Jiyan Dai

A nanotube-based field-emission flat panel display

A matrix addressable diode flat panel display has been fabricated using a carbon nanotube–epoxy composite as the electron emission source. Field-emission uniformity has been confirmed by measuring the I–V curves of pixels across the panel. This prototype display demonstrates well-lit pixels under ±150 V biasing signals. The “on” and “off” of the pixels are well controlled by the half voltage “off-pixel” method. Further improvement of this technology may lead to easy-to-make and inexpensive flat panel displays.

Field emission from nanotube bundle emitters at low fields

The fabrication of nanotube field emitters with an onset field as low as 0.8 V/μm is described and the low-field electron emission mechanism is discussed. These emitters are made using nanotube cathode deposit with the addition of epoxy resin. The preferred orientation of nanotubes in nanotube bundles of the deposit is preserved. The nanotube tips are sharpened by exposing the nanotube bundle surface to a microwave oxygen plasma. The local-field enhancement factor is estimated to be 8000 by using the Fowler–Nordheim equation. The low onset field is attributed to the well-distributed, highly orientated sharp tips at the sample surface.

Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films

A semiconductor laser whose cavities are “self-formed” due to strong optical scattering in highly disordered gain media is demonstrated. The lasers are made of zinc oxide polycrystalline films grown on amorphous fused silica substrates. Lasing occurs at an ultraviolet wavelength of ∼380 nm under optical pumping. Actual images of the microscopic laser cavities formed by multiple scattering have been captured. These results suggest the possibility of using disordered semiconductor microstructures as alternative sources of coherent light emission.

Enhancement of thermal stability of NiSi films on (100)Si and (111)Si by Pt addition

The effect of a small amount of Pt (5 at. %) on the thermal stability of NiSi film on (100) and (111) Si substrates has been investigated both by in situ annealing inside an x-ray photoelectron spectroscopy system and by ex situ rapid thermal annealing. The addition of platinum increases the disilicide nucleation temperature to 900 °C leading to a better stability of NiSi at high temperatures. In the presence of Pt, NiSi films on both (111)Si and (100)Si substrates develop a texture with the relationship (100)NiSi∥(111)Si and (010)NiSi∥(100)Si. The increase in thermal stability has been explained in terms of the nucleation concept.

Second harmonic generation in laser ablated zinc oxide thin films

We have observed large second-order nonlinear optical response from zinc oxide (ZnO) thin films deposited on sapphire substrates by pulsed laser ablation. By comparing the second harmonic signal generated in a series of ZnO films with different crystallinity and thickness, we conclude that a significant part of the second harmonic signal is generated at the grain boundaries and interfaces.

Determination of the optical constants of zinc oxide thin films by spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) has been used to determine the complex pseudo dielectric functions, e1(E)+ie2(E), of ZnO films on (0001) Al2O3 substrates over the spectral range of 1.33 and 4.96 eV at room temperature. The SE measurements are carried out with E⊥c at angles of incidence of 60° and 65° with respect to the surface normal. Below the band gap, the refractive index n is found to follow the first order Sellmeir dispersion relationship n2(λ)=1+1.881λ2/(λ2−0.05382). A free excitonic structure located at the band edge of 3.32 eV is clearly observed in the pseudo absorption spectrum. Elliott expression with Lorentzian broadening is used to model the pseudo absorption coefficient above the band edge.

Broad temperature plateau for high ZTs in heavily doped p-type SnSe single crystals

Excellent thermoelectric performance is obtained over a broad temperature range from 300 K to 800 K by doping single crystals of SnSe. The average value of the figure of merit ZT, of more than 1.17, is measured from 300 K to 800 K along the crystallographic b-axis of 3 at% Na-doped SnSe, with the maximum ZT reaching a value of 2 at 800 K. The room temperature value of the power factor for the same sample and in the same direction is 2.8 mW mK−2, which is an order of magnitude higher than that of the undoped crystal. Calculations show that Na doping lowers the Fermi level and increases the number of carrier pockets in SnSe, leading to a collaborative optimization of the Seebeck coefficient and the electrical conductivity. The resultant optimized carrier concentration and the increased number of carrier pockets near the Fermi level in Na-doped samples are believed to be the key factors behind the spectacular enhancement of the average ZT.

Synthesis and piezoresponse of highly ordered Pb(Zr0.53Ti0.47)O3 nanowire arrays

We report the synthesis and characterization of ferroelectric lead zirconate titanate PbsZr0.53Ti0.47dO3 (PZT) nanowires. The PZT nanowires, with diameters of about 45 nm and lengths of about 6 mm, were fabricated by means of a sol-gel method utilizing nanochannel alumina templates. After postannealing at 700 ° C, the PZT nanowires exhibit a polycrystalline microstructure, and x-ray diffraction and transmission electron microscopy study revealed their perovskite crystal structure. The piezoelectric characteristics of individual PZT nanowires were demonstrated by piezoresponse force microscopy.

High-sensitivity fiber-tip pressure sensor with graphene diaphragm

A miniature fiber-tip pressure sensor was built by using an extremely thin graphene film as the diaphragm. The graphene also acts as a light reflector, which, in conjunction with the reflection at the fiber end-air interface, forms a low finesse Fabry-Perot interferometer. The graphene based sensor demonstrated pressure sensitivity over 39.4 nm/kPa with a diaphragm diameter of 25 μm. The use of graphene as diaphragm material would allow highly sensitive and compact fiber-tip sensors.

Synthesis and ferroelectric properties of multiferroic BiFeO3 nanotube arrays

We report the synthesis and characterization of ordered multiferroic BiFeO3 (BFO) nanotube arrays. BFO nanotubes with diameters of about 250nm and lengths of about 6μm were fabricated by means of a sol-gel method utilizing nanochannel alumina templates. After postannealing at 700°C, the BFO nanotubes exhibited a polycrystalline microstructure, and x-ray diffraction and transmission electron microscopy study revealed that they are of a perovskite crystal structure. Significant ferroelectric and piezoelectric characteristics of BFO nanotubes have been demonstrated by means of piezoresponse force microscopy measurement.