Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. We have synthesized for the first time, mm-scale crystals of a hydrogen-terminated germanium multilayered graphane analogue (germanane, GeH) from the topochemical deintercalation of CaGe2. This layered van der Waals solid is analogous to multilayered graphane (CH). The surface layer of GeH only slowly oxidizes in air over the span of 5 months, while the underlying layers are resilient to oxidation based on X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) measurements. The GeH is thermally stable up to 75 o C, however, above this temperature amorphization and dehydrogenation begin to occur. These sheets can be mechanically exfoliated as single and few layers onto SiO2/Si surfaces. This material represents a new class of covalently terminated graphane analogues and has great potential for a wide range of optoelectronic and sensing applications, especially since theory predicts a direct band gap of 1.53 eV and an electron mobility ~five times higher than that of bulk Ge.iii Acknowledgements

Two-dimensional van der Waals materials have shown great promise for a variety of electronic, optoelectronic, sensing and energy conversion applications. Since almost every atom in these two-dimensional crystals is exposed to the surface, covalent surface termination could provide a powerful method for the controlled tuning of material properties. Here we demonstrate a facile, one-step metathesis approach that directly converts CaGe 2 crystals into mm-sized crystals of methyl-terminated germanane (GeCH 3 ). Replacing -H termination in GeH with -CH 3 increases the band gap by B0.1 eV to 1.7 eV, and produces band edge fluorescence with a quantum yield of B0.2%, with little dependence on layer thickness. Furthermore, the thermal stability of GeCH 3 has been increased to 250°C compared with 75°C for GeH. This one-step metathesis approach should be applicable for accessing new families of two-dimensional van der Waals lattices that feature precise organic terminations and with enhanced optoelectronic properties.

As elemental main group materials (i.e., silicon and germanium) have dominated the field of modern electronics, their monolayer 2D analogues have shown great promise for next‐generation electronic materials as well as potential game‐changing properties for optoelectronics, energy, and beyond. These atomically thin materials composed of single atomic variants of group III through group VI elements on the periodic table have already demonstrated exciting properties such as near‐room‐temperature topological insulation in bismuthene, extremely high electron mobilities in phosphorene and silicone, and substantial Li‐ion storage capability in borophene. Isolation of these materials within the postgraphene era began with silicene in 2010 and quickly progressed to the experimental identification or theoretical prediction of 15 of the 18 main group elements existing as solids at standard pressure and temperatures. This review first focuses on the significance of defects/functionalization, discussion of different allotropes, and overarching structure–property relationships of 2D main group elemental materials. Then, a complete review of emerging applications in electronics, sensing, spintronics, plasmonics, photodetectors, ultrafast lasers, batteries, supercapacitors, and thermoelectrics is presented by application type, including detailed descriptions of how the material properties may be tailored toward each specific application.

With first principles calculations, we predict a novel stable 2D layered structure for group VI elements Se and Te that we call square selenene and square tellurene, respectively. They have chair-like buckled structures similar to other layered materials such as silicene and germanene but with a square unit cell rather than hexagonal. This special structure gives rise to anisotropic band dispersions near the Fermi level that can be described by a generalized semi-Dirac Hamiltonian. We show that the considerably large band gap (∼0.1 eV) opened by spin-orbit coupling makes square selenene and tellurene topological insulators, hosting non-trivial edge states. Therefore, square selenene and tellurene are promising materials for novel electronic and spintronic applications. Finally, we show that this new type of 2D elemental material can potentially be grown on proper substrates, such as a Au(100) surface.The isolation of graphene in 2004 [1] opened up a new avenue in condensed matter physics: two-dimensional (2D) materials research.Following the success of graphene [2], intensive efforts have been devoted to exploring other 2D materials [3]. Among them, elemental 2D materials (composed of only one element) have attracted much attention because of their simple composition and intriguing properties [4]. A number of elemental 2D materials beyond graphene have been predicted and synthesized, such as silicene [5-8], germanene [9], stanene [10,11], phosphorene [12], and borophene [13,14], with elements ranging from group III to group V. However, no studies on group VI elemental 2D materials have ever been reported. Unlike the aforementioned elements, at ambient conditions, most of the group VI elements have 3D bulk structures composed of 1D atomic chains or 0D atomic rings with only two-fold coordination bonding. The question remains whether these elements can form 2D atomic layers as elements from groups III-V do.During the exploration of 2D materials, some are predicted to be topological insulators (TIs), a new quantum state of matter recently discovered [15][16][17][18][19]. TIs have different band topology than normal insulators, giving rise to non-trivial gapless surface states at the interface between TIs and normal insulators or vacuum. These nontrivial surface states are protected by time-reversal symmetry and the spin of these states is locked with their momentum, significantly reducing back-scattering. For a 2D TI, these non-trivial symmetry-protected edge states even give rise to spin-polarized conduction channels without dissipation as back-scattering is strictly prohibited [16,17]. This special property makes 2D TIs extremely appealing in novel electronic and spintronic applications.In this study, we report for the first time the theoretical prediction of a new type of 2D layered structures for group VI elements Se and Te, which we call square selenene and tellurene, following the convention of graphene and the symmetry of the unit cell. We confirm their thermal stability with ab initio phonon calculations and...

Strange metal behavior is ubiquitous in correlated materials ranging from cuprate superconductors to bilayer graphene. There is increasing recognition that it arises from physics beyond the quantum fluctuations of a Landau order parameter which, in quantum critical heavy fermion antiferromagnets, may be realized as critical Kondo entanglement of spin and charge. The dynamics of the associated electronic delocalization transition could be ideally probed by optical conductivity, but experiments in the corresponding frequency and temperature ranges have remained elusive. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh 2 Si 2 , a model strange metal compound. We observe frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery implicates critical charge fluctuations as playing a central role in the strange metal behavior, thereby elucidating one of the longstanding mysteries of correlated quantum matter. arXiv:1808.02296v1 [cond-mat.str-el]