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

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

Nanowire FET based neural element for robotic tactile sensing skin

This paper presents novel Neural Nanowire Field Effect Transistors (υ-NWFETs) based hardware-implementable neural network (HNN) approach for tactile data processing in electronic skin (e-skin). The viability of Si nanowires (NWs) as the active material for υ-NWFETs in HNN is explored through modeling and demonstrated by fabricating the first device. Using υ-NWFETs to realize HNNs is an interesting approach as by printing NWs on large area flexible substrates it will be possible to develop a bendable tactile skin with distributed neural elements (for local data processing, as in biological skin) in the backplane. The modeling and simulation of υ-NWFET based devices show that the overlapping areas between individual gates and the floating gate determines the initial synaptic weights of the neural network - thus validating the working of υ-NWFETs as the building block for HNN. The simulation has been further extended to υ-NWFET based circuits and neuronal computation system and this has been validated by interfacing it with a transparent tactile skin prototype (comprising of 6 × 6 ITO based capacitive tactile sensors array) integrated on the palm of a 3D printed robotic hand. In this regard, a tactile data coding system is presented to detect touch gesture and the direction of touch. Following these simulation studies, a four-gated υ-NWFET is fabricated with Pt/Ti metal stack for gates, source and drain, Ni floating gate, and Al2O3 high-k dielectric layer. The current-voltage characteristics of fabricated υ-NWFET devices confirm the dependence of turn-off voltages on the (synaptic) weight of each gate. The presented υ-NWFET approach is promising for a neuro-robotic tactile sensory system with distributed computing as well as numerous futuristic applications such as prosthetics, and electroceuticals

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

Nanoribbon‐based flexible high‐performance transistors fabricated at room temperature

Si‐nanoribbon‐based high‐performance field‐effect transistors (FETs) with room temperature (RT)‐deposited dielectric are presented. The distinct feature of these devices is that the high‐quality SiNx dielectric deposition at RT, directly on the transfer‐printed nanoribbons, is compatible with most flexible substrates. The performance of these FETs (mobility ≈656 cm2 V−1 s−1 and on/off ratio >106) is on par with the highest performance of similar devices reported with high‐temperature processes, and significantly higher than devices reported with low‐temperature processes. The transfer and output characteristics of nanoribbon‐based field‐effect transistors under planar, tensile, and compressive bending and multiple bending cycles (100) show excellent mechanical stability of the devices as they retain performance. The device characteristics are also compared with the equivalent simulation data. The excellent response of nanoribbon‐based FETs and the fabrication compatibility with diverse flexible substrates makes the presented approach attractive for flexible electronics applications such as conformal tactile active matrix sensors for e‐skin, where high performance is needed

VLS growth mechanism of Si-nanowires for flexible electronics

Nanowires (NWs) are promising building blocks for flexible electronics and sensors and a number of approaches have been used to develop them. Among these, the vapor-liquid-solid (VLS) mechanism has been most appealing as it provides the electronic quality NWs at low fabrication cost. For these reasons, this method plays an important role in many applications including NWs based flexible electronics. The performance of NWs based electronics and sensors depend on their quality and the underlying growth mechanism, which thus far has not attracted much attention. In this paper, we present the physical chemistry model that explains the atomistic aspects of the growth mechanism of silicon nanowires. The mechanistic equations have been derived for various steps involved in a standard VLS growth process. The supersaturation under the steady state conditions has been calculated and utilized to estimate the growth rate of Si-NWs under different temperature conditions. The estimated values are found to be consistent with the reported measured values. The results from our study indicate that the Si-NW growth rate is directly related to the temperature. High-temperatures (~900°C) lead to longer Si-NWs (tens of microns length). This knowledge about growth conditions for Si-NW will enable better control of Si-NW dimensions and hence will have significant positive impact on using Si-NW in flexible electronics - especially the contact printing of NWs based electronic layers on flexible substrates

Nanomaterials processing for flexible electronics

Inorganic nanomaterials such as nanowires (NWs) and nanotubes (NTs) are explored for future flexible electronics applications due to their attributes such as high aspect ratio, enhanced surface-to-volume ratio, prominent mobility and ability to integrate on non-conventional substrates. Device performance of semiconducting NWs are demonstrated to be superior compared to the organic counterparts. Among the synthesis methods, bottom-up vapour-liquid-solid (VLS) growth mechanism playing central role for preparing wide variety of high crystal quality semiconducting NWs. However, the high temperature synthesis process prevents fabrication of NW devices directly over flexible substrates which imply the investigation of efficient transfer techniques such as dry contact printing and electric field assisted assembly. Currently, many efforts are directed to study the integration techniques of NWs from growth substrates to non-conventional receiver substrates and parameters such as transfer-yield, alignment and density. These efforts will help to utilize NWs as building blocks in future flexible electronic devices and circuits. This work focuses on VLS growth of semiconducting NWs and their transfer-printing over large area substrate to fabricate flexible electronics

Metal-assisted chemical etched Si nanowires for high-performance large area flexible electronics

Silicon (Si) nanowires (NWs) are considered important building blocks for high-performance flexible and large-area electronics (LAE). Attributes such as bendability, mobility, ability to achieve high on/off current ratio and suitability for device fabrication make Si-NWs suitable candidates for applications in electronics, optoelectronics, photonics, photovoltaics, sensing and wearable technologies [1-3]. Functionalized or non-functionalized Si-NWs based large area arrays over flexible substrates could be used both as sensing material as well as switching devices. Synthesis of single crystalline doped Si-NWs, controlled NW transfer process and the fabrication of NW field-effect transistors (FETs) are the key steps to realize these applications. Here we present the fabrication and characterisation of flexible NWs based FETs using a cost-effective Si-NWs synthesis and transfer process. Metal-assisted chemical etching (MACE) is considered as one of the cost-effective techniques for the synthesis of single crystalline Si-NWs. This top-down approach uses bulk single crystalline wafer as a starting material for the synthesis of Si-NWs. First, the catalyst metals with nanosized circular patterns are prepared over Si wafer surface and then the wafer was immersed in an etching solution consisting of HF and H2O2. The advantage of this technique is the ability to synthesize Si-NWs at wafer scale, with good control over doping, NW size and NW-to-NW spacing. This approach is favourable for printing of Si-NWs over large areas and non-conventional surfaces. In the current work, Si NWs were synthesised using Nano Sphere Lithography (NSL) patterning followed by MACE process (Fig. 1(e, f)). Close-packed assembly of silica nanospheres (NSs), deposited by dip-coating method, act as a mask for Ag catalyst. The initial dimension of NSs determines the pitch of the nano-mesh (Fig. 1(c,d)). Reactive ion etching (RIE) is carried out subsequently to shrink the NSs to desired dimensions which eventually determines the diameter of resulting NW. Si NWs are synthesised in the diameter range of ~100 nm, lengths up to hundreds of microns, and printed over flexible substrates at defined locations. NW FETs were fabricated (Fig.1(g)) and their performance was studied through current-voltage (I-V) characteristics. This research sets a platform to realize high performance electronics over flexible large-area materials using inorganic nanostructures

Energy generating electronic skin with intrinsic tactile sensing without touch sensors

Electronic skin (eSkin) with various types of sensors over large conformable substrates has received considerable interest in robotics. The continuous operation of large number of sensors and the readout electronics make it challenging to meet the energy requirements of eSkin. In this article, we present the first energy generating eSkin with intrinsic tactile sensing without any touch sensor. The eSkin comprises a distributed array of miniaturized solar cells and infrared light emitting diodes (IRLEDs) on soft elastomeric substrate. By innovatively reading the variations in the energy output of the solar cells and IRLEDs, the eSkin could sense multiple parameters (proximity, object location, edge detection, etc.). As a proof of concept, the eSkin has been attached to a 3-D-printed hand. With an energy surplus of 383.6 mW from the palm area alone, the eSkin could generate more than 100 W if present over the whole body (area ∼1.5 m2). Further, with an industrial robot arm, the presented eSkin is shown to enable safe human−robot interaction. The novel paradigm presented in this article for the development of a flexible eSkin extends the application of solar cell from energy generation alone to simultaneously acting as touch sensors