Small Hydrogen Clusters Research Articles

ΔNO and the complexities of electron correlation in simple hydrogen clusters.

The Δ natural orbital (ΔNO) two-electron density matrix (2-RDM) and energy expression are derived from a multideterminantal wave function. The approximate ΔNO 2-RDM is combined with an on-top density functional and a double-counting correction to capture electron correlation. A trust-region Newton's method optimization algorithm for the simultaneous optimization of ΔNO orbitals and occupancies is introduced and compared to the previous iterative diagonalization algorithm. The combination of ΔNO and two different on-top density functionals, Colle-Salvetti (CS) and Opposite-spin exponential cusp and Fermi-hole correction (OF), is assessed on small hydrogen clusters and compared to density functional, single-reference coupled-cluster, and multireference perturbation theory (MRMP2) methods. The ΔNO-CS and ΔNO-OF methods outperform the single-reference methods and are comparable to MRMP2. However, there is a distinct qualitative error in the ΔNO potential energy surface for H4 compared to the exact. This discrepancy is explained through analysis of the ΔNO orbitals, occupancies, and the two-electron density.

The Resonant Principle of Creation and Structure of the Universe

A model of the origin and structure of the Universe is presented. It is shown that the laws of nature are identical and evident both in the microscopic world and the macrocosm. Already Pythagoras paid special attention to the unit as a sacred number in the language of mathematics, and D.I. Mendeleev assigned special importance to hydrogen, the first element of the periodic table, from which the formation of our solar system started; thus, as one is the origin of numbers, so hydrogen is the basic building material of all other elements. Hydrogen is a diatomic gas in the normal state. Hydrogen molecules can form complex associates of a much larger number of atoms under certain conditions, such as low temperature or very rarefied media. The stability (and therefore, the lifetime) of these structures is lower than that of an ordinary hydrogen molecule. Nevertheless, such clusters are common in natural environments, for example, in the interstellar space. Therefore, ab initio calculations of small hydrogen clusters were performed in the present work, and the quantum chemical method was used to calculate the entropy and frequencies of zero vibrations of hydrogen isotopes.

Molecular dynamics simulations of anisotropic explosions of small hydrogen clusters in intense laser pulses

The explosion dynamics of small hydrogen clusters irradiated by intense femtosecond infrared laser pulses is investigated by classical molecular dynamics simulations. We find a spatial anisotropy in these explosions with proton energies enhanced along the laser polarization direction. Our simulations can identify the origin of this anisotropy: the interplay between the space charge separation in the early stage of cluster ionization and the Coulomb attraction between the rescattered electrons and protons during cluster explosion.

Adsorption configurations and scanning voltage determined STM images of small hydrogen clusters on bilayer graphene

By density functional theory calculations, the scanning tunneling microscopy (STM) images of various hydrogen clusters adsorbed on bilayer-graphene are systematically simulated. The hydrogen configurations of the STM images observed in the experiments have been thoroughly figured out. In particular, two kinds of hydrogen dimers (ortho-dimer, para-dimer) and two kinds of tetramers (tetramer-A, -B) are determined to be the hydrogen configurations corresponding to the ellipsoidal-like STM images with different structures and sizes. One particular hexamer (hexamer-B) is the hydrogen configuration generating the star-like STM images. For each hydrogen cluster, the simulated STM images show unique voltage-dependent features, which provides a feasible way to determine hydrogen adsorption states on graphene or graphite surface in the experiments by varying-voltage measurements. Stability analysis proves that the above determined hydrogen configurations are quite stable on graphene, hence they are likely to be detected in the STM experiments. Consequently, through systematic analysis of the STM images and the stability of hydrogen clusters on bilayer graphene, many experimental observations have been consistently explained.

Hydrogen uptake by graphene and nucleation of graphane

Reactions of hydrogen with electronic materials are important for the operation of related devices. Here we use first-principles density-functional theory calculations to describe hydrogen reactions on pristine and defective graphene. We show that small hydrogen clusters on defect-free graphene are unstable against emission of hydrogen molecules and that the associated reaction energies and barriers have a subtle dependence on the type of the clusters. In contrast, chemisorption of hydrogen in the vicinity of graphene vacancies leads to progressively larger clusters of adatoms and, eventually, to formation of graphane. The results are relevant to the optimization of graphene- and graphane-based devices, as well to the creation of graphene–graphane hybrid systems.

Structure and stability of small H clusters on graphene

The structure and stability of small hydrogen clusters adsorbed on graphene is studied by means of density functional theory (DFT) calculations. Clusters containing up to six H atoms are investigated systematically, with the clusters having either all H atoms on one side of the graphene sheet (cis-clusters) or having the H atoms on both sides in an alternating manner (trans-clusters). The most stable cis-clusters found have H atoms in ortho- and para-positions with respect to each other (two H's on neighboring or diagonally opposite carbon positions within one carbon hexagon), while the most stable trans-clusters found have H atoms in ortho-trans-positions with respect to each other (two H's on neighboring carbon positions, but on opposite sides of the graphene). Very stable trans-clusters with 13--22 H atoms were identified by optimizing the number of H atoms in ortho-trans-positions and thereby the number of closed, H-covered carbon hexagons. For the cis-clusters, the associative ${\mathrm{H}}_{2}$ desorption was investigated. Generally, the desorption with the lowest activation energy proceeds via para-cis-dimer states, i.e., involving somewhere in the H clusters two H atoms that are positioned on opposite sites within one carbon hexagon. ${\mathrm{H}}_{2}$ desorption from clusters lacking such H pairs is calculated to occur via hydrogen diffusion causing the formation of para-cis-dimer states. Studying the diffusion events showed a strong dependence of the diffusion energy barriers on the reaction energies and a general odd-even dependence on the number of H atoms in the cis-clusters.

Experiments on small (H2)N clusters

The nucleation process of small hydrogen clusters has been studied in detail with space, time, and number size resolution on the basis of supersonic expansions of the gas through a cryogenic nozzle operating in the thermal range 24 < T <110 K. The diagnostic of the jet medium is based on high sensitivity (few photons/sec) Raman spectroscopy with very high spatial resolution (\(\approx 2~\mu\)m), enabling for quantitative measurements with time resolution in the domain of nanoseconds. Temperatures down to 0.1 K have been attained in the jet at a collisional rate low enough to avoid sudden freezing of the gas. Clusters between two and eight molecular units are observed size-resolved. Features of the ortho-ortho and ortho-para hydrogen dimers, and the role of three-body collisions for nascent dimers and trimers are reported.

Magnetic moments of small hydrogen clusters

Molecular beams of small normal hydrogen (nH 2) clusters ( N≈20) produced in a low temperature free jet expansion are investigated for their magnetic properties by deflecting them in an inhomogeneous Stern–Gerlach type magnetic field. The mass distributions of the clusters are measured by diffraction from a nano-structured transmission grating. With their masses known the Gaussian shaped symmetric deflection patterns of the pure nH 2 clusters can be simulated by a random distribution of both the nuclear and rotational magnetic moments of the 75% ortho J=1 component. Upon cooling the nH 2 clusters to below 1 K by expanding a mixture of 5% nH 2 in 95% 4He gas the deflection pattern is split into three central peaks. This observation is consistent with a partial orientational ordering of the ortho H 2 molecules leading to a reduction of the total rotational magnetic moments of the clusters.

Vicinage forces between molecular and atomic fragments dissociated from small hydrogen clusters and their effects on energy distributions

In this paper we analyze the dynamic evolution of molecular and atomic fragments of small hydrogen clusters interacting with thin solid foils. We compare the vicinage forces, calculated within the dielectric formalism, for H 1 ,H 0 , and H2 1 fragments. Using a molecular dynamics numerical code we determine the energy distribution of the fragments after interacting with the target. This distribution is compared to experimental results for protons coming from the fragmentation of v52.02 a.u. H2 1 ions impinging on an aluminum foil; a fraction of neutral H 0 is needed to be included in the simulation to get a good agreement with the experimental results. The H2 1 energy spectra for v55.42 a.u. H3 1 interacting with amorphous carbon is also determined. The asymmetry in the Coulomb peaks appearing in the energy spectra both experimentally and in our calculation is opposite for H2 1 than in H 1 ; kinematic effects and differences in the electronic stopping are enough to reproduce the difference in the alignment of H 2 1 and H 1 fragments.

AFM studies of hydrogen implanted silicon

The effects of hydrogen implantation on the surface microtopography of 〈100〉 silicon single crystals were analyzed by means of atomic force microscopy (AFM) technique. The main objective of the study was to get a new insight into the processes occurring during the early stages of hydrogen blister formation in SmartCut® technology used in Silicon-on-Insulator devices manufacturing. The results revealed that hydrogen blistering sets in at doses about 4×10 16 H 2 +/cm 2 by a formation of small, few tens of nanometer high bubbles. The increase of the implantation dose results first in the growth of the bubbles up to few hundreds nm and then in the formation of the next generation of small hydrogen clusters. The detailed analysis of the SiO 2 surface layer formed on the silicon crystals indicated the presence of small, few nm in size, hillock-like structures which can be responsible for the difficulties in SiO 2–SiO 2 face-to-face bonding. A possible use of implantation induced microtopography changes in Microelectronics and Micromechanical Systems applications is also discussed.

Hydrogen microstructure in amorphous hydrogenated silicon.

Using nuclear magnetic resonance (NMR) and infrared (ir) absorption spectroscopy, this paper demonstrates the effects of deposition temperature, annealing temperature, and dopant-atom incorporation on the hydrogen microstructure of plasma-deposited amorphous silicon. Multiple-quantum NMR studies indicate clusters of five to seven hydrogen atoms in films prepared at temperatures ranging from 113 to 324 \ifmmode^\circ\else\textdegree\fi{}C. In the range from 270 to 324 \ifmmode^\circ\else\textdegree\fi{}C, only these small clusters exist, but lower-temperature films also contain larger clusters. A comparison of the ir and NMR results allows the assignment of the 2080-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ absorption to small hydrogen clusters. Annealing studies indicate hydrogen rearranges prior to evolution. Deposition temperature and annealing temperature have similar effects on hydrogen concentration, but deposition temperature was found to control the density and microstructure of the film. The addition of dopant atoms also affects the hydrogen micro- structure, with phosphorous causing larger hydrogen clusters to form, and boron reducing clustering in the film. This perturbation of the film's microstructure suggests that the effects of dopant addition are much more complex in amorphous than in crystalline semiconductors. We surmise that hydrogen microstructure, rather than hydrogen content, determines amorphous-silicon properties.