Fengyuan Liu

Large-area self-assembly of silica microspheres/nanospheres by temperature-assisted dip-coating

This work reports a temperature-assisted dip-coating method for self-assembly of silica (SiO2) microspheres/nanospheres (SPs) as monolayers over large areas (∼cm2). The area over which self-assembled monolayers (SAMs) are formed can be controlled by tuning the suspension temperature (Ts), which allows precise control over the meniscus shape. Furthermore, the formation of periodic stripes of SAMs, with excellent dimensional control (stripe width and stripe-to-stripe spacing), is demonstrated using a suitable set of dip-coating parameters. These findings establish the role of Ts, and other parameters such as withdrawal speed (Vw), withdrawal angle (θw), and withdrawal step length (Lw). For Ts ranged between 25 and 80 °C, the morphological analysis of dip-coatings shows layered structures comprising of defective layers (25-60 °C), single layers (70 °C), and multilayers (>70 °C) owing to the variation of SP flux at the meniscus/substrate assembling interface. At Ts = 70 °C, there is an optimum Vw, approximately equal to the downshift speed of the meniscus (Vm = 1.3 μm/s), which allows the SAM formation over areas (2.25 cm2) roughly 10 times larger than reported in the literature using nanospheres. Finally, the large-area SAM is used to demonstrate the enhanced performance of antireflective coatings for photovoltaic cells and to create metal nanomesh for Si nanowire synthesis.

Modelling of nanowire FETs based neural network for tactile pattern recognition in E-skin

This paper presents device, circuit and system modelling to validate the use of neural nanowire FETs (u-NWFETs) towards a hardware-realizable Neural Network. Hardware neural networks are promising for neuromorphic computing and have many prospective applications for bi-directional interface in prosthetics, and electroceuticals etc. Device simulation of a u-NWFET has been carried out followed by circuit implementation to validate the use of silicon nanowires (Si-NWs) as neuronal elements. A system level simulation of 258 neurons (225 sensor neurons, 50 hidden layer neurons and 3 output layer neurons) has been performed to demonstrate tactile pattern recognition. Training has been carried out and validation of the trained network gives an accurate classification of a database of 50 tactile images into 3 classifiers.

Heterogeneous integration of contact-printed semiconductor nanowires for high-performance devices on large areas

In this work, we have developed a contact-printing system to efficiently transfer the bottom-up and top-down semiconductor nanowires (NWs), preserving their as-grown features with a good control over their electronic properties. In the close-loop configuration, the printing system is controlled with parameters such as contact pressure and sliding speed/stroke. Combined with the dry pre-treatment of the receiver substrate, the system prints electronic layers with high NW density (7 NWs/μm for bottom-up ZnO and 3 NWs/μm for top-down Si NWs), NW transfer yield and reproducibility. We observed compactly packed (~115 nm average diameters of NWs, with NW-to-NW spacing ~165 nm) and well-aligned NWs (90% with respect to the printing direction). We have theoretically and experimentally analysed the role of contact force on NW print dynamics to investigate the heterogeneous integration of ZnO and Si NWs over pre-selected areas. Moreover, the contact-printing system was used to fabricate ZnO and Si NW-based ultraviolet (UV) photodetectors (PDs) with Wheatstone bridge (WB) configuration on rigid and flexible substrates. The UV PDs based on the printed ensemble of NWs demonstrate high efficiency, a high photocurrent to dark current ratio (>104) and reduced thermal variations as a result of inherent self-compensation of WB arrangement. Due to statistically lesser dimensional variations in the ensemble of NWs, the UV PDs made from them have exhibited uniform response.Nanosystems: Contact-Printing System Developed for Semiconductor NanowiresA contact-printing system has been developed that allows the printing of semiconductor nanowires (NWs), such that electronic layers made from printed NWs can lead to devices with uniform response over large areas. With many attractive features, the NWs have gained attention in meeting demands to integrate low-power miniaturized devices over large areas and with flexible substrates. However, achieving response uniformity with nanoscale devices made from NWs has proved challenging. The team from Bendable Electronics and Sensing Technologies (BEST) group headed by Professor Ravinder Dahiya at the University of Glasgow, United Kingdom has developed a contact-printing system that overcome this challenge and efficiently prints NWs to various substrates while preserving the key characteristics and good control over their electronic properties. The technology has high reproducibility and reliability. The team believes its system holds promise for heterogenous integration of different semiconductor and metal NWs, allowing large-area scalability and and possibly the 3-D integration on flexible substrates.

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.

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 utlilize 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.

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.

Towards flexible magnetoelectronics for robotic applications

This paper presents the technological advancements in the field of flexible magnetic sensors for robotics applications. Various magnetic devices (e.g. Hall, GMR, AMR and TMR) have been studied and their suitability for flexible application has been presented. Further, the system level integration of magnetic sensors in robotics is briefly discussed. With rapid development in flexible electronics, a robot with multi-functional conformable electronic skin will be possible in the foreseeable future. This will also open new avenues for a wide range of other applications including wearable electronics and interactive electronic-skin for robots and prosthesis.