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 transition 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. 19-25 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
We report on structural and electronic properties of defects in chemical vapor-deposited monolayer and few-layer MoS2 films. Scanning tunneling microscopy, Kelvin probe force microscopy, and transmission electron microscopy were used to obtain high resolution images and quantitative measurements of the local density of states, work function and nature of defects in MoS2 films. We track the evolution of defects that are formed under heating and electron beam irradiation. We observe formation of metastable domains with different work function values after annealing the material in ultra-high vacuum to moderate temperatures. We attribute these metastable values of the work function to evolution of crystal defects forming during the annealing. The experiments show that sulfur vacancies formed after exposure to elevated temperatures diffuse, coalesce, and migrate bringing the system from a metastable to equilibrium ground state. The process could be thermally or e-beam activated with estimated energy barrier for sulfur vacancy migration of 0.6 eV in single unit cell MoS2. Even at equilibrium conditions, the work function and local density of states values are strongly affected near grain boundaries and edges. The results provide initial estimates of the thermal budgets available for reliable fabrication of MoS2-based integrated electronics and indicate the importance of defect control and layer passivation.
We introduce switching magnetization magnetic force microscopy based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within this approach the sum of the scanned data with reversed tip magnetization depicts local van der Waals forces, while their differences map the local magnetic forces. Here we implement this method by fabricating low-momentum magnetic probes that exhibit magnetic single domain state, which can be easily reversed in low external field during the scanning. Measurements on high-density parallel and perpendicular magnetic recording media show enhanced spatial resolution of magnetization.
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