High-temperature Electron Microscopy Research Articles

Charge density wave and superconductivity in 6R-TaS[formula omitted

The layered transition metal dichalcogenide compounds 1T-TaS2 and 4H-TaS2 are well known for their exotic properties, which include charge density wave, superconductivity, Mott transition, etc., and lately quantum spin liquid. Here, we report the magnetic, transport and transmission electron microscopy study of the charge density wave and superconductivity in 6R-TaS2 which is a relatively less studied polymorph of this dichalcogenide TaS2. Our high temperature electron microscopy reveals multiple charge density wave transitions between room temperature and 660K. Magnetization, and the electrical resistivity measurements in the temperature range of 2-400 K reveal that 6R-TaS2 undergoes a charge density wave transition around 305 K and is followed by a transition to a superconducting state around 3.5 K. The low temperature specific heat measurement exhibits anomaly associated with the superconducting transition around 2.4 K. The estimated Ginzburg Landau parameter suggests that this compound lies at the extreme limit of type-II superconductivity.

Spatially Controlled Fabrication and Mechanisms of Atomically Thin Nanowell Patterns in Bilayer WS2 Using in Situ High Temperature Electron Microscopy.

We show controlled production of atomically thin nanowells in bilayer WS2 using an in situ heating holder combined with a focused electron beam in a scanning transmission electron microscope (STEM). We systematically study the formation and evolvement mechanism involved in removing a single layer of WS2 within a bilayer region with 2 nm accuracy in location and without punching through to the other layer to create a hole. Best results are found when using a high temperature of 800 °C, because it enables thermally activated atomic migration and eliminates the interference from surface carbon contamination. We demonstrate precise control over spatial distributions with 5 nm accuracy of patterning and the width of nanowells adjustable by dose-dependent parameters. The mechanism of removing a monolayer of WS2 within a bilayer region is different than removing equivalent sections in a monolayer film due to the van der Waals interaction of the underlying remaining layer in the bilayer system that stabilizes the excess W atom stoichiometry within the edges of the nanowell structure and facilitates expansion. This study offers insights for the nanoengineering of nanowells in two-dimensional (2D) transitional metal dichalcogenides (TMDs), which could hold potential as selective traps to localize 2D reactions in molecules and ions, underpinning the broader utilization of 2D material membranes.

In situ high-temperature electron microscopy of 3DOM cobalt, iron oxide, and nickel

High-temperature electron microscopy was used to follow how the structure of two specimens of three-dimensionally ordered macroporous (3DOM) materials, also known as inverse opals, and one specimen of a precursor to a 3DOM material changed with temperature. The change in grain size with temperature of 3DOM cobalt and 3DOM iron oxide (as magnetite) was monitored in situ in the TEM by heating in stages to 900 and 1,000 °C, respectively. The two materials studied by TEM showed contrasting grain growth behavior. For 3DOM cobalt, carbon surrounding the nanometer-size grains led to slower grain growth in thinner sample areas than in areas in closer contact with other grains; a bimodal grain-size distribution was observed after heating above 700 °C for 90 min. The grains of the 3DOM iron oxide had no carbon coating and coarsened more evenly to give a unimodal size distribution. Line scans from selected-area diffraction (SAD) patterns were used for phase analysis and showed that traces of cobalt oxide present in the 3DOM cobalt sample at room temperature disappeared when the sample was heated above 500 °C. The transformation of a 3DOM precursor material, nickel(II) oxalate–polystyrene (PS) latex composites, was followed in situ by variable-temperature environmental scanning electron microscopy (ESEM) from room temperature to ca. 700 °C in 0.5–0.7 kPa O2. The ESEM examination of the 3DOM precursors permitted real-time observation of the polymer template decomposition and the shrinkage that occurs upon calcination of these precursor materials.

Evolution of an epitaxial boundary in gold-silver thin film couples

Abstract The microstructure of a gold-silver thin film couple, grown under conditions expected to yield no epitaxy, was characterized by the conventional TEM as well as STEM. Large gold grains were found to be grown epitaxially on the single crystalline silver layer; the areas between the individual grains consisted of fine crystallites of random orientations, which accommodated most of the misfit and misorientation between two layers. In situ high temperature electron microscopy was used to observe directly the development of a complete epitaxial condition. The mode of this structural evolution was qualitatively similar to that for grain growth in bulk metals.

Hafnium II. Oxidation

The oxidation of hafnium has been studied at 760 mm Hg oxygen pressure in the temperature range 600 °–1250 °C using the thermogravimetric technique. A primarily linear oxidation rate was determined for hafnium from 600 ° to 800 °C. For as-received hafnium, an activation energy of oxidation of 0.90 eV with the rate equation, K L = 2500 e −10.400 T was determined. For 3-pass electron-beam-melted hafnium, an activation energy of 0.42 eV with the oxidation rate equation, K L = 2.0 e −4840 T was calculated. High-temperature X-ray diffraction showed that oxidation results in the formation of monoclinic HfO 2 above 550 °C in air and oxygen atmosphere. High-temperature electron microscopy studies showed preferential and random oxidation behavior of hafnium at a pressure of 10 −5 mm Hg above 600 °C.