Thermoelectric Limitations of Graphene Nanodevices at Ultrahigh Current Densities.

Graphene is atomically thin, possesses excellent thermal conductivity, and is able to withstand high current densities, making it attractive for many nanoscale applications such as field-effect transistors, interconnects, and thermal management layers. Enabling integration of graphene into such devices requires nanostructuring, which can have a drastic impact on the self-heating properties, in particular at high current densities. Here, we use a combination of scanning thermal microscopy, finite element thermal analysis, and operando scanning transmission electron microscopy techniques to observe prototype graphene devices in operation and gain a deeper understanding of the role of geometry and interfaces during high current density operation. We find that Peltier effects significantly influence the operational limit due to local electrical and thermal interfacial effects, causing asymmetric temperature distribution in the device. Thus, our results indicate that a proper understanding and design of graphene devices must include consideration of the surrounding materials, interfaces, and geometry. Leveraging these aspects provides opportunities for engineered extreme operation devices

Nanoscale Visualization of Elastic Inhomogeneities at TiN Coatings Using Ultrasonic Force Microscopy

Ultrasonic force microscopy has been applied to the characterization of titanium nitride coatings deposited by physical vapor deposition dc magnetron sputtering on stainless steel substrates. The titanium nitride layers exhibit a rich variety of elastic contrast in the ultrasonic force microscopy images. Nanoscale inhomogeneities in stiffness on the titanium nitride films have been attributed to softer substoichiometric titanium nitride species and/or trapped subsurface gas. The results show that increasing the sputtering power at the Ti cathode increases the elastic homogeneity of the titanium nitride layers on the nanometer scale. Ultrasonic force microscopy elastic mapping on titanium nitride layers demonstrates the capability of the technique to provide information of high value for the engineering of improved coatings

AFM and UFM surface characterization of rubber-toughened poly(methyl methacrylate) samples

The microstructure of a series of injection-molded and extruded rubber-toughened poly(methyl methacrylate) (RTPMMA) samples was investigated. Atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) were used to study surface topography and local elastic properties. AFM topography measurements combined with UFM can reveal the distribution and orientation of the rubber particles in the PMMA matrix. UFM, in particular, reveals the core-shell structure of the particles as well as the presence of particles immediately under the surface, otherwise invisible. In some cases the particles appear to be covered by a thin PMMA layer, whereas in other cases they appear to have broken, thereby exposing parts of their internal structure. Generally, the particles are elongated in the skin region of the injection-molded samples. On the other hand, the particles in the surface region of the extruded samples appear to be almost spherical. The observed difference is attributed to the fountain flow phenomenon, which takes place during injection molding. © 2001 John Wiley and Sons, Inc. J Appl Polym Sci

Ultrasound induced lubricity in microscopic contact

A physical effect of ultrasound induced lubricity is reported. We studied the dynamic friction dependence on out-of-plane ultrasonic vibration of a sample using friction force microscopy and a scanning probe technique, the ultrasonic force microscope, which can probe the dynamics of the tip-sample elastic contact at a submicrosecond scale. The results show that friction vanishes when the tip-surface contact breaks for part of the out-of-plane vibration cycle. Moreover, the friction force reduces well before such a break, and this reduction does not depend on the normal load. This suggests the presence on the surface of a layer with viscoelastic behavior. (C) 1997 American Institute of Physics

Evaluation Of Polishing Damage In Alumina

Grazing incidence X-ray scattering and line focus acoustic microscopy have been applied to the study of grinding, lapping and polishing processes on alumina. Changes as a function of polishing time ill the near-surface density, measured from the critical angle for total external reflection of X-rays, were related to changes in the beating area and surface morphology. No relation was found between the micrometre-scale roughness measured by a stylus instrument and the integrated diffuse scatter, the latter being related to the nanometre-scale roughness on the top of the polished mesas remaining after grain pull-out during the grinding and lapping process. The surface acoustic wave velocity was also found to vary with polishing time. These changes cannot be ascribed to changes in roughness and are believed to measure changes in the residual crack density

Efficient heating of single-molecule junctions for thermoelectric studies at cryogenic temperatures

The energy dependent thermoelectric response of a single molecule contains valuable information about its transmission function and its excited states. However, measuring it requires devices that can efficiently heat up one side of the molecule while being able to tune its electrochemical potential over a wide energy range. Furthermore, to increase junction stability, devices need to operate at cryogenic temperatures. In this work, we report on a device architecture to study the thermoelectric properties and the conductance of single molecules simultaneously over a wide energy range. We employ a sample heater in direct contact with the metallic electrodes contacting the single molecule which allows us to apply temperature biases up to DT ¼ 60 K with minimal heating of the molecular junction. This makes these devices compatible with base temperatures Tbath < 2 K and enables studies in the linear (DT Tmolecule) and nonlinear (DT Tmolecule) thermoelectric transport regimes

Ultrasonic force microscopies

Ultrasonic Force Microscopy, or UFM, allows combination of two apparently mutually exclusive requirements for the nanomechanical probe—high stiffness for the efficient indentation and high mechanical compliance that brings force sensitivity. Somewhat inventively, UFM allows to combine these two virtues in the same cantilever by using indention of the sample at high frequency, when cantilever is very rigid, but detecting the result of this indention at much lower frequency. That is made possible due to the extreme nonlinearity of the nanoscale tip-surface junction force-distance dependence, that acts as “mechanical diode” detecting ultrasound in AFM. After introducing UFM principles, we discuss features of experimental UFM implementation, and the theory of contrast in this mode, progressing to quantitative measurements of contact stiffness. A variety of UFM applications ranging from semiconductor quantum nanostructures, very large scale integrated circuits, and reinforced ceramics to polymer composites and biological materials is presented via comprehensive imaging gallery accompanied by the guidance for the optimal UFM measurements of these materials. We also address effects of adhesion and topography on the elasticity imaging and the approaches for reducing artefacts connected with these effects. This is complemented by another extremely useful feature of UFM ultrasound induced superlubricity that allows damage free imaging of materials ranging from stiff solid state devices and graphene to biological materials. Finally, we proceed to the exploration of time-resolved nanoscale phenomena using nonlinear mixing of multiple vibration frequencies in ultrasonic AFM—Heterodyne Force Microscopy, or HFM, that also include mixing of ultrasonic vibration with other periodic physical excitations, eg. electrical, photothermal, etc. Significant section of the chapter analyses the ability of UFM and HFM to detect subsurface mechanical inhomogeneities, as well as describes related sample preparation methods on the example of subsurface imaging of nanostructures and iii–v quantum dots

Heparan sulfates upregulate regeneration of transected sciatic nerves of adult guinea pigs

An acoustic microscope has been proven to be a very effective tool for visualization and characterization of small internal defects in solids[l]. The distinction of internal defects such as cracks and voids from solid inclusions is sometimes necessary for material evaluation. For example in case of light metal casting alloys ultrasonic scattered echo from pores and heavy metal inclusions used for strengthening purposes can give the ultrasonic signal of the same order of magnitude [2]. In this paper it is shown how the phase information of the reflected echo can be used to distinguish void signals from solid inclusion signals. Conventional acoustic imaging techniques that use only amplitude information and ignores the phase information can not distinguish between voids and inclusions