Nanowire Grid Polarization and Polarized Excitonic Emission Observed in Multilayer GaTe

Temperature Dependence of the Band Gap and Exciton Photoreflectance in Layered Gallium Telluride

Among Group III-A metal monochalcogenides, gallium telluride (GaTe) is one of the less studied materials in terms of applications and optical characterization. For the temperature dependence of the energy transitions in GaTe, photoluminescence (PL) spectroscopy is commonly used, and photomodulated reflectance (PR) is yet to be reported. In this work, layered monoclinic GaTe single crystals were synthesized by the Bridgman technique and used for the investigation of the conduction band (CB) edge and free-exciton (FX) state transitions using PR spectroscopy. Both energy transitions (i.e., absorption and emission) were present at room temperature at 1.656 and 1.647 eV for the CB edge transition (≡Eg) and for the FX state transition, respectively, and show a blueshift at cryogenic temperatures that can be fitted with Varshni's equation. The estimated E(0) is 1.794 eV for Eg and 1.776 eV for the FX transitions at 0 K. The energy of the FX state transition is ∼18 meV lower than that of the band gap (Eg) at 0 K. PL spectroscopy confirms that the PL emission is only the FX state transition that is lower than Eg. The temperature-induced band-gap shifting is related to performing temperature-dependent photodetector experiments using various incident light wavelengths. At 80 K, the responsivity of the single-crystal GaTe photodetector to the energies of wavelengths (735 and 845 nm) smaller than Eg is relatively smaller than that to 630 nm incident light. This indicates that the low-temperature band-gap shift plays a role in applications of GaTe in optoelectronics.

Axially-Polarized Excitonic Series and Anisotropic van der Waals Stacked Heterojunction in a Quasi-1D Layered Transition-Metal Trichalcogenide

Anisotropic optical 2D materials are crucial for achieving multiple-quanta functions within quantum materials, which enables the fabrication of axially polarized electronic and optoelectronic devices. In this work, multiple excitonic emissions owning polarization-sensitive orientations are clearly detected in a multilayered quasi-1D ZrS3 nanoribbon with respect to the nanostripe edge. Four excitons denoted as AS1, AS2, AS, and A2 with E ⊥ b polarized direction and one prominent A1 exciton with E || b polarized emission are simultaneously detected in the polarized micro-photoluminescence (µPL) measurement of 1.9-2.2 eV at 10 K. In contrast to light emission, polarized micro-thermoreflectance (µTR) measurements are performed to identify the polarization dependence and verify the excitons in the multilayered ZrS3 nanoribbon from the perspective of light absorption. At 10 K, a prominent and broadened peak on the lower-energy side, containing an indirect resonant emission (DI) observed by µPL and an indirect defect-bound exciton peak (AInd) observed by both µPL and µTR, is simultaneously detected, confirming the existence of a quasi-direct band edge in ZrS3. A van der Waals stacked p-GaSe/n-ZrS3 heterojunction solar cell is fabricated, which demonstrates a maximum axially-polarized conversion efficiency up to 0.412% as the E || b polarized light incident onto the device.

Phosphorus-Doped Multilayer In6Se7: The Study of Structural, Electrical, and Optical Properties for Junction Device

This work investigates the characteristic of layered In6Se7 with varying phosphorus (P) dopant concentrations (In6Se7:P) from P = 0, 0.5, 1, to P = 5%. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses indicate that the structure and morphology of the In6Se7:P series compounds remain unchanged, exhibiting a monoclinic structure. Room-temperature micro-Raman (μRaman) result of all the compositions of layered In6Se7:P reveals two dominant peaks at 101 ± 3 cm-1 (i.e., In-In bonding mode) and 201 ± 3 cm-1 (i.e., Se-Se bonding mode) for each P composition in In6Se7. An extra peak at approximately 171 ± 2 cm-1 is observed and it shows enhancement at the highest P composition of In6Se7:P 5%. This mode is attributed to P-Se bonding caused by P doping inside In6Se7. All the doped and undoped In6Se7:P showed n-type conductivity, and their carrier concentrations increased with the P dopant is increased. Temperature-dependent resistivity revealed a reduction in activation energy (for the donor), as the P content is increased in the In6Se7:P samples. Kelvin probe measurement shows a decrease in work function (i.e., an energy increase of Fermi level) of the n-type In6Se7 multilayers with the increase of P content. The indirect and direct band gaps for all of the multilayer In6Se7:P of different P composition are identical. They are determined to be 0.732 eV (indirect) and 0.772 eV (direct) obtained by microtransmittance and microthermoreflectance (μTR) measurements. A rectified n-n+ homojunction was formed by stacking multilayered In6Se7/In6Se7:P 5%. The built-in potential is about Vbi ∼ 0.15 V. It agrees well with the work function difference between the two layer compounds.

Internal Built-In Electric Fields at Organic-Inorganic Interfaces of Two-Dimensional Ruddlesden-Popper Perovskite Single Crystals

Two-dimensional (2D) organic-inorganic hybrid Ruddlesden-Popper perovskites (OIRPPs), which consist of naturally formed "multiple quantum well (MQW)-like" structure, have received considerable interest in optoelectronic applications, owing to their outstanding optical properties and tailorable functionalities. While the quantum-confined electrons and holes at an MQW structure are under an applied electric field, the tilt of the energy bands may cause a significant influence on their optical properties. This work demonstrates the presence of internal built-in electric fields (BIEFs) at the as-synthesized 2D OIRPP single crystals. Spontaneous Franz-Keldysh oscillations, which usually act as the fingerprint to account for the presence of BIEFs in the MQW-like structures, are observed at 2D OIRPPs by the highly sensitive differential technique of modulated thermoreflectance spectroscopy. The strength of BIEFs at 2D OIRPP single crystals reduces with increased n values due to the increased width of the quantum well. The origin of the presence of BIEFs at 2D OIRPPs is further unveiled by atomically resolved scanning tunneling microscopy on their electronic band structures at organic-inorganic interfaces. Unlike the conventional III-V MQW semiconductors with the BIEFs, which are dominated by the spatial concentration gradients at heterointerfaces, the presence of BIEFs at the 2D OIRPP single crystals is attributed to the molecular dipoles within organic spacers pointing to the organic-inorganic interfaces. The discovery of internal BIEFs at the 2D OIRPPs may provide deep insight into understanding the fundamental optical properties for the future design of large-area and low-cost perovskite optoelectronic devices.

Exploration of exciton dynamics in GaTe nanoflakes via temperature- and power-dependent time-resolved photoluminescence spectra

GaTe nanoflakes have been receiving much research attention recently due to their applications in optoelectronic devices, such as anisotropic non-volatile memory, solar cells, and high-sensitivity photodetectors from the ultraviolet to the visible region. Further applications, however, have been impeded due to the limited understanding of their exciton dynamics. In this work we perform temperature- and power-dependent time-resolved photoluminescence (PL) spectra to comprehensively investigate the exciton dynamics of GaTe nanoflakes. Temperature-dependent PL measurements manifest that spectral profiles of GaTe nanoflakes change dramatically from cryogenic to room temperature, where the bound exciton and donor-to-acceptor pair transition normally disappear above 100 K, while the charged exciton survives to room temperature. The lifetimes of these excitons and their evolution vs temperature have been uncovered by time-resolved PL spectra. Further measurements reveal the entirely different power-dependent exciton behaviors of GaTe nanoflakes between room and cryogenic temperatures. The underlying mechanisms have been proposed to explore the sophisticated exciton dynamics within GaTe nanoflakes. Our results offer a more thorough understanding of the exciton dynamics of GaTe nanoflakes, enabling further progress in engineering GaTe-based applications, such as photodetectors, light-emitting diodes, and nanoelectronics.

Ga2Se3 Defect Semiconductors: The Study of Direct Band Edge and Optical Properties

Direct band edge is a crucial factor for a functional chalcogenide to be applied in luminescence devices, photodetectors, and solar-energy devices. In this work, the room-temperature band-edge emission of III-VI Ga2Se3 has been first observed by micro-photoluminescence (μPL) measurement. The emission peak is at 1.85 eV, which matches well with the band-edge transition that is measured by micro-thermoreflectance (μTR) and micro-transmittance (μTransmittance) for verification of the direct band edge of Ga2Se3. The temperature-dependent μTR spectra of Ga2Se3 show a general semiconductor behavior with its temperature-energy shift following Varshni-type variation. With the well-evident direct band edge, the peak responsivities of photovoltaic response (∼6.2 mV/μW) and photocurrent (∼2.25 μA/μW at f = 30 Hz) of defect zincblende Ga2Se3 can be, respectively, detected at ∼2.22 and ∼1.92 eV from a Cu/Ga2Se3 Schottky solar cell and a Ga2Se3 photoconductor. On the basis of experimental analysis, the optical band edge and photoresponsivity properties of a III-VI Ga2Se3 defect semiconductor are thus realized.