Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces

The energy scales of interactions that control molecular adsorption and assembly on surfaces can vary by several orders of magnitude, yet the importance of each contributing interaction is not apparent a priori. Tetracyanoquinodimethane (TCNQ) is an archetypal electron acceptor molecule and it is a key component of organic metals. On metal surfaces, this molecule also acts as an electron acceptor, producing negatively charged adsorbates. It is therefore rather intriguing to observe attractive molecular interactions in this system that were reported previously for copper and silver surfaces. Our experiments compared TCNQ adsorption on noble metal surfaces of Ag(100) and Ag(111). In both cases we found net attractive interactions down to the lowest coverage. However, the morphology of the assemblies was strikingly different, with two-dimensional islands on Ag(100) and one-dimensional chains on Ag(111) surfaces. This observation suggests that the registry effect governed by the molecular interaction with the underlying lattice potential is critical in determining the dimensionality of the molecular assembly. Using first-principles density functional calculations with a van der Waals correction scheme, we revealed that the strengths of major interactions (i.e., lattice potential corrugation, intermolecular attraction, and charge-transfer-induced repulsion) are all similar in energy. The van der Waals interactions, in particular, almost double the strength of attractive interactions, making the intermolecular potential comparable in strength to the diffusion potential and promoting self-assembly. However, it is the anisotropy of local intermolecular interactions that is primarily responsible for the difference in the topology of the molecular islands on Ag(100) and Ag(111) surfaces. We anticipate that the intermolecular potential will become more attractive and dominant over the diffusion potential with increasing molecular size, providing new design strategies for the structure and charge transfer within molecular layers.

Figure 1

(a) Constant-current STM images of low-coverage TCNQ grown on Ag(100). (b) A schematic of the TCNQ island unit cell and a close-up image of a single TCNQ island. (c) Constant-current STM images of TCNQ following deposition on the Ag(111) at low (c) and higher coverages (d). (e) A schematic of a TCNQ chain on the Ag(111) surface and a close-up STM image of representative chains and junctions between molecules. All images were taken at 77 K. DFT-optimized configurations of TCNQ on Ag(100) (f), (g) and Ag(111) (h), (i).

Figure 2

(a) $Z-V$ tunneling spectra of TCNQ/Ag(100) (blue, $I_t$ = 15 pA) and of clean Ag(100) surface (black, $I_t$ = 10 pA). (b) $Z-V$ spectrum of TCNQ/Ag(111) (red, $I_t$ = 20 pA) and of clean Ag(111) (black, $I_t$ = 8 pA). (c), (d) Gaussian fits to electronic states on TCNQ/Ag(100) (c) and TCNQ/Ag(111) (d). All spectra shown are averages over at least 20 individual spectra. (f) Energy region around Fermi level probed with dI/dV tunneling spectroscopy. The first harmonic of the modulated signal was recorded with the lock-in amplifier with 5 mV ac modulation at 600 Hz.

Figure 3

(a) Corrugation of lattice potential of TCNQ molecules on Ag(100) and Ag(111) surface. Thick isolines correspond to 0.1 eV energy spacings. (b) Gas-phase intermolecular interactions are plotted as a function of relative displacement $V$ when they form dimer (left) and trimer (right), where thick isolines correspond to 0.1 eV energy spacings. (c) Difference charge density upon the adsorption of TCNQ on the Ag surface is illustrated by the isosurface of 4.8 × $10{}^{-7}$ $e/{\AA }^3$ in top and side view, where red represents charge accumulation and blue is for charge depletion. Difference charge density profiles along the $y$ and $z$ axes are shown on the right: Δρ is plane-summed charge along the axes. These charge differences are mapped into the arrays of point charges.

Figure 4

Potential landscapes of a TCNQ molecule introduced into the TCNQ assembly on Ag(100). The displacement directions are indicated by the arrows (a)–(c) and the origin of position is defined by the minimum of lattice potential which lies on the tip of the arrows. The reference of potential energy (i.e., energy zero) is also set to the minimum of lattice potential. (d)–(f) The total potential (black) energy consists of three energy components: lattice potential (red), intermolecular interaction without the substrate (blue), and additional charge-transfer-induced electrostatic intermolecular interaction between negatively charged molecules (green). Difference charge density of TCNQ upon the adsorption on Ag is illustrated by the isosurface of 4.8 × $10{}^{-7}$ $e/{\AA }^3$ in (b), where red means a charge accumulation and blue means charge depletion.

Figure 5

Potential landscapes of a TCNQ molecule introduced into the TCNQ assembly on Ag(111). The displacement directions are indicated by the arrows (a), (b) and the origin of position is defined by the minimum of lattice potential which lies on the tip of the arrows. The reference of potential energy (i.e., energy zero) is also set to the minimum of lattice potential. (c), (d) The total potential (black) energy consists of three energy components: lattice potential (red), intermolecular interaction without the substrate (blue), and additional charge-transfer-induced electrostatic intermolecular interaction between negatively charged molecules (green). Difference charge density of TCNQ upon the adsorption on Ag is illustrated by the isosurface of 4.8 × $10{}^{-7}$ $e/{\AA }^3$ in (b), where red means a charge accumulation and blue means charge depletion.