, L. H.; Peng, Z. A.; Peng, X. G. Alternative Routes Toward High Quality CdSe Nanocrystals. Nano Lett. 2001, 1, 333–337. (25) Tchebotareva, A. L.; de Dood, M. J. A.; Biteen, J. S.; Atwater, H. A.; Polman, A. Quenching of Si Nanocrystal Photoluminescence by Doping with Gold or Phosphorous. J. Lumin. 2005, 114, 137–144. (26) Roy, S.; Tuinenga, C.; Fungura, F.; Dagtepe, P.; Chikan, V.; Jasinski, J. Progress toward Producing n-Type CdSe Quantum Dots: Tin and Indium Doped CdSe Quantum Dots. J. Phys. Chem. C 2009, 113, 13008–13015. (27) Talapin, D. V.; Rogach, A. L.; Haase, M.; Weller, H. Evolution of an Ensemble of Nanoparticles in a Colloidal Solution: Theoretical Study. J. Phys. Chem. B 2001, 105, 12278–12285. (28) Marqusee, J. A.; Ross, J. Theory of Ostwald Ripening — Competitive Growth and Its Dependence on Volume Fraction. J. Chem. Phys. 1984, 80, 536–543. (29) Petropoulos, J. P.; Cristiani, T. R.; Dongmo, P. B.; Zide, J. M. O. A Simple Thermodynamic Model for the Doping and Alloying of Nanoparticles. Nanotechnol. 2011, 22, 245704. (30) Dalpian, G. M.; Chelikowsky, J. R. Self-Purification in Semiconductor Nanocrystals. Phys. Rev. Lett. 2006, 96, 226802. (31) Du, M.-H.; Erwin, S. C.; Efros, A. L. Trapped-Dopant Model of Doping in Semiconductor Nanocrystals. Nano Lett. 2008, 8, 2878–2882. (32) Tuinenga, C.; Jasinski, J.; Iwamoto, T.; Chikan, V. In situ Observation of Heterogeneous Growth of CdSe Quantum Dots: Effect of Indium Doping on the Growth Kinetics. ACS Nano 2008, 2, 1411–1421. (33) Andersen, K. E.; Fong, C. Y.; Pickett, W. E. Quantum Confinement in CdSe Nanocrystallites. J. Non-Cryst. Solids 2002, 299, 1105– 1110. (34) Pradhan, N.; Peng, X. G. Efficient and Color-Tunable MnDoped ZnSe Nanocrystal Emitters: Control of Optical Performance via Greener Synthetic Chemistry. J. Am. Chem. Soc. 2007, 129, 3339–3347. (35) Bryan, J. D.; Schwartz, D. A.; Gamelin, D. R. The Influence of Dopants on the Nucleation of Semiconductor Nanocrystals from Homogeneous Solution. J. Nanosci. Nanotechnol. 2005, 5, 1472–1479. (36) Filipchenko, A. S.; Bolshakov, L. P.; Naurizbaev, A.; Braginskaya, A. G.; Popkov, A. N. Moss-Burstein Effect in N-Type Gallium Antimonide Crystals. Phys. Status Solidi A 1978, 48, K115–K117. (37) Filipchenko, A. S.; Bolshakov, L. P.; Naurizbaev, A. Moss
Burstein Effect in Normal-Type Indium Arsenide Crystals Doped with Different Donor Impurities. Phys. Status Solidi B 1978, 87, K1–K3. 2788

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(38) Zhao, Y.; Pan, H.; Lou, Y.; Qiu, X.; Zhu, J.; Burda, C. Plasmonic Cu2
xS Nanocrystals: Optical and Structural Properties of CopperDeficient Copper(I) Sulfides. J. Am. Chem. Soc. 2009, 131, 4253–4261. (39) Fu, H. Y.; Tsang, S. W.; Zhang, Y. G.; Ouyang, J. Y.; Lu, J. P.; Yu, K.; Tao, Y. Impact of the Growth Conditions of Colloidal PbS Nanocrystals on Photovoltaic Device Performance. Chem. Mater. 2011, 23, 1805–1810. (40) Lopez-Luke, T.; Wolcott, A.; Xu, L.-p.; Chen, S.; Wcn, Z.; Li, J.; De La Rosa, E.; Zhang, J. Z. Nitrogen-Doped and CdSe QuantumDot-Sensitized Nanocrystalline TiO2 Films for Solar Energy Conversion Applications. J. Phys. Chem. C 2008, 112, 1282–1292. (41) Robel, I.; Kuno, M.; Kamat, P. V. Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 4136–4137.

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