Khan A. Alim

Micro-Raman investigation of optical phonons in ZnO nanocrystals

We have measured nonresonant and resonant Raman-scattering spectra from ZnO nanocrystals with an average diameter of 20nm. Based on our experimental data and comparison with the recently developed theory, we show that the observed shifts of the polar optical-phonon peaks in the resonant Raman spectra are not related to the spatial phonon confinement. The very weak dispersion of the polar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phonon peaks for 20-nm nanocrystals. The observed phonon shifts have been attributed to the local heating effects. We have demonstrated that even the low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The latter causes significant (up to 14cm−1) redshift of the optical-phonon peaks compared to their position in bulk crystals. Nonresonant Raman excitation does not produce noticeable local heating. The obtained results can be used for identification ...

Origin of the optical phonon frequency shifts in ZnO quantum dots

We carried out nonresonant and resonant Raman spectroscopy of ZnO quantum dots with diameter of 20nm. On the basis of our measurements and comparison with a recently developed theory, we were able to clarify the origin of the observed phonon peak shifts in quantum dots as compared to bulk ZnO. It has been found that the spatial confinement of optical phonons in 20-nm-diameter dots leads to only few cm−1 peak shifts. At the same time, we have demonstrated, that even a low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy of ZnO, leads to strong local heating of the ZnO quantum dots, which results in very large (∼14cm−1) redshifts of the optical phonon peaks. We have estimated from the observed redshift that the local temperature of the quantum dot ensemble is about 700°C. The obtained results are important for identification of phonon peaks in the Raman spectra of ZnO nanostructures.

Electrical and Thermal Conductivity of Ge ∕ Si Quantum Dot Superlattices

Recently proposed thermoelectric applications of quantum dot superlattices made of different material systems depend crucially on the values of the electrical and thermal conductivities in these nanostructures. We report results of the measurements of Hall mobility and thermal conductivity in a set of Ge0.5Si0.5/Si quantum dot superlattices. The average measured in-plane Hall mobility for the undoped Ge/Si quantum dot superlattices on a p-type substrate is 233.5 cm 2

Assembly and characterization of hybrid virus-inorganic nanotubes

Recently, rod-shaped viruses have attracted attention as biological templates for assembly of nanostructures. Tobamoviruses such as the type strain of Tobacco mosaic virus (TMV-U1, or -common) have a cylindrical shape and dimensions suitable for nanoelectronic applications: 300nm long and 18nm in diameter with a 4nm axial channel. TMV particles can be coated with metals, silica, or semiconductor materials and may also form end-to-end assemblies to be used as interconnects or device channels. In this letter, we report the preparation of TMV-U1 templated organic-metal nanotubes, and their structural characterization using transmission electron microscopy and micro-Raman spectroscopy. Reproducible phonon signatures different from that of native TMV-U1 were observed from the metal-coated TMVs. Our results indicate that Raman spectroscopy can be used for monitoring of the bio-assisted nanostructure assembly and for analyzing the vibrational modes of the resulting bio-inorganic junctions.

Micro-Raman spectroscopic characterization of ZnO quantum dots, nanocrystals and nanowires

Nanostructures, such as quantum dots, nanocrystals and nanowires, made of wurtzite ZnO have recently attracted attention due to their proposed applications in optoelectronic devices. Raman spectroscopy has been widely used to study the optical phonon spectrum modification in ZnO nanostructures as compared to bulk crystals. Understanding the phonon spectrum change in wurtzite nanostructures is important because the optical phonons affect the light emission and absorption. The interpretation of the phonon peaks in the Raman spectrum from ZnO nanostructures continues to be the subject of debates. Here we present a comparative study of micro-Raman spectra from ZnO quantum dots, nanocrystals and nanowires. Several possible mechanisms for the peak position shifts, i.e., optical phonon confinement, phonon localization on defects and laser-induced heating, are discussed in details. We show that the shifts of ~2 cm-1 in non-Resonant spectra are likely due to the optical phonon confinement in ZnO quantum dots with the average diameter of 4 nm. The small shifts in the non-Resonant spectra from ZnO nanowires with the diameter ~20 nm - 50 nm can be attributed to either defects or large size dispersion, which results in a substantial contribution from nanowires with smaller diameters. The large red-shifts of ~10 cm-1 in the resonant Raman spectrum from nanocrystals were proved to be due to local laser heating.

Interpretation of the Phonon Frequency Shifts in ZnO Quantum Dots

Nanostructures made of zinc oxide (ZnO), a wide-bandgap semiconductor, have recently attracted attention due to their proposed applications in low-voltage and short-wavelength (368 nm) electro-optical devices, transparent ultraviolet (UV) protection films, gas sensors, and varistors. Raman spectroscopy presents a powerful tool for identifying specific materials in complex structures and for extracting useful information on properties of nanoscale objects. At the same time the origin of Raman peak deviation from the bulk values is not always well understood for new material systems. There are three main mechanisms that can induce phonon shifts in the free-standing undoped ZnO nanostructures: (i) phonon confinement by the quantum dot boundaries; (ii) phonon localization on defects and (iii) the laser-induced heating in nanostructure ensembles. Here, we report results of the combined non-resonant and resonant Raman scattering studies of an ensemble of ZnO quantum dots with diameter 20 nm. Based on our experimental data, we have been able to identify the origin of the observed phonon frequency shifts. It has been found that the ultraviolet laser heating of the ensemble induces a large red shift of the phonon frequencies. It is calculated that the observed red shift of 14 cm -1 corresponds to the local temperature of the quantum dot ensemble of about 700°C.

High-speed nano-optical photodetector for free space communication

An inexpensive, easily integrated, sensitive photoreceiver operating in the communications band with a 50-GHz bandwidth would revolutionize the free-space communication industry. While generation of 50-GHz carrier AM or FM signals is not difficult, its reception and heterodyning require specific, known technologies, generally based on silicon semiconductors. We present a 50 GHz photoreceiver that exceeds the capabilities of current devices. The proposed photoreceiver is based on a technology we call Nanodust. This new technology enables nano-optical photodetectors to be directly embedded in silicon matrices, or into CMOS reception/heterodyning circuits. Photoreceivers based on Nanodust technology can be designed to operate in any spectral region, the most important to date being the telecommunications band near 1.55 micrometers. Unlike current photodetectors that operate in this spectral region, Nanodust photodetectors can be directly integrated with standard CMOS and silicon-based circuitry. Nanodust technology lends itself well to normal-incidence signal reception, significantly increasing the reception area without compromising the bandwidth. Preliminary experiments have demonstrated a free-space responsivity of 50 &mgr;A/(W/cm2), nearly an order of magnitude greater than that offered by current 50-GHz detectors. We expect to increase the Nanodust responsivity significantly in upcoming experiments.