Quantum-Confined Emission and Fluorescence Blinking of Individual

Oct 24, 2014 - Also the fluorescence shows a pronounced complex blinking behavior with very different blinking dynamics of different emission lines in...
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Quantum-Confined Emission and Fluorescence Blinking of Individual Exciton Complexes in CdSe Nanowires Dennis Franz, Aina Reich, Christian Strelow, Zhe Wang, Andreas Kornowski, Tobias Kipp,* and Alf Mews Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany ABSTRACT: One-dimensional semiconductor nanostructures combine electron mobility in length direction with the possibility of tailoring the physical properties by confinement effects in radial direction. Here we show that thin CdSe quantum nanowires exhibit low-temperature fluorescence spectra with a specific universal structure of several sharp lines. The structure strongly resembles the pattern of bulk spectra but show a diameter-dependent shift due to confinement effects. Also the fluorescence shows a pronounced complex blinking behavior with very different blinking dynamics of different emission lines in one and the same spectrum. Time- and space-resolved optical spectroscopy are combined with highresolution transmission electron microscopy of the very same quantum nanowires to establish a detailed structure−property relationship. Extensive numerical simulations strongly suggest that excitonic complexes involving donor and acceptor sites are the origin of the feature-rich spectra. KEYWORDS: Quantum nanowires, CdSe, blinking, exciton complexes, donor−acceptor pairs

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of PL peaks. On the high energy side, typically three relatively sharp peaks are observed. We label this group as near-bandedge emission (NBE). The second group of peaks is red-shifted with respect to the NBE. The peaks are generally broader than the NBE peaks and their energy separation matches the energy of the longitudinal optical (LO) phonon of CdSe (26.3 meV). We label these peaks as T emission because we attribute their origin to trap states. The T peak of highest energy is the zerophonon T emission, while the attached red-shifted series of peaks is a sequence of phonon replicas. The appearance of such feature rich spectra is peculiar for low-temperature measurements; room-temperature spectra exhibit a single broad peak with a typical width of 80−90 meV whose shape sometimes deviates from a Gaussian but for which no specific bands can be assigned. Furthermore, the appearance of such systematic spectra as shown in Figure 1a is in vast contrast to previously reported low-T spectra of CdSe QNWs with random spectral positions of emission peaks.10 Instead, the fundamental regular progression of peaks strikingly resembles the spectra of CdSe bulk.12−14 In contrast to the bulk, with decreasing diameter of the QNWs, both sets of peaks clearly shift toward higher energy through size quantization effects. Figure 1b shows the evolution of the spectra of a QNW (d = 7.6 nm) over a time period of about 17 min. The false-color scale represents the PL intensity of 500 consecutive spectra, each integrated for tint = 2 s. Linked to that, Figure 1c shows six of these individual spectra, recorded at points in time marked by the horizontal dashed lines in panel b. The most striking feature disclosed by Figure 1b,c is that both the NBE and the T

ost of the nanowires (NWs) investigated nowadays are prepared in the gas phase by the vapor−liquid−solid technique (VLS).1 In recent years, also wet-chemical methods have been developed to prepare high quality nanowires in solution. The so-called solution−liquid−solid (SLS) method, pioneered by Buhro et al.,2 particularly allows for the preparation of very thin NWs,3−6 which exhibit a strong twodimensional confinement for charge carriers and can thus be called quantum nanowires (QNWs). In order to investigate the detailed electronic and optical properties, measurements on individual NWs are necessary to avoid ensemble-averaging of structural or compositional inhomogeneities. Optical spectroscopy, for example, photoluminescence (PL) spectroscopy, gives insight into the charge carrier dynamics of free-standing NWs7−11 and hence can be seen as an ideal tool to explore the properties of NWs for optoelectronic applications. In this work, individual CdSe QNWs with diameters between 4 and 12 nm, which were grown from CdSe clusters as a singlesource precursor,6 were investigated by energy-, space-, and time-resolved PL spectroscopy at low temperature T ≈ 5 K. The SLS-grown QNWs were deposited onto SiO2 membranes that allow to measure both the optical and structural properties of the very same QNWs by spectroscopy and transmission electron microscopy (TEM), respectively. All spectroscopic data presented in the following has been measured on individual QNWs whose diameters have been determined by TEM after the optical experiments. Figure 1a shows typical low-temperature PL spectra of four different isolated and individual CdSe QNWs. The corresponding diameters determined from TEM range from 8.3 nm (QNW #1) to 5.2 nm (QNW #4). Spectra #2a−c have been recorded from three different positions, more precisely, close to both ends and in the middle, of a QNW 1.5 μm in length. All spectra have a similar structure consisting of two distinct groups © 2014 American Chemical Society

Received: August 29, 2014 Revised: October 9, 2014 Published: October 24, 2014 6655

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Nano Letters

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fluctuations clearly complicate the distinct determination of parameters such as homogeneous line widths or spectral positions, even if individual QNWs are investigated. To minimize this complication we have evaluated the spectra of many (about 50) different QNWs by determining the energy position of the first (i.e., highest in energy) clearly resolvable NBE peak as well as the energy positions of the three T peaks. Figure 2a correlates the PL peak positions and the respective

Figure 1. PL spectra. (a) Representative PL spectra of four individual CdSe QNWs of different diameter d. Spectra #2a,b stem from the same QNW but from different positions. All spectra have been recorded at low temperature (T ≈ 5 K), with cw excitation at wavelength λ = 532 nm, powers Pexc between 100 and 200 nW, and integration times tint ≤ 10 s. The coloring serves as a guide to the eye. (b) Temporal evolution of the PL emission of another QNW (d = 7.6 nm). The PL intensity is encoded in a color scale, and the horizontal (vertical) axis gives the wavelength (time). (c) Six representative 2 s spectra belonging to the six horizontal dashed lines in panel b. Vertical lines are guides to the eyes.

Figure 2. Diameter-dependent emission energies: experiments and calculations. (a) Energy of the first NBE peak (blue crosses) and of the corresponding T peaks (T + phonons, connected red dots) vs diameter of the belonging QNW. The gray curve has been calculated in ref 11. (b) Energy separation between first NBE and first T peak vs diameter. (c) Calculated energies of the free exciton X (blue circles) and D+A−X complexes (black, red, and green squares) vs diameter. The corresponding spatial D+ and A− configurations are depicted in the inset. The gray curve is the same as that in panel a. (d) Energy separation between X and D+A−X complexes as depicted in panel c vs diameter. Crosses represent measured data as depicted in panel b. See text for further explanations.

emission peaks are not constant over time. Instead, the peaks slightly shift in energy and strongly fluctuate in intensity. This is particularly obvious for the T peaks, where abrupt changes between time periods of strong emission (on state) and no emission (off state) occur (cf., the five uppermost spectra in Figure 1c). Here the longest off-state duration of the T peaks was more than 50 s, while the shortest off time observed is limited by the integration time of tint = 2 s. A similar blinking behavior of the fluorescence and its spectral wandering is wellknown for individual nanocrystal quantum emitters.15 Remarkably, the blinking of the T peaks in our QNWs demonstrates that they originate from an individual quantum system inside the segment of about 450 nm of the QNW that is confocally probed by the PL measurement. The blinking and spectral wandering of the NBE emission is not as obvious as for the T emission. In particular, no such long off times are observed. This suggests that the blinking of the NBE emission occurs on a shorter time scale than that of the T emission. The discussion of the temporal evolution of the emission spectra of a single QNW also shines a new light on the discussion of the spectra of different QNWs shown in Figure 1a. The observed changes in the peaks’ intensities and line widths in the spectra for different QNWs as well as for different positions on one and the same QNW can all be affected, if not induced, by temporal fluctuations of the emission and finite integration times. In any case the spectral and intensity

diameter of the very same QNWs as determined by TEM. Blue crosses represent the NBE peaks, whereas each set of red dots represent the T peaks. For comparison, the gray line represents the diameter dependence of the optical band gap as we have calculated in ref 11 within the effective-mass approximation, assuming finite barriers, including Coulomb interaction, and considering a mismatch of permittivity inside and outside the QNW. Here, we assumed a fundamental bulk band gap of E5K g,bulk = 1.775 eV, in between low-temperature values reported for wurtzite (Eg,W = 1.841 eV16) and zincblende CdSe (Eg,ZB = 1.762 eV, estimated from refs 17 and 16). Figure 2a nicely demonstrates that the NBE peaks follow the calculated optical band gap and that both the NBE and T peaks are affected by size quantization. In Figure 2b the difference in energy between the first NBE and the first T peaks are depicted versus the corresponding QNW diameter. Obviously, this energy difference is not constant for the different spectra but varies between 14 and 90 meV. We now want to further elaborate the new aspects in the physics of our QNWs, which strikingly exhibit PL features known either from strongly confined systems (like diameterdependent shifts, blinking, and spectral wandering) or from bulk material (like the overall spectral shape), but which also have features not yet observed at all (like the individual blinking 6656

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Figure 3. Model calculations for exciton comlexes in W/ZB QNWs. (a) High-resolution TEM micrograph of a representative QNW. The yellow line is a guide for the eye. A lattice plane is assigned to wurtzite (W) or zincblende (ZB) when it forms together with its next neighbors an ABA = “zigzag” or ABC = “zigzig” sequence, respectively. Some segments are colored accordingly. (b,c) Results of model calculations for a QNW 5.2 nm in diameter. The colored surface plots represent the electron and hole wave functions evaluated on a central cross-section along the QNW for a potential landscape as given by the gray ragged wells. The corresponding eigenenergies can be read from the vertical axes. In panel b, the external potential landscape corresponding to the W/ZB segment sequence as read out from panel a is cladded between artificial binary W/ZB sequences. In panel c, additional positive and negative point charges on the QNW axis have been taken into account.

of different peaks in a spectrum of a single QNW). For sure the different peaks in the spectra have their origin in individual excitonic complexes experiencing potential variations along the QNWs. We can exclude that radius modulations are the reason for the two regular NBE and T emission bands. A locally increased radius would indeed red-shift the emission like in the T case, but it would not come along with a decreased wave function overlap that is necessary to explain the strongly enhanced phonon coupling. High-resolution TEM images like in Figure 3a prove that our QNWs exhibit alternating wurtzite (W) and zincblende (ZB) crystal modifications. Such segments are discussed to build up a type-II potential as a consequence of the staggered respective band gaps7 that might lead to a trapping of electrons in ZB sections and holes in W sections and by that to a lowering of the excitonic energy and an increase of the exciton−phonon coupling by a decreased electron−hole overlap. Thus, they might explain the T emission. In order to estimate the extent of localization in our QNWs we performed self-consistent calculations of electron−hole pairs in quasi-one-dimensional systems by solving the single-particle Schrödinger equations on a threedimensional spatial grid. For the external potential, barriers of 5 eV were assumed. The W and ZB segments were translated into an axial potential landscape by assuming conduction and valence band offsets of 144 and 59 meV, respectively.18 Results of the calculations are shown in Figure 3b,c, where the vertical axes display the energy, the horizontal axes represent the axial direction, and the third axes represent the radial direction (the diameter of the QNW was set to 5.2 nm). The upper and lower gray ragged wells represent the potential landscapes for electrons and holes, respectively. These potentials are composed of the external QNW potential plus the mutual attractive Coulomb potential of the electron and hole, respectively. The colored surface plots represent the calculated electron and hole wave functions and the horizontal planes represent their calculated eigenenergies. Panel b treats with the situation where the extensions of the respective W and ZB sections are directly taken from the high-resolution TEM image of Figure 3a. Obviously, the length of the W and ZB sections of up to 5 monolayers (MLs) are too small for an effective axial confinement of electrons or holes within individual segments. Larger segment lengths can lead to a more pronounced

localization and to a decrease of the exciton energy. For example, if we artificially extend a ZB segment to 11 MLs, we calculate a shift in emission energy of 20 meV. Larger shifts, which would be necessary to explain the measured NBE−T separation of 50 meV and more, could only be calculated for considerably larger segment lengths that, upon statistics, by far do not occur with the frequency necessary to explain the regularly observed occurrence of the T emission. Hence, we can also exclude W and ZB modifications as the origin of the regular NBE and T emission bands. Interestingly, the general structure of the PL spectra of our QNWs reveal strong similarities to the low-temperature spectra of bulk CdSe13,14,19,20 as well as thick CdSe NWs with a diameter of 200 nm.21 There, a group of two or more relatively sharp peaks close to the bulk band gap energy (corresponding to our NBE peaks) is typically assigned to the recombination of free excitons (X) and excitons bound to neutral donors (D0X) or acceptors (A0X). Some tens of meV lower in energy (corresponding to our T peaks), a set of broader peaks with energy separations matching the LO phonon has always been assigned to the recombination of donor−acceptor pairs (D+A−X or DAPs). The analogy between QNW and bulk spectra is striking. While an exact assignment of the various closely lying NBE peaks to the corresponding exciton complexes like X, D0X, and A0X is difficult because of the diameter dependence and temporal fluctuation of our QNW emission, the assignment of the T emission to DAPs in our QNWs is strongly supported as we will elaborate in the following. To include the effect of ionized donors and acceptors into our theoretical model, we placed a positive and a negative point charge (in the following called D+ and A−, respectively) inside the QNW, with their Coulomb potentials contributing to the external QNW potential. Figure 3c shows the results for the same QNW as addressed in panel b but with D+ and A− sites placed 2.6 nm apart from each other on the QNW axis. Obviously the A− attracts and localizes the hole and significantly lowers its energy. As a consequence the energy of the whole D+A−X complex is about 65 meV smaller than the energy of the exciton described in panel b. Hence, charged defect sites can explain the observed energy separation between 6657

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in accordance with the assignment of our T peaks to D+A−X complexes. It is still puzzling what the physical nature of these charged sites are in the QNWs. For CdSe bulk, acceptors are typically assigned to alkali-metal impurities, while donators are discussed, e.g., with respect to alkali-metal interstitials or chlorine impurities.19,33 Further defects that can induce charged sites can be vacancies, interstitials, or dangling bonds, which might well be located at the surface of the QNWs. Finally we want to discuss the blinking behavior of the NBEand T-emission channels. Obviously both channels exhibit drastically different blinking dynamics, which hints to two fundamentally different emission processes. To the best of our knowledge no temporal blinking, flickering, or jittering of the emission lines of any excitonic complex has been reported for bulk measurements, even in the rare case in which individual complexes have been investigated.26 In contrast, for zerodimensional nanocrystals the blinking phenomenon is wellknown and was intensively investigated.34 Also in onedimensional SLS-grown QNWs blinking has been reported before but only for room-temperature measurements.7,9 Protasenko et al.7 observed temporal intensity fluctuations by 30−80%, which was attributed to exciton localization in a typeII potential emerging from W and ZB segments and by Augerinduced quenching of the emission. In another study, Glennon et al.9 observed a synchronous PL blinking that spans the entire length of some QNWs. Here, the blinking occurs between a constant PL level and intensity bursts with fluctuating maximum intensity levels and durations as long as seconds. The authors explain their findings by a model based on the dynamic, transient filling of surface-trap sites.35 The PL blinking behavior at low temperatures as observed in this study is fundamentally different and more similar to the blinking of nanocrystals, where nonradiative Auger recombinations are discussed to be the origin of the PL blinking. Such Auger quenching should occur when excess charges in the vicinity of the confined excitonic complexes can take up the recombination energy. The different blinking dynamics of fast intensity fluctuations of the NBE compared to the long on/off periods of the T peaks might be explained by differently pronounced bonding of the excess charge to the spatial position, where the corresponding excitonic complex is localized. One might speculate that charged D+ or A− sites lead to deeper traps for the relaxing Auger carrier such that it returns to its origin with a higher probability rendering the D+A−X complex dark for a longer time. In summary, we have shown that the low-temperature PL spectra of SLS-grown CdSe QNWs consist of two groups of emission peaks. One of them is built-up of sharp peaks close to the band edge energy, while the other is red-shifted and consists of a broader peak with pronounced LO-phonon replicas. Features induced by the one-dimensionality of the QNWs are diameter dependent emission energies of both groups, a variable energy spacing between both groups, and their complex blinking behavior. The particular spectral structure cannot be explained with concepts like radius modulations or wurtzite/zincblende phase modulations that were applied in previous works on NWs. Instead, the findings are consistent with concepts of donor and acceptor sites. In particular, the red-shifted phonon-affected group of peaks is assigned to excitons bound to pairs of charged donor and acceptor sites. Calculations modeling these sites as fixed

NBE and T peaks, which is on average of the order of 50 meV, as shown in Figure 2b. Figure 2c compares the calculated diameter dependence of D+A−X energies to the one-dimensional free-exciton energy EX in QNWs. For the calculations we disregarded any crystal-phase induced axial band offsets for a better comparability. The values of EX (blue circles) in Figure 2c qualitatively agree very well with the gray curve as calculated in ref 11 and already depicted in panel a. Deviations occur mainly because in the selfconsistent calculation changes of the effective mass at the QNW surface, i.e., Ben Daniel-Duke boundary conditions, have not been taken into account. The black squares in Figure 2c represent results of the calculations for the D+A−X complex with the D+ and A− sites separated by 10 nm on the axis of the QNW. Obviously, also the D+A−X complex is strongly affected by confinement effects just like the observed T peaks. Following this argumentation, several experimental observations can readily be explained by the different locations and distances of D+ and A− within the QNW. For example, it is well-known from bulk that D+A−X complexes exhibit increasing recombination energies with a decreasing D+−A− distance because of the increasing Coulomb energy stored in the remaining D+A− complex after emission.13 Accordingly, the line that connects the black squares in Figure 2c roughly represents the lower boundary of possible D+A−X transition energies since a further D+−A− separation in axial direction does not lower the energy significantly. With respect to the radial direction one might argue that the D+ and/or A− sites might well represent surface traps since the QNWs have a high specific surface area. Therefore, in Figure 2c we also calculated diameter-dependent energies of the D+A−X complexes where either the D+ or the A− is placed at the QNW surface (red and green squares, respectively). In general, as a new degree of freedom in QNWs compared to bulk, placing charged defect sites close to the surface reduces the energy separation between NBE and T peaks. Consequently, the D+A−X emission energy falls within the yellow area marked in Figure 2c for any radial location. For clarity we plotted in Figure 2d the calculated relative energy differences EX − Ecomplex as compared to the measured energy differences ENBE1 − ET1 from Figure 2b. Obviously, all measured values fully fall into an energy range that can be explained by D+A−X transitions of different geometry. Thus, also the variation of the measured data can readily be explained by different spatial configurations of the D+ and A− sites. Beside the observed energy shifts, the presence of D+A− pairs also modifies the electron and hole wave functions and in particular their overlap. For example, in the configuration examined in Figures 3b,c, the wave function overlap is decreased from about 96% to 64%. It further decreases for larger D+−A− separation. The reduced wave function overlap can generally explain the measured larger recombination lifetimes of the T emission as compared to the NBE emission (not shown). Also the particular form of the wave functions and the configuration of the D+ and A− sites strongly influence the LO-phonon coupling of the exciton. In bulk, D+A−X complexes typically exhibit a stronger LO-phonon coupling than other excitonic complexes. This is the case not only in CdSe12−14,19−21 but also in GaAs,22 CdS,19,23,24 CdTe,25 and ZnSe.26 In contrast, for nanostructures the concept of D+A−X is seldom used. Instead, a pronounced exciton−phonon coupling is often discussed in terms of a decreased electron−hole wave function overlap27−32 generated by polarization effects through localized charges at surface traps. Both argumentations are fully 6658

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positive and negative point charges directly support the diameter dependence and the variable energy spacing. Methods. Sample Preparation. QNWs were chemically synthesized exploiting the SLS method as reported in ref 6. QNWs with TOPO ligands highly diluted in toluene were drop cast onto a free-standing 40 nm thick SiO2 membrane that allows for atomic-force microscopy (AFM), optical spectroscopy, and TEM on one and the same individual QNW. Experimental Procedure. AFM measurements were first carried out to locate presumably single QNWs on the sample. Then the optical measurements were performed using a homebuilt confocal laser scanning microscope equipped with a closed-cycle cryostat (attocube systems AG). For excitation during the steady-state measurements, a diode-pumped solidstate laser operating at 532 nm was used. A microscope objective (100× , 0.8 NA) was used for both focusing the excitation laser onto the sample (temperature T ≈ 5 K) and collecting the emission light. Spectra were recorded using a grating spectrometer equipped with a CCD detector. All TEM investigations had to be done after the optical measurements since electron microscopy quenches the PL emission of our QNWs. Calculations. The self-consistent calculations have been performed using a code written in MATLAB. Within the framework of the effective-mass approximation, they rely on the discretization of the single-particle Schrödinger equation on a three-dimensional spatial grid and a diagonalization of the emerging system of linear equations. Typical numbers of grid point were 43 × 43 × 85, the nearest distance of grid points was typically set to 3.03 Å, representing the thickness of a ML. Effective masses of electrons and holes were set to me = 0.118m0 and mh = 0.5m0, respectively. The iterative calculation starts with determining the hole wave function within the external QNW potential that exhibit finite barriers in radial direction, W/ZB-induced steps in axial direction (optional), and Coulomb contributions from additional point charges (D+ and A− sites, also optional). In further steps the electron (hole) wave functions and eigenenergies have been calculated in the total potential consisting of the external potential as well as the Coulomb potential generated by the hole (electron) wave function as calculated in the respective previous step. The iteration has been stopped at the earliest time when the energy difference for two succeeding steps was below 0.1 meV.

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REFERENCES

(1) Lu, W.; Lieber, C. M. J. Phys. D: Appl. Phys. 2006, 39, R387. (2) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791−1794. (3) Yu, H.; Li, J.; Loomis, R. A.; Gibbons, P. C.; Wang; Buhro, W. E. J. Am. Chem. Soc. 2003, 125, 16168−16169. (4) Wang, F.; Dong, A.; Sun, J.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45, 7511−7521. (5) Puthussery, J.; Kosel, T. H.; Kuno, M. Small 2009, 5, 1112−1116. (6) Li, Z.; Kurtulus, O.; Fu, N.; Wang, Z.; Kornowski, A.; Pietsch, U.; Mews, A. Adv. Funct. Mater. 2009, 19, 3650−3661. (7) Protasenko, V. V.; Hull, K. L.; Kuno, M. Adv. Mater. 2005, 17, 2942−2949. (8) Robel, I.; Bunker, B. A.; Kamat, P. V.; Kuno, M. Nano Lett. 2006, 6, 1344−1349. (9) Glennon, J. J.; Tang, R.; Buhro, W. E.; Loomis, R. A. Nano Lett. 2007, 7, 3290−3295. (10) Glennon, J. J.; Tang, R.; Buhro, W. E.; Loomis, R. A.; Bussian, D. A.; Htoon, H.; Klimov, V. I. Phys. Rev. B 2009, 80, 081303. (11) Myalitsin, A.; Strelow, C.; Wang, Z.; Li, Z.; Kipp, T.; Mews, A. ACS Nano 2011, 5, 7920−7927. (12) Kokubun, Y.; Watanabe, H.; Wada, M. Jpn. J. Appl. Phys. 1974, 13, 1393−1398. (13) Yu, P. Y.; Hermann, C. Phys. Rev. B 1981, 23, 4097−4106. (14) Perna, G.; Capozzi, V.; Ambrico, M. J. Appl. Phys. 1998, 83, 3337−3344. (15) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802−804. (16) Gross, E. F.; Razbirin, B. S.; Fedorov, V. P.; Naumov, Y. P. Phys. Status Solidi B 1968, 30, 485−494. (17) Samarth, N.; Luo, H.; Furdyna, J. K.; Qadri, S. B.; Lee, Y. R.; Ramdas, A. K.; Otsuka, N. Appl. Phys. Lett. 1989, 54, 2680−2682. (18) Li, J.; Wang. Nano Lett. 2003, 3, 1357−1363. (19) Henry, C. H.; Nassau, K.; Shiever, J. W. Phys. Rev. B 1971, 4, 2453−2463. (20) Silberstein, R. P.; Tomkiewicz, M. J. Appl. Phys. 1983, 54, 5428− 5435. (21) Fasoli, A.; Colli, A.; Martelli, F.; Pisana, S.; Tan, P. H.; Ferrari, A. C. Nano Res. 2011, 4, 343−359. (22) Bogardus, E. H.; Bebb, H. B. Phys. Rev. 1968, 176, 993−1002. (23) Ekimov, A.; Kudryavtsev, I.; Ivanov, M.; Efros, A. J. Lumin. 1990, 46, 83−95. (24) Xu, X.; Zhao, Y.; Sie, E. J.; Lu, Y.; Liu, B.; Ekahana, S. A.; Ju, X.; Jiang, Q.; Wang, J.; Sun, H.; Sum, T. C.; Huan, C. H. A.; Feng, Y. P.; Xiong, Q. ACS Nano 2011, 5, 3660−3669. (25) Soltani, M.; Certier, M.; Evrard, R.; Kartheuser, E. J. Appl. Phys. 1995, 78, 5626−5632. (26) Woggon, U.; Lüthgens, E.; Wenisch, H.; Hommel, D. Phys. Rev. B 2001, 63, 073205. (27) Nomura, S.; Kobayashi, T. Phys. Rev. B 1992, 45, 1305−1316. (28) Nirmal, M.; Murray, C. B.; Bawendi, M. G. Phys. Rev. B 1994, 50, 2293−2300. (29) Alivisatos, A. P.; Harris, T. D.; Carroll, P. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 90, 3463−3468. (30) Empedocles, S. A.; Bawendi, M. G. Science 1997, 278, 2114− 2117. (31) Sagar, D. M.; Cooney, R. R.; Sewall, S. L.; Kambhampati, P. J. Phys. Chem. C 2008, 112, 9124−9127. (32) Kelley, A. M. J. Phys. Chem. Lett. 2010, 1, 1296−1300. (33) Arora, A. K.; Ramdas, A. K. Phys. Rev. B 1987, 35, 4345−4350. (34) Frantsuzov, P.; Kuno, M.; Jankó, B.; Marcus, R. A. Nat. Phys. 2008, 4, 519−522. (35) Glennon, J. J.; Buhro, W. E.; Loomis, R. A. J. Phys. Chem. C 2008, 112, 4813−4817.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

D.F. and A.R. performed the spectroscopic experiments. C.S. developed the MATLAB code. Z.W. synthesized the QNWs. A.K. performed the high-resolution TEM studies. D.F., A.R., C.S., and T.K. analyzed the data. T.K. and A.M. conceived the experiments, supervised the project, and wrote the paper, with input and discussion from all coauthors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Tobias Redder for TEM measurments. The work was supported by the Deutsche Forschungsgemeinschaft via Grant Nos. KI 1257/2 and ME 1380/16. 6659

dx.doi.org/10.1021/nl503331t | Nano Lett. 2014, 14, 6655−6659