On the electronic structure and chemical bonding of titanium tetraauride: TiAu 4 and TiAu 4 −

Electronic Structure and Anion Photoelectron Spectroscopy of Uranium-Gold Clusters UAun-, n = 3-7

A collaborative effort between experiment and theory toward elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAun-(n = 3-7) were taken at the third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193 nm) photon energies, where the experimental adiabatic electron affinities and vertical detachment energies values were measured. Complementary first-principles calculations were subsequently carried out to corroborate experimentally determined electron detachment energies and to determine the geometry and electronic structure for each cluster. Except for the ring-like neutral isomer of UAu6 where one unpaired electron is spread over the Au atoms, all other neutral and anionic UAun clusters (n = 3-7) were calculated to possess open-shell electrons with the unpaired electrons localized on the central U atom. The smaller clusters closely resemble the analogous UFn species, but significant deviations are seen starting with UAu5 where a competition between U-Au and Au-Au bonding begins to become apparent. The UAu6 system appears to mark a transition where Au-Au interactions begin to dominate, where both a ring-like and two heavily distorted octahedral structures around the central U atom are calculated to be nearly isoenergetic. With UAu7, only ring-like structures are calculated. Overall, the calculated electron detachment energies are in good agreement with the experimental values.

Predicting the stability and electronic structure of alkali metal aurides

Density functional theory calculations of phonon modes predict that some compounds of the alkali metal aurides family, general formulaA₂MAu₆(A= K, Rb or Cs;M= Ti, Zr, Hf, Sn or Pb), have stable three-dimensional phase with a double perovskite-type structure and cubicFm3¯mspace group (K₂PtCl₆-type). Bader's charge analysis shows that most electron density is located within the six atoms at the octahedra vertices like double perovskite halides. However, the short spacing between Au anions enables d-orbital interactions between them. Compounds of this family, with group 4 metals only, carry conduction states around the Γ point (k= 0). On the other hand, compounds with group 14 metals possess more conduction states around all the Brillouin zone and have electron pockets in their band structures. These compounds provide further insights into the unusual anionic behavior of gold and present other alternatives for the construction of divergent nanodevices.

Anion Photoelectron Spectroscopy and High Level Ab Initio Calculations of the Halide–Nitric Oxide Dimer Complexes

Anion photoelectron spectra are presented for gas phase complexes formed between halide anions and nitric oxide, X−⋯NO where X− = Cl−, Br−, and I−. Electron binding energies are experimentally determined to be 3.82, 3.51, and 3.17 eV. Results from CCSD(T)/aug-cc-pVTZ calculations are presented for the anion species, whereby a single minimum of Cs symmetry is predicted. Binding energies (D0) of 15.3, 13.3, and 11.7 kJ mol−1 are predicted from complete basis set limit extrapolation, and are found to be in line with previous experimental studies.

Strong Influence of Ti Adhesion Layer on Electron-Phonon Relaxation in Thin Gold Films: Ab Initio Nonadiabatic Molecular Dynamics

Electron-phonon relaxation in thin metal films is an important consideration for many ultrasmall devices and ultrafast applications. Recent time-resolved experiments demonstrate a significant, more than a factor of five, increase in the electron-phonon coupling and acceleration in the electron-phonon relaxation rate when a narrow Ti adhesion layer is introduced between an Au film and a nonmetal substrate. Using nonadiabatic molecular dynamics combined with real-time time-dependent density functional theory, we identify the reasons that give rise to this strong effect. First, the Ti layer greatly enhances the density of states (DOS) in the energy region starting at 1 eV below the Fermi level and extending several electronvolts above it. Second, Ti atoms are four times lighter than Au atoms and therefore generate larger nonadiabatic (NA) electron-phonon coupling. Because the transition rates depend on coupling and DOS, both the factors accelerate the dynamics. Showing good agreement with the experiments, the time-domain atomistic simulations provide a detailed mechanistic understanding of the electron-phonon relaxation dynamics in thin gold films and related nanomaterials.

Influence of Ti and Cr Adhesion Layers on Ultrathin Au Films

Efficient adhesion of gold thin films on dielectric or semiconductor substrates is essential in applications and research within plasmonics, metamaterials, 2D materials, and nanoelectronics. As a consequence of the relentless downscaling in nanoscience and technology, the thicknesses of adhesion layer and overlayer have reached tens of nanometers, and it is unclear if our current understanding is sufficient. In this report, we investigated how Cr and Ti adhesion layers influence the nanostructure of 2-20 nm thin Au films by means of high-resolution electron microscopy, complemented with atomic force microscopy and X-ray photoelectron spectroscopy. Pure Au films were compared to Ti/Au and Cr/Au bilayer systems. Both Ti and Cr had a striking impact on grain size and crystal orientation of the Au overlayer, which we interpret as the adhesion layer-enhanced wetting of Au and the formation of chemical bonds between the layers. Ti formed a uniform layer under the Au overlayer. Cr interdiffused with the Au layer forming a Cr-Au alloy. The crystal orientation of the Au layers was mainly [111] for all thin-film systems. The results showed that both adhesion layers were partially oxidized, and oxidation sources were scrutinized and found. A difference in bilayer electrical resistivity between Ti/Au and Cr/Au systems was measured and compared. On the basis of these results, a revised and more detailed adhesion layer model for both Ti/Au and Cr/Au systems was proposed. Finally, the implications of the results were analyzed, and recommendations for the selection of adhesion layers for nano-optics and nanoelectronics applications are presented.

Gas phase anion photoelectron spectroscopy and theoretical investigation of gold acetylide species

We conducted gas phase anion photoelectron spectroscopy and density functional theory studies on a number of gold acetylide species, such as AuC₂H, AuC₂Au, and Au₂C₂H. Based on the photoelectron spectra, the electron affinities of AuC₂H, AuC₂Au, and Au₂C₂H are measured to be 1.54(±0.04), 1.60(±0.08), and 4.23(±0.08) eV, respectively. The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of AuC₂H and AuC₂Au are measured to be about 2.62 and 2.48 eV, respectively. It is interesting that photoelectron spectra of AuC₂H^- and AuC₂Au^- display similar spectral features. The comparison of experimental and theoretical results confirms that the ground-state structures of AuC₂H^-, AuC₂Au^-, and their neutrals are all linear with Au-C≡C-H and Au-C≡C-Au configurations. The similar geometric structures, spectral features, HOMO-LUMO gaps, and chemical bonding between AuC₂H^-/0 and AuC₂Au^-/0 demonstrate that Au atom behaves like H atom in these species. The photoelectron spectrum of Au₂C₂H^- shows that Au₂C₂H has a high electron affinity of 4.23(±0.08) eV, indicating Au₂C₂H is a superhalogen. Further, we found an unusual similarity between the terminal Au atom of Au₂C₂H^- and the iodine atom of IAuC₂H^-.