Nivasan Yogeswaran

New materials and advances in making electronic skin for interactive robots

Flexible electronics has huge potential to bring revolution in robotics and prosthetics as well as to bring about the next big evolution in electronics industry. In robotics and related applications, it is expected to revolutionise the way with which machines interact with humans, real-world objects and the environment. For example, the conformable electronic or tactile skin on robot’s body, enabled by advances in flexible electronics, will allow safe robotic interaction during physical contact of robot with various objects. Developing a conformable, bendable and stretchable electronic system requires distributing electronics over large non-planar surfaces and movable components. The current research focus in this direction is marked by the use of novel materials or by the smart engineering of the traditional materials to develop new sensors, electronics on substrates that can be wrapped around curved surfaces. Attempts are being made to achieve flexibility/stretchability in e-skin while retaining a reliable operation. This review provides insight into various materials that have been used in the development of flexible electronics primarily for e-skin applications.

Flexible FETs using ultrathin Si microwires embedded in solution processed dielectric and metal layers

This work presents a novel manufacturing route for obtaining high performance bendable field effect transistors (FET) by embedding silicon (Si) microwires (2.5 μm thick) in layers of solution-processed dielectric and metallic layers. The objective of this study is to explore heterogeneous integration of Si with polymers and to exploit the benefits of both microelectronics and printing technologies. Arrays of Si microwires are developed on silicon on insulator (SOI) wafers and transfer printed to polyimide (PI) substrate through a polydimethylsiloxane (PDMS) carrier stamp. Following the transfer printing of Si microwires, two different processing steps were developed to obtain top gate top contact and back gate top contact FETs. Electrical characterizations indicate devices having mobility as high as 117.5 cm2 V−1 s−1. The fabricated devices were also modeled using SILVACO Atlas. Simulation results show a trend in the electrical response similar to that of experimental results. In addition, a cyclic test was performed to demonstrate the reliability and mechanical robustness of the Si μ-wires on flexible substrates.

Stretchable resistive pressure sensor based on CNT-PDMS nanocomposites

Flexible pressure sensors attached conformably to skin are of great interest for wearable electronics and robotic applications. However, effective utilization of such devices often requires them to be stretchable. Herein we report a stretchable pressure sensor based on carbon nanotube - polydimethylsiloxane (CNT-PDMS) nanocomposite. The sensors are based on interdigitated silver (Ag) patterns as bottom electrodes which are connected by a top conductive polymer made of CNT-PDMS composite. The sensors are developed on a PDMS substrate to achieve the required elasticity. The performance of the sensors is assessed by measuring change in the resistance of the device for applied mechanical stimuli. The minimal detectable pressure by our sensor is 500Pa. It is noted that the conductivity of CNT-PDMS composites and Ag electrode spacing are the two critical factors significantly influencing the performance of the sensors.

Simulation study of junctionless silicon nanoribbon FET for high-performance printable electronics

High-performance electronics on flexible substrates along with low-cost fabrication by printing has gained interest recently. For this purpose, the printing of inorganic semiconductors based micro/nanostructures such as nanowires etc. are being explored. However, due to thermal budget, the controlled selective source/drain doping needed to obtain transistors from such structure remains a bottleneck post transfer printing. This paper presents an attractive solution to address this challenge. The solution is based on junctionless FETs (JLFET), which do not require selective doping. Unlike conventional JLFETs, which use nanowires, the devices presented here are based on nanoribbons as this enable larger channel width and hence high drive current. Studied through simulation, the JLFETs presented here show high-performance with current high enough to drive micro-LED. The TCAD simulation has been carried out to study the effect of single and dual metal gate (top and bottom side) of JLFETs as well as that of doping and nanoribbon thickness on the electrical characteristics. The simulation results indicate that the proposed devices will be suitable for high performance printable electronics applications.

Tuning electrical conductivity of CNT-PDMS nanocomposites for flexible electronic applications

This paper presents a study into the electrical conductivity of multi-wall carbon nanotube-polydimethylsiloxane (MWNT-PDMS) nanocomposite and their dependence on the filler concentration. It is observed that the electrical conductivity of the composites can be tailored by altering the filler concentration. Accordingly, the nanocomposites with filler weight ratio ranging from 1% to 8% were prepared and tested. Finally, the significance of results presented here for flexible pressure sensors and stretchable interconnects for electronic skin applications have been discussed.

Si microwires based FETs on flexible substrates

This paper presents the new route for fabricating field effect transistors (FETs) on flexible or bendable substrates using a combination of transfer printing and screen printing technologies - the former is used to transfer single crystal silicon (Si) microwires on to flexible substrate and the latter to print the metal lines. A back-gated FET structure has been realized with screen printed silver (Ag) lines as the gate electrode and the transfer-printed arrays of Si microwires are utilized as semiconductor layers of the device. Epoxy based negative resist SU-8 has been used as the adhesive as well as the dielectric for the FET device. An array of 20 wires with variable widths and lengths was developed by using standard photolithography and etching techniques. A custom made micro-spotting technique has been used to obtain the source and drain contacts on top of Si microwires. Good mechanical and electrical responses of the FETs have been observed during the cyclic tests and current-voltage (I-V) measurements respectively.

Piezoelectric graphene field effect transistor pressure sensors for tactile sensing

This paper presents graphene field-effect transistor (GFET) based pressure sensors for tactile sensing. The sensing device comprises GFET connected with a piezoelectric metal-insulator-metal (MIM) capacitor in an extended gate configuration. The application of pressure on MIM generates a piezo-potential which modulates the channel current of GFET. The fabricated pressure sensor was tested over a range of 23.54–94.18 kPa, and it exhibits a sensitivity of 4.55 × 10−3 kPa−1. Further, the low voltage (∼100 mV) operation of the presented pressure sensors makes them ideal for wearable electronic applications.

Graphene gold nanoparticle hybrid based near infrared photodetector

This paper presents novel and simplistic approach towards the development of graphene based near infrared (NIR) photodetectors. The developed device comprises of Au nanoparticles integrated within the channel of the back-gated graphene field effect transistors. The introduction of Au nanoparticles enhanced response of the device under IR illumination due improved NIR absorption. Further, dynamic response of the device under IR illumination is presented. This study will trigger the development of novel hybrid graphene device for graphene based photodetectors in IR regime.

Towards graphene based flexible force sensor

Monolayer graphene transferred over flexible polyvinyl chloride (PVC) substrate combined with closely packed layer of nano-spheres (NSs) is fabricated for force sensing application. The force was applied from vertical direction through NSs which acts as lateral strain enhancers. The stack persuades lateral in-plane strain in the monolayer graphene for the applied vertical pressure through NSs. The electrical measurements demonstrate that the graphene layer is able to respond for soft touch range commonly perceived by human beings. The sensing stack was fabricated using simple approaches such as hot lamination graphene transfer process and drop casting of NSs. The device structure is flexible to conformably cover the nonplanar surface for applications such as large area pressure sensing and robotic e-skin.

Transforming the short-term sensing stimuli to long-term e-skin memory

A Tantalum Pentoxide (Ta2O5) based resistive nonvolatile memory device with bipolar switching behaviour was developed to demonstrate the new concept of memory in e-skin. The memory device showed stable switching behavior under preprogrammed voltage stimuli after an initial forming process. The memory cell was then integrated with a commercial tactile sensor with a new interface circuit, which enabled the switching of the memory cell through the electrical output from the sensor. This study provides a novel method for handling the transport and storage of large tactile data and will trigger advances towards memorable e-skin.