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Pittsburgh, PA, 1998.
Received: March 28, 2003 [Z 767]
On the Covalency of Silver ± Fluorine
Bonds in Compounds of Silver(I),
Silver(II) and Silver(III)
Wojciech Grochala,*[a, b] Russ G. Egdell,[c] Peter
P. Edwards,[b] Zoran Mazej,[d] and Boris Zœemva[d]
Dedicated to Professor Roald Hoffmann.‡
KEYWORDS:
electronic structure ¥ fluorine ¥ photoelectron spectroscopy ¥
silver
Introduction
There has been a recent upsurge of interest in the nature of silver
fluorides in the oxidation states II and III. Chemically, these
compounds are among the strongest oxidizing and fluorinating
agents known.[1] Furthermore, it has recently been predicted that
due to a pronounced covalency in the Ag F bonds in these
compounds, holes introduced by p-type self-doping might
appear in the anion F(2p) bands, rather than in the metal Ag(4d)
bands.[2, 3]
In the present study, we report high-resolution X-ray photoelectron spectra (XPS) of several silver fluorides in different
oxidation states; the investigated compounds include AgF (an
Ag(I) fluoride), AgF2 , KAgF3 (Ag(II) fluorides), and KAgF4 (an Ag(III)
fluoride). From analysis of both the core and valence regions of
the spectra, we will quantitatively evaluate the mixing of the
Ag(4d) and F(2p) states in these compounds, and compare the
results to data obtained from previous density-functional-theory
computations.[2] Our goal is to experimentally verify the
hypothesis of significant covalency in the Ag F bonds in these
compounds, and a covalency that increases substantially as the
silver oxidation state increases. This also leads to the introduction of hole states in the F anion band, which is unprecedented
in the chemistry of the transition metal compounds. We are not
conscious of any previous XPS studies for KAgF3 and KAgF4 ,
although AgF and AgF2 , have been the subject of previous
work.[4, 5]
[a] Dr. W. Grochala
Department of Chemistry, University of Warsaw
Pasteur 1, 02093 Warsaw (Poland)
Fax: (‡ 48) 22-8222309
E-mail: [email protected]
[b] Dr. W. Grochala, Prof. P. P. Edwards
Schools of Chemistry and Metallurgy and Materials
University of Birmingham, Edgbaston
Birmingham B15 2TT (United Kingdom)
[c] Prof. R. G. Egdell
Inorganic Chemistry Laboratory, University of Oxford
South Parks Road, Oxford, OX1 3QR (United Kingdom)
[d] Dr. Z. Mazej, Prof. B. Zœemva
Department of Inorganic Chemistry and Technology
Jozœef Stefan Institute, 1000 Ljubljana (Slovenia)
[‡] The teacher of deep insights, intuition and a charming simplicity.
CHEMPHYSCHEM 2003, 4, 997 ± 1001
DOI: 10.1002/cphc.200300Z777
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
997
Results and Discussion
The recorded positions of the Ag(3d5/2) and F(1s) peaks are
presented in Table 1.
Table 1. Position of the Ag(3d5/2) and F(1s) peaks (eV) in the XPS of several
silver fluorides.
Ag(3d5/2)
F(1s)
AgF
AgF2
comm-AgF2
KAgF3
KAgF4
368.20
683.35
367.85
683.35
368.95
683.49
685.79
368.70
683.47
368.35
686.25
We observed relatively large differences between the core XPS
for freshly prepared and for commercially available AgF2 (commAgF2), for example, the referenced positions of the Ag(3d5/2) peak
for these two samples differ by more than 1 eV, and the singlet
F(1s) signal present in AgF2 is substituted by a broad doublet in
comm-AgF2 . In addition, the comm-AgF2 was characterized by a
five times larger surface contamination by carbon and by a
similar level of surface oxygen as a freshly prepared compound.
It will be further deduced on the basis of the valence region
spectra that comm-AgF2 is partially reduced on the surface, in
contrast to the freshly prepared compound.[6] Thus, in the
subsequent analysis we will concentrate exclusively on the
spectra of the freshly prepared compound.
Interestingly, the referenced position of the Ag(3d5/2) peak lies
at smaller binding energies for AgF2 than for AgF, the difference
being 0.35 eV. The comparison for KAgF3 and KAgF4 brings
analogous findings, the signal of KAgF4 being further downshifted by 0.35 eV. This is perhaps counterintuitive, since the
position of the Ag(3d5/2) peak would be expected to shift to
higher binding energies with the increase of the oxidation state
of silver. Similar observation for AgF and AgF2 ( 0.5 eV difference) were however made in a previous study.[4] Our study
confirms this unusual feature of silver fluorides, which was
explained in terms of differences in the screening of the core
hole in the different oxidation states.[7, 8]
It can be seen from Table 1 that the F(1s) binding energies for
AgF, AgF2 and KAgF3 are very similar (centered around 683.4 eV),
and significantly smaller than for KAgF4 (approximately
686.3 eV).[9] We attribute it to an unusually large electronegativity
of the Ag(III) center, which can strongly attract electrons (possibly
also the core ones) from the very weakly polarizable, hard
fluoride anion. Confirmation of this exceptional feature of KAgF4
is found in the NMR spectra of this compound[10] and its
unprecedented oxidative power.[11] An attraction of both core
and valence electrons from the fluoride anion toward the silver
core, increasing in the order Ag(I) < Ag(II) < Ag(III) might even
compensate for the presumed hardness of bare silver cations
(increasing in the same direction), and thus help to explain the
anomalous shifts of the Ag(3d5/2) peaks.
The valence region X-ray photoemission spectrum of AgF is
shown at the bottom of Figure 1, below the computed densityof-states profile for this material.[2] The experimental photoemission spectrum is in good agreement with that measured by
998
Figure 1. a) Total density-of-states profile for AgF from LDA calculations.[2]
b) F(2p) partial density-of-states profile. c) Ag(4d) partial density-of-states porfile.
d) Al Ka valence band photoemission spectrum for AgF. Note the resemblance of
profiles c and d.
Mason[12] on in-situ evaporated materials saving that the
experimental resolution and the definition of the spectral
features is now enhanced in the present work. Figure 1 also
shows the Ag(4d) and F(2p) partial densities-of-states profiles.
Within the density-of-states profile it is possible to identify an
electronic band of predominant Ag(4d) atomic character at low
binding energies and a band of dominant F(2p) atomic character
at higher binding energies.[13] The former band displays a
prominent shoulder on its low binding energy side. Due to the
Ag F covalency there is a degree of mixing of the Ag(4d)
character into the high binding energy band of F(2p). Overall,
the experimental spectral profile is seen to be in good agreement with the partial density of the Ag(4d) state. We note that
the Ag(4d) one-electron ionization cross-section of 2.1 10 2 Mb
is over two orders of magnitude larger than the F(2p) oneelectron ionization cross-section of 1.36 10 4 Mb under Al Ka
excitation.[14, 15] Thus, within the framework of the Gelius
model[16] one anticipates that the experimental Al Ka spectrum
will be entirely dominated by the Ag(4d) contribution to the
overall density-of-states profile. The experimental separation
between the maxima of the F(2p) and Ag(4d) bands is about
2.5 eV, as compared with a separation of 3.3 eV in the theoretical
density-of-states profile. The distinctive low binding energy
shoulder to the Ag(4d) band clearly present in the calculations, is
also observed experimentally.[17] Thus the photoemission data
are strong evidence and support the theoretical findings that in
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org
CHEMPHYSCHEM 2003, 4, 997 ± 1001
AgF the ligand F(2p) states lie at higher binding energy than the
metal Ag(4d) states. Importantly, this ordering is different to that
found in the other silver halides (AgCl, AgBr and AgI[18] ), where
the states at lowest binding energies are of dominant halogen
np character.[19±21] In contrast, the ordering found for AgF prevails
in the copper(I) halides CuCl, CuBr and CuI, where the states at
lowest binding energies are clearly of dominant Cu(3d) character.[19] It is therefore interesting to note the correlation between
the ordering of the halogen and metal levels and the ability to
prepare metal halides in the oxidation state II. When the halide p
levels (the ™ligand band∫) are above the metal d states (the
™metal band∫) at the metal oxidation state II, automatic depopulation of the halide levels takes place, the redox reaction results,
and the hypothetical metal dihalide is not stable. This is nicely
exemplified by the following oxidation ± reduction couple
[Equation (1)]
Ag2‡ ‡ Cl ! Ag1‡ ‡ 1³2 Cl2
F(2p) electronic states. Thus the F(2p) band now appears as a
high binding energy shoulder to the Ag(4d) band but, importantly, with higher intensity (peak area) than found for the F(2p)
feature in AgF (Figure 1).
This trend of increasing intensity for the F(2p) band continues
also for KAgF3 and KAgF4 ,[22] whose valence band photoemission
spectra are shown in Figure 3. To quantify the changes, the
(1)
The corresponding comparison between the experimental
valence band X-ray photoemission data and the total and partial
densities-of-states profile for AgF2 is shown in Figure 2. In this
compound the separation between F(2p) and Ag(4d) states is
much smaller than for AgF–and more difficult to discern. At the
same time, the overall ™spread∫ of the F(2p) states is much bigger
than in AgF reflecting a stronger mixing between the Ag(4d) and
Figure 3. Al Ka valence band photoemission spectrum for KAgF3 (a) and
KAgF4 (b).
valence band photoemission spectra for all four compounds
have been fitted to a pair (or a trio in the case of AgF and AgF2) of
Voigt functions, hence allowing us to extract the intensity ratio
between the F(2p) ligand band and the dominant Ag(4d) metal
band. The results of this analysis is presented in Table 2. Since the
spectra are completely dominated by the partial density of the
Ag(4d) state, the ratio between integrated band areas can be
compared directly with the relative contribution of the Ag(4d)
orbitals to the two bands derived from the LDA (local density
approximation) calculations.[2, 23, 24]
This comparison is given in Table 3, where we note that the
experimental Ag(4d) contribution to the density-of-states profile
Table 2. Parameters of the component bands in the valence region of the XPS
spectra for several silver fluorides: position of the band (referenced to the C(1 s)
signal) (eV), its half width (eV), and intensity at maximum (counts). These are
results of the peak-fitting procedure using Voigt peak profiles.
Compound
Figure 2. a) Total density-of-states profile for AgF2 from LDA calculations.[2]
b) F(2p) partial density-of-states profile. c) Ag(4d) partial density-of-states.
profile d) Al Ka valence band photoemission spectrum for AgF2 . Note the
resemblance of profiles c and d.
CHEMPHYSCHEM 2003, 4, 997 ± 1001
AgF
comm-AgF2
AgF2
KAgF3
KAgF4
Band 2 (™ligand band∫)
Band 1 (™metal band∫)
Half
Relative
Relative
Posi- Half
Posiwidth intensity
tion
tion [eV 1] width intensity
[eV 1] (at maximum) [eV 1] [eV 1] (at maximum)
7.77
7.29
6.97
6.85
7.27
1.58
2.52
3.46
3.12
3.42
17
25
28
33
41
5.26
5.57
4.90
5.13
5.21
1.45
1.98
1.82
1.86
1.62
www.chemphyschem.org ¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
83
75
72
67
59
999
Table 3. Ratio of the contributions of the Ag(4d) states to the ™metal band∫
and to the ™ligand band∫ for two silver fluoride. Comparison of experimental
and theoretical results. The integrated XPS area is presented here.
Compound
Theory/DFT[2]
Experiment/XPS
AgF
comm-AgF2
AgF2
KAgF3[1]
KAgF4
88:12
66:34
66:34
53:47
40:60
82:18
70:30
58:42
51:49
40:60
matches well the one obtained from theoretical calculations,[2]
and confirms the large intrinsic covalency of the Ag F bonds in
these compounds. The ratio of the Ag(4d) contributions to the
™metal∫ and ™ligand∫ bands varies from 88:12 for AgF (weakly
covalent Ag F bond), through 66:34 to 53:47 for AgF2[25] and
KAgF3 (moderately covalent bonds), and surprisingly passes
through the 50:50 borderline, reaching 40:60 for KAgF4 (which is
indicative of significant covalency). In addition, the difference in
energy between the ™metal band∫ and ™ligand band∫ decreases
with the increase of the oxidation state of silver [some 2.51 eV for
Ag(I), and 1.72 ± 2.07 eV for Ag(II) and Ag(III)], as expected, and in
agreement with calculations.[2] These findings mean that covalency of the Ag F chemical bond increases with the increase of
the oxidation state of the silver atom. Remarkably, for KAgF4
more Ag(4d) states go to the ™ligand∫ band than to the ™metal∫
band; a similar feature has been observed in computations for
this compound.[26] It clearly indicates that formulation of AgF4
ion as Ag(III)(F )4 is very far from realistic. KAgF4 indeed releases
elemental F2 upon slight heating, confirming that the equilibrium of the reaction [Equation (2)]
Ag(III) ‡ F !Ag(II) ‡ 1³2 F2
(2)
can easily be shifted to the product side via the entropy factor,
TDS.[27]
In conclusion, we have shown here that covalency of the Ag F
bonds is significant in the fluorides of Ag(II) and Ag(III). The Ag(III)
ion is an oxidizing agent of unprecedented power; concomitant
with this covalency one sees the intrinsic property of holes being
introduced into the F(2p) band. These findings open an
interesting new perspective for the design of a novel family of
metallic,[28] perhaps even superconducting materials based on
these fascinating compounds as first suggested by theory.[2]
Experimental Section
High-resolution X-ray photoemission spectra were measured in a
Scienta ESCA 300 spectrometer[29] located in the Research Unit for
Surfaces, Transforms and Interfaces (RUSTI) at Daresbury Laboratory.
This incorporates a rotating anode X-ray source, a 7 crystal X-ray
monochromator and a 300 mm mean radius spherical sector electron
energy analyser with parallel electron detection system. The X-ray
source was run with 200 mA emission current and 14 kV anode bias,
whilst the analyser operated at 150 eV pass energy with 0.5 mm slits.
Gaussian convolution of the analyser resolution with a linewidth of
260 meV for the X-ray source gives an effective instrument resolution
1000
of 350 meV. During accumulation of spectral data samples were
subject to low energy electrons from a flood gun in order to
compensate for sample charging. Spectra were referenced to the
C(1s) line due to adventitious contamination. Because of the
enormous fluorinating properties of KAgF4 , the position of the
peaks in the XPS of this compound were referenced to the C(1s)
signal under the assumption that all carbon-containing species
present at the surface of the sample are fully fluorinated to give a
reference binding energy of 296.7 eV for CF4 .[30] In contrast, the
spectra of AgF, comm-AgF2 , AgF2 and KAgF3 were referenced to the
usual C(1s) signal typical of the nonfluorinated species with a binding
energy of 285.2 eV for cyclohexane.[30]
AgF (99.9 ‡ %) was obtained from Aldrich, while AgF2 ,[31] KAgF3[32]
and KAgF4[33] were synthesized according to published procedures.
We have also studied commercially available AgF2 (98.5 ‡ %, AlfaAesar), hereafter denoted as comm-AgF2 . The freshly-prepared AgF2 ,
KAgF3 and KAgF4 were evacuated from the prefluorinated teflon
ampoules and inserted to the spectrometer chamber in the inert
atmosphere provided by an Ar-filled glove bag. The surface of the
samples was not ion sputtered, in order to avoid a selective
depletion of the ions (and thus stoichiometry changes) at the
surface.
We thank to Daresbury National Laboratory for awarding time at
the Scienta ESCA300 spectrometer. W.G. thanks The Crescendum
Est-Polonia Foundation for the research grant No. 6, and The Royal
Society (UK). B.Zœ. and Z.M. gratefully acknowledge the financial
support of the Ministry of Education, Science and Sport of the
Republic of Slovenia. W.G. holds at present the Fellowship of the
Foundation for Polish Science.
[1] a) N. Bartlett, G. Lucier, C. Shen, W. J. Casteel, Jr., L. Chacon, J. M¸nzenberg, B. Zœemva, J. Fluorine Chem. 1995, 71, 163; b) B. Zœemva, R. Hagiwara,
W. J. Casteel, Jr., K. Lutar, A. Jesih, N. Bartlett J. Am. Chem. Soc. 1990, 112,
4846; c) B. Zœemva, Compt. Rend. Acad. Sci. Ser. II Fasc. C 1998, 1, 151.
[2] W. Grochala, R. Hoffmann Angew. Chem. 2001, 113, 2816, ; Angew. Chem.
Int. Ed. 2001, 40, 2743.
[3] Thus modified fluorides of Ag(III) and Ag(II) might possibly emerge as a
novel family of superconducting materials, analogous in some ways to
the well known oxocuprate superconductors. Indeed, sudden drops in
the magnetic susceptibility of a large number of samples in the Be-Ag-F
system have been observed recently, and attributed to superconductivity
at temperatures ranging from 8.5 K to 64 K (W. Grochala, P. P. Edwards,
unpublished results).
[4] J. T. Wolan, G. B. Hoflund, Appl. Surf. Sci. 1998, 125, 251.
[5] M. Romand, M. Roubin, J. P. Deloume, J. Electron Spectrosc. Relat. Phenom.
1978, 13, 229.
[6] This is of importance since previous studies for AgF2 typically utilized the
commercially available, partially decomposed compound.
[7] G. Schon, Acta Chem. Scand. 1973, 27, 2623.
[8] S. W. Gaarenstroom, N. Winograd, J. Chem. Phys. 1977, 67, 3500.
[9] Analyzing the spectra of this inherently unstable and very reactive
compound, one needs to realize that the surface of KAgF4 may (to some
degree) decompose to KAgF3 and F2 in the ultrahigh vacuum conditions
during the XPS experiment.
[10] R. Eujen, B. Zœemva, J. Fluorine Chem. 1999, 99, 139.
[11] In fluorides is true that the compound in anionic form, such as AgF4 , is
not so strong oxidizer as neutral binary fluoride (for example thermodynamically unstable AgF3). Of course, the strongest oxidizer is Ag(III) in
cationic form (for example AgF2‡). AgF4 is a strong oxidizer and can
oxidize Xe to XeF2 , while AgF2‡ is much stronger, and can oxidize PtF6
into PtF6 . Cationic Ag(III) and cationic Ni(IV) are at present the strongest
known oxidizers.
[12] M. G. Mason, Phys. Rev. B 1975, 11, 5094.
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org
CHEMPHYSCHEM 2003, 4, 997 ± 1001
[13] DFT computations indicate that the ™ligand band∫ lies below the ™metal
band∫ in silver fluorides;[2] for example for AgF the energy difference is
some 3 eV, similar to the experimental value of 2.5 eV.
[14] J. J. Yeh, I. Lindau, Atomic Data and Nuclear Data Tables 1985, 32, 1.
[15] The ionization cross-section of the F(2s) electrons is one order of
magnitude smaller than that for the Ag(4d) ones, and the contribution
from the former to the valence band is small, according to the
calculations[2] .
[16] U. Gelius, in Electron Spectroscopy (Ed. D. Shirley) North Holland,
Amsterdam, 1972.
[17] This band is centered at 3.68 eV, and has a small half width of 0.74 eV and
integrated intensity of 7 % of the whole valence band. It has been
erroneously assigned either to valence orbitals baring some O(2p)
character.[4] The same peak shows at 3.78 eV (half width of 0.70 eV,
integrated intensity of 7 % of the whole valence band) in the spectrum of
comm-AgF2 , and it certainly belongs to AgF (comm-AgF2 is partially
reduced on the surface).
[18] The ™ligand band∫ progressively shifts to lower binding energies in the
order AgF > AgCl > AgBr AgI, and simultaneously gains intensity, thus
reflecting the increasing energy of the np orbitals of the nonmetal and
their stronger mixing with the Ag(4d) orbitals.
[19] A. Goldmann, J. Tejeda, N. J. Shevchik, M. Cardona, Phys. Rev. B 1974, 10,
4388.
[20] J. Tejeda, N. J. Shevchik, W. Braun, A. Goldmann, M. Cardona, Phys. Rev. B
1975, 12, 1557.
[21] R. Matzdorf, A. Goldmann, J. Electron Spectrosc. Relat. Phenom. 1993, 63,
167.
[22] Note, also that the dispersion of the ™ligand band∫ usually increases with
the increase of the silver oxidation state (0.79 eV for Ag(I), 1.56 eV for
KAgF3 , and 1.71 ± 1.73 eV for AgF2 and KAgF4), due to the decreasing
(shortest) F ± F distance in these compounds (3.493 ä, 3.026 ä, 2.843 ä,
and 2.639 ä, respectively).
[23] The values listed in Table 6 of ref.[4] were normalized so that the sum of
contributions is 100 %. Contributions from all silver valence orbitals (4d ‡
5s ‡ 5p) was in fact taken in ref.[2]. However the contribution from
Ag(5s ‡ 5p) states to the valence band is computed to be small and can
be neglected in the comparison that follows.
[24] In fact, the computation was for CsAgF3 and not for KAgF3 , but it should
not have significant impact on the quantitative comparison.
[25] The corresponding value for the comm-AgF2 is 70:30, in between the
values for AgF and for the freshly prepared AgF2 ; it suggests that commAgF2 is partially reduced, at least on the surface.
[26] Thus it proves not to be only a (computational) artifact of the choice of
the Weigner ± Seitz radii.
[27] More detailed considerations should involve the difference of the lattice
energies of substrate and products.
[28] KAgF3 studied here, proves to be metallic: W. Grochala, P. P. Edwards,
unpublished results.
[29] http://www.dl.ac.uk/RUSTI/xps/esca300.htm
[30] Practical Surface Analysis (Ed. D. Briggs, M. P. Seah), Wiley, Chichester
1990.
[31] B. Zœemva, R. Hagiwara, W. J. Casteel, Jr., K. Lutar, A. Jesih, N. Bartlett J. Am.
Chem. Soc. 1990, 112, 4846. AgF2 was prepared by the reaction between
AgNO3 and elemental fluorine at 250 8C.
[32] R. H. Odenthal, R. Hoppe, Monatsh. Chem. 1971, 102, 1340. The procedure
was modified by use of AgF2 instead of AgF and F2 as substrates.
[33] K. Lutar, S. Milic¬ev, B. Zœemva, B. G. Mueller, B. Bachmann, R. Hoppe, Eur. J.
Solid State Inorg. Chem. 1991, 28, 1335. KAgF4 was prepared by the
reaction between AgF2 , KF and KrF2 in anhydrous HF as a solvent.
Received: April 7, 2003 [Z 777]
Polarization-Dependent SurfaceEnhanced Raman Spectroscopy of
Isolated Silver Nanoaggregates
Hongxing Xu*[a] and Mikael K‰ll[b]
KEYWORDS:
nanoparticles ¥ polarization ¥ silver ¥ surface-enhanced Raman
scattering
Surface-enhanced Raman scattering (SERS)[1] is a vibrational
spectroscopy technique based on surface plasmon enhanced
optical interactions at noble-metal nanostructures. With a
detection limit at the single-molecule level[2] and a power to
provide structural information through the Raman ™vibrational
fingerprint∫, SERS has a unique potential for ultrasensitive
molecular identification and analysis. A recent example is the
detection of single-DNA and RNA strands labeled by SERS-active
dye molecules.[3] Nanostructured substrates for SERS are expected to be anisotropic in terms of the local surface plasmon
resonances,[4] which imply that the enhancement factor should
extrinsically depend on the incident polarization. This effect is
expected to be particularly strong for nanometric gaps between
nanoparticles,[5] which have been implicated as the most likely
sites for single-molecule SERS.[6, 7] However, most SERS studies
have been performed at fixed polarization configuration and/or
using macroscopic sampling volumes, for which a detailed
comparison between local morphology, SERS activity and theory
is difficult. Herein, we report on the polarization-dependent SERS
from isolated aggregates of silver nanoparticles characterized by
scanning electron microscopy (SEM). The results are found to be
consistent with electrodynamic theory and strongly support the
idea that nanogaps are a key ingredient of ultrasensitive SERS
analysis.
SERS-active structures were prepared by aggregating colloidal
Ag particles (35 pM) through incubation with excess hemoglobin (5 nM), followed by immobilization on polymer-coated Si
substrates. The role of the protein is twofold: to induce
nanogaps through aggregation[6] and to uniformly cover the
nanoparticles with SERS-active species. The SERS structures soformed were well separated and allowed for polarizationdependent Raman studies of isolated nanoparticle aggregates
using a Renishaw 2000 micro-Raman setup operated in either
spectroscopy or imaging mode.
Figure 1 gives examples of polarization-dependent SERS from
two Ag dimers shown in the inset. As reported elsewhere,[6, 8, 9]
[a] Dr. H. Xu
Division of Solid State Physics
Lund University, Box 118 22100 (Sweden)
Fax: (‡ 46) 46-222-3637
E-mail: [email protected]
[b] Prof. Dr. M. K‰ll
Department of Applied Physics
Chalmers University of Technology
41296, Gˆteborg (Sweden)
CHEMPHYSCHEM 2003, 4, 1001 ± 1005
DOI: 10.1002/cphc.200200544
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1001