Letter pubs.acs.org/JPCL
Magneto-Optical Properties of CuInS2 Nanocrystals William D. Rice,† Hunter McDaniel,‡ Victor I. Klimov,*,‡ and Scott A. Crooker*,† †
National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
‡
ABSTRACT: We compare the absorption, photoluminescence, and magneto-optical properties of colloidal CuInS2 (CIS) nanocrystals with two closely related and wellunderstood binary analogs: Cu-doped ZnSe nanocrystals and CdSe nanocrystals. In contrast with conventional CdSe, both CIS and Cu-doped ZnSe nanocrystals exhibit a substantial energy separation between emission and absorption peaks (Stokes shift) and a marked asymmetry in the polarizationresolved low-temperature magneto-photoluminescence, both of which point to the role of localized dopant/defect states in the forbidden gap. Surprisingly, we find evidence in CIS nanocrystals of spin-exchange coupling between paramagnetic moments in the nanocrystal and the conduction/valence bands of the host lattice, a behavior also observed in Cu-doped ZnSe nanocrystals, where the copper atoms incorporate as paramagnetic Cu2+ ions. SECTION: Physical Processes in Nanomaterials and Nanostructures prior PL studies of CIS.16,18,20,22−24 Importantly, these localized defect states may possess extra (or fewer) electrons as compared with the stoichiometric atomic site, raising the asyet-unexplored possibility that they may also exhibit a form of magnetic behavior (paramagnetism, ferromagnetism, etc.). The PL mechanism is even less transparent for zerodimensional CIS nanocrystals, which typically exhibit a single broad PL band occurring several hundred millielectronvolts below the apparent absorption edge. The broad PL profile and substantial shift between PL and absorption edge has led many CIS nanocrystal studies to attribute PL to the same general DAP or conduction-band−deep-acceptor transition mechanism observed in bulk CIS.2,3,8,9,25−27 In this work, we compare and contrast the optical and magneto-optical properties of CIS nanocrystals with those of two closely related and well-understood binary analogs: conventional CdSe nanocrystals and Cu-doped ZnSe (Cu:ZnSe) nanocrystals. In contrast with CdSe nanocrystals, both CIS and Cu:ZnSe nanocrystals exhibit a substantial PL that is peaked at energies well below the onset of strong absorption and also a marked polarization asymmetry in lowtemperature (T) magneto-PL studies. Both of these findings highlight the role of localized dopant or defect levels in the forbidden gap. Additionally, we find evidence in CIS nanocrystals of spin-exchange coupling between paramagnetic moments in the nanocrystal and the conduction/valence bands of the host lattice, a behavior also observed in Cu:ZnSe nanocrystals, where the copper atoms incorporate as para-
I−III−VI ternary nanocrystals have attracted considerable attention as low-toxicity, biocompatible alternatives1,2 to their II−VI binary analogs such as CdSe. CuInS2 (CIS) and CuInSe2 (CISe) nanocrystals are a particular focus for optical applications because they have a bandgap that can be varied with particle size across a wide, technologically relevant wavelength range (500−1000 nm).2−7 The importance of CIS is further enhanced by its large global Stokes shift of the photoluminescence (PL), greatly inhibiting reabsorption, which is a critically important attribute for technological devices. Furthermore, CIS can be chemically synthesized with high efficacy8 to produce efficiently emitting nanocrystals.8,9 The high PL yield, bandgap tunability across the visible optical range, and large global Stokes shift make ternary nanocrystals highly desirable for photovoltaic4,10−12 and lighting applications.13−15 Despite the strong recent interest in CIS and CISe-based nanocrystals, the nature of the PL emission is not well understood. Previous studies of bulk and thin-film CIS observed not only sharp PL lines from direct exciton recombination16−21 but also significant PL at much lower energies occurring over a ∼700 meV span attributable to a variety of defect-related radiative transitions18−20,22,23 including donor−acceptor pair (DAP) transitions, donor−valence band recombination, and conduction-band−acceptor recombination. Assigning specif ic transitions to the various subgap emission features in CIS has proven difficult, owing to the large number of possible emission pathways that arise from the many defect states that can exist in ternary semiconductors, including vacancy, interstitial, and antisite defects, which may behave as donors (e.g., VS, InCu) or acceptors (e.g., VCu, CuIn, VIn).18 These and other defect states have been discussed in many © XXXX American Chemical Society
Received: October 9, 2014 Accepted: November 10, 2014
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magnetic Cu2+ ions,28,29 which may suggest the presence of similar processes in CIS nanocrystals. The optical and magnetooptical similarities between CIS and Cu:ZnSe nanocrystals suggest that ternary compositions can achieve similar material attributes as lightly doped binary compounds. Pyramidal-shaped CIS nanocrystals with an estimated height of 2.5 nm were synthesized using previously described procedures.8,12 Several CIS batches, with Cu/In ratios between 1.25 and 1.45, were produced from which thin films were made either by spin-coating or drop-casting onto glass substrates. For comparison, magnetically doped Cu:ZnSe29 and nonmagnetic CdSe nanocrystals were also synthesized and studied. Figure 1 shows a comparison of the low-T absorption and PL spectra for CIS, Cu:ZnSe, and CdSe nanocrystals. The global
redistribution of carrier population obeying Boltzmann statistics, and, in accord with the optical selection rules in these materials, to B-dependent intensities for RCP and LCP emission.31 For small B, where gexμBB/kBT < 1, the intensity of RCP (LCP) emission is expected to increase (decrease) approximately linearly with B, which is precisely the behavior seen in CdSe nanocrystals (Figure 2d).
Figure 2. (a) Polarization-dependent PL of RCP (teal) and LCP (pink) emission at B = +6 and 0 T (dark gray) for CIS nanocrystals (Cu/In = 1.26). Integrated PL from 0 to 1 T for both RCP and LCP emission versus B for (b) CIS, (c) Cu:ZnSe, and (d) CdSe nanocrystals at 3 K.
Figure 1. Photoluminescence (PL) and absorption spectra (red and black curves, respectively) at 3 K and 0 T for (a) CIS nanocrystals (Cu/In = 1.26), (b) Cu:ZnSe nanocrystals (adapted from Pandey et al.28), and (c) CdSe nanocrystals.
In contrast, the polarization-resolved emission from both CIS and Cu:ZnSe nanocrystals exhibits a markedly different and decidedly nonlinear behavior as a function of B (Figure 2b,c). The intensity of RCP (LCP) emission rapidly increases (decreases) with small applied fields up to ∼0.25 T but thereafter varies much more slowly. In Cu:ZnSe nanocrystals, where the PL originates from radiative transitions of the photogenerated electron in the conduction band to a localized deep acceptor state on a Cu2+ dopant, this nonlinear magnetoPL behavior likely arises from the complex and B-dependent level structure of the local paramagnetic Cu2+ (3d9) ion, which stem from crystal field and spin−orbit effects, as studied in Cu:ZnS and Cu:CdS single crystals.32 CIS nanocrystals exhibit qualitatively similar nonlinear trends in magneto-PL (compare Figure 2b,c), which is consistent with the notion that emission from CIS nanocrystals involves localized dopant or defect states that possess a nontrivial level structure and splitting in applied magnetic fields arising from detailed crystal field and spin−orbit effects. Whereas PL studies provide insight into the nature of radiative recombination and final electronic states in a material, B-dependent absorption studies tell us about the spin and magnetic properties of the fundamental valence-to-conduction band absorption transitions. One such method is magnetic circular dichroism (MCD) spectroscopy (Figure 3a), where the field-induced Zeeman splittings of the valence and conduction bands are inferred from the normalized transmission difference between LCP (TL) and RCP light (TR):33−35 MCD = (TR − TL)/(TR + TL). In conventional undoped binary semi-
Stokes shift between the onset of strong exciton absorption and the PL is very large for both CIS and Cu:ZnSe nanocrystals (510 and 640 meV, respectively). In contrast, conventional CdSe nanocrystals show a much smaller Stokes shift (80 meV) and a narrower PL line from direct conduction-to-valence band recombination of 1S excitons (Figure 1c). Pure ZnSe nanocrystals also show direct band-to-band excitonic emission, but as Cu2+ dopants are added, the PL shifts to the much lower energy and much broader emission arising from conduction band to copper acceptor emission30 seen in Figure 1b. The similarities between CIS and Cu:ZnSe PL suggest that related dopant- or defect-related emission processes are responsible for the PL in CIS nanocrystals. Additional similarities between CIS and Cu:ZnSe nanocrystals were seen in polarization-resolved magneto-PL studies. Using unpolarized 375 nm (∼3.3 eV) excitation light, we measured the intensity of right and left circularly polarized (RCP/LCP) emission in magnetic fields, B, applied in the ̂ Faraday geometry (k∥B). In the simplest picture for nonmagnetic semiconductors such as CdSe and ZnSe, B generates a linear Zeeman splitting of the conduction and valence bands due to the intrinsic diamagnetism of the host semiconductor. For excitonic recombination, the resulting Zeeman energy splitting between the spin-up (Jz = +1) and spin-down (Jz = −1) band-edge excitons, ΔEZ = gexμBB (where μB is the Bohr magneton and gex is the intrinsic exciton g factor), leads to a 4106
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with the average spin polarization of the local moments along B, ⟨SZ⟩. ⟨SZ⟩ is typically modeled by a Brillouin function, BJ(x) with x = gPMμBBJ/kBT, which describes the average spin polarization of paramagnetic atoms at temperature T and field B. Here J is the total angular momentum (J = L + S) of the paramagnetic species and gPM is their g factor. Importantly, ΔEZ, and therefore the corresponding MCD signals, are strongly T-dependent when paramagnetic dopants and Jsp−d coupling are present; this behavior has been observed in several magnetically doped nanocrystal systems.28,33−35 Here we apply MCD spectroscopy to CIS nanocrystals for the first time. The MCD spectra in Figure 3b exhibit a weak feature in the range between 2.0 and 2.2 eV. As shown in Figure 3b, the MCD profile corresponds to the first derivative of the low-energy onset of the nanocrystals’ strong absorption, consistent with a Zeeman splitting of this strong absorption feature. The lack of clear features above 2.25 eV in the MCD spectrum directly follows from the absence of sharp, highenergy optical transitions in the absorption profile of CIS. However, the most important aspect of these data is that the MCD peak at 2.07 eV changes with T, increasing by over a factor of 2 as the temperature is dropped from 50 to 3 K. This behavior is clearly shown in the inset of Figure 3b, where the Tindependent contribution has been subtracted from the MCD data. As previously discussed, T-dependent Zeeman splitting strongly suggests the presence of paramagnetic moments in the nanocrystals that are coupled via sp−d exchange to the conduction and valence bands of the host CIS lattice. In this regard, CIS nanocrystals share some similarities with DMS and intentionally magnetically doped nanocrystals such as Cu:ZnSe.28 In contrast, the T-independent component of the MCD signal measures the CIS exciton without exchange interactions (i.e., no preferential alignment of the magnetic dopant moments); we can extract this contribution from the high-T behavior of the Zeeman splitting. To determine the dependence of ΔEZ on T and B, we monitored the MCD signal at its peak (2.07 eV). The variation of ΔEZ with T and B, shown in Figure 3c,d, can be fit remarkably well to eq 1 (orange lines) with a fixed set of parameters. The excellent fits to eq 1 notwithstanding, we point out that a strict analogy with conventional DMS is not wholly appropriate here because of the significant d-like character of the valence band in CIS. This arises from the substantial (∼40%) hybridization of the nominally p-like valence band with the Cu d orbitals.16,19,37,38 Regardless, these MCD data provide additional evidence of the presence of localized, paramagnetic dopants or defects in CIS nanocrystals, consistent with the PL and magneto-PL measurements shown in Figures 1 and 2. The fits in Figure 3c,d were obtained using gex = 0.2 for the “intrinsic” exciton g factor and ΔEspd = 135 μeV. The quantities J and gPM enter as a product in the Brillouin function of eq 1 and therefore cannot be fit independently from the data, precluding a definitive identification of the paramagnetic state (or states) in the nanocrystals. As discussed in the introduction, potential candidates include the many vacancy, antisite, and interstitial defects that can occur in CIS. Crucially, depending on the position of the Fermi level, these defect states may possess extra (or fewer) localized unpaired electrons, which can lead to paramagnetic behavior and nonzero Jsp−d coupling to the conduction and valence bands of the host CIS lattice. For example, given the similarity between the magneto-optical behavior of CIS and Cu:ZnSe nanocrystals, it may be that the
Figure 3. (a) MCD experiment. (b) MCD of CIS (Cu/In = 1.45) at 6 T for different T (left axis) and film absorption at 3 K (right axis). Inset: MCD of CIS with the T-independent contribution subtracted. (c) T dependence of ΔEZ for CIS obtained from continuously probing MCD at 2.07 eV (black trace) and from the individual scans shown in (b) (purple dots). A Brillouin function (orange curve) describes the data well.(d) B dependence of ΔEZ (black trace) fit with a Brillouin function (orange trace) using the same fit parameters as in panel c.
conductors and semiconductor nanocrystals such as those based on CdSe or ZnSe, MCD has been used to measure the small linear Zeeman splitting of the conduction and valence bands and their associated band-edge (1S) excitons. This exciton splitting (ΔEZ = gexμBB ≈ 116 μeV/T for gex = 2) arises from the intrinsic diamagnetism of the host material and therefore is typically temperature-independent and small, being characterized by exciton g factors gex of order unity.35 In contrast, II−VI binary semiconductor nanocrystals that are lightly doped with magnetic atoms (e.g., Mn:ZnSe, Mn:CdSe, or Cu:ZnSe) are examples of diluted magnetic semiconductors (DMS),36 which are interesting because of the large spinexchange (Jsp−d) interaction that exists between the s-like (plike) conduction (valence) bands of the host semiconductor and the localized d electrons of the embedded, paramagnetic local moments.28,33−35 This sp−d exchange interaction can be very large, generating substantial Zeeman splittings (equivalent to effective B fields of ∼100 T)33 of the host conduction and valence bands that follow the Brillouin-function-like magnetization of the embedded paramagnetic ions. In this case, the associated band-edges are split by a total amount ΔEZ = gex μB B + ΔEspd⟨SZ⟩
(1)
where gexμBB is the intrinsic diamagnetic term discussed above, and the second term describes the additional splitting arising from Jsp−d interactions with the embedded paramagnetic moments. This additional splitting, which has maximum value ΔEspd when all the paramagnetic moments are aligned, scales 4107
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Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (9) Nam, D.-E.; Song, W.-S.; Yang, H. Facile, Air-Insensitive Solvothermal Synthesis of Emission-Tunable CuInS2/ZnS Quantum Dots with High Quantum Yields. J. Mater. Chem. 2011, 21, 18220− 18226. (10) Lewerenz, H. J.; Goslowsky, H.; Husemann, K.-D.; Fiechter, S. Efficient Solar Energy Conversion with CuInS2. Nature 1986, 321, 687−688. (11) Scheer, R.; Walter, T.; Schock, H. W.; Fearheiley, M. L.; Lewerenz, H. J. CuInS2 Based Thin Film Solar Cell with 10.2% Efficiency. Appl. Phys. Lett. 1993, 63, 3294−3296. (12) McDaniel, H.; Fuke, N.; Pietryga, J. M.; Klimov, V. I. Engineered CuInSexS2−x Quantum Dots for Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 355−361. (13) Song, W.-S.; Yang, H. Efficient White-Light-Emitting Diodes Fabricated from Highly Fluorescent Copper Indium Sulfide Core/ Shell Quantum Dots. Chem. Mater. 2012, 24, 1961−1967. (14) Zhong, H.; Bai, Z.; Zou, B. Tuning the Luminescence Properties of Colloidal I-III-VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 2012, 3, 3167− 3175. (15) Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908−918. (16) Tell, B.; Shay, J. L.; Kasper, H. M. Electrical Properties, Optical Properties, and Band Structure of CuGaS2 and CuInS2. Phys. Rev. B 1971, 4, 2463−2471. (17) Yoshino, K.; Ikari, T.; Shirakata, S.; Miyake, H.; Hiramatsu, K. Sharp Band Edge Photoluminescence of High-Purity CuInS2 Single Crystals. Appl. Phys. Lett. 2001, 78, 742−744. (18) Binsma, J. J. M.; Giling, L. J.; Bloem, J. Luminescence of CuInS2: I. The Broad Band Emission and its Dependence on the Defect Chemistry. J. Lumin. 1982, 27, 35−53. (19) Binsma, J. J. M.; Giling, L. J.; Bloem, J. Luminescence of CuInS2: II. Exciton and Near Edge Emission. J. Lumin. 1982, 27, 55−72. (20) Wakita, K.; Fujita, F.; Yamamoto, N. Photoluminescence Excitation Spectra of CuInS2 Crystals. J. Appl. Phys. 2001, 90, 1292− 1296. (21) Yakushev, M. V.; Martin, R. W.; Mudryi, A. V.; Ivaniukovich, A. V. Excited States of the A Free Exciton in CuInS2. Appl. Phys. Lett. 2008, 92, 111908. (22) Massé, G.; Lahlou, N.; Butti, C. Luminescence and Lattice Defects in CuInS2. J. Phys. Chem. Solids 1981, 42, 449−454. (23) Ueng, H. Y.; Hwang, H. L. The Defect Structure of CuInS2. Part I: Intrinsic Defects. J. Phys. Chem. Solids 1989, 50, 1297−1305. (24) Hofhuis, J.; Schoonman, J.; Goossens, A. Elucidation of the Excited-State Dynamics in CuInS2 Thin Films. J. Phys. Chem. C 2008, 112, 15052−15059. (25) Zhong, H.; Zhou, Y.; Ye, M.; He, Y.; Ye, J.; He, C.; Yang, C.; Li, Y. Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals. Chem. Mater. 2008, 20, 6434− 6443. (26) Nose, K.; Omata, T.; Otsuka-Yao-Matsuo, S. Colloidal Synthesis of Ternary Indium Diselenide Quantum Dots and Their Optical Properties. J. Phys. Chem. C 2009, 113, 3455−3460. (27) DeTrizio, L.; Prato, M.; Genovese, A.; Casu, A.; Povia, M.; Simonutti, R.; Alcocer, M. J. P.; D’Andrea, C.; Tassone, F.; Manna, L. Strongly Fluorescent Quaternary Cu-In-Zn-S Nanocrystals Prepared from Cu1−x InS2 Nanocrystals by Partial Cation Exchange. Chem. Mater. 2012, 24, 2400−2406. (28) Pandey, A.; Brovelli, S.; Viswanatha, R.; Li, L.; Pietryga, J. M.; Klimov, V. I.; Crooker, S. A. Long-Lived Photoinduced Magnetization in Copper-Doped ZnSe-CdSe Core-Shell Nanocrystals. Nat. Nanotechnol. 2012, 7, 792−797. (29) Viswanatha, R.; Brovelli, S.; Pandey, A.; Crooker, S. A.; Klimov, V. I. Copper-Doped Inverted Core/Shell Nanocrystals with “Permanent” Optically Active Holes. Nano Lett. 2011, 11, 4753−4758. (30) Gul, S.; Cooper, J. K.; Corrado, C.; Vollbrecht, B.; Bridges, F.; Guo, J.; Zhang, J. Z. Synthesis, Optical and Structural Properties, and
local bonding configuration of defects in CIS can alter the oxidation state of neighboring copper atoms from Cu+ to Cu2+ and thereby lead to the observed paramagnetic behavior, by analogy to the paramagnetism observed in Cu:ZnSe.28 Local charge compensation within the doubled unit cell of I−III−VI compounds (relative to II−VI’s) may be favored, for example, if Cu2+ exists on an In site (charge-balanced by a Cu2+ on an adjoining Cu site) or if a well-known22,23 Cu vacancy forms (compensated by an adjacent Cu2+). It is emphasized, however, that Cu2+ is only one of many potential paramagnetic species or defects that are possible in CIS nanocrystals. In summary, CIS nanocrystals exhibit absorption, magnetoPL, and MCD behavior comparable to intentionally Cu2+doped ZnSe nanocrystals but clearly divergent with conventional undoped (and nonmagnetic) CdSe nanocrystals. From our MCD results, we find evidence of paramagnetic-localized moments in CIS nanocrystals. All together, these observations are consistent with the existence of localized paramagnetic states in the CIS bandgap and their important role in determining the PL from CIS nanocrystals. The similarities between ternary CIS and Cu2+-doped binary nanocrystals suggest that interesting optical and magnetic properties may be obtained by exploring ternary structures, such as CIS, CISe, and related compounds.
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AUTHOR INFORMATION
Corresponding Authors
*S.A.C.: E-mail:
[email protected]. *V.I.K.: E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS W.D.R and S.A.C were supported by the Los Alamos LDRD program and H.M. and V.I.K. were supported by the Chemical Sciences, Biosciences, and Geosciences Division of the Office of Science, U.S. DOE.
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REFERENCES
(1) Pons, T.; Pic, E.; Lequeux, N.; Cassette, E.; Bezdetnaya, L.; Guillemin, F.; Marchal, F.; Dubertret, B. Cadmium-Free CuInS2/ZnS Quantum Dots for Sentinel Lymph Node Imaging with Reduced Toxicity. ACS Nano 2010, 4, 2531−2538. (2) Aldakov, D.; Lefrançois, A.; Reiss, P. Ternary and Quaternary Metal Chalcogenide Nanocrystals: Synthesis, Properties and Applications. J. Mater. Chem. C 2013, 1, 3756−3776. (3) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Synthesis and Characterization of Colloidal CuInS2 Nanoparticles from a Molecular Single-Source Precursor. J. Phys. Chem. B 2004, 108, 12429−12435. (4) Klenk, R.; Klaer, J.; Scheer, R.; Lux-Steiner, M. C.; Luck, I.; Meyer, N.; Rühle, U. Solar Cells Based on CuInS2−An Overview. Thin Solid Films 2005, 480−481, 509−514. (5) Allen, P. M.; Bawendi, M. G. Ternary I-III-VI Quantum Dots Luminescent in the Red to Near-Infrared. J. Am. Chem. Soc. 2008, 130, 9240−9241. (6) Xie, R.; Rutherford, M.; Peng, X. Formation of High-Quality I-IIIVI Semiconductor Nanocrystals by Tuning Relative Reactivity of Cationic Precursors. J. Am. Chem. Soc. 2009, 131, 5691−5697. (7) Bose, R.; Jana, S.; Manna, G.; Chakraborty, S.; Pradhan, N. Rate of Cation Exchange and Change in Optical Properties during Transformation of Ternary to Doped Binary Nanocrystals. J. Phys. Chem. C 2013, 117, 15835−15841. (8) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper 4108
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The Journal of Physical Chemistry Letters
Letter
Charge Carrier Dynamics of Cu-Doped ZnSe Nanocrystals. J. Phys. Chem. C 2011, 115, 20864−20875. (31) Furis, M.; Hollingsworth, J. A.; Klimov, V. I.; Crooker, S. A. Time- and Polarization-Resolved Optical Spectroscopy of Colloidal CdSe Nanocrystal Quantum Dots in High Magnetic Fields. J. Phys. Chem. B 2005, 109, 15332−15338. (32) Telahun, T.; Scherz, U.; Thurian, P.; Heitz, R.; Hoffmann, A.; Broser, I. Nonlinear Zeeman Behavior of Cu2+ Centers in ZnS and CdS Explained by a Jahn-Teller Effect. Phys. Rev. B 1996, 53, 1274. (33) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. HighQuality Manganese-Doped ZnSe Nanocrystals. Nano Lett. 2001, 1, 3− 7. (34) Archer, P. I.; Santangelo, S. A.; Gamelin, D. R. Direct Observation of sp-d Exchange Interactions in Colloidal Mn2+- and Co2+-Doped CdSe Quantum Dots. Nano Lett. 2007, 7, 1037−1043. (35) Bussian, D. A.; Crooker, S. A.; Yin, M.; Brynda, M.; Efros, A. L.; Klimov, V. I. Tunable Magnetic Exchange Interactions in ManganeseDoped Inverted Core-Shell ZnSe-CdSe Nanocrystals. Nat. Mater. 2009, 8, 35−40. (36) Furdyna, J. K. Diluted Magnetic Semiconductors. J. Appl. Phys. 1988, 64, R29−R64. (37) Shay, J. L.; Tell, B.; Kasper, H. M.; Schiavone, L. M. p-d Hybridization of the Valence Bands of I-III-VI2 Compounds. Phys. Rev. B 1972, 5, 5003. (38) Jaffe, J. E.; Zunger, A. Electronic Structure of the Ternary Chalcopyrite Semiconductors CuAlS2, CuGaS2, CuInS2, CuAlSe2, CuGaSe2, and CuInSe2. Phys. Rev. B 1983, 28, 5822−5847.
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