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Although high-entropy alloys (HEAs) have shown tremendous potential for elevated temperature, anticorrosion, and catalysis applications, little is known on how HEA materials behave under complex service environments. Herein, we studied the high-temperature oxidation behavior of Fe0.28Co0.21Ni0.20Cu0.08Pt0.23HEA nanoparticles (NPs) in an atmospheric pressure dry air environment by in situgas-cell transmission electron microscopy. It is found that the oxidation of HEA NPs is governed by Kirkendall effects with logarithmic oxidation rates rather than parabolic as predicted by Wagner’s theory. Further, the HEA NPs are found to oxidize at a significantly slower rate compared to monometallic NPs. The outward diffusion of transition metals and formation of disordered oxide layer are observed in real time and confirmed through analytical energy dispersive spectroscopy, and electron energy loss spectroscopy characterizations. Localized ordered lattices are identified in the oxide, suggesting the formation of Fe2O3, CoO, NiO, and CuO crystallites in an overall disordered matrix. Hybrid Monte Carlo and molecular dynamics simulations based on first-principles energies and forces support these findings and show that the oxidation drives surface segregation of Fe, Co, Ni, and Cu, while Pt stays in the core region. The present work offers key insights into how HEA NPs behave under high-temperature oxidizing environment and sheds light on future design of highly stable alloys under complex service conditions.

Structural defects and heterogeneities play an enormous role in the formation of localized hot spots in 2D materials used in a wide range of applications from electronics to energy systems. In this report, we employ scanning thermal microscopy (SThM) to spatially map the temperature rise across various defects and heterogeneities of titanium carbide (Ti3C2Tx; T stands for surface terminations) MXene nanostructures under high electrical bias with sub-50 mK temperature resolution and sub-100 nm spatial resolution. We investigated several Ti3C2Txflakes having different thicknesses as well as heterogeneous MXene structures incorporating line defects or vertical heterojunctions. High-resolution temperature rise maps allow us to identify localized hot spots and to quantify the nonuniformity of the temperature fields across various morphological features. The results show that the local heating is most severe in vertical junctions of MXene flakes and is highly affected by nonuniform conduction due to the presence of line defects. These results provide a direct insight into the power dissipation of MXene-based devices and the roles of various heterogeneities that are inherent to the material synthesis process. This study provides a guideline for how a better understanding of the structure–property–processing correlations and further optimization of the synthesis routes could improve the lifetime, safety, and operation limits of the MXene-based devices.

Monolayers of transition metal dichalcogenides (TMDs) are attractive for various modern semiconductor devices. However, the limited control over the location, yield, and size distribution of the products using current synthesis methods has severely limited their large-scale applicability. Herein, we identify the ability to use metal (e.g., Au) nanoparticles to seed the growth of MoS2monolayers and thereby provide a means to achieve programmable and controllable synthesis. In this study, prepatterned Au seeds are used as heterogeneous nucleation sites to induce the formation of desired geometries of MoS2monolayers viachemical vapor deposition. Our experimental and theoretical results shed light on the growth mechanism driving the formation of MoS2monolayers at these sites, revealing that the seeding effect originates from the favorable formation energy of MoS2on the Au surface. A field-effect transistor with a predesigned channel geometry exhibits electronic performance that compares nicely with previously reported MoS2monolayer devices. We believe this study contributes fundamental insights into controlled synthesis of TMD monolayers, making integration of these materials into emerging electronic devices more attainable.

The crystallographic orientation of battery electrode materials can significantly impact electrochemical performance, such as rate capability and cycling stability. Among the layered transition metal dichalcogenides, rhenium disulfide (ReS2) has the largest anisotropic ratio between the two main axes in addition to exceptionally weak interlayer coupling, which serves as an ideal system to observe and analyze anisotropy of electrochemical phenomena. Here, we report anisotropic lithiation and sodiation of exfoliated ReS2at atomic resolution using in situtransmission electron microscopy. These results reveal the role of crystallographic orientation and anisotropy on battery electrode electrochemistry. Complemented with density functional theory calculations, the lithiation of ReS2is found to begin with intercalation of Li-ions, followed by a conversion reaction that results in Re nanoparticles and Li2S nanocrystals. The reaction speed is highly anisotropic, occurring faster along the in-plane ReS2layer than along the out-of-plane direction. Sodiation of ReS2is found to proceed similarly to lithiation, although the intercalation step is relatively quicker. Furthermore, the microstructure and morphology of the reaction products after lithiation/sodiation show clear anisotropy along the in-plane and out-of-plane directions. These results suggest that crystallographic orientation in highly anisotropic electrode materials can be exploited as a design parameter to improve battery electrochemical performance.

Graphene oxide decorated with oxygen functional groups is a promising candidate as an active layer in resistive switching devices due to its controllable physical-chemical properties, high flexibility, and transparency. However, the origin of conductive channels and their growth dynamics remain a major challenge. We use in situtransmission electron microscopy techniques to demonstrate that nanoscale graphene oxide sheets bonded with oxygen dynamically change their physical and chemical structures upon an applied electric field. Artificially engineered bilayer reduced graphene oxide films with asymmetric oxygen content exhibit nonvolatile write-once-read-many memory behaviors without experiencing the bubble destruction due to the efficient migration of oxygen ions. We clearly observe that a conductive graphitic channel with a conical shape evolves from the upper oxygen-rich region to the lower oxygen-poor region. These findings provide fundamental guidance for understanding the oxygen motions of oxygen-containing carbon materials for future carbon-based nanoelectronics.

Macrophages are key immune cells that perform various physiological functions, such as the maintenance of homeostasis, host defense, disease progression, and tissue regeneration. Macrophages adopt distinctly polarized phenotypes, such as pro-inflammatory M1 phenotype or anti-inflammatory (pro-healing) M2 phenotype, to execute disparate functions. The remotely controlled reversible uncaging of bioactive ligands, such as Arg-Gly-Asp (RGD) peptide, is an appealing approach for temporally regulating the adhesion and resultant polarization of macrophages on implants in vivo. Here, we utilize physical and reversible uncaging of RGD by a magnetic field that allows facile tissue penetration. We first conjugated a RGD-bearing gold nanoparticle (GNP) to the substrate and then a magnetic nanocage (MNC) to the GNP viaa flexible linker to form the heterodimeric nanostructure. We magnetically manipulated nanoscale displacement of MNC and thus its proximity to the GNP to reversibly uncage and cage RGD. The uncaging of RGD temporally promoted the adhesion and subsequent M2 polarization of macrophages while inhibiting their M1 polarization both in vitroand in vivo. The RGD uncaging-mediated adhesion and M2 polarization of macrophages involved rho-associated protein kinase signaling. This study demonstrates physical and reversible uncaging of RGD to regulate the adhesion and polarization of host macrophages in vivo. This approach of magnetically regulating the heterodimer conformation for physical and reversible uncaging of RGD offers the promising potential to manipulate inflammatory or tissue-regenerative immune responses to the implants in vivo.

Integrating plasmonic materials into semiconductor media provides a promising approach for applications such as photosensing and solar energy conversion. The resulting structures introduce enhanced light–matter interactions, additional charge trap states, and efficient charge-transfer pathways for light-harvesting devices, especially when an intimate interface is built between the plasmonic nanostructure and semiconductor. Herein, we report the development of plasmonic photodetectors using Au@MoS2heterostructuresan Au nanoparticle core that is encapsulated by a CVD-grown multilayer MoS2shell, which perfectly realizes the intimate and direct interfacing of Au and MoS2. We explored their favorable applications in different types of photosensing devices. The first involves the development of a large-area interdigitated field-effect phototransistor, which shows a photoresponsivity ∼10 times higher than that of planar MoS2transistors. The other type of device geometry is a Si-supported Au@MoS2heterojunction gateless photodiode. We demonstrated its superior photoresponse and recovery ability, with a photoresponsivity as high as 22.3 A/W, which is beyond the most distinguished values of previously reported similar gateless photodetectors. The improvement of photosensing performance can be a combined result of multiple factors, including enhanced light absorption, creation of more trap states, and, possibly, the formation of interfacial charge-transfer transition, benefiting from the intimate connection of Au and MoS2.

Recently, Ag–Ag2S hybrid nanostructures have attracted a great deal of attention due to their enhanced chemical and thermal stability, in addition to their morphology- and composition-dependent tunable local surface plasmon resonances. Although Ag–Ag2S nanostructures can be synthesized viasulfidation of as-prepared anisotropic Ag nanoparticles, this process is poorly understood, often leading to materials with anomalous compositions, sizes, and shapes and, consequently, optical properties. In this work, we use theory and experiment to investigate the structural and plasmonic evolution of Ag–Ag2S nanoprisms during the sulfidation of Ag precursors. The previously observed red-shifted extinction of the Ag–Ag2S hybrid nanoprism as sulfidation occurs contradicts theoretical predictions, indicating that the reaction does not just occur at the prism tips as previously speculated. Our experiments show that sulfidation can induce either blue or red shifts in the extinction of the dipole plasmon mode, depending on reaction conditions. By elucidating the correlation with the final structure and morphology of the synthesized Ag–Ag2S nanoprisms, we find that, depending on the reaction conditions, sulfidation occurs on the prism tips and/or the (111) surfaces, leading to a core(Ag)–anisotropic shell(Ag2S) prism nanostructure. Additionally, we demonstrate that the direction of the shift in the dipole plasmon is a function of the relative amounts of Ag2S at the prism tips and Ag2S shell thickness around the prism.

Field effect transistors using ultrathin molybdenum disulfide (MoS2) have recently been experimentally demonstrated, which show promising potential for advanced electronics. However, large variations like hysteresis, presumably due to extrinsic/environmental effects, are often observed in MoS2devices measured under ambient environment. Here, we report the origin of their hysteretic and transient behaviors and suggest that hysteresis of MoS2field effect transistors is largely due to absorption of moisture on the surface and intensified by high photosensitivity of MoS2. Uniform encapsulation of MoS2transistor structures with silicon nitride grown by plasma-enhanced chemical vapor deposition is effective in minimizing the hysteresis, while the device mobility is improved by over 1 order of magnitude.