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Chemical Physics Letters 612 (2014) 45–50

Contents lists available at ScienceDirect

Chemical Physics Letters

jou rn al hom epage: www.elsev ier .com/ locate /cp le t t

dsorption of tetrathiafulvalene (TTF) on Cu(1 0 0): can �-stacked 1-Dggregates be formed at low temperature?

ne Sarasolaa,b,∗, Sara Barjac, Amadeo L. Vázquez de Pargac, Andrés Arnaub,d

Dpto. Física Aplicada I, UPV/EHU, Bilbao, SpainDonostia International Physics Center, San Sebastián, SpainInstituto Madrileno de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia) and Dpto. Física de la Materia Condensada, UAM, Cantoblanco 28048,adrid, SpainDpto. Física de Materiales, Facultad de Química UPV/EHU and Centro de Física de Materiales CFM Centro Mixto, CSIC-UPV, San Sebastián, Spain

r t i c l e i n f o

rticle history:eceived 6 March 2014

n final form 28 July 2014

a b s t r a c t

We present a combined experimental and theoretical study that shows the formation of one-dimensionalaggregates of tetrathiafulvalene (TTF) molecules along high symmetry directions of the Cu(1 0 0) metal

vailable online 4 August 2014surface at low temperatures. Our first principles calculations permit to explain the observations. We findan adsorption geometry of the molecules in a tilted configuration that is explained as the result of adelicate balance between intermolecular and molecule–surface attractive forces of comparable strength.The inclusion of van der Waals (vdW) forces in the description is instrumental to arrive at this conclusion.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

The possibility of growing one-dimensional (1-D) chains of �-tacked molecules on surfaces represents a challenge that wouldermit the study of fundamental electronic properties in conduct-

ng 1-D systems as an alternative to, for example, those formed by-D rows of Au atoms on Si vicinal surfaces [1]. Usually, the adsorp-ion geometry of planar organic molecules, like polyarenes, oningle crystal surfaces is essentially parallel to the surface [2], as theonding is typically due to the interaction between the � electronloud and the surface electrons. As a consequence of this adsorptioneometry, there is no �-stacking between the molecules, contraryo the case of three dimensional molecular crystals [3]. However,here are cases in which the molecules are adsorbed tilted, or evenertically, on the surface, no matter whether it is a single crystal sur-ace, due to the formation of a particular molecule–surface bond [4]r the existence of an unusual intermolecular bonding [5–7] thatay appear at short intermolecular distances or, equivalently, higholecular coverages [8,9].So far, its practical realization using semiconductor or insulat-

ng surfaces does not seem to be an easy task but metal surfacesepresent a first alternative to explore although, in principle, its unclear which type of surface is better: a highly anisotropic

∗ Corresponding author at: Dpto. Física Aplicada I, UPV/EHU, Bilbao, Spain.E-mail address: ane.sarasola@ehu.es (A. Sarasola).

ttp://dx.doi.org/10.1016/j.cplett.2014.07.067009-2614/© 2014 Elsevier B.V. All rights reserved.

quasi-one-dimensional surface, like O(2 × 1)/Cu(1 1 0) [10], or amore isotropic two-dimensional surface, like Cu(1 0 0).

Density Functional Theory (DFT) has been the most used methodto treat hybrid organic/inorganic systems and has demonstratedits efficiency to describe those that have strong chemical bonding.However, in the case of weakly bound or physisorbed systems sev-eral studies [11–21] have shown the relevance of non-local vdWforces in the formation and stabilization of molecular overlayerson surfaces. Therefore, the inclusion of dispersion forces to describethe adsorption of organic molecules on metal surfaces is clearly nec-essary, as described for PTCDA on (1 1 1) noble metal surfaces [15],thiophene adsorbed in the more reactive Cu(1 1 0) surface [22] or,more recently, for azobenzene on Ag(1 1 1) [23].

Among all the methods that have been developed with the aimof including dispersion forces [24], different levels of accuracy exist,from entirely empirical corrections to self-consistently calculateddispersive functionals within DFT, or even more refined methods,like Random Phase Approximation (RPA) or GW approximation,these latter limited to small systems due to its high computationalcost [25].

However, only a few of them [26–31] have been extensivelytested by using their implementations in the available ab initioDFT codes with almost zero additional computational cost: the

DFT-D semiempirical method [11,14,17], the vdW-DF method,which is entirely self-consistent, used in its different exchangeflavors [11,18,21,32,33], and the recently developed vdWsurf,natural extension of the previous DFT + vdW that accounts for

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he screening of the substrate electrons [20,23,33]. In particular,ptB88-vdW exchange functional [34] and the more recent vdWsurf

ethod [31] have stood out due to their accuracy describing, forxample, organic molecule/metal systems [16,23] or graphene onetal surfaces [35].In this work we use first principles calculations, including vdW

ispersion forces within the vdW-DF method with two differentxchange functionals to study the adsorption of TTF moleculesn Cu(1 0 0). We explore the possible reasons why at low tem-eratures scanning tunneling microscopy (STM) topographical

mages seem to show the formation of one dimensional aggre-ates of �-stacked molecules along high symmetry directions. Inarticular, we are interested in the role that the different interac-ions, adsorbate–adsorbate and adsorbate–substrate, play [36,37]n determining the adsorption geometry. We also consider theependence of our results on the local molecular coverage [38–40]hat determines the intermolecular distances but we do not con-ider adsorbate phases that could appear at high coverages, i.e.,lose to a monolayer.

We find an energetically favorable adsorption geometry for theTF monomer that corresponds to planar TTF molecules parallel tohe Cu(1 0 0) surface for low coverages, while TTF aggregates formue to the existence of a metastable phase. This latter correspondso a tilted adsorption geometry that appears to be stabilized byntermolecular van der Waals forces as the local coverage increases.

. Methods

Experiments have been performed in an ultra-high vacuumUHV) chamber with a base pressure of 3 × 10−11 Torr equippedith a low temperature STM. The Cu(1 0 0) crystal has been cleaned

ccording to usual cycles of Ar+ ion sputtering (1.5 keV) followedy annealing to 900 K. TTF molecules (solid compound) have beenublimated at 300 K from a glass crucible heated to room temper-ture by switching off the refrigeration that kept the evaporator atow temperatures. STM images have been measured at low temper-ture (4.6 K) in the constant current mode using etched tungstenips cleaned in UHV [41].

DFT calculations are performed using the plane-wave basedode VASP [42–44]. The projector augmented wave [45] (PAW)ethod has been used to describe the interaction between the

alence electrons and the ion cores. vdW calculations have beenarried out self-consistently using the functional of Dion et al. [27],sually called vdW-DF, and implemented in VASP [16] with thecheme of Román-Pérez and Soler [46]. For exchange and corre-ation, two different DFT-DF options have been compared: one, in

hich the GGA of Perdew, Burke, and Ernzernhof [47] (PBE) is usedor exchange and a second one, in which optB88 functional is used.hus, the exchange-correlation energy functional, in this case, isefined as:

xc[n] = EGGA(x)[n] + ELDA(c)[n] + Enl(c)[n] (1)

here EGGA(x)[n] is the exchange energy obtained with the PBE andptB88 functionals, ELDA(c)[n] is local density approximation corre-ation energy and Enl(c)[n] is the non-local correlation energy parthat accounts for the dispersion interaction.

We use periodic supercells with slabs that include four Cu lay-rs and five and a half layers of vacuum. Different surface unitells were employed to vary the TTF–TTF intermolecular distance.he longitudinal axis of the molecule is always set parallel to theong dimension of the surface unit cell, composed by five atomic

u rows of different widths, ranging from two to five Cu atoms.

n practice, we have assembled five molecule–substrate configura-ions, that we name 1TTF/5 × 2, 1TTF/5 × 3, 1TTF/5 × 5, 2TTF/5 × 4nd 2TTF/5 × 5 and are shown in Figure 3. For consistency, different

s Letters 612 (2014) 45–50

k-point samplings for k-space integrations are used with differentMonkhorst–Pack meshes equivalent to 10 × 10 × 1 in the 1 × 1 sur-face unit cell. The plane-wave energy cut-off was 500 eV in all thecases. The relaxation of all the proposed different configurations inthe distinct functional descriptions are made. All the atoms of themolecule(s) and the top most Cu(1 0 0) layer were relaxed until theforces were smaller than 0.02 eV/A.

3. Results

Figure 1a and b shows two topographic STM images of a Cu(1 0 0)surface at low TTF coverage. They have been taken at two differ-ent tunneling conditions and tip preparation conditions. Therefore,they show different resolution and apparent corrugation. In panel(a) two kind of features are observed: (i) faint round protrusionsthat we tentatively ascribe to TTF monomers, as no molecular crack-ing is expected during deposition [2,49,50], although we cannotneglect the possibility of TTF dissociative adsorption, and (ii) linearchains along a high symmetry direction of the crystal surface thatwe assign to TTF aggregates of up to four TTFs. These latter show amuch brighter contrast and suggest a different adsorption geome-try. The higher resolution image in panel (b) is probably affected bytip effects that distort the appearance of TTF molecules but, on thecontrary, it permits to achieve atomic resolution on the Cu(1 0 0)surface and, thus, reveals: (i) TTF molecules forming the aggregatefrom chains along high symmetry directions, and (ii) a separationdistance of 2d0 between TTF molecules forming the aggregate alonghigh symmetry directions, where d0 is the Cu(1 0 0) in-plane latticeconstant. Recall that, as described in the methods section, molec-ular deposition is done at room temperature but STM images aretaken at 4.6 K. However, STM images taken at room temperaturealso show the formation of TTF aggregates. By further increasingthe temperature to 400 K the TTF aggregates disappear.

In order to reinforce our interpretation of the STM images show-ing the formation of TTF aggregates in which TTF molecules areadsorbed in a tilted adsorption geometry forming chains alonghigh symmetry directions of the Cu(1 0 0) surface, now we com-pare them with STM simulations that correspond to the relevantoptimized adsorption geometries described below. In Figure 2aand b we show two Tersoff–Hamann simulated STM images andtheir corresponding profiles, panel (a) corresponds to the TTFchain with TTF tilted molecules and panel (b) to a TTF monomerin planar adsorption geometry. Although the calculated corruga-tions are significantly higher than the measured ones, typical ofTersoff–Hamann simulations, the overall agreement is quite rea-sonable for the TTF aggregates in both scan directions, across andalong the TTF chains. However, for the “tentative” TTF monomerin planar adsorption geometry the simulated STM image shows amuch higher corrugation than the measured profile and a differ-ent line shape at the boundaries. Therefore, we cannot completelydiscard the possibility of disociative adsorption or the presenceof impurities on the surface. Anyway, what we consider moreimportant is the fact that the formation of TTF aggregates seemsto be consistent with our previous observation. Now, we proceedwith a detailed analysis of the adsorption energetics of the severalcalculated TTF adsorption geometries that permit to support thisinterpretation. We start by studying the van der Waals intermolec-ular forces between TTF molecules in the absence of metal surfaceand, later on, we account for the presence of Cu(1 0 0).

We have calculated the interaction energy between two TTFmolecules as a function of the distance between them but keep-

ing the orientation of both planar molecules parallel and withoutany tilting. Apart from the PBE-vdW and optB88-vdW functionalsdescribed in the Methods section, PBE and DFT-D results are alsogiven. As shown in Figure 3, the intermolecular interaction between

A. Sarasola et al. / Chemical Physics Letters 612 (2014) 45–50 47

Fig. 1. (a) STM image (7 nm × 7 nm) measured at 4.6 K (Vb = −1.0 V, It = 50 pA). TTFaggregates made of three or four monomers appear as bright protrusion on theimages. Insets: Profiles measured along the white lines crossing the TTF monomer(1) and chain (2 and 3). (b) High resolution STM image (Vb = +100 mV, It = 0.20 nA)showing the TTF tetramer aligned along the high symmetry [011] direction simul-taneously with atomic resolution on the Cu(1 0 0) terrace. The monomers formingthe chains are separated by two Cu(1 0 0) in plane lattice constants. The STM is pre-sented in a 3D view with lateral illumination to be able to distinguish simultaneouslytt

TvaitmA

Fig. 2. Tersoff–Hamann simulated STM images: (a) for tilted TTF molecules two in-plane lattice constants apart and (b) for planar molecules. Insets: Profiles along the

he atomic corrugation on the Cu(1 0 0) terrace and the internal structure of the TTFetramer.

TF molecules can only be correctly described when includingan der Waals forces, as otherwise only a short range repulsionppears when using standard DFT-GGA methods. The strongestntermolecular vdW interaction is described by the PBE-vdW func-

ional, followed by optB88-vdW and DFT-D method of Grimme. The

inima appear at separation distances of 3.7 A(PBE-vdW) and 4.0˚ (optB88-vdW). At an intermolecular distance of 5.2 A [2d0] the

numbered lines indicated in the (a) and (b) panels. All images are done using WSxM[48].

energies amount 200 meV (PBE-vdW) and 120 meV (optB88-vdW),respectively.

Next, we explore the way the presence of the Cu(1 0 0) metalsurface changes the adsorption geometry of the TTF molecules atdifferent intermolecular distances. For each of the optimal adsorp-tion geometries shown in Figure 4 (the corresponding geometricalparameters of the optimized configurations are given in Table 1)we calculate the corresponding adsorption energy and adsorp-tion energy per unit area. We do so by following the procedure

described in the Methods section, based on the use of differentperiodic supercell size that correspond to different intermolecu-lar distances between the TTFs. In all cases, we have started by

48 A. Sarasola et al. / Chemical Physics Letters 612 (2014) 45–50

Fsn

ar

T

E

ws

ttwqmsbecsadtmi

htmtw∼naSfA

Fig. 4. Schematic representation of the adsorption geometries for the different con-figurations, according to our DFT-vdW calculations. Qualitatively, both approaches(PBE-vdW and optB88-vdW) predict similar geometries of adsorption, even if thebonding distances and tilting angles change. The parameters indicated in the figure

TG

ig. 3. Intermolecular distance dependence of interaction energy between two �-tacked TTFs using different DFT approaches. The inclusion of dispersive forces isecessary to obtain attractive interaction between the molecules.

n initial configuration in which TTF molecules are 20◦ tilted withespect to the surface plane.

Adsorption energies of these relaxed structures are given inable 2, and are defined as follows:

ads = ETTF+Cu − ETTF − ECu (2)

here ETTF+Cu, ETTF and ECu are the relaxed energies of the totalystem and the isolated constituents, respectively.

Concerning these adsorption energies, they are higher whenhe optB88-vdW functional is used, compared to PBE-vdW andhe corresponding TTF–Cu bond distances are significantly shorterhen using the optB88-vdW functional, particularly in planar o

uasi-planar adsorption geometries. Therefore, based on this infor-ation and the previous analysis of results shown in Figure 3, we

ee that optB88-vdW gives stronger molecule–surface attractionut weaker molecule–molecule attraction. Apart from these differ-nces, in both cases the largest adsorption energy per unit areaorresponds to the case 1TTF/5 × 2. We have also checked (nothown) that at closer intermolecular distances (2TTF/5 × 3) thedsorption energy per unit area decreases, as intermolecular vaner Waals forces are not in the range of distance that correspondso attraction (see Figure 3). This is consistent with the observed for-

ation of TTF aggregates: a higher adsorption energy per unit areas a first hint for the preference to form local 1TTF/5 × 2 structures.

In order to understand the reasons for this behavior, weave performed a more careful analysis and have arrived athe conclusion that a competition between intermolecular and

olecule–substrate van der Waals attraction forces determineshe final adsorption geometry. Before arriving at this conclusion,e have also checked, based on a Bader charge analysis, that the50◦ tilted optimal adsorption geometry shown in Figure 4 isot stabilized by electrostatic forces between charged S and Cu

toms. Actually, the calculated values of the Bader charges for two

atoms (+0.23) bond to Cu differs only by 0.11 electron chargesrom the Bader charges of the two other S atoms (+0.12) in the TTF.t large separations (1TTF/5 × 5 and 1TTF/5 × 3 configurations) the

able 1eometrical parameters of the optimized configurations in both DFT-vdW approaches as

Configuration 1TTF/5 × 5 1TTF/5 × 3

Bonding distances (A)/angles d1/� d2/� d3/ϕ1

PBE-vdW 3.40/0◦ 3.40/0◦ 3.25/9optB88-vdW 2.55/0◦ 2.51/0◦ 2.42/1

are given in Table 1 for the PBE-vdW and optB88-vdW cases.

TTF on Cu(1 0 0) adsorption geometry is planar with respect to thesurface, while at short distances between the TTFs (1TTF/5 × 2 andits equivalent 2TTF/5 × 4) a tilted configuration is preferred. In theintermediate case (2TTF/5 × 5), however, the tilt angles are rathersmall. When minimizing the energy during the relaxation processfor the 2TTF/5 × 5 configuration, we find first what we consider ametastable state or local minimum in the energy landscape thatcorresponds to two TTFs in a ∼45◦ tilted configuration at a separa-tion distance of 5.2 A[2d0]. By further relaxing the system, the twomolecules tend to increase the separation distance between themand tilt to a configuration closer to planar adsorption geometry,

probably due to the existence of a very small energy barrier.Therefore, the optimal geometry of the TTF periodic array corre-sponds to a tilted configuration with a tilt angle that depends on

defined in Figure 4.

2TTF/5 × 5 1TTF/5 × 2 2TTF/5 × 4

d4/ϕ2 d5/� d6/˛1 d7/˛2

◦ 2.59/21◦ 2.65/44◦ 2.68/52◦ 2.68/57◦

5◦ 2.41/27◦ 2.40/50◦ 2.42/50◦ 2.42/49◦

A. Sarasola et al. / Chemical Physics Letters 612 (2014) 45–50 49

Table 2Energies of adsorption and energies of adsorption per area for the different conformations obtained after having relaxed the structures starting from an equivalent 20◦-tiltedposition of the molecule.

Configuration 1TTF/5 × 5 1TTF/5 × 3 2TTF/5 × 5 1TTF/5 × 2 2TTF/5 × 4

Eads (eV) PBE-vdW −0.506 −0.846 −0.915 −0.759 −0.731optB88-vdW −1.959 −1.966 −1.702 −1.477 −1.410

Eads/area (eV/nm2) PBE-vdW −0.30 −0.84 −1.09 −1.12 −1.09optB88-vdW −1.19 −2.00 −2.07 −2.25 −2.15

Table 3Displacement vectors and magnitudes of the numbered atoms in Figure 5 due to the recontruction of the surface upon the adsorption of the TTF on top of it. All the valuesare given in Aand correspond to the relaxations done with the optB88-vdW functional.

Configuration 1TTF/5 × 5 2TTF/5 × 4

Atom # �x �y �z |��r| �x �y �z |��r|1 −0.01 0.05 0.06 0.08 0.00 −0.01 0.10 0.112 −0.03 0.04 0.06

3 0.03 −0.04 0.06

4 0.01 0.05 0.07

Fig. 5. Reconstruction of the surface for the low and high adsorbate-density cases(ta

tb

rvgt0ttatc

planar and tilted adsorption, respectively). The atoms of the top most layer closesto the S atoms of the TTF are the ones that suffer the largest displacements (redtoms in the figure).

he separation distance between the TTF molecules, the tilt angleeing larger the shorter the intermolecular distance is.

The adsorption of TTF on Cu(1 0 0) induces a weak surfaceeconstruction. This is shown in Figure 5 and Table 3 for two rele-ant cases of planar (1TTF/5 × 5) and tilted (2TTF/5 × 4) adsorptioneometries. Only Cu atoms in the Cu(1 0 0) top most layer belowhe S atoms of TTF suffer significant displacements, of the order of.1 A. In planar adsorption geometry, there are four Cu atoms belowhe four S atoms of the TTF molecule, while only two in the case of

ilted adsorption geometry. Additionally, the TTF molecules suffer

slight distortion, with respect to their gas phase geometry, whenhey adsorb on the Cu(1 0 0) surface. We have estimated the energyost of both the Cu(1 0 0) surface reconstruction and TTF distortion

0.08 0.00 −0.01 0.10 0.110.08 0.05 0.01 0.08 0.100.08 0.05 0.01 0.08 0.10

by comparing with the optimized pristine Cu(1 0 0) surface and iso-lated TTF molecules, respectively, and have found them to be ratherlow (of the order of 100 meV) compared to the adsorption energies.Therefore, we can say that the major contribution to the adsorptionenergy of TTF on Cu(1 0 0) is due to S–Cu bond formation.

4. Conclusions

Our energetic analysis of the TTF/Cu(1 0 0) system based onfirst principles calculations is consistent with the formation of TTFaggregates, as observed in the experiment.

A detailed inspection of the different contributions permit us toconclude that there exist both a weakly attractive inter-molecularand molecule–surface interactions giving rise to a tilted adsorptiongeometry.

At the observed optimal TTF separation distance intermolecularvdW forces compete with a somewhat stronger molecule–surfaceattraction. As a consequence of this competition, there exist a deli-cate energy balance giving rise to the formation of tilted TTF chains.However, for temperatures slightly above room temperature, thisequilibrium is broken due to thermal vibrations and the TTF aggre-gates are no longer stable.

Acknowledgments

We thank Sergio Monturet for his help in the early stages ofthe development of this work. A. S. and A. A. acknowledge sup-port from the Basque Departamento de Educación, UPV/EHU (GrantNo. IT-756-13) and the Spanish Ministerio de Ciencia e Innovación(Grant No. FIS2010-19609-C02-01). S.B. and A.L.V.P. acknowledgesupport from the Ministerio de de Ciencia e Innovación (GrantsNo. CONSOLIDER-INGENIO 2010 on Molecular Nanoscience andFIS2010-18847) and Comunidad de Madrid through the programNANOBIOMAGNET S2009/MAT1726.

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