Yanming Wang

Highly stretchable polymer semiconductor films through the nanoconfinement effect

Trapping polymers to improve flexibility Polymer molecules at a free surface or trapped in thin layers or tubes will show different properties from those of the bulk. Confinement can prevent crystallization and oddly can sometimes give the chains more scope for motion. Xu et al. found that a conducting polymer confined inside an elastomer—a highly stretchable, rubber-like polymer—retained its conductive properties even when subjected to large deformations (see the Perspective by Napolitano). Science, this issue p. 59; see also p. 24 A high-performance conjugated polymer is combined with an elastomer to produce a fully stretchable transistor. Soft and conformable wearable electronics require stretchable semiconductors, but existing ones typically sacrifice charge transport mobility to achieve stretchability. We explore a concept based on the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. The fully stretchable transistors exhibit high biaxial stretchability with minimal change in on current even when poked with a sharp object. We demonstrate a skinlike finger-wearable driver for a light-emitting diode.

Shape‐Controlled, Self‐Wrapped Carbon Nanotube 3D Electronics

The mechanical flexibility and structural softness of ultrathin devices based on organic thin films and low‐dimensional nanomaterials have enabled a wide range of applications including flexible display, artificial skin, and health monitoring devices. However, both living systems and inanimate systems that are encountered in daily lives are all 3D. It is therefore desirable to either create freestanding electronics in a 3D form or to incorporate electronics onto 3D objects. Here, a technique is reported to utilize shape‐memory polymers together with carbon nanotube flexible electronics to achieve this goal. Temperature‐assisted shape control of these freestanding electronics in a programmable manner is demonstrated, with theoretical analysis for understanding the shape evolution. The shape control process can be executed with prepatterned heaters, desirable for 3D shape formation in an enclosed environment. The incorporation of carbon nanotube transistors, gas sensors, temperature sensors, and memory devices that are capable of self‐wrapping onto any irregular shaped‐objects without degradations in device performance is demonstrated.

A three-dimensional phase field model for nanowire growth by the vapor?liquid?solid mechanism

We present a three-dimensional multi-phase field model for catalyzed nanowire (NW) growth by the vapor?liquid?solid (VLS) mechanism. The equation of motion contains both a Ginzburg?Landau term for deposition and a diffusion (Cahn?Hilliard) term for interface relaxation without deposition. Direct deposition from vapor to solid, which competes with NW crystal growth through the molten catalyst droplet, is suppressed by assigning a very small kinetic coefficient at the solid?vapor interface. The thermodynamic self-consistency of the model is demonstrated by its ability to reproduce the equilibrium contact angles at the VLS junction. The incorporation of orientation dependent gradient energy leads to faceting of the solid?liquid and solid?vapor interfaces. The model successfully captures the curved shape of the NW base and the Gibbs?Thomson effect on growth velocity.

Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor–Liquid–Solid Nanowire Growth

Kinking, a common anomaly in nanowire (NW) vapor-liquid-solid (VLS) growth, represents a sudden change of the wires axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liquid droplets of defined size. Statistical analysis of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diameter and the growth temperature. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to ⟨110⟩, was observed in Ge NWs with a nominal diameter of ∼ 20 nm. This size coincides with a critical diameter at which a spontaneous transition from ⟨111⟩ to ⟨110⟩ growth occurs in the phase field simulations. Larger diameter NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined ⟨111⟩ axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liquid droplet at the tip of the NW.

Reliability of Single Crystal Silver Nanowire-Based Systems: Stress Assisted Instabilities

Time-dependent mechanical characterization of nanowires is critical to understand their long-term reliability in applications, such as flexible-electronics and touch screens. It is also of great importance to develop a theoretical framework for experimentation and analysis on the mechanics of nanowires under time-dependent loading conditions, such as stress-relaxation and fatigue. Here, we combine in situ scanning electron microscope (SEM)/transmission electron microscope (TEM) tests with atomistic and phase-field simulations to understand the deformation mechanisms of single crystal silver nanowires held under constant strain. We observe that the nanowires initially undergo stress-relaxation, where the stress reduces with time and saturates after some time period. The stress-relaxation process occurs due to the formation of few dislocations and stacking faults. Remarkably, after a few hours the nanowires rupture suddenly. The reason for this abrupt failure of the nanowire was identified as stress-assisted diffusion, using phase-field simulations. Under a large applied strain, diffusion leads to the amplification of nanowire surface perturbation at long wavelengths and the nanowire fails at the stress-concentrated thin cross-sectional regions. An analytical analysis on the competition between the elastic energy and the surface energy predicts a longer time to failure for thicker nanowires than thinner ones, consistent with our experimental observations. The measured time to failure of nanowires under cyclic loading conditions can also be explained in terms of this mechanism.

Evaluation of the Surface Tension of Silicon-Gold Binary Liquid Alloy

The gold-catalyzed vapor-liquid-solid (VLS) method is widely used for silicon nanowire (Si NW) fabrication. As the VLS process is influenced by the physical properties of the catalytic silicon-gold (Si-Au) droplet, quantifying the surface tension of the liquid alloy is important to achieve better control of the wire growth. Because the experimental measurement of the surface tension is difficult, it is necessary to obtain reasonable estimates from computational models. In this work, we conducted molecular dynamics simulations with a modified embedded-atom potential developed for the Si-Au binary system, and evaluated the surface tension γ based on the Virial stress expression. The dependence of surface tension γ on the Si fraction χ and temperature T is predicted. The entropy of the liquid-vapor interface was extracted from the slope of the γ-T curve. The Si concentration and stress distributions near the surface are also predicted. Our surface tension evaluation enables theoretical predictions of droplet and nanowire shape, and provides physical inputs for continuum phase field models of VLS growth.

Discrete shear band plasticity through dislocation activities in body-centered cubic tungsten nanowires

Shear band in metallic crystals is localized deformation with high dislocation density, which is often observed in nanopillar deformation experiments. The shear band dynamics coupled with dislocation activities, however, remains unclear. Here, we investigate the dynamic processes of dislocation and shear band in body-centered cubic (BCC) tungsten nanowires via an integrated approach of in situ nanomechanical testing and atomistic simulation. We find a strong effect of surface orientation on dislocation nucleation in tungsten nanowires, in which {111} surfaces act as favorite sites under high strain. While dislocation activities in a localized region give rise to an initially thin shear band, self-catalyzed stress concentration and dislocation nucleation at shear band interfaces cause a discrete thickening of shear band. Our findings not only advance the current understanding of defect activities and deformation morphology of BCC nanowires, but also shed light on the deformation dynamics in other microscopic crystals where jerky motion of deformation band is observed.

Anisotropy effect on strain-induced instability during growth of heteroepitaxial films

AbstractThe use of misfit strain to improve the electronic performance of semiconductor films is a common strategy in modern electronic and photonic device fabrication. However, pursuing a favorable higher strain could lead to mechanical instability, on which systematic and quantitative understandings are yet to be achieved. In this paper, we investigate the anisotropy effects on strain-induced thin-film surface roughening by phase field modeling coupled with elasticity. We find that compared with films grown along {111} and {100} surfaces, the instability of {110} film occurs at a much lower strain. Our simulations capture the evolution of interface morphology and stress distribution during the roughening process. Similar characterizations are performed for heteroepitaxial growth from a surface pit. Finally, from 3D simulations, we show that the surface roughening pattern on {110} film exhibits a clear in-plane orientation preference, consistent with experimental observations.

Atomistic mechanisms of orientation and temperature dependence in gold-catalyzed silicon growth

Gold-catalyzed vapor-liquid-solid (VLS) growth is widely used in the synthesis of silicon-based low-dimensional nano-structures. However, its growth mechanisms are not fully understood yet. In this paper, we systematically study the orientation and temperature dependences in the VLS process, by means of long molecular dynamics (MD) simulations up to 100 ns using an MEAM potential that well reproduces the binary phase diagram. The crystal growth velocities are extracted from the simulations under various conditions for 〈110〉 and 〈111〉 orientations, respectively. Our data suggest a linear dependence of the growth velocity on the Si supersaturation for 〈110〉 growth, in contrast to a non-linear dependence for 〈111〉 growth. By analyzing the surface morphologies, this difference is linked to the continuous growth mechanism on the {110} substrate and the island nucleation controlled growth on the {111} substrate. Furthermore, we find that the 〈111〉 growth in our MD simulations operates in the regime where the nu...