Bonding and Charge Transfer in Metal–Organic Coordination Networks on Au(111) with Strong Acceptor Molecules

Abstract

The geometric and electronic structure of two structurally similar metal–organic networks grown on the Au(111) surface is investigated by scanning tunnelling microscopy (STM) and spectroscopy (STS) combined with density functional theory (DFT) calculations. The networks are composed of (i) F4TCNQ (C12F4N4, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino–dimethane) molecules and Au adatoms segregated from the pristine metal surface, and (ii) TCNQ (C12H4N4, 7,7,8,8-tetracyanoquinodimethane) and codeposited Mn atoms. In both cases, the strong electron acceptor character of the molecules results in metal–to-ligand charge transfer to the lowest unoccupied molecular orbital (LUMO). The amount of electrons donated from the 4-fold coordinated Mn atoms to TCNQ is higher compared to the 2-fold coordinated Au adatoms to F4TCNQ. This behavior is reflected in the appearance of distinct spectral features in STS data in the energy region close to the Fermi level resulting from the intricate interplay between surface states, adatom states, and molecular orbitals. These observations are consistent with a picture in which the LUMO of the TCNQ acceptor molecule hybridizes with Mn and Au substrate metal states becoming practically filled, while the LUMO of F4TCNQ is only partially filled despite being the stronger electron acceptor. Our results reveal the importance of the type of bonding between the strong acceptor and the metal center (Au or Mn) as well as its coordination in the determination of the charge transfer to the adlayer, which is important for its electronic properties.

Temperature-controlled metal/ligand stoichiometric ratio in Ag-TCNE coordination networks

“…4 The inclusion of surface atoms in coordination networks with organic ligands has been thoroughly investigated on copper surfaces, where the high reactivity of the spontaneously formed surface adatoms facilitates the formation of metal-organic networks. Similar coordination networks with substrate adatoms have also been observed on other noble metal surfaces, such as Au(111), [5][6][7] Ag(110), 8 and Ag(111). 9,10 For silver and gold surfaces, the inclusion of the relatively unreactive surface adatoms into coordination networks is achieved by using very reactive molecules such as surface radicals arising from on-surface dehalogenation of halogenated compounds, [8][9][10] or very strong organic acceptors such as fluorinated tetracyanoquinodimethane (F4-TCNQ).…”

“…9,10 For silver and gold surfaces, the inclusion of the relatively unreactive surface adatoms into coordination networks is achieved by using very reactive molecules such as surface radicals arising from on-surface dehalogenation of halogenated compounds, [8][9][10] or very strong organic acceptors such as fluorinated tetracyanoquinodimethane (F4-TCNQ). 7 While the participation of adatoms arising from the metal surface facilitates the preparation of coordination networks, it also limits our ability to tune their physico-chemical properties. Of course, using surface atoms as metal centers in coordination networks determines the chemical nature of the metal centers, but moreover the coordination networks with substrate adatoms reported so far in previous studies show a unique value for the metal/ligand stoichiometric ratio.…”

“…Of course, using surface atoms as metal centers in coordination networks determines the chemical nature of the metal centers, but moreover the coordination networks with substrate adatoms reported so far in previous studies show a unique value for the metal/ligand stoichiometric ratio. [1][2][3][4][5][6][7][8][9][10] Actually, raising the temperature should enhance the availability of surface adatoms, thereby enabling the formation of coordination networks with different metal/ligand stoichiometries. Different metal/ligand stoichiometries have readily been observed by codepositing Fe atoms and carboxylic acid molecules in different relative amounts.…”

Recent advances in computational studies of organometallic sheets: Magnetism, adsorption and catalysis

“…Based on the recent progress in 2D metal-7,7,8,8-tetracyanoqui nodimethane (TCNQ) coordination networks assembled from TCNQ molecules [46][47][48][49][50], Ma et al present a systematic theoretical study on the structural, electronic, and magnetic properties of the novel tetragonal TM-based TCNQ molecule coordination single sheets (referred to as TM@TCNQ, TM = CrÀCo) [51]. Their results unveiled that, in TM@TCNQ, two valence electrons would transfer from one TM atom to TCNQ molecules, making them more stable.…”

Breaking time-reversal symmetry at the topological insulator surface by metal-organic coordination networks

“…Namely, to achieve the long-range FM order in the system of adatoms at the TI surface one has to organize their ordered arrays [14,34,35] and prevent them from diffusion [44,47] or intercalation inside the bulk or the van der Waals gaps [34,[51][52][53][54]. Lately, a type of system which may provide these conditions were successfully grown on metallic substrates-selfassembled metal-organic coordination networks (MOCNs) [55][56][57][58][59][60][61][62][63][64]. In such experiments, strong electron acceptors like tetracyanoethylene (TCNE, C 6 H 4 ), tetracyanobenzene (TCNB, C 10 H 2 N 4 ), and tetracyanoquinodimethane (TCNQ, C 12 H 4 N 4 ) are the most frequently used molecules.…”