Scanning mass spectrometer for quantitative reaction studies on catalytically active microstructures

We describe an apparatus for spatially resolving scanning mass spectrometry which is able to measure the gas composition above catalytically active microstructures or arrays of these microstructures with a lateral resolution of better than 100 mum under reaction conditions and which allows us to quantitatively determine reaction rates on individual microstructures. Measurements of the three-dimensional gas composition at different vertical distances and separations between active structures allow the evaluation of gas phase mass transport effects. The system is based on a piezoelectrically driven positioning substage for controlled lateral and vertical positioning of the sample under a rigidly mounted capillary probe connecting to a mass spectrometer. Measurements can be performed at pressures in the range of <10(-2)-10 mbars and temperatures between room temperature and 450 degrees C. The performance of the setup is demonstrated using the CO oxidation reaction on Pt microstructures on Si with sizes between 100 and 300 mum and distances in the same order of magnitude, evaluating CO(2) formation and CO consumption above the microstructures. The rapidly decaying lateral resolution with increasing distance between sample and probe underlines the effects of (lateral) gas transport in the room between sample and probe. The reaction rates and apparent activation energy obtained from such measurements agree with previous data on extended surfaces, demonstrating the feasibility of determining absolute reaction rates on individual microstructures.     

Low temperature ultrahigh vacuum noncontact atomic force microscope in the pendulum geometry

A noncontact atomic force microscope (nc-AFM) operating in magnetic fields up to ±7 T and liquid helium temperatures is presented in this article. In many common AFM experiments the cantilever is mounted parallel to the sample surface, while in our system the cantilever is assembled perpendicular to it; the so called pendulum mode of AFM operation. In this mode measurements employing very soft and, therefore, ultrasensitive cantilevers can be performed. The ultrahigh vacuum conditions allow to prepare and transfer cantilevers and samples in a requested manner avoiding surface contamination. We demonstrate the possibility of nc-AFM and Kelvin force probe microscopy imaging in the pendulum mode. Ultrasensitive experiments on small spin ensembles are presented as well.     

In situ site-specific specimen preparation for atom probe tomography

Techniques for the rapid preparation of atom-probe samples extracted directly from a Si wafer are presented and discussed. A systematic mounting process to a standardized microtip array allows approximately 12 samples to be extracted from a near-surface region and mounted for subsequent focused-ion-beam sharpening in a short period of time, about 2h. In addition, site-specific annular mill extraction techniques are demonstrated that allow specific devices or structures to be removed from a Si wafer and analyzed in the atom-probe. The challenges presented by Ga-induced implantation and damage, particularly at a standard ion-beam accelerating voltage of 30 keV, are shown and discussed. A significant reduction in the extent of the damaged regions through the application of a low-energy "clean-up" ion beam is confirmed by atom-probe analysis of the damaged regions. The Ga+ penetration depth into {100} Si at 30 keV is approximately 40 nm. Clean-up with either a 5 or 2 keV beam reduces the depth of damaged Si to approximately 5 nm and <1 nm, respectively. Finally, a NiSi sample was extracted from a Si wafer, mounted to a microtip array, sharpened, cleaned up with a 5 keV beam and analyzed in the atom probe. The current results demonstrate that specific regions of interest can be accessed and preserved throughout the sample-preparation process and that this preparation method leads to high-quality atom probe analysis of such nano-structures.     

Tapping mode scanning force microscopy in water using a carbon nanotube probe

An improved technique for obtaining tapping mode scanning force microscopy (TMSFM) images of soft samples submerged in water is described. This technique makes use of a carbon nanotube several microns in length mounted on a conventional silicon cantilever as the TMSFM probe. The sample is covered by a shallow water layer and during imaging only a portion of the nanotube is submerged. This mode of operation largely eliminates the undesirable effects of hydrodynamic damping and acoustic excitation that are present during conventional tapping mode operation in liquids and leads to high-quality TMSFM images. Because of their low bending force constants, carbon nanotubes are ideal for gentle imaging of soft samples. Because of their small (5–20 nm) diameter and cylindrical shape they provide excellent lateral resolution and are ideal for scanning high aspect ratio objects.     

Measurement of magnetic susceptibility in pulsed magnetic fields using a proximity detector oscillator

We present a novel susceptometer with a particularly small spatial footprint and no moving parts. The susceptometer is suitable for use in systems with limited space where magnetic measurements may not have been previously possible, such as in pressure cells and rotators, as well as in extremely high pulsed fields. The susceptometer is based on the proximity detector oscillator, which has a broad dynamic resonant frequency range and has so far been used predominantly for transport measurements. We show that for insulating samples, the resonance frequency behavior as a function of field consists of a magnetoresistive and an inductive component, originating, respectively, from the sensor coil and the sample. The response of the coil is modeled, and upon subtraction of the magnetoresistive component the dynamic magnetic susceptibility and magnetization can be extracted. We successfully measure the magnetization of the organic molecular magnets Cu(H(2)O)(5)(VOF(4))(H(2)O) and [Cu(HF(2))(pyz)(2)]BF(4) in pulsed magnetic fields and by comparing the results to that from a traditional extraction susceptometer confirm that the new system can be used to measure and observe magnetic susceptibilities and phase transitions.     

Computational models of a nano probe tip for static behaviors

It is difficult to predict the measurement bias arising from the compliance of the atomic force microscope (AFM) probe. The issue becomes particularly important in this situation where nanometer uncertainties are sought for measurements with dimensional probes composed of flexible carbon nanotubes mounted on AFM cantilevers. We have developed a finite element model for simulating the mechanical behavior of AFM cantilevers with carbon nanotubes attached. Spring constants of both the nanotube and cantilever in two directions are calculated using the finite element method with known Young's moduli of both silicon and multiwall nanotube as input data. Compliance of the nanotube-attached AFM probe tip may be calculated from the set of spring constants. This paper presents static models that together provide a basis to estimate uncertainties in linewidth measurement using nanotubes. In particular, the interaction between a multiwall nanotube tip and a silicon sample is modeled using the Lennard-Jones theory. Snap-in and snap-out of the probe tip in a scanning mode are calculated by integrating the compliance of the probe and the sample-tip interacting force model. Cantilever and probe tip deflections and points of contact are derived for both horizontal scanning of a plateau and vertically scanning of a wall. The finite element method and the Lennard-Jones model provide a means to analyze the interaction of the probe and sample and measurement uncertainty, including actual deflection and the gap between the probe tip and the measured sample surface.     

Measurement and estimation of temperature rise in TEM sample during ion milling

The actual temperature rise was measured during ion-milling process used in the transmission electron microscopy (TEM) sample preparation. Special probes were fabricated for the measurements, one with shielded, floating thermocouple mounted onto a 3mm grid to compute the thermal load at the sample, and the other, a bare probe with a polymer coating to measure the maximum temperature attained. The temperature measured in the most commonly used ion-milling system reached up to 295 degrees C when the typical milling conditions, 6keV ion-energy and an incident angle of 80 degrees, were used. Based on the temperature profiles that were obtained by the shielded probe, two unknown parameters, the amount of heat deposited by the energetic ions/neutrals to the sample and the thermal conductivities between the materials, were estimated and used to compute the temperature rise in commonly adopted materials. The calculation showed that the temperature of the glass sample reached more than 300 degrees C under typical ion-milling conditions. The calculated value was confirmed with the experimental result of the crystallization of an amorphous Si on the glass under the typical ion-milling condition, which gave the same extent as annealing at 350 degrees C.     

Fiber deflection probe for small hole metrology

This paper presents the development of a new probing method for coordinate measuring machines (CMM) to inspect the diameter and form of small holes. The technique, referred to as fiber deflection probing (FDP), can be used for holes of approximately 100 μm nominal diameter. The expanded uncertainty obtained using this method is 0.07 μm (k = 2) on diameter. The probing system consists of a transversely illuminated fiber (with a ball mounted on the end) whose shadows are imaged using a camera. We can infer the deflection of the probe from the motion of the image seen by the camera, and we infer the position of the measured surface by adding the fiber deflection along x- and y-directions to the machine scale readings. The advantage of this technique is the large aspect ratio attainable (5 mm deep for a 100 μm diameter hole). Also, by utilizing the fiber as a cylindrical lens, we obtain sharp crisp images of the fiber position, thus enabling high resolution for measured probe deflection. Another potential advantage of the probe is that it exerts an exceptionally low force (ranging from a few micronewtons down to hundreds of nanonewtons). Furthermore, the probe is relatively robust, capable of surviving more than 1 mm over-travel, and the probe should be inexpensive to replace if it is broken. In this paper, we describe the measurement principle and provide an analysis of the imaging process. Subsequently, we discuss data obtained from characterization and validation experiments. Finally, we demonstrate the utility of this technique for small hole metrology by measuring the internal geometry of a 129 μm diameter fiber ferrule and conclude with an uncertainty budget.     

A novel approach for x-ray scattering experiments in magnetic fields utilizing trapped flux in type-II superconductors

We introduce a novel approach to x-ray scattering studies in applied magnetic fields by exploiting vortices in superconductors. This method is based on trapping magnetic flux in a small disk-shaped superconductor (known as a trapped field magnet, TFM) with a single-crystal sample mounted on or at close proximity to its surface. This opens an unrestricted optical access to the sample and allows magnetic fields to be applied precisely along the x-ray momentum transfer, facilitating polarization-sensitive experiments that have been impractical or impossible to perform to date. The TFMs used in our study remain stable and provide practically uniform magnetic fields for days, which are sufficient for comprehensive x-ray diffraction experiments, specifically x-ray resonance exchange scattering (XRES) to study field-induced phenomena at a modern synchrotron source. The TFM instrument has been used in a "proof-of-principle" XRES study of a meta-magnetic phase in a rare-earth compound, TbNi(2)Ge(2), in order to demonstrate its potential.     

A single gold particle as a probe for apertureless scanning near-field optical microscopy

We report on the fabrication, characterization and application of a probe consisting of a single gold nanoparticle for apertureless scanning near-field optical microscopy. Particles with diameters of 100 nm have been successfully and reproducibly mounted at the end of sharp glass fibre tips. We present the first optical images taken with such a probe. We have also recorded plasmon resonances of gold particles and discuss schemes for exploiting the wavelength dependence of their scattering cross-section for a novel form of apertureless scanning near-field optical microscopy.     

A reproducible method for damage-free site-specific preparation of atom probe tips from interfaces

Atom probe tomography (APT) is a mass spectrometry method with atomic-scale spatial resolution that can be used for the investigation of a wide range of materials. The main limiting factor with respect to the type of problems that can be addressed is the small volume investigated and the randomness of common sample preparation methods. With existing site-specific specimen preparation methods it is still challenging to rapidly and reproducibly produce large numbers of successful samples from specifically selected grain boundaries or interfaces for systematic studies. A new method utilizing both focused ion beam (FIB) and transmission electron microscopy (TEM) is presented that can be used to reproducibly produce damage-free atom probe samples with features of interest at any desired orientation with an accuracy of better than 50 nm from samples that require very little prior preparation.     

X-ray excited optical luminescence detection by scanning near-field optical microscope: a new tool for nanoscience

Investigations of complex nanostructured materials used in modern technologies require special experimental techniques able to provide information on the structure and electronic properties of materials with a spatial resolution down to the nanometer scale. We tried to address these needs through the combination of x-ray absorption spectroscopy (XAS) using synchrotron radiation microbeams with scanning near-field optical microscopy (SNOM) detection of the x-ray excited optical luminescence (XEOL) signal. This new instrumentation offers the possibility to carry out a selective structural analysis of the sample surface with the subwavelength spatial resolution determined by the SNOM probe aperture. In addition, the apex of the optical fiber plays the role of a topographic probe, and chemical and topographic mappings can be simultaneously recorded. Our working XAS-SNOM prototype is based on a quartz tuning-fork head mounted on a high stability nanopositioning system; a coated optical fiber tip, operating as a probe in shear-force mode; a detection system coupled with the microscope head control system; and a dedicated software/hardware setup for synchronization of the XEOL signal detection with the synchrotron beamline acquisition system. We illustrate the possibility to obtain an element-specific contrast and to perform nano-XAS experiments by detecting the Zn K and W L(3) absorption edges in luminescent ZnO and mixed ZnWO(4)-ZnO nanostructured thin films.     

Dry fracturing and cutting for techniques for scanning electron microscopy of poly-vinyl chloride particles: Application to internal structure observations

Two methods for examining the internal structure of poly-vinyl chloride (PVC) particles are described. Very small PVC particles, polymerized by the emulsion process, were mounted on an aluminium adhesive tape and pressed with a similar tape. Both tapes were then pulled apart so that the specimen was broken in two fractions, which were observed by scanning electron microscopy. On the other hand, bigger PVC particles manufactured by following the suspension polymerization process, were frozen and hand sectioned with a razor blade under liquid nitrogen vapor. The results were highly satisfactory. These methods were easy to implement, the cost of materials for sample preparation was negligible, and they offered the ability to obtain multiple information from a single sample.     

Controlled microaspiration for high-pressure freezing: a new method for ultrastructural preservation of fragile and sparse tissues for TEM and electron tomography

High‐pressure freezing is the preferred method to prepare thick biological specimens for ultrastructural studies. However, the advantages obtained by this method often prove unattainable for samples that are difficult to handle during the freezing and substitution protocols. Delicate and sparse samples are difficult to manipulate and maintain intact throughout the sequence of freezing, infiltration, embedding and final orientation for sectioning and subsequent transmission electron microscopy. An established approach to surmount these difficulties is the use of cellulose microdialysis tubing to transport the sample. With an inner diameter of 200 μm, the tubing protects small and fragile samples within the thickness constraints of high‐pressure freezing, and the tube ends can be sealed to avoid loss of sample. Importantly, the transparency of the tubing allows optical study of the specimen at different steps in the process. Here, we describe the use of a micromanipulator and microinjection apparatus to handle and position delicate specimens within the tubing. We report two biologically significant examples that benefit from this approach, 3D cultures of mammary epithelial cells and cochlear outer hair cells. We illustrate the potential for correlative light and electron microscopy as well as electron tomography.     

基于Bootstrap方法的小子样试验评估方法研究

Bootstrap方法是一种小子样试验评估方法。该方法在产生随机样本方面有不足之处,即产生的随机样本受到原始样本范围限制。因此本文在研究了用指数分布函数、Boltzm ann函数和三次多项式函数拟合修正样本经验分布函数的可行性之后,讨论用修正的样本经验分布函数替换传统Bootstrap方法中的经验分布函数,提出了基于Bootstrap方法的小子样试验评估方法。结论表明:采用Boltzm ann和三次多项式函数拟合修正样本经验分布函数后可使产生的随机样本不受原始样本范围限制,提高试验评估结果的精度,并附有算例说明所提出的方法。     

Development of probe-to-probe approach method for an independently controlled dual-probe scanning tunneling microscope

We developed a method of fast probe-to-probe approach for an independently controlled dual-probe scanning tunneling microscope (STM), which is essential to measure the transport property of nanostructures, without scanning electron microscopy (SEM). In the approach method, inchworm motors are used as the coarse positioning devices, which are controlled with a personal computer. The method enables an automatic approach of the probe to the other probe within a short time (typically 30 min). After the approach, a real distance between contact points of each probe tip to a sample can be measured from the overlapped part of the STM images obtained with individual probe. The approach method without SEM is also useful to measure the charge transport in the atmosphere, which will be essential for measurement of the bio molecules.     

Controlling tip–sample interaction forces during a single tap for improved topography and mechanical property imaging of soft materials by AFM

We introduce a method that exploits the “active” nature of the force-sensing integrated readout and active tip (FIRAT), a recently introduced atomic force microscopy (AFM) probe, to control the interaction forces during individual tapping events in tapping mode (TM) AFM. In this method the probe tip is actively retracted if the tip–sample interaction force exceeds a user-specified force threshold during a single tap while the tip is still in contact with the surface. The active tip control (ATC) circuitry designed for this method makes it possible to control the repulsive forces and indentation into soft samples, limiting the repulsive forces during the scan while avoiding instability due to attractive forces. We demonstrate the accurate topographical imaging capability of this method on suitable samples that possess both soft and stiff features.     

Structural integrity of polypropylene prosthetic sockets manufactured using the polymer deposition technique

Rapid prototyping (RP) technology has been used recently as a means for automated socket fabrication. Although the technology has proven to be promising and has truly automated the socket fabrication process, the structural integrity of RP sockets remains questionable. For the long term, unsupervised use of these 'unconventional' sockets, their material properties and structural integrity must be determined. This study investigated the structural integrity of polypropylene sockets manufactured using a polymer deposition technique, in which a socket is formed by a continuous strand of partially melted polypropylene that is spirally deposited according to the socket's cross-sectional contour. To investigate the problem of delamination of the socket, the tensile properties of the socket material were determined according to ASTM D638-99. The ultimate tensile strength was found to be approximately 13-23 per cent lower than that of polypropylene sheets that are at present normally used for socket fabrication. In order to improve the load-bearing capacity of the socket, it was reinforced using a double-wall arrangement at the distal region, where failure normally occurs. The structural integrity of the complete prosthesis was then investigated according to ISO 10328 (loading condition II). The prosthesis passed the static loading test registering only 12 mm permanent deformation, and it successfully completed a preliminary cyclic test of 250,000 cycles with no observable failure.     

Combined ultramicrotomy for AFM and TEM using a novel sample holder

A new sample holder that allows combined microtomy for atomic force microscopy (AFM) and transmission electron microscopy (TEM) is described. The main feature of this sample holder is a small central part holding the sample. This central part fits into the head of an atomic force microscope. AFM measurements can be performed with a sample mounted in this central part of the sample holder. This makes the alignment of a microtomed bulk sample unnecessary, and offers the opportunity of an easy and fast combined sample preparation for AFM and TEM.