We introduce a variable-height scanning tunneling spectroscopy ͑VH-STS͒ method that provides a good level of recovery of the combined surface and tip density of states ͑DOS͒ in any bias range without the complexity of previous methods. The combined local electron DOS ͑LDOS͒ recovery was performed using a simplified algorithm based on the one-dimensional Wentzel-Kramers-Brillouin approximation already known in the literature. On the basis of this VH-STS approach we derive three separate methods for the determination of tunnel barrier height and absolute tip-surface separation, and which are critical to enabling LDOS recovery. We report experimental results on polycrystalline-Pt and Si͑100͒ surfaces using this scheme. Experimental and simulated spectra are compared to investigate the limits of the various methods and to demonstrate that this approach can be applied successfully to all kind of probes and surfaces.

We demonstrate how the local density of electronic states evolves as the size and dimensionality of surface dangling bond nanowires are modified. These wires were fabricated using the probe of a scanning tunneling microscope on a hydrogen passivated n-type Si(100)-(2 × 1) surface. We demonstrate that by varying the number and arrangement of dangling bonds on the surface it is possible to arbitrarily engineer the electronic characteristic of a surface nanowire from that of a semiconductor with a controllable band gap to that of a metal.

Precision current measurements are recorded at 5 K during the approach and contact between a Pt-inked probe and the carbon-carbon double-bond region of an isolated 1,3-cyclohexadiene molecule chemisorbed on a Si(100) surface. Scanning tunneling spectroscopic data reveal systematic features in the current at specific probe-molecule separations. Aided by density functional theory calculations, we show that these features arise from interaction forces between the probe and molecule, which can be interpreted as the relaxation of the probe-molecule system prior to and during contact. DOI: 10.1103/PhysRevLett.97.098304 PACS numbers: 82.37.Gk, 39.25.+k, 68.43.ÿh, 81.16.Ta There is enormous interest in the potential use of molecules in future electronic device technologies [1][2][3]. Although molecular electronics has the potential for the highest possible integration densities, the major difficulty in advancing this technology is establishing reliable contacts with molecules [1,4]. Not only is the influence of the contacting metal not well understood but the manner in which the contact perturbs the properties of molecules is a complete unknown [2,5], and precludes the rational design of molecular devices. In this Letter we introduce a novel STM-based method to measure the contact forces and interactions between a metallic probe and a single molecule. We show that, for a Pt-metal-''inked'' STM probe and a 1,3-cyclohexadiene (1,3-CHD) molecule on the Si(100) surface, there is a repulsive barrier at large separations followed by an attractive interaction associated with contact and chemical bond formation. This method is quite general and applicable to a wide range of molecular systems.To study the interaction between a single molecule and a STM probe it is necessary to develop a reliable method to control the chemical composition of the probe. This is accomplished by placing a single crystal metal sample [in this case Pt(111)] and the Si(100) substrate s[n -type As, <5 m cm] together in a sample holder that allows each to be heated and prepared. Each sample was then characterized by LEED, exposed at room temperature to less than 0.1% of a monolayer of 1,3-CHD, cooled to 80 K, transferred into a Createc LT-STM and cooled to 5 K for imaging [see Figs. 1(a) and 1(b)] and spectroscopic analysis [see Figs. 1(c)-1(e)]. Tungsten STM probes were cleaned by electron bombardment and then inked by drawing Pt atoms from the surface onto the end of the probe (see below). Figure 1(c) shows that the current increases exponentially during the approach to the Pt(111) surface, after which there is a sudden jump associated with contact formation [6 -8]. The exponential behavior of the current I follows the well-known distance dependence of the tunneling current [9-11]:where A is the apparent tunneling barrier height, Z is the probe-surface separation, and A 2 8m e 1=2 =h. The Pt surface is locally melted in the contact area and when the probe is withdrawn there is a clear hysteresis in the current due to the formation of a Pt metal nec...

We demonstrate the use of nonlocal scanning tunneling spectroscopic measurements to characterize the local structure of adspecies in their states where they are significantly less perturbed by the probe, which is accomplished by mapping the amplitude and phase of the scattered surface charge density. As an example, we study single-H-atom adsorption on the n-type Si(100)-(4 × 2) surface, and demonstrate the existence of two different configurations that are distinguishable using the nonlocal approach and successfully corroborated by density functional theory.

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