Monolayer Molybdenum Disulfide Nanoribbons with High Optical Anisotropy

Two-dimensional Molybdenum Disulfide (MoS2) has shown promising prospects for the next generation electronics and optoelectronics devices. The monolayer MoS2 can be patterned into quasi-one-dimensional anisotropic MoS2 nanoribbons (MNRs), in which theoretical calculations have predicted novel properties. However, little work has been carried out in the experimental exploration of MNRs with a width of less than 20 nm where the geometrical confinement can lead to interesting phenomenon. Here, we prepared MNRs with width between 5 nm to 15 nm by direct helium ion beam milling. High optical anisotropy of these MNRs is revealed by the systematic study of optical contrast and Raman spectroscopy. The Raman modes in MNRs show strong polarization dependence. Besides that the E' and A'1 peaks are broadened by the phonon-confinement effect, the modes corresponding to singularities of vibrational density of states are activated by edges. The peculiar polarization behavior of Raman modes can be explained by the anisotropy of light absorption in MNRs, which is evidenced by the polarized optical contrast. The study opens the possibility to explore quasione-dimensional materials with high optical anisotropy from isotropic 2D family of transition metal dichalcogenides

Quantum Conductance Oscillations in Metal/Molecule/Metal Switches at Room Temperature

Conductance switching has been reported in many molecular junction devices, but in most cases has not been convincingly explained. We investigate conductance switching in Pt/stearic acid monolayer/Ti devices using pressure-modulated conductance microscopy. For devices with conductance G>>G_Q or G<<G_Q, where GQ=2e^2/h is the conductance quantum, pressure-induced conductance peaks <30 nm in diameter are observed, indicating the formation of nanoscale conducting pathways between the electrodes. For devices with G~ 1- 2 G_Q, in addition to conductance peaks we also observed conductance dips and oscillations in response to localized pressure. These results can be modeled by considering interfering electron waves along a quantum conductance channel between two partially transmitting electrode surfaces. Our findings underscore the possible use of these devices as atomic-scale switches

Lattice dynamics localization in low-angle twisted bilayer graphene

A low twist angle between the two stacked crystal networks in bilayer graphene enables self-organized lattice reconstruction with the formation of a periodic domain. This superlattice modulates the vibrational and electronic structures, imposing new rules for electron-phonon coupling and the eventual observation of strong correlation and superconductivity. Direct optical images of the crystal superlattice in reconstructed twisted bilayer graphene are reported here, generated by the inelastic scattering of light in a nano-Raman spectroscope. The observation of the crystallographic structure with visible light is made possible due to lattice dynamics localization, the images resembling spectral variations caused by the presence of strain solitons and topological points. The results are rationalized by a nearly-free-phonon model and electronic calculations that highlight the relevance of solitons and topological points, particularly pronounced for structures with small twist angles. We anticipate our discovery to play a role in understanding Jahn-Teller effects and electronic Cooper pairing, among many other important phonon-related effects, and it may be useful for characterizing devices in the most prominent platform for the field of twistronics.Comment: 9 pages, 8 figure

Ordered Arrays of Rare Earth Silicide Nanowires on Si(001)

Rare earth silicides have been demonstrated to self-assemble during epitaxial growth as one-dimensional nanostructures with preferred orientation along Si '100' on Si[111] and Si[001]. The evolution of the one-dimensional structure during epitaxial growth has been attributed to an anisotropic lattice mismatch of the two orthogonal axis of the hexagonal unit cell with respect to Si. On highly oriented Si[001] substrates, nanowires align their long axis along '100' Si: thus, two orientations of nanowires were obtained having their long axes orthogonal to one another. We have now demonstrated that alignment of rare earth silicide nanowires can be achieved along a single direction by growth on vicinal Si[001] substrates. Self-assembled ErSi2 and DySi2 wires aligned along Si [110] have been grown at 600°C with aspect ratios exceeding 100 and feature heights on the order of 1 atomic layer. The nanowires were characterized in situ with scanning tunneling microscopy. These rare earth silicide nanowires may have applications as non-lithographically fabricated small scale interconnects due to high electrical conductivity and low Schottky barrier to n-type Si

Ordered arrays of rare-earth silicide nanowires on Si(001)

Ordered arrays of self-assembled ErSi2-x, SmSi2-x, and DySi2-x nanowires aligned along Si [110] have been grown at 600°C on Si(001) vicinal substrates. The nanowires grow perpendicular to the Si dimer rows and are 1.5-5nm wide, approximately 2 monolayers high and up to 1μm long. These nanowires were characterized in situ with scanning tunneling microscopy. © 2002 Elsevier Science B.V. All rights reserved

Memristive switching mechanism for matal/oxide/metal nano-devices

Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron –ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance