2023.09.28

Professor Michael Hsuan-Yi Huang's (黃暄益) research team from the Department of Chemistry at National Tsing Hua University (NTHU) in Taiwan has provided experimental evidence to the observed “facet effects” in cuprous oxide (Cu2O) crystals. Despite having the same chemical composition, crystals with different shapes or exposed surfaces can produce unique electrical, optical, and photocatalytic characteristics due to tiny variations in the bulk and surface layer lattices. That is, each polyhedral shape can have distinct bulk and surface lattice dimensions.
 
This research discovery, which was featured in the prestigious journal Small, is expected to propel significant advancements in the field of semiconductor research in the foreseeable future.
 
In the past, it was probably not well recognized that crystals with the same composition should possess considerably different physical and chemical properties, including conductivity and optical characteristics.
 
However, Huang's earlier findings revealed that a Cu2O octahedron shows significant conductivity, a cube exhibits moderate conductivity, while a rhombic dodecahedron is insulating.
 
Furthermore, Cu2O rhombic dodecahedra are far more photocatalytically active than octahedra, while cubes are simply inert. Intriguingly, Cu2O crystals of different shapes display varying colors.
 
While most researchers showed little interest in exploring these differences, Huang was inspired by these results. He aimed to provide solid experimental evidence of the thin surface layer causing various facet-dependent behaviors in Cu2O crystals.
 
Huang proposed the existence of a thin layer on the surface of Cu2O crystals. With extremely subtle discrepancies in atomic positions, the structure of the surface lattice should differ from that of the interior lattice. If this is the case, it can explain differences both in how charges move and in the absorption of light.
 
Huang provided an analogy. “It's like gently searing the surface of sashimi with a flame. While the appearance may appear similar, the aroma and texture have changed.”
 
To provide experimental evidence of this structural feature, Huang's team employed synchrotron X-ray sources at the National Synchrotron Radiation Research Center (NSRRC) to obtain the X-ray diffraction (XRD) patterns of polyhedral Cu2O crystals.
 
As expected, they found distinct lattice arrangements in octahedral, cubic, and rhombic dodecahedral crystals, providing evidence for the notion of “facet effects”. This revelation suggests that semiconductors should also exhibit surface-related piezoelectric and magnetic characteristics.
 
Bo-Hao Chen (陳柏豪), a Ph.D. candidate in the Department of Chemistry at NTHU and also a research assistant at the NSRRC, explained that the high-resolution powder X-ray diffraction station at Taiwan Photon Source boasts a powerful and focused light source that exhibits minimal dispersion. Consequently, the experiments yield exceptionally precise results.
 
Chen placed Cu2O nanocrystal powder into a capillary tube with a mere 0.3 mm diameter and subjected it to X-ray exposure. The resulting diffraction patterns displayed two diffraction peaks, indicating the presence of both bulk and surface lattices.
 
Huang believes that understanding the physical reality of crystals behind the facet effects will broaden the potential uses of semiconductor materials. This could involve the precise engineering of semiconductor crystals to enable the manipulation of charge transfer or the strategic selection of crystal face combinations for transistor fabrication.
 
Huang extended his appreciation for the technical assistance provided by the NSRRC, which has ultimately validated his long-held hypothesis.
 
Huang's cross-disciplinary team is composed of various members from NTHU as well as other universities and research institutes. The team members from the Department of Chemistry at NTHU include: Bo-Hao Chen (陳柏豪), a Ph.D. candidate; Gautam Kumar (高達姆), a Ph.D. graduate; Professor I-Chia Chen (陳益佳); Yu-Jung Wei (魏佑容), a master's graduate; Hsueh-Heng Ma (馬學恆), a master's graduate; and Po-Jung Chou (周柏融), also a master's graduate.
 
The team members affiliated with external institutions include: Associate Professor Yu-Chieh Lo (羅友杰) and Ph.D. candidate Jui-Cheng Kao (高睿成) from the Department of Materials Science and Engineering at National Yang Ming Chiao Tung University (NYCU); Associate Professor Jyh-Pin Chou (周至品) from the Department of Physics at National Changhua University of Education (NCUE); and Associate Researcher Yu-Chun Chuang (莊裕鈞) from the NSRRC.