Twinning has been recognized to be an important microstructural defect in nanoscale materials. Periodically twinned SiC nanowires were largely synthesized by the carbothermal reduction of a carbonaceous silica xerogel prepared from tetraethoxysilane and biphenyl with iron nitrate as an additive. The twinned β-SiC nanowires, with a hexagonal cross section, a diameter of 50-300 nm and a length of tens to hundreds of micrometers, feature a zigzag arrangement of periodically twinned segments with a rather uniform thickness along the entire growth length. Computer simulation has been used to generate three-dimensional atomic structures of the zigzag columnar twin structure by the stacking of hexagonal discs of {111} planes of SiC. A minimum surface energy and strain energy argument is proposed to explain the formation of periodic twins in the SiC nanowires. The thickness of the periodic twinned segments is found to be linearly proportional to the nanowire diameter, and a constant volume model is proposed to explain the relation.
We report an five-fold overall enhacement of upconvertion emission in NaYF4:Yb/Er nanocryatals when coupled with gold island films. Spectroscopic studies show that the enhancement factors are hlighly depdent on the exact spectral positions and excitation power density, with a largest enhancement factor more than 12 observed at slected spectral position, which may be attributed to different upconversion processes involved.
Nanotwinned copper has been shown to greatly improve the yield strength while maintaining good electrical conductivity. It has great potential to be incorporated in the very-large-scale integration of Cu interconnect technology. The influence of stress/strain on nanotwin formation is studied using first principles calculations of the total crystal binding energy. Under biaxial stress, the total energy of strained Cu can be larger than that of strain-relaxed periodic nanotwinned Cu. We propose that, during pulse electrodeposition of Cu films, highly strained Cu can undergo recrystallization and grain growth to relax stress and form strain-relaxed nanotwins.
In situ stress measurements were performed during high frequency pulse electrodeposition of nanotwinned Cu thin films. Periodic stress changes during pulse-on and pulse-off periods were observed. The stress profile showed an abrupt increase in tensile stress to about 400 MPa during the pulse-on period and a stress relaxation during the pulse-off period. First-principles calculations predict that a complete relaxation of the tensile stress allows the formation of nanotwins separated by 28 nm or more. This is in good agreement with the results obtained from microstructural analysis of the Cu films fabricated during in situ stress measurements.
Pulse-electroplated copper that contains a high density of {111}/⟨112⟩ nanotwins has been found to greatly improve the yield strength while maintaining good electrical conductivity. The thermal stability of nanotwins is a concern and has been studied by in situ transmission electron microscopy (TEM) characterization from 200 to 350 °C in the present work. It was found that the (112¯) twin boundary in a junction of (111)/(112¯)/(111) twins migrates to eliminate the (111) twin boundaries. We propose that it is the dominant mechanism that reduces the twin density in the range of temperature studied. The driving force is provided by the elimination of the two (111) boundaries. The inverse migration of the (112¯) twin boundary driven by a high strain is possible if enough stress has been applied to the copper, e.g., the strain introduced during pulsed electroplating. On the other hand, the migration of (111) twin boundary in the direction normal to the twin plane was not found. However, we propose that it can happen if a (112¯) step migrates on the (111) surface, provided that there exists a driving force. The structure and mobility of (112) twin boundary has been characterized by high resolution TEM. We observed that the (112) step on (111) twin plane has a height of three atomic layers. It is a unique structure unit of the (112) twin.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.