Electromigrated nanoscale gaps for surface-enhanced Raman spectroscopy
Electromigrated nanoscale gaps for surface-enhanced Raman
spectroscopy
Daniel R. Ward1, Nathaniel K. Grady2, Carly S. Levin3, Naomi J.
Halas3,4, Yanpeng Wu2, Peter Nordlander1,4, Douglas Natelson1,4
1Department of Physics and Astronomy,
2Applied Physics Graduate Program, 3Department of Chemistry,
4Department of Electrical and Computer Engineering,
and the Rice Quantum Institute, Rice University,
6100 Main St., Houston, TX 77005, USA
(Dated: October 23, 2018)
Abstract
Single-molecule detection with chemical specificity is a powerful and much desired tool for bi-
ology, chemistry, physics, and sensing technologies. Surface-enhanced spectroscopies enable single
molecule studies, yet reliable substrates of adequate sensitivity are in short supply. We present
a simple, scaleable substrate for surface-enhanced Raman spectroscopy (SERS) incorporating
nanometer-scale electromigrated gaps between extended electrodes. Molecules in the nanogap
active regions exhibit hallmarks of very high Raman sensitivity, including blinking and spectral
diffusion. Electrodynamic simulations show plasmonic focusing, giving electromagnetic enhance-
ments approaching those needed for single-molecule SERS.
http://arxiv.org/abs/0704.0451v1
Multifunctional sensors with single-molecule sensitivity are greatly desired for a vari-
ety of sensing applications, from biochemical analysis to explosives detection. Chemi-
cal and electromagnetic interactions between molecules and metal substrates are used in
surface-enhanced spectroscopies[1] to approach single molecule sensitivity. Electromagnetic
enhancement in nanostructured conductors results when incident light excites local elec-
tronic modes, producing large electric fields in a nanoscale region, known as a “hot spot”,
that greatly exceed the strength of the incident field. Hot spots can lead to particularly
large enhancements of Raman scattering, since the Raman scattering rate is proportional to
|E(ω)|2|E(ω′)|2 at the location of the molecule, where E(ω) is the electric field component
at the frequency of the incident radiation, and E(ω′) is the component at the scattered
frequency.
It has been an ongoing challenge to design and fabricate a substrate for systematic SERS
at the single molecule level. Single-molecule SERS sensitivity was first clearly demon-
strated using random aggregates of colloidal nanoparticles[2, 3, 4, 5]. Numerous other
metal substrate configurations have been used for SERS, including chemically engineered
nanoparticles[6, 7, 8], nanostructures defined by bottom-up patterning[9, 10], and those
made by traditional lithographic approaches[11]. In the most sensitive substrate geome-
tries, incident light excites adjacent subwavelength nanoparticles or nanostructures, result-
ing in large field enhancements within the interparticle gap[12, 13]. Fractal aggregates of
nanoparticles[14] can further increase field enhancements by focusing plasmon energy from
larger length scales down to particular nanometer-scale hotspots[15]. However, precise and
reproducible formation of such assemblies in predetermined locations has been extremely
challenging. An alternative approach is tip-enhanced Raman spectroscopy (TERS), in which
the incident light excites an interelectrode plasmon resonance localized between a sharp,
metal scanned probe tip and an underlying metal substrate. Recent progress has been made
in single-molecule TERS detection[16, 17, 18]. A similar approach was recently attempted
using a mechanical break junction[19]. While useful for surface imaging, TERS requires
feedback to control the tip-surface gap, and is not scalable or readily integrated with other
sensing modalities.
We demonstrate a scaleable and highly reliable method for producing planar extended
electrodes with nanoscale spacings that exhibit very large SERS signals, with each electrode
pair having one well-defined hot spot. Confocal scanning Raman microscopy demonstrates
the localization of the enhanced Raman emission. The SERS response is consistent with
a very small number of molecules in the hotspot, showing blinking and spectral diffusion
of Raman lines. Sensitivity is sufficiently high that SERS from physisorbed atmospheric
contaminants may be detected after minutes of exposure to ambient conditions. The Raman
enhancement for para-mercaptoaniline (pMA) is estimated from experimental data to exceed
108. Finite-difference time-domain (FDTD) modeling of realistic structures reveals a rich
collection of interelectrode plasmon modes that can readily lead to SERS enhancements as
large as 5× 1010 over a broad range of illumination wavelengths. These structures hold the
promise of integration of single-molecule SERS with electronic transport measurements, as
well as other near-field optical devices.
Our structures are fabricated on a Si wafer topped by 200 nm of thermal oxide. Electron
beam lithography is used to pattern “multibowtie” structures as shown in Fig. 1A. The
multibowties consist of two larger pads connected by multiple constrictions, as shown. The
constriction widths are 80-100 nm, readily within the reach of modern photolithography.
After evaporation of 1 nm Ti and 15 nm Au followed by liftoff in acetone, the electrode sets
are cleaned of organic residue by exposure to O2 plasma for 1 minute. The multibowties are
placed in a vacuum probe station, and electromigration[20] is used to form nanometer-scale
gaps in the constrictions in parallel, as shown in Fig. 1B. Electromigration is a nonthermal
process whereby momentum transfer from current-carrying electrons is transferred to the
lattice, rearranging the atomic positions. Electromigration has been studied thoroughly[21,
22, 23, 24] as a means of producing electrodes for studies of single-molecule conduction.
We have performed manual and automated electromigration at room temperature, with
identical results. The number of parallel constrictions in a single multibowtie is limited
by the output current capacity of our electromigration voltage source. A post-migration
resistance of ∼ 10 kΩ for the structure in Fig. 1A appears optimal.
Post-migration high resolution scanning electron microscopy (SEM) shows interelectrode
gaps ranging from too small to resolve to several nanometers. There are no detectable
nanoparticles in the gap region or along the electrode edges. Based on electromigration of
283 multibowties (1981 individual constrictions), 77% of multibowties have final resistances
less than 100 kΩ, and 43% have final resistances less than 25 kΩ. We believe that this yield,
already high, can be improved significantly with better process control, particularly of the
lithography and liftoff.
The optical properties of the resulting nanogaps are characterized using a WITec
CRM 200 scanning confocal Raman microscope in reflection mode, with normal illumination
from a 785 nm diode laser. Using a 100× objective, the resulting diffraction-limited spot
is measured to be gaussian with a full-width at half-maximum of 575 nm. Fig. 1C shows a
spatial map of the integrated emission from the 520 cm−1 Raman line of the Si substrate.
The Au electrodes are clearly resolved. Polarization of the incident radiation is horizontal
in this figure. Rayleigh scattered light from these structures shows significant changes upon
polarization rotation, while SERS response is approximately independent of polarization.
Freshly cleaned nanogaps show no Stokes-shifted Raman emission out to 3000 cm−1.
However, in 65% of clean nanogaps examined, a broad continuum background (see Support-
ing Information) is seen, decaying roughly linearly in wavenumber out to 1000 cm−1 before
falling below detectability. This background is spatially localized to a diffraction-limited re-
gion around the interelectrode gap and is entirely absent in unmigrated junctions. The origin
of this continuum, similar to that seen in other strongly enhancing SERS substrates[5], is
likely inelastic electronic effects in the gold electrodes[25]. In samples coated with molecules,
this background correlates strongly with visibility of SERS. No junctions without this back-
ground displayed SERS signals. Like the SERS signal, this background is approximately
polarization independent. Temporal fluctuations of this background in clean junctions are
minimal, strongly implying that fluctuations of the electrode geometry are not responsible
for SERS blinking.
The SERS enhancement of the junctions has been tested using various molecules. The
bulk of testing utilized pMA, which self-assembles onto the Au electrodes via standard thiol
chemistry. Particular modes of interest are carbon ring modes at 1077 cm−1 and 1590 cm−1.
Fig. 1D shows a map of the Raman emission from the 1077 cm−1 line on the same junction
as Fig. 1B,C, after self-assembly of pMA. This emission is strongly localized to the position
of the nanogap. No Raman signal is detectable either on the metal films or at the edges of
the metal electrodes. Fig. 1E shows the spatial localization of the continuum background
mentioned above.
Fig. 2 shows a more detailed examination of the SERS response of the gap region of a
typical junction after self-assembly of pMA. Fig. 2A,B are time series of the SERS response,
with known pMAmodes indicated. The modes visible are similar to those seen in other SERS
measurements of pMA on lithographically fabricated Au structures[11]. Each spectrum was
acquired with 1 s integration time, with the objective positioned over the center of the
nanogap hotspot. Temporal fluctuations of both the Raman intensity (“blinking”) and
Raman shift (spectral diffusion), generally regarded as hallmarks of few- or single-molecule
SERS sensitivity[26], are readily apparent. Fig. 2C shows a comparison of the wandering
of the 1077 cm−1 pMA line with that of the 520 cm−1 Raman line of the underlying Si
substrate over the same time interval. This demonstrates that the spectral diffusion is due
to changes in the molecular environment, rather than a variation in spectrometer response.
Fig. 2D shows the variation in the peak amplitudes over that same time interval.
This blinking and spectral diffusion are seen routinely in these junctions. We have ob-
served such Raman response from several molecules, including self-assembled films of pMA,
para-mercaptobenzoic acid (pMBA), a Co-containing transition metal complex[28], and spin-
coated poly(3-hexylthiophene) (P3HT). These molecules all have distinct Raman responses
and show blinking and wandering in the junction hotspots.
Another indicator of very large enhancement factors in these structures is sensitivity to
exogenous, physisorbed contamination. Carbon contamination has been discussed[29, 30, 31]
in the context of both SERS and TERS. This substrate is sensitive enough to examine such
contaminants (see Supporting Information). While clean junctions with no deliberately
attached molecules initially show only the continuum background, gap-localized SERS sig-
natures in the sp2 carbon region between 1000 cm−1 and 1600 cm−1 are readily detected
after exposure to ambient lab conditions for tens of minutes. Nanojunctions that have been
coated with a self-assembled monolayer (SAM) (for example, pMA) do not show this carbon
signature, even after hours of ambient exposure. Presumably this has to do with the ex-
tremely localized field enhancement in these structures, with the SAM sterically preventing
physisorbed contaminants from entering the region of enhanced near field.
Recently arrived contaminant SERS signatures abruptly disappear within tens of seconds
at high incident powers (1.8 mW), presumably due to desorption. SERS from covalently
bound molecules is considerably more robust, degrading slowly at high powers, and persisting
indefinitely for incident powers below 700 µW. SEM examination of the nanogaps shows
no optically induced damage after exposure to intensities that would significantly degrade
nanoparticles[32]. The large extended pads likely aid in the thermal sinking of the nanogap
region to the substrate.
Estimating enhancement factors rigorously is notoriously difficult, particularly when the
hotspot size is not known. Although SERS enhancement volume measurements are possible
using molecular rulers[33], this is not feasible with such small nanogaps. Junctions made
directly on Raman-active substrates (Si with no oxide; GaAs) show no clearly detectable
enhancement of substrate modes in the gap region, suggesting that the electromagnetic
enhancement is strongly confined to the thickness of the metal film electrodes. Figure 3
shows a comparison between a typical pMA SERS spectrum acquired on a junction with
a 600 s integration time at 700 µW incident power, and a spectrum acquired over one of
that device’s Au pads, for the same settings. The pad spectrum shows no detectable pMA
features and is dark current limited.
We use FDTD calculations to understand the strong SERS response in these structures
and roughly estimate enhancement factors. It is important to note that the finite grid size
(2 nm) required for practical computation times restricts the quantitative accuracy of these
calculations. However, the main results regarding spatial mode structure (allowing assess-
ment of the localization of the hotspot) and energy dependence are robust to these concerns,
and the calculated electric field enhancement is an underestimate[34]. Fig. 4 shows a cal-
culated extinction spectrum and map of |E|4 in the vicinity of the junction. Calculational
details and additional plots are presented in the Supporting Information. These calculations
predict that there should be large SERS enhancements across a broad bandwidth of excit-
ing wavelengths because of the complicated mode structure possible in the interelectrode
gap. Nanometer-scale asperities from the electromigration process break the interelectrode
symmetry of the structure. The result is that optical excitations at a variety of polariza-
tions can excite many interelectrode modes besides the simple dipolar plasmon commonly
considered. For extended electrodes, a continuous band of plasmon resonances coupling to
wavelengths from 500-1000 nm is expected[35]. This broken symmetry also leads to much
less dependence of the calculated enhancement on polarization direction, as seen experi-
mentally. The calculations confirm that the electromagnetic enhancement is confined in
the normal direction to the film thickness. Laterally, the field enhancement is confined to
a region comparable to the radius of curvature of the asperity. For gaps and asperities in
the range of 2 nm, purely electromagnetic enhancements can exceed 1011, approaching that
sufficient for single-molecule sensitivity.
Using the data from the device in Fig. 3, we estimate the total enhancement in that
device. To be conservative, we assume a hotspot effective radius of 2.5 nm with dense
packing of pMA, giving N ≈ 100 molecules. Blinking and wandering suggest that the true
N value is much closer to one. The integrated Raman signal over a gaussian fit to the
1077 cm−1 Raman line is 2.0 counts/sec/molecule when the incident power is 700 µW. For
our apparatus the count rate from imaging a bulk crystal at the same equivalent power
(see Supporting Information) is 4.2× 10−9 counts/sec/molecule, so that we estimate a total
enhancement of at least 5× 108.
We have demonstrated a SERS substrate capable of extremely high sensitivity for trace
chemical detection. Unlike previous substrates, these nanojunctions may be mass fabricated
in controlled positions with high yield using a combination of standard lithography and
electromigration. The resulting hotspot geometry is predicted to allow large SERS enhance-
ments over a broad band of illuminating wavelengths. Other nonlinear optical effects should
be observable in these structures as well. The extended electrode geometry and underly-
ing gate electrode are ideal for integration with other sensing modalities such as electronic
transport. Tuning molecule/electrode charge transfer via the gate electrode may also enable
the direct examination of the fundamental nature of chemical enhancement in SERS.
DW acknowledges support from the NSF-funded Integrative Graduate Research and Ed-
ucational Training (IGERT) program in Nanophotonics. NH, PN, and DN acknowledge
support from Robert A. Welch Foundation grants C-1220, C-1222, and C-1636, respectively.
DN also acknowledges the National Science Foundation, the David and Lucille Packard
Foundation, the Sloan Foundation, and the Research Corporation. C.S.L. was supported by
a training fellowship from the Keck Center Nanobiology Training Program of the Gulf Coast
Consortia, NIH 1 T90 DK070121-01. YP and NKG are supported by US Army Research
Office grant W911NF-04-1-0203.
Supporting Information Available: Detailed examination of continuum background
and adsorption of exogenous contaminants; extended discussion of FDTD calculations; more
detailed discussion of SERS enhancement calculation. This material is available free of
charge via the Internet at http://pubs.acs.org.
http://pubs.acs.org
FIG. 1: (A) Full multibowtie structure, with seven nanoconstrictions. (B) Close-up of an individual
constriction after electromigration. Note that the resulting nanoscale gap (<∼ 5 nm at closest
separation, as inferred from closer images) is toward the right edge of the indicated red square. (C)
Map of Si Raman peak (integrated from 500-550 cm−1) in device from (B), with red corresponding
to high total counts. The attenuation of the Si Raman line by the Au electrodes is clear. (D) Map of
pMA SERS signal for this device based on one carbon ring mode (integrated from 1050-1110 cm−1).
(E) Map of integrated low energy background (50-300 cm−1) for this device.
FIG. 2: (A) Waterfall plot (1 s integration steps) of SERS spectrum at a single nanogap that had
been soaked in 1 mM pMA in ethanol. Identified pMA peaks include the ring modes at 1077 cm−1
and 1590 cm−1, as well as an 1145 cm−1 δCH mode with b2 symmetry, an 1190 cm
−1 mode
identified as δCH of a1 symmetry, a mode near 1380 cm
−1 identified as δCH+νCC of b2 symmetry,
and a mode near 1425 cm−1 identified as νCC+δCH of b2 symmetry. Mode assignments are based
on Ref. [27]. (B) Close-up of (A) to show correlated wandering and blinking of 1077 cm−1 and
1145 cm−1 modes. (C) Comparison of 1145 cm−1 mode position (blue) with the Si Raman peak
(red), which shows no such wandering. The jitter in the Si peak position is 1 pixel in the detector.
(D) Comparison of 1145 cm−1 peak height (green, found by a gaussian fit to the peak) fluctuations
with those of the Si peak (blue).
Figure 3:
Comparison of pMA at hotspot (blue,left axis) and 5um over on Au 
pad (green,right axis).  Prominent features include the 1077,1590 a1 
symmetry mode peaks, and the less strong 1180 (1160 in moerner
paper) b2 symmetry mode peak.  Additionally the 520 Si peak is 
visible.  The Si peak is significantly reduced in the Au film spectra 
because the film is only partially transparent at 785 nm.  B2 
symmetry modes appear weaker because of blinking on and off 
which reduces the measured intensity when averaged over long time 
periods.  There are no pMA features in the Au pad spectrum dark 
current noise limited.  Estimated enhancement factor is ~1e6.
0 500 1000 1500 2000
0 500 1000 1500 2000
Raman Shift (cm-1)
FIG. 3: Blue curve (left scale): pMA SERS spectrum at hotspot center of one nanojunction densely
covered by pMA, integrated for 10 minutes at incident power of 700 µW. Green curve (right scale):
integrated signal under same conditions on middle of Au pad on the same nanojunction. The
feature near 960 cm−1 is from the Si substrate. No Raman features are detectable on either the
Au pads or their edges under these conditions.
FIG. 4: (A) FDTD-calculated extinction spectrum from the model electrode configuration shown in
(B). (B) Mock-up electrode tips capped with nanoscale hemispherical asperities, with |E|4 plotted
for the 937 nm resonance of (A). Constriction transverse width at narrowest point is 100 nm. Gap
size without asperities is 8 nm. Asperity on left (right) electrode has radius of 6 nm (4 nm). Au
film thickness is 15 nm, SiO2 underlayer thickness is 50 nm. Radiation is normally incident, with
polarization oriented horizontally. Grid size for FDTD calculation is 2 nm. (C) Close-up of central
region of (B), showing extremely localized enhancement at asperities. (D) Cross-section indicated
in (C), showing that enhancement in this configuration does not penetrate significantly into the
substrate. Predicted maximum electromagnetic Raman enhancement in this mode exceeds 108.
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Supporting Information: Electromigrated nanoscale gaps for surface-enhanced
Raman spectroscopy
I. CONTINUUM BACKGROUND
The strong continuum background observed at the nanogaps is shown in Fig. S1A.
The continuum slopes down linearly in intensity from 0 cm−1 to almost 1500 cm−1. This
continuum, seen only in the presence of the nanogap, is compared with the Au film and
Si substrate spectra taken using the same microscope configuration. The 300 cm−1 and
520 cm−1 peaks are from the Si substrate. The spectrum shown in Fig. S1A also shows
a small peak at 1345 cm−1 which is indicative of absorbates from the air settling at the
nanogap. The continuum is localized to the nanogap as seen in Fig. S1B, where a spatial
plot has been made by integrating the SERS spectrum from 600 cm−1 to 800 cm−1 at each
point. This wavenumber range was chosen to avoid any of the Si substrate Raman active
modes. Additionally a comparison of Fig. S1B with the spatial plot of the Si 520 cm−1
peak, Fig. S1C, shows that the continuum background of the nanogap is indeed located at
the center of the bowtie, as expected. Although blinking of SERS at the nanogap has been
observed for pMA and pMBA, the continuum itself does not blink in the absence of molecules.
This is clear from the data in Fig S1D showing the time evolution of the SERS spectrum at
the clean nanogap. No fluctuations are observed, and the continuum background remains
constant.
FIG. S5: (A) Raman spectra at hotspot (blue) of a clean bowtie, Au pad(green), and over Si
substrate(red). The continuum is very strong at the hotspot and shows linear slope from 0 to ∼
1500 cm−1. Also visible are the 300 cm−1 peak and 520 cm−1 peaks of the Si substrate and a weak
peak at 1345 indicating the onset of atmospheric contamination after approximately 15 minutes of
air exposure. Curves have been offset by 150 counts/s (green) and 200 counts/s (red) for clarity.
(B) Spatial plot of integrated signal over 600-800 cm−1 showing the localization of continuum to
the center of the device when compared to the Si plot (C). Yellow is strong signal; blue is no signal.
(C) Spatial plot of integrated Si signal over 500-540 cm−1. Red indicates strong Si signal the blue
areas show where the Au pads are. (D) Time spectra of clean bowtie. The intensity is reported
in CCD counts/second. No blinking of the continuum is observed. The lowest wavenumbers are
reported to have zero counts/second due to the low pass filter used to block out the laser.
II. DEPENDENCE ON INCIDENT POLARIZATION
The SERS signal from the nanogap does not have significant polarization dependence, as
shown in Fig S2A. The two spectra are from the same nanogap with the polarization at 0 and
90 degrees to the gap. Although slightly different due to positioning and actual time variation
of the spectrum, the two spectra do not show any strong differences in the intensities of the
pMA signal or the continuum. The nanogap does exhibit a strong polarization dependence
for Rayleigh scattering, as shown in Fig S2B-S2E. Figures S2B and S2D show a spatial map
of the Rayleigh scattering for the polarization across the gap (B) and parallel with the gap
FIG. S6: (A) Raman spectra at hotspot of bowtie with pMA assembled on surface. The blue
spectrum is with polarization at 0 deg. (direction shown in (C)). The green spectrum has been
shifted 50 counts/s for clarity and is at the same hotspot but with the polarization rotated 90
degrees relative to the substrate (direction shown in (D)). (B) Spatial plot of integrated signal
over -40 to 40 cm−1 showing the Rayleigh scattering from the center of the device. Red is high
intensity blue is low intensity. The large pads are at the left/right. Polarization direction indicated
in (C). (C) Spatial plot of integrated Si signal over 500-540 cm−1. Red indicates Si, blue is Au
pads. Polarization direction is indicated by the arrow. (D) Spatial plot of integrated signal over
-40 to 40 cm−1 showing the Rayleigh scattering from the center of the device for the sample rotated
90 deg. relative to (B),(C). Red is high intensity blue is low intensity. Polarization direction is
indicated in (E). There is a local maximum in the Rayleigh scattering at the center of the gap.
(E) Spatial plot of integrated Si signal over 500-540 cm−1. Red indicates Si, blue is Au pads.
Polarization direction is indicated by the arrow.
III. DEPENDENCE ON SOLUTION CONCENTRATION
We have examined SERS spectra for varying supernatant solution concentration during
the assembly procedure. Ideally, successive dilutions of the solution should vary surface cov-
erage of the assembled molecules. While molecular coverage on the edges of polycrystalline
Au films is not readily assessed, we observe reproducible qualitative trends as concentration
is reduced. For pMBA molecules assembled from solutions in nanopure water, we have var-
ied concentrations from 1 mM down to 100 pM. The fraction of junctions showing SERS
distinct from carbon contamination remains roughly constant down to concentrations as low
as 1 µM. For our volumes and electrode areas, this is still expected to correspond to a dense
coverage of 1 molecule per 0.19 nm2. At concentrations below 1 µM, SERS spectra change
significantly, while remaining distinct from those of carbon contamination: blinking occurs
more frequently; modes of b2 symmetry rather than a1 symmetry appear more frequently;
and the molecular peaks can be more than 100× larger than the high coverage case for the
same integration times. These observations are qualitatively consistent with the molecules
exploring different surface orientations at low coverages, and charge transfer/chemical en-
hancement varying with orientation. However, the actual coverage at the edges remains
unknown.
The concentration of the solution used for assembling molecules on the nanogap surface
strongly influences the form of the observed Raman spectrum as well as the rate and intensity
of the mode blinking. Raman spectra of pMBA were taken by soaking samples in 2 mL of
different concentrations of pMBA. Although for all of these concentrations there are enough
molecules in solution to form a monolayer over the bowtie surface, significant differences in
the spectra were observed. Fig. S3A shows a representative Raman spectrum for pMBA
at the nanogap for 1mM concentrations. The two carbon ring modes at 1077 cm−1 and
1590 cm−1 are clearly present along with a third peak at 1463 cm−1. The time spectra for
this nanogap in Fig S3B. The 1077 cm−1 and 1590 cm−1 peaks are relatively stable and always
present while other modes, such as the 1463 cm−1 mode, blink on and off for a few seconds
at a time. As the concentration is decreased to 1 µM, the pMBA signal tends to be stronger
with more intense blinking. Additionally the 1077 cm−1 mode is observed to disappear while
the 1590 cm−1 mode remains. Additional modes begin to become more visible such as the
1265 cm−1 and 1480 cm−1 modes seen in Fig S3C. At even lower concentrations such as 1
nM, the pMBA signal is again more intense with even more blinking as seen in Fig. S3F
(which has been plotted with intensity on a log scale). The 1077 cm−1 mode is again unseen
while the 1590 cm−1 mode begins to fluctuate in intensity even more. The blinking becomes
much more intense with the intensity of the signal periodically reaching close to ten times
the maximum intensity observed for pMBA at 1 µM. We suggest that the increased blinking
and larger amplitude signals are a result of the molecules not being as tightly packed on the
surface in the 1 µM and 1 nM cases as in the 1 mM case. As a result of looser packing,
the molecules are free to explore more surface conformations, including those with more and
different charge transfer with the Au surface.
We point out that these pMBA spectra are distinct from those seen in physisorbed carbon
contamination on initally clean junctions. These data persist at high incident powers and do
not show “arrival” phenomena as described in the subsequent section. Furthermore, they are
unlikely to originate from photodecomposition of pMBA, since the illumination conditions
are identical for all coverages.
FIG. S7: (A) Raman from pMBA at 1 mM concentration taken at t = 10 s. (B) Corresponding
time spectrum for 1 mM. (C) Raman from pMBA at 1 µM concentration taken at t = 251 s. (D)
Corresponding time spectrum for 1 µM. (E) Raman from pMBA at 1 nM concentration taken at
t = 24.5 s (F) Corresponding time spectrum for 1 nm, plotted on log intensity scale.
IV. DETECTION OF ADSORBED CONTAMINANTS
Due to the large enhancements possible with the nanogaps, contamination from airborne
absorbates occurs readily in the absence of assembled molecules on the nanogap surface. We
have observed the absorption of contaminants onto the surface of clean nanogaps in as little
as 10 minutes. Collecting Raman spectra every 4 seconds, we can observe the appearance
of contaminants on the surface as seen in Fig. S4A and S4B. It is difficult to identify the
contaminants, as the spectra observed have large variations, although carbon ring modes
are often observed in conjunction with other modes. Furthermore the Raman signal from
contaminants often blinks very strongly, with periods of no or weak signal followed by several
seconds of intense blinking, as seen in Fig S4C. The changes in intensity can be more than a
factor of 100. Again we suggest that the strong blinking is a result of the weak attachment
of the contaminants to the nanogap surface, allowing them to move considerably and explore
many interactions with the Au surface. As previously mentioned, these contamination spec-
tra are not observed when molecules of interest have been preassembled deliberately on the
electrode surface. The likely explanation for this is that the self-assembled later sterically
prevents contaminants from arriving at the nanogap region of maximum field enhancement.
FIG. S8: (A) Raman spectra for clean bowtie (blue) and clean bowtie after a few minutes exposed
to the air (green). This change in the Raman spectrum is indicative of contamination for surface
absorbed molecules from the air. (B) Raman spectra for a clean bowtie showing the onset of a
contaminant signal at 900 cm−1 as time progresses. (C) Waterfall plot showing the extremely strong
blinking observed for adsorbed contamination. The fluctuations are much larger than the those
observed for dense coverage of pMA, pMBA, or P3HT. Notice the scale relative to the 520 cm−1
Si peak seen at t = 340 s.
V. FDTD CALCULATIONS
The optical properties of the bowtie structure were calculated using the Finite-Difference
Time-Domain method (FDTD) using a Drude dielectric function with parameters fitted
to the experimental data for gold. This fit provides an accurate description of the optical
properties of gold for wavelengths larger than 500 nm [S1]. These calculations do not account
for reduced carrier mean free path due to surface scattering in the metal film, nor do they
include interelectrode tunneling. However, such effects are unlikely to change the results
significantly.
The bowtie is modeled as a two finite triangular structures as illustrated in Fig. 4A
of the manuscript. Our computational method requires the nanostructures to be modeled
to be of finite extent. The plasmon modes of a finite system are standing modes with
frequencies determined by the size of the sample and the number of nodes of the surface
charge distribution associated with the plasmon. For an extended system such as the bowties
manufactured in this study, the plasmon resonances can be characterized as traveling surface
waves with a continuous distribution of wavevectors.
A series of calculations of bowties with increasing length reveals that the optical spectrum
is characterized by increasingly densely spaced plasmon resonances in the wavelength regime
500-1000 nm and a low energy finite-size induced split-off state involving plasmons localized
on the outer surfaces of the bowtie. For a large bowtie, we expect the plasmon resonances
in the 500-1000 nm wavelength interval to form a continuous band [S2].
The electric field enhancements across the bowtie junction for the plasmon modes within
this band are relatively similar with large and uniform enhancements in the range of 50-150.
The magnitudes of the field enhancements were found to increase with increasing size of the
bowtie structure. For instance, the maximum field enhancement factor was found to be 115
for a 200 nm bowtie (Each half of the bowtie is modeled as a truncated triangle 200 nm
long.) and 175 for a 400 nm bowtie. Our use of a finite gridsize also underestimates the
electric field enhancements[S3]. Thus our calculated electric field enhancements are likely
to significantly underestimate the actual electric field enhancements in the experimentally
manufactured bowties.
For a perfectly symmetric bowtie, significant field enhancements are only induced for
incident light polarized across the junction. If the mirror symmetry is broken, for instance
by making one of the structures thicker or triangular, large field enhancements are induced
for all polarizations of incident light.
To investigate the effects of nanoasperities, FDTD calculations were performed for a
bowtie with two semi-spherical protrusions in the junction as shown in Fig. 4 of the main
text, and Figs. S5-S7 of the Supporting Online Material. As expected, the presence of these
protrusions does not influence the optical spectrum. However, the local field enhancements
around the protrusions become very large, typically three or four times higher than for the
corresponding structure without the defect. The physical mechanism for this increase is
an antenna effect caused by the coupling of plasmons localized on the protrusion with the
extended plasmons on the remaining bowtie structure [S4].
FIG. S9: Maps of FDTD-calculated |E| for the 1535 nm mode indicated in the main manuscript’s
Fig. 4A. Color scale is logarithmic in |E|/|Einc|. Illumination direction is normal incidence, with
electric field polarization oriented horizontally in (A)-(C). Maximum field enhancements are shown.
(A) Overall view. (B) Close-up of interelectrode gap showing asperities. (C) Side-view of section
indicated in (B) in red. (D) Side view of section indicated in (B) in blue.
FIG. S10: Maps of FDTD-calculated |E| for the 937 nm mode indicated in the main manuscript’s
Fig. 4A. Color scale is logarithmic in |E|/|Einc|. Illumination direction is normal incidence, with
electric field polarization oriented horizontally in (A)-(C). Maximum field enhancements are shown.
(A) Overall view. (B) Close-up of interelectrode gap showing asperities. (C) Side-view of section
indicated in (B) in red. (D) Side view of section indicated in (B) in blue.
FIG. S11: Maps of FDTD-calculated |E| for the 746 nm mode indicated in the main manuscript’s
Fig. 4A. Color scale is logarithmic in |E|/|Einc|. Illumination direction is normal incidence, with
electric field polarization oriented horizontally in (A)-(C). Maximum field enhancements are shown.
(A) Overall view. (B) Close-up of interelectrode gap showing asperities. (C) Side-view of section
indicated in (B) in red. (D) Side view of section indicated in (B) in blue.
VI. ENHANCEMENT ESTIMATE
To estimate an enhancement based on the data of Fig. 3 in the main text, it was necessary
to understand the effective count rate per molecule of Raman scattering from bulk pMA in
our measurement setup. This requires knowing the effective volume probed by the WITec
system when the laser is focused on a bulk pMA crystal.
The full-width-half-maximum (FWHM) of the laser spot size was found to be 575 nm.
This was determined by measuring the count rate of the Rayleigh scattering peak (at zero
wavenumbers) as a function of position as the beam was scanned over the edge of a Au film
on a Si substrate. Averaging 16 such scans, the Rayleigh intensity was fit to the form of an
integrated gaussian to determine the FWHM of the gaussian beam. The 575 nm figure is
likely an overestimate due to systematic noise in the flat regions of the fit.
For a gaussian beam with intensity of the form ∝ e−
2σ2 , the FWHM = 2
2 ln 2σ. The
effective radius of an equivalent cylindrical beam is 2σ, or 346 nm in this case. The effective
confocal depth [S5] was determined by measuring the 520 cm−1 Si Raman peak as a function
of vertical displacement of a blank substrate. The effective depth profile was determined
by numerical integration of the Si data using matlab. The effective volume probed by the
beam is 1.92× 10−12 cm3. From the bulk properties of pMA, this corresponds to 1.09× 1010
molecules.
The count rate for the bulk pMA 1077 cm−1 line, corrected by the ratio of (Si SERS
rate/Si bulk rate) to accomodate for the difference in laser powers, is 46 counts/s, compared
with 203 counts/s for the SERS data of Fig. 3. This leads to the enhancement estimate
quoted in the main text of 5× 108.
[S1] Oubre, C.; Nordlander, P. J. Phys. Chem. B 108, 108, 17740-17747.
[S2] Nordlander, P.; Le, F. Appl. Phys. B 2006, 84, 35-41.
[S3] Oubre, C.; Nordlander, P. J. Phys. Chem. B 2005, 109, 10042-10051.
[S4] Hao, F.; Nehl, C. L.; Hafner, J. H.; Nordlander, P. Nano Lett. 2007, 7,
10.1021/nl062969c.
[S5] Cai, W.B.; Ren, B.; Li, X. Q.; Shi, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Surf. Sci.
1998, 406, 9-22.
	References
	Continuum background
	Dependence on incident polarization
	Dependence on solution concentration
	Detection of adsorbed contaminants
	FDTD calculations
	Enhancement estimate
