A hidden phase uncovered by ultrafast carrier dynamics in thin Bi2O2Se.

Bi2O2Se has attracted intensive attention due to its potential in electronics, optoelectronics, and ferroelectric applications. Despite that, there have only been a handful of experimental studies based on ultrafast spectroscopy to elucidate the carrier dynamics in Bi2O2Se thin films. Besides, different groups have reported various ultrafast timescales and associated mechanisms across films of different thicknesses. A comprehensive understanding in relation to thickness and fluence is still lacking. In this work, we have systematically explored the thickness-dependent Raman spectroscopy and ultrafast carrier dynamics in chemical vapor deposition (CVD)-grown Bi2O2Se thin films on a mica substrate with thicknesses varying from 22.44 nm down to 4.62 nm in both low and high pump fluence regions. Combining the thickness dependence and fluence dependence of the slow decay time, we demonstrate a hidden photoinduced ferroelectric transition in the thinner (<8 nm) Bi2O2Se films below the material damage thresholds, influenced by substrate-induced compressive strain and far-from-equilibrium excitation. Moreover, this transition can be manifested at high electronic excitation densities. Our results deepen the understanding of the interplay between the ferroelectric phase and semiconducting characteristics of Bi2O2Se thin films, offering potential applications in optoelectronic devices that benefit from the ferroelectric transition.

Terminal Trajectory Planning for Synthetic Aperture Radar Imaging Guidance Based on Chronological Iterative Search Framework.

Synthetic aperture radar (SAR) is capable of obtaining the high-resolution 2-D image of the interested target scene, which enables advanced remote sensing and military applications, such as missile terminal guidance. In this article, the terminal trajectory planning for SAR imaging guidance is first investigated. It is found that the guidance performance of an attack platform is determined by the adopted terminal trajectory. Therefore, the aim of the terminal trajectory planning is to generate a set of feasible flight paths to guide the attack platform toward the target and meanwhile obtain the optimized SAR imaging performance for enhanced guidance precision. The trajectory planning is then modeled as a constrained multiobjective optimization problem given a high-dimensional search space, where the trajectory control and SAR imaging performance are comprehensively considered. By utilizing the temporal-order-dependent property of the trajectory planning problem, a chronological iterative search framework (CISF) is proposed. The problem is decomposed into a series of subproblems, where the search space, objective functions, and constraints are reformulated in chronological order. The difficulty of solving the trajectory planning problem is thus significantly alleviated. Then, the search strategy of CISF is devised to solve the subproblems successively. The optimization results of the preceding subproblem can be utilized as the initial input of the subsequent subproblems to enhance the convergence and search performance. Finally, a trajectory planning method is put forward based on CISF. Experimental studies demonstrate the effectiveness and superiority of the proposed CISF compared with the state-of-the-art multiobjective evolutionary methods. The proposed trajectory planning method can generate a set of feasible terminal trajectories with optimized mission performance.

Optical Effect Modulation in Polarized Raman Spectroscopy of Transparent Layered α-MoO3.

Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO3 as an excellent platform. It is found that there are several anomalous ARPRS responses in α-MoO3 that cannot be reproduced by the real Raman tensor widely used in non-absorbing materials; however, they can be well explained by considering the birefringence-induced Raman selection rules. Moreover, the systematic thickness-dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α-MoO3 is analyzed, giving the physical origins of the Raman anisotropy. This study employs α-MoO3 as an example, although it is generally applicable to all transparent birefringent ALMs.

Delamination of MoS2/SiO2 interfaces under nanoindentation.

Molybdenum disulphide (MoS2) mounted on silicon dioxide (SiO2) constitutes the fundamental functional components of many nanodevices, but its mechanical properties, which are crucial for the device design and fabrication, remain almost unexplored. Here, the mechanical properties of the multilayer MoS2/SiO2 system are investigated via nanoindentation experiments and molecular dynamics simulations. In terms of the mechanical properties, a comparative study of MoS2/SiO2 and graphene/SiO2 systems is presented. The MoS2/SiO2 and graphene/SiO2 systems are found to possess comparable Young's modulus and hardness values, but their mechanical responses and failure modes under indentation are totally different. Interface delamination failure accompanied by ring-like through-thickness cracking is observed in the MoS2/SiO2 system with a relatively thin MoS2 layer, while no interface separation is found in indentation experiments for the graphene/SiO2 system using the same layer thickness. The different failure modes observed between the MoS2/SiO2 and graphene/SiO2 systems can be attributed to the comparable interface adhesion energy but very different bending stiffness values of the MoS2 and graphene components. Specifically, compared with graphene, the larger bending stiffness of MoS2 means that a larger bending force is experienced in the indentation process, overcoming the adhesion of the MoS2/SiO2 interface, which makes interface delamination much easier in the MoS2/SiO2 system.

Unambiguous determination of crystal orientation in black phosphorus by angle-resolved polarized Raman spectroscopy.

Angle-resolved polarized Raman spectroscopy (ARPRS) is widely used to determine the crystal orientations of anisotropic layered materials (ALMs), which is an essential step to study all of their anisotropic properties. However, the understanding of the ARPRS response of black phosphorous (BP) as a most widely studied ALM is still unsatisfactory. Here, we clarify two key controversies about the physical origin of the intricate ARPRS response and the determination of crystal orientations in BP. Through systematic ARPRS measurements, we show that the degree of anisotropy of the response evolves gradually and periodically with the BP thickness, eventually leading to the intricate response. Meanwhile, we find that using the Raman peak intensity ratio of the two Ag phonon modes, the crystal orientations of BP can be unambiguously distinguished via a concise inequality . Comprehensive analysis and first-principles calculations reveal that the external anisotropic interference effect and the intrinsic electron-phonon coupling are responsible for the observations.

Controlled Growth of Large-Sized and Phase-Selectivity 2D GaTe Crystals.

GaTe has recently attracted significant interest due to its direct bandgap and unique phase structure, which makes it a good candidate for optoelectronics. However, the controllable growth of large-sized monolayer and few-layer GaTe with tunable phase structures remains a great challenge. Here the controlled growth of large-sized GaTe with high quality, chemical uniformity, and good reproducibility is achieved through liquid-metal-assisted chemical vapor deposition method. By using liquid Ga, the rapid growth of 2D GaTe flakes with high phase-selectivity can be obtained due to its reduced reaction temperature. In addition, the method is used to synthesize many Ga-based 2D materials and their alloys, showing good universality. Raman spectra suggest that the as-grown GaTe own a relatively weak van der Waals interaction, where monoclinic GaTe displays highly-anisotropic optical properties. Furthermore, a p-n junction photodetector is fabricated using GaTe as a p-type semiconductor and 2D MoSe2 as a typical n-type semiconductor. The GaTe/MoSe2 heterostructure photodetector exhibits large photoresponsivity of 671.52 A W-1 and high photo-detectivity of 1.48 × 1010 Jones under illumination, owing to the enhanced light absorption and good quality of as-grown GaTe. These results indicate that 2D GaTe is a promising candidate for electronic and photoelectronic devices.

Effects of interlayer interactions on the nanoindentation response of freely suspended multilayer gallium telluride.

Freestanding indentation is a widely used method to characterise the elastic properties of two-dimensional (2D) materials. However, many controversies and confusion remain in this field due to the lack of appropriate theoretical models in describing the indentation responses of 2D materials. Taking the multilayer gallium telluride (GaTe) as an example, in this paper we conduct a series of experiments and simulations to achieve a comprehensive understanding of its freestanding indentation behaviours. Specifically, the freestanding indentation experiments show that the elastic properties of the present multilayer GaTe with a relatively large thickness can only be extracted from the bending stage in the indentation process rather than the stretching stage widely utilised in the previous studies on thin 2D materials, since the stretching stage of thick 2D materials is inevitably accompanied with severe plastic deformations. In combination with existing continuum mechanical models and finite element simulations, an extremely small Young's modulus of multilayer GaTe is obtained from the nanoindentation experiments, which is two orders of magnitude smaller than the value obtained from first principles calculations. Our molecular dynamics (MD) simulations reveal that this small Young's modulus can be attributed to the significant elastic softening in the multilayer GaTe with increasing thickness and decreasing length. It is further revealed in MD simulations that this size-induced elastic softening originates from the synergistic effects of interlayer compression and interlayer shearing in the multilayer GaTe, both of which, however, are ignored in the existing indentation models. To consider these effects of interlayer interactions in the theoretical modelling of the freestanding indentation of multilayer GaTe, we propose here novel multiple-beam and multiple-plate models, which are found to agree well with MD results without any additional parameters fitting and thus can be treated as more precise theoretical models in characterising the freestanding indentation behaviours of 2D materials.

Barrier-Layer Optimization for Enhanced GaN-on-Diamond Device Cooling.

GaN-on-diamond device cooling can be enhanced by reducing the effective thermal boundary resistance (TBReff) of the GaN/diamond interface. The thermal properties of this interface and of the polycrystalline diamond grown onto GaN using SiN and AlN barrier layers as well as without any barrier layer under different growth conditions are investigated and systematically compared for the first time. TBReff values are correlated with transmission electron microscopy analysis, showing that the lowest reported TBReff (∼6.5 m2 K/GW) is obtained by using ultrathin SiN barrier layers with a smooth interface formed, whereas the direct growth of diamond onto GaN results in one to two orders of magnitude higher TBReff due to the formation of a rough interface. AlN barrier layers can produce a TBReff as low as SiN barrier layers in some cases; however, their TBReff are rather dependent on growth conditions. We also observe a decreasing diamond thermal resistance with increasing growth temperature.

Coherent control of GHz resonant modes by an integrated acoustic etalon.

By carefully tuning the thickness of a compliant thin film placed within an acoustic cavity, we achieve coherent control of the cavity's acoustic resonances, analogous to the operation of an optical etalon. This technique is demonstrated using a supported membrane oscillator in which multiple high-frequency harmonic resonances are simultaneously optoexcited by an ultrafast laser. Theoretical and computational methods are used to analyze the selective strengthening or suppression of these resonances by constructive or destructive interference.