Farnaz Niroui

Nanoelectromechanical tunneling switches based on self-assembled molecular layers

We propose nanoelectromechanical (NEM) switches that operate via electromechanical modulation of tunneling current through several-nanometer-thick switching gaps. In such a device, direct contact between electrodes is avoided by utilizing self-assembled molecular layers to define the switching gap. Electrostatic compression of the molecular layer reduces the tunneling gap leading to an exponential increase in the tunneling current, turning on the switch. With removal of an applied voltage, the compressed layer provides the elastic restoring force necessary to overcome the surface adhesive forces, turning off the switch. Thus, the proposed tunneling NEM switch may enable low-voltage operation while simultaneously mitigating device failure due to stiction. This principle is experimentally investigated using a prototype two-terminal tunneling NEM switch with a switching gap formed by a fluorinated decanethiol layer. In this device, the presence of the molecular film promotes repeatable switching. A comparison of the switch operation with a theoretical model indicates electrostatic compression of the molecular switching gap.

Sub-50 mV NEM relay operation enabled by self-assembled molecular coating

Sub-50 mV operation of nano-electro-mechanical relays is demonstrated for the first time, enabled by an anti-stiction molecular coating. Specifically, a self-assembled monolayer of perfluorodecyltriethoxysilane (PFDTES) is shown to be effective for reducing the switching hysteresis voltage, without dramatically increasing its ON-state resistance, enabling stable device operation at very low voltages.

Controlled fabrication of nanoscale gaps using stiction

Utilizing stiction, a common failure mode in micro/nano electromechanical systems (M/NEMS), we propose a method for the controlled fabrication of nanometer-thin gaps between electrodes. In this approach, a single lithography step is used to pattern cantilevers that undergo lateral motion towards opposing stationary electrodes separated by a defined gap. Upon wet developing of the pattern, capillary forces induce cantilever deflection and collapse leading to permanent adhesion between the tip and an opposing support structure. The deflection consequently reduces the separation gap between the cantilever and the electrodes neighboring the point of stiction to dimensions smaller than originally patterned. Through nanoscale force control achieved by altering device design, we demonstrate the fabrication of nanogaps having controlled widths smaller than 15 nm. We further discuss optimization of these nanoscale gaps for applications in NEM and molecular devices.

Tunneling nanoelectromechanical switches

Nanoelectromechanical (NEM) switches have emerged as a promising competing technology to the conventional metal-oxide semiconductor (MOS) transistors. NEM switches exhibit abrupt switching behavior with large on-off current ratios, near-zero off-state leakage currents and sub-threshold slopes below the 60 mV/decade theoretical limit of conventional MOS devices [1]. However, current NEM switches commonly operate at relatively high actuation voltages exceeding 1 V and suffer from failure due to stiction [1]. Reducing the switching gap is a common approach utilized to lower the operating voltage. However, the decrease in the gap size further increases the surface adhesion forces and consequently the possibility of stiction-induced failure.

Electromechanically actuating molecules

Controlled motion at the nanoscale is an emerging avenue for low powered electronics. The necessity for precision at the nanoscale makes organic chemistry an exciting addition to electronics, as organic synthesis is based upon the design and creation of nanoscale and sub-nanoscale structures. We have recently demonstrated the role of organic materials in the development of a nanoelectromechanical (NEM) switch that operates by electromechanical modulation of tunneling current through a switching gap defined by a few nanometer-thick organic molecular layer sandwiched between conductive contacts [1]. In this device, the molecular layer not only facilitates controlled formation of nanoscale switching gaps, but also avoids direct contact of the electrodes to minimize surface adhesion and provides force control at the nanoscale to prevent device failure due to stiction. Recent work has focused on the compression of the molecular layer by an applied electrostatic force between the two electrodes to reduce the tunneling gap. However, we envision next generation devices can contain advanced materials, which undergo electrochemically stimulated shape changes to modulate the tunneling distance and current. In order to achieve large current on-off ratios, the molecules must be capable of producing significant changes in dimension or shape upon electrical stimuli. Herein, we report a few examples of electromechanically actuating molecules.