31 research outputs found
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
Characterizing Transition-Metal Dichalcogenide Thin-Films using Hyperspectral Imaging and Machine Learning
Atomically thin polycrystalline transition-metal dichalcogenides (TMDs) are
relevant to both fundamental science investigation and applications. TMD
thin-films present uniquely difficult challenges to effective nanoscale
crystalline characterization. Here we present a method to quickly characterize
the nanocrystalline grain structure and texture of monolayer WS2 films using
scanning nanobeam electron diffraction coupled with multivariate statistical
analysis of the resulting data. Our analysis pipeline is highly generalizable
and is a useful alternative to the time consuming, complex, and
system-dependent methodology traditionally used to analyze spatially resolved
electron diffraction measurements
Alternative Stacking Sequences in Hexagonal Boron Nitride
The relative orientation of successive sheets, i.e. the stacking sequence, in
layered two-dimensional materials is central to the electronic, thermal, and
mechanical properties of the material. Often different stacking sequences have
comparable cohesive energy, leading to alternative stable crystal structures.
Here we theoretically and experimentally explore different stacking sequences
in the van der Waals bonded material hexagonal boron nitride (h-BN). We examine
the total energy, electronic bandgap, and dielectric response tensor for five
distinct high symmetry stacking sequences for both bulk and bilayer forms of
h-BN. Two sequences, the generally assumed AA' sequence and the relatively
unknown (for h-BN) AB (Bernal) sequence, are predicted to have comparably low
energy. We present a scalable modified chemical vapor deposition method that
produces large flakes of virtually pure AB stacked h-BN; this new material
complements the generally available AA' stacked h-BN
Blue-Light-Emitting Color Centers in High-Quality Hexagonal Boron Nitride
Light emitters in wide band gap semiconductors are of great fundamental
interest and have potential as optically addressable qubits. Here we describe
the discovery of a new color center in high-quality hexagonal boron nitride
(h-BN) with a sharp emission line at 435 nm. The emitters are activated and
deactivated by electron beam irradiation and have spectral and temporal
characteristics consistent with atomic color centers weakly coupled to lattice
vibrations. The emitters are conspicuously absent from commercially available
h-BN and are only present in ultra-high-quality h-BN grown using a
high-pressure, high-temperature Ba-B-N flux/solvent, suggesting that these
emitters originate from impurities or related defects specific to this unique
synthetic route. Our results imply that the light emission is activated and
deactivated by electron beam manipulation of the charge state of an
impurity-defect complex
Dark-field transmission electron microscopy and the Debye-Waller factor of graphene
Graphene\u27s structure bears on both the material\u27s electronic properties and fundamental questions about long-range order in two-dimensional crystals. We present an analytic calculation of selected area electron diffraction from multilayer graphene and compare it with data from samples prepared by chemical vapor deposition and mechanical exfoliation. A single layer scatters only 0.5% of the incident electrons, so this kinematical calculation can be considered reliable for five or fewer layers. Dark-field transmission electron micrographs of multilayer graphene illustrate how knowledge of the diffraction peak intensities can be applied for rapid mapping of thickness, stacking, and grain boundaries. The diffraction peak intensities also depend on the mean-square displacement of atoms from their ideal lattice locations, which is parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a suspended monolayer of exfoliated graphene and find a result consistent with an estimate based on the Debye model. For laboratory-scale graphene samples, finite size effects are sufficient to stabilize the graphene lattice against melting, indicating that ripples in the third dimension are not necessary
Tunable Electronic Structure in Gallium Chalcogenide van der Waals Compounds
Transition metal monochalcogenides comprise a class of two-dimensional
materials with electronic band gaps that are highly sensitive to material
thickness and chemical composition. Here, we explore the tunability of the
electronic excitation spectrum in GaSe using angle-resolved photoemission
spectroscopy. The electronic structure of the material is modified by
potassium deposition as well as by forming
GaSSe alloy compounds. We find that potassium decouples the
top-most tetra-layer of the GaSe unit cell, leading to a substantial change of
the dispersion around the valence band maximum (VBM). The observed band
dispersion of a single tetralayer is consistent with a transition from the
direct gap character of the bulk to the indirect gap character expected for
monolayer GaSe. Upon alloying with sulfur, we observe a phase transition from
AB to stacking. Alloying also results in a rigid energy
shift of the VBM towards higher binding energies which correlates with a blue
shift in the luminescence. The increase of the band gap upon sulfur alloying
does not appear to change the dispersion or character of the VBM appreciably,
implying that it is possible to engineer the gap of these materials while
maintaining their salient electronic properties
Dark-field transmission electron microscopy and the Debye-Waller factor of graphene
Graphene's structure bears on both the material's electronic properties and
fundamental questions about long range order in two-dimensional crystals. We
present an analytic calculation of selected area electron diffraction from
multi-layer graphene and compare it with data from samples prepared by chemical
vapor deposition and mechanical exfoliation. A single layer scatters only 0.5%
of the incident electrons, so this kinematical calculation can be considered
reliable for five or fewer layers. Dark-field transmission electron micrographs
of multi-layer graphene illustrate how knowledge of the diffraction peak
intensities can be applied for rapid mapping of thickness, stacking, and grain
boundaries. The diffraction peak intensities also depend on the mean-square
displacement of atoms from their ideal lattice locations, which is
parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a
suspended monolayer of exfoliated graphene and find a result consistent with an
estimate based on the Debye model. For laboratory-scale graphene samples,
finite size effects are sufficient to stabilize the graphene lattice against
melting, indicating that ripples in the third dimension are not necessary.Comment: 10 pages, 4 figure