Highly ordered liquid crystalline (LC) phases have important potential for organic electronics. We studied the molecular alignment and domain structure in a columnar LC thin film with nanometer resolution during in situ heating using four-dimensional scanning transmission electron microscopy (4D STEM). The initial disordered vapor-deposited LC glass thin film rapidly ordered at its glass transition temperature into a hexagonal columnar phase with small (<10 nm), well-aligned, planar domains (columns oriented parallel to the surface). Upon further heating, the domains coarsen via bulk diffusion, then the film crystallizes, then finally transforms back to an LC phase at an even higher temperature. The LC phase at high temperature shows straight columns of molecules, which we attribute to structure inherited from the intermediate crystalline phase. Nanoscale 4D STEM offers direct insight into the mechanisms of domain reorganization, and intermediate crystallization is a potential approach to manipulate orientational order and texture at the nano- to mesoscale in LC thin films.

Physical vapor deposition can be used to prepare highly stable organic glass systems where the molecules show orientational and translational ordering at the nanoscale. We have used low-dose four-dimensional scanning transmission electron microscopy (4D STEM), enabled by a fast direct electron detector, to map columnar order in glassy samples of a discotic mesogen using a 2 nm probe. Both vapor-deposited and liquid-cooled glassy films show domains of similar orientation, but their size varies from tens to hundreds of nanometers, depending on processing. Domain sizes are consistent with surface-diffusion-mediated ordering during film deposition. These results demonstrate the ability of low-dose 4D STEM to characterize a mesoscale structure in a molecular glass system which may be relevant to organic electronics.

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180 K below the clearing point TLC-iso (and 50 K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility.

X-ray scattering has been used to characterize the columnar packing and the π stacking in a glass-forming discotic liquid crystal. In the equilibrium liquid state, the intensities of the scattering peaks for π stacking and columnar packing are proportional to each other, indicating concurrent development of the two orders. Upon cooling into the glassy state, the π-π distance shows a kinetic arrest with a change in the thermal expansion coefficient (TEC) from 321 to 109 ppm/K, while the intercolumnar spacing exhibits a constant TEC of 113 ppm/K. By changing the cooling rate, it is possible to prepare glasses with a wide range of columnar and π stacking orders, including zero order. For each glass, the columnar order and the π stacking order correspond to a much hotter liquid than its enthalpy and π-π distance, with the difference between the two internal (fictive) temperatures exceeding 100 K. By comparison with the relaxation map obtained by dielectric spectroscopy, we find that the δ mode (disk tumbling within a column) controls the columnar order and the π stacking order trapped in the glass, while the α mode (disk spinning about its axis) controls the enthalpy and the π-π spacing. Our finding is relevant for controlling the different structural features of a molecular glass to optimize its properties.

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The dynamics near the surface of glasses can be much faster than in the bulk. We studied the surface dynamics of a Pt-based metallic glass using electron correlation microscopy with sub-nanometer resolution. Our studies show an ∼20 K suppression of the glass transition temperature at the surface. The enhancement in surface dynamics is suppressed by coating the metallic glass with a thin layer of amorphous carbon. Parallel molecular dynamics simulations on Ni80P20 show a similar temperature suppression of the surface glass transition temperature and that the enhanced surface dynamics are arrested by a capping layer that chemically binds to the glass surface. Mobility in the near-surface region occurs via atomic caging and hopping, with a strong correlation between slow dynamics and high cage-breaking barriers and stringlike cooperative motion. Surface and bulk dynamics collapse together as a function of temperature rescaled by their respective glass transition temperatures.

Zr65Cu27.5Al7.5 metallic glass thin films with widely varying kinetic stability as a function of deposition rate were synthesized by single-target direct current magnetron sputtering. Fluctuation electron microscopy and angular correlations in coherent electron nanodiffraction show that glasses with increased stability have increased nanoscale structural order, particularly of icosahedral character. The most kinetically stable film's reduced modulus was 22% higher than a glass of the same composition rapidly quenched from the liquid, consistent with increased density and improved thermodynamic stability. These results suggest that enhanced nanoscale icosahedral order contributes both to the kinetic stability of the glass and its resistance to mechanical deformation.

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Polyamorphs are often observed in amorphous matters, and a representative example is the reentrant glass transition in colloid systems. For metallic amorphous alloys, however, the cases reported so far are limited to metallic glasses (MGs) that undergo electronic transitions under gigapascal applied pressure, or the presence of two liquids at the same composition. Here we report the first observation of a reentrant glass transition in MGs. This unusual reentrant glass transition transforms an MG from its as-quenched state (Glass I) to an ultrastable state (Glass II), mediated by the supercooled liquid of Glass I. Specifically, upon heating to above its glass transition temperature under ambient pressure, Glass I first transitions into its supercooled liquid, which then transforms into a new Glass II, accompanied by an exothermic peak in calorimetric scan, together with a precipitous drop in volume, electrical resistance and specific heat, as well as clear evidence of local structural ordering on the short-to-medium-range scale revealed via in-situ synchrotron X-ray scattering. Atomistic simulations indicate enhanced ordering of locally favored motifs to establish correlations in the medium range that resemble those in equilibrium crystalline compounds. The resulting lower-energy Glass II has its own glass transition temperature higher than that of Glass I by as much as 50 degrees. This route thus delivers a thermodynamically and kinetically ultrastable MG that can be easily retained to ambient conditions.

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Journal Article Electron Correlation Microscopy Measurements of Metallic Glass Surface Dynamics Get access Debaditya Chatterjee, Debaditya Chatterjee University of Wisconsin - Madison, Madison, Wisconsin, United States Search for other works by this author on: Oxford Academic Google Scholar Paul Voyles Paul Voyles University of Wisconsin - Madison, Madison, Wisconsin, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 1142–1143, https://doi.org/10.1017/S1431927620017080 Published: 01 August 2020

Most electron microscopists have seen the characteristic fuzzy rings of large-area diffraction from an amorphous material at one time or another, if only as a result of a failure of sample preparation for a crystalline sample.This kind of scattering can be normalized and Fourier inverted to yield a pair distribution function, g(r), that measures the mean number of atoms that sit a distance r away from the average atom in the material.g(r) is foundational data for studying the structure of amorphous materials, but it is not reminiscent of the symmetry-based analyses that form the basis of crystallography.If the same amorphous material is studied with a coherent, nanometer-size probe beam of electrons, the fuzzy rings break up in small, bright speckles, as shown in Fig. 1e, which was acquired from a Zr65Cu27.5Al7.5 metallic glass thin film.Those speckles arise from Bragg diffraction from more-ordered local arrangements in the glass.As the probe is scanned from place to place, the speckles change in position and intensity because the probe illuminates different ordered regions.Fluctuation electron microscopy (FEM) is the study of the normalized variance V(k) of many speckles from many positions.FEM data derived from many patterns like Fig. 1e acquired from different positions is shown in Fig. 1a for films with varying deposition rate.

Journal Article Electron Correlation Microscopy for Studying Fluctuating Systems In Situ Get access Debaditya Chatterjee, Debaditya Chatterjee Department of Materials Science & Engineering, University of Wisconsin-Madison, Madison, WI, USA Search for other works by this author on: Oxford Academic Google Scholar Pei Zhang, Pei Zhang Department of Materials Science & Engineering, University of Wisconsin-Madison, Madison, WI, USA Search for other works by this author on: Oxford Academic Google Scholar Paul M Voyles Paul M Voyles Department of Materials Science & Engineering, University of Wisconsin-Madison, Madison, WI, USA Corresponding author: paul.voyles@wisc.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 1520–1521, https://doi.org/10.1017/S143192761900833X Published: 01 August 2019

High mobility, electrolyte-gated FETs (EGFETs), based on precursor-derived oxide semiconductors, enable the possibility of achieving printed and low voltage (<;2 V) operated circuits. These EGFETs can also be realized with displaced-gate geometries. However, the displaced-gate devices are typically slow due to high electrolyte resistance resulting from the large gate-channel distances. Here, we show that a thin insulating (composite solid polymer electrolyte) layer and a top-gate geometry can largely overcome this limitation, a comprehensive comparison between the displaced-gate and the top-gate devices has been provided. In order to facilitate circuit design, we have successfully developed accurate models to predict the behavior of these top-gate EGFETs. The importance of our modeling approach is further enhanced by the fact that appropriate predictive modeling strategies for printed circuits, especially for those that are based on oxide semiconductors, are largely missing. Unlike existing transistor models that do not cover all voltage regimes (below, near, and above threshold), we propose a new modeling methodology that matches very well with the measured data, is continuous and smooth over the entire voltage range, and can be easily incorporated into SPICE simulators.

Anodized titania, synthesized on oxidized silicon substrate, has been used as oxygen gas sensor. The as-anodized films resulted in a sensitivity of 5756% at 125°C, when exposed to 100% oxygen. The gas-sensing performance of anodized films has been evaluated with post-anodization treatment in de-ionized water and aqueous ammonia solution. The sensitivity increases to 8646% and 16599%, with post-treatment in aqueous ammonia solution and de-ionized water, respectively. This is attributed to increase surface area and activation during the post- anodization treatment. The repeatability of sensor performance has also been evaluated, and it is observed that the de-ionized water treated film is unstable for repeated sensing, possibly due to some structural modifications.