Dihydride Units Research Articles

Ligand Protonation Triggers H2 Release from a Dinickel Dihydride Complex to Give a Doubly “T”‐Shaped Dinickel(I) Metallodiradical

AbstractThe dinickel(II) dihydride complex (1K) of a pyrazolate‐based compartmental ligand with β‐diketiminato (nacnac) chelate arms (L−), providing two pincer‐type {N3} binding pockets, has been reported to readily eliminate H2 and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2Na, 2K) of L− are now synthesized via a direct reduction route. They feature two adjacent T‐shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local type magnetic orbitals are oriented into the bimetallic cleft, enabling metal–metal cooperative 2 e− substrate reductions as shown by the rapid reaction with H2 or O2. X‐ray crystallography reveals distinctly different positions of the K+ in 1K and 2K, suggesting a stabilizing interaction of K+ with the dihydride unit in 1K. H2 release from 1K is triggered by peripheral γ‐C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H2 elimination from the bimetallic cleft.

Ligand Protonation Triggers H2 Release from a Dinickel Dihydride Complex to Give a Doubly "T"-Shaped Dinickel(I) Metallodiradical.

The dinickel(II) dihydride complex (1K) of a pyrazolate‐based compartmental ligand with β‐diketiminato (nacnac) chelate arms (L−), providing two pincer‐type {N3} binding pockets, has been reported to readily eliminate H2 and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2Na, 2K) of L− are now synthesized via a direct reduction route. They feature two adjacent T‐shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local dx2-y2 type magnetic orbitals are oriented into the bimetallic cleft, enabling metal–metal cooperative 2 e− substrate reductions as shown by the rapid reaction with H2 or O2. X‐ray crystallography reveals distinctly different positions of the K+ in 1K and 2K, suggesting a stabilizing interaction of K+ with the dihydride unit in 1K. H2 release from 1K is triggered by peripheral γ‐C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H2 elimination from the bimetallic cleft.

Hydrogen inserted into the Si(100)-2 × 1-H surface: a first-principles study.

Hydrogen can be inserted into Si(100)-2 × 1-H during surface preparation or during the hydrogen desorption lithography used to create atomic-scale devices. Here, a hydrogen atom inserted into a hydrogen monolayer on the Si(100)-2 × 1 surface has been studied using density functional theory. Hydrogen-induced defects were considered in their neutral, negative, and positive charge states. It was found that hydrogen forms a dihydride unit on the surface in the most stable neutral and negative charge states. Hydrogen located in the groove between dimer rows is also one of the most stable negative charge states. In the positive charge state, hydrogen forms a three-center bond inside a Si dimer, Si-H-Si, similar to the bulk case. A comparison of simulated scanning tunneling microscopy (STM) images with the experimental data available in the literature showed that neutral and negatively charged hydrogen-induced defects were already observed in experiments. The results reveal that the H atom inserted into a hydrogen monolayer on the Si(100)-2 × 1 surface can lead to the formation of a positively or negatively charged defect. It is shown that H atoms in the considered configurations can play a role in various surface reactions.

Low-temperature silicon epitaxy on hydrogen-terminated Si(001) surfaces

Si deposition on H terminated $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surfaces at temperatures $300--530\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is studied by scanning tunneling microscopy. Hydrogen apparently hinders Si adatom diffusion and enhances surface roughening. The post-growth annealing effect is analyzed. Hydrogen is shown to remain on the growth front up to at least $10\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$. Si deposition onto the $\mathrm{H}∕\mathrm{Si}(001)\text{\ensuremath{-}}3\ifmmode\times\else\texttimes\fi{}1$ surface at $530\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ suggests that dihydride units further suppress Si adatom diffusion and increase surface roughness.

Structural characterization of the hydrogen-covered C(100) surface by density functional theory calculations

We present the results of a density functional theory (generalized gradient approximation) study of the hydrogenated C(100) surfaces. We have analyzed the formation energy of phases with different hydrogen coverages $(\ensuremath{\theta}=1.0,$ 1.2, 1.33, 1.5, and 2.0) as a function of the hydrogen chemical potential. As the hydrogen chemical potential increases, the stable phase changes from the bare surface through all the hydrides considered, in order of increasing coverage. The value of the hydrogen chemical potential beyond which dihydride units are stabilized on the surface nearly coincides with the potential at which the the formation energy of methane is zero. However, since the desorption of hydrocarbons is an activated process, dihydride units can appear in metastable surface phases. To investigate this possibility, we have calculated vibrational frequencies for the $(2\ifmmode\times\else\texttimes\fi{}1):\mathrm{H},$ $(5\ifmmode\times\else\texttimes\fi{}1):1.2\mathrm{H},$ and $(3\ifmmode\times\else\texttimes\fi{}1):1.33\mathrm{H}$ phases of the H/C(100) surface. The presence of a dihydride unit on the surface leads to a H-C-H bending mode with a frequency near $1475{\mathrm{cm}}^{\ensuremath{-}1}.$ Because there is no vibrational mode with a frequency in this region of the spectrum for the monohydride surface, this peak may be viewed as evidence that dihydride units are present on the surface.

First-principles study of vibrational spectra on dihydride-terminated Si(001)/H surfaces

The vibrational spectra on the dihydride-terminated Si(001) surfaces are calculated with a density-functional theory in combination with the finite-element method. On the 1×1 surface, the stretching modes of the SiH bonds are found to split as a result of the HH interaction between adjacent dihydride units. The width of this splitting is estimated to be about 50 cm −1. Especially the upper peak corresponds to the stretching mode of the lying SiH bond of the canted dihydride unit. Its frequency is expected to be similar to that of the trihydride unit on the atomically rough Si(001)/H surface.

A quasi-equilibrium model for the uptake kinetics of hydrogen atoms on Si(100)

A quasi-equilibrium model has been developed to describe the uptake kinetics of atomic hydrogen on Si(100) at 373 and 635 K. A new model is required because a simple consideration of only adsorption at dangling bond sites and Eley-Rideal abstraction cannot be reconciled with a saturation coverage of one monolayer (ML) at 635 K. We argue that although diffusion at low temperatures can be explained by hot precursor dynamics, slow vibrational relaxation in the chemisorption potential does not lead to an almost coverage independent sticking probability. Instead, we propose that a high sticking probability is maintained by reaction with doubly occupied dimers to give dihydride species which then migrate across the surface by an isomerization reaction. Subsequently, dihydride units either react with unoccupied dimer sites or molecular hydrogen is lost by desorption from two dihydride units (the β 2 desorption channel). At 635 K the dihydride species may be regarded as a mobile chemisorbed precursor, although only the bonding arrangement is propagated by isomerization. When the atomic hydrogen source is turned off, the small steady-state dihydride coverage is rapidly lost to leave a saturated monohydride phase. A saturation coverage of 1.5 ML is obtained when abstraction and adsorption reactions are in dynamic equilibrium at 373 K. It is shown that the rate constant for abstraction from monohydride is essentially the same as for abstraction from dihydride. The measured initial rate constant for loss of adsorbed hydrogen during exposure to atomic deuterium is (1.8 ± 0.1) times larger than that for loss of adsorbed deuterium during atomic hydrogen exposure at 635 K. At this temperature, the initial rate constant for loss of deuterium during exposure to atomic hydrogen is found to be the same regardless of initial coverage, but the rate decreases more slowly than expected for first-order kinetics as the reaction proceeds. The uptake model also implies that a small amount of deuterium is lost as D 2 and HD via the β 2 thermal desorption channel at 635 K.

Atomic scale extraction of hydrogen atoms adsorbed on Si(001) with the scanning tunneling microscope

Atomic scale extraction of hydrogen atoms on Si(001)-(2 × 1)-H and -(3 × 1)-H surfaces between 300 and 610 K have been studied using a scanning tunneling microscope. Hydrogen atoms were extracted at positive and negative sample bias voltage. Site-selective desorption and reconstruction from the dihydride phase to the monohydride phase are found in the desorption of hydrogen atoms from Si(001)-(3 × 1)-H surfaces. STM-induced desorption of H atoms on a Si(001)-(3 × 1)-H surface occurs preferentially at dihydride units rather than monohydride units. The results support an electric field induced desorption.

Hydrogen desorption from ion-roughened Si(100)

Low energy Kr+ ions were used to create rough Si(100) surfaces. Hydrogen adsorption and desorption on these surfaces were studied using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). A comparison of hydrogen TPD data from smooth and ion-roughened Si(100) surfaces showed no significant shift for the dihydride and monohydride related desorption features, revealing that ion impact roughening does not change the activation barriers for hydrogen desorption. In contrast to smooth Si(100), HREELS spectra from rough Si(100) surfaces with different hydrogen coverages showed a dihydride scissor mode at 0.8 ML and dihydride desorption below monolayer coverage in the corresponding TPD experiment. The vibrational spectra of hydrogen covered rough surfaces generally showed a smaller ratio of the scissor mode/bending mode intensity than the smooth Si(100) surfaces, suggesting a lower dihydride/monohydride coverage ratio. The corresponding TPD results on the other hand showed increased dihydride desorption and higher total coverages up to 2 ML. Although different dihydride units on terrace sites and at edge sites could not be unambiguously identified in the vibrational spectra, they provide a reasonable explanation for this inconsistency.

Hydrogen elimination and phase transitions in pulsed-gas plasma deposition of amorphous and microcrystalline silicon

Removal of hydrogen from the growth surface during silane plasma deposition of silicon is correlated with the transition from amorphous to microcrystalline film structure. Plasma deposition experiments were performed using a pulsed gas technique, where repeated steps of thin amorphous silicon film deposition, and atomic hydrogen (or deuterium) exposure are used to form microcrystalline and polycrystalline thin films at substrate temperatures below 250 °C. Infrared absorption and Raman spectroscopy are used to estimate the silicon-hydrogen bonding concentrations, and characterize crystal structure, respectively. Hydrogen elimination probed using real-time differentially pumped mass spectroscopy demonstrates that during atomic deuterium exposure, hydrogen abstraction by deuterium, rather than silicon etching, is the primary mechanism for hydrogen removal from the depositing surface. Polycrystalline material, with no shoulder at 480 cm−1 in the Raman spectrum, and grain sizes greater than 1000 Å, as determined by transmission electron microscopy, have been formed at temperatures below 250 °C. The amorphous to crystal transition is observed at substrate temperatures as low as 25 °C, with longer hydrogen exposure required at lower temperatures. Hydrogen is shown to be preferentially abstracted from monohydride (Si–H) units as compared to dihydride (SiH2) units at or near the depositing growth surface, consistent with ab initio energy calculations of hydrogen interactions with silicon hydrides. A transition in hydrogen removal kinetics is observed upon film crystallization, where the rate of hydrogen removal is reduced for more crystalline materials. These results are valuable for understanding surface reactions in low temperature crystalline silicon deposition, for example, for fabrication of high mobility thin film transistor structures on glass.

Atomic and electronic structure of the diamond (100) surface: Reconstructions and rearrangements at high hydrogen coverage

The atomic and electronic structure of the diamond (100) surface has been investigated theoretically via a semiempirical tight-binding model for a range of hydrogen coverages. Model parameters for C-C interactions have been taken from the work of Goodwin [J. Phys. Condens. Matter 3, 3869 (1993)], while new parameter sets have been determined for C-H and H-H interactions. The model gives results for the clean and monohydrogenated surfaces in good agreement with previous studies, but different features have been identified for higher H coverages. When the H coverage is sufficiently high, the substrate lattice is found to distort in order to reduce steric repulsions between dihydride units. As an important example, we obtain two structures for the dihydrogenated surface that are significantly more stable than those proposed previously. For H coverages intermediate between the monohydrogenated and dihydrogenated surfaces, stable geometries consisting of monohydrogenated dimer units and dihydride units are found. In contrast, geometries that include isolated monohydride units, such as have been previously investigated, are found to be thermodynamically and kinetically unstable. Tight-binding molecular dynamics is used to illustrate a mechanism for the rapid removal of isolated monohydride units. The electronic structures of the surfaces are described via the total and partial electronic densities of state, which are obtained directly from the tight-binding Hamiltonian. For the monohydrogenated surface and higher coverages, the stable geometries are found to yield no states in the bulk band gap.

Ab initio calculation of hydrogen abstraction energetics from silicon hydrides

In this article, we present calculated energies for the abstraction of hydrogen from silicon monohydride and silicon dihydride surface bonding units by atomic hydrogen obtained using ab initio configuration interaction theory. Three and four silicon atom clusters are used to model the dihydride and monohydride units, respectively. Heats of reaction and activation energy barriers are calculated, including the vibrational energies of the initial, final, and transition states. Hydrogen abstraction from a Si–H unit (H+Si4H10→Si4H9+H2) is found to be exothermic by 9.4 kcal/mol with a transition state energy barrier of 5.5 kcal/mol when H approaches along the surface normal. The dihydride abstraction reaction, H+Si3H8→Si3H7+H2, is exothermic by 7.7 kcal/mol and has an energy barrier of 7.3 kcal/mol when H is approaching along Si–H axis. The barrier is larger for hydrogen atom approaching along the surface normal. The larger barrier for abstraction from a dihydride unit is consistent with our experimental observation of a preferential reduction in monohydride bond concentrations when hydrogenated silicon films are exposed to atomic hydrogen during plasma deposition.

Scanning tunneling microscopy of the phase transition between H/Si(100)-(2 x 1) and H/Si(100)-(3 x 1).

The transition from H/Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) to H/Si(100)-(3\ifmmode\times\else\texttimes\fi{}1) that occurs upon dosing atomic hydrogen at 400 K has been studied using scanning tunneling microscopy. It is characterized by a series of interactions between hydrogen and the strained silicon bonds via local adsorption and thermal decomposition of dihydride units. The transition starts from a paired H adsorption on a monohydride dimer on the 2\ifmmode\times\else\texttimes\fi{}1 surface, followed by the formation of local 3\ifmmode\times\else\texttimes\fi{}1 domains. The growth of 3\ifmmode\times\else\texttimes\fi{}1 domains is found anisotropic along the dimer row direction due to characteristic antiphase boundaries. A lateral dihydride-row diffusion has been observed, indicating that the local 3\ifmmode\times\else\texttimes\fi{}1 domains are correlated in their expansions, suggesting a plausible pathway for conversion of the spreading 3\ifmmode\times\else\texttimes\fi{}1 domains into a well-defined phase. Etching species have always been found on the surface, which are related to defects on the initially clean surface and breaking of back bonds during atomic hydrogen dosing. \textcopyright{} 1996 The American Physical Society.

Role of bond-strain in the chemistry of hydrogen on the Si(100) surface

A scanning tunneling microscopy (STM) study of the chemistry of hydrogen on the Si(100) surface is presented. The structure and stability of the hydrogenated 2 × 1, 3 × 1 and 1 × 1 surfaces are studied. The 2 × 1 and 3 × 1 surfaces are shown to be well ordered and stable in the presence of atomic hydrogen under their respective formation conditions. In contrast, the 1 × 1 structure is poorly ordered and susceptible to spontaneous etching by hydrogen atoms. No uniform 1 × 1 dihydride phase was observed under any conditions. The existence of this intermediate 3 × 1 phase and the susceptibility of the 1 × 1 structure to etching are both shown to be due to the strain associated with the dihydride units on the 1 × 1 surface. The repulsive steric interaction between dihydride units on this surface weakens the Si-H bonds and stabilizes the 3 × 1 surface observed at 400 K. The Si-Si backbonds of these dihydride units are also strained, resulting in a lower barrier to reaction which is responsible for the etching observed on the 1 × 1 surface.

The a-SiO x:H y thin film system I. Structural study by IR spectroscopy

A systematic study of the trends in the IR spectra of a-SiO x :H y films with both y and x is presented for the first time. The measurements carried out on four series of room temperature rf sputtered films reveal a dramatic decrease in the concentration of bonded hydrogen in the films as x increases beyond 0.7. The spectral features are compared with those of previously reported low-oxygen high-temperature glow-discharged films, and it is found that the x = 0.7 series of films has the monohydride HSi 3O as the predominant structural unit, while the x = 1.1 and 1.5 series contain this units as well as isolated dihydride units. The fine structure in the SiH stretch peak generally supports the above conclusions and explicitly indicates the presence of HSi 3O, H 2Si 2O, HSi 2O 2 and HSiO 3 units in the x = 1.5 series of films.

New ordered structure for the H-saturated Si(100) surface: The (3 x 1) phase.

High-resolution infrared data of Si(100) surfaces saturated with H at room temperature show that the so-called 1\ifmmode\times\else\texttimes\fi{}1 surface is in fact a disordered phase with roughly half the H bonded in monohydride (H-Si-Si-H) and half in dihydride (Si${\mathrm{H}}_{2}$) configurations. Upon saturation exposure at 380 K, the surface is ordered into a newly identified 3\ifmmode\times\else\texttimes\fi{}1 phase, as evidenced by LEED, involving repeated alternating monohydride and dihydride units.