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One of the fastest growing fields in condensed matter physics is that of two-dimensional materials; compounds that promise to revolutionize nanotechnology due to the ability to easily isolate clean atomically thin sheets of conducting material for use in atomic-scale circuits. Since the initial demonstration of the electric-field effect in nanocircuits fabricated from mechanically exfoliated graphene, the number of available compounds that can be isolated and used in atomically thin circuits has exponentially grown to include diverse electrical properties from metals and insulators to superconductors and magnets. The bulk compounds from which flakes are isolated are known as van der Waals materials named for their intrinsic structural anisotropy resulting in weak van der Waals chemical bonds in one dimension. Since this field is relatively young, there are a multitude of branching opportunities for experimental advancement. In this work, we begin by addressing a significant technical challenge within the two-dimensional community; contacting and measuring air-sensitive two-dimensional materials. We developed a novel technique for embedding metal electrodes in atomically thin insulating flakes used to simultaneously contact and preserve a wide-array of air-sensitive two-dimensional materials. Using this technique, we proceed to explore the properties of a diverse set of van der Waals compounds in both three dimensions and two dimensions. We investigate the nature of superconductivity in the two-dimensional limit by quantifying the fragility of the superconducting state in a single atomic sheet of NbSe2. In combination with theoretical time-dependent Ginzburg-Landau simulations, we show that the dissipation in two-dimensional NbSe2 can be accurately described by vortex dynamics, including the poorly understood low-temperature metallic-like state. We examine how superconductors proximitize with normal metals through measurements on atomic-scale normal metal/insulator/superconductor tunnel junctions fabricated from van der Waals materials, demonstrating agreement with Blonder- Tinkham-Klapwijk theory. In addition, in junctions fabricated from graphene and NbN, a high-critical- field superconductor, we gain an understanding of Andreev processes in graphene under large magnetic fields. Finally, we provide a detailed characterization Re6Se8Cl2 and CrSBr, two new van der Waals compounds. In Re6Se8Cl2, we develop a novel strategy for doping in van der Waals compounds with labile ligands, demonstrating a semiconducting to superconducting transition upon electron doping. In CrSBr, we discover a well-developed semiconducting gap along with strong coupling between magnetic order and transport properties, unique among van der Waals magnets. Further, we find the semiconducting and magnetic properties persist down to 2 layers of CrSBr, with the observation of air-stability, establishing it as a promising material platform for increasing the applicability of van der Waals magnets.

The generalized dual-kinetic-balance approach for axially symmetric systems is employed to solve the two-center Dirac problem. The spectra of one-electron homonuclear quasimolecules are calculated and compared with the previous calculations. The analysis of the monopole approximation with two different choices of the origin is performed. Special attention is paid to the lead and xenon dimers, Pb82+–Pb82+–e− and Xe54+–Xe54+–e−, where the energies of the ground and several excited σ-states are presented in the wide range of internuclear distances. The developed method provides the quasicomplete finite basis set and allows for the construction of perturbation theory, including within the bound-state QED.

Van der Waals materials and inorganic nanoparticles are two categories of nanomaterials that have been widely investigated in the past two decades. Both of them have been considered to be promising as candidates for the next generation electrical, optical, and mechanical applications. However, both of them have a few limitations that greatly affect the performance of devices, e.g. zero bandgap for graphene; poor contact quality, low mobility and quantum efficiency for MoS2; and poor interparticle conductivity for nanoparticles. This thesis tries to explore a new way of combining these two categories of material into hybrids, so that the intrinsic limitations of materials from each category will be overcome by the other materials that are introduced into the hybrid. This thesis consists of five parts. The first part (Chapter 1) introduces the background and motivation of the thesis. The second part (Chapters 2, 3, 4, and 5) describes the detailed processes and methods, starting from preparing each element to the assembly of these element into a hybrid structure device. This part also includes understanding the mechanisms of 2D and 3D self-assembly of nanoparticles. The third part (Chapter 6 and 7) describes two examples of hybrid structures, including the investigation of electron or molecule transfer inside the hybrid. The fourth part (Chapter 8) introduces other findings and technical innovations, including alternative ways of thin film nanoparticle self-assembly/deposition, and fabrication methods for the band structure analysis of transition metal dichalcogenides by angle resolved photo-electron spectroscopy. The fifth part (Chapter 9) describes several possible future work directions that could be investigated to improve the understanding of the nanoparticle assembly and translating the conceptual device into real applications.

The weakly-bound van der Waals complex OCS–He has been observed by infrared spectroscopy in the region of the OCS ν₁ vibration (≈2062 cm ⁻¹ ) in a pulsed supersonic jet expansion using a tunable diode laser probe. The results are in good agreement with previous microwave observations of OCS–He, but they extend to the vibrationally excited state, as well as to a considerably larger number of rotational levels with higher J-values. Prominent a-type transitions (ΔK=0) are observed, together with weaker b-type transitions (ΔK=±1). The rotational energy level pattern in the excited vibrational state is quite similar to that in the ground state, and the band origin is slightly blue-shifted (+0.111 cm ⁻¹ ) from that of the free OCS molecule. These results are of special interest due to recent observations in the same spectral region of OCS molecules trapped in ultra cold helium droplets.

In view of further application to the study of molecular and atomic sticking on dust particles, we investigated the capability of the "freeze-and-thaw" cycle of the Kohn–Sham equations with constrained electron density (KSCED) to describe potential energy surfaces of weak van der Waals complexes. We report the results obtained for C6H6[centered ellipsis]X (X = O2, N2, and CO) as test cases. In the KSCED formalism, the exchange-correlation functional is defined as in the Kohn–Sham approach whereas the kinetic energy of the molecular complex is expressed differently, using both the analytic expressions for the kinetic energy of individual fragments and the explicit functional of electron density to approximate nonadditive contributions. As the analytical form of the kinetic energy functional is not known, the approach relies on approximations. Therefore, the applied implementation of KSCED requires the use of an approximate kinetic energy functional in addition to the approximate exchange-correlation functional in calculations following the Kohn–Sham formalism. Several approximate kinetic energy functionals derived using a general form by Lee, Lee, and Parr [Lee et al., Phys. Rev. A. 44, 768 (1991)] were considered. The functionals of this type are related to the approximate exchange energy functionals and it is possible to derive a kinetic energy functional from an exchange energy functional without the use of any additional parameters. The KSCED interaction energies obtained using the PW91 [Perdew and Wang, in Electronic Structure of Solids '91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] exchange-correlation functional and the kinetic energy functional derived from the PW91 exchange functional agree very well with the available experimental results. Other considered functionals lead to worse results. Compared to the supermolecule Kohn–Sham interaction energies, the ones derived from the KSCED calculations depend less on the choice of the approximate functionals used. The presented KSCED results together with the previous Kohn–Sham ones [Weso[barred lower-case ell]owski et al., J. Phys. Chem. A 101, 7818 (1997)] support the use of the PW91 functional for studies of weakly bound systems of our interest.

4 pags. ; 2 figs., Recently, the predissociation lifetimes of the NeBr2(B) complex for different initial vibrational excitation (10 ≤ v′ ≤ 20) have been measured using time-resolved optical pump-probe spectroscopy [ Taylor et al., J. Chem. Phys., 132, 104309 (2010) ]. In the vibrational interval studied, the vibrational predissociation (VP) proceeds by the transfer of a single vibrational quantum and the lifetimes are expected to decrease smoothly with increasing v′, as predicted by the energy gap law. However, the experimental lifetimes show strong oscillations with v′, which were attributed to the occurrence of electronic predissociation into two possible dissociative electronic states of Br2(1g,2g), based on a Franck–Condon spectator model. In this work we reproduce the experimental findings by performing full three-dimensional wave packet calculations for the competition of vibrational and electronic predissociation, including the B(0u+), 2g, and C(1u) electronic states. Model potential energy surfaces were used based on previous theoretical simulations of the VP dynamics on the B state and on ab initio calculations on the NeCl2 related system. Thus, only two parameters, the strength of the electronic couplings, are fit to achieve the excellent theoretical/experimental agreement., This work has been supported by the Ministerio de Ciencia e Innovación under Project No. CTQ2007-62898 and U.S. National Science Foundation Grant No. CHE-0404743. The calculations have been performed in the IFF and CESGA computing centers.

The first three-dimensional ab initio intermolecular potential energy surface of the Ar–CO van der Waals complex is calculated using the coupled cluster singles and doubles including connected triples model and the augmented correlation-consistent polarized valence quadruple zeta (aug-cc-pVQZ) basis set extended with a (3s3p2d1f1g) set of midbond functions. The three-dimensional surface is averaged over the three lowest vibrational states of CO. Rovibrational energies are calculated up to 50 cm−1 above the ground state, thus enabling comprehensive comparison between theory and available experimental data as well as providing detailed guidance for future spectroscopic investigations of higher-lying states. The experimental transitions are reproduced with a root-mean-square error of 0.13 cm−1, excluding states observed around 25 cm−1 above the ground state. The latter states are at variance with the experimentally deduced ordering.

To test the approximate instanton approach to intermolecular proton-transfer dynamics, we report multidimensional ab initio bimolecular rate constants of HH, HD, and DD exchange in the complex of acetic acid with methanol in tetrahydrofuran-d8, and compare them with the NMR (nuclear magnetic resonance) experiments of Gerritzen and Limbach. The bimolecular rate constants are evaluated as products of the exchange rates and the equilibrium rate constants of complex formation in solution. The two molecules form hydrogen-bond bridges and the exchange occurs via concerted transfer of two protons. The dynamics of this transfer is evaluated in the complete space of 36 vibrational degrees of freedom. The geometries of the two isolated molecules, the complex, and the transition states corresponding to double proton transfer are fully optimized at QCISD (quadratic configuration interaction including single and double substitutions) level of theory, and the normal-mode frequencies are calculated at MP2 (Møller-Plesset perturbation theory of second order) level with the 6-31G (d,p) basis set. The presence of the solvent is taken into account via single-point calculations over the gas phase geometries with the PCM (polarized continuum model). The proton exchange rate constants, calculated with the instanton method, show the effect of the structure and strength of the hydrogen bonds, reflected in the coupling between the tunneling motion and the other vibrations of the complex. Comparison with experiment, which shows substantial kinetic isotopic effects (KIE), indicates that tunneling prevails over classic exchange for the whole temperature range of observation. The unusual behavior of the experimental KIE upon single and double deuterium substitution is well reproduced and is related to the synchronicity of two-atom tunneling.