Molecular Assembly Through the Chain Reaction of SubstitutedAcenes on the Si(100)−(2 × 1)−H SurfaceMd. Zakir Hossain,*,† Hiroyuki S. Kato,‡,§ Jaehoon Jung,∥ Yousoo Kim,∥ and Maki Kawai*,‡,⊥

†Advanced Scientific Research Leaders Development Unit, Advanced Engineering Research Team, Faculty of Science andTechnology, Gunma University, Kiryu, Gunma 376-8515, Japan‡Advanced Science Institute (ASI), RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan§Department of Chemistry, Graduate School of Science, Osaka University, Osaka 650-0043, Japan∥Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan⊥Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8501, Japan

ABSTRACT: The formation of a molecular assembly through the chain reaction of somesimple vinyl substituted acenes with the dangling bond site of H-terminated Si(100)-(2 ×1) surface has been studied under ultrahigh vacuum conditions at room temperature. Themolecular assemblies formed on the surface were checked using an in situ scanningtunneling microscope (STM). While styrene and 2-vinylnaphthalene undergo a danglingbond initiated chain reaction forming a perfectly ordered one-dimensional (1D) assemblyas expected, 9-vinylanthracene exhibits unique behavior in forming the molecularassemblies studied to date. Both the perfectly ordered 1D and irregularly shapeddisordered assemblies are formed by the 9-vinylanthracene molecules on the surface, whichallows us to directly compare the interaction between molecules in two differentassemblies. Biased dependent changes of the relative contrast of ordered and disorderedassemblies formed by 9-vinylanthracene indicates a significant π−π interaction between themolecules in 1D assembly, which tends to delocalize and disperse the valence andconduction band of ordered assemblies. This study suggests that an increased π−π interaction is expected in a 1D assembly oflarger molecules of the acene family, although there is a limit of forming such ordered assembly using larger acenes at roomtemperature.

■ INTRODUCTION

Low-dimensional molecular assembly with parallel π-stacking ofphenyl rings is expected to act as a conducting channel forelectric charge and, hence, can be used as a component ofnanoscale electronic devices.1−8 One of the most promising andhighly efficient ways of making such a highly organizednanostructure is the fabrication of a one-dimensional (1D)molecular assembly on a H-terminated Si(100)-(2 × 1) surfacethrough a chain reaction mechanism.1,9 The Si(100)-(2 × 1)-Hsurface consisting of parallel rows of H-terminated silicondimers separated by 3.8 Å is an ideal template to align themolecules with a phenyl π-conjugated system parallel to eachother along the dimer-row. Indeed a variety of molecularassemblies with precise control over the growth direction andintermolecular arrangement have been reported.9−20 Thepractical applications of such an assembly depend on theπ−π interaction among the aligned molecules, which istheoretically predicted to depend on the intermolecularseparation and substituent or the extent of π-conjugation ofthe molecule.2,3,5−8 Since the intermolecular separation withinan assembly on the H-terminated Si(100)-(2 × 1) surface isfixed by the interdimer distance (3.8 Å) along the row, thevariation of substituent or the extent of π-conjugation is theonly way to gain tunability over the π−π interaction.6,8 Hence it

is highly desirable to investigate if the molecules with anextended π-conjugated system such as molecules of acenefamily can form a highly ordered assembly on the H-terminatedSi(100)-(2 × 1) surface.To date, most of the experimental studies were focused on

the growth of molecular assemblies by aromatic hydrocarbonsconsisting of a single phenyl ring (such as styrene), isolated twophenyl rings (such as benzophenone), or aliphatic hydro-carbons (such as allylmercaptan, 1-octene, etc.).9−20 Recently, adirect measurement of valence band dispersion of 1D assemblyformed by benzophenone molecules on the H-terminatedSi(100)-(2 × 1) surface has indicated that such a 1D assemblycan act as a good conductor of electric charge.21 Indeed theconduction modulation of such an assembly can be achievedthrough the substituent in the phenyl group.8 Rochefort et al.have theoretically studied the resonant tunneling transportproperties of 1D molecular assembly of some small moleculesof the acene family such as ethylbenzene, 1-ethylnaphthalene,9-ethylanthrance, and 6-ethylpentacene.6 They have computeda drastic band gap (energy gap between the highest occupied

Received: June 4, 2013Revised: August 30, 2013Published: September 2, 2013

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molecular orbital, HOMO and lowest unoccupied molecularorbital, LUMO) decrease (∼3.5 eV) in isolated acenemolecules with increasing the number of phenyl rings (i.e.,ethylbenzene to ethylpentacene).6 Comparing to an isolatedmolecule, a further decrease of the HOMO−LUMO gap isestimated when the molecules are aligned in 1D with theseparation equal to the interdimer distance (3.8 Å) of theSi(100)-(2 × 1) surface.6 It is expected that the 1D assemblyconsisting of a larger acene molecule such as ethylanthracene orethylpentacene will function as a better charge carrier channelthan that of ethylstyrene. Indeed in spite of its obviousimportance no experimental investigation regarding theformation of 1D assemblies using those acenes has beenperformed so far.Here, we have investigated the formation of molecular

assemblies by some commercially available small molecules ofthe acene family (as shown in Figure 1) through the dangling

bond initiated chain reaction on the H-terminated Si(100)-(2 ×1) surface in ultrahigh vacuum (UHV) at room temperature.The assemblies formed by those molecules on the surface areinvestigated by an in situ scanning tunneling microscope(STM). The 2-vinylnaphthalene molecules undergo a chainreaction similar to that of styrene leading to the 1D molecularassembly along the dimer-row on the surface. Unlike styreneand 2-vinylnaphthalene, the 9-vinylanthracene molecules formboth ordered and disordered assemblies on the surface. Theordered assembly is similar to that of styrene assembly runningalong the dimer-row. Direct comparison of bias dependentcontrast changes in STM images of ordered and disorderedassemblies formed by 9-vinylanthracene indicates that asignificant π−π interaction exit among the aligned molecules,which tends to delocalize and disperse valence and conductionband as theoretically predicted.

■ EXPERIMENTAL SECTIONThe experiments were performed in an ultrahigh vacuum(UHV) chamber with a base pressure better than 5 × 10−11

mbar. The UHV chamber is housed with a variable temperature

scanning tunneling microscope (Omicron VT-STM). The 8mm × 1.5 mm silicon sample was cut from a Boron-doped(0.01 Ωcm) silicon wafer. The surface was cleaned byprolonged annealing at ∼850 K (∼8 h) followed by repeatedflashing up to 1400 K. The H-terminated surface was preparedby exposure to atomic H, generated by a hot W-filament(∼2100 K), at the surface temperature of ∼625 K. The freshlyprepared H-terminated surface normally contains a diluteconcentration of unpaired DB sites resulting from theincomplete H-termination.22 Otherwise DBs in a particulararea are created by injecting an electron from the STM tip.23 9-Vinylanthracene (solid, 97% purity), 2-vinylnaphthalene (solid,98% purity), and styrene (colorless liquid, 99.9% purity)purchased from Sigma-Aldrich were purified by pumping outthe vapor above the liquid or solid phases. The styrene wasdosed onto the silicon surface through electronically controlledpulse-valve dosing system. The 9-vinylanthracene and 2-vinylnaphthalene molecules were dosed by opening a gatevalve between an ampule containing the sample and the siliconsample in a rod-lock chamber. The number of molecules dosedonto the surface is expressed in Langmuir (1 L = 1 × 10−6 Torr·s), which is estimated from the background pressure reading bya B. A. (Bayard-Alpert) gauge during the exposure. Unrestricteddensity functional theory (DFT) calculations were performedto see the spin density distribution of intermediate radical statesformed by styrene, 2-vinylnalphthalene, and 9-vinylanthraceneadsorbed on a simple H-terminated Si9 cluster. We employedthe PBE024 hybrid density functional implemented in theGuassian 0925 program suite. The optimization of molecularstructures and the detailed evaluation of electronic structureswere carried out using 6-31G(d,p) and 6-311G(2d,p) basis sets,respectively.

■ RESULTS AND DISCUSSIONFigure 2 shows the STM images of H-terminated Si(100)-(2 ×1) surfaces after exposure to different molecules shown inFigure 1 at room temperature. In addition to the underlying flatsurfaces with multiple terraces consisting of parallel rows ofmonohydride dimers, different types of molecular assembliesare seen. These molecular assemblies are formed through thechain reaction of different molecules with the dangling bondsites on the surface.1,26,27 We can see in Figure 2a,b that thestyrene and 2-vinylnaphthalene molecules form similar 1Dmolecular assemblies on the surface. In these cases, the 1Dassemblies are aligned along the dimer-row on the H-terminated Si(100)-(2 × 1) surface; that is, the danglingbond initiated chain reaction that 2-vinylnaphthalene under-goes is similar to that of styrene molecules as reported in theearlier studies.1,10 When the H-terminated Si(100)-(2 × 1)surface is exposed to 9-vinylanthracene molecules, therandomly distributed and irregular shaped structure areobserved dominantly throughout the surface as shown inFigure 2c. In addition to those irregularly shaped structures,some 1D structures aligned along the dimer-row are also seenas indicated by the arrow (Figure 2c). These 1D structuresformed by the reaction of 9-vinylanthracene molecules arerelatively less stable compared to that of styrene at roomtemperature.An enlarged STM image of a 1D structure formed by 9-

vinylanthracene molecules (indicated by an arrow) is shown inFigure 2d. The uniform appearance of this structure suggeststhat the chemisorbed molecules are equally spaced and thestacking orientations of all molecules in the assembly are

Figure 1. Molecules investigated in the present study.

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identical; that is, the plane of fused phenyl rings are alignedparallel to each other with an intermolecular separation of 3.8 Åsimilar to that of styrene and vinylnaphthalene.4 On the otherhand, the molecules in the disordered structure are randomlydistributed on the surface, and the orientation of thosemolecules are different. One can see that the 1D structureseen in Figure 2d no longer exists in the next image shown asFigure 2e. After the disappearance of the 1D structure, theunderlying H-terminated dimer-row is clearly seen in Figure 2e.This suggests that a reversed chain reaction leading to thedesorption of all chemisorbed molecules in the 1D assemblyhappens even at room temperature.18 This spontaneousreversed chain reaction at room temperature suggests that asignificant repulsive interaction due to steric hindrance existsamong the aligned molecules, which makes such orderedassembly by 9-vinylanthracene less stable. The high tendency ofundergoing reversed chain reaction leading to desorption of theentire 1D assembly is further confirmed by annealing thesurface (Figure 2c) at 250 °C. After the surface containing bothordered and disordered assemblies annealed at 250 °C, only thedisordered assemblies were seen on the surface (not shown).Careful inspection of the ordered and disordered assemblies

in Figure 2c reveals that the 1D assembly appears slightlybrighter than the overall appearance of the disordered assembly.This apparent contrast difference between ordered anddisordered assemblies is confirmed by the bias dependentSTM measurements as shown in Figure 3. Figure 3a−cdemonstrates that the 1D assembly formed by 9-vinyl-anthracene appears much brighter than that of the disorderedassembly at lower sample biases. Some bright spots within thedisordered assembly might be the defects, which are seen evenin the freshly prepared H-terminated surface. Hence we havefocused on the overall contrast of the disordered assemblies and

compared it with that of 1D assemblies in STM images. Therelative contrast changes are clearly visualized in the heightprofiles along the lines through typical disordered and orderedassemblies at different biases as shown in Figure 3d. Forcomparison, height profiles along the step of the surface arealso shown in the inset. Figure 3d indicates that the relative

Figure 2. STM images of molecular assemblies formed on the H-terminated Si(100)-(2 × 1) surface by exposure to (a) ∼20 L of styrene, (b) ∼100L of 2-vinylnaphthalene, and (c) ∼100 L of 9-vinylanthracene molecules at room temperature. Typical one-dimensional (1D) assemblies formed bythese molecules along the dimer-row are indicated. The (d) and (e) are the same area STM images acquired sequentially demonstrating thedisappearance of a 1D molecular assembly formed by 9-vinylanthracene through the reversed chain reaction. (a,c) Vsample = −2.2 V; (b) Vsample =−3.0 V; (d,e) Vsample = −2.5 V; Itunnel = 0.2 nA.

Figure 3. (a−c) Bias depended STM images of molecular assembliesformed by ∼100 L of 9-vinylanthracene molecules on the H-terminated Si(100)-(2 × 1) surface. Scanning sample bias is indicated.Itunnel = 0.2 nA. (d) Line profile indicating the relative contrast changeof ordered and disordered assemblies at different sample biases alongthe lines shown in panel b. For comparison, line profiles at twodifferent sample biases along the step of the surface are also shown. Tovisualize clearly, the lines are vertically shifted.

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height of the ordered and disordered assemblies varies with thesample bias employed during scanning.The height changes in constant current STM images signify

the tunneling current changes due to variation of topographyand/or the density of states near the Fermi level.28 When theSTM image height is purely due to the topographic height ofthe molecule or substrate, it does not change with the samplebiases as observed in the case of step height (Figure 3d). Therelative contrast change of ordered and disordered assemblieswith sample biases suggests the different local density of states(LDOS) of the corresponding assemblies near the Fermi level.At lower sample bias (i.e., −1.7 and −2.1 V), the adsorbedmolecules in disordered assemblies have a little or nocontribution in tunneling current while the STM tip scanover it; that is, the LDOS corresponding to these molecule liesoutside of the tunneling window at this bias.28 On the otherhand, LDOS corresponding to the 1D assembly lies within thetunneling window even at −1.7 V; hence, it contributes to theenhanced tunneling current leading to the bright appearance inthe STM image. Such contrast difference of one-dimensionalmolecular assemblies on H-terminated Si(100) due to thedifferences in LDOS near the Fermi level has been alsoreported in earlier studies.29

The difference in the electronic states corresponding to theordered and disordered assemblies arises from the molecule−molecule interaction within an assembly.6 In the case ofordered assembly, the adsorbed 9-vinylanthracene moleculesalign parallel to each other with an intermolecular separation of3.8 Å (interdimer distance in a row). On the other hand, themolecules in the disordered assembly are randomly distributed,and the geometric orientations of the molecules are expected tobe different. The previous theoretical calculation has estimatedthat the HOMO−LUMO gap (HLG) of ethyl anthracenemolecules is decreased by ∼0.35 eV when the molecules arealigned parallel to each other with the separation of 3.8 Å.6

Furthermore, HLG of perfectly ordered assembly is lower thanthat of deformed assembly.6 The decrease in HLG by theparallel alignment of the molecules suggests that the HOMO ofparallel assembly is closer to the Fermi level than that of thedisordered assembly,3,6 which is in agreement with the presentobservation. Thus based on the previous theoretical calculation,we suggest that the brighter contrast of the ordered assemblycompared to that of the disordered assembly formed by 9-vinylanthracene molecules indicates that a significant π−πinteraction exist between the molecules stacked parallel alongthe dimer-row, which tends to delocalize and disperse thevalence and conduction band of the ordered assembly.3,6

Recently, a direct dispersion measurement of valence statescorresponding to the ordered assembly of benzophenonemolecules on the H-terminated Si(100)-(2 × 1) surface ensurethe one-dimensional delocalized electronic states along theassembly and suggest that such ordered assembly can be usedas a charge carrier.21 As predicted by the theoreticalcalculation,6 the ordered assembly formed by 9-vinylanthracenemolecules on H-terminated Si(100)-(2 × 1) surface is expectedto function as a better charge carrier channel.It is well-known that a number of molecules containing

>CC< or >CO groups undergoes a chain reaction withthe dangling bond sites of the H-terminated silicon surface asshown in Figure 4.1,9,11,17,26 In previous studies, either orderedor disordered molecular assemblies are observed for a particularkind of molecule on a selected surface.1,9,11,27 On the Si(111)-H surface, only disordered assemblies by 1-alkene are observed

because of the isotropic nature of the surface silicon atom.26,27

On the other hand, the anisotropic nature of the surface siliconatom direct the chain reaction to a particular direction on theH-terminated Si(100)-(2 × 1) surface leading to the 1Dassembly for most of the molecules studied to date.1,9,11 Somemolecules such as cyclopropylmethyl ketone form a disorderedstructure on the H-terminated Si(100)-(2 × 1) surface.30 Thedifferences in forming either ordered or disordered assemblydepend on the characteristics of intermediate C radical [as inFigure 4(ii)]. In a normal chain reaction mechanism, if theintermediate C radical abstracts the surface H atom from thesame dimer-row resulting in a new radical site [as in Figure(iii)], the reaction proceed along the dimer-row directionforming a 1D assembly as observed in the case of styrene, 2-vinylnaphthalene, and ordered assembly of 9-vinylanthracenemolecules (Figure 2).Note that the C radical in (ii) can only reach to the nearest H

of the same row (blue colored H).31,32 Indeed the formation ofa disordered assembly cannot be accounted for by the reactionproceeding through the abstraction of H by the C radicalshown in Figure 4(ii). It has been reported that abstraction ofthe surface H leading to the formation of the nanostructuredepends on the stabilization of the C-centered intermediateradical.14 Hence we suggest that the initial intermediate Cradical is transferred on the phenyl C atom through theresonance structure in the case of 9-vinylanthracene; that is, a Cradical may be localized at ‘7, 5, 10, or other’ positions ofanthracene skeletal [Figure 4(iv)]. The DFT calculation resultsas shown in Figure 5 suggest that, although the relative spindensity is dominant at the vinyl C atom for styrene and 2-vinylnaphthalene, it is at the phenyl C(10) position in the case

Figure 4. (i−iv) Possible chain reaction mechanism for 9-vinyl-anthracene molecules with the dangling bond site of the Si(100)-(2 ×1)-H surface leading to the ordered and disordered patterns. (i) Initialreaction of the 9-vinylanthracene molecule with the dangling bond(BD) site of the surface. (ii) The C-centered intermediate radicalgenerated after initial reaction. (iii) A new DB site is created throughabstraction of the nearby surface H by the C-centered intermediateradical. (iv) Three of several possible C radicals that can form throughthe resonance structure of anthracene.

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of 9-vinylanthracene molecules. Now it can be realized that theC(10) radical can reach the surface H even from the nearbydimer-row and , hence, can abstract any of the several surface Hatoms around the anchored Si−C bond leading to theformation of disordered assembly.

■ CONCLUSIONSThe formation of molecular assemblies by small substitutedacenes such as styrene, 2-vinylnaphthalene, and 9-vinyl-anthracene through the dangling bond initiated chain reactionon H-terminated Si(100)-(2 × 1) surface was investigated inultrahigh vacuum (UHV) at room temperature. The styreneand 2-vinylnaphthalene molecules undergo chain reaction withthe DB site in the usual manner forming 1D molecularassembly along the dimer-row on the surface. Unlike styreneand 2-vinylnaphthalene, the 9-vinylanthracene molecules formboth ordered and disordered pattern on the surface. Theformation of both ordered and disordered assemblies by 9-vinylanthracene molecules is ascribed to the resonancestructure of the C centered intermediate radical generated bythe reaction of the molecule with a DB site of the surface. Theordered assembly by 9-vinylanthracene is similar to that ofstyrene assembly aligned along the dimer-row. The muchbrighter appearance of ordered assembly compared to that ofdisordered assembly of 9-vinylanthracene in STM imagesacquired at lower sample biases indicates that a significant π−πinteraction exist among the molecules in ordered assemblyalong the dimer row. Though it is well established thatincreasing the number of phenyl rings in acenes increases theextent of intermolecular interaction in a 1D assembly, there is alimit of forming such assemblies using such molecules at roomtemperature.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: zakir@gunma-u.ac.jp. Tel. +81-277-30-1625.*E-mail: maki@k.u-tokyo.ac.jp. Tel. +81-471-36-3787.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the Grant-in Aid for Nanoscienceand Technology Program in RIKEN. M.Z.H. acknowledgessupport by the Program to Disseminate Tenure-Track System

of the Ministry of Education, Culture, Sports, Science andTechnology of Japan (MEXT) and Element Innovation (EI)project granted to Gunma University. J.J. and Y.K. are gratefulto the computational resources of the RIKEN IntegratedCluster of Clusters supercomputer system.

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