Tailoring Topological Transitions of Anisotropic Polaritons by Interface Engineering in Biaxial Crystals.

Polaritons in polar biaxial crystals with extreme anisotropy offer a promising route to manipulate nanoscale light-matter interactions. The dynamic modulation of their dispersion is of great significance for future integrated nano-optics but remains challenging. Here, we report tunable topological transitions in biaxial crystals enabled by interface engineering. We theoretically demonstrate such tailored polaritons at the interface of heterostructures between graphene and α-phase molybdenum trioxide (α-MoO3). The interlayer coupling can be modulated by both the stack of graphene and α-MoO3 and the magnitude of the Fermi level in graphene enabling a dynamic topological transition. More interestingly, we found that the wavefront transition occurs at a constant Fermi level when the thickness of α-MoO3 is tuned. Furthermore, we also experimentally verify the hybrid polaritons in the graphene/α-MoO3 heterostructure with different thicknesses of α-MoO3. The interface engineering offers new insights into optical topological transitions, which may shed new light on programmable polaritonics, energy transfer, and neuromorphic photonics.

Hyperbolic polaritonic crystals with configurable low-symmetry Bloch modes.

Photonic crystals (PhCs) are a kind of artificial structures that can mold the flow of light at will. Polaritonic crystals (PoCs) made from polaritonic media offer a promising route to controlling nano-light at the subwavelength scale. Conventional bulk PhCs and recent van der Waals PoCs mainly show highly symmetric excitation of Bloch modes that closely rely on lattice orders. Here, we experimentally demonstrate a type of hyperbolic PoCs with configurable and low-symmetry deep-subwavelength Bloch modes that are robust against lattice rearrangement in certain directions. This is achieved by periodically perforating a natural crystal α-MoO3 that hosts in-plane hyperbolic phonon polaritons. The mode excitation and symmetry are controlled by the momentum matching between reciprocal lattice vectors and hyperbolic dispersions. We show that the Bloch modes and Bragg resonances of hyperbolic PoCs can be tuned through lattice scales and orientations while exhibiting robust properties immune to lattice rearrangement in the hyperbolic forbidden directions. Our findings provide insights into the physics of hyperbolic PoCs and expand the categories of PhCs, with potential applications in waveguiding, energy transfer, biosensing and quantum nano-optics.

Self-trapped excitons in soft semiconductors.

Self-trapped excitons (STEs) have attracted tremendous attention due to their intriguing properties and potential optoelectronic applications. STEs are formed from the lattice distortion induced by the strong electron (exciton)-phonon coupling in soft semiconductors upon photoexcitation, which features in broadband photoluminescence (PL) emission spectra with a large Stokes shift. Recently, significant progress has been achieved in this field but many remain challenges that need to be solved, including the understanding of the underlying physical mechanism, tuning of the performance, and device applications. Along these lines, for the first time, systematic experimental characterizations and advanced theoretical calculations are presented in this review to shed light on the physical mechanism. The possibility of tuning the STEs through multiple degrees of freedom is also presented, along with an overview of the STE-based emerged applications and future research perspectives.

Scalable Production of Boron Quantum Dots for Broadband Ultrafast Nonlinear Optical Performance.

A simple and effective approach based on the liquid phase exfoliation (LPE) method has been put forward for synthesizing boron quantum dots (BQDs). By adjusting the interactions between bulk boron and various solvents, the average diameter of produced BQDs is about 7 nm. The nonlinear absorption (NLA) responses of as-prepared BQDs have been systematically studied at 515 nm and 1030 nm. Experimental results prove that BQDs possess broadband saturable absorption (SA) and good third-order nonlinear optical susceptibility, which are comparable to graphene. The fast relaxation time and slow relaxation time of BQDs at 515 nm and 1030 nm are about 0.394-5.34 ps and 4.45-115 ps, respectively. The significant ultrafast nonlinear optical properties can be used in optical devices. Here, we successfully demonstrate all-optical diode application based on BQDs/ReS2 tandem structure. The findings are essential for understanding the nonlinear optical properties in BQDs and open a new pathway for their applications in optical devices.

Evolution of low-dimensional material-based field-effect transistors.

Field-effect transistors (FETs) have tremendous applications in the electronics industry due to their outstanding features such as small size, easy fabrication, compatibility with integrated electronics, high sensitivity, rapid detection and easy measuring procedures. However, to meet the increasing demand of the electronics industry, efficient FETs with controlled short channel effects, enhanced surface stability, reduced size, and superior performances based on low-dimensional materials are desirable. In this review, we present the developmental roadmap of FETs from conventional to miniaturized devices and highlight their prospective applications in the field of optoelectronic devices. Initially, a detailed study of the general importance of bulk and low-dimensional materials is presented. Then, recent advances in low-dimensional material heterostructures, classification of FETs, and the applications of low-dimensional materials in field-effect transistors and photodetectors are presented in detail. In addition, we also describe current issues in low-dimensional material-based FETs and propose potential approaches to address these issues, which are crucial for developing electronic and optoelectronic devices. This review will provide guidelines for low-dimensional material-based FETs with high performance and advanced applications in the future.

Two-Dimensional Platinum Diselenide: Synthesis, Emerging Applications, and Future Challenges.

In recent years, emerging two-dimensional (2D) platinum diselenide (PtSe2) has quickly attracted the attention of the research community due to its novel physical and chemical properties. For the past few years, increasing research achievements on 2D PtSe2 have been reported toward the fundamental science and various potential applications of PtSe2. In this review, the properties and structure characteristics of 2D PtSe2 are discussed at first. Then, the recent advances in synthesis of PtSe2 as well as their applications are reviewed. At last, potential perspectives in exploring the application of 2D PtSe2 are reviewed.

Recent Progress of Two-Dimensional Thermoelectric Materials.

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power. Moreover, the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades. Among these compounds, layered two-dimensional (2D) materials, such as graphene, black phosphorus, transition metal dichalcogenides, IVA-VIA compounds, and MXenes, have generated a large research attention as a group of potentially high-performance thermoelectric materials. Due to their unique electronic, mechanical, thermal, and optoelectronic properties, thermoelectric devices based on such materials can be applied in a variety of applications. Herein, a comprehensive review on the development of 2D materials for thermoelectric applications, as well as theoretical simulations and experimental preparation, is presented. In addition, nanodevice and new applications of 2D thermoelectric materials are also introduced. At last, current challenges are discussed and several prospects in this field are proposed.

"One-Step" Carbonization Activation of Garlic Seeds for Honeycomb-like Hierarchical Porous Carbon and Its High Supercapacitor Properties.

In this paper, a simple "one-step" route is introduced to prepare a kind of novel honeycomb-like hierarchical porous carbon (h-HPC) by carbonizing and activating garlic seeds. Due to its special microstructure, h-HPC shows excellent electrochemical properties and high supercapacitor performances. The experimental results reveal the following: (1) There exists an optimal condition for synthesizing h-HPC, i.e., 700 °C carbonization temperature and 1:1 mass ratio of KOH and garlic seeds. (2) h-HPC has a three-dimensional interconnected porous structure and exhibits a specific surface area as high as 1417 m2/g with a narrow pore size distribution. (3) When h-HPC is employed as an electrode material in supercapacitors, its specific capacitance reaches a value up to 268 F/g at a current density of 0.5 A/g and excellent rate capability. (4) The h-HPC-based symmetric supercapacitor shows a high energy density of 31.7 Wh/kg at a power density of 500 W/kg and retains 99.2% of the initial capacitance after 10,000 charge/discharge cycles at 200 mV/s. When compared with similar works, these data are competitive, which demonstrates that the garlic-derived h-HPC is a kind of promising electrode material for the next-generation high-energy-density supercapacitors.

Band structure tuning of α-MoO3 by tin intercalation for ultrafast photonic applications.

van der Waals (vdW) transition metal oxides have attracted extensive attention due to their intriguing physical and chemical properties. However, primary drawbacks of these materials are the lack of band structure tunability and substandard optical properties, which severely hinder their implementation in nanophotonic applications. Atomic intercalation is an emerging structural engineering approach for two-dimensional vdW materials to engineer the atomic structure and modify the optical properties, thereby broadening their range of applications. Herein, we synthesized tin-intercalated ultrathin α-MoO3 (Sn-MoO3) nanoribbons via chemical intercalation method and then investigated the broadband nonlinear optics (NLO) of stable few-layer α-MoO3 by performing a Z-scan laser measurement and femtosecond-resolved transient absorption (TA) spectroscopy. Sn-MoO3 showed a stable structure of Mo-O-Sn-O-Mo and a shorter relaxation time than pristine MoO3, indicating the accelerated recombination process of electrons and holes. Furthermore, Sn-MoO3 nanoribbons were used as an optical saturable absorber for ultrafast photonics; a highly stable femtosecond laser with a pulse width of 467 fs was generated from a single-mode fiber in the telecommunication band (1550 nm). These results indicate that atomic intercalation is an effective way to modulate the band structure and nonlinear optical properties of α-MoO3, which hold a great potential in the generation of ultrafast mode-locked laser pulses for optical communication technologies.

Sub-kT/q switching in In2O3 nanowire negative capacitance field-effect transistors.

Limited by the Boltzmann distribution of electrons, the sub-threshold swing (SS) of conventional MOSFETs cannot be less than 60 mV dec-1. This limitation hinders the reduction of power dissipation of the devices. Herein, we present high-performance In2O3 nanowire (NW) negative capacitance field-effect transistors (NC-FETs) by introducing a ferroelectric P(VDF-TrFE) layer in a gate dielectric stack. The fabricated devices exhibit excellent gate modulation with a high saturation current density of 550 µA µm-1 and an outstanding SS value less than 60 mV dec-1 for over 4 decades of channel current. The assembled inverter circuit can demonstrate an impressive voltage gain of 25 and a cut-off frequency of over 10 MHz. By utilizing the self-aligned fabrication scheme, the device can be ultimately scaled down to below 100 nm channel length. The devices with 200 nm channel length exhibit the best performances, in which a high on/off current ratio of >107, a large output current density of 960 µA µm-1 and a small SS value of 42 mV dec-1 are obtained at the same time. All these would not only evidently demonstrate the potency of NW NC-FETs to break through the Boltzmann limit in nanoelectronics, but also open up a new avenue to low-power transistors for portable products.