Ultimate Charge Transport Regimes in Doping-Controlled Graphene Laminates: Phonon-Assisted Processes Revealed by the Linear Magnetoresistance.

Understanding and controlling the electrical properties of solution-processed 2D materials is key to further printed electronics progress. Here, we demonstrate that the thermolysis of the aromatic intercalants utilized in nanosheet exfoliation for graphene laminates allows for high intrinsic mobility and the simultaneous control of doping type (- and -) and concentration over a wide range. We establish that the intraflake mobility is high by observing a linear magnetoresistance of such solution-processed graphene laminates and using it to devolve the interflake tunneling and intralayer magnetotransport.

Thermoelectric Performance of Tetrahedrite (CuSbS) Thin Films: The Influence of the Substrate and Interlayer.

In the present work, tetrahedrite CuSbS thin films were deposited on various substrates via aerosol-assisted chemical vapor deposition (AACVD) using diethyldithiocarbamate complexes as precursors. A buffer layer of SbO with a small lattice mismatch to CuSbS was applied to one of the glass substrates to improve the quality of the deposited thin film. The buffer layer increased the coverage of the CuSbS thin film, resulting in improved electrical transport properties.

Tunnel junctions based on interfacial two dimensional ferroelectrics.

Van der Waals heterostructures have opened new opportunities to develop atomically thin (opto)electronic devices with a wide range of functionalities. The recent focus on manipulating the interlayer twist angle has led to the observation of out-of-plane room temperature ferroelectricity in twisted rhombohedral bilayers of transition metal dichalcogenides. Here we explore the switching behaviour of sliding ferroelectricity using scanning probe microscopy domain mapping and tunnelling transport measurements.

Exceptional Thermoelectric Performance of Cu(Zn,Fe,Cd)SnS Thin Films.

High-quality Cu(Zn,Fe,Cd)SnS (CZFCTS) thin films based on the parent CZTS were prepared by aerosol-assisted chemical vapor deposition (AACVD). Substitution of Zn by Fe and Cd significantly improved the electrical transport properties, and monophasic CZFCTS thin films exhibited a maximum power factor (PF) of ∼0.22 μW cm K at 575 K.

Enhanced Thermoelectric Performance of Tin(II) Sulfide Thin Films Prepared by Aerosol Assisted Chemical Vapor Deposition.

Orthorhombic SnS exhibits excellent thermoelectric performance as a consequence its relatively high Seebeck coefficient and low thermal conductivity. In the present work, polycrystalline orthorhombic SnS thin films were prepared by aerosol-assisted chemical vapor deposition (AACVD) using the single source precursor dibutyl-(diethyldithiocarbamato)tin(IV) [Sn(CH)(SCN(CH))]. We examined the effects of the processing parameters on the composition, microstructure, and electrical transport properties of the SnS films.

Interfacial ferroelectricity in marginally twisted 2D semiconductors.

Twisted heterostructures of two-dimensional crystals offer almost unlimited scope for the design of new metamaterials. Here we demonstrate a room temperature ferroelectric semiconductor that is assembled using mono- or few-layer MoS. These van der Waals heterostructures feature broken inversion symmetry, which, together with the asymmetry of atomic arrangement at the interface of two 2D crystals, enables ferroelectric domains with alternating out-of-plane polarization arranged into a twist-controlled network.

High- dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures.

The anisotropy of hexagonal boron nitride (hBN) gives rise to hyperbolic phonon-polaritons (HPhPs), notable for their volumetric frequency-dependent propagation and strong confinement. For frustum (truncated nanocone) structures, theory predicts five, high-order HPhPs, sets, but only one set was observed previously with far-field reflectance and scattering-type scanning near-field optical microscopy. In contrast, the photothermal induced resonance (PTIR) technique has recently permitted sampling of the full HPhP dispersion and observing such elusive predicted modes; however, the mechanism underlying PTIR sensitivity to these weakly-scattering modes, while critical to their understanding, has not yet been clarified.

Atomic reconstruction in twisted bilayers of transition metal dichalcogenides.

Van der Waals heterostructures form a unique class of layered artificial solids in which physical properties can be manipulated through controlled composition, order and relative rotation of adjacent atomic planes. Here we use atomic-resolution transmission electron microscopy to reveal the lattice reconstruction in twisted bilayers of the transition metal dichalcogenides, MoS and WS. For twisted 3R bilayers, a tessellated pattern of mirror-reflected triangular 3R domains emerges, separated by a network of partial dislocations for twist angles θ < 2°.

Composite super-moiré lattices in double-aligned graphene heterostructures.

When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials.

Ultra-thin van der Waals crystals as semiconductor quantum wells.

Control over the quantization of electrons in quantum wells is at the heart of the functioning of modern advanced electronics; high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. However, this avenue has not been explored in the case of 2D materials. Here we apply this concept to van der Waals heterostructures using the thickness of exfoliated crystals to control the quantum well dimensions in few-layer semiconductor InSe.

Gate-Defined Quantum Confinement in InSe-Based van der Waals Heterostructures.

Indium selenide, a post-transition metal chalcogenide, is a novel two-dimensional (2D) semiconductor with interesting electronic properties. Its tunable band gap and high electron mobility have already attracted considerable research interest. Here we demonstrate strong quantum confinement and manipulation of single electrons in devices made from few-layer crystals of InSe using electrostatic gating.

Nanoscale Mapping and Spectroscopy of Nonradiative Hyperbolic Modes in Hexagonal Boron Nitride Nanostructures.

The inherent crystal anisotropy of hexagonal boron nitride (hBN) provides the ability to support hyperbolic phonon polaritons, that is, polaritons that can propagate with very large wave vectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subdiffractional dimensions, support three-dimensionally confined optical modes in the mid-infrared. Because of optical selection rules, only a few of the many theoretically predicted modes have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy (s-SNOM).

High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices.

Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temperatures are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillation that does not rely on Landau quantization.

Edge currents shunt the insulating bulk in gapped graphene.

An energy gap can be opened in the spectrum of graphene reaching values as large as 0.2 eV in the case of bilayers. However, such gaps rarely lead to the highly insulating state expected at low temperatures.

Imaging of Anomalous Internal Reflections of Hyperbolic Phonon-Polaritons in Hexagonal Boron Nitride.

We use scanning near-field optical microscopy to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium. The obtained images are dominated by a series of "hot" rings that occur on the sloped sidewalls of the nanocones. The ring positions depend on the incident laser frequency and the nanocone shape.

Quality Heterostructures from Two-Dimensional Crystals Unstable in Air by Their Assembly in Inert Atmosphere.

Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest react and decompose in air, which has severely hindered their investigation and potential applications. Here we introduce a remedial approach based on cleavage, transfer, alignment, and encapsulation of air-sensitive crystals, all inside a controlled inert atmosphere.

Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing.

Hyperbolic materials exhibit sub-diffractional, highly directional, volume-confined polariton modes. Here we report that hyperbolic phonon polaritons allow for a flat slab of hexagonal boron nitride to enable exciting near-field optical applications, including unusual imaging phenomenon (such as an enlarged reconstruction of investigated objects) and sub-diffractional focusing. Both the enlarged imaging and the super-resolution focusing are explained based on the volume-confined, wavelength dependent propagation angle of hyperbolic phonon polaritons.

Detecting topological currents in graphene superlattices.

Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from graphene's two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow. We observed this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances as large as several micrometers away from the nominal current path.

Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride.

Strongly anisotropic media, where the principal components of the dielectric tensor have opposite signs, are called hyperbolic. Such materials exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional length scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces size-dependent limitations on optical confinement.

Electronic properties of graphene encapsulated with different two-dimensional atomic crystals.

Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micrometer-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulfides and hBN are found to exhibit consistently high carrier mobilities of about 60 000 cm(2) V(-1) s(-1).

Unintentional high-density p-type modulation doping of a GaAs/AlAs core-multishell nanowire.

Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of considerable technological importance but remains a challenge due to the amphoteric behavior of the dopant atoms. Here we show that placing a narrow GaAs quantum well in the AlAs shell effectively getters residual carbon acceptors leading to an unintentional p-type doping. Magneto-optical studies of such a GaAs/AlAs core-multishell NW reveal quantum confined emission.

Influence of metal deposition on exciton-surface plasmon polariton coupling in GaAs/AlAs/GaAs core-shell nanowires studied with time-resolved cathodoluminescence.

The coupling of excitons to surface plasmon polaritons (SPPs) in Au- and Al-coated GaAs/AlAs/GaAs core-shell nanowires, possessing diameters of ~100 nm, was probed using time-resolved cathodoluminescence (CL). Excitons were generated in the metal coated nanowires by injecting a pulsed high-energy electron beam through the thin metal films. The Purcell enhancement factor (FP) was obtained by direct measurement of changes in the temperature-dependent radiative lifetime caused by the nanowire exciton-SPP coupling and compared with a model that takes into account the dependence of FP on the distance from the metal film and the thickness of the film covering the GaAs nanowires.

High-efficiency Cooper pair splitting demonstrated by two-particle conductance resonance and positive noise cross-correlation.

Entanglement is at the heart of the Einstein-Podolsky-Rosen paradox, where the non-locality is a necessary ingredient. Cooper pairs in superconductors can be split adiabatically, thus forming entangled electrons. Here, we fabricate such an electron splitter by contacting an aluminium superconductor strip at the centre of a suspended InAs nanowire.

Wide-band current preamplifier for conductance measurements with large input capacitance.

A wide-band current preamplifier based on a composite operational amplifier is proposed. It has been shown that the bandwidth of the preamplifier can be significantly increased by enhancing the effective open-loop gain. The described 10(7) V/A current gain preamplifier had the bandwidth of about 100 kHz with the 1 nF input shunt capacitance.

Magnetoresistance oscillations of superconducting Al-film cylinders covering InAs nanowires below the quantum critical point.

When odd multiples of half flux quanta thread a cylindrical superconducting shell with a diameter d shorter than the zero temperature coherence length ξ(0), superconductivity is predicted to be destroyed. We show here that as d is reduced in comparison to ξ(0) the resistance attains the normal state value, which seems to be temperature independent in the vicinity of half flux quanta. The data are in agreement with recent theoretical results.