Michael Lucking

Graphene Oxide as an Ideal Substrate for Hydrogen Storage

Organometallic nanomaterials hold the promise for molecular hydrogen (H(2)) storage by providing nearly ideal binding strength to H(2) for room-temperature applications. Synthesizing such materials, however, faces severe setbacks due to the problem of metal clustering. Inspired by a recent experimental breakthrough ( J. Am. Chem. Soc. 2008 , 130 , 6992 ), which demonstrates enhanced H(2) binding in Ti-grafted mesoporous silica, we propose combining the graphene oxide (GO) technique with Ti anchoring to overcome the current synthesis bottleneck for practical storage materials. Similar to silica, GO contains ample hydroxyl groups, which are the active sites for anchoring Ti atoms. GO can be routinely synthesized and is much lighter than silica. Hence, higher gravimetric storage capacity can be readily achieved. Our first-principles computations suggest that GO is primarily made of low-energy oxygen-containing structural motifs on the graphene sheet. The Ti atoms bind strongly to the oxygen sites with binding energies as high as 450 kJ/mol. This is comparable to that of silica and is indeed enough to prevent the Ti atoms from clustering. Each Ti can bind multiple H(2) with the desired binding energies (14-41 kJ/mol-H(2)). The estimated theoretical gravimetric and volumetric densities are 4.9 wt % and 64 g/L, respectively.

Absolute redox potential of liquid water: a first-principles theory

A first-principles molecular dynamics method is proposed to calculate the absolute redox potentials of liquid water. The key of the method is the evaluation of the difference between the vacuum level and the average electrostatic potential inside liquid water, which employs an average over both space and time. By avoiding the explicit use of the Kohn–Sham levels, such as the position of the valence band maximum, as the reference energy for the excited electrons, we are able to calculate water redox potentials accurately using a semi-local density functional and an entropic contribution estimated from experimental data.

Traditional Semiconductors in the Two-Dimensional Limit

Interest in two dimensional materials has exploded in recent years. Not only are they studied due to their novel electronic properties, such as the emergent Dirac Fermion in graphene, but also as a new paradigm in which stacking layers of distinct two dimensional materials may enable different functionality or devices. Here, through first-principles theory, we reveal a large new class of two dimensional materials which are derived from traditional III-V, II-VI, and I-VII semiconductors. It is found that in the ultra-thin limit all of the traditional binary semiconductors studied (a series of 26 semiconductors) stabilize in a two dimensional double layer honeycomb (DLHC) structure, as opposed to the wurtzite or zinc-blende structures associated with three dimensional bulk. Not only does this greatly increase the landscape of twodimensional materials, but it is shown that in the double layer honeycomb form, even ordinary semiconductors, such as GaAs, can exhibit exotic topological properties.Interest in two-dimensional materials has exploded in recent years. Not only are they studied due to their novel electronic properties, such as the emergent Dirac fermion in graphene, but also as a new paradigm in which stacking layers of distinct two-dimensional materials may enable different functionality or devices. Here, through first-principles theory, we reveal a large new class of two-dimensional materials which are derived from traditional III-V, II-VI, and I-VII semiconductors. It is found that in the ultrathin limit the great majority of traditional binary semiconductors studied (a series of 28 semiconductors) are not only kinetically stable in a two-dimensional double layer honeycomb structure, but more energetically stable than the truncated wurtzite or zinc-blende structures associated with three dimensional bulk. These findings both greatly increase the landscape of two-dimensional materials and also demonstrate that in the double layer honeycomb form, even ordinary semiconductors, such as GaAs, can exhibit exotic topological properties.

Characterization of second-order nonlinear optical properties of two-dimensional transition metal dichalcogenides (Conference Presentation)

Two-dimensional transition metal dichalcogenides (TMD), such as WS2 and MoS2, have been shown to exhibit large second order optical nonlinearity due to their non-centrosymmetric crystalline symmetry in few odd- and mono-layers, and resonance enhancement. Here we study the second-order nonlinear susceptibility of 2D TMDs through second harmonic generation (SHG) and sum frequency generation (SFG). Using a wavelength-tunable femtosecond laser, we can characterize SHG of TMDs to obtain the second-order nonlinear susceptibility at multiple wavelengths. Along with the experimental studies, theoretical investigation of the second-order nonlinear susceptibility is also performed. With this we explore the estimation of the second-order nonlinear susceptibility of 2D TMD layered materials based on their first-order susceptibility through the experimental and theoretical verification of Miller’s Rule for these materials. Additionally, we characterize the second-order nonlinear susceptibility of 2D TMD alloys through the SFG spectroscopy.

Large second harmonic generation in alloyed TMDs and boron nitride nanostructures

First principles methods are used to explicitly calculate the nonlinear susceptibility (χ(2)(2ω, ω, ω)) representing the second harmonic generation (SHG) of two dimensional semiconducting materials, namely transition metal dichalcogenides (TMDs) and Boron Nitride (BN). It is found that alloying TMDs improves their second harmonic response, with MoTeS alloys exhibiting the highest of all hexagonal alloys at low photon energies. Moreover, careful examination of the relationship between the concentration of Se in MoxSeySz alloys shows that the SHG intensity can be tuned by modifying the stoichiometry. In addition, materials with curvature can have large second harmonic susceptibility. Of all the calculated monolayer structures, the hypothetical TMD Haeckelites NbSSe and Nb0.5Ta0.5S2 exhibit the highest χ(2), while one of the porous 3D structures constructed from 2D hBN exhibits a larger χ(2) than known large band gap 3-D materials.

Resonant Raman and Exciton Coupling in High-Quality Single Crystals of Atomically Thin Molybdenum Diselenide Grown by Vapor-Phase Chalcogenization

We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, high-quality single-crystal atomically thin molybdenum diselenide samples. Measurements were performed in samples with four different incident laser excitation energies ranging from 1.95 eV ⩽ Eex ⩽ 2.71 eV, revealing rich spectral information in samples ranging from N = 1-4 layers and a thick, bulk sample. In addition to previously observed (and identified) peaks, we specifically investigate the origin of a peak near ω ≈ 250 cm-1. Our density functional theory and Bethe-Salpeter calculations suggest that this peak arises from a double-resonant Raman process involving the ZA acoustic phonon perpendicular to the layer. This mode appears prominently in freshly prepared samples and disappears in aged samples, thereby offering a method for ascertaining the high optoelectronic quality of freshly prepared 2D-MoSe2 crystals. We further present an in-depth investigation of the energy-dependent variation of the position of this and other peaks and provide evidence of C-exciton-phonon coupling in monolayer MoSe2. Finally, we show how the signature peak positions and intensities vary as a function of layer thickness in these samples.