96 research outputs found
Two-dimensional magnetotransport in Bi2Te2Se nanoplatelets
Single-crystalline Bi2Te2Se nanoplates with thicknesses between 8 and 30 nm
and lateral sizes of several micrometers were synthesized by a vapour-solid
growth method. Angle-dependent magnetoconductance measurements on individual
nanoplates revealed the presence of a two-dimensional weak anti-localization
effect. In conjunction with gate-dependent charge transport studies performed
at different temperatures, evidence was gained that this effect originates from
the topologically protected surface states of the nanoplates
Tunneling spectroscopy of localized states of barriers in vertical van der Waals heterostructures
In transition metal dichalcogenides, defects have been found to play an
important role, affecting doping, spin-valley relaxation dynamics, and
assisting in proximity effects of spin-orbit coupling. Here, we study localized
states in and how they affect tunneling through van der Waals
heterostructures of h-BN/graphene//metal. The obtained
conductance maps as a function of bias and gate voltage reveal single-electron
transistor behavior (Coulomb blockade) with a rich set of transport features
including excited states and negative differential resistance regimes. Applying
a perpendicular magnetic field, we observe a shift in the energies of the
quantum levels and information about the orbital magnetic moment of the
localized states is extracted
Correlation of nanoscale electromechanical and mechanical properties of twisted double bi-layer graphene via UFM, PFM, and E-HFM
Recently, multiple theoretical and experimental studies have been published regarding the properties of stacked two-dimensional (2D) layers forming a twisted heterostructure. This field (known as twistronics) shows that properties of 2D materials can be modified to a great degree, including bandgap modulation and creating superconductive structures. Given the versatility that these structures have, many exciting engineering is being applied to them resulting in promising properties. In this study, we investigated a heterostructure composed of two twisted graphene bi-layers with a small angle between them (1.1º), where an atomic reconstruction is induced changing the lattice symmetry and creating a Moiré pattern. The electrical and mechanical properties of the 2D nanostructure are affected by this symmetry reconstruction, generating relaxation-induced strain gradients. We compared nanomechanical mapping via Ultrasonic Force Microscopy (UFM) and electromechanical response probed by Piezoresponse Force Microscopy (PFM) and Electrical Heterodyne Force Microscopy (E-HFM). These allowed us to assign Moiré patterns of the heterostructure to the particular crystallographic arrangements and to quantify the local Young’s modulus variation between single and double domain walls. Moreover, by measuring these domain walls specifically with PFM, it is possible to extract evidence of non-uniform strain in stretched triangular domains in the Moiré pattern. The phase images from the E-HFM allow us to observe a fast time-domain nanoelectromechanical relaxation in the order of picoseconds with nanoscale lateral resolution
Growth of High-Mobility Bi2Te2Se Nanoplatelets on hBN Sheets by van der Waals Epitaxy
The electrical detection of the surface states of topological insulators is
strongly impeded by the interference of bulk conduction, which commonly arises
due to pronounced doping associated with the formation of lattice defects. As
exemplified by the topological insulator Bi2Te2Se, we show that via van der
Waals epitaxial growth on thin hBN substrates the structural quality of such
nanoplatelets can be substantially improved. The surface state carrier mobility
of nanoplatelets on hBN is increased by a factor of about 3 compared to
platelets on conventional Si/SiOx substrates, which enables the observation of
well-developed Shubnikov-de Haas oscillations. We furthermore demonstrate the
possibility to effectively tune the Fermi level position in the films with the
aid of a back gate
Efficient heating of single-molecule junctions for thermoelectric studies at cryogenic temperatures
The energy dependent thermoelectric response of a single molecule contains valuable information about its transmission function and its excited states. However, measuring it requires devices that can efficiently heat up one side of the molecule while being able to tune its electrochemical potential over a wide energy range. Furthermore, to increase junction stability, devices need to operate at cryogenic temperatures. In this work, we report on a device architecture to study the thermoelectric properties and the conductance of single molecules simultaneously over a wide energy range. We employ a sample heater in direct contact with the metallic electrodes contacting the single molecule which allows us to apply temperature biases up to ΔT = 60 K with minimal heating of the molecular junction. This makes these devices compatible with base temperatures Tbath < 2 K and enables studies in the linear (Δ T ≪ T molecule) and nonlinear (Δ T ≫ T molecule) thermoelectric transport regimes
Single-material MoS2 thermoelectric junction enabled by substrate engineering
To realize a thermoelectric power generator, typically, a junction between two materials with different Seebeck coefficients needs to be fabricated. Such differences in Seebeck coefficients can be induced by doping, which renders it difficult when working with two-dimensional (2d) materials. However, doping is not the only way to modulate the Seebeck coefficient of a 2d material. Substrate-altered electron–phonon scattering mechanisms can also be used to this end. Here, we employ the substrate effects to form a thermoelectric junction in ultrathin, few-layer MoS2 films. We investigated the junctions with a combination of scanning photocurrent microscopy and scanning thermal microscopy. This allows us to reveal that thermoelectric junctions form across the substrate-engineered parts. We attribute this to a gating effect induced by interfacial charges in combination with alterations in the electron–phonon scattering mechanisms. This work demonstrates that substrate engineering is a promising strategy for developing future compact thin-film thermoelectric power generators
Distinguishing Lead and Molecule States in Graphene-Based Single-Electron Transistors
Graphene provides a two-dimensional platform for contacting individual molecules, which enables transport spectroscopy of molecular orbital, spin, and vibrational states. Here we report single-electron tunneling through a molecule that has been anchored to two graphene leads. Quantum interference within the graphene leads gives rise to an energy-dependent transmission and fluctuations in the sequential tunnel-rates. The lead states are electrostatically tuned by a global back-gate, resulting in a distinct pattern of varying intensity in the measured conductance maps. This pattern could potentially obscure transport features that are intrinsic to the molecule under investigation. Using ensemble averaged magneto-conductance measurements, lead and molecule states are disentangled, enabling spectroscopic investigation of the single molecule
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