Ionic Disproportionation of Charge Transfer Salt Driven by Surface

Aug 19, 2013 - Changwon Park , Geoffrey A. Rojas , Seokmin Jeon , Simon J. Kelly , Sean C. Smith , Bobby G. Sumpter , Mina Yoon , Petro Maksymovych...
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Ionic Disproportionation of Charge Transfer Salt Driven by Surface Epitaxy Geoffrey A. Rojas,† P. Ganesh,† Simon J. Kelly,† Bobby G. Sumpter,† John A. Schlueter,‡ and Petro Maksymovych*,† †

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

‡

ABSTRACT: Epitaxial growth of organic charge-transfer salts composed of molecular cations and anions will potentially allow the synthesis of thin films of molecular conductors with strong electron correlations and electron−phonon interactions, highly distinct properties compared to thin organic semiconductors and traditional self-assembled monolayers. Here, we report on ionic decomposition of the charge transfer salt β″-(BEDTTTF)2SF5CH2CF2SO3 into two surface-supported phases on Ag(111), each of which has a cation/anion ratio different from the 2:1 stoichiometry of the bulk. The films were grown by thermal evaporation of the bulk crystal. Subsequent reassembly of the constituent bis(ethylenedithio)tetrathiafulvalene molecules (BEDT-TTF) and the SF5CH2CF2SO3 anions into long-range ordered structures on the Ag(111) surface produced well-ordered molecular islands with either 1:1 or 3:1 stoichiometric ratios of BEDTTTF:SF5CH2CF2SO3. Tunneling spectroscopy revealed that both surface structures could be considered insulating. However, the charge-transfer interaction between cations and anions still persists, producing electronic states that are distinct from those of pure BEDT-TTF molecules on a silver surface. Density functional theory calculations of adsorbed molecules show that they remain ionic, with adsorption and intermolecular binding energies comparable to that of bulk cohesive energies. The diversity of surface-supported multicomponent molecular structures derived from charge-transfer salts and the perseverance of cation and anion molecules despite thermal decomposition of the bulk all provide rich opportunity toward achieving correlated electron properties in ultrathin molecular films.

1. INTRODUCTION Charge transfer ionic salts (CTIS) with molecular cations and anions exhibit a fascinating diversity of electronic ground states including metallic, charge-ordered, Mott-insulating, superconducting, and spin-liquid phases.1−3 The valence band in these compounds derives from the overlap of highest occupied molecular orbitals (HOMO) in the cationic sublattice, while the filling of the band is determined by the stoichiometric ratios between cations and anions. In the case of a 2:1 cation/anion ratio, found in the majority of charge-transfer salts based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) cations,3 the valence band is 3/4 or 1/2 filled giving rise to competing electronic ground states. By far the dominant approach to tune the electronic properties of a charge transfer salt in the bulk crystalline form is external pressure. Doping beyond stoichiometric composition is hardly possible, much in contrast to inorganic correlated electron materials.4 There arises a question whether other approaches to control ionic salts can be found, for example, in the low-dimensional epitaxial form where the electronic properties can be tuned by the proximity to the interface. Moreover, epitaxially grown films may open up brand new approaches to fundamental understanding of these intriguing compounds and their practical applications. The question of primary importance is whether the methods of epitaxial growth will produce structures that still maintain the centrally important charge-transfer. Thermal evaporation © 2013 American Chemical Society

breaks the bonds in the parent compounds, and they would have to be recreated and reassembled upon bonding to the surface, all of which requires complicated surface dynamics to occur. Furthermore, molecule−surface interactions will compete with strong chemical interaction between the cation molecules, possibly giving rise to novel structural and electronic changes in the epitaxial layer. As such, departure from 3/4 filling of the HOMO states-derived electronic band is very likely, as are changes of the Coulomb repulsion (U)5 and electronic bandwidth (W). In the case of completely filled states, correlated electron properties will be lost. In recent works, monolayers of TTF-TCNQ (TTF = tetrathiafulvalene; TCNQ = 7,7,8,8-tetracyanopuinodimethane) and derivative molecules6,7 have been convincingly shown to be nonmetallic. However, in a most striking recent example of the epitaxial growth of (BETS) 2 GaCl 4 (BETS = bis(ethylenedithio)tetraselenafulvalene) on Ag(111), the conducting properties of the bulk appear to be preserved even in bilayer films.8 The reported occurrence of superconductivity with a Tc of 8 K implies conductivity within the molecular layer above Tc. Even more striking is that this Tc is among the highest values reported for the bulk crystals of this salt.9 Yet the electronic Received: May 9, 2013 Revised: July 11, 2013 Published: August 19, 2013 19402

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dimensionality is reduced from 2D in the λ-phase of the bulk10 to quasi-1D in both two-dimensional islands and 1D chains on the Ag(111) surface. Clearly, further insight into the epitaxial growth of molecular ionic compounds is necessary, particularly given the tremendous variety of chemical compositions of ionic salts. We have deposited a charge transfer salt of β″-(BEDTTTF)2SF5CH2CF2SO3 on the Ag(111) surface and investigated ultrathin films with cryogenic scanning tunneling microscopy and spectroscopy. In the crystalline form, β″-(BEDTTTF)2SF5CH2CF2SO3 is superconducting with a Tc of ∼5 K. It was the first CTS-based superconductor where both cations and anions were molecular (as opposed to single or polymerized metal−organic complexes).9 The interest in β″(BEDT-TTF)2SF5CH2CF2SO3 is also motivated by the proximity of the superconducting state to a charge-ordered insulating phase, owing to a 3/4 filled electron band.11 In this case, superconductivity could be mediated by charge-order fluctuations rather than a more common hypothesis of antiferromagnetic spin-order fluctuations found in 1/2 filled dimerized charge-transfer salts.12

F i gure 1 . Constant-current STM images of (BEDT TTF)2SF5CH2CF2SO3 deposited on the Ag(111) surface. (a) A 160 × 55 nm2 image of the adsorbate shows large, ordered islands. (b) A close-up image of a representative island of the 3:1 stoichiometry (12 × 12 nm2) with a unit cell composed of three BEDT-TTF molecules (red) and one SF5CH2CF2SO3 anion (blue). (c) A 10 × 10 nm2 closeup image of a representative 1:1 island.

the scanning tunneling microscopy (STM) images (Figure 1b,c). The first group comprised (a) elongated molecular features with an apparent dimension of 1.45 nm × 0.5 nm, stacked along the short side (red in Figure 1b), and (b) small bright dumbbell-like features composed of two round shapes ∼0.3 nm in diameter and ∼1 nm in length separating the rows of stacked molecules (blue in Figure 1b). The unit-cell dimensions were a = 1.93 ± 0.03 nm and b = 1.99 ± 0.04 nm with an enclosing angle of 81° ± 3° and b-vector ∼30° off the ⟨11̅0⟩ direction of the Ag(111) surface (Figure 1b, inset). The large dimension of 1.45 nm allows us to assign stacked molecules to BEDT-TTF (gas-phase van der Waals length ≈ 1.4 nm)17 leaving SF5CH2CF2SO3 as the most likely origin of the smaller spherical components separating the stacks. With this in mind, SF5CH2CF2SO3 appears only every third BEDT-TTF molecule, with a net stoichiometry 3 (BEDT-TTF):1 (SF5CH2CF2SO3). At the same time, stacked rows of BEDT-TTF exhibit nonuniform neighbor spacing along the rows, from ∼0.6 to 0.7 nm, as well as dimerization along the b axis of the unit cell and weak STM contrast between the anion-bound and dimerized cations. Such close proximity of nearest-neighbors rules out the flat-lying bonding geometry of BEDT-TTF molecules, implying that the molecular plane is either normal or significantly tilted relative to the silver surface. The conclusions derived from the STM images are consistent with the density functional theory (DFT) calculations presented below (Figure 2a). The strong preference for edge-on orientation of the BEDT-TTF molecules is unlike recent studies of the smaller TTF molecule18 and TTF-based charge-transfer compounds6,7 where TTF molecules were oriented parallel to the surface. Edge-on orientation may allow for weak orbital overlap between adjacent BEDT-TTF molecules, and thus a possibility for a dispersive electronic band along the stacked molecular rows. All other differences in STM contrast, outside of the regular difference between the anion-bound and dimerized cations, are attributed to defects of a neighboring anion (i.e., vacancies or decomposition), which are directly visible in the STM image (e.g., Figure 1b). In the second group, the molecular islands had unit cell vectors a = 0.66 ± 0.02 nm and b = 1.58 ± 0.04 nm that enclosed an angle of 82° ± 5°, with the a-vector rotated 15° off

2. EXPERIMENTAL SECTION AND COMPUTATIONAL DETAILS Ag(111) surface was prepared by Ar+ sputtering of the appropriately oriented single crystal followed by annealing to ∼700 K in ultrahigh vacuum (po < 5 × 10−10 mTorr). The crystal was cooled to 87 K and subsequently exposed to a heated crucible containing either BEDT-TTF or the CTIS of (BEDT-TTF)2SF5CH2CF2SO3. BEDT-TTF was purchased from Sigma Aldrich, while the CTIS was prepared ex situ as described previously.13 The crucible was heated to achieve a low deposition rate of ∼1 ML/h to limit any dissociation of the composite molecules (BEDT-TTF and SF5CH2CF2SO3). The sample was subsequently transferred to a cryogenically cooled scanning tunneling microscope (JT-STM by SPECS), with an operating temperature of 4.5 K. All STM measurements were taken using a Pt−Ir tip and at submonolayer coverage. This allowed simultaneous observation of the clean, exposed Ag(111) surface surrounding molecular islands as a spectroscopic and geometric reference. All calculations were performed using a projector-augmented wave potential as implemented in VASP.14,15 Bulk calculations employed a converged k-point mesh (4 × 4 × 4) for Brillouin zone integration. A converged k-point mesh (2 × 4 × 1 for 1:1) was used to perform band-structure, partial density-of-states (pDOS), and Bader analysis.16 To model the 1:1 molecular ordering, a 6 × 1 × 3 Ag(111) slab was used, with the a-axis along the crystallographic ⟨110̅ ⟩ direction and the b-axis (i.e., direction of molecular stacking) along the ⟨112̅⟩ direction. A 7 × 4 × 3 slab with the same orientation was constructed to model the 3:1 phase. A single k-point, the Γ point, was used. All silver atoms were fixed during structural optimization. 3. RESULTS AND DISCUSSION 3.1. Epitaxial Growth. β″-(BEDT-TTF)2SF5CH2CF2SO3 was deposited from a heated crucible onto a Ag(111) surface at 90 K. The adsorbates self-assembled into large ordered molecular islands (Figure 1a). Each island could be assigned to one of two distinct groups, based on the arrangement and stoichiometric ratios of molecular building blocks as seen from 19403

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approximately the same within imaged areas on the Ag(111) surface, the adsorption reaction starting from the bulk can be represented as 2(BEDT−TTF)2 SF5CH 2CF2SO3 → (BEDT−TTF)1SF5CH 2CF2SO3 + (BEDT−TTF)3 SF5CH 2CF2SO3

(1)

Notably, the 1:1 stoichiometric ratios could nominally allow for a 1/2 filled electronic band within the 1:1 structure (assuming a 1e transfer toward the anion, as in the bulk), whereas the bulk crystal with 2:1 stoichiometry has a 3/4 filled band.19 However, the 3:1 ratio observed would result in 5/6 filled bands, while the alternating spacing might result in subsequent charge ordering.20 However, in light of the large adsorption energies, we need also consider explicitly the effect of the silver surface on the local electronic properties. 3.2. Tunneling Spectroscopy of Adsorbed Molecules. The energies of molecular orbitals within the islands with respect to the Fermi level were measured using scanning tunneling spectroscopy. The lowest empty state of the BEDTTTF molecule within the island with 1:1 stoichiometry is registered as a broad peak centered at 0.7 V and with a full width half-maximum (FWHM) of ∼0.4 V (Figure 3a). The

Figure 2. Top- and side-views of structural models of the 3:1 (a) and 1:1 (b) phases obtained by DFT calculations. The corresponding unit cells are shown as black squares. Center-to-center distances between the anion-bound and free cations of the 3:1 phase are included, indicating dimerization. The Brillouin zone of 1:1 phase is shown for reference.

the ⟨110̅ ⟩ direction of the Ag(111) surface (Figure 1c). Identifying SF5CH2CF2SO3 within this structure is less straightforward, but we believe they are likewise located between rows of BEDT-TTF molecules and are imaged as noisy patches (see red arrow). This relative arrangement is consistent with the ground state geometry of the structures subsequently calculated (Figure 2b) and means the local surface coverage of SF5CH2CF2SO3 is the same as that of BEDT-TTF, with a net stoichiometry of BEDT-TTF:SF5CH2CF2SO3. We attribute the difference in contrast and symmetry of the anion in Figure 1b,c to the complexity of the molecule, its surface bonding and the markedly different unit cell geometry of the 3:1 and 1:1 structures. The models presented below confirm our interpretation of the general location of the anions within the unit cells. The assembled structures and stoichiometric ordering are both indicative of net attractive intermolecular interactions. The small unit cell size and close-packing arrangement of the 1:1 structure indicate both an attractive cation−cation and anion− cation potential. The stability of the anion-bound cations and dimerization of nonbound cations in the 3:1 structure are consistent with this notion. At present, we cannot rule out partial decomposition of the SF5CH2CF2SO3 upon adsorption. However, it is difficult to argue in favor of the decomposition, given the good ordering of the molecular monolayer, the necessity for identical decomposition of all the constituent anions, the lack of other observable features on the surface apart from the molecular islands and the apparent stability of the SF5CH2CF2SO3 in DFT calculations. We also believe that both groups of molecular islands are one molecule tall. This assignment is confirmed by the fact that all BEDT-TTF molecules not bound within the network have the same apparent height, regardless of orientation (see, for example, misaligned molecules on the edge of the structure in Figure 1c and those in the disordered island within the island in Figure 1b). Moreover, no molecules of significantly smaller apparent height are observed, unlike those seen in the (BETS)2GaCl4, where a local bilayer coverage was inferred from the significant contrast in the STM images.7 In the case of one monolayer (ML) local coverage, the net stoichiometric ratio between BEDT-TTF and SF5CH2CF2SO3 corresponds to that directly seen in the STM images, 3:1 for the first group and 1:1 for the second, as labeled in Figure 1. As the surface density of the 3:1 and 1:1 structures are

Figure 3. Scanning tunneling spectra (dI/dV) taken over the (a) 1:1 stoichiometry cation, (b) 1:1 stoichiometry anion, (c) anion-bonded cation from the 3:1 structure, (d) dimerized cation of the 3:1 structure, (e) anion from the 3:1 structure, (f) bare BEDT-TTF, and (g) clean Ag(111). SS is the surface state.

spectrum over the SF5CH2CF2SO3, however, appeared featureless over the range scanned (Figure 3b). The spectrum taken over the BEDT-TTF within the 3:1 islands exhibited two peaks at 0.4 and 0.7 V, respectively (Figure 3c,d). Only a single state at 0.4 V was observed in the spectrum taken over the SF5CH2CF2SO3 (e). The peaks were about half as wide as those in the 1:1 islands. The empty states of the anion-bound BEDT-TTF (Figure 3c) and the dimerized BEDT-TTF (Figure 3d) show distinct contrast in magnitude. The 0.4 V state in the anion-bonded molecule is greater than the dimerized molecule, whereas the reverse is true for the 0.7 V state, explaining the 19404

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silver surface perturbs the redox chemistry of the chargetransfer salt: (1) is there still a charge-transfer interaction between the BEDT-TTF and SF5CH2CF2SO3 species; (2) why do well-ordered surface aggregates form in the apparent absence of the redox process on the surface, i.e., what other interactions exist between BEDT-TTF and SF5CH2CF2SO3 and how strong they are; (3) by extension, what is the relative role of the redox processes in stabilizing the bulk form? 3.3. DFT Modeling of Molecular Adsorbates. To understand what interactions stabilize salt-like condensates, we carried out DFT calculations on several model structures with and without van der Waals corrections. We optimized the molecular structures on the Ag(111) surface using the Purdew−Berke−Ernzerhof generalized-gradient approximation (Figure 2a,b), with van der Waals corrections within the Grimme parametrization (vdW-Grimme) as well as the recently formulated self-consistent van der Waals interactions (vdWDF2). The structures produced by the two vdW correction schemes were very comparable. In the 1:1 relaxed vdW− Grimme structure, the BEDT-TTF molecule is 3.61 Å above the silver atoms (measured to the CC bond center). The lowest energy was obtained with the SO 3 group of SF5CH2CF2SO3 bonded upright to the Ag atoms between the BEDT-TTF-molecules (Figure 2b), consistent with the bonding location in the STM images obtained (Figure 1c). In the ground-state geometry of the 3:1 structure, the BEDT-TTF molecule that is closest to SF5CH2CF2SO3 is less upright and rotated off the stacking direction of the neighboring dimerized molecules (Figure 2a). The dimerized molecules are spaced 5.28 Å apart (measured from center C atoms), whereas the BEDT-TTF closest to SF5CH2CF2SO3 and its nearest neighbor are spaced by 5.91 Å. The location of the SF5CF2CH2SO3 in the 3:1 phase therefore prevents parallel ordering and uniform spacing between all BEDT-TTF along the stack. Both the slight rotation and decreased spacing are consistent with STM images (Figure 1b). The modeled unit cell has the net C2 symmetry observed experimentally, if the detailed atomistic structure of the anion is not considered (which is likely to be justified given the STM contrast derives from molecular orbitals rather than the atomic structure). The binding energy of the system referenced to the 1:1 molecular layer in the gas-phase is Ea(1:1) = 3.01 eV per unit cell. The binding energy referenced to the individual BEDT-TTF and SF5CH2CF2SO3 molecules in the gas phase is Ec(1:1) = 6.03 eV. As a result, the interactions within the molecular layer amount to a very significant 3.02 eV (Ec(1:1) − Ea(1:1)) per unit cell. Similar calculations for the 3:1 stoichiometry (shown in Figure 2a) revealed Ea(3:1) = 6.3 eV and Ec(3:1) = 9.52 eV, likewise with a difference of 3.22 eV. That is, even in the presence of a substrate, intermolecular interactions are both strong and comparable in the 1:1 and 3:1 stoichiometric structures. To estimate the charge transfer from the adsorbed molecules to the silver surface, we have carried out the Bader charge analysis. For reference, this procedure applied to TTF on the Au(111) surface gives a charge donation of 0.55e from the TTF molecule, in very good agreement with the earlier work of Martinez et al.23 The Bader charge analysis reveals that, while the individual BEDT-TTF molecule loses about 0.4e, SF5CH2CF2SO3 has gained about 0.9e from its interaction with the silver surface. This is qualitatively consistent with the relative change of the tunneling barrier height measured over the 1:1 island in Figure 5a,b using I−Z spectroscopy.24

difference in STM contrast within the unit cell noted above. Filled and empty electronic states were also probed with the constant-current distance-voltage spectra method (Z−V spectroscopy).21,22 The gap between filled and empty states were measured to be 1.8 eV for the 3:1 and 2.2 eV for the 1:1 stoichiometry (Figure 4a,b). The empty states of BEDT-TTF

Figure 4. Z−V spectra taken over the BEDT-TTF molecules within (a) 3:1 stoichoimetry island, (b) 1:1 stoichiometry island, and (c) pure BEDT-TTF island. The spectra show roughly similar HOMO−LUMO gap of BEDT-TTF molecule, of 1.8, 2.2, and 1.9 eV, respectively.

were consistent between the Z−V and I−V spectra (compare Figure 3a,c and Figure 4a,b). For reference, tunneling spectroscopy on pure BEDT-TTF on Ag(111) revealed the electronic gap of ∼1.9 eV, matching that of the salts in the magnitude but electronic states shifted by ∼0.7 eV toward the vacuum level (Figure 4c). The empty states in both types of islands can be attributed to the lowest unoccupied molecular orbital (LUMO) states of the BEDT-TTF molecule. Notably, the LUMO, LUMO+1, and LUMO+2 of BEDT-TTF are nearly (but not strictly) degenerate in the gas-phase. In this case, the differences between the electronic structure of the 3:1 and 1:1 islands is illustrated by the splitting between the aforementioned states, likely due to the significant differences in the detailed adsorption geometry. Both HOMO and LUMO states of the BEDT-TTF molecule originate from the sulfur atoms of the tetrathiafulvalene (TTF) core. Since at least 2 out of 4 sulfur atoms of the TTF core participate in the surface−molecule bonding, the energies of the LUMO states and their interorbital splitting are expected to be very sensitive to the adsorption geometry. No distinct zero bias-tunneling states were apparent in the I−V spectra. Together with the measured transport gap of ∼2 eV, that is very similar to that of pure BEDT-TTF, this indicates that both 3:1 and 1:1 molecular layers are nominally insulating, with completely filled HOMO states of the BEDTTTF molecule. Therefore, the bonding to the silver surface prevents metallic conduction in these monolayer aggregates. However, there remain several questions on how exactly the 19405

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Figure 6. Projected band structure for the relaxed structure of the island with the 1:1 stoichiometry. Molecular states, mainly derived from the S(s,p) orbitals (red) show slightly dispersive bands along the (BEDT-TTF)-stacking direction (XM). The LUMO states are 0.8 eV above the Fermi level. The HOMO−LUMO gap is ∼2 eV, consistent with 1.8 eV measured experimentally on the molecular layer.

Figure 5. A 4 × 7 nm2 map of the tunneling barrier height change (a) next to a constant-current STM image of the same region (b) within the 1:1 stoichoimetry island (STM acquired at VB = 100 mV). A 3 × 7 nm2 map of the tunneling barrier height change (c) next to a constantcurrent STM image (d) of the same region within the 3:1 stoichiometry island (STM acquired at VB = 100 mV). The change in the barrier height relative to bare Ag surface is comparable in both systems, of approximately 1.3 eV between the anion (+0.9 eV) and cation (−0.5 eV). This illustrates a significant variation in local dipoles across the surface.

row there is a band derived from the molecular S(s,p) hybridized orbitals, which has a weak, but parabolic, dispersion across the XM direction (Figure 1e). This band, and others in general, are more flat along the molecules (i.e., the ΓX direction) as expected. Interestingly, surface molecular states open up gaps in the Ag states at high symmetry points, as seen at X̅ , due to the modulation of the electrostatic potential along the silver surface by the molecular overlayer, and thus a reduction in the point group symmetry of the otherwise bare Ag surface. This mechanism is quite common for metal surfaces and was observed in a TTF-TCNQ overlayer on Au(111)6 as well as the herringbone reconstruction on gold.25 The band has a width of ∼0.3 eV and peaks in the density of states at 0.8 eV. In addition, a filled broad band with a peak at −1.2 eV is observed. The calculated transport gap is therefore ∼2.0 eV, rather surprisingly consistent with the experimentally observed gap of 2.2 eV. Without Ag substrate, the roughly 1e donated by the BEDTTTF molecules gives rise to a half filled band with a relatively weak dispersion. Substituting BEDT-TTF with BETS, i.e., substituting some of the sulfur atoms by larger selenium, was found to increase the dispersion of the half filled band slightly. BETS donates 0.52e with the Ag substrate (larger than BEDT) and 0.93e without the Ag substrate. Extra control over the electronic properties of the epitaxial structures can therefore be gained by substitution of all or some of the donor molecules. Finally, we note that the cohesive energy of the β″-(BEDTTTF)2SF5CH2CF2SO3 is quite large, with a DFT calculated value of ∼6.3 eV per unit-cell. Therefore, even though the silver surface may have strong interactions with BEDT-TTF and SF5CH2CF2SO3, the system may be equally prone to dewetting and formation of bilayer and multilayer films. Indeed, the net cohesive energy of the 3:1 and 1:1 phases is 15.55 eV and, per eq 1, only ∼15% larger than the net cohesive energy of the bulk (∼12.6 eV for 2 uc). The slightly large cohesive energy of the phases supports our observation of epitaxial decomposition of the bulk 2:1 stoichiometry. We note, however, that following annealing to ∼325 K for about 20 min, we observed a new structure with a similar 2D lattice structure to the 1:1 stoichiometry (Figure 7a−c) but with a distinct layered contrast along the molecular stacks. The apparent height difference between these layers is ∼35 pm (Figure 7b),

Referring to the bare Ag(111) surface, the local barrier height is reduced by ∼0.5 eV over the BEDT-TTF molecule and increased by ∼0.8 eV over SF5CH2CF2SO3, implying opposite local dipoles, electron depletion on BEDT-TTF, and electron accumulation on SF5CH2CF2SO3. In the same 1:1 monolayer structure without the silver, the anion still retains a calculated excess of about 0.9e with the BEDT-TTF losing the same amount of charge and creating a half-filled band. The Bader analysis for the 3:1 phase reveals that SF5CH2CF2SO3 is again charged by 0.9e, while each of the BEDT-TTF molecules has lost only 0.2e. Without the silver, each BEDT-TTF molecule in the 3:1 structure would lose about 0.3e per molecule to the anion. This is in turn consistent with the work function map of the 3:1 stoichiometry in Figure 5c,d, showing a local barrier height reduction of ∼0.4 eV over the BEDT-TTF molecule and an increase of ∼1 eV over the SF5CH2CF2SO3. For comparison, a single isolated BEDT-TTF binds with Ea = 2.34 eV and a single, isolated SF5CH2CF2SO3 binds to Ag(111) surface with Ea = 3.08 eV (SO3 as the anchor). A similar Bader analysis shows that a single BEDT-TTF molecule lost 0.2e to silver, while an anion gained 0.84e. With these values as a reference, a significant charge-transfer between the molecules is clearly present in the 1:1 phase, as the BEDT-TTF molecule loses an extra 0.2−0.3e compared to the case of the isolated molecule. Intermolecular charge-transfer is also present in the 3:1 phase, but it would be a smaller effect given the reduced stoichiometric ratio and a strong donor character of the silver surface with respect to the anion. The Ag substrate thus strongly influences the charge-transfer within the BEDT-TTFanion complex, by mediating charge-transfer via back-bonding to both cation/anion molecules, as well as acting as an electron reservoir to the more electronegative anion molecule. The strength of the molecule−substrate interaction may therefore provide a unique control of charge transfer within the molecular layer. Figure 6 shows the projected surface electronic band structure of the 1:1 relaxed geometry. Along the molecular 19406

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sulfur with selenium atoms. In combination with the ability to control the filling, this could allow tuning a charge-transfer salt/ metal interface for targeted electron transport properties.

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AUTHOR INFORMATION

Corresponding Author

*(P.M.) E-mail: [email protected]. Phone: 865-5765220. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Work was supported by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.

Figure 7. (a) A 27 × 27 nm2 constant-current STM image of the postannealed surface with the salt-like condensates. The image captures a boundary between two islands, each of which is a bilayer. (b) A line profile taken over the area of an island with an exposed first layer (as highlighted in panel a), implying the bilayer height. (c) A 320 × 320 nm2 image showing numerous single domains of bilayer structures separated by disordered adsorbates and bare Ag(111).

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ABBREVIATIONS CTIS, charge transfer ionic salts; DFT, density functional theory; BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; LDA, local density approximation

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comparable to the apparent height of BEDT-TTF in the monolayer islands. This may suggest the formation of the bilayer, but other interpretations of this high-temperature structure, including a possibility of reconstruction of the underlying silver surface, cannot be ruled out at present. This is promising for the future efforts toward creation of films with bulk-like stoichiometry and weaker interactions with the underlying substrate.

REFERENCES

(1) Wosnitza, J. Perspectives on the Nature of Superconductivity in Organic Metals. J. Low Temp. Phys. 1999, 117, 1701−1710. (2) Singleton, J.; Mielke, C. Quasi-Two-Dimensional Organic Superconductors: A Review. Contemp. Phys. 2002, 43, 63−96. (3) Wosnitza, J. Quasi-Two-Dimensional Organic Superconductors. J. Low Temp. Phys. 2007, 146, 641−667. (4) Mori, H.; Tanaka, S.; Mori, T. Systematic Study of the Electronic State in θ-ET2MM(SCN)4 (MM = RbCo, RbZn, CsZn) by Reflection Spectroscopy. Phys. Rev. B 1998, 57, 12023. (5) Lof, R. W.; van Veenendaal, M. A.; Koopmans, B.; Jonkman, H. T.; Sawatzky, G. A. Band Gap, Excitons, and Coulomb Interaction in Solid C60. Phys. Rev. Lett. 1992, 68, 3924. (6) Gonzalez-Lakunza, N.; Fernandez-Torrente, I.; Franke, K. J.; Lorente, N.; Arnau, A.; Pascual, J. I. Formation of Dispersive Hybrid Bands at an Organic−Metal Interface. Phys. Rev. Lett. 2008, 100, 156805. (7) Umbach, T. R.; Fernandez-Torrente, I.; Ladenthin, J. N.; Pascual, J. I.; Franke, K. J. Enhanced Charge Transfer in a Monolayer of the Organic Charge Transfer Complex TTF-TNAP on Au(111). J. Phys.: Condens. Matter 2012, 24, 354003. (8) Clark, K.; Hassanien, A.; Khan, S.; Braun, K.-F.; Tanaka, H.; Hla, S.-W. Superconductivity in Just Four Pairs of (BETS)2 GaCl 4 Molecules. Nat. Nanotechnol. 2011, 5, 261−265. (9) Su, X.; Zuo, F.; Schlueter, J. A.; Williams, J. M.; Nixon, P. G.; Winter, R. W.; Gard, G. L. Anisotropic Magnetoresistance in the Organic Superconductor β”-(BEDT-TTF)2SF5CH2CF2SO3. Phys. Rev. B 1999, 59, 4376. (10) Mielke, C.; Singleton, J.; Nam, M.-S.; Harrison, N.; Agosta, C. C.; Fravel, B.; Montgomery, L. K. Superconducting Properties and Fermi-Surface Topology of the Quasi-Two-Dimensional Organic Superconductor λ-(BETS)2GaCl4 (BETS ≡ bis(ethylenedithio) tetraselenafulvalene). J. Phys.: Condens. Mater. 2001, 13, 8325−8345.

4. CONCLUSIONS In summary, we have explored the epitaxy of β″-(BEDTTTF)2SF5CH2CF2SO3 on Ag(111) grown by thermal evaporation from a bulk single crystal. Despite the apparent thermal decomposition of the bulk crystal, the ionic salt reassembles on the surface forming two distinct BEDT-TTF:SF5CH2CF2SO3 ratios, 1:1 and 3:1, neither of which is found in bulk phases of this compound. Strong chemical interaction at the epitaxial interface prevents the immediate growth of bilayer structure needed to satisfy the 2:1 bulk-like stoichiometric ratio. The adsorbed system nominally behaves as an insulator as judged from tunneling spectroscopy. Density functional theory analysis suggests that all the relevant interactions (molecule−molecule and molecule−surface) are strong, with binding energies ∼3 eV. The presence of the silver substrate reduces the amount of charge depleted from BEDT-TTF, when comparing to the same structures without the silver. The SF5CH2CF2SO3 anion, however, gains 0.8−0.9e with and without the silver surface. The stoichiometry and the donor/acceptor separation distance on the surface may change the amount of charge donated by the donor to the acceptor. Theory also shows weak dispersion of the HOMO-derived hybrid bands in the 1:1 phase, the dispersion of which can be increased with the substitution of 19407

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(11) Kaiser, S.; Dressel, M.; Sun, Y.; Greco, A.; Schlueter, J. A.; Gard, G. L.; Drichko, N. Bandwidth Tuning Triggers Interplay of Charge Order and Superconductivity in Two-Dimensional Organic Materials. Phys. Rev. Lett. 2010, 105, 206402. (12) Hishiguro, T. Superconductivity in Organic Charge Transfer Salts. Phys. C 1988, 153−155, 1055−1061. (13) Geiser, U.; Schlueter, J. A.; Hau Wang, H.; Kini, A. M.; Williams, J. M.; Sche, J. P.; Zakowicz, H. I.; VanZile, M. L.; Dudek, J. D. Superconductivity at 5.2 K in an Electron Donor Radical Salt of Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with the Novel Polyfluorinated Organic Anion SF5CH2CF2SO3−. J. Am. Chem. Soc. 1996, 118, 9996−9997. (14) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758. (15) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. (16) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved Grid-Based Algorithm for Bader Charge Allocation. J. Comput. Chem. 2007, 28, 899−908. (17) Kobayashi, H.; Kato, R.; Moriba, T.; Kobayashi, A.; Sasaki, Y.; Gunzi, S.; Enoki, T.; Inokuchi, H. Crystal-Structures and ElectricalProperties of BEDT-TTF Compounds. Mol. Cryst. Liq. Cryst. 1984, 107, 33−43. (18) Fernandez-Torrente, I.; Monturet, S.; Franke, K.; Fraxedas, J.; Lorente, N.; Pascual, J. Long-Range Repulsive Interaction Between Molecules on a Metal Surface Induced by Charge Transfer. Phys. Rev. Lett. 2007, 99, 176103. (19) Kontani, H. Origin of Superconductivity in β′-(BEDTTTF)2ICl2 under High Pressure and in β-(BEDT-TTF)2X at Atmospheric Pressure. Phys. Rev. B 2003, 67, 180503. (20) Kuroki, K. The Origin of the Charge Ordering and Its Relevance to the Superconductivity in θ-(BEDT-TTF)2X: The Effect of the Fermi Surface Nesting and the Distant Electron-Electron Interactions. J. Phys. Soc. Jpn. 1994, 75, 114716. (21) Pronschinske, A.; Mardit, D. J.; Dougherty, D. B. Modeling the Constant-Current Distance-Voltage Mode of Scanning Tunneling Spectroscopy. Phys. Rev. B 2011, 84, 205427. (22) Dougherty, D. B.; Maksymovych, P.; Lee, J.; Feng, M.; Petek, H.; Yates, J. T. Tunneling Spectroscopy of Stark-Shifted Image Potential States on Cu and Au Surfaces. Phys. Rev. B. 2007, 76, 125428. (23) Martinez, J. I.; Abad, E.; Gonzalez, C.; Ortega, J.; Flores, F. Theoretical Characterization of the TTF/Au(111) Interface: STM Imaging, Band Alignment and Charging Energy. Org. Electron. 2012, 13, 399−408. (24) Vitali, L.; Levita, G.; Ohmann, R.; Comisso, A.; De Vita, A.; Kern, K. Portrait of the Potential Barrier at Metal−Organic Nanocontacts. Nat. Mater. 2010, 9, 320−323. (25) Bürgi, L.; Brune, H.; Kern, K. Imaging of Electron Potential Landscapes on Au(111). Phys. Rev. Lett. 2002, 89, 176801.

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