Graduate Student Seminar Series - 2020

May 21, 2020

Experimental Exploration of High Mobility Metal Phosphides—Vapor-Phase Synthesis of Two-Dimensional SiP

Zhuoqun Wen, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

In the search for chemically stable two-dimensional (2D) materials with high in-plane mobility, proper bandgap, and compatibility with vapor-based fabrication, van der Waals semiconductor SiP has become a potential candidate as a robust variation of black phosphorous. While bulk SiP crystals were synthesized in the 1970s, the vapor-based synthesis of SiP nanostructures or thin films is still absent. We here report the first chemical vapor growth of SiP nanostructures on SiO2/Si substrate. SiP islands with lateral size up to 20 μm and showing well-defined Raman signals were grown on SiO2/Si substrate or on SiP- containing concentric rings. The presence of SiP phase is confirmed by XRD. The formation of rings and islands is explained by a multiple coffee ring growth model where a dynamic fluctuation of droplet growth front induces the topography of concentric ring surfaces. This new growth method might shed light on the controlled growth of group IV-III high-mobility 2D semiconductors.

Research about a Ternary Nanocomposite System with Thermal Responsiveness

Chen Gong, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

Over the past decades, polymer nanocomposites (PNC) have drawn great attention both from industrial and academic fields due to its superior and tailorable performance. The faction of polymer in the vicinity of inorganic filler surface (the so called “interfacial polymer”) has significantly altered structure and dynamics as compared to the polymer in bulk. With the dimension of filler coming down to nanoscale, interfacial polymer would occupy a considerable volume in the PNC, which may affect, or in many cases, dominate the property modification. Ternary nanocomposite systems in which the nanoparticle reinforcement is shelled with a distinct polymer layer have heterogeneous interface in between the particle and host polymer. It is advantageous over traditional binary nanocomposite systems in many aspects, such as more choices of system composition, flexible manipulation of filler distribution and capability of self-assembly. More recently, it was found that if the heterogeneous interface has high dynamic asymmetry inside, the PNC could show unexpected stiffening behavior upon heating.

Although the mechanical properties of this “thermal-stiffening” PNC have been systematically revealed, some questions still hang in doubt. For instance, modulus vulnerability has been seen in large-amplitude shear test, it cast doubt over the processability in some common processing methods. Additionally, the local thermal behavior within the heterogeneous interface is hard to specifically detect due to the low volume and nonuniformity of the interface. To close the loop of the research about this unique and emerging PNC category, we tested the processability of bulk nanocomposite samples using extrusion and designed/fabricated a multi-layer planar model nanocomposite to simulate the thermal behavior of real PNC counterparts. Preliminary results have provided some insights about how to optimize the processability of the ternary PNC and demonstrated the validity of the model nanocomposite we designed.

April 22, 2020

This special graduate student seminar is delivered by the recipients of the 2020 Norman S. Stoloff Research Excellence Award recipients at 11:00 am remotely over WebEx, at the time of the weekly department seminar.

First-principles identification of localized trap states in polymer nanocomposite interfaces

Abhishek Shandilya, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

Ab initio design of polymer nanocomposite materials for high breakdown strength requires prediction of localized trap states at the polymer–filler interface. Systematic first-principles calculations of realistic interfaces can be challenging, particularly for amorphous polymers and fillers that necessitate the calculation of ensembles of large unit cells with hundreds of atoms. We present a computational approach for automatically generating reasonable structures for amorphous polymer–filler interfaces, combining classical molecular dynamics and Monte Carlo simulations. We identify trap states by analyzing the localization of electronic eigenstates calculated using density functional theory on ensembles of interface structures, clearly distinguishing shallow trap states from delocalized band-edge states. Applying this approach to silica–polyethylene interfaces as an initial example, we find under-coordination and distorted coordination structures at amorphous silica surfaces contribute a combination of deep and shallow traps at these interfaces, whereas polyethylene does not generate localized interfacial states.

Silica Glass Toughened by Consolidation of Glassy Nanoparticles

Yanming Zhang, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The brittleness of oxide glasses has dramatically restricted their practical applications as structural materials despite very high theoretical strength. Herein, using molecular dynamics simulations, we show that silica glass prepared by consolidating glassy nanoparticles exhibit remarkable tensile ductility. Because of dangling bonds at surfaces and high contact stresses, the pressure applied for consolidating glassy nanoparticles to achieve ductility is significantly lower than that required to toughen bulk glass via permanent densification. We have identified 5-fold silicon, with a higher propensity to carry out local shear deformation than 4-fold silicon, as the structural origin for the observed tensile ductility. Interestingly, the work hardening effect has been, for the first time, observed in thus-prepared silica glass, with its strength increasing from 4 GPa to 7 GPa upon cold work. This is due to stress-assisted relaxation of 5-fold silicon to 4-fold during cold work, analogous to transformation hardening.

April 16, 2020

Towards understanding of filament (re) formation mechanisms of valence change memristors

Saurabh Pandey, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

Memristors are devices with multiple resistance states with applications in neuromorphic computing, IOT, and storage-class memory. There is a special focus on valence change memory (VCM) type memristors having a Metal-Insulator (Transition Metal Oxide) Metal (MIM) structure due to their sub-10nm scalability, low power consumption, large number of states and endurance. The different R states can be accessed by application of voltage pulses and are attributed to a nanometer scale conducting O-vacancy filament (re)formation process. These devices, however, show significant variability in device to device and cycle to cycle parameters due to inherent stochasticity in the filament formation process. In-situ TEM can shed light on the variability of the filament (re)formation process. The typical vertical crossbar structure, however, is challenging for observation of dynamic processes inside the oxide due to confounding effects of diffraction contrast from the metal electrodes. We have fabricated a lateral MIM structure using Electron beam lithography to show in principle functioning of the device by cycling between different states and access to 3 different R states. The challenges expected in the future in-situ work are also discussed.

February 20, 2020

Topological Origins of the Mixed Alkali Effect in Glass

Arron Potter, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The mixed alkali effect, the deviation from expected linear property changes when alkali ions are mixed in a glass, remains a point of contention in the glass community. While several earlier models have been proposed to explain mixed alkali effects on ionic motion, models based on or containing discussion of structural aspects of mixed-alkali glasses remain rare by comparison. However, the transition-range viscosity depression effect is many orders in magnitude for mixed-alkali glasses, and the original observation of the effect (then known as the Thermometer Effect) concerned the highly anomalous temperature dependence of stress and structural relaxation time constants. With this in mind, a new structural model based on topological constraint theory is proposed herein which elucidates the origin of the mixed alkali effect as a consequence of network strain due to differing cation radii. Discussion of literature models and data alongside new molecular dynamics simulations and experimental data are presented in support of the model, with good agreement.

Unit-Cell-Thick Oxide Synthesis by Film-Based Scavenging

Saloni Pendse, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

With the influx of flexible electronics as well as the emergence or prediction of unique phenomena in two dimensional forms of materials, epitaxy at weakly-coupled interfaces is gaining momentum as a feasible technique to develop nanostructured and two-dimensional materials. The weak substrate-film chemical interaction expected in this method of epitaxy has been believed crucial in enabling not only the formation of sharp heterostructures but also the mechanical exfoliation of the epilayer. Via growth of VO2 on a layered Dion-Jacobson perovskite, we unravel an unconventional understanding of epitaxy at weakly-coupled interfaces, entailing ions of the van der Waals substrate being scavenged by the growing film, resulting in the formation of a distinct and uniform unit-cell-thick interfacial layer. We show that VO­2 scavenges ions from the substrate and forms an epitaxial vanadate compound. Additionally, the crystal anisotropy of the substrate significantly modifies the energy landscape for diffusion of ions and leads to the creation of a unit-cell-thick epitaxial Aurivillius phase at the interface, predicted to exhibit the ferroelectric Rashba-Dresselhaus effect. The scavenging effect, interfaced with an anisotropic low-dimensional substrate, opens a new window to develop two-dimensional flexible components for future electronics.