Self-assembly of conjugated oligomers and polymers at the interface:structure and properties{NANOSCALE}

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Self-assembly of conjugated oligomer
s and polymers at the interface:structure and properties
Lirong Xu, Liu Yang and Shengbin Lei*

Received 14th January 2012, Accepted 1st May 2012

DOI: 10.1039/c2nr30122a

In this review, we give a brief account on the recent scanning tunneling microscopy investigation of

interfacial structures and properties of p-conjugated semiconducting oligomers and polymers, either at

the solid–air (including solid–vacuum) or at the solid–liquid interface. The structural aspects of the self-

assembly of both oligomers and polymers are highlighted. Conjugated oligomers can form well ordered

supramolecular assemblies either at the air–solid or liquid–solid interface, thanks to the relatively high

mobility and structural uniformity in comparison with polymers. The backbone structure, substitution

of side chains and functional groups can affect the assembling behavior significantly, which offers the

opportunity to tune the supramolecular structure of these conjugated oligomers at the interface. For

conjugated polymers, the large molecular weight limits the mobility on the surface and the distribution

in size also prevents the formation of long range ordered supramolecular assembly. The submolecular

resolution obtained on the assembling monolayers enables a detailed investigation of the chain folding

at the interface, both the structural details and the effect on electronic properties. Besides the ability in

studying the assembling structures at the interfaces, STM also provides a reasonable way to evaluate

the distribution of the molecular weight of conjugated polymers by statistic of the contour length of the

adsorbed polymer chains. Both conjugated oligomers and polymers can form composite assemblies

with other materials. The ordered assembly of oligomers can act as a template to controllably disperse

other molecules such as coronene or fullerene. These investigations open a new avenue to fine tune the

assembling structure at the interface and in turn the properties of the composite materials. To

summarize scanning tunneling microscopy has demonstrated its surprising ability in the investigation

of the assembling structures and properties of conjugated oligomers and polymers. The information

obtained could benefit the understanding of the elements affecting the film morphology and helps the

optimization of device performance.

1. Introduction

p-Conjugated polymers and oligomers have attracted increasing

attention because of their interesting structure, optical and elec-

tronic properties. In view of their potential applications in less

expensive, so-called plastic optoelectronic devices, they are the

most promising functional organic materials.1–4 Currently,

Lirong Xu received her MS in 2011 from Gannan Normal

University and is currently a PhD student in the Academy of

Fundamental and Interdisciplinary Sciences at Harbin Institute of

Technology. Her current research involves the synthesis of func-

tional building blocks and characterization of their assembly at the

solid–liquid interfaces.

Key Laboratory of Microsystems and Microstructures Manufacturing,Ministry of Education, Harbin Institute of Technology, Harbin, 150080,P. R. China. E-mail: [email protected]; Fax: +86 451 86403625; Tel:+86 451 86403625

This journal is ª The Royal Society of Chemistry 2012

thiophene-containing oligomers and polymers are receiving

considerable attention due to their applications in electronic and

optical devices, for example, field-effect transistors (FETs),5

organic light-emitting diodes (OLEDs),6,7 molecular rectifiers,8

organic thin film transistors9,10 and other optical applications.11

Assembling structures of organic semiconductors are of essential

importance to these devices where they are used as active

layers.12–14 One of the main technological attractions for such

devices based on organic materials is that nearly all the functional

layers can be deposited andpatterned at low/room temperature by

a combination of low-cost solution-processing and direct-write

Liu Yang is in her second year of Master degree in the Academy of

Fundamental and Interdisciplinary Sciences at Harbin Institute of

Technology. Her research is in the area of supramolecular

assembly of conjugated oligomers.

Nanoscale, 2012, 4, 4399–4415 | 4399

http://dx.doi.org/10.1039/c2nr30122a






http://pubs.rsc.org/en/journals/journal/NR

http://pubs.rsc.org/en/journals/journal/NR?issueid=NR004015

Scheme 1 Chemical structures of 1–5. (Reprinted with permission from

ref. 26.)

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printing, which makes them ideally suited for the realization of

low-cost, large-area electronic devices on flexible substrates.15 In

optoelectronic devices generally several interfaces are involved

and the interfacial structure always plays an essential role in

determining the performance of the devices. Not only the prop-

erties of individual molecules but also the conformation, orien-

tation and packing structure of thesemolecules in the first layer on

the solid substrate can determine the final properties of such

optoelectronic devices.16 Therefore, understanding the properties

of single molecules and the molecular arrangement at the inter-

faces is important in the fabrication of devices.17–19

Characterization of the thin film structure, especially within

a few nanometer vicinity of the interface, is a challenging task

because several interfaces are involved in this region, such as the

solid–solid interface, the evolution layer from the interface to the

bulk, and the solid–air interface. These complex cases make it

a tough task to fully characterize the interfacial structures and

their evolution by conventional diffraction and microscopic

techniques. Scanning probe microscopy, especially scanning

tunneling microscopy (STM), is a powerful tool for investigating

the self-assembled monolayers adsorbed onto solid substrates

due to its high resolution. A scanning tunneling microscope also

shows surprisingly good adaptability to different environments,

it works well under ultrahigh vacuum (UHV), at the solid–liquid

interface and under ambient conditions.

In this review, we will give a brief account on the recent

scanning tunneling microscopy investigation of the interfacial

structures and properties of p-conjugated semiconducting olig-

omers and polymers, either at the solid–air (including solid–

vacuum) or at the solid–liquid interface. The structural aspects of

the self-assembly are highlighted, including the self-assembly of

both oligomers and polymers, and a composite assembly formed

with other functional building blocks.

2. Assembly of conjugated oligomers

2.1 Effect of the backbone structure

Conjugated oligomers are promising optoelectronicmaterials due

to their promising properties and processability. Their difference

in chemical substitutions and backbones has different effects on

the structure and properties. So far, p-conjugated oligomers

which have been studied by STM at interfaces include oligothio-

phene,20–22 oligo(phenylene ethynylene)23 and oligo(phenylenevi-

nylene) derivatives.24

Shengbin Lei received his MS in 1999 from the Department of

Chemistry, Shandong University and his PhD in 2002 in physical

chemistry at the Institute of Chemistry, CAS (China) under the

supervision of Prof. Chen Wang. After working for four years at

the same institute in the field of surface assembly and scanning

tunneling microscopy, he joined the group of Prof. Steven De

Feyter at K.U. Leuven as a postdoctoral researcher, working in the

field of 2D nanoporous networks and host–guest chemistry. He

joined HIT in September 2009 as a professor. His current research

focuses on 2D crystal engineering at interfaces and characteriza-

tion of nanomaterials.

4400 | Nanoscale, 2012, 4, 4399–4415

Elena Mena-Osteritz et al.25,26 reported the self-assembling

properties ofp-conjugated oligothiophenes and cyclothiophenes.

A series of molecules have been investigated by scanning

tunneling microscopy under ambient conditions, and sub-

molecular resolution STM images are obtained. The structures of

dodecyl-substituted oligothiophenes (1–3), oligothiophenediace-

tylene (4) and cyclo[n]thiophene (5) are all shown in Scheme 1.

Fig. 1 shows the corresponding STM images of the assemblies

formed by the above compounds on the graphite surface. They

can self-assemble at the solid–liquid interface to form well-

ordered 2D structures. Both the linear p-conjugated oligomers 1

Fig. 1 (a) Short-range ordering of the dodecyl substituted quaterthio-

phene 1 at the solution–HOPG interface (10 � 10 nm2, Vbias ¼ 500 mV,

Iset¼ 48 pA). The inset shows the underlying HOPG substrate (Vbias¼ 20

mV, Iset ¼ 50 pA). (b) Long-range ordering of the decithiophene 2 at the

solution–HOPG interface (90 � 90 nm2, Vbias ¼ �600 mV, Iset ¼ 32 pA).

(c) STM image of the short-range ordering of head-to-tail coupled 3 on

HOPG (20 � 20 nm2, Vbias ¼ �120 mV, Iset ¼ 5 pA). (d) The STM image

of the long-range ordering of macrocycle 4 at the solution–HOPG

interface (60� 60 nm2,Vbias¼�542 mV, Iset¼ 54 pA). The inset (bottom

left) shows the underlying graphite. The second inset (top right) shows the

STM image of an individual macrocycle overlaid with a theoretical

calculated molecule. (e) Long-range ordering of the cyclo[12]thiophene 5

adsorbed on HOPG (28 � 28 nm2, Vbias ¼ �430 mV, Iset ¼ 24 pA). The

inset shows the short-range ordering in detail (6.7 � 6.7 nm2,

Vbias¼�430 mV, Iset¼ 24 pA). (Reprinted with permission from ref. 26.)



Scheme 2 Chemical structures of 6 and 7.

Scheme 3 Chemical structures of 4T–tm–8T (8) and 4T–tm–4T (9).

(Reprinted with permission from ref. 28.)

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and 2 form a linear structure, but the packing is different due

to the difference in the substituent pattern. Although in both

cases the alkyl side chains are interdigitated to stabilize the

assembly, the relative orientation of the side chains and back-

bones is different. For polymer 3, the packing between neigh-

boring polymer chains is similar to oligomer 2 and the

interdigitation of side chains results in an interchain distance

determined by the length of side chains. The details of the

assembling structure of polymers will be discussed later. The two

cyclic oligomers spontaneously assemble at the solution–HOPG

interface into a hexagonal pattern (Fig. 1d and e). The 2D lattice

parameters of the assembling monolayer which corresponds to

a sheet in the 3D bulk material agree very well with those

obtained from X-ray analysis of single crystals, this suggests that

STM can be a helpful tool to determine the packing arrangement

of oligomers in the bulk especially when a single crystal suitable

for X-ray analysis is not available.

Wan Jun-Hua et al. have investigated the molecular packing

structure in the self-assembled p–n diblock and n–p–n triblock

heterostructure oligomers (6 and 7) which were based on thio-

phene and 1,3,4-oxadiazole.27 The structures of 6 and 7 are

shown in Scheme 2.

STM images of these two molecules obtained at the liquid–

HOPG interface are shown in Fig. 2. In the self-assembly of 6,

there are many small ordered regions but no large uniform

pattern can be observed, whereas the adlayer of 7 is uniform and

well-ordered. The backbone of 6 is packed with head-to-tail

mode, and the alkyl chains are possibly stacked with an

Fig. 2 (a) High-resolution STM image of the monolayer of 6 (11.9 �11.9 nm2). (b) High-resolution STM image of the monolayer of 7 (15.6 �15.6 nm2). (Reprinted with permission from ref. 27.)


interdigitated model. In contrast, oligomer 7 is clearly resolved

with submolecular resolution and the monolayer structure is

more ordered than that of oligomer 6. So the molecular back-

bone symmetry is expected to play an essential role in 2D self-

assembly.

Alkyl-substituted dual oligothiophenes, unsymmetrical qua-

terthiophene(4T)–trimethylene(tm)–octithiophene(8T) (8) and

symmetrical 4T–tm–4T (9) (Scheme 3), both molecules are linked

by a trimethylene,28 were used to fabricate molecular monolayer

structures on HOPG or Au(111) surfaces.

STM observation shows that 8 forms both quasi-hexagonal

and linear adlayers (Fig. 3). In the quasi-hexagonal adlayers,

there are bright elliptical rings with dark depressions in the

center. Each elliptical ring is composed of two 8 molecules. The

linear adlayer is composed of short strands in a twisted config-

uration. Each strand is composed of two substrands and each

substrand corresponds to one 8molecule. STM images show that

9 which has a higher symmetry than 8 in its chemical structure

forms both patterns with wave- and lamella-like appearance. In

the wave-like adlayers, long-range ordering structures can be

observed. Each strand consists of two substrands and each

substrand corresponds to one 9 molecule. The lamellar adlayer

has straight short ‘‘sticks’’ with a small black gap in the center of

each stick, each stick corresponds to one 9 molecule, the central

black gap was attributed to the trimethylene linker with low

electron density (Fig. 4).

Besides the oligothiophenes discussed above, self-assembly of

p-conjugated oligo(p-phenylenevinylene) (OPV) dimers (10),

tetramers (11) and hexamers (12) has also been investigated

thoroughly.24 The structures of oligo(p-phenylenevinylene)

(OPV) derivatives are shown in Scheme 4.29

STM experiments were carried out on oligo(p-phenyl-

enevinylene) (OPV) derivatives at the 1,2,4-trichlorobenzene–

HOPG interface. Chiral oligomers 11 and 12 form chiral pure

monolayers, while STM images show that achiral compound 10

forms enantiomorphous domains on the graphite surface. The

molecular chirality is expressed in the orientation of the p-

conjugated backbone with respect to the propagation direction

of the lamellae. A noticeable difference in STM images of 11 in

comparison with those of 12 is the orientation of the molecules in

the 2D self-assembled framework. While the packing of 12 seems

to be determined by the p-conjugated backbones, the packing of

the smaller 11 appears to be determined by the alkyl chains. So

these results proved that even small differences in the molecular

structures can have an essential impact on the self-assembling

properties. These observations could be interesting for the

construction of circularly polarized optoelectronic devices

(Fig. 5).

Nanoscale, 2012, 4, 4399–4415 | 4401


Fig. 3 STM images and models of 8. (a1) Large-scale STM image of the adlayer of 8 on HOPG in a quasi-hexagonal symmetry. (b1) High-resolution

STM image of (a1). (c1) Structural model of the quasi-hexagonal adlayer. (a2) Large-scale STM image of the adlayer of 8 on HOPG in a linear pattern.

(b2) High-resolution STM image of (a2). (c2) Proposed structural model of the linear adlayer. (Reprinted with permission from ref. 28.)

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2.2 Effect of side chains

It is conceivable that the amount and distribution of side chains

attached to the molecular backbone have great effects on the 2D

self-assembly process of conjugated molecules. Many investiga-

tions have shown that the interaction between alkyl side chains is

important, sometimes even decisive to self-assembly.30,31 B€auerle

et al. used scanning tunneling microscopy (STM) to investigate

b-alkylated oligothiophenes which have long alkyl side chains

and observed that the formation of an ordered two-dimensional

framework depends on the length of the oligothiophene back-

bone and the hydrocarbon chain.32–34 Olga A. Gus’kova et al.35

have studied the self-assembly of b-alkylated oligothiophenes

using atomistic molecular dynamics simulations, they compared

the unsubsituted tetrathiophene (13) and the b-alkylated oligo-

thiophenes including 3,300 0-dipropyl-quaterthiophene (14), 3,30 00-dihexyl-quaterthiophene (15) (Scheme 5), and 3,30 00-didodecyl-

Fig. 4 STM images andmodels of 9. (a1) Large-scale STM image of 9 onHOP

Structural model of the wave-like adlayer. (a2) Large-scale STM image of 9 on

(c2) Structural model of the lamellar adlayer. (Reprinted with permission fro

4402 | Nanoscale, 2012, 4, 4399–4415

quaterthiophene (1) and they discovered that the presence of long

alkyl side chains can induce a higher degree of order on graphite.

The assembling properties of oligothiophenes were fundamen-

tally due to the b-alkyl substitution pattern, only side chains

longer than propyl can immobilize the molecule on the graphite

surface to form ordered structures.

It is well-known that the hydrogen bond plays an important

role in the self-assemblies, so a series of oligothiophenes with

carboxylic groups were synthesized in order to obtain a control-

lable molecular architecture. The chemical structures of these

compounds are shown in Scheme 6.36

Although 16 has a carboxylic acid group which can form an

intermolecular hydrogen bond, no long-range ordered adlayer of

16 could be formed, possibly due to the weak interaction between

the thiophene backbone and the solid substrate. In the high-

resolution STM images, elliptical shaped features could be

observed and every ellipse corresponds to a thiophene dimer.

G in a wave-like appearance. (b1) High-resolution STM image of (a1). (c1)

HOPG in the lamellar pattern. (b2) High-resolution STM image of (a2).

m ref. 28.)



Scheme 4 The structures of the achiral OPV-dimer (10), the chiral OPV-tetramer (11) and OPV-hexamer (12). (Reprinted with permission from ref. 29.)

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Instead molecule 17, which includes two carboxylic acid groups

and two thiophene rings, forms long-range ordering. Through

the hydrogen bonds between carboxyl groups, molecules initially

connect with each other to form a molecular stripe, then organize

into a 2D domain by the weak van derWaals interaction between

the stripes. 18 has a more complicated asymmetric molecular

structure, the large-scale STM image reveals three types of

ordered packing geometries which were marked by A, B, and C.

In A domains, the molecules form a rectangular structure

through hydrogen bonds. In B domains, molecules form dimers

which are connected by weak hydrogen bonds through the

thiophene rings and alkyl side chains. In domain C, a ‘‘L’’ shape

arrangement can be seen, every ‘‘L’’ consists of two 18molecules,

they interact with each other via hydrogen bonds between the end

carboxylic groups. Molecule 19 can form a long-range ordered

2D framework. In one direction, the molecules form rows

through the interdigitation of the alkyl side chains, and in the

other direction, the molecules interact with hydrogen bonds

between carboxylic groups (Fig. 6).

The polar end groups attached to the conjugated backbone

can influence the self-assembling behavior because of dipole–

dipole interactions. Oligothiophenes substituted by one or two

iodine atoms were investigated by scanning tunneling micros-

copy.37 The chemical structures and the corresponding STM

images are shown in Fig. 7. An ordered monolayer of 1 was self-

assembled at the liquid–HOPG interface. The main driving force

Fig. 5 (a) STM image of 10 at the liquid–graphite interface, image size is 14.5

size is 25.1 � 25.1 nm2. (c) Large scale image of 12 adlayer, image size is 50.0


for the arrangement is the van der Waals interactions between

the interdigitated alkyl side chains. Iodinated quaterthiophene 20

can form self-assembled monolayers spontaneously. The thio-

phene backbone forms an angle of 23 � 3� with respect to the

lamellar axis, this angle is smaller than that in the assembly of

compound 1 (30 � 3�). The oligothiophene backbones of diio-

dinated quaterthiophene 21 form an angle of 44� 1� with respect

to the lamellar axis. The difference in backbone orientation is

attributed to the influence of the iodine atoms on the assembling

behavior.

Gong Jian-Ru et al. have investigated the assembling struc-

ture of two alkoxy-substituted oligo(phenylene ethynylene)s

(OPEs) which have similar structures (Fig. 8). They concluded

that the different arrangement of the two molecules is due to

the change of end groups.38 For molecule 22 the carboxyl

groups possess a head-to-head configuration in the monolayer,

and the hydrogen bond between neighboring carboxyl groups

may contribute to the stability of the monolayer. Molecule 23

can form a less ordered monolayer when they adsorb on the

surface of HOPG, the molecular orientation is also determined

by the registered adsorption of alkoxy chains with respect to

the substrate lattice, but due to the interdigitation of trime-

thylsilyl end groups, and they obtained a more closely packed

structure.

2D assemblies of a series of alkoxy-substituted oligo(pheny-

lene ethynylene)s (OPEs) with a longer backbone and various

� 14.5 nm2. (b) STM image of 11 at the liquid–graphite interface, image

� 50.0 nm2. (Reprinted with permission from ref. 29.)

Nanoscale, 2012, 4, 4399–4415 | 4403


Scheme 5 Chemical structures of 13–15. (Reprinted with permission

from ref. 35.)

Fig. 6 (a) A large-scale image of 16 assembly on the HOPG surface, the

upper right inset is the high-resolution STM image of 16. (b) High-

resolution STM image of 17 assembled onHOPG. (c) STM top view of 18

assembly, A, B, and C indicate three domains. (d) The STM image of the

assembly of 19, and a high-resolution image is shown in the inset.


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substituent patterns were also investigated by STM (Scheme 7).

Through the intermolecular hydrogen bond, especially that

between the carboxylic end groups, well-ordered assemblies were

obtained.39 It was found that different end groups or a biethy-

nylene linkage can change the 2D ordering significantly (Fig. 9).

Molecule 24 adsorbs on HOPG forming a stable monolayer, the

alkyl chains are interdigitated between two adjacent bright

stripes. Molecule 25 with two TMS end groups can also self-

assemble into a well-ordered stable structure on HOPG. Within

a row, adjacent molecules are dislocated with TMS groups

interdigitated with each other. The different 2D ordering of

molecules 24 and 25 arises from the number of TMS end groups.

In molecule 26, the two TMS end groups are replaced by two

Scheme 6 Structures of 3-thiophene acetic acid (16), 2,20-dithiophene-5,50-dicarboxylic acid (17), 30-pentyl-5,20:50,20 0-terthiophene-2,50 0-dicar-boxylic acid (18), 40,30 0-dipentyl-5,20:50,20 0:50 0,20 0 0-quaterthiophene-2,50 0 0-dicarboxylic acid (19). (Reprinted with permission from ref. 36.)

4404 | Nanoscale, 2012, 4, 4399–4415

carboxyl groups. The two carboxyl groups of molecule 26 can

form hydrogen bonds with carboxyl groups of adjacent mole-

cules. Molecule 27 forms a monolayer which contains many

domains. In the STM image, each molecular backbone dislocates

in the same stripe, biethynylene provides enough space for the

alkyl chains to interdigitate with those from the neighboring

rows. Due to the formation of hydrogen bonds, molecule 28

forms a similar monolayer as molecule 27 but without disloca-

tion. In comparison with molecules 25 and 26, the presence of

a biethynylene group in molecules 27 and 28 changes the distance

between neighboring alkyl chains along the backbone and in turn

changes the intermolecular interaction and tunes the 2D assem-

bling structure.

2.3 ‘‘Edge-on’’ against ‘‘face-on’’, the effect of p stacking

In order to maximize the molecule–substrate interaction in most

cases aromatic molecules tend to adsorb with their molecular

plane parallel to the substrate surface, resulting in a ‘‘face-on’’

configuration as shown in the results mentioned above. Even for

molecules that exhibit quite strong tendency of intermolecular p

stacking, for instance tetrathiafulvalene (TTF), other kinds of

non-covalent interactions such as hydrogen bonding are still

necessary to force the molecule to adopt an ‘‘edge-on’’ configu-

ration so that intermolecular p–p stacking becomes possible.

Puigmart�ı-Luis et al. have developed a strategy to introduce H-

bond forming amide groups into the side chains of TTFand found

out that the cis constitutional isomers form well defined supra-

molecular fibres at the solid–liquid interface with the TTF unit

tilted with respect to the substrate surface.40 Scanning tunneling

spectroscopy proved the decrease of the conductance gap due to

the intermolecularp–p stacking. Evenmultilayerswith parallel or



Fig. 7 A comparison of packing patterns observed for compounds 1, 20 and 21. Left, STM image of 1: 16.4� 16.4 nm2; middle, STM image of 20: 22.6�22.6 nm2; right, STM image of 21: 19.2� 19.2 nm2. The parameters (a, b, and a) of the unit cells of the three derivatives are provided, as well as the angle

between the molecular axis and the lamellar axis (g) and the density of molecules in the monolayer (M nm�2). (Reprinted with permission from ref. 37.)

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cross-configuration of these supramolecular fibres can be stabi-

lized with the aid of intermolecular hydrogen bonds which opens

the possibility to interconnect these nanowires.41,42

Noncovalently or covalently connected multiporphyrin arrays

represent both ideal models for the study of the energy transfer

mechanism and potential functional components in nanodevices.

Porphyrin hexamers covalently connected to a benzene ring were

synthesized and STM investigations revealed that the uncoor-

dinated molecule 29 forms kinetically favored ‘‘face-on’’ oriented

structures at the 1-phenyloctane–HOPG interface, which grad-

ually changes to a more thermodynamically favored lamellar

phase.43 The intermolecular distance measured from the sub-

molecularly resolved images suggests that molecule 29 adopts an

‘‘edge-on’’ orientation in the lamellar phase, which is stabilized

by intermolecular p–p stacking. The zinc coordinated

Fig. 8 (a) The chemical structure of molecule 22 and the corresponding

large scale STM image. (b) A large scale STM image of molecule 23, the

chemical structures of molecules 22 and 23 are shown on top of the STM

images. (Reprinted with permission from ref. 38.)


compound 30 forms exclusively a lamellar assembly without

formation of a kinetically favored ‘‘face-on’’ orientation due to

the increased p–p interactions between the molecules of 30. The

orientation of 30 in the assembly can be further manipulated by

the addition of a potential axial ligand of zinc. The addition of

a small bidentate ligand diaza[2,2,2]bicyclooctane (DABCO)

further stabilizes the lamellar phase by coordination to the ZnII

ions while still preserving the p–p stacking between porphyrin

units (Fig. 10b). However, addition of a larger ligand, 4,40-bipyridine, disrupts the p–p stacking, leading to the exclusive

formation of a ‘‘face-on’’ structure, which highlights the impor-

tance of p–p stacking in stabilizing the columnar structure.

Even larger porphyrin oligomers, dodecamers, show a similar

assembling behavior at the solid–liquid interface.44 The metal

free dodecamer 31 forms uniform large lamellar domains with

‘‘edge-on’’ orientation (Fig. 10c). Despite their large diameter,

individual dodecamers can be clearly distinguished. Possibly due

to the axial coordination of water molecules the zinc coordinated

dodecamer 32 did not form stable adlayers. Only after addition

of excess DABCO ligands instantaneous formation of stable

lamellae was observed. The precise control between ‘‘edge-on’’

and ‘‘face-on’’ orientations by addition of appropriate ligands is

of great interest considering the application of such porphyrin

oligomers in nanodevices.

2.4 Composite structures

Most studies on binary mixtures show phase separation or

formation of randomly mixed monolayers. It remains a challenge

to control the ordering of multi-component mixtures at the

molecular level.45–48

Mohamed M. S. et al.37 have used scanning tunneling

microscopy to investigate mixtures of three quaterthiophene

Nanoscale, 2012, 4, 4399–4415 | 4405


Scheme 7 Chemical structures of alkoxy-substituted oligo(phenylene ethynylene)s 24–28. (Reprinted with permission from ref. 39.)

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derivatives at the liquid–solid interface. The chemical structures

of these compounds are shown in Fig. 7. The mixture of mole-

cules 1 and 20 predominantly shows a monolayer of 1 even at

a ratio of 1 : 10, only at much higher ratios molecule 20 can form

monolayers at the liquid–solid interface. For mixtures of mole-

cules 1 and 21, only monolayers of molecule 21 were observed at

the ratio of 5 : 1. This consequence is related to the packing

density of each compound.

The self-assembly of oligo(phenylene ethynylene) (OPE) (29)

can be used as a molecular template; organic molecules such as

coronene (COR) and biomolecules such as tripeptides are con-

trollably monodispersed on HOPG by using this template49

(Fig. 11). At a 1 : 2 molar ratio of COR to 33, each cavity of the

network of 33 is occupied by a single COR molecule (Fig. 11b).

COR molecules are not clearly resolved due to the movement of

the COR molecules in the slightly larger cavity compared to its

Fig. 9 The high-resolution STM images of molecules 24 (a), 25 (b), 2

4406 | Nanoscale, 2012, 4, 4399–4415

size. At a molar ratio of 1 : 1 of COR to 33, CORs form a dimer

in each cavity, and COR molecules are well stabilized in the

framework. At a 3 : 2 molar ratio, as marked by arrows I and II

in the image (Fig. 11d), there are two types of COR molecular

arrays, single COR molecule and COR dimer, which fill within

the space of 33 template in arrays I and II, respectively. At this

ratio, COR not only fills the cavities adjacent to the carboxyl

groups but also those close to the ethynylene end groups. These

results illustrate that COR molecules can be distributed in

a controlled fashion in the 33 template by tuning the concen-

tration ratio. Oligopeptide TGG can also be dispersed within the

33 template.

Additionally, molecule 34 and its coadsorption with C18H37Br

are studied by scanning tunneling microscopy on HOPG50

(Fig. 12). Molecule 34 can self-assemble into ordered helix

structures by itself, but coadsorption with C18H37Br resulted in

6 (c), 27 (d) and 28 (e). (Reprinted with permission from ref. 39.)



Fig. 10 (a) Structures of compounds 29–32. (b) High resolution STM

image of a domain of ‘‘edge-on’’ oriented 30 and DABCO on graphite.

Cartoon demonstrating that the addition of DABCO results in a well-

defined columnar structure by bridging adjacent 30 molecules is shown

below. (c) STM topography of a monolayer of 31 at the phenyloctane–

graphite interface. The inset shows a high resolution image of the

columnar structure. (Reprinted with permission from ref. 43 and 44.)

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the bicomponent lamellar structure. The bright rods were

attributed to the backbones of 34. The bicomponent cocrystal is

stabilized by the van der Waals interaction and the halogen

bonding between 34 and C18H37Br.

Elena Mena-Osteritz and B€auerle have investigated the

complexation of C60 on a cyclothiophene monolayer.51 Chemical

structures of cyclothiophene (35) and C60 are shown in Scheme 8.

Molecule 35 can self-assemble into a highly ordered monolayer

of a hexagonally packed framework. When deposited onto the

macrocycle template from a 1,2,4-trichlorobenzene solution C60

can interact with the p-system of the macrocycle and self-

assemble into a second layer on top of the 35 template (Fig. 13).

At a very low coverage, only a few C60 adsorb in the cavities of

the macrocycle template at different sites. At high coverage, C60

interacts with the 35 molecular template at precisely the same

location and forms nearly perfect domains.

3. Assembling of conjugated polymers

3.1 Polythiophene

Poly(3-alkylthiophene)s (P3ATs) were used as active compo-

nents in field effect transistors (FETs), light-emitting diodes

(LEDs) and photovoltaic devices. The control of the structural

order on the molecular level has important consequences on the

electronic properties. X-ray diffraction (XRD) studies of a solu-

tion-processed film of P3ATs reveal lamellar structures with 2D

sheets resulting from interchain stacking. Crystalline domains


with a typical size of �10 nm were deduced from XRD.52,53

Within these ordered domains, the thiophene rings were deduced

to adopt an all-trans conformation, while it is also reasonable

that polymer folds occur between these crystalline domains and

form a disordered/amorphous matrix. However, concerning the

polymer conformations within the folds there is very limited

information available due to the lack of appropriate techniques

to study these folded conformations at the atomic level. In this

sense, STM and P3ATs represent a perfect model system for the

investigation of conformations of folded polymer chains

considering the atomic resolution of STM and the good

conductivity of P3AT. In one of the pioneer works by Mena-

Osteritz et al.,54 STM was used to study the assembly of head-to-

tail coupled poly(3-alkylthiophene)s (HT-P3ATs) at the

solution–HOPG interface. The compounds investigated, HT-

coupled poly(3-hexylthiophene) (P3HT) and HT-coupled poly-

(3-dodecylthiophene) (P3DT), comprised high regioregularities

of 95% and 98%, respectively. STM images reveal a long range

order on the micrometer scale with molecularly resolved indi-

vidual strands. Ordered domains are oriented according to the

three crystallographic axes of the HOPG substrate. The side

chains prefer to adsorb with fully extended and interdigitated

fashion along the main graphite axes. In most of the surface, the

conjugated backbones are linearly arranged with the thiophene

repeating units adopting an all-trans conformation. However,

a closer look at the images reveals that chain folding also occurs

in the monolayer, in which cis conformations of the thiophene

units are a prerequisite for the fold (Fig. 14). Hair-pin folds,

where the polymer chain change their propogate direction for

180� within a few repeating units, are regularly seen. The calcu-

lation supported analysis of the folds in both polymers reveals

that an intramolecular hair-pin fold is composed of seven thio-

phene units in a cis conformation in the case of P3HT and eight

units for P3DT. Thanks to the atomic resolution of STM images,

a 6.8 �A periodicity can be determined in the dodecyl-substituted

P3DT chains, which corresponds to a ‘‘compressed’’ bithiophene

repeating unit (in the gas phase the length of a bithiophene unit is

calculated to be 7.4 �A). This significant compression was

attributed to the epitaxial effects of the underlying substrate

surface on the alkyl side chains. The submolecular resolution of

STM also allows for the determination of interchain distances in

the lamellae. Typical distances of 13–14�A and 19–20�A for P3HT

and P3DT were found, respectively. These values agree well with

semiempirical calculations with the hexyl and dodecyl side chains

fully interdigitated, but smaller than that determined from XRD

measurements. This difference was attributed to the epitaxial

effects of the graphite surface which promote the interdigitation

between side chains in order to maximize the intermolecular

interactions.

The submolecular resolution of STM images also facilitated

the measurement of the polymer chain contour length, which

allows the determination of the number of repeating units in

individual strands. However, the average degree of polymeriza-

tion estimated from STM images is lower than those from GPC

and MALDI-TOF.

Though XRD studies revealed a lamellar structure with 2D

sheets resulting from interchain stacking in the solution pro-

cessed films, despite the occurrence of self-organization in some

areas, the global microstructure of P3HT films should not be

Nanoscale, 2012, 4, 4399–4415 | 4407


Fig. 11 (a) Chemical structures of 33 and COR. (b) High-resolution STM image of one-by-one distribution of COR. (c) High-resolution STM image of

two-by-two distribution of COR. (d) High-resolution STM image of one-by-two distribution of COR. (Reprinted with permission from ref. 49.)

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regarded as simply polycrystalline. Instead, one should rather

expect polycrystalline domains embedded in a disordered/

amorphous matrix. STM studies on self-assembled P3HT dry

films cast from chloroform to an HOPG surface proved this

hypothesis.55,56 Polycrystalline domains were observed to coexist

with disordered/amorphous polymers, which partially cover the

surface and prevent clear observation of the underlying

substrate. Monodomains were observed to be interconnected by

folded chains. These monodomains follow the three-fold

symmetry of the HOPG substrate due to the epitaxial effects.

However, domains with a small misfit angle of �5� were also

observed.

The chain-to-chain distance in the crystalline domains was

determined to be 1.40 � 0.05 nm, which corresponds well with

a full interdigitation of hexyl side chains. This value is in good

agreement with STM measurements at the solution–substrate

interface while significantly smaller than �1.6 nm from XRD

measurements in ‘‘bulk’’ samples. The domain size in the trans-

verse direction (perpendicular to the conjugated backbone) can

be determined from the full width at half maximum (FWHM) of

the FFT peaks. The value deduced (18 � 2 nm) is in very good

agreement with that determined from the direct space. The

domain size in the parallel (along the conjugated backbone)

direction, which is not accessible by other techniques, can be

estimated directly from the STM images, and an average size of

20 nm is revealed. This verified that STM is a powerful tool for

studying the mesoscopic ordering of conjugated polymers.

4408 | Nanoscale, 2012, 4, 4399–4415

Similar to that at the solid–liquid interface, chain folding

occurring with angles of 120�, 60� and 180� was observed.

Another interesting feature is that since the mono-domains with

different orientations are interconnected by folded chains, the

frontiers between domains cannot be precisely defined, thus the

concept of grain boundary is not totally adequate for describing

P3HT films.

Inclusion of carbon nanotubes can increase the effective

crystallinity of conjugated polymers, and this kind of composite

materials was proved to be promising for use in organic photo-

voltaic and optoelectronic memory devices. Thus the way by

which the polymers are attached to carbon nanotubes becomes

an interesting topic of research. Goh et al. have carried out STM

investigations on the effect of the substrate curvature on the

adsorption of P3HT on single walled carbon nanotubes

(SWCNTs).57 A SWCNTwith a diameter of�1.4 nm was used in

their study. Rod like features with lengths ranging from tens to

200 nm were observed in the STM images, and the diameter of

these features was determined to be 2.1 to 2.5 nm, which is in

good agreement if assuming a 0.38 nm stacking distance of the

polymers and taking into account the van derWaals radius of the

polymer. Besides the diameter, high resolution STM images also

revealed a high degree of P3HT chain organization along the

entire tube axis. The cross-section measurement along the axis

reveals an average periodicity of 1.68 � 0.02 nm, substantially

larger than the polymer chain-to-chain distance of 1.45 nm found

on HOPG, which may indicate that the substrate curvature plays



Fig. 12 Large-scale (a) and high-resolution (b) STM images of the self-

assembly of 34/C18H37Br on HOPG. (c) High-resolution STM image of

C18H37Br with a ‘‘head-to-head’’ configuration on HOPG. (d) Structural

model of the adlayer in (b). The chemical structures of 34 and C18H37Br

are overlaid on top of the STM images. (Reprinted with permission from

ref. 50.)

Scheme 8 Chemical structures of 35 and C60. (Reprinted with permis-

sion from ref. 51.)

Fig. 13 (a) STM image of a monolayer of 35 on HOPG, including a 35–C6

a monolayer of 35 on HOPG and C60-fullerenes adsorbed at different sites, i

some non-complexed macrocycles (deep red) and different domains (A, A0 an


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a fundamental role in the assembly of ordered domains. A chiral

angle was also revealed for the polymer chains in wrapping the

nanotubes, which may indicate that at least for the P3HT–

SWCNT composite the chirality of CNTs could significantly

influence the polymer structures deposited at the surface.

The adsorption of P3HT has also been investigated on other

substrates. Terada et al. deposited P3HT on a H-terminated

Si(100) surface using a pulse-injection method.58 This method

provides a way to deposit large molecules with high molecular

weight and produce contaminant-free surfaces by avoiding direct

exposure of the substrate to air during preparation. Polymers

were adsorbed as isolated species on the surface with no packing

features or self-assembly (Fig. 15). The contour length could be

precisely determined. Chain folds were usually observed along

the polymer chains and the linear strands between folds comprise

an almost all-trans conformation of the repeating units. The

average length of the linear strands was 17.2 nm. The relatively

large length of extended conformation was attributed to the

highly rigid character of the P3HT backbone because of the steric

hindrance of hexyl side chains.

The isolated polymer molecules observed in this study are in

contrast to the XRD and STM results on the solution processed

films where self-assembly of P3HTs was confirmed. The absence

of self-assembly could be due to the rapid vaporization of the

chloroform solvent.

In another work, Kasai et al. have used the same pulse-injec-

tion method to deposit P3HT on the Cu(111) surface.59 STM

images reveal also isolated polymer chains, however, on the

Cu(111) surface P3HT chains adsorb with majority being in rigid

all trans conformation, coexisting with 60 or 120� folds, which

are composed of five or three thiophene rings with continuous cis

conformation. The number of monomer units in one polymer

chain can be definitely counted, allowing the determination of

the absolute molecular weight of the polymer.

Besides imaging, the spectroscopic mode, scanning tunneling

spectroscopy (STS) provides a powerful tool for detecting the

local electronic states, especially when combined with the

imaging mode, it provides a powerful tool to investigate, at

the local scale, the precise relationship between the structural

organization and electronic properties. The simultaneous

acquisition of STM topographic and 2D spectroscopic images

should allow the investigation of the impact of the polymer local

conformation on its electronic properties. For this purpose, Scifo

et al.60 have investigated the local electronic properties of the self-

organized 2D polycrystals of poly(3-dodecylthiophene) (P3DT)

0 complex (white arrow), image area: 11.6 � 8.7 nm2. (b) STM image of

mage area: 20 � 20 nm2. (c) STM image of 35–C60 complexes, including

d B), image area: 79 � 79 nm2. (Reprinted with permission from ref. 51.)

Nanoscale, 2012, 4, 4399–4415 | 4409


Fig. 14 Chemical structures of P3HT, P3DT and the corresponding

STM images. (Reprinted with permission from ref. 54.)

Fig. 15 STM images of pulse-injected regiorandom P3HTs at the H-

terminated Si(100) surface, obtained at room temperature. Image size: (a)

150 � 150 nm2, (b) 43 � 43 nm2, (c) 70 � 70 nm2, and (d) 50 � 50 nm2.

The inset in (d) shows a STM image of a clean H-terminated Si(100)

surface. (Reprinted with permission from ref. 58.)

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drop-cast from CHCl3 solution to a HOPG surface. STM and

STS measurements were conducted under UHV conditions at

room temperature. A typical STM image displays a highly

organized 2D polycrystalline structure with oriented mono-

domains connected by well-defined regular chain folds. Isolated

4410 | Nanoscale, 2012, 4, 4399–4415

polymer chains adsorbed as a second layer were also occasionally

observed, which enables the investigation of the contribution of

p–p electronic coupling on the STS spectra and images. Most of

the P3DT chains in the second layer were randomly oriented,

crossing the underlying chains in the first layer. The chains in the

first and second layers show an apparent height of 0.24 � 0.04

nm and 0.29 � 0.05 nm, respectively. The difference in apparent

height between the first and second layers could be a result of the

variation of electronic properties above the two layers, with

a greater electronic density above the second layer chains

compensated by an increased tip–sample distance.

STS gives more information on the local electronic effects.

Current imaging tunneling spectroscopy (CITS) images give

a sharp contrast within a broad energy range due to the different

electronic properties between the HOPG substrate and the

polymer assembly. Both I–V curves and conductance spectra of

the polymers exhibit an extended plateau and both are asym-

metric: a much higher current was obtained for positive than for

negative bias (Fig. 16). The STS conductance gap was attributed

to the HOMO–LUMO band gap and simulations with ab initio

density functional theory were conducted to further confirm the

nature of the STS conductance gap. Based on the simulation, no

significant charge transfer should be expected.

Although an increase of the HOMO–LUMO gap over chain

folds is expected, whatever the bias applied, CITS images show

constant tunneling current over the polymer monolayer within

statistical fluctuation. This means at room temperature the

electronic properties of a single chain are weakly affected by the

folding. However, CITS images recorded over chains of

the second layer above +0.6 eV show a remarkable contrast,

which arises from the 0.3 eV shift of the HOMO edge as shown in



Fig. 16 (a) Experimental and calculated I–V spectra obtained over the

bare substrate (HOPG), the first polymer monolayer (P1) and the second

polymer layer (P2). The calculated curves are shown in the inset. (b and c)

Conductance dI/dV and normalized conductance spectra, respectively.

The arrows point to the edges of HOMO and LUMO bands. (d) Topo-

graphic (top) and current images acquired during CITS measurements at

different setting bias,�0.61 V (middle) and 0.86 V (bottom). The location

of a chain fold is highlighted with white circles in the top and bottom

images. (Reprinted with permission from ref. 60.)

Fig. 18 STM image of a polydispersed PPE at the phenyloctane–

graphite interface (Vbias ¼ 1.2 V, Iset ¼ 1.0 nA). (Reprinted with

permission from ref. 62.)

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Fig. 16b. This increase in the band gap for the second layer

polymer chains was attributed to polarization effects.

Although the first layer forms relatively ordered arrangements,

the upper layer of P3ATs merely forms a random pattern. For

instance, in the case of P3DT a second layer constituted of iso-

lated strands can be observed at the phenyloctane–HOPG

interface, though most of these strands do not follow the direc-

tion of the underlying polythiophene backbones but rather

intersect with random angles. However, when the alkyl side

chains were brominated at the end, the brominated P3ATs form

a clearly organized second layer, in which the polymer strands lie

aligned following the direction of the first layer and form parallel

bundles (Fig. 17).61 Close inspection of the STM image indicates

that the conjugated backbones are staggered. With the help of an

Fig. 17 Comparison of the self-assembly of P3DT (a) and brominated

P3DT (b) at the phenyloctane–HOPG interface. (Reprinted with

permission from ref. 61.)


especially developed dual-bias imaging technique the position of

bromine end groups can be identified to be located in the center

of two neighboring polymer strands, which confirms an end-to-

end model of the alkyl side chains in the monolayer of bromi-

nated P3ATs. This indicates that the polythiophene backbone of

the second layer is systematically located on top of a row of

bromine atoms, which suggests that the bromine–sulfur weak

bondings are at the origin of a self-templating effect. This opens

an avenue towards the designed self-assembly of conjugated

polymers with 3D geometry.

3.2 Poly(phenyleneethynylene)s

The self-assembly of p-conjugated macromolecules offers

a strategy for the construction of well-defined nanometer struc-

tures with chemical functionalities and physical properties that

are of potential use as active components in electronic devices.

Among all the conjugated polymers, poly(p-phenyl-

eneethynylene)s (PPEs) are of particular interest because of their

rigid-rod structure, strongly anisotropic electronic properties,

electroluminescence in the blue-green region and high and stable

photoluminescence quantum yield. When deposited onto

a HOPG surface from 1-phenyloctane STM reveals a nematic-

like monolayer with both the conjugated skeletons and the hexyl

side chains lying flat on the basal plane of graphite (Fig. 18). The

2D Fourier transformation indicates that the conjugated skele-

tons are aligned according to the three-fold symmetry of the

graphite substrate. The stiffness of the molecular rods and the

low polydispersity play a key role in the formation of stable 2D

structures. The distance between neighboring backbones was

determined to be 1.62 � 0.10 nm, which is significantly smaller

than that expected from fully extended side chains. This indicates

that the side chains are disordered between adjacent parallel

backbones. In comparison, an oligomer composed of three PE

units which has the same substituent skeletons forms a well-

defined 2D epitaxial crystal structure with an interchain distance

of 1.46 � 0.10 nm. The difference in 2D structures of the olig-

omer and polydispersed polymer was attributed to the distribu-

tion of molecular lengths, which prevents the assembly of the

macromolecules into perfect crystals. The time required for

achieving a stable assembly is much longer for the polymer,

Nanoscale, 2012, 4, 4399–4415 | 4411


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which is probably due to the self-segregation and consecutive

adsorption that occurred at the molecular level at the interface.62

The PPE samples with different average lengths were also

measured by SFM on a mica surface, where nanoribbons were

detected for polymers with low molecular weight. In these

nanoribbons the PPE molecules are oriented with their backbone

parallel to the substrate. Statistics on the width of the nano-

ribbons yielded average values consistent with the molecular

length expected, suggesting that the PPEs are packed parallel to

each other with their long molecular axis perpendicular to the

long ribbon axis. The mole-fraction distribution of molecular

weight calculated from the ribbon width shows a very good

agreement with that expected from the Schulz–Zimm distribu-

tion, which theoretically describes the molecular-weight distri-

bution of polycondensation reaction.

In comparison, the statistic of the contour length of adsorbed

PPE molecules from STM images obtained at the HOPG–1-

phenyloctane interface revealed that the experimentally deter-

mined maximum was shifted to higher rod lengths than that

expected from the Schulz–Zimm distribution, indicating

a favorite adsorption of the elongated molecules. Also, the

experimental histograms are narrower than the Schulz–Zimm

plots, which suggest molecular weight fractionation by the

interface.63

The effect of substituents on the 2D assembling and chain

folding of a series of PPE derivatives was studied at the solid–

liquid interface. However, different from the above reports, the

Fig. 19 Molecular structures (top) and the composite assembly (bottom)

of PPE 1 and CuPcBu8. The inset in the STM image is a schematic model

of the composite assembly. (Reprinted with permission from ref. 65.)

4412 | Nanoscale, 2012, 4, 4399–4415

polymers were first dissolved in toluene and then after solvent

evaporation a drop of 1-phenyloctane was added before STM

imaging. In this way, submolecularly resolved STM images were

obtained, which reveal diverse forms of chain folding in the

assembling monolayer.64 The results demonstrate that the

structure and concentration of side chains affect not only

the supramolecular order at the interface but also the chain

folding characteristics in the adsorbed layer. The degree of

polymerization and molecular weight distribution were directly

determined by analysis of the contour length of polymer chains.

The distribution of the polymer contour length agrees quite well

with that expected from the Schulz–Zimm distribution and no

fractionation was observed. This differs from the observation of

P. Samor�ı et al.63 and can be attributed to the difference in the

experimental protocol. These results demonstrate that STM

provides a reasonable evaluation of the molecular weight

distribution for a rigid-rod polymer. For such polymers, it is

difficult to determine the correct molecular-weight distribution

with standard analytical techniques such as gel permeation

chromatography (GPC) due to the aggregation of polymer

chains.

The self-assembled monolayer of PPE could be used as

a template to direct the assembly of other functional building

blocks, for instance phthalocyanines. Coadsorption of PPE 1

with copper(II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-

phthalocyanine (CuPcBu8) yields a composite structure

(Fig. 19). CuPcBu8 adsorbs commensurately atop PPE back-

bone, which leads to an increased intermolecular distance

between CuPcBu8 molecules. The match of structural parame-

ters between the template and subject plays an important role in

the success of templating.65

Fig. 20 Diverse chain foldings observed in the PmPV adlayer. The

corresponding molecular models of the polymer chains indicated by

arrows in the images are also shown. (Reprinted with permission from

ref. 67.)



Fig. 21 (a) Structure of porphyrin oligomer and polymer. (b and c) STM images of the porphyrin polymer containing ca. 40 porphyrin subunits.


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3.3 Poly[(m-phenylenevinylene)-co-(2,5-dioctoxy-p-

phenylenevinylene)] (PmPV)

PmPV is a semi-conjugated luminescent polymer whose structure

is a variation of the more common poly(p-phenylenevinylene)

(PPV) and has been used to form composites with carbon

nanotubes for photovoltaic devices.66 Thus the interfacial

structure and interaction modes of PmPV with carbon nanotubes

are of fundamental and technological interest. Due to the

structural similarity PmPV/HOPG was used as a model system.67

Submolecularly resolved STM images reveal that PmPV tends to

adsorb ‘‘straight’’ on the HOPG surface with its backbone

parallel to the surface. Small ordered domains can frequently be

detected within which the PmPV chains are packed parallel to

each other with the alkyl side chain interdigitated. The flexibility

of the semi-conjugated PmPV backbone results in a high

frequency of chain folding with the 60�, 120� and 180� (hairpin)folds as the basic modes (Fig. 20). The orientation of the PmPV

chains is determined by the registered adsorption of the semi-

conjugated backbone and alkyl side chain with the underlying

substrate lattice. The planar adsorption conformation of the

conjugated backbone of PmPV allows strong overlap of the

highly delocalized electron system of the substrate with the p-

electrons of the PmPV backbone. This is in agreement with the

theoretical simulation on PmPV/SWNTs in which PmPV

adsorbs onto nanotubes with a planar conformation along the

axis of the nanotube.

In photovoltaic devices, PPV derivatives are commonly mixed

with C60 derivatives to form heterojunctions, which is quite

important in order to improve the performance of devices. The

interfacial structure of such composites prepared by the elec-

trochemical method was studied by in situ EC-STM on a Cu(111)

surface. The composite film was prepared by sequential

adsorption of an oppositely charged poly{(2,5-bis(3-bromo-

trimethylammoniopropoxy)-phenylene-1,4-divinylene)-alt-1,4-

(2,5-bis(2-(2-hydroxylethoxy)ethoxy))phenylenevinylene} (BH-

PPV) and hexa(sulfobutyl)C60 (HSC60), which result in a 4 �3O3 adlayer. The density of this adlayer is higher than that

formed by pure BH-PPV (4� 4O3) and is also different from that

of HSC60 (4 � 4).68

Another approach to the preparation of the heterojunctions

involves the application of preferential adsorption. When a 1 : 1

mixture of PmPV and copper(II) 2,3,9,10,16,17,23,24-octaki-

s(octyloxy)-29H,31H-phthalocyanine (CuPcOC8) was applied

onto the basal plane of graphite, PmPV adsorbs preferentially

onto the surface due to its higher affinity, while in the upper layer

these two compounds phase segregated into different domains.


In this way, a vertical heterojunction could be created in the

sequence of graphite–PmPV–CuPcOC8.69 The combined char-

acterization with STM and AFM gives out detailed information

on the interfacial structure of the composite film, which could be

very important for understanding the mechanism of OPV and

OFET devices and also for the improvement of their

performance.

3.4 Other polymers

Conjugated oligo and polyporphyrins are of great interest due to

their potential applications in nonlinear optics and light har-

vesting. Like other polymers, the interfacial properties of these

materials also play an important role in their applications in

molecular electronics. P. H. Beton et al. have used the UHV

electrospray deposition (UHV-ESD) method to prepare the

monolayer of oligo- and polyporphyrins directly from solution.70

Surprisingly despite their large size, the porphyrin oligomers

(hexamer) can diffuse on the surface and form highly ordered

islands, and the porphyrin units can be clearly resolved. The 2.6

� 0.2 nm intermolecular distance indicates that the assembly of

porphyrin hexamers is determined by side chain interdigitation.

In contrast, the porphyrin polymer (contains ca. 40 porphyrin

subunits) forms small quasi-close-packed regions where chains

are parallelly aligned, coexisting with disordered regions

(Fig. 21). In the disordered regions bends and kinks, even points

where polymer chains across occur. The average polymer chain

length determined from the STM images (54 � 12 nm) agrees

with that expected for a 40-unit chain. The angle between the

axes of two crossing polymers is measured to be 91 � 9�, indi-cating that there is a preferred relative orientation for the over-

lapping of polymer chains. The authors have also determined the

angular correlation length, Lc, which is a measure of the flexi-

bility of the polymer chains. The value of Lc (25 � 6 nm) is

significantly larger than the persistent length measured in solu-

tion, which possibly arises from the planar geometry of adsorbed

molecules. The interactions with the surface and between

neighboring molecules also contribute to the increase of rigidity.

A similar phenomenon has also been observed for other poly-

mers such as PPE, where the intermolecular interaction was

found to significantly influence the folding of the polymer

chains.64

4. Conclusions and perspectives

Conjugated oligomers can form well ordered supramolecular

assemblies either at the air–solid or liquid–solid interface, thanks

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to the relatively high mobility and structural uniformity in

comparison with polymers. The controllable preparation of well-

ordered nanoscale self-assemblies especially at the interface is the

precondition for tailoring the performance of nano-devices. The

backbone structure, substitution of side chains and functional

groups can affect the assembling behavior significantly, which

offers the opportunity to tune the supramolecular structure of

these conjugated oligomers at the interface and in turn optimize

the performance of devices. The properties of plastic optoelec-

tronic devices or functional organic materials depend on both the

nature of the constituents of supramolecular assemblies and the

interactions between them. Through tuning the non-covalent

forces, supramolecular assemblies can be designed on purpose.

Besides noncovalent interactions covalent interactions could be

introduced to further stabilize the supramolecular structure and

to improve the lifetime of nanoscale devices.71,72

For conjugated polymers, the large molecular weight limits the

mobility on the surface and the distribution in size also prevents

the formation of a long range ordered supramolecular assembly.

Thus conjugated polymers normally only form small relatively

ordered domains and between these domains chain foldings are

frequently observed. Since the mono-domains with different

orientations are interconnected by such folded chains, the fron-

tiers between domains cannot be precisely defined, thus the

concept of the grain boundary is not totally adequate for

describing the assembled monolayers of conjugated polymers.

The submolecular resolution obtained on the assembling

monolayers enables a detailed investigation of the chain folding

at the interface, both the structural details and the effect on

electronic properties. Diversity of folding modes was observed

with submolecular resolution and structural models of these

foldings were proposed based on the STM observation. The

effect of chain folding on the electronic structure was also

investigated by scanning tunneling spectroscopy.

For conjugated polymers, it is difficult to determine the correct

molecular-weight distribution with standard analytical tech-

niques such as gel permeation chromatography (GPC) due to the

aggregation of polymer chains. Besides the ability in studying the

assembling structure at the interface, STM also provides

a reasonable way to evaluate the distribution of the molecular

weight of conjugated polymers by the statistic of the polymer

contour length. Unlike GPC, which needs calibration via PPP or

PS, the polymer contour length of individual polymer chains is

measured directly from the STM images and no overestimation

should be expected. However, one caution that needs to be kept

in mind is that the molecular weight fractionation could happen

at the solid–liquid interface due to the stronger interaction with

the substrate experienced by the larger molecules.

Both conjugated oligomers and polymers can form composite

assemblies with other materials. The ordered assembly of oligo-

mers can act as a template to controllably disperse other mole-

cules such as coronene or fullerene. Polymer monolayers with

proper tailored structures were also used to template the

assembling of other conjugated molecules such as phthalocya-

nine or self-template the growth of itself to form a controllable

3D assembly. These investigations open a new avenue to fine

tune the assembling structure at the interface and in turn the

properties of the composite materials. To summarize scanning

tunneling microscopy has demonstrated its surprising ability in

4414 | Nanoscale, 2012, 4, 4399–4415

the investigation of the assembling structures and properties of

conjugated oligomers and polymers at the interface. The infor-

mation obtained could benefit the understanding of the elements

affecting the film morphology and help the optimization of

device performance. STM has also proven its ability in the

evaluation of the distribution of polymerization and ‘‘true’’

molecular weight, and can be applicable to other polymer

systems.

Although STM proved its usefulness in characterizing the

interfacial structures of conjugated oligomers and polymers,

there are severe limitations in using STM to investigate multi-

layer formation and that using several techniques would be

desirable. The current investigations are mostly carried out on

traditional STM substrates such as HOPG and metal single

crystals, however, structural information on substrates such as

ITO will be more helpful for the improvement of device perfor-

mance. Graphene, which is an ideal candidate material for

replacing ITO as the transparent conductive electrode in

photovoltaic devices, will also be an interesting substrate.73–75

Information on the interfacial structures and properties on such

systems will directly help to understand the mechanism of related

nanodevices.

Acknowledgements

This work is supported by the start-up funding of HIT, New

Century Excellent Talents in University (NCET) from the

Ministry of Education of P. R. China, the fundamental research

funds for the central universities HIT.BRET2.2010002 and the

National Science Foundation of China (21173061).

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Self-assembly of conjugated oligomers and polymers at the interface:structure and properties{NANOSCALE}