Nanoprocessing of Self-Suspended Monolayer Graphene and Defect Formation by Femtosecond-Laser Irradiation

We demonstrate the femtosecond-laser processing of self-suspended monolayer graphene grown by chemical vapor deposition, resulting in multipoint drilling with holes having a diameter of <100 nm. Scanning transmission electron microscopy revealed the formation of many nanopores on the laser-irradiated graphene. Furthermore, atomic-level defects as well as nanopores were found in the graphene membrane by high-resolution transmission electron microscopy, while the overall crystal structure remained intact. Raman spectroscopy showed an increase in the defect density with an increase in the number of laser shots, suggesting that the nanopore formation triggered the creation of the <100 nm holes. The approach presented herein can offer an experimental insight into the simulation of atomic dynamics in graphene under femtosecond-laser irradiation. The thorough examination of the atomic defect formation and secondary effect of surface cleaning observed in this study would help develop engineering methods for graphene and other two-dimensional materials in the future.

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects

Synchrotron and laboratory-based X-ray imaging techniques have been increasingly used for in situ investigations of alloy solidification and other metal processes. Several reviews have been published in recent years that have focused on the development of in situ X-ray imaging techniques for metal solidification studies. Instead, this work provides a comprehensive review of knowledge provided by in situ X-ray imaging for improved understanding of solidification theories and emerging metal processing technologies. We first review insights related to crystal nucleation and growth mechanisms gained by in situ X-ray imaging, including solute suppressed nucleation theory of α-Al and intermetallic compound crystals, dendritic growth of α-Al and the twin plane re-entrant growth mechanism of faceted Fe-rich intermetallics. Second, we discuss the contribution of in situ X-ray studies in understanding microstructural instability, including dendrite fragmentation induced by solute-driven, dendrite root re-melting, instability of a planar solid/liquid interface, the cellular-to-dendritic transition and the columnar-to-equiaxed transition. Third, we review investigations of defect formation mechanisms during near-equilibrium solidification, including porosity and hot tear formation, and the associated liquid metal flow. Then, we discuss how X-ray imaging is being applied to the understanding and development of emerging metal processes that operate further from equilibrium, such as additive manufacturing. Finally, the outlook for future research opportunities and challenges is presented.

Influence of the Reaction Pathway on the Defect Formation in a Cu2ZnSnSe4 Thin Film

Point defect engineering in Cu₂ZnSnSe₄ (CZTSe) thin films is the main issue to improve its device performance. This study reveals the correlation between the reaction pathway and the point defects in the CZTSe film. The reaction pathway from a metallic precursor (Mo/Zn/Sn/Cu) to a kesterite CZTSe film is varied by changing the annealing process. The synthesized CZTSe films under different reaction pathways induce different device performances with different defect energy levels, although all CZTSe films have similar structural and optical properties (E_g ∼ 1.0 eV). The admittance spectroscopy demonstrates the correlations between point defect types (V_Zn, Zn_Sn, Zn_Cu, Cu_Zn, and V_Cu) and the reaction pathways for the formation of CZTSe films. The different growth rates of binary selenides, such as ZnSe and/or Sn-Se phases, during the annealing process are especially strongly related to the formation of point defects, leading to the different open-circuit voltages (396-451 mV) and fill factors (51-65%). The results of this study suggest that controlling the reaction pathway is an effective approach to adjust the formation of defects in the kesterite CZTSe film as well as to fabricate high-performance solar cell devices.

Kibble-Zurek Scaling during Defect Formation in a Nematic Liquid Crystal

Symmetry-breaking phase transitions are often accompanied by the formation of topological defects, as in cosmological theories of the early universe, superfluids, liquid crystals or solid-state systems. This scenario is described by the Kibble-Zurek mechanism, which predicts corresponding scaling laws for the defect density ρ. One such scaling law suggests a relation ρ≈τ_Q^-1/2 with τ_Q the change of rate of a control parameter. In contrast to the scaling of the defect density during annihilation with ρ≈t^-1 , which is governed by the attraction of defects of the same strength but opposite sign, the defect formation process, which depends on the rate of change of a physical quantity initiating the transition, has only rarely been investigated. Herein, we use nematic liquid crystals as a different system to demonstrate the validity of the predicted scaling relation for defect formation. It is found that the scaling exponent is independent of temperature and material employed, thus universal, as predicted.

Mobility Modulation and Suppression of Defect Formation in Two-Dimensional Electron Systems by Charge-Transfer Management

Electron mobility is one of the most-debated key attributes of low-dimensional electron systems emerging at complex oxide heterointerfaces. However, a common understanding of how electron mobility can be optimized in these systems has not been achieved so far. Here, we discuss a novel approach for achieving a systematic increase in electron mobility in polar/nonpolar perovskite interfaces by suppressing the thermodynamically required defect formation at the nanoscale. We discuss the transport properties of electron gases established at interfaces between SrTiO₃ and various polar perovskites [LaAlO₃, NdGaO₃, and (La,Sr)(Al,Ta)O₃], allowing for the individual variation of epitaxial strain and charge transfer among these epitaxial interfaces. As we show, the reduced charge transfer at (La,Sr)(Al,Ta)O₃/SrTiO₃ interfaces yields a systematic increase in electron mobility, while the reduced epitaxial strain has only minor impact. As thermodynamic continuum simulations suggest, the charge transfer across these interfaces affects both the spatial distribution of electrons and the background distribution of ionic defects, acting as major scatter centers within the potential well. Easing charge transfer in (La,Sr)(Al,Ta)O₃/SrTiO₃ yields an enlarged spatial separation of mobile charge carriers and scattering centers, as well as a reduced driving force for the formation of ionic defects at the nanoscale. Our results suggest a general recipe for achieving electron enhancements at oxide heterostructure interfaces and provide new perspectives for atomistic understanding of electron scattering in these systems.

Modification of graphene/SiO2 interface by UV-irradiation: effect on electrical characteristics

Graphene is a promising material for next-generation electronic devices. The effect of UV-irradiation on the graphene devices, however, has not been fully explored yet. Here we investigate the UV-induced change of the field effect transistor (FET) characteristics of graphene/SiO2. UV-irradiation in a vacuum gives rise to the decrease in carrier mobility and a hysteresis in the transfer characteristics. Annealing at 160 °C in a vacuum eliminates the hysteresis, recovers the mobility partially, and moves the charge neutrality point to the negative direction. Corresponding Raman spectra indicated that UV-irradiation induced D band relating with defects and the annealing at 160 °C in a vacuum removed the D band. We propose a phenomenological model for the UV-irradiated graphene, in which photochemical reaction produces dangling bonds and the weak sp(3)-like bonds at the graphene/SiO2 interface, and the annealing restores the intrinsic graphene/SiO2 interface by removal of such bonds. Our results shed light to the nature of defect formation by UV-light, which is important for the practical performance of graphene based electronics.