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Combined atomic force microscopy-Raman mapping of electric field enhancement and surface-enhanced Raman scattering hot-spots for nanosphere lithography substrates

[+] Author Affiliations
Claire S. Sweetenham, Ioan Notingher

University of Nottingham, Nanoscience Group, School of Physics and Astronomy, University Park, Nottingham, NG7 2RD, United Kingdom

J. Nanophoton. 5(1), 059504 (June 01, 2011). doi:10.1117/1.3595345
History: Received January 21, 2011; Revised May 05, 2011; Accepted May 09, 2011; Published June 01, 2011; Online June 01, 2011
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Surface-enhanced Raman spectroscopy (SERS) substrates formed by nanosphere lithography were investigated for their spatial distribution and magnitude of electric field enhancement. An integrated atomic force microscopy and Raman micro-spectroscopy system was used to establish, with high accuracy, the correlation between the local SERS mappings and substrate topography. Using a monolayer of rhodamine 6G as a probe of the local electric field, the high resolution Raman mappings, showed that the highest electric field enhancement originates from the metallic nanostructures rather than the gaps between them. The enhancement factor of the substrates is calculated from Raman spectra of the substrates covered in a monolayer of p-aminothiophenol and spatial measurements, giving a value on the order of 105. The experimental results were confirmed by theoretical calculations using the finite element method.

Figures in this Article

In the past decade, surface enhanced Raman spectroscopy (SERS) has become a widely used derivative method of Raman spectroscopy. The enhancement it provides mainly originates from increased electric fields that arise from the plasmon resonances of rough metallic surfaces.12 Nanosphere lithography (NSL) is a common technique that has been extensively used to fabricate periodic arrays of nanoparticles upon solid surfaces3 and more recently developed for optical applications.4 Typically, NSL substrates are prepared via the evaporation of a metallic thin film over hexagonally close-packed spheres that are later removed. This type of substrate is now a popular choice for SERS studies, as it can be entirely custom-made to requirements.

NSL substrates are known to provide reliable enhancement of Raman scattering in the range ∼104 to 108.5 However, the distribution of this enhancement and the location of so-called “hot-spots” is not as well established. Theoretical models showed that the SERS activity of these substrates is based on coupling between the excitation laser and the plasmon resonances of metallic surfaces. However, it is yet unclear whether in the case of NSL substrates potential electromagnetic coupling between adjacent metallic nanostructures play any role in the enhancement. It is well established that if two nanoparticles are in close proximity, electron oscillations in a particle can be affected by the neighboring structure producing SERS hot-spots.4 One of the main experimental difficulties in imaging SERS hot-spots is the lack of ability to precisely locate the source of the effect as a function of geometry, as it requires direct comparison of spatial measurements at the nanoscale with chemical mapping. The integration of atomic force microscopy (AFM) into a Raman micro-spectroscopy system can provide the required comparisons between the topographical structure of a SERS substrate and the distribution of electric field enhancement across it. Such experiments have been recently reported for imaging and identification of hot-spots for SERS substrates consisting of dielectric nanospheres covered with a thin metallic film.6 Hot-spots were detected in the gaps between the nanospheres, where coupling between the adjacent spheres led to enhanced electric field and increased Raman signal. Although the spatial resolution of the Raman measurements (∼300 nm) is not sufficient for detailed quantitative measurements of the electric field distribution between nearest metal nanoparticles (100 to 200 nm), direct comparison between Raman maps and AFM topography images can provide information whether maximum enhancement occurs at the surface of the metal structures or between such structures.

In this study, we report mapping of electric field enhancement and location of hot-spots in NSL substrates by comparing experimental SERS mappings and topographical AFM images recorded from the same regions of the substrates. The main advantage of the NSL SERS substrates is that they are optically transparent and thus allow investigations of samples on an inverted optical Raman microscope, which is of particular importance for biological applications as samples need to be maintained in an aqueous environment.

SERS measurements were performed on a purpose-built instrument consisting of an inverted Raman microscope (Olympus IX71) and integrated AFM with a piezoelectric xy-stage (Nanowizard II, JPK Instruments).7 The instrument was equipped with a 20 μW 532 nm continuous wave laser (Laser 2000, UK), a water-immersion 1.2NA 60 × objective (Olympus), and a Czerny–Turner Raman spectrometer with a 1800 l/mm ruled grating and a back-illuminated CCD (Andor Technologies, United Kingdom). The AFM was used in tapping mode with rectangular silicon probes of 70 kHz resonance frequency and 2 N/m spring constant (Veeco).

Large-scale, 2D arrays of truncated tetrahedral nanostructures were formed to create NSL SERS-active substrates.89 An evaporation mask of a nanosphere monolayer was prepared using monodisperse polystyrene spheres with a diameter of 1002 nm, 10 wt% in water (Varian, Inc.) mixed with an equal amount of ethanol (Sigma Aldrich, United Kingdom). About 4 μl of the prepared solutions of polystyrene spheres was applied to the surface of a silicon (111) substrate (PI-KEM, United Kingdom), which was slowly immersed in a 15-cm diameter beaker filled with ultrapure water. Then, 4 μl of 2% sodium dodecyl sulphate (Sigma Aldrich, UK) solution was added to form a large area of highly ordered hexagonally close-packed monolayer of polystyrene spheres that was slowly lifted off the surface of the water onto 22-mm diameter glass coverslips. A ∼25-nm thin film of silver was thermally evaporated in vacuum (Edwards) onto these coverslips at a pressure of 10−7 mbar and an evaporation rate of 0.3 to 0.4 nm/s. Finally, the polystyrene spheres were removed by immersion of the substrates in tetrahydrofuran (Sigma Aldrich, United Kingdom) and an ultrasonic bath for 20 s.

The SERS activity of these substrates was assessed with monolayers of rhodamine 6G (R6G) and p-aminothiophenol (p-ATP) (Sigma Aldrich, United Kingdom) as local probes of the electric fields. Both of these molecules have been commonly used in SERS studies.1012 R6G is a molecule that attaches to the entire surface of the substrate, while p-ATP strongly bonds to the metal regions only due to its thiol group (–SH). The SERS substrates were immersed in 1 mM R6G and 10−4 M p-ATP methanol solutions for 2 h and then rinsed in methanol to ensure a single molecular layer was adsorbed.

The size of individual nanostructures and the extent of ordering across the substrate is apparent from a standard topographical AFM image and line profile [Fig. 1]. A firmer indication of this ordering can be achieved from measuring a 2D fast Fourier transform (FFT) from this image [Fig. 1]; the concentric periodic structure reveals the length scale of the nanostructures within the substrate and more importantly high hexagonal order. This data demonstrates the suitability and potential of these substrates for activating SERS, however, a quantitative measure of this enhancement can be derived from further analysis with Raman micro-spectroscopy, calculations, and simulations.

Grahic Jump LocationF1 :

NSL substrates characterized for SERS. (a) Large AFM image and line profile of a typical substrate. (b) 2D FFT of the presented image. (c) 3D image of an array of truncated tetrahedral silver nanostructures.

The SERS activity of the substrates was investigated with monolayers of R6G as local probes of the electric field. A typical topographical image and chemical mapping recorded from the same area of the substrate is shown in Fig. 2. The prepared substrates were initially imaged with the AFM to locate a large-ordered area of at least 15 μm2 containing a monolayer of molecules, ideal for performing SERS mapping. Within this image, a 1- to 2-μm region was selected to record a chemical mapping. Mappings were constructed by automated scanning of the sample through the focus of the laser, with a spatial resolution of ∼300 nm. The sample was moved in a raster pattern with step sizes of 50 to 100 nm, acquiring a SERS spectrum at each position at a rate of 2 s per pixel. Chemical mappings of R6G monolayers were created from the Raman band at 1363 cm−1 (aromatic C–C and C–N stretching). The area of this Raman band in each spectrum was calculated after subtraction of a local linear baseline and used to build a spectral mapping one pixel at a time. This mapping was then related back to the original AFM image, enabling direct correlation between SERS nanostructures and Raman scattering to reveal the exact position and orientation of enhancement across the substrate.

Grahic Jump LocationF2 :

AFM image of R6G on 1002 nm substrates with SERS spectra and mapping recorded across the highlighted area. Labeled spectra correspond to the scattering measured at the position of the same labeling in the mappings.

The regions of higher SERS intensity correspond to the position and orientation of the silver nanostructures while weaker SERS scattering was detected in between these features. This suggests that for this type of substrate the SERS effect arises from the metallic regions themselves rather than the gaps between them. These results indicate that the electromagnetic coupling between the silver truncated tetrahedrons is negligible and enhancement of electric field is a result of excitation of surface plasmon resonances in individual pyramids.

The magnitude of electric field enhancement across the substrates was investigated with monolayers of p-ATP. For quantitative analysis, p-ATP is preferable over R6G as it does not show any resonances when excited at 532 nm, thus ensuring that the enhancements of the Raman signal is exclusively attributed to SERS. Also, since SERS hot-spots have been identified to be located on the silver truncated tetrahedral nanostructures (Fig. 2), the high chemical affinity to metal of the thiol groups (–SH) of p-ATP ensures that the molecule is exposed to the hot-spots in a single monolayer.

The enhancement factor (EF) of the substrates was calculated by comparing the area of a p-ATP Raman band from the SERS spectra (ISERS) with the area of the same band of unenhanced Raman scattering from a neat solution of the molecule on a bare glass coverslip (IRS). This value was calculated from the “point of view of the substrate,”13 given by Display Formula

1EF=ISERS/(μMμSAM)IRS/(cRSHeff),
where μM is the surface density of the individual nanostructures producing the enhancement, μS is the surface density of molecules on the metal, AM is the surface area of a metallic nanostructure, cRS is the concentration of the solution used for the non-SERS measurements, and Heff is the effective height of the scattering volume of the Raman spectroscopy system.

A typical SERS mapping and corresponding spectra of p-ATP monolayers adsorbed on a substrate are shown in Fig. 3. Two different p-ATP Raman bands and several concentrations of neat solution cRS were used for the intensities ISERS and IRS and then averaged to give the final EF of the substrates. From geometry, μM and AM were calculated to be 2.30 × 10−6 nm−2 and 44227 nm2, respectively.1415 Kinetic studies estimate a coverage area of 22 Å per molecule for a close-packed monolayer of p-ATP,16 giving 4.5 nm−2 for the value of μS for our setup. Heff was directly measured from the instrumental setup by depth profiling, giving a value of 21.2 μm. This results in a value for EF of about 1.6 × 105 for these SERS substrates, which is in good agreement with similar substrates.5 In general, conventional SERS is considered to be capable of an enhancement on the order of 105–106.17

Grahic Jump LocationF3 :

SERS spectra and mapping of p-ATP on 1002 nm substrates, with the spectra used to calculate its enhancement factor (Raman bands used are highlighted).

The distribution and magnitude of electric field enhancement across the SERS substrates was modeled and further analyzed with finite element software (COMSOL Multiphysics), a numerical technique for finding approximate solutions of the Maxwell's equations by dividing a geometric model into domains and smaller elements. To model the substrates, 2D sections through a 3D array of silver truncated tetrahedrons on a glass support in air were constructed in a frequency-domain study. The substrate was positioned parallel to the xz-plane while the silver nanostructures were placed along the y-axis. A transverse electromagnetic wave propagating along the y-axis was considered with incident electric and magnetic fields in the x and z directions, respectively. Each domain was given the appropriate material properties, including optical constants for air, glass, and silver corresponding to the appropriate wavelength of incident light.18 Appropriate boundary conditions were applied at the edge of the system and the silver nanostructures and perfectly matched layers were also employed in the model to simulate open boundaries. Maxwell's equations are then solved for each of these elements to obtain the 2D electric field distribution across the system (Fig. 4). These theoretical results support our experimental SERS mappings. They demonstrate that the SERS effect occurs on the metallic surfaces themselves rather than in the gaps between them. The model also indicates that the greatest enhancement of the electric field occurs at the vertices of the nanostructures, which is in agreement with reported theoretical studies of these substrates.1920 Using this model, the value calculated for the Raman enhancement was 1.35 × 104, which is about an order of magnitude lower than the one calculated from experimental data. Such small discrepancies in the enhancement values calculated by theoretical models are expected, since the model does not consider the 3D nature of the real substrates or any potential enhancement through chemical mechanism.

Grahic Jump LocationF4 :

Finite element model with defined domains, boundary conditions, and PML and solution, displaying the resultant electric field (Vm−1).

An integrated AFM and Raman spectroscopy microscope was used to investigate the distribution and magnitude of electric field enhancement and the location of hot-spots across SERS substrates formed by NSL. Monolayers of R6G were deposited on the substrates to measure the distribution of electric field enhancement across them. Direct correlation of chemical mappings with topographical images of the same areas indicated that the highest SERS signal occurs on the silver nanostructures themselves rather than in the gaps between them, suggesting that the electromagnetic coupling between the silver truncated tetrahedral nanostructures is negligible. Monolayers of p-ATP were deposited on the substrates to measure the magnitude of electric field enhancement across them. Spectra from these samples were combined with unenhanced Raman spectra of the same molecule and spatial measurements of the substrates to calculate their enhancement factor. This was found to be on the order of 105, which is reasonable for this type of SERS substrate and was supported by finite element simulations.

We thank the UK Engineering and Physical Sciences Research Council ( UNSPECIFIED EP/P502632/1 ) for financial support.

Fleischmann  M., , Hendra  P. J., , and McQuillan  A. J., “ Raman spectra of pyridine adsorbed at a silver electrode. ,” Chem. Phys. Lett.. 26, , 163–166  ((1974)).
Jeanmaire  D. L., and Van Duyne  R. P., “ Surface Raman electrochemistry part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode. ,” J. Electroanal. Chem.. 84, , 1–20  ((1977)).
Hulteen  J. C., , Treichel  D. A., , Smith  M. T., , Duval  M. L., , Jensen  T. R., , and Van Duyne  R. P., “ Nanosphere lithography: Size-tunable silver nanoparticle and surface cluster arrays. ,” J. Phys. Chem. B. 103, , 3854–3863  ((1999)).
Haynes  C. L., and Van Duyne  R. P., “ Nanosphere lithography: A versatile nanofabrication tool for studies of size-dependent nanoparticle optics. ,” J. Phys. Chem. B. 105, , 5599–5611  ((2001)).
Haes  A. J., , Haynes  C. L., , McFarland  A. D., , Schatz  G. C., , Van Duyne  R. P., , and Zou  S., “ Plasmonic materials for surface-enhanced sensing and spectroscopy. ,” MRS Bull.. 30, , 368–375  ((2005)).
Farcau  C., and Astilean  S., “ Mapping the SERS efficiency and hot-spots localization on gold film over nanospheres substrates. ,” J. Phys. Chem. C. 114, , 11717–11722  ((2010)).
Sweetenham  C. S., and Notingher  I., “ Raman spectroscopy methods for detecting and imaging supported lipid bilayers. ,” Spectroscopy. 24,  (1–2 ), 113–117  ((2010)).
Rybczynski  J., , Ebels  U., , and Giersig  M., “ Large-scale, 2D arrays of magnetic nanoparticles. ,” Colloids Surf., A. 219, , 1–6  ((2003)).
Baia  L., , Baia  M., , Popp  J., , and Astilean  S., “ Gold films deposited over regular arrays of polystyrene nanospheres as highly effective SERS substrates from visible to NIR. ,” J. Phys. Chem. B. 110, (47 ), 23982–23986  ((2006)).
Schmidt  J. P., , Cross  S. E., , and Buratto  S. K., “ Surface-enhanced Raman scattering from ordered Ag nanocluster arrays. ,” J. Chem. Phys.. 121, (21 ), 10657–10659  ((2004)).
Zhao  J., , Jensen  L., , Sung  J., , Zou  S., , Schatz  G. C., , and Van Duyne  R. P., “ Interaction of plasmon and molecular resonances for Rhodamine 6G adsorbed on silver nanoparticles. ,” J. Am. Chem. Soc.. 129, , 7647–7656  ((2007)).
Farcau  C., and Astilean  S., “ Probing the unusual optical transmission of silver films deposited on two-dimensional regular arrays of polystyrene microspheres. ,” J. Opt. A, Pure Appl. Opt.. 9, , 345–349  ((2007)).
Le Ru  E. C., , Blackie  E., , Meyer  M., , and Etchegoin  P. G., “ Surface enhanced Raman scattering enhancement factors: A comprehensive study. ,” J. Phys. Chem. C. 111, (37 ), 13794–13803  ((2007)).
Hulteen  J. C., and Van Duyne  R. P., “ Nanosphere lithography: A materials general fabrication process for periodic particle array surfaces. ,” J. Vac. Sci. Technol. A. 13, , 1553–1558  ((1995)).
Haynes  C. L., and Van Duyne  R. P., “ Plasmon-sampled surface-enhanced Raman excitation spectroscopy. ,” J. Phys. Chem. B. 107, , 7426–7433  ((2003)).
Mohri  N., , Inoue  M., , Arai  Y., , and Yoshikawa  K., “ Kinetic study on monolayer formation with 4-aminobenzenethiol on a gold surface. ,” Langmuir. 11, , 1612–1616  ((1995)).
Schatz  G. C., , Young  M. A., , and Van Duyne  R. P., “ Electromagnetic mechanism of SERS. ,” Top. Appl. Phys.. 103, , 19–45  ((2006)).
Palik  E. D.,  Handbook of Optical Constants of Solids. ,  Academic ,  New York  ((1998)).
Lin  T-H., , Linn  N. C., , Tarajano  L., , Jiang  B., , and Jiang  P., “ Electrochemical SERS at periodic metallic nanopyramid arrays. ,” J. Phys. Chem. C. 113, , 1367–1372  ((2009)).
Haes  A. J., , Zou  S., , Schatz  G. C., , and Van Duyne  R. P, “ Nanoscale optical biosensor: short range distance dependence of the localized surface plasmon resonance of noble metal nanoparticles. ,” J. Phys. Chem. B. 108, , 6961–6968  ((2004)).
© 2011 Society of Photo-Optical Instrumentation Engineers (SPIE)

Citation

Claire S. Sweetenham and Ioan Notingher
"Combined atomic force microscopy-Raman mapping of electric field enhancement and surface-enhanced Raman scattering hot-spots for nanosphere lithography substrates", J. Nanophoton. 5(1), 059504 (June 01, 2011). ; http://dx.doi.org/10.1117/1.3595345


Figures

Grahic Jump LocationF1 :

NSL substrates characterized for SERS. (a) Large AFM image and line profile of a typical substrate. (b) 2D FFT of the presented image. (c) 3D image of an array of truncated tetrahedral silver nanostructures.

Grahic Jump LocationF2 :

AFM image of R6G on 1002 nm substrates with SERS spectra and mapping recorded across the highlighted area. Labeled spectra correspond to the scattering measured at the position of the same labeling in the mappings.

Grahic Jump LocationF3 :

SERS spectra and mapping of p-ATP on 1002 nm substrates, with the spectra used to calculate its enhancement factor (Raman bands used are highlighted).

Grahic Jump LocationF4 :

Finite element model with defined domains, boundary conditions, and PML and solution, displaying the resultant electric field (Vm−1).

Tables

References

Fleischmann  M., , Hendra  P. J., , and McQuillan  A. J., “ Raman spectra of pyridine adsorbed at a silver electrode. ,” Chem. Phys. Lett.. 26, , 163–166  ((1974)).
Jeanmaire  D. L., and Van Duyne  R. P., “ Surface Raman electrochemistry part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode. ,” J. Electroanal. Chem.. 84, , 1–20  ((1977)).
Hulteen  J. C., , Treichel  D. A., , Smith  M. T., , Duval  M. L., , Jensen  T. R., , and Van Duyne  R. P., “ Nanosphere lithography: Size-tunable silver nanoparticle and surface cluster arrays. ,” J. Phys. Chem. B. 103, , 3854–3863  ((1999)).
Haynes  C. L., and Van Duyne  R. P., “ Nanosphere lithography: A versatile nanofabrication tool for studies of size-dependent nanoparticle optics. ,” J. Phys. Chem. B. 105, , 5599–5611  ((2001)).
Haes  A. J., , Haynes  C. L., , McFarland  A. D., , Schatz  G. C., , Van Duyne  R. P., , and Zou  S., “ Plasmonic materials for surface-enhanced sensing and spectroscopy. ,” MRS Bull.. 30, , 368–375  ((2005)).
Farcau  C., and Astilean  S., “ Mapping the SERS efficiency and hot-spots localization on gold film over nanospheres substrates. ,” J. Phys. Chem. C. 114, , 11717–11722  ((2010)).
Sweetenham  C. S., and Notingher  I., “ Raman spectroscopy methods for detecting and imaging supported lipid bilayers. ,” Spectroscopy. 24,  (1–2 ), 113–117  ((2010)).
Rybczynski  J., , Ebels  U., , and Giersig  M., “ Large-scale, 2D arrays of magnetic nanoparticles. ,” Colloids Surf., A. 219, , 1–6  ((2003)).
Baia  L., , Baia  M., , Popp  J., , and Astilean  S., “ Gold films deposited over regular arrays of polystyrene nanospheres as highly effective SERS substrates from visible to NIR. ,” J. Phys. Chem. B. 110, (47 ), 23982–23986  ((2006)).
Schmidt  J. P., , Cross  S. E., , and Buratto  S. K., “ Surface-enhanced Raman scattering from ordered Ag nanocluster arrays. ,” J. Chem. Phys.. 121, (21 ), 10657–10659  ((2004)).
Zhao  J., , Jensen  L., , Sung  J., , Zou  S., , Schatz  G. C., , and Van Duyne  R. P., “ Interaction of plasmon and molecular resonances for Rhodamine 6G adsorbed on silver nanoparticles. ,” J. Am. Chem. Soc.. 129, , 7647–7656  ((2007)).
Farcau  C., and Astilean  S., “ Probing the unusual optical transmission of silver films deposited on two-dimensional regular arrays of polystyrene microspheres. ,” J. Opt. A, Pure Appl. Opt.. 9, , 345–349  ((2007)).
Le Ru  E. C., , Blackie  E., , Meyer  M., , and Etchegoin  P. G., “ Surface enhanced Raman scattering enhancement factors: A comprehensive study. ,” J. Phys. Chem. C. 111, (37 ), 13794–13803  ((2007)).
Hulteen  J. C., and Van Duyne  R. P., “ Nanosphere lithography: A materials general fabrication process for periodic particle array surfaces. ,” J. Vac. Sci. Technol. A. 13, , 1553–1558  ((1995)).
Haynes  C. L., and Van Duyne  R. P., “ Plasmon-sampled surface-enhanced Raman excitation spectroscopy. ,” J. Phys. Chem. B. 107, , 7426–7433  ((2003)).
Mohri  N., , Inoue  M., , Arai  Y., , and Yoshikawa  K., “ Kinetic study on monolayer formation with 4-aminobenzenethiol on a gold surface. ,” Langmuir. 11, , 1612–1616  ((1995)).
Schatz  G. C., , Young  M. A., , and Van Duyne  R. P., “ Electromagnetic mechanism of SERS. ,” Top. Appl. Phys.. 103, , 19–45  ((2006)).
Palik  E. D.,  Handbook of Optical Constants of Solids. ,  Academic ,  New York  ((1998)).
Lin  T-H., , Linn  N. C., , Tarajano  L., , Jiang  B., , and Jiang  P., “ Electrochemical SERS at periodic metallic nanopyramid arrays. ,” J. Phys. Chem. C. 113, , 1367–1372  ((2009)).
Haes  A. J., , Zou  S., , Schatz  G. C., , and Van Duyne  R. P, “ Nanoscale optical biosensor: short range distance dependence of the localized surface plasmon resonance of noble metal nanoparticles. ,” J. Phys. Chem. B. 108, , 6961–6968  ((2004)).

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