CAPÍTULO 3. DISEÑO Y SIMULACIÓN DEL ARREGLO DE ANTENAS
3.4 Simulación del sistema compuesto por el arreglo y las redes adaptadoras de
STM studies show that when adsorbed on the surface of Au(111), TCNQ packs into an incommensurate ‘brickwork’ structure that promotes the formation of intermolecular N∙∙∙H hydrogen bonds.129, 156 This brickwork structure bears striking resemblance to layers of the TCNQ crystal structure when viewed normal to the (020) direction157 as well as TCNQ adsorbed on an Ir(111)-supported graphene layer, where molecule-substrate interactions are weak.158 This suggests that the observed structure is largely independent of the underlying substrate resulting from the weak molecule-substrate interactions relative to the intermolecular hydrogen bonds. This conclusion is further supported by the Au(111) surface retaining its herringbone reconstruction upon adsorption of TCNQ,129, 156 a feature that is usually lifted in the presence of strongly interacting molecules. UPS and XPS measurements indicate that TCNQ adsorbs in a neutral charge state on Au(111), owing to the relatively high work function of the substrate.57, 159 In contrast, F4-TCNQ, which has a much greater electron affinity than TCNQ, does become negatively charged on Au(111) and subsequently adsorbs in a commensurate structure in which STM shows the lifting of the herringbone reconstruction and also suggests that Au adatoms, abstracted from the underlying substrate, are incorporated into the molecular layer.54, 160 This implies that charge transfer leads to stronger interactions with the substrate.
Figure 3.2.1 – Skeletal representation of TCNQ, TCNQ- and TCNQ2-. Upon uptake of electrons, the central quinoid ring of TCNQ aromatises, which disrupts the conjugated π-system that extends through the molecule. The negative charge is largely accommodated by the electronegative cyano groups.161
with the central ring adsorbing at ~3.0 Å above the surface with the peripheral CN groups pointing down to interact with the substrate at a height of ~2.4 Å.128 This result conflicts with the experimental observations as the calculated adsorption height is indicative of relatively strong TCNQ-Au bonding interactions that have some covalent character. Furthermore, the molecular conformation is not consistent with that expected of a neutral TCNQ molecule as TCNQ has a rigid and planar structure owing to the conjugated π-system that extends throughout the molecule.17, 161 However, upon uptake of one or more electrons, the central quinoid ring of TCNQ aromatises, with the additional electron(s) being accommodated by the electron-withdrawing cyano groups (Figure 3.2.1).161 This aromatisation disrupts the π-conjugation, with the peripheral carbons becoming sp3-hybridised, thus giving the molecule greater flexibility.161
It is thus reasonable to expect that the molecule would only deviate from its planar conformation for substrates on which it accepts electrons, though the published DFT models for TCNQ on Au(111) do not reflect this.128 It should, however, be noted that these calculations do not include corrections for long range dispersion interactions,128 which are likely to have an effect on the adsorption structure.24 Additionally, due to the periodic boundary conditions of DFT codes, the exact periodicity of the incommensurate TCNQ Au(111) structure could not be modelled, which may also affect the calculated structure.128
Figure 3.2.2 – Examples of the two common TCNQ packing regimes found on metal surfaces. The unit mesh for each structure is shown in red. A single ‘windmill’ is highlighted in yellow with a black dot indicating its centre.
In contrast to Au(111), on the low-index surfaces of Ag and Cu, XPS and UPS measurements show that TCNQ becomes negatively charged.57, 162, 163 On these surfaces, TCNQ forms structures that deviate from the optimal hydrogen-bonding arrangement and are, in some cases, also commensurate with the underlying substrate, indicating stronger molecule-substrate interaction.17, 57, 58, 164-167 From STM images, these structures can be split into two types of packing regimes (Examples of these are shown in Figure 3.2.2).
The first of these is a ‘head-to-tail’ arrangement, observed on Cu(100),17 Ag(100)58 and Ag(111),57 in which all of molecules adopt the same orientation, similar to the Au(111) brickwork structure but distorted from the optimal hydrogen-bonding alignment between the rows of molecules. The other type of packing arrangement, which has been reported to form on Cu(111)164-166 and Ag(111),167 features units of four TCNQ molecules spiralling around a central point in a windmill-like structure. From this windmill unit, a diverse range of structures can form depending on their relative spacing and the sharing of TCNQ molecules between windmill units. An example of this is on Cu(111), which, depending on the substrate temperature, forms two distinctly different structures that include windmill motifs.164, 165 This also emphasises how the deposition conditions can influence the resulting structure and is further highlighted by TCNQ on Ag(111), which adopts a windmill-type structure at lower coverages but switches to a head-to-tail structure when the coverage is increased.57, 167 This therefore shows that the phase space of TCNQ on Cu and Ag surfaces is complex and most lkely results from the fine balance of molecule-substrate, intermolecular hydrogen bonding and electrostatic repulsion interactions between the molecules.38, 39
In both packing regimes identified above, negatively charged TCNQ molecules pack together with the cyano groups of adjacent molecules close to each other. This observation is rather unusual as the negative charge is typically localised on the cyano groups161, 168 and thus would seemingly cause unfavourable electrostatic repulsions between molecules. DFT calculations performed for TCNQ on Cu(100)17 and Cu(111)164, 165 offer some possible insight into this. On both substrates, the calculations predict that TCNQ becomes negatively charged and adsorbs in a significantly bent conformation, with the peripheral cyano N atoms more than 1 Å below the central ring of the molecules,17, 164, 165 a conclusion that is qualitatively supported on Cu(100) by the results of near edge X-ray absorption fine structure (NEXAFS) measurements17. This bending enables the molecule to bond with the underlying substrate, which, according to the DFT models, causes the substrate atoms to be lifted out of the surface plane by ~0.3 Å. 17, 164, 165 It is then predicted that a stress field generated around the lifted substrate atoms overcome the electrostatic repulsions between molecules and make it energetically favourable for the negatively charged cyano groups to be in close proximity. 17, 164, 165 However, one other possible
underlying substrate into the molecular layer to which could act as counterions. This hypothesis is supported by the similar TCNQ head-to-tail and windmill assemblies observed when it is codeposited with metal atoms (see section 3.3).54, 57-59, 164 Furthermore, the related molecules TCNE and F4-TCNQ, are both reported to form networks with adatoms from the underlying substrate on coinage metal surfaces. Specifically, low temperature STM images show that Ag adatoms are etched away from the step-edges near to TCNE molecular islands, suggesting that Ag atoms are incorporated into the TCNE adsorption layer.60 Additionally, STM of the ordered overlayer formed by F4-TCNQ on Au(111) features bright protrusions between molecules that are attributed to adatoms.54 Similar bright protrusions have also been observed in STM images for TCNQ on Ag(111), which may also be caused by the presence of adatoms.57, 167 However, as STM images show a convolution of structural and electronic effects,85 it is not possible to confirm the origin of these bright protrusions from STM alone.
DFT calculations may be able to offer some insight into this by comparing the adsorption energies of STM measured assemblies of TCNQ both with and without adatoms, though a suitable dispersion correction for this system would need to be identified. Experimentally obtained quantitative structural measurements may also be able to shed light on whether adatoms are present in TCNQ adsorption structures and furthermore could also provide a reference point to compare to DFT models. Despite this, there are currently no quantitative structural measurements available for TCNQ or its derivatives adsorbed on metal surfaces with the exception of one NIXSW investigation of F4-TCNQ on Cu(111).169 In this NIXSW study, the molecule is reported to bend down towards the surface, though the coherent fractions reported (0.43, 0.28, 0.15 for F, N and C respectively) are so low that attributing the associated coherent positions of F and N atoms to single heights, as reported in this paper,169 is certainly questionable. Also, the lack of information regarding the coverage or molecular ordering for the measured surface169 makes it difficult to identify other possible causes of these low coherent fractions.