S. Chiang

First Direct Observation of a Nearly Ideal Graphene Band Structure

Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.

Atomic force microscopy using optical interferometry

An atomic force microscope using optical interferometry as the lever detection method is presented. Topographic images of graphite with 50‐A lateral resolution have been obtained using repulsive contact forces. Images of the graphite surface showing the atomic periodicity were also recorded; in this case, the image contrast does not represent surface topography, but rather variations in the lateral frictional force on the tip. Images using attractive electrostatic forces to map topographic features are shown. The instrument has also been operated as a scanning tunneling microscope while simultaneously measuring the contact force between the tip and a graphite surface.

Atomic scale friction between the muscovite mica cleavage plane and a tungsten tip

An atomic force microscope using optical lever detection has been used to study the friction between muscovite mica and a tungsten tip. The frictional force is shown to vary laterally with the periodicity of the hexagonal layer of SiO4 units that forms the cleavage plane of mica. The frictional force varies linearly with normal force, giving a coefficient of friction of 0.09.

Ordered nucleation of Ni and Au islands on Au(111) studied by scanning tunneling microscopy

The scanning tunneling microscope reveals that Ni deposited on Au(111) at room temperature forms regular arrays of two‐dimensional islands. The islands grow with spacing 73 A in rows 140 A apart at sites determined by the Au(111) ‘‘herringbone’’ reconstruction. This nucleation at evenly spaced sites yields islands with a narrow size distribution. The apparent Ni island height (1.9 A) is bias‐independent and agrees with a hard‐sphere model of pseudomorphic Ni/Au(111). The behavior of Ni is contrasted with Au deposited on Au(111), for which far fewer islands are formed.

Direct measurement of forces during scanning tunneling microscope imaging of graphite

Abstract The normal force acting on a scanning tunneling microscope tip while imaging a graphite surface in air has been measured directly. Forces in the range of 10 −7 to 10 −6 N are required to achieve tunneling. Further, the force needed to maintain a constant current varies considerably as the tip scans from one part of the graphite unit cell to another. Our results are consistent with a model, originally suggested by Mamin et al., in which the force between the tip and the surface is mediated by a contamination layer, and tunneling occurs at the end of an asperity which pierces this layer. However, we cannot rule out a model where a graphite flake is dragged across the graphite surface to generate an STM image.

Nucleation and growth of ultrathin Fe and Au films on Cu(100) studied by scanning tunneling microscopy

Cu(100) surfaces modified by depositing Au and Fe in the monolayer (ML) thickness regime are studied with the scanning tunneling microscope (STM). STM results confirm that Au deposition of 0.5 ML at 300 K creates a CuAu c(2×2) alloy monolayer, with nearly regular disruptions of structure that relieve misfit strain. The alloy forms by Au replacement of Cu atoms in the surface. Submonolayer Fe deposition at 300 K yields monolayer islands and additional features interpreted as Fe patches in the surface Cu layer. This suggests that Fe atoms replace Cu atoms in the surface. Deposition of ∼4 ML Fe yields a more homogeneous surface with height variations that suggest structural disorder in the Fe layer. Fe deposition at ∼110 K and warming to 300 K yields many monolayer and bilayer islands, with no evidence of Fe incorporation into the surface. Unusual tip behavior is also discussed which appears to yield STM sensitivity to the chemical difference between Fe and Cu areas.

Growth and morphology of partial and multilayer Fe thin films on Cu(100) and the effect of adsorbed gases studied by scanning tunneling microscopy

Fe epitaxy on Cu(100) is investigated for Fe coverages θ≤3.0 ML. The layer filling statistics are quantitatively related to an evolving growth process, which includes intermixing at the substrate overlayer interface. The resulting inhomogeneous substrate surface and first layer affect the processes by which arriving Fe atoms add to the growth front. Our results explain the previously reported covering of initial Fe not as bilayer growth, but instead as the result of island growth on top of Fe incorporated in the top substrate layer. First layer composition and structure influence the nucleation and growth of the second layer. Island coalescence and formation of a first layer percolation network change the connected first layer area thereby changing the nucleation and growth behavior of the second layer. After both first and second layer growth are completed, images show additional growth is much more layer‐by‐layer in nature. Oxygen exposure after Fe deposition changes the layer filling by both promotion ...

Atomic Force Microscopy: General Principles and a New Implementation

Recently, Binnig, Quate, and Gerber developed the atomic force microscope (AFM), an instrument which senses minute (10-12 – 10-8 N) forces between a sharp tip and a sample surface [1], In addition to enabling the study of solid-solid interactions on a unprecedentedly small scale, the AFM provides a general method for doing non-destructive surface profilometry at a resolution better than 10 nm and perhaps down to the atomic level. In this paper we review the principles of the AFM, discuss its potential resolution and data rate, describe our new AFM design, and present some initial results. We have obtained three dimensional surface profiles with 20 nm lateral resolution, which to our knowledge is better than what has been attained previously by stylus profilometry.

Surface alloy formation studied by scanning tunneling microscopy : Cu(100) + Au-c(2×2)

Abstract The ordered surface alloy formed by Au deposited on Cu(100) at room temperature is studied using scanning tunneling microscopy (STM). The formation of a CuAu alloy monolayer is demonstrated by STM images showing c(2 × 2) ordering with two unequal peaks per unit cell. Longer-range images show linear features in which misfit strain is apparently relieved by transverse atomic displacements. The surface topography of islands shows that they also consist of a CuAu alloy layer, which demonstrates that alloy formation arises by Au replacement of Cu atoms in the surface.

Predicting STM images of molecular adsorbates

Abstract Extended Huckel theory forms the basis of a simple computational method for predicting scanning tunneling microscopy images for molecules adsorbed onto metal surfaces. Development of the technique is described, including adsorbate geometry selection, calculation of electronic structure images, and comparison of the modeled images with both experiment and isolated molecule calculations. This technique is shown to reproduce such experimentally observed image features as detailed internal structure and resolution and bias effects. Systems considered include naphthalene, azulene, and a range of methylazulenes on Pt(111), benzene and CO on Rh(111), graphite, and fluorobenzene on Pt(111).