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Relationship between Surface-enhanced Raman scattering (SERS) and surface enhanced hyper Raman scattering (SEHRS) analyzed with single Ag nanoaggregates.

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Presentation on theme: "Relationship between Surface-enhanced Raman scattering (SERS) and surface enhanced hyper Raman scattering (SEHRS) analyzed with single Ag nanoaggregates."— Presentation transcript:

1 Relationship between Surface-enhanced Raman scattering (SERS) and surface enhanced hyper Raman scattering (SEHRS) analyzed with single Ag nanoaggregates adsorbed by dye molecules AIST 1, Kwansei Gakuin Univ. 2 Osaka Univ. 3 Tamitake Itoh 1, Vasudevanpillai Biju 1, Mitsuru Ishikawa 1, Yukihiro Ozaki 2, Hiroyuki Yoshikawa 3, Takuji Adachi 3, Hiroshi Masuhara 3

2 Outline Electromagnetic (EM) mechanism of SERS Relationship between SERS and SEHRS

3 EM mechanism of SERS

4   M   sc   (   D  ) I (  sc ) R 12 1 (M (i )(M (i ) SERS intensity: E(i)E(i) { } 2 Two-fold dipole-dipole coupling  ll hh   ffii gg gg Two-fold EM enhancement in SERS process B. Pettinger, JCP. 85, 7442 (1986). ħ (  I -   ħIħI Adsorbed molecule Plasmon resonance molecular resonance

5 nm Particle-by-particle variations in Plasmon resonance maxima Wavelength / nm Intensity / a.u. Increment and red-shift of plasmon resonance bands induced by increment of particle size

6 100×120  m Raman shift/cm Normalized Intensity (a.u.) Plasmon resonance Rayleigh scattering image SERRS image SERS spectrum Plasmon resonance spectrum Wavelength / nm

7 Experimental set-up Kr laser 568 nm Plasmon resonance spectrum SERS spectrum Dark-field condenser

8 Wavelength /nm Cross section /  m 2 × nm 2.5× × ° 30° 60° 90° 120° 150° Photon energy /eV Normalized intensity [arb.u.] Polarization angle  / degree Comparison of experimental and FDTD calculation results 2.0×

9 Polarization dependence of SERS spectra 0° 30° 60° 90° 120° 150° ° 30° 60° 90° 120° 150° Intensity [count] Photon energy / eV Normalized intensity [arb.u.] Polarization angle  /degree Photon energy /eV Normalized intensity [arb.u.] Polarization angle  / degree Normalized intensity [arb.u.] 2.5×

10 SERS enhancement factor: M 1st enhancement 2nd enhancement Adsorbed molecule Ag nanoaggregate ħ (  I -   ħIħI Inoue and Ohtaka. J. Phys. Soc. Jpn. 52, 3853 (1983) Practical calculation of SERS spectra using Two-fold SERRS EM enhancement model SERS cross section:  SERS (cm -2 ) Resonance Raman cross section:  RRS (cm -2 ) Fluorescence cross section:  FL (cm -2 ), q : quenching factor 1st enhancement2nd enhancement

11 Reproduction of SERS spectra × X cm Wavelength (nm) = X nm Wavelength (nm) X cm 2 M in M sca σ SERRS σ RRS + q σ FL 568 nm x experiment calculation Wavelength (nm) σ SERRS (cm 2 ) X (cm 2 ) X (cm 2 ) Wavelength (nm) × q +

12 568 nm q=5X10 -6 R × × ×10 9 Reproduced nanoaggregate-by-nanoaggregate variations in SERS spectra of R123 Wavelength (nm) M in M sca σ Rayleish (cm 2 ) σ SERRS (cm 2 ) blue : experiment red : calculation x x x x x x x x nm

13 514 nm Wavelength (nm) 568 nm 647 nm q=1X10 -8 σ SERRS (cm 2 ) blue : experiment red : calculation Reproduced variations in SERS spectra of R6G of the same nanoaggregate for three excitation wavelength R6G σ Rayleish (cm 2 ) M in M sca R6G 2.9× ×10 9 Wavelength (nm) M in M sca x x x x10 9 M in M sca =6.3× x x x x x 拡大表示

14 Results (1) We developed quantitative SERS model including excitation wavelength, molecular absorption bands, molecular fluorescence bands, plasmon resonance bands according to 2-fold enhancement theory. (2) The SERS model quantitatively reproduced and explained variations in SERRS spectra. Result (1)-(2) revealed that SERS spectra are simply described as follows; Conclusion Peak values of M in M sca are around 10 9 – Values of q are around –

15 Relationship between SERS and SEHRS,

16 T. Itoh et al, APL 88, , 2006 Background Light-Emission (BLE) of Surface-enhanced hyper Raman scattering (SEHRS) This is a typical SEHRS, BLE, and SEHRlS spectrum of R6G adsorbed on an Ag nanoaggregate. Such spectrum can be measured even using cw NIR laser. 532 nm Intensity [count/2s] Wavelength / nm SEHRS Week SEHRlS BLE G. Brehm, et al, J. Mol. Struct. 735, 85 (2005). 1. BLE is always overlapped with SEHRS using cw NIR laser excitation. We focus on the BLE to elucidate a detailed mechanism of SEHRS.

17 Enhancement factors; M SERRS, M BLE E in : incident electric field E Loc : local electric field : frequency of the incident laser light mol  : vibrational frequency 1st enhancement2nd enhancement Adsorbed molecule h ( I -  h I Ag nanoaggregate 1st enhancement2nd enhancement L  : fluorescence frequency Consideration of enhancement mechanism of SEHRS, BLE, SEHRlS deduced from SERRS two-fold EM enhancement thory 1st enhancement 2nd enhancement Inoue, JPSJ. 52, 3853 (1983) Enhancement factors; M SEHRS, M BLE, M SEHRlS H. Xu. PRL. 93, (2004). M. Moskovits, Rev. Mod. Phys. 57, 783 (1985). B. Pettinger, J. Chem. Phys. 85, 7442 (1986).

18 O1 P L White light C Ag nanoaggregates Optical Fiber Polychromator + CCD O1 P L Laser beam (1064 nm) DM O1 P L N Laser beam (532 nm) O2 Experiment setup Plasmon resonance band Wavelength/nm SERRS and BLE SEHRS, BLE and SEHRlS Laser power density Max 6 MW/cm 2 Laser power density Max 30 W/cm Wavelength/nm pinhole lens 200  m objective

19 Intensity (counts) Wavelength / nm s 2-4 s 4-6 s 6-8 s 8-10 s s s s Temporal fluctuation of SEHRS and BLE spectra from a single Ag nanoaggregate SEHRS, BLE, and SEHRlS SEHRS spectra often show intermittence of intensity on the time scale of several seconds. Intermittence on this time scale is too slow considering diffusion of free molecules crossing a SEHRS-active site because the time scale due to Brownian motion is within a millisecond. However, chemical affinity between R6G and Ag surfaces decreases the intermittence rate and such slow intermittence can be one proof of single molecule detections [A. Weiss, JPCB 105, (2001)]. Following the previous work on SERRS, we consider that the SEHRS signals in the present experiment is also a proof of single molecule detections.

20 Comparison between BLE spectrum of SEHRS and that of SERRS from large number of Ag nanoaggregates Wavelength / nm Residual spectrum after subtracting I SEHRS ( ) from I SERRS ( ) is similar to fluorescence spectrum of monomer R6G SEHRS with BLE Fluorescence of monomer R6G Normalized intensity Wavelength / nm I SEHRS ( ) SERRS and BLE Fluorescence of monomer R6G Wavelength / nm Normalized intensity) I SERRS ( ) I SERRS ( ) - I SEHRS ( ) = 500×500  m The residual spectrum indicates that R6G monomers cannot have SEHRS activity.

21 Spectral variations in BLE of SEHRS from single Ag nanoaggregates (not from larger number of Ag nanoaggregtes) 2-photon fluorescence spectrum of monomer R6G SEHRlS spectra from single Ag nanoaggregate Wavelength / nm Intensity (counts) SEHRS with BLE BLE spectra of SEHRS are different from nanoaggregate to nanoaggregate. These various spectra are composed of three bands indicated by red-lines whose positions are red-shifted from fluorescence maxima of monomer R6G. SEHRS, BLE, and SEHRlS spectra from three single Ag nanoaggregates

22 Comparison of BLE of SEHRS with that of SERRS for identical single Ag nanoaggregates BLE spectra of SEHRS are similar to those of SERRS for the identical nanoaggregates except fluorescence of monomer R6G. Wavelength / nm Identical four Ag nanoaggregates SEHRS with BLE photon fluorescence spectrum of monomerR6G Intensity (counts) SERRS with BLE Intensity (counts) Wavelength / nm 1 photon fluorescence spectrum of monomer R6G 200×200  m

23 1.Why is monomer fluorescence of R6G not observed ? 2. Why is fluorescence of aggregates selectively observed ? 1. BLE spectra of SEHRS are composed of three bands whose maxima are red- shifted from fluorescence maxima of monomer R6G. 2. BLE spectra of SEHRS are similar to those of SERRS for the identical nanoaggregates except monomer fluorescence. Based on the red-shifts and the similarity, we attributed the three BLE bands of SEHRS to two-photon fluorescence of J-like aggregates of R6G molecules. Indeed, several papers concluded such red-shifts arise from linear aggregation of R6G. (e.g. C. T. Lin, et al, CPL 193, 8 (1992), J. Bujdak 2006, 110, 2180 (2006) JPCB) Conclusion 1 Questions 3. Why is SEHRS intensity comparable to SEHRlS intensity ?

24  BLE1 of J-like aggregates of dye molecules is ~8-10 times larger than that of monomers because of increase in transition dipole moment (C. T. Lin, et al, Chem. Phys. Lett. 193, 8 (1992)). Thus,  BLE2 of J-like aggregates is expected to be ~ times larger than  BLE1. This increasing can compensate the smaller  BLE2. Thus, this compensation may be the reason for selective observation of fluorescence from J-like agrgegates. 1. Why is monomer fluorescence not observed ? 2. Why fluorescence of J-like aggregates is selectively observed ?  BLE1 (1.9 x cm 2, J. Opt. Soc. Am. B 13, 481 (1996). ) is 3.0 x 10 7 times larger than the effective two-photon cross-section  BLE2 (6.4 ± 0.6 x cm 2 at (6x10 5 ) MW/cm 2 ). (here, 10 5 is conventional EM field enhancement factor for single molecule detection) However, excitation power for two-photon fluorescence (6 MW/cm 2 ) is only ~ 5.0 x 10 5 times larger than that of one-photon fluorescence (30 W/cm 2 ). Thus, the small  BLE1 of monomer may be the reason for lack of observation of monomer fluorescence. Our answers 3. Why is SEHRS intensity comparable to SEHRlS intensity ? It will be discussed later. Too small one-photon cross-section of monomer R6G:  BLE1 Larger  BLE2 of R6G J-aggregetes than that of monomers

25 Intensity variations of SEHRS, BLE, and SEHRlS intensity Scattering of data points of single nanoaggregate measuremets is much larger than that of large aggregate measurements. BLE intensity (counts) SEHRS intensity (counts) SEHRlS intensity (counts) Wavelength / nm Single Ag nanoaggregate Large number of Ag nanoaggregates 500×500  m SEHRS intensity (counts) SEHRlS intensity (counts) x BLE intensity (counts) SEHRS intensity (counts) Wavelength / nm

26 Origin of Ag nanoaggregate by nanoaggregate variations SEHRS, BLE, and SEHRlS spectra are modulated by plasmon resonance due to 2nd enhancement. In other words, the scattering of data points is indirect evidence of 2nd enhancement. 2st enhancement (plasmon resonance) Wavelength / nm Intensity (a.u.) Wavelength / nm Single Ag nanoaggregateLarge number of Ag nanoaggregates

27 Spectral blue-shifts in plasmon resonance Rayleigh scattering and BLE spectra Wavelength / nm Relative intensity (counts) Intensity (counts) Relative intensity (counts) Intensity (counts) BLE of SEHRSBLE of SERRSPlasmon resonance Blue-shifts in BLE spectra of SEHRS, BLE spectra of SEHRlS, plasmon resonance spectra coincidentally happened.

28 Origin of the spectral blue-shifts of BLE Wavelength / nm Intensity (a.u.) 2st enhancement (plasmon resonance) SEHRS, BLE, and SEHRlS spectra has modulated by plasmon resonance due to 2nd enhancement. In other words, the blue-shift is direct evidence of 2nd enhancement.

29 Conclusion (2) Ag nanoaggregate by nanoaggregte variations in SEHRS, BLE, and SEHRlS spectra support that their signals are enhanced through two-fold EM interactions described as following; (1) Spectral analysis of BLE revealed that J-like aggregates of R6G molecules selectively show SEHRS and BLE because of their larger dipole moment than that of monomers. Unclosed question Why is SEHRS intensity comparable to SEHRlS? (SEHRlS intensity should be several hundred times larger than SEHRS intensity.)

30 Laser power dependence of SEHRS, BLE, and SEHRlS from large number of Ag nanoaggregates SEHRlS shows nonlinear response, but SEHRS and BLE does show nonlinear responses. BLE intensity (counts) SEHRS intensity (counts) Incident laser power (kWcm2)Incident laser power (kW/cm2) SEHRlS intensity (counts) Incident laser power (kW/cm2) Wavelength / nm Intensity (counts/2s) 500×500  m 1. Why does only SEHRlS show nonlinear response? 2. Why does SEHRS and BLE not show nonlinear response?

31 Why does SEHRS and BLE show linear dependence even SEHRlS shows nonlinear dependence? Destruction of R6G molecules by laser excitation We checked SEHRS intensity several times for the same Ag nanoaggregates, but they showed almost same intensity. Thus, we think that destruction of Ag nanoaggregates may not be a reason for lack of nonlinear dependence of SEHRS and BLE. Saturation of nonlinear resonance of R6G by high power excitation We think this is an important candidate to explain the lack of nonlinear dependence. SEHRlS from mainly Ag nanoaggregates Ag nanoaggregates which do not show SEHRS show SEHRlS. Thus, a part of SEHRlS photons is independently generated from directly Ag nanoagregtas. Thus, defines the nonlinear dependence of SEHRlS arises from nonlinear polarization of Ag nanoaggregates Intensity (counts/2s) Wavelength / nm 1. Why does only SEHRlS show nonlinear response? 2. Why does SEHRS and BLE not show nonlinear response?

32  BLE2 of R6G monomer = 2.0 x cm 4 sec/photon. Effective  BLE2 of R6G 6.4 x cm 2 Incident photon density 6 MW/cm 2 = 3.2 x photon/sec cm 2 Expected enhanced local photon density 6 x 10 5 MW/cm 2 = 3.2 x photon/sec cm x photon/sec cm 2 X 6.4 x cm 2 = 2.05 x 10 9 photon/sec Absorption 8.2 photon/molecule Life time of R6G = 4 x sec (R. F.Kubin,. J. Lumin. 1982, 27, 455.) Almost R6G molecules is always photo-excited. Thus, saturation effect may be reasonable from the estimation. Disappearance of two-photon absorption (optical resonance) due to saturation effect. Simple estimation of saturation of nonlinear optical resonance Estimated  BLE2 of R6G monomer = 2.0 x cm 4 sec/photon.

33 SERRS intensity (counts) SEHRS intensity (counts) Relationship between intensity of SERRS and that of SEHRS Intensity of SERRS and SEHRS does not have any correlation even both of them are from identical Ag nanoaggregates. The lack of correlation indicates that intensity of SEHRS depends on enhanced EM fields at both 532 nm and 1064 nm, but intensity of SERRS depends on enhanced EM fields at 532 nm only Wavelength / nm SERRS SEHRS 1st enhancement 532 nm 1064 nm Common 1st enhancement Wavelength / nm C3C4 SERRSSEHRS

34 Second enhancement in SERS StokesAnti-Stokes Laser line Wavelength / nm Intensity (a.u.) Band shape of plasmon resonance Wavelength / nm = The correlation between plasmon resonance and SERRS spectra shows that SERRS bands overlapping with a vicinity of plasmon resonance maximum are selectively enhanced. For example, anomalous anti- Stokes bands are result of coupling SERRS and plasmon having maximum in the anti-Stokes region.

35 Comparison between background light-emission spectrum of SEHRS and that of SERRS from large number of Ag nanoaggregates Residual spectrum after subtracting I SEHRS ( ) from I SERRS ( ) is similar to fluorescence spectrum of monomer R6G SERRS with background light-emission and Fluorescence of R6G Wavelength / nm Normalized intensity) 500×500  m R6G monomers cannot have SEHRS activity?

36 (1) Lack of intensity correlation between SERRS and SEHRS indicates that 1st enhancement is not common for them. (2) Linear intensity correlation among SEHRS, background light-emission, and hyper-Rayleigh scattering indicates that those kinds of light are generated through common 1st enhancement. Conclusion 2 Results (1) and (2) support the comprehensive mechanism of SEHRS, background light-emission, hyper- Rayleigh scattering provided as follows; A. K. Sarychev, et al, PRB 60, 16389(1999). Polarization

37 Intensity (counts) A1A2 A3A4 B1B2 B3B4 C1C Wavelength / nm C3C4 Manners of disappearance of SEHRRS with background light- emission Nanoaggregate A Nanoaggregate B Nanoaggregate C SERRSSEHRS

38 Background light emission intensity (counts) F Peak wavelength / nm SEHRRS intensity (counts) E Peak wavelength / nm Relationship between SEHRRS intensity and peak wavelength of plasmon resonance bands Above Eqs. imply that efficient coupling between incident NIR light and plasmons contributes to stronger first enhancement. This means that Ag nanoaggregates whose plasmon resonance maxima in longer wavelength region are advantageous to get larger first enhancement. Indeed, Figs. 3E and F approximately indicate that Ag nanoaggregates whose plasmon resonance maximum wavelength is longer than 650 nm show larger intensity of SEHRRS and background-light emission than those shorter than 650 nm Wavelength / nm SEHRRS and background light-emission Plasmon resonance maxima 1064 nm

39 SERRS intensity (counts) Background light emission intensity (counts) A Relationship between SERRS and background light-emission The positive correlation indicates that SERRS and background light-emission are generated from common enhanced EM local fields. This indication agrees with the SERRS-EM model which describes that incident EM fields which are coupled with plasmons induce both SERRS and its background light-emission. 532 nm Wavelength / nm SERRS with background light-emission Common 1st enhancement

40 0 Wavelength (nm) Normalized intensity (a.u.) A B C D E F Luminescence maximum (nm) Plasmon resonance maximum (nm) Intensity (counts) Wavelength / nm 0-2 s 2-4 s 4-6 s 6-8 s 8-10 s s s s

41 Origin of SERRS background light-emission (a) E metal surface adsorbed molecule electron EPEP S 1 state S 0 state CT state EFEF hihi hlhl (b) hihi EFEF E metal surface adsorbed molecule electron EPEP hlhl S 1 state S 0 state E CT T. Itoh et al, JPC B, 110, 21536, 2006 Normalized intensity (a.u.) Wavelength (nm) Fluorescence spectrum of R6G in an aqueous solution We attributed the three background light-emission to fluorescence coupled with plasmon and emitted from monomer, dimer, and two kinds of higher-order aggregates of R6G molecules on an Ag surface. SERRS image

42 Incident light Electric field Polarization ① ② ③ 時間領域差分法による電場計算 SERS 発現メカニズム-電磁場増強モデル- 表面プラズモン共鳴 ( SPR ) 10 2 ~ 10 3 程度の電場増強 10 8 ~ 程度の電場増強 SPR によって生じる局所増強電 場が SERS を引き起こしている


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