Gautham Dharuman

Internal Electron Tunneling Enabled Ultrasensitive Position/Force Peapod Sensors

The electron quantum tunneling effect guarantees the ultrahigh spatial resolution of the scanning tunneling microscope (STM), but there have been no other significant applications of this effect after the invention of STM. Here we report the implementation of electron-tunneling-based high sensitivity transducers using a peapod B4C nanowire, where discrete Ni6Si2B nanorods are embedded in the nanowire in a peapod form. The deformation of the nanowire provides a higher order scaling effect between conductivity and deformation strain, thus allowing the potentials of position and force sensing at the picoscale.

A generalized Ewald decomposition for screened Coulomb interactions

Medium-range interactions occur in a wide range of systems, including charged-particle systems with varying screening lengths. We generalize the Ewald method to charged systems described by interactions involving an arbitrary dielectric response function ϵ(𝐤). We provide an error estimate and optimize the generalization to find the break-even parameters that separate a neighbor list-only algorithm from the particle-particle particle-mesh algorithm. We examine the implications of different choices of the screening length for the computational cost of computing the dynamic structure factor. We then use our new method in molecular dynamics simulations to compute the dynamic structure factor for a model plasma system and examine the wave-dispersion properties of this system.

Modeling and simulation of an ultrasensitive electron tunneling position/force nanosensor

An ultrasensitive position/force nanosensor model was constructed and theoretically characterized. This model is based on a core–shell nanostructure with an inter-segment nanogap embedded, which forms an alignment-free metal–insulator–metal (MIM) junction. The occurrence of the tunneling effect enables the exponential scaling of the change of the displacement or force using tunneling current, which guarantees an ultrasensitive transduction. The simulation indicates that the combination of proper core materials and optimized design of the nanostructure could highly enhance the transduction performance. The simulation results provide instructions for the implementation of such ultrasensitive tunneling nanosensors, which in turn open new ground for tunneling-effect-based sub-nanoscale or even picoscale position/force detection.

Nanorobotic end-effectors: Design, fabrication, and in situ characterization

This paper reports three types of nanorobotic end-effectors: m@CNTs-based sphere-on-pillar (SOP) optical nanoantennas, nanotube fountain pen, and m@CNTs-based tunneling nanosensor. The fabrication method of the m@CNTs-based SOP optical nanoantennas we developed has potentials in the investigation of nano-optics and nano-photonics due to its feasibility in preparing such devices. Nanotube fountain pen (NFP) illustrates the practical applications in the direct fabrication of nanostructures from 0 to 3D, which is of critical importance in the future fast-prototype of nanodevices. The m@CNTs-based tunneling nanosensor provides a new design in measuring force/displacement in nanoscale, opening a new ground in developing nanoelectromechanical system (NEMS). These three end-effectors enable new functions for the nanorobotic manipulators and extending peoples ability in exploring the world in nanoscale.

Effective quantum potentials for molecular dynamics simulation of non-ideal plasmas

Molecular dynamics simulation of electron-ion systems are prone to the so-called “Coulomb catastrophe” which is due to the infinitely deep Coulomb potential of the ion without any stabilizing effect in a classical framework. Quantum mechanically this is forbidden by the Heisenberg Uncertainty Principle. In addition, many electron systems need to satisfy the Pauli Exclusion Principle as well. Fermion Molecular Dynamics1 accommodates the two effects in a quasi-classical treatment through the momentum-dependent pseudo-potentials that exclude the regions of phase space forbidden by the quantum principles. The extent of exclusion is determined by a set of parameters which need to be chosen carefully to include the quantum effects specific to the system. Initial attempts to determine these parameters were ad-hoc2 followed by accurate calculation of the Heisenberg parameters for Hydrogen or Helium-like systems3. The results so far do not suffice to study plasmas where multi-electron effects are important in atomic processes. We report the first attempt to accurately determine the Heisenberg and Pauli parameters for a set of atoms. The parameters are found to take a set of correlated values and the correlation is elliptic for all the atoms we studied. It is possible that the coefficients of the ellipse could be functions of atomic number indicating a possible connection with a quantum mechanical treatment in the limiting case.

Scattering and bound-state trajectories with effective quantum potentials

Fermion Molecular Dynamics (FMD)1 has been successful in understanding quantum mechanical processes through a simplified quasi-classical treatment using effective quantum potentials that are momentum dependent. These potentials mimic the Heisenberg Uncertainty and Pauli Exclusion principles by excluding the forbidden regions of phase space. Our study aimed at understanding the reason behind the success of this simplified treatment. As a first step, dynamic effects brought in by these potentials in electronic bound-states and scattering were studied. The trajectories differ considerably from their classical counterparts. We report the evolution of the combined momentum-dependent and Coulomb potential for different bound state trajectories which explain the stabilizing effect possible in this treatment. This could be the reason for longer ionization times predicted by FMD that are comparable to experimental results2. It is known that classical and quantum studies of Rutherford scattering cross sections result in identical results. We tested the effects of the effective quantum potential on the scattering process. The results differ considerably from classical/quantum results in the large angle scattering regime where the effective quantum potential dominates over the repulsive Coulomb. Non-physical oscillations in the scattering angle are observed indicating extreme sensitivity to the impact parameter for small impact parameters. In all the tests, conservation principles were not violated by the effective potentials.

An inter-segment tunneling nanoscale force sensor: Modeling and simulation

A segmented high aspect ratio nanowire employing the tunneling effect is proposed to theoretically realize non-linear effects in nanoscale force sensing. The structure consists of an external insulating shell with metallic core regions separated at the structures geometric center. The separation is considered to be in the order of a few Å for the tunneling effect to be pronounced. As the structure is subjected to an axial force at the ends, buckling occurs causing a change in the separation which results in a non-linear change in the current. Force-separation and current-force relations are studied and the high sensitivity of the structure as a force sensor is established.