267 research outputs found
Orbital character effects in the photon energy and polarization dependence of pure C60 photoemission
Recent direct experimental observation of multiple highly-dispersive C
valence bands has allowed for a detailed analysis of the unique photoemission
traits of these features through photon energy- and polarization-dependent
measurements. Previously obscured dispersions and strong photoemission traits
are now revealed by specific light polarizations. The observed intensity
effects prove the locking in place of the C molecules at low
temperatures and the existence of an orientational order imposed by the
substrate chosen. Most importantly, photon energy- and polarization-dependent
effects are shown to be intimately linked with the orbital character of the
C band manifolds which allows for a more precise determination of the
orbital character within the HOMO-2. Our observations and analysis provide
important considerations for the connection between molecular and crystalline
C electronic structure, past and future band structure studies, and for
increasingly popular C electronic device applications, especially those
making use of heterostructures
Fabrication of Atomically Precise Nanopores in Hexagonal Boron Nitride
We demonstrate the fabrication of individual nanopores in hexagonal boron
nitride (hBN) with atomically precise control of the pore size. Previous
methods of pore production in other 2D materials create pores of irregular
geometry with imprecise diameters. By taking advantage of the preferential
growth of boron vacancies in hBN under electron beam irradiation, we are able
to observe the pore growth via transmission electron microscopy, and terminate
the process when the pore has reached its desired size. Careful control of beam
conditions allows us to nucleate and grow individual triangular and hexagonal
pores with diameters ranging from subnanometer to 6nm over a large area of
suspended hBN using a conventional TEM. These nanopores could find application
in molecular sensing, DNA sequencing, water desalination, and molecular
separation. Furthermore, the chemical edge-groups along the hBN pores can be
made entirely nitrogen terminated or faceted with boron-terminated edges,
opening avenues for tailored functionalization and extending the applications
of these hBN nanopores.Comment: 5 pages, 6 figure
Vibrational spectroscopy at electrolyte/electrode interfaces with graphene gratings.
Microscopic understanding of physical and electrochemical processes at electrolyte/electrode interfaces is critical for applications ranging from batteries, fuel cells to electrocatalysis. However, probing such buried interfacial processes is experimentally challenging. Infrared spectroscopy is sensitive to molecule vibrational signatures, yet to approach the interface three stringent requirements have to be met: interface specificity, sub-monolayer molecular detection sensitivity, and electrochemically stable and infrared transparent electrodes. Here we show that transparent graphene gratings electrode provide an attractive platform for vibrational spectroscopy at the electrolyte/electrode interfaces: infrared diffraction from graphene gratings offers enhanced detection sensitivity and interface specificity. We demonstrate the vibrational spectroscopy of methylene group of adsorbed sub-monolayer cetrimonium bromide molecules and reveal a reversible field-induced electrochemical deposition of cetrimonium bromide on the electrode controlled by the bias voltage. Such vibrational spectroscopy with graphene gratings is promising for real time and in situ monitoring of different chemical species at the electrolyte/electrode interfaces
The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes
We describe the synthesis of very thin sheets (between a few and ten atomic layers) of hexagonal boron nitride (h-BN), prepared either on a SiO2 substrate or freely suspended. Optical microscopy, atomic force microscopy, and transmission electron microscopy have been used to characterize the morphology of the samples and to distinguish between regions of different thicknesses. Comparison is made to previous studies on single- and few-layer graphene. This synthesis opens the door to experimentally accessing the two-dimensional phase of boron nitride
Low pressure chemical vapor deposition synthesis of hexagonal boron nitride on polycrystalline metal foils, Phys
The two-dimensional sp 2 -bonded material hexagonal boron nitride (h-BN) has unique electronic, thermal, mechanical, and chemical properties. It has recently found use as an ideal substrate for graphene-based electronic devices. We here describe synthesis of mono-to few-layer h-BN films using low pressure chemical vapor deposition (LPCVD) from borazine, with nickel, copper and platinum employed as catalytic substrates, and transfer of some of these films using a non-polymer method. Characterization of the films via Raman spectroscopy and transmission electron microscopy (TEM) is performed. Chemical vapor deposition synthesis of hexagonal boron nitride from borazine using metallic substrates
Evolution of Interlayer Coupling in Twisted MoS2 Bilayers
Van der Waals (vdW) coupling is emerging as a powerful method to engineer and
tailor physical properties of atomically thin two-dimensional (2D) materials.
In graphene/graphene and graphene/boron-nitride structures it leads to
interesting physical phenomena ranging from new van Hove singularities1-4 and
Fermi velocity renormalization5, 6 to unconventional quantum Hall effects7 and
Hofstadter's butterfly pattern8-12. 2D transition metal dichalcogenides
(TMDCs), another system of predominantly vdW-coupled atomically thin layers13,
14, can also exhibit interesting but different coupling phenomena because TMDCs
can be direct or indirect bandgap semiconductors15, 16. Here, we present the
first study on the evolution of interlayer coupling with twist angles in
as-grown MoS2 bilayers. We find that an indirect bandgap emerges in bilayers
with any stacking configuration, but the bandgap size varies appreciably with
the twist angle: it shows the largest redshift for AA- and AB-stacked bilayers,
and a significantly smaller but constant redshift for all other twist angles.
The vibration frequency of the out-of-plane phonon in MoS2 shows similar twist
angle dependence. Our observations, together with ab initio calculations,
reveal that this evolution of interlayer coupling originates from the repulsive
steric effects, which leads to different interlayer separations between the two
MoS2 layers in different stacking configurations
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