26 research outputs found
Origin of the Mosaicity in Graphene Grown on Cu(111)
We use low-energy electron microscopy to investigate how graphene grows on
Cu(111). Graphene islands first nucleate at substrate defects such as step
bunches and impurities. A considerable fraction of these islands can be
rotationally misaligned with the substrate, generating grain boundaries upon
interisland impingement. New rotational boundaries are also generated as
graphene grows across substrate step bunches. Thus, rougher substrates lead to
higher degrees of mosaicity than do flatter substrates. Increasing the growth
temperature improves crystallographic alignment. We demonstrate that graphene
growth on Cu(111) is surface diffusion limited by comparing simulations of the
time evolution of island shapes with experiments. Islands are dendritic with
distinct lobes, but unlike the polycrystalline, four-lobed islands observed on
(100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial
graphene on smooth, clean Cu(111) has fewer structural defects than it does on
Cu(100).Comment: Article revised following reviewer comment
Real-time observation of epitaxial graphene domain reorientation.
Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, 'ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials
Correlation between structure and electrical transport in ion-irradiated graphene grown on Cu foils
Graphene grown by chemical vapor deposition and supported on SiO2 and
sapphire substrates was studied following controlled introduction of defects
induced by 35 keV carbon ion irradiation. Changes in Raman spectra following
fluences ranging from 10^12 cm^-2 to 10^15 cm^-2 indicate that the structure of
graphene evolves from a highly ordered layer, to a patchwork of disordered
domains, to an essentially amorphous film. These structural changes result in a
dramatic decrease in the Hall mobility by orders of magnitude while,
remarkably, the Hall concentration remains almost unchanged, suggesting that
the Fermi level is pinned at a hole concentration near 1x10^13 cm^-2. A model
for scattering by resonant scatterers is in good agreement with mobility
measurements up to an ion fluence of 1x10^14 cm^-2
Vapor-liquid-solid growth of highly-mismatched semiconductor nanowires with high-fidelity van der Waals layer stacking
Nanobelts, nanoribbons and other quasi-one-dimensional nanostructures formed
from layered, so-called, van der Waals semiconductors have garnered much
attention due to their high-performance, tunable optoelectronic properties. For
layered alloys made from the gallium monochalcogenides GaS, GaSe, and GaTe,
near-continuous tuning of the energy bandgap across the full composition range
has been achieved in GaSe1-xSx and GaSe1-xTex alloys. Gold-catalyzed
vapor-liquid-solid (VLS) growth of these alloys yields predominantly nanobelts,
nanoribbons and other nanostructures for which the fast crystal growth front
consists of layer edges in contact with the catalyst. We demonstrate that in
the S-rich, GaS1-xTex system, unlike GaSe1-xSx and GaSe1-xTex, the Au-catalyzed
VLS process yields van der Waals nanowires for which the fast growth direction
is normal to the layers. The high mismatch between S and Te leads to
extraordinary bowing of the GaS1-xTex alloy's energy bandgap, decreasing by at
least 0.6 eV for x as small as 0.03. Calculations using density functional
theory confirm the significant decrease in bandgap in S-rich GaS1-xTex. The
nanowires can exceed fifty micrometers in length, consisting of tens of
thousands of van der Waals-bonded layers with triangular or hexagonal
cross-sections of uniform dimensions along the length of the nanowire. We
propose that the low solubility of Te in GaS results in an enhancement in Te
coverage around the Au catalyst-nanowire interface, confining the catalyst to
the chalcogen-terminated basal plane (rather than the edges) and thereby
enabling layer-by-layer, c-axis growth
Heat flow model for pulsed laser melting and rapid solidification of ion implanted GaAs
Some of the authors thank for the support of the Center for Nanoscale Systems (CNS) at Harvard University is acknowledged. Harvard-CNS is a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award No. ECS-0335765. K. M. Yu and J. W. Beeman were supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.Some of the authors thank for the support of the Center for Nanoscale Systems (CNS) at Harvard University is acknowledged. Harvard-CNS is a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award No. ECS-0335765. K. M. Yu and J. W. Beeman were supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231
Extraordinary epitaxial alignment of graphene islands on Au(111)
Pristine, single-crystalline graphene displays a unique collection of
remarkable electronic properties that arise from its two-dimensional, honeycomb
structure. Using in-situ low-energy electron microscopy, we show that when
deposited on the (111) surface of Au carbon forms such a structure. The
resulting monolayer, epitaxial film is formed by the coalescence of dendritic
graphene islands that nucleate at a high density. Over 95% of these islands can
be identically aligned with respect to each other and to the Au substrate.
Remarkably, the dominant island orientation is not the better lattice-matched
30^{\circ} rotated orientation but instead one in which the graphene [01] and
Au [011] in-plane directions are parallel. The epitaxial graphene film is only
weakly coupled to the Au surface, which maintains its reconstruction under the
slightly p-type doped graphene. The linear electronic dispersion characteristic
of free-standing graphene is retained regardless of orientation. That a weakly
interacting, non-lattice matched substrate is able to lock graphene into a
particular orientation is surprising. This ability, however, makes Au(111) a
promising substrate for the growth of single crystalline graphene films
Recommended from our members
Composition dependence of Schottky Barrier Heights and Bandgap Energies of GaNxAs1−x Synthesized by Ion Implantation and Pulsed-Laser Melting
We present a systematic investigation on the band structure of the GaNxAs1−x alloys synthesized using nitrogen ion implantation followed by pulsed-laser melting and rapid thermal annealing. The evolution of the nitrogen-concentration depth profile is consistent with liquid-phase diffusion, solute trapping at the rapidly moving solidification front, and surface evaporation. The reduction of the Schottky barrier height of the T-like threshold at nitrogen composition up to x = 0.016 is studied by ballistic electron emission microscopy (BEEM) and determined quantitatively using the second voltage derivative BEEM spectra to be −191 +/- 63 meV per x = 0.01, which is close to the corresponding slope for samples grown by low-temperature molecular beam epitaxy. This slope is also consistent with the bandgap narrowing measured on the same samples by photomodulated reflectance and is consistent with the band anticrossing model for the splitting of the conduction band in the GaNxAs1−x alloys. Lithographically patterned GaNxAs1−x dots are imaged by BEEM. Analysis of BEEM spectra of the locally confined dots indicates an alloying-induced decrease in the Schottky barrier height of four times the thermal energy at room temperature.Engineering and Applied Science