Nanoscale electrical properties measurements

The first and best known near-field technique to measure electrical properties in the nanoscale is of course Scanning Tunnelling Microscopy (STM). Since its invention by Binnig et al., STM has been used to explore the mechanisms of lots of phenomena on surfaces [289-294], ranging from experiments concerning the local work function to the use of an STM-tip to induce electropolymerisation [295]. Most of all, STM provides us with atomically resolved images of the surface structure. 

These characteristics make CP-AFM ideal for studying electrical transport of nanotubes, nanoparticle assemblies, micro- or nanofabricated semiconductor devices, and individual molecules. Detailed appraisal of these characterizations can be obtained by comparing CP-AFM and STM. Although CP-AFM and STM share high spatial resolution imaging capability (STM 0.1 mn CP-AFM -10 nm, due to larger tip apex) that is critical in linking nanoscale structure to transport properties, an important distinction is the position of the tip with respect to the sample. In the case of CP-AFM, a metal-coated tip is directly contacted to the sample under a controlled load. This means that the measured I V relationship is mainly affected by the electrical properties of the tip-sample contact. 

The main advantages of scanning probe microscopy, on the other hand, are operation modes analyzing simultaneously the local morphology of the photoactive layer and measuring functional (optical and electrical) properties with nanometer resolution at the same spot. Such information is mandatory to better understand operation of PSCs at the nanoscale and to bring such devices to a new level of ultimate performance. 

Sources of current measurements (4) S.J. Tans etal. Individual single-wall carbon nanotubes as quantum wires, Nature, 386, 474 77, 1997 (5) M.A. Reed, Electrical Properties of Molecular Devices, presented at 1997 DARPA ULTRA Program Review Conference, Santa Fe, NM, October, 1997 (6) M.A. Reed, Molecular-scale electronics, Proc. IEEE, 87, 652-658, 1999 (7) C. Thou, M.R. Deshpande, M.A. Reed, and J.M. Tour, Nanoscale metal/self-assembled monolayer/metal heterostructures, Appl. Phys. Lett., 71, 611-613, 1997 (8) C. Zhou, Atomic and Molecular Wires, Ph.D. dissertation, Yale University, 1999. With permission. 

The above measurements all rely on force and displacement data to evaluate adhesion and mechanical properties. As mentioned in the introduction, a very useful piece of information to have about a nanoscale contact would be its area (or radius). Since the scale of the contacts is below the optical limit, the techniques available are somewhat limited. Electrical resistance has been used in early contact studies on clean metal surfaces [62], but is limited to conducting interfaces. Recently, Enachescu et al. [63] used conductance measurements to examine adhesion in an ideally hard contact (diamond vs. tungsten carbide). In the limit of contact size below the electronic mean free path, but above that of quantized conductance, the contact area scales linearly with contact conductance. They used these measurements to demonstrate that friction was proportional to contact area, and the area vs. load data were best-fit to a DMT model. 

There is great interest in the electrical and optical properties of materials confined within small particles known as nanoparticles. These are materials made up of clusters (of atoms or molecules) that are small enough to have material properties very different from the bulk. Most of the atoms or molecules are near the surface and have different environments from those in the interior—indeed, the properties vary with the nanoparticle s actual size. These are key players in what is hoped to be the nanoscience revolution. There is still very active work to learn how to make nanoscale particles of defined size and composition, to measure their properties, and to understand how their special properties depend on particle size. One vision of this revolution includes the possibility of making tiny machines that can imitate many of the processes we see in single-cell organisms, that possess much of the information content of biological systems, and that have the ability to form tiny computer components and enable the design of much faster computers. However, like truisms of the past, nanoparticles are such an unknown area of chemical materials that predictions of their possible uses will evolve and expand rapidly in the future. 

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