Liudi Jiang

Reconfigurable photonic metamaterials

We demonstrate the first temperature driven mechanically reconfigurable photonic metamaterials (RPMs) providing tunability at optical frequencies.

Fluorination of carbon nanotubes in CF4 plasma

The effect of CF4 gaseous plasma exposure to single-wall carbon nanotubes (CNTs) has been studied. Raman spectroscopy results show that CNTs have gained more disordered sp3 bonds associated with functionalization, as both the flow rates of gas in the plasma and exposure time in the plasma are increased. Scanning electron microscopy images indicate the CNTs have been preserved after CF4 plasma exposure. X-ray photoelectron spectroscopy provides evidence of carbon to fluorine bonds (C–F) on the CNTs samples after CF4 plasma exposure. Semi-ionic and covalent C–F bonds are prevalent on the CNTs after CF4 exposure with the intensity ratio of the semi-ionic to covalent C–F bond decreasing as the flow rate of CF4 and exposure time in the CF4 plasma is increased.

Inductively coupled plasma etching of SiC in SF6/O2 and etch-induced surface chemical bonding modifications

4H silicon carbide (SiC) substrates were dry etched in an inductively coupled plasma (ICP) system, using SF6/O2 gas mixtures. Etch rate and etch mechanisms have been investigated as a function of oxygen concentration in the gas mixture, ICP chuck power, work pressure, and flow rate. Corresponding to these etch conditions, surface information of the etched SiC has been obtained by x-ray photoelectron spectroscopy measurements. The fact that no obvious Si–Si and Si–F bonds were detected on the etched surface of SiC in all our etch experiments suggests efficient removal of Si atoms as volatile products during the processes. However, various kinds of C–F bonds have been detected on the etched SiC surface and the relative intensities of these bonds vary with the etch conditions. In addition, the nature of the incorporated F atoms on the etched surface also depends strongly on etch conditions, which was identified by the change of the relative ratio between semi-ionic and covalent carbon fluorine bonds. The electrical behavior for different bond structures on the etched SiC surface can be one of the basic reasons affecting related devices.

Dry etching of SiC in inductively coupled Cl2/Ar plasma

Inductively coupled Cl2/Ar plasma etching of 4H–SiC has been studied. The SiC etch rate has been investigated as a function of average ion energy, Ar concentration in the gas mixtures, inductively coupled plasma power, work pressure and substrate temperature. The etch mechanism has been investigated by correlating the ion current density and relative atomic chlorine content to the etch rate under various etch conditions. For the first time, it has been found that the etch rate of SiC increases by about 50% at lower substrate temperatures (−80°C) than at high substrate temperatures (150°C) with the highest SiC etch rate of 230 nm min−1 being achieved at a substrate temperature of −80°C.

Fabrication of SiC microelectromechanical systems using one-step dry etching

A simple one-step inductively coupled plasma etching technique has been developed for the fabrication of SiC resonant beam structures. Straight cantilever and bridge devices have been made successfully. The structures have been actuated and resonant frequencies ranging from ∼120 kHz to ∼5 MHz have been measured. Comparison of the theoretically simulated and experimentally measured resonant frequencies shows the presence of significant tensile stress in bridge structures while the cantilever beams are free of stress. The degree of the tension in the bridge structures has been found to be independent of the bridge length.

Characterization of frequency tuning using focused ion beam platinum deposition

This paper presents and characterizes focused ion beam (FIB) platinum (Pt) deposition as a novel frequency tuning method for micromechanical resonators. FIB Pt deposited tuning was performed at room temperature and in contrast to other reported methods, frequency changes were achieved without any device failure. To perform the tuning, Pt was deposited on a 13 µm× 5 µm surface area at the free end of 3C silicon carbide (SiC) and polysilicon cantilever resonators in thicknesses ranging from 0.5 µm to 2.6 µm. To determine the amount of tuning, the resonant frequency of SiC and polysilicon devices was measured before and after Pt deposition. Frequency measurements performed before Pt deposition found that SiC resonators operated at higher resonant frequencies and quality (Q)-factors than their polysilicon counterparts. Measurements after Pt deposition on SiC and polysilicon resonators confirmed the predicted maximum frequency change of −15.5% made by FEM simulations and analytical modelling. Due to their lower mass, polysilicon resonators showed a larger frequency change than their SiC counterparts. A Q-factor decrease was observed for SiC and polysilicon resonators due to thermoelastic damping associated with the deposited Pt and surface contamination.

Development and validation of a 3D-printed interfacial stress sensor for prosthetic applications.

A novel capacitance-based sensor designed for monitoring mechanical stresses at the stump-socket interface of lower-limb amputees is described. It provides practical means of measuring pressure and shear stresses simultaneously. In particular, it comprises of a flexible frame (20 mm × 20 mm), with thickness of 4mm. By employing rapid prototyping technology in its fabrication, it offers a low-cost and versatile solution, with capability of adopting bespoke shapes of lower-limb residua. The sensor was first analysed using finite element analysis (FEA) and then evaluated using lab-based electromechanical tests. The results validate that the sensor is capable of monitoring both pressure and shear at stresses up to 350 kPa and 80 kPa, respectively. A post-signal processing model is developed to induce pressure and shear stresses, respectively. The effective separation of pressure and shear signals can be potentially advantageous for sensor calibration in clinical applications. The sensor also demonstrates high linearity (approx. 5-8%) and high pressure (approx. 1.3 kPa) and shear (approx. 0.6 kPa) stress resolution performance. Accordingly, the sensor offers the potential for exploitation as an assistive tool to both evaluate prosthetic socket fitting in clinical settings and alert amputees in home settings of excessive loading at the stump-socket interface, effectively preventing stump tissue breakdown at an early stage.

A review of silicon carbide development in MEMS applications

Due to its desirable material properties, Silicon Carbide (SiC) has become an alternative material to replace Si for Microelectromechanical Systems (MEMS) applications in harsh environments. To promote SiC MEMS development towards future cost-effective products, main technology areas in material deposition and processes have attracted significant interest. The developments in these areas have contributed to the rapid emergence of SiC MEMS prototypes. In this paper, we give an overview of the important developments in SiC material formation and fabrication processes in recent years. Some of the most interesting state-of-the-art SiC MEMS devices are reviewed. This highlights the major progresses in SiC MEMS developed thus far. This paper also looks into the prospect of SiC MEMS drawing attention to potential issues.

Nonpolar resistive switching in Cu/SiC/Au non-volatile resistive memory devices

Amorphous silicon carbide (a-SiC) based resistive memory (RM) Cu/a-SiC/Au devices were fabricated and their resistive switching characteristics investigated. All four possible modes of nonpolar resistive switching were achieved with ON/OFF ratio in the range 106–108. Detailed current-voltage I-V characteristics analysis suggests that the conduction mechanism in low resistance state is due to the formation of metallic filaments. Schottky emission is proven to be the dominant conduction mechanism in high resistance state which results from the Schottky contacts between the metal electrodes and SiC. ON/OFF ratios exceeding 107 over 10 years were also predicted from state retention characterizations. These results suggest promising application potentials for Cu/a-SiC/Au RMs.

Microstructural properties of amorphous carbon nitride films synthesised by dc magnetron sputtering

Amorphous carbon nitride (a-C:N) films have been prepared on silicon(1 0 0) substrates by direct current magnetron sputtering of graphite using a gaseous mixture of Ar and N2. Raman spectra have shown that these a-C:N films have a graphitic structure. The incorporation of nitrogen in the films has been confirmed by Fourier transform infrared (FTIR) spectroscopy. Graphitic and disordered sp2-bonded carbon which are present in Raman spectra and are normally forbidden (not observed) in FTIR become infrared active in our films as the symmetry of the hexagonal carbon rings is broken by nitrogen incorporation. X-ray photoelectron spectroscopy has been used to study the type of chemical bonding in these a-C:N films. The C 1s and N 1s X-ray photoelectron peaks have been deconvoluted and studied. We have found that for the C6-point triple bond; length half of m-dashN and C=N components of the C 1s and N 1s photoelectron peaks, there is a maximum peak intensity ratio of C6-point triple bond; length half of m-dashN:C=N in the films deposited when the gaseous mixture contains 35% N2 in the sputter gas.