Zexian Cao

High-efficiency visible photoluminescence from amorphous silicon nanoparticles embedded in silicon nitride

Confinement of silicon nanoparticles in silicon nitride instead of an oxide matrix might materially facilitate its potential applications as a light-emitting component in optoelectronics. We report in this letter the production of high-density (up to 4.0×1012/cm2 from micrographs) silicon nanoparticles in SiNx thin films by chemical vapor deposition on cold substrates. Strong room-temperature photoluminescence was observed in the whole visible light range from the deposits that were postannealed at 500 °C for 2 min. The Si-in-SiNx films provide a significantly more effective photoluminescence than Si-in-SiOx fabricated with similar processing parameters: for blue light, the external quantum efficiency is over three times as large. The present results demonstrate that the nanostructured Si-in-SiNx system can be a very competitive candidate for the development of tunable high-efficiency light-emitting devices.

Magnetic field-induced martensitic transformation and large magnetoresistance in NiCoMnSb alloys

Magnetic field-induced martensitic transformation was realized in Ni50−xCoxMn39Sb11 alloys. The partial substitution of Co for Ni has turned the antiferromagnetically aligned Mn moments in the starting material Ni50Mn39Sb11 into a ferromagnetic ordering, raising the magnetization at room temperature from 8emu∕g for NiMnSb to ∼110emu∕g for Ni41Co9Mn39Sb11. In the same quaternary sample, a magnetization difference up to 80emu∕g was measured across the martensitic transformation, and the transformation temperature (T0=259K) could be lowered by 35K under a field of 10T. Also a magnetoresistance over 70% was observed through this field-induced transformation.

Stress-driven buckling patterns in spheroidal core/shell structures

Many natural fruits and vegetables adopt an approximately spheroidal shape and are characterized by their distinct undulating topologies. We demonstrate that various global pattern features can be reproduced by anisotropic stress-driven buckles on spheroidal core/shell systems, which implies that the relevant mechanical forces might provide a template underpinning the topological conformation in some fruits and plants. Three dimensionless parameters, the ratio of effective size/thickness, the ratio of equatorial/polar radii, and the ratio of core/shell moduli, primarily govern the initiation and formation of the patterns. A distinct morphological feature occurs only when these parameters fall within certain ranges: In a prolate spheroid, reticular buckles take over longitudinal ridged patterns when one or more parameters become large. Our results demonstrate that some universal features of fruit/vegetable patterns (e.g., those observed in Korean melons, silk gourds, ribbed pumpkins, striped cavern tomatoes, and cantaloupes, etc.) may be related to the spontaneous buckling from mechanical perspectives, although the more complex biological or biochemical processes are involved at deep levels.

Atmospheric pressure plasma jet: Effect of electrode configuration, discharge behavior, and its formation mechanism

Atmospheric pressure plasma jet (APPJ) can protrude several centers into the ambient air; therefore it holds remarkable promise for many innovative applications. The mechanism underlying this nonthermal discharge, however, remains unsettled that it has been often taken as resulting from dielectric barrier discharge or vaguely referred as streamerlike. We generated APPJ by using a quartz capillary tube with three distinct electrode configurations: conventional double dielectric electrodes for making dielectric barrier discharges, single dielectric electrode, and single bare metal electrode attached to the tube orifice. The jets generated by using the double dielectric electrodes were found consisting of three distinct parts and of different origins. The plasma jet starting from the active electrodes is essentially the propagation of streamers induced by corona discharge. With one single electrode, plasma jets can be generated in both downstream and upstream directions simultaneously; and more importantly a...

Giant magnetothermal conductivity in the Ni-Mn-In ferromagnetic shape memory alloys

In this letter the authors present the observation of giant magnetothermal conductivity in NiMnIn single crystals. Upon cooling, a martensitic transformation is accompanied by a ferromagnetic metal→ferrimagnetic poor-metal transition. Most strikingly, this transition can be shifted to lower temperature and even totally suppressed by a magnetic field. The magnetic field-induced phase transition leads to a large magnetoresistance and a large magnetothermal conductivity up to 70% and 120%, respectively. The specific heat measurements indicate that the large magnetotransport properties are due to the increasing the density of free electrons, suggesting existence of superzone gap in the low-temperature, ferrimagnetic martensite.

Microstructure and its effect on field electron emission of grain-size-controlled nanocrystalline diamond films

Nanocrystalline diamond films were grown by microwave plasma assisted chemical vapor deposition using N2 and CH4 as precursors. The microstructure of the films such as the diamond grain size, graphite content, and N incorporation, was controlled by introducing a small amount of hydrogen gas (0–10 sccm) in the growth. Effects of the growth parameters on the film microstructure were investigated using transmission electron microscopy, x-ray diffraction, Raman spectroscopy, and secondary ion mass spectroscopy. A surface stabilizing model is suggested to explain the formation mechanism of the uniformly grain size-controlled nanocrystalline diamond. A systematic investigation on the film microstructure and their field electron emission (FEE) property is presented for various films of different diamond grain sizes and graphite contents. It was found that the FEE property highly depended on the diamond/graphite mixed phase structure. Novel field emission properties (1 V/mum emission threshold and 10 mA/cm2 emiss...

Growth of nanocrystalline diamond protective coatings on quartz glass

Transparent diamond films with a crystallite size well controlled below 70 nm were grown by hydrogen and methane microwave plasma-enhanced chemical vapor deposition on quartz glass substrates, which had been scratched with 0.5 μm diamond powder. A complementary set of analyzing tools was employed to study the microstructure, the optical and mechanical properties of the deposits. Transmission electron microscopy revealed a nucleation density generally larger than 1011/cm2, which is of the same order of magnitude as the spotlike defects on the pretreated surface of the substrates. The Vicker’s hardness of the deposits scatters between 61 and 95 GPa. An optimal transmittance of 65% in the visible light range is achieved in coatings of 1.0 μm in thickness when the surface roughness measures about 10 nm or less. The nanocrystalline diamond films thus prepared can meet the requirements on transparent protective coatings for optical components.

Interplay of discharge and gas flow in atmospheric pressure plasma jets

Interplay of discharge and gas flow in the atmospheric pressure plasma jets generated with three different discharge modes [N. Jiang, A. L. Ji, and Z. X. Cao, J. Appl. Phys. 106, 013308 (2009); N. Jiang, A. L. Ji, and Z. X. Cao, J. Appl. Phys. 108, 033302 (2010)] has been investigated by simultaneous photographing of both plasma plumes and gas flows in the ambient, with the former being visualized by using an optical schlieren system. Gas flow gains a forward momentum from discharge except for the case of overflow jets at smaller applied voltages. Larger applied voltage implies an elongated plasma jet only for single-electrode mode; for dielectric barrier discharge jet the plume length maximizes at a properly applied voltage. These findings can help understand the underlying processes, and are useful particularly for the economic operation of tiny helium plasma jets and jet arrays.

Blue-violet photoluminescence from amorphous Si-in-SiNx thin films with external quantum efficiency in percentages

Bright blue-violet photoluminescence centered at 428nm was obtained in amorphous Si-in-SiNx thin films prepared in a cyclic growth mode on cool substrates by plasma-enhanced chemical vapor deposition, in which the typical size of the silicon particles has been brought down to ∼1.80nm and the number density amounts to 1.07×1013cm2. A maximum external quantum efficiency over 3.0% was measured in the as-deposited samples. Time-resolved photoluminescence spectra revealed decay times within nanosecond even at room temperature, disclosing a fast recombination dynamics in this amorphous system. The excellent efficiency can be attributed to a better passivation of the silicon particles—hence, the conventional rapid thermal annealing has a negligible effect on the photoluminescence intensity—and also to an improved transparency of the film by cyclic growth. The high-efficiency, tunable wavelength and nanosecond decay time at room temperature, achieved via a low-temperature process without invoking any posttreatmen...

Atmospheric pressure plasma jets beyond ground electrode as charge overflow in a dielectric barrier discharge setup

With a proper combination of applied voltage and the width of ground electrode, atmospheric pressure plasma jets extending beyond the ground electrode, whether it sits on the downstream or the upstream side, can be equally obtained with a dielectric barrier discharge setup, which can be ascribed to the overflow of deposited charges [J. Appl. Phys. 106, 013308 (2009)]. Here, we show that, by using narrower ground electrodes, such an overflow jet can be successfully launched at a much reduced voltage (down to below 10 kV). Moreover, by using transparent and triadic ground electrodes, development of charge overflow beneath the ground electrode was temporally and spatially resolved. Temporal evolution of discharge currents measured on the severed ground electrode helps establish the propagation dynamics of discharges along the dielectric surface beneath ground electrode, and also reinforces the conception that the streamer’s head is in connection to the active electrode via a conducting channel. A small propa...