Atomic force microscopy and surface

Transcription

Atomic force microscopy and surface-enhanced
Raman spectroscopy.
I. Ag island films and Ag film over polymer nanosphere surfaces
supported on glass
R. P. Van Duyne,=) J. C. Hulteen, and D. A. Treichelb)
Department of Chemistry and Materials Research Center, Northwestern University, Evanston,
Illinois 60208
(Received 10 February 1993; accepted 8 April
1993)
The surface roughness and nanometer scale structure of Ag films used for surface-enhanced
Raman scattering (SERS) are characterized using atomic force microscopy (AFM). Two
important types of thin film based SERS-active surface have been examined in this study: ( 1) Ag
island films (AgIF’s) on smooth, insulating substrates and (2) thick Ag films evaporated over
both preroughened and smooth substrates. AFM is demonstrated to be capable of quantitatively
defining the three-dimensional (3D) structure of these roughened surfaces. The effects of mass
thickness, d, , and thermal annealing on the nanostructure of AgIF’s are studied in detail.
Particle size histograms are calculated from the AFM images for both “as-deposited” and
annealed IF’s with d,,,= 1.8 and 3.5 nm. Quantitative measurements of the SERS enhancement
factor (EF) are coupled with the AFM data and interpreted within the framework of the
electromagnetic theory of SERS. AFM images for thick evaporated Ag films over a monolayer
of polymer nanospheres (AgFON) shows the clear presence of “random substructure roughness” reducing their utility as controlled roughness surfaces. Similar roughness structures are
observed for thick evaporated Ag films on smooth, insulating substrates. Nevertheless, AgFON
surfaces are demonstrated to be among the most strongly enhancing thin film based surfaces ever
studied with EF’s comparable to those found for electrochemically roughened surfaces. Applications of FON surfaces to ultrahigh sensitivity SERS, anti-Stokes detected SERS, and surfaceenhanced hyper-Raman spectroscopy (SEHRS) are reported.
I. INTRODUCTION
Surface-enhanced Raman spectroscopy (SERS) is a
widely used technique for obtaining vibrational spectra at
some roughened metal surfaces. Several recent review articleslg provide a comprehensive overview of the more
than 2000 papers on SERS dealing with fundamental
mechanistic studies of photon/molecule/surface
interactions as well as applications in electrochemistry, surface
science, materials science, and biochemistry. Recently, fundamental work has been far outpaced by applications development in spite of the circumstance that, in the opinion
of the authors, a full, quantitative, and predictive understanding of all features of the SERS enhancement mechanism(s) has yet to emerge.
Currently, it is believed that two simultaneously operative mechanisms, a long range classical electromagnetic
(EM) effect and a short range “chemical” (CHEM) effect
are responsible for SERS. The partitioning of the total enhancement between these two mechanisms is still a matter
of debate but at least for Ag surfaces excited with visible
laser excitation wavelengths, it appears that the EM mechanism contributes about lo4 with the CHEM mechanism
contributing an additional factor of about 10’. Schatz6 has
reviewed the current state of the EM mechanism while
Ottotp7 has reviewed the recent progress in conceptualizing
. . ..
.._. ‘)Author to whom correspondence should be addressed.
b)Present address: Department of Chemistry, University of Texas at Aus-
tin, Austin, Texas.
the CHEM mechanism as a metal electron-mediated resonance Raman effect that involves increased electronphoton coupling at atomically rough surfaces and transient
charge transfer into affinity levels of the adsorbate.
As a spectroscopic probe of surface chemistry and dynamics, SERS has its strengths.‘” (i) The ability to determine the, identity, bonding, orientation, and monitor the
dynamics of adsorbed molecules. (ii) Extraordinary sensitivity and selectivity to the inter-facial species. (iii) Adsorbate generality. (iv) Operation at solid/UHV, solid/gas,
solid/liquid, and even solid/solid interfaces. (v) Operation
at metal surfaces in addition to Ag, Cu, and Au: Li,8*g
~~
pt
10-14 K
;2-24
,,&
12,14,15 cs
14 R,,
14 4
11,16-18 ~~
11,19 1n 9,11,20,21
a.25
(6) The scope of SERS has been extended to include other surface-enhanced spectroscopies
( SES ) such as the nonlinear processes of surface-enhanced
second harmonic generation22 (SESHG) and surfaceenhanced hyper-Raman scattering (SEHRS ) .26
SERS’also has its weaknesses.‘” (i) Surface roughness
on a nanometer length scale is required thereby excluding
the smooth, low index, single crystal surlaces commonly
used in UHV surface science. (ii) The magnitude of the
surface enhancement factor (EF) and, hence, the sensitivity of SERS is a strong function of the chemical and optical
properties of the metal surface as well as its detailed surface- morphology. (iii) It is not clear whether semiconductor surfaces are intrinsically SERS active or not. (iv) Insulator surfaces are not intrinsically SERS active. (v) The
stability and reproducibility of most SERS-active surfaces
0021-9606/93/99(3)/2101
/15/$6.00
@ 1993 American Institute of Physics
2101
J. Chem. Phys. 99 (3), 1 August 1993
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Van Duyne, Hulteen, and Treichel: Atomic force microscopy.
are far from ideal. (vi) A key weakness in the comparison
between SES experiment and EM theory is the mismatch
between the actual surface nanostructures used in experiments and the model surface nanostructures used for theory.6
One of our longstanding goals has been to ascertain
whether these weaknesses are fundamental impediments to
extending the scope of SERS or whether, through an improved understanding of the SERS enhancement mechanism( s) and/or appropriate experimental design, they can
be overcome. Those problems which derive from the current lack of information concerning the detailed threedimensional (3D) structure of SERS-active surfaces and
its reproducible control will be specifically addressed in
this paper.
Because SERS-active surfaces possess roughness on
several characteristic length scales, x-ray diffraction and
low-energy electron diffraction (LEED) methods are not
generally applicable to surface structure determination. As
a consequence, it has become customary in SERS studies to
characterize surface structure (i.e., surface roughness) using scanning electron microscopy (SEM) . Electrochemitally roughened surfaces,27”4 island films,35,36 thick metal
films over CaF, roughened surfaces,37 thick metal films
over nanosphere (FON) surfaces,38-45and lithographically
fabricated 2D metal particle arrays’6P46947
have all been examined using SEM. While much useful information can be
acquired from qualitative assessments of SEM micrographs
and the measurement of particle size distributions as a
function of surface preparation or SERS operating conditions, there are certain inherent limitations to SEM when it
is applied to the SERS problem. For example, all of the
studies cited above involve removal of the surface from the
SERS operating environment and the placement of it in the
vacuum chamber of the SEM instrument. Thus one must
assume that there are no or minimal structural rearrangements that occur during this environment transfer. It
would be far more desirable to carry out the microscopy
under identical conditions to the spectroscopy. SEM requires electrically conducting samples. Thus SEM studies
of metal island films, which are usually deposited on insulating substrates, requires overcoating the sample with thin
continuous conducting films (e.g., Al). A more desirable
situation would be to perform the microscopy on the “asdeposited” SERS-active surfaces. SEM has a typical resolution of 5-10 nm which is insufficient to resolve the complete distribution of roughness features in SERS which
range from atomic scale roughness (ASR) to large scale
roughness (LSR, particles with dimensions of 10 nm to
3 pm>.
Sufficient spatial resolution not only to resolve the
ASR and LSR of SERS-active particles, but to determine
highly detailed structural information about metal particles, can be obtained by using transmission electron microscopy (TEM) or high resolution electron microscopy
(HREM) . In the context of SERS, TEM has been applied
to metal colloids,48-53 island films,54955 and photographically derived Ag surfaces.2 HREM has been used to examine the internal structure of Ag and Au colloids although
I
not specifically in the context of SERS.56,57 Here- again
there are inherent problems. Both TEM and HREM suffer
from the environmental transfer problem cited above for
SEM. In addition, these techniques require specimens that
are semitransparent to electrons. Therefore, specimen
thicknesses must be < 100 rim or the deposition of thin
islands of SERS-active material must be on low atomic
number substrates. It would be more desirable if such high
resolution information could be obtained without the necessity of sample thinning or special substrates.
Since the invention of the scanning tunneling microscope5’ (STM) , it has been clear that its atomic resolution
capabilities and environmental compatibility with solid/
vacuum, solid/liquid, and solid/gas interfaces5g-63 offered
attractive possibilities for the study of surface enhancement
phenomenon. Recently several research groups have used
the STM to examine the morphology of silver surfaces: ( 1)
50 nm thick vapor deposited Ag films in air,64 (2) cold
deposited and warm deposited Ag films in UHV,65-67 (3)
Ag island films deposited on electrically conducting
indium-tin-oxide
(ITO) glass in UHV,68 and (4) electrochemically roughened surfaces.6g-73 Although the relevance of the STM results to surface-enhanced optical processes was discussed in these papers, no SERS data were
included. Several of these papers, however, have presented
surface-enhanced second harmonic generation (SESHG)
data.67P68P72,73
Byahut and Furtak74 have demonstrated
strong SERS from electrochemically roughened and STManalyzed Ag films; however, the SERS spectra were published without the concurrent STM images. To date, Ferrell and co-workers75’76 are the only researchers to publish
both SERS spectra and STM data from the same surfaces.
Although no STM images were reported, STM-derived
surface roughness spectral power density functions are
given. In their work, 45 nm thick Ag films were deposited
over glass and over glass preroughened with CaF, films.
However, they found no measurable enhancements from
the Ag films on glass, and only weak enhancements could
be obtained from the CaF, roughened surfaces when using
benzoic acid as the adsorbate. The main limitation to STM
for the study of SERS-active surfaces is that like the SEM,
it requires that the specimen be electrically conductive.
Thus, the study of several popular types of SERS-active
surface such as island films on insulator substrates and
isolated particle arrays on insulating substrates are precluded from examination.
In this paper we report the first application of atomic
force microscopy5g~77-80(AFM) to examine the 3D structure of a SERS-active surface. AgIF’s deposited on smooth
glass substrates were chosen as the primary target surface
for this combined AFM and SERS study because: their
oblate structure is well suited to accurate topographic imaging by scanning probe microscopy, they are relatively
stable and easy to fabricate, and they have played an important role in the development of SERS particularly in
elucidating such properties of the EM enhancement mechanism as the long-range distance dependence, coverage dependence, and correlation with optical properties. AFM
structural data on AgIF’s is combined with measurements
J. Chem. Phys., Vol. 99, No. 3, 1 August 1993
of their SERS enhancement factor (EF) and optical extinction spectra. These results are compared with published conclusions from the EM theory of SERS. In addition to the island film experiments,
two other
nanostructured surfaces have been imaged with the AFM:
a thick (viz., about d,=250
nm) Ag film deposited over a
monolayer of polymer nanospheres supported on a glass
substrate and, for comparison, an identical thick Ag film
deposited on a smooth glass substrate. The purpose of this
study was to test the hypothesis that the monolayer of
polymer nanospheres would provide an easy to fabricate,
reproducible, and variable sized, hemispherical template
for the formation of a SERS-active metal over-layer whose
EM enhancement properties could be selectively “tuned”
by changing the diameter of the nanospheres.38A5
II. EXPERIMENTAL
TECHNIQUES
The fabrication of the roughened Ag films that are the
subject of this research was carried out in a modified Consolidated Vacuum Corporation vapor deposition system.
Pressures in the diffusion pumped chamber were < 10m6
Torr during the deposition. The deposition source consisted of a tungsten boat filled with the material to be deposited. Prior to deposition the substrates were cleaned by
washing in 1 M NaOH followed by successive sonications
in deionized water, methanol, and acetone. The temperature of the substrate, T,, was not independently controlled
in this apparatus but was continuously measured during
the deposition by a thermocouple attached to the substrate.
The mass thickness of Ag films was measured with a
homemade quartz crystal microbalance that was calibrated
by two independent methods: cyclic voltammetry and
STM. The cyclic voltammetric
approach followed the
work of Fornarisl STM calibration of mass thickness was
performed by masking half of a silver substrate and depositing silver over the other half. The STM tip was repetitively scanned over the boundary and the average height
calculated.
Monodisperse colloidal suspensions of polystyrene
spheres were purchased from three vendors: Interfacial
Dynamics Corporation (Portland, OR), Bangs Laboratories (Carmel, IN), and Duke Scientific (Palo Alto, CA).
The variation in sphere diameter was less than 4% for all
polystyrene spheres used. Spheres were diluted by a factor
of 2 to 5 in methanol and spin coated onto glass substrates.
In all experiments, sphere surface coverage was roughly 1
monolayer.
The 683 cm-’ line of cobalt pthalocyanine (CoPc)
was used as a SERS intensity standard for calculating the
enhancement factors for the surfaces used in this study.
The standard was fabricated by depositing 0.1 monolayer
of CoPc on a thick Ag film over polymer nanosphere surface (see Sec. III B and the upper panel inset in Fig. IO).
CoPc was chosen as the SERS intensity standard because
of previous experience,82*83 ease of fabrication, specimen
longevity, and its enormous intensity. Depositions of CoPc
were carried out similarly to the method used by Carron.*2183CoPc was sublimed simultaneously onto the SERS
active substrate and six glass slides of known area. The
2103
I
CoPc was subsequently dissolved off each slide with pyridine and diluted to 5 ml and its concentration was measured by UV-visible absorption spectroscopy using a Beckman DU-7 spectrophotometer. Coverages were calculated
using E= 1.17~ lo5 M-’ cm-’ and assuming 4x 1013molecules per monolayer coverage.82,83 Preresonance Raman
spectra of CoPc were acquired from a thick film sublimed
onto a sapphire window. tram- 1,Zbis (4-pyridyl) -ethylene
(BPE) was chosen as an adsorbate because it adsorbs
strongly to Ag in air and it is not resonantly enhanced in
the visible. The BPE was spin coated onto the prepared
substrates from a 2~ 10v4 M solution in methanol:
A Spectra Physics model 171 Arf laser was used both
for Raman excitation, and to pump the tunable lasers used
in determining excitation profiles. Two coherent dye lasers;
a model 490 and model 590, were combined with three
dyes; rhodamine 560, rhodamine 590, and DCM, to provide wavelengths between 535 and 700 nm. A Spectra
Physics model 3900 Ti:A1203 laser provided excitation
wavelengths greater than 700 nm. A Spectra Physics series
3000 cw mode-locked Nd:YAG laser operating at 1064 nm
and its second harmonic at 532 nm were used, respectively,
for the experiment comparing SEHRS and SERS on thick
film over polymer nanosphere surfaces. SER and SEHR
spectra were obtained on two detection systems: a SPEX1400-11 double grating monochromator equipped with a
cooled RCA C31034A-02 photomultiplier
tube, and an
ACTON VM-505 single grating monochromator equipped
with a Photometrics PM-5 12 charge couple device (CCD)
camera.
AFM images were all taken in air using a Digital Instruments Nanoscope II. Commercial Si3N4 cantilever tips
(Digital Instruments) with spring constants of approximately 0.12 nm-’ were used. These tips are pyramidal in
shape with a cone angle of 70” and an effective radius of
curvature at the tip R,=20-50
nm. The images reported
here are raw, unfiltered data collected in the constant force
mode (i.e., the tip-sample separation is adjusted to maintain a constant force equal to 15-25 nN) with a scan speed
of 8 lines s-l. All surfaces were imaged with at least two
different tips to minimize the possibility of tip induced false
images. Two scan heads, with ranges of 12 ,um by 12 pm
for large area scans and 640 nm by 640 nm for smaller
areas, were routinely used.
III. RESULTS AND DISCUSSION
A. Island films on smooth
glass substrates
“As-deposited” Ag island films were formed on glass
substrates with mass thicknesses, d,,,, of 1.8, 3.5, 8, and 16
nm using a fixed deposition rate, rd, of 0.3 nm s-l. Under
these conditions, radiative heating of the substrate was
minimal so that T, was approximately constant at about
300 K. “Annealed” Ag island films were prepared by a
post-deposition heat treatment of the same specimens at
< 10B6 torr vacuum at T,=600 K for 60 min.
Although all the island film surfaces reported in this
paper were prepared with a fixed Ag rd=O.3 nm s-l, a
systematicexamination of the effect of depositionrate on
2104
a.
I
e.
8 nm
15 nm
0 nm
0 nm
100 nm
100 nm
d.
b.
10 nm
12 nm
0 nm
0 nm
100 nm
FIG. 1. AFM images of “as-deposited” Ag island films on glass slides. Island films were prepared at room temperature. rd=0.3 nm s-‘; d,,,= (a) 1.8
nm, (b) 3.5 nm, (c) 8.0 nm, and (d) 16.0 nm.
island film structure was carried out varying the rate from
0.05 <rd<2.0 nm s-l. This study was carried out because,
in previous work by Cotton,54 a strong effect of deposition
rate on film structure as determined by TEM, optical properties of the film determined by electronic extinction spectra, and SERS activity for both electronically resonant and
nonresonant adsorbates was found. They prepared Ag island films of constant d,=5.0
nm on glass substrates at
deposition rates from 0.003 < rd < 0.5 nm s- ‘. Films deposited with slow rates near 0.003 nm s-l were found to have
larger, more well-separated particles and result in 5 or 6
times greater SERS activity than films deposited at high
rates near 0.5 nm s-l. The explanation offered was that at
low deposition rates metal atoms migrate independently to
their final surface sites. At higher deposition rates, additional metal atoms arrive within a capture area on the
surface and influence the aggregate rate of migration to
final surface sites. Thus the structure of the film would be
a function of arrival rate at the substrate surface.
In our work, no such effect of rd on film structure as
determined by atomic force microscopy, or optical properties, or SERS activity was found. Why there should be such
disparity between our results and those of Cotton is not at
all clear at this time. Two possible sources for this effect are
( 1) differences in T, caused by differences in radiative
heating of the specimen in low vs high rd experiments and
(2) differences in substrate surface energy caused by different pretreatments. Cotton et al. assumed that the distance between their sample and evaporation source (12
cm) was sufficient that T, was not influenced by the different radiative heat loads on the sample at different deposition rates and consequently, they did not measure T,. If,
in fact, T, was a fUnCtiOn of rd, the Complex interplay
between the rates of metal atom arrival, surface diffusion,
nucleation of islands, growth of islands, and coalescence of
islands would have to be modeled in detail to understand
this phenomenon. Additional complexity enters this problem because the surface energies of the substrates used in
our experiments are likely to be different. from those in
Cotton’s work as a result of the different surface pretreatment’s used. Consequently, the rates of surface diffusion,
island growth, island coalescence, etc., would be modulated by differences in substrate surface energy. Clearly,
further study is required to unravel the details of these two
different observations concerning the effect of rd on island
film structure and SERS EF values.
1. Atomic
force microscopy
A set of AFM images with (400 nm) 2 field of view is
shown in Fig. 1 for “as-deposited” Ag island films of various d, on glass substrates. These images provide an overview of “as-deposited” island film morphology as a function of mass thickness. For purposes of discussing the
geometry of the particles in these AFM images, as well as
making contact with the EM theory of SERS based on
spheroids,6984-g5these films will be considered as a random
array of individual, oblate spheroidal particles. Each particle is, therefore, characterized by two size metrics: a is
the major axis or particle diameter and b is the minor axis
or particle height. Particle shape is referenced by the
minor-to-major
axis ratio, R = b/a. The random oblate
spheroidal array approximation is clearly well-suited to the
films shown in Figs. l(a) and l(b) with d,=1.8 nm and
3.5 nm since they are composed of regularly shaped, individual particles. The z-contrast scale provides a qualitative
a.
I
2105
c.
10 urn
20 nm
0 nm
0 nm
100 nm
100 nm
d.
b.
$8 nm
3 nm
0 nm
0 nm
100 nm
FIG. 2. AFM imagesof Ag island films annealedat 600 K for
d,,,=(a) 1.8 nm, (b) 3.5 nm, (c) 8.0 nm, and (d) 16.0 nm.
60 min under vacuum.Depositionsperformedat room temperature.r,=O.3 nm s-’ and
indication of the particle heights for these films which
range up to 8 and 10 nm, respectively. When the mass
thickness is increased to 8 nm [Fig. 1 (c)l, the film is still
composed of individual particles but there is now great
irregularity to the particle shape and the distribution of
particle sizes and shapes has increased. The z-contrast scale
shows that particle height now ranges upward of 15 nm.
This is presumably due to the onset of particle-particle
coalescence. At d,n = 16 nm [Fig. 1 (d)], the thickest films
imaged in this series, the Ag particles appear more spherical but now strongly overlap each other. Particle heights,
which really correspond to film thickness now that the
particles overlap, are somewhat lower than those in the 8
nm film with a range up to 12 nm. Particle-particle overlap
and/or coalescence pigs. 1 (c) and 1 (d)] reduces the ease
of particle geometry interpretation in terms of a random
array of spheroids.
A corresponding set of (400 nm)2 AFM images that
reveals the overall behavior of Ag island films subjected to
post-deposition, vacuum annealing to T,=600 K is shown
in Fig. 2. These images show that annealed 1.8 nm [Fig.
2(a)] and 3.5 nm [Fig. 2(b)] island films are still composed of individual spheroidal particles but that they have
grown in diameter and height compared to those in the
corresponding “as-deposited” film. The 8 nm [Fig. 2(c)]
annealed film still shows the irregular particle shapes produced by coalescence but they are larger and higher than
those of the “as-deposited” film. The particles composing
the 16 nm annealed [Fig. 2(d)] film, in contrast to those of
the 8 nm annealed film, are reduced in diameter with particle height or film thickness now much smaller at approximately 3 nm after annealing.
In order to quantitate the effect of mass thickness and
thermal annealing on particle geometry, a series of higher
resolution AFM images for these same films in both their
“as-deposited” and annealed states were acquired. Figures
3 and 4 show AFM images with ( 100 nm)2 and (200 nm)2
fields of view, respectively, for 1.8 nm and 3.5 nm films
along with selected AFM line scans which provide an independent measure of the particle height distribution.
Since these surfaces are composed of isolated, regularly
shaped particles, it was possible to obtain histograms of the
particle diameter distribution which are shown in Fig. 5.
The insets to Fig. 5 schematically illustrate the particle
geometry. Particle diameters, a, are given as the mean for
40-60 particles in the field of view. Particle heights, b, are
derived from the peaks in AFM line scan data and are
reported as average values from at least five randomly se-
b.
10nm
5 nm
ONn
c-
100 run ------+
OllUl
+-----
1oonm----+
FIG. 3. AFM imagesand linescansof Ag island films on glassslides.
Depositionswere performedat room temperature.rd=0.3 nm s-’ and
d,= 1.8 nm (a) unannealedand (b) annealed at 600 K for 60 min.
2106
TABLE I. Hemispheroidal
land films.
b.
a.
Film
thickness
(nm)
1.8
3.5
8.0
16.0
FIG. 4. AFM images and linescans of Ag island films on glass slides.
Depositions performed at room temperature. r,=O.3 nm s-’ and d,=3.5
nm (a) unannealed and (b) annealed at 600 K for 60 min.
lected lines in the field of view. These values and their
standard deviations are also given in Table I. Figures 6 and
7 present higher resolution AFM images and line scans for
the 8 and 16 nm mass thickness films. Particle diameter
histograms were not computed for these surfaces due to the
small number, irregular shape, and/or overlapping of the
particles. Estimates of a were obtained by measuring the
diameter of a circle drawn to circumscribe the particle on
at least ten particles. Particle height was again taken directly from the AFM line scan maxima. Table I summarizes all the particle diameter, height, and shape, data obtained for these films. Several trends in particle structure
are clearly revealed in these studies as a function of increasing d, . In “as-deposited” films the average particle diam-
,
15.0-
.
-
-
. b=5,&
lO.O-
5.00.0
--I
s= 1010nnl.
R 7 0.50
_I’. I-:
r
10.0
a h-n)
I
particle size and shape properties for Ag is-
As-deposited
b (nm)
10.0* 1.3
14.6A2.1
33*4
100* 12
5
8
12
10
Annealed
b (nm)
R
a (nm)
0.50
0.55
0.36
0.10
13.3*2.0
20.8=‘=3.0
63*7
40*5
10
16
18
3
R
0.15
0.77
0.29
0.08
eter increases; distribution of particle diameters is quite
narrow and nearly Gaussian (d,= 1.8 and 3.5 nm films);
particle height increases more rapidly than the diameter
(the 16 nm film is an exception probably due to the effects
of particle-particle
overlap); and particle shape becomes
increasingly oblate with the minor-to-major axis ratio systematically varying by a factor of 5. Thermal annealing to
600 K for 60 min produces the following changes in particle structure: particle diameter increases by about 35%,
45%, and 90% for the 1.8, 3.5, and 8 nm films, respectively, but decreases by a factor of 2.5 for the 16 nm film;
particle diameter distributions are significantly broadened;
particle height doubles for the 1.8 and 3.5 nm films, increases by only 50% for the 8 nm film, and actually decreases for the 16 nm film; oblate particle shape is retained
with particles in the 1.8 and 3.5 nm films becoming significantly more prolate.
This reexamination of the structure of Ag island films
from the new vantage point afforded by atomic force microscopy shows that the AFM is a powerful tool for nonconducting specimens with lateral resolution superior to
that for SEM and comparable to that for TEM. In addition
the clear advantage of the AFM- is its capability of independently measuring both particle diameter and particle
height. Thus, we are able to easily extract the particle
shape parameter, R, that is so important in determining the
optical properties of these surfaces and, in particular, their
SERS activity.
20.0
Particle Diameter (nm.)
FIG. 5. Histograms of size distributions from AFM micrographs for
d,,,= 1.8 and 3.5 nm Ag island films. Schematic illustrations of these
surfaces are shown in the insets.
Depositions performed at room temperature. r,=O.3 nm s-’ and d,,,=8.0
nm (a) unannealed and (b) annealed at 600 K for 60 min.
1.0
+200nm
+
-
I
2107
. .. . .
I a.
.g 0.6 z
.g
$J 0.4 -
200 nm +
b.
15.0
Depositions were performed at room temperature. i-,=0.3 nm SC’ and
dm= 16 nm (a) unannealed and (b) annealed at 600 K for 60 min.
“z
10.0
z
w
5.0
0.0
2. Extinction
spectra
The optical properties of submicron structures composed of metal particles have been extensively studied.35*36*87-g5
It is well known, for example, that the optical
absorbance of Ag island films composed of oblate spheroidal particles change dramatically as a function of 1 nm
< dttl < 16 nm.35p36*96Normal incidence, unpolarized extinction spectra of the “as-deposited” Ag island films studied above by AFM are shown in Fig. 8 (a). The 1.8 nm
thick film shows a maximum extinction at 510 nm. Since
the particles in this film are oblate with R=0.50,
this extinction maximum can be assigned to surface-plasmon excitation along the major axis. As d, increases to 3.5, 8.0,
I.
1.0
0.8 5
0.6 -
8.0 nm.
‘42
“e
*R 04’
w
0.2 :
a&) R
33 0.36 ,.
b.
d,=
40.0- -is- 8Onm
+ 160”rn
“2x
t 3.5l-m
ii
-o- 1e.m
E-4
20.0--A==
1000.10
o.ojLc*::
400
I
500
I
600
,::/
I
700
800
Wavelength (nm.)
FIG. 8. (a) Unpolarized, normal incidence extinction spectra of “asdeposited” Ag island films with d,,,= 1.8, 3.5, 8.0, and 16.0 nm; rd=0.3
rims -‘. (b) SERS EF’s for the same films at laser excitation wavelengths
488.0, 5 14.5, 641.3, and 722.0 nm. The values of d,,, , CI, and R are listed
from top to bottom in order of decreasing EF at /1,x=722 nm.
c
+II+
I=
e.onm.
3snm.
I
:,“,“I.1
~%~s
1
20.8 0.77
=* ;;s x:7;
i
400
I
500
I
600
I
700
800
Wavelength (nm.)
FIG. 9. (a) Unpolarized, normal incidence extinction spectra of Ag island films annealed at 600 K for 60 min with d,,,= 1.8, 3.5, 8.0, and 16.0
nm; rd=0.3 nm s-‘. (b) SERS EF’s for the same films at laser excitation
wavelengths 488.0, 514.5, 641.3, and 722.0 nm. The values of d,,, , a, and
R are listed from top to bottom in order of decreasing EF at A,=722 nm.
,
a.
-I
.,.
and 16 nm, the surface-plasmon resonance energy red
shifts and its width broadens significantly. These results,
when combined with our AFM structural data (Table I)
which showed a systematic decrease in R with increasing
d,, are consistant $th EM theory (see, for example,.Fig.
~~
i3 in Ref. 67).
The corresponding set of normal incidence, unpolarized extinction spectra for Ag island films after postdeposition annealing at T,=600 K for 60 min is shown in
Fig. 9(a). Three features are observed: ( 1) the extinction
maxima for the annealed samples are blue-shifted and reduced in intensity With respect to those for the “asdeposited” samples, (2) the surface-plasmon resonance
widths are -_..
decreased, a@ (3). there is a general loss of
light absorption in the red for all samples. Blue-shifted
extinction or absorption maxima have been previously reported for thermally annealed Ag island films on rock
salt9’ and on g1ass98substrates. McCarthy measured Ag
particle shape and height using the Pt shadow casting technique with TEM observation. He interpreted the blue shift
to be a consequence of thermally induced Ag particle
breakup and regrowth at new nucleation sites resulting in
Ag particles with increased height. Aussenegg did not
characterize annealed AgIF’s using microscopic methods
and did not offer a structural interpretation of the blue
shift. Red-shifted extinction or absorption maxima have
also been observed for thermally annealed Ag island
films.36J87The red shifts were interpreted as being consistent with EM theory based on SEM and TEM micrographs
showing the particles in the annealed film to be larger in
J. Chem. Phys., Vol. 99, No. 3, I August 1993
2108
diameter than those in “as-deposited” films. In order for
this to be the case for oblate spheroidal particles, the particle shape factor, R, would have had to be approximately
constant before and after annealing. No independent measurements of the particle height were made to vary the
constancy of R.
Our results for low d,, thermally annealed Ag island
films on glass are similar to those of McCarthy on rock
salt. The AFM shows that annealed AgIF’s are composed
of larger but significantly less oblate particles than “asdeposited” films. This change in the minor-to-major axis
ratio of the Ag particles accounts for the blue shift in the
extinction spectra according to EM theory.
Why the annealing of Ag island films in some experiments should be dominated by increases in particle height,
whereas, the annealing of similar films should be dominated by increased particle diameter is not clear. One possible explanation centers on substrate energies. Different
substrates and/or pretreatments for the same substrate are
likely to result in different substrate surface energies and
consequently contact angles between the metal overlayer
and the substrate. If, for example, the Ag islands in the
Royer experiments were to “wet” the substrate more effectively at higher temperatures, then they will grow in diameter and decrease in height producing the red shift as they
are annealed. If, on the other hand, the Ag islands in our
experiments or those of McCarthy and Aussenegg were to
“wet” the substrate less effectively at the annealing temperature, the opposite would be expected to occur.
3. SERS enhancement
factor
In this section we examine the dependence of the surface enhancement factor for AgIF’s on film thickness and
laser excitation wavelength in light of the nanostructural
information provided by the AFM. Correlations with optical properties characterized by extinction spectra and
with the results of the EM theory of SERS are also made.
trans- 1,2-bis (4-pyridyl) -ethylene (BPE) , rather than the
more familiar pyridine system, was chosen as the adsorbate
molecule for these island film EF measurements because it
adsorbs to Ag more strongly than pyridine; adsorbs irreversibly on Ag as compared with the reversible adsorption
of pyridine; has a larger spontaneous Raman cross section
than pyridine; and is a nonresonant adsorbatelike pyridine
when excited at excitation wavelengths in the visible and
near infrared. A representative SER spectrum of BPE adsorbed onto a 8.0 nm thick Ag on glass island film is shown
in Fig. 10.
The EF is usually defined and directly measured as the
ratio of the adsorbate’s SERS intensity to its bulk, normal
Raman intensity, where the intensities are in units of
counts s -’ W-i mol-’ to normalize for the effects of data
aquisition time, laser power, and surface vs volume number
density, as shown for 1640 cm-’ symmetric C==C! stretching mode of BPE in Eq. ( 1) . An alternative approach was
adopted here because of the thin film nature of our samples:
I
500
1000
1500
Wavenumbers (cm-‘)
FIG. 10. SER spectrum of 1.0 monolayer of BPE spin coated on a Ag
island film, d,,,=8 nm. 10 mW of A,,=722 nm.
G$m
EFBPE,
1640 cm-l(~>=
g#-)
(1)
*
The EF of BPE adsorbed on AgIF’s was obtained by first
measuring the ratio of its SERS intensity to that of a thin
film SERS intensity standard based on the 683 cm-’ line of
cobalt phthalocyanine (CoPc) .82p83Combining this ratio
with the enhancement factor for the CoPc intensity standard excited at 753 nm and the ratio of the bulk, normal
Raman intensities for BPE and CoPc we get the desired EF
from the following identity:
1&9A.=753
Gi&(N
EFBPE(A)=
Is~$,(A=753
I”z$(,,(n=753
(
nm) )(
nm)
1
&$#>
nm)
(2)
nm) 1 ’
The EF for the 683 cm-’ CoPc standard is calculated to be
1 X 10’ from the spectra shown in Fig. 11. The inset in Fig.
11 (b) shows the electronic absorption spectrum for an 80
monolayer thick film of CoPc on glass and reveals that the
unenhanced Raman reference spectrum is actually an electronically preresonance Raman spectrum.
The EF’s for BPE on the “as-deposited” AgIF’s as a
function of /2,, and d, are listed in Table II as the mean
and standard deviation for 6 replicate surfaces at each film
thickness. The wavelength dependencies of the EF, which
are also plotted in Fig. 8(b) as excitation profiles to facilitate the visualization of the trends. The following characteristics are observed in the 488 < il,, < 722 nm range: ( 1)
a systematic increase in EF as &., is swept to the red, (2)
no well-defined EF maxima are observed, and (3) the largest EF=5.3 X lo5 for d,=8.0
nm and a,,=722 nm. The
film thickness dependence of EF at constant ;1, shows a
systematic increase with d, from 1.8 to 8.0 nm followed by
a falloff as d, is further increased to 16.0 nm. In order to
interpret this behavior within the framework of the EM
theory of SERS for isolated oblate spheroidal particles, it is
x
Igic(,(n=753
a.
52
--~ in -rmrrj
j
3.0 x 10’
counts
s“
I
.--
L
b
G
‘_
I
1500
1000
Wavenumbers
500
(cm-‘)
FIG. 11. Surface enhanced and preresonance Raman spectra of CoPc.
(a) SER spectrum of 0.10 monolayer of CoPc. Inset in (a) shows a
schematic illustration of the surface composed of 200 nm of Ag evaporated over a monolayer of 542 nm polymer nanospheres. Substrate composed of 200 nm of Ag evaporated over 542 mn polystyrene microspheres.
1.0 mW of /1,,=753.2 nm. (b) Preresonance Raman spectrum of an 80
monolayer thick CoPc film. 300 mW of &,=753.2
nm. Inset in (b) is
UV-vis absorption spectrum of 80 monolayer thick CoPc film on glass.
necessary to keep in mind that the EF for each AgIF is
actually characterized by the two independent particle
structure parameters, a and R, rather than a single d,
value. For convenience the a and R structural parameters
from Table I are reproduced in Fig. 8 (b) . The EM theory
of SERS predicts that,6*86J89for a fixed value of the particle’s major axis, a, the plasmon frequency will shift to the
red as the spheroid becomes more oblate (decreasing R)
increasing the Raman EF with red laser excitation compared to blue, and, similarly, for a,fixed particle shape, R,
the plasmon frequency will also shift to the red as the
spheroid’s major axis, a, becomes larger again increasing
the Raman EF with red laser excitation compared to blue.
Thus the systematic increase in EF as ;1,, is swept to the
red observed in Fig. 8(b) for AgIF’s with d,=1.8,
3.5,
and 8.0 nm can be understood as the consequence of in-
I
2109
creasing the major ‘axis length’and decreasing the R bG-- ~rameter for the film’s constituent particles. The EF for the
d, = 16.0 nm film does not follow this trend. This is probably a consequence of the high packing density of the particles in these films. Densely packed particle arrays, either
periodic or random, exhibit surface plasmon resonance features that are broadened considerably by dipole-dipole interactions and may have either red- or blue-shifted excitation
spectra
compared
with
those of isolated
particles.6’99-‘03 Thus, for /2,, that produces a large EF in a
system of well isolated particles, a diminished EF is likely
to result from increased particle-particle interactions in an
otherwise. similar system.
Two previous studies of the SERS enhancement proper$es of AgIF’s provide relevant data for comparison.
Otto’ (see Fig. 8) reported excitation spectra for d,= 2.2,
.6.0]-and 15.6’nm.“AgIF’s prepared in UHV that exhibit EF
-maxima:= 1,.4X $.at
475’ nm, 1.4X 104 at 540 nm, and
0.6~10~. at 575 nm, respectively. Optical transmission
spectra were reported for each of these UHV-prepared
AgIF’s but no nanostructural characterization was carried
out. As far as we are aware, our observation of EF=5.3
X 105, is the largest yet reported for a AgIF. Although
EF’s of this magnitude are commonly found for electrochemically roughened surfaces most AgIF EF’s are about
1 x lo4 as found by Otto. Otto’s results in comparison with
ours clearly attest to the high degree of sensitivity that the
enhancement properties of a SERS-active surface have
with respect to its method of preparation and therefore its
detailed nanostructure. In addition, they suggest that it
would be profitable to examine the 700 <;lex < 1000 nm
range more extensively to search for an optimum excitation
wavelength that would maximize the SERS EF. Cotton54
found a linear relationship between SERS intensity and
optical density for AgIF’s with constant d,,,=5.0 nm but
deposited with varying rd. However, no quantitative characterization of particle structure vs rd was reported. Our
results show that although SERS intensities are generally
stronger for more strongly absorbing films there is no
quantitative linear relationship manifested. Our conclusion
is that one must supplement the measurement of an island
film’s optical absorption with the direct measurement of
nanostructural parameters to provide a firm basis for the
interpretation of its surface enhancement properties.
The EF’s for BPE on the annealed AgIF’s as a function of il, and d, are listed in Table III as the mean and
standard deviation for six replicate surfaces at each film
TABLE II. SERS enhancement factors (EF) for BPE from “as-deposited” Ag island films at various laser
excitation wavelengths (nm) .
Film thickness (nm)
1.8
3.5
8.0
16.0
488.0 (nm)
1.0*0.2x 104
1.0*0.8x lo4
3.8*l.5x104
b
514.5 (nm)
641.3 (nm)
722.0 (nm)
1.9*0.8x IO4
4.8&0.6x lo4
9.5=‘=2.6x104
1.8=!=0.5x1Q4
Oa
7.1*1.9x
104
23.7*5.6x
lo4
8.2* 1.7~ lo4
0”
9.7+3.8x104
52.6*2.3x
lo4
34.8*6.7x
lo4
“EF < ld.
bNo data available.
2110
I
TABLE III. SERS enhancement factors (EF) for BPE from annealed Ag island films at various laser
excitation wavelengths (nm) .
Film thickness (nm)
1.8
3.5
8.0
16.0
488.0 (nm)
0.70*0.18x
0.70*0.20x
b
Oa
514.5 (nm)
lo4
lo4
1.2*0.3x104
1.4&0.4X 104
7.8+2.5x104
Oa
641.3 (nm)
722.0 (nm)
0”
o.66+o.16x104
4.0*2.0x lo4
0=
0”
0.25 ho.07 x lo4
7.8*2.3x104
0”
aEF < 102.
bNo data available.
thickness. Excitation profiles are plotted in Fig. 9 (b). The
EF’s for annealed films are essentially unchanged at the
low d, and blue it,, end of the data set but attenuated by
factors as large as 40 at the high d, and red il,, end of the
data set compared to “as-deposited” films, show very little
wavelength dependence, have no maxima, and have the
largest value equal to 7.8 X lo4 for d,= 8.0 nm. The film
thickness dependence of EF at constant A,, shows a systematic increase with d, from 1.8 to 8.0 nm followed by a
falloff to EF < lo* as d, is further increased to 16.0 nm.
For the d,= 1.8 and 3.5 nm films, the attenuation of EF in
Fig. 9(b) can be understood in terms of an annealing induced change to significantly less oblate particle shapes
with only slightly increased major axis size. This is consistent with the annealing induced blue shift observed in the
extinction spectra. For d,=8.0
nm film, the annealing
doubles the major axis size while leaving R little changed
thus offsetting the blue-shift trend and maintaining a relatively large EF. The lack of SERS activity for the d, = 16.0
nm film is likely to be due to a shift, probably to the red, of
the plasmon resonance caused by a combination of
R=0.08
and the effects of dipole-dipole interactions in
densely packed random particle arrays. So far as we are
aware there have been no previous studies of SERS EF’s
for annealed Ag island films for comparison; however,
Aussenegg9* has reported the effects of island film annealing on the fluorescence intensity and lifetime for adsorbed
dye molecules. One practical consequence of this annealing
study, which should be noted in addition to the important
results connecting AFM structural data, SERS EF’s, and
EM theory, is that AgIF’s are viable candidates for SERSactive surfaces in high temperature applications such as in
situ heterogeneous catalytic studies and combined thermal
desorption/SERS investigations in UHV experiments.
B. Thick films over polymer
nanospheres
It is clear from the experiments presented earlier that
even with the improved structural information afforded by
AFM, island films are still not quite ideal surfaces either
for studying more detailed aspects of the EM theory of
SERS or for preparing optimized, reproducible surfaces for
purposes of chemical analysis. Comparison of theory and
experiment for island films is complicated by the distribution of particle sizes and shapes in thin films with isolated
particles as well as the overlapping particle problem in
thicker films. Analytical studies are complicated by the
need to control the many deposition parameters that control the details of the particle size, shape, and spacing dis-
tribution. Several attempts have been made in the literature
to overcome these problems by using nanofabrication tech~quesWW
to predefine the roughness of the substrate so
that an overlayer film of SERS-active material will adopt
that ~roughness. Significant success in EM theory/
experiment comparison has to be achieved with this approach but it is far too difficult to prepare these samples for
routine use. An approach to nanofabricated surfaces that
would seem to combine the elegance of nanofabrication as
a means of defining surface roughness with the simplicity
of surface preparation associated with island film or CaF2
roughened surfaces, involves depositing a “thick, smooth”
SERS-active metal film over a monolayer of polymer nanospheres supported on glass, quartz, or silicon substrates.38’39’41These studies which relied on SEM to define
the nanostructure of the surface have left the impression
that the electromagnetic enhancement properties of these
surfaces can be selectively “tuned” by changing the size of
the polymer spheres. A schematic representation of such a
surface is given in the inset to Fig. 11 (a).
Ag film over polymer nanosphere surfaces were prepared with a variety of sphere diameters, D, and film thicknesses, d, . Figure 12(a) shows a low-resolution AFM image for a d,= 250 nm. Ag film deposited at a rdZO.3
nms -’ over 542 nm diameter polymer nanospheres supported on glass. The radiative heating of the substrate resulted in T, rising by less than 30 K so that no melting or
deformation of the nanosphere was observed to occur under these deposition conditions. This image with its 2000
nm by 2000 nm field of view clearly shows the well-ordered
( 111 )-type structure of the nanosphere monolayer that defines a roughness scale dictated by the size of the underlying spheres. Figure 12(b) shows a higher-resolution AFM
image of the top of one of the nanospheres that reveals
random substructure roughness (RSR) on the 40-50 nm
scale, superimposed on the roughness scale defined by the
nanospheres. Clearly, the schematic representation of these
surfaces in Fig. 11 (a) is not correct. For comparison purposes, similar AFM images were recorded for d,=250
nm
Ag films deposited at the same rate directly on the glass
substrate. Figures 13(a) and 13 (b) show the surface of the
film to be composed of roughly spherical Ag particles
about 40 nm in diameter, not unlike those in the 16 nm
thick island films and nearly identical to the RSR on the
nanosphere surfaces. Large area images, not shown, reveal
the structure to be consistent over macroscopic regions.
Figure 13 (c) is an AFM line scan image with (50 nm)2
J. Chem. Phys., Vol. 99, NO. 3, 1 August 1993
I
2111
a.
RIO
b.
50 rim
033Ill
FIG. 12. AFM images of d,,,=250 nm thick Ag film vacuum evaporated
over close-packed 542 nm diameter polystyrene nanospheres. rd=O.3
nm s-‘. (a) 2 pm by 2 pm image showing patterning caused by spheres
and (b) 200 nm by 200 nm image of “random substructure roughness” on
a single sphere.
field of view showing that the individual 40 nm particles
are free of substructure roughness down to about the 1 nm
level.
SERS intensities for CoPc adsorbed on sphere coated
substrates were measured using il,=753.2
nm from a cw
Ti:A1203 laser as a function of D, d,, and nanosphere
coverage. The values used were D= 121,250, 542, and 870
nm and 0 <dm <4CQ nm. SERS EF’S were then calculated
from the ratio of the intensities of the 683 cm-’ line in the
surface and bulk samples. Figure 14(a) shows the dependence of SERS intensity on Ag film thickness for various
nanosphere sizes. The maximum intensity was obtained for
d,=200
mn and D=542
nm. This intensity maximum
corresponds to an EF= 1 X 10’. Given that the AFM images for Ag film over polymer nanosphere surfaces clearly
show two distinct roughness scales, the question naturally
arises as to their relative contributions. This question was
addressed in two ways. First, EF’s were compared for the
d,,, =200 nm and D= 542 nm film over polymer nanosphere system with a d,=200
nm film directly deposited
on glass. These numbers were EFtotal= 1 X lo7 and
EFfi~rn only- 1 X 105. Therefore the incremental enhancement
is EFnanospheres = 1 X 102. Next the dependence of
E%al on nanosphere coverage and size were determined.
Figure 14(b) shows the coverage data for two sphere sizes.
This incremental enhancement due to the nanospheres is
approximately linear in sphere coverage and clearly size
dependent. From these studies we conclude that EFtotal= 1
8 nm.
FIG. 13. AFM images of d,,,=250 nm thick Ag film vacuum evaporated
over cleaved mica. rd=0.3 nm s-‘. (a) 400 run by 400 nm image; (b) 200
nm by 200 nm image; (c) 50 nm by 50 nm linescan image.
X lo7 may be partitioned into a lo5 contribution from the
RSR of the thick film and a size dependent contribution
from the roughness defined by the nanospheres. It is likely
that this incremental enhancement due to the large scale
but well-defined roughness of the nanospheres is the consequence of extended or delocalized, propagating surface
plasmon polaritons (SPP). Studies of the wavelength dealong with theoretical and experpendence
of EFnanospheres
imental examination of the gratinglike properties of these
surfaces will be required to further elucidate the role of
SPP’s in the incremental enhancement mechanism.
Before moving on to the section dealing with applications of film over polymer nanosphere surfaces, a comment
is directed to the observation that d,=200
nm Ag films
directly deposited on glass at room temperature support
SERS EF’s on the order of 105. While this may not be the
first observation of this effect, we were unable to readily
locate literature references to previous studies. The closest
was the case of cold-deposited thick Ag films which are
well-known to be SERS active.73 In fact, the usual impression is that thick Ag and Au films are too smooth to support SERS. Clearly this is also highly dependent on depo-
I
2112
q
- WC0 a.
-m
B
8
JO
-o+
-+-la-
= o.mv
100
Dmnm
2W”rn
w2nm
8mnm
x0
I
a. g
300
-400
dm(m.)
0.30 ADU d mW’
I
I
16.96
aol
040
0.80
sition and substrate conditions. That thick films of Ag and
Au support SERS directly may be extremely relevant to
studies of the molecular structure of alkanethiol-based selfassembled monolayers (SAM).
C. Applications
surfaces
for film over polymer
nanosphere
Studying the EM enhancement mechanism is only one
use for nanosphere roughened substrates. Because they
support large enhancements, these surfaces can be used for
a variety of other studies. The fact that the enhancements
are greatest in the red are a further advantage. In many
cases it is desirable to study adsorbates at longer wavelengths to avoid electronic resonances and reduce laser
degradation of adsorbates-particularly
with biological
molecules.
In addition, the introduction of new detectors, most
importantly, the charge coupled device (CCD), have enabled efficient detection of photons into the near IR. Coupled with CCD detections systems, nanosphere surfaces
can be valuable for improving detection limits of SERS,
and studying scattering processes weaker than Stokes Raman; i.e., anti-Stokes Raman, and hyper-Raman processes.
Figure 15 shows a SERS spectra of BPE taken with
only 5 PW of input 753 nm radiation. A data acquisition
time of 1000 s resulted in a S/N> 100. Previously, the
lowest reported laser power for a SERS experiment was 0.7
mW at 632.8 nm for a monolayer of benzoic acid adsorbed
on a stochastic Ag particle array surface yielding a spec-
I
I
1.550
1500
Wavemnmbers (cm-‘)
Nanosphere Coverage
FIG. 14. SERS intensity for 0.1 monolayer of CoPc adsorbed on Ag film
over polymer nanosphere surfaces, (a) d, dependence at 1.0 monolayer
nanosphere coverage for various nanosphere diameters: D=121, 264,
542, and 870 nm; 1.0 mW of &,=753.2 nm. (b) Nanosphere coverage
dependence at optimal d ,,,E0.4 D for nanosphere diameters: D=264 and
542 nm; 14 mW of L-=753.2 nm.
I
~1600
FIG. 15. SER spectra of BPE adsorbed on a 200 nm Ag film over 542 nm
diameter polymer nanosphere surface using a CCD detector. (a) 2.5 nW
of a,=753 nm, 5 s integration. (b) 5 PW of /2,,=753 nm, 1000 s integration.
trum with S/N=4-5
(data acquisition time not given).**
A loo-fold reduction in the minimum laser power requirements for Raman spectroscopy has been achieved. This
suggests that PW power level SERS may permit experiments on adsorbates that readily decompose by phototherma1 or photochemical pathways. In addition, low power
diode lasers or even nonlaser light sources should now be
usable for SERS.
The lo7 enhancements for SERS also increases the intensities of an adsorbate’s anti-Stokes spectrum. Classical
Raman theory predicts that both the Stokes and antiStokes branches contain the same vibrational information,
but the relative intensities are related as
Is(%) (Q--yfJ4
4ew&w (YL--Ye+YK)2+r2
-ZZ
(YL-t2rK)
( (YL--Ye-vfJ2+P
1’ (3)
IAs
where I = intensity, yL = laser frequency, vK= vibration
frequency, h =Planck’s constant, k=Boltzmann’s
constant, and T = temperature. The term in large parentheses
is a correction factor for heating induced by the resonance
Raman effect, lo4 where Y, is the frequency of the absorption-for the resonance and I is the full width at half maximum (FWHM) for the resonance. With the exception of
colloidal systems, the importance of this correction is difficult to quantify. Furthermore, although this correction
factor has been used by Pemberton and co-workers27 to
measure adsorbate temperatures, the correction was small.
SERS spectra of the anti-Stokes bands of BPE at 1609 and
1640 cm-’ demonstrate the large intensities obtainable
I
2113
nificant to note that although comparable SERS intensities
are seen for SERS spectra taken in air and electrochemical
environments, SEHRS was only obtainable when the surface was under electrochemical control. It is possible that
electromagnetic enhancements are greatly increased at
1064 nm excitation for electrochemical systems and not
ones in air. However, SERS spectra taken by FT-Raman
indicate that enhancements actually decrease between 750
and 1064 nm.ro5 Another possibility is that chemical enhancement mechanisms are major contributors to hyperRaman enhancements. Further studies are underway to
determine the relative role of enhancements in SEHRS.
I
1650
I
I
l&O0
I
I
1550
I
I
I
1500 -1500
I
I
-1550
I
,
-1600
8
I
-1650
IV. CONCLUSIONS
Wavenumber (cm-‘)
FIG. 16. SERS of 1.0 monolayer
over 542 nm diameter polymer
range using a CCD detector. (a)
1 s integration. (b) Anti-stokes
integration.
of BPE adsorbed on a 200 nm Ag film
nanosphere surface in the 1600 cm-’
Stokes region, 30 mW of AC,=753 nm,
region, 30 mW of &=753
nm, 30 s
(Fig. 16). At room temperature the anti-Stokes bands are
predicted to be approximately lo3 weaker than the Stokes
intensities for 753 nm excitation.
Large enhancements from nanosphere roughened substrates are not limited to experiments in air or vacuum.
Figure 17 reveals that not only SERS spectra, but also
surface-enhanced hyperRaman spectra (SEHRS), can be
readily acquired in electrochemical environments. It is sig-
a. SEERS
3
ADU
8”mW’
I 0.01
-82
&a
4 gi
L
1600
1300
700
1000
Wavenumbers (cm-‘)
FIG. 17. Electrochemical SERS and SEHRS spectra of 1.0 monolayer of
BPE on a 200 nm. Ag film over 542 nm diameter polymer nanosphere
electrode surface using a CCD detector. (a) 1.0 W of A,,= 1064 nm from
a cw mode-locked Nd:YAG laser, E= -0.6 V vs SCE and (b) 10 mW of
da=532 nm from second harmonic of a cw mode-locked Nd:YAG laser,
E= -0.6 V vs SCE.
This is the first study in which atomic force microscopy has been used to directly probe the three-dimensional
nanostructure of SERS-active surfaces. AFM is clearly
demonstrated to be a powerful new tool for determining
the shape factor, R, for isolated particles. The direct measurement of R afforded by AFM greatly simplifies the interpretation of the SERS EF’s for Ag island films in terms
of the electromagnetic theory of SERS for isolated oblate
spheroids.
This is one of the first AFM studies of island film
formation and nanostructural modification upon thermal
annealing. Since many island films are grown on insulating
substrates, these systems would normally require shadow
coating with conductive metal for conventional scanning
electron microscopy or scanning tunneling microscopy
work.
The SERS activity of Ag island films is not lost when
they are subjected to quite vigorous thermal annealing conditions (viz., 600 K for 60 min.). EF’s with blue excitation
are essentially the same as “as-deposited” surfaces while
EF’s with red excitation are attenuated from the 5 X lo5
level down to the lo4 level.
AFM studies show that Ag film over polymer nanosphere systems do not possess the well-controlled roughness characteristics desired in nanofabricated surfaces.
However, SERS EF measurements show that they are extremely active. Under “optimized” conditions EF’s in the
lo7 range can be obtained. The EFtotal= lo7 is partitioned
into a 10’ contribution from random substructure roughness supporting localized surface plasmons and a lo2 contribution which is nanosphere size dependent and may by
the consequence of delocalized surface plasmon polaritons.
This is the first thin film based SERS-active surface with an
EF comparable to that found for electrochemically roughened surfaces.
Ag film over polymer nanosphere surfaces are shown
to have applications in ultrahigh sensitivity SERS experiments involving only 5 PW of laser power, detecting antiStokes SERS signals for W,ib < 1650 cm-‘, and surfaceenhanced hyper-Raman scattering (SEHRS).
ACKNOWLEDGMENTS
We thank Professor George C. Schatz for helpful comments. This work was supported by the National Science
2114
Foundation through the Solid State Chemistry Program,
Grant No. DMR-8802706 and by the Northwestern University Materials Research Center, Grant No. DMR9120521.
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Atomic force microscopy and surface

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