Formation of carboxy- and amide-terminated alkyl monolayers on silicon(111) investigated by ATR-FTIR, XPS, and X-ray scattering: Construction of photoswitchable surfaces

Article abstract of DOI:10.1021/la402068d

We have prepared high-quality, densely packed, self-assembled monolayers (SAMs) of carboxy-terminated alkyl chains on Si(111). The samples were made by thermal grafting of methyl undec-10-enoate under an inert atmosphere and subsequent cleavage of the est

Full text of DOI:10.1021/la402068d

Article
pubs.acs.org/Langmuir
Formation of Carboxy- and Amide-Terminated Alkyl Monolayers on
Silicon(111) Investigated by ATR-FTIR, XPS, and X‑ray Scattering:
Construction of Photoswitchable Surfaces
Karola Ruck-Braun,*,† Michael Åxman Petersen,†,∥ Fabian Michalik,† Andreas Hebert,†
̈
Daniel Przyrembel,‡ Christopher Weber,§ Saleh A. Ahmed,†,⊥ Stefan Kowarik,§ and Martin Weinelt*,‡
†Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany
̈
̈
‡Fachbereich Physik, Freie Universitat Berlin, Arnimallee 14, 14195 Berlin, Germany
̈
§Institut fur Physik, Humboldt-Universitat zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: We have prepared high-quality, densely packed,
self-assembled monolayers (SAMs) of carboxy-terminated
alkyl chains on Si(111). The samples were made by thermal
grafting of methyl undec-10-enoate under an inert atmosphere
and subsequent cleavage of the ester functionality to disclose
the carboxylic acid end-group. X-ray photoelectron spectros-
copy (XPS) and grazing incidence X-ray diffraction (GIXD)
indicate a surface coverage of about 50% of the initially H-
terminated sites. In agreement, GIXD implies a rectangular
unit mesh of 6.65 and 7.68 Å side lengths, containing two
molecules in a regular zigzag-like substitution pattern for the
ester- and carboxy-terminated monolayer. Hydrolysis of the
remaining H−Si(111) bonds at the surface furnished HO−Si(111) groups according to XPS and attenuated total reflection
Fourier-transform infrared spectroscopy (ATR-FTIR) studies. The amide-terminated alkyl SAM on Si(111) assembled in a 2-(6-
chloro-1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HCTU)-mediated one-pot coupling reaction
under an inert atmosphere, whereby the active ester forms in situ prior to the reaction with an amino-functionalized
photoswitchable fulgimide. ATR-FTIR and XPS studies of the fulgimide samples revealed closely covered amide-terminated
SAMs. Reversible photoswitching of the headgroup was read out by applying XPS, ATR-FTIR, and difference absorption spectra
in the mid-IR. In XPS, we observed a reversible breathing of the amide/imide C1s and N1s signals of the fulgimide. The results
demonstrate the general suitability of HCTU as a reagent for amide couplings to carboxy-terminated alkyl SAMs and the on-chip
functionalization toward photoswitchable Si(111) surfaces.
amide coupling.6,7 NHS-terminated alkyl SAMs can be purified
INTRODUCTION
■
and stored over time. As a drawback, unreacted NHS groups
have to be capped after amide coupling prior to subsequent
transformations and applications, e.g., with ethanolamine.5,6
Only a few alternative protocols have been published, e.g.,
the use of trifluoroacetic anhydride and triethylamine furnishing
anhydride-terminated monolayers prior to the formation of
amide-terminated alkyl SAMs.5 In peptide chemistry, in situ
activation of carboxylic acids using aminium salts for amide
coupling, e.g., HBTU or HCTU,1 is common practice.8 These
coupling reagents effect the formation of an active ester
intermediate that then undergoes aminolysis. This approach has
the advantage of providing the amide-terminated alkyl SAM in
one step without the need to monitor or to isolate and
characterize reaction intermediates.
Silicon is one of the most relevant materials for nanotechnology
and semiconductor devices. In these fields, fine-tuning of the
surface properties by well-defined organic molecules is of
importance.2,3 We are interested in on-chip synthetic routes for
the assembly of organic molecules at semiconductor surfaces
for biosensors and nanotechnology. Progress strongly depends
on a high and reproducible quality of functionalized-alkyl self-
assembled monolayers (SAMs). For these aims and purposes, a
considerable number of wet-chemical multistep procedures
starting from H-terminated Si(111) have been reported in the
past decade.4−6 Avoiding SAM oxidation is a major challenge
during grafting and subsequent steps. Thus, the conditions for
predictable and reproducible multistep transformations of
terminal functional groups at the organic−semiconductor
hybrid interface at ambient conditions have to be carefully
chosen. The formation of active esters of N-hydroxysuccinimide
(NHS) in an independent step is an extensively studied
activation principle for carboxy-terminated alkyl SAMs prior to
Received: May 30, 2013
Revised: July 24, 2013
Published: August 23, 2013
© 2013 American Chemical Society
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with 4 cm−1 resolution. Freshly prepared H-terminated Si(111) or
SiOx samples were used as references, as indicated in the text. IR data
were analyzed using the software OPUS 6.5 (Bruker).
In this paper, we report on a HCTU protocol for the one-pot
coupling of amines to carboxy-terminated alkyl SAMs on
Si(111). The latter resulted from thermal grafting of a methyl
ester-terminated SAM and subsequent on-chip cleavage. The
reproducibility and quality of the SAMs were improved by
performing the grafting step and the amide coupling in a closed
vessel in a glovebox. All steps were analyzed by attenuated total
reflection Fourier-transform infrared spectroscopy (ATR-
FTIR) and X-ray photoelectron spectroscopy (XPS). X-ray
scattering methods were applied for analyzing the ester- and the
carboxy-terminated alkyl SAMs. Photochemical studies were
carried out by alternating irradiation of the fulgimide samples
with ultraviolet (UV) and visible light and readout by ATR-
FTIR and XPS. Fulgimide 1 is most suitable for these studies,
since strong changes of its vibrational modes are observed
following the ultrafast ring-closure and ring-opening reac-
tions.9,10
XPS Measurements. We used an XPS setup in an ultrahigh
vacuum (UHV) mu-metal chamber equipped with a monochromatized
Al KαI X-ray source (VG Scienta MX650; hν = 1486.7 eV, 15 kV, 450
W) and a hemispherical electron analyzer (VG Scienta SES-200). After
preparation of the various SAMs, samples were transported in an inert
atmosphere and transferred into the UHV system via a quick-load lock.
XP spectra were recorded with an overall energy resolution of 0.4
eV. A takeoff angle of the photoelectrons of 65° with respect to the
sample normal was chosen to enhance the surface sensitivity of the
XPS measurements. In this geometry, the X-ray source features a line
focus of about 3 mm × 9 mm, which helps to reduce radiation damage
of the molecular layer below the detection limit. The setup has a
typical operational pressure of 2 × 10−10 mbar, ensuring that surface
contamination by residual gas adsorption is kept at a minimum during
prolonged measurements. All spectra were measured at room
temperature. For the quantitative XPS analyses, Shirley backgrounds
were subtracted and the spectra fitted with Voigt profiles. To get out of
the fits the peak positions of the amidic carbons in the fulgimide layers
with reasonable errors, all other fit peaks were subtracted from the
respective spectra and the remaining single peak spectrum fitted again
with a Voigt profile (cf. Supporting Information, Figure S20).
XRR and GIXD Measurements. All X-ray reflectivity and
diffraction measurements were performed using a lab-based diffrac-
tometer (EFG) with a Cu Kα (λ = 1.54 Å) rotating anode source
(Rigaku). To prevent ozone exposure, the sample was kept in a
vacuum (p ≈ 1 mbar) during X-ray measurements. To exclude beam
damage, the X-ray reflectivity (XRR) was repeatedly measured at
several different spots on the sample. No changes due to X-ray
exposure have been found. For XRR, the angular resolution was 0.05°,
and the diffusely scattered signal, as determined by an offset scan, was
subtracted from the raw XRR data to arrive at the true specular
reflectivity. For the quantitative analysis of the XRR measurements, the
specular reflectivity was fitted using a least-squares routine (Parratt32,
Helmholtz-Zentrum Berlin) based on the Parratt algorithm.12 The
angular resolution of the grazing incidence X-ray diffraction (GIXD)
scans was 0.3° at an angle of incidence of 0.18°.
Photoinduced Switching. For photoswitching, the SAMs were
irradiated in the sample compartment of the ATR-FTIR spectrometer
under nitrogen. Illuminated samples were then measured by ATR-
FTIR or transferred in the dark under argon to the XPS setup. Using
custom-made setups of light-emitting diodes with wavelengths of 365
and 545 nm at intensities of 2 and 5 mW cm−2, the two
photostationary states (PSS) were reached within 30 min of
irradiation. This corresponds to a relatively small fluence of less
than 10 J cm−2.
Fulgimide Synthesis and Characterization. Indolylfulgimide 1
(Scheme 1) was synthesized in three steps starting from fulgide 5,
which was obtained essentially by following the route of Yokoyama et
al.;13 all intermediates were fully characterized. Tosylation of indole 2
furnished compound 3 in 90% yield. The Stobbe condensation of
indole 3 with diethyl isopropylidenesuccinate was carried out with
lithium diisopropylamide (LDA) as base and gave the monoester (E/
Z)-4 in 63% yield. Saponification of the monoester (E/Z)-4 with hot
aqueous ethanolic potassium hydroxide (KOH) solution gave the
crude diacid. Subsequent anhydride formation with dicyclohexylcarbo-
diimide (DCC) afforded fulgide 5 as an 85:15 (E/Z)-mixture in 86%
yield over the two steps after workup and purification by flash
chromatography. Ring-opening of fulgide 5 with tert-butyl 2-
aminoethylcarbamate in DCM gave the regioisomeric succinamic
acids as an inseparable mixture that was treated with carbon-
yldiimidazole (CDI) in a one-pot procedure for the final imide ring
closure. Fulgimide (E/Z)-6 was isolated after workup and purification
in 80% yield over the two steps. Deprotection with 4 M HCl in
dioxane gave (E/Z)-1 in 81% yield after aqueous basic workup and
purification by flash-chromatography as an 85:15 mixture of the (E/
Z)-forms used in the present studies. Compound 1a was isolated as
byproduct in 5% yield because of an acid-catalyzed carbonyl−ene
EXPERIMENTAL SECTION
■
General Remarks. Samples for X-ray photoelectron spectroscopy
(XPS) and X-ray scattering were cut from single side polished Si(111)
(Silchem, n-type, 7.5 Ω cm, thickness 525 μm) in 1 cm × 4 cm pieces.
After each synthesis step a 1 cm × 1 cm sample was broken off from
the main sample and transferred into the XPS or X-ray scattering
apparatus. Samples for attenuated total reflection Fourier-transform
infrared spectroscopy (ATR-FTIR) were cut from double side
polished Si(111) (Siltronix, n-type, 10−30 Ω cm, thickness 500−
550 μm) in 1.1 cm × 2.5 cm dimension, and the sides were cut and
polished at an angle of 45°.
Methyl undec-10-enoate (97%) was purchased from Sigma-Aldrich,
distilled twice under reduced pressure (124 °C, 14 mbar), and stored
in a glovebox (MBraun, Labmaster). Ammonium sulfite [(NH4)2SO3·
H O], HCTU,1 Hunig’s base (diisopropylethylamine, biotech grade),
̈
2
and acetonitrile (amine-free) were purchased from Sigma-Aldrich and
used without further purification. Hydrogen peroxide (H2O2, 31%),
sulfuric acid (H2SO4, 96%), and ammonium fluoride (NH4F, 40%)
were of VLSI grade (very large scale integration grade, trace metal
content <100 ppb) and purchased from BASF. Ultrapure Milli-Q
water (18 MΩ cm) was obtained from a MilliPore four-bowl system.
Monolayer Preparation. The Si(111) wafers were cleaned by
successive sonication in acetone, dichloromethane (DCM), and
methanol, followed by treatment with “piranha” solution (H2O2/
H2SO4, 1/3, v/v) for 30 min at 100 °C and then copiously rinsed with
Milli-Q water. CAUTION: “piranha” solution reacts violently with
organic material and must be handled with extreme care! The wet
sample was directly transferred into the etching solution (40% NH4F
containing 50 mM (NH4)2SO3).11 After 15 min of etching the sample
was quickly rinsed with ultrapure water and dried in a vacuum.
Grafting was conducted in pure methyl undec-10-enoate in a closed
vessel at 160 °C for 2 h in the glovebox. After cooling to room
temperature, the sample was rinsed with hexane and DCM. The
methyl ester in the monolayer was cleaved using 5.5 M hydrochloric
acid (HCl) (degassed with argon) at 40 °C for 3 h followed by
washing with ultrapure water and DCM.
Fulgimide Coupling. The carboxy-terminated surface was
immersed in the glovebox for 2 h in the dark in a solution of an
85:15 mixture of (E/Z)-fulgimide [(E/Z)-1] (5 mM), HCTU (as
coupling agent, 10 mM), and Hunig’s base (10 mM) in acetonitrile.
̈
After the reaction, the samples were rinsed five times with 3 mL of
acetonitrile and two times with 3 mL of DCM, blown dry, and stored
under argon.
ATR-FTIR Measurements. The spectra were recorded with a
Bruker Vertex70v ATR-FTIR spectrometer containing a liquid
nitrogen-cooled mercury−cadmium−telluride (MCT) detector while
the sample compartment was kept under a nitrogen atmosphere. The
nitrogen to purge the spectrometer was dried with an absorption drier
(Specken Drumag SDAT-G70/420) to contain less than 0.1 ppm
water. We investigated the spectral range between 370 and 4000 cm−1
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a
temperature within 30 min and was isolated in 96% yield after workup
and purification by flash chromatography (Supporting Information,
Scheme S1). The ultraviolet−visible spectroscopy (UV/vis) data of
the open-isomers [(E/Z)-mixtures] and the closed-isomers [(C)-
isomers] of compounds 1, 5, 6, and 7 were measured in benzene at
room temperature as previously described14 and are listed in the
Supporting Information (Table S1). The isomer ratios in the PSS at
365 nm (compounds 1, 5, 6, and 7) and at 545 nm (compound 6)
were determined by reversed-phase high-performance liquid chroma-
tography (RP-HPLC). In comparison to the 1-methyl indolylfulgi-
mides described previously,14 similar results were obtained here.
However, the replacement of the methyl group at the indole nitrogen
by a hydrogen atom resulted in an increase of the content of the
colored forms [(C)-isomers] of compounds 1, 5, 6, and 7 in the PSS at
365 nm to 79−83%.
Scheme 1
RESULTS AND DISCUSSION
■
Preparation of High-Quality, Carboxy-Terminated
Alkyl SAMs on Si(111) by Thermal Grafting. Initially,
several series of samples were prepared from H-terminated
Si(111) and methyl undec-10-enoate using thermal grafting
procedures (160 °C, 2 h) in carefully dried and degassed neat
reagents.15 However, it was often not possible to exclude the
presence of water and oxygen from the grafting solutions. As a
result, we observed the formation of SiOx and a partial
hydrolysis of the ester. Furthermore, it proved to be challenging
to obtain reproducible packing densities in the self-assembled
monolayer (SAM). Cleavage of the methyl ester-terminated
alkyl SAM was first attempted in 2 M hydrochloric acid (HCl)
at 70 °C,15,16 but this method was often accompanied by
significant SiOx formation. Milder conditions were required,
and the cleavage was instead performed in 5.5 M HCl
(degassed with argon) at 40 °C for 3 h,17 resulting in a clean
conversion of the ester group to the carboxyl group. Finally, all
thermal reactions were performed in a closed vessel in a
glovebox under nitrogen and anhydrous conditions. This
greatly improved the reproducibility and quality of the grafting
step, furnishing ester-terminated alkyl SAMs.
The synthetic steps described above were validated by
attenuated total reflection Fourier-transform infrared spectros-
copy (ATR-FTIR) and X-ray photoelectron spectroscopy
(XPS) (see also Supporting Information, Figures S4−S18).
Figure 1 shows ATR-FTIR spectra of the ester and acid layer
(black and red lines, respectively) grafted on H−Si(111). The
spectra are dominated by the ν(CO) absorption band, which
shifts from 1747 cm−1 in the ester layer to 1718 cm−1 in the
acid layer. The cleavage reaction is additionally confirmed by
the disappearance of the symmetric and antisymmetric CH3
absorption bands, δsym(CH3) and δas(CH3), at 1366 and 1438
cm−1, which are observed for the ester but not for the acid layer
(Figure 1a). Vice versa, the δ(OH) vibration at 1415 cm−1
occurs only for the acid but not for the ester layer.
The ATR-FTIR data in Figure 1b show the alkyl region of
the ester- (black line) and carboxy-terminated (red line) SAM
with the νas(CH2) and νsym(CH2) absorption at 2924 and 2854
cm−1, respectively.18 Comparing ester- and acid-termination,
these bands only slightly change in wavenumber and intensity,
indicating only small conformational changes of the alkyl-linker
layer. For example, a different twist of the alkyl chains in the
acid-terminated SAM may support the formation of hydrogen
bonds among the carboxy end-groups. The shoulder at 2958
cm−1 corresponds to the asymmetric νas(CH3) vibration and is,
according to this, observed for the ester layer only.
a
Reagents and conditions: (a) TsCl, NaOH, DCM; 90%. (b) Diethyl
isopropylidenesuccinate, LDA, THF; 63%. (c) KOH/EtOH/H2O,
reflux; 97%. (d) DCC, DCM; 86%. (e) tert-Butyl 2-amino-
ethylcarbamate, DCM, reflux and then (f) CDI; one-pot procedure,
80%. (g) 4 M HCl in dioxane; 81%. (h) 365 nm, benzene; 45%. (i)
545 nm, benzene; quant. (j) HCTU, undec-10-enoic acid, DIEA,
acetonitrile; 96%.
reaction during deprotection. This side-reaction takes place next to the
electron-rich indolyl moiety and is followed by a subsequent
interconversion from the enol form to the keto form due to keto−
enol tautomerism. This byproduct is no longer photochromic because
of the disruption of the central hexatriene unit. However, it can be
clearly distinguished from the mixture (E/Z)-1 and pure (E)-1 and
(C)-1, and it is not formed during illumination of the amide-
terminated alkyl SAM with the fulgimide headgroup (Supporting
Information, Figures S1−S3). The pure isomers (E)-1 and (Z)-1 were
applied in an independent study and were derived from the 85:15
mixture of (E/Z)-1.7 The isomer (C)-1 was obtained in 45% yield by
irradiation of the mixture (E/Z)-1 at 365 nm (short-arc lamp, Osram
HBO, 200 W) in benzene for 24 h, followed by purification by flash
chromatography in the dark. Isomer (C)-1 was dissolved in benzene
and irradiated at 545 nm (xenon-arc lamp, Osram XBO, 1000 W) for 2
h to yield pure (E)-1, quantitatively. Photoswitch-linker conjugate (E/
Z)-7 was synthesized as IR reference substance from undec-10-enoic
acid and (E/Z)-1 (85:15 mixture) with HCTU in acetonitrile at room
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Figure 1. (a) ATR-FTIR spectra of the ester layer (black line) and the acid layer (red line) grafted on H−Si(111). Data are referenced to the
spectrum of H−Si(111). (b) Alkyl region with νas(CH2) and νsym(CH2) absorption bands of ester- (black line) and carboxy-terminated (red line)
SAMs. (c) Superposition of the ATR-FTIR spectra of the ester- (black line) and acid-terminated alkyl SAM (red line), referenced to the spectrum of
SiOx to resolve the Si−H contribution.
Even in a densely packed alkyl SAM, steric constraints
suggest about 50% remaining H-terminated silicon sites. They
appear in the ester-terminated SAM when referencing the ATR-
FTIR data against SiOx.11 As highlighted in Figure 1c, we
observed a broad absorption band around 2070 cm−1, which we
assign to the unreacted Si−H sites between the grafted alkyl
chains.19 Upon ester cleavage, this band disappeared,
accompanied by a slight increase in the 1300−1200 cm−1
region (see Figure 1a.). We suggest that these changes originate
from hydrolysis of the remaining Si−H bonds between the alkyl
chains (vide infra).
Corresponding components with equal intensity were also
found in the O1s XP spectra of methyl ester-terminated alkyl
SAMs (Figure 2b, top). The single- and double-bonded oxygen
atoms show chemically shifted XPS lines at 534.2 and 532.7 eV
binding energy, respectively.24,28 The additional shoulder at the
low-energy side is attributed to Si−O. The same O1s binding
energy of 532.2 eV was observed for the Si−O contaminations
at the H-terminated Si(111) surface (Supporting Information,
Figure S6).29
Upon hydrolysis of the methyl ester end-group, the C1s XPS
peak at 287.4 eV, attributed to it, completely disappeared in the
spectrum of the carboxy-terminated alkyl SAM (Figure 2a,
center). This observation supports the above assignment and,
moreover, confirms that the methyl ester cleavage runs till
completion. The C1s line of the carbonyl group shifts to a
slightly higher binding energy and, in particular, its full width at
half-maximum (fwhm) increases by 36%. In line with the small
changes of the alkyl vibrations (cf. Figure 1b), we attribute this
broadening to the formation of acid pairs via hydrogen
bonding, which would also change the intramolecular
orientation, i.e., the conformation of the C11 alkyl-linker chains.
This assumption is corroborated by the shift of the C1s main
peak to lower binding energy (284.8 eV) and the increase of its
fwhm by 7%. As expected, the surface-bound Si−C component
remains at constant binding energy. Cleavage of the methoxy
group reduces the peak splitting in the O1s XP spectra to 1.2
eV with the hydroxyl and carboxyl groups at 534.0 and 532.8
eV, respectively. Note that also in case of hydrogen bond
formation two chemically shifted XPS components are
observed, since on the time scale of the core-level excitation
the oxygen atoms remain unequal. Comparison of the C1s and
O1s intensities consistently reveals that the ester- and the
In the following, the various synthesis steps are confirmed
and further quantified by XPS. The C1s and O1s core-level
spectra of the methyl ester-terminated monolayer are shown at
the top of Figure 2. The C1s spectrum is dominated by the C10
alkyl backbone of the linker, which accounts for the main peak
at 284.9 eV. Its asymmetric peak shape is ascribed to the
excitation of C−H vibrational modes (2800−3000 cm−1
≈
0.35−0.37 eV) upon core ionization. Similar asymmetric XPS
line shapes were found for SAMs of long-chain alkanethiols
chemisorbed on gold and silver.20 The small shoulder seen at
lower binding energy is attributed to the carbon atom directly
bound to the silicon surface (Si−C). Its C1s binding energy of
284.0 eV is compatible with those reported for small
hydrocarbons chemisorbed on Si(100).21 The two components
at higher binding energies of 287.4 and 289.6 eV can be
unambiguously assigned to the C1s core levels of the methoxy
and carbonyl groups of the ester, respectively.24,25 As expected,
both components show equal intensity; however, each signal is
1.7 times larger than that of the Si−C surface component
because of the dampening by the overlying molecular layer.26,27
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Figure 2. XPS scans of (a) the C1s and (b) the O1s core levels recorded with monochromatized Al KαI radiation (hν = 1486.7 eV) at a sample
temperature of 300 K. The sequence follows from top to bottom the molecular grafting from the ester, via the acid, to the amide layer containing
fulgimide (E/Z)-1.
carboxy-terminated monolayer have comparable coverage. This
result supports the existence of an intact monolayer as
suggested by ATR-FTIR.
After cleavage of the methyl ester, we observed a doubling in
intensity of the Si−O component, in both the O1s (Figure 2b)
and Si2p (Figure 3) XP spectra. The oxygen XPS peak of Si−O
accounts for 28% of the total O1s signal for the carboxy-
terminated alkyl SAM (Figure 2b). As we found only Si+1
species in the silicon XPS (Figure 3) after the grafting step,
remaining H-terminated Si(111) sites must have been replaced
by hydroxyl groups, furnishing HO−Si(111). We assume that
the O1s signal of these hydroxyl groups is dampened by the
overlying densely packed alkyl SAM. Comparing the Si2p signal
of H-terminated Si(111) and the carboxy-terminated mono-
layer in Figure 3, we estimate this dampening to be about 25%.
Together, this assessment yields a nearly 1:1 ratio of Si−OH-
and Si−C-terminated surface areas. In other words, every
second H-terminated site of H−Si(111) carries an alkyl chain,
forming a densely packed SAM (Figure 4d).
As an independent proof of the surface coverage, the
thickness and electron density of the carboxy-terminated alkyl
SAM on Si(111) were determined by X-ray reflectivity (XRR)
measurements.31 Figure 4a shows the offset-corrected specular
XRR of a carboxy-terminated alkyl SAM on Si(111) as a
function of momentum transfer qz = (4π/λ) sin(θ), with θ
being the angle of incidence. The deviation from the smooth
Fresnel reflectivity of the bare Si(111) substrate, i.e., the
occurrence of a minimum around 0.22 Å−1, gives evidence for a
thin layer on top of the silicon. The XRR data were fitted using
Potential oxidation of the silicon surface during the
preparation procedure was monitored by the Si2p core-level
spectrum, which exhibits distinct chemical shifts according to
the silicon oxidation state. Figure 3 depicts XP spectra of the
native silicon oxide surface of n-doped Si(111), the H-
terminated Si(111) surface prior to grafting, the methyl ester-
terminated monolayer, and the subsequent carboxy- and amide-
terminated monolayers. A survey of the binding energies is
given in the Supporting Information (Figure S4). The silicon
wafer with its native oxide capping layer showed, besides the 2p
doublet, the typical silicon oxidation states, Si+1, Si+2, and Si+3,
and the dominant Si+4 component gradually shifted to higher
binding energies.30 The shift of the Si2p level with respect to
H−Si(111) is attributed to surface band bending. The H-
terminated surface exhibited none of the higher oxidation states
(Si+2, Si+3, or Si+4), revealing the high efficiency of the etching
procedure. There was, however, a small, second doublet
chemically shifted by 0.90 eV to higher binding energy,
which indicated the presence of Si−O, i.e., silicon atoms in
oxidation state +1. We note that the Si−O signal was too weak
to be detected in normal emission, where the surface sensitivity
of the XPS measurement is reduced (see Supporting
Information, Figure S5).
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In addition to the out-of-plane information, the in-plane
SAM structure, i.e., the spacing d of SAM lattice planes and
their epitaxial orientation with respect to the Si(111) surface,
was analyzed with grazing incidence X-ray diffraction (GIXD, αi
= 0.18°).35 Figure 4a shows the azimuthal orientation between
silicon and SAM lattice planes as obtained from in-plane Bragg
reflections. Besides the Si(220) reflection, additional sharp
̅
peaks are found that can be assigned to second- and third-order
in-plane Bragg reflections of SAM lattice planes that are rotated
−30° (for momentum transfer q∥ = 1.90 0.02 Å−1) and +60°
(for q∥ = 2.43 0.02 Å−1) with respect to the Si(220) planes
̅
(see Figure 4b,c). All in-plane reflections of the SAM exhibit a
6-fold rotational symmetry along the azimuthal angle (only one
of the six symmetry-equivalent reflections is shown for each q∥)
ascribed to the different SAM domains on the quasihexagonal
Si(111) surface. On the basis of the area per molecule and
GIXD results, in Figure 4d we show an in-plane unit mesh for
the carboxy-terminated alkyl SAM on Si(111) consistent with
the derived lattice parameters and azimuthal orientation. The
dimensions of the unit mesh are a = 6.65 0.1 Å, b = 7.68
0.1 Å, and β = 90°. Each contains two molecules. According to
the mesh parameters, the SAM reflections in Figure 4d are
denoted by (20) and (03). The in-plane unit mesh corresponds
to a zigzag substitution pattern as proposed in simulations
performed by Sieval et al.32,33 While we use a rectangular
primitive unit mesh for indexing the GIXD reflections, the
positioning of the alkyl chains with respect to the Si(111)
surface atoms can also be described as a c(2 × 4) overlayer (see
Figure 4e). No Bragg reflections corresponding to a lattice
constant b′ = b/2 = 3.84 Å resulting from a linear substitution
pattern with just one molecule per unit mesh were found.36
Therefore, a predominant ordering with a zigzag substitution
pattern is assumed. From the width Δq of the SAM reflections,
we can furthermore estimate the lower limit of the coherent
domain size within the SAM via dmin = 2π/Δq. In the case of
the SAM(20) reflection, this gives a lower limit for the domain
width of about 16 nm, i.e., more than 50 lattice planes.
Figure 3. XPS scans of the Si2p core level recorded with
monochromatized Al KαI radiation (hν = 1486.7 eV) at a sample
temperature of 300 K. The sequence starts with the native oxide-
capped surface of n-doped silicon, followed by the H-terminated
Si(111) surface. The lower three spectra are representative of the on-
chip synthesis from the ester-, via the acid-terminated, to the amide-
terminated layer using fulgimide (E/Z)-1.
In addition to the carboxy-terminated alkyl SAM, the methyl
ester-terminated monolayer was investigated to check for
structural changes caused by the ester cleavage. Neither XRR
nor GIXD revealed significant structural differences between
the ester-terminated and the carboxy-terminated alkyl SAMs
(Supporting Information, Figures S7−S9).
a stratified homogeneous media approach, where the electron-
density distribution along the z-axis is modeled by three slabs
(vacuum, SAM, silicon) with smooth boundary conditions.12
The SAM thickness as obtained from the fit is 14.25 1.0 Å.
Assuming a molecular length of ∼16 Å, this thickness
Formation of Amide-Terminated Alkyl SAMs by HCTU
Coupling. The formation of amide-terminated alkyl SAMs was
accomplished by coupling fulgimide (E/Z)-1 (85:15 mixture)
to the carboxy-terminated monolayer in a one-step reaction in
corresponds to a molecular tilt angle of 28°
8°, which
would be in good agreement with measurements of similar
acetonitrile over a period of 2 h using HCTU1 and Hunig’s base
SAMs from Sieval et al.32,33
̈
(DIEA, diisopropylethylamine). When the reaction was
conducted under oxygen- and water-free conditions in a
glovebox, ATR-FTIR showed no byproducts or unreacted
reagents and intermediates after workup. The ATR-FTIR
signatures of the amide-terminated alkyl SAM on Si(111)
(Supporting Information, Figures S10, S17, S18) consist of the
characteristic νsym(CO) and νas(CO) absorption bands
due to the imide group of the (E/Z)-forms of the fulgimide
headgroup, which are found at 1739 and 1689 cm−1,
respectively. The ν(CO) amide I band and ν(C−N−H) +
δ(C−NH) amide II bands are observed at 1653 and 1544 cm−1,
respectively, verifying the formation of the amide-terminated
monolayer. The similar νas(CH2) and νsym(CH2) absorption
bands of the methyl ester- (black line), carboxy- (red line), and
The electron density of the monolayer extracted from the fit
is 0.30
0.01 e−1 Å−3. From the values obtained for the
thickness (d) and electron density (ρe) of the monolayer and
the absolute number of electrons per molecule (ne), one can
calculate the surface area per molecule via Amol = ne/dρe.
Assuming the presence of both alkyl-linker chains and Si−OH
groups, as suggested by XPS, gives an area of 24 2 Å2 per
alkyl chain plus hydroxyl group, corresponding to a packing
density of (6.8 0.6) × 10−10 mol cm−2 or (4.1 0.3) × 1014
molecules cm−2. Since the maximum theoretical packing
density of alkyl chains on Si(111) is 12.8 × 10−10 mol cm−2,
the XRR results confirm that indeed every second accessible Si
bond is occupied by an alkyl chain, in agreement with our XPS
data and simulations.34
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Figure 4. (a) XRR scan of a carboxy-terminated alkyl SAM on Si(111), together with a least-squares fit. The fit gives a total thickness of the layer of d
= 14.25 1.0 Å. (b) GIXD scans along q||, showing two in-plane Bragg reflections at q|| = 1.90 and 2.43 Å−1. (c) Azimuthal distribution of the lattice
planes, derived from Bragg reflections in GIXD. (d) Sketch of the in-plane unit mesh with the dimensions a = 6.65 Å, b = 7.68 Å, and β = 90°,
containing two molecules (black circles). Blue hexagons depict the Si(111) surface while black lines stand for crystal lattice planes of the Si(220)
̅
reflection. (e) Sketch showing the alternative c(2 × 4) description of the SAM unit mesh with respect to the primitive unit mesh of the Si(111)
surface atoms.
amide-terminated (blue line) SAMs prove their chemical
stability and indicate a dense packing.
hybrid aliphatic−aromatic backbone one cannot distinguish
between the C1s emission stemming from the aromatic part of
the backbone and that originating from the aliphatic part.22,23
As the concept of the chemical shift is not fully applicable to the
photoemission spectra of SAMs, we refrain from further
decomposition of the main line. The additional C1s component
at a binding energy of 288.4 eV is characteristic for the CONH
In the C1s XP spectra of the amide layer in Figure 2a
(bottom), the main peak shifted to a higher binding energy of
285.1 eV, typical for combinations of aromatic and saturated
entities, i.e., the fulgimide photoswitch and the alkyl-linker
layer. We note that in SAMs comprised of molecules with
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moiety.37,38 This group shows a larger signal than the carboxyl
group of the acid layer, reflecting the additional imide group of
the fulgimide.
bottom in Figure 2a is larger by a factor of ∼1.6 when
comparing the acid- and amide-layer spectrum. Switch and
linker contain 20 and 11 carbon atoms, respectively. If we
neglected the attenuation of the XPS signal in the molecular
layer, we would conclude that every third alkyl linker carries a
fulgimide photoswitch: (20/3 + 11)/11 = 1.6. However, as seen
from the intensity of the Si2p core level in Figure 3, the
dampening of the substrate signal by the aromatic amide-
terminated monolayer is about twice as strong as that by the
aliphatic alkyl-based monolayers, i.e., the methyl ester-
terminated and the carboxy-terminated alkyl SAMs. From the
carboxy- to the amide-terminated monolayer, the Si2p signal is
reduced by a factor of ∼0.6. Assuming an equal damping of the
C1s signal of the alkyl chain, an even denser packing of the
fulgimide headgroups on top of the carboxy-terminated
monolayer is required to explain the XPS intensities. In fact,
the C1s XPS signal of the amide-terminated monolayer requires
∼0.55 fulgimide molecules per carboxy linker: (0.55 × 20 + 0.6
× 11)/11 = 1.6. In conclusion, evaluation of the XPS intensities
reveals a high turnover for the amide coupling; on average, one
fulgimide photoswitch is coupled to every second alkyl linker.
Combining the results from ATR-FTIR, XPS, and X-ray
scattering, we can now deduce reliable models of the various
molecular layers. As sketched in Scheme 2, the rectangular unit
mesh (a = 6.65 Å, b = 7.68 Å) of the ester and acid layer
contains two linker molecules in a zigzag arrangement with a
coverage of about 4 × 1014 molecules cm−2 [∼50% of initially
H-terminated Si(111) sites]. The cleavage of the ester runs till
completion. During this reaction, hydrolysis of the remaining
H−Si(111) sites leads to HO−Si(111) groups between the
molecular rows. There is no further, significant oxidation of the
silicon substrate. The final one-step reaction using HCTU and
The O1s main peak of the amide layer in Figure 2b (bottom)
strongly shifted to a lower binding energy of 531.8 eV,
compatible with the literature value for an amide-terminated
alkyl SAM.37 To properly fit the O1s XP spectrum, we needed,
in addition to the Si−O signal, two small components
representing some remaining, uncoupled carboxyl groups of
the carboxy-terminated alkyl-linker SAM. From steric con-
straints, i.e., considering the respective van der Waals radii, it is
obvious that at most only every second carboxy linker can carry
a fulgimide molecule (cf. Scheme 2). However, when
Scheme 2. Top and Side Views of the H-Terminated Si(111)
Surface, the Grafted Carboxy-Terminated Alkyl-Linker
a
Layer, and the SAM with Coupled Fulgimide Photoswitches
Hunig’s base leads to a densely packed functionalized SAM
̈
with one fulgimide photoswitch per two alkyl-linker molecules,
i.e., 2 × 1014 molecules cm−2 [∼25% of initially H-terminated
Si(111) sites].
Readout of the Amide-Terminated Alkyl SAM Using
Light. The photoswitching between the mixture of the open
(Z)- and (E)-forms and the closed (C)-form with ultraviolet
(UV) and visible (vis) light is illustrated in Scheme 3. We note
that in solution the ratio between (Z)- and (E)-forms is
identical in both photostationary states (PSS), PSS(365 nm)
and PSS(545 nm) (Supporting Information, Table S1).
Therefore, IR and UV/vis difference spectra solely monitor
the E/Z-to-C and C-to-E/Z conversion upon UV and visible
light illumination.9,10 The switching characteristics of the
a
The hard-sphere models of the molecules indicate the van der Waals
radii; O and N atoms are shown in red and blue. The primitive unit
meshes of H−Si(111) and the acid and amide layer are shown as well
as the unit mesh with c(2 × 4) periodicity. The on-chip synthesis leads
to a densely packed SAM. The molecular arrangement and surface
coverage are deduced combining X-ray scattering, XPS, and ATR-
FTIR results.
Scheme 3. Fulgimide Photochromic Equilibria upon Light
Illumination with UV (365 nm) and Visible (Vis) Light (545
nm)
transferring the peak positions of the hydroxyl and carboxyl
groups from the acid layer, the intensity of the uncoupled
components in the amide layer amounts to only 11% compared
to the acid layer. We conclude that the XPS signal of the
uncoupled components is strongly attenuated by the overlaying
fulgimide molecules. A similarly strong dampening is seen for
the C1s peak of the carboxyl group at 289.8 eV, which renders
the C1s signal of the amide and imide groups at 288.4 eV
slightly asymmetric.
The strong reduction of the XPS signals of the remaining,
uncoupled carboxy-linker molecules upon amide formation
with fulgimide (E/Z)-1 indicates a dense packing of the
photoswitches coupled to the SAM. The C1s signal at the
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Figure 5. (a) Switching between PSS(365 nm), enriched in the (C)-form (red line), and PSS(545 nm), enriched in the (E/Z)-form of fulgimide 1
(black line). The ATR-FTIR spectra differ in the characteristic νsym(CO) and νas(CO) absorption bands for the imide moiety of the open and
closed fulgimide headgroup. (b) ATR-FTIR spectra of the amide-terminated monolayer derived from fulgimide (E/Z)-1 (black) and of the PSS(545
nm) after four illumination cycles (red). (c) Steady-state difference-absorption spectra between the photostationary states due to the ring-closure
(red) and ring-opening (black) reactions.
amide-terminated SAM bearing the fulgimide headgroup were
evaluated by alternating irradiation with UV and visible light
and nondestructive readout of the two photostationary states
using ATR-FTIR and XPS.
the photostationary states PSS(365 nm) and PSS(545 nm) of
the fulgimide headgroup due to the ring-closing and ring-
opening reactions remained nearly unchanged.
XPS measurements confirm the expected (E/Z)-to-(C) ring
closure for reaching PSS(365 nm). Subsequent (C)-to-(E/Z)
ring-opening, furnishing PSS(545 nm), as well as successive
ring-closing and -opening cycles all led to weak but clearly
discernible changes in the recorded spectra. The N1s and C1s
signals of the fulgimide layer’s imide and amide groups are
depicted in Figure 6a,b. The oxidation of the Si surface, traced
by the Si+1 2p XPS component, increased only slightly after the
first irradiation. However, even though greatest care was taken,
a continuous, minor buildup in the carbon and oxygen spectra’s
main peaks was observed and attributed to contaminants.
Therefore, switching of XPS samples was stopped after two
cycles each.
The changes of the N1s and C1s peak heights and peak
widths as well as the C1s binding energy of the amide/imide
moiety upon irradiation are summarized in Figure 6c,d. As
mentioned in the Experimental Section, the latter was
determined by fitting the C1s spectra after subtracting all
other peaks (see Figure S20 in the Supporting Information).
Analogously to the ATR-FTIR data, the XPS measurements
show that the original 85:15 ratio of the starting (E/Z)-mixture
was not fully reached after the first irradiation cycle, but only
minor variations were observed. On the molecular level, ring-
closing and -opening reactions are affecting the indolyl ring
As seen from the ATR-FTIR spectra in Figure 5a, upon
irradiation of the amide-terminated monolayer, derived from
fulgimide (E/Z)-1 (85:15 mixture), with 365 nm light, in the
PSS(365 nm) the characteristic νsym(CO) and νas(CO)
absorption bands of the (E/Z)-mixture shifted from 1739 to
1758 cm−1 and from 1689 to 1694 cm−1, respectively. In
accordance to our IR spectroscopy studies in solution
(Supporting Information, Figure S1), these changes indicate
the ring-closure of the (E)-fulgimide to the (C)-fulgimide and
the arrival at a PSS(365 nm) enriched in the (C)-form.
Comparable to the situation in solution, switching between the
(E)- and the (Z)-form in the SAM is not detectable by IR
spectroscopy. Upon irradiation with 545 nm light, a PSS(545
nm) containing the (E)- and (Z)-forms is reached, and the
νsym(CO) and νas(CO) absorption bands shift back to
1739 and 1690 cm−1, respectively.
In Figure 5b, the ATR-FTIR spectrum of the amide-
terminated alkyl SAM derived from fulgimide (E/Z)-1 (85:15
mixture) is superimposed with the ATR-FTIR spectrum of the
photostationary state PSS(545 nm) after four illumination
cycles. This comparison indicates a nearly fully reversible
switching behavior. As illustrated in Figure 5c for four
irradiation cycles, the steady-state difference spectra between
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Figure 6. XP spectra of (a) the N1s and (b) the C1s core-level components of the pristine amide-terminated monolayer bearing fulgimide moieties
and two alternating irradiation cycles at wavelengths of 365 and 545 nm, respectively. (c) N1s and C1s peak heights and widths and (d) binding
energy of the C1s core level of the amide/imide moiety extracted from parts a and b. The weak oscillations of all signals demonstrate two iterative
cycles of switching and back-switching of the fulgimide SAM with UV (365 nm) and vis (545 nm) light, respectively.
with its embedded nitrogen atom. Ring-closure of the (E)-form,
containing a helical hexatriene system with an intact indole ring,
furnishes the planar, conjugated ring-closed (C)-form with its
cyclohexadiene core structure containing two sp3-hybridized
carbon atoms.
Assuming a photostationary state PSS(365 nm) consisting of
70% of the (C)-isomer, i.e., comparable to the molecule in
solution, these exposures correspond to large cross sections and
demonstrate efficient decoupling of the fulgimide photoswitch
from the silicon substrate in the SAM.
Because of these structural variations in the respective
nitrogen atom’s chemical surrounding, changes in the recorded
N1s spectra in a breathing-like fashion were observed, as shown
in Figure 6c. The N1s peak height (filled squares) is clearly
reduced after the first irradiation with UV light (365 nm) and
weakly increases and decreases after each following irradiation
session with visible (545 nm) and UV light, respectively. The
peak’s fwhm varies in the opposite manner, beginning with a
small width that is first increased and then slightly decreased
and increased again (open squares in Figure 6c). Analogously,
in the carbon spectra the modified environment causes the
amide signal to breathe as well. This peak not only varies in
height and width (filled and open circles in Figure 6c) but also
changes its binding energy peak position by about 50 meV
(Figure 6d and Figure S20 in the Supporting Information). We
attribute the small but reversible changes of the XP spectra to
the repeated ring-closing and -opening cycles due to the
associated structural alterations described above and, accord-
ingly, the corresponding changes in bulk properties.
CONCLUSION
■
We prepared densely packed ester-terminated alkyl self-
assembled monolayers on Si(111) by thermal grafting in a
glovebox and subsequently hydrolyzed the methyl ester to give
an acid end-group, while simultaneously the remaining H−
Si(111) sites were hydrolyzed to HO−Si(111). X-ray photo-
electron spectroscopy and X-ray reflectivity measurements
indicate a surface coverage of about 50% for the carboxy-
terminated alkyl SAM [with respect to the H-terminated sites
of H−Si(111)] while grazing incidence X-ray diffraction results
imply a unit mesh with a regular zigzag-like substitution pattern
(Figure 4d and Scheme 2). The coverage corresponds to a
densely packed alkyl SAM. The amounts of alkyl linkers and
hydroxyl groups after thermal grafting and subsequent
hydrolysis of the ester end-group are similar, thereby indicating
a well-defined, neat carboxy-terminated alkyl SAM. The
aminium-based coupling reagent HCTU1 gave, under one-pot
coupling conditions, high-quality, amide-terminated alkyl SAMs
with a fulgimide headgroup. Since the fulgimide is quite
voluminous compared to the packing density of the carboxyl
groups in the linker layer, a maximum coupling yield of 50% is
possible, i.e., every second carboxy linker carries a fulgimide
Both ATR-FTIR and XPS demonstrate reversible photo-
switching of the fulgimide SAM. The photostationary states are
reached after irradiating the SAM with about 1019 photons
cm−2 in the UV or vis range (2 and 5 mW for 30 min).
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(10) Koller, F. O.; Schreier, W. J.; Schrader, T. E.; Malkmus, S.;
(Scheme 2). Photoswitching between the two photostationary
states, PSS(365 nm) and PSS(545 nm), is observed with a
fluence of less than 10 J cm−2. This corresponds to high cross
sections of 10−19 cm2, suitable for practical applications.
Schulz, C.; Dietrich, S.; Ruck-Braun, K.; Braun, M. Ultrafast Ring-
̈
Closure Reaction of Photochromic Indolylfulgimides Studied with
UV-Pump-IR-Probe Spectroscopy. J. Phys. Chem. A 2008, 112, 210−
214.
(11) Faucheux, A.; Gouget-Laemmel, A. C.; Henry de Villeneuve, C.;
Boukherroub, R.; Ozanam, F.; Allongue, P.; Chazalviel, J.-N. Well-
Defined Carboxyl-Terminated Alkyl Monolayers Grafted onto H−
Si(111): Packing Density from a Combined AFM and Quantitative IR
Study. Langmuir 2006, 22, 153−162.
(12) Parratt, L. G. Surface Studies of Solids by Total Reflection of X-
rays. Phys. Rev. 1954, 95, 359−369.
ASSOCIATED CONTENT
* Supporting Information
Experimental data (synthesis), additional ATR-FTIR data, XPS
data, and XRR data. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
(13) Yokoyama, Y.; Takahashi, K. Trifluoromethyl-Substituted
Photochromic Indolylfulgide. A Remarkably Durable Fulgide Towards
Photochemical and Thermal Treatments. Chem. Lett. 1996, 25, 1037−
1038.
AUTHOR INFORMATION
Corresponding Authors
*K.R.-B.: e-mail, karola.rueck-braun@tu-berlin.de.
*M.W.: e-mail, weinelt@physik.fu-berlin.de.
■
(14) Zarwell, S.; Dietrich, S.; Schulz, C.; Dietrich, P.; Michalik, F.;
Ruck-Braun, K. Preparation of an Indolylfulgimide-Adamantane Linker
̈
Present Addresses
∥Department of Chemistry, Technical University of Denmark,
DK-2800 Kgs.-Lyngby, Denmark.
Conjugate with Nitrile Anchoring Groups through Palladium-
Catalyzed Transformations. Eur. J. Org. Chem. 2009, 13, 2088−2095.
(15) Dietrich, P.; Michalik, F.; Schmidt, R.; Gahl, C.; Mao, G.;
⊥Chemistry Department, Faculty of Applied Sciences, Umm-
Al-Qura University, 21955 Makkah, KSA.
Notes
Breusing, M.; Raschke, M. B.; Priewisch, B.; Elsasser, T.; Mendelsohn,
̈
R.; Weinelt, M.; Ruck-Braun, K. An Anchoring Strategy for
̈
Photoswitchable Biosensor Technology: Azobenzene-Modified SAMs
on Si(111). Appl. Phys. A 2008, 93, 285−292.
The authors declare no competing financial interest.
(16) Boukherroub, R.; Wayner, D. D. M. Controlled Functionaliza-
tion and Multistep Chemical Manipulation of Covalently Modified
Si(111) Surfaces. J. Am. Chem. Soc. 1999, 121, 11513−11515.
(17) Touahir, L.; Chazalviel, J.-N.; Sam, S.; Moraillon, A.; Henry de
Villeneuve, C.; Allongue, P.; Ozanam, F.; Gouget-Laemmel, A. C.
Kinetics of Activation of Carboxyls to Succinimidyl Ester Groups in
Monolayers Grafted on Silicon: An in Situ Real-Time Infrared
Spectroscopy Study. J. Phys. Chem. C 2011, 115, 6782−6787.
(18) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D.
Spontaneously Organized Molecular Assemblies. 4. Structural
Characterization of n-Alkyl Thiol Monolayers on Gold by Optical
Ellipsometry, Infrared Spectroscopy, and Electrochemistry. J. Am.
Chem. Soc. 1987, 109, 3559−3568.
ACKNOWLEDGMENTS
■
The SFB 658 (“Elementary Processes in Molecular Switches at
Surfaces”), the Fonds der Chemischen Industrie, The Danish
Research Council for Independent Research | Natural Sciences,
The Carlsberg Foundation, and the Alexander von Humboldt
Foundation are gratefully acknowledged for supporting this
work.
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