Growth and characterisation of titanium sulphide nanostructures by surface-assisted vapour transport methods; from trisulphide ribbons to disulphide nanosheets

Surface Assisted Chemical Vapour Transport (SACVT) methods have been employed to grow nanostructures of titanium disulphide (TiS2) and titanium trisulphide (TiS3). SACVT reactions occur between titanium and sulphur powders to form TiSx species transported in the vapour phase to grow nanometric flower-like structures on titanium-coated silica substrates. The evolution of structure and composition has been followed by powder X-ray diffraction, electron microscopy and Raman spectroscopy. At 1 : 2 Ti : S ratios, the size and shape of the hexagonal 1T-TiS2 titanium disulphide structures formed can be varied from flower-like growths with 'petals' formed from nanosheets 10 nm thick to platelets microns across. Increasing the proportion of sulphur (Ti : S 1 : 4) enables TiS3 flower-like structures composed of radiating nanoribbons to grow at elevated temperatures without decomposition to TiS2. TEM/SAED suggests that individual trisulphide ribbons grow along the [010] direction. Magnetic properties of the disulphide nanomaterials have been determined using SQUID magnetometry and Raman spectra for disulphides suggest that their crystal and electronic structures may be more complex than expected for bulk, stoichiometric, CdI2-structured TiS2

Thermal analysis of submicron nanocrystalline diamond films

The thermal properties of sub-μm nanocrystalline diamond films in the range of 0.37–1.1 μm grown by hot filament CVD, initiated by bias enhanced nucleation on a nm-thin Si-nucleation layer on various substrates, have been characterized by scanning thermal microscopy. After coalescence, the films have been outgrown with a columnar grain structure. The results indicate that even in the sub-μm range, the average thermal conductivity of these NCD films approaches 400 W m− 1 K− 1. By patterning the films into membranes and step-like mesas, the lateral component and the vertical component of the thermal conductivity, k<sub>lateral</sub> and k<sub>vertical</sub>, have been isolated showing an anisotropy between vertical conduction along the columns, with k<sub>vertical</sub> ≈ 1000 W m− 1 K− 1, and a weaker lateral conduction across the columns, with k<sub>lateral</sub> ≈ 300 W m− 1 K− 1

Quantification of atomic force microscopy tip and sample thermal contact

A thermal conduction measurement device was fabricated, consisting of a silicon dioxide membrane with integrated thermal sensors (Pt resistance heater/thermometer and Pt–Au thermocouples) using MEMS technology. Heat transfer between the heated device and a number of unused atomic force microscope and scanning thermal microscope probes was measured. Changes in thermal conduction related to changes in the tip shape resulting from initial contact were observed. The sensors were fabricated by electron beam lithography and lift-off followed by local subtractive processing of a Pt–Au multilayer to form Pt heater–resistance thermometer elements and Pt–Au thermocouples. Thermal isolation from the silicon substrate was provided by dry release of the supporting 50 nm thick SiO2 membrane using an isotropic SF6 inductively coupled plasma etch. The high thermal isolation of the sample combined with the sensitivity of the temperature sensors used allowed the detection of thermal conduction between the tip and the sample with high precision. The measured temperature range of the Pt resistor was 293–643 K. The measured thermal resistance of the membrane was 3 × 105 K/W in air and 1.44 × 106 K/W in vacuum. The tip contact resistance was measured with a noise level of 0.3g0 T at room temperature, where g0 is the thermal resistance quantum

The ultrasonic imaging of plastic deformation

Incl. 1 ficheAvailable from British Library Document Supply Centre- DSC:D73609/87 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Fabrication of sub-5 nm gaps between metallic electrodes using conventional lithographic techniques

The reproducible fabrication of nanoscale gaps below 5 nm between metallic electrodes is key to the study of the electronic characteristics of individual molecules, but is hampered by the resolution limit and mechanical instabilities of commonly used electron-sensitive resists. We describe a fabrication process for the creation of nanoscale gaps between metallic electrodes based on conventional lithographic techniques. The process involves the patterning of a lithographic gap of 5–∼20 nm between metallic electrodes on an oxidized silicon substrate. The SiO<sub>2</sub> not covered by the electrodes is undercut and another metal film is thermally evaporated onto the substrate. Due to the slow buildup of material at the edges of the patterned electrode, the gap size can be reduced in a controllable way, and the final gap size is determined by the thickness of the evaporated metal film. This batch fabrication process is suitable for high-density fabrication of nanoscale gaps with the attractive feature that a self-aligned gate can be formed underneath the gap. We have investigated the effect of annealing samples for a short period at 125 °C in air. Scanning electron microscopy data of a batch of identical gaps is presented which illustrates the variation in gap size and morphology after annealing. Gaps down to 1–∼2 nm can be resolved directly using a scanning electron microscope. For gaps below 1 nm, the separation between the two metallic electrodes cannot be resolved. To determine whether a tunnel gap is present, electrical measurements are required. Use of the Simmons tunnel model to fit an analytical curve to the measured IV characteristics of a gap gives a separation of 1.2±0.2 nm and also verifies the consistency of parameters such as the effective barrier height in air indicating the presence of contaminants on the electrodes

ACOUSTIC MICROSCOPY TECHNIQUES FOR OBSERVING DISLOCATION DAMPING

A unique advantage of scanning acoustic microscopy lies in the ability to image the interaction of acoustic waves with a specimen. This can be made quantitative, so that the velocity and attenuation of surface waves can be measured over a few microns. This technique can be applied to the detection of dislocation damping in the acoustic microscope

Fabrication of metallic tunnel junctions for the scanning single electron transistor atomic force microscope

Metallic tunnel junctions are important in the formation of high temperature single electron devices, which can act as the ultimate electrometer. We present a method for the fabrication of highly defined metallic tunnel junctions based on the step-edge cutoff process. Fabrication involves conventional electron beam lithography and lift-off of metallic thin films. Junctions scaling down to 50 nm linewidth have been achieved. The devices show a spread in impedance at low bias ranging from less than 10 MΩ to more than 100 GΩ. We have investigated the behavior of thin metallic films across a step forming a single tunnel junction. In the case of palladium we find that grain growth during deposition can give rise to multiple junctions across a single step. We illustrate this using one particular example of a junction, where isolated grains form a spontaneous Coulomb blockaded island with a charging energy of 20 meV at 77 K. The single and double junction fabrication process is compatible with our generic atomic force microscope probe technology, which has been shown to be capable of defining ultrasmall metallic structures on cantilevers and pyramidal tips. We demonstrate this by the fabrication of a lithographically defined device on a silicon cantilever with an integrated Ti/Au thin-film strain gauge

High robustness of correlation-based alignment with Penrose patterns to marker damage in electron beam lithography

Correlation-based alignment is an alternative alignment method for electron beam lithography. Using complex marker patterns, such as Penrose patterns, which contain more positional information, greater alignment accuracy can be achieved. Correlation-based alignment with Penrose patterns is less susceptible to marker edge defects, such as rat bites, roughness and flagging, since many more edges contribute to determining the marker position. There are however other defects associated with fabricating markers and this paper investigates how defects that result in parts of the pattern being omitted or obscured affect the correlation process when using Penrose pattern markers. We show that in both cases severely damaged markers can be used successfully and demonstrate fabricated structures with sub-5 nm alignment using markers with up to 80% of the marker pattern missing

Direct observation of changes to domain wall structures in magnetic nanowires of varying width

Lorentz microscopy has been used to explore the structure variation of domain walls in thin Permalloy nanowires in the vicinity of symmetric triangular antinotches. The antinotches present a complex potential landscape to domain walls. Walls can be trapped in front of, partly enter, or be trapped inside the antinotches according to the geometry of the latter and, in the case of vortex domain walls, the chirality. In all cases, the magnetization distribution was determined. Of particular note was the structure the wall assumed during depinning from the antinotch, complex forms extending over distances several times the wire width being observe

Improvements to the alignment process in a commercial vector scan electron beam lithography tool

This paper examines the desirable properties of marker patterns for use in correlation-based alignment systems and demonstrates alignment accuracies of better than 1 nm. A framework for evaluating different classes of marker patterns has been developed and a figure of merit for marker patterns used in correlation-based alignment has been defined. We show that Penrose tilings have many desirable properties for correlation-based alignment. An alignment system based on correlation and using marker patterns derived from Penrose tilings has been developed and implemented on a commercial Vistec VB6 UHR EWF electron beam lithography tool. A new method of measuring alignment at the sub-nm level using overlaid gratings and a Fourier Transform based analysis scheme is introduced. (C) 2008 Elsevier B.V. All rights reserve