Scanning tunneling microscopy (STM) images of the (3x3) superlattice of benzene and carbon monoxide coadsorbed on the Rh(l 11) surface reveal a well-ordered array of ringlike features associated with individual benzene molecules, while CO is not resolved. Images further show translational domain boundaries, step-edge structures, and evidence for surface diffusion. The origin of the STM image contrast of these molecules and implications for STM imaging of other molecular adsorbates are briefly discussed.PACS numbers: 68.35.Bs, 61.16.Di Scanning tunneling microscopy (STM) has been proven to be a very powerful tool for atomic resolution imaging of surfaces. This technique has been successfully applied to semiconductor and metal surfaces, both with and without atomic adsorbates. l Early results for molecular adsorbates on metal surfaces have been somewhat less encouraging. For example, chemisorbed carbon monoxide molecules have not been resolved on Pt(lOO) surfaces, even though the CO-induced restructuring of the metal surface was observed. 2 Images of Cu phthalocyanine on Ag surfaces 3 showed low symmetry and resolution, which was interpreted in terms of molecular motion induced by the electric field gradients near the tip. The difficulties in imaging of molecular adsorbates have been thought to be due either to rapid surface diffusion, possibly augmented by electric fields, or to the absence of molecular orbitals near the Fermi level (£>), which are accessible by STM.In this Letter, we report the first real-space images of an ordered array of coadsorbed molecules on a clean metal surface. This allows us to study, for the first time, the source of STM image contrast for different molecules under the same experimental conditions. The system studied is the ordered (3x3) superlattice of coadsorbed benzene (C6H6) and CO on a Rh(lll) surface, which has been well characterized by other surface techniques. 4-6 The detailed geometry of this 3x3 overlayer, as determined with a dynamical LEED analysis, 6 is shown in Fig. 1. The unit cell contains one flat-lying benzene molecule and two upright CO molecules, all chemisorbed over hep-type threefold hollow sites which, as opposed to fec-type hollow sites, are directly above second-layer Rh atoms. STM images of this structure reveal individual benzene molecules as threefold ringlike features. Carbon monoxide is not resolved within this structure, as in previous STM work. 2 We analyze the structure of the overlayer at step edges and domain boundaries and find evidence for diffusion of benzene molecules. Our observations are an important step towards real-space imaging of surface-molecule interac-tions.The sample was prepared and analyzed in a surface preparation and analysis chamber connected to the STM chamber by internal transfer mechanisms, which is described elsewhere. 7 The Rh(l 11) sample was cleaned by repeated cycles of 1-keV ion bombardment and heating at 1000°C in the presence of 4xl0" 5 Torr Ar and 8xl0~1 0 Torr O2. The sample was then annealed at 800 °C for 10 min. Azi...

This study investigates the thermal conductivity and viscosity of copper nanoparticles in ethylene glycol. The nanofluid was prepared by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. Volume loadings of up to 2% were prepared. The measured increase in thermal conductivity was twice the value predicted by the Maxwell effective medium theory. The increase in viscosity was about four times of that predicted by the Einstein law of viscosity. Analytical calculations suggest that this nanofluid would not be beneficial as a coolant in heat exchangers without changing the tube diameter. However, increasing the tube diameter to exploit the increased thermal conductivity of the nanofluid can lead to better thermal performance.

Heat conduction mechanisms in nanofluids, fluids seeded with nanoparticles, have been extensively scrutinized in the past decades to explain some experimental observations of their enhanced thermal conductivity beyond the effective medium theory. Although many mechanisms such as Brownian motion, clustering, ballistic transport, and internanoparticle potential are speculated, experimental proof of any of the mechanisms has been difficult. Here, we investigate the mechanisms experimentally by thermal conductivity measurements and structural analysis for the same materials in both liquid and solid states. These studies strongly suggest that clustering holds the key to the thermal conductivity enhancement of nanofluids.

Different from the electrical conductivity of conductive composites, the thermal conductivity usually does not have distinctive percolation characteristics. Here we report that graphite suspensions show distinct behavior in the thermal conductivity at the electrical percolation threshold, including a sharp kink at the percolation threshold, below which thermal conductivity increases rapidly while above which the rate of increase is smaller, contrary to the electrical percolation behavior. Based on microstructural and alternating current impedance spectroscopy studies, we interpret this behavior as a result of the change of interaction forces between graphite flakes when isolated clusters of graphite flakes form percolated structures. Our results shed light on the thermal conductivity enhancement mechanisms in nanofluids and have potential applications in energy systems.