Typically, materials with high electrical conductivity such as metals are opaque, and materials with high optical transparency such as glass are insulating. Finding materials that are both transparent to visible light and electrically conductive has proven to be a challenge. The need for such materials continues to grow, as many of today’s popular devices such as liquid-crystal displays and organic light-emitting diodes in televisions, touch screens in phones or tablet computers, electrophoretic displays in e-readers, or solar cells on a roof require one or more layers to transmit visible light, while simultaneously applying a voltage or conducting a current. Today, the industry’s need for such a material is serviced by various metal oxides, of which indium tin oxide (ITO) is by far the most common. The opto-electronic properties of ITO satisfy industry need for most devices; however, ITO has several drawbacks (e.g., brittle, expensive, and typically applied via costly sputtering techniques). To address these issues, recent advances in solution-processed nanomaterials have enabled several printable alternatives to sputtered ITO. These nanomaterials include conducting polymers, metallic nanostructures, ITO nanostructures, carbon nanotubes, and graphene. The ability to apply nanomaterials from the liquid phase opens the possibility to print these electronic materials roll-to-roll, greatly reducing cost and increasing yield and throughput, while the nanomaterial topology enables truly flexible devices.

Predicting the dislocation nucleation rate as a function of temperature and stress is crucial for understanding the plastic deformation of nanoscale crystalline materials. However, the limited time scale of molecular dynamics simulations makes it very difficult to predict the dislocation nucleation rate at experimentally relevant conditions. We recently develop an approach to predict the dislocation nucleation rate based on the Becker–Döring theory of nucleation and umbrella sampling simulations. The results reveal very large activation entropies, which originated from the anharmonic effects, that can alter the nucleation rate by many orders of magnitude. Here we discuss the thermodynamics and algorithms underlying these calculations in greater detail. In particular, we prove that the activation Helmholtz free energy equals the activation Gibbs free energy in the thermodynamic limit and explain the large difference in the activation entropies in the constant stress and constant strain ensembles. We also discuss the origin of the large activation entropies for dislocation nucleation, along with previous theoretical estimates of the activation entropy.

Nanoindentation and nanoscratching of an indium phosphide (InP) semiconductor surface was investigated via contact mechanics. Plastic deformation in InP is known to be caused by the nucleation, propagation, and multiplication of dislocations. Using selective electrochemical dissolution, which reveals dislocations at the semiconductor surface, the load needed to create the first dislocations in indentation and scratching can be determined. The experimental results showed that the load threshold to generate the first dislocations is twice lower in scratching compared to indentation. By modeling the elastic stress fields using contact mechanics based on Hertz’s theory, the results during scratching can be related to the friction between the surface and the tip. Moreover, Hertz’s model suggests that dislocations nucleate firstly at the surface and then propagate inside the bulk. The dislocation nucleation process explains the pop-in event which is characterized by a sudden extension of the indenter inside the surface during loading.

GaN nanowires are promising building blocks for future nanoelectronics, optoelectronic devices, and nanogenerators. Here, we report on strong piezoelectricity in individual single-crystal GaN nanowires revealed by direct measurement of the piezoelectric constant using piezoresponse force microscopy. Our experimental results show that individual c-axis GaN nanowires, with a characteristic dimension as small as 65 nm, show a shear piezoelectric constant of d15 ~ 10 pm/V, which is several times that measured in bulk. The revealed strong piezoelectricity could open promising opportunities for application of GaN nanowires in nanowire-based sensors and generators for self-powered nanodevices.

Nanoindentation was used to examine the impact of impurities and grain boundaries on the mechanical properties of a “model” (110)/(100) grain boundary (GB) interface prepared using direct silicon bonding via the hybrid orientation technique of (110) and (100) p-type silicon wafers. Remarkable differences were found between the mechanical behavior of Fe- and Cu-contaminated samples. The direct silicon bonded wafers contaminated with either Fe or Cu showed opposite effects on mechanical properties, with Fe enhancing the silicon hardness, while Cu contamination induces a gradual weakening. High-resolution transmission electron microscopy was used to verify that the abrupt hardness changes observed during increasing nanoindentation loading is attributed to local deformation induced by the GB interface, Cu precipitate colony induced dislocations, and the abrupt crystallographic orientation change across the GB. The resulting dislocation loop generation facilitated the deformation process during nanoindentation and therefore softened the material.

Availability of atomic force microscope (AFM) is discussed in the point of view of the measurement standard. For indenter geometry verification in the pyramidal part, the fully mathematical method, Regression Polyhedral Model, is used to avoid the effects of human factors and the misalignment of the indenter axis. As an example, facet angles, angles between opposite faces, and the interior angles of the square base of pyramid of a Vickers indenter are analyzed to demonstrate the efficiency of this method. The area function of the indenter is also calculated as an analytical result of AFM image. The procedure of uncertainty of measurement of those geometrical parameters is developed. For geometrical parameters of pyramid, the uncertainty can be estimated by combining the resolution and linearity of the AFM, the effect of the scanning tip, the uncertainty of the reference standard used to calibrate the AFM, and the regression error which includes flatness and roughness of the pyramid faces; and for the uncertainty of the area function, the uncertainty of AFM coordinates are taken into account. The results show that the uncertainty of facet angles and angles between opposite faces is small; but on the other hand, the uncertainty of the interior angles of square base of pyramid is not negligible. The uncertainty of the area function is almost proportional to the contact depth, but the relative uncertainty is exponentially increased where the contact depth is small. In addition, the observations of indents are shown to verify the indentation hardness HIT of the certified reference materials (CRMs) by comparing with Vickers hardness determined with AFM observations. Through those examples, the possibility to improve certified values of CRMs is suggested.