KEYWORDS Transition metal carbide; 2D material; synthesis; MXene; electronic properties 2 ABSTRACT Introducing point defects in two dimensional, 2D, materials can alter or enhance their properties. Here, we demonstrate how etching a laminated (Nb2/3Sc1/3)2AlC MAX phase (solid solution) of both the Sc and Al atoms, results in a 2D Nb1.33C material (MXene) with a large number of vacancies and vacancy clusters. This method is applicable to any quaternary, or higher, MAX phase wherein one of the

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... nsition metals is more reactive than the other and could be of vital importance in applications such as catalysis and energy storage. We also report, for the first time, on the existence of (Nb2/3Sc1/3)3AlC2 and (Nb2/3Sc1/3)4AlC3 phases. 3 Two-dimensional (2D) materials have shown great promise for many applications. 1-6 The reduced dimension leads to an increase in the surface to volume ratio, and can fundamentally alter the chemical, optical and electronic properties of a material. The properties can be altered further, either chemically via surface functionalization, 7 intercalation 8 or structurally, by introducing defects. [8][9] About 7 years ago, a new class of 2D materials based on transition metal carbides and/or nitrides (MXenes) was discovered. 10-11 MXenes are mainly produced by etching the Mn+1AXn (MAX) phases or related ternary phases. 12 The MAX phases are a family of hexagonal, layered ternary transition metal carbides and/or nitrides where M stands for an early transition metal, A stands for group 13 and 14 elements, X stands for carbon and/or nitrogen and n = 1, 2, or 3. 13 Various acidic solutions, containing fluoride ions are used to selectively etch the A layers (either Al or Ga) and convert MAX to MXene. 10, 14-17 The A layers are replaced with oxygen, hydroxyl and/or fluoride surface terminating (T) groups. 18 MXenes show promise for a large host of applications including batteries, supercapacitors, transparent conducting electrodes, catalytic and photocatalytic applications, water treatment, electromagnetic shielding, gas sensors and biosensors. [19][20] [21] [22] [23] [24] [25] MXene properties can be tuned in at least three ways that involve either altering their: i) composition, ii) surface terminations, Tx and/or, iii) structure/morphology. The composition can be changed by e.g. forming solid solutions though alloying on the M-, 26 and/or X-27 sites in the parent MAX phase. The quaternaries, (Nb0.8,Ti0.2)4C3Tx and (Nb0.8,Zr0.2)4C3Tx 28 are examples Sweden