Understanding of the excitonic perspective on light-driven energy conversions is limited, particularly in higher excited states of excitons in 2D systems arising from nonlocal screening effects. Besides, the anisotropy in the sequence of excited exciton states (Rydberg Spectra) has been completely overlooked in the literature. This work presents a theoretical investigation of the anisotropic non-hydrogenic exciton dynamics in blue-AsP, systematically exploring the origin of the deviation from the conventional hydrogenic Rydberg series. A key insight from our investigation underscores the profound influence of the principal quantum number on effective dielectric screening when probing the binding energies in the Rydberg spectra of excited excitonic states (ns = 1s,2s,3s,...). This study also offers a much-needed systematic comparison between the non-hydrogenic analytical models, complemented by G0W0 + BSE methodology. Importantly, this highlights the exciting possibility of precisely controlling exciton behavior through strategic substrate selection. Our findings extend the anisotropic Rydberg spectra to the "anomalous screening regime" in both armchair and zigzag directions. 

Probing 2D Exciton Dynamics of Non-Hydrogenic Anisotropic Rydberg Spectra in Anomalous Screening Regime

The realization of controlled modulation of ultrashort pulses in the all-solid-state visible ultrafast lasers is a challenge, limited by the development of advanced optical modulation devices. Here, we reported a Ta2PdSe6 photonic device-based controlled visible ultrafast laser that exploits the polarization-dependent optical response of the Ta2PdSe6 photonic device to modulate the pulse width. Ultrashort pulse widths of 33.3 ps and 36.6 ps can be achieved under the horizontal and vertical polarization emission, respectively. This study presents polarization-dependent photonic devices for solid-state lasers, which might lead to the creation of controlled modulation in visible ultrafast lasers.

Controlled visible ultrafast lasers based on polarization-dependent photonic devices

Herein, the density functional theory is used to study the structural, mechanical, electrical, optical, and hydrogen evolution reaction (HER) properties of monolayer structures ( α , β ) – S 2 C 2 N 2 $\left(\right. \alpha , \beta \left.\right) – \left(\text{S}\right)_{2} \left(\text{C}\right)_{2} \left(\text{N}\right)_{2}$ and ( α , β ) – SCN / CN $\left(\right. \alpha , \beta \left.\right) – \text{SCN} / \text{CN}$ . They are claimed to be stable monolayer structures by phonon dispersion and ab initio molecular dynamics (AIMD). Compared to graphene (≈340 N m−1), the ( α , β ) – SCN / CN $\left(\right. \alpha , \beta \left.\right) – \text{SCN} / \text{CN}$ shows a higher Young's modulus (≈429, 414 N m−1). Moreover, the piezoelectric stress coefficient of α- SCN / CN $\text{SCN} / \text{CN}$ ( d 11 $d_{11}$ =2.65 pm V−1) is comparable with bulk piezoelectric α-quartz ( d 11 $d_{11}$ =2.3 pm V−1). Exciton effect at M point causes redshift and enhancement of light absorption of α- SCN / CN $\text{SCN} / \text{CN}$ and β – SCN / CN $\beta – \text{SCN} / \text{CN}$ . The overpotential of β – SCN / CN $\beta – \text{SCN} / \text{CN}$ is only –0.072 V. Besides, the ( α , β ) – SCN / CN $\left(\right. \alpha , \beta \left.\right) – \text{SCN} / \text{CN}$ gives a small effective electron mass of about 0.2(m0). It is worth mentioning that the monolayer structures ( α , β ) – S 2 C 2 N 2 $\left(\right. \alpha , \beta \left.\right) – \left(\text{S}\right)_{2} \left(\text{C}\right)_{2} \left(\text{N}\right)_{2}$ show the semimetal phases. Due to the unique semimetal phase, the absorption of β – S 2 C 2 N 2 $\beta – \left(\text{S}\right)_{2} \left(\text{C}\right)_{2} \left(\text{N}\right)_{2}$ has an excellent performance in the near-infrared (NIR) region. It can be enhanced at the NIR region and extended to the middle infrared region when applying tensile stress. These results provide more choices for 2D materials in electrocatalysis, piezoelectric sensors, visible light devices, and infrared photodetectors. 

Hydrogen evolution, Piezoelectricity, and Optical properties of monolayer semimetal (α,β)‐S
 2
 C
 2
 N
 2
 and Janus (α,β)‐SCN/CN

Two-dimensional (2D) layered materials have sparked the interest of researchers because of their interesting properties drawn from quantum confinement effects, elevated density of states (DOS), increased surface area, increased active sites, etc. Phosphorene, an interesting addition to the 2D material family in 2014, has interesting properties such as in-plane anisotropy, elevated carrier mobility and thickness-dependent bandgaps that have made it a material of intense interest. Furthermore, there have been theoretical studies predicting that phosphorene could be made even more versatile by intercalating alkali atoms into its layers. Intercalation has proven to be a veritable tool for synthesis, modification, transformation, and phase transitions in 2D layered materials. This dissertation focuses on the experimental the mechanism behind cesium and sodium intercalation of phosphorene through spectroscopic studies and electrical transport measurements. Additionally, alloying phosphorene with grey arsenic was achieved and investigated as a solution to the stability problems of phosphorene that has prevented its large-scale deployment in devices and commercialization.Phosphorene degrades within hours when exposed to ambient conditions. A photo-assisted oxidation process occurs which yields phosphorous oxide species ( that etches away its surface; ultimately degrading it). The first project of this dissertation was focused on studying the effects of cesium (Cs) vapors on black phosphorene (BP) flakes at varied times and at a temperature gradient of 150oC. The x-ray diffraction (XRD) measurements of these samples suggest a reduction in the strength of the van der Waals interactions between BP layers leading to the loss of coherence of out-of-plane peaks. At the same time, the three main Raman modes of BP (, , and ) steadily redshifted as exposure times were increased, with modes and shifting faster than . After initial rapid downshifts of active BP phonon modes, this intercalation strategy showed its limits following prolonged exposure times. Saturation of BP flakes by Cs vapors ensued and the kinetics was fitted with an exponential decay function. Furthermore, the thermoelectric power (TEP) of cesiated BP exhibited an inversion in sign from positive to negative around 400 K, lending credence to the transformation of as-prepared BP which is a p-type semiconductor to an n-type equivalent due to Cs atom intercalation driven shifting of the Fermi level toward the conduction band of BP and the donation of electrons from Cs. The next project was on the electrochemical intercalation of sodium (Na) into BP flakes which eventually produced phosphorene nanoribbons (PNRs). PNRs have inspired strong research interests to explore their exciting properties that associate with the unique 2D structure of phosphorene as well as the additional quantum confinement of the nanoribbon morphology, providing new materials strategy for electronic and optoelectronic applications. Despite several attempts at their fabrication, the production of PNRs with narrow widths has been a great challenge. The study conducted in this project lead to the development of a facile and straightforward approach to synthesize PNRs via electrochemical process. The anisotropic Na+ diffusion barrier in black phosphorus (BP) along the [001] zigzag direction against the [100] armchair direction proved to be beneficial in achieving this feat. The produced PNRs displayed widths of good uniformity (10.3 3.8 nm) observed by high resolution transmission electronic microscopy (HR-TEM), and the suppressed B2g vibrational mode from Raman spectroscopy results. More interesting, when used in field-effect transistors (FETs), the synthesized bundles of PNRs exhibit the n-type behavior, which is dramatically different with from bulk BP flakes which are p-type. This work provides insights into a new synthesis approach of PNRs with confined width, paving the way towards to the development of phosphorene and other highly anisotropic nanoribbon materials for high quality electronic applications. Finally, the effects of oxygen and humidity on black phosphorous (BP) and black arsenic phosphorous () flakes using Raman spectroscopy and in-situ electric transport measurements (four-probe resistance and thermoelectric power, TEP) was investigated. The results show that the incorporation of arsenic into the lattice of BP renders it more stable, with the degradation times for BP, , and being 4, 5, and 11 days, respectively. The P-P Raman peak intensities were found to decrease with exposure to oxygen and moisture. The TEP measurements confirmed that both BP and are p-type semiconductors with the TEP of stabilizing more slowly than that of BP. In addition, the four-probe resistance of BP and stabilized significantly faster when exposed to air after being degassed in a vacuum. This was attributed to the charge transfer between the oxygen redox potential of air and the Fermi energy (EF) of the semiconductors. 

Synthesis, property tuning and degradation studies of phosphorene-based materials.

Intrinsic Polarity extend β-GaO/Janus-XP (X=P, As) Heterostructures Potential in UV/IR Dual-band Photodetector: A Theoretical Study

Two-dimensional (2D) arsenic–phosphorus (AsP), as a derivative of black phosphorus (BP), has achieved great progress in regards to preparation methods, property modulation, and front application, which can be attributed to the following two points. The first is that a method has been developed of alloying BP with the congener element arsenic to produce high-quality AsP; the second is that stable AsP possesses unique electronic and optical properties. To conclude the continuous and extensive research, this review focuses on synthesis details, modulation strategies, and application advances of stable AsP. Firstly, several pathways to prepare AsP with different phases are listed. Secondly, multiple solutions to optimize the electronic properties of AsP are discussed, such as strain regulation and composition tuning, and especially composition tuning of AsP including element modification, atomic substitution, and dopant participation, which can bring about adjustments of the lattice structure, bandgaps, and electronic properties. Based on the regulated AsP, applications in infrared photodetectors, high-performance transistors, and efficient-energy storage devices and so on have been widely developed. Although there are challenges ahead, this review may bring new insights into and inspirations for further development of 2D AsP-based materials and devices.

Recent advances in stable arsenic–phosphorus: preparation, properties, and application

Chiral transition metal oxides (TMOs) are in the forefront of research as potential active materials in various optoelectronic applications. However, the nonlinear optical (NLO) properties of the chiral TMOs have not been fully understood. Here, several kinds of copper oxide nanosheets capped with different chiral amino acids are synthesized. Notably, we investigate the NLO activities of these materials, including broadband second harmonic generation and transformation of nonlinear optical properties from saturable absorption to reverse saturable absorption. This work will broaden the use of chiral TMO materials in nonlinear photonic devices.

Nonlinear optical properties in chiral copper oxide nanosheets

The design of polarization-sensitive, stable self-powered, and broadband photoresponse optoelectronic devices remains a big challenge. Here, the influence of vacancy defects on the electronic structure properties of CsCu2I3 has been studied by density functional theory, which reveals the feasibility of their application in the field of optoelectronic devices, and then, their photogalvanic effects have been investigated based on quantum transport simulations. The results show that the pristine CsCu2I3 and I-vacancy devices indeed generate robust photocurrents under irradiation with linearly polarized light at the near ultraviolet to the visible wavelength without bias, demonstrating the self-powered and broadband response of the devices. The extinction ratios of the pristine CsCu2I3 and I-vacancy devices were 9.84 and 33.02 at zero bias, respectively. In addition, the I-vacancy device exhibits an ultra-high extinction ratio of up to 69.7 at 0.2 V. These results demonstrate potential applications of CsCu2I3-based devices in high performance, low power, and polarization detection.

Perovskite CsCu2I3-based optoelectronic device with exceptional polarization sensitivity via point vacancy modulation

We present a vanadium dioxide (VO2) metasurface capable of tuning the radiative properties through the insulator-to-metal transition in the atmospheric window. Theoretically, we demonstrate that the hemispherical emissivity approaches near-unity due to the opening of the radiation channel and the Fabry–Pérot mode formed by the multilayer structure. Moreover, when the incident angle is 60°, the average emissivity in the insulating state reaches 0.45, and that in the metallic state reaches 0.64. Additionally, we integrate photothermal conversion and optical tweezers techniques to experimentally demonstrate the generation of thermofluid. The VO2 metasurface generates closed-loop thermal convection vortices with a maximum fluid velocity of ∼10 nm/s. The thermophoretic force (∼100 pN) is three orders of magnitude larger than the optical force (100–400 fN). The thermofluid convection was caused by the asymmetric thermal distribution resulting from the photons absorbed in the heater. The structure we proposed has the potential to be applied in various fields related to thermal control, biochemistry, microwave hyperthermia, nanoparticle transportation, and so on.

Dynamic microfluidic control with VO2-based metasurfaces

Nonreciprocal transport is indispensable for many modern optical devices. However, traditional nonreciprocal devices heavily depend on external bias. Achieving significant nonreciprocity in practice remains challenging. In this work, we propose a Kerr-nonlinearity-based design to achieve strong nonreciprocal transmission. By leveraging high-Q guided-mode resonances (GMRs) in a Si waveguide nanostructure decorated with periodic grooves, we demonstrate magnetic-free bidirectional nonreciprocal transmission and isolation. A large nonreciprocal intensity range (NRIR ≈ 3.7) is achieved through parameter tuning under the low pump intensity. Simultaneously, high-performance bidirectional isolation behaviors are realized including an isolation of 71.16 dB with 2.59 dB insertion loss for forward transmission and an isolation of 80.98 dB with 1.62 dB insertion loss for reverse transmission. This design offers a versatility for applications in optical communication, optical switches, and bidirectional nonreciprocal devices.

Bidirectional nonreciprocal transmission in a silicon metasurface with high-Q guided-mode resonances.

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