Science: Tsinghua team proposes 3D laser assembly technology of nanoparticles

Recently, the research group of Professor Sun Hongbo and Associate Professor Lin Linhan from the Department of precision instruments of Tsinghua University proposed a new technology of three-dimensional laser assembly of nanoparticles based on chemical bonding by using photogenerated high-energy carriers to regulate the surface chemical activity of nanomaterials. Based on this technology, the research team demonstrated the complex three-dimensional structure and heterostructure of a variety of different nanoparticles, realized high-precision laser micro nano manufacturing beyond the optical diffraction limit, and provided a new idea for the preparation of micro nano functional devices. This achievement was published in the journal Science on September 2, entitled "3D nanoprinting of semiconductor quantum dots by photo excitation induced chemical bonding".

As the source of the generation and development of high-tech in the 21st century, nanoscience and technology originate from a series of strange new physical and chemical effects caused by the reduction of material size to the nanometer scale, including the quantum confinement effect and quantum tunneling effect in semiconductor materials, and the surface plasmon resonance in metal materials. The existing preparation of nano devices is mainly based on micro nano manufacturing technologies such as photolithography and electron beam exposure, which is only applicable to a limited number of nano materials, and as a planarization preparation process, it is difficult to achieve three-dimensional manufacturing of nano materials. On the other hand, chemical synthesis can achieve the preparation and precise cutting of colorful (different sizes, morphologies and compositions) nanoparticles, and these nanomaterials have high crystal quality, good surface quality and excellent optical, electrical and magnetic properties. However, these chemically synthesized nanoparticles lack an effective device preparation process, which has become the technical bottleneck of their widespread application.

In view of the above problems, the research team proposed a new principle of photo excitation induced chemical bonding, realized the laser three-dimensional assembly technology of nanoparticles, and assembled three-dimensional nano devices using various nanoparticles as raw materials. Taking the semiconductor quantum dot with core-shell structure as an example (as shown in Fig. 1), the laser is used to excite the quantum dot to generate electron hole pairs. Through energy level matching, the tunneling and surface migration of photogenerated holes are driven, which promotes the desorption of ligands on the surface of the quantum dot and forms active chemical sites, thereby inducing chemical bonding on the surface of the quantum dot and achieving efficient assembly between the quantum dots.

Figure 1. Schematic diagram of the principle of photoexcitation induced chemical bonding

Based on the above principles, the research team further focused and programmed the laser beam to achieve the precise molding of the complex three-dimensional structure of nano materials. Compared with the existing micro nano fabrication technology, this technology has the following distinctive features. High purity of printing materials: compared with the existing laser 3D nano printing technology, this technology breaks through the principle of photopolymerization, does not require any optical bonding components, and realizes 3D printing of nearly 100% functional nano particle components; Strong three-dimensional processing ability: it can realize nano printing of multiple three-dimensional structures such as complex linear, curved and bulk structures (Fig. 2); High printing resolution: using nonlinear light excitation, the printing resolution breaks through the optical diffraction limit, the printing dot array density exceeds 20000ppi, the printing limit resolution reaches 77nm, and maintains excellent uniformity in large-scale array processing; With multi-component printing function: using quantum dots of different sizes as raw materials, this technology also demonstrates the heterogeneous composite printing ability of multi-component (Fig. 3).

Figure 2. 3D nano printed structure and morphology of quantum dots

Figure 3. Quantum dot RGB tricolor printing and heterogeneous patterning

In this paper, the high-sensitivity and light responsive quantum dot micro photodetectors based on the above technologies are demonstrated. It is worth pointing out that the micro nano fabrication principle of photo excitation induced chemical bonding has a wide range of material and structure adaptability.

Through energy level design, high-precision micro nano fabrication of a variety of semiconductors and metal materials can be realized, which opens up a new way of nano device fabrication technology and has important application prospects in the fields of on-chip optoelectronic device integration and high-performance near eye display.

The co first authors of this paper are Liu Shaofeng, a 2019 doctoral student in the Department of precision instruments of Tsinghua University, and Hou Zhengwei, a 2021 doctoral student in the school of materials. Tsinghua University is the first unit to complete the paper, and the corresponding authors of the paper are Professor Sun Hongbo and Associate Professor Lin Linhan of the Department of precision instruments of Tsinghua University. Professor Li Zhengcao from the school of materials of Tsinghua University, Associate Professor Zhang Hao from the Department of chemistry, Dr. Li Fu, and Associate Professor Fang Honghua from the Department of precision instruments have made important contributions to the work of the paper.

This research was supported by the national key R & D program, the National Natural Science Foundation of China, the Tsinghua Foshan innovation special fund and the State Key Laboratory of precision testing technology and instruments.

Paper link：https://www.science.org/doi/10.1126/science.abo5345

Source: Tsinghua University