Communications
Antimony Tetramers
Crystalline Structures of Sb4 Molecules in
Antimony Thin Films**
Thorsten M. Bernhardt, Bert Stegemann,
Bernhard Kaiser,* and Klaus Rademann
Elemental antimony occurs in the solid bulk in a metallic as
well as an amorphous modification.[1] Over a wide temperature range the gas phase of elemental antimony comprises
almost exclusively of tetrameric molecules;[2] however, a
solid-state modification of antimony consisting of Sb4 molecules, comparable to white phosphorus and yellow arsenic, is
not known to date. This is explained by the fact that the Sb4
molecule is so unstable that even quenching with liquid
nitrogen leads to the polymeric amorphous phase.[3]
Herein we demonstrate that scanning tunneling microscopy (STM) can be used to identify unexpected crystalline
ordered areas with nearly cubic symmetry in antimony thin
films that are prepared under appropriate conditions from Sb4
molecules. The lattice constants of these structures are clearly
different from the lattice constants of metallic a-antimony
and are in agreement with the known structural data of
tetrahedral Sb4 molecules.[4±6] The symmetry of the wellordered areas in the antimony films is also in accordance with
the structures of the modifications of the lighter homologous
elements arsenic and phosphorus which are likewise composed of tetrameric units (As4 and P4). The present STM data
therefore indicate that a new allotropic modification of
antimony composed of Sb4 molecules exists on the nanometer
scale. Substrate surfaces on which such well-ordered antimony structures can be observed, are the (0001) basal plane of
MoS2 and the (100) plane of the surface alloy AuSb2.
Atomically resolved STM images of both substrates are
depicted in Figure 1.
Antimony thin films on various surfaces and their
amorphous, nonmetallic structure have been described in
the literature.[7, 8] On the MoS2(0001) surface the condensation
of Sb4 at a substrate temperature of 190 8C also results in
macroscopically amorphous antimony films without apparent
structural order. This can be seen from Figure 2 a, which
displays a two-monolayer-thick film. If such a film is investigated at the highest resolution, local, well-ordered areas
[*] Priv.-Doz. Dr. B. Kaiser, Dipl.-Phys. B. Stegemann,
Prof. Dr. K. Rademann
Institut f¸r Chemie
Humboldt Universit‰t zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax: (þ 49) 30-2093-5559
E-mail: [email protected]
Dr. T. M. Bernhardt
Institut f¸r Experimentalphysik
Freie Universit‰t Berlin
Arnimallee 14, 14195 Berlin (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
We thank S. Rogaschewski and A. Laws for the preparation of gold
films.
Angew. Chem. Int. Ed. 2003, 42, No. 2
Figure 1. STM images of the substrate surfaces (39 î 39 ä2) on which
the local crystalline areas can be observed after deposition of 1±2
monolayers of antimony. a) MoS2(0001) with hexagonal structure. Tunneling parameters: 0.1 V, 2.8 nA. b) AuSb2(100) with quadratic surface structure. Tunneling parameters: 0.5 V, 2.0 nA. Both images were
high-frequency-filtered to minimize noise. The schematic drawings of
the lattice structures enable the assignment of the bright maxima of
the local density of states in the STM images: In a) the hexagonal arrangement of S atoms in the uppermost layer is shown,[13] in b) the
two quadratic sublattices of Au atoms and Sb dimers, respectively, are
visible. In the case of the Sb dimers the scanning tunnel microscope
images show a superposition of the two atoms.[15]
(marked by arrows in Figure 2 b) can be identified at coverages of 1±2 monolayers. These local structures always exhibit
a simple, rectangular symmetry. In general no epitaxial
Figure 2. STM data of a two-monolayer-thick antimony film on MoS2 at
different magnifications. a) The 2480 î 2480 ä2 image showing the generally amorphous structure of the film. b) The areas marked by arrows
in the 310 î 310 ä2 image exhibit a well-ordered structure. c) The
100 î 100 ä2 magnification of the structures A and B in b). Along the
line marked by arrows a grain boundary between two neighboring ordered areas is visible. The orientations of these structures are rotated
by 59 18 with respect to each other. d) A 39 î 39 ä2 excerpt of area A.
The image was high-frequency-filtered to emphasize the well-ordered
structure. Tunneling parameters of all images: 0.6 V, 1.0 nA.
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Communications
orientation between film and substrate could be observed.
Figure 2 c displays a magnification of the areas marked with A
and B. A grain boundary highlighted by the two arrows
separates structure A from structure B. The lattice orientation
in both areas (white lines) is rotated relative to each other by
59 18. This could, however, point toward an alignment of the
areas along the hexagonal substrate symmetry (see Figure 1 a). In Figure 2 d a stronger magnification of structure A
is displayed. Close analysis of this image reveals significant
differences between the perpendicular lattice vectors a1 and
a2. The statistical analysis of the lattice distances of 14
investigated, local well-ordered areas (depicted in Figure 3 a)
can be seen in Figure 4 c. The edges and corners of the
uppermost layer of the well-ordered structure are well
resolved. The high-resolution image in Figure 4 d shows the
corrugation of the crystalline area in Figure 4 c in a magnification comparable to that in Figure 2 d. Close inspection
reveals that also on AuSb2 the antimony structures appear
with clearly rectangular symmetry. In total 31 local, crystalline
areas of antimony on AuSb2 have been investigated, and the
statistical evaluation is given in Figure 3 b. In this case, the
average lattice constants are a1 ¼ 4.65 0.13 and a2 ¼ 4.27 0.16 ä. These values are somewhat smaller than in the case of
the MoS2 surface. However, the lattice parameters a1 and a2 of
the antimony structures are consistently in the ratio of 10:9 to
each other on both substrates. The comparison of the size of
the local well-ordered areas on both substrates in Figure 3 c
reveals that the average size of the nanostructures on MoS2 is
« ) than on AuSb2 (maximum at
smaller (maximum at 0±39 nm
« ). In addition the size distribution on AuSb2 is
40±79 nm
clearly broader than on MoS2.
The local crystalline areas display a three-dimensional
structure. Although in both cases only the nominal amount of
two-monolayer antimony was deposited, no ideal two-dimensional but a heavily structured film is formed (see Figure 2 a
and 4 a). The observed crystalline areas appear to generally
have a height of several monolayers, as can be seen from
Figure 5. These images were recorded without an electronic
difference filter for measuring the vertical distance. Several
layers can be identified of a structure on MoS2 (Figure 5 a) as
well as on AuSb2 (Figure 5 c), which are separated by steps.
Figure 3. Statistical analysis of the lattice distances and sizes of the
crystalline ordered areas. Displayed are the lattice parameters a1 ( ! )
and a2 ( ! ) of all analyzed structures n on a) MoS2 and b) AuSb2. The
numbers given are the data averages (marked by the dashed lines)
and their standard deviations. c) Size distribution of the crystalline,
well-ordered areas on both substrates MoS2 and AuSb2. The relative
abundance P is plotted as a function of the size A of the crystalline
areas.
illustrates this observation. Average lattice parameters of a1 ¼
5.06 0.13 ä and a2 ¼ 4.51 0.16 ä are obtained for the
crystalline antimony structures on MoS2(0001).
On a Au(111) substrate covered with the surface alloy
AuSb2, ordered areas can be observed even at room temperature. From Figure 4 a the generally amorphous structure of a
two-monolayer-thick antimony film on this substrate can be
recognized. With increasing magnification, again local, wellordered structures with rectangular symmetry appear. These
are highlighted by arrows in Figure 4 b. A preferred alignment
of the lattice orientation in these areas with respect to the
quadratic symmetry of the AuSb2(100) substrate cannot be
identified. Further magnification of the structure in area A
200
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Figure 4. STM image of a two-monolayer-thick antimony film on a
Au(111) surface covered by the surface alloy AuSb2 : a) The
2480 î 2480 ä2 overview over the generally amorphous structure of the
film. Tunneling parameters: 0.8 V, 1.8 nA. b) The 310 î 310 ä2 STM image displays well-ordered areas in the film at the positions marked by
arrows. Tunneling parameters: 0.96 V, 1.7 nA. c) Magnification of
area A in b) (100 î 100 ä2). d) A further enlarged view of area A
(39 î 39 ä2). The image was high-frequency-filtered, to enhance visibility of the well-ordered structure.
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Angewandte
Chemie
Figure 5. STM images (120 î 120 ä2) of the three-dimensional structure
in the well-ordered areas on a) MoS2 (tunneling parameters: 0.6 V,
1.0 nA) and c) AuSb2 (tunneling parameters: 0.25 V, 7.9 nA). The data were taken without electronic difference filter so that the gray scale
represents the absolute sample height. Vertical profiles (z direction)
along the white lines (y direction) are depicted below the corresponding STM images in b) and d). The given numbers are the heights of
monomolecular steps at the positions marked by A and B.
dron.[4] Ab initio calculations gave a bond length in the
tetrahedron of 2.687 ä.[5]
Starting from the measured distances between the corrugation maxima in the local, well-ordered areas of the
antimony thin films and based on an atomic radius of half
the bond length (1.34 ä), a molecular lattice arrangement is
obtained as displayed in Figure 6. Other arrangements in
accord with the observed symmetry are not possible on both
substrates. The observed crystalline ordered antimony structures can hence be explained, if each protrusion measured
with STM, that is, the maxima of the local electronic density
of states, is assigned to one undissociated Sb4 tetrahedron
molecule. This assignment of the observed corrugation is also
supported by STM measurements of single Sb4 molecules on
Si(001) which gave an Sb4 diameter of about 5 ä.[6] Taking
into consideration the convolution with the STM tip, this
value represents an upper limit for the lateral size of Sb4 and
thus agrees well with the measured distances.Although the
detailed structures of yellow arsenic and white phosphorus,
which are built of As4 and P4 molecules, respectively, have not
been resolved yet, crystals of these modifications with cubic
symmetry could be obtained.[1, 11] The step heights of the
observed antimony modification are comparable to the
distances between the Sb4 molecules in the lattice layers
(Figure 5) and thus demonstrate the three-dimensional character of the crystalline structures. It is therefore reasonable to
assume that the antimony modification presented here has a
nearly cubic lattice structure.
In this respect, the well-ordered areas on AuSb2 in general
show besides the lateral also a stronger vertical extension. As
at larger coverages (above 3 ML) no well-ordered Sb4 areas
can be identified, the smaller distances between the Sb4
molecules on AuSb2 indicate that the more extended structures in comparison to MoS2 are closer to the critical size, at
which the transition to the amorphous phase takes place.[12] A
further possible explanation for the different intermolecular
distances on the two substrates might lie in a different
interaction of Sb4 with MoS2 and AuSb2. Future experiments
and a detailed theoretical modeling of the Sb4 structures on
the substrates will hopefully give more insight into these
aspects.
The rectangular symmetry of several consecutive layers of the
antimony structures in Figure 5 c is particularly well resolved.
Vertical profiles along the white lines are depicted below the
STM images (Figure 5 b and d). In both images two particular
steps are marked with A and B. The measured step heights on
both substrates range between 4.5 and 5.0 ä and are thus
comparable to the lattice distances in the layers. Based on
these results, the atomic arrangement in the local, wellordered areas will now be discussed.
The crystal structure of metallic a-antimony is rhombohedral (space group R3m). Each Sb atom in a lattice plane has
three next neighbors at a distance of 2.908 ä. The smallest
interatomic distance between the adjacent lattice planes is
3.355 ä.[1] The evidently larger measured distances as well as
the different symmetry of the local, crystalline areas on both
substrates prove that the corrugation maxima in the STM
images cannot be assigned to single antimony atoms
in a lattice plane of a-antimony. The presence of
nanocrystallites of metallic antimony can therefore
be ruled out.
However, the measured lattice distances fit well
the known dimensions of the vapor-deposited Sb4
molecules. Collision-induced fragmentation of the
impinging Sb4 molecules can be excluded at thermal
deposition energies.[9] A solid-state modification of
antimony based on Sb4 molecules has not been
described before, as mentioned above; however,
data obtained by X-ray diffraction indicate the
existence of chains and layers of tetrahedrons in
antimony thin films.[8] Sb4 was detected in liquid Figure 6. Structure model for the arrangement of the Sb4 tetrahedrons in the uppermost layer of the Sb4 antimony modification on a) the MoS2 substrate and on
antimony by neutron scattering[10] and individual Sb4
b) the AuSb2 substrate according to the observed symmetry and lattice distances.
molecules were observed by STM on a Si(001) The scanning tunnel microscope images a superposition of the four atoms in the
surface.[6] Sb4 has the structure of a regular tetrahe- tetrahedron. Consequently, these arrangements are the only possible variations of
rectangular lattices with the measured distances.
Angew. Chem. Int. Ed. 2003, 42, No. 2
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Communications
Experimental Section
Conformationally Restricted Peptides
The antimony thin films were synthesized in ultra-high vacuum (base
pressure: 4 î 1011 mbar) by evaporation of Sb4 molecules from a
resistance-heated effusive oven (temperature: 330 8C; deposition
rate: 0.1 nm s1; Sb: 99.9999 %, Johnson-Matthey). The layer thickness is given in monolayers of Sb atoms. The MoS2 substrates were
prepared by cleavage according to the procedure given in reference
[13]. The AuSb2 surface alloy was prepared starting from Au(111).[14]
On this substrate the surface alloy AuSb2 with the (100) orientation
forms spontaneously, if less than one monolayer of antimony is
deposited at room temperature.[15] The beetle-type STM[16] is located
in an analysis chamber directly attached to the preparation chamber.[17] To increase image contrast, the STM data were electronically
differentiated directly during acquisition. Therefore the STM images
appear as if they are illuminated from the side.
Received: April 8, 2002
Revised: October 11, 2002 [Z19057]
[1] J. Donohue, The structures of the elements, Wiley-Interscience,
New York, 1974.
[2] J. M¸hlbach, P. Pfau, E. Recknagel, K. Sattler, Surf. Sci. 1981,
106, 18.
[3] A. F. Hollemann, N. Wiberg, Lehrbuch der Anorganischen
Chemie, 101. ed., Walter de Gruyter, Berlin, 1995.
[4] H. Sontag, R. Weber, Chem. Phys. 1982, 70, 23.
[5] V. Kumar, Phys. Rev. B 1993, 48, 8470.
[6] Y. W. Mo, Phys. Rev. Lett. 1992, 69, 3643.
[7] J. Cohen, J. Appl. Phys. 1954, 25, 798; M. Hashimoto, K.
Umezawa, R. Murayama, Thin Solid Films 1990, 188, 95; A.
Hoareau, J. X. Hu, P. Jensen, P. Melinon, M. Treilleux, B.
Cabaud, Thin Solid Films 1992, 209, 161; H. Murmann, Z. Phys.
1929, 54, 741; J. A. Prins, Nature 1933, 131, 760; H. Levinstein, J.
Appl. Phys. 1949, 20, 306.
[8] H. Richter, H. Berckhemer, G. Breitling, Z. Naturforsch. A 1952,
9, 236.
[9] B. Kaiser, T. M. Bernhardt, B. Stegemann, J. Opitz, K. Rademann, Nucl. Instrum. Methods Phys. Res. Sect. B 1999, 157, 155;
T. M. Bernhardt, B. Kaiser, K. Rademann, Phys. Chem. Chem.
Phys. 2002, 4, 1192.
[10] P. Lamparter, S. Steeb, W. Knoll, Z. Naturforsch. A 1976, 31, 90.
[11] J. Eiduss, R. Kalendarev, A. Rodionov, A. Sazonov, G. Chikvaidze, Phys. Status Solidi B 1996, 193, 3.
[12] G. Fuchs, P. Melinon, F. Santos Aires, M. Treilleux, B. Cabaud,
A. Hoareau, Phys. Rev. B 1991, 44, 3926.
[13] J. G. Kushmerick, S. A. Kandel, P. Han, J. A. Johnson, P. S. Weiss,
J. Phys. Chem. B 2000, 104, 2980.
[14] J. A. DeRose, T. Thundat, L. A. Nagahara, S. M. Lindsay, Surf.
Sci. 1991, 256, 102.
[15] B. Stegemann, T. M. Bernhardt, B. Kaiser, K. Rademann, Surf.
Sci. 2002, 511, 153.
[16] K. Besocke, Surf. Sci. 1987, 181, 145.
[17] B. Kaiser, T. M. Bernhardt, K. Rademann, Nucl. Instrum.
Methods Phys. Res. Sect. B 1997, 125, 223; T. M. Bernhardt,
Dissertation thesis, Humboldt-University, Berlin, 1997.
Analogues of Neuropeptide Y Containing
b-Aminocyclopropane Carboxylic Acids are the
Shortest Linear Peptides That Are Selective for
the Y1 Receptor**
Norman Koglin, Chiara Zorn, Raphael Beumer,
Chiara Cabrele, Christian Bubert, Norbert Sewald,
Oliver Reiser,* and Annette G. Beck-Sickinger*
Dedicated to Professor Peter Welzel
on the occasion of his 65th birthday.
Neuropeptide Y (NPY) is one of the most abundant neuropeptides in the mammalian central nervous system. It consists
of 36 amino acids and has an amide group at the C terminus.
To date NPY is the strongest known stimulator of food intake
in rat and mice models. Other important biological functions
are vasoconstriction in the periphery, regulation of behavior
and modulation of pain and epileptic seizures.[1]
In mammals, the activities of NPY are mediated by at least
three different G-protein-coupled receptors (Y1, Y2, and Y5).
Because NPY shows sub-nanomolar affinity towards all of
them, it is still difficult to distinguish the physiological roles of
each receptor in vivo. To address this problem, the knowledge
of the particular bioactive conformation at each receptor is
indispensable for the further development of subtype-selective ligands. Substitutions of single amino acids have revealed
that especially the highly conserved C-terminal part of NPY,
with its two positively charged arginine side chains in
positions 33 and 35 and the tyrosine amide in position 36,
plays a crucial role during the recognition process by the
respective receptor. Because of its flexibility, no defined
structure could be assigned to this important part of the
molecule to date. It is assumed that different secondary
structure motifs of the C terminus are responsible for
receptor subtype selectivity.
[*] Prof. Dr. O. Reiser, Dr. C. Zorn, Dr. R. Beumer, Dr. C. Bubert
Institut f¸r Organische Chemie
Universit‰t Regensburg
Universit‰tsstrasse 31, 93040 Regensburg (Germany)
Fax: (þ 49) 941-9434-121
E-mail: [email protected]
Prof. Dr. A. G. Beck-Sickinger, Dipl.-Biochem. N. Koglin,
Dr. C. Cabrele
Institut f¸r Biochemie
Universit‰t Leipzig
Talstrasse 33, 04103 Leipzig (Germany)
Fax: (þ 49) 341-9736-998
E-mail: [email protected]
Prof. Dr. N. Sewald
Institut f¸r Organische und Bioorganische Chemie
Universit‰t Bielefeld
Postfach 100131, 33501 Bielefeld (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(BE1264/3-1 and RE948-4/1) and the Fonds der Chemischen
Industrie and through generous chemical gifts from BASF AG, Bayer
AG, and Degussa AG.
202
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