Mandar M. Deshmukh

Nanofabrication using a stencil mask

We describe tests of a technique to fabricate nanostructures by the evaporation of metal through a stencil mask etched in a suspended silicon nitride membrane. Collimated evaporation through the mask gives metal dots less than 15 nm in diameter and lines 15–20 nm wide. We have investigated the extent of hole clogging and the factors which determine the ultimate resolution of the technique.

Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene nanoelectromechanical systems resonators.

We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300 and 30 K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With a lowering of temperature, we find that the positively dispersing electromechanical modes evolve into negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature-compensated sensors.

Schottky barrier heights for Au and Pd contacts to MoS2

The search of a p-type metal contact on MoS2 has remained inconclusive, with high work function metals such as Au, Ni, and Pt showing n-type behavior and mixed reports of n as well as p-type behavior for Pd. In this work, we report quantitative Schottky barrier heights for Au and Pd contacts to MoS2 obtained by analysing low temperature transistor characteristics and contact resistance data obtained using the transfer length method. Both Au and Pd exhibit n-type behavior on multilayer as well as monolayer MoS2 transistors with Schottky barrier heights of 0.126 eV and 0.4 eV, and contact resistances of 42 Ω.mm and 18 × 104 Ω.mm respectively. Scanning photocurrent spectroscopy data is in agreement with the resulting energy band alignment in Au-MoS2-Pd devices further reinforcing the observation that the Fermi-level is pinned in the upper half of MoS2 bandgap.

Solving rate equations for electron tunneling via discrete quantum states

We consider the form of the current-voltage curves generated when tunneling spectroscopy is used to measure the energies of individual electronic energy levels in nanometer-scale systems. We point out that the voltage positions of the tunneling resonances can undergo temperature-dependent shifts, leading to errors in spectroscopic measurements that are proportional to the temperature. We do this by solving the set of rate equations that can be used to describe electron tunneling via discrete quantum states, for a number of cases important for comparison to experiments, including (1) when just one spin-degenerate level is accessible for transport, (2) when two spin-degenerate levels are accessible, with no variation in electron-electron interactions between eigenstates, and (3) when two spin-degenerate levels are accessible, but with variations in electron-electron interactions. We also comment on the general case with an arbitrary number of accessible levels. In each case we analyze the voltage positions, amplitudes, and widths of the current steps due to the quantum states.

TUNNELING VIA INDIVIDUAL ELECTRONIC STATES IN FERROMAGNETIC NANOPARTICLES

We measure electron tunneling via discrete energy levels in ferromagnetic cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due to magnetic anisotropy, the energy of each tunneling resonance shifts as an applied magnetic field rotates the particles magnetic moment. We see both spin-increasing and decreasing tunneling transitions, but we do not observe the spin degeneracy at small magnetic fields seen previously in non-magnetic materials. The tunneling spectrum is denser than predicted for independent electrons, possibly due to spin-wave excitations.

Field-effect modulation of conductance in VO2 nanobeam transistors with HfO2 as the gate dielectric

We study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (~ 6 percent) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins.

Fabrication of Asymmetric Electrode Pairs with Nanometer Separation Made of Two Distinct Metals

We report a simple and reproducible method to fabricate two metallic electrodes made of different metals with a nanometer-sized gap. These electrodes are fabricated by defining a pair of gold electrodes lithographically and electrodepositing a second metal onto one of them. This method enables the fabrication of pairs of metallic electrodes that exhibit distinct magnetic properties or work functions. The utility of this technique is demonstrated by making single-electron tunneling devices incorporating 2-nm gold nanocrystals.

A facile process for soak-and-peel delamination of CVD graphene from substrates using water

We demonstrate a simple technique to transfer chemical vapour deposited (CVD) graphene from copper and platinum substrates using a soak-and-peel delamination technique utilizing only hot deionized water. The lack of chemical etchants results in cleaner CVD graphene films minimizing unintentional doping, as confirmed by Raman and electrical measurements. The process allows the reuse of substrates and hence can enable the use of oriented substrates for growth of higher quality graphene, and is an inherently inexpensive and scalable process for large-area production.

Dense Electron System from Gate-Controlled Surface Metal–Insulator Transition

Two-dimensional electron systems offer enormous opportunities for science discoveries and technological innovations. Here we report a dense electron system on the surface of single-crystal vanadium dioxide nanobeam via electrolyte gating. The overall conductance of the nanobeam increases by nearly 100 times at a gate voltage of 3 V. A series of experiments were carried out which rule out electrochemical reaction, impurity doping, and oxygen vacancy diffusion as the dominant mechanism for the conductance modulation. A surface insulator-to-metal transition is electrostatically triggered, thereby collapsing the bandgap and unleashing an extremely high density of free electrons from the original valence band within a depth self-limited by the energetics of the system. The dense surface electron system can be reversibly tuned by the gating electric field, which provides direct evidence of the electron correlation driving mechanism of the phase transition in VO(2). It also offers a new material platform for implementing Mott transistor and novel sensors and investigating low-dimensional correlated electron behavior.

Magnetic Anisotropy Variations and Nonequilibrium Tunneling in a Cobalt Nanoparticle

We present detailed measurements of the discrete electron-tunneling level spectrum within nanometer-scale cobalt particles as a function of magnetic field and gate voltage, in this way probing individual quantum many-body eigenstates inside ferromagnetic samples. Variations among the observed levels indicate that different quantum states within one particle are subject to different magnetic anisotropy energies. Gate-voltage studies demonstrate that the low-energy tunneling spectrum is affected dramatically by the presence of nonequilibrium spin excitations.