Session B06: Materials I

How to Predict What to Measure

Experimentally produced alloys, with useful properties for applications, are often composed of many elements. Computational efforts to study many element systems are inhibited by the vast search space as the number of possible configurations scales dramatically with the number of elements. We present a machine learning approach which allows us to identify a set of configurations in the CoNbV system for training of our model and predicting configurations of interest among the rest of the space. We present the process, for training the potential and identifying structures of interest. We show how the identified ternary configurations can then be extended to higher element models to guide the search for what to measure in the many element systems of experimental interest.   

Synthesis of Carbon Nanotube Forests on Deep-Etched Patterns for Light Absorption

An ideal black surface should have low reflectance uniformly across the spectrum of electromagnetic radiation. Black paints are not ideal because they have specific absorption peaks and bands. Vertically aligned nanopillars of proper shapes and physical properties are good candidates, but the fabrication and oxidation in air are challenging. Carbon nanotube (CNT) forests could be a cheap alternative but the optical properties are sensitive to the density, length, and alignment of the CNTs in a forest. Additionally, the reflectance spectra of nanopillars always rise sharply from a cutoff wavelength. Our approach to extend the cutoff wavelength is to create a modulation length scale. The fabrication of deep-etched Si substrates and findings of CNT growth on these patterned surfaces will be presented.   

Low-Temperature Infiltration of Carbon Nanotubes to Improve Mechanical Robustness

Infiltrated carbon nanotubes (CNT) forests, fabricated in a chemical vapor deposition (CVD) furnace, are a good material for MEMS for their combination of relatively low density and high mechanical strength. However, for larger scale forests, the infiltration process is poor at improving the mechanical strength of CNT since the ethylene gas is diffusion limited moving through CNT forests. At high temperatures ($\gtrsim$900 $^\circ$C), this causes the amorphous carbon to cap the outside of CNT forests quickly preventing deeper infiltration into the forest. At lower temperatures ($\sim$850 $^\circ$C), the overall infiltration proceeds slower making the capping process take longer which allows for better infiltration of the CNT forests. Scanning electron microscopy (SEM) images were taken of cross-sections of CNT forests infiltrated at 850 $^\circ$C to determine the infiltration time required to achieve maximal infiltration. Mechanical testing of infiltrated CNT beams was conducted using 3-point bending to determine the strength and modulus of the beams.   

BNA as a broadband THz emitter

Terahertz spectroscopy is an emerging field with varied applications, such as imaging of biological samples, probing vibrational modes in crystals, studying interfaces, and identification of a large number of materials. These applications are limited by the THz source: the smaller the frequency range produced, the fewer observables. BNA is an organic nonlinear optical crystal which produces a broadband spectrum from 0.5--6 THz when pumped with a 100-femtosecond laser with a central wavelength of 1200 nm. We have found that pumping very thin (\textgreater 300 microns) BNA crystals with an 800-nm laser also produces a broadband spectrum in the THz region. We analyze the difference between a 1200-nm and 800-nm pump and find the optimal length for BNA when using each pump. Finally, we demonstrate the use of BNA to determine the refractive index of lithium niobate using THz time-domain spectroscopy and compare our results with the use of DAST, a common organic crystal used for THz generation. Because of the broadband nature of BNA as a THz emitter, BNA is better suited for studying materials from 0--6~THz than other crystals commonly in use.   

Ab Initio Studies of Magnetoelectric Coupling at PbZr$_{0.2}$Ti$_{0.8}$O$_3$/La$_{0.8}$Sr$_{0.2}$MnO$_{3}$ and PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$/La$_{0.5}$Sr$_{0.5}$MnO$_{3}$ Multiferroic Interfaces

The magnetic properties of multiferroic materials can be controlled by the applied electric field. In this work, we use {\it ab initio} methods based on density functional theory (DFT) to study the magnetoelectric coupling at the (0,0,1) interface between PZT (PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$) and LSMO at two different doping levels (La$_{0.8}$Sr$_{0.2}$MnO$_{3}$ and La$_{0.5}$Sr$_{0.5}$MnO$_3$). A $\pm1\%$ uniaxial strain is applied to modeled system in the direction orthogonal to the LSMO/PZT interface. We observe a transition from the ferromagnetic (FM) to antiferromagnetic (AFM) state at the PbZr$_{0.2}$Ti$_{0.8}$O$_3$/La$_{0.5}$Sr$_{0.5}$MnO$_{3}$ interface under the applied strain. In contrast, no FM/AFM transition is observed at the PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$/La$_{0.8}$Sr$_{0.2}$MnO$_{3}$ interface. The results of our calculations are consistent with the available experimental data.   

The Crystal Structure of $\mathrm{Zn}_3 \mathrm{As}_2$

In the course of trying to produce doped semiconductors, the Colton lab happened upon a sample of ZnO that had been doped with $\mathrm{Zn}_3 \mathrm{As}_2$. In the difficulty to reproduce such a sample, they became interested in studying $\mathrm{Zn}_3 \mathrm{As}_2$ itself, as the literature disagrees on what the structure is. It is likely that $\mathrm{Zn}_3 \mathrm{As}_2$ has a structure isomorphic to $\mathrm{Cd}_3 \mathrm{As}_2$ due to their number of valence electrons. It is generally agreed upon that these isomorphic structures are a 2x2x4 super structure of anti-fluorite with a 25\% cation deficiency. In this work, two different purity samples of $\mathrm{Zn}_3 \mathrm{As}_2$ are studied and compared to several candidate structures obtained from the literature and additional symmetry arguments. The method of studying the crystalline structure is X-ray diffraction (XRD). The structure was solved using single crystal (SC) XRD, and is presented in this work. The structure found using SC-XRD is also compared to powder XRD data.