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Understanding tapping-mode atomic force microscopy data

on the surface of soft block copolymers

You Wang a,*, Rui Song b, Yingshun Li b, Jingshu Shen b

a Applied Chemistry Department, Harbin Institute of Technology, Harbin 150001, PR Chinab Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China

Received 12 September 2002; accepted for publication 4 March 2003

Abstract

In this paper, we focus on better understanding tapping-mode atomic force microscopy (AFM) data of soft block

copolymer materials with regard to: (1) phase attribution; (2) the relationship between topography and inside structure;

(3) contrast-reversal artifacts; (4) the influence of annealing treatment on topography. The experiments were performed

on the surface of poly(styrene–ethylene/butylene–styrene) (SEBS) triblock copolymer acting as a model system. First,

by coupling AFM with transmission electron microscopy (TEM) measurements, the phase attribution for AFM images

was determined. Secondly, by imaging an atomically flat SEBS surface as well as an AFM tip-scratched SEBS surface, it

was confirmed that the contrast in AFM height images of soft block copolymers is not necessarily the result of surface

topography but the result of lateral differences in tip-indentation depth between soft and hard microdomains. It was

also found that there is an enlarging effect in AFM images on the domain size of block copolymers due to the tip-

indention mechanism. Thirdly, based on the tip-indention mechanism, tentative explanations in some detail for the

observed AFM artifacts (a reversal in phase image followed by another reversal in height image) as function of imaging

parameters were given. Last, it was demonstrated that the commonly used annealing treatments in AFM sample

preparation of block copolymers may in some cases lead to a dramatic topography change due to the unexpected order-

to-order structure transition.

Ó 2003 Elsevier Science B.V. All rights reserved.

Keywords: Atomic force microscopy; Surface structure, morphology, roughness, and topography; Electron microscopy

1. Introduction

Tapping-mode atomic force microscopy (TM-

AFM) measures topography by tapping the sur-

face with an oscillating probe tip so that the tip

makes contact with the sample only for short du-

ration in each oscillation cycle. The method of

operation results in lower lateral forces com-pared to conventional contact mode in which the

probe slides across the surface, so the irrevers-

ible destruction on soft surface can be eliminated.

For this reason, TM-AFM has been established in

recent years as a standard tool to investigate sur-

faces of soft materials [1]. Specifically, model block

copolymer materials like poly(styrene–butadiene–

styrene) (SBS) [2–5] and its derivate poly(styrene–

ethylene/butylene–styrene) (SEBS) [6,7] have

been well studied using the technique. Its spatial

* Corresponding author. Tel./fax: +86-451-6412516.

E-mail address: [email protected] (Y. Wang).

0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0039-6028(03)00388-1

Surface Science 530 (2003) 136–148

www.elsevier.com/locate/susc

http://mail%20to:%[email protected]/

http://mail%20to:%[email protected]/



nanometer resolution together with its potential to

distinguish different materials without further

staining has made TM-AFM an attractive alter-

native to the established technique transmissionelectron microscopy (TEM) [8]. Despite its great

promise in the study of block copolymers, however

(1) the contradictory reports concerning phase

attribution in the AFM images [3,6,9]; (2) the

confusion between real surface topography and tip

indention [2,10]; (3) the frequently occurring AFM

artifacts of contrast reversal as function of imaging

parameters [2,5,6]; and (4) the underestimation of

annealing effect on sampleÕs structure give rise to

the notion that the results may be subject to var-

ious uncontrolled factors and raise the question

whether and how reproducible the imaging con-

ditions can be established [2]. Following is an in-

troduction of the existing problems and relevant

research.

2. Background

2.1. Phase attribution for AFM images

Since AFM data do no provide direct informa-

tion concerning which part of microdomains maycorrespond to certain block chains, the determi-

nation of phase attribution becomes the predomi-

nant task. In the earlier studies, ‘‘composition

correlation’’ method [3,9] is used to determine

phase attribution by comparing the composition of

one phase of block copolymer with corresponding

percentage of the area in the AFM image. How-

ever, contradictory results were reported based on

this method. Motomatsu et al. [9] deemed that

higher spots in the AFM topography images

should correspond to the hard phase polystyrene(PS), while Dijk and coworkers [3,4] regarded the

other way round.

Later, McLean and Sauer [6] attempted to use

TM-AFM phase imaging technique to determine

the phase attribution. Because it is generally ac-

cepted [11,12] that the brighter domains in phase

image usually correspond to the hard materials

when the interaction between tip and sample is

dominated by repulsive force, the author drew a

same conclusion as Motomatsu et al. [9] did.

However exceptions were reported in these litera-

tures [13–15] where the brighter domains in phase

image were assigned to the soft materials. Since

changes in phase angle are related to energy dis-sipation [16,17], and can be due to changes in

tip–sample molecular interactions [1], and defor-

mation at the tip–sample contact [15,18], Ragha-

van et al. [14] pointed out that the evaluation of

phase data is not always straightforward. In ad-

dition, the frequently occurring contrast inversion

artifacts in phase images [1,19] further increase the

difficulty of using this strategy.

Recently, Knoll et al. [2] try to solve this prob-

lem through a new approach amplitude–phase–

distance curve measurement (referred to as APD

curves [20,21]). Surprisingly, it was found that the

conventional AFM height images are not neces-

sarily reflecting the surface topography of SBS but

reflect lateral differences in tip-indentation depth.

In other word, deeper tip indentation on the soft

rubber phase than the polystyrene (PS) hard phase

leads to the result that the PS phase corresponds to

the high spots in AFM topography image (see Fig.

1b). However, some assumptions have to be in-

troduced in the experiment to draw the above

conclusion [2].

From fundamental point of view, to answer thequestion of phase attribution for AFM images

requires a thoroughly understanding of the rela-

tionship between surface topography and phase

separation structure inside. So, the following

question in fact needs to be answered first.

2.2. How to correlate the surface topography with

the phase separation structure inside?

So far, three models (see Fig. 1) based on the

assignment of the higher domains to the PS phasehave been put forward in attempt to establish the

relationship. Cross-sectional TEM studies of block

copolymer surface have shown [22] that the lower

surface-energy constituent preferentially locates

itself at the free surface. So model one [5,23] at-

tributes the presence of surface protrusions ob-

served by AFM to the periodical elevation of the

rubber thin layer by the underlying PS domain

(Fig. 1a). Unlike model one, model two [2,6] stresses

that the untouched real surface is in fact relative

Y. Wang et al. / Surface Science 530 (2003) 136–148 137



flat, and the lateral tip-indention difference be-

tween different domains gives rise to the contrast

in height image (Fig. 1b). Departing from the

general case where the free surface is occupied by

one rubber phase, cross-sectional TEM [24] has

shown both the PS and the rubber phases could

appear on the surface. For this case, the PS do-

mains will elevate on the surface according to the

study of PS/PB blend system by Raghavan et al.

[14]. Model three [9,24] was thus presented (Fig.

1c). Obviously, there are many unanswered ques-

tions and disputations concerning how to correlate

topography with structure for block copolymers.

2.3. TM-AFM contrast-reversal artifacts

One notable TM-AFM artifact for soft block

copolymers such as SBS and SEBS is the contrast

inversion in height image depending on imaging

conditions. Important experimental parameters of

TA-AFM are the amplitude A0 of free oscillation,

the set-point amplitude ratio r sp ¼ Asp= A0, where

Asp is the set-point amplitude, and the operating

frequency x. With a decreasing r sp, Magonov et al.

[5] and McLean and Sauer [6] reported a contrast

inversion in height image but found phase image isrelative stable. The same results have also been

reported by Knoll et al. [2] but in their APD curve

measurements, the inversions of both height and

phase images took place. The author attributed the

difference in observation between TM-AFM and

APD curve measurements to a very high r sp for

the inversion of phase image, at which a stable

TM-AFM imaging is not possible. The study of

Magonov et al. [5] also revealed that changing

operating frequency form low-frequency side to

high-frequency side will lead to a contrast inver-

sion in height image. It is a crucial issue of TM-

AFM study for block copolymers when and why

such AFM artifacts could happen, because in case

of an artifact image, the interpretation of AFM

data like phase attribution according to the com-

mon rules will be completely wrong.

2.4. The effect of annealing treatment on sample’s

topography

On the one hand, annealing treatments are

commonly used in preparation of AFM samples of

block copolymers because it is effective to enhance

the contrast [4], sharpen the phase boundary, andorder the domains. On the other hand, much less

attention has been placed so far on the possibility

of dramatic topography change caused by an-

nealing due to the thermally induced ‘‘order-to-

order’’ structure transition [25,26]. Although both

Motomatsu et al. [9] and van Dijk and coworkers

[3] have reported that the higher discrete domains

of block copolymers are getting continuous upon

annealing in their pioneering AFM studies, the

phenomenon is not connected to the possibility of

structure change inside the film. It is not surprisingfor the situation because it is generally believed

[27] that the structure transition in block copoly-

mers is difficult due to the large activation energy

barrier, especially given that little has been re-

ported on such transition in a block copolymer

thin film system.

In the current research, we focus on the above

existing problems and confusing research results.

First, by comparing AFM and TEM data for the

samples prepared under the similar conditions, the

Fig. 1. Schematic diagrams showing the different correlations between the surface topography and the phase structure inside the SEBS

film: (a) model one, (b) model two, and (c) model three.

138 Y. Wang et al. / Surface Science 530 (2003) 136–148



phase attribution problem for AFM images was

solved; next, using special strategies and ordinary

TM-AFM imaging technique we investigated the

relationship between topography and structure fora soft block copolymer thin film system. Further-

more, based on the understanding of the rela-

tionship tentative explanations in some detail for

the observed AFM artifacts of contrast reversal

were given. Last, we investigated the influence of

annealing treatment on AFM imaging results. We

hope through this work, a better understanding of

TM-AFM data on the soft block copolymer sur-

face could be made.

3. Experimental section

3.1. Materials

The SEBS triblock copolymer Kraton G 1650

was produced by Shell company. The molecular

weight M n, polydispersity and styrene content are

7.5Â 104, 1.36 and 27% respectively [28]. Three

solvents xylene, cyclophexane and toluene were

used in the experiment.

3.2. Sample preparation

Kraton G-1650 was dissolved into the three

solvents to make 0.2 and 0.5 wt.% solutions re-

spectively. All the sample films in the experiment

were prepared by drop-casting technique and dried

at ambient conditions. The film-forming process

takes about 2 or 3 min.

The TEM samples were prepared as the fol-

lowing sequence: (1) 0.5% SEBS solutions were

cast on a freshly cleaved mica surface (for theannealing samples, the treatment was then per-

formed in a vacuum oven at temperature of

140Æ 2 °C. Liquid nitrogen gas atmosphere was

used to quench the samples after annealing); (2)

the mica substrate with the casting film on its top

was immersed slowly at an angle into the water

and the film was freed from the mica surface and

floated onto water surface; (3) the film on the

water surface were next transferred to copper grids

and stained by RuO4 vapor.

The ordinary AFM samples for comparison

with TEM data were prepared by casting of 0.2

wt.% SEBS solutions on freshly cleaved mica

surface. The thickness for AFM samples wasdetermined from profile images like Fig. 4d to be

16–30 nm. For TEM samples, the thickness was

estimated to be 70–80 nm. As-cast samples with

different thicknesses ranging from 16 to 80 nm

were comprehensively checked by AFM and the

imaging results show film-thickness indepen-

dence.

In order to tell whether the lateral differences of

tip indentation or the real surface topography

gives rise to the contrast in AFM height image,

two special SEBS samples for AFM measurement

were made. One is the replica film of atomically

flat mica surface [29]. The sample film was cast

from 0.5 wt.% SEBS solution in xylene on freshly

cleaved mica surface. The mica substrate was im-

mersed slowly at an angle into the water and the

casting film was freed and floated onto water

surface. Note that the surface contacting the water

is the replica surface. A small piece of mica sub-

strate (about 1Â 1 cm2) was put onto the top free

surface of the floating film and then a large piece

of filter paper was placed onto the mica and the

outside film surfaces. The paper will adhere on themica and the outside film surface after socking by

water. Next, the paper was lift up along with the

film and mica and turned up-side down so that the

film is on the top of mica and mica on the top of

the paper. The film was allowed to dry and a razor

was used to separate the film around the mica

substrate. In such a way, the replica surface was

turned to top. This sample preparation strategy is

just borrowed from a standard method to prepare

specimen-support film on TEM grids.

In the other case, a special sample with tip-scratched surface was prepared. According to

previous reports [30–32], by carefully choosing

spring constant of tip, controlling loading force

and scanning times, it is possible to scrape the film

without damaging its beneath substrate by AFM

tip scanning in contact mode so that the thickness

of the film could also be obtained by imaging the

tip-scratched area in tapping mode. A thin film

was made by casting of 0.2 wt.% SEBS solution in

xylene on freshly cleaved mica surface. In contact




mode, scan operation is performed for one or two

times within an area of 300Â 300 nm2 and care has

been taken not to damage the mica surface. After

tip-scratched treatment, the special sample wasstored at room temperature for about two weeks

to let the scratched area completely relax. It is of

interest that although a square area of the film was

scraped, its boundary quickly relaxed into a round

shape, which is likely due to the elastic property of

the block copolymer material.

The annealing treatment conditions for AFM

samples are the same as those for TEM ones. In

AFM measurements, the same sample was used

before and after annealing treatment. In particu-

lar, for the tip-scratched sample, the tip-scratched

area has also been used as a mark for relocating

the tip to the same imaging area after such treat-

ments as relaxation and annealing, an ex situ ob-

servation strategy [33].

3.3. Characterization

Digital Instrument multimode SPM III AFM

was performed at ambient conditions in tapping-

mode for imaging the surface and in contact mode

for scraping the film. Commercial silicon cantile-

vers with spring constant ranging from 25 to 50N mÀ1 were used. For consistence with other TM-

AFM literature [14], three tapping force levels

corresponding to r sp of 0.80–0.99 (light tapping),

0.60–0.80 (moderate tapping), and 0.40–0.60 (hard

tapping) were defined. Without special mention,

the height and phase images were recorded si-

multaneously under a moderate tapping condi-

tions (r sp % 0:70) with a free amplitude of 60Æ 10

nm. The operating frequency was readjusted after

engaging the tip on the surface such that the op-

erating frequency was on the low-frequency side of the resonance during the imaging [14].

For comparison purpose, Seiko Instruments

Inc. SPA400 AFM with SPI3800N controller was

also used only for the samples cast from toluene

(as-cast and annealed). The dynamic force mode

was performed to obtain height and phase data.

No difference concerning the imaging results was

found between the two different AFM systems.

The TEM equipment was H-800 from Hitachi

Company using an accelerating voltage of 100 kV.

4. Results and discussion

4.1. Determination of the phase attribution

It is well known [34] that a block copolymer can

exhibit a variety of microphase separation patterns

depending on the casting solvent used. Taking

advantage of this characteristic, three kinds of

morphologies were established by using three dif-

ferent solvents for both AFM and TEM mea-

surements. Shown in Fig. 2 are AFM (left) and

TEM (right) data for the SEBS films cast from

solutions in xylene, cyclohexane, and toluene

(from top to bottom) respectively. The discussions

begin with the TEM results because much knowl-

edge has been accumulated on the technique. The

dark regions in TEM micrographs (Fig. 2d–f)

correspond to polystyrene (PS) microdomains se-

lectively stained with RuO4 vapor [35], and the

bright region corresponds to the polyethylene/

butylene (PEB) rubber matrix. It can be seen that

in the above three micrographs, the shape of dis-

crete PS microdomains are coexistence of cylinder

(with branches in some cases) and sphere, sphere,

and coexistence of cylinder (without branch) and

sphere corresponding to xylene, cyclohexane, and

toluene casting solvents respectively.In Fig. 2a–c, the AFM height data captured

under the moderate tapping conditions also show

discrete microdomains (bright), which are in rea-

sonable agreement with both shape and size (see

Table 1) of the PS phase shown in the corre-

sponding TEM micrographs. Obviously, the

higher regions (bright) in the AFM height images

correspond to the PS phase. Two slight differences

between AFM and TEM data have also been ob-

servable. One is that the PS domains in TEM mi-

crographs look a little bit denser (see differencebetween Fig. 2a and d) and can be overlapped at

some points (Fig. 2f). This is due to the reason that

AFM data usually just provide top layer infor-

mation but the TEM data are projections through

the specimen and can be crowded and overlapped

when it contains multiple layers of microdomains

[36], particularly when the microdomains are in

disordered state between layers like our case. The

other difference is the diameter values of PS do-

mains are larger in AFM images than in TEM




micrographs for the three cases (Table 1). The

cause of the phenomenon will be discussed later.

As for the reason of why previous literatures

[3,4] assigned lower regions in AFM height image

to the PS phase, we deduce that it may result from

the ‘‘composition correlation’’ method [3,9] they

used. In fact, there is a precondition to be met for

using this method that is the phase separation

Fig. 2. AFM height images of the as-cast SEBS films from (a) xylene, (b) cyclohexane, and (c) toluene. Corresponding TEM mi-

crographs of the as-cast SEBS films from (d) xylene, (e) cyclohexane, and (f) toluene. The size of these images is 300 Â 300 nm

2

. Theheight scale is 10 nm for image (a), (b) and 3 nm for (c).

Table 1

Average diameter d of the discrete microdomains, obtained

from AFM height images (bright area) and TEM micrographs

(dark area) for the different solvent-casting samples

Casting solvent d AFM (nm) d TEM (nm)

Xylene 20 13

Cyclohexane 21 12

Toluene 22 15




structure of the system must be perpendicular

lamella but not parallel cylinder to the film sur-

face. Because in case of parallel cylinders embed in

matrix, any two-dimensional AFM image couldnot reflect the real three-dimensional composition

in bulk. For example, in Fig. 2a–c, the low com-

position (27%) PS domain usually accounts for

more than half of the total area. This is due to the

projection effect enlarges the real cylinder com-

position. Moreover, based on AFM data alone, it

is difficult to tell structure between perpendicular

lamella and parallel cylinder. Taking Fig. 6 for

example, from AFM data (Fig. 6a), it is reasonable

to regard the microphase structure to be perpen-

dicular lamella. But the existing of many over-

lapped spots shown by TEM (see white circled

areas in Fig. 6b) demonstrates that it belongs to

the parallel cylinder network structure. So using

‘‘composition correlation’’ method to determine

phase attribution is not always reliable.

4.2. Establishment of the correlation between to-

pography and structure

For SEBS block copolymers, previous re-

searches have shown that there is a thin PEB

rubber layer located on the top surface by water-contact-angle measurements [6,7]. So model 3 (Fig.

1c) is ruled out for our case. To tell which of the

left two models (Fig. 1a and b) is reasonable, we

design the following two experiments. By repli-

cating mica surface, an atomically flat SEBS

sample was prepared. Fig. 3 is TM-AFM height

image of such surface in which the microphase

separation pattern can be clearly seen. The mor-

phology is basically accordant with that of the as-

cast film (Fig. 2a). The result suggests for soft

block copolymer materials, the surface contrast isnot necessary the result of topography which in the

case is atomically flat.

Now, let us presume that the tip-indention dif-

ferences on the surface give rise to the contrast in

AFM height image. If so, by increasing the loading

force it can be expected: first, the measured

thickness of films will decrease; second, the tip-

indention differences between soft and hard phase

regions will increase; last, higher domains on the

surface should correspond to the hard PS do-

mains. To check these presumption, a small area

of the film surface was scraped by AFM tip to

expose the surface of mica substrate. So the

thickness of the film could be obtained by imaging

the tip-scratched area. All expected results have

been found in our designed experiment. Shown in

Fig. 4a–c are AFM height images captured con-

tinuously with decreasing r sp of 0.95, 0.85 and 0.65

respectively. The intersection in each height image

is phase data corresponding to the white squaredarea of the height image. The central round darker

area (about 300 nm in diameter) in each height

image is the tip-scratched area. The cross-sectional

profiles along the central black lines at the same

position in the three images are given in Fig. 4d

denoted as curves a, b and c. It can be seen that

with increasing the tapping force, the thickness of

the film decrease from 20 nm (curve a) to 14 nm on

average (curve c); surface contrast in curve c shows

the maximum value of 3–4 nm; The higher domain

in Fig. 4c corresponds to the PS domains; the slopeedge of tip-scratched area become wider in curves

a, b and c. All these results suggest the tip-inden-

tion effect on the film surface gives rise to the

height contrast. Further increasing the tapping

force level could not decrease the thickness of the

film nor increase the height contrast on the surface

(result not shown here), which means the inden-

tion reaches its limitation. The maximum of tip

indention on PS phase determined from the dif-

ference between curve a and c in our research is

Fig. 3. AFM height image for the atomically flat SEBS film.




about 5 nm which smaller than the value of about

15 nm in APD curve experiment [2]. The reason

can be explained as the difference in definition of z -

position the tip touches the sample surface. In ourcase, curve a is regard as z -position the tip touch

the surface which involves a negative systematic

error. In APD curve measurement, the kink in

phase signal is regard as the tip touches the surface

that may involve positive error [2]. Another pos-

sible reason could be the difference in thickness of

sample (20 nm for our case and estimated 100 nm

for theirs). At this point, we like to report on the

enough evidence for the assigning the bottom lines

in profile curves (see Fig. 4d) to the mica surface,

which is the basis of our discussion. First, in the

cross-sectional profile images, it can be seen that

the bottom lines are level and very flat matching

the characteristic of mica surface. Second, theheight contrast at bottom line area did not increase

with increasing tapping force level like the SEBS

surface area. Third, under the moderate tapping

conditions, the tip-scraped area shows an even

brighter color than the PS microdoamins in the

phase image (see left area of the intersection in Fig.

4c) suggesting a very hard surface. Last, the pro-

files checked for different tip-scratched areas on

the same film surface under light tapping condi-

tions show a good reproducibility of 20Æ 4 nm.

Fig. 4. AFM images (800Â 800 nm2) captured continuously with degreasing set-point ratio: (a) 0.95, (b) 0.85 and (c) 0.65. The in-

tersection in each height image is phase data corresponding to the white squared area of the height image. (d) Cross-sectional profiles

along the central black lines at same position in height image (a), (b) and (c) are shown in curves a, b and c respectively. A height scale

of 10 nm and phase scale of 30° are used.




It is noteworthy that the tip-indention contrast

forming mechanism has determined that usually

there is a positive systematic error existing in AFM

images about the domain size of hard PS phase,which comes from the existence of top rubber layer

together with the possible deformation on PS do-

mains by AFM tip in tapping operation. This en-

larging effect is obvious in the tip-indention

scheme (Fig. 1b) and fully supported by our earlier

finding that the diameter of PS domains is larger in

AFM images than in TEM micrographs (Table 1).

In addition, Motomatsu et al. [9] also reported

that the domain size of PS phase obtained from

AFM images is always larger than the calculated

value. Further evidence could come from curve c

that if the maximum thickness of film is only 16.3

nm, the maximum PS diameter of about 21 nm

from top view as shown in Fig. 4c must be an

enlarged value. A reasonable description of the

cross-sectional structure of the film should be the

PS cylinder with diameter of 13 nm on average

(TEM result in Table 1) embed in a film of about

20 nm in thickness (curve a). So the thickness of

top PEB layer is estimated to be 3.5 nm. By the

way, the enlarging factor of domain size that de-

pends on the sharpness of the tip, the level of

tapping force and the quality of the image, mayvary from one image to another.

In principle, the contrast in TM-AFM height

images should be determined between the interplay

of real surface topography and tip indention.

However, due to the surprising large value (3–4

nm) of indention differences found on the SEBS

surface, the tip-indention effect seems to play a

dominating role. From this point, we deduce that

whether there is slight height difference on surface

according to model one and model three is not

important, model two perhaps represent the com-mon situation for soft block copolymers when

using TM-AFM imaging technique.

4.3. Tentative explanations for contrast-reversal

AFM artifacts

According to tip-indention mechanism and in

agreement with previous reports [2,5,6], the bright

area in both the height and the phase images should

correspond to hard PS phase. We call these images

normal while the other way round (inverted ones)

artifact. By comparing Fig. 4a and c, AFM con-

trast reversal can be clearly seen in both height and

phase images. To our knowledge, the reversals of both height and phase contrast for block copoly-

mer TM-AFM images were first reported by this

study. More interesting, we always found in our

experiments that the contrast reversal in phase

image takes place ahead of that in height image. As

we can see by comparing Fig. 4a and b, the inver-

sion of phase contrast completed while the inver-

sion of height contrast is just on the way.

Surprisingly, the left part of Fig. 4b shows a posi-

tive contrast like Fig. 4c while the right part (white

squared area) shows a negative contrast like Fig.

4a. The delay for inversion of height image can also

be seen in KnollÕs APD curves [2]. Their result

shows r sp of about 0.99 for inversion of phase im-

age which is higher than 0.95 for inversion of height

image. Based on the obtained tip-indention mech-

anism, tentative explanations for these somewhat

puzzling phenomena were given as follows.

In tapping mode, as the tip approaches the

sample, the tip–sample interaction alters the am-

plitude and phase angle of the oscillating cantile-

ver. During imaging, the amplitude is maintained

at a constant level by adjusting the vertical posi-tion of the sample. For example, for a surface re-

gion of larger amplitude damping, the feedback

control will move the sample downward to keep

the amplitude constant and thus this area is re-

corded as higher in topography. Meanwhile, the

phase shift D/ with respect to the freely oscillating

cantilever is recorded as a phase image. At present

stage, the theoretical understanding of phase data

is still underway. Three approaches through en-

ergy [16], force [19], and combination of both en-

ergy and force [15] have been put forward toexplain the phase contrast. Recommended by a

recent review article [37] the energy model suggests

that the phase angle / and the amplitude A of a

tapping cantilever are related to the average tip–

sample power dissipation, P , by expression:

P ¼k 0 A

2x0

2Q0

A0

Asin/

À 1

ð1Þ

where Q0 is quality factor and x0 is natural reso-

nant frequency.




The two main aspects [16] on explanation phase

contrast by energy model are (1) with the constant

amplitude scanning feedback control, the sine/ is

directly proportional to changes in the energydissipation. For a two-phase system, if the phase

stays either above or below the ‘‘free phase’’ (90°)

for both domains, the domain with the phase angle

closer to the ‘‘free phase’’ has the higher energy

dissipation; (2) since sine is a symmetric function

about 90°, symmetric jumps from phase >90° to

<90° are not due to changes in energy dissipation

but are caused by a switching between net attrac-

tive and net repulsive interaction forces. The sec-

ond aspect may lead to a even better phase contrast

and deserve more attention [38]. Moreover, it

worth mention that the Nanoscope III software

use the phase shift D/ rather than the phase angle

given in Eq. (1) to plot the phase image. With a

‘‘zero phase’’ function, the ‘‘free phase’’ is set to be

zero by the software resulting in positive and

negative phase shift value used in some literature

[2].

So far, most research has attributed the contrast

flips to changes in the tip–sample interaction be-

tween attractive and repulsive force [1,2]. For

convenience of discussion, the total force rt be-

tween the tip and sample is written as rt ¼ ra þ rr

where ra is attractive force caused by contamina-

tion layer composed of water and ambient debris

on the sample surface, rr is repulsive force. In our

case, it can be imagined that when the tip begins to

touch a virtually flat SEBS surface, it always gives

a deeper indention on soft PEB phase than on

hard PS phase for a given tapping force level.

Under the light tapping conditions where the tip–

sample interaction rt is dominated by attractive

force, the deeper tip indention on PEB phase re-

gion will lead to a higher attractive force, since thetip contacts a larger area with the contamination

layer (the adhesive top PEB surface should also be

regarded as part of the contamination layer for the

case). The larger adhesion force together with a

longer contact time gives rise to larger amplitude

damping of the tip on PEB phase, which in turn

makes this region brighter in the height image.

Note that based on our discussion the situation of

tip indention now is just opposite to the topogra-

phy given by AFM data. As for the phase data, it

is reasonable to believe that the phase angles for

both domains stay above 90° under extremely light

tapping conditions (r sp ¼ 0:95) [17]. So the domain

with larger energy dissipation (PEB) has a phaseangle closer to 90° (less negative phase shift) and

therefore looks brighter in the phase image (see

Fig. 4a).

Further increasing tapping force, the contribu-

tion of repulsive force rr increases. However its

increasing speed is much faster on PS phase than

on the PEB phase given that the PS phase is harder

and less deformable, and that on PEB phase the

increase of ra along with further deformation

(more contact area with the contamination layer)

counterparts the increase of rr. Then, it can be

expected when the total interaction force rt on PS

phase becomes repulsive, it is still attractive on

PEB phase. So, the inversion of phase image

happened, since phase for PS domain jump from

attractive phase to repulsive phase while phase for

PEB domain stay in attractive phase regime. Our

explanation on inversion of phase image is in good

agreement with Knoll et al.Õs D/ vs r sp quantitative

measurement on the similar SBS surface (Fig. 4 of

Ref. [2]). Their results showed that with decreasing

r sp the D/ values for the both domains increase

from negative to positive but for PS domain itincreases much faster. At this moment, for the

surface area where the amplitude damping by re-

pulsive force on PS phase is smaller than that by

the attractive force on PEB phase, inversion of

height image will not happen (see white squared

area in Fig. 4b). Finally under the moderate tap-

ping conditions, the repulsive forces will eventually

dominated on both PS and PEB phases, the harder

material (PS) will show larger decrease in ampli-

tude and inversion of height image completed (see

Fig. 4c).The effect of operating frequency on contrast-

reversal artifacts is also investigated. The same

result (not shown here) has been found as re-

ported by Magonov et al. [5] that increasing op-

erating frequency form lower-frequency side to

high-frequency side under moderate tapping con-

ditions will lead to a change of normal image like

Fig. 4c to an artifact image like white squared

area in Fig. 4b (reversal in height image only). On

the basis of theoretical study [39] which shows




high-frequency side driving will increase contri-

bution of attractive force, it is likely that by in-

creasing operating frequency, the tip–sample

interaction on PEB phase fall into attractive forcewhile on PS phase keep on repulsive force, and

that the repulsive force damping is smaller than

the attractive force damping.

Generally speaking, the uses of moderate tap-

ping condition, large free oscillation amplitude

and frequency at low-frequency side are recom-

mended to avoid contrast-reversal artifacts [14].

However, artifacts can also happen due to the

contamination [2] and/or bluntness of tip when

imaging on soft block surfaces and there is no

absolute regularity could follow according to our

experience. For example, good tips could also

provide the right image at light tapping condi-

tions while bad tips give you the artifacts even if

you use moderate or hard tapping force level. The

result can be explained as that the sharp and clean

tips have a lower capillary attraction with the

sample, since they have a small area of contact

within the contamination layer. They can also be

moved in and out of the layer more readily than

these contaminated or dull tips. Since AFM arti-

facts for the soft block copolymer system is very

common, it is necessary to comprehensively checkimaging parameters such as, set-point ratio, op-

erating frequencies, and free oscillation ampli-

tude, and check the image with different tips or

even use other imaging technique such as TEM

wherever possible. Only for a non-artifact image,

which is usually captured under moderate tappingconditions, can we assign the bright area in height

and phase images to hard phase of block co-

polymers.

4.4. Study on the effect of annealing on topography

Fig. 5 is the morphology of SEBS film cast from

xylene and annealed at 140 °C. By comparing the

AFM images recorded at the same area before

(Fig. 4c) and after (Fig. 5a) annealing treatment,

we provide a unambiguous evidence that anneal-

ing treatment will lead to the discrete higher

domains (PS) getting continuous, which is in

agreement with the previously reports [3,9]. The

corresponding TEM result (Fig. 5b) demonstrated

the reason for this change is thermally induced

order-to-order structure transition beneath the

surface. Such transition, which reduces the inter-

facial energy between the two phases, is expected

from the equilibrium consideration. Annealing

treatments of SEBS films cast from toluene show

the similar trend (see the difference between Figs.

2c, f and 6a, b). So the question of why the discretephase domains in AFM image become continuous

upon annealing is answered. In addition, the result

Fig. 5. Morphology (800Â 800 nm2) of SEBS film cast from xylene and annealed at 140 °C: (a) AFM height image for 0.5 h annealing

treatment. The height scale is 10 nm. (b) TEM micrograph for 1 h annealing treatment.




suggests that the use of annealing treatment in

AFM sample preparation of block copolymer

materials deserves special attention because it may

completely change sampleÕs original morphology.

Besides annealing treatment, solvent evaporation

speed [27] and solution-casting temperature [40]

also have influence on morphology of block co-

polymers, which could bring unwanted artifacts

during the sample preparation process.By the way, the previous report [6] tends to

attribute Ôthe assignment of lower domains in

AFM height images to the PS phaseÕ [3,4] to the

possible artifact of contrast inversion. But in Fig. 1

of Ref. [3], the observed phenomenon that higher

discrete domains getting continuous upon an-

nealing, which is the same as our finding suggests

that it was the ‘‘composition correlation method’’

rather than the artifact of contrast inversion cre-

ates the problem of phase attribution for their

case. In Fig. 1 of Ref. [4], it is also difficult to say if the images have an inversed contrast because as

the author pointed later [41] that what they be-

lieved PS cylinders perpendicular to the surface

structure where darker domains were assigned to

PS phase is possible in fact cartenoid lamellar

structure if brighter domains were assigned to PS

phase. In a word, by a single AFM image it is

often difficult to judge the phase separation

structure as well as whether it belongs to an arti-

fact.

5. Concluding remarks

Based on our research work, we like to give

following comments on interpreting AFM data of

soft block copolymer materials:

(1) Using special strategies and ordinary TM-

AFM imaging technique, it was confirmed that

lateral tip-indentation differences between dif-ferent phase domains rather than real surface

topography plays a key role to provide con-

trast in AFM height image.

(2) Reasonable agreement between AFM and

TEM data was reached in all cases, from which

we provide convincing evidence that under the

moderate tapping conditions, the higher spots

in AFM height image and the brighter do-

mains in phase image correspond to the hard

PS phase. However, due to the common exis-

tence of contrast reversal artifact, the explana-tion of phase separation structure for soft

block copolymers based solely on AFM data

should be very careful.

(3) TM-AFM artifacts of contrast reversal first

in phase image and then in height image were

observed in the research. Based on the tip-

indention mechanism, the phenomena were

attributed not only to the changes for the

tip–sample interaction from attractive force

to repulsive force but also to the difference of

Fig. 6. Morphology (300Â 300 nm2) of SEBS film cast from toluene and annealed at 140 °C for an hour: (a) AFM height image. Theheight scale is 3 nm. (b) TEM micrograph.




changing speed on soft and hard phase do-

mains.

(4) It was also found that the diameter value of PS

domains is larger in AFM images than in TEMmicrographs. The result suggests that usually

there seems to be a positive systematic error

existing in AFM images about the domain size

of block copolymers due to the tip-indention

contrast forming mechanism.

(5) An unexpected order-to-order structure transi-

tion caused by annealing in the block copoly-

mer thin film system was demonstrated by

both ex situ AFM and TEM characterization.

The result shows that the use of annealing

treatment for the block copolymer sample

preparation may in some cases changes the

original morphology of the system.

Acknowledgements

This work was partly supported by NSFC un-

der Grant 50243019, ‘‘The Research Fund for the

Doctoral Program of Higher Education, under

Grant 20010213029’’ and ‘‘Scientific Research

Foundation for the Returned Overseas Chinese

Scholars, State Education Ministry’’.

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