Graduate Student Seminar Series - 2019

November 21, 2019

Direct visualization of functional crosslinkers in swollen PNiPAm based microgels

Gopal Kenath, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The spatial distribution of cross-links in bulk gels and colloidal gel particles impacts their mechanical and transport properties. Here we report on the spatial distribution of dye tagged crosslinkers of colloidal n-isopropylacrylamide (PNIPAM) microgels revealed by super-resolution microscopy at the level of individual particles as well as at the ensemble level. Using a W-4PiSMSN microscope we demonstrate for the first time, the presence of higher crosslink nanodomains within the already dense cores of the microgels. Additionally, we show that the average probability density profile, extracted from super-resolution, can be used to extract quantitative relative reaction rate constants. We prove this by coupling the localization probability density profile with the temporal volumetric evolution of the particles to accurately predict the consumption of the dye tagged species with time. These predicted consumption curves are fit to accepted kinetic models for precipitation polymerization of microgels to extract relative rate constants for the functional cross-linker to n-isopropylacrylamide and N,N'-methylene-bis-acrylamide.

Synthesis of Unimolecular Polyolefins via Templated Ring-Opening Metathesis (TROM)

Zhe Zhou, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

We developed a fully abiotic approach to template the synthesis of discrete unimolecular polyolefins. Discrete,unimolecular oligo(thiophene)s with alternating sequence were obtained by iterative convergent/divergent couplings and then functionalized with pendant cyclic olefin monomers in the side chains. Upon treatment with the Grubbs third-generation catalyst in dilute solution (0.15 mM in DCM at 0 °C), the pendant monomers undergo templated ring-opening metathesis (TROM). Then, the daughter olefin is liberated from the parent thiophene by hydrolysis. Cyclooctenes undergo TROM to afford macrocyclic products that exactly replicate the chain length of the parent oligomer, as evidenced by MALDI MS/MS and NMR. Norbornene derivatives also undergo TROM and replicate unimolecular chain lengths, but in contrast, they exclusively form the linear oligomeric products with styrenic end groups. A template that was functionalized with one norbornene unit at the α chain end, followed by five cyclooctene units along the template, underwent TROM to afford the macrocyclic daughter olefin. Intertemplate metathesis is suppressed by tuning the concentration and reaction time. Using this strategy, we can effectively replicate the unimolecular nature of a template, made by labor-intensive iterative synthesis, to produce a discrete daughter oligomer by chain growth. We also demonstrate that the templates are recyclable upon hydrolytic cleavage of daughter oligomer, attachment of fresh daughter monomer, and repetition of the TROM process.

October 17, 2019

A Reconfigurable Remotely Epitaxial VO2 Electrical Heterostructure    
Yuwei Guo, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The reconfigurability of the electrical heterostructure featured with external variables, such as temperature, voltage and strain, enabled electronic/optical phase transition in functional layers has great potential for future photonics, computing and adaptive circuits. VO2 has been regarded as an archetypal phase transition building block with superior metal-insulator transition characteristics. However, reconfigurable VO2-based heterostructure and the associated devices are rare due to the fundamental challenge in integrating high quality VO2 in technologically important substrates. In this report, for the first time, we show the remote epitaxy of VO2 and the demonstration of a vertical diode device in graphene/epitaxial VO2/single crystalline BN/graphite structure with VO2 as a reconfigurable phase change material and hexagonal boron nitride (h-BN) as an insulating layer. By diffraction and electrical transport studies, we show that the remote epitaxial VO2 films exhibits higher structural and electrical quality than direct epitaxial one. By high resolution transmission electron microscopy and Cs- corrected scanning transmission electron microscopy we show that graphene buffered substrate leads to less strained VO2 film than bare substrate. In the reconfigurable diode, we find that the Fermi level change and spectral weight shift along with metal-insulator transition of VO2 could modify the transport characteristics. The work suggests the feasibility of developing single crystalline VO2-based reconfigurable heterostructure with arbitrary substrates and sheds light on designing novel adaptive photonics and electrical devices and circuits.

September 19, 2019

Trap state distribution in polymer nanocomposite interphases from first principles

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

Polymer nano-composites are promising dielectric materials with the possibility of achieving high dielectric breakdown strength. Electron and hole traps at the interface of polymer matrix and inorganic filler are probable candidates for mitigating breakdown. So, it is important to investigate the trap state distribution and find its correlation with breakdown strength. However, a single DFT calculation is inconclusive for a composite system where both filler and matrix are amorphous in nature. To capture the entropic contribution, an ensemble of atomic structures are analyzed, and a programmatic approach to create atomic structures is adopted to do so. Differences in coordination environment of matrix and filler makes artificial synthesis of the interface challenging. Different procedures are used to generate atomic structures of matrix and filler before stitching them together. Amorphous silica is produced through simulated quench, whereas the polymer chains are generated using self-avoiding random walk. Dangling bonds on the matrix surface are satisfied by surface modifiers. After DFT analysis of an ensemble of such structures, major structural and electronic features in the composite system which shape the trap state distribution are narrowed down by post-processing the local density-of-states near the interface.

Modeling surface birefringence in SiO2 glass fibers

Bronson Hausmann, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

The retardance of silica glass fibers was evaluated using photoelastic techniques. Here, surface birefringence in glass fibers is shown to be a consequence of surface stress relaxation for as-received fibers drawn from Suprasil II. The surface features of the birefringent fibers were compared to a model of the residual axial stress profile resulting from a diffusion-controlled surface stress relaxation. Additionally, a uniform birefringence in the fiber equivalent to a constant tensile stress was recognized and attributed to structural anisotropy produced during fiber drawing. The contribution of structural anisotropy to the observed birefringence remained constant as the surface features were successively etched away. Significant features of the retardance profile in as-received silica glass fibers, with a thin surface compressive stress layer and compensating interior tensile stress, agreed with the residual stress profiles predicted by the surface stress relaxation model after correcting for this observed structural anisotropy.

April 18, 2019

2D Electron Transport in CrN(001) Films

Mary E. McGahay, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

CrN is a relatively unexplored Mott-Hubbard type semiconductor with magnetic ordering and a predicted vanishing bandgap at reduced length-scales, promising a unique two-dimensional (2D) electron gas and dilute magnetic semiconductor properties. Initial results of epitaxial CrN(001) films deposited of MgO(001) by reactive DC magnetron sputtering suggest a size dependent transport with a two-orders-of-magnitude reduction in the resistivity as the layer thickness is reduced from 2500 to 2 nm. In situ transport measurements during controlled oxygen exposure from <10-6 to 240 Pa show an increase in sheet conductance that is independent of the CrN thickness but is absent for control samples that are capped with insulating AlN prior to oxygen exposure, suggesting that the size-effect may be attributed to n-type surface doping of semiconducting CrN through substitutional replacement of N surface atoms with O. These results demonstrate the existence of a conductive 2D surface oxide with a sheet conductance of 6.0×10-5 [Ω/□]-1, which is promising for the realization of 2D electron transport devices in refractory transition metal nitrides.

Effect of strain on the Curie temperature and band structure of low-dimensional SbSI

Yang Hu, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

Photoferroelectric materials show great promise for developing alternative photovoltaics and photovoltaic-type non-volatile memories. However, the localized nature of the d orbital and large bandgap of most natural photoferroelectric materials lead to low electron/hole mobility and limit the realization of technologically practical devices. Antimony sulpho-iodide (SbSI) is a photoferroelectric material which is expected to have high electron/hole mobility in the ferroelectric state due to its non-local band dispersion and narrow bandgap. However, SbSI exhibits the paraelectric state close to room temperature. In this report, as a proof of concept, we explore the possibility to stabilize the SbSI ferroelectric phase above room temperature via mechanical strain engineering. We synthesized thin low-dimensional crystals of SbSI by chemical vapor deposition, confirmed its crystal structure with electron diffraction, studied its optical properties via photoluminescence spectroscopy and time-resolved photoluminescence spectroscopy, and probed its phase transition using temperature-dependent steady-state photoluminescence spectroscopy. We found that introducing external mechanical strain to these low-dimensional crystals may lead to an increase in their Curie temperature (by ∼60 K), derived by the strain-modified optical phase transition in SbSI and quantified by Kern formulation and Landau theory. The study suggests that strain engineering could be an effective way to stabilize the ferroelectric phase of SbSI at above room temperature, providing a solution enabling its application for technologically useful photoferroelectric devices.

April 17, 2019

This special graduate student seminar is delivered by the recipients of the 2019 Norman S. Stoloff Research Excellence Award recipients at 11:00 am in LOW 3051, at the time and location of the weekly department seminar.

Surface Shear Stress Relaxation of Silica Glass

Emily M. Aaldenberg, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

A constant angle of twist was applied to silica glass rods in order to produce a torsional shear strain and a reduction in torque representative of the stress state in the glass was measured as a function of time when rods were heat-treated in air at temperatures, 550–700°C, far below the glass transition temperature. The monotonic decrease of torque with time was explained by surface stress relaxation, which can be described by a relaxation of stress at the surface of glass which is promoted by water. The obtained surface stress relaxation diffusion coefficients were consistent with those obtained earlier from silica glass fiber bending under a similar water vapor pressure. The observed relaxation in torsion supports the mechanism of surface stress relaxation over the swelling-based mechanism for applications including glass fiber strengthening.

Towards 3D Nanopatterning in polymeric gels using photochromic switches

Harikrishnan Vijayamohanan, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute

Rapid, high throughput patterning in bulk polymeric systems with nanoscale resolution in three dimensions has long remained a coveted target for materials scientists. State of the art fabrication techniques capable of nanoscale resolution like electron beam lithography are not capable of being used on soft polymer targets, besides being associated with high setup and usage costs. Optical interference lithography has long been an attractive technique to cheaply and rapidly pattern three dimensional features in polymer photoresists despite both the resolution and feature size being limited by diffraction. In the past few years, Stimulated Emission Depletion Microscopy (STED) inspired lithography schemes have shown the ability to direct-write features well below the diffraction limit using visible light. However, the high light thresholds required for effective photoinhibition renders them unsuitable to be used for interference lithography and limits their use to point by point writing. In this talk, I will highlight the development of a new super-resolution writing system with the desired low light thresholds for parallel nanopatterning by combining the reversibly saturable isomerization of photochromic spirothiopyran with the thiol-Michael addition reaction. This chemistry can easily be adapted to synthesize photoswitchable polymer gels and functional surfaces. The kinetics of photopatterning using covalently bound spirothiopyran and the resultant resolution obtained can be effectively modulated by controlling the chemical environment of the photoswitch. Using a dual-color interference lithography setup, large area nanopatterning with sub-diffraction feature sizes can be obtained. Utilizing this setup on photochromic glass surfaces allows for the direct writing of single molecule thin high density nanoarrays. These experiments mark important steps toward realizing a highly parallelized fabrication technique with nanoscale resolution, over large volumes in three dimensions.

March 14, 2019

Excitonic Fine Structure in Two-dimensional Layered Materials

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

The unique properties of two-dimensional (2D) layered semiconductors have attracted immense research interest and promise new device applicaitons in electronics and optoelectronics. Amongst the 2D family, atomically thin transition metal dichalcogenides (TMDCs) have extensively been investigated for potential applications in valleytronics, field-effect transistors, logic circuits, phototransistors, and quantum computing. With reduced dimension in the out-of-plane direction, 2D TMDCs show strong Coulomb interactions compared with bulk materials. The enhanced electron-electron interaction enables a new platform to study fine excitonic structures of quasi-particles. New excitonic quasi-particles states can be revealed from both optical and electronic measurements. 

A phonon-assisted circularly-polarized replica is also unveiled by gate-tunable photoluminescence (PL) and magneto PL measurements in hexagonal boron nitride (hBN) encapsulated monolayer tungsten diselenide (WSe2). The exciton-phonon replica inherits large magneto-tunability and long lifetime from dark exciton, from which the exciton valley information can be extracted efficiently. We also investigated the interlayer excitonic states in the 2D heterostructure and an exciton excited states to Landau level transition is exploited through a photocurrent spectroscopy method.

Toughen Silica glass by Consolidation of Silica Nanoparticles

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

The brittleness of oxide glass has dramatically restricted its practical applications as structural materials despite high theoretical strength. Herein, using molecular dynamics simulations, we show silica prepared by consolidating glassy nanoparticles exhibit significant tensile ductility. Using silica nanoparticles as the starting materials for consolidation, instead of bulk silica glass, significantly reduces the consolidation pressure to achieve ductility, due to dangling bonds near surface and high local contact stress. We have identified five-fold silicon, which has a higher propensity for shear deformation than four-fold silicon as the structural origin for the observed tensile ductility. Interestingly, work hardening effect has been, for the first time, observed in thus-prepared silica, where strength increases from 4 GPa to ~ 8 GPa as deformation proceeds. This is due to the stress-assisted relaxation of five-fold silicon to four-fold during deformation, analogous to transformation hardening.

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