The first order curvature correction to the surface tension of a drop is given by Tolman’s length. Up until now, no accurate estimates of this length existed for realistic fluids. Recently Blokhuis and Bedeaux proposed a new relation that expresses Tolman’s length as an integral over the pair distribution function of a planar liquid–vapor interface. We have used this relation to obtain estimates of Tolman’s length from molecular dynamics simulations of a Lennard-Jones liquid–vapor interface. We found it to be positive and small for a range of temperatures running from the triple-point temperature to the critical temperature.
Room-temperature deposited submonolayers of silicon on Si͑001͒ are investigated using STM. The observed structures and the mechanisms leading to their formation are discussed. Isolated ad-dimers in different geometries are described and a kinetic model for their formation is deduced. It is shown how further growth occurs via the formation of 3-atom clusters, which act as nucleation centers for the formation of two types of linear structures. One of the line types is formed in the ͓110͔ direction, and has been observed before. The other is in the ͓310͔ direction. At a coverage of nearly 0.2 ML a kind of random network consisting of segments of the two types of atomic lines is formed. Above 0.2 ML coverage these lines are converted into epitaxial dimer rows. A pathway for this conversion is proposed on the basis of experimental observations.
We investigate by first-principles molecular dynamics the structural properties of liquid GeSe 4 , i.e., Ge x Se 1Ϫx at xϭ0.2. This composition is very close to the so-called stiffness threshold composition, at which dramatic changes in a series of experimental properties occur. The calculated total neutron structure factor is in very good agreement with experiment. The results show that liquid GeSe 4 is a good prototype of a chemically ordered network. It consists of GeSe 4 tetrahedra that are connected by either shared Se atoms or Se chains.
The interactions between adsorbed Si dimers on Si͑001͒ have been studied using scanning tunneling microscopy. These interactions determine the formation of clusters from diffusing dimers. We show that by increasing the tip-sample voltage, we induce transitions between the clusters. These transitions are used to clarify the dimer-dimer interactions and to determine the pathway for the formation of multiple-dimer clusters. ͓S0163-1829͑96͒01628-1͔Homoepitaxial growth on Si͑001͒ is a good model system for studying epitaxial growth of semiconductors. A large amount of information about the statics and diffusion of adatoms and adsorbed dimers on Si͑001͒ is known. The diffusion of isolated adatoms has been calculated with model potentials 1 and with ab initio methods. 2,3 These calculations predict the adatom mobility to be so high that scanning tunneling microscopy ͑STM͒ measurements cannot reveal isolated adatoms at room temperature. This is consistent with the fact that single adatoms have only been observed at 160 K. 4 Adsorbed dimers are observed to be immobile at room temperature and for observing their diffusion with STM the sample temperature has to be raised to about 340 K.5 Recently the observation of rotations of adsorbed dimers at room temperature has been reported, 6,7 in accordance with predictions on the basis of ab initio calculations. 8 The dynamic behavior of adsorbed dimers and especially the interactions between them are crucial for the formation of larger clusters from diffusing dimers. However, experimental observations of the processes leading to multiple-dimer clusters are not yet available.This report describes the interactions between Si dimers adsorbed on top of the substrate dimer rows of a Si͑001͒ surface. These interactions have been deduced from STM observations of transitions between multiple-dimer clusters. We start with a discussion of the rotations of isolated dimers on top of the dimer rows, as it will turn out that these elementary transitions are useful for understanding the more complex transitions in multiple-dimer clusters. Our observations reveal a pronounced tip-sample voltage dependence of the rotation activity of these isolated dimers. This field enhancement of transitions is used as a tool for studying transitions in multiple-dimer clusters. It enables us not only to determine the structures, it also gives insight about how they are formed. It is shown that the interaction of two dimers on top of two neighboring substrate dimer rows can yield two different tetramers. Observations are presented to demonstrate that the interactions in one of these tetramers can be extended to bind more dimers, thus revealing the formation process of large linear clusters.Experiments are performed in an UHV system with a base pressure of about 5ϫ10 Ϫ11 Torr. Silicon ͑001͒ surfaces are prepared by flashing to 1250°C, yielding the (2ϫ1) reconstructed surface with monolayer height steps. Silicon is deposited at room temperature from a commercial miniature electron-beam evaporator. A commercial Be...
The stability and electronic structure of a nanowire are studied by first-principles calculations. The wire consists of a single depassivated silicon dimer row on the hydrogen passivated Si͑001͒ 2ϫ1 surface. We predict that sodium atoms evaporated onto this surface stick preferentially at the depassivated row and partially fill the empty one-dimensional states of this row. This leads to a thin metallic wire of atomic size dimensions. At room temperature the sodium atoms are mobile along the depassivated row; they become immobile at temperatures below ϳ120 K. ͓S0163-1829͑97͒50328-6͔Nanowires have attracted considerable attention as such small one-dimensional ͑1D͒ structures are expected to exhibit intriguing physical properties such as a quantized conductance or a transition to a Tomonaga-Luttinger liquid at low temperatures.1 In recent years rapid progress has been made in manipulating atoms with the scanning tunneling microscope ͑STM͒, which makes it possible to build 1D structures atom by atom.2 On metallic substrates the 1D electronic states of such structures are usually strongly mixed with the bulk metal states. In order to study unperturbed 1D effects, one would therefore prefer to use semiconducting ͑or insulating͒ substrates. Recent examples of stable 1D structures on semiconductor surfaces constructed using STM techniques, are atomic scale grooves on the Si͑111͒ surface 3 and depassivated single dimer rows on the 2ϫ1 monohydride Si͑001͒ surface.4 Both these structures are semiconducting, however, and a necessary condition for the occurrence of the low-temperature effects mentioned above is that the structures have 1D metallic character at higher ͑e.g., room͒ temperature. The challenge therefore is to construct an atomic scale metallic wire on a semiconducting substrate. There are a number of possible approaches. One could, for instance, modify a specific surface in order to create a wire which is intrinsically metallic. An example of this was explored in recent theoretical work where the possibility of a stable ''dangling bond wire'' on hydrogen passivated Si͑111͒ was discussed.5 Another possibility is to use semiconducting 1D structures as templates for metal atoms evaporated onto the surface, the experimental feasibility of which has recently been demonstrated quite convincingly by Shen et al. 6 The microscopic limit of a single row of metal atoms ͑which interact strongly with the substrate͒ will not necessarily result in a conducting wire. For example, first-principles calculations by Brocks et al. show that lines of aluminum atoms on Si͑001͒ are semiconducting.7 A third approach is to dope semiconducting 1D structures. Some of these structures have states inside the bulk band gap, which have a pure 1D character. Transferring electrons ͑holes͒ from dopant atoms to empty ͑filled͒ 1D states would result in conducting wires.In this paper we propose a specific realization of this last approach and study it by means of first-principles calculations. We start from a single depassivated dimer ͑DD͒ row on th...
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