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Kanazawa, Japan, June 12, 2019 / PRNewswire / – Using optical and electrical measurements, it has been found that a two-dimensional anisotropic crystal of rhenium disulfide exhibited opposite piezoresistant effects along two main axes, namely positive along One axis and negative along another. Piezoresistance was also reversible; it appeared during the application of a constraint, but the relative resistance returned to its initial value during the elimination of the stress. This new discovery should lead to widespread application of rhenium disulfide.
When applying mechanical stresses such as pressure on crystals and certain types of ceramics, a surface load proportional to the applied stress is induced; this phenomenon is called the piezoelectric effect. The piezoelectric effect has been known since the mid-eighteenth century and has found use, for example, in the ignition device of lighters. Today, it is widely used in sensors, actuators, and so on. On the other hand, when mechanical stresses are applied to semiconductor materials, some of them exhibit a change in electrical resistance, called a piezoresistive effect. Materials with piezoresistive effect are used in pressure sensors, stress sensors, and so on.
Rhenium disulfide (ReS2) is a two-dimensional material (2D) crystallizing into a lamellar structure, in the form of a black platelet (lamellar crystal), having direct band gap properties independent of the thickness * 1) and anisotropic physical properties. It is classified in the sub-group of transition metal dichalcogenides * 2). According to theoretical calculations, it has two anisotropic directions along different main axes. Two anisotropic directions should react differently to a uniaxial strain. When validating this property, ReS2 should be useful for the precise detection and recognition of stress / effort and multidimensional gestures, which will have many applications in the fields of electronic skin * 3), man-machine interfaces, stress sensors, etc.
This international research team from China and Japan, in which Dr. Liu from Tianjin The University and Dr. Yang of Kanazawa University (WPI-NanoLSI) played an important role in confirming the anisotropic piezoresistive effect of rhenium disulfide, but also in discovering a new phenomenon which, according to the direction of the deformation applied along two crystalline axes, is a 2D ReS device2 shown opposite, that is, piezoresistance positive and negative.
A 2D ReS device2 was manufactured. After examining its configuration using atomic force microscopy (AFM), the anisotropic properties have been studied by both optical and electrical methods.
Firstly, the optical measurements were performed using Differential Reflectance * (4) microscopy (RDM) developed by this research team. A ReS device2 with a thickness of 8 nm was irradiated with a polarized light of different directions to determine the two axial directions (principal) of the 2D crystal.
Then, the electrical anisotropy was measured with the same sample for optical measurements in 12 directions with a spacing of 30 degrees. These measurements also determined the two main directions with a difference of 110 degrees. The same measurements were made with another ReS device2but with a different thickness (70 nm). The latter also gave a very similar anisotropic behavior, indicating the independent character of the thickness of the phenomenon. These results are consistent with previous work.
The crystal 2D ReS2 device whose main axes were determined as above was clamped at one end along a main axis and the other end was moved to the fixed end at a specified speed, c & c. That is to say that a compressive stress has been applied. The device generated piezoresistance due to stress. With a fixed end, the piezoresistance is fully restored when the compression stress of the other end has returned to its original state.
On the other hand, when the same experiment was performed along the other major axis, piezoresistance due to deformation was smaller when greater stress was applied and increased when the stress applied was smaller. The same experiment was repeated with different ReS2 devices, but the results have always been consistent. So, ReS2 The 2D crystalline devices showed a positive or negative piezoresistance opposite, that is to say along the main axes.
In addition, when the same experiment using a single device was repeated 28 times, the results were almost identical. This indicates that after applying a constraint to the ReS2 By releasing the tension, the piezoresistive effect was able to recover its initial state.
While the piezoresistive effect results from constraint-induced gap adjustment, the piezoelectric effect is the result of stress-dependent crystal lattice distortion. Various electrical measurements were made, which also demonstrated that the observed phenomenon was piezoresistance and not the piezoelectric effect.
This study demonstrates that the ReS2 The 2D devices exhibited an opposite positive and negative piezoresistance, i.e., the principal axes in which a stress was applied. Such positive and negative piezoresistive effects as a function of the main axes have not been observed in previous studies. The present study is therefore the first to identify such an effect. This study is expected to lead to many applications of ReS2 to electronics, such as electronic skin, man-machine interfaces, stress sensors, and so on.
Title: The opposite anisotropic piezoresistive effect of ReS2
Authors: Chunhua AN, Zhihao XU, SHEN Wanfu, ZHANG Rongjie, SUN Zhaoyang, Shuijing TANG, Yun-Feng XIAO, Daihua ZHANG, SUN Dong, Xiaodong HU, Chunguang HU, Lei YANG and Jing LIU.
Newspaper: ACS Nano 2019, 13, 3310-3319
Posted online on March 6, 2019
DOI: 10.1021 / acsnano.8b09161
https://pubs.acs.org/doi/10.1021/acsnano.8b09161
[Glossary]* 1) Direct Bandgap
The band gap of a semiconductor is a range of energy, between the valence band and the conduction band, where no electronic state can exist. The direct band gap is a band gap in which the bottom of the conduction band and the top of the valence band exist on the same wave vector.
* 2) Transition metal dichalcogenide
A chalcogenide is a chemical compound consisting of at least one chalcogen anion (member of the oxygen family including sulfur) and at least one additional electropositive element. ReS2 has two sulfur atoms bonded to a Re atom, a transition metal. Since the molecule has two sulfur atoms, it is called a dichalcogenide.
* 3) electronic skin
Electronic skin refers to flexible, stretchable and self-healing electronic components capable of mimicking the functionality of human or animal skin, such as the ability to respond to environmental factors such as heat and pressure changes.
* 4) Difference of reflectance microscopy
Reflectance difference microscopy (RDM) measures the difference in reflectance of two light beams with a different linear polarization. The current international research team has developed the material used in this study.
About Kanazawa University
As the first global university on the sea of Japan Kanazawa University has contributed greatly to higher education and university research Japan since its founding in 1949. The University has three colleges and 17 schools offering courses in fields such as medicine, computer engineering and humanities.
The university is located on the coast of the sea Japan in Kanazawa – a city rich in history and culture. The city of Kanazawa has a highly respected intellectual profile since the time of the fief (1598-1867). The University of Kanazawa is divided into two main campuses: Kakuma and Takaramachi for its some 10,200 students, including 600 foreigners.
Website of Kanazawa University: http://www.kanazawa-u.ac.jp/e/
Further information:
Kanazawa University
Kakuma, Kanazawa
Ishikawa 920-1192, Japan
E-mail: [email protected]
Tel: +81 (76) 264-5963
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