58 research outputs found
Structural phase transition and material properties of few-layer monochalcogenides
GeSe and SnSe monochalcogenide monolayers and bilayers undergo a
two-dimensional phase transition from a rectangular unit cell to a square unit
cell at a temperature well below the melting point. Its consequences on
material properties are studied within the framework of Car-Parrinello
molecular dynamics and density-functional theory. No in-gap states develop as
the structural transition takes place, so that these phase-change materials
remain semiconducting below and above . As the in-plane lattice transforms
from a rectangle onto a square at , the electronic, spin, optical, and
piezo-electric properties dramatically depart from earlier predictions. Indeed,
the and points in the Brillouin zone become effectively equivalent at
, leading to a symmetric electronic structure. The spin polarization at
the conduction valley edge vanishes, and the hole conductivity must display an
anomalous thermal increase at . The linear optical absorption band edge
must change its polarization as well, making this structural and electronic
evolution verifiable by optical means. Much excitement has been drawn by
theoretical predictions of giant piezo-electricity and ferroelectricity in
these materials, and we estimate a pyroelectric response of about here. These results uncover the fundamental role of
temperature as a control knob for the physical properties of few-layer group-IV
monochalcogenidesComment: Supplementary information included. Published versio
Ultra-tuning of nonlinear drumhead MEMS resonators by electro-thermoelastic buckling
Nonlinear micro-electro-mechanical systems (MEMS) resonators open new
opportunities in sensing and signal manipulation compared to their linear
counterparts by enabling frequency tuning and increased bandwidth. Here, we
design, fabricate and study drumhead resonators exhibiting strongly nonlinear
dynamics and develop a reduced order model (ROM) to capture their response
accurately. The resonators undergo electrostatically-mediated thermoelastic
buckling which tunes their natural frequency from 4.7 to 11.3 MHz, a factor of
2.4x tunability. Moreover, the imposed buckling switches the nonlinearity of
the resonators between purely stiffening, purely softening, and even
softening-to-stiffening. Accessing these exotic dynamics requires precise
control of the temperature and the DC electrostatic forces near the resonator's
critical-buckling point. To explain the observed tunability, we develop a
one-dimensional physics-based ROM that predicts the linear and nonlinear
response of the fundamental bending mode of these drumhead resonators. The ROM
captures the dynamic effects of the internal stresses resulting from three
sources: The residual stresses from the fabrication process, the mismatch in
thermal expansion between the constituent layers, and lastly, the applied
electrostatic forces. The ROM replicates the observed tunability of linear
(within 5.5% error) and nonlinear responses even near the states of critical
buckling. These remarkable nonlinear and large tunability of the natural
frequency are valuable features for on-chip acoustic devices in broad
applications such as signal manipulation, filtering, and MEMS waveguides
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Heterostructures based on inorganic and organic van der Waals systems
The two-dimensional limit of layered materials has recently been realized through the use of van der Waals (vdW) heterostructures composed of weakly interacting layers. In this paper, we describe two different classes of vdW heterostructures: inorganic vdW heterostructures prepared by co-lamination and restacking; and organicinorganic hetero-epitaxy created by physical vapor deposition of organic molecule crystals on an inorganic vdW substrate. Both types of heterostructures exhibit atomically clean vdW interfaces. Employing such vdW heterostructures, we have demonstrated various novel devices, including graphene/hexagonal boron nitride (hBN) and MoS2 heterostructures for memory devices; graphene/MoS2/WSe2/graphene vertical p-n junctions for photovoltaic devices, and organic crystals on hBN with graphene electrodes for high-performance transistorsPhysic
Graphene growth on h-BN by Molecular Beam Epitaxy
The growth of single layer graphene nanometer size domains by solid carbon
source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is
demonstrated. Formation of single-layer graphene is clearly apparent in Raman
spectra which display sharp optical phonon bands. Atomic-force microscope
images and Raman maps reveal that the graphene grown depends on the surface
morphology of the h-BN substrates. The growth is governed by the high mobility
of the carbon atoms on the h-BN surface, in a manner that is consistent with
van der Waals epitaxy. The successful growth of graphene layers depends on the
substrate temperature, but is independent of the incident flux of carbon atoms.Comment: Solid State Communications, 201
Atomically thin p–n junctions with van der Waals heterointerfaces
Semiconductor p–n junctions are essential building blocks for electronic and optoelectronic devices. In conventional p–n junctions, regions depleted of free charge carriers form on either side of the junction, generating built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes influenced by the spatial extent of this depletion region. With the advent of atomically thin van der Waals materials and their heterostructures, it is now possible to realize a p–n junction at the ultimate thickness limit3, 4, 5, 6, 7, 8, 9, 10. Van der Waals junctions composed of p- and n-type semiconductors—each just one unit cell thick—are predicted to exhibit completely different charge transport characteristics than bulk heterojunctions10, 11, 12. Here, we report the characterization of the electronic and optoelectronic properties of atomically thin p–n heterojunctions fabricated using van der Waals assembly of transition-metal dichalcogenides. We observe gate-tunable diode-like current rectification and a photovoltaic response across the p–n interface. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic p–n junction between graphene layers enhances the collection of the photoexcited carriers. The atomically scaled van der Waals p–n heterostructures presented here constitute the ultimate functional unit for nanoscale electronic and optoelectronic devices.Physic
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