The deformation behavior of bulk ZnO single crystals is studied by a combination of spherical nanoindentation and atomic force microscopy. Results show that ZnO exhibits plastic deformation for relatively low loads (≳4–13 mN with an ∼4.2 μm radius spherical indenter). Interestingly, the elastic–plastic deformation transition threshold depends on the loading rate, with faster loading resulting, on average, in larger threshold values. Multiple discontinuities (so called “pop-in” events) in force–displacement curves are observed during indentation loading. No discontinuities are observed on unloading. Slip is identified as the major mode of plastic deformation in ZnO, and pop-in events are attributed to the initiation of slip. An analysis of partial load–unload data reveals values of the hardness and Young’s modulus of $\text{5.0±0.1}$ and $\text{111.2±4.7\hspace{0.17em}}\mathrm{GPa},$ respectively, for a plastic penetration depth of 300 nm. Physical processes determining deformation behavior of ZnO are discussed.

We study the evolution of lattice defects in single-crystal ZnO bombarded with 60-keV ${}^{28}\mathrm{Si}$ and 300-keV ${}^{197}\mathrm{Au}$ ions at 77 and 300 K. To characterize ion-beam-produced structural defects, we use a combination of Rutherford backscattering/channeling (RBS/C) spectrometry, cross-sectional transmission electron microscopy (XTEM), x-ray photoelectron spectroscopy, and atomic force microscopy. Results show that ZnO exhibits strong dynamic annealing, and even high-dose bombardment with heavy ${(}^{197}\mathrm{Au})$ ions at 77 K does not render ZnO amorphous. However, a crystalline-to-amorphous phase transition can be induced by irradiation with relatively light ${}^{28}\mathrm{Si}$ ions. In this latter case, amorphization is attributed to strong chemical effects of Si atoms implanted into the ZnO lattice, resulting in the stabilization of an amorphous phase. High-dose heavy-ion bombardment also results in a strong stoichiometric imbalance (loss of O) in the near-surface region. A variation in irradiation temperature from 77 up to 300 K has a minor effect on the damage buildup behavior in ZnO bombarded with Au ions. Data analysis also shows that a variation in the density of collision cascades by increasing ion mass from ${}^{28}\mathrm{Si}$ up to ${}^{197}\mathrm{Au}$ has a negligible effect on the damage buildup behavior. For both light- ${(}^{28}\mathrm{Si})$ and heavy- ${(}^{197}\mathrm{Au})$ ion bombardment regimes, XTEM reveals that ion irradiation produces energetically favorable planar defects which are parallel to the basal plane of the wurtzite structure of ZnO. Interestingly, our RBS/C study also reveals the formation of a middle defect peak between the surface and bulk peaks of disorder in Au-implanted ZnO, but not in Si-bombarded samples. The formation of this middle peak, most likely to be related to complex defect agglomeration processes, is rather unexpected and, to our knowledge, has not been observed in any other material. Physical mechanisms of defect formation in ZnO under ion bombardment are discussed based on these experimental findings.

Spherical indentation of crystalline silicon has been studied using cross-sectional transmission electron microscopy (XTEM). Indentation loads were chosen below and above the yield point for silicon to investigate the modes of plastic deformation. Slip planes are visible in the XTEM micrographs in both indentation loads studied. A thin layer of polycrystalline material has been identified (indexed as Si-XII from diffraction patterns) on the low-load indentation. The higher-load indentation revealed a large region of amorphous silicon. The sequence of structural deformation by indentation in silicon has been observed with the initial deformation mechanism being slip until phase transformations can take place.

Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties.