In Situ Scanning Tunneling Microscopy of MolecularAssemblies of Cobalt(II)- and Copper(II)-Coordinated

Tetraphenyl Porphine and Phthalocyanine on Au(100)

Soichiro Yoshimoto,† Akinori Tada,† Koji Suto,† Shueh-Lin Yau,‡ andKingo Itaya*,†,‡

Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,Aoba-yama 04, Sendai 980-8579, Japan, and Core Research Evolutional Science and

Technology organized by Japan Science and Technology Agency (CREST-JST), KawaguchiCenter Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

Received October 20, 2003. In Final Form: January 16, 2004

Molecules of copper(II) and cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphine (CuTPP and CoTPP)and cobalt(II) phthalocyanine (CoPc) are spontaneously adsorbed onto reconstructed Au(100) substratefrom a benzene solution containing each individual complex. In situ scanning tunneling microscopy (STM)was used to examine the real-space arrangement and the internal molecular structure of each of theindividual molecules in 0.1 M HClO4 under potential control. The adsorption of CuTPP and CoTPP producedthe same highly ordered square array with an intermolecular spacing of 1.44 nm on a reconstructedAu(100) surface. These molecular superlattices and the underlying reconstructed Au(100) predominatedbetween 0 and 0.9 V, but lifting of the reconstructed Au(100) surface and elimination of the orderedadlayers occurred at more positive potentials. Molecular resolution STM revealed propeller-shapedadmolecule with its center imaged as a protrusion for Co(II) and a depression for Cu(II). In contrast, thespontaneous adsorption of CoPc molecules resulted in a rapid phase transition from the reconstructedAu(100) surface to the (1 × 1) phase, coupled with the production of locally ordered, square-shaped arrayswith an intermolecular distance of 1.65 nm. This molecular adlayer and the Au(100)-(1 × 1) remainedunchanged when the potential was modulated between 0 and 1.0 V. These results indicate that the subtlevariation in the molecular structure of adsorbate influenced not only its spatial arrangement but also thestructure of the underlying Au(100) substrate.

Introduction

It is now recognized that the precise control of molecularassembly on a substrate is one of the key foundations toharness nanomolecular devices. Consequently, there hasbeen an upsurge of interest recently to prepare andcharacterize the molecular architecture on an atomicallyflat substrate such as gold and highly oriented pyrolyticgraphite (HOPG). There are many reports describing thepreparation of ordered molecular assemblies from vaporphase or solution phase. For example, porphyrins andphthalocyanines have found applications in such diversi-fied fields as biology,1 photosynthesis,1 electrocatalysis,2,3

and molecular devices.4 More specifically, copper(II)phthalocyanine (CuPc) is involved in research on light-emitting diodes (LED)5 and field effect transistors (FET).4,5

In addition, the use of metallophthalocyanines (MPcs) forcatalyzing the electroreduction of dioxygen representsanother well-known research subject needed for thedevelopment of an efficient cathode for fuel cells.2,3,6-10

Intensive effort in the past decade has made sub-stantial progress in the research on molecular assemblieson the surfaces of Au,11-21 Ag,11,22 and Cu11,23,24 in avacuum (UHV) and solution phases. The adsorption of5,10,15,20-tetrakis(3,5-di-tertiarybutylphenyl)porphine cop-per(II) (CuTBPP) on Cu(100), Au(110), and Ag(110)surfaces has been examined to elucidate the role ofsubstrate.11,12 The adsorption of 5,10,15,20-tetrakis(3,5-

* To whom correspondence should be addressed. Phone/Fax:+81-22-214-5380. E-mail: itaya@atom.che.tohoku.ac.jp.

† Tohoku University.‡ Core Research Evolutional Science and Technology.(1) Electron Transfer in Chemistry; Balzani, V., Ed.; WILEY-VCH:

New York, 2001; Vol. 3.(2) Yeager, E. Electrochim. Acta 1984, 29, 1527.(3) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem.,

Int. Ed. Engl. 1994, 33, 1537 and references therein.(4) Molecular Electronics; Jortner, J., Ratner, M., Eds.; IUPAC:

Oxford, 1997.(5) Guillaud, G.; Simon, J.; Germain, J. P. Coord. Chem. Rev. 1998,

178-180, 1433.(6) Jasinski, R. J. Electrochem. Soc. 1965, 112, 526.(7) Alt, H.; Binder, H.; Sandstede, G. J. Catal. 1973, 28, 8.

(8) Savy, M.; Andro, P.; Bernard, C.; Magner, G. Electrochim. Acta1973, 18, 191.

(9) Mho, S.-i.; Ortiz, B.; Park, S.-M.; Ingersoll, D.; Doddapaneni, N.J. Electrochem. Soc. 1995, 142, 1436.

(10) Cardenas-Jiron, G. I.; Gulppi, M. A.; Caro, C. A.; del Rıo, R.;Paez, M.; Zagal, J. H. Electrochim. Acta 2001, 46, 3227 and referencestherein.

(11) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K.; Tang, H.; Joachim,C. Science 1996, 271, 181.

(12) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K. Nature 1997, 386,696.

(13) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Mashiko, S. J. Chem.Phys. 2001, 115, 3814.

(14) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko,S. Nature 2001, 413, 619.

(15) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000,104, 11899.

(16) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am.Chem. Soc. 2001, 123, 4073.

(17) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc.1996, 118, 7197.

(18) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem.1996, 100, 11207.

(19) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391.(20) Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 5993.(21) Chizhov, I.; Scoles, G.; Kahn, A. Langmuir 2000, 16, 4358.(22) Lackinger, M.; Hietschold, M. Surf. Sci. 2002, 520, L619.(23) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Woll, C.; Chiang, S.

Phys. Rev. Lett. 1989, 62, 171.(24) Yanagi, H.; Mukai, H.; Ikuta, K.; Shibutani, T.; Kamikado, T.;

Yokoyama, S.; Mashiko, S. Nano Lett. 2002, 2, 601.

3159Langmuir 2004, 20, 3159-3165

10.1021/la0359474 CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 03/09/2004

di-tertiarybutylphenyl)porphine platinum (PtTBPP) ona Cu(100) substrate is reported to shed insight into themechanism of molecular organization.24 Yokoyama et al.found that CN-substituted TBPP molecule is adsorbed onAu(111) to produce one-dimensional supermolecular as-semblies such as monomers, trimers, tetramers, andextended wire-like structures.14 Hipps and co-workersinvestigated various metal-coordinated tetraphenyl-21H,-23H-porphine (MTPP) such as NiTPP, CuTPP, and CoTPPon the reconstructed Au(111) surface.15,16 Among thesemacrocyclic molecules, only the Co(II) metal centerexhibited a pronounced protrusion in the STM image. Thisdifference is presumably due to the presence of a half-filled dz2 orbital at the Co(II) site, which acts as theelectron-tunneling mediator between the tip and the goldsubstrate. Furthermore, molecular resolution STM hasrevealed the internal molecular structure of MPc adlayers,allowing detailed characterization of their adsorptionconfigurations.17-24 Furthermore, STM has been exten-sively used to probe the molecular assembly of variousmetal-coordinated Pc’s such as CuPc,17-19 CoPc,17,18 NiPc,19

FePc,19 and VOPc20 on Au(111). The metal centers appearto be unimportant in the organization of the admoleculesas they are arranged similarly on the Au(111) substrate.These metal centers yield uneven corrugations, presum-ably due to the difference in their electronic structures.15-20

The above highlighted studies were conducted in avacuum environment, showing that the vapor-phasedeposition is useful to produce ordered overlayers ofmacrocyclic molecules.10-23 On the other hand, solution-phase dosing can be an inexpensive and convenientalternative for preparing highly ordered molecular ad-lattices on gold and HOPG substrates. An aqueous dosingsolution works well if the admolecule is water soluble.However, it is obviously difficult to use for most organicmolecules in aqueous solutions. To continue to develop atechnique for producing molecular assemblies from asolution phase, we explored the use of organic solventssuch as benzene and ethanol as the medium for thedeposition of organic compounds. Preliminary results havealready been reported.25-27 For example, water-insolubletetraphenyl-21H,23H-porphine cobalt(II) (CoTPP) andcopper(II) (CuTPP) formed highly ordered adlayers onAu(111) from their benzene solutions, as indicated bymolecular resolution STM.25,27 The as-prepared adlayersof CoTPP and CuTPP possess the same structure as thosefound in UHV.15,16 On the other hand, it was noted thatCoPc and CuPc molecules adapt different spatial struc-tures on reconstructed Au(111). To further explore therole of substrate in the formation of molecular assemblies,we examined CoTPP, CuTPP, and CoPc on the Au(100)surface. Chart 1 illustrates their molecular structures.Meanwhile, the reconstruction of Au(100) surface, apopular subject in the study of electrified interfaces, isalso of our present interest.28-30

Experimental Section

Compounds of 5,10,15,20-tetraphenyl-21H,23H-porphine co-balt(II) (CoTPP) and copper(II) (CuTPP) and cobalt(II) phtha-

locyanine (CoPc) were purchased from Aldrich and used withoutfurther purification. Benzene was obtained from Kanto ChemicalCo. (spectroscopy grade). The supporting electrolyte for volta-mmetric and STM measurements was 0.1 M HClO4 prepared bymixing ultrapure perchloric acid obtained from (Cica-Merck) andMilli-Q water (resistivity g 18.2 MΩ cm).

The Au(100) single-crystal electrode was prepared by theClavilier method, and the size of the crystal was roughly 2 mmin diameter.31 As reported previously, the surface structure ofAu(100) is critically dependent on details in the preparationprocess,andweemployedtheprocedureweestablishedpreviouslyto make a reconstructed or ideal (1 × 1) surface.28-30 Specifically,the reconstructed Au(100) surface was prepared by annealing ina hydrogen flame for several seconds, followed by cooling in airfor 3-5 min.28 This Au(100) electrode was immersed in a benzenedosing solution containing a saturated amount of CoPc or 10 µMCoTPP or CuTPP. (The former was only slightly soluble inbenzene, and its concentration was lower than 10 µM.) In situSTM imaging (vide infra) showed that an immersion time of 5min or longer enabled the formation of a full monolayer of CoPc.In contrast, CoTPP and CuTPP readily dissolved in benzene,and a dipping time as short as 10 s was sufficient to produce afull monolayer of these molecules on Au(100).25 The Au(100)electrode coated with one of the molecules of CoTPP, CuTPP,and CoPc was then rinsed with ultrapure water before it wastransferred into the electrochemical or STM cell to performsubsequent experiments. The elapsed time was about 10 minbefore the STM imaging was actually started.

Cyclic voltammetry was carried out at 20 °C with a potentiostat(HOKUTO HAB-151, Tokyo). The Au(100) electrode configuredby the hanging meniscus method was placed in a three-compartment electrochemical cell blanketed with N2. The refer-ence and counter electrodes were a reversible hydrogen electrode(RHE) and Pt wire, respectively. The electrochemical STMmeasurement was performed with a Nanoscope E (DigitalInstruments, Santa Barbara, CA). The tip was made of a tungstenwire etched in 1 M KOH, and a thin layer of nail polish wasapplied to minimize residual faradic currents. All STM imageswere recorded in the constant-current mode. A Pt wire was usedas a quasi-reference electrode in the STM experiments, but allpotentials reported are referred to the reversible hydrogenelectrode (RHE).

Results and Discussion

Voltammetry. Parts a, b, and c of Figure 1 show cyclicvoltammograms (CVs) obtained with an Au(100) electrodemodified with CoTPP, CuTPP, and CoPc in 0.1 M HClO4at a scan rate of 50 mV s-1, respectively. The dotted tracesin Figure 1 represent the CV for a bare Au(100). It ismostly featureless between 0 and 1.0 V, except for theanodic peak at 0.85 V. This feature stems from thestructural transition from the reconstructed to the (1 ×1) phase, but the process is so slow that the CV appearsto be irreversible at the scan rate used. These CVs areidentical to those reported for a well-ordered Au(100)

(25) Yoshimoto, S.; Tada, A.; Suto, K.; Narita, R.; Itaya, K. Langmuir2003, 19, 672.

(26) Yoshimoto, S.; Narita, R.; Wakisaka, M.; Itaya, K. J. Electroanal.Chem. 2002, 532, 331.

(27) Yoshimoto, S.; Tada, A.; Suto, K.; Itaya, K. J. Phys. Chem. B2003, 107, 5836.

(28) Magnussen, O. M.; Hotlos, J.; Behm, R. J.; Batina, N.; Kolb, D.M. Surf. Sci. 1993, 296, 310.

(29) Gao, X.; Edens, G. J.; Hamelin, A.; Weaver, M. J. Surf. Sci.1993, 296, 333.

(30) Kolb, D. M. Surf. Sci. 2002, 500, 722.(31) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal.

Chem. 1980, 107, 205.

Chart 1. Molecular Structure of CoTPP (or CuTPP)and CoPc

3160 Langmuir, Vol. 20, No. 8, 2004 Yoshimoto et al.

electrode.32,33 Because the molecular adlayers were de-posited in benzene solutions, the possibility of interferenceby benzene adsorption was scrutinized by running vol-tammograms of a Au(100) electrode after soaking inbenzene for 10-60 s. The CV (not shown here) wasessentially the same as that of a bare Au(100), indicatingthat benzene molecule was not adsorbed on Au(100). Thisfinding agrees with the results obtained in UHV.

The CV profiles obtained with an Au(100) electrodemodified with CoTPP (a) and CoPc (c) were mostlyfeatureless between 0 and 1.0 V. The lack of any featureof these profiles was somewhat unexpected, because thesemolecules contain an electroactive metal center, Co(II);so they were expected to give a peak near 0.22 V in thenegative-going scan, as observed previously at Au(111).25,27

This phenomenon appears to indicate that the Co3+/2+

reduction reaction was kinetically slow at Au(100).However, the reason behind this effect of electrodeorientation on the rate of electron transfer is not clear.Another peculiar feature of these profiles is the increasein the reduction current at the negative end, which shouldresult from the evolution of hydrogen and the possibledesorption of the admolecules. However, it is not clearwhy CoPc resulted in the most pronounced increase ofcurrent among these molecules.

Furthermore, the peak at 0.85 V, associated with thelifting of reconstructed Au(100) surface, is either sub-stantially reduced (Figure 1a and b) or totally disappeared(Figure 1c). These results imply that the Au(100) surfaceafter being modified with these molecules remained ineither the reconstructed (5 × 20) or the unreconstructed(1 × 1) state within the potential range between 0 and 1.0

V. Results from in situ STM imaging (vide infra) supportthis view, and it was the reconstructed (5 × 20) structurethat predominated within this potential regime. A similarphenomenon was also noted in the case of Au(111), wherethe adsorption of a CoTPP molecular adlayer favors thereconstructed (x3 × 22) over the (1 × 1). Ultimately, themolecular adlayer might block the adsorption of anions,frequently a necessary condition for lifting the recon-struction. The adsorption of organics on gold electrodehasbeenextensivelyexamined,where theadsorbateexertsa strong impact on the stability of the atomic structureof the Au(100) substrate.34,35 It is concluded that therelative adsorption energy of an organic molecule on thereconstructed and ideal (1 × 1) surfaces dominates thestructure of a gold electrode. If an organic adsorbate bindsmore strongly to the reconstructed phase, the lifting ofreconstruction would shift positively, as compared to thatof a bare gold electrode.

In situ STM. Well-Defined Au(100) Surface. We firstconducted STM imaging to ensure that the Au(100)substrate was structured as expected. The STM resultsare sufficient to show that the Au(100) surface was initiallyunreconstructed but changed into the reconstructed phaseat more negative potentials.28-30 This potential-inducedphase transition of Au(100) reflects the effects of chargedensity on the bonding energy between surface gold atomsand on the adsorption of anions.28-30 It is emphasized thatone can differentiate between the two atomic structuresof the Au(100) substrate by a simple examination of thesurface morphology. The reconstructed Au(100) surfacewould be flat with the prevalent strand features with 0.07nm corrugation. In contrast, the (1 × 1) surface containsa high density of mesas resulting from the 24% differencein the density of surface gold atoms between these twophases. An even higher resolution STM scan was able toreveal atomic details of the surface structure. However,we will not describe further details because they havealready been reported in the literature.28-30

CoTPP Adlayer. Figure 2 shows STM images of a CoTPPadlayer on Au(100) prepared by immersing into a benzenedosing solution for 10 s. The potential of the gold electrodewas held at 0.75 V, which is slightly more negative thanthe open-circuit potential, and the imaging parameters,such as feedback current and bias voltage, are typically5 nA and -0.3 V, respectively. Figure 2a is an STMtopography scan intended to reveal the gross surfacemorphology of the as-prepared Au(100) sample. Withinthe 250 × 250 nm2 area two uniform terraces separatedby a step line marked by a white arrow in the diagonaldirection of the image are clearly seen with only a fewminor defects of dots and cracks. This homogeneousappearance of the surface clearly indicates that a recon-structed Au(100) surface existed under the monolayer ofCoTPP. A faint modulation of intensity was found on theterrace. The higher resolution STM scan shown in Figure2b indicates the formation of a highly ordered moleculararray spanning 75 × 75 nm2 with only a few vacancydefects. The bright dots forming the periodical squarearray are ascribed to the CoTPP molecules. Occasionally,this structure was found to be near on a reconstructedAu(100) domain, for example, as observed in the upperportion of the STM image in Figure 2c. Although theunderlyingAu(100)wasnot imaged, the island-freesurfacemorphology indicates that it was in the form of areconstructed phase. Figure 2c further allows extrapola-

(32) (a) Hamelin, A.; Weaver, M. J. J. Electroanal. Chem. 1987, 223,171. (b) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1.

(33) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929.

(34) Holzle, M. H.; Kolb, D. M. Ber. Bunsen-Ges. Phys. Chem. 1994,98, 330.

(35) Skoluda, P. Electrochem. Commun. 2003, 5, 142.

Figure 1. Typical cyclic voltammograms of bare Au(100)(dashed line) and CoTPP (a), CuTPP (b), and CoPc (c) adsorbed(solid line)Au(100)electrodes inpure0.1MHClO4.Thepotentialscan rate was 50 mV s-1.

Co(II)- and Cu(II)-Coordinated TPP and Pc Langmuir, Vol. 20, No. 8, 2004 3161

tion of the gross registry of CoTPP admolecule with respectto the corrugated strands, if the reconstruction pattern isextended into the molecular arrays. It appears that theCoTPP molecules are located on the peaks of the cor-rugated strands.

A further close-up view of the CoTPP array on thereconstructed Au(100) is shown in Figure 3a, where eachindividual CoTPP molecule is seen as a propeller with abright dot at the center, which are ascribed, respectively,

to the peripheral porphyrin molecule and the Co(II) cation.This configuration resembles that of CoPc and CoTPPadmolecules on Au(111) in UHV17,18 and in solution.25,27

The intermolecular spacing is measured to be 1.44 ( 0.05nm. Figure 3b reveals a proposed model of this molecularsquare array superimposed on the image of a reconstructedAu(100) substrate. This assignment is in line with theresult shown in Figure 3b. On the other hand, selectiveimaging with different STM parameters may be oneapproach for elucidating the registry of an admolecule, asdemonstrated previously.36

Modulation of potential between 0 and 0.85 V did notaffect the structure of this molecular adlayer. However,at potentials positive of 0.95 V, the reconstructed Au(100)was lifted, resulting in disordering of the adlayer. It isnoteworthy that the lifting of reconstruction occurred ata potential more positive than that in 0.1 M HClO4,suggesting that the CoTPP overlayer expanded the stablerange of potential for the reconstructed phase. Thisphenomenon unambiguously observed by the in situ STMis also in agreement with the voltammetric result, in whichthe phase transition feature at 0.85 V was largelysuppressed (Figure 1a). The effect of organic adsorptionon the structure of Au(100) has been extensively examined.Ultimately, the relative adsorption energy of organicadsorbate on the two phases of Au(100) determines theatomic structure of Au(100). For example, coumarin,cyclohexanone and γ-butyrolactone are shown to bind morestrongly to the hex phase so that the lifting of reconstructedsurface occurs at more positive potentials.35 As a corollary,CoTPP could be more strongly held at the reconstructedAu(100) surface, rendering the phase transition moredifficult to proceed. Alternatively, these organic adlayersmight inhibit the adsorption of anions, such as perchloratein the present case. In fact, a similar phenomenon wasnoted for the adsorption of porphyrin on Au(111), wherethe (x3 × 22) reconstruction is lifted at 0.8 V, which is0.2 V more positive than the potential of bare Au(111).25

On the other hand, the reason for the preference towardthe structure of Au(100) is not clear.

CuTPP Adlayer. We then studied the adsorption of aCuTPP adlayer on the reconstructed Au(100) to unveilthe role of the metal cation in the molecular adsorption.The same procedure was used to prepare a CuTPP adlayer,and the in situ STM result (Figures 4 and 5) showed thatthis molecule also formed a long-range ordered moleculararray. Although these images were obtained at 0.8 V in0.1 M HClO4, the same structure predominated also at

(36) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337.

Figure 2. Large-scale (250 × 250 nm2) (a), middle-scale (75 × 75 nm2) (b), and small-scale (20 × 20 nm2) (c) STM images of theCoTPP adlayer on the Au(100)-(hex) surface in 0.1 M HClO4 acquired at 0.75 V versus RHE. The tip potentials and the tunnelingcurrents were, respectively, (a and b) 0.45 V and 5.0 nA and (c) 0.35 V and 3.5 nA.

Figure 3. High-resolution (9 × 9 nm2) STM image (a) andstructural model (b) of the CoTPP adlayer superimposed on theSTM image of atomically resolved Au(100)-(hex) surface in0.1 M HClO4, acquired at 0.75 V versus RHE. The tip potentialand the tunneling current were 0.45 V and 3.5 nA, respectively.

3162 Langmuir, Vol. 20, No. 8, 2004 Yoshimoto et al.

other potentials, as long as it is not more positive than0.95 V. Further close-up views of the molecules in regionsI and II in Figure 4 are shown in Figure 5a and b,respectively. The internal molecular structure is clearlyseen. The four spots located at the four corners of molecularsquares are assigned to the four phenyl rings lying roughlyperpendicularly to the porphyrin ring. The unit length ofthe as-marked square lattice is 1.45 nm, the same as thatof CoTPP. Each CuTPP molecule was also found to be ofa propeller shape, indicating the parallel adsorptionconfiguration of CuTPP molecule. It is likely that the mostefficient molecular packing on the surface is obtained whenall molecules are rotated with their C2 axis intersectingthe substrate atomic rows at an angle of about 30°(clockwise for Figure 5a or anticlockwise for Figure 5b).These STM images are almost the same as those observedon Au(111), presented in our recent paper.25

The adlattices of CoTPP and CuTPP are practiallyindistinguishable, except in their molecular resolutionimages. More specifically, the center of a CuTPP moleculeor the location of the Cu2+ cation appears as a depression,whereas the cobalt ion in CoTPP is apparently protruded.This difference in three-dimensional configuration be-tween these two molecules was previously noted also onAu(111).14,15 This result is explained by an electronic effectarising primarily from the difference in the occupation ofthe d orbitals.14-19 The electronic configuration of CoPc isdxz

2, dyz2, dxy

2, dz21, whereas that of CuPc is dxz2, dyz

2, dxy2,

dz22. The half-filled dz2 orbital in CoTPP is believed tomediate electron tunneling, which leads to the protrudedappearance in the STM image.16 This electron-mediatingchannel apparently does not exist for CuTPP, giving thedepressed appearance in the STM image.

The structure of CuTPP adlayer was also very stableon the reconstructed Au(100) surface; no structural change

Figure 4. STM image (20 × 20 nm2) of the CuTPP adlayer onreconstructed Au(100)-(hex) in 0.1 M HClO4 acquired at 0.8V versus RHE. The tip potential and the tunneling currentwere 0.35 V and 15.0 nA, respectively.

Figure 5. High-resolution STM images (8 × 8 nm2) of CuTPPadlayer on reconstructed Au(100)-(hex) in 0.1 M HClO4acquired at 0.8 V versus RHE. Images a and b correspond tothose of regions I and II in Figure 4, respectively. The tippotential and the tunneling current were 0.35 V and 15 nA,respectively.

Figure 6. Large-scale (50 × 50 nm2) STM images of CuTPPadlayer on Au(100)-(1 × 1) surface in 0.1 M HClO4 acquiredat 0.95 V (a) and at 0.75 V stepped from 0.95 V (b) versus RHE,respectively. The tip potential and the tunneling current were0.35 V and 10 nA, respectively. The set of two arrows indicatethe close-packed directions of the Au(100) substrate.

Figure 7. Large-scale (50 × 50 nm2) STM image of the CoPcadlayer on Au(100)-(1 × 1) surface in 0.1 M HClO4 acquiredat 0.75 V versus RHE. The tip potential and the tunnelingcurrent were 0.43 V and 15 nA, respectively. The two arrowsindicate close-packed directions of the Au(100) substrate.

Co(II)- and Cu(II)-Coordinated TPP and Pc Langmuir, Vol. 20, No. 8, 2004 3163

within the adlayer and the gold substrate was observedin the potential range between -0.1 and 0.85 V. However,applying a potential equal to or more positive than 0.9 Vdestroyed the ordered molecular overlayer and formedmany monatomically high mesas on the terraces, asrevealed by the STM image in Figure 6a. The emergingmesas are indicative of the lifting of reconstruction of Au-(100) to the (1 × 1) phase. This phase transition wasreversible with respect to potential, as locally orderedarrays of CuTPP appeared again at a more negativepotential (0.75 V), as seen in the STM image of Figure 6b.These two images were not acquired at the same locationon the Au(100), and therefore they have rather differentmorphologies. Also, it is believed that the admoleculeswere not desorbed at 0.9 V, and they simply becamedisordered. This is indicated by the fact that the orderedmolecular overlayer was restored during the prolongedimaging at 0.75 V. On the other hand, it is not clear whyCuTPP did not form an ordered 2D structure on the(1 × 1) phase.37

CoPc Adlayer. To explore the effect of molecularstructure on the spatial arrangement and the atomicstructure of the supporting gold substrate, we investigatedthe adsorption of CoPc on Au(100). The Au(100) electrodewas made to be reconstructed, and the dosing of CoPc wasdone in a benzene solution, as described earlier. Figure7 shows a large-scale STM image of a CoPc adlayer onAu(100) acquired at 0.8 V in 0.1 M HClO4. The CoPcadmolecules imaged as bright spots were found on theterraces and islands. The size and shape of the island areevidently poorly defined, but they all exhibit the sameheight (∆z ) 0.23 nm). These areas of the protruded mesasadd up to account for ca. 24% of the surrounding substrate,which roughly matches the excess in atomic density ofthe hex phase with respect to the (1 × 1) phase. In otherwords, the CoPc adlayer, prepared by using the sameprocedure as was used to produce CoTPP and CuTPPadlayers, was formed on (1 × 1), suggesting that theadsorption of CoPc simultaneously caused the liftingof reconstruction. Intriguingly, a similar phenomenonwas also noted when CoPc was adsorbed on Au(111)-(x3 × 22), as reported in our recent paper.27 This resultis in marked contrast with the results obtained withCoTPP and CuTPP, which formed long-range orderedadlattices on reconstructed Au(100).

A modulation of potential was applied to the Au(100)electrode to investigate the stability of the CoPc adlayerand the Au(100)-(1 × 1) phase. Remarkably, the CoPcadlayer was so stable that no change occurred between 0and 1.0 V. In other words, the Au(100)-(1 × 1) surface,which is normally reconstructed into the hexagonal phaseat potentials negative of 0.45 V in 0.1 M HClO4, remainedstable at potentials as negative as 0 V in the presence ofa CoPc adlayer. To the best of our knowledge, such a highstability of the reconstructed phase of Au(100) has notbeen observed thus far in an electrochemical environment.This result implies that CoPc molecule binds much morestrongly on the reconstructed Au(100) surface than onthe unreconstructed phase.

The evenly distributed mesas on the terrace madeit difficult to form a well-ordered molecular array andalso to achieve high-quality molecular resolution STM.Occasionally, CoPc molecule was imaged as a propeller-shaped object, as seen in some portions (e.g., the areacircled by a dotted line in Figure 8a). Interpretation of theSTM result is straightforward. The center protrusion andthe four weaker spots at the four corners of squares arereadily associated with the Co ions and phenyl moietiesof the Pc molecules, respectively. It is stressed, however,that details in the sample preparation procedure caninfluence the spatial molecular arrangements of CoPc.For example, CoPc adlayer was found to be square-like onreconstructed Au(111) at an immersion time less than 5min, but a nearly close-packed hexagonal phase wasproduced when the dosing time was increased to 10 min.27

The adsorption of CoPc on reconstructed Au(100) wasattempted by reducing the dosing time to less than 3 min,but the results showed that the (1 × 1) phase alwaysprevailed even with a dosing time as short as 1 min. Thisresult indicates that the adsorption of CoPc monolayerincreased the mobility of gold atoms.

The intermolecular distance between nearest neighborCoPc molecules in this adlayer was about 1.65 nm, whichis greater than the adlattice constant of CoTPP or CuTPPon reconstructed Au(100) as described above. The inter-molecular spacings between CoPc molecules are 2.05 (0.05 and 2.61 ( 0.07 nm, respectively, in the [011h] and the[01h1h] directions, which correspond to 7 and 9 times the Aulattice constants. This local structure is thus assigned asc(7 × 9)rect, whose real-space structural model is shownin Figure 8b. This model features a high symmetry of thePc molecules with respect to the Au(100) substrate, where(37) Yoshimoto, S.; Tada, A.; K.; Itaya, K. Manuscript in preparation.

Figure 8. STM image (25 × 25 nm2) (a) of the CoPc adlayer on Au(100)-(1 × 1) surface in 0.1 M HClO4 acquired at 0.75 V versusRHE and the corresponding structural model (b). The tip potential and the tunneling current were 0.43 V and 15 nA, respectively.

3164 Langmuir, Vol. 20, No. 8, 2004 Yoshimoto et al.

all phenyl moieties can occupy 2-fold bridge sites. Thisassignment is supported by the two possible rotationaldomains imaged in Figure 8a. The fact that the samestructure was also observed for CuPc on Au(100)-(1 × 1)suggests that the real-space structures of Pc complexesare largely determined by the framework of the organicportion of the molecule rather than by the center cations.Furthermore, in view of the known fact that vapordeposition of CuPc on reconstructed Au(100) surface inUHV produces a (5 × 5) structure,38 it is clear that detailsin the sample preparation procedure can control theamounts and structures of these adsorbate molecules.

ConclusionsImmersing a Au(100) electrode in a benzene solution

containing CoTPP, CuTPP, or CoPc molecules producesa highly ordered adlayer of CoTPP or CuTPP on thereconstructed Au(100) surface and a local ordering of CoPcon the unreconstructed Au(100)-(1×1) surface. Molecularresolution STM has revealed the internal molecularstructure of each individual admolecule in 0.1 M HClO4underpotential control.CoTPPandCuTPPadapt thesamespatial arrangement on the reconstructed Au(100) surfacewith both molecules located on the peak of corrugatedstrands of gold atoms. The spacing of 1.44 nm betweentwo neighboring molecular rows equals to the distancebetween two neighboring strands of reconstructed Au-

(100). The metal centers of Co(II) and Cu(II) in the CoTPPand CuTPP admolecules appear as a protrusion anddepression, respectively, in the molecular resolution STM.The highly ordered CoTPP arrays stabilize reconstructedAu(100) at potentials up to 0.85 V, whereas those of CuTPPsustain the structure even at a potential as positive as 0.9V. The adsorption of CoPc molecule results in a rapidphase transition from the hexagonal phase to the (1 × 1)phase of the Au(100) substrate, leading to the formationof an only locally ordered adlayer. Molecular structureapparently governs the interaction between the admol-ecule and the gold substrates. This study further il-lustrates the feasibility and readiness of using the solution-phase dosing to produce a well-ordered molecular adlayer,which may be important in the development of molecularelectronics.

Acknowledgment. This work was supported in partby the Core Research for Evolutional Science and Tech-nology (CREST) of the Japan Science and TechnologyAgency (JST) and by the Ministry of Education, Culture,Sports, Science and Technology, a Grant-in-Aid for theCenter of Excellence (COE) Project, Giant Molecules andComplex Systems, 2003. The authors acknowledge Dr. Y.Okinaka for his assistance in writing this manuscript andDr. J. Inukai of Tohoku University for his usefuldiscussion.

LA0359474(38) Auerhammer, J. M.; Knupfer, M.; Peisert, H.; Fink, J. Surf. Sci.

2002, 506, 333.

Co(II)- and Cu(II)-Coordinated TPP and Pc Langmuir, Vol. 20, No. 8, 2004 3165