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.

Electron mobilities limited by phonon and ionized impurity scattering have traditionally been modeled by suppressing atomic-scale detail, relying on empirical deformation potentials and either effective-mass theory or bulk energy bands to describe electron velocities. Parameter fitting to experimental data is needed. As modern technologies require modeling of transport at the nanoscale and unprecedented materials are introduced, predictive parameter-free mobility modeling becomes necessary. Here we report the development of first-principles quantum-mechanical methods to calculate scattering rates and electronic mobilities limited by phonon and ionized-impurity scattering. We report results for n-doped silicon that are in good agreement with experiment.

The mean-free-paths (MFPs) of energy carriers are of critical importance to the nanoengineering of better thermoelectric materials. Despite significant progress in the first-principlesbased understanding of the spectral distribution of phonon MFPs in recent years, the spectral distribution of electron MFPs remains unclear. In this work, we compute the energy dependent electron scatterings and MFPs in silicon from first-principles. The electrical conductivity accumulation with respect to electron MFPs is compared to that of the phonon thermal conductivity accumulation to illustrate the quantitative impact of nanostructuring on electron and phonon transport. By combining all electron and phonon transport properties from first-principles, we predict the thermoelectric properties of the bulk and nanostructured silicon, and find that silicon with 20 nm nanograins can result in more than five times enhancement in their thermoelectric figure of merit as the grain boundaries scatter phonons more significantly than that of electrons due to their disparate MFP distributions.Nanostructuring has proven to be an effective strategy to improve the figure of merit of thermoelectric materials [1][2][3][4][5][6][7][8][9][10][11]. The figure of merit is proportional to the electrical conductivity (), the square of the Seebeck coefficient (S) and inversely proportional to the thermal conductivity consisting of both phonon (k p ) and electron (k e ) contributions. The most effective nanostructuring approach so far has been reducing the phonon thermal conductivity while maintaining the electronic performance. For this strategy to be effective, the nanostructures should be smaller than the phonon mean free path (MFP) but larger than the electron MFP so that phonons are more strongly scattered than electrons. It is understood that both electron and phonon MFPs have a distribution over certain energy range. There has been good progress in predicting the spectral phonon MFPs for a range of bulk single crystals and alloys [12][13][14][15][16][17][18][19][20][21][22][23][24]. However, there has been no discussion on the spectral electron MFPs from first-principles. Surprisingly, this status exists even for silicon, one of the most important materials. Existing knowledge on electron scattering, relaxation time, and MFP, is mostly based on analytical models derived from Fermi's golden rule assuming ideal electron and phonon dispersions [25,26]. Past work on the phonon MFP distributions based on first-principles simulations, however, shows that such semi-empirical treatments on scattering lead to large error [13,21,23,27].In this work, we compute the electron scattering rates and MFPs in silicon from first-principles and examine their dependence on energy, doping concentration, and their contributions to electronic conductivity and Seebeck coefficient. We demonstrate quantitatively the large disparity in the electron and phonon MFP distributions in silicon, and use the information obtained to predict that nanostructures with size of 20 nm can...

The insulating, fully ordered, double perovskite Sr2CoOsO6 undergoes two magnetic phase transitions. The Os(VI) ions order antiferromagnetically with a propagation vector k = (1/2, 1/2, 0) below TN1 = 108 K, while the high-spin Co(II) ions order antiferromagnetically with a propagation vector k = (1/2, 0, 1/2) below TN2 = 70 K. Ordering of the Os(VI) spins is accompanied by a structural distortion from tetragonal I4/m symmetry to monoclinic I2/m symmetry, which reduces the frustration of the face centered cubic lattice of Os(VI) ions. Density functional theory calculations show that the long-range Os-O-Co-O-Os and Co-O-Os-O-Co superexchange interactions are considerably stronger than the shorter Os-O-Co interactions. The poor energetic overlap between the 3d orbitals of Co and the 5d orbitals of Os appears to be responsible for this unusual inversion in the strength of short and long-range superexchange interactions.