Quantum-confinement-effect-driven type-I-type-II transition in inhomogeneous quantum dot structures

We investigate the electronic structures of the inhomogeneous quantum dots within the framework of the effective mass theory. The results show that the energies of electron and hole states depend sensitively on the relative magnitude 77 of the core radius to the capped quantum dot radius. The spatial distribution of the electrons and holes vary significantly when the ratio eta changes. A quantum-confinement-driven type-II-type-I transition is found in GaAs/AlxGa1-xAs-capped quantum dot structures. The phase diagram is obtained for different capped quantum dot radii. The ground-state exciton binding energy shows a highly nonlinear dependence on the innner structures of inhomogeneous quantum dots, which originates from the redistribution of the electron and hole wave functions.

On detection wavelength and electron-hole wave function overlap of type II InAs/In-x Ga1-x Sb superlattice infrared photodetector

In this paper, the detection wavelength and the electron-hole wave function overlap of InAs/IrxGa1-xSb type II superlattice photodetectors are numerically calculated by using the envelope function and the transfer matrix methods. The band offset is dealt with by employing the model solid theory, which already takes into account the lattice mismatch between InAs and InxGa1-xSb layers. Firstly, the detection wavelength and the wave function overlap are investigated in dependence on the InAs and InxGa1-xSb layer thicknesses, the In mole fraction, and the periodic number. The results indicate that the detection wavelength increases with increasing In mole fraction, InAs and InxGa1-xSb layer thicknesses, respectively. When increasing the periodic number, the detection wavelength first increases distinctly for small periodic numbers then increases very slightly for large period numbers. Secondly, the wave function overlap diminishes with increasing InAs and InxGa1-xSb layer thicknesses, while it enhances with increasing In mole fraction. The dependence of the wave function overlap on the periodic number shows the same trend as that of the detection wavelength on the periodic number. Moreover, for a constant detection wavelength, the wave function overlap becomes greater when the thickness ratio of the InAs over InxGa1-xSb is larger.

Short range scattering mechanism of type-II GaSb/GaAs quantum dots on the transport properties of two-dimensional electron gas

We have studied the scattering process of AlGaAs/GaAs two-dimensional electron gas with the nearby embedded GaSb/GaAs type-II quantum dots (QDs) at low temperature. Quantum Hall effect and Shubnikov-de Haas oscillation were performed to measure the electron density n(2D), the transport lifetime tau(t) and the quantum lifetime tau(q) under various biased gate voltage. By comparing measured results of QDs sample with that of reference sample without embedded QDs, mobilities (transport mobility mu(t) and quantum mobility mu(q)) dominated by GaSb QDs scattering were extracted as functions of n(2D). It was found that the ratios of tau(t) to tau(q) were varying within the range of 1-4, implying the scattering mechanism belonging to the sort of short-range interaction. In the framework of Born approximation, a scattering model considering rectangular-shaped potential with constant barrier height was successfully applied to explain the transport experimental data. In addition, an oscillating ratio of tau(t)/tau(q) with the increasing n(2D) was predicted in the model.

DETERMINATION OF THE X-CONDUCTION-SUBBAND ENERGIES IN TYPE-II GAAS/ALAS/GAAS QUANTUM-WELL BY DEEP-LEVEL TRANSIENT SPECTROSCOPY

Using deep level transient spectroscopy (DLTS) the X conduction-subband energy levels in an AlAs well sandwiched by double GaAs layers were determined. Calculation gives eight subbands in the well with well width of 50 Angstrom. Among them, five levels and the other three remainders are determined by using the large longitudinal electron effective mass m(1)(1.1m(0)) and transverse electron effective mass m(t)(0.19m(0)) at X valley, respectively. Two subbands with the height energies were hardly detectable and the other six ones with lower energies are active in the present DLTS study. Because these six subbands are close to each other, we divided them into three groups. Experimentally, we observed three signals induced from the three groups. A good agreement between the calculation and experiment was obtained. (C) 1995 American Institute of Physics.

Type-II SiGe/Si MQWS (multi-quantum wells) and self-organized Ge/Si islands grown by UHV/CVD system

Type-II SiGe/Si MQWs (Multi-Quantum Wells) and Self-Organized Ge/Si Islands were successfully grown by a homemade ultra-high vacuum/chemical vapor deposition (UHV/CVD) system. Growth characteristics and PL (photoluminescence) spectra at different temperature were measured. It demonstrated that some accumulation of carriers in the islands results in the increase of the integrated PL intensity of island-related at a certain temperature range.

Optical transitions and type-II band lineup of MBE-grown GaNAs/GaAs single-quantum-well structures

We have investigated transitions above and below band edge of GaNAs/GaAs and InGaNAs/GaAs single quantum wells (QWs) by photoluminescence (PL) as well as by absorption spectra via photovoltaic effects. The interband PL peak is observed to be dominant under high excitation intensity and at low temperature. The broad luminescence band below band edge due to the nitrogen-related potential fluctuations can be effectively suppressed by increasing indium incorporation into InGaNAs. In contrast to InGaNAs/GaAs QWs, the measured interband transition energy of GaNAs/GaAs QWs can be well fitted to the theoretical calculations if a type-II band lineup is assumed. (C) 2001 Elsevier Science B.V. All rights reserved.

Type I-type II transition of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots

The photoluminescence (PL) of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots has been measured at 15 and 80 K under hydrostatic pressure. The lateral size of the dots ranges from 7 to 62 nm. The emissions from the dots with 26, 52 and 62 nm size have a blue shift under pressure, indicating that these quantum dots have the normal type-I structure with lowest conduction band at the Gamma -valley. However, the PL peak of dots with 7 nm diameter moves to lower energy with increasing pressure. It is a typical character for the X-related transition. Then these small dots have a type-II structure with the X-valley as the lowest conduction level. An envelope-function calculation confirms that the Gamma -like exciton transition energy will rise above the X-like transition energy in the In0.55Al0.45As/Al0.5Ga0.5As structure if the dot size is small enough.

Type I collagen-templated assembly of silver nanoparticles and their application in surface-enhanced Raman scattering

Silver nanoparticles (Ag NPs) are one of the active substrates that are employed extensively in surface-enhanced Raman scattering (SERS), and aggregations of Ag NPs play an important role in enhancing the Raman signals. In this paper, we fabricated two kinds of SERS-active substrates utilizing the electrostatic adsorption and superior assembly properties of type I collagen. These were collagen-Ag NP aggregation films and nanoporous Ag films.

Fabrication, characterization, and application in surface-enhanced Raman spectrum of assembled type-I collagen-silver nanoparticle multilayered films

In this paper, we report a facile method for the fabrication of type-I collagen-silver nanoparticles (Ag NPs) multilayered films by utilizing type-I collagen as a medium. These samples were characterized by UV-vis spectra photometer, atomic force microscopy, scanning electron microscopy, and Fourier transform IR spectrum. Experimental results show that collagen molecules serve as effective templates to assemble Ag NPs into multilayer films. These samples exhibit high surface-enhanced Raman scattering (SERS) enhancement abilities.

Type I collagen-mediated synthesis of noble metallic nanoparticles networks and the applications in Surface-Enhanced Raman Scattering and electrochemistry

In this paper, we demonstrated an effective enviromentally friendly synthesis route to prepare noble metallic (Au, Ag, Pt and Pd) nanoparticles (NPs) networks mediated by type I collagen in the absence of any seeds or surfactants. In the reactions, type I collagen served as stabilizing agent and assembly template for the synthesized metallic NPs. The hydrophobic interaction between collagen and mica interface as well as the hydrogen bonds between inter- and intra-collagen molecules play important roles in the formation of collagen-metallic NPs networks. The noble metallic NPs networks have many advantages in the applications of Surface-Enhanced Raman Scattering (SERS) and electrochemistry detection. Typically, the as-prepared Ag NPs networks reveal great Raman enhancement activity for 4-ATP, and can even be used to detect low concentration of DNA base, adenine.

Electrodeposition of platinum nanoclusters on type I collagen modified electrode and its electrocatalytic activity for methanol oxidation

We firstly reported a novel polymer matrix fabricated by type I collagen and polymers, and this matrix can be used as nanoreactors for electrodepositing platinum nanoclusters (PNCs). The type I collagen film has a significant effect on the growth of PNCs. The size of the platinum nanoparticles could be readily tuned by adjusting deposition time, potential and the concentration of electrolyte, which have been verified by field-emitted scanning electron microscopy (FE-SEM). Furthermore, cyclic voltammetry (CV) has demonstrated that the as-prepared PNCs can catalyze methanol directly with higher activity than that prepared on PSS/PDDA film, and with better tolerance to poisoning than the commercial E-TEK catalyst. The collagen-polymer matrix can be used as a general reactor to electrodeposit other metal nanostructures.

Type I collagen-mediated synthesis and assembly of UV-photoreduced gold nanoparticles and their application in surface-enhanced Raman scattering

We reported a simple method to synthesize gold nanoparticles (NPs) by photoreducing HAuCl4 in acetic acid solution in the presence of type I collagen. It was found that the collagen takes an important role in the formation of gold NPs. The introduction of collagen made the shape of the synthesized gold nanocrystals change from triangular and hexangular gold nanoplates to size-uniform NPs. On the other hand, thanks to the special characters of collagen molecules, such as its linear nanostructure, are positively charged when the pH < 7, and the excellent self-assembly ability, photoreduced gold NPs were assembled onto the collagen chains and formed gold NPs films and networks. A typical probe molecule, 4-aminothiophenol, was used to test the surface-enhanced Raman scattering activity of these gold NPs films and networks and the results indicated good Raman activity on these substrates.