Lattice Structure and Bandgap Control of 2D GaN Grown on Graphene/Si Heterostructures

2D group‐III nitride materials have shown a great promise for applications in optoelectronic devices thanks to their thickness‐dependent properties. However, the epitaxial growth of 2D group‐III nitrides remains a challenge. In this work, epitaxial growth of 2D GaN with well‐controlled lattice structures and bandgaps is achieved by plasma‐enhanced metal organic chemical vapor deposition via effective regulation of plasma energy and growth temperature. The structure of graphene/2D GaN/Si heterostructures is carefully investigated by high‐resolution transmission electron microscopy. The formation mechanism of the 2D GaN layer is clearly clarified by theoretical calculations. Furthermore, a bandgap for 2D GaN ranging from ≈4.18 to ≈4.65 eV varying with the numbers of layers is theoretically calculated and experimentally confirmed. 2D GaN with well‐controlled lattice structure and bandgap holds great potential for the development of deep ultraviolet light‐emitting diodes, energy conversion devices, etc.     

2D Antimony–Arsenic Alloys

Alloying in group V 2D materials and heterostructures is an effective degree of freedom to tailor and enhance their physical properties. Up to date, black arsenic‐phosphorus is the only 2D group V alloy that has been experimentally achieved by exfoliation, leaving all other possible alloys in the realm of theoretical predictions. Herein, the existence of an additional alloy consisting of 2D antimony arsenide (2D‐As_xSb_1?x) grown by molecular beam epitaxy on group IV semiconductor substrates and graphene is demonstrated. The atomic mixing of As and Sb in the lattice of the grown 2D layers is confirmed by low‐energy electron diffraction, Raman spectroscopy, and X‐ray photoelectron spectroscopy. The As content in 2D‐As_xSb_1?x is shown to depend linearly on the As₄/Sb₄ deposition rate ratio and As concentrations up to 15 at% are reached. The grown 2D alloys are found to be stable in ambient conditions in a timescale of weeks but to oxidize after longer exposure to air. This study lays the groundwork for a better control of the growth and alloying of group V 2D materials, which is critical to study their basic physical properties and integrate them in novel applications.     

Enabling Flexible Heterostructures for Li‐Ion Battery Anodes Based on Nanotube and Liquid‐Phase Exfoliated 2D Gallium Chalcogenide Nanosheet Colloidal Solutions

2D metal chalcogenide (MC) nanosheets (NS) have displayed high capacities as lithium‐ion battery (LiB) anodes. Nevertheless, their complicated synthesis routes coupled with low electronic conductivity greatly limit them as promising LiB electrode material. Here, this work reports a facile single‐walled carbon nanotube (SWCNT) percolating strategy for efficiently maximizing the electrochemical performances of gallium chalcogenide (GaX, X = S or Se). Multiscaled flexible GaX NS/SWCNT heterostructures with abundant voids for Li⁺ diffusion are fabricated by embedding the liquid‐exfoliated GaX NS matrix within a SWCNT‐percolated network; the latter improves the electron transport and ion diffusion kinetics as well as maintains the mechanical flexibility. Consequently, high capacities (i.e., 838 mAh g^?1 per gallium (II) sulfide (GaS) NS/SWCNT mass and 1107 mAh g^?1 per GaS mass; the latter is close to the theoretical value) and good rate capabilities are achieved, which can be majorly attributed to the alloying processes of disordered Ga formed after the first irreversible GaX conversion reaction, as monitored by in situ X‐ray diffraction. The presented approach, colloidal solution processing of SWCNT and liquid‐exfoliated MC NS to produce flexible paper‐based electrode, could be generalized for wearable energy storage devices with promising performances.     

Dynamic contact strain measurement by time‐resolved stroboscopic energy dispersive synchrotron X‐ray diffraction

Recent developments of synchrotron X‐ray sources and dedicated high‐energy beamlines are now enabling strain measurements from large volumes of industrially relevant metallic materials. Such capability is allowing the validation of novel and alternative nondestructive experimental methods of strain measurement or computational models of complex deformation processes. This study describes the first dynamic contact strain measurement of a ball bearing using stroboscopic energy dispersive X‐ray diffraction. The experiment probed the dynamic contact strain in the outer raceway of a test bearing. The inner raceway of the bearing was attached to a shaft rotating at 150 revolutions per minute, and the outer raceway, where the measurements were made, was fixed in a stationary bearing housing. A triggering system was used to synchronise the data acquisition of the energy dispersive X‐ray diffraction detector with the bearing rotation. Specifically, diffraction data were acquired, stroboscopically, from the material volume within the raceway, in a known location, when the ball was positioned directly below it. A total of 20 s of accumulated diffraction signal was recorded, acquiring 2 ms of data per revolution, providing diffraction patterns of sufficient quality for the dynamic contact strain to be measured. Macromechanical stress field was calculated from the micromechanical strains measured from five lattice planes. This allowed a comparison of the experimentally measured stress field and that of finite element simulations. Good agreement was observed between the finite element results and experimental measurements indicating the applicability of this novel dynamic strain measurement technique for tribological systems.     

Efficient and Stable CsPbI3 Solar Cells via Regulating Lattice Distortion with Surface Organic Terminal Groups

All‐inorganic cesium lead iodide perovskites (CsPbI₃) are promising wide‐bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner‐sharing octahedral framework ([PbI₆]^4?). Here, a novel strategy to suppress the octahedral tilting of [PbI₆]^4? units in cubic CsPbI₃ by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X‐ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted‐structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.     

Untersuchung dünner Schichten mittels Röntgenreflektometrie

Thin film characterization by means of X‐ray reflectometry X‐ray reflectometry and diffractometry are widely used non‐destructive methods to characterize thin films in the total thickness range which is typically between 2nm and approximately 500nm. On special arrangements a resolution up to 1000nm layer thickness has been demonstrated. Layer stack morphology, surface topography, layer structure, material density, single layer or period thickness and surface and interface roughness are the typical structural parameters both of single layers and of multilayers which can be described by the measured data. The performance of the measurement setup is mainly influenced by the parameters of the incident X‐ray beam like beam divergence, monochromatism and photon energy. In the following the influence of the optical components in the beam path to angle and energy resolution of X‐ray reflectometry is discussed.     

How Does Graphene Grow? Easy Access to Well‐Ordered Graphene Films

The selective formation of large‐scale graphene layers on a Rh‐YSZ‐Si(111) multilayer substrate by a surface‐induced chemical growth mechanism is investigated using low‐energy electron diffraction, X‐ray photoelectron spectroscopy, X‐ray photoelectron diffraction, and scanning tunneling microscopy. It is shown that well‐ordered graphene layers can be grown using simple and controllable procedures. In addition, temperature‐dependent experiments provide insight into the details of the growth mechanisms. A comparison of different precursors shows that a mobile dicarbon species (e.g., C₂H₂ or C₂) acts as a common intermediate for graphene formation. These new approaches offer scalable methods for the large‐scale production of high‐quality graphene layers on silicon‐based multilayer substrates.     

Anatase Mesoporous TiO2 Nanofibers with High Surface Area for Solid‐State Dye‐Sensitized Solar Cells

Mesoporous nanofibers (NFs) with a high surface area of 112 m²/g have been prepared by electrospinning technique. The structures of mesoporous NFs and regular NFs are characterized and compared through scanning electron microscope (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD) and selected area electron diffraction (SAED) studies. Using mesoporous TiO₂ NFs as the photoelectrode, solid‐state dye‐sensitized solar cells (SDSCs) have been fabricated employing D131 as the sensitizer and P3HT as the hole transporting material to yield an energy conversion efficiency (η) of 1.82%. A J_sc of 3.979 mA cm^?2 is obtained for mesoporous NF‐based devices, which is 3‐fold higher than that (0.973 mA cm^?2) for regular NF‐based devices fabricated under the same condition (η = 0.42%). Incident photon‐to‐current conversion efficiency (IPCE) and dye‐desorption test demonstrate that the increase in J_sc is mainly due to greatly improved dye adsorption for mesoporous NFs as compared to that for regular NFs. In addition, intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) measurements indicate that the mesopores on NF surface have very minor effects on charge transport and collection. Initial aging test proves good stability of the fabricated devices, which indicates the promise of mesoporous NFs as photoelectrode for low‐cost SDSCs.     

Anatomy of Skyrmionic Textures in Magnetic Multilayers

Room temperature magnetic skyrmions in magnetic multilayers are considered as information carriers for future spintronic applications. Currently, a detailed understanding of the skyrmion stabilization mechanisms is still lacking in these systems. To gain more insight, it is first and foremost essential to determine the full real‐space spin configuration. Here, two advanced X‐ray techniques are applied, based on magnetic circular dichroism, to investigate the spin textures of skyrmions in [Ta/CoFeB/MgO]_n multilayers. First, by using ptychography, a high‐resolution diffraction imaging technique, the 2D out‐of‐plane spin profile of skyrmions with a spatial resolution of 10 nm is determined. Second, by performing circular dichroism in resonant elastic X‐ray scattering, it is demonstrated that the chirality of the magnetic structure undergoes a depth‐dependent evolution. This suggests that the skyrmion structure is a complex 3D structure rather than an identical planar texture throughout the layer stack. The analyses of the spin textures confirm the theoretical predictions that the dipole–dipole interactions together with the external magnetic field play an important role in stabilizing sub‐100 nm diameter skyrmions and the hybrid structure of the skyrmion domain wall. This combined X‐ray‐based approach opens the door for in‐depth studies of magnetic skyrmion systems, which allows for precise engineering of optimized skyrmion heterostructures.     

In‐Plane Isotropic/Anisotropic 2D van der Waals Heterostructures for Future Devices

Mono‐ to few‐layers of 2D semiconducting materials have uniquely inherent optical, electronic, and magnetic properties that make them ideal for probing fundamental scientific phenomena up to the 2D quantum limit and exploring their emerging technological applications. This Review focuses on the fundamental optoelectronic studies and potential applications of in‐plane isotropic/anisotropic 2D semiconducting heterostructures. Strong light–matter interaction, reduced dimensionality, and dielectric screening in mono‐ to few‐layers of 2D semiconducting materials result in strong many‐body interactions, leading to the formation of robust quasiparticles such as excitons, trions, and biexcitons. An in‐plane isotropic nature leads to the quasi‐2D particles, whereas, an anisotropic nature leads to quasi‐1D particles. Hence, in‐plane isotropic/anisotropic 2D heterostructures lead to the formation of quasi‐1D/2D particle systems allowing for the manipulation of high binding energy quasi‐1D particle populations for use in a wide variety of applications. This Review emphasizes an exciting 1D–2D particles dynamic in such heterostructures and their potential for high‐performance photoemitters and exciton–polariton lasers. Moreover, their scopes are also broadened in thermoelectricity, piezoelectricity, photostriction, energy storage, hydrogen evolution reactions, and chemical sensor fields. The unique in‐plane isotropic/anisotropic 2D heterostructures may open the possibility of engineering smart devices in the nanodomain with complex opto‐electromechanical functions.     

Laser heat treatment of titanium GRADE 1 with iron‐nickel powder

Titanium alloys thanks to their low density and high mechanical properties are a group of materials that are being used willingly nowadays. A promising method of titanium heat treatment is laser alloying. Process parameters like laser beam power, its traverse speed, amount of alloying elements and shield gas, have influence on the resulting material. Different chemical composition and morphology can be achieved resulting in a change of properties on the surface of the material. The paper presents the investigation of titanium GRADE 1 processed with iron‐nickel powder using laser alloying. The treatment was performed using a high power diode laser. Different laser beam power values were used. Treatment resulted in obtaining good‐adhere, porous‐free, uniform composite material with no cracks. Formation of new phases and solid solution was the reason why it was possible to achieve an increase in microhardness on the surface. Light microscopy, scanning electron microscopy with energy dispersive X‐ray spectroscopy examination, microhardness results and X‐ray diffraction are presented within the paper.     

The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications

Transparent conducting oxides (TCO) have integral and emerging roles in photovoltaic, thermoelectric energy conversion, and more recently, photocatalytic systems. The functional properties of TCOs, and thus their role in these applications, are often mediated by the bulk electronic band structure but are also strongly influenced by the electronic structure of the native surface 2D electron gas (2DEG), particularly under operating conditions. This study investigates the 2DEG, and its response to changes in chemistry, at the (111) surface of the model TCO In₂O₃, through angle resolved and core level X‐ray photoemission spectroscopy. It is found that the itinerant charge carriers of the 2DEG reside in two quantum well subbands penetrating up to 65 Å below the surface. The charge carrier concentration of this 2DEG, and thus the high surface n‐type conductivity, emerges from donor‐type oxygen vacancies of surface character and proves to be remarkably robust against surface absorbents and contamination. The optical transparency, however, may rely on the presence of ubiquitous surface adsorbed oxygen groups and hydrogen defect states that passivate localized oxygen vacancy states in the bandgap of In₂O₃.     

Indium Nitride at the 2D Limit

The properties of 2D InN are predicted to substantially differ from the bulk crystal. The predicted appealing properties relate to strong in‐ and out‐of‐plane excitons, high electron mobility, efficient strain engineering of their electronic and optical properties, and strong application potential in gas sensing. Until now, the realization of 2D InN remained elusive. In this work, the formation of 2D InN and measurements of its bandgap are reported. Bilayer InN is formed between graphene and SiC by an intercalation process in metal–organic chemical vapor deposition (MOCVD). The thickness uniformity of the intercalated structure is investigated by conductive atomic force microscopy (C‐AFM) and the structural properties by atomic resolution transmission electron microscopy (TEM). The coverage of the SiC surface is very high, above 90%, and a major part of the intercalated structure is represented by two sub‐layers of indium (In) bonded to nitrogen (N). Scanning tunneling spectroscopy (STS) measurements give a bandgap value of 2 ± 0.1 eV for the 2D InN. The stabilization of 2D InN with a pragmatic wide bandgap and high lateral uniformity of intercalation is demonstrated.     

Intercalation of Layered Materials from Bulk to 2D

Intercalation in few‐layer (2D) materials is a rapidly growing area of research to develop next‐generation energy‐storage and optoelectronic devices, including batteries, sensors, transistors, and electrically tunable displays. Identifying fundamental differences between intercalation in bulk and 2D materials will play a key role in developing functional devices. Herein, advances in few‐layer intercalation are addressed in the historical context of bulk intercalation. First, synthesis methods and structural properties are discussed, emphasizing electrochemical techniques, the mechanism of intercalation, and the formation of a solid‐electrolyte interphase. To address fundamental differences between bulk and 2D materials, scaling relationships describe how intercalation kinetics, structure, and electronic and optical properties depend on material thickness and lateral dimension. Here, diffusion rates, pseudocapacity, limits of staging, and electronic structure are compared for bulk and 2D materials. Next, the optoelectronic properties are summarized, focusing on charge transfer, conductivity, and electronic structure. For energy devices, opportunities also emerge to design van der Waals heterostructures with high capacities and excellent cycling performance. Initial studies of heterostructured electrodes are compared to state‐of‐the‐art battery materials. Finally, challenges and opportunities are presented for 2D materials in energy and optoelectronic applications, along with promising research directions in synthesis and characterization to engineer 2D materials for superior devices.     

3D Nanoprinted Plastic Kinoform X‐Ray Optics

High‐performance focusing of X‐rays requires the realization of very challenging 3D geometries with nanoscale features, sub‐millimeter‐scale apertures, and high aspect ratios. A particularly difficult structure is the profile of an ideal zone plate called a kinoform, which is manufactured in nonideal approximated patterns, nonetheless requires complicated multistep fabrication processes. Here, 3D fabrication of high‐performance kinoforms with unprecedented aspect ratios out of low‐loss plastics using femtosecond two‐photon 3D nanoprinting is presented. A thorough characterization of the 3D‐printed kinoforms using direct soft X‐ray imaging and ptychography demonstrates superior performance with an efficiency reaching up to 20%. An extended concept is proposed for on‐chip integration of various X‐ray optics toward high‐fidelity control of X‐ray wavefronts and ultimate efficiencies even for harder X‐rays. Initial results establish new, advanced focusing optics for both synchrotron and laboratory sources for a large variety of X‐ray techniques and applications ranging from materials science to medicine.     

Site‐Selective and van der Waals Epitaxial Growth of Rhenium Disulfide on Graphene

The surface property of growth substrate imposes significant influence in the growth behaviors of 2D materials. Rhenium disulfide (ReS₂) is a new family of 2D transition metal dichalcogenides with unique distorted 1T crystal structure and thickness‐independent direct bandgap. The role of growth substrate is more critical for ReS₂ owing to its weak interlayer coupling property, which leads to preferred growth along the out‐of‐plane direction while suppressing the uniform in‐plane growth. Herein, graphene is introduced as the growth substrate for ReS₂ and the synthesis of graphene/ReS₂ vertical heterostructure is demonstrated via chemical vapor deposition. Compared with the rough surface of SiO₂/Si substrate with dangling bonds which hinders the uniform growth of ReS₂, the inert and smooth surface nature of graphene sheet provides a lower energy barrier for migration of the adatoms, thereby promoting the growth of ReS₂ on the graphene surface along the in‐plane direction. Furthermore, patterning of the graphene/ReS₂ heterostructure is achieved by the selective growth of ReS₂, which is attributed to the strong binding energy between sulfur atoms and graphene surface. The fundamental studies in the role of graphene as the growth template in the formation of van der Waals heterostructures provide better insights into the synthesis of 2D heterostructures.     

Materialbeanspruchungsanalyse und ihre Anwendung auf Prüfstandsversuche zum Oberflächenausfall (Nierlich‐Schadensmodus) von Wälzlagern

Material Response Analysis and its Application to Rig Tests for the Surface Failure (Nierlich Damage Mode) of Rolling Bearings The material response analysis according to Nierlich using X‐ray diffraction represents an important physical examination technique for the evaluation of material stressing and the lifetime estimation of rolling bearings and other highly loaded machine parts. The method is presented and employed for the evaluation of automobile gearbox rig tests. The extensively described damage modes of the practically predominating surface and the classical sub‐surface failure of rolling bearings can be distinguished that way. In gearboxes, lubricating oil contaminated by metal abrasion of the cogwheels usually appears. Penetrating foreign particles produce indentations at the ring raceways and rolling elements of the rolling bearings, which promotes surface fatigue. The results of the X‐ray diffraction measurements confirm this damage mode. Evaluation of the occurred material stressing permits a more detailed characterization of the surface failure of rolling bearings.     

2D Materials Decorated with Ultrathin and Porous Graphene Oxide for High Stability and Selective Surface Activity

2D black phosphorus (BP) and MXenes have triggered enormous research interest in catalysis, energy storage, and chemical sensing. Unfortunately, the low stability of these materials under practical operating conditions remains a critical bottleneck, particularly as they are prone to oxidization under moisture. In this work, the design and application of stable 2D heterostructures obtained from decorating BP and MXene (Ti₃C₂T_x) with few-layer holey graphene oxide (FHGO) membranes are presented. In the resulting heterostructured systems, FHGO serves as a multifunctional passivation layer that shields BP or MXene from oxidative degradation, while allowing the selective diffusion of target gas molecules through its micropores and toward the underlying 2D material. Through a case study of dilute NO₂ sensing, it is demonstrated that these heterostructures show a greatly enhanced sensing performance under humid conditions, where fast sensing speed and response are consistently observed, and high stability is impressively retained upon repetitive sensing cycles for 1000 min. These results corroborate the efficacy of material decoration with porous FHGO membranes and suggest that this is a generalizable strategy for reliable high-performance applications of 2D materials.     

Heterostructures Based on 2D Materials: A Versatile Platform for Efficient Catalysis

The unique structural and electronic properties of 2D materials, including the metal and metal‐free ones, have prompted intense exploration in the search for new catalysts. The construction of different heterostructures based on 2D materials offers great opportunities for boosting the catalytic activity in electo(photo)chemical reactions. Particularly, the merits resulting from the synergism of the constituent components and the fascinating properties at the interface are tremendously interesting. This scenario has now become the state‐of‐the‐art point in the development of active catalysts for assisting energy conversion reactions including water splitting and CO₂ reduction. Here, starting from the theoretical background of the fundamental concepts, the progressive developments in the design and applications of heterostructures based on 2D materials are traced. Furthermore, a personal perspective on the exploration of 2D heterostructures for further potential application in catalysis is offered.     

Eigenspannungsmessungen an oberflächenbearbeiteten Proben aus einem Gamma‐Titanaluminidwerkstoff

Residual Stress Measurements on Machined Specimens of a Gamma Titanium Aluminide Material Residual stress measurements by x‐ray diffraction and hole drilling were carried out on specimens made of the commercially available gamma titanium aluminide alloy ABB‐2. The specimens had been ground and milled by three various methods in each case. Evaluation of X‐ray measurements displayed significantly different stress profiles in the surface layers and enabled qualitative ratings of the applied machining techniques. Evaluation of the hole drilling results showed agreements as well as discrepancies with the X‐ray findings. Determination of X‐ray elastic constants indicated similar values as obtained in a previous investigation on a different gamma titanium aluminide alloy.