New Flexible Channels for Room Temperature Tunneling Field Effect Transistors.

Tunneling field effect transistors (TFETs) have been proposed to overcome the fundamental issues of Si based transistors, such as short channel effect, finite leakage current, and high contact resistance. Unfortunately, most if not all TFETs are operational only at cryogenic temperatures. Here we report that iron (Fe) quantum dots functionalized boron nitride nanotubes (QDs-BNNTs) can be used as the flexible tunneling channels of TFETs at room temperatures. The electrical insulating BNNTs are used as the one-dimensional (1D) substrates to confine the uniform formation of Fe QDs on their surface as the flexible tunneling channel. Consistent semiconductor-like transport behaviors under various bending conditions are detected by scanning tunneling spectroscopy in a transmission electron microscopy system (in-situ STM-TEM). As suggested by computer simulation, the uniform distribution of Fe QDs enable an averaging effect on the possible electron tunneling pathways, which is responsible for the consistent transport properties that are not sensitive to bending.

Evidence of a graphene-like Sn-sheet on a Au(111) substrate: electronic structure and transport properties from first principles calculations.

Two dimensional nanostructures of group IV elements have attracted a great deal of attention because of their fundamental and technological applications. A graphene-like single layer of tin atoms, commonly called stanene, has recently been predicted to behave like a quantum spin Hall insulator. Here we report the atomic structure, stability and electron transport properties of stanene stabilized on a gold substrate. The optimization of geometry and electronic structure was carried out using a plane-wave based pseudo-potential approach. This work is divided into three parts: (i) the nature of chemical interaction between tin atoms and the gold support, (ii) the geometrical shape and electronic structure of the tin layer on the gold support and (iii) the electron transport behavior of the gold supported tin layer. The results show that tin atoms bind to the gold support through strong chemical bonds and significant electronic charge transfer occurs from tin to the gold support. Remarkably, for a layer of tin atoms, while a buckled structure is preferred in the free state, a planar graphene-like atomic arrangement is stabilized on the gold support. This structural change corroborates the metal-like band structure of the planar stanene in comparison to the semi-metallic buckled configuration. The tunneling current of the supported tin layer shows Ohmic-like behavior and the calculated STM pattern of the supported tin layer shows distinct images of 'holes', characteristic of the hexagonal lattice.

Electronic stability and electron transport properties of atomic wires anchored on the MoS2 monolayer.

The stability, electronic structure, and electron transport properties of metallic monoatomic wires anchored on the MoS2 monolayer are investigated within the density functional theory. The anchoring of the atomic wires on the semiconducting monolayer significantly modifies its electronic properties; the metallic characteristics of the assembled monolayers appear in the density of states and band structure of the system. We find that Cu, Ag and Au wires induce the so-called n-type doping effect, whereas Pt wires induce a p-type doping effect in the monolayer. The distinctly different behavior of Pt-MoS2 compared to the rest of the metallic wires is reflected in the calculated current-voltage characteristics of the assembled monolayers with a highly asymmetric behavior of the out-of-the-plane tunneling current with respect to the polarity of the external bias. The results of the present study are likely to extend the functionality of the MoS2 monolayer as a candidate material for the novel applications in the areas of catalysis and optoelectronic devices.

Effect of Si doping on the electronic properties of BN monolayer.

The effect of Si doping on the stability, electronic structure, and electron transport properties of boron nitride (BN) monolayer has been investigated by density functional theory method. Unique features in the electron transport characteristics consisting of a significant enhancement of current at the Si site, diode-like asymmetric current-voltage response, and negative differential resistance are noted for the doped BN monolayer. These features are found to result from new "tunnel" channels induced by the substitutional Si atom near Fermi level in the band gap. The calculated position-projected tunneling currents providing scanning tunneling micrograph clearly discern the site-dependence of the Si atom and can be used to distinguish substitutional sites of atomic dopants in the monolayer.

Room-temperature tunneling behavior of boron nitride nanotubes functionalized with gold quantum dots.

One-dimensional arrays of gold quantum dots (QDs) on insulating boron nitride nanotubes (BNNTs) can form conduction channels of tunneling field-effect transistors. We demonstrate that tunneling currents can be modulated at room temperature by tuning the lengths of QD-BNNTs and the gate potentials. Our discovery will inspire the creative use of nanostructured metals and insulators for future electronic devices.

Electron tunneling characteristics of a cubic quantum dot, (PbS)32.

The electron transport properties of the cubic quantum dot, (PbS)32, are investigated. The stability of the quantum dot has been established by recent scanning tunneling microscope experiments [B. Kiran, A. K. Kandalam, R. Rallabandi, P. Koirala, X. Li, X. Tang, Y. Wang, H. Fairbrother, G. Gantefoer, and K. Bowen, J. Chem. Phys. 136(2), 024317 (2012)]. In spite of the noticeable energy band gap (~2 eV), a relatively high tunneling current for (PbS)32 is predicted affirming the observed bright images for (PbS)32. The calculated I-V characteristics of (PbS)32 are predicted to be substrate-dependent; (PbS)32 on the Au (001) exhibits the molecular diode-like behavior and the unusual negative differential resistance effect, though this is not the case with (PbS)32 on the Au (110). Appearance of the conduction channels associated with the hybridized states of quantum dot and substrate together with their asymmetric distribution at the Fermi level seem to determine the tunneling characteristics of the system.

NMR chemical shifts as a tool to analyze first principles molecular dynamics simulations in condensed phases: the case of liquid water.

We present (1)H NMR chemical shift calculations of liquid water based on first principles molecular dynamics simulations under periodic boundary conditions. We focus on the impact of computational parameters on the structural and spectroscopic data, which is an important question for understanding how sensitive the computed (1)H NMR resonances are upon variation of the simulation setup. In particular, we discuss the influence of the exchange-correlation functional and the size of the basis set, the choice for the fictitious electronic mass and the use of pseudopotentials for the nuclear magnetic resonance (NMR) calculation on one hand and the underlying Car-Parrinello-type molecular dynamics simulations on the other hand. Our findings show that the direct effect of these parameters on (1)H shifts is not big, whereas the indirect dependence via the structural data is more important. The (1)H NMR chemical shifts clearly reflect the induced structural changes, illustrating once again the sensitivity of (1)H NMR observables on small changes in the local chemical structure of complex hydrogen-bonded liquids.