H. C. Yang

Mechanism of luminescence in InGaN/GaN multiple quantum wells

We report a firm evidence of luminescence from InN clusters in InGaN/GaN multiple quantum wells. Photoluminescence, photoluminescence excitation, and Raman scattering measurements have been employed to study the optical properties of InGaN/GaN multiple quantum wells. A careful examination of the low energy shoulders of the main peak luminescence reveals the fact that their separation is in good agreement with the longitudinal optical phonon energy of pure InN film measured by Raman scattering. A large Stokes-like shift between the emission peak energy and the absorption edge is found; it increases with increasing indium content. All these observations can be explained in a consistent way by the effect of localization due to self-organized InN clusters within InGaN layers. Our results thus strongly suggest that the emission mechanism of InGaN/GaN quantum wells originates from radiation recombination within the localized states of self-organized InN clusters.

Biofunctionalized magnetic nanoparticles for high-sensitivity immunomagnetic detection of human C-reactive protein

In this work, we developed immunomagnetic detection techniques for detecting human C-reactive protein (CRP). To enhance the technique’s sensitivity to human CRP, biofunctionalized magnetic nanoparticles were used as markers, and a superconductive quantum interference device gradiometer system was adapted to measure the saturated magnetization of magnetically labeled CRP–anti-CRP immune complexes. Sensitivity for human CRP was found to be as high as 1ng in 0.1ml, i.e., 10ng∕ml in concentration. This sensitivity is much higher than that of conventional enzyme linked immunosorbent assay by one order of magnitude.

Wash-free immunomagnetic detection for serum through magnetic susceptibility reduction

In this study, a platform for wash-free immunomagnetic detections is discussed. The reagent is a magnetic fluid composed of biofunctionalized magnetic particles dispersed in water. The assay instrument is a mixed-frequency magnetosusceptometer. The mixed-frequency ac magnetic susceptibility χac of the magnetic reagent is reduced, as a result of the formation of magnetic clusters via the association among magnetic nanoparticles and a specific biotarget. The immunoassay detection of proteins in water, such as aqueous c-reactive protein, is characterized and is compared to that in serum. These observations demonstrate the advantages of high convenience, high sensitivity, and high specificity for χac-reduction immunomagnetic detection.

Optical quenching of the photoconductivity in n-type GaN

Results of optical quenching of photoconductivity measurements in undoped n-type and Se-doped GaN epitaxial thin films are presented. The spectral distribution of quenching phenomena shows a broadband centered around 1.26 eV. Transient changes in photoconductivity on application or removal of the quenching radiation are shown to exhibit a metastable behavior. The results reveal that the origin of the optical quenching phenomena is closely related to the defects corresponding to the persistent photoconductivity effects and the yellow luminescence band observed in most n-type GaN. In addition, this result indicates that these defects can have multiple charge states. It is found that the quenching ratio increases with increasing Se-doping concentration. We point out that the origin of the defects responsible for the optical quenching can be attributed to nitrogen antisite and/or Ga vacancy.

Persistent photoconductivity in InGaN/GaN multiquantum wells

Optical properties of undoped InGaN/GaN multiquantum wells (MQWs) have been investigated by photoconductivity, photoluminescence, and photoluminescence excitation measurements. We report the observation of persistent photoconductivity (PPC) in InGaN/GaN MQWs and show that the PPC effect arises from In composition fluctuations in the InGaN well layer. From the analysis of the decay kinetics, the localization depth caused by composition fluctuations has been determined. Compared with the results of complementary absorption and photoluminescence measurements, it is found that the quantum-confined Stark effect due to piezoelectric field and composition fluctuations both exist in the InGaN/GaN MQWs. These two effects are responsible for the photoluminescence Stokes’ shift in the InGaN well layers. Here, we provide a unique way to distinguish the individual contribution to the Stokes’ shift for the piezoelectric field and composition fluctuations.

Optical properties of Si-doped GaN films

Results of the front-side and back-side photoluminescence (PL) measurements in a set of Si-doped GaN epifilms are presented. From the back-side PL spectrum, the enhancement of the yellow emission implies that most of the intrinsic defects responsible for the yellow band exist mainly near the interface between the buffer layer and the epilayer. We also found that the intensity of the yellow luminescence decreases with increasing Si dopants, which is consistent with the fact that the microscopic origin of the yellow emission can be attributed to gallium vacancies VGa. In additions, our investigations reveal that the potential fluctuations, that give rise to the effect of band-gap narrowing and linewidth broadening, are mainly caused by randomly distributed doping impurities instead of other defects.

Digitized Charge Transfer Magnitude Determined by Metal–Organic Coordination Number

Well-ordered metal-organic nanostructures of Fe-PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer.

High-Tc SQUID gradiometer system for magnetocardiography in an unshielded environment

We have designed a magnetocardiography (MCG) system that is capable of measuring magnetocardiograms in an unshielded environment. In order to carry out such a measurement, one has to contend with various ambient noise sources. These include power line and RF interference, microphonics pickup, fluctuations in the earth’s magnetic field, and electrostatic pickup. Earlier solutions devised to overcome these problems have entailed the use of a second-order gradiometer inside a deep mine or inside a magnetically shielded enclosure [1]. Apart from the above problems which are common to both low-Tc and high-Tc SQUIDs, high-Tc SQUIDs exhibit additional 1/f noise [2]. This noise is the result of hopping of flux vortices that are trapped in the body of the field cooled high-Tc SQUID, and can only be eliminated by zero-field cooling the SQUID.

Magnetically modulated optical transmission of magnetic fluid films

Abstract The optical transmission of the magnetic fluid film under perpendicular magnetic fields was investigated by measuring the transmitted intensity of the normally incident light through the film. The transmittance I / I 0 of the magnetic fluid film was observed to decrease with the increase of the magnetic field strength H due to the reduction in the area occupied by the relatively optic transparent liquid phase. Moreover, under a given H , the transmittance decreased exponentially with the increasing film thickness L , i.e. I / I 0 =exp(− L / δ ), where the δ denotes the attenuation distance. It was further found that the δ varies with the magnetic field strength and the concentration of the fluid, but is independent of the sweep rate of the field.

Spin-Dependent Molecule Symmetry at a Pentacene–Co Spinterface

Incorporating spin-polarized scanning tunneling microscopy (SP-STM) measurements and first-principles calculations, we resolve spin-polarized states and consequent features in a pentacene(PEN)-Co hybrid system. Symmetry reduction of PEN clarifies the PEN adsorption site and the Co stacking methods. Near the Fermi energy, the molecular symmetry is spin-dependently recovered and an inversion of spin-polarization in PEN with respect to Co is observed. The experimental findings and calculation results are interpreted by a pz-d hybridization model, in which spin-dependent bonding-antibonding splitting of molecular orbitals happens at metal-organic spinterfaces.