Monolayer structure of tetracene on Cu (100) surface: Parallel geometryWeidong Dou, Jiabao Zhu, Qing Liao, Hanjie Zhang, Pimo He, and Shining Bao Citation: The Journal of Chemical Physics 128, 244706 (2008); doi: 10.1063/1.2940335 View online: http://dx.doi.org/10.1063/1.2940335 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/128/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effects of intrinsic defects on methanthiol monolayers on Cu(111): A density functional theory study J. Chem. Phys. 138, 134708 (2013); 10.1063/1.4799557 1,2-Dibromoethane on Cu(100): Bonding structure and transformation to C2H4 J. Chem. Phys. 135, 064706 (2011); 10.1063/1.3624348 The initial growth behavior of perylene on Cu(100) J. Chem. Phys. 134, 194702 (2011); 10.1063/1.3591968 Scanning tunneling microscopy study of metal-free phthalocyanine monolayer structures on graphite J. Chem. Phys. 127, 114702 (2007); 10.1063/1.2770732 Formation of hydrogen-bridged cytosine dimers on Cu(110) J. Chem. Phys. 124, 204704 (2006); 10.1063/1.2190225

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Monolayer structure of tetracene on Cu „100… surface: Parallel geometryWeidong Dou,1,2,a� Jiabao Zhu,1 Qing Liao,1 Hanjie Zhang,1,b� Pimo He,1 andShining Bao1

1Physics Department, Zhejiang University, Hangzhou, 310027, China2Department of Physics, Shaoxing College of Arts and Science, Shaoxing, 312000, China

�Received 6 March 2008; accepted 14 May 2008; published online 24 June 2008�

The geometrical arrangement of tetracene on Cu �100� surface at monolayer coverage is studied byusing scanning tunneling microscopy measurement and density functional theory �DFT�calculations. Tetracene molecule is found to be oriented with its molecular plane parallel to thesubstrate surface, and no perpendicular geometry is observed at this coverage. The molecule is

aligned either in the �011� or �011̄� direction due to the fourfold symmetry of the Cu �100� surface.DFT calculations show that the molecule with the “flat-lying” mode has larger adsorption energythan that with the “upright standing” mode, indicating that the former is the more stable structure.With the flat-lying geometry, the carbon atoms prefer to be placed between surface Cu atoms. Themolecular center prefers to be located at the bridge site between two nearest surface Cu atoms.© 2008 American Institute of Physics. �DOI: 10.1063/1.2940335�

I. INTRODUCTION

Organic molecular thin films are currently studied exten-sively both because of their application potential �e.g., fororganic thin film field-effect transistors �OTFTs�� and be-cause of fundamental interest in molecular self-assembly atsurfaces.1–3 Technically, the fabrication processes of OTFTsare much less complex compared to those of conventional Sitechnology, which involves high-temperature and high-vacuum deposition processes and sophisticated photolitho-graphic patterning methods. In general, low-temperaturedeposition and solution processing can replace the morecomplicated processes involved in conventional Si technol-ogy. In addition, the mechanical flexibility of organic mate-rials makes them naturally compatible with plastic substratesfor lightweight and foldable products. OTFTs have beendemonstrated to have great potential for a wide variety ofapplications, especially for new products that rely on theirunique characteristics, such as electronic newspapers, inex-pensive smart tags for inventory control, and large-areaflexible displays.

One of the limitations of current organic technologies isclearly the performance of the active layer, e.g., the low con-ductivity or the low mobility of the charge carriers.4,5 Theperformance of OTFTs can be improved by controlling thedeposition rate and the temperature of the substrate, whichaffect the morphology of the semiconductor, e.g., the order-ing of the organic semiconductor system. A mobility up to35 cm2 /V s has been reported recently for high-purity pen-tacene single crystal.6 The ordering of the molecular orien-tation in organic thin film is affected by various factors, e.g.,the deposition rate, the postdeposition treatment, such as an-nealing, the size,7 the type,8 and even the molecular weight.9

In general, the determination of the ordering formation canbe summarized as a balance of interactions between mol-ecules and that with the substrates. Ordering of the firstmonolayer is believed to drive the epitaxy in the upper layersby governing the formation of domains and grain boundaries.Due to the primary role, the structure of the organic molecu-lar layer in contact with the substrate, especially metal sub-strate, has been the subject of several detailed studies.10–12

The polycyclic aromatic molecules, such as pentacene,tetracene, and perylene, are among the most extensivelystudied materials since they can be grown as single crystalswith remarkably high charge carrier mobility. In the case oftetracene �C12H18� �see Fig. 1�, the planar structure makes ita popular candidate for the studies of ordered structures inthe first overlayer and in thin films. The growth behavior oftetracene on metal and inert substrates has attracted extensivestudies. On inert surfaces, such as H-passivated Si�001�-2�1,13 the weak van der waals �vdW�-like interactions be-tween adsorbed molecules can play a major role in the pro-cess of forming ordered structures. While on active metalsubstrates, such as Cu �110�,3 Ru �10-10�,14 Ag �110�,15 andAg�111�,16 strong interaction between organic molecule andsubstrate may govern the formation of different patterns inthe first monolayer and determine the subsequent three-dimensional �3D� crystalline structure.17 Therefore, the na-ture of the substrate surface plays a very important role in thedesigning of one- and two-dimensional self-assembled mo-lecular �SAM� structures. An appropriately treated surfacecan serve as template for nanofabrication. For example, on along-range �2�1� reconstructed surface of Cu �110�, theO–Cu nanotemplate has been used to drive self-assembly ofrubrene into a variety of highly ordered structures.18 At loworganic coverage, rubrene molecules adsorb preferentially onCu stripes, while they occupy the O–Cu region at multilayercoverage, forming a flowerlike pattern. The difference in mo-lecular arrangements in low and high coverage regimes can

a�Electronic mail: vgstone@sohu.com.b�Author to whom correspondence should be addressed. Electronic mail:

zhj�fox@zju.edu.cn.

THE JOURNAL OF CHEMICAL PHYSICS 128, 244706 �2008�

0021-9606/2008/128�24�/244706/6/$23.00 © 2008 American Institute of Physics128, 244706-1

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be understood by studying the adsorbate-substrate interac-tion. In other cases, e.g., thiophene adsorbed on Cu �110�surface,19 vdW interactions between molecules is found to begoverning. In these cases, the adsorbed molecules are onlyphysisorbed, and the interaction between adsorbates andmetal surfaces is weak, which indicates a different role ofadsorbate-substrate interactions as in the cases of some aro-matic molecules on metal surfaces. Although a rich varietyof structures of organic semiconductor films upon growingon inorganic substrates have been observed, a detailed under-standing of the principles governing these film structures hasnot yet been achieved. Of particular importance in this re-spect is, however, that the molecular interaction with metal-lic electrode also decisively determines the charge carrierinjection in a device.

In this paper, we report single layer growth of tetraceneon Cu �100�. Comparing with Cu �110� surface, Cu �100� hasfourfold symmetry and a little weaker interaction with theadsorbed molecules,20 which indicates different structures inthe first overlayer and on Cu �110� surface. Yannoulis andKoch21 studied the tetracene/Cu �100� system using angularresolved ultraviolet photoemission spectroscopy �ARUPS�and reported a standing-up configuration of tetracene withthe molecular plane oriented perpendicular to the Cu �100�substrate, namely “upright standing” mode. However, a dif-ferent geometry configuration is found from our experiment.

II. EXPERIMENTAL SETUP AND CALCULATIONMETHOD

The experiments were performed using a multitechniquesurface analysis system �Omicron instrument for surface sci-

ence� with a base pressure better than 2�10−10 mbar. Thesystem is described in detail elsewhere.22 In brief, it consistsof a fast-loading chamber, a preparation chamber, an analysischamber, and a scanning tunneling microscopy �STM� cham-ber. The system is equipped with a resistive-heating Ta boattetracene evaporator, a low-energy electron diffraction�LEED� attachment, an x-ray photoemission spectrometer,etc.

Before the tetracene was deposited, a clean 1�1 Cu�100� surface was obtained by standard Ar+ sputtering andannealing procedures. This procedure results in a clean andvery flat target surface with a mean width of terraces largerthan 100 nm. The cleanness of the surface was checked byLEED and STM measurements. The tetracene �Sigma, 99%�was purified thoroughly with preheating at 330 K, and thedeposition was done at a source temperature of about 440 Kwith a deposition rate of 0.2 ML /min. The substrate waskept at room temperature �RT� during the molecular beamdeposition and STM measurements.

Density functional theory �DFT� calculations have beenperformed by using the DMOL

3 �e.g., density functional formolecules and 3D periodic solids�23,24 code within general-ized gradient approximation �GGA�.25 As for the computa-tional details, Perdew–Wang functional26 �PW91� and theDFT semicore pseudopotential were used to describeelectron-electron interaction and electron-ion interaction, re-spectively. The Monkhorst–Pack scheme27 was used tosample the Brillouin zone. A localized atom-centered doublenumeric polarized basis set was used for functional expan-sion, with a finite basis-set cutoff radius of 4.0 Å.

The Cu �100� surface was modeled by a slab with fourlayers of Cu atoms and a vacuum region of 10 Å �for “flat-lying” mode� or 25 Å �for upright standing mode�. To studythe interactions between tetracene and the substrate surface,a unit cell with dimensions of 10.2�15.3 Å2 for planar ad-sorption was used. The unit cell contains only one moleculeand is large enough to avoid the interaction between mol-ecules. For perpendicular geometry, the surface unit cell wasbuilt according to the existing experimental value,21 say, ap�2�2� periodicity. In addition, a larger unit cell with p�4�4� periodicity was also used for the structural optimiza-tion. This unit accommodates only one molecule and is largeenough to avoid molecule-molecule �M-M� interactions.During the process of structural optimization, the moleculeand Cu atoms in the upper two layers of the substrate areallowed to be relaxed. The convergence criteria of the geo-metrical optimizations are set as 2.0�10−5 hartree for en-ergy, 4.0�10−3 Å /hartree for energy gradient, and 5.0�10−3 Å for displacement of the ionic step.

III. RESULTS AND DISCUSSIONS

The atomic resolved STM image of the Cu �100� surfaceis shown in Fig. 2�a�. The substrate is clean and the atomlines along the �011� direction of the substrate can be easilyidentified. Figure 2�b� shows a typical STM image of thesurface at 1.0 ML coverage. As shown in Fig. 2�b�, tetracenedid not form a long-range ordered structure under our experi-mental conditions. For pentacene, which is very similar to

FIG. 1. �Color online� Molecular structure of tetracene in the gas phase andits highest occupied molecular orbital �HOMO� and lowest unoccupied mo-lecular orbital �LUMO�. The long and short axes are indicated by the dashedand dotted lines, respectively.

244706-2 Dou et al. J. Chem. Phys. 128, 244706 �2008�

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tetracene, it was found that multilayer growth started beforethe completion of monolayer when the molecules were de-posited onto the Cu �110� surface.28 The well-defined mono-layer films can be grown at a temperature of 430 K, wherebilayer formation is thermodynamically unfavorable.29 How-ever, we observed that tetracene can form a well-definedmonolayer on Cu �100� surface at RT. The long axis of the

molecular plane is oriented either along the �011� or �011̄�direction of the substrate surface, which can be attributed tothe fourfold symmetry of the Cu �100� surface. In Figs. 2�c�and 2�d�, one molecule has a “footprint” of 1.37�0.72 nm2. This value is in good agreement with the tet-racene molecule in the gas phase, which has a rectangularmolecular plane with vdW dimensions of 1.37�0.70 nm2.The agreement between these values indicates that the tet-racene molecules are adsorbed with the molecular planesparallel to the substrate surface, maximizing vdW interac-

tions with the substrate surface. This conclusion agrees withthe widely accepted view for adsorption of aromatics onmany metal surfaces. However, it is in clear contradiction toearlier studies of tetracene adsorption on Cu �110� and Cu�100� surfaces using near edge x-ray absorption fine structurespectroscopy and ARUPS,21,30,31 which showed a perpen-dicular orientation of the tetracene molecular plane withrespect to the substrate surfaces.

The “shape” and dimension of the tetracene moleculewere constant as the bias voltage was change during STMmeasurements, which indicates that the features in STM im-ages are true topographic images of the tetracene, rather thanthe electronic ones. Figure 3�a� is a STM image obtained at adifferent tunneling condition as oppose to that in Fig. 2�b�.The molecules are arranged almost randomly on the sub-strate surface with their long axis aligning along the twoequivalent directions. In Fig. 3�b�, the diagram which iscounted from Fig. 3�a� shows the number of the molecules

arranged in the �011� and �011̄� directions. It contains almostthe same number of molecules in the two equivalent direc-tions, indicating a dominant mechanism of the molecule-substrate �M-S� interaction for the organic SAM. The M-Sinteraction is so strong that the molecular alignment is notchanged even when the substrate was annealed up to 400 K.Upon adsorption on the Cu �100� surface, the molecularplane of tetracene is slightly bended upward. The end phenylrings are decoupled from the substrate surface due to thedeformation of the molecular plane, resulting in two protru-sions �indicated by the bright regions� in the STM image ofone molecule �see Figs. 2�c� and 2�d��. The distribution ofthe bending values is shown in Fig. 2�c�. The typical valuesare found in the scope of 0.10–0.15 Å.

Another feature in the STM image is the local orderedstructures in the regions indicated by the ellipse in Fig. 3�a�.The coexistence of long-range disordering and local orderedstructure is an indicator of the competition between M-S andM-M interactions. The structural quality of the tetracenemonolayer may be improved when the sublimation is carriedout at elevated substrate temperature since the high vibra-tional energy of tetracene will help it overcome the bindingforce imposed by substrate Cu atoms. In that case, the mo-lecular geometry will mainly be driven by M-M interactions.

FIG. 2. �Color online� �a� STM image of Cu substrate �10�10 nm2, Vt=−0.002 V, It=50 nA�. The �011� direction of the substrate is indicated by thewhite arrow. �b� STM image of a monolayer tetracene on Cu �100� surface�30�30 nm2, Vt=−0.380 V, It=0.712 nA�. �c� An enlarged image of anarea marked in �B� �4.63�2.58 nm2�. The rectangular area indicates theplanar dimension of a molecule. �d� Line profiles along the directions shownin �c� by the solid and dotted lines.

FIG. 3. �Color online� �A� STM image of a monolayertetracene on Cu �100� surface �15�15 nm2,Vt=−1.32 V, It=0.712 nA�. �b� Molecular alignmentand �c� molecular bending, counting from �a�.

244706-3 Monolayer structure of tetracene on Cu �100� surface J. Chem. Phys. 128, 244706 �2008�

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While at RT, adsorbates/substrate interactions play a veryimportant role. One question that needed to be answered ishow strong the M-S interaction is. To answer this question,we have carried out a series of theoretical calculations basedon DFT.

Due to the fourfold symmetry of the Cu �100� surface,

adsorption geometries along the �011̄� direction will be iden-tical to the corresponding geometries along the �011� direc-tion. Therefore, only structures with the long axis of the mol-ecule aligning along the �011� direction are calculated. InFig. 4�a�, four possible structures from which the optimiza-tion is started are presented. The adsorption sites are definedas the positions at the first layer of the substrate where thecenter of the tetracene molecule is located. In Fig. 4�a�, S1

and S2 represent the geometrical structures with the moleculebeing aligned between the two copper atom lines along the�011� direction at the first layer of the substrate, where S1

represents the hollow site and S2 represents the bridge site

between two adjacent Cu atoms along the �011̄� direction. Inaddition, S3 and S4 represent the structures on which themolecule is aligned on top of the Cu atom lines. S3 refers tothe bridge site between two adjacent Cu atoms along the�011� direction, while S4 refers to the top site with the mo-lecular center being located just above the Cu atom at thefirst layer of the substrate.

In Fig. 4�b�, the potential energy surface �PES� of theadsorbed system is shown as a function of the angles � be-tween the �011� direction of the substrate surface and thelong axis of the adsorbed molecule. The PES is obtained bycalculating the total energy of the tetracene/Cu �100� systemswith different angles �. From the PES of the adsorbed sys-tem, it is obvious that the adsorbed structure with the longaxis of the molecule aligning along the �011� direction hasthe lowest energy, which indicates the possibility of the moststable structure.

To find the most stable structure, the adsorption energy�AE� of several structures was compared. The AE is definedas Eads= �Esubs+Emol�−Esys, where Esubs indicates the totalenergy of the Cu �100� substrate, Emol indicates the totalenergy of single tetracene molecule in the gas phase and Esys

indicates the total energy of the tetracene/Cu �100� system.The most stable structure is found to be S2 �see Fig. 4�c��.Some calculated results are summarized in Table I. The posi-tive values in the AE indicate the exothermic reaction duringthe process of adsorption. For the most stable structure, themolecular plane remains almost planar although the two endphenyl rings are slightly bended upward, contributing to amaximum molecular bending of 0.20 Å, which agrees withthe experimental value qualitatively. The hydrogen atoms,which play an important role in the self-assemblymechanics,3 are raised upward, showing a repulsive interac-tion between H atoms and Cu substrate. It may be attributedto the slight bending of the molecular plane.

In order to get further understanding of the M-S interac-tion, the redistribution of valence electrons was studied bycalculating the difference in charge density before and afterthe adsorption. The charge density was calculated usingCASTEP �Ref. 32� with a GGA/PW91 level, and an ultrasoftpseudopotential was used for the electron-ion interaction.The energy cutoff for this calculation is set as 400 eV, whichis sufficient for convergence. The electron density differencemap of the substrate with one organic molecule is shown inFig. 4�c� �lower panel�. The map shows a transfer of spilledout electrons from the Cu substrate into the tetracene mo-lecular state �mainly into the � state�33 and the interfaceregion between the organic molecules and substrate. The re-distribution of the electrons indicates a strong M-S interac-tion.

From Table I, the optimized structure that started from S1

�e.g., on hollow site� is always consistent with that started

FIG. 4. �a� Possible adsorbed geometry of tetracene on Cu �100� surface with flat-lying mode. �b� PES as a function of angle �. Here, S3� is the identical

structure with S3, except for the aligning direction of the molecular long axis, which is in the �011̄� direction in the former case. �c� The optimized structure�upper panel� and countermap of electron density difference upon adsorption �lower panel� of S2, marked in �a�. The electron density difference map is cutacross the molecular center along the long axis, indicated by the dashed line in �a�. M represents tetracene molecule and SL represents the second layer of thesubstrate.

TABLE I. Calculated results of the optimized structures of tetracenemolecule with flat-lying geometry on Cu �100� surface.

Initialstructure

Optimizedstructure AE �eV� M-S distance ��

Molecularbending ��

S1 S2 2.44 2.19 0.20S2 S2

S3 S3 1.46 3.42 0.18S4 S4 1.51 3.33 0.18

244706-4 Dou et al. J. Chem. Phys. 128, 244706 �2008�

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from S2 �e.g., on bridge site�, indicating that the S1 is anunstable structure since the carbon atoms are directly overthe surface Cu atoms. The calculated result is consistent withthe common feature of naphthalene and anthracene on Pt�111� surface34 that the carbon atoms tend to locate betweenthe surface metal atoms and not directly over them in orderto reduce the repulsive interaction and increase the attractiveinteraction between the adsorbed molecule and the substrate.Comparing with the structures with the molecule lying overthe Cu atom lines �e.g., S3 and S4�, the structure with themolecule located between the Cu atom lines �e.g., S2� haslarger AE and shorter M-S distance, indicating a strongerM-S interaction at this site. This rather strong interactioncauses planar adsorption of the molecules in the first mono-layer and a small distance to the substrate of only 2.19 Å,which is significantly smaller than a typical layer separationin planar aromatic hydrocarbon crystals.

The molecular geometry concluded from the STM dataand DFT calculations is in conflict with the earlier ARUPSand LEED results21 taken from Cu �100� surface coveredwith 2–3 ML of tetracene molecules. The structures with themolecule standing up on the substrate surface are also calcu-lated in order to compare to the results of flat-lying geom-etry. The optimized structures are presented in Fig. 5 and thecalculated results are summarized in Table II. The AEs aremuch smaller than those of structures with flat-lying geom-etry, indicating a weaker M-S interaction. This is expectedsince the center of the molecule is far away from the sub-strate surface.

For the monolayer, planar arrangement is commonly ob-

served for the adsorption of planar aromatic molecules onmetal surfaces.35,36 The interaction of aromatic molecule ad-sorbates with metal substrates is primarily expected to bebetween the molecule � system and the substrate d electrons,causing the molecule backbone rings to be close to the sub-strate to maximize dispersion force.37 For tetracene on Cu�100� surface, the interaction between organic adsorbate andmetal substrate is strong due to the active nature of the Cu�100� surface. Therefore, flat-lying geometry is more favor-able than upright standing ones in the submonolayer to 1 MLregime. The preference of flat-lying arrangement in low cov-erage regime is supported by the much higher AE than thatof the upright standing mode. Considering the face-to-face �

stacking of tetracene with upright geometries, the intermo-lecular noncovalent �-� interaction may decrease the totalenergy of the adsorbate/substrate system, which will stabilizethe upright structures. The �-� interactions are caused by theintermolecular overlapping of p orbitals in the p-conjugatedsystem and are described by the PW91 functional used in thepresent calculations. To maximize the overlapping of the porbitals, calculations of upright geometries using a cutoffradius of 10.0 Å have been carried out. Compared to calcu-lations using a cutoff radius of 4.0 Å, the increase in thecutoff radius only lowers the AE with a small value of0.05 eV.

Tetracene molecule can be adsorbed with upright stand-ing geometry on the inert surface, such as H-passivated Si�001� surface13,38 or surface over the first monolayer whereintermolecular forces are dominant. It is reported that planararomatic molecules experience a characterized reorientationof molecular plane relative to the substrate surface. This re-orientation of the molecular plane has been observed fornaphthalene on Ag �100�,39 perylene on Cu �110�,40 and pen-tacene on Cu �110� surface.29 In the monolayer regime, themolecules form a highly ordered structure with planar ad-sorption geometry, while in the multilayer regime, they re-orient their molecular planes with an upright or tilted mo-lecular orientation. This molecular reorientation is normalsince the influence of the metal substrate �e.g., �-d interac-tion� is weakens, and the interaction between adsorbates�e.g., �-� interaction� becomes dominant.

For tetracene on Cu �110� surface, actually, recent evi-dence from UPS measurements suggests a structural phasetransition involving reorientation of the molecule in the firstand subsequent layers.33 This may suggest that perpendiculararrangement is possible beyond 1 ML. Therefore, we assumethat the reason for the discrepancy is the difference in cov-erage. In the earlier work, 2–3 ML are detected based on thequartz crystal microbalance measurement. If 1 ML of tet-

FIG. 5. Optimized structure of tetracene on Cu �100� surface with uprightstanding mode. ��a� and �b�� Top and side views of the structure with �4�4� surface unit cell, respectively. ��c� and �d�� Top and side views �view inthe �010� direction� of the structure with �2�2� cell, respectively. The blackballs represent the Cu atoms at the first layer of the substrate, and the darkballs represent that of the second layer.

TABLE II. Calculated structure of the optimized structures of tetracenemolecule with upright standing geometry on Cu �100� surface.

Optimized structure Adsorbed site � �deg� AE �eV�

�2�2� cell Bridge 17 1.22�4�4� cell Hollow 0 1.12

244706-5 Monolayer structure of tetracene on Cu �100� surface J. Chem. Phys. 128, 244706 �2008�

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racene on Cu �100� surface was derived from a �2�2� LEEDpattern, the coverage of 2–3 ML would be much higher thanthat observed in the present work.

For the optimized structures with upright standing mode,tetracene molecule is perpendicular to the substrate surfacein the case with �4�4� periodicity �see Figs. 5�a� and 5�b��,while it is tilted in the case that with �2�2� periodicity �seeFigs. 5�c� and 5�d��. The tilted angle � which is defined asthe angle between the molecular plane and the Cu �100� sur-face normal, is �17°. This value is in good agreement withthe existing experimental value of pentacene in the thin filmregime �� phase�.29 In the later case, pentacene was depos-ited on the SiO2 surface. Considering the similarity betweentetracene and pentacene, the agreement between the theoret-ical result and experimental ones indicates that the moleculararrangement is less likely to be affected by the Cu substrateif tetracene is adsorbed with upright geometry. In tilted ge-ometry, the short axis of the tetracene molecule is parallel tothe �010� direction of the fcc crystal, while in perpendiculargeometry �the �4�4� model�, the short axis of the tetracene

molecular is aligned in the �011̄� direction. The difference ingeometrical arrangements in the �2�2� and �4�4� casescan be attributed to the intermolecular �-� interaction,which is negligible in the latter case since it has a largerintermolecular distance and a smaller overlapping of p orbit-als between adjacent phenyl rings. The tilted structure showsa more stable nature than the perpendicular structure since ithas larger AE. In addition, the intensity of the �-� interac-tion between tetracene molecules is weaker than the M-Sinteraction since the former interaction only increases the AEwith a value of 0.1 eV compared to that in the p�4�4� case.However, in the multilayer regime where the influence fromthe substrate is dramatically decreased, the interaction be-tween molecules is dominant. Therefore, the tilted structureprovides a clue to understand this interaction.

IV. SUMMARY

This work gives a clear picture of the geometrical ar-rangement of tetracene adsorbed on Cu �100� surface in themonolayer regime. STM measurements show that tetracenemolecules are adsorbed on Cu �100� surface with the mo-lecular plane parallel to the substrate surface at monolayercoverage. The long axis of the molecule is aligned in twoidentical directions. DFT calculations show that the moleculewith the flat-lying mode has larger AE than that with theupright standing mode, indicating that the former is the morestable structure. With the flat-lying geometry, the carbon at-oms prefer to be placed between surface Cu atoms. The mo-lecular center prefers to be located at the bridge site betweentwo nearest Cu atoms of adjacent Cu rows in the �011� �or

�011̄�� direction. The conclusion about tetracene arrangementon Cu �100� surface in low coverage regime is different fromthat in the earlier UPS experiments. Tetracene does not forma long-range ordered structure at RT on Cu �100� surface.However, it can form a well-defined monolayer with the

molecules equally aligned in �011� and �011̄�, without begin-ning of multilayer growth before the completion of the first

layer. This work gives a clear idea that adsorbate-substrateinteraction plays a governing role in the monolayer filmgrowth of tetracene on Cu �100� surface.

ACKNOWLEDGMENTS

The work is supported by National Science Foundationof China with Grant. Nos. 60506019, 10674118, and10774129.

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