John A. Jaszczak

Giant intrinsic carrier mobilities in graphene and its bilayer

We have studied temperature dependences of electron transport in graphene and its bilayer and found extremely low electron-phonon scattering rates that set the fundamental limit on possible charge carrier mobilities at room temperature. Our measurements show that mobilities higher than 200 000 cm2/V s are achievable, if extrinsic disorder is eliminated. A sharp (thresholdlike) increase in resistivity observed above approximately 200 K is unexpected but can qualitatively be understood within a model of a rippled graphene sheet in which scattering occurs on intraripple flexural phonons.

Landau-Level Splitting in Graphene in High Magnetic Fields

The quantum Hall (QH) effect in two-dimensional electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors nu = 0, +/-1, +/-4 are discovered at magnetic fields B > 20 T, indicating the lifting of the fourfold degeneracy of the previously observed QH states at nu = +/-4(absolute value(n) + 1/2), where n is the Landau-level index. In particular, the presence of the nu = 0, +/-1 QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at nu = +/-4 is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the n = 1 Landau level.

Naturally occurring graphite cones

Abstract Carbon, boron nitride, and other materials that form nanotubes are also able to form conical shapes. Even though the potential applications of cone arrays as electron emitters and other devices are very promising, understanding of their structure and formation mechanisms is still very limited compared to nanotubes and other carbon structures. Moreover, the cones have only been synthesized in a mixture with other shapes, but never as continuous arrays. It appears, however, that we can learn from nature how to produce large carbon cone arrays. We here report the first-known natural occurrence of large arrays of conical graphite crystals. These occur on the surfaces of millimeter-sized polycrystalline spheroidal aggregates of graphite. Cone heights range from less then a micron to 40 μm, which is larger than any other carbon cones reported in the literature. They are also observed to dominate sample surfaces. The surface topography of the cones and petrologic relations of the samples suggest that the cones formed from a metamorphic fluid. Unlike most laboratory produced cones, the natural cones have a wide distribution of apex angles, which supports a disclination model for cone-helix structures.

Room‐Temperature Tunneling Behavior of Boron Nitride Nanotubes Functionalized with Gold Quantum Dots

One-dimensional arrays of gold quantum dots (QDs) on insulating boron nitride nanotubes (BNNTs) can form conduction channels of tunneling field-effect transistors. We demonstrate that tunneling currents can be modulated at room temperature by tuning the lengths of QD-BNNTs and the gate potentials. Our discovery will inspire the creative use of nanostructured metals and insulators for future electronic devices.

Multiple length scale growth spirals on metamorphic graphite {001} surfaces studied by atomic force microscopy

Abstract The microtopography of {001} surfaces on single crystals of graphite from a Neoproterozoic marble of the Swakop group, near Wlotzkas Baken, western Namibia, has been studied using differential interference contrast (DIC) microscopy and atomic force microscopy (AFM). A unique aspect of the observed surface microtopography is the presence of growth spirals and hillocks on three different length scales. The largest spirals are polygonized and can be seen without magnification. Steps on this feature are roughly 4 μm high and 90 μm apart. The second-order features are hexagonal growth hillocks with an average step height of 1.5 nm and total lateral dimensions of 5-40 μm. The apex of these hillocks coincides directly with the apex of reentrants in the macrosteps of the large spiral. Morphology suggests the formation of these polygonized hillocks by some mechanism other than simple spiral growth. We speculate that these features may be due to pinning of the macrostep by impurities and subsequent formation of the second-order hillocks. The third length-scale features are spirals found on terraces forming the vicinal faces of the second-order hillocks. These spirals have steps that are 6.7 Å high (unit-cell length along [001]) and an average step spacing of 900 Å. These double-layer steps also show some regions with partial step separation into 3.3 Å high monolayer steps. The observed microtopographic features give us insight into the conditions in, and mechanisms by which these graphite crystals formed during carbonate metamorphism. Crystal growth, unrestricted by the surrounding calcite, was from a fluid phase at low graphite supersaturation and was dominated by the spiral growth mechanism. Comparison with theoretical and simulation studies suggests a critical radius for two-dimensional nucleation on the (001) surface on the order of 100 Å.

A Monte Carlo simulation method for {111} surfaces of silicon and other diamond-cubic materials

Abstract We introduce a modified, solid-on-solid model for Monte Carlo computer simulation studies of equilibrium, growth and etching phenomena on the {111} surfaces of silicon and other diamond-cubic materials. Incorporation of the actual diamond-cubic crystal structure allows for the inclusion of possible effects of the structures rotational symmetry, coordination and the layer-stacking periodicity (normal to the surface) on the surface properties and dynamics. Results of extensive equilibrium simulations, including surface energies, surface specific heats, step energies and surface roughness, are presented to characterize the basic model. A broad peak in the surface specific heat and the universal behavior of the height-difference correlation function indicate a roughening transition at temperature TR. A preroughening transition is indicated by a sharp peak in the surface specific heat and a dramatic decrease in the step energy at a temperature TPR ≈ 0.43TR. The energies of stable and unstable steps differ significantly below TPR but become isotropic above TPR. Initially straight, unstable steps are observed to spontaneously microfacet below TPR. Preliminary simulations of growth and etching of surfaces, both with and without intersecting dislocations, are also presented.

On the interaction between steps in vicinal fcc surfaces: I. Steps along 〈001〉

Atomistic computer simulations are used to study the zero-temperature interaction between steps in vicinal free fcc surfaces in order to test the continuum-elastic theory of Marchenko and Parshin (MP). Two different types of surface steps, both parallel to 〈001〉 but situated, respectively, on the (100) and (110) planes, are investigated using two qualitatively different interatomic potentials, one of the embedded-atom-method type and the other the Lennard-Jones potential. In agreement with the MP theory, for the largest step separations δ the energy due to the step-step repulsion is found to decrease as δ−2, with a strength given by the surface-stess tensor and the elastic moduli of the material. Surprisingly, for both potentials and for both types of steps the δ−2 power law appears to be obeyed even for the smallest step separations, albeit with a 2–3 times smaller interaction strength. The relationship of our simulation results with a nearest-neighbor broken-bond model is also explored.

On the elastic behavior of composition-modulated superlattices

Atomistic computer simulations are used to systematically investigate the role of interfacial disorder on the elastic behavior of composition-modulated superlattices of fcc metals, represented by simple Lennard-Jones potentials. The structures, energies, and average elastic properties of four types of superlattices with various degrees of interfacial disorder are computed as a function of the modulation wavelength along (001). The four superlattice types studied include perfectly coherent, incoherent, and two types derived from these by introducing relative twists about (001) between alternating layers. A 20% lattice-parameter mismatch between the two modulating materials is assumed. Results are compared with our earlier work on unsupported thin films, grain-boundary superlattices, and incoherent superlattices with a 10% lattice-parameter mismatch. The degree of structural disorder at the interfaces is found to correlate well with the magnitude of the elastic anomalies, which cannot be accounted for by anisotropic lattice-parameter changes alone. The grain-boundary superlattices studied earlier are found to provide a good model limit for the elastic behavior of interfacially disordered dissimilar-material superlattices.

Role of coherency in the elastic behavior of composition-modulated superlattices

We investigate the role of coherency in the elastic behavior of composition‐modulated superlattices of fcc metals by atomistic computer simulations using Lennard–Jones potentials. Structures, energies, and elastic properties of incoherent superlattices are computed as a function of the compositional modulation wavelength along [001] and compared with those of coherent superlattices. Both superlattice types were taken to have a 10% lattice parameter mismatch between the two materials. The incoherent superlattices, as compared to coherent superlattices, were found to be more structurally disordered and exhibited greater elastic anomalies, which cannot be accounted for by the overall dimensional changes of the superlattices alone. High‐ and low‐frequency elastic constants are briefly compared. It is proposed that increasing the structural disorder in the superlattices by increasing the lattice‐parameter mismatch or by introducing a relative rotation between the two materials will enhance all of the elastic a...

Graphite: Flat, Fibrous and Spherical

The paradigm for the structure of graphite is that of a staggered stacking of flat layers of carbon atoms (Figure 6-1). Individual layers, sometimes referred to as graphene sheets,1 are weakly bonded to each other and are composed of strongly bonded carbon atoms at the vertices of a network of regular hexagons in a honeycomb pattern.2 Both the properties and the morphology of graphite reflect its highly anisotropic structure. Due to the strong bonding within layers and the weak bonding between layers, the growth of graphite takes place predominantly along the edges of the layers (perpendicular to the c axis) and only very slowly normal to the layers (parallel to the c axis). As a result of the growth rate anisotropy, the anisotropic surface energy, and the crystallographic symmetry, the expected morphology for graphite crystals is that of tabular hexagonal prisms.3 However, well-formed natural crystals, such as shown in Figure 6-2, are rare,4 and it has been said that near-ideal crystals of graphite may be rarer than diamonds.5 Well-formed, laboratory-grown crystals of graphite are also uncommon. During the 1960s, graphite crystals from Ticonderoga, New York, and Sterling Hill, New Jersey, became a standard of perfection for experiments and for comparison with laboratory-grown crystals.6,7