Bendable Electronics and Sensing Technology (BEST) group focusses on the multidisciplinary field of flexible electronics, especially based on inorganic and high-mobility semiconductors. Our Vision is to develop cost-effective high-performance flexible and large area electronics and sensing systems. In this regard, direct printing of electronics on various flexible substrates is being explored.
A summary of our research and vision is given in Dr. Dahiya TEDxGlasgow talk:
Graphene Sensors for Electronic-skin
Nowadays, there is a great demand for electronics, optoelectronics and sensors developed on unconventional substrates such as plastic, fabric and paper. Graphene is a promising material due to its advanced mechanical, electrical and optical properties for applications in bendable, and stretchable electronics. Even if it is a one-atom-thick material, it can be seen on certain dielectric layer thicknesses and provides an easy optical characterization even with the optical microscope. The main optical properties of the graphene such as the transmittance and reflectance are defined by the fine structure constant. At BEST group, we are exploiting these properties to develop Graphene based touch sensors for electronic skin applications. As an example, following movies show BEST group ongoing research about energy autonomous transparent and flexible electronic skin based on single layer Graphene capacitive sensors. See more details in our recent publication in Advanced Functional Materials.
Stretchable Interconnects for Conformable Integration of Sensors
The electronics market is evolving towards flexible, conformable and stretchable electronics. The stretchable interconnects research at BEST group bridges the electrical and mechanical properties of rigid chips and skin-like chip arrays. This technology not only enable rigid chips to be “Stretchable and conformable” but also maintains the high efficiency of integrated chips.
The stretchability is obtained not only by shaping the interconnects (wavy shapes as shown in the Fig below ), but also using the materials that are intrinsically stretchable.
Ultra-thin Flexible CMOS Chips & Device Modeling
Ultra-thin Flexible CMOS Chips
The current organic-based flexible electronics is insufficient to meet high performance requirements and device stability needed in many applications, including displays. In this regard, the ultra-thin flexible chips is a promising alternative as it enables compact and bendable electronics.
Si chips are traditionally built on wafers whose thicknesses are about 500µm. However, these wafers are intrinsically brittle and therefore cracks easily when we try to bend. At BEST group, we overcame this challenge (results shown in the figure) and obtained bendable Si chips by reducing the thickness of wafers to about 20μm. We are now extending this work to obtain bendable ultra-thin tactile sensing chips. This work is funded through EU and EPSRC grants.
Compact Modelling of Electronic Devices on Flexible Substrates
The bending of flexible substrates induces stresses in the electronics integrated on the chips, which leads to changes in the I-V characteristics. The standard device models available today are insufficient to capture this piezoresistive behavior and hence the circuit design becomes challenging. At BEST group, we are addressing this issue by analyzing the effect of uniaxial stress on the MOS devices on ultra-thin chips and developing new models based on the observations. By taking into account the bendability induced changes (e.g. in mobility, threshold voltage etc.), we are advancing the state of the art in device modelling to enable designing of circuits over flexible substrates and also advance the commercial circuit simulation tools.
This work is funded by EPSRC.
Large-area Flexible Electronics Based on Semiconductor Nanowires
Nanowires (NWs) are promising building blocks for the development of high-performance flexible electronics on large areas. Due their large surface-to-volume ratio and nanometric dimensions, NWs present more functionality than their thin film and bulk counterparts. However, high-crystal quality semiconductor NWs cannot be grown directly on flexible substrates due to thermal budget issues. In order to overcome this challenge, BEST group is investigating novel techniques such as contact-printing and dielectrophoresis to transfer semiconductor NWs from planar substrates to flexible substrates.
This work is funded by EPSRC.
Metal-assisted chemical etching (MACE) route is one of the most cost-effective top-down based techniques to synthesize silicon (Si) nanowires (NWs) from bulk Si wafer. Initially, bare Si wafer is nano-patterned using silica or polymer nano-spheres (NSs), aiming to mask the Si wafer surface; this process is followed by catalyst metals such as silver (Ag), platinum (Pt) or gold (Au) deposited over NSs using standard thin film techniques. The NSs are removed from the substrate, leaving the catalyst nano-mesh for etching process. Etching solution consists of HF+H2O2 is prepared and the sample is immersed in the solution for the MACE process. The areas under the catalyst nano-mesh are etched away, resulting in Si NWs vertically aligned on the Si wafer. MACE is one of the economic methods to produce large area Si NWs.
Vapour-liquid-solid (VLS) growth mechanism by chemical vapour deposition (CVD) technique is one of the popular method to synthesize high crystal quality semiconductor nanowires (NWs). The animation below shows VLS-CVD growth of silicon (Si) NWs using gold (Au) catalyst. Firstly, Au nano-droplets are deposited atop a Si wafer; then, Si gas source such as silane is introduced into the growth chamber at a specific gas flow and using an appropriate substrate temperature which lead to the formation of a eutectic Au-Si alloy. When the alloy is supersaturated with Si, there is a precipitation of Si towards the droplet-substrate interface. NW growth proceeds further with Au-Si on the tip of the wire. The diameter of the resultant NW is decided by the initial size of the catalyst particles. Various atomistic process such as adsorption, surface diffusion, desorption and crystallization are painted in the animation.
Nanowire Transfer Techniques
BEST group actively investigates new approaches for the development of high-performance flexible electronics based on both MACE and VLS semiconductor nanowires using techniques such as dielectrophoresis and contact-printing. Dielectrophoresis uses non-uniform electric fields to align and attract nanowires to specific places, allowing to control the positioning and assembly of nanowires in complex circuits in both rigid and flexible substrates. On the other hand, contact-printing allows to assemble nanowires on both rigid and flexible substrates, forming a two dimensional layer at large area. This technique presents an accurate control over the transfering performance, including nanowire-to-nanowire spacing, nanowire density, nanowire alignment, etc. These features make contact-printing to be a promising technique for nanowire electronics on non-conventional substrates and the fabrication of three dimensional stacked structures.
Prosthetic/Robotic Touch Interface
Touch interfaces are increasingly becoming a major way through which we interact with electronic gadgets especially with mobile phones and tablets. The use of touch, tactile input and gestures to interact with electronics or in turn everyday objects will further increase in the future. In this regard, at BEST group we are building large area touch screens, flexible touch screens and/or 3D touch based on various technology namely resistive, capacitive, optical etc. for seamless interface with the world around us.
The bendable version of some of these touch interfaces, including in the form of smart glove, have been developed by the group to enable gesture based control of artificial limbs.