Aluminum scandium nitride on 8-inch Si wafers: material characterization and photonic device demonstration.

The anisotropic optical properties of aluminum scandium nitride (Al1-xScxN) thin films for both ordinary and extraordinary light are investigated. A quantitative analysis of the band structures of the wurtzite Al1-xScxN is carried out. In addition, Al1-xScxN photonic waveguides and bends are fabricated on 8-inch Si substrates. With x = 0.087 and 0.181, the light propagation losses are 5.98 ± 0.11 dB/cm and 8.23 ± 0.39 dB/cm, and the 90° bending losses are 0.05 dB/turn and 0.08 dB/turn at 1550 nm wavelength, respectively.

The evolution of atomistic structure and mechanical property of coke in the gasification process with CO2 and H2O at different temperatures: A ReaxFF molecular dynamics study.

CONTEXT: An atomistic coke carbon model was constructed to simulate the structural evolution in the gasification and stretching process. The coke model was placed in a box with different CO2/H2O content to investigate the evolution of the atomistic structure of coke during the gasification. It was found that different atmospheric concentrations had different effects on the structure and reaction sites of the coke model. The CO2 molecules tended to dissolve on the surface of coke and disrupt its surface structure, while H2O molecules were more likely to enter the coke model to disrupt the internal structure. For tensile simulation, it was found that CO2 and H2O had different effects on the tensile resistance of the coke model. Controlling the composition content of the reaction gas can effectively influence the tensile strength of the coke model. By revealing the behavior of coke model at the micro scale, it provides a theoretical basis for the industrial coke application process. METHODS: Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) was used to conduct the molecular dynamics using the reactive force field (ReaxFF). The atomistic model of coke carbon was constructed using the well-known annealing and quenching method, and its composition is determined according to the element analysis of industrial coke. The structural evolution in the gasification with CO2/H2O and the stretching process were analyzed in detail. Molecular dynamics simulations with reactive force field (ReaxFF-MD) were used to simulate the coke dissolution reaction under CO2/H2O atmosphere and the coke stretching process. The atmosphere ratio was modified to investigate the changes in coke structure under different atmosphere conditions. The Packmol software was used to place gas and coke models into the same box. During the reaction process, the Ovito software was used to perform corresponding visualization analysis on the changes in the atomic structure of coke.

Effect of Different sp2 Bond Contents on the Reactivity and Mechanical Properties of Coke Carbon: A ReaxFF Molecular Dynamics Study.

In this study, ReaxFF-MD was used to construct a large-molecule model of coke containing 3000 atoms, and the sp2 bond content of the model was controlled by changing the heating and cooling rates. The increase of the sp2 bond content led to a significant difference in the reactivity of coke. The presence of the sp2 bond caused the carbon atoms inside the coke to change into a circular structure, making it more difficult for the gaseous atoms to adsorb on the surface of the coke. It significantly reduced the gasification reaction rate of coke in the CO2 and H2O atmospheres. In the tensile simulation experiment, it was found that the stretching process of coke was mainly divided into three stages: an elastic stretching stage, a plastic stretching stage, and a model fracture stage. During the stretching process, the carbon ring structure would undergo a C-C bond fracture while generating carbon chains to resist stress. The results indicated that the presence of sp2 bonds can effectively reduce the phenomenon of excessive local stress on coke to improve its tensile resistance. The method developed in this paper may provide further ideas and platforms for the research on coke performance.

Water nanolayer facilitated solitary-wave-like blisters in MoS2 thin films.

Solitary waves are unique in nonlinear systems, but their formation and propagation in the nonlinear fluid-structure interactions have yet to be further explored. As a typical nonlinear system, the buckling of solid thin films is fundamentally related to the film-substrate interface that is further vulnerable to environments, especially when fluids exist. In this work, we report an anomalous, solitary-wave-like blister (SWLB) mode of MoS2 thin films in a humid environment. Unlike the most common telephone-cord and web buckling deformation, the SWLB propagates forward like solitary waves that usually appear in fluids and exhibits three-dimensional expansions of the profiles during propagation. In situ mechanical, optical, and topology measurements verify the existence of an interfacial water nanolayer, which facilitates a delamination of films at the front side of the SWLB and a readhesion at the tail side owing to the water nanolayer-induced fluid-structure interaction. Furthermore, the expansion morphologies and process of the SWLB are predicted by our theoretical model based on the energy change of buckle propagation. Our work not only demonstrates the emerging SWLB mode in a solid material but also sheds light on the significance of interfacial water nanolayers to structural deformation and functional applications of thin films.

Unconventional and Dynamically Anisotropic Thermal Conductivity in Compressed Flexible Graphene Foams.

Although a variety of methods to predict the effective thermal conductivity of porous foams have been proposed, the response of such materials under dynamic compressive loading has generally not been considered. Understanding the dynamic thermal behavior will widen the potential applications of porous foams and provide insights into methods of modifying material properties to achieve desired performance. Previous experimental work on the thermal conductivity of a flexible graphene composite under compression showed intriguing behavior: the cross-plane thermal conductivity remained approximately constant with increasing compression, despite the increasing mass density. In this work, we use molecular dynamics (MD) simulations and finite element analysis to study the variation in both the cross-plane and in-plane thermal conductivities by compressing isotropic graphene foams. We have found that, interestingly, the cross-plane thermal conductivity decreases with compression while the in-plane thermal conductivity increases; hence, the dynamic thermal transport of the graphene foam becomes anisotropic with a significant anisotropy ratio. Such observations cannot be explained by the conventional effective medium theory, which describes the increase of thermal conductivity to be proportional to mass density. Thus, we propose a model that can describe such anisotropic effective thermal conductivity of highly porous open-cell media during compression. The model is validated against the MD simulations as well as a larger-scale finite element simulation of an open-cell foam geometry.

Humidity-Controlled Dynamic Engineering of Buckling Dimensionality in MoS2 Thin Films.

Dynamic engineering of buckling deformation is of vital importance as it provides multiphase modulation of thin film devices. In particular, dynamic switch of buckles between one-dimensional (1D) and two-dimensional (2D) configurations in a single film system on rigid substrates is intriguing but very challenging. The current approach to changing buckling configuration is mainly achieved by varying the built-in stress at the film-substrate interface, but it is difficult to realize dynamic engineering on rigid substrates. Herein, we report a dynamic engineering of buckling deformation in MoS2 thin films by humidity-tuned interfacial adhesion. With the change of humidity, the MoS2 thin films deform from 1D telephone-cord buckles to 2D web-like buckles due to the hydrophilic nature of both MoS2 and substrate. Such 1D-to-2D evolution of buckles is attributed to the weakened interfacial adhesion of mixed deformation modes induced by humidity, which is verified by finite-element modeling. These buckled films further find potential applications as patterned templates for liquid condensation and sensing units for tactile sensors. Our work not only demonstrates the humidity-controlled dimensionality engineering of buckles in MoS2 thin films but also sheds light on the functional applications of buckled films based on their profile features.

Wide range continuously tunable and fast thermal switching based on compressible graphene composite foams.

Thermal switches have gained intense interest recently for enabling dynamic thermal management of electronic devices and batteries that need to function at dramatically varied ambient or operating conditions. However, current approaches have limitations such as the lack of continuous tunability, low switching ratio, low speed, and not being scalable. Here, a continuously tunable, wide-range, and fast thermal switching approach is proposed and demonstrated using compressible graphene composite foams. Large (~8x) continuous tuning of the thermal resistance is achieved from the uncompressed to the fully compressed state. Environmental chamber experiments show that our variable thermal resistor can precisely stabilize the operating temperature of a heat generating device while the ambient temperature varies continuously by ~10 °C or the heat generation rate varies by a factor of 2.7. This thermal device is promising for dynamic control of operating temperatures in battery thermal management, space conditioning, vehicle thermal comfort, and thermal energy storage.

Structural Defects, Mechanical Behaviors, and Properties of Two-Dimensional Materials.

Since the success of monolayer graphene exfoliation, two-dimensional (2D) materials have been extensively studied due to their unique structures and unprecedented properties. Among these fascinating studies, the most predominant focus has been on their atomic structures, defects, and mechanical behaviors and properties, which serve as the basis for the practical applications of 2D materials. In this review, we first highlight the atomic structures of various 2D materials and the structural and energy features of some common defects. We then summarize the recent advances made in experimental, computational, and theoretical studies on the mechanical properties and behaviors of 2D materials. We mainly emphasized the underlying deformation and fracture mechanisms and the influences of various defects on mechanical behaviors and properties, which boost the emergence and development of topological design and defect engineering. We also further introduce the piezoelectric and flexoelectric behaviors of specific 2D materials to address the coupling between mechanical and electronic properties in 2D materials and the interactions between 2D crystals and substrates or between different 2D monolayers in heterostructures. Finally, we provide a perspective and outlook for future studies on the mechanical behaviors and properties of 2D materials.

Watching Dynamic Self-Assembly of Web Buckles in Strained MoS2 Thin Films.

Thin films with large compressive residual stress and low interface adhesion can buckle and delaminate from relatively rigid substrates, which is a common failure mode of film/substrate interfaces. Current studies mainly focused on the geometry of various buckling patterns and related physical origins based on a static point of view. However, fundamental understanding of dynamic propagation of buckles, particularly for the complicated web buckles, remains challenging. We adopt strained two-dimensional MoS2 thin films to study the phenomenon of web buckling because their interface adhesion, namely van der Waals interaction, is naturally low. With a delicately site-controlled initiation, web buckles can be triggered and their dynamic propagation is in situ observed facilely. Finite element modeling shows that the formation of web buckles involves the propagation and multilevel branching of telephone-cord blisters. These buckled semiconducting films can be patterned by spatial confinement and potentially used in diffuse-reflective coatings, microfluidic channels, and hydrogen evolution reaction electrodes. Our work not only reveals the hidden mechanisms and kinematics of propagation of web buckles on rigid substrates but also sheds light on the development of semiconducting devices based on buckling engineering.