Supporting information
Reactive ion etching-assisted
surface-enhanced Raman scattering
measurements on the single nanoparticle level
Si-Yi Wang,1 Xiang-Xu Jiang,1 Ting-Ting Xu,1,2 Xin-Pan Wei,1 Shuit-Tong Lee,
and Yao He*1
1
Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for
Carbon-Based Functional Materials & Devices, and Devices Collaborative Innovation
Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu
215123, China
2
Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong,
Hong Kong SAR, China and Department of Physics and Materials Science, City University of
Hong Kong, Hong Kong SAR, China
E-mail: yaohe@suda.edu.cn (Y. H)
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1. Experimental methods
The silicon wafer is well patterned with microhole markers via standard
photolithography technique and reactive ion etching (RIE) process (Figure S1 shows a
schematic of this procedure).1,2 Basically, the silicon wafer (p-(100), 0.01-0.05 Ω•cm)
or quartz glass slide was cleaned with acetone by ultrasonic treatment for 10 min, and
then washed with Milli-Q water (Millipore) for three times. Afterwards, the Si wafer
was immersed in an oxidant solution containing H2SO4 (98%) and H2O2 (30%,) with a
volume ration of 3:1 for 20 min, and then rinsed with Milli-Q water for three times to
remove organics. The as-prepared samples were further immersed in ethanol by
ultrasonic treatment for 10 min, followed by rinsed with Milli-Q water (Millipore) for
three times, and finally dried under a stream of N2, producing the cleaned silicon
wafer for following photolithography and RIE treatment.
In particular, the pattern on the as-prepared silicon wafer was fabricated by
photolithography using a positive photoresist layer (AR-P 5350, GermanTech Co.,Ltd)
with thickness of 1 μm via spin-coating procedure. Afterwards, the resultant sample
was etched in a RIE system (Plasmalab 80 Plus, Oxford Instruments) with O2 and SF6
as etching gases to produce the silicon wafer with microhole arrays. The detailed
parameters are as follow, radial frequency (RF) power = 120 W, flow rate = 6 sccm
(O2) and 36 sccm (SF6), pressure = 275 mTorr, time = 10 min. Thereafter, the sample
was further treated with acetone solution, and then rinsed with Milli-Q water to fully
remove the residual photoresist layer. In the final step, oxygen plasma (Ion 40, PVA
TePla) (power = 300 W, flow rate = 200 sccm, time = 5 min) was employed to
eliminate the organic residue and solvent, yielding the cleaned silicon wafer with
microholes pattern.
For preparation of patterned glass substrates, the quartz glass slide was used
instead of the silicon wafer. Ar and CHF3 were employed as the etching gases. The
parameters for RIE treatment are as follow, RF power = 150 W, flow rate = 30 sccm
(Ar) and 20 sccm (CHF3), pressure = 30 mTorr, time = 30 min. Other manipulations
were identical to those of patterned silicon wafers.
The gold nanoparticles (AuNPs) were prepared using our previously reported
protocal.3 In details, the silicon wafer was washed with acetone and Milli-Q water for
three times, followed by immersing in an mixture solution containing H2SO4 (98%)
and H2O2 (30%) (volume ratio: 3:1) for 30 min. Afterwards, the resultant wafer was
washed by Milli-Q water, and then treated with hydrogen fluoride (HF, 5%) to
produce the H-terminated silicon wafer. Thereafter, the as-prepared wafer was placed
into a freshly prepared reduction solution containing silver nitrate (AgNO3) and HF
(10%) with slowly stirring for 3 min, followed by treatment of the mixture solution
containing 10% HF and 0.4 M H2O2, producing the aligned silicon nanowires (SiNWs)
with 100~200 nm in diameter and ~4.5 μm in length. The resultant SiNWs were
reacted with HAuCl4 (1 mM) for 10 min at 60 ̊C, yielding the AuNPs through a
reduction reaction between Au+ and Si-H bonds. The as-prepared AuNPs was
collected from the precipitation via centrifugation (5000 rpm, 15 min), and then was
ready for following experiments.
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Based on previously reported protocol [Xia et al. Angew. Chem. Int. Ed. 50,
5473 (2011); J. Phys. Chem. Lett. 1, 696 (2010); Phys. Chem. Chem. Phys. 15, 5400
(2013); etc], the as-prepared AuNPs were first drop-cast over the microholes patterned
silicon substrate and allowed to dry under ambient conditions. Noted that, to realize
deposition of single AuNP in each hole, the concentration of prepared AuNPs was
extremely low (e.g. 1 nM), similar to that (1-5 nM) reported in previous reports.
Thereafter, the individual gold nanoparticles on patterned silicon and glass substrates
were characterized by scanning electronic microscopy (SEM) and re-located the
relative position for surface-enhanced Raman scattering (SERS) measurements. The
SEM overview images were recorded using scanning electron microscopy (FEI
Quanta 200F) equipped with energy-dispersive X-ray (EDX) spectroscopy. The SERS
analysis was performed using a HR800 Raman microscope instrument produced by
HORIBA Jobin Yvon (France). The instrument is equipped with a standard 633 nm
HeNe 20mW laser, and other two external lasers (514 nm Ar ion laser and 785 nm
Laser diode). Four kinds of objectives (10 × NA 0.25, 50 × NA 0.7, 100 × NA 0.9, and
long work distance) are available in the instrument. In the range between 450 nm and
850 nm, the wavenumber accuracy is ± 1 cm-1. Under normal conditions of
temperature stability (± 1 oC), the repeatability is 1 pixel with a standard
charge-coupled device (CCD). LabSpec 5 software is used for Raman data acquisition
and data analysis. Enhancement factor (EF) calculation is similar to the established
methods described elsewhere. In details, the EF value was calculated using the
following equation,1
EF = (Isers × Nbulk) / (Ibulk × Nsers) (1)
where Isers and Ibulk represent the intensities of the same band of the SERS spectra
and the Raman spectra (non-SERS) respectively; Nsers and Nbulk represent the number
of molecules on the substrates within the laser spot. Herein for SERS examination, a
certain volume Vsers R6G aqueous solution was dispersed to an area of Ssers at a
concentration of Csers on the patterned silicon/glass substrates with individual AuNPs.
For non-SERS Raman spectra, a certain volume Vbulk R6G solution was dispersed to
an area of Sbulk at a concentration of Cbulk on a clean Si wafer or glass slide. Both the
substrates were dried in the air. The foregoing equation thus becomes,
EF = (Isers / Ibulk) × (Ssers × Vbulk × Cbulk) / (Sbulk × Vsers × Csers) (2)
Raman measurements were conducted under identical experimental conditions
(laser wavelength, laser power, microscope objective/lenses, spectrometer, etc.). In
our experiment, 10 μL of 1 × 10-6 M Rhodamine 6G (R6G) aqueous solution (the
detection limit of R6G) was dispersed to the single AuNP at Si substrate and the
single AuNP at glass substrate, respectively, while 10 μL 1 × 10-2 M R6G solution was
dispersed to the bare Si wafer and bare glass slide, respectively. The beam size (d) of
the laser spot was 700 nm (d = 1.22 × (λ/N.A) = 700 nm (the 1.22 factor accounts for
the deviation of the laser beam from a perfect Gaussian profile)). As a result, the area
of laser beam is 0.38 μm2 (Sbulk). Besides, the penetration depth of 514 nm laser beam
is about 2 μm. Hence, Vsers = 0.77 μm3. When determining Nsers in the illuminating
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volume of our laser, we assumed that R6G molecules were adsorbed as a monolayer
on the surface of a single AuNP (1 R6G molecule occupies 1.5 nm2). Thus, Ssers and
Nsers equal to 0.03 μm2 and 2.0 × 104, respectively. The R6G molecules within a laser
spot on bulk substrate is determined by the follow formula： Nbulk = Cbulk × VR6G × NA
× Sbulk / Ssubstrate. As a result, Nbulk = 2.3 × 108. The intensities for the C-C stretching
mode at 1507 cm−1 are 1453 a.u. for the single AuNP dispersed on the silicon
substrates, and 109 a.u. for the bare silicon substrate, respectively. We calculated the
average EF from the SERS substrate point of view under plausible assumption that all
the probe molecules within the laser spot are illuminated and contribute to the SERS
spectra and the Raman spectra. Therefore, the average EFs are calculated to be 2.8 ×
105 for the single AuNP at Si substrate. Similarly, the intensities for the C-C stretching
mode at 1507 cm−1 are 243 a.u. for the single AuNP at glass substrate, and 70 a.u. for
the bare glass substrate, respectively. Therefore, the average EFs are calculated to be
3.4 × 104 for the single AuNP at glass substrate.
The simulations were carried out using the commercial Finite Difference Time
Domain (FDTD) software package Lumerical® FDTD Solutions 8.5. The geometries
of the AuNPs and the substrate were estimated according to the SEM images (Figure
1f-h). Cross section views of simulation region from XY and XZ plane are shown in
Figure 4a and b, respectively. Complex electric permittivity of the silicon, glass and
gold was adapted from the literatures.4,5 To avoid simulation artifacts from the
meshing of the thin dielectric layer, a 2 nm air gap was used between the substrate and
the single AuNP. The simulation volume was 2 μm (x) by 2 μm (y) by 2 μm (z) with
periodic boundaries along the x-axis and perfectly matched layer boundaries along the
y- and z-axes. A plane wave of 514 nm propagating along the -z direction was used as
the excitation source.
Reference:
1Z.
Y. Jiang, X. X. Jiang, S. Su, X. P. Wei, S. T. Lee, and Y. He, Appl. Phys. Lett. 100, 203104
(2012).
2K.
Seeger and R. E. Palmer, Appl. Phys. Lett. 74, 1627 (1999).
3X.
P. Wei, S. Su, Y. Y. Guo, X. X. Jiang, Y. L. Zhong, Y. Y. Su , C. H. Fan, S. T. Lee, and Y. He,
Small 9, 2493 (2013).
4E.
D. Palik, Handbook of Optical Constants of Solids (Elsevier, Amsterdam, 1998).
5P.
B. Johnson, R. W. Christy, Phys. Rev. B 6, 4370 (1972).
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2. Additional data
Figure S1. Schematic representation for fabricating patterned silicon/glass substrate
with microhole arrays.
Figure S2. Measurement setup for the RIE-assisted correlated SERS with SEM
(RIE/SERS/SEM) strategy.
Figure S3. Extinction spectra and electromagnetic field (EM-field) profiles of a free
single AuNP, as well as a single AuNP on glass and silicon substrate by FDTD
simulation.
Figure S4. Extinction spectra and EM-field profiles of a single AuNP on plane
silicon and silicon microhole with different diameters by FDTD simulation.
Figure S5. SERS spectra of R6G modified with three individual AuNPs located in
each microhole with different position.
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Figure S1. Schematic representation for fabricating the patterned silicon/glass
substrate with microhole arrays.
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Figure S2. Measurement set-up for the RIE/SERS/SEM strategy. In particular,
photolithography and RIE were serially employed for patterning microholes (diameter:
~6 μm) array on the clean substrate (Step 1 and 2), followed by deposition of AuNPs
in the resultant microholes. Afterwards, SEM characterization was carried out to
identify the exact position of each single AuNP in the microholes (Step 3). In the
fourth step, e-beam-induced amorphous carbon contamination was fully removed by
using plasma cleaner. The resultant sample was then modified with R6G molecules
and finally employed for SERS investigation in the fifth step (beam size: ~700 nm).
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Figure S3. (a) Extinction spectra and (c-e) EM-field profiles of a free single AuNP, as
well as a single AuNP on glass and silicon substrate by FDTD simulation. (b)
Schematic illustration of the FDTD simulation. The blue box represents Discrete
Fourier transform (DFT) monitor, the blue arrows represent a plane incident wave by
using the total field/scattered-field (TFSF) technique, the grey box refers to perfectly
matched layer (PML) as the simulation boundary. The simulation results of EM-field
profiles (XZ plane) in the blue box region are shown in (c) free single AuNP, (d)
single AuNP on glass substrate and (e) single AuNP on silicon substrate (incident
wavelength: 526 nm).
As show in Figure S3 (a), the localized surface plasmon resonance (LSPR) peak
(526 nm) of the single AuNP on glass substrate is almost the same as the counterpart
of the free single AuNP, while there is an another plasmon mode at 666 nm which
may possibly due to adjacent dielectric substrate induced energy splitting effect [Nano
Lett. 9, 2188 (2009); J. Phys. Chem. C 114, 7302 (2010)]. In comparison, a higher
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LSPR peak is observed when the AuNP is dispersed on the silicon substrate.
Moreover, the enhancement in EM-field profiles (XZ plane) of a single AuNP on
silicon (e) is much stronger than those of a free single AuNP (c) and a single AuNP on
glass (d). These results indicate that hot spots can be efficiently coupled through the
semiconducting silicon substrate, leading to the highest EM-field enhancement value,
well consistent with our experiment data (Figure 3) and previously reported results
[Nano Today, 6, 122 (2011); Appl. Phys. Lett. 100, 203104 (2012); Small 9, 2493
(2013); etc]. Besides, compared to that (~2.1) of glass, the larger permittivity value of
silicon (εs, ~17.5) is considered as another significant contributor to the observed
silicon substrate-induced EM-field enhancement (e.g., Nordlander et al. has revealed
that increase of the permittivity of substrate facilitates enhancement of EM-filed.
[Nano Lett. 9, 2188 (2009); J. Phys. Chem. C 114, 7302 (2010); etc.]).
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Figure S4. (a) Extinction spectra and (c-e) EM-field profiles of a single AuNP on
plane silicon and silicon microhole with different diameters (i.e., 1 and 2 μm) by
FDTD simulation. (b) Schematic illustration of the FDTD simulation. The blue box
represents discrete Fourier transform (DFT) monitor, the blue arrows represent a
plane incident wave by using the total field/scattered-field (TFSF) technique, the grey
box refers to perfectly matched layer (PML) as the simulation boundary. The
simulation results EM-field profiles (XZ plane) in the blue box region are shown in (c)
single AuNP on plane silicon, (d) single AuNP on 1 μm silicon microhole and (e) 2
μm silicon microhole (incident wavelength: 526 nm). As shown in Figure S4(a), the
extinction spectra and the EM-field profiles (XZ plane) of the AuNP dispersed on the
plane silicon and silicon microholes are nearly identical, indicating that effect of
microhole is negligible in the FDTD simulation.
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Figure S5. SERS spectra of R6G modified with three individual AuNPs located in
each microhole with different position. In particular, the SERS spectra of R6G
modified with three individual AuNPs located in each microhole with different
position are similar, with an adaptable variability of SERS intensity from hole to hole.
Typically, the relative standard deviation (RSD) values of six Raman peaks of R6G
are as follows: 26.7% (1186 cm-1), 28.5% (1310 cm-1), 17.9% (1362 cm-1), 14.1%
(1507 cm-1), 12.6% (1575 cm-1), and 15.2% (1647 cm-1). The intensities of all the
three SERS spectra (blue, green, and red) of R6G-modified AuNPs dispersed in
silicon substrate are significantly stronger that those of R6G-modified AuNPs
dispersed in glass substrate (black line), demonstrating silicon wafers as superior
SERS substrates.
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