Direct Observation of Band Gap Renormalization in Layered Indium Selenide

Study of conduction mechanisms of InSeSb nano-chalcogenide alloys

The electrical conduction mechanisms for bulk samples of In0.1Se0.9-xSbx(x= 0, 0.04, 0.08 and 0.12) nano-chalcogenide system, synthesized by the melt-quenching technique are investigated through current-voltage (I-V) characteristics. For the detailed study of conduction mechanism pellets of bulk samples are prepared. A thorough examination of electrical conductivity is done in the temperature range of 295-318 K and 0-50 V voltage range. FromI-Vmeasurements it is observed that samples are showing ohmic nature at lower field and non-ohmic nature at relatively higher field values. The temperature dependence of DC conductivity is analyzed using the Arrhenius relationship which is found to increase with Sb content. The value of activation energy and pre-exponential factor are calculated, which revealed that the conduction is due to the hopping of charge carriers among the localized states. Different parameters of Mott's variable range hopping such as degree of disorderT0, density of localized statesN(EF), hopping distance (Rhop), and hopping energy (W) are calculated. For the high field conduction process Poole-Frenkel, and Schottky processes are studied.

Gate-Tunable Renormalization of Spin-Correlated Flat-Band States and Bandgap in a 2D Magnetic Insulator

Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are largely governed by the interplay between exchange and Coulomb interactions. The ability to precisely tune the Coulomb interaction enables the control of spin-correlated flat-band states, band gap, and unconventional magnetism in such strongly correlated materials. Here, we demonstrate a gate-tunable renormalization of spin-correlated flat-band states and bandgap in magnetic chromium tribromide (CrBr3) monolayers grown on graphene. Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that the interflat-band spacing and bandgap of CrBr3 can be continuously tuned by 120 and 240 meV, respectively, via electrostatic injection of carriers into the hybrid CrBr3/graphene system. This can be attributed to the self-screening of CrBr3 arising from the gate-induced carriers injected into CrBr3, which dominates over the weakened remote screening of the graphene substrate due to the decreased carrier density in graphene. Precise tuning of the spin-correlated flat-band states and bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not only provides effective strategies for optimizing the spin transport channels but also may exert a crucial influence on the exchange energy and spin-wave gap, which could raise the critical temperature for magnetic order.

Engineering an Indium Selenide van der Waals Interface for Multilevel Charge Storage

As the continuous miniaturization of floating-gate transistors approaches a physical limit, new innovations in device architectures, working principles, and device materials are in high demand. This study demonstrated a nonvolatile memory structure with multilevel data storage that features a van der Waals gate architecture made up of a partially oxidized surface layer/indium selenide (InSe) van der Waals interface. The key functionality of this proof-of-concept device is provided through the generation of charge-trapping sites via an indirect oxygen plasma treatment on the InSe surface layer. In contrast to floating-gate nonvolatile memory, these sites have the ability to retain charge without the help of a gate dielectric. Together with the layered structure, the surface layer with charge-trapping sites facilitates continual electrostatic doping in the underlying InSe layers. The van der Waals gating effect is further supported by trapped charge-induced core-level energy shifts and relative work function variations obtained from operando scanning X-ray photoelectron spectroscopy and Kelvin probe microscopy, respectively. On modulating the amount of electric field-induced trapped electrons by the electrostatic gate potential, eight distinct storage states remained over 3000 s. Moreover, the device exhibits a high current switching ratio of 10⁶ within 11 cycles. The demonstrated characteristics suggest that the engineering of an InSe interface has potential applications for nonvolatile memory.

Ultrafast dynamics of hot carriers in a quasi-two-dimensional electron gas on InSe

Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas-Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high-κ dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides.

Phase transition and thermal stability of epitaxial PtSe2 nanolayer on Pt(111)

This work relates to direct synthesis of the two-dimensional (2D) transition metal dichalchogenide (TMD) PtSe₂ using an original method based on chemical deposition during immersion of a Pt(111) surface into aqueous Na₂Se solution. Annealing of the sample induces significant modifications in the structural and electronic properties of the resulting PtSe₂ film. We report systematic investigations of temperature dependent phase transitions by combining synchrotron based high-resolution X-ray photoemission (XPS), low temperature scanning tunnelling microscopy (LT-STM) and low energy electron diffraction (LEED). From the STM images, a phase transition from TMD 2H-PtSe₂ to Pt₂Se alloy monolayer structure is observed, in agreement with the LEED patterns showing a transition from (4 × 4) to (√3 × √3)R30° and then to a (2 × 2) superstructure. This progressive evolution of the surface reconstruction has been monitored by XPS through systematic de-convolution of the Pt4f and Se3d core level peaks at different temperatures. The present work provides an alternative method for the large scale fabrication of 2D transition metal dichalchogenide films.

LEED, STM and XPS techniques were used to systematically study a temperature-dependent phase transition on a PtSe₂ film grown on the surface of Pt(111) by a chemical deposition method.