Electronic States and Exciton Fine Structure in Colloidal CdTe

May 19, 2009 - Department of Chemistry, 80 St. George Street, Institute for Optical Sciences, and ... which is consistent with the theoretical predict...
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J. Phys. Chem. C 2009, 113, 10465–10470

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Electronic States and Exciton Fine Structure in Colloidal CdTe Nanocrystals Haizheng Zhong, Michelle Nagy, Marcus Jones, and Gregory D. Scholes* Department of Chemistry, 80 St. George Street, Institute for Optical Sciences, and Center for Quantum Information and Quantum Control, UniVersity of Toronto, Toronto, Ontario, M5S 3H6 Canada ReceiVed: March 4, 2009; ReVised Manuscript ReceiVed: April 27, 2009

We prepared colloidal zinc-blend CdTe nanocrystals (NCs) with sizes ranging from 1.85 to 6.6 nm. Temperature-controlled two-dimensional photoluminescence excitation spectroscopy was applied to investigate their size-dependent electronic properties and enabled the examination of up to nine electronic excited states. At low temperatures, the exciton fine structure of the first excited state was significantly enhanced, which allowed an accurate measurement of the exchange splitting energy for the CdTe NCs via estimations of the resonant Stokes shift. It was found that the exchange splitting energy in zinc blend CdTe NCs decreases linearly with R-3.17, which is consistent with the theoretical prediction of R-3. 1. Introduction Semiconductor nanocrystals (NCs) have attracted a significant amount of interest over recent years for their size-dependent properties, as well as their potential application in a variety of fields.1-4 Size-dependent quantum confinement effects have been an area of particular focus. It is well-known that quantum confinement affects the energy levels in semiconductor NCs that are physically smaller than the exciton Bohr radius of the constituent material, yielding discrete energy states rather than bands. The confinement also enhances the electron-hole direct and exchange Coulomb interactions, thus amplifying signatures of the exciton,5 such as its fine structure.6 In addition to the fundamental scientific interest, their size-dependent electronic properties have inspired potential applications of semiconductor NCs in lasers, light-emitting diodes, and photovoltaic devices.5 For example, the wavelength of an NC laser can be tuned by choosing the NC size in the gain medium.7,8 CdTe NCs are of particular interest for their applications in optical switches, light emission diodes, solar cells, chemical sensors, and in vivo biomedical imaging.9-14 Accurate frequency-domain measurements of the size-scaling of NC electronic transitions are complicated by size and shape distributions in colloidal ensembles. Specialized spectroscopic techniques are therefore typically required to remove this inhomogeneous line broadening. Photoluminescence excitation (PLE) and fluorescence line narrowing (FLN) measurements have been successfully applied to study the size-dependent fine structure in colloidal wurtzite CdSe NCs.15-17 Size-dependent energy levels in colloidal InP, InAs, CdTe, and ZnSe NCs were also determined by similar methods.18-21 Two-dimensional photoluminescence excitation (2D PLE) spectroscopy, obtained by measuring photoluminescence (PL) spectra over a range of excitation energies, contains more information than onedimensional (1D) PLE and FLN spectra. It has previously been used to great effect in the determination of single-walled carbon nanotube exciton states.22 Here we apply the method to CdTe semiconductor NCs. The basic electronic structure of cadmium chalcogenide semiconductors is described by k.p theory, which predicts S-type * To whom correspondence should be addressed. E-mail: gscholes@ chem.utoronto.ca.

conduction band orbitals and P-type upper valence band orbitals, split into the topmost J ) 3/2 band and the split-off J ) 1/2 band.23 CdTe has the largest spin-orbit splitting 0.927 eV and the smallest band gap energy, Eg ) 1.606 eV, among CdS, CdSe, and CdTe. As a result, the split-off band (J ) 1/2) is expected to mix weakly with the topmost valence band.20 The valence band of zinc-blend CdTe is less complicated than that of wurtzite materials owing to the absence of crystal field splitting. Size-dependent spectra of CdTe NCs have mainly been reported for samples prepared in doped silica glasses.20,24,25 Recent developments in the chemical synthesis of high-quality solution-phase CdTe colloidal NCs make it possible to investigate their properties in more detail.26 In this paper, we report the preparation of colloidal CdTe NCs with PL wavelengths ranging from 490 to 750 nm (size ranges from 1.8 to 6.6 nm). Temperature-controlled 2D PLE spectroscopy was applied to investigate the size-dependent energy levels and exciton fine structures in these colloidal CdTe NCs. By using this technique, the size dependence of up to nine exciton states was identified in CdTe nanocrystals. 2. Experiments 2.1. Synthesis of CdTe NCs. Colloidal CdTe NCs were synthesized using a modified published method.26 Typically, a mixture of 0.0256 g (0.2 mmol) of CdO, 0.116 g (0.4 mmol) of tetradecylphosphonic acid (TDPA), and 10 mL of octadecene (ODE) was heated in a three-neck flask to 300 °C to obtain a clear solution. At this temperature, Te solution, prepared by dissolving 0.1 mmol of Te powder in 0.5 mL trioctylphosphine (TOP) and 4 mL ODE, was quickly injected into the hot mixture. The reaction was allowed to cool to 260 °C, allowing further growth to a specific size. To synthesize CdTe NCs with diameters smaller than 2.5 nm, the injection temperature was set at 270 °C. After the injection, the heater was removed and an additional 4 mL of ODE was injected into the reaction system to stop the growth. All the samples were purified following a published procedure.26 The ODE solution of as-prepared NCs was combined with an equal-volume mixture of hexane and methol (1/2, v/v) by vigorous shaking and subsequent centrifugation. The ODE phase was then extracted. This procedure was repeated once, and then the NCs in the ODE phase were precipitated with

10.1021/jp901995c CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009

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Figure 1. (a) Absorption and PL spectra at room temperature for colloidal CdTe NCs with band-edge emissions of 493 nm (S1, 1.85 nm), 516 nm (S2, 2.1 nm), 546 nm (S3, 2.8 nm), 598 nm (S4, 3.5 nm), 631 nm (S5, 3.8 nm), 660 nm (S6, 4.4 nm), and 743 nm (S7, 6.6 nm). (b) Room temperature Stokes shifts for different colloidal ensembles of CdTe NCs plotted as a function of the emission peak energy.

excess acetone. The precipitate was redissolved into a mixture of isopentane and methylcyclohexane (6/1, v/v) for optical measurements. 2.2. Optical Measurements. Room temperature absorption spectra and PL spectra were obtained on a CARY100 BIO UV/ vis spectrophotometer and CARY Varian florescence spectrophotometer. 2D PLE spectroscopic measurements were performed on a commercial a J-Y Spex Fluorolog 3-22 spectrofluorimeter with a 450 W xenon arc light source and a Peltier-cooled R928 PMT (Hamamatsu) photodetector. The sample was placed between two sapphire plates mounted on a closed-cycle helium cryostat, CTI-Cryogenics model-22 (Helix Technology Corp.). The emission/excitation slit widths were adjusted to optimize the resolution and signal-to-noise ratio, resulting in a 2 nm bandpass for the 1.85 nm CdTe NCs, 1 nm for the 2.1 nm NCs, and 0.75 nm for the 2.8 nm-6.6 nm NCs. Individual PL spectra, collected over a series of excitation wavelengths, were combined to yield 2D PLE maps. 3. Results and Discussion Figure 1a shows the absorption and PL spectra of the assynthesized CdTe NCs. Typically each absorption spectrum has

Zhong et al. several features relating to exciton states, which indicates that our samples are of high quality. The smallest sized CdTe NCs have a first absorption peak at 470 nm, and with increasing diameter of the NCs, the first absorption peak shifts to 494, 525, 580, 616, 650, and 740 nm. Average sizes of the as-synthesized NCs were determined from the wavelength of the first absorption peak.27 The PL spectra of the CdTe NCs show peaks at 493, 516, 546, 598, 631, 660, and 743 nm, respectively. These peaks were all red-shifted in relation to the first absorption peak by the Stokes shift energy, which was found to increase linearly with PL energy, as shown in Figure 1b. Similar phenomena have been observed in CdSe and InP.28,29 2D PLE spectra were recorded at 9 K, and four typical CdTe 2D PLE maps are shown in Figure 2. In these images, the PL intensity is represented by the color at a particular excitation and emission energy. Analysis of PL intensity modulations with excitation and emission energies within the 2D PLE spectra can yield more information about the ensemble than typical 1D PLE and FLN spectra. Additionally, CdTe excited states are identifiable and correspond to the maxima in PL intensity. In Figure 2, the CdTe NC excited states are indicated using dashed white lines. By examination of the apparent ridges and valleys in each of the PL landscapes, the position and gradient of each of these lines can easily be identified. The energies of these states depend on the degree of quantum confinement,30 and the clear slope of the white lines indicates the distribution of NC transition energies and therefore sizes within the ensemble. The size-dependent energies of these states are shown in Figure 3a, where we plot the excitation and emission energies of points along the PL ridges in seven CdTe NC preparations, each depicted in a different color. Solid lines, numbered 1-8, are overlaid to demonstrate the states’ linear dependence on the PL energy and their persistence in different NC samples across a broad size range. The first exciton state consists of two substates: bright (optically active) and dark (optically forbidden).29 Line 1, in Figure 3a, corresponds to the first bright exciton state. Additionally, line 1 is slightly shifted by longitudinal optical (LO) phonon excitation, which is a complication previously reported for low-temperature measurements.29 In Figure 3b, the size-dependent energies of CdTe NCs are replotted with the excitation energy of the states in relation to the first dark excited state versus the PL energy. The red lines in Figure 3b are the results of effective mass calculations extracted from a paper by Efros et al.31 Models based on the effective mass approximation are often used to describe the energy levels in semiconducting NCs.6,31 This theoretical calculation does not take into account the first exciton substates, which accounts for the difference in the theoretical and experimental first exciton state. There is a good correspondence between the lowest four CdTe NC states and the effective mass results, so these spectroscopic features may be assigned to (1) 1S3/2(h)-1S(e), (2) 2S3/2(h)-1S(e), (3) 1P3/2(h)-1P(e), and (4) 1S1/2(h)-1S(e) states. It was also noted that the PL intensity, and hence the oscillator strength, of the transition to 2S3/2(h)-1S(e), in each of the samples, was weaker than the transition to 1S3/2(h)-1S(e). The fifth state shows a nearly constant offset of ∼1.0 eV with the first excited state (1S3/2(h)-1S(e)), which is consistent with the CdTe split-off coupling energy of 0.927 eV. The fifth transition is therefore assigned to photoexcitation of the 1Sso-1S(e) state. We can assign the sixth transition to the 1P1/2(h)-1P(e) state, but higher energy transitions are obscured by the broad overlapping PLE features, making them difficult to identify and assign.

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Figure 2. Typical 2D PLE spectra of colloidal CdTe NCs at 9 K: (a) 2.8 nm (S3), (b) 3.5 nm (S4), (c) 3.8 nm (S5), (d) 4.4 nm (S6).

Figure 3. (a) Size-dependent energy levels of seven CdTe NCs, shown in different colors. Straight lines overlie the data to illustrate the correspondence between different samples. (b) Dark line: size-dependent energy levels replotted with the excitation energy of the states in relation to the first dark excited state versus the PL energy. Red line: effective mass calculations published by Efros et al.31

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Figure 4. High-resolution 2D PLE spectra of colloidal CdTe NCs at 9 K: (a) 2.1 nm (S2), (b) 3.5 nm (S4), (c) 3.8 nm (S6), (d) 6.6 nm (S7).

From the 2D PLE spectra, several substates of the first 1S3/2(h)-1S(e) excited state can be resolved. These substates arise due to electron-hole correlation effects that are neglected in the effective mass calculations. In zinc-blend CdTe NCs, the exciton fine structure is determined completely by the electron-hole exchange interaction. Efros’s theory regarding the size dependence of the band-edge exciton structure states that for spherical CdTe NCs the ground exciton is split into two levels by the electron-hole exchange interaction.6,29 The higher energy level is 3-fold degenerate and optically active (bright) with a total angular momentum of 1, and the lower energy group is forbidden 5-fold degenerate and optically passive (dark) with a total angular momentum of 2. The energy offset between dark and bright states is referred to as the exchange splitting energy (∆E). At 9 K, ∆E . kT, meaning that the Boltzmann population of the bright states is very small. This means that the Stokes shift, observed in low-temperature PL studies, originates from the energy difference between predominant absorption to the bright state and subsequent photoemission from the dark state and is therefore equivalent to the exchange splitting energy in cubic NCs. To elucidate accurately the exciton fine structure, highresolution 2D PLE spectra were recorded. Figure 4 shows four high-resolution 2D PLE spectra for the CdTe samples with average diameters of 2.1, 3.5, 3.8, and 6.6 nm. Taking horizontal slices from 2D PLE spectra, FLN spectra can be extracted. Figure 5 shows the FLN spectra for six CdTe NC

sizes. The diameters of the NCs are 2.1, 2.8, 3.5, 3.8, and 6.6 nm. The peak of the zero-LO-phonon line is observed to be shifted with respect to the band-edge excitation energy. Features in the PL spectra are observed to narrow as the excitation energy (labeled) decreases, which is equivalent to FLN measurements. All the samples in Figure 5, except for the 4.4 and 6.6 nm CdTe NCs (Figure 5e,f), have one or two LO-phonon-assisted excitations in addition to the zero-phonon PL peak from the dark exciton. The Stokes shift can be obtained from the energy difference between the dark state, which we identify as the PL peak closest to the scattered excitation peak (found by fitting the spectrum with a multicomponent Gaussian function), and the bright state (excitation energy). The bright-dark splitting energy in the largest NCs is expected to be very small. Therefore, for the larger CdTe NCs (4.4 and 6.6 nm), the band-edge emission from the fine structure was hidden within the excitation beam and only one or two LO-phonon-assisted transitions are resolved. For the largest CdTe NCs with an average size of 6.6 nm, it is noted that all the emissions from the dark state with one LO photon have a constant energy shift of 20 ( 2 meV with respect to the excitation energy, which is similar to Morello’s recent report.32 This means the exchange splitting of 6.6 nm CdTe NCs is not obvious, which is consistent with theory.29 Assuming the dominant phonon-assisted transition is coupled to the dark exciton, low-temperature Stokes shifts for the largest samples were determined by subtracting the LO phonon energy

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Figure 5. FLN spectra with different excitation energies of CdTe NCs [(a) 2.1 nm (S2), (b) 2.8 nm (S3), (c) 3.5 nm (S4), (d) 3.8 nm (S5), (e) 4.4 nm (S6), (f) 6.6 nm (S7)] extracted from corresponding high-resolution 2D PLE spectra: dotted line, emission from dark state; dashed line, emission from dark state with one LO phonon; dashed and dotted line, emission from dark state with two LO phonons.

(∼20 meV) from the energy shift of the LO phonon transition. Figure 6a shows the extracted low-temperature Stokes shifts of CdTe NCs plotted versus the energy of the first excited state. The Stokes shifts are in the range of ∼1-40 meV and decrease with decreasing particle size, which is in agreement with previous experimental results in CdTe NC doped glasses and calculated results of the effective mass and tight binding.24,29 It is also worth noting that the room temperature nonresonant Stokes shifts (6-128 meV, Figure 1b) are much larger than the low-temperature Stokes shifts (∼1-40 meV, Figure 6a). This is because the line-narrowing effect of low temperatures and subensemble photoexcitation allows the zero-phonon dark exciton transition to be resolved. Figure 6b plots the low-temperature bright-dark exciton splitting (for spherical CdTe NCs, this is equivalent to the electron-hole exchange interaction energy) versus the NC radius. The electron-hole exchange for bulk CdTe is 0.045 meV;6,29 it is significantly enhanced by quantum confinement in NCs. The exchange splitting energy in CdTe NCs of radius R was found to decrease as R-3.17, which is in good agreement with the theoretically predicted R-3 dependence.5 Our experimental results confirm that the exchange interaction in CdTe NCs is predominantly a short-range electron-hole exchange interaction.

4. Conclusions In summary, colloidal CdTe NCs with PL wavelengths ranging from 490 to 750 nm (with diameters of 1.85-6.6 nm, respectively) were prepared. 2D PLE spectra were recorded and analyzed to determine the excited-state energies and diameter dependence of up to nine states. Most of these energies agreed well with recent multiple-band effective mass calculations, and we were able to identify and assign the first six excited states. Furthermore, high-resolution 2D PLE spectra of CdTe NCs at 9 K enabled us to resolve the exciton fine structure and thereby determine the exchange interaction energies, which are in good agreement with theoretical predictions. Importantly, these results demonstrate the convenience of 2D PLE spectroscopy in determining the electronic structures of NCs. The utility of the low-temperature 2D PLE technique arises primarily from its ability to reveal a sample’s entire PL landscape, thereby simplifying the identification of excited states in inhomogeneously broadened samples that would be difficult to identify from absorption spectra alone. In addition, as we demonstrate for CdTe NCs, FLN spectra can be extracted that reduce inhomogeneous line broadening and allow exciton fine structure states to be identified. Using this simple technique, we can explore the electronic structure of more complex NCs,

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Figure 6. (a) Dark squares: experimental Stokes shifts from FLN curves extracted by multiple-peak Gaussian fitting. Dotted line: calculation results from the tight binding (TB) method.24 (b) Size dependence of bright-dark exciton splitting of CdTe NCs.

such as rods, wires, tetrapods, etc. and type I and type II core/shell heterostructured NCs. The insight gained from these materials will make it possible to identify future NC materials with potential applications in a variety of fields such as displays, lasers, and energy conversion. Acknowledgment. The Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged for support of this research. G.D.S. acknowledges the support of an EWR Steacie Memorial Fellowship. References and Notes (1) Rogach, A. L. Semiconducotor Nanocrystal Quantum Dots: Synthesis, Assembly, Spectroscopy and Applications; Springer: New York, 2008.

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