Scanning Thermal Microscopy and Ballistic Phonon Transport in Lateral Spin Valves.

Using scanning thermal microscopy, we have mapped the spatial distribution of temperatures in an operating nanoscale device formed from a magnetic injector, an Ag connecting wire, and a magnetic detector. An analytical model explained the thermal diffusion over the measured temperature range (2-300 K) and injector-detector separation (400-3000 nm). The characteristic diffusion lengths of the Peltier and Joule heat differ remarkably below 60 K, a fact that can be explained by the onset of ballistic phonon heat transfer in the substrate.

Thermal and electrical signatures of a hydrodynamic electron fluid in tungsten diphosphide.

In stark contrast to ordinary metals, in materials in which electrons strongly interact with each other or with phonons, electron transport is thought to resemble the flow of viscous fluids. Despite their differences, it is predicted that transport in both conventional and correlated materials is fundamentally limited by the uncertainty principle applied to energy dissipation. Here we report the observation of experimental signatures of hydrodynamic electron flow in the Weyl semimetal tungsten diphosphide. Using thermal and magneto-electric transport experiments, we find indications of the transition from a conventional metallic state at higher temperatures to a hydrodynamic electron fluid below 20 K. The hydrodynamic regime is characterized by a viscosity-induced dependence of the electrical resistivity on the sample width and by a strong violation of the Wiedemann-Franz law. Following the uncertainty principle, both electrical and thermal transport are bound by the quantum indeterminacy, independent of the underlying transport regime.

Length-dependent thermal transport along molecular chains.

We present heat-transport measurements conducted with a vacuum-operated scanning thermal microscope to study the thermal conductance of monolayers of nine different alkane thiols self-assembled on Au(111) surfaces as a function of their length (2 to 18 methylene units). The molecular thermal conductance is probed in a confined area with a diameter below 10 nm in the contact between a silicon tip and the self-assembled monolayer. This yields a pWK(-1) sensitivity per molecule at a tip temperature of 200-300 °C versus the gold at room temperature. We found a conductance variance of up to a factor of 3 as a function of alkane chain length, with maximum conductance for a chain length of four carbon atoms.

Full thermoelectric characterization of InAs nanowires using MEMS heater/sensors.

Precise measurements of a complete set of thermoelectric parameters on a single indium-arsenide nanowire (NW) have been performed using highly sensitive, micro-fabricated sensing devices based on the heater/sensor principle. The devices were fabricated as micro electro-mechanical systems consisting of silicon nitride membranes structured with resistive gold heaters/sensors. Preparation, operation and characterization of the devices are described in detail. Thermal decoupling of the heater/sensor platforms has been optimized reaching thermal conductances as low as 20 nW K(-1) with a measurements sensitivity below 20 nW K(-1). The InAs NWs were characterized in terms of thermal conductance, four-probe electrical conductance and thermopower (Seebeck coefficient), all measured on a single NW. The temperature dependence of the parameters determining the thermoelectric figure-of-merit of an InAs NW was acquired in the range 200-350 K featuring a minor decrease of the thermal conductivity from 2.7 W (m K)(-1) to 2.3 W (m K)(-1).

Inducing a direct-to-pseudodirect bandgap transition in wurtzite GaAs nanowires with uniaxial stress.

Many efficient light-emitting devices and photodetectors are based on semiconductors with, respectively, a direct or indirect bandgap configuration. The less known pseudodirect bandgap configuration can be found in wurtzite (WZ) semiconductors: here electron and hole wave-functions overlap strongly but optical transitions between these states are impaired by symmetry. Switching between bandgap configurations would enable novel photonic applications but large anisotropic strain is normally needed to induce such band structure transitions. Here we show that the luminescence of WZ GaAs nanowires can be switched on and off, by inducing a reversible direct-to-pseudodirect band structure transition, under the influence of a small uniaxial stress. For the first time, we clarify the band structure of WZ GaAs, providing a conclusive picture of the energy and symmetry of the electronic states. We envisage a new generation of devices that can simultaneously serve as efficient light emitters and photodetectors by leveraging the strain degree of freedom.

Quantized thermal transport across contacts of rough surfaces.

Heat transport across interfaces is often discussed in terms of the transmission probability of the heat-carrying phonons through the contact zone. Although interface roughness influences the true contact area and affects phonon scattering within the contact zone, its effect on nanoscale heat transport remains poorly understood. Here, we report experimental data on the pressure dependence of thermal transport across polished nanoscale contacts. The data can be quantitatively explained by a model of thermal conductance across interfaces that incorporates the effect of nanoscale roughness through the quantized thermal conductance across individual atomic-scale contacts within the contact zone.

Probe based surface modification of polymers below 30 nm pitch.

Heated probes are used to modify the surface of polymeric thin films by thermomechanical indentation and local evaporation of material. The resolution of the processes is discussed for probe-storage and surface patterning-applications. As storage densities exceed 1 Tbit/in2, the depth of the indents becomes comparable to the natural surface roughness of the polymer. By templating an atomically flat surface this limitation can be overcome, enabling storage densities of up to 4 Tbit/in2, corresponding to an indentation half-pitch of 7.5 nm.

Wear-less floating contact imaging of polymer surfaces.

An atomic force microscopy (AFM) technique is described combining two operating modes that previously were mutually exclusive: gentle imaging of delicate surfaces requiring slow dynamic AFM techniques, and passive feedback contact mode AFM enabling ultra-fast imaging. A high-frequency force modulation is used to excite resonant modes in the MHz range of a highly compliant cantilever force sensor with a spring constant of 0.1 N m( - 1). The high-order mode acts as a stiff system for modulating the tip-sample distance and a vibration amplitude of 1 nm is sufficient to overcome the adhesion interaction. The soft cantilever provides a force-controlled support for the vibrating tip, enabling high-speed intermittent contact force microscopy without feedback control of the cantilever bending. Using this technique, we were able to image delicate polymer surfaces and to completely suppress the formation of the ripple wear patterns that are commonly observed in contact AFM.

Multi Tbit/in(2) storage densities with thermomechanical probes.

Exploiting the spatial resolution of scanning probes presents an attractive approach for novel data storage technologies in particular for large-scale data repositories because of their inherent potential for high storage density. We show that multi-Tbit/in(2) density can be achieved by means of thermomechanically embossing the information as indentation marks into a polymer film. The data density is determined by the nonlinear interaction between closely spaced indents and the fundamental scaling relations governing the shape and size of the indents. We find that cooperative effects in polymers give rise to a minimum indentation radius on the order of the correlation length of the cooperatively rearranged region even if formed by an infinitely sharp indenter. Thus, cooperativity coupled to alpha-transitions in polymers is evinced in a real space geometrical experiment. Furthermore, we predict that indentation marks cannot be made smaller than 5 nm in diameter, which limits the feature resolution for embossing technologies in general.

Relaxation kinetics of nanoscale indents in a polymer glass.

Nanometer scale indents have been written in a cross-linked polystyrene sample, and their relaxation has been studied at annealing temperatures well below the glass transition of the polymer. The indents represent a highly nonequilibrium state of the polymer which is subjected to mechanical stress of up to 0.4 GPa and thermal quench rates on the order of 10{8} K/s during writing. It is shown that the relaxation towards equilibrium evolves logarithmically over more than 10 orders of magnitude in time. The relaxation kinetics are accurately described in terms of a thermally activated process with an energy barrier whose magnitude decreases linearly with the distance from equilibrium.

Nanoscale shape-memory function in highly cross-linked polymers.

Topographic engraving of structures in polymer surfaces attracts widespread interest for application in imprint lithography and data storage. We study the nonlinear interaction of nanoindents written in close proximity, 20-100 nm, to one another in a highly cross-linked polystyrene matrix. The indents are created thermomechanically by applying heat and force stimuli of 10 micros duration to a tip, thereby raising the polymer temperature to 250 degrees C and exerting contact pressures of up to 1 GPa. We show that on the nanoscale plastic deformation is highly reversible providing outstanding shape-memory functionality of the material.

Self-similarity and finite-size effects in nano-indentation of highly cross-linked polymers.

The scalability of thermomechanical polymer deformations in the sub-10 nm regime is of particular importance for nano-imprint techniques, hardness measurements of thin films by nano-indentations, and scanning-probe-based thermomechanical data storage. We investigate nano-indentation in the sub-10 nm regime performed on highly cross-linked polymer films of different thicknesses. It is shown that the lateral and vertical geometric characteristics of the indents independently scale down to an indent depth of 1 nm and that the scaling parameters are functions of the film thickness and the temperature of the indenter. However, in the limit of shallow indents the scaling of the cross-coupling between lateral and vertical dimensions is lost. It is argued that the breakdown of self-similarity is due to a minimum strain requirement originating from the co-operative nature of the polymer response induced by α transitions which lock the indent in the deformed state. The results shed new light on the fundamental processes and size effects involved in nanoscale plastic replication, in general.

Nanowear on polymer films of different architecture.

In this paper, we describe atomic force microscope (AFM) friction experiments on different polymers. The aim was to analyze the influence of the physical architecture of the polymer on the degree and mode of wear and on the wear mode. Experiments were carried out with (1) linear polystyrene (PS) and cycloolefinic copolymers of ethylene and norbornene, which are stabilized by entanglements, (2) mechanically stretched PS, (3) polyisoprene-b-polystyrene diblock copolymers, with varying composition, (4) brush polymers consisting of a poly(methyl methacrylate) (PMMA) backbone and PS side chains, (5) PMMA and PS brushes grafted from a silicon wafer, (6) plasma-polymerized PS, and (7) chemically cross-linked polycarbonate. For linear polymers, wear depends critically on the orientation of the chains with respect to the scan direction. With increasing cross-link density, wear was reduced and ripple formation was suppressed. The cross-linking density was the dominating material parameter characterizing wear.

Dynamic force spectroscopy of conservative and dissipative forces in an Al-Au(111) tip-sample system.

The conservative and dissipative interaction between an aluminum tip and a gold (111) surface were investigated using dynamic force spectroscopy in UHV. Complete force vs distance curves and friction coefficient vs distance curves were obtained quantitatively. The force curves were compared to the model by Muller, Yushenko, and Derjaguin, and long and short range interactions were subsequently quantified without fit parameters. A short range conservative interaction was separated from longer range van der Waals forces. The long range behavior of the damping coefficient obeys an inverse power law of third order.