N. O. V. Plank

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.

Rare-earth mononitrides

Abstract When the rare earth mononitrides (RENs) first burst onto the scientific scene in the middle of last century, there were feverish dreams that their strong magnetic moment would afford a wide range of applications. For decades research was frustrated by poor stoichiometry and the ready reaction of the materials in ambient conditions, and only recently have these impediments finally been overcome by advances in thin film fabrication with ultra-high vacuum based growth technology. Currently, the field of research into the RENs is growing rapidly, motivated by the materials demands of proposed electronic and spintronic devices. Both semiconducting and ferromagnetic properties have been established in some of the RENs which thus attract interest for the potential to exploit the spin of charge carriers in semiconductor technologies for both fundamental and applied science. In this review, we take stock of where progress has occurred within the last decade in both theoretical and experimental fields, and which has led to the point where a proof-of-concept spintronic device based on RENs has already been demonstrated. The article is organized into three major parts. First, we describe the epitaxial growth of REN thin films and their structural properties, with an emphasis on their prospective spintronic applications. Then, we conduct a critical review of the different advanced theoretical calculations utilized to determine both the electronic structure and the origins of the magnetism in these compounds. The rest of the review is devoted to the recent experimental results on optical, electrical and magnetic properties and their relation to current theoretical descriptions. These results are discussed particularly with regard to the controversy about the exact nature of the magnetic state and conduction processes in the RENs.

The etching of silicon carbide in inductively coupled SF6/O2 plasma

The etching mechanisms of silicon carbide in an inductively coupled plasma (ICP) reactor using a SF6/O2 gas mixture, have been investigated using optical emission spectroscopy (OES) and Langmuir probe measurements. The etching is shown to be ion induced with a high degree of anisotropy. An optimum etch rate is achieved with 20% oxygen content within the gas mixture. By studying the independent influence of the ICP power and the substrate bias voltage on the ion current density, as well as the fluorine and oxygen radical densities in the plasma, the etch mechanism is found to be dominated by the number of ions bombarding the SiC surface. The steady state sputter yield observed at P>0.7?Pa, despite the increase in F radical concentration indicates the dominant role of ion bombardment in this etch regime, while at P<0.7?Pa, the etch mechanism is limited by the number of F radicals in the plasma. The OES results have shown that the etch rate is dependent upon the concentration of reactive radicals present with the [F]/[0] ratio = 8 at the optimum. Whilst using the optimum gas composition, the parameters which dominate the physical side of the reaction, ICP power and bias voltage, produce an increase of the etch rate as the potential difference between the substrate and the plasma is increased.

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.

Thiolation of single-wall carbon nanotubes and their self-assembly

A method for the thiolation of single-wall carbon nanotubes has been developed by exposing a sulfur/carbon nanotubes mixture to an argon/hydrogen gaseous plasma. X-ray photoelectron spectroscopy provides evidence of the existence of sulfur attached to carbon on the carbon nanotubes samples and Raman spectroscopy results show that the carbon nanotubes’ structure has been preserved after the treatment. One added advantage of the reported method is that excess oxygen is not present on the nanotubes. The thiolated carbon nanotubes are shown to self-assemble onto gold electrodes. Our method for thiolating carbon nanotubes provides a reliable and simple way for preparing functionalized tubes for nanoelectronic circuits based on carbon nanotubes.

Enhanced Curie temperature in N-deficient GdN

Polycrystalline GdN thin films have been grown at room temperature with varying N2 pressure. By varying the nitrogen pressure during growth we alter the carrier concentrations of the films. Films grown at low nitrogen pressures display onset of magnetization at temperatures as high as 200 K and a resistivity of 0.3 mΩ cm, whereas films grown at high nitrogen pressures all show a Curie temperature very close to 70 K and resistivity ranges over 1–1000 Ω cm are observed. For all GdN films a peak in the resistivity occurs at TC.

An investigation into the growth conditions and defect states of laminar ZnO nanostructures

The variation in morphology of zinc oxide when it crystallises is one of the fascinating aspects of this important semiconductor. Alterations in hydrothermal growth conditions can allow the synthesis of a wide range of shapes and structures, including nanowires and layered structures, formed via a secondary growth regime but, to date, little work has investigated the effect changes in growth conditions can have and their influence on the optical properties. Here, systematic changes in growth conditions and reactants were carried out and the results studied using scanning electron microscopy, X-ray diffraction, optical transmission, cathodoluminescence and Raman spectroscopy and atomic force microscopy. We demonstrate that the choice of the reactants can have a significant effect not just on the morphologies of the structure, but on the fundamental properties of the crystalline state such as alterations into the defect states within the system. Furthermore, secondary growth is shown to be dependent on the underlying primary growth morphology, and general reaction conditions for laminar growth are suggested.

Review of hydrothermal ZnO nanowires: Toward FET applications

In this short review, the authors put forward the case that ZnO nanowires grown by low temperature ( 600  °C) vapor phase methods. The hydrothermal synthesis route displays many advantages over vapor phase synthesis routes, the foremost being the compatibility of hydrothermal synthesis with flexible and vulnerable substrates. However, most hydrothermally synthesized ZnO nanowires require annealing at temperatures above 400 °C in order to exhibit field dependent transport necessary for FET devices. Effort should be directed toward understanding the mechanism causing some as-grown hydrothermal ZnO nanowires to display field dependence without the need for annealing. If these mechanisms are understood, great strides can be made in achieving integrated nanodevices using lateral arrays on technologically relevant substrates and understanding the fundamental cause of doping leading to n-type behavior in ZnO.In this short review, the authors put forward the case that ZnO nanowires grown by low temperature ( 600  °C) vapor phase methods. The hydrothermal synthesis route displays many advantages over vapor phase synthesis routes, the foremost being the compatibility of hydrothermal synthesis with flexible and vulnerable substrates. However, most hydrothermally synthesized ZnO nanowires require annealing at temperatures above 400 °C in order to exhibit field dependent transport necessary for FET devices. Effort should be directed toward understanding the mechanism causing some as-grown hydrothermal ZnO nanowires to display field dependence without the need for annealing. If these mechanisms are understood, great strides can be made in achieving i...

Direct measurement of charge transport through helical poly(ethyl propiolate) nanorods wired into gaps in single walled carbon nanotubes

We report the direct measurement of electrical transport through rod-like polymer molecules, of poly(ethyl propiolate) (PEP), utilizing single walled carbon nanotubes (SWNTs) as electrodes. The electrical properties of the devices were measured (i) before cutting a SWNT, (ii) when a SWNT was cut and (iii) after PEP deposition into the nanoscale gap in a cut SWNT. The gate-dependent electrical properties showed a reduction in current from I(on) = 2.4 x 10(-7) A for SWNT devices to I(on) = 3.6 x 10(-9) A for PEP bridge devices, both with the ON/OFF ratio of 10(4). Similarly, metallic SWNT devices showed a reduction in current from a few hundreds of microA for a SWNT device to a few nA for a PEP-SWNT structure. The current density of a single PEP molecule is 10(5)-10(6) A cm(-2), which is relatively high, indicating that the PEP molecule can carry significant current. Use of SWNT electrodes was seen to be an effective method of contacting PEP nanorods to facilitate electrical measurements.

Carbon nanotube field effect transistor aptasensors for estrogen detection in liquids

The authors demonstrate a small molecule 17 β-estradiol (E2) sensor based on aptamer functionalized carbon nanotube network film field effect transistors (CNT FETs). The real time current response for the 35-mer E2 aptamer functionalized CNT FET shows a clear increase in current over the range of 50 nM to 1.6 μM of E2. The E2 response using a longer 75-mer version of the aptamer functionalized CNT FETs, where the aptamer/E2 binding occurs beyond the Debye length, shows no obvious evidence of sensing. The CNT FET sensing platform has been fabricated via a simple surfactant free solution processing route, compatible with further carbon nanotube functionalization to develop a versatile sensing platform. The CNT FET aptasensors are able to perform real time monitoring of E2 levels for selective and quantitative detection of E2 in liquids.