Silicon nanostructures such as small clusters, superclusters, elongated chains, and tube- or rod-like structures with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) exhibits that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized spherical structures of 0.6 +/- 0.2 mn, 5.1 +/- 1.2 mn, and 15.4 +/- 3 nm diameter. Surface regions with defects such as foldings, pits and craters descending a few layers into the graphite surface, are sparsely covered with silicon. In such defect rich regions most of the deposited material is found to be attached to the monatomic step edges forming the crater rims. The average diameter of the silicon nanoparticles that are attached to these steps is 2 nm +/- 0.5 nm. A simulation of the growth process, i.e., deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, reproduces convincingly most of the Si nanostructures observed in the STM topographs.

In a series of recent experiments, the HOMO-LUMO energy gaps of small Si clusters deposited on a graphite substrate have been determined by Scanning Tunneling Microscopy (STM). The values obtained were found to be substantially smaller than the energy gaps of corresponding passivated clusters. This work considers dimensional reduction as a possible mechanism for a sizeable energy gap narrowing by the example of the system Si5. The impact of the graphite substrate on the deposited species is investigated in the framework of a pseudocluster model.

Scanning tunneling spectroscopy was performed on small pristine silicon clusters on graphite. For the investigated clusters with sizes between 2.5 and 40 Angstrom, two distinct size regimes, were found: below 15 Angstrom, Si clusters show energy gaps up to 450 meV, above that size only zero gaps are observed. We conclude that a structural transition occurs at the size of 15 Angstrom. Larger clusters assume the diamond structure, and the zero gaps originate from the dangling bonds on the cluster surface. Smaller clusters appear in high-coordination structures, which have eliminated the dangling bonds.

The present work deals with the interpretation of recent high-resolution scanning tunneling microscopy (STM) measurements in which Si7 clusters were assembled through quasi-free growth on a clean highly oriented pyrolytic graphite (HOPG) surface. It was found that at low bias, some clusters exhibited the Coulomb blockade phenomenon, acquiring a negative charge that blocks the tunneling current from the microscope tip to the cluster. However, upon a switch of polarity, conceivably neutralizing the charged system, the clusters proved to be detectable within a wide range of positive and negative values of the STM bias. We attempt to understand this effect in terms of an electronic structure change of the Si7 unit associated with a transition from a singly charged Si7 anion in a spin quartet state to a neutral Si7 cluster in a spin triplet state, performing density functional computations for a Si7C54H18 cluster which simulates the combined system of the Si7 unit and the graphite layer.

Studies of the fundamental energy gap of pristine silicon clusters have been performed using STM and STS. Clusters with sizes between 2.5 and 40 Å are studied. The size dependence of the gap is determined. For particles below 15 Å, gaps up to 450 meV are found. Larger particles exhibit zero gaps. The results are explained in terms of a transformation from diamond to compact structure occurring at 15 Å (∼44 atoms per cluster). For clusters with diamond structure the surface dangling bond density is high leading to electronic states filling the energy gap. On the other hand, the compact arrangement of the smaller clusters tends to eliminate dangling bonds. Therefore, finite-gap values are observed for clusters with less than ∼44 atoms.

Silicon nanostructures such as small clusters, superclusters, and elongated chains, with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) reveals that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized silicon clusters of 0.6±0.2nm, 5.1±1.2nm, and 15.4±3nm diameter. Surface regions with defects such as pits and craters, descending a few layers into the graphite surface, are sparsely covered with silicon. Most of the deposited material, with an average diameter of 2nm, is found to be attached to the monatomic step edges forming the crater rims. A simulation of the growth process, i.e. deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, convincingly reproduces most of the Si nanostructures observed in the STM topographs.

Electronic properties of carbon clusters with diameters d between 4 and 1100 Å were studied by scanning tunneling spectroscopy (STS). Large-sized clusters all showed the zero band gap of graphite. A gap opening was observed for clusters with diameters less than about 15 Å. Energy gaps up to 650 meV were measured. The experimental results were modeled using a finite barrier potential for confined electrons, with consideration of the effective mass.