Abstract: Three-dimensional topological insulator (3D TI) nanowires display remarkable magnetotransport properties that can be attributed to their spin-momentum-locked surface states such as quasiballistic transport and Aharonov-Bohm oscillations. Here, we focus on the transport properties of a 3D TI nanowire with a gated section that forms an electronic Fabry-Perot (FP) interferometer that can be tuned to act as a surface-state filter or energy barrier. By tuning the carrier density and length of the gated section of the wire, the interference pattern can be controlled and the nanowire can become fully transparent for certain topological surface-state input modes while completely filtering out others. We also consider the interplay of FP interference with an external magnetic field, with which Klein tunneling can be induced, and transverse asymmetry of the gated section, e.g. due to a top-gated structure, which displays an interesting analogy with Rashba nanowires. Due to its rich conductance phenomenology, we propose a 3D TI nanowire with gated section as an ideal setup for a detailed transport-based characterization of 3D TI nanowire surface states near the Dirac point, which could be useful towards realizing 3D TI nanowire-based topological superconductivity and Majorana bound states.

Abstract: Following an extensive investigation of various monolayer transition metal dichalcogenides (MX2), research interest has expanded to include multilayer systems. In bilayer MX2, the stacking order strongly impacts the local band structure as it dictates the local confinement and symmetry. Determination of stacking order in multilayer MX(2)domains usually relies on prior knowledge of in-plane orientations of constituent layers. This is only feasible in case of growth resulting in well-defined triangular domains and not useful in-case of closed layers with hexagonal or irregularly shaped islands. Stacking order can be discerned in the reciprocal space by measuring changes in diffraction peak intensities. Advances in detector technology allow fast acquisition of high-quality four-dimensional datasets which can later be processed to extract useful information such as thickness, orientation, twist and strain. Here, we use 4D scanning transmission electron microscopy combined with multislice diffraction simulations to unravel stacking order in epitaxially grown bilayer MoS2. Machine learning based data segmentation is employed to obtain useful statistics on grain orientation of monolayer and stacking in bilayer MoS2.

Abstract: There is an increasing need to understand interfaces between two-dimensional materials to realize an energy efficient boundary with low contact resistance and small heat dissipation. In this respect, we investigated the impact of charge and substitutional atom doping on the electronic transport properties of the hybrid metallic-semiconducting lateral junctions, formed between metallic (1T and 1T(d)) and semiconducting (1H) phases of MoS2 by means of first-principles and non-equilibrium Green function formalism based calculations. Our results clearly revealed the strong influence of the type of interface and crystallographic orientation of the metallic phase on the transport properties of these systems. The Schottky barrier height, which is the dominant mechanism for contact resistance, was found to be as large as 0.63 eV and 1.19 eV for holes and electrons, respectively. We found that armchair interfaces are more conductive as compared to zigzag termination due to the presence of the metallic Mo zigzag chains that are directed along the transport direction. In order to manipulate these barrier heights we investigated the influence of electron doping of the metallic part (i.e. 1T(d) -MoS2). We observed that the Fermi level of the hybrid system moves towards the conduction band of semiconducting 1H-MoS2 due to filling of 4d-orbital of metallic MoS2, and thus the Schottky barrier for electrons decreases considerably. Besides electron doping, we also investigated the effect of substitutional doping of metallic MoS2 by replacing Mo atoms with either Re or Ta. Due to its valency, Re (Ta) behaves as a donor (acceptor) and reduces the Schottky barrier for electrons (holes). Since Re and Ta based transition metal dichalcogenides crystallize in either the 1T(d) or 1T phase, substitutional doping with these atom favors the stabilization of the 1T(d) phase of MoS2. Co-doping of hybrid structure results in an electronic structure, which facilities easy dissociation of excitons created in the 1H part.

Abstract: Pd-C core–shell nanoparticles with independently controllable core size and shell thickness are grown by gas phase synthesis. First, the core size is selected by electrical mobility values of charged particles, and second, the shell thickness is controlled by the concentration of carbon precursor gas. The carbon shell grows by adsorption of carbon precursor gas molecules on the surface of nanoparticles, followed by sintering. The presence of a carbon shell on Pd nanoparticles is potentially important in hydrogen-related applications operating at high temperatures or in catalytic reactions in acidic/aqueous environments.

Abstract: Recently, black phosphorus quantum dots were fabricated experimentally. Motivated by these experiments, we theoretically investigate the electronic and optical properties of rectangular phosphorene quantum dots (RPQDs) in the presence of an in-plane electric field and a perpendicular magnetic field. The energy spectra and wave functions of RPQDs are obtained numerically using the tight-binding approach. We find edge states within the band gap of the RPQD which are well separated from the bulk states. In an undoped RPQD and for in-plane polarized light, due to the presence of well-defined edge states, we find three types of optical transitions which are between the bulk states, between the edge and bulk states, and between the edge states. The electric and magnetic fields influence the bulk-to-bulk, edge-to-bulk, and edge-to- edge transitions differently due to the different responses of bulk and edge states to these fields.

Abstract: Synthesis of 1D-polymer nanowires by a self-assembly method using marginal solvents is an attractive technique. While the formation mechanism is poorly understood, this method is essential in order to control the growth of nanowires. Here we visualized the time-dependent assembly of poly (3-hexyl-thiophene-2,5-diyl) (P3HT) nanowires by atomic force microscopy and scanning tunneling microscopy. The assembly of P3HT nanowires was carried out at room temperature by mixing cyclohexanone (CHN), as a poor solvent, with polymer solution in 1,2-dichlorobenzene (DCB). Both pi-pi stacking and planarization, obtained at the mix volume ratio of P3HT (in DCB):CHN (10:7), were considered during the investigation. We find that the length of nanowires was determined by the ordering of polymers in the polymer repetition direction. Additionally, our density functional theory calculations revealed that the presence of DCB and CHN molecules that stabilize the structural distortions due to tail group of polymers was essential for the core-wire formation.

Abstract: We report the controlled self-organization and switching of newly designed Schiff base (E)-4-((4-(phenylethynyl) benzylidene) amino) benzenethiol (EPBB) molecules on a Au (111) surface at room temperature. Scanning tunneling microscopy and spectroscopy (STM/STS) were used to image and analyze the conformational changes of the EPBB molecules. The conformational change of the molecules was induced by using the STM tip while increasing the tunneling current. The switching of a domain or island of molecules was shown to be induced by the STM tip during scanning. Unambiguous fingerprints of the switching mechanism were observed via STM/STS measurements. Surface-enhanced Raman scattering was employed, to control and identify quantitatively the switching mechanism of molecules in a monolayer. Density functional theory calculations were also performed in order to understand the microscopic details of the switching mechanism. These calculations revealed that the molecular switching behavior stemmed from the strong interaction of the EPBB molecules with the STM tip. Our approach to controlling intermolecular mechanics provides a path towards the bottom-up assembly of more sophisticated molecular machines.

Abstract: Quasi-two-dimensional gallium chalcogenide heterostructures are created by transferring exfoliated few-layer GaSe onto bulk GaTe sheets. Luminescence spectroscopy measurements reveal that the light emission from underlying GaTe layers drastically increases on heterojunction regions where GaSe layers make contact with the GaTe. Density functional theory (DFT) and band offset calculations show that conduction band minimum (CBM) (valance band maximum (VBM)) values of GaSe are higher (lower) in energy compared to GaTe, forming type-I band alignment at the interface. Consequently, GaSe layers provide photo-excited electrons and holes to GaTe sheets through relatively large built-in potential at the interface, increasing overall exciton population and light emission from GaTe. Observed results are not specific to the GaSe/GaTe system but observed on GaS/GaSe heterolayers with type-I band alignment. Observed experimental findings and theoretical studies provide unique insights into interface effects across dissimilar gallium chalcogenides and offer new ways to boost optical performance by simple epitaxial coating.

Abstract: We propose systems that allow a tuning of the phonon transmission function T(omega) in graphene nanoribbons by using C-13 isotope barriers, antidot structures, and distinct boundary conditions. Phonon modes are obtained by an interatomic fifth-nearest neighbor force-constant model (5NNFCM) and T(omega) is calculated using the non-equilibrium Green's function formalism. We show that by imposing partial fixed boundary conditions it is possible to restrict contributions of the in-plane phonon modes to T(omega) at low energy. On the contrary, the transmission functions of out-of-plane phonon modes can be diminished by proper antidot or isotope arrangements. In particular, we show that a periodic array of them leads to sharp dips in the transmission function at certain frequencies omega(nu) which can be pre-defined as desired by controlling their relative distance and size. With this, we demonstrated that by adequate engineering it is possible to govern the magnitude of the ballistic transmission functions T(omega) in graphene nanoribbons. We discuss the implications of these results in the design of controlled thermal transport at the nanoscale as well as in the enhancement of thermo-electric features of graphene-based materials.

Abstract: The fabrication and integration of low-resistance carbon nanotubes (CNTs) for interconnects in future integrated circuits requires characterization techniques providing structural and electrical information at the nanometer scale. In this paper we present a slice-and-view approach based on electrical atomic force microscopy. Material removal achieved by successive scanning using doped ultra-sharp full-diamond probes, manufactured in-house, enables us to acquire two-dimensional (2D) resistance maps originating from different depths (equivalently different CNT lengths) on CNT-based interconnects. Stacking and interpolating these 2D resistance maps results in a three-dimensional (3D) representation (tomogram). This allows insight from a structural (e.g. size, density, distribution, straightness) and electrical point of view simultaneously. By extracting the resistance evolution over the length of an individual CNT we derive quantitative information about the resistivity and the contact resistance between the CNT and bottom electrode.

Abstract: We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

Abstract: We theoretically investigate resonant tunneling through S- and U-shaped nanostructured graphene nanoribbons. A rich structure of resonant tunneling peaks is found emanating from different quasi-bound states in the middle region. The tunneling current can be turned on and off by varying the Fermi energy. Tunability of resonant tunneling is realized by changing the width of the left and/or right leads and without the use of any external gates.

Abstract: The influence of lateral asymmetry on the electronic structure and optical transitions in elliptical strained InAs nanorings is analyzed in the presence of a perpendicular magnetic field. Two-dimensional rings are assumed to have elliptical inner and outer boundaries oriented in mutually orthogonal directions. The influence of the eccentricity of the ring on the energy levels is analyzed. For large eccentricity of the ring, we do not find any AharonovBohm effect, in contrast to circular rings. Rather, the single-particle states of the electrons and the holes are localized as in two laterally coupled quantum dots formed in the lobes of the nanoring. Our work indicates that the control of shape is important for the existence of the AharonovBohm effect in semiconductor nanorings.

Abstract: Hybrid titania films have been prepared using an adapted sol-gel method for obtaining well-dispersed hydrogen plasma-treated multiwall carbon nanotubes in either pure titania or Nb-doped titania. The drop-coating method has been used to fabricate resistive oxygen sensors based on titania or on titania and carbon nanotube hybrids. Morphology and composition studies have revealed that the dispersion of low amounts of carbon nanotubes within the titania matrix does not significantly alter its crystallization behaviour. The gas sensitivity studies performed on the different samples have shown that the hybrid layers based on titania and carbon nanotubes possess an unprecedented responsiveness towards oxygen (i.e. more than four times higher than that shown by optimized Nb-doped TiO(2) films). Furthermore, hybrid sensors containing carbon nanotubes respond at significantly lower operating temperatures than their non-hybrid counterparts. These new hybrid sensors show a strong potential for monitoring traces of oxygen (i.e. <= 10 ppm) in a flow of CO(2), which is of interest for the beverage industry.

Abstract: Nanocrystalline powders of undoped and lanthanide (Pr3+, Tm3+)- doped gadolinium gallium garnet, Gd3Ga5O12 (GGG), were prepared by propellant synthesis and studied by x-ray powder diffraction (XRD), electron diffraction (ED), high-resolution electron microscopy (HREM) and luminescence spectroscopy. The x-ray diffraction patterns of the GGG samples were analysed using the Rietveld method. The Rietveld refinement reveals the existence of two garnet-type phases: both are cubic (space group Ia $(3) over bar $d) with a slightly different lattice parameter and probably a slightly different composition. Electron diffraction and electron microscopy measurements confirm the x-ray diffraction results. EDX measurements for lanthanide-doped samples show that stable solid solutions with composition Gd(3-x)Ln(x)Ga(5)O(12), x approximate to 0.3 ( Ln = Pr; Tm) have been obtained. The luminescence properties of the Tm3+ -doped nanocrystalline GGG samples were measured and analysed.