Session N36: Focus Session: Optical Properties of Nanostructures with S, Se, Te, and Ge

Symmetry considerations for semiconductor nanocrystals

Semiconductor nanocrystals or quantum dots show a wide range of physical properties with respect to their size or shape. In this paper we show that symmetry is also an important characteristic that can lead to different electronic and optical properties, mainly for small nanocrystals. This means that two spherical nanocrystals with similar sizes but different symmetries have different optical and electronic signatures, which should be accessible experimentally. We use pseudopotential density- functional theory, on a real space approach, to address the differences between spherical nanocrystals with similar sizes but different symmetries. We will report differences in the energy gap, the crystal field splitting and the absorption spectra for CdSe nanocrystals. The symmetry of the nanocrystal is also important when studying doping of nanocrystals.   

Optical transitions and the nature of Stokes shift in spherical CdS quantum dots

Resonant Stokes shift observed in CdS quantum dots (QDs) has been previously studied theoretically using {\bf k$\cdot$p} approach. The large values of measured Stokes shift along with the structure of the excitonic levels obtained by the {\bf k$\cdot$p} calculations have suggested an optically forbidden $P$ envelope valence state, thus forming a spatial symmetry induced ``dark exciton'' in CdS QDs, in contrast with the spin-forbidden exchange interaction induced ``dark exciton'' found in CdSe QDs. Since the {\bf k$\cdot$p} method has been known to incorrectly predict the energy levels in other QDs, here we apply {\sl ab initio} accuracy methods to study this problem. Using the LDA-based charge patching method to generate the Hamiltonian, combined with the folded spectrum method to solve the single particle states of thousand-atom nanostructures, we find that the top of the valence band state is $S$-like, thus optically bright, in contrast with all the previous {\bf k$\cdot$p} calculations. Our results also indicate the range of applicability of the {\bf k$\cdot$p} method. The calculated electron-hole exchange splitting suggests that the spin-forbidden valence state may explain the nature of the ``dark exciton'' in CdS quantum dots.   

Bright Exciton Fine Structure Observed in Single CdSe Nanocrystal Quantum Dots

The fine structure splitting of bright excitons in epitaxial quantum dots provides a basis for many quantum computation and entanglement schemes. We demonstrate the existence of a similar splitting in single colloidal CdSe nanocrystals through high- resolution, polarization-resolved, low-temperature photoluminescence (PL) experiments. At 4K, single-dot spectra reveal emission from two distinct, linearly- (and orthogonally- ) polarized bright exciton states. This splitting of the nominally degenerate spin $\pm1$ bright excitons ranges from 1 to 2 meV, depending on nanocrystal size. These values agree well with the splitting recently inferred from spin-polarized resonant PL of nanocrystal ensembles measured in high magnetic fields to 33 Tesla [1]. Similarly to epitaxially-grown quantum dots, the observed fine structure likely results from shape anisotropy of the nanocrystal (i.e. a reduction of axial symmetry), leading to a long-range, anisotropic electron-hole exchange. [1] M. Furis et al., cond-mat/0511567.   

The Peculiar electronic structure of PbSe quantum dots

PbSe quantum dots have recently emerged as promising systems that may realize direct carrier multiplication (DCM) for solar cell applications. We have calculated the underlying electronic/optical structure of PbSe nanocrystals with an atomistic pseudopotential method, finding that the electronic structure is more subtle than k$\cdot$p or tight-binding calculations have previously suggested. The following two effects emerge from our calculations: (i) The bulk-degenerate L states forming the VBM and CBM are split due to (1) valley-valley coupling, (2) valence-conduction interband coupling, and (3) the strong anisotropy of the bulk L valleys. Optical absorption is dictated by transitions among anisotropic dot states characteristic of transverse and longitudinal effective masses. Our calculated optical absorption spectrum is in good agreement with experiment. In particular, our calculation reproduces the measured second obsorption peak that had previously been attributed to forbidden transitions 1S$_{h}\rightarrow$1P$_e$ or 1P$_{h}\rightarrow$1S$_e$ on the basis of k$\cdot$p and tight-binding calculations. (ii) Using our calculated single-particle states, we evaluate DCM mechanism, showing that the rate of X-to-XX (exciton to biexciton) transitions far exceeds the reverse, XX-to-X rate, thus opening the way to efficient DCM.   

Study of colloidal quantum dot surfaces using an innovative thin-film positron 2D-ACAR method

Despite a wealth of information, many fundamental questions regarding the nature of the surface of nanosized inorganic particles and its relationship with the electronic structure remain unsolved. We have investigated the electron momentum density (EMD) of colloidal CdSe quantum-dots via depth-resolved positron 2D angular correlation of annihilation (2D-ACAR) spectroscopy at the Delft intense variable-energy positron beam. This method, in combination with first-principles calculations of the EMD, shows that implanted positrons are trapped at the surface of CdSe nanocrystals. They annihilate mostly with the Se electrons and monitor changes in composition and structure of the surface while hardly sensing the ligand molecules. We thus unambiguously confirm [1] the strong surface relaxation predicted by first-principles calculations [2]. Work supported by the USDOE.\\ \mbox{[1] S.W.H. Eijt {\em et al.}, Nature Materials (in press).}\\ \mbox{[2] A. Puzder, {\em et al.}, Phys. Rev. Lett. 92, 217401 (2004).}   

Fluorescence blinking statistics from single CdSe nanorods

We report that room temperature fluorescence from single colloidally synthesized CdSe nanorods exhibits intermittency (blinking) with truncated power-law off-time and on-time statistics. The nanorods have cross-sectional diameter 5 nm and length 20 nm and are deposited on mica substrates. The aggregated off-time statistics from 67 single nanorods follow a power law: $P(t_{off})\sim t_{off}^{-\alpha }$, with $\alpha \approx $1.1. Power-law behavior extends to off-times of roughly 10 s; longer-time probabilities fall below the best-fit power law. Individual nanorods also show power-law off-time statistics with 1$\le \alpha \le $1.3. On-time probabilities drop below a power law after only $\sim $0.6 s; no on-times longer than $\sim $3 s are observed. These results differ somewhat from those observed with spherical CdSe or CdSe/ZnS core-shell nanocrystals, for which power-law statistics persist to much longer on- and off-times.   

Time-resolved photoluminescence of individual CdS nanowires

We study photoluminescence (PL) dynamics of single VLS-prepared CdS nanowires. AFM imaging reveals that while some nanowires are straight and uniform, the others show significant morphological irregularities. Low temperature PL of uniform nanowires displays a single near band edge (NBE) peak. Spectra of the irregular nanowires exhibit a broad PL band with a high energy shoulder in the same energy range as the NBE peak of the uniform nanowires, as well as an array of narrow peaks at the lower energy. Spatially-resolved PL images indicate that the narrow lines originate at specific locations along the nanowire. Time-resolved PL (TRPL) measurements show that NBE emission in all nanowires is short-lived (lifetime $<$ 50 ps), indicating the presence of non-radiative recombination channels. On the other hand, TRPL of the localized states exhibit are significantly longer (400 ps to 1 ns) and vary from line to line. At room temperature, the PL spectra of all nanowires, regardless of the morphology, consist of the single short-lived NBE emission peak. We acknowledge the support of ACS through the PRF, and NSF through grants 0071797, 0216374, and a graduate fellowship (JLL).   

Morphology and temperature dependence of single CdS nanowire photoluminescence

We study the optical properties of single CdS nanowires (grown by VLS method using 50 nm catalysts) using the technique of micro-PL. We studied ten wires, several that were straight and uniform, and others with morphological irregularities. At room temperature, the PL spectra of all wires are alike and consist of a single line around 2.41 eV. At low temperature (5 K), the PL properties of these two groups of wires differ significantly: the spectra of the uniform wires display a single peak near the band edge, and the spectra of the irregularly shaped wires exhibit a series of sharp lines at lower energies. Detailed PL imaging reveals that the sharp lines are emitted only from particular positions along the wires. Moreover, most of the photons emitted at low temperatures occur at energies below the band edge PL of bulk CdS. This suggests that the sharp lines result from defects or surface states which rapidly trap carriers from the bulk of the wires. As the temperature increases, the sharp lines begin to weaken at about 30 K and completely disappear at 85 K, while a peak which emerges from the high energy shoulder of the low-T emission band becomes dominant and survives up to room temperature. We acknowledge the support of ACS through PRF, and NSF through grants 0071797, 0216374, and a graduate fellowship (JLL).   

Probing the Electronic and Vibronic Structure of Single and Ensemble CdS Nanowires using Resonant Raman Scattering.

Semiconductor nanostructure electronic and vibrational states can be sensitively probed using resonant Raman scattering (RRS) even when such states are not accessible through photoluminescence or transport techniques. We present an investigation of the electronic and vibrational states in both a single CdS nanowire and in an ensemble of CdS nanowires using RRS at room temperature. The CdS nanowire samples were grown using a chemical vapor deposition and gold-catalyzed vapor liquid solid growth technique. We observe strong 1-LO and 2-LO Raman resonances within the broader photoluminescence emission. The energy separation between the peaks of the 1-LO and 2-LO resonance of an ensemble of CdS nanowires was found to be 34 meV. Raman scattering from a single nanowire exhibits similar behavior but with a narrower resonance. These results demonstrate that RRS is a powerful tool for probing the electronic and vibrational properties of semiconductor nanostructures. We acknowledge the support of ACS through PRF, and NSF through grants 0071797, 0216374, and a graduate fellowship (JLL).   

Investigation of the optical gap in Ge nanowires.

We investigate the role of quantum confinement for the optical and electronic properties of Ge nanowires.~ Real space pseudopotentials constructed within density functional theory were used to solve the electronic structure problem.~ We predict the quasi-particle and optical gaps as a function of the diameter up to approximately 3 nm for wires oriented along the (110) and (111) directions.~ We compare our results to previous work on Si wires.~   

Exciton-polariton emission and absorption in inorganic-organic hybrid crystals ZnTe(en)$_{0.5}$

The ultimate accuracy and quality test of nanotechnologies based on either MBE or MOCVD growth is perhaps to make an ultra-short-period superlattice with one monolayer thick alternating components. However, neither artificially nor spontaneously ordered monolayer superlattices (e.g., GaAs/AlAs or GaP/InP) that have been grown by either MBE or MOCVD have been shown to have the desired perfection [1,2]. Recently, we have successfully synthesized a group of II-VI based inorganic-organic crystalline hybrid superlattices with single atomic-layer thick inorganic slabs and single molecular-length organic spacers [3]. We will report experimental and/or theoretical studies on the exciton-polariton emission and absorption, exciton binding energies, and dielectric properties for a prototype hybrid superlattice ZnTe(en)$_{0.5}$ [4,5]. [1] J. Li et al., PRL 91, 106103 (2003). [2] Spontaneous Ordering in Semiconductor Alloys, edited by A. Mascarenhas. [3] X. Huang et al., JACS 122, 8789 (2000); 125, 7049 (2003). [4] B. Fluegel et al., PRB 70, 205308 (2004). [5] Y. Zhang et al., PRL (in press).