Manoharan Muruganathan

Room temperature detection of individual molecular physisorption using suspended bilayer graphene

Researchers detect individual CO2 physisorption with suspended bilayer graphene based on induced Coulomb impurity scattering. Detection of individual molecular adsorption, which represents the ultimate resolution of gas sensing, has rarely been realized with solid-state devices. So far, only a few studies have reported detection of individual adsorption by measuring the variation of electronic transport stemming from the charge transfer of adsorbate. We report room-temperature detection of the individual physisorption of carbon dioxide molecules with suspended bilayer graphene (BLG) based on a different mechanism. An electric field introduced by applying back-gate voltage is used to effectively enhance the adsorption rate. A unique device architecture is designed to induce tensile strain in the BLG to prevent its mechanical deflection onto the substrate by electrostatic force. Despite the negligible charge transfer from a single physisorbed molecule, it strongly affects the electronic transport in suspended BLG by inducing charged impurity, which can shut down part of the conduction of the BLG with Coulomb impurity scattering. Accordingly, we can detect each individual physisorption as a step-like resistance change with a quantized value in the BLG. We use density functional theory simulation to theoretically estimate the possible resistance response caused by Coulomb scattering of one adsorbed CO2 molecule, which is in agreement with our measurement.

Low pull-in voltage graphene electromechanical switch fabricated with a polymer sacrificial spacer

A simple bottom-up procedure using a polymer sacrificial spacer is presented to fabricate graphene electromechanical contact switch devices without using acid etching. Low pull-in voltage of below 2 V is achieved with good consistency on a run-to-run basis, which is compatible with the conventional, complementary metal-oxide-semiconductor circuit requirements. In addition, the formation of carbon-gold bonds at the contact position is proposed as another important mechanism for the irreversible switch—other than the well-known irreversible static friction.

Lateral plasma etching enhanced on/off ratio in graphene nanoribbon field-effect transistor

Opening the transport gap in graphene by minimizing its width is highly desirable to achieve outstanding switching performance, i.e., the high on/off ratio, in its field effect transistors (FETs). In this letter, we propose a simple method to open a comparable transport gap in graphene by narrowing down it into graphene nanoribbon (GNR) via the conventional nanofabrication procedure. In the process, GNR capped with a 50-nm-wide hydrogen-silsesquioxane mask is trimmed down from the edges by lateral plasma etching. The on/off ratio of the FET device is dramatically enhanced by two orders of magnitude as etching duration increases. The large on/off ratios of ∼47 and ∼105 are achieved at room temperature and 5.4 K, respectively. The electrical measurement reveals a transport gap opening of ∼145 meV in GNR, which corresponds to a resulting width of <10 nm.

Electrically Tunable van der Waals Interaction in Graphene–Molecule Complex

van der Waals (vdW) interactions play a central role in the surface-related physics and chemistry. Tuning of the correlated charge fluctuation in a vdW complex is a plausible way of modulating the molecules interaction at the atomic surface. Here, we report the vdW interaction tunability of the graphene-CO2 complex by combining the first-principles calculations with the vdW density functionals and the time evaluation measurements of CO2 molecules adsorption/desorption on graphene under an external electric field. The field-dependent charge transfer within the complex unveils the controllable tuning of CO2 from acceptor to donor. Meanwhile, the configuration of the adsorbed molecule, the equilibrium distance from graphene and O-C-O bonding angle, is modified accordingly. The range of electrical tunability is a unique feature for each type of molecule.

3D Finite Element Simulation of Graphene Nano-Electro-Mechanical Switches

In this paper, we report the finite element method (FEM) simulation of double-clamped graphene nanoelectromechanical (NEM) switches. Pull-in and pull-out characteristics are analyzed for graphene NEM switches with different dimensions and these are consistent with the experimental results. This numerical model is used to study the scaling nature of the graphene NEM switches. We show the possibility of achieving a pull-in voltage as low as 2 V for a 1.5-μm-long and 3-nm-thick nanocrystalline graphene beam NEM switch. In order to study the mechanical reliability of the graphene NEM switches, von Mises stress analysis is carried out. This analysis shows that a thinner graphene beam results in a lower von Mises stress. Moreover, a strong electrostatic force at the beam edges leads to a mechanical deflection at the edges larger than that around the center of the beam, which is consistent with the von Mises stress analysis.

Precise milling of nano-gap chains in graphene with a focused helium ion beam

A focused helium ion beam was used to introduce nano-sized gap chains in graphene. The effect of beam scanning strategies in the fabrication of the nano-gap chains was investigated. The tuning of graphene conductivity has been achieved by modulating the magnitude and uniformity of the ion dose and hence the morphology of the nano-gap chains. A model based on the site-specific and dose-dependent conductivity was built to understand the tuning of the conductivity, taking into account the nanoscale non-uniformity of irradiation.

Tunneling in Systems of Coupled Dopant-Atoms in Silicon Nano-devices.

Following the rapid development of the electronics industry and technology, it is expected that future electronic devices will operate based on functional units at the level of electrically active molecules or even atoms. One pathway to observe and characterize such fundamental operation is to focus on identifying isolated or coupled dopants in nanoscale silicon transistors, the building blocks of present electronics. Here, we review some of the recent progress in the research along this direction, with a focus on devices fabricated with simple and CMOS-compatible-processing technology. We present results from a scanning probe method (Kelvin probe force microscopy) which show direct observation of dopant-induced potential modulations. We also discuss tunneling transport behavior based on the analysis of low-temperature I-V characteristics for devices representative for different regimes of doping concentration, i.e., different inter-dopant coupling strengths. This overview outlines the present status of the field, opening also directions toward practical implementation of dopant-atom devices.

Locally-Actuated Graphene-Based Nano-Electro-Mechanical Switch

The graphene nano-electro-mechanical switches are promising components due to their outstanding switching performance. However, most of the reported devices suffered from a large actuation voltages, hindering them from the integration in the conventional complementary metal-oxide-semiconductor (CMOS) circuit. In this work, we demonstrated the graphene nano-electro-mechanical switches with the local actuation electrode via conventional nanofabrication techniques. Both cantilever-type and double-clamped beam switches were fabricated. These devices exhibited the sharp switching, reversible operation cycles, high on/off ratio, and a low actuation voltage of below 5 V, which were compatible with the CMOS circuit requirements.

Interaction study of nitrogen ion beam with silicon

Focused ion beam technology with light gas ions has recently gained attention with the commercial helium and neon ion beam systems. These ions are atomic, and thus, the beam/sample interaction is well understood. In the case of the nitrogen ion beam, several questions remain due to the molecular nature of the source gas, and in particular, if and when the molecular bond is split. Here, the authors report a cross-sectional scanning transmission electron microscopy (STEM) study of irradiated single crystalline silicon by various doses and energies of nitrogen ionized in a gas field ion source. The shape and dimensions of the subsurface damage is compared to Monte Carlo simulations and show very good agreement with atomic nitrogen with half the initial energy. Thus, it is shown that the nitrogen molecule is ionized as such and splits upon impact and proceeds as two independent atoms with half of the total beam energy. This observation is substantiated by molecular dynamics calculations. High resolution STEM ...

Single-electron quantization at room temperature in a-few-donor quantum dot in silicon nano-transistors

Quantum dots formed by donor-atoms in Si nanodevices can provide a breakthrough for functionality at the atomic level with one-by-one control of electrons. However, single-electron effects in donor-atom devices have only been observed at low temperatures mainly due to the low tunnel barriers. If a few donor-atoms are closely coupled as a molecule to form a quantum dot, the ground-state energy level is significantly deepened, leading to higher tunnel barriers. Here, we demonstrate that such an a-few-donor quantum dot, formed by selective conventional doping of phosphorus (P) donors in a Si nano-channel, sustains Coulomb blockade behavior even at room temperature. In this work, such a quantum dot is formed by 3 P-donors located near the center of the selectively-doped area, which is consistent with a statistical analysis. This finding demonstrates practical conditions for atomic- and molecular-level electronics based on donor-atoms in silicon nanodevices.