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Suppression of Surface-Enhanced Raman Scattering on Gold Nanostructures by Metal Adhesion Layers Loan Le Thi Ngoc, Tao Yuan, Naoto Oonishi, Jan W. van Nieuwkasteele, Albert van den Berg, Hjalmar P. Permentier, Rainer Bischoff, and Edwin T. Carlen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05375 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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The Journal of Physical Chemistry

Suppression of Surface-Enhanced Raman Scattering on Gold Nanostructures by Metal Adhesion Layers Loan Le Thi Ngoc,†,§ Tao Yuan,‡ Naoto Oonishi, ¶ Jan van Nieuwkasteele,§ Albert van den Berg,§ Hjalmar Permentier,‡ Rainer Bischoff,‡ and Edwin T. Carlen*,¶ †

Physics Department, Quy Nhon University, Quy Nhon City, Binh Dinh, Vietnam

‡

Analytical Biochemistry Group, University of Groningen, Groningen, 9713 AV, The Netherlands

§

BIOS Lab on a Chip Group, University of Twente, Enschede, 7522 NH, The Netherlands

¶

Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki, 305-8573, Japan

1

ACS Paragon Plus Environment


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ABSTRACT

We report on the suppression of localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS) of 60 nm thick gold nanostructures by titanium adhesion layers. Significant plasmon damping is observed for titanium layer thicknesses up to tTi = 5 nm, which cannot be fully explained by simulation models that account for the overlap of the electric field in the bimetal layer with the titanium layer. Furthermore, the relationship between the SERS enhancement factor and the LSPR quality-factor G ∝ Q4 breaks down as tTi is increased beyond 1 nm. We attribute the LSPR and SERS suppression to plasmon damping and spectral broadening of interband absorption due to the field overlap with the titanium layer and titanium impurities that interdiffuse through the grain boundaries of the polycrystalline gold layer during the deposition of the Au-Ti bimetal. These observations and analyses serve as a guide for improving the reliability of gold nanoplasmonic sensors.

KEYWORDS Nanoplasmonics, LSPR, SERS, Ti, dephasing, virtual bound states

INTRODUCTION Nanoplasmonic resonators concentrate incident optical energy into nanoscale dimensions as localized surface plasmon resonance (LSPR) modes on metal nanostructures. The LSPR modes are free-electron oscillations driven by the incident optical field that penetrates the metal nanostructure. The LSPR modes produce enhanced electric fields in the near-zone of the nanostructure, which is a key property for applications such as Raman spectroscopy based on surface-enhanced Raman scattering (SERS). However, since the penetration depth of the incident field into the metal is comparable to the nanostructure dimensions, plasmon dephasing due to the intrinsic properties of the metal, such as 2


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electron interactions, intraband transitions, and interband transitions limit the near-field enhancement and SERS. Despite significant effort to develop new fabrication technologies and plasmonic materials, top-down fabrication of polycrystalline gold (PC-Au) thin films is still widely used to realize nanoplasmonics systems.1 Moreover, PC-Au is an important material for nanoplasmonic biosensors and SERS due to its chemical stability, compatibility with self-assembled monolayers (SAMs), and availability of wellestablished fabrication techniques. However, intrinsic plasmon damping strongly limits the near-field enhancement of PC-Au nanoplasmonic resonators. Furthermore, the experimental near-field enhancement is usually much less due to extrinsic factors such as radiation damping and increased plasmon dephasing due to impurities and structural inhomogeneities, which are typically related to the fabrication technology. One of the major problems with top-down fabricated PC-Au nanostructures, prepared by physical vapor deposition, is their poor adhesion to most surfaces, thus requiring an adhesion layer to prevent delamination.2,3 Metals such as titanium (Ti) and chromium (Cr) are commonly used to improve the adhesion of PC-Au; however, both metals are known to cause significant plasmon damping.4-10 Moreover, both Ti and Cr impurities have been observed in PC-Au layers of as-deposited bimetal thin films, as well as intermetallic phases, such as Au4Ti, Au2Ti, and AuTi.11,12 The presence of these impurities and intermetallic phases in PC-Au can significantly increase the plasmon dephasing rate and broaden interband absorption.13-15 Therefore, managing the fabrication technologies for nanoplasmonic systems is of crucial importance in order to optimize their performance. The damping of surface plasmon resonance (SPR) in PC-Au thin films by Ti and Cr adhesion layers was addressed by the biosensing community over twenty years ago.4 It is now common practice to manufacture SPR biosensors with ultrathin (1-2 nm) Ti adhesion layers with approximately 50-nm thick PC-Au layers, which offers a reasonable trade-off between plasmon damping and adhesion strength of 3



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the metal layer to glass substrates.5 Plasmon damping by metal adhesion layers has been recently rediscovered in PC-Au nanostructures.6-10 However, the mechanisms responsible for plasmon damping in PC-Au nanostructures by metal adhesion layers have not been fully explored. In this article, we investigate the effects of Ti adhesion layers on LSPR and SERS of nanostructured PC-Au surfaces. The resonance quality-factor and wavelength are extracted from reflectance measurements and compared to numerical simulations of the near-fields to assess plasmon damping by the Ti layer. SERS spectra from benzenethiol (BT) SAMs chemisorbed on the nanostructured PC-Au surfaces are used to further evaluate the effects of Ti impurities.

METHODS Reagents. Benzenethiol (BT), 30% H2O2, and concentrated H2SO4 are analytical grade and purchased from Sigma-Aldrich (Munich, Germany). Water was purified by a Maxima Ultrapure water system (ELGA, High Wycombe, Bucks, UK). Nanostructured gold surface fabrication. Conventional (100) silicon substrates were used for all template fabrication. First, a thin (~30 nm) low stress silicon nitride (SiN) layer is deposited onto the silicon substrate by low-pressure chemical vapor deposition (pressure: 100 mTorr; 77.5 sccm H2SiCl2, 20 sccm NH3; temperature: 850 °C; deposition rate: 4 nm min–1; refractive index: 2.2). A 100 nm thick polymethyl methacrylate (PMMA, MicroChem Corp.) electron-sensitive photoresist was spin-coated on a silicon substrate and exposed to a 95 pA electron beam (acceleration voltage: 10 kV; aperture size: 20 m) with the area dose in the range of 90-120 μA cm–2 (Raith150-TWO). The electron-beam exposure along the length of the SiN template was aligned to the [110] direction of the (100) silicon wafers using the sample edge as a reference. The total template surface area of 1 mm2 was written in 100×100 µm2 sections. The exposed regions were developed in a 1:3 methyl isobutyl ketone:isopropanol solution for 30 s, and followed by immersion in isopropyl alcohol. The exposed SiN regions were removed using 4


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reactive ion etching (parallel plate reactor; pressure: 10 mTorr; RF (13.56 MHz) power: 60 W; electrode voltage: –500 V; electrode temperature: 10 °C; 25 sccm CHF3, 5 sccm O2; etch rate: 60 nm min–1) and followed by removal of the remaining PMMA and surface cleaning with oxygen plasma. Prior to silicon etching, the native oxide on the exposed silicon regions was removed by immersion in 1% hydrofluoric acid solution for 1 min and subsequently rinsed with deionized water. The silicon was etched in a 1% KOH solution at 55 °C with stirring for 45 s and rinsed with deionized water for 2 min. The different crystal planes etch anisotropically by hydroxide ions in an alkaline solution where (111) planes have the lowest etch rate and (100) and (110) planes both have higher etch rates. The surfaces were then cleaned in a 3:1 piranha solution (H2SO4:H2O2) for 15 min, rinsed with deionized water for 2 min, and dried with N2. The Ti-Au film stack was deposited by electron-beam evaporation. The deposition rate of Ti: 0.06 nm s–1 and deposition times: 17 s (tTi ≈ 1 nm), 50 s (≈ 3 nm), and 83 s (≈ 5 nm). The deposition rate of Au: 0.6 nm s–1 and deposition time: 100 s for a total Au thickness of 60 nm. The deposition rate was controlled by a quartz crystal microbalance in the electron-beam evaporation system. Reflectance spectroscopy. Normal incidence reflectance measurements were performed with a TMpolarized (LPVISB100, Applied Laser Technology) white light (100 W tungsten halogen lamp, HLX 64625, Osram) focused on the surfaces with a microscope objective (10×/0.3 NA, Leica). The reflected beam was directed through a multimode fiber (QP450-1-XSR, Ocean Optics) to the spectrometer (HR4000, Ocean Optics). Measurements were calibrated with an unpatterned PC-Au surface. Raman spectroscopy. The reflectance minimum was measured to confirm that the plasmon resonance is λ0 ≈ 633 nm. Benzenethiol (BT) SAMs were prepared by immersion of clean nanostructured PC-Au surfaces into 4 mM BT-ethanol solution for 4 hours. Raman spectra of BT SAMs were measured in ambient air with an integration time of 100 ms. A HeNe laser with 1 mW power was focused on the sample with a 100×/NA 0.9 microscope objective.

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Raman spectroscopy instrumentation. A confocal Raman microscope system (alpha300R, WITec GmbH) was used for the Raman measurements, which consists of a TE-cooled charge coupled device (DU970P-BV, Andor Technology, Belfast, Northern Ireland) and UHTS300 spectrometer (f/4 300 mm FL; grating: 600 lines mm–1). A backscatter configuration, with HeNe laser (632.8 nm) and 100×/NA 0.9 microscope objective (MPlan FLN, Olympus), was used for all measurements. Elastically scattered light was removed with an edge filter and the laser polarization was aligned perpendicular to the length of the nanogaps by manual rotation of a λ/2 rotator phase plate. The spectrometer was calibrated with the first order Raman vibration of a silicon (520 cm–1) sample prior to each measurement. The laser poser was measured at the entrance of the microscope objective. Simulations. 2D finite-difference time-domain (FDTD) calculations performed with Fullwave (RSoft, Inc.). Periodic boundary conditions were used in the x-direction. Perfectly matched layer (PML) boundary conditions of 8 nm were set at the grid edges in the z-direction. A nonuniform grid with nominal 1 nm grid spacing was used. The complex frequency-dependent dielectric function ε(ω) of Au is represented by the Brendel-Borman model that is included in the simulation code. Simulations at all incident frequencies were checked for sufficient convergence. A time step of 2×10–4 cT units (c = 2.99792458×108) and simulation time of 30 cT were used. Data processing. All measured Raman bands were modeled with a Lorentzian band shape after removal of the background. The background was modeled with a cubic polynomial function. The decrease of the normalized quality-factor as a function of the titanium layer thickness tTi was modeled with an exponential decay expression (Qn/Q0) = A + Bexp(–tTi/α), where A and B were determined from the measured (Qn/Q0) at tTi = 0 nm and tTi = 5 nm, respectively. For the measured spectra A = 0.203, B = 0.793, and α = 2.309 nm and for the calculated spectra A = 0.473, B = 0.521, and α = 1.811 nm.

RESULTS AND DISCUSSION 6


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The nanostructured PC-Au surfaces are comprised of arrays of nanowires separated by sub-20 nm nanocavities, recently reported by our group.16,17 Figure 1(a) shows a scanning electron micrograph (SEM) image of a nanostructured PC-Au surface with pitch Λg = 250 nm and nanocavity spacing g = 10 nm. Each Au nanowire has an elliptical cross-section supported by a silicon nitride (SiN) template on a silicon (Si) substrate, shown in Fig. 1(b). The PC-Au layer thickness is tAu = 60 nm at the center of the nanowire.

(a)

(b)

nanocavity

g

Ti Au SiN

(c)

tTi = 0 nm

0

−50

−100

−10

0

x

10

100

(d)

tTi = 0 nm

tTi = 0 nm 80

|Em0/Ei|2

0

10

z

100 nm

x

|E(x=0, z)/Ei|

|E / Ei|

tAu

Si

y

Λg

250 nm z / nm

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5

60

(e)

TM

40 20

0 -100 100

-50 50

x / nm

00

50 −50

TE 0 500 550 600 650 700 750

z / nm

Wavelength / nm

Figure 1. Nanostructured PC-Au surface. (a, b) SEM images. (a) Top view with Λg = 250 nm and g = 10 nm nanocavity spacing. (b) Cross section view. (c) FDTD simulation of |E/Ei| generated in the nanocavity with a TM-polarized incident plane wave (|Ei| = 1) at the plasmon resonance wavelength (tTi = 0 nm). (d) Profile of |E/Ei| along the center of the nanocavity (white dashed line at x = 0 in (c)) in the z-direction (– 50 nm ≤ z ≤ 100 nm). (e) Maximum electric field spectrum in the nanocavity |Em0/Ei|2 excited with TM (solid blue line) and TE (dashed blue line) polarized incident waves.

The Au and Ti layers thicknesses were precisely controlled with a quartz-crystal microbalance in the electron-beam evaporation system. A TM-polarized (x-direction) incident wave at the plasmon resonance frequency results in an enhanced electric field |E/Ei| in the nanocavity (with incident field |Ei| = 1); shown in the 2D finite-difference time-domain (FDTD) simulation of the nanocavity region 7



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(white dashed box in Fig. 1(b)) in Fig. 1(c). Figure 1(d) shows the electric field |E(x = 0, z)/Ei| generated at the plasmon resonance frequency along the z-direction corresponding to the white dashed line in Fig. 1(c). Figure 1(e) shows a spectrum of the maximum electric field generated in the nanocavity |Emn/Ei|2, with n = 0 for tTi = 0 (no adhesion layer) for both TM and TE-polarized incident plane waves. The LSPR is generated with the TM polarized incident waves. Surfaces have been fabricated with three Ti layer thicknesses (tTi = 1 nm, 3 nm, and 5 nm) and Figs. 2(a)-(d) show reflectance spectra from the nanostructured PC-Au surfaces measured using TM-polarized (x-direction, Fig. 1(a)) white light. The experimental quality-factor Q = λ0/2ζ is estimated from the plasmon resonance wavelength λ0 and linewidth 2ζ, which are extracted by fitting the reflectance to R(λ) = [R m – (R m – R 0)Λ( λ)]×100%, where R m is the maximum reflectance far from resonance, R0 is the minimum reflectance at resonance, and Λ(λ) = ζ2/[(λ – λ0)2 + ζ2] with damping term ζ. The experimental quality-factor without an adhesion layer is Q0 ≈ 14. However, the spectral linewidth 2ζ broadens significantly for tTi = 5 nm with 71% decrease of the quality-factor. Furthermore, the resonance wavelength redshifts by ∆λ0 ≈ 11 nm. The changes in the reflectance spectra and surface plasmon resonance characteristics are not caused by shape changes of the PC-Au nanowires (Supporting Information, Section 1).

8


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Photon energy, ħω / eV 2.4

Reflectance, R / %

100 80

2.2

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tAu=60 nm tTi=0 nm

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Reflectance, R / %

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tAu=60 nm tTi=3 nm

Q1=10

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60

40

40

tAu=60 nm tTi=5 nm

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20

Q3=6

Q5=4

0 0 500 550 600 650 700 750 500 550 600 650 700 750

Wavelength, λ / nm

Figure 2. Reflectance from nanostructured PC-Au surfaces with different tTi. (a) No titanium layer (n = 0): λ0 = 636 nm, ζ = 45 nm. (b) tTi = 1 nm (n = 1): λ0 = 637 nm, ζ = 65 nm. (c) tTi = 3 nm (n = 3): λ0 = 644 nm, ζ = 114 nm. (d) tTi = 5 nm (n = 5): λ0 = 647 nm, ζ = 146 nm.

Figure 3(a) shows 2D FDTD simulated results of the normalized field |Emn/Em0|2 spectra in the nanocavity for different tTi. As tTi is increased, the LSPR band changes with decreases in the peak |Emn/Em0|2 by 20%, 43%, and 52% for tTi = 1 nm, 3 nm, and 5 nm, respectively, and with broadening of the LSPR linewidth 2ζ by 24%, 41%, and 50%, respectively. The simulations also indicate a resonance wavelength redshift of ∆λ0 = 4 nm for tTi = 5 nm. In Fig. 3(b), the decrease of the LSPR quality-factor with increasing tTi is shown for the experimental (red circles) and simulated (blue squares) cases. The solid lines represent fits to an exponential model (see Methods section). The experimental quality-factor (Fig. 2) decreases more than the simulated quality-factor as tTi is increased, with a decay length of α = 2.31 nm, while the decay length of the simulated quality-factor is α = 1.81 nm. We attribute the greater LSPR 9



damping in the experimental samples to small amounts of Ti impurities that diffuse in the Au layer during the deposition step. This can be explained by first considering the properties of PC-Au materials at optical frequencies. 1.0 1.0

1.0

(a)

0.6 0.6 0.4 0.4

0.2 0.2

(b)

0.8

Qn/Q0

0.8 0.8

|Emn/Em0|2

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

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t Ti 0 nm (n = 1 nm (n = 3 nm (n = 5 nm (n =

0) 1) 3) 5)

0.6 0.4 Sim.

0.2

0.0 0.0 500 550 600 650 650 700 700 750 500 550 600 750

Wavelength / nm

0.0 00

Exp.

11

22

33

44

55

tTi / nm

Figure 3. (a) 2D FDTD simulations of normalized maximum electric field |Emn/Em0|2 spectra generated in the nanocavity (with TM polarization and |Ei| =1) of a nanostructured PC-Au surface with Λg = 250 nm, g = 10 nm and different titanium layer thicknesses. No titanium layer (n = 0): λ0 = 667 nm, ζ = 97 nm; tTi = 1 nm (n = 1): λ0 = 668 nm, ζ = 128 nm; tTi = 3 nm (n = 3): λ0 = 669 nm, ζ = 165 nm; tTi = 5 nm (n = 5): λ0 = 671 nm, ζ = 195 nm. (b) Experimental (red circles) and simulated (blue squares) normalized qualityfactor Qn/Q0 as a function of tTi. Solid red and blue lines are fits to an exponential decay model.

The excitation of LSPR modes on metal nanostructures depends on the metal dielectric function ε(ω) = ε1(ω) + iε2(ω), where ε1(ω0) < 0 is required for a dispersionless surrounding medium εd > 0 at resonance frequency ω0. The large negative ε1 of PC-Au (Fig. 4(b)) facilitates the generation of LSPR modes with an enhanced electric field in the near-zone, as shown in the nanocavity in Fig. 1(c). However, intrinsic plasmon damping, represented in ε2, limits the quality-factor and near-field enhancement. Furthermore, the components of the dielectric function can be expressed as ε1(ω) ≈ ωp2/ω2 and ε2(ω) ≈ ωp2γ/ω3, where ωp is the bulk resonance frequency, and γ is the electron damping rate that accounts for intrinsic plasmon damping.18 The effects of interband transitions are not included in these expressions. The 10


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quality-factor can be expressed as Q ≡ ω0/2γ ≈ (ω∂ε1/∂ω)/2ε2.19 Both ∂ε1/∂ω and ε2 affect the qualityfactor of nanoplasmonic resonators. For wavelengths above 600 nm, the upper limit of the qualityfactor of PC-Au nanoplasmonic resonators is Q ~ 20.20 The experimental quality-factor is typically much less than this upper limit due to radiation damping, material impurities, and structural inhomogeneities.21 12

20

4 2

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0 0

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10 15 20 25 30 35 40

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Au Ti Cr

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Au

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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


SiN

10 15 20 25 30 35 40

x / nm

Figure 4. (a,b) Dielectric data of PC-Au (triangle),22 Ti (circle),23 and Cr (square).23 (c,d) Profiles of the electric field in the nanocavity |E(x,zi)/Ei| and the field in the PC-Au nanostructure |EAu(x,zi)/Ei| from 2D FDTD simulations. (c) z1 = 5 nm (d) z2 = −20 nm. Insets: dashed white line indicates the location of the profile scan.

The electric field that penetrates into a planar metal surface has an exponential length dependence, i.e. EAu(x) ∝ exp(x/δ), where x is the coordinate normal to the surface and δ is the skin depth24, which is the distance that the amplitude of EAu(x) decreases to 1/e of its value at the surface and is defined as δ = c/ωκ, where c is the speed of light and 2κ = (ε12 + ε22)1/2 – ε1.18 The simulated field profile, shown in Fig. 4(c) (red trace), decreases exponentially into the metal and a fit to an exponential decay model gives δ ≈ 11



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16 nm, which is consistent with the penetration depth of planar PC-Au thin films in the visible spectrum (Supporting Information, Section 2). From the simulated profiles shown in Figs. 4(c) and 4(d), the electric field in the PC-Au layer |EAu/Ei|, the penetration depth of is sufficient to overlap with the Ti layer in the narrowest regions of the nanostructure (Fig. 1(b)). Such significant plasmon damping by the interaction of the plasmon field with Ti occurs because ∂ε1/∂ω is smaller by a factor of 14× and ε2 is larger by factor of 10× compared to PC-Au. As tTi is increased, the field overlap with the Ti layer increases, and therefore, plasmon damping further increases leading to a further reduction of |E/Ei|2, broadening of the LSPR band, and redshift of the resonance wavelength, as shown in Fig. 3. However, the reduction of the field in the nanocavity, LSPR band broadening, and redshift of the resonance wavelength are greater for the experimental measurements compared to those indicated by the simulation results. These observations are consistent with previous reports.6-10 For PC-Au nanostructures smaller than the skin depth, thin Ti layers (tTi ~ 1-2 nm) strongly dampen the LSPR.9 Plasmon damping was reported to be less severe for 50 nm thick PC-Au split-ring resonators.8 Siegfried et al. reported that plasmon damping decreased as the PC-Au thickness was increased up to 80 nm.10 Plasmon damping was further reduced for 200 nm thick PC-Au layers.6 Moreover, the Ti layer can increase the rate of plasmon dephasing through charge transfer across the metal-metal interface.9 Since plasmon damping has been observed for a wide range of thicknesses beyond the skin depth, it is likely that interdiffusion of adhesion-metal impurities into the as-deposited PC-Au layer contribute to plasmon damping. Especially, given the extensive evidence of interdiffusion of Ti impurities into PC-Au layers prepared by physical vapor deposition.11,12 Tisone and Drobek reported the presence of Ti and AuxTi impurities in the grain boundaries of as-deposited Au thin films with diffusivity of DTi ≈ 5×10–15 cm2/s.11 This high diffusion rate of 10 nm/min has been attributed to intense grain boundary diffusion, and would result in the distribution of Ti impurities about 14 nm from the Au-Ti interface (Supporting 12


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Information, Section 3).12 Therefore, a far greater volume of Ti overlaps with the electric field in the PCAu layer, compared to the simulated results, which can account for the greater plasmon damping and resonance wavelength redshift observed in the experimental measurements (Fig. 2). There is further evidence that Ti impurities are present in the PC-Au layer in SERS measurements.

Figure 5. SERS of BT SAM on nanostructured PC-Au surfaces with different tTi. (a) No Ti layer (b) tTi = 1 nm. (c) tTi = 3 nm. (d) tTi = 5 nm. Blue traces: raw data; Red traces: modeled vibration bands; Green traces: background model. Note the different scales in (a,b) and (c,d). Figures 5(a)-(d) show SERS spectra from BT SAMs on nanostructured PC-Au surfaces with three different Ti layers, which are summarized in Table 1. It is important to note that the SERS measurements from the nanostructured PC-Au surfaces are highly uniform with less than 10% variation in the SERS enhancement over large areas (Supporting Information, Section 4).16 First, we consider the decrease of the vibration band intensities as tTi is increased using the ratio defined as In/I0, where In is the integrated intensity with an adhesion layer and I0 is the integrated intensity without an adhesion layer. Siegfried et 13



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al. reported the reduction of SERS enhancements by Ti, Cr, and TiO2 adhesion layers by monitoring SERS of the ring-breathing mode of benzene-ethanethiol; however, no spectral details were provided.10

Table 1. SERS of BT SAM for different tTi. BT Band assignments: 7a(a1), v(C-S) + β(C-C-C); 16b(b1), γ(C-CC); 6a(a1), v(C-S) + β(C-C-C); 12(a1), β(C-C-C); 18a(a1), β(C-H); 6a(a1)+7a(a1), [ν(C-S) + β(C-C-C)]]; 15(b2),

β(C-H); 9a(a1), β(C-H); 8a(a1), ß(C-C).25 Labels γ, β, and ν indicate out-of-plane bending, in-plane bending, and stretching modes, respectively. ∆ν is the Raman shift, In/I0 is the ratio of integrated band intensities for a particular tTi, and ∆n is the full-width half-maximum linewidth of each modeled vibration band. †Band intensity very weak. ‡See Supporting Information, Section 6.

SERS

tTi = 0 nm 2 (Q0/Q0) = 1.0 ∆ν ν / cm

–1

∆0 / cm

tTi = 1 nm 2 (Q1/Q0) = 0.49 –1

∆ν ν / cm

–1

I1/I0

tTi = 3 nm 2 (Q3/Q0) = 0.18

∆1 / cm

–1

∆ν ν / cm

–1

I3/I0

∆3 / cm

7a(a1)

416

12

418

0.59

11

419

0.009

11

16b(b1)

473

11

474

0.58

10

-

-

-

6a(a1)

691

9

693

0.60

9

695†

0.046

43‡

12(a1)

994

7

997

0.56

6

998

0.006

4

18a(a1)

1018

8

1021

0.52

7

1020

0.008

8

1(a1)

1068

12

1070

0.58

11

1072

0.008

12

6a(a1)+7a(a1)

1107

14

1109

0.56

14

-

-

-

15(b2)

1152

7

1152

0.48

6

-

-

-

9a(a1)

1173

8

1176

0.55

9

-

-

-

19a(a1)

1463

12

1466

0.56

13

-

-

-

8a(a1)

1565

11

1568

0.67

10

1569

0.012

9

–1

For tTi = 1 nm, eleven vibration bands of the BT SAM are present with similar reductions in band intensity and vibration linewidths ∆n consistent with the SERS spectrum without an adhesion layer. For tTi = 3 nm, five of the vibration bands are present, and vibrations with b1 and b2 symmetries, i.e. 16b(b1) and 15(b2), and weak a1 modes, i.e. 6a(a1)+7a(a1), 9a(a1), and 19a(a1) have vanished. For tTi = 5 nm, only one broad band near 712 cm–1 is observed, which we do not associate with BT alone (Supporting Information, Section 6). If the quality-factor reduction is due to plasmon damping caused by the overlap of the field with Ti in the PC-Au layer, then the SERS enhancement G should decrease proportionally 14


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with Q4, i.e. based on G ∝ Q4/Vm2, where Vm is the effective LSPR mode volume in the nanocavity.26 Furthermore, since G ∝ I2, where I is the integrated SERS intensity, then Q2 ∝ I. Assuming that Vm is the same for each measurement, the quality-factor ratio (Qn/Q0)2 should decrease proportionally with In/I0 as tTi is increased.

Figure 6. Comparison of normalized integrated intensities In/I0 of several vibration modes and (Qn/Q0)2 as a function of tTi.

From Table 1, the ratios (Q1/Q0)2 and I1/I0 decrease similarly for tTi = 1 nm. However, as the thickness is increased to tTi = 3 nm, the ratios diverge; I3/I0 decreases by up to 30× greater than (Q3/Q0)2, with the exception of the weak 6a(a1) vibration at 695 cm–1 (Supporting Information, Section 6). As the Ti layer is increased to tTi = 5 nm, the ratios diverge further, where (Q5/Q0)2 ≈ 0.08, but no BT spectra can be observed (Fig. 5(d)). It is important to note that we have neglected the chemical enhancement effect, however; it is generally accepted that the electromagnetic enhancement is the dominant effect in the SERS effect. Thus, we expect that these general relationships will hold in this case. The comparisons of In/I0 (five vibration bands) and (Qn/Q0)2 as a function of tTi are summarized in Figure 6. Furthermore, for the laser power used (1 mW) and integration time (100 ms), no degradation of the SAM has been observed.16 15



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Based on this, we believe there is an additional loss mechanism responsible for the strong suppression of the SERS enhancement as tTi is increased beyond 1 nm that is related to Ti impurities in PC-Au layer. When a transition metal, such as Ti, is located in a Au host, the impurity electrons form virtual bound states with a density of states near the Fermi energy of the host.13 (Supporting Information, Section 7) The virtual bound states of dilute Ti concentrations in PC-Au can significantly change the optical absorption characteristics of PC-Au by further increasing the plasmon dephasing rate and by broadening interband transitions to lower energies.14,15 Although, the hybridized sp bands of Au resemble freeelectron behavior, there are two interband d-sp transitions in the visible spectrum near ∆X ≈ 1.9 eV and ∆L ≈ 2.4 eV. Tangamonsiri et al. reported that with 1.7 at.% Ti in an Au host, the optical absorption band, near ∆X and ∆L, were changed to a broad absorption spectrum extending down to 1.6 eV.15 This broadening of the absorption band of Au results in a strong overlap with the energy of the HeNe laser (1.96 eV), which explains the strong SERS suppression as the amount of Ti impurity increases. For tTi = 5 nm, the maximum impurity concentration that can interdiffuse to the Au layer is 12 at.%. Thus, a small fraction of Ti in the PC-Au during deposition can account for the suppression of SERS where a large fraction of incident laser power is strongly absorbed by the virtual bound states of the Ti impurities in the PC-Au layer.

CONCLUSIONS In summary, we showed that Ti adhesion layers attenuate, broaden, and redshift the LSPR band, as well as strongly suppress SERS on PC-Au nanostructures. We attribute the LSPR attenuation, spectral broadening and wavelength redshift to the overlap of the electric field in the PC-Au layer with the Ti layer at the interface, as well as Ti impurities in the PC-Au layer. Titanium impurities are known to interdiffuse through the grain boundaries of PC-Au during the deposition of the Au-Ti bimetal thin film, which can increase the plasmon dephasing rate. We attribute the strong suppression of SERS to the 16


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plasmon resonance broadening caused by the Ti layer and impurities, and to virtual bound states formed by the Ti impurities that further increase the plasmon dephasing rate, while also broadening the interband absorption of PC-Au. For Au nanostructures with dimensions more than 50 nm, ultrathin Ti layers (~1-2 nm) do not significantly dampen the LSPR and SERS and provide a good trade-off between SERS enhancement and substrate reliability.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. SEM images of nanostructured Au surfaces with and without the Ti adhesion layers, decay of electric field into the Au layer from simulated data, discussion of titanium interdiffusion into gold, uniformity of the SERS measurements, discussion of the gold dielectric function, discussion of the 6a(a1) vibration band, gold band structure and virtual bound states, and details of numerical simulation model

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Notes † L. Le Thi Ngoc and T. Yuan contributed equally to this work. ‡ This research was performed while L. Le Thi Ngoc was at the University of Twente, Enschede, 7522 NH, The Netherlands. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Vietnamese Overseas Scholarship Program (Project number 322) and the Dutch Technology Foundation (STW, Project 11957) for the support of this work. We also thank Johan Bomer for help with fabrication of the plasmonic surfaces.

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10. Siegfried, T.; Ekinci, Y.; Martin, O. J. F.; Sigg, H. Engineering Metal Adhesion Layers That Do Not Deteriorate Plasmon Resonances. ACS Nano 2013, 7, 2751-2757. 11. Tisone, T. C.; Drobek, J. Diffusion in Thin-Film Ti-Au, Ti-Pd, and Ti-Pt Couples. J. Vac. Sci. Technol. 1972, 9, 271-275. 12. Martinez, W. E.; Gregori, G.; Mates, T. Titanium Diffusion in Gold Thin Films. Thin Solid Films 2010, 518, 2585-2591. 13. Anderson, P. W. Localized Magnetic States in Metals. Phys. Rev. 1961, 124, 41-53. 14. Beaglehole, D. A Study of the Virtual Bound State Dielectric Constant. J. Phys. F: Met. Phys. 1975, 5, 657-668. 15. Tangamonsiri, S.; Gilberd, P. W.; Kaiser, A. B. Virtual Bound States in AuTi Alloys. Solid State Commun. 1977, 24, 125-127. 16. Le Thi Ngoc, L.; Jin, M.; Wiedemair, J.; van den Berg, A.; Carlen, E. T. Large Area Metal Nanowire Arrays with Tunable Sub-20 nm Nanogaps. ACS Nano 2013, 7, 5223-5234. 17. Yuan, T.; Le Thi Ngoc, L.; van Nieuwkasteele, J.; Odijk, M.; van den Berg, A.; Permentier, H.; Bischoff, R.; Carlen, E. T. In Situ Surface-Enhanced Raman Spectroelectrochemical Analysis System with a Hemin Modified Nanostructured Gold Surface. Anal. Chem. 2015, 87, 2588-2592. 18. Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Berlin, 1995; pp 14-22. 19. Wang, F.; Shen, Y. R. General Properties of Local Plasmons in Metal Nanostructures. Phys. Rev. Lett. 2006, 97, 206806. 20. Olmon, R. L.; Slovick, B.; Johnson, T. W.; Shelton, D.; Oh, S.-H.; Boreman, G. D.; Raschke, M. B. Optical Dielectric Function of Gold. Phys. Rev. B 2012, 86, 235147. 21. Heilweil, E. J.; Hochstrasser, R. M. Nonlinear Spectroscopy and Picosecond Transient Grating Study of Colloidal Gold. J. Chem. Phys. 1985, 82, 4762-4770. 22. Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370-4379. 23. Palik, E. D. The Handbook of Optical Constants; Academic Press: San Diego, CA, 1998; pp 240-249 and pp 374-385.

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24. Jackson, J. D. Classical Electrodynamics, Third Edition, John Wiley & Sons, Inc., New Jersey, 1999, p. 354. 25. Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Elsevier: Amsterdam, 1969; pp 142-393. 26. Maier, S. A. Plasmonic Field Enhancement and SERS in the Effective Mode Volume Picture. Opt. Express 2006, 14, 1957-1964.

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TOC

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Suppression of Surface-Enhanced Raman ... - ACS Publications

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