(a) Crystal structure of MoS2: S atoms in gold and Mo in black with unit cell parameter a and b, (b) top view of a single-layer MoS2 structure in honeycomb shape; (c) HRTEM of 2H-MoS2 (top insert is the fast Fourier transform (FFT) image showing the planes; slight distortion is due to the tilting of the flake in the TEM; a1, a2 and a3 presented HRTEM image is the Mo-Mo interatomic distance of 2.8 Å for 2H-MoS2 (d–e) schematic of the indirect electron transition in bulk MoS2 and direct electron transition in single layer MoS2 respectively.A team of researchers at the Worcester Polytechnic Institute has developed a revolutionary 2-D layered transition metal (TDM) nanomaterial that can directly convert the photonic energy of light into mechanical motion. This light-activated semiconductor can be used in applications ranging from surgical robots to power micro mirrors for optical communication.
"This is a new area of science," said Balaji Panchapakesan, associate professor of mechanical engineering at WPI. "Very few materials are able to convert photons directly into mechanical motion. In this paper, we present the first semiconductor nanocomposite material known to do so. It is a fascinating material that is also distinguished by its high strength and its enhanced optical absorption when placed under mechanical stress.” The new research was published in Scientific Reports, an open access journal associated with Nature.
"Tiny grippers and actuators made with this material could be used on Mars rovers to capture fine dust particles," added Panchapakesan. "They could travel through the bloodstream on tiny robots to capture cancer cells or take minute tissue samples. The material could be used to make micro-actuators for rotating mirrors in optical telecommunications systems; they would operate strictly with light, and would require no other power source."
The material in question is molybdenum disulfide (MoS2), a semiconductor material. The researchers discovered that the atomic orbits of the molybdenum and sulfur atoms are arranged in a unique way that permits excitons in the conducting band to interact with the p-orbits electrons of the sulfur atoms. (An exciton is a bound electron and a hole that are attracted by Coulomb forces, producing a concentration of energy in the material.) This interaction of the excitons with the p-orbit electrons—called exciton resonance —generates heat within the material, which is responsible for the mechanical response.
The researchers created thin films one to three layers of MoS2 thick and exposed the films to different wavelengths of light. This caused the material to expand or contract, depending on the wavelength used. Panchapakesan and his team, which included graduate students Vahid Rahneshin and Farhad Khosravi, and colleagues at the University of Louisville and the University of Warsaw Pasteura, in Poland, fabricated nano grippers that open and close when subjected to pulses of light. These grippers, they found, can capture objects as small as a human cell.
They also discovered that when the material is stretched its ability to absorb light increases. "This is something that cannot be done with conventional thin-film semiconductors, because when you stretch them, they will prematurely break,” Panchapakesan said. “But with its unique material strength, molybdenum disulfide can be stretched. And its increased optical absorption under strain makes it a good candidate for more efficient solar cells, photodetectors, and detectors for thermal and infrared cameras. The exciton resonance, photomechanical response, and increased optical absorption under strain make this an extraordinary material and an intriguing subject for further investigation.”
