Search

Molecular junctions exhibit a rich and tunable set of thermal transport phenomena. However, the predicted high thermoelectric efficiencies, phonon quantum interference effects, rectification, and nonlinear heat transport properties of organic molecules are yet to be verified because suitable experimental techniques have been missing. Here, by combining the break junction technique with suspended heat-flux sensors with picowatt per Kelvin sensitivity, we measured the thermal and electrical conductance of single organic molecules at room temperature simultaneously. We used this method to study the thermal transport properties of two model systems, namely, dithiol-oligo(phenylene ethynylene) and octane dithiol junctions with gold electrodes. In agreement with our density functional theory and phase-coherent transport calculations, we show that heat transport across these systems is governed by the phonon mismatch between the molecules and the metallic electrodes. This work represents the first measurement of thermal transport through single molecules and opens new opportunities for studying heat management at the nanoscale level.

We report heat transport measurements on suspended single-layer graphene disks with radius of 150–1600 nm using a high-vacuum scanning thermal microscope. The results of this study revealed a radius-dependent thermal contact resistance between tip and graphene, with values between 1.15 and 1.52 × 108 KW−1. The observed scaling of thermal resistance with radius is interpreted in terms of ballistic phonon transport in suspended graphene discs with radius smaller than 775 nm. In larger suspended graphene discs (radius >775 nm), the thermal resistance increases with radius, which is attributed to in-plane heat transport being limited by phonon–phonon resistive scattering processes, which resulted in a transition from ballistic to diffusive thermal transport. In addition, by simultaneously mapping topography and steady-state heat flux signals between a self-heated scanning probe sensor and graphene with 17 nm thermal spatial resolution, we demonstrated that the surface quality of the suspended graphene and its connectivity with the Si/SiO2 substrate play a determining role in thermal transport. Our approach allows the investigation of heat transport in suspended graphene at sub-micrometre length scales and overcomes major limitations of conventional experimental methods usually caused by extrinsic thermal contact resistances, assumptions on the value of the graphene's optical absorbance and limited thermal spatial resolution.

The search for hard materials to extend the working life of sharp tools is an age-old problem. In recent history, sharp tools must also often withstand high temperatures and harsh chemical environments. Nanotechnology extends this quest to tools such as scanning probe tips that must be sharp on the nanoscale, but still very physically robust. Unfortunately, this combination is inherently contradictory, as mechanically strong, chemically inert materials tend to be difficult to fabricate with nanoscale fidelity. Here a novel process is described, whereby the surfaces of pre-existing, nanoscale Si tips are exposed to carbon ions and then annealed, to form a strong silicon carbide (SiC) layer. The nanoscale sharpness is largely preserved and the tips exhibit a wear resistance that is orders of magnitude greater than that of conventional silicon tips and at least 100-fold higher than that of monolithic, SiO-doped diamond-like-carbon (DLC) tips. The wear is well-described by an atom-by-atom wear model, from which kinetic parameters are extracted that enable the prediction of the long-time scale reliability of the tips. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The atomic force microscope (AFM) tip is often used as a model of a single sliding asperity in order to study nanotribological phenomena including friction, adhesion, and wear. In particular, recent work has demonstrated a wear regime in which surface modification appears to occur in an atom-by-atom fashion. Several authors have modeled this atomic-scale wear behavior as a thermally activated bond breaking process. The present article reviews this body of work in light of concepts from formal transition state theory (also called reaction rate theory). It is found that this framework is viable as one possible description of atomic-scale wear, with impressive agreements to experimental trends found. However, further experimental work is required to fully validate this approach. It is also found that, while the Arrhenius-type equations have been widely used, there is insufficient discussion of or agreement on the specific atomic-scale reaction that is thermally activated, or its dependence on stresses and sliding velocity. Further, lacking a clear picture of the underlying mechanism, a consensus on how to measure or interpret the activation volume and activation energy is yet to emerge. This article makes suggestions for measuring and interpreting such parameters, and provides a picture of one possible thermally activated transition (in its initial, activated, and final states). Finally, directions for further experimental and simulation work are proposed for validating and extending this model and rationally interrogating the behavior of this type of wear. © 2010 Springer Science+Business Media, LLC.