Ultrashort, Angstrom-Scale Decay of Surface-Enhanced Raman

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Ultrashort, Angstrom-Scale Decay of Surface Enhanced Raman Scattering at Hot Spots Gayatri K. Joshi, Sarah L. White, Merrell A Johnson, Rajesh Sardar, and Prashant K. Jain J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08242 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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The Journal of Physical Chemistry

Ultrashort, Angstrom-Scale Decay of Surface Enhanced Raman Scattering at Hot Spots Gayatri K. Joshi,1,# Sarah L. White,2,# Merrell A. Johnson,3 Rajesh Sardar1,4,* and Prashant K. Jain2,* 1

Department of Chemistry and Chemical Biology, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana 46202 2

Department of Chemistry, University of Illinois-Urbana Champaign, Urbana, Illinois 61801

3

Department of Physics, Indiana University-Purdue University Fort Wayne, Fort Wayne, Indiana 46805

4

Integrated Nanosystems Development Institute, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana 46202

#

These authors contributed equally

*Corresponding author

ACS Paragon Plus Environment

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Abstract Anisotropic plasmonic nanostructures are known to exhibit large enhancements of surfaceenhanced Raman scattering (SERS) of adsorbed molecules at their sharp tips or edges, where the near-field is intense. We show that the SERS enhancement at such field hotspots decays over a distance of ca. 4 Å, much shorter than the typical decay length reported for SERS. The finding is made in SERS sensors constructed from chemically synthesized triangular nanoprisms with azobenzene reporter molecules linked to the nanoprism surface using variable chain length alkanethiol spacers. With the aid of electrodynamic simulations, the ultrashort decay length, the shortest reported till date, is explained by solely an electromagnetic field effect. Our work provides a key design consideration for the use of hot spots of anisotropic nanostructures for SERS. The Å length-scale effect may also allow the achievement of intra-molecular spatial resolution in SERS probing.


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INTRODUCTION The localized surface plasmon resonances (LSPR) and associated electromagnetic field enhancements exhibited by metallic nanostructures1-5 have found utility in photocatalysis,6-11 medical diagnostics,12-14 biological and molecular sensing,15-30 and in surface-enhanced Raman scattering (SERS).30-37 For SERS, anisotropic plasmonic nanostructures have been particularly attractive.33, 38, 39 Raman scattering cross-sections vary as the fourth power of the electric field amplitude, |E|4. As a result, sharp edges and tips of an anisotropic nanostructure, where the electric field amplitude can be orders-of-magnitude larger than in the ambient medium, constitute “hotspots” for enhancement of Raman scattering from adsorbed molecules. For instance, at the sharp tips of silver nanoprims, the electric field intensity |E|2 has been simulated to be as much as 3500-fold enhanced relative to the applied field, promising a large electromagnetic (EM) field enhancement of SERS.40 Enormous enhancement factors (~108 to 109) in SERS41-43 have allowed detection limits to be pushed down to single-molecule levels.44-48 While large EM enhancements are promising, EM fields decay with distance away from the nanostructure surface.26, 31, 49 As a result, the SERS enhancement falls with distance, which is important to characterize for a number of reasons. Often, the analyte being probed is not in contact with the surface. In many cases, a spacer is used to separate the metal surface from the analyte, either to protect the metal from oxidation/chemical degradation or to prevent conformational changes in the analyte induced by interaction with the metal surface. In other cases, such as binding-based assays, large capture probes are utilized to bind analyte molecules. Knowledge of the distance decay of SERS enhancement is therefore crucial for determining the optimal separation of the analyte from the surface.50-54 Too large a distance would result in little to no SERS enhancement of vibrational modes in the analyte molecule. Motivated by this need, we experimentally characterized the distance-dependence of SERS enhancement on anisotropic nanostructures, specifically triangular nanoprisms (TNPs) of Au. There have been past studies of distance-dependent SERS.55-58,59 For instance, Murray et al.55 found that a 1000-fold SERS enhancement persisted up to a distance of 100 Å from the film


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surface. However, these past studies were performed on Ag island films and also yielded considerable variation in the measured distance range, which leaves open the question of how SERS decays with distance at the hot spots of anisotropic nanostructures. In our study, designed to address this question, we find a SERS decay length of ca. 4 Å, approaching the typical size of molecules and the shortest reported till date. With the aid of electrodynamic simulations, we determined that the ultrashort Å-scale decay length is a result of the steeper EM field drop-off at the sharply curved surfaces of hot spots, unlike the nanometer length-scale decay on planar surfaces, which dominate in contribution in film-based SERS. This steep decay of SERS on anisotropic nanostructures may be exploited for achieving Å-scale spatial resolution in probing of molecular structure, for instance, for determining molecules

the

binding

mode

of

(e.g.,

proteins,

DNA,

or

adsorbed intermediates in nanocatalysis) on solid surfaces.35, 60-62

A

B

Figure 1. Model system for SERS studies. (A) Au TNPs bound to a silanized glass substrate were functionalized with a combination of AXT and XT (XT = HT, NT, DDT, and HDT). (B) The terminal azobenzene groups were transformed predominantly to the trans configuration through blue light irradiation. Extinction and Raman spectra were collected for each type (n = 1, 3, 6, and 10) of AXT/XT self-assembled monolayer (SAM). Atomic force microscopy (AFM) images shown are enlarged for the purpose of the schematic and are not to scale. (B) Scanning electron microscopy (SEM) image of Au TNPs. The scale bar is 100 nm. The TNPs have an average edge length of 35.5 ± 4.1 nm (see histogram in Figure S2).


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EXPERIMENTAL SECTION Materials: Chloro(triethylphosphine) gold (I) (Et3PAuCl, 97%), poly(methylhydrosiloxane) (PMHS, Mw ~ 2000, 33-35 Si-H units), trioctylamine (TOA, 98%), (3-mercaptopropyl)triethoxysilane (MPTES, ≥80%), 4-(phenyldiazenyl) phenol, 1,6-dibromohexane (96%), 1,9dibromononane (97%), 1,12-dibromododecane (98%), 1,16-dibromohexadecane (97%), 1hexanethiol (HT, 95%), 1-nonanethiol (NT, 99%), 1-dodecanethiol (DDT, ≥98%), 1hexadecanethiol (HDT, 99%), dimethylformamide (DMF), thiourea, triphenylphosphine (PPh3, 99%),

bromine,

anhydrous

acetonitrile

(CH3CN),

methanol,

hexane,

ethyl

acetate,

dichloromethane (CH2Cl2), and ethanol were purchased from Sigma Aldrich and were used as received. Hydrochloric acid was obtained from Acros Organics. Sodium hydroxide (NaOH), sodium sulfate (Na2SO4), sodium chloride (NaCl), silica gel, potassium hydroxide (KOH), RBS 35 detergent, and glass coverslips were purchased from Fisher Scientific. All water was purified using a Thermo Scientific Barnstead Nanopure system. Super sharp silicon cantilevers for atomic force microscopy (AFM) were purchased from NanoSensors. The syntheses of 6-(4(phenyldiazenyl)phenoxy)hexane-1-thiol (AHT), 9-(4-(phenyldiazenyl)phenoxy)nonane-1-thiol (ANT), 12-(4-(phenyldiazenyl)phenoxy)dodecane-1-thiol

(ADDT),

and

16-(4-(phenyldiazenyl)phenoxy)

hexadecane-1-thiol (AHDT) are described in the Supporting Information.

Synthesis and characterization of Au TNPs. Au TNPs were chemically synthesized according to our published procedure.63,64 PMHS was used as a reducing agent and TOA as a stabilizing ligand. In a 250 mL Erlenmeyer flask, 0.02 mmol (0.008 g) of Et3PAuCl was dissolved in 20 mL of acetonitrile and the solution was allowed to stir for 30 min at room temperature. Then 0.18 mmol (0.08 mL) of TOA was injected followed by another 30 min of stirring. Next, 5.0 mmol (0.3 mL) of PMHS was added and the reaction mixture was stirred at room temperature for 30 min followed by heating at 40-42 oC. After 4 h of heating, the solution became dark blue and displayed a stable absorption spectrum with a peak maximum (λLSPR) at 750 nm. Construction of SERS sensors. Glass coverslips were functionalized with MPTES using our published procedure.65, 66 Briefly, glass coverslips were incubated in a 20% (v/v) aqueous RBS 35 detergent solution at 90 oC for 30 min and then sonicated for 5 min. The coverslips were rinsed with Nanopure water and placed in a solution of hydrochloric acid and methanol (1:1 v/v)


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for 30 min. After rinsing with Nanopure water multiple times, coverslips were dried in a vacuum oven at 60 oC overnight. Afterwards, coverslips were placed in a solution of 10% MPTES in ethanol for 30 min followed by 10 min of sonication and rinsing with anhydrous ethanol. The ethanol rinse and sonication steps were repeated at least 5 times. The coverslips were then placed in a vacuum oven for 3 h at 120 oC. The MPTES-functionalized coverslips were then immersed in a freshly prepared TNP solution. After 40 min of incubation, the coverslips were rinsed with ethanol and dried under nitrogen. The substrates, with attached TNPs were cleaned to selectively remove non-prismatic nanostructures, according to our published procedure.64,67,68 Briefly, adhesive tape (Scotch) was applied on the surface of the coverslip, pressed gently with a finger, and slowly removed at an approximately 90o angle. These glass substrates with attached TNPs were characterized by UV-visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). SERS sensors (see Figure 1) with different chain-length thiol self-assembled monolayers (SAMs) were fabricated by functionalizing the Au TNPs bound to the silanized glass substrate with AXT and XT (where XT = HT, NT, DDT, HDT). Glass substrates with attached Au TNPs were incubated overnight in an ethanolic solution of AXT: XT in a 3:7 mole ratio. This procedure formed SAMs of mixed AXT/XT thiols on the surface of the Au TNPs. The SAMmodified Au TNPs were rinsed with copious amounts of ethanol to remove any loosely-bound thiols, dried under nitrogen flow, and used for SERS studies. The SERS sensors were prepared at room temperature under normal laboratory conditions and white light, which typically results in a mixture of cis and trans isomers of azobenzene. Therefore, the SERS sensors were exposed first to UV light (365 nm) for 1 h and then to blue light for 1 h. The blue light irradiation transforms the terminal azobenzene groups to predominantly the trans form. Microscopy and spectroscopy measurements. SEM micrographs were acquired using a Hitachi S-4700 FESEM at 20 kV. The average edge lengths of the Au TNPs were calculated from the SEM image using Image J software. Edge lengths for 300 Au TNPs were used to determine the average edge-length and standard deviation. AFM images were obtained using a Bioscope AFM instrument. The AFM was operated in tapping mode using beam-shaped super sharp silicon cantilevers having an average force constant of 42 N/m. The operation frequency of the cantilevers for all measurements was 330 kHz. Thickness values of 50 Au TNPs were used to


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determine the average thickness and standard deviation. The sample for TEM analysis was prepared by drop-casting 10 µL of a freshly prepared Au TNP solution in acetonitrile onto a formver-coated copper grid (Electron Microscopy Science). The grid was allowed to air dry. Images were acquired on a Tecnai G2 instrument operating at 80 kV. Absorption and extinction spectra in the range of 300-1100 nm were collected with a Varian Cary 50 Scan UV-visible spectrophotometer. All solution spectra were collected using 0.3 mL of reaction mixture in 2.0 mL of acetonitrile placed in a 1-cm path-length quartz cuvette. All extinction spectra of glass substrates with bound Au TNPs were collected at room temperature and in air. SERS measurements were performed using a CRAIC technology Raman system with a 785 nm diode laser excitation source and 20 mW laser power. SERS spectra were acquired for SERS sensors with different chain-length SAMs. We collected 5 scans in the spectral range of 400-2000 cm-1 and an integration time of 16 s. Automatic baseline correction was performed in OMNIC software before acquired spectra were plotted. Determination of decay length. From each acquired Raman spectrum, we determined the intensities of Raman peaks at 1138 cm-1, 1184 cm-1, 1415 cm-1, 1442 cm-1, 1465 cm-1, and 722 cm-1, the first five of which correspond to stretching modes of azobenzene and the last of which corresponds to the C-S stretch of the thiol. The peak intensity (i.e., height) was measured from the base of each peak (see Table S1). Raman peak intensities were then corrected for differences in ligand concentration by dividing the Raman peak intensity for each mode by the C-S stretch peak intensity to obtain the normalized SERS intensity. The thiolate bond is directly at the surface in all SAMs and its SERS intensity has no chain-length dependence; therefore, the C-S stretch intensity serves as an internal reference for the number density of ligands in each chainlength SAM. The azobenzene SERS intensity normalized by the C-S stretch intensity provides the pure chain-length dependence of the SERS enhancement free of any artifacts from variations in the number density of ligands, and therefore of azobenzene molecules, in different chain length alkanethiol SAMs. Normalized SERS intensity was plotted as a function of the SAM chain length. The chain length from the surface Au atom to the N atom of the azobenzene group was determined by using ChemBioDraw 14.0 software, as shown for ANT below:

x = 15.698 Ao N O

Au

N

S ACS Paragon Plus Environment

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The decay length was determined from these plots by fitting the normalized SERS intensity as a function of chain length to a single-exponential decay function of the type: Normalized SERS Intensity = A0 .exp(-x/d1/2)

(1)

where x is the chain length in Å and d1/2 is the decay length in Å. Simulations of EM field enhancement. The discrete dipole approximation (DDA) method was used to simulate the electric near-field for a model Au TNP structure. The DDA method numerically solves Maxwell’s equations for arbitrarily shaped objects by discretizing each object into a cubic array of N polarizable point dipoles and self consistently solving for the polarizability of each dipole interacting with the incident field and all other N-1 dipoles. We employed the open source DDSCAT 7.3.0 code from Draine and Flatau.69 The validity of the DDA method for the simulation of the steep variation in near-field enhancement at hot-spots is discussed in the Supporting Information. The Au TNP target was generated via tools available on nanoHUB.org. The TNP structure (7.5 nm thickness, 34.5 nm edge length) was generated using the Blender component of the nanoDDSCAT+ tool70 which takes a shape input in the form of a triangulated mesh and converts it into a cubic array of points, the input for DDSCAT. An inter-dipole spacing of 0.5 nm was used. In all simulations, the refractive index of the ambient medium was set at nm = 1.5 to account for the ligand environment at the TNP surface. The bulk experimental dielectric function of Au from Johnson and Christy71 was utilized for all calculations without any corrections. In order to compare our model Au TNP with those used in the experiment, an optical absorption spectrum was generated using the DDSCAT program. The LSPR energy maxima of the simulated and experimental spectrum of Au TNPs match well. The experimental spectrum is broader due to sample heterogeneity. The target structure was excited with unpolarized 780 nm light, simulated by a circularly polarized plane wave. Light, incident normal to the face of the Au TNP, propagates along the thickness of the Au TNP. Near-field calculations were performed utilizing the DDSCAT 7.3 subroutine NEARFIELD. The macroscopic field amplitude (E normalized to the incident field amplitude E0, since the latter is set to a value of 1) along various axes through the target structure was obtained. Values of the real and imaginary components of E


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were extracted as , , , , , and . The SERS enhancement factor was determined as: ||⁄| | = + + + + +

(2)

These theoretical SERS enhancement factors, (|E|/|E0|)4, were plotted as a function of distance from the surface of the Au TNPs along various axes. Each enhancement factor plot was fit in the distance range from 15 to 29 Å (similar to that probed experimentally) to a single-exponential decay function: ||⁄| | = // where is the distance from the surface in Å and

/

(3)

is the decay length. The lack of an offset

in this decay function implies that the enhancement factor reaches a value of 0 at infinite distance; however this introduces a small error because in principle, (|E|/|E0|)4 tends to a value of 1 in the infinite-distance limit. Simulations of near-field enhancements under unpolarized 780-nm excitation and extinction spectra were also run for a non-ideal TNP structure possessing some degree of rounding or truncation at the vertices and edges. We started with an ideal TNP target of 7.5 nm thickness and an edge length of 35 nm. The sharp edges and vertices of this TNP were rounded using the three-segment beveling option available in Blender. Each vertex of the TNP was offset by an amount of 3.0 nm. The top and bottom faces of the TNP were also beveled in the same manner. The beveled TNP had a thickness of 7.5 nm. The edge length was 31.5 nm for the faces of the beveled TNP but it was 33.5 nm at the widest location. The volume of this rounded TNP was only marginally (< 3%) lower than the ideal TNP structure, ensuring that size-effects do not complicate the study of the effect of rounding. All other parameters and settings for the simulations of the rounded TNP were identical to those used for the ideal TNP structure. RESULTS AND DISCUSSION Our model system consists of azobenzene molecules covalently attached through alkanethiol SAMs to Au TNPs. The large Raman scattering cross section of azobenzene72 makes it an ideal Raman reporter. Au TNPs were selected as SERS substrates because they possess sharp corners and edges that serve as hot spots for SERS enhancement.2,40,73,74 Unlike poorly


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controlled morphologies of SERS-active Ag island films,55-59 a potential cause of the large variability in measured decay lengths, our chemically synthesized Au TNPs are well-defined in shape.27,64,67,75 Finally, high-quality, pinhole free SAMs of variable chain length alkanethiols can be formed on Au TNPs, which allowed systematic control of the distance between the Raman reporter and the TNP surface. Recently, our group76 and Weiss and co-workers77 demonstrated that a mixed SAM of ANT:NT with a 3:7 mole ratio can effectively isolate the azobenzene from the surface and maintain it at a controlled distance from the surface. A distance-dependent study becomes possible with SERS sensors constructed using variable chain-length SAMs of AXT:XT (XT = HT, NT, DDT, and HDT).

Figure 2. SERS spectrum from a sensor with AHT-HT SAM. Spectra for sensors with ANT-NT, ADDT-DDT, and AHDT-HDT SAMs are shown in Figures S3-S6. Spectrum is shown with baseline subtracted. The SERS intensity of the 1138 cm-1 azobenzene mode normalized to the C-S stretch intensity is shown as a function of the alkanethiol chain length in (B). The chain length dependence is fit to a single-exponential decay function, from which a decay length, d1/2, is determined. Chain length-dependent SERS intensity plots for other Raman modes of azobenzene are shown in Figure S7. Each plot was fit to a single-exponential decay function, from which the decay length d1/2 was found to range from 4.2 Å to 4.7 Å. This range serves as a measure of the uncertainty in the measured distance-decay trend.

As-synthesized Au TNPs displayed a dipolar LSPR mode at λLSPR ~732 nm in air (Figure S1) and were determined to have an average edge-length of 35.5 ± 4.1 nm and an average height 8.5 ± 0.9 nm (Figure S2). Following SAM functionalization, the dipolar LSPR mode of the TNPs exhibited a red-shift of 10s of nm depending on the chain-length (Figures S3-S6). Representative SERS spectra of the AHT:HT SAM, shown in Figure 2A, show five characteristic Raman peaks


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(CN stretching modes at 1138 and 1184 cm-1; N=N ring bending modes at 1415 and 1442 cm-1; N=N stretching mode at 1465 cm-1) of azobenzene, predominantly in the trans form.76,78 By analysis of SERS spectra for different chain length SAMs (n = 1, 3, 6, and 10), we were able to determine how the SERS intensity of azobenzene modes changes as a function of the distance between the azobenzene and the TNP surface (Figure 2B and Figure S7). We assumed this distance to correspond to the chain length in Å from the surface Au atom to the N atom of the azobenzene group. It must be noted that, in practice, the actual chains in an alkanethiol SAM are tilted with respect to the normal to the surface, e.g., ɵ = 25-30o for Au(111);79 thus the actual surface-to-azobenzene distances may be shorter than the idealized chain length values used here by a magnitude of cosɵ. Moreover, this tilt angle could be variable for SAMs of different chain lengths, leading to an additional contribution to the actual distance-decay trend. These factors introduce small errors in the decay lengths measured from the trends plotted in terms of idealized chain lengths. The normalized SERS intensity for the 1138 cm-1 azobenzene mode plotted as a function of the chain length (Figure 2B) was fit to a single-exponential decay of the type: !"# $%&' (( )*)+&*, = ⁄/

(4)

where is the chain length in Å, is the intensity extrapolated at the surface of the Au TNP, and

/

is the decay length in Å, which signifies the distance at which the SERS intensity

decays to 1/e of its maximum value. Such plots for the other azobenzene modes are shown in Figure S7. While the analytical distance dependence of SERS at the TNP surface is expected to be quite complex due to multipolar contributions (as discussed later), the fit to a singleexponential decay is a phenomenological choice26 that allows quantification of the distancedecay rate of the SERS enhancement, in terms of a single physical parameter, d1/2. The measured d1/2 was found (for all five stretching modes) to be in the 4-5 Å range. Thus, the distance range of SERS enhancement measured here is remarkably short, when compared to that measured previously on Ag island films,41,55-59 where decay lengths ranging from ca. 10 Å to tens of Å have been measured. The few Å decay length found here is the shortest reported till date.


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Figure 3. Simulated (|E|/|E0|)4, plotted as a function of distance from the surface at various locations and along different axes (indicated in the schematic) of a representative Au TNP. The distance-dependence of (|E|/|E0|)4 is fit to a single-exponential decay (eq. 3), resulting in a decay length, d1/2, shown for each plot. DDA simulations were performed for an ideal (unrounded) Au TNP excited by unpolarized light of 780-nm wavelength.

To rationalize the ultrashort decay length of SERS enhancement observed here, we performed electrodynamics simulations of the field enhancement at the surface of Au TNPs. SERS enhancement can be either EM or chemical in origin.52,56 Chemical enhancement is


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thought to result from charge or energy transfer between the metal surface and adsorbed chemical species, or coupling between electronic energy levels of the metal surface and the chemisorbed molecule. Due to the lack of electronic contact between azobenzene and the TNP surface in our studies, we focus on EM enhancement alone and find it to fully account for our observations. In the EM mechanism, there is an enhancement of both the incident field intensity and Raman scattered field intensity at the surface of the nanostructure under plasmonic excitation. For reasonably low wavenumber Raman modes, the overall EM enhancement of SERS scales as (|E|/|E0|)4, where E is the near-field-intensity at the TNP surface and E0 is the applied field. From discrete dipole approximation (DDA) simulations performed for a representative Au TNP at the excitation wavelength of 780 nm, we obtained the magnitude of (|E|/|E0|)4 as a function of distance away from the Au TNP surface (Figure 3 A-I). The model Au TNP structure used in the simulations was 7.5 nm in thickness and had an edge-length of 34.5 nm. The dimensions were obtained from a coarse-refinement performed until the simulated extinction spectrum matched that of the experimental sample (Figure S8). Similar to experimental plots of normalized SERS intensity, plots of (|E|/|E0|)4 as a function of distance from the Au TNP surface were fit to single-exponential decay functions: ||⁄| | = ||⁄| |-./0123 ⁄/ from which ||⁄| |-./0123 and

/

(5)

were determined. The simulated decay length,

/

was

found to depend on the location on the Au TNP surface. Measured normal to the flat (top or side) surfaces of the Au TNP, the decay length was in the 20 Å range, (Figure 3 B, C, E, F), which is interestingly similar to the typical value measured in film-based studies. However, at the vertices of the TNP (Figure 3 A, G, H, I), the decay length of (|E|/|E0|)4 was much shorter (4.4 - 4.8 Å) irrespective of the direction along which it was measured. From an inspection of |E|⁄|E |56789:; (Table S2), the vertices also appear to be sites where EM enhancement was the highest and two-orders of magnitude higher than at the flat surfaces. In other words, high-index sites, specifically the vertices are the hotspots of EM enhancement: the SERS signal in experiments is predominately from azobenzene-terminated ligands located at the vertices of the Au TNPs. The relative contribution from ligands at other locations on the TNPs, specifically low-index flat surfaces, is considerably small in the measured SERS spectrum. As a result, the


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experimentally measured decay length for SERS enhancement is mostly representative of the behavior at the Au TNP hotspots. It must be acknowledged that the DDA simulations discussed above considered a TNP model of ideal geometry with perfectly sharp vertices and edges. In practice however, the experimental TNPs have vertices and edges with some degree of rounding/truncation, as seen in TEM images (Figure S2 D, E). It is necessary to consider how such rounding, especially at the vertices influences the distance decay of SERS enhancement, as characterized by the decay constant d1/2. In line with the comparison of highly curved sites vs. flat surfaces made above, one may expect apriori the distance decay to be less steep for TNPs with vertices that are not perfectly sharp. Additional DDA simulations were performed with a TNP with sufficient degree of rounding at the vertices, representative of the experimental TNP geometry (see Experimental Methods and Figure S8). Such a rounded TNP exhibits a bluer LSPR maximum (λLSPR = 758 nm) as compared to an ideal, unrounded TNP model (λLSPR = 780 nm), which is a geometric effect of the reduction in the surface curvature. The simulated EM enhancement, (|E|/|E0|)4, at a vertex site of the rounded TNP is shown as a function of distance from the TNP surface in Figure S9. At the corner of the vertex,

/

= 5.1 Å for the rounded TNP (Figure S9 A). Thus, when

compared to the ideal TNP, which has a

/

of 4.6 Å (Figure 3G), the SERS distance decay for

the rounded TNP appears less steep, but only by a factor of 10%. In other words, the effect of rounding on the distance decay is not drastic despite the large drop in the local curvature at the corner caused by rounding. It appears that the field enhancement and its distance dependence are dictated by a spatially averaged radius of curvature rather than the local point curvature at the corner. The latter would explain why simulations performed for an ideal TNP geometry do not deviate considerably from the experimentally measured d1/2. There is, however, an additional reason described below for the close agreement between experiments and simulations performed with an ideal geometry. The experimental sample, as seen from TEM images, consists of TNPs with variable degrees of rounding at the vertices (Figure S2 D, E). In fact, the ensemble extinction spectrum is heterogeneously broadened (Figure S8) due to variability in the degree of rounding between individual TNPs. For TNPs within the ensemble with a large degree of rounding, it is expected that the SERS distance decay is considerably more gradual, i.e.,


/

is significantly larger than

14

Page 15 of 23

the value of 4-5 Å simulated for the ideal geometry. However, as shown in Figure S8, TNPs with sufficient rounding are expected to possess a much bluer LSPR, which would decrease resonant overlap with the 785 nm laser excitation and thereby reduce their contribution to the overall SERS signal measured in an ensemble manner. TNPs within the ensemble closest in geometry to the ideal TNP structure are expected to exhibit LSPRs (maxima ~ 780 nm as shown in Figure S8) strongly overlapped with the laser excitation. Therefore, minimally rounded TNPs contribute the most to the ensemble-averaged SERS signal and the experimentally measured

/

is closely

representative of the behavior of a TNP of near-ideal geometry.

120

Normalized SERS Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1415 cm-1 stretch Fit with a = 22 + 3Å

100

80

60

40

20

0 16

18

20

22

24

26

28

Chain length (Å)

Figure 4. Normalized SERS intensity of the 1415 cm-1 mode plotted as a function of the SAM

chain length. The plot was fit to the form: (( )ℎ$)= )* = >?1@ where is the

distance from the surface and $ is radius of curvature of the surface feature. The fit (with R2 = 0.99) yields a = 2.2 nm. The ultrashort decay length measured at the vertices is associated with the high surface curvature at these sites. While, in the dipolar approximation, near-fields decay with distance d from the center-of-mass of the dipole as 1/d3,80 at sharply curved surfaces, the plasmonic oscillation is significantly multi-polar in character: higher-order modes such as quadrupoles and octupoles contribute significantly. Since, these higher order modes decay more rapidly with distance (~1/d5 for quadrupoles and ~1/d7 for octopoles), the EM enhancement falls off much


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Page 16 of 23

more steeply at highly curved surfaces.49 In fact, from past work on roughened metal surfaces consisting of nanoscale metal protrusions,81 there exists a phenomenological relationship for the geometry-dependent distance decay of SERS enhancement: 41 (( )ℎ$)= )* = >

?1

@

(6)

where is the distance from the surface and $ is radius of curvature of the surface feature. Smaller the radius of curvature, a, of the feature, steeper is the distance decay of the SERS enhancement. When fit to the form expressed in eq. 6 (Figure 4), our chain-length dependent SERS data yielded a = 2.2 nm. As a comparison, Kennedy et al.41 found from SERS data on roughened Ag surfaces a value of 9.3 nm, which was in line with the measured radius of curvature of the roughness features. In our case, we obtain a radius of curvature much smaller than the 10s of nm dimensions of the TNP, which phenomenologically captures the strong effect of the sharp, highly curved vertices of the TNPs that serve as the primary sites or “hotspots” of SERS enhancement. It must be acknowledged that for the short (n < 8 or 9) alkanethiols employed here, SAMs may not be perfectly crystalline. In addition, at the TNP vertices, the SAM spacer layer is likely to be defective. Thus, the surface-to-azobenzene distances may be shorter in practice than the idealized ones represented by the SAM chain-length. In such a scenario, the fall-off of SERS enhancement can appear more gradual than its actual distance-dependence,59 resulting in a longer apparent decay length. Thus, our measured decay length may be somewhat overestimated due to the presence of defects in the spacer layer. The actual SERS decay length may in fact be shorter than our measured value of 4 Å. CONCLUSION In summary, we have experimentally measured the distance dependence of SERS enhancement on Au TNPs. We found an ultra-short decay length of ca. 4 Å arising mainly from steep distance decay of EM fields at the vertices of the Au TNP, which constitute the hot spots of SERS enhancement. We are also able to rationalize the longer, more variable decay lengths measured on island films, where the SERS enhancement is contributed by a mixture of sites ranging from low-index flat surfaces with shallower (tens of Å) decay to higher-curvature surfaces with steep (few Å) decay; the relative contribution and effective decay length depends


16

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on the island morphology. In the case of shape-controlled Au TNPs studied here and other such anisotropic structures, the hot spots are precisely defined and form a dominant fraction of the nanostructure surface, resulting in the observed decay length of few Å. Since nanostructures with engineered hot spots are popular for SERS detection, the steep fall-off of enhancement has important implications. For instance, binding-assays based on nanostructure-enhanced SERS must be designed with this short decay length in consideration. Analyte capture probes (e.g. DNA strands) must be shorter than a few Å for successful SERSbased detection of the analyte. It is also interesting to note that the few Å decay length at hot spots approaches intramolecular distances in many systems of interest. For instance, a glucose molecule is 9 Å in length and width. Therefore, for a molecule adsorbed to a nanostructure, the vibrational peaks in the SERS spectra are expected to be dominated by the end of the molecule closest to the nanostructure surface, whereas the further end of the molecule would contribute negligibly to the SERS spectrum. Such intra-molecular spatial resolution in SERS would allow the steady state or dynamic probing of the binding mode, spatial orientation, and/or conformation of molecular/biomolecular adsorbates on metal nanostructures engineered with hot spots.

ASSOCIATED CONTENT Supporting Information. Synthesis of AHT, ANT, ADDT, and AHDT; UV-visible extinction spectra; SEM, TEM, and AFM images; SERS spectra; experimental SERS decay plots with fits; simulated extinction spectra; simulated EM enhancement factors for rounded TNP structure; table of experimental SERS peak intensities for various SERS sensors; and table showing fit parameters obtained from simulations performed with the ideal TNP structure. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author e-mails and telephone numbers: [email protected] ; +1 (217) 333-3417


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[email protected] ; +1 (317) 278-2511

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT R.S. thanks the National Science Foundation (CBET-1604617) for financial support. P. K. J. acknowledges financial support from the Arnold and Mabel O. Beckman Foundation. REFERENCES 1.

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TOC Graphic

d1 /2 = 4.4 Å 8.0E4

(|E|/|E0|)4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6.0E4

4.0E4

2.0E4

16 18 20 22

24 26 28

Distance From Surface (Å )


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Ultrashort, Angstrom-Scale Decay of Surface-Enhanced Raman

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