Ss

YC

a

ARRAA

KSSE

1

sdtptiih�toctntUis

t

0h

Applied Surface Science 263 (2012) 73–78

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

elf-assembly of dendronized non-planar conjugated molecules on a HOPGurface

ang Yang, Xinrui Miao ∗, Gang Liu, Li Xu, Tingting Wu, Wenli Dengollege of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China

r t i c l e i n f o

rticle history:eceived 18 July 2012eceived in revised form 31 August 2012ccepted 31 August 2012vailable online 13 September 2012

eywords:elf-assembly

a b s t r a c t

Two-dimensional self-assembly of a series of dendronized molecules with different functional groupswere observed on the highly oriented pyrolitic graphite (HOPG) surface by scanning tunneling microscopy(STM). The solution evaporation of these molecules on HOPG surfaces under ambient conditions resultsin the formation of self-organized monolayers. STM images demonstrate that the dendronized conju-gated moiety in these molecules all adopt an edge-on arrangement on the HOPG surface, revealing theimportance of �–� stacking interactions. The molecules with a hydroxyl as the substituent group adsorbon the HOPG surface with an ordered lamellar nanopattern resulting from the intermolecular hydrogen

canning tunneling microscopydge-on

bonding. The molecules without alkyl chain are perpendicular to the HOPG surface, in a face-to-facecard-stack fashion by �–� stacking. Alkyl chain assisted molecules adsorb on the HOPG surface withone side chain arrangement by tail-to-tail fashion. Our results demonstrate that the balance betweendifferent molecule–molecule and molecule–substrate interactions can be easily influenced by a smallstructural change in one of the components of the supramolecular assemblies resulting in different

solid

organized patterns on the

. Introduction

It is now full recognized that, in addition to the chemicaltructure of the molecules, the two-dimensional (2D) or three-imensional (3D) supramolecular organization and morphology inhe solid states has a significant influence on those opto-electronicroperties [1–4]. Therefore, solution processing and depositionechniques have been developed to improve the molecular ordern organic thin films [2,5,6]. Dendronized conjugated moleculesn terms of unequivocal structure and the luminous propertiesave been investigated [7–9]. The aggregates obtained from the–� interactions between conjugated molecules provide charge

ransport pathways and thus can be used as an active material inptoelectronic devices. In order to better understand the complexharge transport processes in thin films, a detailed observation ofhe morphology of the active layer is highly desirable. Scanning tun-eling microscopy (STM) has proven to be especially well suitedo investigate molecular arrangement on solid surfaces [10–13].p till now, to our knowledge, the surface structure and order-

ng of non-planar conjugated molecules have not been extensively

tudied or imaged with STM [14].

Herein, with the aim of developing self-assembled regulationhat combine the advantages of dendronized conjugated molecules,

∗ Corresponding author.E-mail address: msxrmiao@scut.edu.cn (X. Miao).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.08.124

surface.© 2012 Elsevier B.V. All rights reserved.

we have synthesized a series of molecules as shown in Scheme 1,and investigated the self-assembly on the HOPG surface at thegas/solid interface by STM. To probe the strength of the interactionsbetween an adsorbate with a particular lattice, the functionali-ties of the molecule may likewise be varied from the groups thathave different interactions between the adsorbate–adsorbate andadsorbate–substrate. Moreover, higher ordered structures at inter-faces have been realized by changing the nature of the pendantsubstituent group. The increase in the bulkiness of the molecules,due to adding one or two C18H37 tails when compared to D1,should have implications on the 2D mobility of the molecules onthe surface and influence the relationship of the ordered adlayersto the HOPG substrate. We find that self-assembly of the newlydesigned molecules on the HOPG surface indeed leads to differ-ent well-ordered stacks as revealed by our detailed studies usingSTM. The edge-on orderings of all the molecules indicate that theenergy transfer property is associated with the molecular self-assembled morphologies. The STM results can contribute to anincreased understanding in issues related to the intermolecularinteractions (packing) between molecules and the performance offor instance photovoltaic devices.

2. Experimental

Dendronized conjugated molecules used in this study weresynthesized by Gang Liu and provided as a gift. All experimentswere performed at ambient conditions using a Nanoscope IIIa

74 Y. Yang et al. / Applied Surface Science 263 (2012) 73–78

D1, R = H

D , R = C H

OR

RO

D3, R = C18H37

OR

y

x2.0 nm

1.5

nm

onized

MnmwdafAwoi

bsd

3

sfi

Fp

2 18 37

Scheme 1. Chemical structures of dendr

ulti-mode SPM (Digital Instruments, Santa Barbara, CA). Scan-ing tunneling microscopy (STM) was operated in constant-currentode. STM tips were prepared by mechanical cutting from Pt/Irire (80:20, diameter 0.2 mm). Prior to image, the molecule wasissolved (concentration = ∼10−6 M) in dichloromethane solventnd a drop of this solution was applied on a freshly cleaved sur-ace of HOPG (quality ZYB, Digital Instruments, Santa Barbara, CA).fter the solvent evaporated, the stable physisorbed monolayersere spontaneous formed. Then the STM experiment was carried

ut. The images were corrected for drift using the recorded HOPGmages for calibration purposes.

Molecular models of the observed assembled structure wereuilt by Materials Studio 4.4. The model of monolayer was con-tructed by placing the molecules according to the intermolecularistances and angles obtained from the analysis of STM images.

. Results and discussion

D1 molecule consists of a conjugated moiety and a hydroxylubstituent. The self-assembly of such molecule on the HOPG sur-ace is expected to be governed by a delicate balance betweenntermolecular interactions and molecule–substrate interactions.

ig. 1. High-resolution STM image of the D1 monolayer on HOPG surface. It = 480 mV, Vb

attern and enlarged structural model showing hydrogen bonds by green dashed lines. (c

conjugated molecules (D1, D2, and D3).

Hydroxyl substituent on the phenyl ring possible involves in thehydrogen-bonding between adsorbed molecules. Fig. 1a shows asub-molecularly resolved STM image of D1 self-assembled at thegas/HOPG interface. The molecules form a lamellar structure andadopt an edge-on arrangement, where the aromatic discs are nearlyoriented perpendicularly to the surface plane. The edge-on patternsuggests the �–� stacking from the conjugated groups.

It is known that intermolecular spacing is about 0.4 nm along�–� stacking directions in three-dimensional (3D) crystals [15].The slightly larger intermolecular spacing (0.50 ± 0.05 nm) sug-gests that the conjugated moieties are tilted from the HOPG surfaceand adopt an edge-on arrangement due to the stronger �–� inter-actions between the conjugated groups as well as O. . .H hydrogenbonding (Fig. 1b and 1c). Hydrogen-bonds depicted in the mag-nified image of Fig. 1b cause D1 molecules to be connected eachother in one lamella, which make the columns more easily beingconstructed and effectively stabilized. The unique characteristics ofD1 hence manifest the significant contributions of �–� interactions

and hydrogen bonding to the formation of the nearly defectless tec-tonic assembly. The lamellar width is (2.0 ± 0.1) nm, which is in theapproximation of a rectangular shaped molecule packed edge-oncalculated by Materials Studio. Note that all D1 molecules in Fig. 1a

ias = 674 mV. (b) Schematic model of the D1 monolayer adopting edge-on stacking) Side-view model of individual D1 molecule on the HOPG surface.

rface S

att[p

jctetimcDcs

ootTcpai

Fe

Y. Yang et al. / Applied Su

dsorb on the HOPG surface along its x direction. The ability of elec-ron tunneling to occur through a layer that is more than 1.5 nm inhickness highlights the remarkable conductivity of the molecules16]. It can be concluded that the hydroxyl group on the phenyl ringlays an important role in the 2D organization of D1.

To probe the competitive effect between �–� stacking of con-ugated moieties together with van der Waals interactions of sidehains on the self-assembled morphology, the adsorption of D2 onhe HOPG surface was investigated by STM. Because of the pres-nce of one C18-alkyl chain on the conjugated core, it was expectedhat the lateral intermolecular interactions of the long chain and itsnteraction with the hydrophobic HOPG substrate would drive the

olecules to form well-ordered monolayer assemblies. Two con-entration ranges (high 10−5 M to 10−6 M, low 10−6 M to 10−7 M) of2 after the solvent evaporation were investigated. We find that theoncentration influences the 2D self-assembly of D2 on the HOPGurface.

Interestingly, during the initial scans an STM image wasbtained at relative high concentration showing the coexistencef different molecular domains (Fig. 2a). The domains consist ofwo different polymorphs which are labeled 1 and 2 on the image.he structural transition from polymorph 2 to polymorph 1 underontinuous scanning was observed as shown in Fig. 2a and b. This

henomenon can be explained that in the presence of a disturb-nce by a STM tip, the molecules are subjected not only to the �–�nteractions between the conjugated moieties but also to the van

ig. 2. (a and b) Consecutive STM images of D2 at the gas-HOPG interface under relative hdge-on stacking pattern. It = 450 pA; Vbias = 620 mV. (d) Suggested model for the D2 mole

cience 263 (2012) 73–78 75

der Waals interactions between the side chain and substrate. Thecompetition of two kinds of factors forces the molecules to adopt anedge-on arrangement, and orient them in its direction. At the sametime, it could possibly be attributed to a conformational change ofthe conjugated cores from non-planar to planar, which causes achange by the �–� staking of the conjugated cores. Hypothesizingthat the voltage at the STM tip induces the structural transition of D2and electrical fields could be used to increase the order and controlthe arrangement of the molecules on the surface [17,18]. Further-more, we suggest that the competition of the molecule–substrateinteractions and the �–� staking interactions plays another keyrole in determination of the structural transition. The columnaroligomers would act as seeds for the structural transition.

The strips can be attributed to D2 columns within ordereddomains labeled 1 in Fig. 2a. From the highly resolved STM image(Fig. 2c), the spacings within the strips and perpendicular tothem are (0.43 ± 0.05) nm and (2.1 ± 0.1) nm, respectively. Theyare attributed to �–� stacked conjugated cores, assembled inan edge-on arrangement along x direction of conjugated groupon the HOPG surface. No side chain adsorbs on the surface. Theformation mechanism of the D2 nanocolumn would then be theformation of individual aggregates followed by their adsorptionon the surface and subsequent self-assembly into long uniaxial

columns. The molecular shift in one lamella reflects the kineti-cally driven nature of the adsorption process for the D2 moleculecompared to the thermodynamically favored pairwise interaction

igh concentration (∼10−5 M). (c) Typical high resolution STM image of D2 adoptingcule showing the edge-on pattern.

76 Y. Yang et al. / Applied Surface Science 263 (2012) 73–78

F er lowS struc

oalcgs

caittFtsa

kwiaics

ttsttcDas

ig. 3. (a) Large-scale STM image of the self-assembly of D2 on HOPG surface undTM image showing the self-assembly of D2. It = 375 pA; Vbias = 659 mV. (c) Proposed

f adjacent molecules. Unlike the seemingly limitless and straightrray of D1, the assembly of D2 appear dislocate and the out-ine of the conjugated moiety appears jagged and somewhaturvy. The results indicate the packing pattern of the conju-ated group in different derivatives is strongly dominated by theubstituent.

Note that when the concentration of D2 molecule is low (typi-ally <10−6 M), column patterns are not observed, probably becauseggregates do not form anymore in dichloromethane. So far theres no experimental evidence that such molecule aggregates in solu-ion at this concentration. It is therefore reasonable to assume thathe self-assembly of D2 columns occurs on the surface. As shown inig. 3a, only the lamellar structure is observed under low concentra-ion. Some apparent disordered regions exist among the domains,howing a highly dynamic feature of the monolayer [19] or nodsorbed molecules.

High resolution STM images as shown in Fig. 3b reveals twoinds of ordered row structures with different widths. They arrangeith each other alternately. Row structures are a result of the

ncreasing influence of tails interactions both between other tailsnd between the tails and substrate. The importance of these tailnteractions increases as one alkyl chain is added, resulting in ahange from edge-on orientation to row structures on the HOPGurface.

As is well known, the STM image contrast is dominated by bothopographic and electronic coupling factors. For the D2 molecule,he conjugated moieties are identified as bright contrast protru-ions in an STM image since it has a larger electronic coupling thanhe methylene groups, reflecting high electron densities aroundhem [15]. Therefore, in the narrower double-lamella structure, we

an assign the bright protrusions to the conjugated moieties of the2 molecule. Adjacent conjugated moieties in different lamellaere arranged staggered by shifting by half of the intermolecularpacing due to intermolecular lateral repulsion. The length of the

concentration (typically <10−6 M). It = 420 pA; Vbias = 580 mV. (b) High-resolutiontural model of the D2 adlayer.

bright strip is about 1.2 nm, which indicates the conjugated groupof D2 stacks along its y direction on the HOPG surface. The high-resolution image (Fig. 3b) reveals the conjugate moieties as thinlines 1–2 A in width and ∼15 A in length, with interlamellar con-jugated moiety distances of 3–4 A. In the wider double-lamellaribbon structure, individual strips exhibit a darker image contrast.The average length of the strips is 2.1 ± 0.05 nm, being in agree-ment with the length of octadecyl group. In the lamella, the sidechains arrange with their long axes parallel to each other. Theconjugated moieties in the narrower lamellae rotate ∼60◦ withrespect to the alkyl side chain in the wider lamellae. Further-more, unlike the long-chain alkane self-assembled monolayer, nodistinct interlamellar trough is observed between two lamellae.The methyl groups at the ends of the molecular chains betweentwo lamellae tend to maximally approach each other to enhancetheir interactions [20]. One possible explanation for this assem-bly of D2 is that the side chains lie flat on the surface and theconjugated moieties stand on edge, perpendicular to the HOPGsurface, in face-to-face card-stack fashion. If the conjugated moi-ety adsorbs as a straight rod, the alkyl chains can adsorbed onthe underlying substrate, thus maximizing the adsorption energy[21]. This packing model is shown in Fig. 3c. The resulting struc-ture has possibility of enjoying a strong �–� interaction of theconjugated cores as well as significant tail–graphite and tail–tailinteractions.

In the case of the D3 with two C18-alkyl side chains, the 2Dadsorbed structure is similar with that for the more symmetricmolecule (D2) with one side chain. Fig. 4a and b presents typicalSTM images of D3 adlayer on the HOPG surface at various scales.The large-scale STM image shows the self-assembled structure with

the domain of several hundred nanometers. A visible dark troughexists in the middle of the double lamellae.

High-resolution STM (Fig. 4b) obtained by scanning indicatetwo kinds of ordered ribbon structures with different widths. The

Y. Yang et al. / Applied Surface Science 263 (2012) 73–78 77

F (b) him of brig

aeFsfis(sstidobatact

mepoiaoaOcapp

ig. 4. STM images of the self-assembly of D3 on HOPG surface. (a) Large-scale;olecular lamella structure. (d) Surface profile indicated in (b) showing the length

lkyl chains in the dark lamellae tilt from the conjugated moi-ties in the bright lamellae by an angle of 40 ± 1◦ as depicted inig. 4b. The slightly larger intermolecular spacing (0.50 ± 0.05 nm)uggests that the conjugated moieties are tilted from the HOPG sur-ace and adopt an edge-on arrangement due to the stronger �–�nteractions between the molecular conjugated groups. From theymmetric shape of the molecule in the high-resolution STM imageFig. 4b), it can be concluded that stacks mostly lie inclined on theurface, in which the conjugated plane is not perpendicular to theubstrate surface, but rather is titled from it at an angle of 70◦. Thisitled packing feature of D3 molecule is schematically illustratedn Fig. 4c. The angle between the conjugated moieties axis and theirection of the lamella is almost 90◦ in contrast to the 60◦ anglebserved for D2. Moreover, the distance between the two adjacentright lamellae is larger than that of D2, which demonstrates thatnother side chain of D3 is protruding from the surface and directedoward the gas phase at the position. The length of the bright strips the red arrows indicated in Fig. 4d is about 1.2 nm, which indi-ates the conjugated group of D3 also stacks along its y direction onhe HOPG surface.

Our results demonstrate that the balance between differentolecule–molecule and molecule–substrate interactions can be

asily influenced by a small structural change in one of the com-onents of the supramolecular assemblies resulting into differentrganized patterns on the solid surface. The molecular �–� stack-ng is so strong that the conjugated groups in different moleculesll adopt the edge-on pattern on the HOPG surface. The changef substituent group of dendronized molecule influences the self-ssembled pattern due to different intermolecular interactions.wing to the presence of one C18-alkyl chain on the conjugated

ore, the molecule and substrate interactions are enhanced. Inddition, molecular dynamic simulations of ordered alkane chainshysisorbed on HOPG surface demonstrated that a subtle inter-lay of packing and entropic effects dominated the orientation and

gh-resolution. It = 350 pA; Vbias = 620 mV. (c) A possible packing pattern of the D3

ht strips.

order of the zigzag carbon skeletons of the alkane molecules rela-tive to the graphite surface [22]. The chain dynamics in combinationwith the high packing density may cause communal titling of themolecular axes inside a lamella. It is probably the reason why theangle is 60◦ and 40◦ between the side chain and the conjugatedmoiety of D2 and D3, respectively.

4. Conclusions

We have observed the self-assembly of a series of dendronizedconjugated molecules on the HOPG surface by STM. Due to theexistence of a hydroxyl group, an ordered edge-on structure ofD1 is formed due to the hydrogen bonding. The presence of oneC18-alkyl chain on the conjugated core in D2 and D3 results in a uni-form lamellar nanopattern due to the van der Waals interactionsbetween the side-chain and the substrate. Note that the conjugatedmoiety in different molecules all adsorbs on the HOPG surface withthe edge-on pattern. We believe that our observed system perhapsgive a guidance to future work on the fabricated monolayer of otherphotovaltaic materials.

Acknowledgements

Financial supports from the National Program on Key BasicResearch Project (2012CB932900 and 2009CB930604), the NationalNatural Science Foundation of China (21103053, 91023002, and51073059), the Fundamental Research Funds for the Central Uni-versities (2011ZM0004) are gratefully acknowledged.

References

[1] P. Keg, A. Lohani, D. Fichou, Y.M. Lam, Y. Wu, B.S. Ong, S.G. Mhaisalkar, Macro-molecular Rapid Communications 29 (2008) 1197.

7 rface S

[[

[

[

[

[

[

[

[

[

[

8 Y. Yang et al. / Applied Su

[2] A.L. Briseno, J. Aizenberg, Y.J. Han, R.A. Penkala, H. Moon, A.J. Lovinger, C. Kloc,Z.A. Bao, Journal of the American Chemical Society 127 (2005) 12164.

[3] A.M. Gao, X.R. Miao, J. Liu, P. Zhao, J.W. Huang, W.L. Deng, ChemPhysChem 11(2010) 1951.

[4] V. Palermo, P. Samorì, Angewandte Chemie International Edition 46 (2007)4428.

[5] J.L. Chen, V. Leblanc, S.H. Kang, P.J. Benning, D. Schut, M.A. Baldo, M.A. Schmidt,V. Bulovic, Advanced Functional Materials 17 (2007) 2722.

[6] X.R. Miao, L. Xu, Z.M. Li, W.L. Deng, Journal of Physical Chemistry C 115 (2011),3358.

[7] L.-H. Chan, R.-H. Lee, C.-F. Hsieh, H.-C. Yeh, C.-T. Chen, Journal of the AmericanChemical Society 124 (2002), 6469.

[8] L.H. Chan, H.C. Yeh, C.T. Chen, Advanced Materials 13 (2001) 1637.[9] J. Dai, S.W. Chang, A. Hamad, D. Yang, N. Felix, C.K. Ober, Chemistry of Materials

18 (2006), 3404.

10] S. De Feyter, Nature Chemistry 3 (2011) 14.11] X. Zhang, T. Chen, H.-J. Yan, D. Wang, Q.-H. Fan, L.-J. Wan, K. Ghosh, H.-B. Yang,

P.J. Stang, ACS Nano 4 (2010) 5685.12] X.R. Miao, L. Xu, Y.J. Li, Z.M. Li, J. Zhou, W.L. Deng, Chemical Communications

46 (2010), 8830.

[

[

cience 263 (2012) 73–78

13] X.R. Miao, L. Xu, C.Y. Liao, Z.M. Li, J. Zhou, W.L. Deng, Applied Surface Science257 (2011) 4559.

14] K.S. Mali, D. Wu, X. Feng, K. Mullen, M. Van der Auweraer, S. De Feyter, Journalof the American Chemical Society 133 (2011) 5686.

15] M. Mas-Torrent, P. Hadley, S.T. Bromley, N. Crivillers, J. Veciana, C. Rovira,Applied Physics Letters 86 (2005), 012110.

16] M.C. Lensen, J.A.A.W. Elemans, S.J.T. van Dingenen, J.W. Gerritsen, S. Speller,A.E. Rowan, R.J.M. Nolte, Chemistry – A European Journal 13 (2007) 7948.

17] A. Cristadoro, M. Ai, H.J. Rader, J.P. Rabe, K. Mullen, Journal of Physical ChemistryC 112 (2008) 5563.

18] L. Piot, C. Marie, X.L. Feng, K. Mullen, D. Fichou, Advanced Materials 20 (2008)3854.

19] P.A. Lewis, Z.J. Donhauser, B.A. Mantooth, R.K. Smith, L.A. Bumm, K.F. Kelly, P.S.Weiss, Nanotechnology 12 (2001) 231.

20] M. Zhao, P. Jiang, K. Deng, S.S. Xie, G.L. Ge, C. Jiang, Nanotechnology 20 (2009)

1.

21] S.S. Sheiko, F.C. Sun, A. Randall, D. Shirvanyants, M. Rubinstein, H. Lee, K. Maty-jaszewski, Nature 440 (2006) 191.

22] R. Hentschke, B.L. SchOrmann, J.P. Rabe, Journal of Chemical Physics 96 (1992)6213.