Adsorption and two-dimensional phases of a large polar molecule: Sub-phthalocyanine on Ag„111…
S. Berner,1,* M. de Wild,1 L. Ramoino,1 S. Ivan,2 A. Baratoff,1 H.-J. Guntherodt,1 H. Suzuki,3 D. Schlettwein,4
and T. A. Jung51National Center of Competence in Research on Nanoscale Science, Department of Physics and Astronomy, University of B
Klingelbergstrasse 82, CH-4056 Basel, Switzerland2Institute of Organic Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
3Kansai Advanced Research Center, C.R.L., 588-2 Iwaoka, Nishi-ku, Kobe 651-2492, Japan4Fachbereich Chemie, University of Oldenburg, 26129 Oldenburg, Germany
5Laboratory for Micro- and Nanostructures, Paul Scherrer Institut, CH-5232 Villigen, Switzerland~Received 12 March 2003; revised manuscript received 4 June 2003; published 11 September 2003!
The adsorption and the two-dimensional~2D! ordering of chloro@subphthalocyaninato#boron~III ! ~SubPc! onAg~111! has been studied in detail by combined scanning tunneling microscopy and photoelectron spectros-copy at room temperature. SubPc is a polar, highly symmetric molecule, consisting of an extended aromaticsystem and a central B-Cl bond. When growing on Ag~111! an interesting phase behavior is observed for thefirst molecular layer of SubPc. At low coverage, below'0.2 monolayer~ML !, a 2D lattice gas is present,whereas at medium coverage~on the order of 0.2–0.5 ML!, 2D condensed molecular islands are observed incoexistence with the 2D lattice gas. In these condensed islands, the molecules assemble into a well-orderedhoneycomb pattern. At higher coverage~approximately 0.5–0.9 ML! the molecules organize into a 2D hex-agonal close-packed~hcp! pattern, in equilibrium with a dense 2D gas phase. In the honeycomb and in the hcppattern, individual molecules are imaged with submolecular resolution, giving information on their orientation.For both the honeycomb and hcp patterns, islands with two different orientations of the superstructures withrespect to the Ag~111! substrate are observed. In case of the honeycomb pattern, the two superstructures areenantiomorphic. The chirality of these layers originates in the loss of the symmetry of the metal surface uponadsorption of SubPc, while the molecules alone are intrinsically achiral. Based on different photoelectronspectroscopy experiments we conclude that the SubPc molecule is adsorbed on Ag~111! with its Cl atomtowards the substrate and that the molecule remains intact. Finally, several aspects of the observed 2D con-densed phases and the thermodynamic phase behavior are discussed with respect to the charge distribution andthe adsorption physics and chemistry of the SubPc molecules.
The adsorption physics and chemistry of functionalganic molecules on metallic surfaces are of great intebecause of their electronic1,2 and optic properties. Fundamental efforts to explore and understand these propertieadsorbed molecules are also driven by the emergence oganic semiconductors in technological applications3,4 withactive volumes shrinking to the nanometer scale, where ctact properties can no longer be ignored. Intermolecularmolecule-surface interactions of adsorbates on metal surfaffect diffusion, island nucleation and growth, and therefinfluence the structure and properties of overlayers.5–10 Or-dered molecular layers can show chirality in the casechiral11 and achiral molecules.10,12,13 The latter are formedwhenever intermolecular interactions favor arrangemewhich break the symmetries of the underlying substrate.
Metal phthalocyanines are organic molecules whichbe used as functional dyes.14 Phthalocyanines were amonthe first organic molecules to be studied by scanning tuning microscopy15,16 ~STM! and were also investigated bmeans of photoelectron spectroscopy.17,18 In the presentwork, chloro@subphthalocyaninato#boron~III ! ~SubPc!, a po-lar molecule with an aromatic 14-p-electron system, habeen investigated in detail on Ag~111!. SubPc differs in sev-eral ways from the usual fourfold symmetric phthalocy
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nines: In the case of SubPc the central metal atom is replaby boron which is sp3 coordinated to an apex chlorine andthree instead of four isoindoline rings.19–23 In contrast to themostly planar phthalocyanines, the SubPc molecule is cshaped.
The adsorption and growth of SubPc on Ag~111! in thesubmonolayer to monolayer regime are investigated. Thesorption chemistry is analyzed by photoelectron spectrcopy, and the arrangement of the molecular layers is invtigated by STM at room temperature. The combinationthese techniques provides a detailed picture of the coverdependent molecular adsorption, growth, and twdimensional~2D! phase behavior.
II. EXPERIMENT
All experiments were performed in a multichambultrahigh-vacuum system, providing differentin situ prepa-ration and characterization methods. Ag~111! films were pre-pared by evaporation of Ag onto cleaved mica.24 Prior tomolecular deposition, the quality of the Ag~111! substrateswas checked by means of STM, x-ray photoelectron sptroscopy~XPS!, and low-energy electron diffraction~LEED!.From the observed LEED pattern it is apparent that the figrows in the~111! direction but exhibits domains with different rotational orientations. The prepared and character
S. BERNERet al. PHYSICAL REVIEW B 68, 115410 ~2003!
FIG. 1. SubPc molecule:~a! Side view of the calculated geometric structure. The scale bar is valid for~a!. ~b! Chemical Structure.~c!Effective atomic charges determined by a CHelpG population analysis.
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Ag~111! substrates were used for several experimentsrow. For this purpose molecular layers were removedcycles of argon ion etching and annealing toT5570 K inorder to regain atomically clean Ag~111! surfaces.
The organic molecules were deposited by sublimatfrom a resistively heated tantalum crucible. During depotion the substrate was kept at room temperature. A custbuilt quartz microbalance was used to measure the deption rates. Molecular evaporation rates ranging from 0.20.8 nm per minute were used. In various exposures, smonolayer coverages down to a few percent of a monolahave been prepared with high reproducibility. All SubPc coerages refer to the full monolayer of the hexagonal clopacked structure~Sec. III B 2!. The error in the thickness othe molecular layers is on the order of 10%–20%. By alyzing the molecular layers before and after annealing toK, no characteristic changes could be observed in the Simages as well as in photoelectron spectroscopy expments.
Figure 1 shows the chemical and geometric structurethe chloro@subphthalocyaninato#boron~III ! ~SubPc! adsorbateinvestigated here. According to x-ray diffraction data frothe bulk molecular crystal analyzed by Kietaibl, the bolength between the central B and axial Cl is 1.8 Å anddistance between the centers of the peripheral benzeneis 7.6 Å.20 The geometric and electronic structures of tSubPc molecule have been calculated with the semiemical AM1 method as well as withab initio density functionaltheory ~DFT!.22,25 We repeatedab initio DFT calculationswith the B3LYP exchange-correlation function and t6-31Gd basis set using theGAUSSIAN98 program package.26
The effective atomic charges displayed in Fig. 1~c! were de-termined from the computed electrostatic potential accordto the CHELPG option implemented inGAUSSIAN98. Thesecharges are least-squares fitted to reproduce the electropotential at a large number of points within a shell surrouing the molecule. In SubPc an excess of negative chargfound on the electronegative atoms which surroundelectron-deficient boron@Fig. 1~c!#. The negative charge icompensated by an electron deficit localized mainly atsix most central carbon atoms. As a consequence the Sis a polar molecule with negative charge at the Cl and p
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tive charge in the conjugated core. This results in a callated axial permanent dipole moment of 1.0 eÅ, whiagrees well with the experimental value of 1.1 eÅ.22
The molecular arrangements on the substrate were stuby means of a homebuilt STM, operating at room tempeture. All STM images were obtained in the constant-currmode by recording the vertical tip movement. Analysis of telectronic structure of the molecular layers were performby photoelectron spectroscopy in a VG ESCALAB MKinstrument. The XPS experiments were conducted withnonmonochromatized Mg/Al twin anode as an x-ray sourUltraviolet photoelectron spectroscopy~UPS! was performedusing a nonmonochromatized He gas discharge line souThe main line of the lamp is HeI a with a photon energy of21.2 eV. The electrons emitted from the sample are detein a hemispherical 150° analyzer with three channeltelectron counters.
III. RESULTS AND DISCUSSION
Using photoelectron spectroscopy the electronic strucof a sample can be analyzed. With XPS the binding energof the atomic core levels can be determined. Different checal environments of an adsorbate, or a species in genlead to slightly different binding energies of the atomic colevels. This energy difference in the binding energy is cala ‘‘chemical shift.’’27 UPS is used to obtain informatioabout the density of states~DOS! of the valence band andabout the DOS close to the Fermi energy. Consequently, Xand UPS reveal details of both the adsorbate and subselectronic structure and therefore of the molecular bondiThese experiments are complemented by series of STMages which provide a real-space map of topographicelectronic features on the atomic scale related to the adbate overlayers. Therefore, detailed insight into the geomric and electronic structures of the samples is achieved bycombination of photoelectron spectroscopy and STM, psented below.
A. Adsorption chemistry and electronic structureof SubPc on Ag„111…
XPS measurements of molecular layers with thicknranging from submonolayer to multilayer coverage ha
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been performed. Figure 2 shows the evolution of the Clpcore level peak for increasing film thickness. A large shift1.6 eV towards lower binding energy was observed forCl 2p peak when changing from a coverage of 13 monola~ML ! to submonolayer coverage. Due to the surface sens
FIG. 2. XPS spectra of the Cl 2p peak for increasing SubPcoverage measured with MgKa excitation. A chemical shift to-wards lower binding energy is observed for submonolayer coage.
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ity of XPS,28 the spectrum for 13 ML is dominated by bulklike SubPc features, whereas for submonolayer coverageXPS measurements are sensitive to the SubPc-Ag intetion. In contrast to the large shift of the Cl 2p peak, the C 1sand the N 1s core level peaks shifted only 0.2–0.3 eV twards lower binding energies for submonolayer coveraThus, the observed Cl 2p core level shift is interpreted as‘‘chemical shift’’27 due to the interaction of the Cl atom othe SubPc with the Ag substrate. The measured bindingergy of the Cl 2p core level for submonolayer coverage coresponds well to the binding energy obtained for ClAg~110! as measured by Briggs and co-workers.29 We there-fore conclude that the Cl in SubPc interacts with thesubstrate and the SubPc is adsorbed with the Cl towardssubstrate. Since the peak shifts to lower binding energy,chemical shift is attributed to a partial electron charge trafer from the Ag substrate to the Cl of the SubPc.
Additional information on the SubPc layers was gainedcomplementary UPS measurements. In Fig. 3 the evoluof the UPS spectra measured with He I excitation is psented for increasing coverage. Besides the highest occumolecular orbital~HOMO! located at a binding energy oaboutEB51.8 eV ~denoted as HOMO in Fig. 3! additionalmolecular orbitals with higher binding energies are visibEspecially the peak aroundEB59 eV ~labeled as MO4 inFig. 3! is easy to identify even for submonolayer coverasince no features of the Ag~111! valence band are located ithis energy range. Below 1 ML the features of the Ag~111!substrate contribute significantly to the UPS spectra. Ab1 ML coverage the Ag features vanish and the peaks assated with SubPc orbitals shift to higher binding energiHowever, the shape of the molecular features in these speremains almost unchanged. Because of the high sur
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FIG. 3. UPS spectra of the valence band of SubPc on Ag~111! for increasing SubPc coverage measured with He I excitation. For the cAg~111! substrate, the Ag 4d valence states dominate.~a! A shift of the spectra to higher binding energies is observed at higher cove~b! The same spectra as in~a! shifted to match the HOMO position for each film thickness. Apart from the disappearance of the Ag feno significant changes are observed with increasing film thickness. In particular, the energy difference between the HOMO and thequal for all layers. The binding energies are given with respect to the Fermi energyEF of the substrate.
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sensitivity of UPS,28 only SubPc molecules with bulk-likeinteraction are probed in the case of 13 ML. Different occpied molecular orbitals corresponding to bulk SubPc candistinguished in this spectrum. In Fig. 3~b! the shift of thedifferent spectra is compensated by matching the energysition of the HOMO for all coverages. With increasing filthickness no significant change in the electronic structureSubPc is observed. In particular, the energy differenDEHOMO-MO4 between the HOMO and the MO4 are equwithin their errors~Table I!. This indicates that the frontieorbitals of thep-electron system appear not to be affectedthe charge transfer which is observed in the XPS measments, as evidenced above. Therefore, the Ag-Cl chatransfer seems to be localized at the Cl, not affectingextended ring system of the SubPc molecule.
However, there is a shift of the binding energy of tmolecular orbitals to higher energies, which needs to beplained. In general, such a shift can occur either due tcharge transfer between the substrate and the adsorbedecules, due to screening effects of the photoemission ho30
or even due to charging of the molecular layer upon irradtion. In the following we discuss the possible influencesthese three mechanisms on our results.Charge transfer: thecharge transfer between a substrate and the adsorbate ispronounced for coverages up to one monolayer. Usuallyadditional shift is observed for different multilayer coveragdue to the localization of the charge transfer to the interfand because of the high surface sensitivity of the Umeasurements.28 Therefore, the shift we observed in the UPspectra by increasing the coverage from 2.3 ML to 13 Mcan hardly be explained by a charge transfer between thesubstrate and the SubPc layers.Screening effects: for a mo-
TABLE I. Binding energies (EB) of the HOMO and the MO4peaks for different SubPc coverage deduced from the UPS spand compared to numerical calculations. The experimentallyduced energy differenceDEHOMO-MO4 between the HOMO and theMO4 is constant, independent of the coverage. For the@SubPc#1 ashift of 3.36 eV to higher binding energy and a decrease of 0.36has been calculated for the HOMO andDEHOMO-MO4, respectively.In the UPS measurements neither such a shift to higher binenergy nor a change inDEHOMO-MO4 was observed for the layerwith submonolayer coverage compared to multilayers. This istrong evidence against dissociation of SubPc upon adsorptionbinding energies of the UPS measurements are related to the FenergyEF of the Ag~111!, whereas the binding energies for thSubPc and@SubPc#1 are calculated for the free molecule and thefore related to the vacuum energyEV .
EB HOMO@eV#
EB MO4@eV#
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0.4 ML SubPc 1.7860.03 8.9060.10 7.1260.101.2 ML SubPc 1.8360.02 8.9860.03 7.1460.042.3 ML SubPc 1.9360.02 9.0960.03 7.1660.0413 ML SubPc 2.1660.02 9.2760.03 7.1160.04
lecular layer in close proximity to a metal, a photoemissihole in the molecular layer is screened by the induced imcharge in the metallic substrate. Bulk SubPc is noncondtive, and therefore a photoemission hole created nearsurface of a multilayer is less effectively screened than a hcreated in a molecular monolayer on a metal substrate. Aconsequence, higher binding energies of the molecular oals are observed for a thick film compared to a thin film onmetallic substrate. Shifts due to screening of a photoemishole have, for instance, been described for C60 monolayerson metallic substrates.30,31Irradiation charging: in the case ofa nonconductive film, the emitted photoelectrons leadpositive charge at the sample surface. With increasingface charge densities, the electron binding energies asserved in UPS are shifted by the Coulomb potential. Thefore, the shift we observed in the UPS spectra of increasmultilayer coverage is attributed to a less effective screenof the photoemission hole at higher coverage and to chargof the molecular layer.
There is no evidence for decomposition of the SubPc mecules upon adsorption on the Ag~111! as only little changesare observed in the UPS spectra for different film thicnesses. Nevertheless, in a ‘‘Gedanken experiment’’ adstion of SubPc, accompanied by a splitting off of the chlorinmight be a possible process. Indeed, dissociative chemistion with a sticking coefficient close to unity occurs durinthe exposure of Cl2 to Ag substrates at room temperature29
Furthermore, ourab initio DFT calculations revealed that thSubPc molecule without the Cl is stable as a cation—i.e.a 11e charged ion~denoted in the following as@SubPc#1).The calculations revealed that an energy of 5.6 eV is neeto split the free SubPc molecule in Cl2 and @SubPc#1. In atheoretical study of the chlorine adsorption on the Ag~111!surface the adsorption energy is calculated to be on the oof 3 eV per Cl atom.32 Therefore, the calculations support aintact adsorption of the SubPc molecule on the Ag~111! sur-face. According to the calculations of the@SubPc#1, all mo-lecular orbitals shift to higher binding energies comparedSubPc; e.g., the shift of the HOMO is 3.36 eV. In the casethe @SubPc#1, we calculated a decrease in the energy diffenceDEHOMO-MO4 of 360 meV. In the UPS spectra neithesuch a dramatic shift of the spectra to higher binding enenor a decrease ofDEHOMO-MO4 was observed~Table I!. Inaddition to the theoretical argumentation from above, thisstrong experimental evidence against a dissociation of Suupon adsorption on Ag~111!.
The work function change of the sample upon SubPcsorption could also be inferred from UPS measurements.work function was measured by applying a negative voltato the sample in order to cover the whole width of the sptrum including the low-energy cutoff of the emitted photelectrons. The obtained values are listed in Table II showa decrease of the work function. A linear extrapolation of tvalues for 0.4 ML and 0.5 ML leads to a decrease inwork function ofDf'21 eV for 1 ML. Such a reduction ofthe sample work function is expected for the adsorptionSubPc with the Cl atom towards the Ag substrate. In tconfiguration the permanent dipole moment of the SubPperpendicular to the Ag surface, pointing away fromTherefore, the surface dipole of the metal is reduced wh
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ADSORPTION AND TWO-DIMENSIONAL PHASES OF A . . . PHYSICAL REVIEW B68, 115410 ~2003!
leads to a reduction of the work function.33 According to thesimplest model where the dipoles of the adsorbed molecare treated as a uniform dipole layer, the change of the wfunction is given by34
Df5epNA /e0 . ~1!
In this equationp is the molecular dipole moment,NA thenumber of dipoles per surface area, ande0 the permittivity offree space. Using the SubPc dipole moment of 1.0 eÅ,~1! leads to a decrease ofDf520.58 eV for 1 ML. Screen-ing by surface charges induced in the metallic substratebe adequately represented by an image dipole, providedthe charge distribution which produces the original diplittle overlaps with the electron density of the substrate. Asuming this to be the case, the image dipole has the sorientation, because the SubPc dipole is perpendicular tosurface. Therefore, the total effective dipole momentmolecule is doubled,33 leading to a change in the work function of Df52230.58 eV521.16 eV. This value com-pares with the measured reduction of the work functiDf'21 eV. For a more accurate treatment of the callated work function change, one has to take into accountan opposite electric field is generated at the site of anyticular dipole by all the surrounding parallel dipoles.34 How-ever, the deviation between the experimental and the calated effective dipole moment and change in work functcould also be affected by the partial charge transfer fromAg substrate to the Cl atom of the SubPc molecule asserved in the XPS measurements. This charge transfer wlead to an induced dipole moment pointing against the sface which would also reduce the effective dipole momenthe adsorbed molecule. In view of the reasonable agreemachieved with the simple equation~1!, our work functionmeasurements give additional evidence that the SubPc issorbed intact with the Cl towards the Ag~111! surface asillustrated in Fig. 4.
B. Two-dimensional superstructures and phase behaviorof SubPc on Ag„111…
While photoelectron spectroscopy experiments providedetailed picture of the average chemical bonding ofSubPc molecules in a close-to-complete monolayerAg~111!, by STM individual molecules in their 2D arrangement can be addressed and analyzed. In time-lapsed imasequences slow dynamical processes~msec to min! are alsoaccessible. In our STM images of the SubPc layers differoverlayer patterns and phases can be distinguished.
TABLE II. Measured work functions of SubPc layers oAg~111!. A decrease of the work function is observed upon Subadsorption.
f @eV# Df @eV#
Ag~111! 4.6060.020.4 ML SubPc 4.2260.02 20.3860.030.5 ML SubPc 4.1360.02 20.4760.03
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1. Honeycomb pattern: A chiral structure madeof achiral molecules
STM measurements on Ag~111! with a SubPc coverage oapproximately 0.2–0.5 ML show substrate terraces particovered by 2D ordered islands. Within these islands the mecules are assembled in a 2D overlayer with a hexagohoneycomb pattern~Fig. 5!. Clearly, the periodic vacanciewhich give rise to the low surface packing density of theislands can be observed in these images. The moleculeimaged as protrusions with a height of'4.5 Å for positivesample bias voltages around 1 V. This apparent heightflects the increased tunneling current due to the presencmolecular levels which are significantly coupled to electrostates of the substrate and the STM tip. In high-resolutSTM images individual SubPc molecules are resolved asangular structures with trefoil shape. In Fig. 5~a! the sche-matic structures of six SubPc molecules are outlined witthe STM image. The intermolecular distance in the honcomb pattern measured from the STM images is 161.0 Å. This leads to a packing density o0.24 molecules/nm2 for the 2D honeycomb pattern.
In the STM experiments domains or islands with two dferent orientations of the honeycomb pattern with respecthe Ag~111! substrate have been observed. The angletween these two orientations is 9°61°. This experimentalfinding—that only two different orientations of the honecomb pattern exist—implies that besides the intermolecuinteraction also molecule-substrate interactions are crufor establishing the particular registries of the overlayer.the case of a dominant intermolecular interaction and a nligible molecule-substrate interaction, many more differedomain orientations of the overlayer pattern should habeen found.
A close inspection of the high-resolution images revethe orientation of individual SubPc molecules. Each mecule is rotated with respect to the principal axis of the heycomb pattern. The angle between the phenyl ring andline joining the center of a molecule with the center of eahoneycomb is about 23°65°. In the honeycomb pattern oFig. 5~a! each SubPc molecule is rotated counterclockwiwhile in the pattern of Fig. 5~b! the molecules are rotateclockwise. As a result, the two different orientations of t
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FIG. 4. Schematic sketch of the inferred adsorption geometrSubPc on Ag~111! at monolayer coverage. Each SubPc is adsorwith the Cl atom towards the Ag surface.
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honeycomb structure have different chirality, althoughSubPc molecules and the Ag~111! surface alone are intrinsically achiral. The 2D self-organized domains or islands henantiomorphic character.
FIG. 5. STM images of the 2D ‘‘honeycomb’’ overlayer oSubPc on Ag~111!. ~a! Scan range 14316 nm, I 510 pA, U50.7 V. Single molecules are observed with submolecular restion. The internal structure of the SubPc molecules is outlined atbottom right~drawn to scale!. The dark region in the center of eachoneycomb represents the underlying silver substrate. The phrings of the molecules point to the left side of the center ofhoneycomb, as indicated by the arrow.~b! Scan range 56356 nm2, I 510 pA, U51.2 V. The observed honeycomb ordeing exhibits a very high perfection. The inset shows a magnifiimage of one ‘‘honeycomb.’’ For this orientation of the honeycompattern, the phenyl rings of the molecules point to the right sidethe center of the honeycomb. Therefore, the two honeycomb stures from~a! and ~b! are enantiomorphic.
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In order to recognize the interactions responsible forobserved structures, it is instructive to consider possmodels of the overlayer structure. However, it was not psible to simultaneously image the atomic structure ofsubstrate together with the molecular overlayer which woallow us to determine directly the exact adsorption regisNevertheless, the experimental data available togethersymmetry arguments allow us to propose a feasible moAll molecules in the honeycomb pattern show the same ctrast and the same submolecular structure for both orietions. It is therefore reasonable to assume that all molecare located in equivalent adsorption positions. The supstructure of SubPc on Ag~111! which best fits the experimental data is depicted in Fig. 6. It has aA1113A111R64.7° ~2SubPc! unit cell and the center of each molecule is locatabove a silver atom~on top site! and the phenyl rings approximately above threefold hollow sites. In this model, eaSubPc molecule is rotated by 23.8° with respect to the lijoining their centers with the center of each honeycomb. Ttwo orientations of the pattern are rotated with respect tohigh-symmetry@110# direction of the Ag~111! substrate by64.7°. This proposed model agrees very well with the eperimental data: The intermolecular distance is 17.7 Å athe angle between the two domains is 9.4°, compared17.961.0 Å and 9°61° in the STM images, respectivelyFurthermore, the rotation of each molecule with respecthe honeycomb lattice corresponds to the experimentallyserved orientation.
The intermolecular distance of 17.961.0 Å in the case ofSubPc on Ag~111! is larger than the intermolecular distancof 13 Å measured for the square lattice of SubPc on Cu~100!~Ref. 35! and the distance of 12 Å for SubPc on Au~111!~Ref. 36!. This larger intermolecular distance on Ag~111! to-gether with geometrical arguments excludes the fact thatrotation of the SubPc molecules with respect to the honcomb pattern is caused by steric repulsion. On the othand, through the rotation of the molecules in the Subhoneycomb pattern the distance between the phenyl ringtwo adjacent SubPc molecules is enlarged. This is mlikely caused by the repulsive electrostatic interactiontween the phenyl rings, since the H atoms carry positpartial charges, whereas the outer N atoms of the phthalonine macrocycle carry negative partial charges@Fig. 1~c!#.Thus, judging from the charge distribution of the SubPc mecule a rotation of the molecules with respect to the axethe honeycomb pattern is energetically favored becausthe existence of electrostatic intermolecular interactions.
2. Hexagonal close-packed pattern
If the coverage is increased to 0.5–0.9 ML, the Submolecules self-organize in a 2D hcp pattern~Fig. 7!. Theintermolecular distance measured from STM images is 161.0 Å and therefore slightly larger than the one of the hoeycomb pattern. This leads to a packing density0.32 molecules/nm2 for the hcp pattern which is 35% highecompared to the honeycomb pattern. Individual molecuare imaged with submolecular resolution as to the honcomb pattern. Furthermore, for the hcp pattern two orien
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ADSORPTION AND TWO-DIMENSIONAL PHASES OF A . . . PHYSICAL REVIEW B68, 115410 ~2003!
tions with respect to the Ag~111! substrate were observed awell. The angle between these two orientations is 15°61°.
The difference between the honeycomb and the hcptern is more profound than just a missing molecule in e‘‘honeycomb.’’ In contrast to the honeycomb pattern, tSubPc molecules in the hcp pattern are aligned in rowhere each molecule is pointing with its phenyl rings toouter N atom of the neighboring phthalocyanine. Therefo
FIG. 6. Proposed model for the honeycomb pattern. The bconsisting of two SubPc and the corresponding Bravais vectorsdrawn into the model for both enantiomorphic orientations. Tarrow drawn to the molecule at the bottom right indicates theferent chirality.
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FIG. 7. STM images of the 2D hcp overlayer of SubPcAg~111!. ~a! Scan range 86382 nm2, I 520 pA, U51.0 V. A largenumber of defects is observed in the hcp layer.~b! Zoom imagetaken with a scan range of 20318 nm2, I 510 pA, U51.0 V. In-dividual SubPc molecules are imaged with submolecular resolutThe internal structure and the orientation of SubPc is outlinedthe right-hand side. Two characteristically different defects canidentified. Two vacancy defects are visible; one has been markea white circle. The lower apparent height of some moleculesattributed to a local change of their electronic structure, ascussed in the text.~c! Cross section between the two arrowsimage ~b!. The difference in the apparent height for the ‘‘lowmolecules is clearly visible.
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S. BERNERet al. PHYSICAL REVIEW B 68, 115410 ~2003!
in the hcp pattern the distance between the phenyl ringadjacent molecules is increased and the phenyl ringspointing to the electron-rich outer N of the phthalocyanineneighboring molecules. This configuration is expected toenergetically favored due to the charge distribution ofSubPc molecules@Fig. 1~c!# and the resulting electrostatiinteraction between adjacent molecules.
Overview [email protected]., Fig. 7~a!# show a surprisinglylarge number of defects in the hcp layer. However, higresolution STM images@Fig. 7~b!# revealed that these defecare not vacancies, but rather molecules of different appaheight. These molecules have the same shape as themolecules, but appear in STM at approximately half thheight. Therefore, these entities are denoted in the followas ‘‘low’’ molecules. It is expected that these low moleculare different, in the sense that they represent places of lolocal density of states~LDOS! within the 2D layer. Conse-quently, constant-current STM is imaging these moleculea lower apparent height.37,38 The difference in the electronistructure is attributed to a slightly different bonding of tmolecule to the Ag substrate. Alternative explanations sas STM image artifacts or chemical modifications in conquence of the sublimation process are considered unlifor the following reasons. The comparison of the apparheight of the SubPc molecules for the hcp and honeycopatterns excludes the possibility of a double layer in theages of Fig. 7. The absence of low molecules in the honcomb pattern gives evidence against a decomposition ofmolecules. Furthermore, the amount of low molecules inhcp pattern is in the order of 5%–12%, whereas the puritythe molecules is.99%. From our experimental data no drect evidence for a reconstruction or local modification ofmetal surface was observed upon SubPc adsorption. Sinclow molecules are observed in the honeycomb pattern,also not likely that the low molecules in the hcp patternlocated on Ag-vacancy sites which would result in topgraphically lowered molecules. On the other hand, the tographic height difference between low and ‘‘normal’’ Submolecules is in the order of a Ag~111! monoatomic stepheight. Thus changes in the adsorption mechanism due tohigher molecular coverage cannot be ruled out completTo analyze the origin of the different electronic structuresthe low molecules, it is worth noting that these moleculesslightly rotated with respect to the molecular rows of the hpattern. This rotation of the low SubPc molecules alongB-Cl axis leads to a slightly different adsorption geometwhich might account for the lower apparent height. Hoever, to gain more insight into this interesting phenomenmore detailed investigations beyond the scope of this paare needed. In the case of C60 on various metals, differenapparent heights for the C60 have been observed.39–42 Thisdifference in the apparent height might be due to substreconstructions39 or due to electronic differences causedsurface interactions, where a mixture of subtle differencechemical bonding and adsorption geometry takes place.40–43
On the basis of the assumption that all molecules witthe layer are located at equivalent adsorption sites, a mfor the hcp superstructure is proposed. The superstructuSubPc on Ag~111! which fits best to the experimental data
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a A433A43R67.6° overlayer as shown in Fig. 8. The tworientations of the pattern are67.6° rotated with respect tothe @110# direction of the Ag~111! substrate. This model isin good agreement with the experimental data: The intermlecular distance is 19.1 Å and the angle between thedomains is 15.2°, while a distance of 18.961.0 Å and anangle of 15°61° have been measured in the STM imagesis interesting to note that both proposed models for the heycomb and hcp patterns are consistent because in bothels each molecule is located at exactly the same adsorpsite: The Cl on an on-top site and the phenyl rings on hollsites.
FIG. 8. Proposed model of the hcp pattern. The Bravais vecfor the two possible orientations~a! and ~b! are indicated.
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ADSORPTION AND TWO-DIMENSIONAL PHASES OF A . . . PHYSICAL REVIEW B68, 115410 ~2003!
A single SubPc molecule has threefold symmetry alothe B-Cl bond, whereas the Ag~111! surface has sixfold symmetry. This allows the nucleation of the hcp pattern in foinequivalent domains. For each of the two experimentaobserved orientations of the pattern, two orientations difing by a rotation of the molecules of 60° or equivalen180° are possible. These different orientations have beenserved experimentally.
Next to the ordered 2D islands, characteristic ‘‘streakpatterns were observed in our STM data~Fig. 9!. From adetailed analysis of individual scan lines, these streaks hbeen identified as mobile molecules which form a 2D ‘‘latice gas.’’44 The molecules in this gas phase are stablysorbed for a certain time but tend to hop to nearby adsorp
FIG. 9. ~a! STM image of SubPc on Ag~111! with a molecularcoverage of 0.3 ML ~scan range 51341 nm2, I 512 pA, U50.85 V). On the left-hand side of the image a condensed islwith a honeycomb pattern~c! is present, whereas on the same trace next to the condensed island a noisy streak pattern~g! is vis-ible. This noisy pattern is due to mobile molecules which form alattice gas. Bunched step edges of the Ag~111! substrate~s! crossthe image at the left bottom corner and in the top right. These sare decorated by SubPc molecules which form an irregular patThe white line represents the location of the scan line shown in~b!.~b! Height profile in the fast scanning direction (x direction!. Singlemolecules are clearly visible and exhibit a characteristic crosstion which is similar for molecules in the condensed island awithin the regions denoted by the noisy pattern.
11541
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sites. The 2D gas phase is observed on all terraces andtends over the whole terrace whether there are nucleatelands or not. Molecular diffusion across step edges is signcantly reduced or even completely suppressed as observextended time lapse image series. Below a coverage ofML the SubPc molecules form a 2D gas phase which coists with ordered 2D islands assembling in a honeycombhcp pattern above 0.2 ML or 0.5 ML, respectively. The dtails of this system, which can be described as a 2D therdynamic equilibrium where molecules are exchangedtween the 2D lattice-gas and 2D ‘‘solid’’ phases have bepublished earlier.44
In areas with monoatomic substrate steps bunchedgether, most of the SubPc molecules are stably adsorbedremain at their adsorption sites. This leads to the irregulaof the adsorption pattern of the molecules at the step ed~Fig. 9!. In addition, these molecules have a slightly differeshape in STM images. This may indicate that the chemicaphysical bonding of the SubPc at these adsorption sitedifferent. This is potentially due to the lower coordinatioand higher chemical reactivity of the step edges, but coalso be a consequence of electronic properties of theedges.45
Compared to lower coverage, the hcp pattern is obserin coexistence with mobile molecules forming a 2D gphase which appears to have a higher ‘‘streak density’’ ththe one coexisting with the honeycomb phase. In generalincreasing molecular coverage, not only the condensedlands increased in size but a higher density of the molec2D gas was observed. Therefore, a higher molecular coage leads to a higher 2D ‘‘vapor pressure’’ of the Submolecules and thus to an increase of the condensed arevery similar behavior was recently observed and analyzethe case of coadsorption of N and O on Ru~0001!.46
4. Two-dimensional phase behavior
The different 2D lattice patterns which have been dcussed above can be parametrized by the coverage of Smolecules: For coverage below'0.2 ML only mobile mol-ecules forming a 2D lattice gas are observed. A coexisteof the honeycomb pattern with a 2D lattice gas is obserfor coverage on the order of 0.2–0.5 ML, whereas for highcoverage on the order of 0.5–0.9 ML the hcp pattern isserved in coexistence with a dense 2D lattice gas. The heycomb pattern together with the hcp pattern was neverserved on the same terrace. For either pattern, honeycomhcp, no changes in the pattern were observed upon anneto 360 K; nor have new patterns appeared. Consequentlycan identify these patterns as two distinctly different 2solid phases, characterized by their different 2D densitwhich are observed in thermodynamic equilibrium with tneighboring lattice gas phase. The molecular density of2D gas phase is observed to increase with increasing coage.
It remains an interesting question to which extent theserved 2D solid and gas phases are thermally excited—to which extent molecular rotations and vibrations taplace. In the case of phosphangulene, a similar cone-shmolecule like SubPc, some isolated molecules are stan
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S. BERNERet al. PHYSICAL REVIEW B 68, 115410 ~2003!
upright and are ‘‘balancing’’ on the Ag~111! surface at a tem-perature of 6 K.47 In the condensed SubPc phases the oritation of individual molecules is visible and molecular rottion is therefore frozen. However, in the SubPc gas pharotation of the molecules cannot be ruled out.48 In addition, itmight be that only the SubPc molecules embedded in a laare standing upright, whereas isolated molecules on theface are possibly tilted or wobbling with the B-Cl axisdifferent sides. Internal degrees of freedom of individuSubPc molecules in the gas phase might be hinderedincreasing molecular coverage. One could also specuabout correlated motion of nearest-neighbor molecules fcoverage close to one monolayer.
The phase diagrams of 2D lattice gas models have bstudied in detail by Monte Carlo simulations for differetypes of pair and triple interactions.49–52 In models with ei-ther attractive or repulsive nearest-neighbor interactioonly one ordered phase is observed for either case.49,50 Or-dered phases with distinctly different symmetries areserved for nearest-neighbor repulsion in conjunction wnext-nearest-neighbor attraction and three-body interactio52
In the case of SubPc it is expected that a combinationattractive and repulsive interactions leads to the obsernontrivial phase behavior. In addition to the attractive vder Waals interaction, a repulsive interaction due to the pallel dipole moments of adsorbed molecules is present.thermore, there is a delicate interplay of repulsive or attrtive interactions depending on the relative angle of tneighboring molecules originating in the inhomogeneocharge distribution. In other words, the interaction betwetwo molecules depends on their relative angle, since thatoms of the phenyl rings are charged positive, whereasouter N atoms are charged negative@Fig. 1~c!#. Conse-quently, the phenyl rings repel each other andelectrostatic-favored configuration is obtained if the pherings point to the N of the neighboring phthalocyanine mrocycle. This tendency is observed for the honeycombwell as for the hcp pattern.
IV. CONCLUSIONS
SubPc, a large polar molecule with an aromap-electron system, shows a versatile phase behaviorincreasing coverage of the first molecular layer grownAg~111!. At room temperature, two different rather opeself-organized structures, honeycomb and hcp, are obsein coexistence with a 2D lattice gas.
From XPS and UPS measurements in conjunction wcalculations of the SubPc molecule it is concluded thatSubPc adsorbs intact with the Cl atom towards the Ag~111!substrate. The molecular orbitals of the SubPc molecule
*Electronic address: [email protected]. Joachim, J. Gimzewski, and A. Aviram, Nature~London! 408,
541 ~2000!.2R. Carroll and C. Gorman, Angew. Chem., Int. Ed. Engl.41, 4378
~2002!.3J. Burroughes, D. Bradley, A. Brown, R. Marks, M. Mackay,
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only slightly influenced by adsorption on Ag~111!, allowingthe conclusion that the SubPc is essentially physisorbed.observed electron transfer from the substrate to the Cl oleads to a small overall shift of the electronic structure ofmolecule. This is compatible with the relatively high mobity which enables self-organization and thermodynamequilibration of different phases at room temperature.analogy to ‘‘self-assembled monolayers’’~SAM’s!,53 themolecules spontaneously orient themselves upon adsorpand interaction with the substrate. However, compared tostrong chemisorption of thiol derivatives in SAM, the Ag-Cbond of SubPc on Ag~111! is weak. Nevertheless, the Ag-Cbond seems to be crucial for the orientation of the Submolecule on the Ag~111! substrate since it is reported thupon adsorption on Au~111! the molecules are attached wittheir isoindolyl groups to the surface.36 This characteristicdifference most probably originates from the different eletronic structures and the resulting different binding forcesthe two substrates. Furthermore, the different adsorptionometry could account for the different overlayer structuresSubPc on these substrates.
A very striking feature of the ordered patterns describhere is that the molecules are well ordered although the cest atoms of nearest neighbors are separated by aboutlattice spacings of the substrate. This excludes any dichemical interactions, even weak hydrogen bonds like threcently invoked to explain ordered patterns of molecuwith a p-conjugated core adsorbed flat on Ag~111! ~Ref. 54!or Au~111! ~Refs. 9, 54 and 55!. Nevertheless, the arrangement of the molecules in the honeycomb and the hcp patcan be related to the charge distribution of the individuSubPc molecule and the resulting electrostatic interactThe balance between intermolecular interactions, molecsubstrate interactions, and thermal energy governs the dsion of single molecules, island diffusion, nucleation, aself-organization in general. All these effects have beenserved by analyzing the behavior of SubPc on Ag~111!. Thisis a unique possibility to study the delicate interplay of itermolecular and molecule-substrate interactions at work
ACKNOWLEDGMENTS
We thank H. Kliesch and D. Wo¨hrle ~University of Bre-men, Germany! for synthesizing and providing the SubPmolecules. Helpful discussions with A. Alkauskas,Schintke, M. von Arx~University of Basel, Switzerland!, andH. Yanagi ~Kobe University, Japan! are gratefully acknowl-edged. Financial support from the Swiss National ScieFoundation, the Swiss Top Nano 21 program, the NCCRNanoscale Science, and the University of Basel is gratefacknowledged.
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