Effects of Ion Irradiation and Temperature on Mechanical Properties of GaN Single Crystals under Nanoindentation.

Understanding the impact of irradiation and temperature on the mechanical properties of GaN single crystals holds significant relevance for rational designs and applications of GaN-based transistors, lasers, and sensors. This study systematically investigates the influence of C-ion irradiation and temperature on pop-in events, hardness, Young's modulus, and fracture behavior of GaN single crystals through nanoindentation experiments. In comparison with unirradiated GaN samples, the pop-in phenomenon for ion-irradiated GaN samples is associated with a larger critical indentation load, which decreases with increasing temperature. Both unirradiated and ion-irradiated GaN samples exhibit a decline in hardness with increasing indentation depth, while Young's moduli do not exhibit a clear size effect. In addition, intrinsic hardness displays an inverse relationship with temperature, and ion-irradiated GaN single crystals exhibit greater intrinsic hardness than their unirradiated counterparts. Our analysis further underscores the significance of Peierls stress during indentation, with this stress decreasing as temperature rises. Examinations of optical micrographs of indentation-induced fractures demonstrate an irradiation embrittlement effect. This work provides valuable insights into the mechanical behavior of GaN single crystals under varying irradiation and temperature conditions.

Accurate Measurement of Defect Generation Rates in Silicon Carbide Irradiated with Energetic Ions.

In this work, we obtained the Si vacancy generation rates η in SiC nanowire samples irradiated with 1, 3 MeV protons, and 2.8 MeV helium ions using the electrical resistivity measurement, which further indicated an intuitive linear function correlation between η and the nuclear stopping power of the incident ions at a low dpa level with a coefficient of 2.15 × 10-3 eV-1. Prediction through this correlation is consistent with previous work. Besides, the measured value is about 1/2 of the simulation results with the popular SRIM code. Overall, our work provides a feasible way to get the generation rate of a certain irradiation-induced defect by electric measurements, and the correlation obtained is practically useful in various applications.

Environment-Dependent Radiation Tolerance of Graphene Transistors under Proton Irradiation.

Electronic devices based on two-dimensional materials are promising for application in space instrumentation because of their small size and low power consumption, and irradiation tolerance of these devices is required because of the existence of energetic particles in aerospace conditions. We investigate the performance degradation of graphene field effect transistors (GFETs) with 3 MeV protons by using an in situ irradiation facility. Our results indicate that GFET performance degraded severely at the ion fluence of 8 × 1011 cm-2. Surprisingly, although the performance of the proton-irradiated GFETs is difficult to recover in vacuum, it can nearly completely recover within hours when the GFET is moved into an air environment, indicating that the performance change is due to the charge accumulation in SiO2 under proton irradiation rather than the lattice damage of graphene. Our results have great importance for the application of 2D devices in aerospace and other radiative environments.

Multifunctional graphene heterogeneous nanochannel with voltage-tunable ion selectivity.

Ion-selective nanoporous two-dimensional (2D) materials have shown extraordinary potential in energy conversion, ion separation, and nanofluidic devices; however, different applications require diverse nanochannel devices with different ion selectivity, which is limited by sample preparation and experimental techniques. Herein, we develop a heterogeneous graphene-based polyethylene terephthalate nanochannel (GPETNC) with controllable ion sieving to overcome those difficulties. Simply by adjusting the applied voltage, ion selectivity among K+, Na+, Li+, Ca2+, and Mg2+ of the GPETNC can be immediately tuned. At negative voltages, the GPETNC serves as a mono/divalent ion selective device by impeding most divalent cations to transport through; at positive voltages, it mimics a biological K+ nanochannel, which conducts K+ much more rapidly than the other ions with K+/ions selectivity up to about 4.6. Besides, the GPETNC also exhibits the promise as a cation-responsive nanofluidic diode with the ability to rectify ion currents. Theoretical calculations indicate that the voltage-dependent ion enrichment/depletion inside the GPETNC affects the effective surface charge density of the utilized graphene subnanopores and thus leads to the electrically controllable ion sieving. This work provides ways to develop heterogeneous nanochannels with tunable ion selectivity toward broad applications.

Mechanical Properties of GaN Single Crystals upon C Ion Irradiation: Nanoindentation Analysis.

Mechanical properties of gallium nitride (GaN) single crystals upon carbon ion irradiation are examined using nanoindentation analysis at room temperature. Pop-in events in the load-depth curves are observed for unirradiated and irradiated GaN samples. A statistical linear relationship between the critical indentation load for the occurrence of the pop-in event and the associated displacement jump is exhibited. Both the slope of linear regression and the measured hardness increase monotonically to the ion fluence, which can be described by logistic equations. Moreover, a linear relationship between the regression slope as a micromechanical characterization and the hardness as a macroscopic mechanical property is constructed. It is also found that the maximum resolved shear stress of the irradiated samples is larger than that of the unirradiated samples, as the dislocation loops are pinned by the irradiation-induced defects. Our results indicate that the nanoindentation pop-in phenomenon combined with a statistical analysis can serve as a characterization method for the mechanical properties of ion-irradiated materials.