Characterization of nanometer-sized, mechanically exfoliated graphene on the H-passivated Si(100) surface using scanning tunnelling microscopy

We have developed a method for depositing graphene monolayers and bilayers with minimum lateral dimensions of 2-10 nm by the mechanical exfoliation of graphite onto the Si(100)-2x1:H surface. Room temperature, ultra-high vacuum (UHV) tunnelling spectroscopy measurements of nanometer-sized single-layer graphene reveal a size dependent energy gap ranging from 0.1-1 eV. Furthermore, the number of graphene layers can be directly determined from scanning tunnelling microscopy (STM) topographic contours. This atomistic study provides an experimental basis for probing the electronic structure of nanometer-sized graphene which can assist the development of graphene-based nanoelectronics.Comment: Accepted for publication in Nanotechnolog

STM induced hydrogen desorption via a hole resonance

We report STM-induced desorption of H from Si(100)-H(2$\times1$) at negative sample bias. The desorption rate exhibits a power-law dependence on current and a maximum desorption rate at -7 V. The desorption is explained by vibrational heating of H due to inelastic scattering of tunneling holes with the Si-H 5$\sigma$ hole resonance. The dependence of desorption rate on current and bias is analyzed using a novel approach for calculating inelastic scattering, which includes the effect of the electric field between tip and sample. We show that the maximum desorption rate at -7 V is due to a maximum fraction of inelastically scattered electrons at the onset of the field emission regime.Comment: 4 pages, 4 figures. To appear in Phys. Rev. Let

Towards the fabrication of phosphorus qubits for a silicon quantum computer

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices however requires atomic scale manipulation - an immense technological challenge. We demonstrate that it is possible to fabricate an atomically-precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a silicon-based quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth.Comment: To Appear in Phys. Rev. B Rapid Comm. 5 pages, 5 color figure

Scanning Tunneling Microscopy Study and Nanomanipulation of Graphene-Coated Water on Mica

We study interfacial water trapped between a sheet of graphene and a muscovite (mica) surface using Raman spectroscopy and ultra-high vacuum scanning tunneling microscopy (UHV-STM) at room temperature. We are able to image the graphene-water interface with atomic resolution, revealing a layered network of water trapped underneath the graphene. We identify water layer numbers with a carbon nanotube height reference. Under normal scanning conditions, the water structures remain stable. However, at greater electron energies, we are able to locally manipulate the water using the STM tip.Comment: In press, 5 figures, supplementary information at Nano Letters websit

Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

When alkanethiolate self-assembled monolayers on Au{111} are exchanged with alkaneselenols from solution, replacement of thiolates by selenols is rapid and complete, and is well described by perimeter-dependent island growth kinetics. The monolayer structures change as selenolate coverage increases, from being epitaxial and consistent with the initial thiolate structure to being characteristic of selenolate monolayer structures. At room temperature and at positive sample bias in scanning tunneling microscopy, the selenolate-gold attachment is labile, and molecules exchange positions with neighboring thiolates. The scanning tunneling microscope probe can be used to induce these place-exchange reactions

A STM study of surface reconstructions on Si(111):B

The scanning tunneling microscope is used to study the boron-doped Si(111)surface as a function of annealing times and temperatures. The surface structure is found to be determined by the concentration of B. When the substitutional B concentration is less than 1% of the top 1 X 1 bilayer atoms, the surface is largely 7X7 but surrounded by adatom-covered 1X1 regions (which have higher B concentration). When the B concentration is more than 3\u27, the whole surface will be adatom-covered 1X1 regions including (v 3 Xv 3)R30\u27 structures. The (&3Xv 3)R30\u27 domains will increase with the B concentration. Because 7 X7 can only exist in the region with low B concentration, the growth of 7 X7 is slowed down. Further annealing at 560\u27C can convert 2X2, c(4X2) into 7X7 and 9X9. Sides of the 7X7 domain preferentially grow along the three equivalent [112]directions. The adatom-covered 1 X 1 regions are bounded by faulted halves of the 7 X7 domains. The dark sites of 7 X7 are observed and counted. They are further interpreted in terms of a B substitution model. The pattern of bright and dark atoms in (V 3 X&3)R30\u27 domains is analyzed and a criterion for a B stabilized Si-1&3X v 3)R30\u27 structure is obtained

Nanoscale oxide patterns on Si (100) surfaces

Ultrathin oxide patterns of a linewidth of 50 Å have been created on Si(100)‐2×1 surfaces by a scanning tunneling microscope operating in ultrahigh vacuum. The oxide thickness is estimated to be 4–10 Å. The morphology and spectroscopy of the oxide region are obtained. Hydrogen passivation is used as an oxidation mask. The defects caused by oxidation in the passivated region before and after the hydrogen desorption are compared and discussed. The multistep silicon processings by an ultrahigh vacuum scanning tunneling micropscope is thus demonstrated

Ion irradiation effects on graphite with scanning tunneling microscope

Scanning tunneling microscope is used to create local surface modifications by means of ion impact damage. Graphite has been used as a test case to demonstrate this local surface sputtering. Using a 0.1‐μs voltage pulse of −30 to −140 V applied to the sample in a rough vacuum of 10−2 Torr, a confined area of damage (typically about 100 Å in diameter) is usually obtained. The damaged area consists of several layers of terraces. Defects of the size of a few atoms can also be found. Electronic perturbations caused by defects can form superlattices with a spacing three times that of the graphite lattice. From measurements of the threshold voltage for the discharge, the minimum radius of curvature of the tip can be estimated. The potential applications of this technique and comparison with previous results are discussed

Nanoscale STM-patterning and chemical modification of the Si(100) surface

Nanoscale patterning of the Si(100)-2x1:H monohydride surface has been achieved using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM). The monohydride surface, prepared in UHV by exposure of a heated sample (650 K) to an atomic hydrogen flux, serves as an effective resist for STM patterning and exposure to O 2 and NH 3. Operating the STM in field emission causes hydrogen to be desorbed from the surface, exposing atomically clean silicon. There is no evidence for repassivation of the surface after patterning, suggesting that hydrogen may desorb as H 2. Hydrogen desorption can also be achieved at tunneling biases (~3-4 V) by using larger currents. Nanometer-scale linewidths can be achieved with this technique; single dimer rows have in fact been depassivated. The patterned areas display the same chemical reactivity as clean Si, suggesting the possibility of selective chemical modification of the surface at nanometer scales. This STM-depassivation technique shows considerable potential as a means for nanostructure fabrication