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Molecular Imaging of Alkanol Monolayers on Graphite.

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Molecular Imaging of Alkanol Monolayers
on Graphite**
By Stefan Buchholz and Jurgen P.Rube*
The adsorption behavior of long-chain alkanes and alkyl
derivatives on the basal plane of graphite has been of considerable interest as a model for molecular adsorption. Measurements of the adsorption isotherms have shown that alkanes, alkanols, and fatty acids adsorb from nonpolar solvents
as densely packed monolayers with the molecules parallel to
the basal plane of graphite. The monolayers are stable
against further adsorption over a broad concentration
range." - 31 With scanning tunneling microscopy (STM) the
direct in situ structure characterization of
and similar['- 1' monolayers has become possible recently (see the
review on STM investigations of molecular systems)." We
report here on the structure and dynamics of monomolecular
adsorbate layers of long-chain alkanols on graphite.
Figure 1 displays highly ordered areas in monolayers of
octadecanol (C,,H,,OH), tetracosanol (C,,H,,OH),
and
1,12-dodecanediol (HOC, 2H,,0H). The molecules are
addition, small differences in the interaction energy of the
monolayer with the substrate may influence the packing behavior.
In the case of the tetracosanol (Fig. 1 b) in particular,
neighboring lamellae boundaries clearly exhibit different image contrast, indicating a bilamellar structure. This difference is less obvious in the case of octadecanol but the bilamellar structure is also likely there, since only in that case
can all hydroxyl groups partcipate in hydrogen bonds. In
general hydroxyl groups and oxygen functionalities (e.g. carboxyl groups) do not produce a distinctly different image
contrast from alkyl chains.
The monolayers of the monofunctional alkanols exhibit
many defects-the areas shown in Figure 1 a,b are the largest
defect-free domains that have been observed. The reason is
the tilted arrangement of the molecules in the lamellae,
which allows a dense packing also at the domain boundaries.
Hence, the line tensions at the domain boundaries are relatively small, similar to the case of 1,4-didodecylben~ene.[~I
Figure 2a,b shows a domain boundary of a monolayer of
tetracosanol and octadecanol, respectively. At the domain
Fig. 1. STM images of alkanol monolayers on graphite: a) octadecanol (image size: 9.7 nmx 7.8 nm); b) tetracosanol (image size: 8.1 nm x 7.6 nm);
c) 1,12-dodecanediol (image size:
8.8 nm x 7.4 nm).
packed parallel to each other in lamellae. From the width of
the lamellae it can be deduced that they exhibit all-trans
conformations. At the lamellae boundaries a displacement
of half the molecular width is observed. The comparison of
STM images before and after the adsorption of the monolayers shows that both the long axes of the molecules and the
lamallae boundaries are oriented parallel to basis vectors of
the graphite lattice. The angle of 60" between the molecular
axes and the lamellae boundaries corresponds very well to a
displacement of neighboring alkyl chains by two methylene
units. Since the plane containing the carbon backbone is
almost perpendicular to the substrate['] only every second
methylene unit is in direct contact with the graphite surface.
The repeating unit of two methylenes amounts to 254 pm
and agrees well with the periodicity observed along the alkyl
chains by STM. In the octadecanol and the dodecanediol
(Fig. 1 a,c) predominantly herringbone structures are observed, while for the longer chain alkanols tetracosanol
(Fig. 1 b) and triacontanol (C30H610H,Fig. 4) the long axes
of the molecules in neighboring lamellae are parallel to each
other. The fact that the shorter chain alkanol and the alkanediol exhibit the herringbone structure indicates that the tilted
arrangement favors hydrogen bonding whereas the parallel
arrangement allows a stable packing of the alkyl chains. In
[*] Dr. J. P. Rabe, Dr. S . Buchholz
[**I
Max-Planck-Institut fur Polymerforschung
Postfach 3148, D-W-6500 Mainz (FRG)
This work was supported by the European Science Foundation (Additional Activity: "Chemistry and Physics of Polymer Surfaces and Interfaces")
and the Fonds der Chemischen Industrie (Kekule scholarship for S . B).
A n g m . Cheni. Int. Ed. Engl. 31 11992) No. 2
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boundary of the tetracosanol, the direction of the displacement of neighboring molecules parallel to each other is reversed. In the octadecanol case shown here every second
lamella encounters the lamella of a different domain at an
obtuse angle, due to the herringbone structure. The CPK
Fig. 2. STM images of domain boundaries in a a) tetracosanol and b) octadecanol monolayer (image sizes: a) 10.4 nm x 7.8 nm; b) 10.6 nm x 12.9 nm).
model displayed in Figure 3 shows that also in this case the
molecules are able to pack densely without breaking hydrogen bonds. 1,12-Dodecanediol also exhibits a herringbone
pattern, but in this case such a domain boundary would
involve a loss of hydrogen bonds. Therefore such defects
would be highly energetic and should occur rarely, if the
development of the defects is not kinetically controlled. Indeed in the case of the diol, whose packing is highly stabilized
Verlagsgesellschajl mbH. W-6940 Weinheim, 1992
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Often the domain boundaries described above are observed in combination with each other resulting in very complicated defect patterns. As an example, Figure 6 a shows an
image of a tetracosanol monolayer with a domain that consists of only two lamellae in one and probably seven molecules in the other direction. In such defect-rich areas of the
monolayers reorientations occur very often, resulting in
noisy pictures. Occasionally we succeeded in imaging the
metastable states with high resolution before (Fig. 6a) and
after (Fig. 6 b) the reorientation (see also Ref. [6, 71). Such
reorganizations of the monolayers are probably combined
with a desorption of at least part of the affected molecules.
The reorientation processes are too fast to allow their details
to be followed.
Fig. 3. CPK model of the domain boundary of the octadecanol monolayer
shown in Figure 2 b.
through the hydrogen bonds, not a single domain boundary
has been observed on an area of several pm2.
Another type of domain boundary often observed is
shown in Figure 4a, which displays the STM image of a
monolayer of triacontanol. At these domain boundaries the
lamellae of one domain encounter those of another at an
obtuse angle. Here also a dense packing of the molecules is
observed. Corresponding to the lamella boundaries in the
Fig. 6. STM images of a defect-rich area in a tetracosanol monolayer. Image b)
was recorded only a few seconds after image a). In this time the monolayer was
reorganized in the upper part of the image. (Image sizes: a) 11.5 nm x 12.3 nm;
b) 11.5nmx10.9nm).
Long-chain alkyl derivatives are often contaminated with
homologous impurities, if no extensive purification is performed. As shown in Figure 7 such impurities can be included in the monolayers. Above one lamella (A) of triacontanol
Fig. 4. STM images of domain boundaries in a triacontanol monolayer (image
size: 13.9 nm x 10.3 nm).
left domain, the molecules in the right domain are shifted by
one graphite repeat unit, thereby allowing adaptation to the
contour of the left domain. This displacement in the right
domain is extended over a few lamellae. Figure 5 shows an
enlargement of such a displacement within the domain.
Close to the region shown in Figure 4 a a corner of the left
domain could be recorded (Fig. 4 b).
Fig. 7. STM image o f a triacontanol monolayer containing a longer chain
impurity. Whereas the width of lamella A is as expected for triacontanol, lamella B is significantly broader. The molecules in lamella B have a length of 40
methylene units. Furthermore, imbedded in lamella A are two longer molecules.
Adjacent to them in lamella B are two shorter triacontanol molecules allowing
a dense packing despite the defect (image size: 10.5 nm x 9.3 nm).
Fig. 5. STM image of a displacement at a Iamella boundary in a triacontanol
monolayer. The image has been recorded close to the domain boundary shown
in Figure 4 a (image size: 2.9 nm x 2.3 nm).
190
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molecules there is a broader lamella (B),
. ,. in which the immrities are enriched. Whereas in lamella A 15 repeat units are
Observed along the molecules,correspondingto the length Of
the triacontanol molecules, in lamella B the molecules con-
057(~-o833/9Z/o202-0190X 3 . 5 0 t ,2510
Angew. G e m . In[. Ed. En'?/. 31 (1992) No. 2
sist of 20 repeating units corresponding to a length of 40
methylene units. In addition, one can recognize that two
molecules of the longer chain impurity are included in
lamella A. Adjacent to them in lamella B are two triacontanol molecules allowing a regular packing in the following
lamellae.
The present investigation has shown what detailed information on structure and dynamics of monomolecular alkano1 adsorbate layers can be obtained by scanning tunneling
microscopy. The packing pattern in monodomains as well as
at domain boundaries indicate the importance of hydroxyl
end groups for the packing of primary alkanols with up to 30
carbon atoms. In particular the patterns at the many domain
boundaries are consistent with a tendency to form as many
hydrogen bonds as possible. Reorganizations of the domain
structure could be followed on a millisecond timescale, and
finally the inclusion of single molecules of an impurity into
the monolayer could be observed for the first time.
Experimental
For the basics of scanning tunneling microscopy see [I 1- 131 and for technical
details of the instrument used here, [14]. All images were recorded in the variable current mode with a tunneling current of 1 nA and a bias of 1 +/- 0.5 V
(tip positive). In this regime no pronounced dependence of the image contrast
on the bias was observed. Tunneling currents higher than 1 nA resulted generally in images with higher noise levels, whereas with lower currents only poor
image contrast was obtained. The scanning frequencies in x and y direction
were 200 and 0.3 Hz, respectively, equivalent to 1.5 s per image at a resolution
of about 200 x 200 pixels. The data was recorded continuously on video tape
[14]. Tunneling tips were mechanically formed from platinum/iridium wire.
Only those allowing atomic resolution on graphite were used for the studies of
the monolayers.
Imaging of the monolayers was performed at the interface of graphite with a
solution of the appropriate alkanol in 1-phenyloctane. The alkanol solutions
were prepared from concentrated solutions after removing undissolved material by filtration and subsequent dilution by 10% with pure solvent.
I-octadecanol(99%), I-tetracosanol (99%), l-triacontanol(96%), 1.12-dodecanediol (99%) were purchased from Aldrich and used without further purification.
Received: July 16, 1991 [Z 4797 IE]
German version: Angew. Chem. 1992, 104, 188
CAS Registry numbers :
Ci,H,,OH,
112-92-5; Cz4HwOH, 506-51-4; HOClzHz,OH, 5675-51-4;
C,,H,,OH, 593-50-0; grahite, 7782-42-5.
[I] A. J. Groszek, Nature 1962, 196, 531 -533; ibid. 1964, 204, 680; Proc. R .
Soc. London Ser. A 1970,314,473-498.
[2] G. H. Findenegg. M. Liphard, Carbon 1987, 25, 119-128.
[3] U. Bien-Vogelsang, G. H. Findenegg, Colloids Surf 1986, 21, 469-481.
[4] G. C . McGonigal, R. H. Bernhardt, D. J. Thomson. Appl. Phys. Lezt.
1990. 57. 28-30.
[5] G . C. McGonigal, R. H. Bernhardt, Y. H. Yeo, D. J. Thomson, J. Vac. Sci.
Technol. B 1991, 9. 1107-1110.
[6] J. P. Rabe. S . Buchholz, Phys. Rev.Lett. 1991, 66, 2096-2099.
[7] J. P. Rabe. S . Buchholz, Science 1991. 253, 424-427.
[8] J. P. Rabe. S . Buchholz, Makromol. Chem. Macromol. Symp. 1991, 50,
261 -268.
[9] D. P. E. Smith. J. K. H. Horber, G. Binnig, H. Nejoh, Nature 1990, 344,
641 -644.
[lo] R. Kuroda, E. Kishi, A. Yamano, K. Hatanaka. H. Matsuda, K. Egushi,
T. Nakagiri, J. Yac. Sci. Technol. B 1991, 9. 1180-1183.
[I 11 J. P. Rahe, Ultramicroscopy 1992, in press.
1121 G. Binnig, H. Rohrer, Angew. Chem. 1987. 99, 622-631; Angew. Chem.
I n t . Ed. Engl. 1987, 26, 606-615.
[13] H. Rohrer, Scanning Tunneling Microscopy and Related Methods (Eds.:
R. J. Behm, N. Garcia, H. Rohrer), NATO ASI Ser., Ser. E : Appl. Sci.,
Vol. 184. Dordrecht, 1990. p. 1-25.
[14] J. P. Rabe. M. Sano, D. Batchelder, A. A. Kalatchev, J. Microsc. (Oxf o r d ) . 1988, 152. 573-583.
Angew. Chem. fni. Ed. Engl. 31 (1992) N o . 2
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The Trapping of Iron Hydroxide Units by the
Ligand “heidi”: Two New Hydroxo(oxo)iron
Clusters Containing 19 and 17 Iron Atoms
By Sarah L. Heath and Anne K . Powell*
Compounds containing extended arrays of iron centers
linked by oxide and hydroxide ions are common in nature.
Examples include the core of the iron-storage protein f e d in, which can accommodate up to 4500iron atoms and a
variety of iron minerals as well as rust. The importance of
such materials has fueled efforts to synthesize and characterize smaller hydroxo(oxo)polyiron complexes to serve as
models. Although a variety of interesting polyiron complexes has resulted, including structures with eight,”’ ten,[21and
elevenL3]iron centers, as well as the heterometal cluster
[Fe,,MO,,(OH),o(O,CPh),,l, M = Mn or Co,I4l none has
exhibited the known features of the naturally occurring compounds.
We describe here the synthesis and characterization of
three iron complexes with the ligand heidi (H,heidi =
N(CH,COOH),(CH,CH,OH)); two of these complexes are
examples of a new type of hydroxo(oxo)iron cluster, in
which the close-packed cluster core is enclosed in a shell of
ligands which are complexed to further iron atoms. X-ray
Fig. 1. The crystal structures of I (left) and 2 (right); Fe shown in green, 0 in
red.
crystallography[’] reveals these clusters to contain 19 and
17 iron atoms; thus 1 and 2 are the largest polyiron complexes of this type characterized to date (Fig.
The number of iron atoms in clusters 1 and 2 is determined
by the arrangement of Fe/heidi units around the core structure common to both compounds. This cluster core consists
of an [Fe,(p,-OH),(p,-OH)4{(p3-O)Fe}2]’3+
unit. In 1 this
is surrounded by ten iron heidi units linked to the core by
p 3 - 0 , p2-OH, and alkoxo bridge units from the heidi ligands.
This shell, formulated as [Fe,o(heidi),o(H,0),,(p,-O)4(p,gives an overall charge of 1 + to 1. The vacant
sites on the peripheral iron centers are taken up with water
molecules so that each iron atom is octahedrally coordinat(‘1
Dr A. K. Powell, S . L. Heath
School of Chemical Sciences
University of East Anglia
Norwich NR4 7TJ (UK)
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0570-0833/92/0202-0191$3.50+ ,2510
191
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