Nathan P. Guisinger

Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs

Borophene: Boron in two dimensions Although bulk allotropes of carbon and boron differ greatly, small clusters of these elements show remarkable similarities. Boron analogs of two-dimensional carbon allotropes such as graphene have been predicted. Now Mannix et al. report the formation of two-dimensional boron by depositing the elemental boron onto a silver surface under ultrahigh-vacuum conditions (see the Perspective by Sachdev). The graphene-like structure was buckled, weakly bonded to the substrate, and metallic. Science, this issue p. 1513; see also p. 1468 A two-dimensional boron allotrope forms after depositing its elemental vapor on a silver surface in vacuum. [Also see Perspective by Sachdev] At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.

Epitaxial Graphene on Cu(111)

The growth of graphene on single crystal Cu(111) has been achieved by thermal decomposition of ethylene in an ultrahigh vacuum chamber for the first time. The structural and electronic properties of graphene on Cu(111) have been investigated by scanning tunneling microscopy and spectroscopy. The nucleation of monolayer islands and two predominant domain orientations have been observed, which lead to the formation of numerous domain boundaries with increasing coverage. These results reveal that reducing the density of domain boundaries is one challenge of growing high-quality graphene on copper.

Atomic-Scale Investigation of Graphene Grown on Cu Foil and the Effects of Thermal Annealing

We have investigated the effects of thermal annealing on ex-situ chemically vapor deposited submonolayer graphene islands on polycrystalline Cu foil at the atomic-scale using ultrahigh vacuum scanning tunneling microscopy. Low-temperature annealed graphene islands on Cu foil (at ∼430 °C) exhibit predominantly striped Moiré patterns, indicating a relatively weak interaction between graphene and the underlying polycrystalline Cu foil. Rapid high-temperature annealing of the sample (at 700-800 °C) gives rise to the removal of Cu oxide and the recovery of crystallographic features of the copper that surrounds the intact graphene. These experimental observations of continuous crystalline features between the underlying copper (beneath the graphene islands) and the surrounding exposed copper areas revealed by high-temperature annealing demonstrates the impenetrable nature of graphene and its potential application as a protective layer against corrosion.

Exposure of Epitaxial Graphene on SiC(0001) to Atomic Hydrogen

Graphene films on SiC exhibit coherent transport properties that suggest the potential for novel carbon-based nanoelectronics applications. Recent studies suggest that the role of the interface between single layer graphene and silicon-terminated SiC can strongly influence the electronic properties of the graphene overlayer. In this study, we have exposed the graphitized SiC to atomic hydrogen in an effort to passivate dangling bonds at the interface, while investigating the results utilizing room temperature scanning tunneling microscopy.

Direct oriented growth of armchair graphene nanoribbons on germanium

Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribbons narrower than 10 nm with controlled crystallographic orientation and well-defined armchair edges. However, the scalable synthesis of nanoribbons with this precision directly on insulating or semiconducting substrates has not been possible. Here we demonstrate the synthesis of graphene nanoribbons on Ge(001) via chemical vapour deposition. The nanoribbons are self-aligning 3° from the Ge〈110〉 directions, are self-defining with predominantly smooth armchair edges, and have tunable width to <10 nm and aspect ratio to >70. In order to realize highly anisotropic ribbons, it is critical to operate in a regime in which the growth rate in the width direction is especially slow, <5 nm h−1. This directional and anisotropic growth enables nanoribbon fabrication directly on conventional semiconductor wafer platforms and, therefore, promises to allow the integration of nanoribbons into future hybrid integrated circuits.

Structural and Electronic Decoupling of C60 from Epitaxial Graphene on SiC

We have investigated the initial stages of growth and the electronic structure of C(60) molecules on graphene grown epitaxially on SiC(0001) at the single-molecule level using cryogenic ultrahigh vacuum scanning tunneling microscopy and spectroscopy. We observe that the first layer of C(60) molecules self-assembles into a well-ordered, close-packed arrangement on graphene upon molecular deposition at room temperature while exhibiting a subtle C(60) superlattice. We measure a highest occupied molecular orbital-lowest unoccupied molecular orbital gap of ∼3.5 eV for the C(60) molecules on graphene in submonolayer regime, indicating a significantly smaller amount of charge transfer from the graphene to C(60) and substrate-induced screening as compared to C(60) adsorbed on metallic substrates. Our results have important implications for the use of graphene for future device applications that require electronic decoupling between functional molecular adsorbates and substrates.

Graphene Induced Surface Reconstruction of Cu

An atomic-scale study utilizing scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) is performed on large single crystalline graphene grains synthesized on Cu foil by a chemical vapor deposition (CVD) method. After thermal annealing, we observe the presence of periodic surface depressions (stripe patterns) that exhibit long-range order formed in the area of Cu covered by graphene. We suggest that the observed stripe pattern is a Cu surface reconstruction formed by partial dislocations (which appeared to be stair-rod-like) resulting from the strain induced by the graphene overlayer. In addition, these graphene grains are shown to be more decoupled from the Cu substrate compared to previously studied grains that exhibited Moiré patterns.

Silicon Growth at the Two-Dimensional Limit on Ag(111)

Having fueled the microelectronics industry for over 50 years, silicon is arguably the most studied and influential semiconductor. With the recent emergence of two-dimensional (2D) materials (e.g., graphene, MoS2, phosphorene, etc.), it is natural to contemplate the behavior of Si in the 2D limit. Guided by atomic-scale studies utilizing ultrahigh vacuum (UHV), scanning tunneling microscopy (STM), and spectroscopy (STS), we have investigated the 2D limits of Si growth on Ag(111). In contrast to previous reports of a distinct sp(2)-bonded silicene allotrope, we observe the evolution of apparent surface alloys (ordered 2D silicon-Ag surface phases), which culminate in the precipitation of crystalline, sp(3)-bonded Si(111) nanosheets. These nanosheets are capped with a √3 honeycomb phase that is isostructural to a √3 honeycomb-chained-trimer (HCT) reconstruction of Ag on Si(111). Further investigations reveal evidence for silicon intermixing with the Ag(111) substrate followed by surface precipitation of crystalline, sp(3)-bonded silicon nanosheets. These conclusions are corroborated by ex situ atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Even at the 2D limit, scanning tunneling spectroscopy shows that the sp(3)-bonded silicon nanosheets exhibit semiconducting electronic properties.

Cryogenic variable temperature ultrahigh vacuum scanning tunneling microscope for single molecule studies on silicon surfaces

The design and performance of a variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) is presented. This STM operates from 8.2 to 300 K in a UHV environment with a base pressure of less than 6×10−11 Torr. Cooling is achieved from 300 to 80 K within 3.5 h and from 80 to 8.2 K within 4.5 h. The base temperature of 8.2 K is maintained at a liquid helium consumption rate of 0.9 l/h. This design allows for direct optical access to the tip-sample interface and direct line-of-sight dosing while the sample is mounted in the STM. The STM tip may be coarse translated laterally in two dimensions through a 6-mm-diam area at all temperatures. With the feedback loop off, the drift in the tip-sample spacing is approximately 0.008 A/min at 8.2 K. Atomic resolution feedback controlled lithography is performed on hydrogen passivated Si(100) and differential tunneling conductance maps are gathered for isolated cyclopentene molecules on unpassivated Si(100), thus demonstrating that this system is we...

Room temperature nanofabrication of atomically registered heteromolecular organosilicon nanostructures using multistep feedback controlled lithography

Ultrahigh vacuum scanning tunneling microscopy is employed for the nanofabrication and characterization of atomically registered heteromolecular organosilicon nanostructures at room temperature. In the first fabrication step, feedback controlled lithography (FCL) is used to pattern individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical molecules at opposite ends of the same dimer row on the Si(100)‐2×1:H surface. In atomic registration with the first pattern, FCL is subsequently applied for the removal of a single hydrogen atom. The resulting dangling bond templates the spontaneous growth of a styrene chain that is oriented along the underlying dimer row. The styrene chain growth is bounded by the originally patterned TEMPO molecules, thus resulting in a heteromolecular organosilicon nanostructure. The demonstration of multistep FCL suggests that this approach can be widely used for fundamental studies and fabricating prototype devices that require atomically registered organic molecules mou...