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Transcript of Surface Science 03
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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
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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
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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.
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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
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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
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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
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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.
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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.
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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.
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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
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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.
146 Y. Wang et al. / Surface Science 530 (2003) 136–148
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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.
Y. Wang et al. / Surface Science 530 (2003) 136–148 147
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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’’.
References
[1] R. Brandsch, G. Bar, M.-H. Whangbo, Langmuir 13
(1997) 6349.
[2] A. Knoll, R. Magerle, G. Krausch, Macromolecules 34
(2001) 4159.
[3] R. van den Berg, H. de Groot, M.A. van Dijk, Polymer 35
(1994) 5778.
[4] M.A. van Dijk, R. van den Berg, Macromolecules 28
(1995) 6773.
[5] S.N. Magonov, J. Cleveland, V. Elings, D. Denley, M.-H.
Whangbo, Surf. Sci. 389 (1997) 201.[6] R.S. McLean, B.B. Sauer, Macromolecules 30 (1997) 8314.
[7] I. Luzinov, D. Julthongpiput, V.V. Tsukruk, Macromol-
ecules 33 (2000) 7629.
[8] J. Hahm, W.A. Lopes, H.M. Jaeger, S.J. Sibener, J. Chem.
Phys. 109 (1998) 10111.
[9] M. Motomatsu, W. Mizutani, H. Tokumoto, Polymer 38
(1997) 1779.
[10] R. Hoper, T. Gesang, W. Possart, O.-D. Henneman, S.
Bosek, Ultramicroscopy 60 (1995) 17.
[11] S.N. Magonov, V. Elings, V.S. Papkov, Polymer 38 (1997)
297.
[12] R. Garcia, J. Tamayo, M. Calleja, F. Garcia, Appl. Phys.
A 66 (1998) S309.
[13] G. Bar, Y. Thomann, R. Brandsch, H.J. Cantow, M.-H.Whangho, Langmuir 13 (1997) 3807.
[14] D. Raghavan, X. Gu, T. Nguyen, M. VanLandingham, A.
Karim, Macromolecules 33 (2000) 2573.
[15] G. Bar, M. Ganter, R. Brandsch, L. Delineau, M.-H.
Whangho, Langmuir 16 (2000) 5702.
[16] J.P. Cleveland, B. Anczykowski, A.E. Schmid, V.B. Elings,
Appl. Phys. Lett. 72 (1998) 2613.
[17] J. Tamayo, R. Garcia, Appl. Phys. Lett. 71 (1997) 2394.
[18] J. Tamayo, R. Garcia, Langmuir 12 (1996) 4430.
[19] S.N. Magonov, V. Elings, M.-H. Whangbo, Surf. Sci. 375
(1997) L385.
[20] X. Chen, M.C. Davies, C.J. Roberts, S.J.B. Tendler, P.M.
Williams, J. Davies, A.C. Dawkes, J.C. Edwards, Ultra-
microscopy 75 (1998) 171.
[21] X. Chen, M.C. Davies, C.J. Roberts, S.J.B. Tendler, P.M.
Williams, N.A. Burnham, Surf. Sci. 460 (2000) 292.
[22] H. Hasegawa, T. Hashimoto, Polymer 33 (1992) 475.
[23] D.W. Schwark, D.L. Vezie, J.R. Reffner, E.L. Thomas,
B.K. Annis, J. Mater. Sci. Lett. 11 (1992) 392.
[24] E. Gattiglia, A. Turturro, D. Ricci, A. Bonfiglio, Macro-
mol. Rapid Commun. 16 (1995) 919.
[25] C.Y. Ryu, T.P. Lodge, Macromolecules 32 (1999) 7190.
[26] N. Sakamoto, T. Hashimoto, C.D. Han, D. Kim, N.Y.
Vaidya, Macromolecules 30 (1997) 5321.
[27] G. Kim, M. Libera, Macromolecules 31 (1998) 2569.
[28] J.M.G. Cowie, D. Lath, I.J. McEwen, Macromolecules 12
(1979) 52.[29] M. Hegner, P. Wagner, G. Semneza, Surf. Sci. 39 (1994)
291.
[30] C. Ton-That, A.G. Shard, R.H. Bradley, Langmuir 16
(2000) 2281.
[31] K. Emoto, Y. Nagasaki, K. Kataoka, Langmuir 16 (2000)
5738.
[32] T. Serizawa, K. Nanameki, K. Yamamoto, M. Akashi,
Macromolecules 35 (2002) 2184.
[33] Y. Wang, S. Dong, J.S. Shen, IUPAC World Polymer
Congress 2002, China, Beijing, 2002, 10e-4p-54.
[34] T. Inoue, H. Soen, T. Hashimoto, H.J. Kawai, Polym. Sci.
A 2 (7) (1969) 1283.
[35] R.A. Weiss, A. Sen, L.A. Pottic, C. Willis, Polymer 32
(1991) 2785.[36] C. Harrison, M. Park, P. Chaikin, R.A. Register, D.H.
Adamson, N. Yao, Macromolecules 31 (1998) 2185.
[37] R. Garcia, R. Perez, Surf. Sci. Rep. 47 (2002) 197.
[38] X. Chen, C.J. Roberts, J. Zhang, M.C. Davies, S.J.B.
Tendler, Surf. Sci. 519 (2002) L593.
[39] R. Garcia, A. San Paulo, Phys. Rev. B 60 (1999) 4961.
[40] Y. Wang, J.S. Shen, C.F. Long, Polymer 42 (2001) 8443.
[41] H.P. Huinink, J.C.M. Brokken-Zijp, M.A. van Dijk,
G.J.A. Sevink, J. Chem. Phys. 112 (2000) 2452.
148 Y. Wang et al. / Surface Science 530 (2003) 136–148