Current Issue : July - September Volume : 2012 Issue Number : 3 Articles : 6 Articles
Nanoelectromechanical systems are characterized by an intimate connection between electronic and mechanical degrees of\r\nfreedom. Due to the nanoscopic scale, current flowing through the system noticeably impacts upons the vibrational dynamics of the\r\ndevice, complementing the effect of the vibrational modes on the electronic dynamics. We employ the scattering-matrix approach\r\nto quantum transport in order to develop a unified theory of nanoelectromechanical systems out of equilibrium. For a slow mechanical\r\nmode the current can be obtained from the Landauerââ?¬â??BÃ?¼ttiker formula in the strictly adiabatic limit. The leading correction to\r\nthe adiabatic limit reduces to Brouwerââ?¬â?¢s formula for the current of a quantum pump in the absence of a bias voltage. The principal\r\nresults of the present paper are the scattering-matrix expressions for the current-induced forces acting on the mechanical degrees of\r\nfreedom. These forces control the Langevin dynamics of the mechanical modes. Specifically, we derive expressions for the (typically\r\nnonconservative) mean force, for the (possibly negative) damping force, an effective ââ?¬Å?Lorentzââ?¬Â force that exists even for timereversal-\r\ninvariant systems, and the fluctuating Langevin force originating from Nyquist and shot noise of the current flow. We\r\napply our general formalism to several simple models that illustrate the peculiar nature of the current-induced forces. Specifically,\r\nwe find that in out-of-equilibrium situations the current-induced forces can destabilize the mechanical vibrations and cause limitcycle\r\ndynamics....
Background: Noncontact atomic force microscopy (NC-AFM) now regularly produces atomic-resolution images on a wide range\r\nof surfaces, and has demonstrated the capability for atomic manipulation solely using chemical forces. Nonetheless, the role of the\r\ntip apex in both imaging and manipulation remains poorly understood and is an active area of research both experimentally and\r\ntheoretically. Recent work employing specially functionalised tips has provided additional impetus to elucidating the role of the tip\r\napex in the observed contrast.\r\nResults: We present an analysis of the influence of the tip apex during imaging of the Si(100) substrate in ultra-high vacuum\r\n(UHV) at 5 K using a qPlus sensor for noncontact atomic force microscopy (NC-AFM). Data demonstrating stable imaging with a\r\nrange of tip apexes, each with a characteristic imaging signature, have been acquired. By imaging at close to zero applied bias we\r\neliminate the influence of tunnel current on the force between tip and surface, and also the tunnel-current-induced excitation of\r\nsilicon dimers, which is a key issue in scanning probe studies of Si(100).\r\nConclusion: A wide range of novel imaging mechanisms are demonstrated on the Si(100) surface, which can only be explained by\r\nvariations in the precise structural configuration at the apex of the tip. Such images provide a valuable resource for theoreticians\r\nworking on the development of realistic tip structures for NC-AFM simulations. Force spectroscopy measurements show that the\r\ntip termination critically affects both the short-range force and dissipated energy....
Self-assembled monolayers (SAMs) of 4''-methylbiphenyl-4-thiol (MBP0) adsorbed on polycrystalline gold substrates served as\r\ntemplates to control electrochemical deposition of Cu structures from acidic solution, and enabled the subsequent lift-off of the\r\nmetal structures by attachment to epoxy glue. By exploiting the negative-resist behaviour of MBP0, the SAM was patterned by\r\nmeans of electron-beam lithography. For high deposition contrast a two-step procedure was employed involving a nucleation phase\r\naround 0.7 V versus Cu2+/Cu and a growth phase at around 0.35 V versus Cu2+/Cu. Structures with features down to 100 nm\r\nwere deposited and transferred with high fidelity. By using substrates with different surface morphologies, AFM measurements\r\nrevealed that the roughness of the substrate is a crucial factor but not the only one determining the roughness of the copper surface\r\nthat is exposed after lift-off....
Plant surfaces showing hierarchical structuring are frequently found in plant organs such as leaves, petals, fruits and stems. In our\r\nstudy we focus on the level of cell shape and on the level of superimposed microstructuring, leading to hierarchical surfaces if both\r\nlevels are present. While it has been shown that epicuticular wax crystals and cuticular folds strongly reduce insect attachment, and\r\nthat smooth papillate epidermal cells in petals improve the grip of pollinators, the impact of hierarchical surface structuring of plant\r\nsurfaces possessing convex or papillate cells on insect attachment remains unclear. We performed traction experiments with male\r\nColorado potato beetles on nine different plant surfaces with different structures. The selected plant surfaces showed epidermal\r\ncells with either tabular, convex or papillate cell shape, covered either with flat films of wax, epicuticular wax crystals or with cuticular\r\nfolds. On surfaces possessing either superimposed wax crystals or cuticular folds we found traction forces to be almost one\r\norder of magnitude lower than on surfaces covered only with flat films of wax. Independent of superimposed microstructures we\r\nfound that convex and papillate epidermal cell shapes slightly enhance the attachment ability of the beetles. Thus, in plant surfaces,\r\ncell shape and superimposed microstructuring yield contrary effects on the attachment of the Colorado potato beetle, with convex or\r\npapillate cells enhancing attachment and both wax crystals or cuticular folds reducing attachment. However, the overall magnitude\r\nof traction force mainly depends on the presence or absence of superimposed microstructuring....
Noncontact atomic force microscopy provides access to several complementary signals, such as topography, damping, and contact\r\npotential. The traditional presentation of such data sets in adjacent figures or in colour-coded pseudo-three-dimensional plots gives\r\nonly a qualitative impression. We introduce two-dimensional histograms for the representation of multichannel NC-AFM data sets\r\nin a quantitative fashion. Presentation and analysis are exemplified for topography and contact-potential data for graphene grown\r\nepitaxially on 6H-SiC(0001), as recorded by Kelvin probe force microscopy in ultrahigh vacuum. Sample preparations by thermal\r\ndecomposition in ultrahigh vacuum and in an argon atmosphere are compared and the respective growth mechanisms discussed....
Particle lithography offers generic capabilities for the high-throughput fabrication of nanopatterns from organosilane self-assembled\r\nmonolayers, which offers the opportunity to study surface-based chemical reactions at the molecular level. Nanopatterns of\r\noctadecyltrichlorosilane (OTS) were prepared on surfaces of Si(111) using designed protocols of particle lithography combined\r\nwith either vapor deposition, immersion, or contact printing. Changing the physical approaches for applying molecules to masked\r\nsurfaces produced OTS nanostructures with different shapes and heights. Ring nanostructures, nanodots and uncovered pores of\r\nOTS were prepared using three protocols, with OTS surface coverage ranging from 10% to 85%. Thickness measurements from\r\nAFM cursor profiles were used to evaluate the orientation and density of the OTS nanostructures. Differences in the thickness and\r\nmorphology of the OTS nanostructures are disclosed based on atomic force microscopy (AFM) images. Images of OTS nanostructures\r\nprepared on Si(111) that were generated by the different approaches provide insight into the self-assembly mechanism of\r\nOTS, and particularly into the role of water and solvents in hydrolysis and silanation....
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