Crystalline–Crystalline Phase Transformation in Two-Dimensional In₂Se₃ Thin Layers

We report, for the first time, the fabrication of single-crystal In₂Se₃ thin layers using mechanical exfoliation and studies of crystalline–crystalline (α → β) phase transformations as well as the corresponding changes of the electrical properties in these thin layers. Particularly, using electron microscopy and correlative in situ micro-Raman and electrical measurements, we show that, in contrast to bulk single crystals, the β phase can persist in single-crystal thin layers at room temperature (RT). The single-crystal nature of the layers before and after the phase transition allows for unambiguous determination of changes in the electrical resistivity. Specifically, the β phase has an electrical resistivity about 1–2 orders of magnitude lower than the α phase. Furthermore, we find that the temperature of the α → β phase transformation increases by as much as 130 K with the layer thickness decreasing from ∼87 nm to ∼4 nm. These single-crystal thin layers are ideal for studying the scaling behavior of the phase transformations and associated changes of the electrical properties. For these In₂Se₃ thin layers, the accessibility of the β phase at RT, with distinct electrical properties than the α phase, provides the basis for multilevel phase-change memories in a single material system

Quantitative Heat Dissipation Characteristics in Current-Carrying GaN Nanowires Probed by Combining Scanning Thermal Microscopy and Spatially Resolved Raman Spectroscopy

Using an approach combining scanning thermal microscopy (SThM) and spatially revolved Raman spectroscopy, we have investigated quantitatively the heat dissipation characteristics in substrate-supported and suspended (with asymmetric type of contacts) current-carrying GaN nanowires with diameters of ∼40−60 nm, where the phonon confinement is expected to play an important role in thermal transport. In particular, this approach allows direct measurements of nanowire−substrate/electrode interface thermal resistances and the nanowire thermal conductivity. On the basis of these results, the nanowire−substrate thermal transfer was suggested to be the main heat dissipation route, counting for ∼80−93% of the total dissipated heat, whereas the nanowire−electrode interface plays a minor role. The relative significance of nanowire-substrate/electrode interfaces in dissipating heat was further demonstrated in suspended nanowire devices. The measured nanowire thermal conductivity (∼40−60 W/mK) is lower than that in bulk GaN, possibly due to the phonon confinement and boundary scattering effects. Besides providing quantitative insight into heat dissipation characteristics, our results also reveal aspects, particularly the topography-related thermal signals and the relative significance of various tip−sample thermal transfer processes, that are important to advancing the applications of SThM technique in nanoscale thermal characterizations

Electronically Driven Amorphization in Phase-Change In₂Se₃ Nanowires

We show that the amorphization process in phase-change In₂Se₃ nanowires grown by chemical vapor deposition can be driven by electronic effects and does <i>not</i> require the conventional thermal melt-quench process. In particular, using transmission electron microscopy, in situ single-nanowire Raman spectroscopy, scanning Kelvin probe microscopy, and finite-element simulations, we demonstrate that the electronic amorphization can be achieved under optical excitations at temperatures far below the thermal melting point. The mechanism of this electronic amorphization is likely related to the presence of atomic bonds with different strengths in the crystalline phase In₂Se₃ and the weakening of the weaker bonds by nonequilibrium electrons. Our findings suggest that In₂Se₃ is a promising candidate for phase-change memory applications, with potential advantages including energy-efficient memory switching due to the electronic amorphization process and highly stable data storage as a result of a high melting point compared to Ge/Sb–Te alloys. On a more general level, these results indicate the need to take into account the electronic effects in modeling and understanding the phase transition processes in phase-change memories

Synthesis and Ultrafast Carrier Dynamics of Single-Crystal Two-Dimensional CuInSe₂ Nanosheets

We report, for the first time, the synthesis of single-crystal two-dimensional (2D) CuInSe₂ nanosheets and the studies of ultrafast carrier dynamics and transport in this 2D material. Particularly, single-crystal 2D CuInSe₂ with various thicknesses in the nanometer regime were fabricated by a solid-state chemical reaction between Cu and single-crystal exfoliated In₂Se₃ nanosheets. Characteristics of transient optical reflectivity, obtained from femtosecond optical pump–probe measurements on single CuInSe₂ nanosheets, suggest that the hot carrier cooling process dominates the carrier dynamics within a few picoseconds following the optical excitation. Spatially resolved pump–probe measurements, coupled to simple model calculations, were used to obtain the ambipolar hot carrier diffusion coefficient in single nanosheets. The dependence of the hot carrier diffusion coefficient on the nanosheet thickness provides insight into the limiting mechanisms of hot carrier transport and can be used to gauge the possibility of efficient hot carrier collection in nanostructured CuInSe₂ solar cells

Imaging Charged Exciton Localization in van der Waals WSe₂/MoSe₂ Heterobilayers

Exciton localization in transition-metal dichalcogenide monolayers is behind a variety of interesting phenomena and applications, including broad-spectrum solar cells and single-photon emissions. Strain fields at the periphery of topographically distinct features such as nanoscopic bubbles were recently associated with localized charge-neutral excitons. Here, we use tip-enhanced photoluminescence (PL) to visualize excitons in WSe2/MoSe2 heterobilayers (HBL). We find strong optical emission from charged excitons, particularly positively charged trions, in HBL supported by interlayer charge transfer. Our results reveal strong trion confinement, with a localization length scale comparable to the trion size, at the apex region inside individual nanoscopic bubbles. Nano-PL mapping also shows sub-10-nm spatial variations in the localized trion emission spectra, which stem from atomic-scale potential energy fluctuations. These findings demonstrate the possibility of confining charged exciton complexes that are electrically tunable, opening up further opportunities to probe many-body exciton physics and to explore additional possible sites for strong exciton localization that can lead to quantum emission

Diameter-Dependent Surface Photovoltage and Surface State Density in Single Semiconductor Nanowires

Based on single-nanowire surface photovoltage measurements and finite-element electrostatic simulations, we determine the surface state density, <i>N</i>_s, in individual n-type ZnO nanowires as a function of nanowire diameter. In general, <i>N</i>_s increases as the diameter decreases. This identifies an important origin of the recently reported diameter dependence of the surface recombination velocity, which has been commonly considered to be independent of the diameter. Furthermore, through the determination of the surface carrier lifetime, we suggest that the diameter dependence of the surface state density accounts for the rather abrupt transition from bulk-limited to surface-limited carrier transport over a narrow nanowire diameter regime (∼30–40 nm). These findings are supported by the comparison between bulk-limited and surface-dependent minority carrier diffusion lengths measured at various diameters

Multivariate analysis of RFS in classical PTC patients.

Multivariate analysis of RFS in classical PTC patients.</p