Recent Advances in Strain-Induced Piezoelectric and Piezoresistive Effect-Engineered 2D Semiconductors for Adaptive Electronics and Optoelectronics

The development of two-dimensional(2D)semiconductors has attracted widespread attentions in the scientific community and industry due to their ultra-thin thickness,unique structure,excellent optoelectronic properties and novel physics.The excellent flexibility and outstanding mechanical strength of 2D semiconductors provide opportunities for fabricated strain-sensitive devices and utilized strain tuning their electronic and optic–electric performance.The strain-engineered one-dimensional materials have been well investigated,while there is a long way to go for 2D semiconductors.In this review,starting with the fundamental theories of piezoelectric and piezoresistive effect resulted by strain,following we reviewed the recent simulation works of strain engineering in novel 2D semiconductors,such as Janus 2D and 2D-Xene structures.Moreover,recent advances in experimental observation of strain tuning PL spectra and transport behavior of 2D semiconductors are summarized.Furthermore,the applications of strain-engineered 2D semiconductors in sensors,photodetectors and nanogenerators are also highlighted.At last,we in-depth discussed future research directions of strain-engineered 2D semiconductor and related electronics and optoelectronics device applications.

Piezoresistive effect in MoO3 nanobelts and its application in strain-enhanced oxygen sensors

MoO3 nanobelts （NBs） having different properties have been synthesized via a physical vapor deposition （PVD） method. The crystallographic structures and morphologies of the NBs were characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. Electrical measurements were performed and the profound piezoresistive effect in MoO3 experimentally studied and verified. Factors that influence the gauge factor, such as NB size, doping concentration and atmosphere composition, are discussed and analyzed. Gas sensing performance was also tested in devices and it was demonstrated that by applying strain to the gas sensor, its sensing performance could be effectively tuned and enhanced. This study provides the first demonstration of significant piezoresistivity in MoO3 NBs and the first illustration of a generic mechanism by means of which this effect can be coupled with other electronic modulation measures to afford better device performance and broader material functionality.

Tunnelling piezoresistive effect of grain boundary in polysilicon nano-films

The experiment results indicate that the gauge factor of highly boron doped polysilicon nanofilm is bigger than that of monocrystalline silicon with the same doping concentration, and increases with the grain size decreasing. To apply the unique properties reasonably in the fabrication of piezoresistive devices, it was expounded based on the analysis of energy band structure that the properties were caused by the tunnel current which varies with the strain change forming a tunnelling piezoresistive effect. Finally, a calculation method ofpiezoresistance coefficients around grain boundaries was presented, and then the experiment results ofpolysilicon nanofilms were explained theoretically.

Piezoresistive Characteristic of Conductive Rubber for Flexible Tactile Sensor

In the research of 2D flexible tactile sensor matrix,pressure-sensitive conductive rubber was developed and tested in which carbon black was used as its conductive phase and silicon rubber as its matrix layer.Experiments were undertaken and the resultant data were used for its piezoresistive characteristics investigation for two kinds of electrode connection configurations,the surface directive connection and embedded connection.It is found that due to the rather strong nonlinearity of the piezoresistive characteristic curves obtained,a higher correlation relationship can be obtained by means of quadratic polynomial fitting.It also showed that the embedded electrode assembling has higher fitting accuracy while the surface directive connection has better mechanical sensitivity.

High-performance piezoresistive sensors based on transfer-free large-area PdSe_(2) films for human motion and health care monitoring

Two-dimensional transition metal dichalcogenides(TMDs)are needed in highperformance piezoresistive sensors due to their strong strain-induced bandgap modification and thereby large gauge factors.However,integrating a conventional high-temperature chemical vapor deposition(CVD)-grown TMD with a flexible substrate necessitates a transfer process that inevitably degrades the sensing properties of the TMDs and increases the overall fabrication complexity.We present a high-performance piezoresistive strain sensor that employs largearea PdSe_(2) films grown directly on polyimide(PI)substrates via plasma-assisted selenization of a sputtered Pd film.The reliable strain transfer from the substrate to the PdSe_(2) film ensures an outstanding strain-sensing capability of the sensor.Specifically,the sensors have a gauge factor of up to315±2.1,a response time under 25 ms,a detection limit of 8×10^(-6),and an exceptional stability of over 104 loadingunloading cycles.By attaching the sensors to the skin surface,we demonstrate their application for measuring physiological parameters in health care monitoring,including motion,voice,and arterial pulse vibration.Furthermore,using the PdSe_(2) film sensor combined with deep learning technology,we achieved intelligent recognition of artery temperature from arterial pulse signals with only a 2%difference between predicted and actual temperatures.The excellent sensing performance,together with the advantages of low-temperature fabrication and simple device structure,make the PdSe_(2) film sensor promising for wearable electronics and health care sensing systems.

RTD’s Relaxation Oscillation Characteristics with Applied Pressure

The relaxation oscillation characteristics of a resonant tunneling diode （RTD） with applied pressure are reported. The oscillation circuit is simulated and designed by Pspice 8. 0, and the measured oscillation frequency is up to 200kHz. Using molecular beam epitaxy （MBE） ,AIAs/lnx Ga1-x As/GaAs double barrier resonant tunneling structures （DBRTS） are grown on （100） semi-insulated （SI） GaAs substrate,and the RTD is processed by Au/Ge/Ni/Au metallization and an airbridge structure. Because of the piezoresistive effect of RTD,with Raman spectrum to measure the applied pressure, the relaxation oscillation characteristics have been studied, which show that the relaxation oscillation frequency has approxi- mately a - 17.9kHz/MPa change.

Early-age Hydration Characteristics of Composite Binder Containing Graphite Powder

The early-age hydration characteristics of composite binder containing graphite powder(GP)with two different finenesses were investigated by determining the hydration heat,thermo gravimetric,morphology of hardened paste as well as the compressive strength of mortar.The experimental results show that:replacing 2%-6%of cement with graphite powder significantly improves the piezoresistive effect of early age mortar,can be used to monitor accidental loads caused by dropped objects,collisions,or other accident events,and thus avoids initial damage.Some GP provides additional nucleation sites that lead to a fast formation of hydration products(nucleation-site effect).However,due to the almost hydrophobic water contact angle,most of the GP causes a large number of micro-cracks in the hydrated paste(gap effect).Because of the lamellar shape and high surface energy,GP is easily balled and can not be uniformly distributed in the composite,resulting in clumping together and wrapping some of the cement particles(barrier effect).Due to nucleation-site effect,when the dosages of coarse and fine GP reached 2%and 4%,1 d strength were increased by 9.1%and 9.6%,respectively.At 3 days,as the interior damage caused by the gap effect gradually increased,and the retarding effect on cement hydration caused by barrier effect was enhanced.GP has an obvious negative effect on compressive strength.However,micro-cracks caused by fine GP are less,so its negative effect on 3 d compressive strength is lower.

A novel micro-accelerometer with adjustable sensitivity based on resonant tunnelling diodes

Resonant tunnelling diodes （RTDs） have negative differential resistance effect, and the current-voltage characteristics change as a function of external stress, which is regarded as mesc-piezoresistance effect of RTDs. In this paper, a novel micro-accelerometer based on AlAs/GaAs/In0.1Ga0.9As/GaAs/AlAs RTDs is designed and fabricated to be a four-beam-mass structure, and an RTD-Wheatstone bridge measurement system is established to test the basic properties of this novel accelerometer. According to the experimental results, the sensitivity of the RTD based micro-accelerometer is adjustable within a range of 3 orders when the bias voltage of the sensor changes. The largest sensitivity of this RTD based miero-accelerometer is 560.2025 mV/g which is about 10 times larger than that of silicon based micro piezoresistive accelerometer, while the smallest one is 1.49135 mV/g.

A design and application of compound multi-functional sensor in wood-based panel processing

A compound multi-functional sensor was designed by the study on the on-line testing technology of wood-based panels, and its properties of shape, functions, size, resistance to special environment were studied in details. The operational principles of different sensors, technical flow of manufacturing, development of software systems of special functions, and the assessments of technical specification were also be introduced. This sensor adopted many new technologies, such as the applications of piezoresistant effect and heat sensitive effect can effectively measure the pressure and temperature, digital signal processing technology was used to extract and treat signals, and resist interference, encapsulation technology was used to keep the normal run of sensor under a harsh environment. Thus, the on-line compound multi-functional temperature/pressure sensor can be applied better to supervise the production of wood-based panels. All technical specifications of the compound multi-functional sensor were tested and the results met the requirements of the equipments.

A tunneling piezoresistive model for polysilicon

Based on the trap model, the band structure and the conductive mechanism ofpolysilicon were analyzed, and then an equivalent circuit used to interpret the tunneling piezoresistive effect was proposed. Synthesizing the piezoresistive effect of the grain boundary region and grain neutral zone, a new piezoresistive model--a tunneling piezoresistive model is established. The results show that when the doping concentration is above 10^20 cm^-3, the piezoresistive coefficient of the grain boundary is higher than that of the neutral zone, and it increases with an increase in doping concentration. This reveals the intrinsic mechanism of an important experimental phenomena that the gauge factor of heavily doped polysilicon nano-films increases with an increase in doping concentration.

Mechanic-Electric Coupling Characteristics of a Resonant Tunneling Diode

This paper reports the piezoresistive effect of a resonant tunneling diode （RTD） in a microstructure. The fourbeam structure is analyzed and fabricated by positing RTDs at the stress sensitive regions. Stress along the [110] orienta- tion and [110]ientation induces a change in the RTD＇s current-voltage （I-V） curves,i, e., the meso-piezoresistance variety,mainly in its negative different resistance （NDR） region. By different methods,the mechanic-electric coupling characteristic of RTD is studied and the consistent 10^9Pa^1 piezoresistive coefficients are discovered.

Strain engineering of two-dimensional materials:Methods,properties,and applications

Two dimensional(2D)materials have attracted extensive research interests due to their excellent properties related to unique structure.Strain engineering,as an important strategy for tuning the lattice and electronic structure of 2D mate-rials,has been widely used in the modulation of physical properties,which broadens their applications in flexible nanoelectronic and optoelectronic devices.In this review,we fist summari ze the methods of inducing strain to 2D materials and discuss the advantages and problems of various methods.We then introduce the strain induced effects on optical,electrical,and magnetic proper-ties,together with the phase transition of 2D materials.Finally,we ilustrate the potential applications of strained 2D materials and further look forward to their opportunities and challenges in practical applications in the future.