Douglas Banyai

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.

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

We present 1H 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 1H 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 1H shifts is not big, whereas the indirect dependence via the structural data is more important. The 1H NMR chemical shifts clearly reflect the induced structural changes, illustrating once again the sensitivity of 1H NMR observables on small changes in the local chemical structure of complex hydrogen‐bonded liquids. Copyright

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.

Simulation of charge transport in multi-island tunneling devices: Application to disordered one-dimensional systems at low and high biases

Although devices have been fabricated displaying interesting single-electron transport characteristics, there has been limited progress in the development of tools that can simulate such devices based on their physical geometry over a range of bias conditions up to a few volts per junction. In this work, we present the development of a multi-island transport simulator, MITS, a simulator of tunneling transport in multi-island devices that takes into account geometrical and material parameters, and can span low and high source-drain biases. First, the capabilities of MITS are demonstrated by modeling experimental devices described in the literature, and showing that the simulated device characteristics agree well with the experimental observations. Then, the results of studies of charge transport through a long one-dimensional (1D) chain of gold nano-islands on an insulating substrate are presented. Current-voltage (IV) characteristics are investigated as a function of the overall chain-length and temperature. Under high bias conditions, where temperature has a minimal effect, the IV characteristics are non-Ohmic, and do not exhibit any Coulomb staircase (CS) structures. The overall resistance of the device also increases non-linearly with increasing chain-length. For small biases, IV characteristics show clear CS structures that are more pronounced for larger chain-lengths. The Coulomb blockade and the threshold voltage (Vth ) required for device switching increase linearly with the increase in chain length. With increasing temperature, the blockade effects are diminished as the abrupt increase in current at Vth is washed out and the apparent blockade decreases. Microscopic investigations demonstrate that the overall IV characteristics are a result of a complex interplay among those factors that affect the tunneling rates that are fixed a priori (island sizes, island separations, temperature, etc.), and the evolving charge state of the system, which changes as the applied source-drain bias (VSD ) is changed. In a system of nano-islands with a broad distribution of sizes and inter-island spacings, the applied bias is divided across the junctions as one would expect of a voltage divider, with larger potential drops across the wider junctions and smaller drops across the narrower junctions. As a result, the tunneling resistances across these wider junctions decrease dramatically, relative to the other junctions, at high VSD thereby increasing their electron tunneling rates. IV behavior at high VSD follows a power-law scaling behavior with the exponent dependent on the length of the chain and the degree of disorder in the system.

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.