Rainbow Emission from an Atomic Transition in Doped Quantum Dots

Letter pubs.acs.org/JPCL

Rainbow Emission from an Atomic Transition in Doped Quantum Dots Abhijit Hazarika,† Anshu Pandey,*,† and D. D. Sarma*,†,§,¶ †

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Council of Scientific and Industrial Research − Network of Institute for Solar Energy (CSIR-NISE), Anusandhan Bhawan, New Delhi 110001, India

§

S Supporting Information *

ABSTRACT: Although semiconductor quantum dots are promising materials for displays and lighting due to their tunable emissions, these materials also suffer from the serious disadvantage of self-absorption of emitted light. The reabsorption of emitted light is a serious loss mechanism in practical situations because most phosphors exhibit subunity quantum yields. Manganesebased phosphors that also exhibit high stability and quantum efficiency do not suffer from this problem but in turn lack emission tunability, seriously affecting their practical utility. Here, we present a class of manganese-doped quantum dot materials, where strain is used to tune the wavelength of the dopant emission, extending the otherwise limited emission tunability over the yellow−orange range for manganese ions to almost the entire visible spectrum covering all colors from blue to red. These new materials thus combine the advantages of both quantum dots and conventional doped phosphors, thereby opening new possibilities for a wide range of applications in the future. SECTION: Physical Processes in Nanomaterials and Nanostructures

M

manganese center. The manganese emission band corresponds to an onsite 4T1 → 6A1 radiative transition.9 This transition, being a spin-forbidden one, has a long radiative lifetime and a negligible absorption cross section, making the manganese impurity a potentially useful phosphor material. Additionally, the states involved in the emission are essentially localized at the Mn site. As is the case for some other dopant-based phosphors,26 this site-localized character makes the emission particularly insensitive to environmental effects. Unfortunately, the insensitivity of this transition to its environment also makes Mn emission particularly difficult to tune. This drawback seriously limits its utility as a marker as well as in devices. Previous efforts to tune the manganese emission band have relied on pressure or dopant position to achieve a rather limited range of tunability (∼0.2 eV) within the orange−red part of the visible spectrum,27,28 rather insignificant when compared to the size tunability of CdSe QDs covering the entire visible range.29,30 A further limitation is that all of these approaches only lead to an overall red shift of the manganese emission band. The rather limited tunability observed in all earlier studies at the ensemble level does not, however, reflect the true nature of manganese-doped QDs at a single-particle level. In a recent study, we were able to show that isolated manganese

anganese-doped semiconductors have been studied for over 100 years.1−13 A paramagnetic impurity, manganese also exhibits exchange interactions with band edge carriers in several semiconductor hosts.14−17 In III−V semiconductors in particular, manganese doping can give rise to ferromagnetic diluted magnetic behavior.18−25 Besides their unusual magnetic properties, manganese-doped semiconductors have also been investigated for their peculiar luminescence. In II−VI materials with a sufficiently wide band gap, manganese impurities have been known, for over a century, to give rise to a characteristic yellow−orange emission band that has been a subject of numerous studies in quantum dots (QDs) as well as in bulk semiconductors.9 Only recently has it been shown13 by singlecluster spectroscopy that isolated Mn-doped QDs in such an ensemble with yellow−orange emission may randomly exhibit emissions ranging all the way to green, though it has not been possible so far to obtain ensemble-averaged emission of Mndoped QDs emitting at anything outside of the yellow−orange window. In this Letter, we design and demonstrate the tunability of ensemble-averaged manganese emission over a wide spectral window ranging from blue to red. These materials are the first examples of a novel class of phosphors with virtually no self-absorption that combine the efficiency of inorganic phosphors with the easy tunability of semiconductor QDs. In manganese-doped semiconductors, photoexcitation of the host leads to the eventual migration of the excitation into the © XXXX American Chemical Society

Received: May 11, 2014 Accepted: June 9, 2014

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Figure 1. (a) Schematic of the synthetic route followed to synthesize the Mn-doped ZnSe/CdSe/ZnSe QDs; (b) luminescence from Mn-doped NCs upon 405 nm excitation; (c) Mn emission band from different QDs. The numbers 1−9 indicate emission patterns from samples with Mn located at different distances from the ZnSe/CdSe interface. In samples 1−4, Mn is situated exactly at the interface. The CdSe thickness is different in each case. Samples 5−8 have Mn doped at different positions. Sample 9 is a Mn-doped ZnSe QD (zero CdSe thickness). (d) TEM images of Mn-doped nanocrystals.

Figure 2. (a,b) Absorption and photoluminescence emission from typical samples of Mn-doped QDs; (c,d) decay kinetics of the Mn emission band shows a fast and a slow component; (e,f) band edge photoluminescence decay occurs on a much faster time scale of 2−3 ns.

centers may have emissions distributed over a wide spectral range from 540 (green) to 630 nm (red).13 This study suggests that the apparent invariance of the Mn emission band at an ensemble level does not reflect its much narrower, tunable emission at the single-particle level. In this Letter, we show that the seemingly impossible target of tuning the Mn emission over the entire visible spectral range at an ensemble scale can be achieved by controlling the strain generated due to an epitaxial growth of a semiconductor shell over a semiconductor core. We start from ZnSe cores (Figure 1a) of size 1.6 nm for all of the samples. In order to introduce strain into the ZnSe host, a thin epitaxial shell of CdSe is overgrown on the ZnSe core. The CdSe shell is made thin enough to ensure that the QD band edge has a sufficiently high energy so as to not interfere with

the dopant emission. We then proceed with the growth of a thick outer ZnSe shell. The lattice mismatch between CdSe and ZnSe ensures a well-controlled strain gradient within the ZnSe shell. Manganese ions are doped at various distances from the CdSe/ZnSe interface within the ZnSe shell. The details of the structural information regarding thicknesses and volumes of different layers can be found in the Supporting Information (see in the section “Details of Fig. 4c” of the Supporting Information). This approach allows us to tune the position of Mn emission (Figure 1b and c) over almost the entire visible spectrum (480−580 nm) while ensuring uniform, epitaxial growth of the shell material (Figure 1d). The peak at the shortest wavelength (∼485 nm) in Figure 1c represents the emission from Mn ions 2209

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Figure 3. (a) Mn lifetime versus the blue shift in its emission position relative to 2.11 eV. (b) The same data plotted versus the inverse square of the blue shift. (c) The absolute quantum yields (QYs) of Mn-doped QDs (solid circles) are independent of concentration. This is unlike conventional QDs (diamonds), where the QYs fall sharply with increasing concentration.

We measured the QYs as well as the decay kinetics of various Mn-doped samples. In each case, the decay is fit to a biexponential given by A1 exp(−t/τ1) + A2 exp(−t/τ2). In this situation, the total number of photons emitted by the sample is given by A1τ1 + A2τ2. The electronic structure of the atomic Mn center is well-understood, and there is no reason to expect the existence of two different sets of selection rules. We therefore consider the possibility that the shorter time constant τ1 originates due to a nonradiative process, while τ2 is entirely radiative. Under this assumption, the QY is expected to be (A1τ1 + A2τ2)/((A1 + A2)τ2). We find that, in general, there is an excellent agreement between the expected and measured QYs. For example, a sample with a band edge at around 580 nm exhibits a measured absolute QY of 25%. Its kinetics suggests that A1/A2 = 8.2 with τ1 and τ2 values of 92 and 580 μs. The expected QY (25%) is in excellent agreement with the experimentally observed one. This correlation is observed in all other cases as well, thus justifying this partitioning of the kinetics into radiative and nonradiative channels. We note that the Mn emission band formally originates from a spin and orbital angular momentum forbidden transition. The transition becomes weakly allowed because of mixing of spin and angular momenta that arise due to spin−orbit coupling as well as strain effects. We assume that these low-energy processes have a perturbative effect on the Mn center. Under this assumption, the blue shift of the emission band is expected to vary linearly with the strain. The mixing of various onsite electronic energy levels is also expected to show a linear dependence on the lattice strain. The radiative rate that is dependent upon the square of the dipolar matrix element of the excited- and the ground-state wave functions is thus expected to vary with the square of the strain. These considerations allow us to write 1/τ ∝ Rate ∝ (Strain)2 ∝ Δ2 or τ ∝ Δ−2, where τ is the Mn emission lifetime. A more detailed explanation is presented in the Supporting Information. We fit the lifetimes of Mn to biexponentials in order to extract a longer and shorter decay component. Figure 3b shows a plot of both components against Δ−2. The longer decay time constants are found to vary linearly with Δ−2, in excellent agreement with these considerations. More surprisingly, we note that even the shorter component that is ascribed to nonradiative decay is seen to vary linearly with Δ−2. The biexponential nature of Mn emission kinetics has been observed previously in the literature, and the shorter component is typically ascribed to trapping.31 The linear relationship between nonradiative decay and Δ−2 however

doped exactly at the CdSe/ZnSe interface. The data shown represent different thicknesses of CdSe, with the thickest shells, grown by adding 0.020 mmol Cd2+ precursors, leading to the bluest emission. Addition of a smaller amount of Cd2+ precursors leads to redder emission, for example, 0.010 mmol leads to a peak at 505 nm. The peak at 585 nm corresponds to Mn in ZnSe only. Alternatively, this effect can also be realized by doping manganese farther away from the interface for a single thickness of CdSe. This is discussed later in this Letter. We noted that even mild annealing of the samples at high temperature leads to the disappearance of the shifted Mn emission. This is consistent with our view that Mn is doped into a very select region of the QD. Because the strain field in our QDs is generated by a carefully controlled interface located within the QD, an additional, significant advantage of our synthesis is that the Mn emission band is narrower (typically 160 meV) compared to conventional Mn-doped QDs (for example, typically 250 meV for ZnCdS). The line widths observed by us are however broader than the reported 60 meV homogeneous line width associated with manganese emission,13 indicating the important possibility of improving the synthesis further to obtain a greater color purity. Aside from the spectral tunability, our QDs still exhibit optical characteristics typically associated with other Mn-doped materials. Figure 2 shows the optical properties of two typical Mn-doped QDs that are produced through our synthesis; the QDs in Figure 2a correspond to the sample synthesized by adding 0.010 mmol of Cd precursors on the ZnSe core and doping Mn at the ZnSe/CdSe interface, while in the QDs in Figure 2b, Mn is located at a distance of 0.78 nm from the interface. Both samples have very similar S exciton energies (Figure 2a and b). Emission from the QD band edge and the substantially Stokes-shifted Mn dopant emission are clearly observable. In both samples, the Mn emission exhibits a biexponential decay. The longer time constants of 200 μs in Figure 2c and 600 μs in Figure 2d clearly establish the nature of these emission features as Mn d−d transitions. In contrast, the QD band edge decays with much shorter time constants of 3 ns in Figure 2e and 2 ns in Figure 2f. As is evident from Figure 2c and d, the emission kinetics of the Mn impurity varies from sample to sample. Figure 3a shows that the variation of the Mn decay kinetics is correlated with its emission energy. In this figure, Mn ions in an unstrained ZnSe lattice are taken to emit at 2.11 eV (588 nm). The blue shift Δ is estimated by subtracting this energy from the sample emission maximum in each case. 2210

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suggests that this component arises due to a nonradiative intersystem crossing event at the Mn center itself and not due to the coupling of this center to defects. Regardless of the existence of an intrinsic decay mechanism, Mn-doped materials are in fact far superior phosphors than normal QDs. A major problem of conventional QD phosphors is their strong tendency for self-absorption of the emitted light. Practical devices employ films or dispersions of QDs with optical densities at the band edge S exciton approaching unity. Due to the problem of self-absorption of band edge, the effective emission QYs of optically dense QD films and dispersions are much lower. Self-absorption also makes QD devices susceptible to catastrophic failures because even the degradation of a small percentage of QDs in a device causes a very substantial fall in the device efficiency. CdSe/CdS heterojunction nanorods and tetrapods ameliorate these problems somewhat by presenting significantly larger absorption cross sections while retaining the usual oscillator strengths at the band edge.32 Although these materials have very high QYs, the unavailability of large quantum confinements in these geometries limits their tunability. Doped QDs do not exhibit self-absorption and have been shown to perform better in devices.33 The QDs synthesized in this work are seen to exhibit negligible self-absorption (Figure 3c). This figure compares the absolute emission QYs of four different samples as a function of the optical density. At high optical densities, the emission QYs of conventional QDs such as CdSe and CdSe/ZnS fall drastically, while the QYs of Mn-doped QDs, prepared by our synthesis or by standard routes, are invariant with concentration. This indicates that even Mn-doped QDs with very low starting QYs, for example, 1%, can readily outperform conventional QDs with far higher QYs in practical devices. Mndoped QDs thus offer a promising alternative to conventional QD and nanorod phosphors. The origin of this spectacular spectral tuning may be understood by modeling the nanocrystal as an elastic continuum with the QD divided into three regions corresponding to the three distinct semiconductor layers. The pressure outside of each QD is taken to be zero and is continuous across each interface. The boundary conditions for the displacement vector at the semiconductor interfaces are similar to those used previously in other studies.27 On the basis of bulk results, a 7% lattice mismatch is assumed between CdSe and ZnSe. The radial strain in the outermost ZnSe shell is of the form (A/r3) + B, where A and B are constants and r is the radial position. Within a continuum model, the net change in volume of the outer shell material is simply related to the sum of the angular and radial strain components. Figure 4a shows the radial strain profile for a 4 nm QD with a 1.6 nm ZnSe core and an intermediate 0.1 nm CdSe shell. Because the radial compression of the shell is exactly balanced by its lateral enlargement, there is no net volume compression or expansion of the shell material. The tuning of the manganese emission band thus arises due to changes in its local geometry that are highlighted in Figure 4b. Despite conservation of unit cell volume, the size of the Mn coordination sphere does increase with radial strain. Neglecting shear effects, the ratio of the Mn−Se center distance before and after distortion is given by [(2/e) + e2]1/2. Here, e = a/c, is the ratio of the sides of the cuboid enclosing the Mn coordination sphere after and before the deformation (see Figure 4b). We assume the undistorted intercenter separation to be 31/2/4ao or 0.245 nm. Here ao is the lattice constant of ZnSe. An approximate connection to the

Figure 4. (a) Cross section of a 4 nm QD showing the radial strain; (b) schematic of the strain-induced structural deformation expected at a Mn site (right) along with the undistorted geometry (left); (c) variation of the emission maximum of Mn as a function of dopant distance from the CdSe/ZnSe interface. The dashed line is a visual best fit estimated from a hydrostatic model described in the text. Y error bars represent a 0.01 eV error in determining the position of emission maximum. X error bars represent a 20% error in determining the distance of the dopant from the CdSe interface. (Inset) Schematic of the system.

continuum model may be made by expressing e = 1 + err, where err is the radial strain. As the position of the Mn center is moved away from the CdSe/ZnSe interface, the coordination around the Mn center becomes closer to the tetrahedral coordination that is observed in an unstrained ZnSe QD. The emission maximum of the Mn center thus red shifts continuously as its distance from the CdSe/ZnSe interface is increased. This is highlighted in Figure 4c. The dashed line is a guide to the eye generated from the hydrostatic model described above. The blue shift in Mn emission is assumed to vary linearly with the Mn−Se bond distance. The system is modeled as a 5 nm QD with a 1.6 nm ZnSe core and a 0.3 nm CdSe shell. An exact correlation between the Mn coordination geometry and its emission maximum may be made in several different ways. It could be done for example through atomistic simulations34 or very approximately through a crystal field treatment. Here, we make a very rough estimate of the expected tuning by using results obtained through diamond anvil cell experiments on Mn-doped semiconductors.31 In a ZnS matrix, for example, the position of the Mn emission band has been shown to shift by −33 meV per GPa of applied pressure; this translates to a 300 meV blue shift per 0.01 nm increase in the intercenter distance. On the basis of the strain estimates made from the continuum model, this suggests a tuning range of 0.1 eV, in reasonable agreement with the experimentally observed tuning range. The synthetic scheme suggested here thus offers a potential route to generating materials with rather unnatural interatomic separations. Strain couples directly to the electronic degrees of freedom, allowing for the regulation of a host of phenomena. We further note that the technique adopted by us allows for the realization of conditions that are not readily sampled by controlling pressure, for example, in diamond anvil cell experiments. Most notably, our technique can allow for the realization of both higher as well as lower than normal interatomic separations, while experiments that rely on pressure can sample only a small subset of this parameter space. In conclusion, precise control over strain in QDs enables the tuning of emissions from dopants, such as Mn2+ ions in the present study, over almost the entire visible spectrum. This 2211

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(10) Kamat, P. V. Semiconductor Nanocrystals: To Dope or Not to Dope. J. Phys. Chem. Lett. 2011, 2 (21), 2832−2833. (11) Eilers, J.; Groeneveld, E.; de Mello Donegá, C.; Meijerink, A. Optical Properties of Mn-Doped ZnTe Magic Size Nanocrystals. J. Phys. Chem. Lett. 2012, 3 (12), 1663−1667. (12) Santra, P. K.; Kamat, P. V. Mn-Doped Quantum Dot Sensitized Solar Cells: A Strategy to Boost Efficiency over 5%. J. Am. Chem. Soc. 2012, 134 (5), 2508−2511. (13) Hazarika, A.; Layek, A.; De, S.; Nag, A.; Debnath, S.; Mahadevan, P.; Chowdhury, A.; Sarma, D. D. Ultranarrow and Widely Tunable Mn2+-Induced Photoluminescence from Single MnDoped Nanocrystals of ZnS−CdS Alloys. Phys. Rev. Lett. 2013, 110 (26), 267401. (14) Erwin, S. C.; Zu, L. J.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Doping Semiconductor Nanocrystals. Nature 2005, 436 (7047), 91−94. (15) Hoffman, D. M.; Meyer, B. K.; Ekimov, A. I.; Merkulov, I. A.; Efros, A. L.; Rosen, M.; Couino, G.; Gacoin, T.; Boilot, J. P. Giant Internal Magnetic Fields in Mn Doped Nanocrystal Quantum Dots. Solid State Commun. 2000, 114 (10), 547−550. (16) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. HighQuality Manganese-Doped ZnSe Nanocrystals. Nano Lett. 2001, 1 (1), 3−7. (17) Feng, Y.; Badaeva, E.; Gamelin, D. R.; Li, X. Excited-State Double Exchange in Manganese-Doped ZnO Quantum Dots: A TimeDependent Density-Functional Study. J. Phys. Chem. Lett. 2010, 1 (13), 1927−1931. (18) Dunsiger, S. R.; Carloe, J. P.; Goko, T.; Nieuwenhuys, G.; Prokscha, T.; Suter, A.; Morenzoni, E.; Chiba, D.; Nishitani, Y.; Tanikawa, T. Spatially Homogeneous Ferromagnetism of (Ga, Mn)As. Nat. Mater. 2010, 9 (4), 299−303. (19) Ohno, H. Making Nonmagnetic Semiconductors Ferromagnetic. Science 1998, 281 (5379), 951−956. (20) Dietl, T. A Ten-Year Perspective on Dilute Magnetic Semiconductors and Oxides. Nat. Mater. 2010, 9 (12), 965−974. (21) Hegde, M.; Farvid, S. S.; Hosein, I. D.; Radovanovic, P. V. Tuning Manganese Dopant Spin Interactions in Single GaN Nanowires at Room Temperature. ACS Nano 2011, 5 (8), 6365− 6373. (22) Somaskandan, K.; Tsoi, G. M.; Wenger, L. E.; Brock, S. L. Isovalent Doping Strategy for Manganese Introduction into III−V Diluted Magnetic Semiconductor Nanoparticles: InP:Mn. Chem. Mater. 2005, 17 (5), 1190−1198. (23) Erwin, S. C.; Petukhov, A. G. Self-Compensation in ManganeseDoped Ferromagnetic Semiconductors. Phys. Rev. Lett. 2002, 89 (22), 227201. (24) Stowell, C. A.; Wiacek, R. J.; Saunders, A. E.; Korgel, B. A. Synthesis and Characterization of Dilute Magnetic Semiconductor Manganese-Doped Indium Arsenide Nanocrystals. Nano Lett. 2003, 3 (10), 1441−1447. (25) Sapra, S.; Sarma, D. D.; Sanvito, S.; Hill, N. A. Influence of Quantum Confinement on the Electronic and Magnetic Properties of (Ga,Mn)As Diluted Magnetic Semiconductor. Nano Lett. 2002, 2 (6), 605−608. (26) Wu, S. W.; Han, G.; Milliron, D. J.; Aloni, S.; Altoe, V.; Talapin, D. V.; Cohen, B. E.; Schuck, P. J. Non-Blinking and PhotostableUpconverted Luminescence from Single Lanthanide-Doped Nanocrystals. Proc. Natl. Acad. Sci. U.S.A. 2009, 106 (27), 10917− 10921. (27) Ithurria, S.; Guyot-Sionnest, P.; Mahler, B.; Dubertret, B. Mn2+ as a Radial Pressure Gauge in Colloidal Core/Shell Nanocrystals. Phys. Rev. Lett. 2007, 99 (26), 265501. (28) Nag, A.; Cherian, R.; Mahadevan, P.; Gopal, A. V.; Hazarika, A.; Mohan, A.; Vengurlekar, A. S.; Sarma, D. D. Size-Dependent Tuning of Mn2+ d Emission in Mn2+-Doped CdS Nanocrystals: Bulk vs Surface. J. Phys. Chem. C 2010, 114 (43), 18323−18329. (29) Chestnoy, N.; Hull, R.; Brus, L. E. Higher Excited Electronic States in Clusters of ZnSe, CdSe, and ZnS  Spin−Orbit, Vibronic, and Relaxation Phenomena. J. Chem. Phys. 1986, 85 (4), 2237−2242.

result is especially significant because such dopant emission bands have been generally thought to have very limited tunability due to their essentially atomic nature. The doped QDs produced in this work have the same tunability as conventional CdSe-based QD phosphors but share none of their disadvantages, such as the self-absorption and surface sensitivity.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details including synthesis procedures of the samples and characterization techniques. Additional details regarding the figures and details of computational methodologies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.P.). *E-mail: [email protected] (D.D.S.). Present Address ¶

D.D.S.: Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560054, India and Department of Physics and Astronomy, Uppsala University, Box-516, SE75120, Uppsala, Sweden.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.D.S. thanks the Department of Science and Technology, Government of India for financial support. A.P. thanks the Indian Institute of Science for generous funding including seed funds. A.H. thanks the Council of Scientific and Industrial Research for financial support.

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REFERENCES

(1) Pauli, W. E. Ü ber Ultraviolette und Ultrarote Phosphoreszenz. Ann. Phys. 1911, 339 (4), 739−779. (2) 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 (11), 2532−2540. (3) Pradhan, N.; Peng, X. Efficient and Color-Tunable Mn-Doped ZnSe Nanocrystal Emitters: Control of Optical Performance via Greener Synthetic Chemistry. J. Am. Chem. Soc. 2007, 129 (11), 3339−3347. (4) Nag, A.; Chakraborty, S.; Sarma, D. D. To Dope Mn2+ in a Semiconducting Nanocrystal. J. Am. Chem. Soc. 2008, 130 (32), 10605−10611. (5) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319 (5871), 1776−1779. (6) Acharya, S.; Sarma, D. D.; Jana, N. R.; Pradhan, N. An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals. J. Phys. Chem. Lett. 2010, 1 (2), 485−488. (7) Srivastava, B. B.; Jana, S.; Karan, N. S.; Paria, S.; Jana, N. R.; Sarma, D. D.; Pradhan, N. Highly Luminescent Mn-Doped ZnS Nanocrystals: Gram-Scale Synthesis. J. Phys. Chem. Lett. 2010, 1 (9), 1454−1458. (8) Chen, H.-Y.; Maiti, S.; Son, D. H. Doping Location-Dependent Energy Transfer Dynamics in Mn-Doped CdS/ZnS Nanocrystals. ACS Nano 2011, 6 (1), 583−591. (9) Pradhan, N.; Sarma, D. D. Advances in Light-Emitting Doped Semiconductor Nanocrystals. J. Phys. Chem. Lett. 2011, 2 (21), 2818− 2826. 2212

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(30) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115 (19), 8706−8715. (31) House, G. L.; Drickamer, H. G. High-Pressure Luminescence Studies of Localized Excitations in ZnS Doped with Pb-2+ and Mn-2+. J. Chem. Phys. 1977, 67 (7), 3230−3237. (32) Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P. Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies. Nano Lett. 2007, 7 (10), 2951−2959. (33) Wang, X.; Yan, X.; Li, W.; Sun, K. Doped Quantum Dots for White-Light-Emitting Diodes Without Reabsorption of Multiphase Phosphors. Adv. Mater. 2012, 24 (20), 2742−2747. (34) Yang, S.; Prendergast, D.; Neaton, J. B. Strain-Induced Band Gap Modification in Coherent Core/Shell Nanostructures. Nano Lett. 2010, 10 (8), 3156−3162.

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