Study of Alkyl Organic Monolayers with Different Molecular ChainLengths Directly Attached to Silicon

Takahiro Ishizaki,*,† Nagahiro Saito,‡ Lee SunHyung,§ Kaoru Ishida,§ and Osamu Takai†

EcoTopia Science Institute, Nagoya UniVersity, Furo-cho, Chikusa, Nagoya 464-8603, Japan, Departmentof Molecular Design and Engineering, Graduate School of Engineering, Nagoya UniVersity, Furo-cho,

Chikusa, Nagoya 464-8603, Japan, and Department of Materials, Physics and Energy Engineering,Graduate School of Engineering, Nagoya UniVersity, Furo-cho, Chikusa, Nagoya 464-8603, Japan

ReceiVed August 27, 2005. In Final Form: August 1, 2006

Alkyl organic monolayers with different alkyl molecular chain lengths directly attached to silicon were preparedat 160°C from 1-decene (C10), 1-dodecene (C12), 1-tetradecene (C14), 1-hexadecene (C16), and 1-octadecene (C18).These monolayers were characterized on the basis of water contact angle measurement, ellipsometry, X-ray reflectivity(XR), X-ray photoelectron spectroscopy (XPS), and grazing incidence X-ray diffraction (GIXD) to elucidate the effectof the molecular chain length on the molecular arrangement and packing density of the monolayers. Water contactangle and XPS measurements showed that C12, C14, and C16 monolayers have a comparably higher quality, whilethe quality of C10 and C18 monolayers is worse. GIXD revealed that the alkyl monolayers directly attached to theSi were all amorphously structured regardless of their alkyl chain length. The amorphous structure of the alkylmonolayers could be attributed to the rigid Si-C bonding, low quality of hydrogen-terminated silicon substrate, and/orlow mobility of physisorbed molecules.

Introduction

There has been growing interest in organic monolayers directlyattached to silicon through Si-C bonds due to their applicationto semiconductor technology.1,2 Such organic monolayers offerthe potential for effective electron transfer at the silicon/organiclayer interface, since there is no insulating layer, that is, no siliconoxide layer.3-5 It has been reported that organic monolayers canbe employed for silicon surface passivation6,7 and for theincorporation of biochemical functionality at interfaces.8,9 Inparticular, among the several types of organic monolayers, alkylmonolayers directly attached to silicon are promising as resistfilms for nanofabrication.10,11Moreover, since alkyl monolayersfunction as insulator layers,12,13they are expected to be appliedto organic field-effect transistors (OFET).

It is vital for the practical application of OFETs to control themolecular arrangement and packing density of the organicmonolayer directly attached to the silicon substrate.14 Thus, itis necessary to understand in detail the structure of such

monolayers through characterization by various experimentaltechniques. Nonetheless, there is still little information availableon the properties of such films. In the case of alkyl monolayerson hydrogen-terminated silicon surfaces, the possible structureof the assembled molecules has been inferred from theoreticalstudies15,16 and experimental results17-20 obtained by infrared(IR) spectroscopy, ellipsometry, and X-ray reflectivity. Sievalet al. reported that the maximum coverage of alkyl moleculeson Si(111) is about 0.5-0.55 on the basis of molecular modelingsimulation.15,16 Bansal et al. demonstrated by XPS and ellip-sometric measurements that the thickness of an alkyl monolayervaried monotonically with the length of the alkyl group in thereactant.6 However, no attempt has yet been made to directlyinvestigate the effect of the molecular chain length on themolecular arrangement of alkyl monolayers directly attached tosilicon by grazing incidence X-ray diffraction (GIXD).

In this study, alkyl monolayers with different molecular chainlengths were prepared on hydrogen-terminated Si substrates bythe radical-initiated reaction of Si-H bonds with olefins. Theirmolecular arrangements were investigated on the basis of grazingincidence X-ray diffraction (GIXD), X-ray reflectivity (XR),X-ray photoelectron spectroscopy (XPS), ellipsometry, and watercontact angle measurement.

Experimental Section

As raw materials for alkyl monolayers with different molecularchain lengths, 1-decene (C10; Aldrich, 94%), 1-dodecene (C12;

* Corresponding author. E-mail: ishizaki@plasma.numse.nagoya-u.ac.jp.† EcoTopia Science Institute.‡ Department of Molecular Design and Engineering.§ Department of Materials, Physics and Energy Engineering.(1) Sze, S. M.Physics of Semiconductor DeVices, 2nd ed.; Wiley-Interscience:

New York, 1981.(2) Buczkowski, A.; Radzimski, Z. J.; Rozgonyi, G. A.; Shimura, F.J. Appl.

Phys.1991, 69, 6495.(3) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudho¨ter, E. J. R.AdV. Mater.2000,

12, 1457.(4) Bansal, A.; Lewis, N. S.J. Phys. Chem. B1998, 102, 4058.(5) Bansal, A.; Li, X.; Zi, S. I.; Weinberg, W. H.; Lewis, N. S.J. Phys. Chem.

B 2001, 105, 10266.(6) Bansal, A.; Lewis, N. S.J. Phys. Chem. B1998, 102, 1067.(7) Royea, W. J.; Juang, A.; Lewis, N. S.Appl. Phys. Lett.2000, 77, 1988.(8) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M.J. Am. Chem.

Soc.2000, 122, 1205.(9) Strother, T.; Hamers, R. J.; Smith, L. M.Nucleic Acids Res. 2000, 28,

3535.(10) Ara, M.; Graaf, H.; Tada, H.Appl. Phys. Lett.2002, 80, 2565.(11) Sondaghuethorst, J.; Vanhelleputte, H.; Fokkink, L.Appl. Phys. Lett.

1994, 64, 285.(12) Kobayashi, S.; Nishikawa, T.; Takenobu, T.; Mori, S.; Shimoda, T.; Mitani,

T.; Shimotani, H.; Yoshimoto, N.; Ogawa, S.; Iwasa, Y.Nat. Mater.2004, 3, 317.(13) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K.AdV. Mater. 1999, 11, 605.

(14) Park, J. S.; Vo, A. N.; Barriet, D.; Shon, Y-.S.; Lee, T. R.Langmuir2005,21, 2902.

(15) Sieval, A. B.; Hout, B.; Zuilhof, H.; Sudho¨ter, E. J. R.Langmuir2000,16, 2987.

(16) Sieval, A. B.; Hout, B.; Zuilhof, H.; Sudho¨ter, E. J. R.Langmuir2001,17, 2172.

(17) Linford, M. R.; Chidsey, C. E. D.J. Am. Chem. Soc.1993, 115, 12631.(18) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D.J. Am.

Chem. Soc.1995, 117, 3145.(19) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Lindord, M. R.; van der

Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨ter, E. J. R.Langmuir1998, 14,1759.

(20) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M.Angew. Chem.,Int. Ed. Engl.1998, 37, 2462.

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10.1021/la052342u CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 10/21/2006

Aldrich, 95%), 1-tetradecene (C14; Sigma, 99%), 1-hexadecene (C16;Tokyo Kasei, 99%), and 1-octadecene (C18; Fulka, 97%) were usedas received. The alkyl monolayers were prepared from each rawmaterial on p-type Si(111) wafers with electrical resistivity of 10-20Ω cm. First, the silicon substrates were cleaned in acetone, ethanol,and ultrapure water, in that order. Next, the silicon substrates werecleaned in a piranha solution (H2SO4:H2O2 ) 3:1) at 100°C for 10min,17then rinsed thoroughly with ultrapure water (18.2 MΩ). Afterrinsing, the oxidized silicon substrates were dried with N2 gas andimmersed in a 40% aqueous ammonium fluoride solution (NH4F)at room temperature for 15 min in order to remove the native siliconoxide layer, resulting in hydrogen-termination of the surface.21Eachhydrogen-terminated Si substrate was placed in a solution containingone of the raw materials for 5 h at thetemperature of 160°C. Alkylmonolayers with different molecular chain lengths were formed onthe silicon substrates through Si-C bonds.22-26 Figure 1 shows aschematic reaction diagram of this synthesis. Alkyl monolayerformation was confirmed by X-ray photoelectron spectroscopy(Shimadzu-Kratos, AXIS), water contact angle measurement (KRUSS,DSA10-MK2), and ellipsometry (Philips, PZ 2000). XPS wasperformed with Al KR (1486.3 eV) radiation operating at 10 mAand 12 kV with a takeoff angle of 30°. All binding energies werereferenced to the Si 2p peak of bulk silicon (99.3 eV). Water contactangles were collected at room temperature under ambient humidity.The values at five different points on each sample were averaged.

Film thickness was estimated by ellipsometry and X-ray reflectivity(XR) (Rigaku, ATX-G). The ellipsometer was equipped with a He-Ne laser (λ ) 632.8 nm) and a 45° polarizer. The incident angle ofthe laser was 70° from the surface normal. The refraction index andthe absorption coefficient for the silicon substrate were 3.865 and-0.020, respectively. The respective constants for silicon oxide were1.465 and 0, which were applied to those of the alkyl monolayers.27

In XR, the angular dependence of the specular reflectivity wasmeasured by a series ofRRi-2RRi scans, whereRRi indicates theincident angle of the monochromatic X-ray beam.28,29The reflectedbeam was detected by a scintillation counter through a receiving slit.

GIXD (Rigaku, ATX-G) was conducted to evaluate the moleculararrangement of the different alkyl monolayers. Each sample wasplaced on the sample stage of a three-axis goniometer for surfaceX-ray diffraction. The sample surface was then irradiated by amonochromatic X-ray beam at an incident angle of 0.2°.30,31 Thewave vector is defined asqxy () 4π(sinθ)/λ), whereθ andλ are theangle and the wavelength of the X-ray (Cu KR, λ ) 0.154 nm),respectively.

The surfaces of the hydrogen-terminated Si(111) substrate andthe alkyl monolayers directly attached to the Si(111) substrate were

observed in air with an atomic force microscope (AFM; SeikoInstruments, SPA-300HV+SPI-3800N). AFM images were acquiredat the scan rate of 1 Hz in contact mode using a sharpened siliconnitride tip with a force constant of 0.08 N/m and a resonance frequencyof 34 kHz (Seiko Instruments Inc., Micro Cantilever, type SI-AF01).

Results and Discussion

Figure 2, parts a and b, shows XPS Si 2p spectra of siliconsubstrate surfaces before and after immersion in a 40% ammoniumfluoride aqueous solution, respectively. In the spectrum of thesample surface before immersion, a peak attributed to silicondioxide was observed at the binding energy of 104 eV. On theother hand, no or little peak attributed to silicon dioxide wasobserved in the spectrum after immersion. This indicates that thesilicon dioxide layer on the Si substrate had been almost removed.Figure 2c shows an AFM topographic image of the Si substratesurface confirming terrace and step structures of silicon. Thein-plane intervals of the structures were 180 nm, and the differencein height between adjacent terraces was 3.2( 0.3 Å.

Densely packed methyl-terminated monolayers generally showwater contact angles greater than 110°. The water contact anglebecomes smaller with decreasing packing density.17,18,32,33Thus,the water contact angle strongly depends on the molecularstructure of the monolayer. Figure 3 shows the water contactangles on the alkyl monolayers as a function of the number ofcarbon atoms in their alkyl chains,n. In the present case, thealkyl chain length is proportional to the number of carbon atoms.Henceforth, the respective monolayers are referred to by their

(21) Saito, N.; Youda, S.; Hayashi, K.; Sugimura, H.; Takai, O.Surf. Sci.2003, 532, 970.

(22) Hurley, P. T.; Ribbe, A. E.; Buriak, J. M.J. Am. Chem. Soc.2003, 125,11334.

(23) Buriak, J. M.; Allen, M. J.J. Am. Chem. Soc.1998, 120, 1339.(24) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M.Langmuir

1999, 15, 3831.(25) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D.Langmuir2000, 16,

7429.(26) Munford, M. L.; Maroun, F.; Cortes, R.; Allongue, P.; Pasa, A. A.Surf.

Sci.2003, 537, 95.(27) In Annual Book of ASTM Standards; 1990; p F 576.(28) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides,

G. M. Phys. ReV. B 1990, 41, 1111.(29) Kojio, K.; Takahara, A.; Omote, K.; Kajiyama, T.Langmuir2000, 16,

3932.(30) Kojio, K.; Takahara, A.; Kajiyama, T.Bull. Chem. Soc. Jpn.2001, 74,

1397.(31) Takahara, A.; Kojio, K.; Kajiyama, T.Ultramicroscopy2002, 91, 203.

(32) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D.J. Am. Chem.Soc.1987, 109, 3559.

(33) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A.N.; Nuzzo, R. G.J. Am. Chem.Soc.1991, 113, 7152.

Figure 1. Schematic diagram of the reaction between 1-alkenesand a hydrogen-terminated Si surface.

Figure 2. XPS Si 2p spectra of Si sample surfaces obtained (a)before and (b) after NH4F treatment. (c) Topographic image of thehydrogen-terminated Si surface.

Alkyl Organic Monolayers Directly Attached to Si Langmuir, Vol. 22, No. 24, 20069963

number of carbon atoms, that is, as C10, C12, C14, C16, andC18 monolayers. The water contact angles of the C12, C14, andC16 monolayers were greater than 110°, while those of the C10and C18 monolayers were less than 110°. This indicates that thepacking density of the C10 and C18 monolayers was lower thanthat of the C12, C14, and C16 monolayers. In studies on alkylthiolmonolayers on gold and alkyl monolayers prepared from radicalreaction of 1-alkenes on (111)-oriented silicon, it has beenreported5,17,18 that the alkyl monolayers with a close packingstructure had water contact angles in the range of 110-115°.The water contact angles of the C12, C14, and C16 monolayersagreed well with this report. This indicates that the surface ofthe C12, C14, and C16 monolayers was densely covered withmethyl groups.

XPS scans of the monolayers with different alkyl chain lengthsshowed signals originating from Si, C, and O. The C 1s/Si 2pratio monotonically increased with increasing alkyl chain length.XPS spectra were acquired in narrow ranges to obtain moredetailed information on the alkylated silicon surfaces. Figures4 and 5 show XPS C 1s and Si 2p spectra, respectively. The XPSC 1s spectra showed a peak located at 285.0 eV, which is assignedto C-H34 bonding. There was an inflection point at 283.8 eVin the spectra, indicating a peak at this binding energy. This peakmay correspond to a chemical bonding state originating fromSi-C bonds.35 The Si 2p spectra of the C10-C16 monolayerswere nearly identical to that of the hydrogen-terminated Si(111)surface. Although silicon oxide was hardly detected in the narrowspectra of these monolayers, little silicon oxide might be formedon the Si(111) surface. However, the C 1s spectra revealed thatthe hydrogen-terminated Si(111) surface was chemically pas-sivated by the C10-C16 alkyl monolayers. These results providestrong evidence that the alkylation of the hydrogen-terminatedsilicon substrate was accomplished. In contrast, two peaks wereclearly observed in the Si 2p spectrum of the C18 monolayer.One was a strong peak at 99.3 eV attributed to metallic silicon,while the other was a weak peak at 103.4 eV attributed to siliconoxide. This indicates that silicon oxide formed on the siliconsubstrate. The presence of this silicon oxide would inhibit theformation of the alkyl monolayer and may have led to the lowerpacking density of the C18 monolayer. From another respect,the reaction temperature of 160°C was not high enough to preparea highly packed C18 monolayer. The water contact angle andXPS results indicate that C12, C14, and C16 monolayers havea comparably higher quality, while the quality of C10 and C18monolayers is worse.

Table 1 shows ellipsometrically determined thicknesses ofthe alkyl monolayers. The film thicknesses monotonicallyincreased with increasing alkyl chain length. The film thicknessesof the C10-C18 monolayers were 1.2, 1.3, 1.5, 1.6, and 1.9 nm,respectively. We also calculated the molecular length bysemiempirical molecular orbital (MO) calculation using the AM1Hamiltonian. Here, the calculation was made on (CH3)(CH2)n-1-SiH3 for simplicity. The molecular chain length was defined asthe distance between the Si atom and the C atom of the methylgroup. The film thicknesses were obtained by considering boththe molecular chain lengths and the chain tilt angles. Thesecalculations showed that the film thicknesses of the C10, C12,C14, C16, and C18 monolayers were 1.0, 1.2, 1.4, 1.6, and 1.8nm, respectively. The measured film thicknesses were greaterthan the calculated ones. However, this difference is veryreasonable since it is likely due to contributions from adventitiouscarbon and the chain tilt angle. We applied a value of 1.465 asthe refractive index to the alkyl monolayers. However, therefractive index value might have been considerably influencedby both the surface coverage and the chain length. Thus, it wasnecessary to confirm the measured ellipsometric thicknesses withother analytical data. We therefore measured the X-ray reflectivity(XR) of the monolayers to precisely determine their film thickness.XR measurements are intrinsically more precise than ellipsometry,since the X-ray probe has a wavelength comparable to themonolayer thickness. Figure 6 shows the XR profiles for ahydrogen-terminated silicon surface and for the alkyl monolayerswith different chain lengths. The first minimum seen in the profileswould originate from destructive interference between X-rayreflections from the top and bottom regions of the alkyl

(34) Saito, N.; Youda, S.; Hayashi, K.; Sugimura, H.; Takai, O.Chem. Lett.2002, 31, 1194.

(35) Muehlhoff, L.; Choyke, W. J.; Bozack, M. J.; Yates, J. T., Jr.J. Appl.Phys.1986, 60, 2842.

Figure 3. Static contact angles of water on the alkyl monolayersas a function of the linear alkyl molecular chain length,n. n indicatesthe number of carbon atoms in the alkyl monolayer (CnH2n+1).

Figure 4. XPS C 1s spectra of alkyl monolayers directly attachedto Si obtained by XPS.

9964 Langmuir, Vol. 22, No. 24, 2006 Ishizaki et al.

monolayers. The value ofqz for the first minimum in the XRcurves was defined asqz,min. The film thickness,L, was calculatedusing the equationL ) π/qz,min.28 At grazing incidence, thereflectivity for all the samples was high, although it rapidlydecreased as the angle between the surface and the incident X-raybeam increased. Unlike the smooth Fresnel-like decay in the XRprofile of the hydrogen-terminated Si(111) surface, all the alkylmonolayers showed a local minimum in reflectivity at theqz of1-3. Compared to the hydrogen-terminated Si substrate, drasticdecays in reflectivity were observed with the alkyl monolayers.The film thicknesses of the C10-C18 monolayers weredetermined to be 1.11, 1.28, 1.47, 1.63, and 1.81 nm, respectively,as shown in Table 1. These values agree well with theellipsometrically calculated ones.

To roughly estimate the chain tilt angles of the alkylmonolayers, the thicknesses obtained through XR measurements

were corrected for the contribution of adventitious carbon,assuming from the XPS results that it was constant at ap-proximately 0.3 nm for the series of alkyl monolayers.5 Thechain tilt angles of the C10-C18 monolayers derived from thecorrected film thicknesses were 43.1, 40.0, 37.3, 35.3, and 33.5°,respectively. The predicted chain tilt angles on the basis of MOcalculation were 41.6, 40.8, 40.3, 39.9, and 38.2°, respectively.The respective differences were less than 5°. Assumptions madein the estimations would be taken into consideration; however,all the tilt angles for the alkyl monolayers derived from the XRdata were very close to those reported derived from IRmeasurements.18

GIXD measurements were performed to evaluate the in-planearrangement of the alkyl monolayers. Figure 7 shows GIXDpatterns of the alkyl monolayers with different alkyl chain lengths.If the alkyl monolayer has a periodic molecular arrangement, apeak corresponding to that periodic arrangement would appearon its GIXD pattern at the wave vectorqxy between 14 and 16nm-1. However, no peak related to periodic arrangement wasobserved in the pattern of any of the alkyl monolayers regardless

Figure 5. XPS Si 2p spectra of alkyl monolayers directly attachedto Si obtained by XPS.

Table 1. Physical Properties of Alkyl-Terminated Si(111)Surfaces

-R

observedellipsometric

thickness(Å)

analytical thicknessfrom XR measurement

(Å)

calculatedfilm thicknessa

(Å)

-C10H21 12.4( 2 11.1 10.2-Ì12H25 13.0( 1 12.8 12.2-C14H29 15.2( 1 14.7 14.2-C16H33 16.4( 1 16.3 16.2-C18H37 19.2( 2 18.1 18.2

a Film thickness was calculated on the basis of the assumption thatthe alkyl groups are robustly attached to the silicon surface and theSi-C bonds are normal to the surface.

Figure 6. X-ray reflectivity profiles of (a) hydrogen-terminated Siand the alkyl monolayers directly attached to Si: (b) C10, (c) C12,(d) C14, (e) C16, and (f) C18.

Figure 7. GIXD profiles for the alkyl monolayers directly attachedto Si: (a) C10, (b) C12, (c) C14, (d) C16, and (e) C18.

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of the alkyl molecular chain length. This indicates that all thealkyl monolayers directly attached to the Si substrate hadamorphous structure. This is most likely due to low in-planeorientational order, that is, the presence of defects in the alkylmonolayer directly attached to the Si substrate, since such defectsinhibit the formation of a lattice with long-range periodicity.Matthew et al. reported that actual organic monolayers couldhave a significant number of such defects.18 In addition, the loworientational order would lead to low conformational order,resulting in thepresenceofgauchedefects.Thesestructural defectscould arise from two factors as follows: (i) a low quality of theH-Si substrate, initially having a high inhomogeneity and ahigh density of defects, and (ii) a low mobility of the physisorbedmolecules, which cannot assemble in a proper way after they geta grip onto the substrate. In addition, the van der Waals radiusof a methylene unit is sufficiently large. Thus, packing an alkylchain onto every Si atom would be highly unlikely due to thesteric hindrance.

On the other hand, self-assembled monolayers (SAMs), suchas octadecyltrichlorosilane (OTS)-SAM, are known to becomposed of a lattice with long-range periodicity. The Si-Cbonds in alkyl monolayers are more rigid than the Si-O bondsin alkylsilane SAMs. Moreover the siloxane layer under the OTS-SAM is amorphous with some degree of flexibility. Due to thisflexibility, the formation of the OTS monolayer is influenced byvan der Waals attraction between the alkyl chains. This is theorigin of “self-assembling.” In the case of our alkyl monolayers,the structure at the interface between the organic monolayer andthe silicon substrate was mainly determined by the Si-C-Cbond angle. This bond angle remained approximately constantsince it originated from the sp3 chemical bonding state. Theresults of our tilt angle evaluation agree with the molecular bond

angle. Thus, the agreement originates from the rigid Si-C-Cbond. On the other hand, there are many gauche defects in thealkyl monolayers due to the rigid bonding, low quality ofhydrogen-terminated silicon substrate, and/or low mobility ofphysisorbed molecules although the defects decrease with theincrease of the number of carbon atoms. These factors, whichlead to the structural defects, prevent the adsorbed molecules onthe surface from self-assembling transforming the arrangementsduring the formation process unlike the organosilane SAMs.

ConclusionAlkyl monolayers with different molecular chain lengths from

C10 to C18 were prepared at 160°C on hydrogen-terminatedsilicon substrates through a radical-initiated reaction. The effectof the chain length on the alkyl monolayers was investigated bywater contact angle measurement, ellipsometry, X-ray reflectivity,X-ray photoelectron spectroscopy, and grazing incidence X-raydiffraction. XPS and ellipsometry indicated that the thickness ofthe alkyl monolayers monotonically increased with increasinglength of the alkyl chains. Water contact angle measurementsindicated that the alkyl monolayers from C12 to C16 were verydensely packed, whereas GIXD measurements showed no peakrelated to periodic arrangement for any of the alkyl monolayers.This disordered in-plane structure could be due to the rigid Si-Cbonding, low quality of hydrogen-terminated silicon substrate,and/or low mobility of physisorbed molecules.

Acknowledgment. This study was partially supported by theMinistry of Education, Science, Sports and Culture, Grant-in-Aid for Young Scientists (B) (No. 17760577), and the AichiScience and Technology Foundation.

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