Surface-enhanced Raman scattering and imaging of Langmuir-Blodgett monolayers of bis(phenethylimido)perylene on silver island films

Article abstract of DOI:10.1366/0003702001950913

The optical spectra of bis(phenethylimido)perylene (PhPTCD) are discussed. Surface-pressure area isotherms of floating Langmuir monomolecular layers have been obtained, and Langmuir-Blodgett (LB) molecular monolayers of the material have been fabricated on silver island substrates for surface-enhanced vibrational studies. The electronic absorption and emission spectra of solutions and thin solid films are described. The vibrational spectra, infrared and Raman for the bulk, and the surface-enhanced Raman (SERS) and resonance Raman scattering (SERRS) spectra of LB monolayers have been obtained. Surface-enhanced fluorescence (SEF) for LB films is also demonstrated. Given the unique properties of the LB coated silver surfaces, the mapping of the SERS/SERRS signal and global Raman images, at a particular vibrational wavenumber, were obtained by using the 780 and 514.5 nm laser lines. The images give a visual picture of the variation of the SERRS and SERS signal intensity on the rough metal surface.

Full text of DOI:10.1366/0003702001950913

Surface-Enhanced Raman Scattering and Imaging of
Langmuir–Blodgett Monolayers of
Bis(phenethylimido)perylene on Silver Island Films
R. F. AROCA,* C. J. L. CONSTANTINO, and JAMES DUFF
Materials and Surface Science Group, School of Physical Sciences, University of Windsor, Windsor, Ontario N9B 3P4 Canada
(
R.F.A., C.J.L.C.); and Xerox Research Centre of Canada 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada (J.D.)
The optical spectra of bis(phenethylimido)perylene (PhPTCD) are
discussed. Surface-pressure area isotherms of  oating Langmuir
monomolecular layers have been obtained, and Langmuir–Blodgett
recorded. The chromophore dominates not only the fea-
tures of the absorption and emission spectra but also the
inelastic scattering spectra in and out of resonance with
the excitation laser line. Correspondingly, the surface-en-
hanced Raman scattering is also characteristic of the PTC
moiety. The fact that the spectra of PTC derivatives are
those characteristic of the chromophore is advantageous
when examining the aggregation and stacking properties
(
LB) molecular monolayers of the material have been fabricated on
silver island substrates for surface-enhanced vibrational studies.
The electronic absorption and emission spectra of solutions and thin
solid Ž lms are described. The vibrational spectra, infrared and Ra-
man for the bulk, and the surface-enhanced Raman (SERS) and
resonance Raman scattering (SERRS) spectra of LB monolayers
have been obtained. Surface-enhanced  uorescence (SEF) for LB
9
of these materials in thin solid Ž lms.
Ž
lms is also demonstrated. Given the unique properties of the LB
LB monolayers deposited onto silver island Ž lms pro-
vide a unique sample to study the surface-enhanced scat-
tering signal from a single monolayer coating of the
rough metal surface. In particular, the spatial mapping of
the relative intensity of the SERS/SERRS spectra with
coated silver surfaces, the mapping of the SERS/SERRS signal and
global Raman images, at a particular vibrational wavenumber,
were obtained by using the 780 and 514.5 nm laser lines. The images
give a visual picture of the variation of the SERRS and SERS signal
intensity on the rough metal surface.
;
1 mm spatial resolution can be recorded. Complemen-
Index Headings: SERS; SERRS; Imaging; Surface-enhanced vibra-
tional spectroscopy; Langmuir–Blodgett Ž lms; Perylene; Surface-
enhanced  uorescence.
tary information is obtained by ‘‘global’’ Raman imaging
1
0
using Ž lters. In the present report the SERRS and SERS
spectra of a single Langmuir–Blodgett Ž lms are collected
point by point to produce spectral maps. Wide-Ž eld or
global imaging of the LB Ž lms obtained using the im-
aging Ž lter technique implemented in the Renishaw Ra-
INTRODUCTION
1
1
The objectives of the present report are twofold. The
man imaging spectrograph on silver is also reported.
Ž
rst goal was the fabrication of Langmuir–Blodgett (LB)
molecular monolayers of bis(phenethylimido)perylene
PhPTCD), a member of the class of materials deriving
EXPERIMENTAL
(
from the perylene tetracarboxylic unit, to be followed by
the electronic and vibrational spectroscopic characteriza-
tion. The second part of the work was the investigation
of the surface-enhanced spectra obtained from a single
LB molecular monolayer on silver islands, which pro-
vides a unique sample for the study of surface-enhanced
Raman spectroscopy/surface-enhanced resonance Raman
spectroscopy (SERS/SERRS) imaging. The material
PhPTCD belongs to the family of perylene tetracarbox-
ylic (PTC) derivatives, organic dyes whose electrical and
optical properties have attracted interest for applications
Bis(phenethylimido)perylene. Anthra[2,1,9-def:
6,5,10-d9e 9f 9]diisoquinoline-1,3,8,10(2H,9H)-tetrone,
2,9-bis(2-phenylethyl), also known as C. I. Pigment
Black 31, was prepared at high purity as follows: A dis-
persion of 3,4,9,10-perylene tetracarboxylic acid dian-
hydride (7.8 g, 0.020 mole) in 400 mL of 1-methyl-2-
pyrrolidineone was treated with 12.2 g (0.10 mole) of 2-
phenethylamine. The mixture was stirred under argon and
was heated to re ux (202 8C) for 30 min. The mixture
was cooled to 150 8C and was Ž ltered through a medium-
porosity 350 mL sintered glass funnel, which had been
preheated with boiling dimethylformamide. The solid
was washed in the funnel with 4 3 100 mL portions of
boiling DMF, then with 50 mL of cold DMF and 3 3 50
mL of methanol. The initial boiling DMF wash was dark
brown; the Ž nal wash was a faint orange. The product
was dried at 60 8C for 16 h to give 10.9 g (91% yield)
of the required product as shiny black needle crystals,
[M.P., 420 8C (DSC)].
1
–3
in the optoelectronic area. The chromophore moiety is
derived from the PTC molecule, which has been the ob-
4
,5
ject of careful spectroscopic, scanning tunneling mi-
6
7,8
croscopy, and theoretical studies. The PhPTCD mole-
cule contains 72 atoms, 38 of which form the chromo-
phore moiety. However, the spectroscopic properties of
PhPTCD are for the most part those of the central chro-
mophore. The material is characterized by a deep red
coloration due to a strong absorption in the visible with
The Raman scattering, mapping, and global imaging
were obtained with a Renishaw Research Raman micro-
scope system RM2000 equipped with a computer-con-
trolled 3-axis encoded (XYZ) motorized stage with a
minimum step of 0.1 mm. The RM2000 uses a Leica
microscope (DMLM series). The spectrum is measured
4
21
3
21
an extinction coefŽ cient of ; 10 (mol dm cm ). The
absorption spectrum of a single LB monolayer can be
Received 12 November 1999; accepted 27 March 2000.
*
Author to whom correspondence should be sent.
0
003-7028 / 00 / 5408-1120$2.00 / 0
1120
Volume 54, Number 8, 2000
APPLIED SPECTROSCOPY
q 2000 Society for Applied Spectroscopy

FIG. 1. Surface-pressure area isotherm of PhPTCD on water.
FIG. 2. Electronic absorption spectrum of PhPTCD. The absorbance
scale is that of the Ag island Ž lm on glass and the LB coated Ag Ž lm.
The scale is arbitrary for all other traces.
with a Peltier-cooled (270 8C) charge-coupled device
(
CCD) array. The high throughput of the instrument per-
mits the use of very low powered lasers. Typically, 5 mW
or less of the 514.5 nm argon-ion laser line at the sample
was used. For the 780 nm laser line, the power at the
sample location was varied from 40 mW to a maximum
of 10 mW. The spectrograph was equipped with a 1200
grooves/mm grating with additional angle-tuned band-
pass Ž lter optics. Raman spectra were recorded at room
accelerating voltage and the secondary electron emission
with a 20 0003 magniŽ cation.
RESULTS AND DISCUSSION
The Absorption Spectra of PhPTCD and its LB
Monolayers. The Langmuir monolayers were prepared
by spreading the solution on the water subphase by a
microsyringe. Thermal equilibrium is achieved after sol-
vent evaporation (; 30 min) to begin Ž lm compression at
a speed 65 mm/min. Reproducible surface-pressure area
isotherms were obtained, as shown in Fig. 1. The curves
given in Fig. 1 also show the hysterisis of the thermo-
dynamic process and the isotherm obtained after expan-
sion. From the isotherms it can be determined that the
 oating layer is rather rigid, and that feature would ex-
plain the hysterisis for the compression–expansion pro-
cesses. It is also important to notice that a stable solid
phase is obtained for the Langmuir monolayer and that
transferring at 25 mN/m is well within the solid phase.
The area per molecule for the neat  oating layer is 65
2
1
temperature with ; 4 cm resolution. The maps reported
here were recorded with 514.5 and 780 nm laser lines,
using a Leica microscope and a 503 microscope objec-
2
tive to focus the laser beam onto a spot of ; 1 mm and
mapping an area of 40 by 40 mm. For global imaging,
recorded with angled-tuned dielectric Ž lters at the Raman
wavenumber of the SERRS spectrum (1293 or 1378
2
1
cm ) and 514.5 nm laser line, the laser spot size is de-
focused and the Raman scattered light from a large sur-
face area is collected. However, a 40 mm by 40 mm area
was used here to facilitate a visual comparison of the
resulting images and mapping plots. Data acquisition and
analysis was carried out with the WIRE software for win-
dows and Galactic Industries GRAMS/32 C software in-
cluding the 3D package. Infrared spectra were recorded
with a Nicolet Magna-IR 550 spectrometer Series II with
Ê
2
A .
The absorption spectra of perylene tetracarboxylic de-
2
1
DTGS detector and 4 cm resolution. The chamber was
purged with dry air.
rivatives containing one or two perylene chromophores
A monolayer of PhPTCD was transferred to Corning
059 glass slides coated with 6 nm silver, in a Lauda
usually consist of one electronic transition with the cor-
1
2
7
responding vibronic structure. The absorption spectrum
of a dilute solution, shown in Fig. 2, has a characteristic
vibronic structure associated with the p–p* transition of
the perylene moiety with a 0–0 band at 540 nm followed
by subsequent transitions at 503 and 469 nm. The non-
resonant 780 nm laser line and the 514.5 nm laser line
in resonance with the absorption spectrum of PhPTCD
are also displayed in Fig. 2.
Langmuir–Blodgett Ž lms of PhPTCD were deposited
onto glass and silver substrates. The absorption spectrum
of the LB on glass presented in Fig. 2 shows an intense
red-shifted component with broad maximum at 618 nm
and a blue-shifted component at 461 nm. Kasha’s point-
dipole model permits rationalization of the observed band
shifts and splits. The extreme cases are represented by a
head-to-tail arrangement of the dipoles, which results in
a red-shifted band. A blue-shifted band arises from an
Langmuir Ž lm balance equipped with an electronically
controlled dipping device, Lauda Filmlift FL-1. The sub-
phase was ultrapure water (18.2 MV cm) containing cad-
2
4
mium chloride (2.5 10
PhPTCD were prepared with 10% tri uoroacetic acid
TFA) in dichloromethane. The LB Ž lm PhPTCD was
M) at 15 8C. Solutions of
(
fabricated with the use of Z-deposition and maintenance
of a constant pressure which corresponded to the con-
densed phase of the Langmuir Ž lm (around 25–30 mN/
m in both the cases). Glass substrates were covered with
2
7
silver islands deposited by vacuum evaporation (at 10
torr) where the thickness (6 nm mass thickness) was con-
trolled by quartz crystal balance. UV-visible spectra were
recorded in Cary 50 Scan UV/VIS spectrophotometer.
Scanning electron micrographs of the islands Ž lms (not
shown) were obtained with a Jeol 5800LV using a 15 kV
APPLIED SPECTROSCOPY
1121

TABLE I. Observed SERS and SERRS wavenumbers for an LB
monolayer of PhPTCD. Numbers in parentheses are the full width
2
1
at half-maximum (FWHM) in cm
.
SERS SERRS
2
1
21
(
cm
)
(cm
)
Interpretation
5
5
34 (6) w
55 (7) m
Perylene deformation
Perylene deformation
C–H bending
554 (18) w
1075 (22) w
1
087 (8) w
1
256 (22) sh
Alkyl C–H bending
C–H bend 1 ring stretch
Perylene ring stretch
Perylene ring stretch
Perylene ring stretch
Alkyl C–H bending
Perylene ring stretch
Perylene ring stretch
C5C stretch
1
1
1
1
1
293 (9) vs
302 (9) s
374 (10) sh
381 (9) s
427 (6) w
1290 (20) vs
1378 (30) vs
1417 (19) sh
1450 (19) sh
1518 (13) sh
1572 (19) s
1586 (40) vs
FIG. 3. Excimer emission of the LB monolayer on glass and surface-
enhanced  uorescence of the LB monolayer on silver islands.
1456 (8) m
1
1
571 (9) s
588 (11) s
C5C stretch
unfavorable arrangement (H aggregate formation).13 The
strong red- and blue-shifted band component observed in
the absorption spectrum of the LB Ž lm and bulk samples
is an indication of an intermediate case that results in
band splitting. The mid point between the two maxima
is in close agreement with the v0–0 transition observed in
the solution spectrum. The emission obtained with 514.5
nm excitation is also typical of PTCD derivatives with a
very strong broad and structureless band at ; 680 nm,
which is attributed to the excimer emission, as shown in
Fig. 3. The observation of a very strong excimer emission
is due to the formation of organized structures or molec-
ular stacking in the LB Ž lms, which requires the chro-
mophores to be arranged with parallel and overlapping
ring systems, as has been observed in a-perylene crys-
surface normally diminishes the  uorescence signal.
Fluorescence enhancement is observed to increase with
the separation from the metal surface to reach a maxi-
mum at an optimum distance where SEF enhancement
1
8
overpowers quenching due to surface-induced damping.
The large SEF enhancement observed for PhPTCD on
silver islands, as shown in Fig. 3, demonstrates that the
orientation of the PTC chromophore on the metal surface
is edge-on or head-on but not  at-on with respect to the
metal surface. The latter molecular orientation would
probably have resulted in  uorescence quenching rather
than enhancement.
1
4
Surface-Enhanced Raman and Resonance Raman
Scattering of PhPTCD Monolayers. The vibrational
spectra of PTCD molecules have been discussed for a
tals. Excitation with the 633 nm laser line produces an
extremely strong  uorescence that completely masks the
Raman scattering. The latter is also valid for neat LB
monolayers of PhPTCD on silver.
Surface-Enhanced Fluorescence. The plasmon ab-
sorption of the silver island Ž lm on glass is broad (Fig.
1
9
series of related materials. The Raman scattering con-
tains a small number of characteristic fundamentals that
can be attributed to the chromophore moiety. Wave-
numbers listed in Table I are observed by using the res-
onant Raman effect (514.5 nm excitation) or the spon-
taneous Raman scattering excited at 780 nm. Numbers
given in parentheses in Table I are the FWHM (full width
at half-maximum), obtained by using curve Ž tting in
Grams/32 with mixed Gaussian–Lorentzian band shapes,
and relative intensities are indicated by the Ž rst letter(s)
of very strong, strong, medium, weak, very weak, or a
shoulder. Both the 514.5 and 780 nm laser lines are in
resonance with the broad surface plasmon absorption of
the silver Ž lm shown in Fig. 2, and thereby, surface-en-
hanced resonance Raman scattering and surface-en-
hanced Raman scattering spectra are obtained for a single
LB monolayer on silver island Ž lms. The middle infrared
spectrum of PhPTCD dispersed in a KBr pellet was re-
corded and is shown for comparison with the SERS spec-
trum in Fig. 4. It can be seen that, despite the lack of a
center of symmetry of the entire molecule, the local sym-
metry of the chromophore makes the IR and the Raman
spectra fully complementary. The characteristic IR wave-
numbers are listed separately in Table II. For instance,
2
), indicating a wide range of sizes of metal particles.
The plasmon absorption is strongly affected by the LB
coating, as can be seen in Fig. 2. The optical properties
of the monolayer are also affected by the silver islands.
As can be seen in Fig. 3, the excimer emission of the
monolayer is enhanced by a factor of ; 400. The reso-
nance Raman scattering (RRS) signal is augmented by an
4
enhancement factor of at least 10 . The SERRS spectrum
in Fig. 3 was recorded by using 1% of the laser power
used in the RRS experiment. Since the surface-enhanced

uorescence (SEF) and SERRS of the monolayer are ob-
served in the same spectrum, the relative enhancement
factors are directly comparable. The difference in en-
hancement factor for the two phenomena is in full agree-
ment with the electromagnetic enhancement mechanism.
As a reminder, according to an early description by Weitz
1
5
et al., on average, the hierarchy of enhancement would
5
3
be as follows: RS: ; 10 ; RRS ; 10 ; SEF ; 10. Al-
though the actual orders of magnitudes are not exact, the
trend is valid. According to the physical model, the Ra-
2
2
man cross section in SERS is enhanced twice, L (v ) L
0
2
1
(
v ), where v and v are the Rayleigh and the inelasti-
the carbonyl stretching vibrations at 1660 and 1694 cm
are not seen in the Raman spectrum. Similarly, the out-
s
0
s
1
6,17
cally scattered frequencies respectively.
However, sig-
2
1
nal damping plays an important role in both the reso-
nance Raman process and  uorescence. In the latter case,
quenching for molecules directly adsorbed onto the metal
of-plane C–H wagging modes at 745 and 810 cm are
well deŽ ned in the IR spectrum, but they carry no relative
intensity in the Raman. In summary, the PhPTCD mol-
1122
Volume 54, Number 8, 2000

TABLE II. Characteristic fundamental vibrations observed in the
infrared spectrum of PhPTCD.
IR band
2
1
(
cm
)
Interpretation
4
48 w
40 w
97 m
45 m
10 m
52 w
75 vw
Ring deformation
Ring deformation
Ring deformation
Perylene C–H wagging
Perylene C–H wagging
Perylene C–H wagging
C–H bending
6
6
7
8
8
9
1
1
1
1
1
1
1
088 vw
180 vw
263 m
340 s
374 m
404 m
438 m
C–H bending
C–H bending
Alkyl C–H bending
C–N stretching
Perylene ring stretch
Perylene ring stretch
CH
Perylene ring stretching
O stretch
2
scissors
FIG. 4. FT-IR spectrum of the material dispersed KBr and the SERS
spectrum obtained with the 780 nm laser line.
1593 vs
1660 vs
1
2
2
2
3
C
5
694 vs
851 w
922 sh
955 m
025 w
C5O stretch
C–H Alkyl stretch
C–H
C–H
2
Alkyl stretch
Alkyl stretch
ecule follows the trend of the PTC series with respect to
characteristic vibrational wavenumbers.
3
Perylene C–H stretch
The SERS (at 780 nm) and SERRS (at 514.5 nm) spec-
tra of a single monomolecular layer of PhPTCD are
shown in Fig. 5. Although the intensity scale is the same
for both, the intensities are not directly comparable. Since
there is no observable Raman signal for a single mono-
layer on glass even with the use of longer accumulation
times and higher laser power, the relative enhancement
factor for SERS was not directly attainable. However, the
produced by ; 2 attomoles of the material. The global
image shown in Fig. 6 shows the peculiar topography of
the SERRS signal. Scanning electron microscopy indi-
cated that the average particle size in our work is less
than 100 nm in diameter. Therefore there are several sil-
ver particles participating in the SERS/SERRS enhance-
ment at each point of the surface mapping. Nevertheless,
there are ‘‘hot spots’’ in the silver island Ž lm, as has been
4
enhancement factor for SERRS (see Fig. 3) is at least 10
and, considering that the extinction coefŽ cient of Ph-
4
PTCD is ; 10 , the SERS enhancement factor at 780 nm
21
pointed out by the work of Nie and Emory, where a
8
could be about 10 . Notably, the FWHMs in the SERRS
small number of nanoparticles exhibited unusually high
enhancement efŽ ciencies. Therefore, it is evident that the
SERRS activity is not homogeneous and that, in the mi-
crometer scale of the present experiments, the presence
of hot spots seems to be characteristic of the silver island
spectrum are much wider than the corresponding values
found in SERS.
SERS and SERRS Imaging. Two distinct Raman im-
aging experiments were carried out with Raman micros-
copy; the results are illustrated in Fig. 6. Area mapping
of the SERS and SERRS signal for a single LB mono-
layer of PhPTCD on silver island Ž lms was obtained with
the 780 and 514.5 nm laser lines, respectively. The global
image from the same sample was captured for the sur-
face-enhanced resonance Raman scattered light of the
Ž
lms prepared by vacuum evaporation.
The point-by-point area mapping of a 40 3 40 mm area
recorded with a 3 mm step for the monolayer sample on
silver using the 780 nm laser line (SERS) is presented in
Fig. 6 (top right). The bright coloration represents higher
intensity of the SERS signal for the peak area of the
2
1
fundamental vibrational wavenumber at 1378 cm . From
both the SERS and SERRS spectra described above, it
can be concluded that there is no spectral evidence of
chemical interaction of the PhPTCD molecule with the
silver surface.20 Therefore, the SERRS and SERS signals
are mainly due to the electromagnetic enhancement pro-
vided by the metal roughness. The SERRS and SERS
spectra are seen on the background of a weak  uores-
cence. Consequently, for the area mapping we have se-
lected a baseline-corrected intensity for the most intense
21
baseline-corrected peak at 1381 cm . The line scanning
2
1
21
peak at 1378 cm in SERRS and 1381 cm in SERS.
It should be pointed out that the Langmuir–Blodgett tech-
nique provides one of the best ways of fabricating ho-
mogeneous coatings of rough metal surfaces. The transfer
ratio provides a control to monitor the quality of the coat-
ing with a well-characterized molecular monolayer. For
analytical applications, it should be recognized that, as-
Ê
2
suming an area of 65 A per molecule (from the p-iso-
2
therm) and 1 mm for the spot probed by the laser in the
FIG. 5. SERS of the PhPTCD monolayer obtained with the 780 nm
laser line is compared with the SERRS spectrum excited at 514.5 nm.
sample, the SERRS signal and the SERS intensities are
APPLIED SPECTROSCOPY
1123

FIG. 6. Line and area mapping of a 40 3 40 mm surface of the silver islands coated with one LB monolayer of PhPTCD and using the use of
2
1
the 780 nm laser line. Bright coloration correlates with higher signal intensity. The global image of the PhPTCD monolayer recorded at 1378 cm
is shown (bottom right corner) as a three-dimensional plot of the collected data. The projected map of the global image is also given (bottom left
corner).
for a section of the SERS spectrum using the same spatial
resolution of mapping is also shown in Fig. 6 (left cor-
ner). The most important aspect of the SERS results com-
pared with the SERRS (at 514.5 nm) results is the fact
that the SERS signal presents a more homogeneous dis-
tribution. Further studies are underway with molecules of
the same series and other macrocycles where both SERS
and SERRS are observable, which should help to shed
light on SERS/SERRS properties.
formation of aligned aggregates, while the emission spec-
tra in LB and solids are due to excimers. Comparison of
mapping and global images of the SERS and SERRS
signals permits detection of ‘‘hot spots’’ in the spatial
distribution of the SERRS intensities; i.e., stronger than
average SERRS intensities are spread randomly within
the 40 3 40 mm square. The mapping of SERS spectra
(at 780 nm) shows more homogeneous values of vibra-
tional intensities. In summary, a new dye (PhPTCD) has
been synthesized and characterized spectroscopically. It
has been shown that Langmuir and LB Ž lm of PhPTCD
can be fabricated. Finally, the LB Ž lms of the new ma-
terial were used to illustrate the high sensitivity and to-
pographic applications of SERS/SERRS imaging.
CONCLUSION
The synthesis of electronic and vibrational character-
ization of a novel bis(phenethylimido)perylene material
has been carried out. Floating Langmuir monomolecular
layers have been characterized by their surface-pressure
area isotherms. The rigid character of the layer is con-
ACKNOW LEDGMENTS
Ž
rmed by the hysterisis observed in the compression–
Financial assistance from the Natural Science and Engineering
Research Council of Canada is gratefully acknowledged. C. J. L.
Constantino acknowledges a fellowship from the Funda c¸ a˜ o de Am-
paro a` Pesquisa do Estado de S a˜ o Paulo (FAPESP), Brazil.
decompression experiments. Langmuir–Blodgett molec-
ular monolayers were fabricated onto silver island sub-
strates to be used in surface-enhanced spectroscopic stud-
ies. SERS, SERRS, and surface-enhanced  uorescence of
the LB monolayer on silver were measured. The EF for
SEF was found to be about 400, while the RRS signal is
4
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1124
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