Tailoring Electronic and Optical Properties of MXenes through Forming Solid Solutions

Abstract

Alloying is a long-established strategy to tailor properties of metals for specific applications, thus retaining or enhancing the principal elemental characteristics while offering additional functionality from the added elements. We propose a similar approach to the control of properties of two-dimensional transition metal carbides known as MXenes. MXenes (M n+1 X n ) have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites presents promising routes for tailoring the chemical, optical, electronic, or mechanical properties of MXenes. Herein, we systematically investigated three interrelated binary solid-solution MXene systems based on Ti, Nb, and/or V at the M-site in a M 2 XT x structure (Ti 2-y Nb y CT x , Ti 2-y V y CT x , and V 2-y Nb y CT x , where T x stands for surface terminations) showing the evolution of electronic and optical properties as a function of composition. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. This work greatly increases the number of nonstoichiometric MXenes reported to date and opens avenues for controlling physical properties of different MXenes with a limitless number of compositions possible through M-site solid solutions.

Improving oxidation stability of 2D MXenes: synthesis, storage media, and conditions

“…With the addition of different M and X elemental compositions, synthesized MXenes possess electrical conductivity ranging from 1 S cm −1 to as high as ~ 20,000 S cm −1 under controlled synthesis conditions [24,31]. The intrinsic properties of MXenes, including metal-like electrical conductivity, are determined not only by the type and composition of M and X constituents in MXenes [6], but also by many other factors including defect density [32], flake size [33], surface functional groups of the synthesized MXene flakes, and type of intercalants [9,34]. These factors directly affect the oxidation kinetics of the synthesized MXenes.…”

“…Unlike other 2D materials, Ti 3 C 2 T x MXene, which is the representative of the MXene family, exhibits outstanding electrical, electrochemical, optoelectronic, and mechanical properties and has prompted the exploration of other MXenes with different elemental compositions, namely: M 5 X 4 , M 4 X 3 , M 3 X 2 , and M 2 X. Subsequently, binary transition metal compositions (M 1 and M 2 ) and solid solutions (X 1 and X 2 ) further expanded the list of conventional single M and X layer MXenes, offering a wide range of controllable electronic and optical properties [5][6][7][8].…”

The Broad Chromatic Range of Two‐Dimensional Transition Metal Carbides

“…[ 30 ] In the solid state, the visible absorption coefficient of V 2 C was half that of Ti‐based MXenes, another hint at the vast opportunity to identify and design optical properties. [ 31,32 ] With the diversity in composition, including solid solutions on M and X sites, [ 33 ] characterization of light–matter interactions remains critical to advancing MXenes for photonic and plasmonic applications. The present study provides the first quantitative juxtaposition of the spectroscopic properties of M n +1 C n MXenes of systematically varying n and M ( Figure ) and expands the palette of optical phenomena possible for MXenes.…”

High-Entropy 2D Carbide MXenes

“…While we have not characterized the composition of the undesired multiphases in the non-equimolar mixtures (Figure S1), they can possibly be a mixture of solid solution phases. For example, M2AlC may be comprised of a previously reported solid solution MAX phase of Nb and V 17,50 and the M4AlC3 can contain either Ti-Nb 51 or Ti-V 50 phases. The M4AlC3 phase might contain a non-equimolar ratio of all elements as well.…”

“…[12][13][14][15] DTM MXenes provide a limitless range of possible compositions of MXenes in the form of random solid solutions of transition metals, where two transition metals randomly occupy the different M layers, which provides control and tunability of MXenes' properties. [15][16][17] In addition to adding a second transition metal in previously seen structures of MXene, DTM MXenes also provide novel structures which are not seen in mono-transition metal MXenes, such as out-of-plane ordered M3C2Tx and M4C3Tx 12 or in-plane ordered M2CTx, 13 and a new MXene structure of M5C4Tx solid solution. 4 DTM MXenes have opened new avenues for application specific compositional tailoring which are otherwise impossible in 2D nanomaterials or mono-transition metal MXenes, such as semiconductive, magnetic, or to be topological materials in out-of-plane M3C2Tx and M4C3Tx MXenes.…”