Hessam Ghassemi

In situ environmental transmission electron microscopy study of oxidation of two-dimensional Ti3C2 and formation of carbon-supported TiO2

Two-dimensional Ti3C2, also known as “MXene”, was oxidized in air under two different oxidizing regimes in order to produce carbon-supported TiO2. In situ TEM analysis coupled with Raman spectroscopy revealed the formation of either anatase nanoparticles or planar rutile nanocrystals, which were controlled by the time, temperature and heating rate.

Atomic-Scale Characterization of Oxide Thin Films Gated by Ionic Liquid

Ionic liquids (ILs) have received considerable interest for use in electrostatic gating in complex oxide systems. Understanding the ionic liquid/oxide interface, and any bias-induced electrochemical degradation, is critical for the interpretation of transport phenomena. The integrity of the interface between ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate and La1/3Sr2/3FeO3 under various biasing conditions was examined by analytical transmission electron microscopy, and we report film degradation in the form of an irreversible chemical reaction regardless of the applied bias. This results in an intermixing region of 4-6 nm at the IL/oxide interface. Electron energy loss spectroscopy shows La and Fe migration into the ionic liquid, resulting in secondary phase formation under negative bias. Our approach can be extended to other ionic liquid/oxide systems in order to better understand the electrochemical stability window of these device structures.

In-situ TEM study of the Corrosion Behavior of Zry-4

This research investigates the effect of texture and grain boundary character on the corrosion response of Zircaloy-4 (Zry-4) using an in situ environmental cell in a transmission electron microscope (TEM). The corrosion response was studied in an oxygen rich environment at elevated temperatures, and monitored in real time using TEM bright field imaging and diffraction to observe the transition of Zry-4 from base metal to oxide.

Simultaneous Structural and Electrical Analysis of Vanadium Dioxide Using In Situ TEM

Vanadium dioxide (VO2), a correlated electron material, has received significant attentions due to its metal-insulator transition (MIT) at ~ 67 °C [1]. This transition is associated with structural phase transition from the monoclinic (M1), an insulating phase, to rutile (R), a metallic phase. This metalinsulator transision is accompanied by a noticeable resistivity, optical transparency and magnetic changes. These distinctive properties have inspired many applications such as thermo/electrochromics, Mott transistors, memristors, thermal actuators, gas sensors, strain sensors and temperature sensors. Recent efforts focus on controlling of phase transition and domain structures in finite size VO2, which results in different material properties and play a critical role in device applications.

The Perfect Cut: Focused Ion Beam Preparation for In Situ TEM

In situ TEM techniques have improved considerably in recent years with respect to their ability to understand materials behavior with high temporal and spatial resolution. While significant advances have been made in elucidating atomic-scale mechanisms that control properties of materials for a wide range of applications, geometric compromises made to accommodate in situ TEM experiments could play a detrimental role in the ability to apply data to “real-life” structures or devices.

In-Situ Characterization of the Evolution of Defects in AlGaN\GaN HEMTs in the On-state and Off-state condition

Nitride semiconductors offer many unique and beneficial properties for new generation electronic devices [1]. GaN-based HEMTs are contenders for replace existing Si and GaAs devices in high-power RF applications. AlGaN/GaN High Election Mobility Transistors (HEMTs) are devices designed for applications where high-power and high-frequency devices are needed. AlGaN/GaN HEMTs take advantage of a two-dimensional electron gas (2DEG) that forms at the AlGaN/GaN interface to create a layer of highly mobile electrons that are easily modulated by an applied bias. Unfortunately, high-power operating conditions often result in unpredictable and catastrophic device degradation [2]. In our previous ex-situ work, we observed the formation of defects under the drain side edge of the gate during bias. The amount of defects present increased with bias duration and this was related to changes in ID and strain near the AlGaN/GaN interface [3]. However, the formation mechanism of these defects has not been fully investigated as a function of operating time. As such, quantitative analysis on the evolution of defects is needed to further understand device failure mechanisms.

In Situ TEM Investigation on the Effect of Atmosphere on Anatase-To-Rutile Phase Transformation

This research investigates the effect of atmosphere and temperature on the kinetics of the anatase-torutile phase transformation in titanium dioxide (TiO2) nanoparticles using an in situ gas cell in a transmission electron microscope (TEM). High-resolution (HR) TEM images and diffraction patterns were captured in various gaseous environments (argon, oxygen, and nitrogen) and at different annealing rates up to 1000°C in situ to study the transformation phenomenon.