Yongchang Fan

The role of C2 in nanocrystalline diamond growth

This paper presents findings from a study of nanocrystalline diamond (NCD) growth in a microwave plasma chemical vapor deposition reactor. NCD films were grown using Ar∕H2∕CH4 and He∕H2∕CH4 gas compositions. The resulting films were characterized using Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. Analysis revealed an estimated grain size of the order of 50nm, growth rates in the range 0.01–0.3μm∕h, and sp3- and sp2-bonded carbon content consistent with that expected for NCD. The C2 Swan band (dΠg3↔aΠu3) was probed using cavity ring-down spectroscopy to measure the absolute C2(a) number density in the plasma during diamond film growth. The number density in the Ar∕H2∕CH4 plasmas was in the range from 2to4×1012cm−3, but found to be present in quantities too low to measure in the He∕H2∕CH4 plasmas. Optical emission spectrometry was employed to determine the relative densities of the C2 excited state (d) in the plasma. The fact that similar NCD material was grown whether using Ar or He as the carrier gas suggests that C2 does not play a major role in the growth of nanocrystalline diamond.This paper presents findings from a study of nanocrystalline diamond (NCD) growth in a microwave plasma chemical vapor deposition reactor. NCD films were grown using Ar∕H2∕CH4 and He∕H2∕CH4 gas compositions. The resulting films were characterized using Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. Analysis revealed an estimated grain size of the order of 50nm, growth rates in the range 0.01–0.3μm∕h, and sp3- and sp2-bonded carbon content consistent with that expected for NCD. The C2 Swan band (dΠg3↔aΠu3) was probed using cavity ring-down spectroscopy to measure the absolute C2(a) number density in the plasma during diamond film growth. The number density in the Ar∕H2∕CH4 plasmas was in the range from 2to4×1012cm−3, but found to be present in quantities too low to measure in the He∕H2∕CH4 plasmas. Optical emission spectrometry was employed to determine the relative densities of the C2 excited state (d) in the plasma. The fact that similar NCD material was grown whether usin...

Umbravirus-encoded movement protein induces tubule formation on the surface of protoplasts and binds RNA incompletely and non-cooperatively.

Various functions of the cell-to-cell movement protein (MP) of Groundnut rosette virus (GRV) were analysed. The GRV ORF4-encoded protein was shown by immunofluorescence microscopy to generate tubular structures that protrude from the surface of the protoplast. The protein encoded by ORF4 was assessed also for RNA-binding properties. This protein was tagged at its C terminus with six histidine residues, produced in Escherichia coli using an expression vector and purified by affinity chromatography. Gel retardation analysis demonstrated that, in contrast to many other viral MPs, including the 3a MP of Cucumber mosaic virus (CMV), the ORF4-encoded protein bound non-cooperatively to viral ssRNA and formed complexes of low protein:RNA ratios. Competition binding experiments showed that the ORF4-encoded protein bound to both ssRNA and ssDNA without sequence specificity, but did not bind to dsDNA. UV cross-linking and nitrocellulose membrane-retention assays confirmed that both the GRV and the CMV MPs formed complexes with ssRNA and that these complexes showed similar stability in NaCl. Probing the MP-RNA complexes by atomic force microscopy demonstrated that the ORF4-encoded protein bound RNA incompletely, leaving protein-free RNA segments of varying length, while the CMV 3a protein formed highly packed complexes. The significance of the two properties of limited RNA binding and tubule formation of the umbraviral MP is discussed.

Low temperature plasma chemical vapour deposition of carbon nanotubes

Carbon nanotubes have been grown on alumina-supported iron compound catalysts by 2.45 GHz microwave plasma chemical vapour deposition, without additional substrate heating, using methane/argon gas mixtures with no added hydrogen, and with microwave powers typically up to 100 W. The conditions that produced a stable plasma were investigated by statistical selection of the values of power, pressure and gas flow rates. The conditions for abundant multi-walled nanotube formation were determined within this parameter space by SEM and TEM observations of the deposited material. The temperatures of the plasma and of the substrate in the reactor were investigated by optical emission spectroscopy and melting point samples, respectively.

Scanning probe microscopy and spectroscopy of CVD diamond films

Abstract. The surface morphology and electronic properties of as-deposited CVD diamond films and the diamond films which have been subjected to boron ion implantation or hydrogen plasma etching have been systematically studied by high resolution scanning probe microscopy and spectroscopy techniques. AFM and STM image observations have shown that (a) both the as-deposited CVD diamond films and the boron ion implanted films exhibit similar hillock morphologies on (100) crystal faces and these surface features are formed during the deposition process; (b) boron ion implantation does not cause a discernible increase in surface roughness; (c) atomic flatness can be achieved on crystal faces by hydrogen plasma etching of the film surface. Scanning tunnelling spectroscopy analysis has indicated that (a) the as-deposited diamond films and the hydrogen plasma etched diamond films possess typical p-type semiconductor surface electronic properties; (b) the as-deposited diamond films subjected to boron implantation exhibit surface electronic properties which change from p-type semiconducting behaviour to metallic behaviour; (c) the damage in the boron implanted diamond films is restricted to the surface layers since the electronic properties revert to p-type on depth profiling.

Analysis of metal features produced by uv irradiation of organometallic films

Gold and platinum patterns have been produced by UV-induced decomposition of a range of organometallic films. A nanosecond-pulsed excimer laser operating at 193 or 248 nm was used as the source of illumination. The metal thereby formed has been extensively studied using a wide range of analytical techniques. These include scanning and transmission electron microscopy (SEM, TEM), atomic force microscopy (AFM), and energy dispersive X-ray microanalysis (EDX). This analysis has demonstrated that the metal is polycrystalline with a grain size of typically 10 nm. This technique for producing high-resolution metal patterns has wide ranging applications, especially in microelectronics.

Study of the Interface Microstructures of CVD Diamond Films by TEM

Abstract. The characteristics of the interface microstructures between a CVD diamond film and the silicon substrate have been studied by transmission electron microscopy and electron energy loss spectroscopy. The investigations are performed on plan-view TEM specimens which were intentionally thinned only from the film surface side allowing the overall microstructural features of the interface to be studied. A prominent interfacial layer with amorphous-like features has been directly observed for CVD diamond films that shows a highly twinned defective diamond surface morphology. Similar interfacial layers have also been observed on films with a <100> growth texture but having the {100} crystal faces randomly oriented on the silicon substrate. These interfacial layers have been unambiguously identified as diamond phase carbon by both electron diffraction and electron energy loss spectroscopy. For the CVD diamond films that exhibit heteroepitaxial growth features, with the {100} crystal faces aligned crystallographically on the silicon substrate, such an interfacial layer was not observed. This is consistent with the expectation that the epitaxial growth of CVD diamond films requires diamond crystals to directly nucleate and grow on the substrate surface or on an epitaxial interface layer that has a small lattice misfit to both the substrate and the thin film material.

A novel STM-based depth profiling technique for the electronic characterisation of thin film materials

Abstract Material removal from a sample surface by operating a scanning tunneling microscope (STM) in the scanning tunneling spectroscopy (STS) mode can be controlled at the rate of a few angstroms per bias voltage ramping cycle. Monitoring the modified sample surface by tunneling spectroscopy allows determination of the electronic properties of the material. By combining these two capabilities, a novel type of depth profiling based on surface electronic properties has been proposed and studied. This depth profiling technique is based on the removal of small amounts of material obtained by operating the STM in the surface modification mode while simultaneously acquiring tunneling spectra from the material revealed by the tunneling electrons. The I – V curve profile is monitored on a pulse-by-pulse basis which allows the correlation of electronic properties with the etching depth. By this technique, the surface damage on the boron ion-implanted CVD diamond films and argon ion-etched CVD diamond films has been investigated. It has also been demonstrated that this technique can be used to measure thin film thickness. It is envisaged that this experimental technique could find applications in the characterisation of shallow-doped semiconductor devices.

SPM Study of YBCO Films Prepared by Plasma Assisted Laser Ablation

High T c and J c YBa2Cu3O7-x (YBCO) superconducting thin films have been prepared by DC oxygen discharge plasma assisted excimer laser ablation on MgO (100), SrTiO3 (100) and yttria-stabilized ZrO2 (100) substrates. The main improvement obtained with this laser ablation scheme is that laser deposition can be carried out in the oxygen discharge environment. This is achieved by two ring-shaped electrodes which have been placed in the middle between the target and substrate. X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) techniques have been used to characterise the as-deposited films. A set of narrow (001) peaks in the x-ray diffraction patterns from these films indicate film growth with a preferential c-axis orientation. AFM image observations show that the number of the particulates observed on thin films prepared by this new laser ablation scheme is reduced by comparison with films prepared by the conventional laser ablation method. The majority of growth islands have merged and connected with each other and have formed much larger, denser and smoother flat plateaus. Similar to films fabricated by the conventional laser ablation scheme, STM surface characterisation reveals that the films prepared by oxygen plasma assisted laser ablation are also formed mainly by screw dislocation generated spiral growth.

Surface Characterisation and Modification of YBCO Thin Films by STM

The optimum conditions required to obtain a stable image and reliable morphological information for YBa2Cu3O7-x (YBCO) high Tc superconducting thin films are summarised. Some typical morphological images of YBCO thin films prepared by the conventional laser ablation technique are presented. By analysing the STM images, three growth modes i.e. spiral growth, layer-by-layer growth and step propagation growth have been identified. Scanning tunneling spectroscopy has been used to study the local area tunneling behaviour for different regions of the YBCO thin films. Distinct and representative I-V curves for spiral growth crystal grains, particulates and non-spiral growth islands have been obtained. Some initial experiments show nanometre scale modification and structure fabrication on the YBCO thin film surface obtained by scanning the tip.

The synthesis of novel polynuclear organogold complexes

The synthesis and peirastic characterisation of two new organogold(III) complexes with a high molar ratio of gold content is described. The proposed formulae for the complexes in the present study are [C 2 H 2 Au 4 F 8 (PPh 3 ) 4 ] n and [C 2 H 2 Au 4 F 8 (NCCH 3 ) 4 ] n (Figure 1). These complexes were found to be stable to the atmosphere. The aim of this work was to demonstrate that such materials have a high metal to ligand ratio, suitable for physical vapour deposition process (PVD) and, hence, can be used as precursors for the deposition of pure metallic features. Physical and chemical characterisation methods were employed to obtain information about i) the structures, ii) the thermal and chemical stability, iii) the volatility and iv) the adhesion of these materials to specific substrates. These include n.m.r. ( 1 H, 19 F, 13 C and 31 P) IR spectroscopy, EDX (Energy Dispersive X-ray analysis), DSC ( Differential Scanning Calorimetry), TGA (Thermogravimetric Analysis), Powder X-ray Diffraction and Electron Microscopy (Scanning, Transmission and Atomic Force).