stm investigation of the dependence of alkane and alkane (c18h38, c19h40) derivatives self-assembly...

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Surface Science 602 (2008) 1256–1266

STM investigation of the dependence of alkane and alkane(C18H38, C19H40) derivatives self-assembly on molecular

chemical structure on HOPG surface

Qing Chen a,b, Hui-Juan Yan a, Cun-Ji Yan a, Ge-Bo Pan a, Li-Jun Wan a,*,Guo-Yong Wen a, De-Qing Zhang a,*

a Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, Chinab Graduate School of CAS, Beijing 100064, China

Received 7 December 2007; accepted for publication 25 January 2008Available online 6 February 2008

Abstract

Alkane molecules and their derivatives are composed of alkyl chain and functional groups that dominate the interactions of intermol-ecule and molecule/substrate and result in different self-assemblies on solid surface. In the present paper, the self-assemblies of alkanesand alkane (C18H38, C19H40) derivatives are studied on HOPG surface at room temperature in ambient conditions by scanning tunnelingmicroscopy (STM) to understand the structure and stability of these self-assemblies and to reveal the dependence of their self-assemblieson their chemical structures. It is found that the alkane molecules with short alkyl chains such as tridecane (C13H28), tetradecane(C14H30) and pentadecane (C15H32) can form lamellar structures on HOPG and be steadily imaged by STM. On the other hand, linearor herringbone structures can be observed in the self-assemblies formed by a series of derivatives of C18H38 and C19H40. The results arediscussed based on STM results to understand the structures and structural stability of alkane molecular self-assemblies.� 2008 Elsevier B.V. All rights reserved.

Keywords: Alkanes; Alkyl derivatives; Self-assembly; Structure and stability; Scanning tunneling microscopy (STM)

1. Introduction

As a natural, spontaneous and effective process in theformation of materials and living organisms, self-assemblyis widely applied in fabricating supramolecular architec-ture, materials science, crystal engineering and surfacenanostructure [1–3]. In particular, two-dimensional (2D)self-assembly of organic molecules on solid surfaces hasbeen well studied during past decades [4–9]. Among vari-ous molecules, alkanes and their derivatives (CH3(CH2)nX,X = CH3, OH, COOH, Cl, Br, I, NH2, SH) are employedas model systems for the study of 2D molecular self-assem-bly on HOPG and other solid surfaces [10–16]. The alkylmolecules are saturated linear molecules with carbon atoms

0039-6028/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2008.01.030

* Corresponding authors. Tel./fax: +86 10 6255 8934.E-mail address: [email protected] (L.-J. Wan).

arranged in a zigzag structure. The length of C–C–C zigzagis measured to be 0.251 nm (see Fig. 1). These linear com-pounds are typical molecules with stable and simple con-formation in their chemical structures and are wellstudied as model systems. As a result of the study, it isknown that the formation of the self-assembly is domi-nated by the interactions including van der Waals force,hydrogen bonding and dipole–dipole interaction [17–19].

Scanning probe microscopy (SPM) such as scanningtunneling microscopy (STM) and atomic force microscopy(AFM) is a powerful tool for characterizing surface struc-tures with submolecular or atomic resolution [20,21]. Byusing SPM, information about molecular arrangement, ori-entation and stability of n-alkanes and alkyl derivatives onsolid surfaces can be achieved [22–24]. Our group previ-ously reported that the alkane derivatives with alkanoland thiol groups form ordered monolayers on HOPG at

mailto:[email protected]

Fig. 1. Schematic diagram of alkane molecules (C18H38) adsorbed on HOPG. The alkanes lie in all-trans geometry with the molecular backbone planeparallel to the graphite surface. The length of C–C–C zigzag is a = 0.251 nm. The hollow distance of graphite lattice is b = 0.246 nm.

Q. Chen et al. / Surface Science 602 (2008) 1256–1266 1257

ambient conditions [17,25,26]. However, although the re-sults are accumulated, it is lack of a systematic study onthe self-assemblies of alkanes and their derivatives. There-fore, it is important to achieve comprehensive and system-atic information on alkane and their derivatives assembliesand understand the effect of molecular structure on molec-ular self-assemblies.

Recently, we have performed a series of experiments onthe self-assembly of alkanes and alkane (C18H38, C19H40)derivatives on HOPG at room temperature in ambient con-dition. The experiments are concentrated on (1) the struc-ture and stability of alkane molecular self-assembly and(2) the effect of chemical structure of alkane (C18H38,

C19H40) derivatives on their self-assemblies. The alkanemolecules were found to pack side by side in adjacent lam-ellas with an angle of 90� between the molecular long axisand the row direction. The inter-distance between theneighboring lamellas is quantificationally proposed. The ef-fect of functional groups and chain length on the structuresof alkane derivatives has been investigated. It is found thatthe alkane molecules with short alkyl chains such as tridec-ane (C13H28), tetradecane (C14H30) [27] and pentadecane(C15H32) can also adsorb orderly on HOPG and be steadilyimaged by STM at room temperature in ambient condi-tion. All n-alkanes and their derivatives investigated inthe present paper are briefly listed in Table 1. Althoughthe experiment is limited in these molecules, the influenceof chemical structure on their self-assemblies is clearly ob-

Table 1A summary of the alkyl molecules investigated

Alkanes C12H26, C13H28, C14H30

C15H32, C18H38, C19H40 [28]

Functionalized alkyl molecules C18H37OH [44], C19H39OHC18H37Cl [29], C19H39ClC18H37Br, C19H39BrC18H37I [29]C18H37SH [26], C19H39SHC18H37NH2 [29]

served, which is an important preliminary step for fullyunderstanding and predicting 2D self-assembly.

2. Experimental section

Tetradecane (C14H30, 99%), pentadecane (C15H32, 99%)and octadecane (C18H38, 99%) were purchased from Acros.1-Bromooctadecane (C18H37Br, 96%) was purchased fromAldrich. 1-Bromononadecane (C19H39Br, 97%) was pur-chased from Fluka. Dodecane (C12H26, 99%), tridecane(C13H28, 99%) and 1-nonadecanol (C19H39OH, 98%) werepurchased from TCI. All the compounds were used as re-ceived. C19H39X (X = Cl, SH) were synthesized fromC19H39OH.

The samples of C12H26, C13H28, C14H30, C15H32 andC18H38 for STM observation were prepared by directlydepositing a drop of the samples on freshly cleaved surfacesof HOPG (quality ZYB, Digital Instruments). As for theothers, the molecules were first dissolved into toluene(HPLC grade, Tianjin Jinke Chemical Factory) with a con-centration less than 10�4 M. Then, a drop of toluene solu-tion containing the corresponding molecules was depositedon HOPG. All the toluene solvent evaporated thoroughlybefore STM measurement.

STM experiments were performed with a NanoScope IIISPM (Digital Instruments, Santa Barbara, USA) at ambi-ent conditions. The tunneling tips were prepared bymechanically cutting Pt/Ir wire (90/10). All the images wererecorded in the constant-current mode. The specific tunnel-ing parameters are given in the corresponding figurecaptions.

3. Results

3.1. Molecules of alkanes (Cn: n = 12, 13, 14, 15, 18, 19)

In the past two decades, the films of n-alkanes ad-sorbed on solid surfaces have been studied by varioustechniques including LEED, neutron diffraction, X-rayscattering, IR and SPM. For instance, Firment and Som-

1258 Q. Chen et al. / Surface Science 602 (2008) 1256–1266

orjai studied normal paraffins and cyclohexane ontoPt(111) [30] and Ag(111) [31] by using LEED. The IRspectrum showed that normal paraffin formed monolay-ers and C7H16 to C14H30 might form 2–3 layers [32]. Re-cent efforts have been made by using STM to study theself-assembly of alkanes (usually in the range of Cn:16 6 n 6 50) on HOPG [22,28,33–38]. However, thereare few reports on STM study of the alkane self-assem-bly, whose alkyl chains are shorter than that of C16H34

at ambient conditions on HOPG. Recently, Chen et al.found that C14H30 might form ordered self-assembly onHOPG observed by STM [27]. These results encourageus to investigate whether n-alkanes with shorter alkylchains can form ordered and stable self-assembly. As aresult in the present study, it is found that C13H28,C14H30 and C15H32 molecules can form ordered mono-layers on HOPG and the so-prepared assemblies can beimaged by STM at room temperature, although we failedto get ordered monolayers of C12H26.

Fig. 2 shows typical STM images of C13H28 on HOPG.The molecules pack side by side in adjacent lamellas withan angle of 90� between the molecular alkyl chain axisand the molecular alignment row direction. The width ofeach row extracted from the image is ca. 1.78 nm, closeto the theoretical length of a fully extended C13H28

[35,36]. It is important to point out that below room tem-perature the herringbone structures was also observed formonolayers of shorter even-carbon-number alkanes andrectangular–centered structures for shorter odd-carbon-number alkanes by neutron as well as X-ray diffractionmeasurements [39,40]. The difference may be induced bythe different interactions between molecule and HOPGsubstrate at different temperature.

Efforts have also been made to investigate the self-assemblies of C14H30 and C15H32 on HOPG. Fig. 3 showstypical large-scale and high-resolution STM images ofC14H30 on HOPG. It is clear that a well-ordered monolayer

Fig. 2. Self-assembly of C13H28 on HOPG. (a) Large scale STM image. Vbias

It = 829 pA. A molecule is indicated by a white bar in (b).

has also been formed. The molecules are assembled in alamellar structure.

Fig. 4 shows typical STM image of the C18H38 mono-layer on HOPG, displaying a characteristic lamellar struc-ture as that in Figs. 2 and 3. Obviously, C18H38 adopts aside-by-side structure within the lamellas. The angle be-tween the molecular axis and the direction of lamella is90�. The width L of lamella is measured to be2.42 ± 0.1 nm, consistent with the theoretical length ofthe fully extended C18H38. In the STM image, a ‘‘half-mol-ecule width” shift in neighboring lamellas is also observedand labeled as ‘‘DL”. The measured DL is 0.22 ± 0.05 nm.The above values match very well with the theoretical re-sults (Fig. 1) within experimental errors and in good agree-ment with that of n-C32D66 obtained by neutron diffraction[41,42]. In addition, the arrangement of C19H40 (not shownhere) is the same as C18H38 previously reported in the liter-ature [28].

The adsorption and self-assembled structure of alkanesstrongly depends on the molecular chemical structure andHOPG substrate. From Groszek model, [32] it is knownthat every methylene in alkane molecules locates in a car-bon hexagonal ring of HOPG. On the basis of the self-assembled structure from STM result, a correlation be-tween L and the carbon atoms in an alkyl chain exists,when molecules pack side by side in adjacent lamellas withan angle of 90� between the molecular alkyl chain axis andthe molecular row direction. The relation can be describedas:

L ¼ nþ 2

2� 0:246 ðnmÞ ð1Þ

Here, L is the width of a lamella formed by alkane mol-ecules and n is the number of carbon atoms in an alkylchain. 0.246 (nm) corresponds to the hollow distance ofHOPG lattice [36]. In addition, with minor modification,

= 646 mV; It = 829 pA. (b) High-resolution STM image. Vbias = 646 mV;

Fig. 3. Self-assembly of C14H30 on HOPG. (a) Large scale STM image. Vbias = 740 mV; It = 412 pA. (b) High-resolution STM image. Vbias = 881 mV;It = 473 pA. A molecule is indicated by a white bar in (b).

Fig. 4. STM image of C18H38 adlayer on HOPG. Vbias = 802 mV;It = 672 pA. The displacement between the neighboring lamellae is labeledas ‘‘DL”.


this correlation can be extended for long chain alkyl deriv-atives such as fatty acids [43].

3.2. Alkane (C18H38, C19H40) derivatives

Previous studies on alkyl derivatives demonstrated thatdifferent functional groups resulted in different self-assem-bled structures on HOPG [29]. However, the length ofthe alkyl chains of these derivative molecules is differentand will affect their assemblies. To exclude the influenceof alkyl chains, we concentrated the self-assemblies of n-al-kane (C18H38, C19H40) derivatives with alkanol, halide,thiol and amine functional groups.

3.2.1. Alkanol

C18H37OH molecules form herringbone structure onHOPG reported previously [44]. Fig. 5a shows a schematicmodel for the C18H37OH adlayer on the basis of STMobservation [44]. For C19H39OH, we prepared an orderedself-assembly on HOPG shown in Fig. 5b. It can be seenthat the molecular axis in neighboring lamella are parallelto each other. However, there exists an angle of 60 ± 2� be-tween the molecular axis and lamellar direction. This struc-ture is different from the herringbone structure ofC18H37OH [44]. Interestingly, a trough can be seen in a la-mella in Fig. 5b. It indicates that alkanol molecules lie in a‘‘head-to-head” configuration on HOPG, forming hydro-gen bondings between adjacent hydroxyls. Moreover, theinfluence of temperature on the adlayer structures has beentaken into account. There is no structural transformationwhen the temperature lowers to 277 K [45,46].

3.2.2. Alkyl halides

Because of special electronic and geometric properties ofhalides, alkyl halides are of great importance in studyingSTM contrast mechanism of adsorbate. The polarity, elec-tronegativity, ionization energy, and atom radius changegradually from F to I. These properties have significant ef-fect on STM contrast as well as molecular self-assembly.The self-assemblies of C18H37Cl, C22H45Br and C18H37Iwere reported previously [29]. Herein, we have investigatedthe self-assemblies of C19H39Cl, C18H37Br and C19H39Br.

3.2.2.1. Cl-substituted alkanes. The self-assembly ofC18H37Cl (not shown here) is similar to that of n-alkanes[29]. The angle between the molecular axis and the rowdirection is 90�. The location of the chlorine atom cannot be discerned from STM images since similar contrastis observed on both ends of C18H37Cl.

Fig. 6a shows a typical STM image of C19H39Cl onHOPG. The chlorine atoms can not be distinguished from

Fig. 5. (a) A schematic model of C18H37OH adlayer structure on the basis of STM observation [45]. (b) STM image of C19H39OH on HOPG.Vbias = 650 mV; It = 790 pA. The trough is indicated by an arrow. (c) Proposed structural model for the C19H39OH self-assembly on HOPG shown in (b).


carbon atoms from image contrast difference. However, atrough can be seen in a molecular lamella. From this fea-ture it is deduced that the rows of C19H39Cl moleculesmay form a ‘‘head-to-head” configuration. On the basisof above discussion, a structural model is tentatively pro-posed in Fig. 6b for the ordered C19H39Cl adlayer.

3.2.2.2. Br-substituted alkanes. C18H37Br molecules formordered self-assembly on HOPG shown in Fig. 7a. Anobvious trough in a molecular lamella of C18H37Br canbe seen from this image. A ‘‘head-to-head” configuration

Fig. 6. (a) High-resolution STM image of C19H39Cl adlayer on HOPG. Vbias =structural model for the ordered adlayer in (a).

is proposed for the molecular arrangement in this assem-bly. This arrangement can be demonstrated by the STM re-sult in Fig. 7b. With the change of imaging condition, it isnoted that bromine groups in C18H37Br molecules appear‘‘bright” as shown in Fig. 7b. This appearance may bedue to a trans-gauche isomerization around the terminalC–C bond [29]. Because of small energy difference betweenthe two conformers at room temperature, it is easy forC18H37Br to change its conformation. When changing togauche conformer, electronic wave function of brominesgets out of molecular plane. Because of the larger steric

600 mV; It = 944 pA. The trough is indicated by an arrow. (b) Proposed

Fig. 7. Self-assembly of C18H37Br on HOPG. (a) STM image showing bromine atoms in ‘‘dark” contrast. Vbias = 750 mV; It = 900 pA. (b) STM imageshowing bromine atoms in ‘‘bright” contrast. Vbias = 1.56 V; It = 592 pA. (c) Structural model for the C18H37Br adlayer in (a). (d) Structural model for theC18H37Br adlayer in (b).


overlap between tip and bromines, the tunneling current isenhanced. Another possibility is that the whole moleculerotates around its long axis, proposed for the bulk alkanerotator phases [47,48]. Fig. 7c and d are the proposed struc-tural models for the self-assemblies in Fig. 7a and b, respec-tively, with top and side view.

For C19H39Br, molecules pack densely in lamellas withthe molecular axis parallel to each other. From the STMimages in Fig. 8, the bromine atoms in C19H39Br moleculesappear in bright contrast at the end of alkyl chains.

Fig. 8a is a typical large-scale STM image of theC19H39Br monolayer on HOPG. The random distributionof bright lines reflects the position of Br atoms in the mol-ecules. The structural details are revealed by a higher reso-lution STM image in Fig. 8b. These brighter and biggerspots in the image are corresponded to Br atoms. In the cir-cle A of Fig. 8b, a ‘‘head-to-head” configuration is corre-sponded, while ‘‘head-to-tail” configuration is displayedin the circle B of Fig. 8b. Fig. 8c shows a height-shaded sur-face plot, which is clear to show the two opposite configu-rations. Moreover, the angle between the molecular alkylaxis and the lamella direction is a = 60 ± 2�. Fig. 8d showsa composite STM image with molecular adlayer and under-lying HOPG substrate lattice. The top part is the image ofthe underlying graphite lattice, and the bottom is theC19H39Br monolayer. The image shows that the directions

of the molecular alkyl axis (indicated with arrow b) and thelamellar boundary (indicated with arrow a) are parallel tothe close packed direction of the underlying HOPG lattice.It is also discernible that the alternation of image contrastintensity from dark to bright over four or five molecules ina lamella. On the basis of above analysis, a tentative struc-tural model for the two different arrangements of ‘‘head-to-head” and ‘‘head-to-tail” are proposed in Fig. 8e.

3.2.2.3. I-substituted alkane. C18H37I was previously imagedon HOPG [29]. According to the previous results of STMobservation, equidistance bright lines are parallel each other,and the distance between two lines is double length of mole-cule, indicating that C18H37I molecules are arranged in a‘‘head-to-head” configuration on HOPG surface.

3.2.3. Alkyl thiols

On HOPG two different self-assembled structures of alkylthiols are reported. One is linear structure with parallelmolecular lamellas, in which the molecular alkyl axis andthe lamella direction cross each other at an angle of 90�[17,29]. The molecules are preferentially paired in a ‘‘head-to-head” configuration (the SH groups facing each other),while ‘‘head-to-tail” (SH group to CH3 terminated end) con-figuration was rarely observed. Another structure is V-type(herringbone) configuration. Xu et al. found a herringbone

Fig. 8. Self-assembly of C19H39Br on HOPG. (a) Large-scale STM image. Vbias = 660 mV; It = 900 pA. (b) High-resolution STM image. Vbias = 625 mV;It = 816 pA. In the circle A, a ‘‘head-to-head” configuration is corresponded, while ‘‘head-to-tail” configuration is displayed in the circle B. (c) Height-shaded surface plot of (b). (d) Composite STM image of C19H39Br adlayer and underlying HOPG lattice. The directions of the molecular alkyl axis(indicated with b) and the lamellar boundary (indicated with a) are parallel to the close packed direction of the underlying HOPG lattice. (e) A schematicpresentation of C19H39Br adlayer. The left is ‘‘head-to-head” configuration, and the right is ‘‘head-to-tail” configuration.


configuration of C18H37SH self-assembly on HOPG withtwo kinds of hydrogen bondings. From STM images andtheoretical simulation, they proposed two different struc-tural models. The S��S distance between two thiol moleculesis 0.56 nm and 0.34 nm, respectively, consistent with thewidth of the bright bands in STM images [25,26]. However,only V-type (herringbone) structure is found for the self-assembly of C19H39SH on HOPG shown in Fig. 9. The con-trast of S atoms in STM image appears brighter than that ofcarbon skeleton. The angle between molecular alkyl axis andthe lamella direction is 60�. Obviously, the molecules lie in a‘‘head-to-head” configuration.

3.2.4. AlkylaminesAlthough the STM investigation on alkylamines was

rarely reported, C18H37NH2 was previously studied [29].

From the literature, [29] C18H37NH2 molecules form orderedlamella structure. C18H37NH2 molecules lie in a ‘‘head-to-head” configuration on HOPG surface. The angle betweenmolecular axis and lamella direction is 60�. The nitrogenatoms present bright contrast in STM image. Several brightlines parallel to the direction of lamella are observed. In addi-tion, there is a clear Moire pattern, ‘‘bright and black” con-trast variation repeated in every four or five molecules.

4. Discussion

On the basis of STM observation, it is concluded that allof the alkanes and their derivatives studied in the presentpaper form highly ordered 2D lamellar structures. Tounderstand and analyze the results more comprehensively,a figure (Fig. 10) containing structural models for alkane

Fig. 9. Self-assembly of 1-C19H39SH on HOPG. (a) Large scale STM image. Vbias = 650 mV; It = 800 pA. (b) High-resolution STM image.Vbias = 637 mV; It = 850 pA. (c) Proposed structural model for the ordered adlayer.


and their derivatives with 18 and 19 carbon atoms is pre-sented, which shows molecular arrangement is apparentlyinfluenced by the functional groups and the number of car-bon atoms in alkyl chain (length and odd–even effect).

4.1. Functional groups

From the STM results, it is seen that different functionalgroups with alkane molecules result in different self-assem-bled structures due to the different geometric and electronicproperties of the functional groups [18,49]. Table 2 is a listof all the functional groups involved in the present researchand the influence of them on the structures of self-assem-blies including the angle between the molecular axis andlamellar alignment direction, and the configuration offunctional groups in the molecular arrangement.

The self-assembly formation mainly depends on inter-molecular reaction and molecule/substrate interaction.Hydrogen bonding and van der Waals force play importantroles in forming self-assembly of alkyl molecules. When al-kyl chains adsorb on HOPG surface in a parallel confor-mation with an angle of 90� between the molecular axisand the lamella direction, van der Waals force betweenneighboring molecules is maximized for the largest contactarea. For the alkyl derivatives that can form hydrogen

bonding, the angle between molecular axis and the direc-tion of lamella alignment is 60� except fatty acids. How-ever, owing to structure of carboxyl group, morehydrogen bondings could be formed between neighboringmolecules. On the other hand, dipole–dipole interactionis important in forming the self-assembly of alkyl halides.For alkyl thiols, there are two types of self-assemblies: lin-ear and V-type structures. Yin et al. proposed that hydro-gen bondings in thiols were fairly weak [17]. Thus,hydrogen bonding is not always dominant for the self-assembling process. If the adsorbate–graphite and the al-kyl–alkyl interactions control the adsorption process, thelinear structure is formed with both ‘‘head-to-head” and‘‘head-to-tail” arrangements. If the S��H–S bondings arestrong, their directionality results in an angle of 60� be-tween molecules axis and lamella alignment direction. Inthis case, it is easy to understand that the longer the alkylchains are, the less V-type structure appears. This hypoth-esis could be verified by the STM results of C18H37SHadlayer that shows both V-type and linear structure, butC22H45SH only shows linear structure [17,29]. In addition,the steric repulsion between substituents also influences thepacking structure. In the structure of fatty acid self-assem-bly, driven by reducing intermolecular repulsions an inter-digitated arrangement is observed [43].

Fig. 10. List of structural models for alkanes and their derivatives with 18 and 19 carbon atoms.

Table 2Influence of the functional groups on molecular configuration and anglebetween the molecular axis and lamellar alignment direction

Groups CH3 OH Cl, Br, I NH2 SH

Angle 90� 60� 90� 60� 60� 90� 60�Configuration – A A A&B A A&B A

‘‘A” represents ‘‘head-to-head” configuration and ‘‘B” represents ‘‘head-to-tail” configuration.


4.2. Alkyl chain length

All the molecules investigated here have alkyl chains.For the alkanes with short alkyl chains, the molecules willbe very mobile on substrate at room temperature due to thethermal effect. It will be difficult for these molecules to formordered monolayer and to be imaged by STM at ambient

conditions. C13H28 molecule is found to be capable offorming ordered adlayer on HOPG in the present experi-mental conditions. Longer chain molecule assembly onHOPG will be stabler than shorter chain molecule. Whenalkyl chains become longer, van der Waals force would ex-ert more influence on adsorption process and the patternformation. Research shows the adsorption energy increasesproportionally along with the length of alkyl chain in-creases. The adsorption energy per CH2 on HOPG is�12.1 and �10.4 kJ/mol in the flat and vertical orientation,respectively [16].

4.3. Odd–even effect

Odd–even (the number of carbon atoms in alkyl chain)effect on self-assembly is an interesting phenomena, and


it was observed on metal and HOPG surfaces previously[50]. On Au(11 1) the odd alkanes are reported to adopta perpendicular lamellar structure, while perpendicularand tilted lamellae are imaged for even alkanes. The effectis attributed to the difference in the symmetry of the mole-cule, which is determined by the direction of the terminalmethyl groups [51–53]. On HOPG surface, saturated andunsaturated acids were reported for odd–even effect [43,54].

In the present research, STM results show that the self-assembled structures of Br-terminated alkanes varied withthe number of carbon atoms. For example, the self-assem-bly of C19H39Br is different from C18H37Br and C22H45Br[29]. It is observed that the bromine atoms in C19H39Br ap-pear in ‘‘bright” contrast in STM images, while both‘‘dark” and ‘‘bright” contrasts are observed for C18H37Brand C22H45Br assemblies. The angle formed between themolecular axis and the lamella direction is 60� and 90�for C19H39Br and C18H37Br, respectively. Only ‘‘head-to-head” configuration is observed for C18H37Br andC22H45Br. Both ‘‘head-to-head” and ‘‘head-to-tail” config-urations have been observed for C19H39Br. The odd–eveneffect would originate from intermolecular reaction andthe mismatch between underlying HOPG lattice and adsor-bate. Owing to the complicacy, the detailed study on odd–even effect is now in progress.

5. Conclusion

The self-assembly of n-alkanes and n-alkane (C18H38,

C19H40) derivatives on HOPG have been studied by STMat ambient conditions. All the molecules form well-orderedlamellar structures. The molecules of tridecane (C13H28),tetradecane (C14H30) and pentadecane (C15H32) can formstable lamellar structures and be clearly imaged by STM.The structures of various self-assemblies are influenced byalkyl chain length, different functional groups, and odd/even number of carbon atoms contained in the molecules.The resulting structures are dependent on a balance ofthe intermolecular interaction and molecule/substrate reac-tion. The systematic study of the simple alkyl moleculesand their derivatives will be useful to understand morecomplicated assembly components and surface nanostruc-ture accurately.

Acknowledgments

This work was supported by the National Natural Sci-ence Foundation of China (Nos. 20575070, 20673121,and 20733004), National Key Project on Basic Research(Grant Nos. 2006CB806100 and 2006CBON0100), andthe Chinese Academy of Sciences.

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stm investigation of the dependence of alkane and alkane (c18h38, c19h40) derivatives self-assembly...

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