The isolation of graphene in 2004 from graphite was a defining moment for the “birth” of a field: two-dimensional (2D) materials In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement Here, we review significant recent advances and important new developments in 2D materials “beyond graphene” We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (ie, silicene, phosphorene, etc) and transition metal carbide- and carbon nitride-based MXenes We then discuss the doping and functionalization of 2

Recent Advances in Two-Dimensional Materials beyond Graphene

Transition metal dichalcogenides (TMDs) are being considered for a variety of electronic and optoelectronic devices such as beyond complementary metal-oxide-semiconductor (CMOS) switches, light-emitting diodes, solar cells, as well as sensors, among others. Molybdenum disulfide (MoS2) is the most studied of the TMDs in part because of its availability in the natural or geological form. The performance of most devices is strongly affected by the intrinsic defects in geological MoS2. Indeed, most sources of current transition metal dichalcogenides have defects, including many impurities. The variability in the electrical properties of MoS2 across the surface of the same crystal has been shown to be correlated with local variations in stoichiometry as well as metallic-like and structural defects. The presence of impurities has also been suggested to play a role in determining the Fermi level in MoS2. The main focus of this work is to highlight a number of intrinsic defects detected on natural, exfoliated MoS...

Surface Defects on Natural MoS2

Room temperature X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICPMS), high resolution Rutherford backscattering spectrometry (HR-RBS), Kelvin probe method, and scanning tunneling microscopy (STM) are employed to study the properties of a freshly exfoliated surface of geological MoS2 crystals. Our findings reveal that the semiconductor 2H-MoS2 exhibits both n- and p-type behavior, and the work function as measured by the Kelvin probe is found to vary from 4.4 to 5.3 eV. The presence of impurities in parts-per-million (ppm) and a surface defect density of up to 8% of the total area could explain the variation of the Fermi level position. High resolution RBS data also show a large variation in the MoSx composition (1.8 < x < 2.05) at the surface. Thus, the variation in the conductivity, the work function, and stoichiometry across small areas of MoS2 will have to be controlled during crystal growth in order to provide high quality uniform materials for future device fa...

Impurities and Electronic Property Variations of Natural MoS2 Crystal Surfaces.

The atomic and electronic structures and stability of native defects in a single-layer ${\mathrm{MoS}}_{2}$ are investigated, based on density-functional theory calculations. Native defects such as a S vacancy (${V}_{\mathrm{S}}$), a S interstitial (${\mathrm{S}}_{i}$), a Mo vacancy (${V}_{\mathrm{Mo}}$), and a Mo interstitial (${\mathrm{Mo}}_{i}$) are considered. The ${\mathrm{S}}_{i}$ is found to have S-adatom configuration on top of a host S atom, and the ${\mathrm{Mo}}_{i}$ has Mo-${\mathrm{Mo}}_{i}$ split interstitial configuration along the $c$ direction. The formation energies of the native defects in neutral and charged states are calculated. For the charged states, the artificial electrostatic interactions between image charges in supercells are eliminated by a supercell size scaling scheme and a correction scheme that uses a Gaussian model charge. It is found that the ${V}_{\mathrm{S}}$ has a low formation energy of 1.3--1.5 eV in the Mo-rich limit condition, and the ${\mathrm{S}}_{i}$ has 1.0 eV in the S-rich limit condition. The ${V}_{\mathrm{S}}$ is found to be a deep single acceptor with the (0/$\ensuremath{-}$) transition level at 1.7 eV above the valence-band maximum (VBM). The ${\mathrm{S}}_{i}$ is found to be an electrically neutral defect. The Mo-related native defects of ${V}_{\mathrm{Mo}}$ and ${\mathrm{Mo}}_{i}$ are found to be high in formation energy above 4 eV. The ${V}_{\mathrm{Mo}}$ is a deep single acceptor and the ${\mathrm{Mo}}_{i}$ is a deep single donor, of which the (0/$\ensuremath{-}$) acceptor and (+/0) donor transition levels are found at 1.1 and 0.3 eV above the VBM, respectively.

Stability and electronic structures of native defects in single-layer MoS 2

> “The Answer to the Great Question... Of Life, the Universe and Everything... 
> 
> Is... Forty-two,” said Deep Thought, with infinite majesty and calm… 
> 
> “I checked it very thoroughly,” said the computer, “and that quite definitely is the answer.” 
> 
> — Douglas Adams, The Hitchhiker’s Guide to the Galaxy 
Molybdenum (Mo)—the element with atomic number 42—possesses unique properties that make it the answer to many questions in the geosciences, life sciences, and industry.

In the geosciences, the redox sensitivity of Mo makes it particularly useful for answering questions about environmental redox conditions. In particular, it was first suggested as an ocean paleoredox proxy over 30 years ago (Holland 1984; Emerson and Huested 1991)—an application that finally came to fruition in the late 1990s and 2000s when understanding of Mo geochemical behavior in modern environments improved significantly (e.g., Crusius et al. 1996; Helz et al. 1996, 2011; Morford and Emerson 1999; Erickson and Helz 2000; Barling et al. 2001; Siebert et al. 2003, 2005; Arnold et al. 2004; Vorlicek et al. 2004; Morford et al. 2005; Nagler et al. 2005; Algeo and Lyons 2006; McManus et al. 2006; Poulson et al. 2006; Anbar et al. 2007; Wille et al. 2007; Pearce et al. 2008; Archer and Vance 2008; Neubert et al. 2008; Scott et al. 2008; Gordon et al. 2009; Poulson Brucker et al. 2009).

In the life sciences, nature settled on Mo as the answer to the challenge of biological-N2 fixation at least ~ 2 billion years ago (Boyd et al. 2011), with the evolution of the Mo-dependent nitrogenase enzyme. Molybdenum is also at the heart of nitrate reductase enzymes, which are essential for assimilatory and dissimilatory nitrate reduction (Glass et al. 2009). Therefore, Mo is central …

/pdf/the-stable-isotope-geochemistry-of-molybdenum-5eblplscoa.pdf

The Stable Isotope Geochemistry of Molybdenum

http://www.geo.arizona.edu/xtal//group/pdf/EPSL366_1.pdf

Melissa McMillan

Papers

Rhenium variations in molybdenite (MoS2): Evidence for progressive subsurface oxidation