저차원 나노구조체의 크기 의존적 나노역학특성에 관한 연구

Abstract

Due to the excellent and unique properties, one-dimensional nanomaterials (e.g., nanowires and nanorods) have gathered attentions from the interdisciplinary areas in science and engineering. Especially, inorganic semiconducting nanomaterials, such as Si nanowires and ZnO nanorods (which are mainly dealt in this thesis), have been considered as one of the most important building blocks for emerging bottom-up nanotechnology. For this reason, significant efforts have been made to apply them into broad range of electric and optoelectric systems and, more recently, the development of near-future devices that is moving toward flexible, bendable, rollable and stretchable ones. To optimize the design, ensure the reliable operation, and improve the lifetime of those nanomaterial-based new devices, better understating of the materials’ mechanical behavior is essential. Especially, considering that the components of such devices will be exposed small but long-lasting external stresses, the information about both time-dependent and time-independent mechanical behavior should be required. With these in mind, this thesis is devoted to investigate the time-independent and time-dependent mechanical behavior of nanomaterials, the size effects on the behavior, and the influence of time-dependent deformation on electrical properties.

As a first step, the most appropriate testing and analyzing method is suggested for accurately determining the time-independent mechanical behavior of nanomaterials and their size effects were investigated. Elastic modulus and strength of Si nanowires were estimated from the two most popular nanomechanical tests, atomic force microscopy (AFM) bending and nanoindentation. A variety of nanomechanical experiments led to the suggestion that AFM bending based on the line tension model was the most appropriate and reliable testing method for mechanical characterization of silicon nanowires. This result was supported by additional finite element simulations, and an experimental guideline was suggested. In the examined range of nanowires radius from 15 to 70 nm (this may be the widest range ever reported in this research field), elastic modulus is maintained at about 185 GPa which is similar to that of bulk Si, whereas nanowires strength increases from 2 to 10 GPa as radius is reduced. This size effect in strength is discussed in terms of the less possibility of the existence of surface flaws leading catastrophic failure and the higher fracture strength based on fracture mechanics in smaller nanowires.

As a preliminary experiment for observing the nanoscale time-dependent deformation of nanomaterials, spherical nanoindentation creep tests were performed on single crystal bulk specimens. During the constant-load indentation creep tests in elastic strain and elastic-plastic strain regimes, (0001)-oriented ZnO single crystal indeed experienced the creep deformation even at ambient temperature, despite much pronounced creep in elastic-plastic strain regimes. With the observed quasi-steady-state creep rate and mean contact pressure, the stress exponent which is directly related with creep mechanism was found as ~1.36 for elastic contact regime and ~3.08 for elastic-plastic regime, suggesting suggesting dominant creep mechanism of diffusion and dislocation, respectively. This strain-dependent transition of creep mechanism was discussed in terms of possible factors affecting the creep in each regime. From these experiments, we could expect that the diffusion creep may occur in the compression creep tests on single crystal ZnO nanorods in elastic strain regime.

Then, compression creep experiments were carried out on vertically oriented (0001) single crystal ZnO nanorods to investigate the effect of nanorods size on the nanoscale creep behavior. As observed from indentation creep tests on bulk sample, it was revealed that creep indeed occurred in the nanorods at low stress level in elastic regime and ambient temperature. Analyzing the stress exponent (which was independent of nanorods size as ~1) and the activation volume suggested that the creep may be controlled by the diffusion creep (through the space-charge layer near the surface and/or along the interface between the punch and the top surface of the rod). Diffusional creep behavior in elastic strain regime (which is also agreed by the results of bulk creep tests) was additionally supported by the results from in-situ creep tests under electron-beam irradiation and in-situ electric measurements. Nanoscale creep was more pronounced in smaller nanorods; i.e., increased creep amount and rate with reduced equivalent diameter of nanorods from ~2000 to ~200 nm. This size effect here was discussed in terms of the excellent diffusion paths of surface and interface and the ratio of their area to nanorod volume.

Finally, the influence of nanoscale creep behavior on electrical behavior was systematically explored through a series of compression creep tests combined with in-situ electrical measurement. Continuous measurement of current-voltage curves before, during and after creep tests showed that electrical current can be non-negligibly enhanced by the creep deformation, which was also confirmed by in-situ electrical current measurement at a fixed voltage during the test. Amount of reduction in nanorods resistance, caused by the creep deformation, was extracted from the analysis of experimental current-voltage data based on the thermionic emission-diffusion theory. A simple analytical model was suggested to predict the creep-induced reduction in nanorods resistance in terms of creep strain and the predicted value was in a reasonably good agreement with those experimentally determined.