Surface studies in a UHV field emission gun scanning electron microscope

Recent instrumental advances have enabled scanning electron microscopy using a field emission gun (FEG) to be combined with an ultra high vacuum specimen chamber. The resulting instrument, the UHV-FEG-SEM, allows high spatial resolution microstructural and micro-analytic information to be obtained reliably from clean surfaces for the first time. Such an instrument has been installed and developed at the University of Sussex. It incorporates SEM at ～5 nm resolution, scanning Auger microscopy at ～30 nm resolution, and various diffraction techniques, including RHEED and electron backscattering pattern (EBSP) techniques, at (lateral) resolutions which can approach 10 nm. The work function can also be measured microscopically as demonstrated in this paper. Combined with facilities for in situ cleaning and molecular beam deposition, these techniques represent a considerable improvement in our ability to study surface and near-surface phenomena on a microscopic scale. Examples of recent work on technique development, including work-function measurement and backscattering patterns, are described. Recent applications to metal and semiconductor systems are described, including the crystal growth of Ag on Si (111) and Mo (100), the composition profiles of GaAs lasers, and adsorbed Cs layers on tungsten.     

Ultrasound Doppler system for two-dimensional flow mapping in liquid metals

A novel ultrasound Doppler measurement system for investigating liquid metal flows is presented. It employs an array of 25 transducer elements allowing a fast electronic traversing with concurrently high spatial resolution and therefore overcomes the limitations of commercially available ultrasound Doppler devices. For a high temporal resolution investigations were performed to parallelize the measurements as much as possible. Their results proved this parallel processing technique allowing a four times higher measurement rate compared to a serial processing for our specific ultrasound Doppler system. Therewith, a first two-dimensional one-componential flow mapping of liquid metal flows driven by a rotating magnetic field was successfully performed. In objective, this measurement system will be extended to a two-componential flow mapping.     

Scanning magnetic microscope system utilizing a magneto-impedance sensor for a nondestructive diagnostic tool of geological samples

We have developed a scanning magneto-impedance (MI) magnetic microscope to image surface stray magnetic fields of room-temperature geological samples with submillimeter resolution. The instrument consists of a small, 30 microm diameter, 5 mm length amorphous wire-based magneto- impedance (MI) sensor without any cooling mechanisms. The spacing between the sensor head and the sample was less than 300 microm. The length of the amorphous wire and sample-to-wire distance limits the spatial resolution. We have achieved a spatial resolution of 400 microm with a magnetic resolution of 10 nT. This instrument enables us to map a two-dimensional out-of-page component of a stray magnetic field of a natural remanent magnetization over a millimeter-thick slab of a primitive ordinary chondrite meteorite, documenting dipolelike features. A comparison of element mapping images with the stray field of the meteorites reveals what individual metals carry the dipolar remanences in the meteorites. These results suggest that the scanning MI microscope offers a room-temperature operable, small, low-maintenance alternative to the scanning SQUID microscope, and can aid in the interpretation of the magnetic remanence acquisition process of a meteorite.     

Quantitative phase imaging of nanoscale electrostatic and magnetic fields using off-axis electron holography

Off-axis electron holography in the transmission electron microscope is a powerful interferometric technique that enables electrostatic and magnetic fields to be imaged and quantified with spatial resolution often approaching the nanometer scale. Here, we demonstrate the capabilities of the technique for phase quantification at the nanoscale by briefly reviewing some of our recent studies of nanostructured materials. Examples that are described include determination of the electrostatic potential profiles associated with doped Si- and GaAs-based semiconductor devices, measurement of hole accumulation in Ge quantum dots, mapping of polarization fields in III-nitride heterostructures, and observation of the remanent states and reversal mechanisms of lithographically patterned magnetic nanorings. Some issues associated with sample preparation for doped semiconductor heterostructures are also briefly discussed.     

Contour mapping of precision ball defects with laser Doppler vibrometry

Laser Doppler vibrometry is presented as an effective inspection technique for contour mapping of defects on precision roller bearing balls. Localized defects are characterized by measuring the Doppler shift of a laser beam incident on a rotating ball. Contour maps are obtained by scanning the ball at equally spaced azimuths with the laser beam as the ball rotates about a vertical axis. A technique for characterizing the repeatable runout of a ferrofluid spindle is discussed. Tests were performed on two different grades of ceramic ball bearings. With the experimental setup, a height resolution of 5.5 nm could be attained. Defects of 22 μm in width could be detected. Use of the contour mapping technique greatly enhances visualization of defects on ball bearing surfaces.     

Off-axis and inline electron holography: Experimental comparison

Electron holography is a very powerful technique for mapping static electric and magnetic potentials down to atomic resolution. While electron holography is commonly considered synonymous with its off-axis variant in the high energy electron microscopy community, inline electron holography is widely applied in low-energy electron microscopy, where the realization of the off-axis setup is still an experimental challenge. This paper demonstrates that both inline and off-axis holography may be used to recover amplitude and phase shift of the very same object, in our example latex spheres of 90 and 200 nm in diameter, producing very similar results, provided the object does not charge under the electron beam.     

Determination of radial location of rotating magnetic islands by use of poloidal soft x-ray detector arrays in the STOR-M tokamak

A technique is presented for determining the radial location of the rotating magnetic islands in the STOR-M tokamak by use of soft x-ray (SXR) detector arrays. The location is determined by examining the difference in the integrated SXR emission intensities through two adjacent lines of sight. A model for calculating dependence of the line integrated SXR emission intensity on the radius, the mode numbers and the magnetic island geometry, has been developed. The SXR difference signal shows phase inversion when the impact parameter of the line of sight sweeps across the magnetic islands. Experimentally, the difference SXR signals significantly reduce noise and suppress the influence of background plasma fluctuations through common mode rejection when a dominant mode exists in the STOR-M tokamak. The radial locations of the m = 2 magnetic islands have been determined under several experimental conditions in the STOR-M discharges. With the decrease in the tokamak discharge current and thus the increase of the safety factor at the edge, the radial location of the m = 2 magnetic islands has been found to move radially inward.     

Resonance enhanced dynamic light scattering

We present a novel light scattering setup that enables probing of dynamics near solid surfaces. An evanescent wave generated by a surface plasmon resonance in a metal layer is the incident light field in the dynamic light scattering experiment. The combination of surface plasmon resonance spectroscopy and dynamic light scattering leads to a spatiotemporal resolution extending a few hundred nanometers from the surface and from microseconds to seconds. The comparison with evanescent wave dynamic light scattering identifies the advantages of the presented technique, e.g., surface monitoring, use of metal surfaces, and biorelevant systems. For both evanescent wave geometries, we define the scattering wave vector necessary for the analysis of the experimental relaxation functions.     

Sliding behavior of dual phase steels in vacuum and in air

While the importance of fatigue failure is well established in rolling systems, the evidence for a correlation of fatigue with sliding wear is less convincing. McEvily has reported striking differences in the fatigue properties of two dual phase steels, one containing martensite islands in a ferrite matrix (type A) and one containing ferrite islands in a martensite matrix (type B). We have examined the sliding behavior of these same materials in vacuum and in air for various sliding distances. The pin-on-disk tests included self-mated pairs and tests with molybdenum as a counterface. Post-test analysis included the use of microhardness testing, scanning electron microscopy (including local chemical analysis) and X-ray diffraction analysis of the wear surfaces, sample cross-sections and wear debris. Transitions from low to high friction were associated with transfer processes and mechanical mixing. The wear rates of the type A and B samples were similar in vacuum. However, in air, the wear rates of the type B samples were increased by about 100 times. The lack of correlation with fatigue properties is not yet understood. Any of the following could be responsible: short crack (initiation) phenomena; large local stress intensities; surface material associated with transfer.     

2D soft x-ray system on DIII-D for imaging the magnetic topology in the pedestal region

A new tangential two-dimensional soft x-ray imaging system (SXRIS) is being designed to examine the edge island structure in the lower X-point region of DIII-D. Plasma shielding and/or amplification of the calculated vacuum islands may play a role in the suppression of edge-localized modes via resonant magnetic perturbations (RMPs). The SXRIS is intended to improve the understanding of three-dimensional (3D) phenomena associated with RMPs. This system utilizes a tangential view with a pinhole imaging system and spectral filtering with beryllium foils. SXR emission is chosen to avoid line radiation and allows suitable signal at the top of a H-mode pedestal where T(e)～1-2?keV. A synthetic diagnostic calculation based on 3D SXR emissivity estimates is used to help assess signal levels and resolution of the design. A signal-to-noise ratio of 10 at 1 cm resolution is expected for the perturbed signals, which are sufficient to resolve most of the predicted vacuum island sizes.     

Studies of polytetrafluoroethylene transfer layers produced by rubbing in ultrahigh vacuum using a relatively simple apparatus

A simple apparatus is described which enables studies to be made on transfer layers produced by rubbing under ultrahigh vacuum conditions. Initial results on the transfer of polytetrafluoroethylene (PTFE) layers to atomically clean tin and lead surfaces are reported. These layers have been examined using X-ray photoelectron spectroscopy. The results indicate the presence of two different forms of PTFE on the metal surfaces.     

Unique Solid Lubricating Materials for High Temperature-Air Applications

A technique has been developed, whereby mechanical strength and excellent oxidation resistance is imparted to self-lubricating bodies of high lubricant content. By an “amalgamation” of powdered solid lubricants, such as tungsten diselenide, with a gallium alloy, followed by a subsequent compression-curing cycle, self-lubricating surfaces have been formed that resist oxidation at a temperature of 1500 F. This paper presents friction-wear characteristics of these materials at temperatures to 950 F. They offer potential as load-bearing surfaces and seals in high temperature applications for both oxidizing and inert or vacuum environment.     

Experiments on magnetic fluid rotary seals operating under vacuum conditions

Magnetic fluid rotary vacuum seals have been shown to be effective in machinery operating in a vacuum chamber. Such seals have the advantages of simple design, zero leakage at almost any rotation speed, and low friction. They have no wear and require no maintenance. This paper presents results obtained from experimental investigations of the operation of magnetic fluid rotary seals under vacuum conditions. The paper discusses the test apparatus and the seals used, the test conditions, and the procedure. The experimental results show characteristic phenomena observed in magnetic fluid rotary vacuum seals, including changes in vacuum pressure, temperature, and frictional moment dependent on the rotation speed of the shaft, number of sealing stages, height of the sealing gap, and mean magnetic flux density in the sealing gap.     

High-precision detection of focal position on a curved surface for laser processing

A new technique for detecting the focal position of a curved surface provides several advantages both in research and industrial applications. The quality of patterns lasered on a roll surface is determined by the precision of the focus detection, and surfaces of the massive rolls used in laser fabrication can be difficult to adjust properly using conventional technologies. Here, a unique method for detecting the focal position of a curved surface based on the reflected profile of a laser beam is presented. The versatility of the proposed technique results from being able to adjust the laser beam based on changes in the shape and diameter of the beam spot when the specimen surface deviates from the focal plane. A theoretical model based on three-axis movement is proposed, and experimental setups are developed based on the model. Analysis of the obtained results enables high precision positioning of the specimen and identification of the focal point. Furthermore, the presented technique can be used to locate the focal point on any curved surface. Therefore, the theoretical model, analysis results, and focal detection method can be combined in an algorithm for a novel auto-focusing system that can be applied to laser processing of curved surfaces, such as fabricating microgrooves, or engraving roll surfaces in printed electronics.     

Full characterization and optimization of a femtosecond ultraviolet laser source for time and angle-resolved photoemission on solid surfaces

A novel experimental apparatus for time and angle-resolved photoemission on solid surfaces is presented. A 6.28 eV laser source operating at 250 kHz repetition rate is obtained by frequency mixing in nonlinear beta barium borate crystals. This UV light source has a high photon flux of 10(13) photons/s with relatively low number of photons/pulse so that Fermi surface mapping over a wide region of the Brillouin zone is possible while mitigating space charge effects. The UV source has been fully characterized spatially, spectrally, and temporally. Its potential for time and angle-resolved photoemission is demonstrated through Fermi surface mapping and photoexcited electron dynamics in Bismuth. True femtosecond time resolution <65 fs is obtained while the energy resolution of 70 meV appears to be mainly limited by the laser bandwidth.     

复杂曲面上机器人自动喷涂路径规划方法

针对目前含有岛屿或空洞的复杂曲面上机器人喷涂路径规划中存在的问题,提出一种能够保持轮廓平行的机器人喷涂路径设计方法。利用最小二乘坐标映射原理,将被喷涂曲面映射到二维平面域,建立目标曲面与二维平面域之间的一一映射关系,然后对平面映射域的内外边界进行连续偏置,构造能够保持轮廓平行的二维偏置曲线;进而以被喷涂曲面到二维平面域的坐标映射为向导,将无干涉二维偏置曲线逆映射到被喷涂曲面上,生成能够保持轮廓平行的喷涂路径,实现多岛屿或空洞复杂曲面上三维喷绘路径设计到二维平面的降维处理。实例仿真实验结果表明,所提方法简单实用,可在含有岛屿或空洞的复杂曲面上快速地生成能够保持轮廓平行的机器人喷涂路径。     

Enhancing image contrast of carbon nanotubes on cellular background using helium ion microscope by varying helium ion fluence

Carbon nanotubes (CNTs) have become an important nano entity for biomedical applications. Conventional methods of their imaging, often cannot be applied in biological samples due to an inadequate spatial resolution or poor contrast between the CNTs and the biological sample. Here we report a unique and effective detection method, which uses differences in conductivities of carbon nanotubes and HeLa cells. The technique involves the use of a helium ion microscope to image the sample with the surface charging artefacts created by the He⁺ and neutralised by electron flood gun. This enables us to obtain a few nanometre resolution images of CNTs in HeLa Cells with high contrast, which was achieved by tailoring the He⁺ fluence. Charging artefacts can be efficiently removed for conductive CNTs by a low amount of electrons, the fluence of which is not adequate to discharge the cell surface, resulting in high image contrast. Thus, this technique enables rapid detection of any conducting nano structures on insulating cellular background even in large fields of view and fine spatial resolution. The technique demonstrated has wider applications for researchers seeking enhanced contrast and high‐resolution imaging of any conducting entity in a biological matrix – a commonly encountered issue of importance in drug delivery, tissue engineering and toxicological studies.     

Nucleation and formation of oxide film with the magnetic field on dry sliding contact of ferromagnetic steel

The nucleation and formation of oxide film in unidirectional dry sliding contact has been studied in a vacuum chamber with and without the application of a magnetic field in the intention of identifying the role of magnetic intensity on the oxidation wear. The wear tests of the steel AISI 1045/steel AISI 1045 couple are investigated on a pin‐disc configuration under three various gas environments: in ambient air, under oxygen at 10⁵ Pa and in vacuum at 5.10⁻⁵ Pa. The formation of oxide layer strongly depends on oxygen partial pressure and magnetic field intensity. These took the form of protective raised ‘islands’ of compacted debris which is developed gradually but rapidly increased as the oxygen partial pressure is increased and which could persist for extended periods during subsequent evacuation. Evidence from various experimental techniques indicates that the compacted debris is a mixture of iron oxides in the form of oxide‐covered particles, although the depth of oxide film has not yet been fully elucidated. Copyright © 2010 John Wiley & Sons, Ltd.     

Time resolved cathodoluminescence in the scanning electron microscope by use of the streak technique

Minority carrier lifetime is one of the basic material properties in optoelectronic devices and material. Both the micrometer range dimensions of the devices and lifetime variations around defects in materials require a lifetime measurement technique with both high spatial and high temporal resolution. In order to meet these requirements a highly efficient cathodoluminescence (CL) measurement system has been developed consisting of a commercial scanning electron microscope extended for integral and spectral CL-measurements and a streak camera with subnanosecond time resolution as time resolving detector. The lifetime is determined by evaluation of CL-decay time after excitation of the specimen by an electron beam pulse, which is blanked in less than 50 ps by an adjustable plate capacitor. The CL-light is collected by an adjustable, ellipsoidal mirror and can be dispersed in a vacuum monochromator. The monochromator exit slit is imaged on to the photocathode of the streak camera, which transforms the temporal distribution of the photon intensity into a lateral distribution on the camera phosphor screen after amplification by an integrated microchannel plate. By this technique it is possible to record the complete CL-decay simultaneously, thus avoiding all measurement falsifications by system instabilities. The resulting intensity distribution is read out by a SIT vidicon camera with subsequent multichannel analyser, providing an intensity plot versus streak time in less then 1 min for each beam spot location. The technique is therefore well suited for lifetime mapping experiments. The best time resolution of the complete system achieved today is about 100 ps. Its performance is here demonstrated by measurements of the temperature dependence of the CL-decay in a highly Se-doped GaAs specimen in the temperature range from 90 K to 300 K.     

Near-field optics on silicon–electrolyte junctions

A technique allowing near-field photocurrent (PC) mapping of silicon surfaces in contact with an electrolyte is presented. The illumination source is an optical fibre tip with a 100-nm aperture. A shear force detection system controls the tip–sample distance while scanning the tip across the silicon–electrolyte interface. Topographic and PC images on SiO₂/Si mesas both show 300 nm resolution. It is shown that this PC contrast is induced by the tip–topography interaction and hence the PC resolution is limited by the resolution of the topography. Indeed, PC mapping on topography-less patterned porous-silicon/silicon samples shows that the lateral resolution is only limited by the aperture size which is of the order of 100 nm.