Abu Riduan Md Foisal

Environment-friendly carbon nanotube based flexible electronics for noninvasive and wearable healthcare

Flexible and stretchable electronics have a wide variety of wearable applications in portable sensors, flexible electrodes/heaters, flexible circuits and stretchable displays. Spinnable carbon nanotubes (CNTs) constructed on flexible substrates are potential materials for wearable sensing applications owing to their high thermal and electrical conductivity, low mass density and excellent mechanical properties. Here, we demonstrate a wearable thermal flow sensor for healthcare using lightweight, high strength, flexible CNT yarns as hotwires, pencil graphite as electrodes, and lightweight, recyclable and biodegradable paper as flexible substrates, without using any toxic chemicals. The CNT-based sensor which could be utilized to monitor respiratory diseases is comfortably affixed to human skin and detects real-time human respiration. We also successfully demonstrate the temperature detecting functionality integrated in the same sensor, which can measure body temperature using a non-contact mode. The results indicate that the CNT yarn can be used to develop a wide range of environment-friendly, low-cost and lightweight paper-based flexible devices for wearable applications in temperature and respiratory monitoring, and personal healthcare.

3C–SiC on glass: an ideal platform for temperature sensors under visible light illumination

This letter reports on cubic silicon carbide (3C–SiC) transferred on a glass substrate as an ideal platform for thermoresistive sensors which can be used for in situ temperature measurement during optical analysis. The transfer of SiC onto an insulating substrate prevents current leakage through the SiC/Si junction, which is significantly influenced by visible light. Experimental data shows that the 3C–SiC on glass based sensor possesses a large temperature coefficient of resistance (TCR) of up to −7508 ppm K−1, which is about 10 times larger than that of highly doped Si. Moreover, the 3C–SiC based temperature sensor also outperforms low doped Si in terms of stability against visible light. These results indicate that 3C–SiC on glass could be a good thermoresistive sensor to measure the temperature of cells during optical investigations.

Isotropic piezoresistance of p-type 4H-SiC in (0001) plane

In this work, the isotropic piezoresistance in the (0001) plane of p-type 4H-SiC was discovered by means of the hole energy shift calculation and the coordinate transformation. These results were also confirmed by the measurement of the piezoresistance using a bending beam method. The fundamental longitudinal and transverse piezoresistive coefficients π11 and π12 were found to be 6.43 × 10−11 Pa−1 and −5.12 × 10−11 Pa−1, respectively. The isotropy of the piezoresistance in the basal plane of p-type 4H-SiC is attributed to the isotropic hole energy shift under uniaxial strain. This interesting phenomenon in p-type 4H-SiC is promising for the design and fabrication of mechanical sensors and strain-engineered electronics since high sensitivity and consistent performance can be achieved regardless of the crystallographic orientation.

Self-sensing paper-based actuators employing ferromagnetic nanoparticles and graphite

Paper-based microfluidics and sensors have attracted great attention. Although a large number of paper-based devices have been developed, surprisingly there are only a few studies investigating paper actuators. To fulfill the requirements for the integration of both sensors and actuators into paper, this work presents an unprecedented platform which utilizes ferromagnetic particles for actuation and graphite for motion monitoring. The use of the integrated mechanical sensing element eliminates the reliance on image processing for motion detection and also allows real-time measurements of the dynamic response in paper-based actuators. The proposed platform can also be quickly fabricated using a simple process, indicating its potential for controllable paper-based lab on chip.

Highly sensitive p-type 4H-SiC van der Pauw sensor

This paper presents for the first time a p-type 4H silicon carbide (4H-SiC) van der Pauw strain sensor by utilizing the strain induced effect in four-terminal devices. The sensor was fabricated from a 4H-SiC (0001) wafer, using a 1 μm thick p-type epilayer with a concentration of 1018 cm−3. Taking advantage of the four-terminal configuration, the sensor can eliminate the need for resistance-to-voltage conversion which is typically required for two-terminal devices. The van der Pauw sensor also exhibits an excellent repeatability and linearity with a significantly large output voltage in induced strain ranging from 0 to 334 ppm. Various sensors aligned in different orientations were measured and a high sensitivity of 26.3 ppm−1 was obtained. Combining these performances with the excellent mechanical strength, electrical conductivity, thermal stability, and chemical inertness of 4H-SiC, the proposed sensor is promising for strain monitoring in harsh environments.

Pushing the Limits of Piezoresistive Effect by Optomechanical Coupling in 3C-SiC/Si Heterostructure

This letter reports a giant opto-piezoresistive effect in p-3C-SiC/p-Si heterostructure under visible-light illumination. The p-3C-SiC/p-Si heterostructure has been fabricated by growing a 390 nm p-type 3C-SiC on a p-type Si substrate using the low pressure chemical vapor deposition (LPCVD) technique. The gauge factor of the heterostructure was found to be 28 under a dark condition; however, it significantly increased to about -455 under illumination of 635 nm wavelength at 3.0 mW/cm2. This gauge factor is over 200 times higher than that of commercial metal strain gauge, 16 times higher than that of 3C-SiC thinfilm, and approximately 5 times larger than that of bulk Si. This enhancement of the gauge factor was attributed to the opto-mechanical coupling effect in p-3C-SiC/p-Si heterostructure. The opto-mechanical coupling effect is the amplified effect of the photoconductivity enhancement and strain-induced band structure modification in the p-type Si substrate. These findings enable extremely high sensitive and robust mechanical sensors, as well as optical sensors at low cost, as no complicated nanofabrication process is required.

High-temperature tolerance of the piezoresistive effect in p-4H-SiC for harsh environment sensing

4H-silicon carbide based sensors are promising candidates for replacing prevalent silicon-based counterparts in harsh environments owing to their superior chemical inertness, high stability and reliability. However, the wafer cost and the difficulty in obtaining an ohmic contact in the metallization process hinders the use of this SiC polytype for practical sensing applications. This article presents the high-temperature tolerance of a p-type 4H-SiC piezoresistor at elevated temperatures up to 600 °C. A good ohmic contact was formed by the metallisation process using titanium and aluminium annealed at 1000 °C. The leakage current at high temperatures was measured to be negligible thanks to a robust p–n junction. Owing to the superior physical properties of the bulk 4H-SiC material, a high gauge factor of 23 was obtained at 600 °C. The piezoresistive effect also exhibits good linearity and high stability at high temperatures. The results demonstrate the capability of p-type 4H-SiC for the development of highly sensitive sensors for hostile environments.

A rapid and cost-effective metallization technique for 3C–SiC MEMS using direct wire bonding

This paper presents a simple, rapid and cost-effective wire bonding technique for single crystalline silicon carbide (3C–SiC) MEMS devices. Utilizing direct ultrasonic wedge–wedge bonding, we have demonstrated for the first time the direct bonding of aluminum wires onto SiC films for the characterization of electronic devices without the requirement for any metal deposition and etching process. The bonded joints between the Al wires and the SiC surfaces showed a relatively strong adhesion force up to approximately 12.6–14.5 mN and excellent ohmic contact. The bonded wire can withstand high temperatures above 420 K, while maintaining a notable ohmic contact. As a proof of concept, a 3C–SiC strain sensor was demonstrated, where the sensing element was developed based on the piezoresistive effect in SiC and the electrical contact was formed by the proposed direct-bonding technique. The SiC strain sensor possesses high sensitivity to the applied mechanical strains, as well as exceptional repeatability. The work reported here indicates the potential of an extremely simple direct wire bonding method for SiC for MEMS and microelectronic applications.

Pseudo-Hall Effect in Graphite on Paper Based Four Terminal Devices for Stress Sensing Applications

A cost effective and easy to fabricate stress sensor based on pseudo-Hall effect in Graphite on Paper (GOP) has been presented in this article. The four terminal devices were developed by pencil drawing with hand on to the paper substrate. The stress was applied to the paper containing four terminal devices with the input current applied at two terminals and the offset voltage observed at other two terminals called pseudo-Hall effect. The GOP stress sensor showed significant response to the applied stress which was smooth and linear. These results showed that the pseudo-Hall effect in GOP based four terminal devices can be used for cost effective, flexible and easy to make stress, strain or force sensors.

Optical and Electrical Characterizations of Nanoscale Robust 3C-SiC Membrane for UV Sensing Applications

This paper presents the fabrication and optical characterization of an ultrathin 3C-SiC membrane for UV light detection. SiC nanoscale film was grown on Si substrate and subsequently released to form a robust membrane with a high aspect ratio of about 5000. Transmission measurements were performed to determine the thickness of the film with a high accuracy of 98%. We also employed a simple and highly effective direct wirebonding technique to form electrical contacts to the SiC membrane. The considerable change in the photocurrent of the SiC membrane was observed under UV illumination, indicating the potential of using 3C-SiC membranes for UV detection.