Nanotwin Detection and Domain Polarity Determination via Optical Second Harmonic Generation Polarimetry

We demonstrate that optical second harmonic generation (SHG) can be utilized to determine the exact nature of nanotwins in noncentrosymmetric crystals, which is challenging to resolve via conventional transmission electron or scanned probe microscopies. Using single-crystalline nanotwinned CdTe nanobelts and nanowires as a model system, we show that SHG polarimetry can distinguish between upright (Cd–Te bonds) and inverted (Cd–Cd or Te–Te bonds) twin boundaries in the system. Inverted twin boundaries are generally not reported in nanowires due to the lack of techniques and complexity associated with the study of the nature of such defects. Precise characterization of the nature of defects in nanocrystals is required for deeper understanding of their growth and physical properties to enable their application in future devices

Observing Oxygen Vacancy Driven Electroforming in Pt–TiO₂–Pt Device via Strong Metal Support Interaction

Oxygen vacancy formation, migration, and subsequent agglomeration into conductive filaments in transition metal oxides under applied electric field is widely believed to be responsible for electroforming in resistive memory devices, although direct evidence of such a pathway is lacking. Here, by utilizing strong metal–support interaction (SMSI) between Pt and TiO₂, we observe via transmission electron microscopy the electroforming event in lateral Pt/TiO₂/Pt devices where the atomic Pt from the electrode itself acts as a tracer for the propagating oxygen vacancy front. SMSI, which originates from the d-orbital overlap between Pt atom and the reduced cation of the insulating oxide in the vicinity of oxygen vacancies, was optimized by fabricating nanoscale devices causing Pt atom migration tracking the moving oxygen vacancy front from the anode to cathode during electroforming. Experiments performed in different oxidizing and reducing conditions, which tune SMSI in the Pt-TiO₂ system, further confirmed the role of oxygen vacancies during electroforming. These observations also demonstrate that the noble metal electrode may not be as inert as previously assumed

Direct Observation of Metal–Insulator Transition in Single-Crystalline Germanium Telluride Nanowire Memory Devices Prior to Amorphization

Structural defects and their dynamics play an important role in controlling the behavior of phase-change materials (PCM) used in low-power nonvolatile memory devices. However, not much is known about the influence of disorder on the electronic properties of crystalline PCM prior to a structural phase-change. Here, we show that the application of voltage pulses to single-crystalline GeTe nanowire memory devices introduces structural disorder in the form of dislocations and antiphase boundaries (APB). The dynamic evolution and pile-up of APBs increases disorder at a local region of the nanowire, which electronically transforms it from a metal to a dirty metal to an insulator, while still retaining single-crystalline long-range order. We also observe that close to this metal–insulator transition, precise control over the applied voltage is required to create an insulating state; otherwise the system ends up in a more disordered amorphous phase suggesting the role of electronic instabilities during the structural phase-change

Direct Observation of Metal–Insulator Transition in Single-Crystalline Germanium Telluride Nanowire Memory Devices Prior to Amorphization

Structural defects and their dynamics play an important role in controlling the behavior of phase-change materials (PCM) used in low-power nonvolatile memory devices. However, not much is known about the influence of disorder on the electronic properties of crystalline PCM prior to a structural phase-change. Here, we show that the application of voltage pulses to single-crystalline GeTe nanowire memory devices introduces structural disorder in the form of dislocations and antiphase boundaries (APB). The dynamic evolution and pile-up of APBs increases disorder at a local region of the nanowire, which electronically transforms it from a metal to a dirty metal to an insulator, while still retaining single-crystalline long-range order. We also observe that close to this metal–insulator transition, precise control over the applied voltage is required to create an insulating state; otherwise the system ends up in a more disordered amorphous phase suggesting the role of electronic instabilities during the structural phase-change

Highly Reactive TiO₂ Anatase Single Crystal Domains Grown by Atomic Layer Deposition

Anatase TiO₂ films with unusual domains-like morphology were obtained by postdeposition annealing of amorphous TiO₂ films deposited by atomic layer deposition (ALD). Such particular morphology was observed only for TiO₂ films deposited using TiCl₄ precursor in a nonconventional ALD regime where the reaction byproducts or nonreacted precursors are incorporated into the film and induce an explosive recrystallization upon annealing. This recrystallization leads to the formation of micrometric single crystal domains. The investigation of domains by electron backscatter diffraction shows the formation of a significant amount of highly reactive anatase crystalline facets such as (111) that contradicts fundamental crystal growth rules. The stabilization of (111) facets in films without additional seed layers has a strong interest for photocatalysis-based applications for environmental remediation or hydrogen production