Solar "funnel" research progresses

Collecting solar energy and converting it into electrical energy has always been a very important issue. Feng Ji of the Quantum Material Science Center of the School of Physics, Peking University, proposed in the recent work that the construction of the so-called “solar funnel” provides a new perspective for this subject. This work was published in Nature-Photonics [Ji Feng, et al. Nature Photonics 6, 886-872 (2012); doi:10.1038/nphoton.2012.285].

The tensile elastic strain that conventional materials can withstand does not exceed 0.2%. Recently, a new class of low-dimensional materials, such as graphene and monolayered molybdenum disulfide, have been able to withstand large elastic stretches. Feng Ji and his collaborators consider how to use elastic strains to bring unprecedented performance to materials. They envisage that the probe presses on the suspended elastic membrane to construct an uneven strain field. The elastic strain corresponds microscopically to the stretching or compression of a chemical bond, which changes the energy of electrons (or carriers) in the material. An uneven strain distribution can cause an effective electric field to the carrier. If stress concentration is achieved in one device, it is possible to concentrate carriers through the stress field. This is like a carrier "funnel". The geometries of the devices proposed by Feng Ji and his collaborators are just like funnels (pictured).


Solar "funnel" schematic

As a conceptual demonstration, Feng Ji and his collaborators conducted a numerical simulation of monolayered molybdenum disulfide. The monolayer of molybdenum disulfide is currently the most concerned quantum material. In his previous cooperation, Feng Ji and Wang Enge demonstrated that a single layer of molybdenum disulfide has a quantum freedom termed “valley,” reflecting the “valley” circular dichroism selectivity and quantum transport properties [Ting Cao et al. Nature Communications 3, 887 (2012)]. In addition to its specific optical properties, molybdenum disulfide is an ultra-strength elastic crystal film that is only 0.6 nm thick and can carry 11% of elastic strain. Feng Ji and his collaborators solved the Bethe-Salpeter equation by using the GW approximation based on the density functional theory to obtain the quasi-particle energy (electrons, holes, and excitons) in molybdenum disulfide. Calculations show that the quasi-particle energy in the molybdenum disulfide is very sensitive to the change, and its exciton energy can change by as much as 0.7 eV within the material's intensity range. The strain distribution of monolayer molybdenum disulfide under top pressure was calculated using classical molecular dynamics. Feng Ji and his collaborators demonstrated the feasibility of this design.

Strain and strain fields have a profound effect on many properties of the material at the quantum level. Feng Ji's work shows the unique ability of non-uniform strain fields in the regulation of carriers. The elastic strain has d(d+1)/2 dimensions (d is the dimension of the material), and the corresponding strain field is a d(d+3)/2-dimensional continuous variable, which has rich and varied regulation of material properties. ability. With the emergence of super-strength materials, it is not difficult to foresee that the elastic strain engineering will be a research direction with theoretical and technological value. Professor James Hone from Columbia University commented on Feng Ji's work in Nature-View's News and Views [Nature Photonics 6, 804-806 (2012)]. He also spoke highly of the potential of elastic strain engineering.

This work was done in collaboration with Professor Li Ju, Dr. Qian Xiaofeng and Cheng-Wei Huang from MIT Department of Materials and Nuclear Engineering, and was supported by the National Natural Science Foundation of China, the 973 Program, the US NSF, and the Air Force.

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