Lateral and Vertical Stiffness of the Epitaxial <i>h</i>‑BN Monolayer on Rh(111)

The response to strain in covalently bound single layers has a large impact on the growth and properties. We investigate the quasi-two-dimensional hexagonal boron nitride on Rh(111), which is interesting due to its high intrinsic corrugation. We use combined atomic force and scanning tunneling microscopy to measure the response of this monolayer to probing forces. Three-dimensional force maps and the atomic resolution of the layer enable us to determine lateral and vertical stiffness of this prototypical system with unprecedented spatial resolution. Extremely low stiffnesses ≈1 N/m are derived. Our experiments give insights into the mechanical properties of corrugated incommensurate layers that buckle into the third dimension to relieve strain

Efficient Charge Extraction out of Nanoscale Schottky Contacts to CdS Nanowires

Charge recombination dynamics in semiconductor nanostructures is of vital importance for photovoltaic or photodetector device applications. We use local photocurrent measurements to explore spatially separated drift- and diffusion-currents close to the edge of gold contacts on top of cadmium sulfide nanowires. By theoretical modeling of the experimental photocurrent profiles, the electron diffusion length and lifetime in the wires are obtained to 0.8 μm and 1 ns, respectively. In contrast to bulk devices, the nanoscale dimensions of the involved Schottky contacts enable a highly efficient charge carrier extraction from below the electrodes. This finding paves the way for designing nanostructured optoelectronic devices of improved performance

Local Conformational Switching of Supramolecular Networks at the Solid/Liquid Interface

We use the electric field in a scanning tunneling microscope to manipulate the transition between open and close packed 2D supramolecular networks of neutral molecules in nonpolar media. We found that while the magnitude of the applied field is not decisive, it is the sign of the polarization that needs to be maintained to select one particular polymorph. Moreover, the switching is independent of the solvent used and fully reversible. We propose that the orientation of the surface dipole determined by the electric field might favor different conformation-depended charge transfer mechanisms of the adsorbates to the surface, inducing open (closed) structures for negative (positive) potentials. Our results show the use of local fields to select the polymorphic outcome of supramolecular assemblies at the solid/liquid interface. The effect has potential to locally control the capture and release of analytes in host–guest systems and the 2D morphology in multicomponent layers

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/SiO_<i>x</i> 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 Photothermoelectric Conversion in Lateral Topological Insulator Heterojunctions

Tuning the electron and phonon transport properties of thermoelectric materials by nanostructuring has enabled improving their thermopower figure of merit. Three-dimensional topological insulators, including many bismuth chalcogenides, attract increasing attention for this purpose, as their topologically protected surface states are promising to further enhance the thermoelectric performance. While individual bismuth chalcogenide nanostructures have been studied with respect to their photothermoelectric properties, nanostructured p–n junctions of these compounds have not yet been explored. Here, we experimentally investigate the room temperature thermoelectric conversion capability of lateral heterostructures consisting of two different three-dimensional topological insulators, namely, the n-type doped Bi₂Te₂Se and the p-type doped Sb₂Te₃. Scanning photocurrent microscopy of the nanoplatelets reveals efficient thermoelectric conversion at the p–n heterojunction, exploiting hot carriers of opposite sign in the two materials. From the photocurrent data, a Seebeck coefficient difference of Δ<i>S</i> = 200 μV/K was extracted, in accordance with the best values reported for the corresponding bulk materials. Furthermore, it is in very good agreement with the value of Δ<i>S</i> = 185 μV/K obtained by DFT calculation taking into account the specific doping levels of the two nanostructured components

Graphene Sublattice Symmetry and Isospin Determined by Circular Dichroism in Angle-Resolved Photoemission Spectroscopy

The Dirac-like electronic structure of graphene originates from the equivalence of the two basis atoms in the honeycomb lattice. We show that the characteristic parameters of the initial state wave function (sublattice symmetry and isospin) can be determined using angle-resolved photoemission spectroscopy (ARPES) with circularly polarized synchrotron radiation. At a photon energy of <i>h</i>ν = 52 eV, transition matrix element effects can be neglected allowing us to determine sublattice symmetry and isospin with high accuracy using a simple theoretical model

Landing Proteins on Graphene Trampoline Preserves Their Gas-Phase Folding on the Surface

Molecule–surface collisions are known to initiate dynamics that lead to products inaccessible by thermal chemistry. These collision dynamics, however, have mostly been examined on bulk surfaces, leaving vast opportunities unexplored for molecular collisions on nanostructures, especially on those that exhibit mechanical properties radically different from those of their bulk counterparts. Probing energy-dependent dynamics on nanostructures, particularly for large molecules, has been challenging due to their fast time scales and high structural complexity. Here, by examining the dynamics of a protein impinging on a freestanding, single-atom-thick membrane, we discover molecule-on-trampoline dynamics that disperse the collision impact away from the incident protein within a few picoseconds. As a result, our experiments and ab initio calculations show that cytochrome c retains its gas-phase folded structure when it collides onto freestanding single-layer graphene at low energies (∼20 meV/atom). The molecule-on-trampoline dynamics, expected to be operative on many freestanding atomic membranes, enable reliable means to transfer gas-phase macromolecular structures onto freestanding surfaces for their single-molecule imaging, complementing many bioanalytical techniques

Active Conformation Control of Unfolded Proteins by Hyperthermal Collision with a Metal Surface

The physical and chemical properties of macromolecules like proteins are strongly dependent on their conformation. The degrees of freedom of their chemical bonds generate a huge conformational space, of which, however, only a small fraction is accessible in thermal equilibrium. Here we show that soft-landing electrospray ion beam deposition (ES-IBD) of unfolded proteins allows to control their conformation. The dynamics and result of the deposition process can be actively steered by selecting the molecular ion beam’s charge state or tuning the incident energy. Using these parameters, protein conformations ranging from fully extended to completely compact can be prepared selectively on a surface, as evidenced on the subnanometer/amino acid resolution level by scanning tunneling microscopy (STM). Supported by molecular dynamics (MD) simulations, our results demonstrate that the final conformation on the surface is reached through a mechanical deformation during the hyperthermal ion surface collision. Our experimental results independently confirm the findings of ion mobility spectrometry (IMS) studies of protein gas phase conformations. Moreover, we establish a new route for the processing of macromolecular materials, with the potential to reach conformations that would be inaccessible otherwise

Landing Proteins on Graphene Trampoline Preserves Their Gas-Phase Folding on the Surface

Molecule–surface collisions are known to initiate dynamics that lead to products inaccessible by thermal chemistry. These collision dynamics, however, have mostly been examined on bulk surfaces, leaving vast opportunities unexplored for molecular collisions on nanostructures, especially on those that exhibit mechanical properties radically different from those of their bulk counterparts. Probing energy-dependent dynamics on nanostructures, particularly for large molecules, has been challenging due to their fast time scales and high structural complexity. Here, by examining the dynamics of a protein impinging on a freestanding, single-atom-thick membrane, we discover molecule-on-trampoline dynamics that disperse the collision impact away from the incident protein within a few picoseconds. As a result, our experiments and ab initio calculations show that cytochrome c retains its gas-phase folded structure when it collides onto freestanding single-layer graphene at low energies (∼20 meV/atom). The molecule-on-trampoline dynamics, expected to be operative on many freestanding atomic membranes, enable reliable means to transfer gas-phase macromolecular structures onto freestanding surfaces for their single-molecule imaging, complementing many bioanalytical techniques

Raman Characterization of the Charge Density Wave Phase of 1T-TiSe₂: From Bulk to Atomically Thin Layers

Raman scattering is a powerful tool for investigating the vibrational properties of two-dimensional materials. Unlike the 2H phase of many transition metal dichalcogenides, the 1T phase of TiSe₂ features a Raman-active shearing and breathing mode, both of which shift toward lower energy with increasing number of layers. By systematically studying the Raman signal of 1T-TiSe₂ in dependence of the sheet thickness, we demonstrate that the charge density wave transition of this compound can be reliably determined from the temperature dependence of the peak position of the E_g mode near 136 cm^–1. The phase transition temperature is found to first increase with decreasing thickness of the sheets, followed by a decrease due to the effect of surface oxidation. The Raman spectroscopy-based method is expected to be applicable also to other 1T-phase transition metal dichalcogenides featuring a charge density wave transition and represents a valuable complement to electrical transport-based approaches