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Atomically Resolved Structure of Fracture Surfaces of a BaSiOC Glass with Atomic Force Microscopy.

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a) W. Uh1,Angew. Chem. 1993,105,1449;Angew. Chem. Int. Ed. Engl. 1993,
32,1386; b) C. Dohmeier, D. Loos, H. Schnockel, ibid. 1996,108, 141 and
1996,35,129; c) N. Wiberg, K. Amelunxen, H. Noth, H. Schwenk, W. Kaim,
A. Klein, T. Scheiring, ibid. 1997, 109, 1258 and 1997,36, 1213.
a) X.-W. Li, W. T. Pennington, G. H. Robinson, J. Am. Chem. Soc. 1995,117,
7578: b) X.-W. Li, Y. Xie, P. R. Schreiner, K. D. Gripper, R. C. Crittendon,
C. F. Campana, H. F. Schaefer, G. H. Robinson, Organometallics 1996, 15,
D. Loos, H. Schnockel, D. Fenske, Angew. Chem. 1993,105, 1124: Angew.
Chem. Int. Ed. Engl. 1993,32, 1059.
C. U. Doriat, E. Baum, A. Ecker, H. Schnockel, Angew. Chem. 1997, 109,
2057; Angew. Chem. Int. Ed. Engl. 1997 36, 1969.
a) W. Uhl, W. Hiller, M. Layh, W. Schwarz, Angew. Chem. 1992,104, 1378;
Angew. Chem. Int. Ed. Engl. 1992,31,1364; b) G. Linti, J. Orgunomet. Chem.
1996, 520, 107.
0.T. Beachley, Jr., J. C. Pazik, M. J. Noble, OrgmometaNics 1994,13,2885.
M. L. H. Green, P. Mountford. G. J. Smout, S. R. Speel, Polyhedron 1990.9,
a) H. Gilman, C. L. Smith, J. Organomet. Chem. 1968, 14, 91; b) G.
Gutekunst, A. G. Brook, ibid. 1982, 225, 1; c) A. Heine. R. Herbst-lrmer,
G. M. Sheldnck, D. Stalke, Inorg. Chem. 1993,32,2694.
M. C. Kuchta, J. B. Bonnano, G. Parkin, J. Am. Chem. SOC.1996,118,10914.
X-ray crystal structure analysis: STOE IPDS, Mo,, radiation. Structure
solution with direct methods, full-matrix least-squares refinement against
F', hydrogen atoms as riding model. Program used: Siemens SHELXTL 5.0
(PC). 2: C52H140Ga413Li04Si,6.0.25C4H100,
crystal dimensions: 0.20 x
0.15 x 0.15 mm3, monoclinic, space group P2,/c, u = 1763.3(2), b =
2377.3(3), c=2517.4(5) pm, p=99.91(2)", V = 10.395(3) nm', 2 = 4 , pcalFd
1.255 g ~ m - p
, 2.13 mm-I, 41 561 measured reflections in 28 = 4-50",
16324 [13847 with F > 4o(F)] independent reflections, 945 parameters. R, =
D.043, wR2= 0.135 (all data), max. residual electron density 0.82 e i k 3 . Two
hypersilyl groups are disordered in a manner, that for each of the three
trimethylsilyl groups two silicon positions are observed, but 213 of the
carbon atoms do not have split positions. 3: C,oHl,4Ga,Li0,Siz4, crystal
dimensions: 0.20 x 0.20 x 0.15 mm3, monoclinic, space group C2k,
a=2060.0(4). b=2280.4(5), c=2929.0(6) pm, ,B= 108.25(3)", V =
13.067(5) nm', Z = 4, pcalcd
= 1.224 gem--', p = 2.078 mm-', F(000) = 5056,
38397 measured reflections in 28=4-48", 10081 (6456 with F>4o(F))
independent reflections, 830 parameters. R, = 0.044, wRI = 0.127 (all data),
max. residual electron density 0.82 e,&,. The extreme disorder oi Li(thf):
is described only inadequately by a split model. The Ga, framework shows
also slight disorder (96:4), which equals a rotation around a C, axis
orthogonal to the Ga, plane. Further details of the crystal structure
investigations are available on request from the Fachinformationszentrum
Karlsruhe, D-76344 Eggenstein-Leopoldshafen,on quoting the depository
numbers CSD-407072 (2) and CSD-407017 (3)
[Ill G. Lmti, W. Kostler, Angew. Chem. 1996,108, 593; Angew. Chem. Int. Ed.
Engl. 1996,35,550.
[12] a) J. C. Beamish, M. Wilkinson, I. J. Worrall, Inorg. Chem. 1978.17,2026; b)
G. Gerlach, W. Honle, A. Simon, 2. Anorg. Allg. Chem. 1982,486,7.
[13] G. L i d , R. Frey, A. Appel, Chem Ber. 1996,129, 561.
[14] R. Nesper, Angew. Chem. 1989,101,99; Angew. Chem. lnt. Ed. Engl. 1989,
28, 58.
[15] DFT calculations with RI-approximation using the program package
TURBOMOLE: B-P86 functional, SV(P)-basis, R. Ahlrichs, 1996.
[16] R. L. Wells, S. Shafieezad, A. T. McPhail, C. G. Pitt, J. Chem. Soc. Chem.
Commun. 1987,1823.
Atomically Resolved Structure of
Fracture Surfaces of a Ba/Si/O/C Glass with
Atomic Force Microscopy**
Wolfgang Raberg, Volker Lansmann, Martin Jamen,*
and Klaus Wandelt"
As sharp discrepancy with the importance of amorphous
solids[']-which include on one hand the oxide glasses as one
of the largest and most versatile material classes and on the
other the amorphous
or the amorphous high
performance ceramics[3] as materials of the future-the
knowledge about their structures is far from
Indeed, the highly developed repertoire of methods for
structural analysis of molecules and crystalline solids fails
when applied to samples which do not exhibit any translational symmetry.
Spectroscopic methods yield quite reliable information on
the nearest neighbors (first coordination sphere) of a given
atomic ~pecies.1~1
Despite the enormous progress, for instance
in nuclear magnetic resonance techniques (spin-echo and
double-resonance techniques)l6I or in diffraction methods
(determination of partial structure factors)
I ' [ , only averaged
information about the medium- and long-range order is
obtainable. For this reason only in specific cases (e.g., in the
amorphous modification of elements)[81has it been possible to
develop concrete and detailed structural models that explain
all experimental data.
A potential alternative to structurally sensitive probes,
which reach their limits in applications on amorphous
substances and therefore cannot give definite information
on this topic, is the use of as many different methods as
possible in order to obtain complementary as well as overlapping structural information. By combining this data with
theoretical simulations concrete structural models may be
developed. But again such a model gives the amorphous
structure only indirectly. The better approach would be a
direct imaging of the structure, which has recently become
possible with scanning probe methods. In this paper we
present the first image of the fracture surface of a glass with
atomic resolution obtained with the atomic force microscope
(AFM);Iyl this image gives direct insight into the glass
A barium silicate glass was chosen as test system for this
investigation, since a good contrast and differentiated distance spectrum was expected from this material. Furthermore,
the crystalline structures of several barium silicates are well
which may facilitate classification of the observed
structural units. In order to strengthen the glass network,
carbon was introduced in the melting process. That the carbon
indeed was inserted into the silicate network was proved by
2ySiMAS-NMR spectroscopy.["] The concomitant increase of
the glass transition temperature indicates a higher degree of
crosslinking in the oxycarbide glass than in the oxide glass. A
[*I Prof. Dr. M. Jansen, DipLChem. V. Lansmann
Institut fur Anorganische Chemie der Universitat
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax: Int. code + (228)73-5660
e-mail :
Prof. Dr. K. Wandelt, Dipl. Phys. W. Raberg
Physikalische Chemie der Universitat
Wegelerstrasse 12,53115 Bonn (Germany)
Fax: Int. code + (228)73-2551
[**] This work was supported by the Deutsche Forschungsgemeinschaft (DFG)
through Sonderforschungsbereich 408 (Inorganic solids without translational symmetry).
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
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Angew. Chem. fnt. Ed. Engl. 1997,36, No. 23
fragment of this oxycarbide glass was introduced into an ultra
high vacuum chamber (UHV). The fracture surface of the
sample was cleaned in an outgassing procedure and then
investigated with the atomic force microscope in the contact
On a 1000 nm x 1000 nm scale (Figure 1) of the untreated
fracture surface one can clearly recognize a distinct granular
structure comprising grains 150 nm to 200 nm in diameter.
Figure 2. Topographic AFM image (constant force mode. F = 2 5 n N ) of a
3.9 nm x 3.9 nm section of the BalSiiOiC glass after an outgasing procedure of
several hours at around 400 K. The white circles indicate the atomic positions
used for the calculation of the pair distance distribution. The grey scale
corresponds to a height difference of 0.3 nm.
Figure 1. AFM image of the unprepared glass surface (contact force mode;
maximum height difference about 50 nm).
These structures couid not be observed in an earlier investigation with a conventional scanning electron microscope
(usual lateral resolution in the range of 200 nm). The reason
for the development of the grain boundaries and their
composition is not known yet. An obvious possibility is a
beginning phase separation.
Since the surface roughness between the domain walls is
very small, the atomic structure of the fracture surface can be
imaged. Figure 2 shows a topographic image of a 3.9 nm x
3.9 nm section after an outgasing procedure of several hours
at around 400 K. The distances between the brighter regions,
which correspond to elevations on the surface, are of the order
of the expected interatomic distances and suggest that in this
case atomic resolution was indeed obtained. There are no
hints of periodic structures, only several more regular units
like rings or chains.
In Figure 3 the pair distribution function corresponding to
Figure 2 is shown, which in the following section is compared
with the structural properties of a crystalline barium silicate,
with experimental data (X-ray and neutron-scattering measurements[”]) on a barium disilicate glass, and with results
from electron diffraction experiment^"^] on the identical
oxycarbide glass. The pair distribution function in Figure 3 is
representative for the pair distribution functions corresponding to images of other areas of the fracture surface.
In the case of a totally disordered distance distribution one
would expect a linear increase in the pair distribution
function. This is indeed observed in Figure3. In addition,
several intensity peaks are superimposed upon the linearly
Angew. Chrm. Inr. Ed. Engl. 1997.36.No. 23
.. -
pair distance I nmFigure 3 Interatomic pair distribution function determined from Figure 2
rising background, which indicates increased occurence of
characteristic pair distances.
The shortest distance of 0.16 nm in the crystalline reference
material is also observed in electron diffraction experiments
with this glass, and in the X-ray and neutron scattering
measurements of the barium disilicate glass, but does not
appear in the AFM measurements (Table 1). Since this
distance is attributed to Si - 0 bonding, this observation
suggests that no Si atoms are on the surface and that the Si04
tetrahedra are intact, This is supported by the absence of a
peak at 0.3 nm, corresponding to a Si-Si distance in the
crystalline reference material.
The distance of 0.27 nm can be attributed to the Ba - 0 and
the 0 - 0 distance (tetrahedral edge), in full agreement with
the results from the crystalline barium disilicate and from the
diffraction measurements with glasses. In contrast, the peak at
0.21 nm is not found in the reference data; this peak might
represent an 0 - C distance (edge of an Si0,C tetrahedron).
In the same way the other distances (0.34 nm, 0.41 nm, and
0.44nm) can be attributed to particular atomic pairs. The
distance of 0.34 nm seems to be assignable only if Si atoms are
considered. This is, however, not in contradiction to the
interpretation that no silicon is at the surface, since this
distance might result from a bond between an oxygen atom
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
0570-083319713623-2647$ 17.50+.SOIO
Table 1. Comparison of interatomic distances [nm] and their assignment
extracted from pair distribution functions of the BdSiiOiC glass with those in
crystalline BaSi,O, and amorphous barium disilicate.
From AFM of From electron
BaiSilOiC glass diffraction of
From neutron
and X-ray diffraction of a
In crystalline
BaSi,O, [14]
c-0 ( ? )
A fragment of this sample was clamped to the sample holder with a small vice.
The images were obtained under ultrahigh vacuum after an outgassing procedure
of several hours at about 400 K with a combined atomic forcelscanning tunneling
microscope (Omicron Vakuumphysik GmbH). The AFM measurements were
carried out in the contact mode immediately after transfer with single crystal
silicon cantilevers (Nanosensors) and constant loading forces in the range
25 nN-50 nN [ 171.
Received: June 5,1997 [ZlO511IE]
German version: Angew. Chem. 1997,109,2760-2762
Keywords: amorphous materials
glasses structure elucidation
[a] Distances above 0.45 nm are not assigned to a certain atomic distance pair
and are attributed to medium range order effects (mro).
situated perpendicular (with respect to the surface) to a Si
atom and a neighboring atom.
At larger distances further peaks clearly indicate a mediumrange order in the investigated glass. However, a definite
assignment has yet to be done. The appearance of the 0.77 nm
peak both in AFM and diffraction measurements is particularly remarkable, since this is a lattice constant of crystalline
barium disilicate.
Two possible problems arise with the direct imaging of
fracture surfaces of amorphous solids by atomic force microscopy, if one wants to resolve the bulk structure of glasses.
First, the structures on the fracture surface might undergo a
relaxation and therefore not correspond exactly to the
position of the atoms in the bulk. Second, as the fracture will
take place predominantly at the weaker bonds in the glass,
these areas will dominate the images. However, since the
relaxation in glasses with a highly developed network and low
crystallization tendency will only result in displacements and
not in reconstructions one can expect that the principles
(repetition of structures, ring dimension, residues of translational symmetry) of medium- and long-range order will be
preserved and reproduced in the image. In the case of the Bal
SilOlC glass the justification for this assumption is proved by
the good agreement of the AFM results with data obtained
from completely independent measurements.
Atomic force microscopy is superior to all other methods of
studying solids without translational symmetry in that it
allows direct imaging of the atomic structure and therefore
can be used to develop a reliable model of the structure of
glasses. In the present case the classical controversy concerning the structure of glass between the hypothesis of Zachariasen (“network hyp~thesis”)~’~]
and that of Lebedew (“crystallite
can be settled. The AFM images show
unambiguously that the barium silicate glass consists of a
tetrahedral network.
Experimental Section
The BalSiiOiC glass was made under argon atmosphere by melting a mixture of
BaCO,, SiO,, and S i c in a high frequency furnace in a BN crucible placed in a
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
carbon crucible to ensure inductive coupling. The melt was held at 1650°C for
30 min and after switching off the furnace cooled down in a strong stream of
argon. The average cooling rate from maximum temperature to around 700°C
was about 10 Ks-’. The cation ratio Ba:Si in the starting mixture was 37:63 with
3 atom % carbon in form of Sic.
- atomic force microscopy
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Sergeants-and-Soldiers Principle in Chiral
Columnar Stacks of Disc-Shaped Molecules
with C3Symmetry**
Ania R. A. Palmans, Jef A. J. M. Vekemans,
Edgko E. Havinga, and E. W. Meijer*
Seminal studies by Green and co-workers to understand the
underlying principles of chirality in stiff helical polymers such
as the polyisocyanates have led to observations referred to as
the “Majority Rule” and the “Sergeants-and-Soldiers Principle”.[’] Poly(n-alkyl isocyanates) adopt an extended helical
conformation in which long stretches of one helical sense, P o r
M , are separated by high-energy reversals. To favor one
[*] Prof. Dr. E. W. Meijer, A. R. A. Palmans, Dr. J. A. J. M. Vekemans,
Dr. E. E. Havinga
Laboratory of Organic Chemistry
Eindhoven University of Technology
P.O. Box 513, NL-5600 MB Eindhoven (The Netherlands)
Fax: Int. code +(40)245-1036
[**I We would like to thank Dr. H. Fischer and Dr. R. Hikmet for stimulating
discussions. Unrestricted research grants from Philips Research and DSM
Research are gratefully acknowledged.
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Angew. Chem. Int. Ed. Engl. 1997.36, No. 23
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resolved, atomically, structure, microscopy, basioc, surface, atomic, force, fractured, glasn
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