Thermodynamic Control of Two-Dimensional Molecular Ionic

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Thermodynamic Control of Two-Dimensional Molecular Ionic Nanostructures on Metal Surfaces Seokmin Jeon,† Peter W. Doak,† Bobby G. Sumpter,†,‡ Panchapakesan Ganesh,† and Petro Maksymovych*,† †

Center for Nanophase Materials Sciences and ‡Computer Science & Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: Bulk molecular ionic solids exhibit fascinating electronic properties, including electron correlations, phase transitions, and superconducting ground states. In contrast, few of these phenomena have been observed in lowdimensional molecular structures, including thin films, nanoparticles, and molecular blends, not in the least because most of such structures have been composed of nearly closedshell molecules. It is therefore desirable to develop lowdimensional ionic molecular structures that can capture potential applications. Here, we present detailed analysis of monolayer-thick structures of the canonical TTF−TCNQ (tetrathiafulvalene 7,7,8,8-tetracyanoquinodimethane) system grown on low-index gold and silver surfaces. The most distinctive property of the epitaxial growth is the wide abundance of stable TTF/TCNQ ratios, in sharp contrast to the predominance of a 1:1 ratio in the bulk. We propose the existence of the surface phase diagram that controls the structures of TTF−TCNQ on the surfaces and demonstrate phase transitions that occur upon progressively increasing the density of TCNQ while keeping the surface coverage of TTF fixed. Based on direct observations, we propose the binding motif behind the stable phases and infer the dominant interactions that enable the existence of the rich spectrum of surface structures. Finally, we also show that the surface phase diagram will control the epitaxy beyond monolayer coverage. Multiplicity of stable surface structures, the corollary rich phase diagram, and the corresponding phase transitions present an interesting opportunity for low-dimensional molecular systems, particularly if some of the electronic properties of the bulk can be preserved or modified in the surface phases. KEYWORDS: phase diagram, molecular ion, charge transfer complex, self-assembly, electrostatics, scanning tunneling microscopy, density functional theory

I

phase diagrammatic approach for categorizing self-assembled structures of decanethiol on the Au(111) surface.12 This was motivated by observing as many as six structural phases of a decanethiol molecule and their phase transitions as a function of increasing temperature and molecular coverage (“surface pressure”). Self-assembly becomes intriguingly complicated with respect to structures and their transitions.13 The attractive interactions of alkanethiols are enabled by two distinct functional groups and a macroscopic dipole moment. More generally, attractive interactions can be found in a multicomponent mixture of simple molecules, preferably with a large difference of electronegativity. As early as in 2002, de Wild

n general, any molecule on the surface should exhibit phase transitions, for example, condensation from a “lattice gas” phase upon cooling or increasing coverage.1 The associated order−disorder transformation is the reason why most molecules adsorbed on solid surfaces will eventually assemble into ordered patterns at some temperature. However, wellordered phases will typically emerge at high surface coverage and are rarely subject to control in the few-molecule regime.2−4 The main reasons are repulsive contributions of electrostatics5−7 and Pauli repulsion (“pillow effect”),8 both of which can efficiently counter short-range van der Waals and chemical interactions,9−11 particularly in unimolecular systems. The phase diagram will span a much wider range of coverages and will become much more intricate if the attractive interactions between the molecules become comparable to or exceed repulsion. In the early 2000s, Poirier advocated the © 2016 American Chemical Society

Received: May 26, 2016 Accepted: July 26, 2016 Published: July 26, 2016 7821

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Figure 1. Unimolecular and bimolecular assemblies of TTF and TCNQ molecules on the Ag(111) surfaces. (a−g) Constant current STM images taken at 4.3 K. (a) Pure TTF (Vs = 0.1 V; It = 100 pA), TTF−TCNQ (b) 2:1 phase (Vs = −0.1 V; It = 40 pA), (c) 1:1 phase (Vs = 0.59 V; It = 500 pA), (d) 1:2 phase (Vs = 0.02 V; It = 100 pA), (e) 2:8 phase (Vs = 0.2 V; It = 16 pA), (f) 2:13 phase (Vs = 0.1 V; It = 100 pA), and (g) TCNQ (Vs = 1.5 V; It = 100 pA) on Ag(111). Blue boxes overlaid on the STM images represent each unit cell. Insets display ball-and-stick structure models of the unit cells. Blue, orange, gray, and white circles represent N, S, C, and H atoms, respectively. Blue arrows indicate the unit cell vectors b1 and b2 in (b−f) and b1 in (a,g). Red arrows in (c−g) represent linear or zigzag chain of TCNQ molecules. Green dashed lines in (c,d) mark the vacancy-ordered domain boundaries within 1:1 and 1:2 phases, respectively. (h) Stick models of TTF and TCNQ molecules.

Table 1. Experimental and Theoretical Lattice Parameters, Gas-Phase Formation Energy, and Adsorption-Phase Molecular Charge of TTF−TCNQ Multiphase on Ag(111) 2:1 phase experimental data b1 (Å) b2 (Å) angle (deg) theoretical models b1 (Å)b b2 (Å)b angle (deg)b superstructure matrixc packing densityd gas-phase formation energy (meV)e adsorption-phase chargef TTF TCNQ Ag

1:1 phase

1:2 phase

2:8 phase

2:13 phase

hypothetical 2D TCNQa

15.27 11.62 92.5

17.29 7.06 86.5

16.96 12.32 65.5

30.31 23.39 97.6

36.40 31.37 91.7

15.05 (−1.4) 11.78 (+1.4) 92.4 (−0.1) ⎛ 5 − 1⎞ ⎟ ⎜ ⎝3 5 ⎠

16.88 (−2.4) 7.15 (+1.3) 84.8 (−2.0)

⎛ 7 5⎞ ⎟ ⎜ ⎝− 1 2 ⎠

16.81 (−0.9) 12.34 (+0.1) 65.2 (−0.4) ⎛ 7 5⎞ ⎟ ⎜ ⎝ 1 5⎠

30.10 (−0.7) 23.41 (+0.1) 98.9 (+1.4) ⎛10 − 2 ⎞ ⎟ ⎜ ⎝ 5 10 ⎠

35.76 (−1.8) 31.87 (+1.6) 91.8 (+0.1) ⎛10 − 5⎞ ⎟ ⎜ ⎝10 13 ⎠

10.81 19.49 70.9 ⎛8 6⎞ ⎜ ⎟ ⎝ 4 0⎠

0.417 −706 (−698) +0.24e −1.49e +1.01e

0.441 −795 (−804) +0.34e −1.41e +1.07e

0.448 −775 (−759) +0.46e −1.32e +2.18e

0.428

0.402

0.475 −581 (−534) −1.18e +3.54e

a

Hypothetical 2D TCNQ phase on Ag(111) is modeled after an experimentally observable structure on Au(111). The 2D TCNQ phase on Ag(111) is relaxed in the DFT calculations according to the relaxation procedures described in the Methods section. bNumbers in parentheses represent ⎛ b1 ⎞ ⎛ a1 ⎞ percentage errors in epitaxy models from the experimentally determined values. c⎜⎜ ⎟⎟ = (superstructure matrix) ⎜ ⎟, where b1 and b2 are the unit ⎝a2 ⎠ ⎝ b2 ⎠

cell vectors defined in Figure 1; a1 and a2 are the surface unit cell vectors of Ag(111). dPacking density is calculated from sum of molecular areas divided by the unit cell area defined by the b1 and b2 vectors. The molecular area is defined by the rectangular model cornered by N in TCNQ and H and S in TTF (TTF: 24.11993 Å2, TCNQ: 36.88524 Å2). eCorresponding molecular structures are displayed in Figure 4 without substrate atoms. The height of each unit cell is 15 Å. Numbers in parentheses represent formation energy calculated without restriction on the cell volume. fMolecular or substrate charge corresponds to the sum of the constituent atomic charges calculated in the Bader scheme. For the molecule which has multiple equivalent ones within a unit cell, the value represents the average of them.

et al. reported two stable phases in the coassembly of C60 and proposed that these follow the phase diagram.10 Multiplicity of

structural phases was further illustrated among phthalocyanine derivatives14 and a blend of phthalocyanine derivatives and 7822

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Figure 2. Phase coexistence. Constant current STM images of multiphase TTF−TCNQ on Ag(111) at 4.3 K: (a) 1:1 and 1:2 phases (Vs = 1 V; It = 100 pA). (b) Surface topography after additional dose of estimated 0.24L of TTF−TCNQ precursor (1L = 10−6 Torr·s) with a high TTF/TCNQ flux ratio (source temperature at 50 °C) on the sample in (a). (c) Schematic phase diagram representing all the observed phases. Dashed lines are schematics aimed to illustrate phase equillibria at finite temperatures.

RESULTS AND DISCUSSION Figure 1 shows various surface structures formed by TTF and TCNQ on the Ag(111) surface. We will refer to them as 2:1 (Figure 1b), 1:1 (Figure 1c), 1:2 (Figure 1d), 2:8 (Figure 1e), and 2:13 (Figure 1f) phases, corresponding to the relative ratio of TTF to TCNQ molecules within each unit cell. The main point of our discussion is to represent the various phases, as well as pure TTF and TCNQ molecules as instances of a single phase diagram of TTF and TCNQ coassembly (or “molecular alloy”) rather than random or metastable structures that happen to form under some specific growth condition. We first observe that well-defined TTF−TCNQ phases form on the surface even in the limit of very small net molecular coverage (≪1 ML)essentially at “zero pressure”. Each of the structures in Figure 1 is found on Ag(111) in the shape of an island surrounded by either a clean Ag(111) surface, residual TTF, or TCNQ molecules or proximate to another phase. The structure within the island, on the other hand, is regular and saturated to its corresponding monolayer density (Table 1). It is a classical manifestation of the nucleation and growth model, and we are likely observing islands that have grown out of a single nucleus in most cases. In contrast, single-component systems such as most polyaromatic molecules, and even parent TTF and TCNQ molecules, exist as a sparse array of molecules more or less uniformly distributed on the surface. Both TTF

where large islands of TTF−TCNQ comprise both 1:1 and 1:2 phases, separated by sharp phase boundaries. Dosing extra TTF−TCNQ with a high TTF to TCNQ ratio increased the size of the islands and also repartitioned the ratio between TTF−TCNQ, so that most of the area covered by the 1:2 phase is switched to 1:1 (Figure 2b). The phase coexistence already implies that there is a phase transition occurring between 1:2 and 1:1 phases. Moreover, it appears that the transition occurs within individual islands (see close-up view in Figure 2b). We could intentionally trigger the phase transitions by a separate control over TTF and TCNQ precursors. The images in Figure 3 were acquired by dosing progressively increasing amounts of TCNQ onto the surface precovered with only TTF. The new TCNQ molecules become progressively intermixed with all the present TTF molecules, nucleating first the 2:1 phase (Figure 3a) and upon additional TCNQ dose two new phases, 2:8 (Figure 3b) and 2:13 (Figure 3c), and pure TCNQ made up of excess molecules, as shown in Figure 3b. We therefore propose that the formation of TTF−TCNQ adlayers on the Ag(111) surface is governed by a phase diagram, schematically shown in Figure 2c. At 0 K, the phases constitute stable surface molecular “alloys” along with pure TTF and TCNQ end members. Coexistence of the phases in Figure 2 and phase transformation in Figures 2 and 3 directly

and TCNQ form chain-like structures, where molecules are densely arranged along the chains (Figure 1a,g). However, the chains do not form a regular arrangement; instead, they are randomly distributed over the surface. The second argument in favor of the phase equilibrium is the coexistence of phases (Figure 2). This is shown in Figure 2a,

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Figure 3. Phase transitions controlled by addition of TCNQ. (a−d) Constant current STM images of TTF−TCNQ on Ag(111) at 4.3 K. (a) Pure TTF and 2:1 phase (Vs = −1.5 V; It = 40 pA) obtained by dosing TCNQ onto TTF-covered Ag(111), as shown in Figure 1a; (b,c) 2:8 and 2:13 phases and pure TCNQ obtained upon subsequent TCNQ dosing and postannealing up to ∼300 K for 10 min (Vs = 1.5 V; It = 22 pA). (d) Disordered mixture of TTF and TCNQ molecules on the sample held at ∼100 K (Vs = 1 V; It = 100 pA). Annealing this mixture to 300 K produced (b,c).

Figure 4. Atomic structures of individual molecules and TTF−TCNQ phases obtained from DFT calculations. Top view of relaxed geometries of (a−d) gas molecules in fixed cell volumes and (e−j) adsorbed molecules on Ag(111); 2:1 phase (a,f), 1:1 phase (b,g), 1:2 phase (c,h), hypothetical 2D TCNQ (d,i), single TTF (e), and a single TCNQ (j). Light navy, blue, orange, gray, and white circles represent Ag, N, S, C, and H atoms, respectively. Black boxes represent unit cells; b1 and b2 represent the edge length of the unit cell in each model. Unit cell sizes as well as the included angles and formation energies are displayed in Table 1. Red arrows in (d,e) represent a linear or zigzag chain of TCNQ molecules. Green arrows represent surface primitive vectors.

molecules on the surface. In the future, it is intriguing also to consider the existence of intermediate phases at finite temperatures and the corresponding reactions. Indeed, a finite temperature diagram of decanethiol on the Au(111) surface is as rich as it is surprising with respect to the total number of phase transitions.12,13 The immediate advantage of the existence of multiple phases is the high degree of structural ordering within the 2D layers and deterministic control of their composition, which is particularly important given “imperfections” of the source material. Indeed, the evaporation of molecules from TTF− TCNQ crystals should always be nonstoichiometric, even if the crystals themselves were stoichiometric. That is because TTF has much higher vapor pressure than TCNQ even at room temperature (sublimation temperature of TTF and TCNQ is around 300 and 393 K, respectively).23 In general, the TTF− TCNQ source should be producing a TTF-rich bimolecular flux at all times. Yet, in most of our growth conditions, where the temperature of the source was intentionally kept relatively low, the majority of the grown phases were either 1:1 or 1:2 or a coexistence thereof.

confirms this notion. The existence of multiple phases starkly contrasts the predominance of the 1:1 bulk stoichiometry. When adsorbed on the surface, the intermolecular interactions are strongly modified as are the degrees of freedom for molecular motion. Therefore, one should not expect the bulk stoichiometry to persist on the surface. At the same time, we also note that bulk compositions other than 1:1 may also exist, but kinetics may easily prevent their synthesis in significant amounts. We have not directly observed finite-temperature properties of TTF−TCNQ above ∼100 K. We presume that at 300 K the phases will melt into a 2D “liquid” or “gas” states where adsorbed molecules have high mobility and long-range order is absent. Supporting evidence comes from the molecular structure obtained by quenching the silver crystal to below 100 K (estimated). In this case, the arrangement of molecules is much more irregular, as seen in Figure 3d. Careful annealing of this structure produced the 2:8 and 2:13 phases shown in Figure 3b,c. As a corollary, this result implies that the regular structures in Figure 1−3 correspond to stable and equilibrated structures corresponding to a specific ratio of TTF and TCNQ 7824

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Figure 5. Madelung energy calculations and structural motif analysis. (a) Representative edge structures of TTF−TCNQ islands of the 1:1 phase. The edges are labeled as T1−T5. Constant current STM images are shown next to the respective ball-and-stick models. T4 and T5 edges are more rarely observed. Green dashed lines denote charge-neutral edges; blue denotes negatively charged and red denotes positively charged edges. The charge of the edge is judged by the alignment of the centroids of the respective molecules. (b) Average binding energy per molecule as a function of charge within electrostatically stable island structures. Solid and dashed lines correspond to island T1T2 and island T1T3 in Figure S2, respectively. Colors correspond to different charge models: blue, atom-in-molecule model with no screening; green, centroid model with no screening; red, atom-in-molecule model with screening; cyan, centroid model with screening. (c) Gaussian probability density estimates describing the distribution of electrostatic energies in island models shown in Figure S2 and those take into account image charge screening with an assumed adsorption distance of 0.3 nm for both molecules. “Tiling” models for 1:1 (left) and 1:2 (right) phases (d) and those for the domain boundaries within respective islands (e). Colors represent different tiles: green, TCNQ; orange, TTF inside domains; yellow, TTF at domain boundaries. The “cross” motif discussed in the main text is outlined by a box within each image.

We now turn to the question of why so many phases can be found on the Ag(111) surface. The main contributing energies that stabilize the phases are hydrogen bonding and electrostatic interactions. Hydrogen bonding among TCNQ molecules has been discussed previously.24,25 Hydrogen bonding between TTF and TCNQ is less straightforward, simply because TTF is not known to be a strong Lewis acid or base. We calculated structures and energies for 2:1, 1:1, and 1:2 phases as well as those of a hypothetical 2D TCNQ monolayer phase using density functional theory (DFT), as shown in Figure 4. The lattice vectors of the unit cell and the displacements and rotational angles of individual molecules with respect to the lattice vectors were determined in high-resolution scanning tunneling microscopy (STM) images, as shown in Figure 1. For the gas phases in Figure 4a−d, DFT structure optimizations were carried out with and without restriction on the unit cell volume, as shown in Table 1. In spite of slight differences in geometries, we obtained the same trend in formation energies in both calculations. When ignoring molecule−surface interactions in gas-phase calculations, the 1:1 phase is the most energetically stable followed by 1:2 and 2:1 phases. The least stable arrangement is the hypothetical 2D TCNQ monolayer phase. We note that the long-range-ordered 2D TCNQ monolayer on Ag(111) has neither been observed in our experiment nor reported to our knowledge. Lack of this phase on Ag(111) was also supported by theoretical calculation.24 On the other hand, the densely packed 2D TCNQ monolayer phase was observed on Au(111). This is because the electrostatic repulsion between TCNQ molecules is much stronger on Ag(111) than on Au(111). In DFT calculations, despite nearly the same intermolecular distance, the TCNQ molecular charge is −1.18e on Ag and −0.47e on Au. We built a hypothetical 2D TCNQ phase based on the molecular configuration from the monolayer 2D TCNQ phase on Au(111).25

It is quite surprising that the hypothetical 2D TCNQ monolayer phase does not have as high formation energy as those in the TTF−TCNQ phases despite of the stronger hydrogen bonding. For instance, the average hydrogen bond distance, which is defined by the distance between H of the C− H bond in TTF and N of the CN bond in TCNQ, is 2.47−2.66 Å, while that in 2D TCNQ is 2.34 Å (2.31−2.58 and 2.21 Å, respectively, when the unit cell volumes are not fixed during relaxation). One possible reason why the TTF−TCNQ mixed structure will be preferred over those of pure end members is electrostatics. To model electrostatics in a realistic island, with well-defined boundaries as seen in the experiment, we turned to summation of pairwise Coulomb interactions. We constructed four TTF− TCNQ islands, each with different edge terminations (Figure 5a and Supporting Information Section A). Each molecular ion was described by a set of atomic charges within the atoms-inmolecule approximation26 (Supporting Information Section B), while the metal substrate was represented by image charges at ∼0.6 nm (twice the assumed adsorption distance)24 from the molecular ionic charges normal to the surface. For completeness, we also analyzed the electrostatic interactions between centroids of molecular ions (Supporting Information Section C). Figure 5b compares the average Madelung energy of the 1:1 phase as a function of ionic charge. The energy is negative for any amount of partial charge on TTF and TCNQ molecules if the charge is the same. The Madelung energy increases to ∼1 eV/molecule already for a moderate charge of 0.5e and exceeds 2 eV/molecule for 1e. Although these values likely represent the upper bound due to efficient charge screening on metal surfaces (our model only assumes a simplistic image charge model to account for screening), the energies compare favorably to that of van der Waals (∼0.2 eV/molecule)24,27 and hydrogen bonding (0.02−0.06 eV/bond)28,29 interactions. These values 7825

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vacancy ordering. The main difference to the X-unit of the parent phases is that the TTF molecule (and its tile) is rotated by ∼90° at the boundary. At the same time, as seen in Figure 5e, the boundaries of the 1:1 and 1:2 phases are structurally differentiated only by the stacking of the X-units. Finally, the propensity toward the X-unit also explains the peculiar shape of the T2 edge structure in Figure 5a, which is essentially the phase boundary of the 1:1 phase with half of the TCNQ missing. Finally, one of the main advantages of coassembly of TTF and TCNQ is that we already know that the two molecules do coexist in the bulk phase. Other molecular blends, such as C60 and pentacene32 derivatives, can most likely only coexist when supported on a substrate. The natural question is the effect of coverage. Figure 6 shows the result of two experiments on the

are also consistent with our qualitative assessment on the contributions of each potential component in the DFT calculations (Supporting Information Section D). In the case of unequal charging of the molecules, we observed that TCNQ−1 becomes stable if TTF+k is charged by at least k ∼ +0.5e (Figure S9). The energy of TTF−TCNQ islands with neutral TTF is prohibitively large, however, as shown in Figure S9. Finally, the specific edge structure does influence the Madelung energy even in the screened case (Figure 5c), particularly for the molecules proximate to the edges (see also Supporting Information Section C for details). If the islands could be stabilized on an insulating substrate, the edge structures could even cause a “polar catastrophe”, that is, reconstruction of the islands to avoid polar terminations, qualitatively similar to Tasker’s argument for a polar crystal surface.30 We then calculated the binding energies of adsorbed molecules using density functional theory. The 1D chain structure of TCNQ molecules on Ag(111) in Figure 1g results from a subtle balance of lattice and intermolecular potentials24 and is the preferred binding motif on the Ag(111) surface. This is why linear or zigzag TCNQ chain structures are commonly observed in most phases (red arrows in Figure 1). Inclusion of TTF molecules between the TCNQ chains further lowers the total system energy due to positive intermolecular interactions including electrostatic, hydrogen bonding, and van der Waals potentials. Moreover, in terms of adsorption states, both TTF and TCNQ molecules maintain their most stable adsorption configurations as a part of 1:1 and 1:2 phase structures (Figure 4e,g,h,j). On the other hand, the TTF-rich 2:1 phase and pure 2D TCNQ structures are expected to be less stable than 1:1 and 1:2 phases on Ag(111) because the four potentials do not minimize the total system energy for a given periodicity. For instance, in the 2:1 phase, one of TTF molecules has adsorption structures different than the adsorption minimum structure (Figure 4e). Whereas detailed analysis of binding structures is always preferred to quantitatively determine the stability of the corresponding phases, it would also be desirable to have a simpler, phenomenological picture of the connection between the phases, which would also provide the structural “selection” criteria for the existence of a specific phase. Indeed, from a direct inspection of 1:1 and 2:1 phases, a clear common structural motif can be derived (Figure 5d). Its existence becomes more apparent by considering the actual connectivity of the molecules within the layer, rather than the corresponding unit cells. To this end, we represent the molecules as stretched hexagonal tiles, following the general arguments of ref 31. This shape naturally predicts the preference of the zigzag and linear arrangements of TCNQ molecules (Figure 5d,e) because of favorable space-filling by the tiles. Moreover, the tile with an almost similar shape can also describe the TTF molecule. In this case, it is clear that the “X-shaped” motif involves one TTF molecule coordinated by four TCNQ neighbor molecules (outlined by boxes in Figure 5d). Specific phases are differentiated by the detailed shape of the X-unit, subsequent stacking, and density of these units. In the TTF-rich phase, the X-unit does not exist due to scarcity of TCNQ molecules, although their absence is compensated by extra bonding to the additional TTF molecule within the unit cell. The X-unit also explains the morphology of phase boundaries seen in Figure 1c,d, which exhibit regular TTF

Figure 6. Relative stability of pure 2D TCNQ phase versus a 1:1 TTF−TCNQ phase on Au(111). (a−d) Constant current STM images. (a) 2D TCNQ monolayers grown on Au(111) (Vs = 0.6 V; It = 100 pA; 77 K, inset: Vs = 1 V; It = 96 pA; image size of 3 nm3). (b) Partial replacement of 2D TCNQ monolayers by a monolayer TTF−TCNQ 1:1 phase after additionally dosing at 0.02L (1L = 10−6 Torr·s) of TTF−TCNQ on the sample in (a) at around 100 K (Vs = 2.0 V; It = 40 pA). (c) Low-coverage 1:1 TTF−TCNQ phase islands on Au(111) (Vs = 2.0 V; It = 80 pA; 4.3 K). (d) Second-layer 2D TCNQ phase grown on top of the full-coverage 1:1 TTF− TCNQ phase monolayers on Au(111) (Vs = 1.0 V; It = 50 pA).

Au(111) surface, where a dense TCNQ monolayer is readily formed. We explored the possibility of using the TCNQ monolayer as a decoupling layer for subsequent TTF−TCNQ growth. The TCNQ monolayer was first grown on the Au(111) surface (Figure 6a) and subsequently overdosed with TTF− TCNQ. From inspection of Figure 6b, TTF from the TTF− TCNQ source replaces TCNQ molecules from within the first monolayer. At the same time, TTF that penetrated into the first layer forms 1:1 domains of 1:1 TTF−TCNQ, as evidenced from comparison of panels b and c of Figure 6 (the latter is 1:1 TTF/TCNQ on the Au(111) at submonolayer coverage that is qualitatively similar to the 1:1 phase on Ag(111)). The greater stability of TTF/TCNQ over TCNQ in the monolayer phase is 7826

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The adsorbed molecular structures of a single TTF (TCNQ) molecule, 2:1, 1:1, and 1:2 TTF−TCNQ phases, and hypothetical 2D monolayer TCNQ on the Ag(111) surface were built based on the STM apparent features, as displayed in Figure 1. The molecules were relaxed with the three layers of Ag slabs as the bottom layers were fixed until the atomic forces were smaller than 0.01 eV/Å. The 2D gas-phase monolayers were built in a similar way as in the adsorbed phases. Relaxation was carried out with and without restriction on the cell volume. For the restricted volume calculation, the periodicity along the surface-normal direction was set to 15 Å. For the unrestricted calculation, after obtaining relaxed structures, volume-restricted relaxation was additionally carried out by fixing the cell height of 15 Å. Formation energy at 0 K was calculated using the following equation:

further confirmed by facile growth of TCNQ over the saturated monolayer of TTF−TCNQ, as seen in Figure 6d. Here, indeed, TTF−TCNQ acts as a buffer layer, enabling nucleation of wellordered TCNQ crystallites. We note that the preference of the 1:1 phase is not apparent from simply packing density considerations. As seen in Table 1, the packing density of all phases is quite comparable, with 2D TCNQ being marginally larger than the 1:1 phase on the Au(111) surface. However, it is quite clear that electrostatic contribution to the cohesive energy will be biased toward equal ratios of the two molecules, which is why the 1:1 phase is likely the most stable under equal external potential conditions.

CONCLUSIONS In summary, we have shown that self-assembly of TTF and TCNQ on the Ag(111) surface yields a rich spectrum of surface phases that grow by nucleation and growth even in the limit of zero coverage. The multiplicity of observed TTF/TCNQ ratios is in stark contrast to the predominance of the 1:1 ratio in the bulk. The connectivity of the molecules within each phase points to a common bonding motif of TTF and TCNQ molecules, whose permutations and coverage dependence are responsible for the diversity of the observed phases. We were able to induce phase transitions between most of the phases by selectively dosing one or the other molecular components of the 2D TTF−TCNQ layer. Computational analysis reveals that molecular bonding within the layers has a significant electrostatic component, which is always repulsive in the case of adsorption of pure end members and is likely always attractive (which we confirmed for the 1:1 phase) for the mixture of TTF/TCNQ. The attractive nature of net intermolecular interactions enables phase equilibrium at deep submonolayer coverages, where single molecules would simply form sparse or weakly ordered layers. The studies form a natural stepping stone to predictive statistical modeling (i.e., using accelerated Cluster Monte Carlo methods) of such arrangements that also include dynamics of how the assembly occurs, which could be applied to a variety of important molecular systems given the vast diversity of charge transfer salts known today. At the same time, it is interesting to consider the tunability of molecular electronic properties in TTF−TCNQ, such as the Kondo resonance, within such structures. Furthermore, molecular epitaxy beyond the first layer may potentially be controlled or at the very least sensitively depend on the relative stability of the surface phases that will tend to template follow-on layers.

Ef =

Etot − (n TTF × E TTF + n TCNQ × E TCNQ ) n TTF + n TCNQ

(1)

where Etot, ETTF/TCNQ, and nTTF/TCNQ represent DFT total energy of the unit cell structure and each individual component whose geometry is optimized in a 30 Å3 vacuum box and the number of individual components in each unit cell. Numbers in parentheses represent energy calculated without restriction on the cell volume. Bader analysis was performed using the Bader charge analysis code37 and was used to provide the approximate point charges for each atom at a particular molecular charge state for TTF or TCNQ. In cases of short and strong bonds, we find that this can result in larger amounts of charge assigned to atoms in comparison to other methods such as Voronoi deformation density, Hirshfeld, or natural population analysis.38 Since “atom charge” is not an experimental observable that can be measured, it is difficult to determine if any of these methods is superior to another. We therefore use the Bader analysis to provide an atom-in-molecule point charge approximation of the charge distributions of the variously charged TTF and TCNQ molecules. Classical electrostatic energy of TTF−TCNQ monolayer islands was calculated numerically using the above point charge approximation for the charge distribution of the constituent molecules and the image charge approximation for the screening of the adsorbed layer by the metal substrate (see text for details). We modeled TTF−TCNQ islands to mimic experimentally observed structures. Electrostatic energy (or finite Madelung energy) of a molecule in the modeled island, Emol, was calculated as the sum of Coulomb energies of the atomic point charges of this molecule interacting with the point charges of all the other molecules in the system: i ∈ mol N − 1

Emol =

∑ ∑ i

j≠i

Q iQ je 2 1 × − 4πε0 |ri − rj|

i ∈ mol j ∈ mol

∑ ∑ i

j≠i

Q iQ je 2 1 × 4πε0 |ri − rj| (2)

METHODS

where Qi is the Bader point charge of an atom, i at its position of ri; N is the total number of point charges representing all the molecules in the island; e is electron charge; and ε0 is vacuum permittivity. The last term is the self-energy of the distributed charges within the molecule. To model structures adsorbed on a metal surface, we introduced image charges at a distance of 2ra from each of the atom-projected Bader charge, where ra is taken to be 0.3 nm (approximately, the adsorption distance of the studied molecules). This is favored over using the simplified dipole−dipole approximations because intermolecular distances and the depth of the adsorption well have comparable length scales. The probability distribution plots for Madelung energy were estimated using a probability density estimate routine with a Gaussian kernel. Although the Gaussian distribution is not warranted in our case, this description is not overly sensitive to the kernel type and it yields a much clearer picture than histogram distributions due to finite size of the islands in our simulation.

All experiments were performed in an ultrahigh vacuum (UHV) system at background pressure of