Y.S. Tung

GaAs nanocrystals formed by sequential ion implantation

Sequential ion implantation of As and Ga into SiO2 and α‐Al2O3 followed by thermal annealing has been used to form zinc‐blende GaAs nanocrystals in these two matrices. In SiO2, the nanocrystals are nearly spherical and randomly oriented, with diameters less than 15 nm. In Al2O3, the nanocrystals are three dimensionally aligned with respect to the crystal lattice. Infrared reflectance measurements show evidence for surface phonon modes in the GaAs nanocrystals in these matrices.

Annealing behavior of tin implanted silica

Abstract Tin ions were implanted into silica substrates at 275 keV. The samples were annealed at 300–1000°C. The as-implanted and annealed samples were studied by the infrared specular reflectance technique. Kramers–Kronig transformation (KKT) was carried out on the reflectance spectra to obtain n, k, transverse optical (TO), and longitudinal optical (LO) spectra. Based on these spectra, we can understand the effects of ion implantation and annealing on the Si–O–Si bond angle, Si–O–Si bond breaking, and density change of implanted silica.

Optically measured diffusion constants of oxygen vacancies in MgO

We report the estimation of diffusion constants for oxygen vacancies in MgO based on optical absorption spectra. MgO single crystals were annealed in a reducing atmosphere (H2 + Ar) to create oxygen vacancies and the samples were annealed in an oxidizing atmosphere (O2 + Ar) to annihilate them. Fe3+ impurities in the MgO samples were utilized to monitor the degree of reduction or oxidation of the samples during the annealing cycle. An activation energy of 3.08 eV for the diffusion of oxygen vacancies (F-centres) in MgO was obtained, based on the absorption measurements of Fe3+ band.

Surface-phonon characterization of InP, GaP and GaAs quantum dots confined in dielectric hosts

The proper combination of sequential ion implantation and thermal annealing can successfully fabricate binary semiconductor quantum dots (QDs), such as InP, GaP and GaAs QDs. Far-infrared reflectance techniques can be effectively employed to identify the species formed before and after various thermal treatments via probing molecular vibrations and surface phonons of the QDs. This technique is very useful for the characterization of an ultra-thin layer of foreign species on or in the near surface of the bulk substrate materials. In addition, this technique is non-destructive, simple and versatile. The examples given in the present report are InP and GaP QDs formed in fused silica and GaAs QDs formed in fused silica and in [0001] substrates.

Medium energy ion beam analysis and modification of Langmuir-Blodgett thin films

Abstract The erosion behavior of cadmium arachidate Langmuir-Blodgett films is studied under 270 keV He and Ar ion irradiation. Initial characterization by medium energy time-of-flight backscattering yields a thickness and stoichiometry consistent with an unmodified film. In addition, time-of-flight elastic recoil detection is used to monitor the hydrogen evolution from the films as a function of the cumulative dose. After a dose of 5 × 10 14 Ar ions/cm 2 , the film is reduced to less than 50% of its initial thickness and depleted in hydrogen. Smaller losses are measured for He at a dose of nearly 1 × 10 15 ions/cm 2 . Dramatic changes are observed in the interference color of 50-layer films under ion erosion. Measurements by ellipsometry yield thicknesses which are consistent with the backscattering estimates and indicate an increase in the refractive index of the film. The response of cadmium arachidate to energetic ions is compared with the cadmium salt of ω-tricosenoic acid, which can polymerize due to the presence of a CC double bond.

The Study of Sublimation Rates and Nucleation and Growth of TNT and Petn on Slica and Graphite Surfaces by Optical and Atomic Force Microscopy and Ellipsometry

Optical and atomic force microscopy (AFM) were used to study the structure and crystallization kinetics of 2,4,6-trinitrotoluene (TNT) and pentaerythritol tetranitrate (PETN) vapor deposited on silica, Muscovite mica and highly oriented pyrolytic graphite (HOPG) surfaces. Under ambient conditions, the deposited TNT and PETN on these surfaces are in supercooled liquid droplet forms. Dendritic-like crystal nucleation and growth are found for both TNT and PETN on a silica surface after the exposure to the air for 16 and 48 h, respectively. A plate-like solid TNT is also observed in the depletion zone between the grown crystals and the supercooled liquid droplet region. Both AFM and DSC measurements suggest that these plate-like TNT films are amorphous.

A New Approach to Examine Interfacial Interaction Potential between a Thin Solid Film or a Droplet and a Smooth Substrate

Atomic force microscopy (AFM) technique has been successfully used to study the sublimation rate of explosive solid film on a smooth surface. Based on the experimental results, a dipole-induced dipole propagation potential is employed to explain a nonlinear sublimation rate of solid TNT thin film very close to the interface. In this model, three important physical parameters, bulk TNT sublimation rate δ0, surface interaction potential Uo and the effective range of the surface potential h o , are introduced with no arbitrary constants. It is argued that this type of model reflects a general phenomena rather than a special case. This model has been further extended to describe the evaporation rate of liquid droplets resting on a smooth surface. It is also demonstrated that tapping mode AFM is capable of imaging highly viscous explosive droplets under ambient conditions.

Stark effects on band gap and surface phonons of semiconductor quantum dots in dielectric hosts

The authors have investigated quantum-confined Stark effect (QCSE) on GaAs and CdSe nanocrystals and the electric field effect on surface phonons of GaAs nanocrystals isolated in sapphire substrates. For a strongly quantum-confined system, GaAs quantum dots illustrated no exciton energy shift. When the excitons are weakly confined in CdSe, a {approximately} 2 meV red-shift was observed. On the other hand, the results of the electric field effect on surface phonon are dramatic both phonon oscillator strength and frequency. As the strength of the electric field increases, the total intensity of the surface phonon decreases. At the same time, an additional peak was also observed at 277 cm{sup {minus}1}, which is about 3 cm{sup {minus}1} above the center frequency of the surface phonon mode of GaAs nanocrystals embedded in a sapphire host.