Recently, Associate Professor Liu Can of Chinese Minmin University, Professor Liu Kaihui of Peking University and their collaborators have published two Science papers, which have been evaluated by reviewers as "milestone work in the field of low-dimensional material manufacturing", which is behind the team's efforts and continuous technical research in the past four years.
The researchers have innovatively designed a strategy of "multi-interface coupled atom fabrication", which not only realizes a specific chiral structure, but also enables the fabrication of tungsten disulfide strips in the direction of coherent polarization, thus enabling the "homogeneous" control of one-dimensional semiconductor arrays [1].
In another study, they proposed for the first time a new paradigm for the preparation of rhombic phase two-dimensional crystals of "lattice mass transfer-interface epitaxy" [2], which breaks through the limitations of the self-limitation of the number of layers and the uncontrollable stacking structure in the surface growth of two-dimensional materials.
For example, it is able to grow 2D single crystals quickly at a rate of 50 layers/minute; In addition, it can also control the single direction of each layer, so as to ensure that the layers are perfectly parallel, and finally achieve a thick rhomboidal two-dimensional crystal structure.
"This is a key technical barrier that has been difficult to break through for a long time by traditional surface growth methods, and this research provides a new idea for the precise atom fabrication of new functionalized crystals." Liu Can said.
Photo丨Liu Can (Source: School of Physics, Chinese Minmin University)
Evaluated as "a milestone work in the field of low-dimensional material manufacturing"
Over the past 30 years, one-dimensional materials such as carbon nanotubes and nanoribbons have been developed, but due to their structural diversity and complexity, it is extremely difficult to control their precise structures, especially at the atomic scale, such as chiral indices and helical structures.
In this study, the strategy of "multi-interface coupled atom fabrication" is as follows:
The lattice arrangement of tungsten disulfide (WS2) is directly related to the coupling of WS2-sapphire epitaxial interface. The axial direction of the strip is limited by the atomic-step coupling of sodium tungstate (Na2WO4) precursor-sapphire. The single polarization direction of the bands is locked under the coupling of the WS2-Na2WO4 precursor.
In order to control the interrelationship between the atomic-scale steps and lattice arrangements on the surface, the researchers precisely designed sapphire substrates for specific crystal planes, enabling precise atomic-scale fabrication of WS2 strips, including armchair shapes, zigzag shapes, and chiral structures (including left-handed chirality and right-handed chirality).
(Source: Science)
In the early days of the study, researchers were excited when they saw an array of perfectly parallel bands.
"Inspired by the early research on one-dimensional carbon nanotube arrays, when we see that the WS2 arrays arranged in parallel are always accompanied by cutting-edge particles, we immediately think of a carbon tube-like growth pattern that starts with catalyst particles," said Liu. ”
Moreover, their orientation is always limited by the atomic steps of the A-plane sapphire oxygen-dense array, which is also the arrangement law of the ultra-dense carbon tube array.
After chiral structure identification, the WS2 bands obtained by the researchers were all zigzag-shaped. But in fact, it's not surprising that this achiral array alone is.
It was not until later, with the in-depth exploration of the mechanism of controlled preparation of epitaxial single crystals in two-dimensional systems by scientists in the field, that researchers realized that the design idea of this two-dimensional material could be "reversed" to solve the problem of one-dimensional materials.
So, they tried to use the substrate lattice to "lock" the lattice direction and the step to "guide" the band direction. After trying it, I found that the effect was very remarkable, and I was able to control the various structures as expected.
"When we saw the results that day, we were all very excited and we talked about it until late at night." Liu Can recalled.
Figure丨WS2 strip array fabrication with controllable chirality and coherent polarity and integrated output of self-luminous current (source: Science)
It is important to understand that the "homogeneous" manufacturing in this study is not the same as the concept of "homogeneity" in quantum mechanics.
In physics, "identical" means that different particles are identical in all observable physical properties.
The study borrowed the concept of "identical" to express that the characteristic physical properties of WS2 bands are consistent, including: the orientation, chiral structure, and polarity direction of all WS2 bands in the array.
Previously, controlling so many parameters at the same time was extremely challenging and difficult to achieve in the one-dimensional field.
日前，相关论文以《具有可控手性与相干极性的二硫化钨条带阵列》（WS2 ribbon arrays with defined chirality and coherent polarity）为题，发表在 Science 上 [1]。
Peking University Ph.D. students Xue Guodong and Guo Quanlin, postdoctoral fellow Zhou Ziqi, and Associate Professor Zuo Yonggang of Kunming University of Science and Technology are the co-first authors, and Professor Liu Kaihui of Peking University, Associate Professor Liu Can of Chinese Renmin University, Wei Zhongming, researcher of the Institute of Semiconductors, Chinese Academy of Sciences, and Ding Feng, researcher of the Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences are the co-corresponding authors.
Figure丨Related papers (source: Science)
The high-density arrangement of parallel one-dimensional arrays is the prerequisite for the efficient energy harvesting and conversion of integrated optoelectronic chips.
WS2 strip arrays with controllable chirality and coherent polarity are expected to promote the development of technologies in the fields of self-driven photoelectric detection and new mechanisms of solar photovoltaics.
In the future, researchers will pay more attention to the study of the relevant states and properties of one-dimensional materials, such as rich edge states, limit-size transistors, and unconventional photovoltaic devices.
It provides a new idea for the precise atomic fabrication of new functionalized optical crystals
In another study, the researchers developed a new method for the growth of two-dimensional tandem crystals by "lattice mass transfer-interface epitaxy".
First, the atoms are transported through the lattice of the substrate to the surface of the substrate, forming the "first layer of crystal". After that, the newly added atoms continue to be transported to the interface space between the substrate and the first layer of the crystal through the crystal lattice.
Finally, it grows on top of the previously formed layer of crystal that is located above, and in this way continues to form new layers of crystals.
It is worth noting that various surface manipulation methods in the epitaxial growth of 2D single crystal surfaces, such as the single orientation of step-controlled crystal domains and the seamless splicing between different crystal domains, can be retained in this interface.
In other words, each layer of crystals is processed on the same substrate surface, which is equivalent to a "sibling" copied from the same template.
These layers are arranged parallel to each other, and the combined action of the upper crystals will form a special rhombocubic crystal structure.
"The fabrication of this structure is very demanding and often cannot be obtained directly by surface growth, and our 'lattice mass transfer-interfacial epitaxy' provides the possibility for this." Liu Can said.
Fig丨Development of a new growth paradigm of "lattice mass transfer-interface epitaxy" to prepare wafer-level rhombosquare phase two-dimensional transition metal chalcogenide single crystals (Source: Science)
Two-dimensional transition metal dichalcogenides (TMDs) have the characteristics of ultra-thin thickness, layer-dependent band structure, and high carrier mobility, and show application potential in the new generation of advanced process electrical devices.
TMD materials ranging from a single layer to 15,000 layers were prepared, and most importantly, the TMD layers were arranged in a completely parallel orientation, forming a stacked structure of 3R rhombosquare phase.
The few-layer of these 3R-TMDs has the advantages of high carrier mobility and high current density in terms of electrical aspects, making them ideal channel materials for integrated circuits at sub-5nm technology nodes.
The single-, double-, and triple-layer molybdenum sulfide in this study has mobility rates of up to 137 cm2 V−1 s−1, 155 cm2 V−1 s−1, and 190 cm2 V−1 s−1, respectively, meeting the 2028 semiconductor device mobility target of the International Devices and Systems Roadmap.
Compared to the common hexagonal phase 2H-TMDs, the unique stacking structure of 3R-TMDs brings more diverse characteristics.
For example, the ferroelectric properties of the inversion of the interface polarization, the photovoltaic effect of high-efficiency bodies, and the coherently enhanced nonlinear optical response.
Under the condition of quasi-phase matching, the micron-scale thick-layer 3R-TMDs achieve high-efficiency differential frequency conversion efficiency in the near-infrared band, which is 5 orders of magnitude higher than that of a single layer.
最终，相关论文以《多层菱方相过渡金属硫族化合物单晶的界面外延》（Interfacial epitaxy of multilayer rhombohedral transition metal dichalcogenide single crystals）为题发表在 Science 上 [2]。
Qin Biao and Guo Quanlin, postdoctoral fellow Ma Chaojie of Peking University, and Li Xiuyao, doctoral student of the Institute of Physics of the Chinese Academy of Sciences, are the co-first authors, and Professor Liu Kaihui of Peking University, Associate Professor Liu Can of Chinese People's University, and Zhang Guangyu, researcher of the Institute of Physics, Chinese Academy of Sciences, are the co-corresponding authors.
Figure丨Related papers (source: Science)
In the future, the researchers will continue to explore the possibility of this new paradigm of material preparation, and continue to explore the direction of crystal preparation and technological innovation in high-quality two-dimensional crystals, especially for nonlinear frequency conversion applications of optical crystals.
Combined with the angle phase matching theory previously proposed by the research group and the advantages of this material, they will further explore the possibility of developing miniaturized and integrated optoelectronic devices and photonic chips.
Resources:
1.Xue,G. et al. WS2 ribbon arrays with defined chirality and coherent polarity. Science 384, 6700,1100-1104(2024). https://doi/10.1126/science.adn9476
2.Qin,B. et al. Interfacial epitaxy of multilayer rhombohedral transition metal dichalcogenide single crystals. Science 385, 6704，99-104(2024). https://doi.org/10.1126/science.ado6038
Typesetting: Liu Yakun