Graduate Student Seminar Series - 2018

October 18, 2018 

Electron Scattering at Transition Metal Surfaces 

Erik Milosevic, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

A significant increase in the resistivity of metallic conductors is observed as their physical dimensions are decreased to the nanoscale. This phenomenon, termed the “resistivity size effect”, stands as a major roadblock to the continued downscaling of cutting-edge integrated circuits, which contain copper interconnects with widths rapidly approaching the sub-10 nm regime. Due to their high resistivity, these wires are now the dominant cause of signal delay and drastically decrease chip efficiency due to high losses from resistive heating. Understanding the physical origins of the resistivity size effect is key to combating its many negative outcomes and this talk will discuss the two primary mechanisms that contribute to the effect: electron-surface and electron-grain boundary scattering. The classical models that describe the resistive effects of these two scattering mechanisms predict an increase in resistivity proportional to the product of the metal’s bulk resistivity and its bulk electron-phonon mean free path ρo×λ. Thus, in the limits of narrow wires where electron-surface and electron-grain boundary scattering dominate, a metal with a low ρo×λ should have the lowest resistivity. From first principles simulations, the metals ruthenium and cobalt are expected to achieve this goal. While the resistivities of the elemental metals are well known, the mean free path remains uncharacterized for most, necessitating experimental investigation and validation. Is this talk, the fabrication and characterization of single crystal, epitaxial Ru and Co films will be presented. The key advantage of using epitaxial layers is the absence of grain boundaries, which allows deconvolution of the effects of surface and grain boundary scattering, resulting in more confident determination of mean free path values. From in situ and ex situ transport measurements, the mean free paths are determined to be λ = 6.7 ± 0.3 nm and λ = 19.5 ± 1.0 nm for Ru and Co, respectively. From these results, the conductance of polycrystalline Ru interconnects in 2:1 aspect ratio interconnect trenches is predicted to exceed that of Cu for half-pitch widths w < 19 nm, while the conductance of Co lines becomes comparable to that of Cu at w = 10 nm. Time permitting, the large predicted anisotropy for the resistivity size effect in Ru will be discussed. 

Effect of water on topological constraints in silica glass 

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

A fundamental understanding of how water interacts with glass is valuable for many practical concerns due to the myriad effects of water on glass properties. Topological constraint theory, which has been shown in prior studies to be an excellent model for various thermomechanical properties, is expanded herein to account for strain on the glass network, in this application due to unbonded interstitial species. Literature Tg data have been collected for silicate glasses containing various alkali and water contents and an expanded constraint theory model has been successfully applied to describe the effects of water on the glass transition temperature. 

Reference: Arron R. Potter, Collin J. Wilkinson, Seong H. Kim, John C. Mauro*. Effect of water on topological constraints in silica glass, Scripta Materialia, Volume 160, February 2019, Pages 48-52

September 27, 2018

Radiation damage in nuclear fuel

Tiankai Yao, Department of Mechanical, Nuclear, and Aerospace Engineering, Rensselaer Polytechnic Institute

Under neuron irradiation, there is an enormous energy release within nuclear fuel in a form of heat by fission chain reaction. Around 200 MeV of energy was deposited to a small region of radiation cascade (column of 10nm long and 1 nm wide) per fission reaction, generating a huge temperature gradient of 1000 K/cm and a high level of radiation damage by the fission fragments. Those fragments are heavy noble metal atoms with ~ 170 MeV energy flying before being stopped in the fuel matrix in a couple of micron distance, knocking out the fuel matrix atoms from their equilibrium sites into interstitials, leaving those sites vacant and forming Frankel pair of interstitial and vacancy. A direct impact on fuel performance by those defects is the degradation of fuel properties, such as thermal conductivity and mechanical integrity, which ultimately leads to the replacement of fuel elements every year in nuclear reactors. Different fuel matrix, however, reacts to these extreme conditions differently. Post-irradiation examination is the most direct way to study the related fuel degradation phenomena by neutron irradiation from reactor conditions, but it is expensive and time-consuming to reach the required radiation dose (yearly). Heavy ion irradiation offers a complementary technique that allows the researcher to achieve a high level of radiation dose within hours and being possibly monitored by in-situ transmission electron microscopy (TEM) observation. In this talk, the author is going to give a general picture of radiation damage in different fuel matrix (UO2 and U3Si2). The radiation-induced crystalline to amorphous transition, radiation-induced gas bubble formation, and radiation-induced grain subdivision will be covered in detail to depict an overall picture of the radiation damage in nuclear fuel. 

Nontrivial van der Waals interactions in layered halide perovskite (C4H9NH3)2PbI4

Zhizhong Chen, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Despite their weak nature, van der Waals interactions have been shown to be effective for controlling the optoelectronic and vibrational properties of layered materials like graphene and transition metal dichalcogenides. However, how such van der Waals effects exist in Ruddlesden-Popper layered halide perovskites remains unclear. In this work, we reveal for the first time the role of the interlayer van der Waals force in Ruddlesden-Popper phase perovskite in regulating its phase transition kinetics and carrier dynamics, based on the high quality epitaxial single crystalline (C4H9NH3)2PbI4 flakes with controlled dimensions. Based on spectroscopy studies, we show that both the substrate-perovskite epitaxial interaction and the interlayer van der Waals interaction play a significant role in suppressing the structural phase transition. It is demonstrated that with reducing flake thickness from ~100 nm to ~20 nm, electron-phonon coupling strength decreases by around 30%. These discoveries show that the conventional understanding that Ruddlesden-Popper halide perovskite is equivalent to a multiple quantum well structure has to be substantially amended due to the existence of the significant nonlocal phononic effects in the layered crystal in which intra-layer interaction is not drastically different from the interlayer force.

April 25, 2018

This special graduate student seminar is delivered by the recipients of the 2018 Norman S. Stoloff Research Excellence Award recipients at 11:00 am in LOW 3051, at the time and location of the weekly department seminar.

Distributions of Kinetic Pathways In Strain Relaxation of Heteroepitaxial Films

Dustin Andersen, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

In semiconductor heteroepitaxy, misfit dislocations relieve mismatch strain at the interface between the substrate and layer, and the epilayer growth parameters control the kinetics of dislocation array evolution. In this work, the kinetic relaxation pathways for GexSi1-x/Si(100) films are mapped using a continuum level process simulator that integrates experimental and model descriptions of the energetic and kinetic parameters that define the nucleation, propagation and interaction of strain relieving dislocations. The kinetic pathways for strain evolution are plotted for film growth as functions of the primary kinetic parameters: growth temperature, growth rate, and initial lattice mismatch, generating relaxation surfaces for parameter pairs. Sensitivity analyses are presented of how deviations from mean parameters disperse the resultant relaxation surfaces. Additionally, discrete dislocation arrays are generated from the output of the continuum simulator to illustrate the “fingerprint” of fundamental kinetic mechanisms – particularly dislocation nucleation mechanisms – on the final dislocation array. These discrete dislocation arrays allow for statistical analysis of the distribution of dislocation array parameters, such as dislocation length and inter-dislocation spacing, whose distributions may both describe and influence the evolution of the array. Finally, experimental electron channeling contrast imaging (ECCI) data is shown which corroborates the shape of the spacing distribution. The overarching goal is to establish a robust framework for predicting, interrogating and optimizing strain relaxation pathways, and underlying mechanisms, for misfit dislocations in strained heteroepitaxial films.

Understanding and Exploring the Hidden Photo Physics and Carrier Dynamics in Epitaxial Halide Perovskite Nanostructure and Thin Film

Yiping Wang, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The versatility of vapor phase epitaxy enables control of various semiconductor morphologies that can meet the geometrical requirements for a designated application as well as for the understanding of intrinsic physical properties. Here, by utilizing van der Waals and ionic epitaxy, we grow halide perovksite, an emerging class of new semiconductors into both high quality nanostructures and thin film through which some intrinsic and previously unnoticed photo physics and carrier dynamics can be revealed. The unique geometry in halide perovksite nanowire network provides us with a good platform to understand the photon absorption, transport and waveguiding effect along the wire direction. Photo induced ion migration, previously a disturbing phenomenon, can also be taken advantage of in this system to facilitate the fabrication of halide perovskite double heterojunctions. On the other hand, the single crystal high quality thin film by high temperature growth enables a minimization of defect density that can fully reveal its intrinsic optical properties. Surprisingly, we found very significant hot photoluminescence in pure inorganic CsPbBr3 which is previously believed to be non-existing. As a step further, we identified a possible photo Dember effect and Gigahertz emission through an anomalous photoluminescence decay curve that results from a discrepancy of electron/hole mobility. Our study on both nanostructured and thin film halide perovskite provides knowledge on both the fundamental physics and the possible underlying application ideas in this emerging material family.

April 19, 2018

Viscoelastic and Dynamic Properties of Nanocomposite Systems: A Molecular Dynamics Simulation Study

Wei Peng, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

It is well recognized that adding fillers into polymers can result in significant improvement of thermal, mechanical, optical and dielectric properties. The enhancement of the mechanical performance by the addition of fillers draws much attention in particular. Recently, an unusual and unique property was observed in polymer nanocomposite systems by Senses et al. (Senses, E.; Isherwood, A.; Akcora, P. ACS Appl. Mater. Interfaces 2015, 7, 14682). These nanocomposite systems show stiffening behavior upon heating that is reversible and repeatable. This unique thermal stiffening behavior was attributed to the dynamic coupling of high glass transition (Tg) grafted chains and low-Tg matrix chains. To better study the stiffening mechanism, we first studied the viscoelasticity and dynamics of a model dynamically asymmetric binary polymer blend which consists of two type of chains with significantly different Tg. Our study shows that the good dispersion of relatively immobile polymer chains with high-Tg in the mobile polymer chains with low-Tg leads to a significant storage modulus increase compared to the phase separated blend. It is suggested that the obstacle effect and the percolated network of the high-Tg polymer chains are responsible for this phenomenon. We later studied the composite system containing nanoparticles grafted with high- Tg chains dispersed in a low- Tg matrix polymer. We found that these nanocomposites exhibited significantly greater storage modulus when the high-Tg grafted chains assumed stretched conformations (and were well-mixed with the low-Tg matrix chains) compared to when they assumed collapsed conformations. Further detailed static and dynamic analysis showed that stretched grafted chains could significantly reduce the mobility of matrix chains, which was ultimately one of the most important factors that were responsible for the stiffening of the whole system.

Growth and properties of epitaxial Ti1−xMgxN(001) layers

Baiwei Wang, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The goal of this research is to grow a completely new semiconductor, Ti0.5Mg0.5N. This compound has never been synthesized yet but has been theoretically predicted to exhibit a 1.3 eV band gap and to have promising properties for thermoelectric and plasmonic devices without compromising its refractory and CMOS-compatible performances. More specifically, in my work, epitaxial Ti1−xMgxN(001) layers were deposited on MgO(001) by reactive magnetron co-sputtering from titanium and magnesium targets in 5 mTorr pure N2 at 600 °C. The combination of X-ray diffraction ω - 2θ, φ-scans, pole figures, and high resolution reciprocal space maps (RSMs) show that rock-salt Ti1−xMgxN layers are epitaxial single crystals which grow cube-on-cube with respect to their substrates: (001)TiMgN║(001)MgO and [100]TiMgN║[100]MgO. Ti1−xMgxN/MgO(001) layers, ~ 50 nm thick, are fully strained after deposition, with in-plane lattice parameters at constant 4.212 ± 0.001 Å and out-of-plane lattice parameters increasing from 4.254 Å (x = 0.00) to 4.308 Å (x = 0.49). The in-plane x-ray coherence length decreases from 212 to 25 nm as x increases from 0.00 to 0.49, while the out-of-plane x-ray coherence length is confined by the layer thickness d for x ≤ 39, but is smaller than d for x ≥ 0.45, indicating a decreasing crystalline quality with increasing Mg content and local strain variations along the film growth direction. The electrical resistivity of Ti1−xMgxN layers increases from 13 to 358 𝜇Ω∙cm for x = 0 and 0.49, respectively. Optical transmission and reflectivity spectra are described by a Drude–Lorentz model. Optical data fitting indicates that the free carrier density drops by a factor of four, from 1.3×1022 cm-3 for TiN to 3.3×1021 cm-3 for Ti0.51Mg0.49N, while the optically determined resistivity agrees well with the values from 4-point probe measurements for alloys with compositions x ≥ 0.26. The real part of the dielectric function indicates that the plasma frequency ωp moves into the infrared region for compositions with x ≥ 0.39, indicating that Ti1−xMgxN is promising for plasmonic applications.

March 20, 2018

Cationic Poly(benzyl ether)s as Self-Immolative Antimicrobial Polymers

Cansu Ergene & Edmund Palermo, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Conventional polymer chemistry has been devoted to link small molecules together to construct macromolecules. Interestingly, “self-immolative” polymers (SIMPs) just do the opposite. They are long chain macromolecules, which can revert to their small components spontaneously upon the cleavage of the stimuli-responsive endcaps. In contrast to conventional degradable polymers, SIMPs can undergo rapid and non-stop depolymerization only at specific sites within the backbone by specific triggering events (light, enzymes, pH change etc.), yet stay stable under normal operational conditions. A library of SIMP backbones and architectures have been emerged for applications such as, tissue engineering, drug delivery, responsive coatings, and microfluidics, but not as antimicrobial agents till recently. Our group developed first generation of antimicrobial SIMPs from poly (benzyl ether)s. inspired by the idea of mimicking antimicrobial peptides. Amphiphilic SIMPs in a range of molecular size with various amine functionalities in the side chains were successfully synthesized by thiol-ene click chemistry. Cationic polymers showed a broad spectrum of activity against bacteria. In later studies, polymer side chains were partially modified with PEG to reduce hemolytic behavior of polymers bearing 100% primary amine groups. Here, we demonstrate a novel class of materials which can be explored as a platform for advanced functional biomaterials including new tissue engineering platforms, and antimicrobial/anti-biofouling surfaces.

Electromigration: A Cumulative Failure Mechanism in High Current Density Conductors

Brent Engler, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Electromigration is a wear-out failure mechanism in conductors which is highly relevant to materials processing and design of industrial microelectronic structures. It occurs in devices which experience high current density over extended periods of time leading to open or short circuits due to the mass transport of the metal. The consideration of electromigration has been critical in improving the reliability of microelectronics in the past and will continue to be relevant as device dimensions continue to shrink. I will discuss the physical origins of electromigration, its study, and how it has impacted design choices. Particular attention will be paid to modern in-situ studies, and I will briefly introduce my own work on the subject.

February 22, 2018

Growth and Characterization of Wide Band Gap Oxides (ZnO and SnO2)

Jie Jiang, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Wide band gap semiconductors, such as ZnO and SnO2, have attracted numerous attentions due to the promising applications in light-emitting diodes and laser diodes operating in the visible and ultraviolet region. In order to design devices, p-type, n-type doping and band gap engineering are important. I have investigated doping, band gap engineering of ZnO alloyed with Cd and S, and quantum well structures using pulsed laser deposition during my PhD study in Zhejiang University in China. I have studied undoped and N-doped SnO2 using chemical vapor deposition during my postdoctoral work in University Giessen in Germany. I will briefly introduce these works. 

Phase Behavior of Two-Component DNA Colloidal Crystals

Neha Chauhan, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

DNA has emerged as a powerful tool to assemble colloidal crystals. The specificity of Watson-Crick base-pairing has enabled us to design DNA grafted particles that crystallize into superlattices, while affording precise and independent control over constituent particle size, crystal lattice parameters and crystallographic symmetry. Thus far, research in the field has primarily focused on mapping the phase space with respect to stoichiometry of the particles, ratio of particle size andvalency differences, resulting in the establishment of rules reminiscent of the Pauling and Hume-Rothery rules for atomic lattices. Looking beyond crystal structures, we are intrigued by the notion of programing DNA interactions to alter the solution phase behavior of such assemblies. This could be achieved by employing blends of self- complementary and complementary strands of DNA in varying ratios to selectively favor like or unlike bonding. Although configurational entropy does not play a large role, the tunable enthalpic interactions are expected to have a strong effect on crystallization and phase behavior. The preliminary results from a very simple model paints a phase space that includes crystallographic symmetries previously unattained by the contemporary approach, and strongly phase separated regions. With the aid of microscopy tools like confocal and stimulated emission depletion (STED) microscopy it is possible to undertake in-situ imaging, and thereby explore the resultant phases, microstructures, and phase transformations. In this talk I will present the background of the field, why it’s worth exploring and the status of our current efforts towards the goals outlined.

January 18, 2018

Electron Microscopy of Biological Samples

Thomas Jordan, Department of Biology, Rensselaer Polytechnic Institute

Historically, electron microscopy (EM) of biologic materials has been complicated by the fragility of biological materials in the face of the electron beam, the weak interaction between electron beams and the light atoms that predominate in biological samples, and incompatibility of native biological structures with the vacuum required for electron microscopy. This talk provides a brief history of electron microscopy in biology and focuses on the sample preparation techniques best suited to overcoming the challenges unique to the study of cells, organelles, molecular complexes, and the structure of single proteins with near atomic resolution and gives an overview some of the biology research using TEM at RPI. TEM studies of cells or organelles often focus on their three-dimensional organization and the tissue must first be chemically cross linked, or fixed, to maintain that organization, then impregnated with heavy metal, or stained, to provide contrast before being dehydrated and embedded in a hard resin so that thin sections can be prepared. Imaging these sections sequentially allows a three-dimensional understanding of cellular architecture. Studies of viruses or small particles are typically interested in their external morphology. These particles are sturdy enough that only staining is required for imaging. Studies of the structure of single proteins or complexes require the highest resolution. These samples must generally be flash frozen to maintain their native structure and can tolerate only low doses of radiation. The resolution is achieved by signal averaging.

Structure and Properties of Crystalline Polymer Nanocomposites: Filler  Dispersion and Crystallization Behavior

Xin Ning, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Polymer nanocomposites (PNC), a mixture of polymer matrixes and inorganic/organic nanoscale particles, have received tremendous attention over the last few decades. Recent work has focused on the assembly of ligand-grafted nanoparticles in amorphous polymer matrices. It has been found that the molecular weight and graft density of the ligands can be used to control spatial dispersion and interface interaction. A new approach to nanofiller organization using crystallization kinetics has been demonstrated but not fully explored. This work will focus on kinetically organizing spherical nanoparticles through polymer crystallization across multiple length scales. The issues that determine NPs dispersion and crystal morphology will be addressed along with aspects of mechanical/dielectric properties.