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.

2D transition metal carbides, carbonitrides, and nitrides, known as MXenes, are a rapidly growing family of 2D materials with close to 30 members experimentally synthesized, and dozens more studied theoretically. They exhibit outstanding electronic, optical, mechanical, and thermal properties with versatile transition metal and surface chemistries. They have shown promise in many applications, such as energy storage, electromagnetic interference shielding, transparent electrodes, sensors, catalysis, photothermal therapy, etc. The high electronic conductivity and wide range of optical absorption properties of MXenes are the key to their success in the aforementioned applications. However, relatively little is currently known about their fundamental electronic and optical properties, limiting their use to their full potential. Here, MXenes' electronic and optical properties from both theoretical and experimental perspectives, as well as applications related to those properties, are discussed, providing a guide for researchers who are exploring those properties of MXenes.

Figure S1. (a) Blocks of Ti3AlC2 (top) and Al-Ti3AlC2 (bottom), (b) mass loss during the washing of Al-Ti3AlC2 with HCl, (c) image of the purple filtrate from the acid washing process, (d) Al-Ti3AlC2 particles after acid washing using HCl, (e) higher magnification of (d).

Lightweight, ultrathin, and flexible electromagnetic interference (EMI) shielding materials are needed to protect electronic circuits and portable telecommunication devices and to eliminate cross-talk between devices and device components. Here, we show that a two-dimensional (2D) transition metal carbonitride, Ti3CNTx MXene, with a moderate electrical conductivity, provides a higher shielding effectiveness compared with more conductive Ti3C2Tx or metal foils of the same thickness. This exceptional shielding performance of Ti3CNTx was achieved by thermal annealing and is attributed to an anomalously high absorption of electromagnetic waves in its layered, metamaterial-like structure. These results provide guidance for designing advanced EMI shielding materials but also highlight the need for exploring fundamental mechanisms behind interaction of electromagnetic waves with 2D materials.

MXenes are a family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides with a general formula of M n+1 X n T x , in which two, three, or four atomic layers of a transition metal (M: Ti, Nb, V, Cr, Mo, Ta, etc.) are interleaved with layers of C and/or N (shown as X), and T x represents surface termination groups such as −OH, O, and −F. Here, we report the scalable synthesis and characterization of a MXene with five atomic layers of transition metals (Mo 4 VC 4 T x ), by synthesizing its Mo 4 VAlC 4 MAX phase precursor that contains no other MAX phase impurities. These phases display twinning at their central M layers which is not present in any other known MAX phases or MXenes. Transmission electron microscopy and X-ray diffraction were used to examine the structure of both phases. Energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and highresolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy were used to study the composition of these materials. Density functional theory calculations indicate that other five transition metal-layer MAX phases (M′ 4 M″AlC 4 ) may be possible, where M′ and M″ are two different transition metals. The predicted existence of additional Al-containing MAX phases suggests that more M 5 C 4 T x MXenes can be synthesized. Additionally, we characterized the optical, electronic, and thermal properties of Mo 4 VC 4 T x . This study demonstrates the existence of an additional subfamily of M 5 X 4 T x MXenes as well as a twinned structure, allowing for a wider range of 2D structures and compositions for more control over properties, which could lead to many different applications.

properties of novel transparent electrode materials include high transmittances (>85%) in the UV and visible regions and low sheet resistances ( R s ), in the range of 0.01-1 kΩ sq −1 . In addition, the new materials should be mechanically robust for fl exible touch screen and organic lightemitting diode applications. On top of excellent optoelectronic and mechanical properties, low cost of production per unit area and feasibility of large-scale fabrication are key factors determining their industrial success. [ 1a , 2 ] Among numerous materials being considered for transparent electrode applications, 2D graphene is arguably the most studied in the past few years. [ 1e , 3 ] Transparent conductive graphene fi lms are mainly produced by two methods: direct chemical vapor deposition (CVD) or solution-processing of reduced graphene oxide (rGO). [ 4 ] Although the CVD method results in high-performance fi lms, their high fabrication cost and size limitations have hindered their commercialization. Several solution-processing methods, such as spray coating, spin coating, and dip coating, have also been employed to fabricate thin fi lms from rGO solutions. [ 5 ] However, the transparent fi lms produced by such methods exhibit high R s (1-1000 kΩ sq −1 ), mainly due to defects (functional groups) and inter-fl ake resistance. [ 6 ] The development of novel materials for transparent conductive fi lms and suitable methods for their large-scale and low-cost production remains a challenge.Recently, a new class of 2D transition metal carbides and/or nitrides, so-called MXenes, was discovered. [ 7 ] Both theoretical and experimental results indicate that most MXenes exhibit metallic conductivity, hydrophilicity, high mechanical strength, and can act as intercalation hosts. [ 8 ] They have shown great promise as electrodes in supercapacitors, Li-ion and other types of batteries, fuel cells, reinforcement for polymers, adsorbents, and sensors. [ 9 ] Upon delamination, colloidal MXene solutions contain large quantities of ≈1 nm-thick 2D fl akes with lateral sizes up to several micrometers, which are perfect for solution processing.

MXenes, an emerging family of 2D transition metal carbides and nitrides, have shown promise in various applications, such as energy storage, electromagnetic interference shielding, conductive thin films, photonics, and photothermal therapy. Their metallic nature, wide range of optical absorption, and tunable surface chemistry are the key to their success in those applications. The physical properties of MXenes are known to be strongly dependent on their surface terminations. In this study we investigated the electronic properties of Ti3C2Tx for different surface terminations, as achieved by different annealing temperatures, with the help of photoelectron spectroscopy, inverse photoelectron spectroscopy, and density functional theory calculations. We find that fluorine occupies solely the face-centered cubic adsorption site, whereas oxygen initially occupies at least two different adsorption sites, followed by a rearrangement after fluorine desorption at high annealing temperatures. The measured electronic structure of Ti3C2Tx showed strong dispersion of more than 1 eV, which we conclude to stem from Ti-O bonds by comparing it to calculated band structures. We further measured the work function of Ti3C2Tx as a function of annealing temperature and found that is in the range of 3.9-4.8 eV, depending on the surface composition. Comparing the experimental work function to detailed density functional theory calculations shows that the measured value is not simply an average of the work function values of uniformly

MXenes are a rapidly growing class of 2D transition metal carbides and nitrides, finding applications in fields ranging from energy storage to electromagnetic interference shielding and transparent conductive coatings. However, while more than 20 carbide MXenes have already been synthesized, TiN and TiN are the only nitride MXenes reported so far. Here by ammoniation of MoCT and VCT MXenes at 600 °C, we report on their transformation to 2D metal nitrides. Carbon atoms in the precursor MXenes are replaced with N atoms, resulting from the decomposition of ammonia molecules. The crystal structures of the resulting MoN and VN were determined with transmission electron microscopy and X-ray pair distribution function analysis. Our results indicate that MoN retains the MXene structure and VC transforms to a mixed layered structure of trigonal VN and cubic VN. Temperature-dependent resistivity measurements of the nitrides reveal that they exhibit metallic conductivity, as opposed to semiconductor-like behavior of their parent carbides. As important, room-temperature electrical conductivity values of MoN and VN are three and one order of magnitude larger than those of the MoCT and VCT precursors, respectively. This study shows how gas treatment synthesis such as ammoniation can transform carbide MXenes into 2D nitrides with higher electrical conductivities and metallic behavior, opening a new avenue in 2D materials synthesis.