MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes’ electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes' Fermi level density of states and thereby MXenes’ electronic conductivity. Here, we directly correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes.

Due to the tremendous importance of electrochemical energy storage, numerous new materials and electrode architectures for batteries and supercapacitors have emerged in recent years. Correctly characterizing these systems requires considerable time, effort, and experience to ensure proper metrics are reported. Many new nanomaterials show electrochemical behavior somewhere in between conventional double-layer capacitor and battery electrode materials, making their characterization a non-straightforward task. It is understandable that some researchers may be misinformed about how to rigorously characterize their materials and devices, which can result in This article is protected by copyright. All rights reserved. 2 inflation of their reported data. This is not uncommon considering the current state of the field nearly requires record breaking performance for publication in high-impact journals. Incorrect characterization and data reporting misleads both the materials and device development communities, and This tutorial aims to clarify the main causes of inaccurate data reporting and to give examples of how researchers should proceed. The best practices for measuring and reporting metrics such as capacitance, capacity, coulombic and energy efficiencies, electrochemical impedance, and the energy and power densities of capacitive and pseudocapacitive materials are discussed.

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) have attracted significant attention due to their electronic, electrochemical, chemical, and optical properties. However, understanding of their thermal stability is still lacking. To date, MXenes are synthesized via top-down wet chemical etching, which intrinsically results in surface terminations. Here, we provide detailed insight into the surface terminations of three carbide MXenes (Ti 3 C 2 T x , Mo 2 CT x , and Nb 2 CT x ) by performing thermal gravimetric analysis with mass spectrometry analysis (TA−MS) up to 1500 °C under a He atmosphere. This specific technique enables probing surface terminations including hydroxyl (−OH), oxy (O), and fluoride (−F) and intercalated species, such as salts and structural water. The MXene hydrophilicity depends on the type of etching (hydrofluoric acid concentration and/or mixed acid composition) and subsequent delamination conditions. We show that the amount of structural water in Ti 3 C 2 T x increases with decreasing O-containing surface terminations. The thermal stability of Ti 3 C 2 T x is improved by employing a low HF concentration or using a mixture of etchant acids, such as H 2 SO 4 /HF or HCl/HF instead of only HF, due to the reduced defect density. When tetramethylammonium hydroxide (TMAOH) is used for delamination, new N-containing species appear on the MXene surface. Moreover, free-standing films produced from Ti 3 C 2 T x etched with different HF concentrations and delaminated using TMAOH have similar TA−MS profiles, indicating that the post-treatment of Ti 3 C 2 T x controls its surface chemistry. The thermal stability of MXenes strongly depends on their chemical composition and structure; Ti 3 C 2 T x is more thermally stable than the fewer-atomic-layered Mo 2 CT x or Nb 2 CT x , and Mo 2 CT x is more/less thermally stable than Nb 2 CT x .

In order to compensate for the loss of performance when scaling resonant sensors down to NEMS, it proves extremely useful to study the behavior of resonators up to very high displacements and hence high nonlinearities. This work describes a comprehensive nonlinear multiphysics model based on the Euler-Bernoulli equation which includes both mechanical and electrostatic nonlinearities valid up to displacements comparable to the gap in the case of an electrostatically actuated doubly clamped beam. Moreover, the model takes into account the fringing field effects, significant for thin resonators. The model has been compared to both numerical integrations and electrical measurements of devices fabricated on 200 mm SOI wafers; it shows very good agreement with both. An important contribution of this work is the provision for closed-form expressions of the critical amplitude and the pull-in domain initiation amplitude including all nonlinearities. This model allows designers to cancel out nonlinearities by tuning some design parameters and thus gives the possibility to drive the resonator beyond its critical amplitude. Consequently, the sensor performance can be enhanced to the maximum below the pull-in instability, while keeping a linear behavior.

MXenes, a large family of 2D transition metal carbides and nitrides, have shown potential in energy storage and optoelectronic applications. Here, the optoelectronic and pseudocapacitive properties of titanium carbide (Ti 3 C 2 T x ) are combined to create a MXene electrochromic device, with a visible absorption peak shift from 770 to 670 nm and a 12% reversible change in transmittance with a switching rate of <1 s when cycled in an acidic electrolyte under applied potentials of less than 1 V. By probing the electrochromic effect in different electrolytes, it is shown that acidic electrolytes (H 3 PO 4 and H 2 SO 4 ) lead to larger absorption peak shifts and a higher change of transmittance than the neutral electrolyte (MgSO 4 ) (Δλ is 100 nm vs 35 nm and ΔT 770 nm is ≈12% vs ≈3%, respectively), hinting at the surface redox mechanism involved. Further investigation of the mechanism by in situ X-ray diffraction and Raman spectroscopy reveals that the reversible shift of the absorption peak is attributed to protonation/deprotonation of oxide-like surface functionalities. As a proof of concept, it is shown that Ti 3 C 2 T x MXene, dip-coated on a glass substrate, functions as both transparent conductive coating and active material in an electrochromic device, opening avenues for further research into optoelectronic and photonic applications of MXenes.