Guo Xuefeng group, Peking University: multi-step nucleation and growth mechanism of organic solid-phase crystallization

It is an important method to grow organic single crystal thin film on the surface of dielectric layer However, the surface crystallization mechanism of organic solids and the debate on the "classical" and "non classical" crystallization theories are still unresolved basic scientific issues in the field of organic electronics For a long time, the crystallization mechanism of organic materials has basically drawn lessons from inorganic materials On the one hand, because organic molecules are relatively fragile, it is difficult to be applied to atomic and molecular level characterization means such as transmission electron microscopy; on the other hand, the constituent unit of organic solids is molecules, and the complexity of the crystallization process is far greater than that of inorganic materials based on atoms To solve this problem, recently, Guo Xuefeng, sun Junliang and Zhu Wenguang from the school of chemistry and molecular engineering, Peking University, have realized real-time in-situ observation of the whole process of organic solid surface crystallization at room temperature for the first time (Figure 1 ）The "nonclassical" crystallization mechanism of long-range mass transfer of organic molecular clusters (NAT Commun 2019, 10, 3872) was revealed by molecular dynamics simulation Figure 1 Five step evolutionary crystallization mechanism diagram (source: nature Communications) brief introduction of Professor Guo Xuefeng's research group Professor Guo Xuefeng's research group has long been committed to the development of molecular electronics and organic electronics In the field of organic electronics, we took the lead in introducing the concept of liquid crystal semiconductor into molecular design (adv mater 2012, 24, 5576; adv mater 2015, 27, 2113), which improves the self-assembly performance of semiconductor molecules, and has obtained highly efficient, stable and cheap organic semiconductor materials; at the same time, the device functionalization is realized by combining the interface engineering strategy, and the highly sensitive and stable multi-functional detection and sensor devices are developed (adv mater 2013, 25, 6752; small 2013, 8, 1144; ACS Nano 2016, 10, 436; Nano Lett 2016 , 16 , 3600; Adv Mater Technol 2019 , 1800358 ）。 Prof Guo Xuefeng, Professor of Peking University, outstanding youth, chief scientist of key R & D plan of the Ministry of science and technology and leader of "ten thousand talents plan" of the central organization department He received his bachelor's and master's degrees from Beijing Normal University in 2001 and his doctor's degree from the Institute of chemistry, Chinese Academy of Sciences in 2004 In 2004-2007, he worked as a postdoctoral researcher at Columbia University He joined Peking University in 2008 Mainly engaged in the research of molecular materials and devices, developed the breakthrough methods of preparing stable single molecular devices, built the first stable and controllable single molecular electronic switch device in the world, developed the new technology of single molecular electrical detection, and opened up the new field of single molecular science and technology research More than 155 SCI papers (if > 10, 67) including 2 science papers have been published, which has attracted wide attention of the scientific and industrial circles Scientific American, nature, science and other journals and media have reported highlights in different forms for more than 25 times, and have been invited to chem Rev., NAT Rev Physics, Many invited reviews have been written in international authoritative journals such as acc.chem.res.and chem.soc.rev., which shows that these works are in the leading position in the world and are one of the few representative research groups that can carry out single molecule electronics research in the world He has applied for 14 international and domestic patents, and has won 100 national excellent doctoral dissertation awards, the first prize of Natural Science Award of the Ministry of education, ten scientific and technological progress awards of Chinese universities and ten scientific progress awards of China As the chief scientist, he has undertaken the key R & D plan of the Ministry of science and technology, and presided over the instrument project, key project and outstanding project of NSFC Cutting edge scientific research achievements: multi-step nucleation and growth mechanism of organic solid-phase crystallization Organic crystal materials play an important role in electronics, optics and even biology (Science 2010, 330, 337; nature 2016, 536, 446) However, the crystallization mechanism of organic molecules and the debate on the "classical" and "non classical" theories are an outstanding basic scientific issue in the field of organic materials (chem Rev 2008, 108, 4899; chem Rev 2016, 116, 7078), which are crucial to the design and regulation of crystal structures and functions For a long time, the crystallization mechanism of organic materials has been based on that of inorganic materials (NAT Rev Mater 2016, 1, 16034), on the one hand, because of the fragility of organic molecules, it is difficult to be applied to the atom and molecule level characterization means such as transmission electron microscope; on the other hand, the component unit of organic solid is molecule, and the complexity of crystallization process is far greater than that of inorganic materials with atom as the basic component unit In recent two years, researchers have observed the crystallization process of organic molecules in solution in ultrafast flow cell atomic force microscope (NAT Commun 2014, 5, 5598; science 2014, 344, 729; science 2015, 347, 1329; science 2018, 360, 897; science 2018, 362, However, the mechanism of solid-phase crystallization on the substrate surface is still poorly understood On the one hand, due to the strong π - π interaction between the molecules, it is difficult for the molecules to migrate on the surface of the substrate, so it is usually necessary to heat the substrate above 100 ℃ to induce the Brownian motion of the molecules However, at high temperature, the image of AFM will drift and the spatial resolution will be reduced To solve this problem, Guo Xuefeng's group and his collaborators designed and synthesized a molecule containing phosphate group and semiconductor framework, which can realize phase transition at about 50 ℃, which makes it possible to observe at room temperature Using the time-resolved atomic force microscope platform, they observed in situ the whole process of organic molecules evolving from the initial droplets to single-crystal micrometres (Fig 2) Fig 2 The growth kinetics of thin film was characterized by time-resolved atomic force microscopy (source: nature Communications) Then, the research team conducted quantitative research on the kinetics of crystal growth process First of all, the newly spun film exists in the form of liquid drop, desolvates and collapses into pancake like film in a very short time, and rapidly increases the area; at the same time, the films in different areas collide and fuse with each other, forming a complete and continuous, but loose film At this time, the film is in metastable state, and then the metastable phase separation occurs, which shows that the film cracks, forming two kinds of island like regions: thicker and thinner After the separation and splitting of the two regions, the thinner island will be "dissolved and eaten" to grow the thicker Island, which is similar to the "Ostwald ripening" in the crystallization process of solution phase This process is related to the distance between the thick island and the thin island When the distance between them is more than 2 μ m, the distance between them exceeds the diffusion distance of the molecular cluster on the surface At this time, after the thick film, the domain self reconstruction is carried out, that is, the self etching of high-energy crystal surface is used for layered growth, and finally the morphology of the single crystal micrometre line is achieved In addition, the solid-phase crystallization mechanism of organic films was studied by time-resolved X-ray diffractometer, which further verified the five-step crystallization theoretical model (Fig 3) Figure 3 Quantitative kinetic characterization of the self-assembly process (source: nature Communications) Another highlight of this work is that the intermediates of mass transfer: Organic nanoclusters (Figure 4), which verify the "non classical" crystallization theory The key step of continuous growth of single crystal thin film is Ostwald ripening, that is, the thick film of stable phase "eats" the thin film of metastable phase The phenomenon shows that the thick film with large area grows continuously, and the thin film with small area disappears gradually Through the rapid scanning of the edge of the evolving film, the team observed the migration of a large number of organic nanoclusters (Fig 4f-m) From the nanometer scale, it is confirmed that the crystallization process of organic solids is a process of "stripping migration fusion" with "clusters" as the carrier This provides a new idea for the design and research of new semiconductor molecules, nucleating agents and inhibitors Fig 4 The "non classical" crystallization mechanism (source: nature Communications) in the mass transport process did not stop here In the process of optimizing the growth conditions, the ultra long single crystal microline array was obtained, which has a very strong diffraction signal (Fig 5) The group further tested the properties of the FET of the micro meter line, which is equivalent to the same single crystal devices reported, which provides a new idea for the development of large-area and high-performance optoelectronic devices in the future Figure 5 Characterization of the electrical properties of microwires formed by solid-phase crystallization (source: nature Communications) The research results are published on nature communications under the title of "multistepjunction and growth mechanisms of organizational crystals from amorphous solidstates" (NAT Commission 2019, 10, 3872) Professor Guo Xuefeng and Professor Sun Junliang of Peking University and Professor Zhu Wenguang of University of science and technology of China are the co correspondents of the work The first co authors of the paper are Dr Chen Hongliang, Dr Li Mingliang, Dr Lu Zheyu and Dr Wang Xiaoge of the research group of Guo Xuefeng and Dr Li Wenguang The work was supported by the Ministry of science and technology, the National Natural Science Foundation of China, the Beijing Municipal Science and Technology Commission and Peking University Nowadays, people and scientific research have been paid more and more attention in the economic life China has ushered in the "node of science and technology explosion" Behind the progress of science and technology is the work of countless scientists In the field of chemistry, in the context of the pursuit of innovation driven, international cooperation has been strengthened, the influence of Returned Scholars in the field of R & D has become increasingly prominent, and many excellent research groups have emerged in China For this reason, CBG information adopts the 1 + X reporting mechanism CBG information website, chembeangoapp, chembeango official micro blog, CBG information wechat subscription number and other platforms jointly launch the column of "people and scientific research", approach the representative research groups in China, pay attention to their research, listen to their stories, record their demeanor, and explore their scientific research spirit.