Session A36: Structural, Optical, and Electronic Properties of Nanostructures

3-D Structure of Semiconductor Nanomaterials

In recent years, nanotechnology has become a key component in the field of materials physics, placing a more rigorous demand on characterization of nanomaterials, ultimately in three dimensions. This has led to the development of electron tomography for inorganic materials using Z-contrast imaging in the scanning transmission electron microscope (STEM). Here we present high resolution 3-D reconstructions of quantum dots and tetrapods. The results from Sn quantum dots in a Si matrix illustrate the ability to identify the size, shape and distribution of embedded dots in a layer in three dimensions, as well as identifying their formation mechanism. Dual axis tomography is demonstrated for the first time for inorganic materials on semiconducting nanorods in the shape of tetrpaods made of CdTe. These results indicate approximately 1nm resolution in all three dimensions using single axis tomography.   

Propagation of coherence in X-ray Optics for Nanocrystallography

Coherent X-ray Diffraction is a powerful tool for studying the internal structure of nanostructures to angstrom precision in three dimensions. A coherent diffraction pattern is a single 2D slice through the 3D Fourier Transform of an object. These diffraction images can be inverted with iterative phasing algorithms to obtain an image of a projection of the physical object. The success and accuracy of the inversion depends strongly on the quality of the initial diffraction pattern and thus on the coherence properties of the incoming X-ray beam. Coherence of synchrotron radiation depends on machine parameters but also on the optics in the beam path. Focusing optics, which are necessary to provide sufficient flux onto the sample, distort the wavefront and can especially alter the coherence of the beam. For a finite object, fringes will result from the interference of opposite sides or facets and the visibility of these fringes is a quantitative measure of the coherence quality. We present a study of the effect of optical elements in altering the coherence of an x-ray beam by using the coherent diffraction images from gold nanoparticles.   

X-ray Studies of Ultrathin Wires and Tubes in Nanoscale Confinement

Nanoporous Anodized Aluminum Oxide (AAO) membranes are composed of self-assembled, densely packed, co-aligned cylindrical pores. The pore diameter of these membranes can be controllably reduced to as little as $\approx$1 nm through Atomic Layer Deposition (ALD) process. AAO pores have been used as templates for formation of metallic nanotubes and nanowires through ALD or thermal vapor deposition. These embedded structures have been characterized by small- and wide-angle x-ray scattering (SAXS, WAXS). The penetrating ability of x rays provides a non- destructive structural characterization technique for materials confined within AAO matrix, on both atomic (WAXS) and nanometer (SAXS) length scales.   

Three-dimensional Structure of Nanoparticles from High-energy X-ray Diffraction and Atomic Pair Distribution Function Analysis

Knowledge of the atomic-scale structure is an important prerequisite to understand and predict the properties of materials. In the case of crystals it is obtained from the positions and intensities of the Bragg peaks in the diffraction patterns. However, materials constructed at the nanoscale lack the translational symmetry and long-range order of perfect crystals. The diffraction patterns of such materials show only a few Bragg peaks, if any, and a pronounced diffuse component. This poses a real challenge to the usual techniques for structure characterization. The challenge can be met by employing the so-called atomic Pair Distribution Function (PDF) technique and high energy x-ray diffraction. This non-traditional experimental approach takes into account both Bragg and diffuse scattering and yields the atomic structure in terms of a small set of parameters such as a unit cell and atomic coordinates. The basics of the technique will be introduced and its potential demonstrated with results from recent structural studies gold, ZrO$_{2}$ and GaN nanoparticles.   

Anomalous scattering and PDF analysis of HgSe nanoclusters in zeolites using synchrotron X-ray radiation

HgSe, has been grown inside LTL and Nd-Y zeolites (tubular and spherical nanosized pores, respectively). A first sharp diffraction peak appears in an X-ray diffraction profile, that we associate with the clustering of HgSe constrained to the zeolite pore diameter. Charge imbalance within the zeolite pores, combined with the guest material properties, affects the cluster-cluster correlation (clusters in separate pores), either in a filled-unfilled-like fashion or through a relative local orientational order. Such correlations are expressed through a broad diffuse modulation, absent in the pure host, upon which the more obvious, Bragg peaks are imposed. This effect might be seen via pair-distribution function (PDF) analysis. Anomalous scattering experiments carried out both below the Se K-edge and the Hg L-edge, as well as off-edge for each zeolite, aided in calculating average crystallographic parameters of HgSe. These results and the PDF analyses promise to reveal an understanding of the structural configuration and cluster allocation mechanisms. A potential technological application of this system should arise, due to the transparency of these zeolites and ease of interaction of visible light with species contained within the pores.* M.C-C. now at U.T. El Paso   

Amorphous-Ge Nanocluster Formation During Ion Implantation

Ge nanocrystals formed by ion implantation followed by thermal annealing have an average diameter of 5.1 nm with a distribution full width at half maximum (FWHM) of 3.4 nm. This contrasts with Kinetic Monte Carlo (KMC) and rate equation predictions which give a much narrower distribution. Here, we show that the difference between theory and experiment is the result of amorphous nanocluster formation during room temperature implantation. Therefore, thermal annealing to form nanocrystals is predominantly a crystallization process rather than a conventional nucleation, growth, and coarsening process during thermal annealing. This work is supported in part by U.S. Department of Energy under contract No. DE-AC02-05CH11231 and in part by U.S. NSF Grant No. DMR-0109844.   

A Computational Investigation of the Structure and Optical Properties of Nanoconfined Semiconducting Polymers

The promise of semiconducting organic polymers lies in their incredible flexibility, both through varied structure types and via chemical modifications of the polymer backbone. One type of structure we have recently explored, both experimentally and theoretically, arises from the use of ink-jet printing methods to produce nanoconfined particles with arbitrary size and composition. We have found substantial evidence indicating the production of highly ordered, rod-shaped, pi-stacked folded chain structures for single molecule PPV-based systems. Results show that chain organization, which is greatly influenced by the solvent, is crucial in determining the photophysical properties. Recent multiscale calculations, including molecular dynamics, molecular mechanics, and both ground-state and excited-state ab initio calculations, have been used to elucidate the role of the substituents in controlling the structure of the nanoconfined polymer, as well as the resultant effect on the optical properties. Through this work, a structure-property relationship is developed that will prove to be of great utility to experimentalists and polymer theorists alike.   

Single-particle energy levels and interparticle Coulomb interactions in CdSe quantum dots measured by Scanning Tunneling Spectroscopy

Semiconductor quantum dots (QDs) provide the opportunity to study the evolution of electronic properties in a size range intermediate between molecular and solid state regime. We report a detailed size evolution analysis of the electronic structure of colloidal CdSe QDs of different sizes, by employing Scanning Tunneling Spectroscopy to individual dots. Tuning the tunnelling-in versus tunnelling-out rates allows one to show the electron occupation in the dot. The energy level structure and electron-hole Coulomb attraction in the dot are obtained by a combination of shell-tunneling spectroscopy and optical spectroscopy. The results are in agreement with tight-binding calculations. The electron-electron interactions are investigated by shell-filling spectroscopy. The tunneling spectra in this regime are analyzed by solving the master equation for electron and hole occupancy of the QD.   

Optical Phonons and Multiphonon Raman Scattering in Spherical PbSe Quantum Dots

The experimentally observed multi-phonon resonant Raman scattering spectra in spherical PbSe quantum dots are interpreted using our non-adiabatic approach [1]. The optical phonons in quantum dots are treated within the multimode dielectric continuum model, taking into account both the electrostatic and mechanical boundary conditions for the relative ionic displacement. Our multimode dielectric continuum model, which includes, as a substantial ingredient, the realistic dispersion of optical phonons, adequately describes the optical-phonon spectra of spherical PbSe quantum dots. The non-adiabaticity leads to a substantial enhancement of the relative intensities of multi-phonon peaks with respect to the intensity of the one-phonon peak. The peak positions and intensities of the calculated Raman scattering spectra for spherical PbSe quantum dots are in good agreement with recent experimental results on the Raman scattering in oleic-acid-capped colloidal PbSe nanocrystals. [1] E. P. Pokatilov, S. N. Klimin, V. M. Fomin, J. T. Devreese, and F. W. Wise, Phys. Rev. B \textbf{65}, 075316 (2002).   

Power dependent spectral diffusion in single CdSe quantum dots

Optical emission from a single CdSe quantum dot was investigated. We studied spectral diffusion effects which were observed earlier[1].We found that the spectral diffusion characteristic frequency increases with the pump power. In addition, increasing laser pump power causes occurrence of additional emission lines. The measured correlations show that the lines correspond to multiparticle complexes. The physics describing these effects is analyzed. [1]S. A. Empedocles, D. J. Norris, and M. G. Bawendi, Phys. Rev. Lett. 77, 3873 (1996).   

Experimental and numerical confirmation of composite diffracted evasnescent-wave (CDEW) model for enhanced transmission of subwavelength apertures

When a subwavelength aperture in an opaque film is surrounded by periodic surface corrugations, its optical transmission can be enhanced or suppressed with respect to that of an identical aperture without surface corrugations. We have proposed a model in which the subwavelength surface structure scatters the incident light into evanescent waves: The total, or composite, diffracted evanescent waves (CDEWs) travel along the surface and their interference with the light directly falling on the aperture leads to the transmission modulation. The CDEW model is valid for metallic as well as non-metallic surfaces, and thus differs qualitatively from the surface plasmon model, which requires a metallic surface. We show that the optical transmission of an embedded periodic array of dots, where the surface is absent altogether, is related to the transmission of hole arrays by Babinet’s principle, underscoring the importance of diffraction. Furthermore, numerical calculations on small- area corrugations verify the functional form of the CDEWs.   

Transport Properties of Gold Nanoparticle Arrays at Low Temperatures

We investigated the effects of elastic and inelastic cotunneling on the electronic transport properties of Au monolayers and multilayers. Highly-ordered monolayers of dodecanethiol-ligated Au particles (6nm in diameter)$^{1}$ were assembled onto Si$_{3}$N$_{4}$ substrates with prefabricated electrodes. Multilayers were created by adding layers to the original monolayers$^{2}$. Current-voltage (IV) measurements were performed with 50fA resolution down to 30mK. TEM images were taken posterior to all measurements. We found that transition from inelastic to elastic cotunneling occurred at T$\sim $5K. In the inelastic cotunneling regime, the IV curves were found to be nonlinear for k$_{B}$T $<$ eV/N $<$E$_{C }$ and linear when k$_{B}$T $>$ eV/N where N is the number of grains across the electrode gap. In the elastic regime below 5K, the IV curves were found to be linear for eV/N $<$ (E$_{C}$ $\delta )^{1/2 }$where $\delta $ is the level spacing and E$_{C}$ is the charging energy. As a consequence of cotunneling, Efros-Shklovskii variable-range-hopping-like conduction (VRH) was observed near zero bias. Above T$\sim $ 80-100K, transition from VRH to nearest-neighbor hopping took place.. X.M. Lin, \textit{et.al.}, J. Nanoparticle Res. \textbf{2} 157 (2000). T.B. Tran, \textit{et.al.}, Phys. Rev. Lett. \textbf{95} 076806 (2005),   

Observation of Optical Signature of the Aharonov-Bohm Phase in Type-II Quantum Dots

Recent theoretical studies$^{1,2}$ on the optical response of type-II excitons in the magnetic field have shown that the excitons will acquire the Aharonov-Bohm (AB) phase as the electrical dipole, formed due to carrier separation, interacts with the field, resulting in the field dependent exciton energy and the emission intensity. Experimentally, the former has been reported$^{3}$; however, the behavior of the intensity is still not fully understood. We present results of magneto-photoluminescence studies on type-II ZnTe/ZnSe quantum dots (QDs) formed in Zn-Se-Te multilayer systems$^{4}$; this ensures that electron move within the $x-y$ plane. The observed strong oscillations in the intensity is explained in terms of the AB effect$^{1,2,5}$ due to the electron motion around a stack of QDs, when the hole is strongly localized in one them. This is in qualitative agreement with the theoretical predictions$^{2}$. 1. Kalameitsev, \textit{et al.}, JETP Lett. \textbf{68}, 669 (1998); Govorov, \textit{et al.}, PRB \textbf{R66}, 081309 (2002); Janssens, \textit{et al.}, PRB \textbf{67}, 235325 (2003). 2. Janssens, \textit{et al.}, PRB \textbf{69}, 235320 (2004). 3. Ribeiro, \textit{et al.}, PRL \textbf{92}, 126402 (2004). 4. Gu, \textit{et al.}., PRB \textbf{71} 045340 (2005). 5. Dias da Silva, \textit{et al.}, PRB \textbf{70}, 155318 (2004).   

Observation of the Aharonov-Bohm Effect in Self-Assembled InGaAs/GaAs Nano-Volcanoes

According to our X-STM data, self-assembled InGaAs/GaAs nano-volcanoes are characterized by an asymmetric rim and a depression rather than an opening at the center. We show that these asymmetric singly connected structures still can effectively manifest the electronic properties, like the Aharonov-Bohm (AB) oscillations, peculiar to the doubly connected geometry of ideal rings. AB oscillations in the persistent current are observed in low temperature magnetization measurements using torsion magnetometry in magnetic fields up to 15 T. The experimental results are in excellent agreement with theoretical predictions for strained nano-volcanoes.   

Mesoscopic conductance fluctuations in a coupled quantum dot system

We study electron transport properties of an Aharonov-Bohm ring containing two quantum dots. One of the dots has well-separated resonant levels and the other dot is chaotic and is treated by random matrix theory. We develop a statistical theory of the conductance through the dot using the supersymmetry method and the numerical calculation, and find that the conductance is significantly affected by mesoscopic fluctuations. The Breit-Wigner resonant peak of the conductance is changed to an antiresonance by increasing the ratio of the level broadening to mean level spacing of the random dot. The asymmetric Fano form turns into a symmetric one and the resonance peak can be controlled by magnetic flux. We also find that the conductance distribution shows rich behavior depending on the fluctuations of the chaotic dot and the position of the resonance peaks.