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Tuning Intermolecular Charge Transfer in DonorAcceptor Two-Dimensional Crystals on Metal Surfaces Jonathan Rodriguez-Fernandez, Maitreyi Robledo, Koen Lauwaet, Alberto Martín-Jiménez, Borja Cirera, Fabian Calleja, Sergio Díaz-Tendero, Manuel Alcami, Luca Floreano, Marcos Dominguez Rivera, Amadeo L. Vazquez de Parga, David Ecija, Jose María Gallego, Rodolfo Miranda, Fernando Martin, and Roberto Otero J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08017 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Tuning Intermolecular Charge Transfer in DonorAcceptor Two-Dimensional Crystals on Metal Surfaces Jonathan Rodríguez-Fernández,†,¶,‡ Maitreyi Robledo, §,‡ Koen Lauwaet, ⊥,‖,‡ Alberto MartínJiménez, ⊥ Borja Cirera, ⊥ Fabián Calleja, ⊥ Sergio Díaz-Tendero, § Manuel Alcamí, § Luca Floreano,▼ Marcos Domínguez-Rivera, ▼ Amadeo L. Vázquez de Parga, † David Écija, ⊥ José M. Gallego,∆ Rodolfo Miranda, †,⊥ Fernando Martín, §,⊥ and Roberto Otero*,†,⊥

†Dep. de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain §Dep. de Química, Universidad Autónoma de Madrid, Madrid, Spain ⊥Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-NANO), Madrid, Spain

▼CNR-IOM, Laboratorio TASC, Basovizza, Trieste, Italy ∆ Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid, Spain

ACS Paragon Plus Environment

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ABSTRACT. Organic charge transfer (CT) compounds display a wide range of exotic electronic properties (Charge-Density Wave stabilization, Peierls transitions, etc.) depending on the amount of charge transferred from the donor (D) to the acceptor (A) species. A complete exploration of the complex electronic phase diagrams for such compounds would thus require methods to systematically tune the amount of charge exchanged in the CT process. This has proven however challenging in the past: chemical functionalization of the constituent molecules can also affect the packing of the molecular units in the crystal, whereas changing D:A stoichiometry is often not possible in the bulk. Interestingly, it was recently found that multiple stoichiometries can actually be achieved by codeposition of different amounts of D- and A-molecules on metal surfaces. The question however of whether CT processes between D- and A- molecules can be tuned with the D:A ratio in such mixtures has not yet been studied, and it is no trivial matter, since competing CT processes between the metal surface and the organic adsorbates might hinder inter-adsorbate charge transfer. Here we demonstrate that the CT process from the organic donor tetrathiafulvalene (TTF) to the acceptor tetracyanoquino-p-dimethane (TCNQ) can be tuned with exquisite accuracy (∼0.1 e-) by controlling the stoichiometry of D:A co-crystals deposited on Ag(111). This control opens new avenues to explore the complex phase diagrams of organic CT compounds and to tailor their electronic properties.

INTRODUCTION Crystals composed of mixtures between donor and acceptor organic molecules constitute a fascinating playground to synthesize materials with exotic electronic behavior1-3. TTF-TCNQ cocrystals are known to behave, at temperatures in excess of 54 K, as organic correlated metals with quasi-1D character3, and have long been considered as examples of Luttinger-liquid


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behavior4. At lower temperatures, two phase transitions can be observed, related to the stabilization of Charge-Density Wave (CDW) states and a Peierls transition leading to the insulating low temperature state3. These interesting phenomena are ultimately related to the transfer of 0.6e- from the donor (TTF) to the acceptor (TCNQ), which creates electrons in the conduction band of TCNQ stacks and holes in the valence band of the TTF stacks3-5. During the last decades a large number of D/A co-crystals have been synthesized, and the investigation of their electronic properties has uncovered a large range of interesting phenomena, from 1D semimetallic conduction to Mott transitions and superconducting behavior1-3. It is generally accepted nowadays that all these properties are defined by the band filling of the solids, which in turn depends on the amount of charge being transferred and, thus, on the electron affinity and ionization potential of the acceptors and donors respectively1-3. Control over the band filling of D/A co-crystals could theoretically be attained by the modification of their D:A stoichiometric ratio, but this option is very often difficult to realize. For example, an excess of TTF or TCNQ during the crystallization of TTF-TCNQ mixtures has only minor effects on the effective stoichiometry of the crystalline blend, nucleating the molecular excess in pure segregated crystals6-10. On the other hand, it is known that the selfassembly of D/A co-crystals on metallic surfaces does indeed allow for a much larger range of D:A ratios11-16, thanks to the screening effect of the metallic electron density at the surface. In particular, 2D TTF:TCNQ co-crystals self-assembled on Ag(111) were recently reported16, showing stoichiometries ranging from 2:1 to 1:4. These co-crystals are stabilized by electrostatic interactions between the molecular species, which become charged due to CT processes with the substrate. CT processes between the adsorbates and the metallic surface, however, should also hinder subsequent intermolecular charge exchange, since they increase the ionization potential


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and decrease the electron affinity of donor and acceptor species respectively. Thus, while D/A CT processes between donor and acceptor species adsorbed on a metallic surfaces have been recently reported, the estimated amount of charge exchanged is low17. Moreover, the role of the stoichiometry in determining the charge state of donors and acceptors in D/A co-crystals has not been addressed. In this study we describe a systematic investigation by Scanning Tunnelling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and Density Functional Theory (DFT) calculations, demonstrating that TTF:TCNQ co-crystals grown on Ag(111) with different D:A stoichiometric ratio, also show different amounts of charge being transferred from TTF to TCNQ adsorbates. To achieve this result, we have developed a new method to distinguish between interadsorbate and adsorbate-substrate CT processes by comparing the shifts in binding energies (BE) of the relevant core-levels with the shifts in the work function, as a function of the D:A ratio. This analysis reveals that the difference between these two quantities is always non-zero and increases with increasing D:A ratio, as should be expected due to intermolecular CT processes. DFT calculations corroborate this finding, showing differences in the charge transferred from TTF to TCNQ of the order of 0.1e-, in agreement with the observed changes in the BE shifts. METHODS The experiments were carried out in UHV conditions (base pressure ~10-10 Torr ). Atomically flat, crystalline Ag(111) surfaces were prepared by standard sputter/anneal procedures (sputter with 1 kV Ar+ ions for 10 min followed by annealing to 800 K for another 10 min), resulting in large terraces (~200 nm wide) separated by monoatomic steps. TCNQ and TTF were sublimated from a glass crucible held at 350 K and room temperature, respectively, onto the clean Ag(111)


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substrate held at room temperature. STM investigations were performed with two different instruments, a low temperature STM by Omicron and a variable temperature ‘Aarhus’ type STM by SPECS. XPS experiments were performed in the ALOISA beamline of the Elettra synchrotron in Trieste, using a photon energy of 500 eV, and in a local XPS system from SPECS using Al-K α radiation (hν=1486 eV), yielding comparable results. The work function measurements were carried out in the local XPS system by subtracting from the photon energy the difference in energy between the minimum (secondary electron cut-off, SECO) and maximum (Fermi level) kinetic energies measured by the analyzer. Density functional theory (DFT) calculations including periodic boundary conditions (PBC) were performed using the Vienna Ab initio Simulation Package (VASP) 18-20, using the Perdew−Wang 91 functional (GGA-PW91)21-23. Van der Waals interactions have been taken into consideration by the use of DFT-D2 method24. The electron density has been described employing a plane wave basis set with a kinetic energy cutoff of 400 eV. Metal surfaces were modeled by a slab consisting of four atomic layers, separated by a vacuum space of 10 Å in the coordinate perpendicular to the surface, z. The lateral dimensions of the unit cell were taken from the experimental sizes of the different molecular unit cells. RESULTS AND DISCUSSION


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Figure 1. High-resolution STM images of the different stoichiometries of mixed TTF-TCNQ on Ag(111) with the following ratio: a) 2:1. b)1:1. c)1:2. d)1:4. And simulated STM image of the 2:1 phase. Figure 1 shows STM images recorded at 4.6 K of 2D crystals obtained by sublimation of TTF and TCNQ molecules on a pristine Ag(111) surface in different ratios at submonolayer coverage. Comparison with simulated STM images based on DFT calculations allows us to distinguish the features corresponding to TCNQ molecules (red oval) and TTF molecules (blue oval). With this information we can extract the D:A stoichiometric ratio of the 2D crystals by direct inspection of the experimental STM images. Although the only observed stoichiometry for TTF-TCNQ cocrystals in bulk is 1:1, Ag-supported 2D co-crystals can have D:A ratios of 2:1, 1:1, 1:2 or 1:4, in good agreement with Ref. [16]. We also observe a well-ordered TCNQ phase on Ag(111), which was also found by other authors25. On a larger scale, coexistence of domains with different local stoichiometries is usually found for a given set of deposition conditions, although when the TTF:TCNQ coverage ratio is close to one of the stable phases identified above, the majority of the surface (≥75%) is covered by that phase. Domain sizes for the majority stoichiometric phase are in excess of 500 nm2. Similar experiments on Au(111) have not revealed such a wide variety of possible stoichiometries, and only the 1:1 phase and a minority presence of the 2:1 stoichiometry were observed26.


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Figure 2. a-b) N1s and S2p region of the XPS spectra (hν= 500 eV) taken on the different TTFTCNQ stoichiometries, respectively (they have been vertically displaced for clarity. The spectra corresponding to the pure TTF and TCNQ layer are also included. The shaded lines correspond to a fitting with Voigt functions using a Shirley background. Vertical lines mark the position of the peak centers. In order to investigate experimentally if interadsorbate CT processes take place on the Ag(111) surface, we have performed high-resolution XPS experiments to characterize the shifts in the N1s core level of TCNQ and in the S2p core level of TTF (see Figure 2) as a function of the D:A ratio. Increasing the number of acceptors per donor (decreasing the D:A ratio) should increase the charge donated by each TTF molecule and/or decrease the charge accepted by each TCNQ molecule. Based on this effect alone, as the D:A ratio decreases, one should expect a shift of the core levels of N1s in TCNQ to higher binding energies, because they are expected to bear a lower negative charge, and a shift of the S2p core levels of TTF also to higher binding energy, since they bear more positive charge. On the contrary, the spectra reveal that decreasing the D:A ratio leads to shifts of S2p (TTF) core levels to lower BE (see Figure 2). The corresponding shift for the N1s core level (TCNQ) is much smaller than that observed for TTF, but if anything, it also shifts to lower binding energies. Similar results were recently reported for different D:A couples13-15,27 and explained in terms of the effect of the electrostatic potential created at each molecular position by the presence of the rest of the molecules at the surface, which bear a partial charge due to charge transfer between the substrate and the adsorbates. In our case, DFT calculations of TCNQ and TTF overlayers on Ag(111) reveal that TCNQ molecules are charged by more than one electron, whereas the positive charge at the TTF molecules is much smaller (see Figure 4). Decreasing the D:A ratio, therefore, leads to a more negative value of the electrostatic potential experienced by both molecules and, thus, is expected to shift the core levels towards lower BE, as observed in our experiments.


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Figure 3. a) Experimental (black squares) and DFT calculated (blue circles) surface potential shifts for different stoichiometric D:A ratios (left axis referenced to TTF, right axis referenced to TCNQ). b) BE shifts for the N1s (red up triangles) and S2p (green down triangles). The shifts are obtained from the fits to the spectra shown in Figure 2 and referenced to pure TCNQ and pure TTF respectively. c) Left Scale: Potential-corrected binding energy shifts (same symbols as in b) for N1s and S2p core levels; Right Scale: Difference between molecular charges in TCNQ and TTF for a D/A co-crystal with a given D:A ratio and the corresponding values for homomolecular films (TCNQ red open circles, TTF green open circles).

An estimation of the average potential experienced by electrons at D:A co-crystals on Ag(111) due to charge-transfer with the substrate, ∆V(θ TTF :θ TCNQ ), can be obtained from the measurement of the work function. Indeed, work function shifts reveal changes in the dipole perpendicular to the surface. Interadsorbate charge-transfer, consisting of a lateral charge rearrangement parallel to the surface, is therefore expected not to produce changes in the work function. In order to obtain ∆V(θ TTF :θ TCNQ ) for different stoichiometries, we have estimated TTF and TCNQ coverages (θ TTF and θ TCNQ ) from the S2p and N1s XPS intensities, and measured the work function of such sample (W S ) and of bare Ag(111) (W AG ). The change of the surface potential associated to an area covered by molecular mixture with θ TTF :θ TCNQ stoichiometric ratio (∆V(θ TTF :θ TCNQ )) can then be obtained from the expression:


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W S = - (θ TTF + θ TCNQ ) ∆V(θ TTF :θ TCNQ ) + (1- θ TTF - θ TCNQ ) W AG

(1)

which takes into consideration the fact that moving from a more positive potential to a more negative potential actually increases the overall work function of the surface. Figure 3a shows the changes in the surface potential of the mixture as a function of the D:A ratio, referenced to the surface potential of pure TTF (left axis) or pure TCNQ (right axis) obtained from the experimental analysis described above (black squares). The work functions for full monolayers obtained from DFT calculations (blue circles) are also shown for comparison in figure 3a. It can be observed that the agreement between theory and experiment is good, showing a decrease in the surface potential of about 1 eV with decreasing D:A ratios, from pure TTF to pure TCNQ. Notice that Equation (1) is just a statement about the dependence of the work function with the coverage of the film. ∆V(θ TTF : θ TCNQ ) calculated from Equation (1) includes thus two different kind of effects: 1- Changes in the electrostatic potential due to the replacement of donor (acceptors) by molecules of different sign: According to our calculations, TCNQ molecules in homomolecular films receive about 1 e- from the metallic surface, thereby creating a dipole moment perpendicular to, and pointing towards the surface. Similarly, adsorption of TTF molecules leads to dipole moments pointing outwards from the surface. Substitution of TTF molecules by TCNQ molecules, i.e. decreasing the D:A ratio, should thus modify the perpendicular dipole moment, thereby affecting ∆V(θ TTF : θ TCNQ ) and the values of the work function28. 2- Changes in the work function due to modification in the interaction between donor (acceptor) molecules and the substrate due to the presence of acceptor (donor) molecules:


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Recent studies have shown that the adsorption height for donors and acceptors on metal surfaces are affected by the presence of molecules of the other sign. In particular, it has been observed that coadsorption of donors and acceptors leads to an increase of the adsorption height of the acceptor molecules compared to the case in which only acceptor molecules are present on the surface, while the adsorption height of the donors is decreased in mixed layers with respect to the adsorption geometry of pure donors17,29-30. This description is coherent with the relaxed geometries obtained from DFT calculations: in the case of TCNQ, N(on top)-Ag distance increases from 2.30 Å in the TCNQ monolayer to 2.66 Å when the D:A ratio is 1:1. For TTF, S-Ag distance changes from 2.93 Å in the pure TTF monolayer to 2.85 Å when the D:A ratio is 1:1. Changes in the adsorption height will determine the electron density spill-out above the metallic surface and, thereby, will have a role in determining the dipole perpendicular to the surface and thus the work function. This contribution to ∆V(θ TTF : θ TCNQ ) can then be interpreted as the change in the potential experienced by electrons at the molecular overlayer due to the variations in the metal electron density upon coadsorption. The values of ∆V(θ TTF : θ TCNQ ) obtained from Equation (1), thus, always gauge the change in the potential experienced by electrons due to charge transfer from the surface to the molecular adsorbates, due to substitution of donor (acceptor) molecules by molecules of the opposite sign.


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Figure 4. Schematic representation of the balance between intermolecular versus moleculesubstrate charge transfer processes. Blue ovals represent TTF molecules and red ovals TCNQ molecules. Red arrows mean transfer from the Ag(111) substrate to the molecular adsorbates, while blue arrows signify electron transfer from the molecules to the substrate. a) and e) represent the charge exchange between pure donor and acceptor molecules and the substrate.

The shift in the BE of the N1s core level with respect to pure TCNQ, and of S2p core levels with respect to pure TTF, as a function of the D:A ratio can be found in Figure 3b. The span of the energy axis has been chosen equal to that in Figure 3a to facilitate comparison between the data. Both shifts follow a similar trend as that of the surface potential, but with significantly smaller variations: 0.13 eV for the shift of the N1s binding energy between pure TCNQ and the 1:1 phase, and 0.5 eV for the shift of the S2p binding energy from pure TTF to the 1:4 phase. Interestingly, if those shifts are corrected for the variation of the surface potential ∆V(θ TTF :θ TCNQ ) referred to monolayers of pure TCNQ or pure TTF on Ag(111) respectively, (Figure 3c) the trends are completely reversed. The corrected shifts for TTF are always positive and those corresponding to the TCNQ are always negative. Moreover, both donor and acceptor core levels shift now to higher corrected BEs with decreasing D:A ratio, and the slopes of both shifts are equal. All these observations require further discussion. The shifts in the BE of the core levels will be determined by changes in the total electron density around the atom of interest and by in the local surface potential. The local surface potential, in turn, can be decomposed in a laterally averaged potential, exclusively related to the presence of dipole moments perpendicular to the surface31, and a laterally inhomogeneous component. Since we have already concluded that the changes in the surface potential due to molecule-substrate charge transfer induced dipoles are given by ∆V(θ TTF : θ TCNQ ) regardless of the bonding strength between the organic molecules and


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the metal surface, a correction such as the one carried out here is expected to partially eliminate the effect of the changing surface potential with the D:A ratio, leaving only two contributions: interadsorbate CT processes and residual electrostatic shifts due to lateral inhomogeneity of the surface potential. The latter contribution, however, must shift the core levels in the same sense as the general electrostatic contribution, i.e. to lower binding energies, and for the same reasons. Thus the fact that we observe a shift to higher BE after our correction, implies that interadsorbate charge transfer must take place, being the shift associated to it larger than that due to the residual electrostatic corrections. Actually, previous studies have shown that, for truly intermixed systems, such lateral corrugation of the local potential is very small28. Notice that this argument is completely general and does not depend on specific assumptions as to the level alignment at the interface, an effect that will be related to molecule-substrate charge transfer and, thus, be included in our determination of ∆V(θ TTF :θ TCNQ ). In particular, if no intermolecular charge transfer occurred, the core level shifts would only be due to the difference in the potential experienced by electrons at the organic layer due to charge-transfer with the substrate, thus matching the change in the work function, as previously found for other donor-acceptor couples on metallic surfaces13-15,27, and the shifts in the corrected BEs would thus be identically zero. This is also the case when similar molecules with different dipole orientations are coadsorbed on a metal surface28: Substitution of molecules with dipole up by molecules with dipole down leads to an overall increase of the work function which matches the shifts in the core level BEs (both in the experiments and in the calculations), consistent with the expectation that no intermolecular charge-transfer takes place between dipole up and down molecules, since both have the electron affinity and ionization potential. It was recently suggested that substrate mediated intermolecular


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charge transfer could mutually enhance the donor and acceptor character of molecular adsorbates in close proximity17, which is consistent with the quantitative trend we determined in Figure3c. From the previous analysis, we thus conclude that, in spite of the rather strong molecule-metal charge transfer, intermolecular charge transfer does occur between TCNQ and TTF molecules when adsorbed on Ag(111), and this transfer can be fine-tuned by control of the D:A stoichiometric ratio. The fact that intermolecular charge-transfer is not completely prevented by adsorption onto the metallic Ag(111) substrate, as in other systems such as PEN/CuPc, might be related to a stronger intermolecular interaction between the donor and acceptor species, mediated by the electronegative peripheral cyano groups of TCNQ,which can form rather strong hydrogen bond, while the C-F—H-C bonds stabilizing the structures of PEN/CuPc are expected to be much weaker due to the low polarizability of fluorine. We have checked these conclusions from DFT calculations performed for the TTF:TCNQ cocrystals on Ag(111) , with the stoichiometries found in the experiment. According to Bader analysis of these DFT calculations (see Figure 4), the adsorption of TCNQ alone leads to the transfer of 1.1e- from Ag(111) to the molecule, whereas the effect of the adsorption of TTF is smaller in absolute value (transfer of 0.1e- from the molecule to the surface). For the 1:2 stoichiometry on Ag(111) the charge localized at the TTF molecule increases by 0.3e- with respect to the pure TTF case, while each one of the two TCNQ molecules receives 0.15e- more than in the absence of TTF. Similarly, the formation of the 1:1 lattice involves the increase of the charge at TCNQ by -0.25e- and at TTF by +0.2e-, and for the 2:1 phase, -0.3e- for TCNQ and +0.1e- for each TTF molecule. Notice that, for the calculated cases, the amount of extra charge given by all the TTF donors compensates quite closely the extra charge accepted by all the TCNQ acceptors. The change in the electron charge located at each TTF or TCNQ molecules for


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a given D:A ratio with respect to the charge for homomolecular films, is plotted in open circles in Figure 3c. Comparison with the corrected BE shifts shows that, within the margin of error, the corrected BE shifts are proportional to the changes in the molecular charges due to intermixing between donors and acceptors. This analysis thus suggest that, in spite of the adsorption-induced charge transfer, TTF and TCNQ still exchange electron charge in different amounts depending on the D:A ratio, changing by about 0.1e- between one phase and the next. Notice that the absolute values of the charges calculated here are in good agreement with Ref. [16]. CONCLUSIONS In conclusion, we have shown that we can control the charge state of molecular donors and acceptors on solid surfaces very accurately (~0.1 e-) by controlling the stoichiometric D:A ratio. Although both TTF and especially TCNQ molecules already exchange charge with the surface, their very strong donor and acceptor character still enables intermolecular charge-transfer between the adsorbed species. This result has been verified both theoretically and experimentally, by means of a method that corrects the binding energy shifts by the surface potential changes obtained from measurements of the work function. These results open promising possibilities for the engineering of new 2D charge-transfer materials and films with tunable properties, which might find application in fields as diverse as molecular electronics, highly conductive/low weight coatings for aerospace vehicles or photovoltaics. ASSOCIATED CONTENT Supporting Information.


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The Supporting Information is available free of charge via the Internet on the ACS Publications website (http://pubs.acs.org). TTF-TCNQ Phase Coexistence (PDF). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Present Addresses ¶Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark. ‖ Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid, Spain Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT The authors acknowledge financial support from the Spanish Ministry for Economy and Competitiveness (grants FIS2012-33011, FIS2015-67367-C2-1-P, FIS2013-42002-R, FIS201677889-R, CTQ2013-43698-P, CTQ2016-76061-P), the regional government of Comunidad de Madrid (grants S2009/MAT1726 and S2013/MIT-3007), Universidad Autónoma de Madrid (UAM/48) and IMDEA Nanoscience. L.F. acknowledges financial support from MIUR (PRIN-


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2010BNZ3F2, project DESCARTES), S. D.-T. acknowledges the ‘‘Ramón y Cajal’’ programme of the MINECO (RYC-2010-07019), and the María de Maeztu Programme for Units of Excellence in R&D of the MINECO (MDM-2014-0377). We thank the Center for Scientific Computing of the Autonomous University (CCC-UAM) and the Spanish Supercomputing Network (RES) for the allocation of computer time. Technical support for the realization of the experiments by Dr. M. Á. Niño and J. Matarrubia is gratefully acknowledged.

REFERENCES 1. Mori, T. Organic conductors with unusual band fillings. Chem Rev. 2004, 104, 4947– 4969. 2. Seo, H.; Hotta, C.; Fukuyama, H. Toward systematic understanding of diversity of electronic properties in low-dimensional molecular solids. Chem. Rev. 2004, 104, 5005– 5036. 3. Jérome, D. Organic conductors: From charge density wave TTF-TCNQ to superconducting (TMTSF) 2 PF 6 . Chem. Rev. 2004, 104, 5565–5591. 4. Ito, T.; Chainani, A.; Haruna, T.; Kanai, K.; Yokoya, T.; Shin, S.; Kato, R. Temperaturedependent luttinger surfaces. Phys. Rev. Lett. 2005, 95, 1–4. 5. Sing, M.; Schwingenschlögl, U.; Claessen, R.; Blaha, P.; Carmelo, J. M. P.; Martelo, L. M.; Sacramento, P. D.; Dressel, M.; Jacobsen, C. S. Electronic structure of the quasi-onedimensional organic conductor TTF-TCNQ. Phys. Rev. B. 2003, 68, 125111.


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Tuning Intermolecular Charge Transfer in Donor ... - ACS Publications

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