Search Results - (Author, Cooperation:C. I. Wu)

2016-05-14

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2012-09-22

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

The electronic structure of, and the effects of cesium (Cs) and oxygen (O) adsorption on, the p-GaN(001) surface are investigated via photoemission spectroscopy. Bulk- and surface-sensitive photoemission measurements, and oxygen exposure of clean surfaces, demonstrate the existence of filled surface states which extend ∼0.6 eV above the valence band maximum. The valence band maximum measured after the removal of the surface states gives a downward band bending and electron affinity equal to 1.2±0.2 and 3.3±0.2 eV, respectively. The surface dipole layer induced by exposure to oxygen followed by Cs deposition lowers the vacuum level by 2.8±0.3 eV with respect to the valence and conduction band edges. Under these conditions, the vacuum level is approximately 0.7 eV below the conduction band minimum of the bulk, corresponding to the effective negative electron affinity at this surface. © 1999 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Current–voltage (I–V) characteristics of Al, Mg, and Au Schottky contacts to atomically clean n-GaN(0001)-1×1 surfaces prepared in an ultrahigh vacuum were investigated. The Al/n-GaN contact is rectifying at room temperature and becomes Ohmic after annealing at 500 °C. Coupled with previous photoemission spectroscopy data, this result demonstrates that the origin of the Ohmicity is the reaction-induced doping of the interface. For nonannealed interfaces, the Schottky barrier heights determined from I–V characteristics are in qualitative agreement with the results obtained by photoemission spectroscopy. We find that the ideality factor of the barrier is close to unity for the unreactive interface i.e. Au/GaN, but significantly higher for the reactive interfaces, i.e., Al/GaN and Mg/GaN. Our experimental results suggest that the reaction-induced defects and thermionic field emission play an important role in the electrical behavior of these interfaces. © 2001 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The effects of cesium (Cs) adsorption on band bending and electron affinity at the AlN(0001)-1×1 surface are investigated via ultraviolet and x-ray photoemission spectroscopy. The movement of the Fermi level indicates an initial interaction between Cs and empty surface states, followed by an increase in band bending presumably linked to metallization. The electron affinity, χ, of the clean AlN surface is positive and equal to 1.9±0.3 eV. The Cs-surface dipole layer decreases χ by 2.6±0.3 eV, leading to evidence of true negative electron affinity at the surface of this important material. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We investigate the electron affinity of aluminum nitride surfaces prepared by nitrogen sputtering and annealing via x-ray, ultraviolet, and inverse photoemission spectroscopy. The combination of these techniques leads to a precise determination of the relative positions of the Fermi level, valence-band maximum, conduction-band minimum, and vacuum level at the semiconductor surface. We demonstrate that, in spite of the presence of a sharp photoemission onset feature previously associated with negative electron affinity, the electron affinity is clearly positive on these surfaces. © 1998 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We use low energy electron diffraction, Auger electron spectroscopy, and ultraviolet and x-ray photoemission spectroscopy to study the surface structure, stoichiometry, and electronic properties of n- and p-type GaN (0001) grown by metal-organic chemical vapor deposition. Ordered (1×1) surfaces with nearly stoichiometric composition are prepared by nitrogen sputtering and annealing. The band bending is found to be 0.75±0.1 eV up and 0.75±0.1 eV down for n- and p-type samples, respectively. The work function, electron affinity, and Ga 3d core level binding energy are also determined. © 1998 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We present an investigation via ultraviolet photoemission spectroscopy of the electronic structure of three organic-organic heterojunctions formed between the standard electron-transport emissive material tris(8-hydroxy-quinoline)aluminum (Alq3) and two hole-transport materials, i.e., 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA), and N,N′-diphenyl-N,N′-bis(l-naphthyl)-1-1′biphenyl-4,4″diamine (α-NPD). We measure directly the energy offsets between highest occupied molecular orbitals during the formation of the interfaces. We show that the relative positions of the highest occupied and lowest unoccupied molecular orbitals across the Alq3/PTCDA and Alq3/α-NPD interfaces are qualitatively different and explain, in part, the difference in the performance of electroluminescent devices based on these heterojunctions. We demonstrate the existence of charge transfer-induced dipoles which shift the molecular levels of one organic with respect to the other and invalidate the usual assumption of vacuum level alignment across organic heterojunctions. Finally, we show that the molecular level alignment is independent of the deposition sequence of the organic films and that transitivity applies to these organic "band offsets." © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We demonstrate the improvement of an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment. Enhanced hole-injection efficiency improves dramatically the performance of single-layer doped-polymer devices: the drive voltage drops from 〉20 to 〈10 V, the external electroluminescence quantum efficiency (backside emission only) increases by a factor of 4 (from 0.28% to 1%), a much higher drive current can be applied to achieve a much higher brightness (maximum brightness ∼10,000 cd/m2 at 1000 mA/cm2), and the forward-to-reverse bias rectification ratio increases by orders of magnitude (from 102 to 106–107). The lifetime of the device is also enhanced by two orders of magnitude. © 1997 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We investigate the electronic structure of aluminum nitride (0001)-1×1 surfaces via direct and inverse photoemission spectroscopy. Bulk and surface sensitive measurements on clean surfaces and surfaces exposed to oxygen or cesium demonstrate the existence of filled and empty surface states which extend more than 1 eV beyond the valence- and conduction-band edges. The filled states are tentatively associated with Al dangling or back bonds. The measurement of the top of the valence band upon removal of the filled states leads to a determination of an electron affinity equal to 1.9±0.2 eV. The empty surface states are presumed to play a role in the pinning of the Fermi level in the upper part of the gap and are consistent with the anticipated metallicity of the surface. © 1999 American Institute of Physics.

Electronic Resource

0040-5809

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Biology

Electronic Resource

1432-1432

Duplicate genes ; Accelerated evolution ; Processed pseudogenes ; Age of pseudogenes ; Phylogenetic relationships

Springer Online Journal Archives 1860-2000

Biology

Summary A statistical analysis of the nucleotide sequences of cytochrome c genes from four species of animals and two of yeast and of cytochrome c pseudogenes from rat, mouse, and human was conducted. It was estimated that animals and yeast diverged 1.2 billion years ago, that the two duplicated genes DC3 and DC4 inDrosophila diverged 520 million years ago, and that the two duplicated genes Iso-1 and Iso-2 in the yeastSaccharomyces cerevisiae diverged 200 million years ago. DC3 is expressed at a low level and has evolved 3 times faster than DC4. This observation supports the neutralist view that relaxation of functional constraints is a more likely cause of accelerated evolution following gene duplication than is advantageous mutation. All the rodent pseudogenes examined appear to be processed pseudogenes derived directly from the functional genes, and most of them apparently arose after the mosue-rat split. No event of gene conversion could be detected between any pair of the rodent pseudogenes. Our analysis suggests that the human cytochrome c gene has evolved at a rate comparable to the average rate for pseudogenes, whereas some human cytochrome c pseudogenes have evolved at an exceptionally low rate.

Electronic Resource