Enhanced Light Emission from Plasmonic Nanostructures by Molecules

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Enhanced Light Emission from Plasmonic Nanostructures by Molecules Yuqing Cheng, Jingyi Zhao, Te Wen, Guantao Li, Jianning Xu, Aiqin Hu, Qihuang Gong, and Guowei Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08179 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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The Journal of Physical Chemistry

Enhanced Light Emission from Plasmonic Nanostructures by Molecules Yuqing Cheng,1 Jingyi Zhao,1 Te Wen,1 Guantao Li,1 Jianning Xu,1 Aiqin Hu,1 Qihuang Gong,1,2 and Guowei Lu1,2,* 1

State Key Laboratory for Mesoscopic Physics & Collaborative Innovation Center of

Quantum Matter, Department of Physics, Peking University, Beijing 100871, China 2

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China

ACS Paragon Plus Environment


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Abstract Light emission from plasmonic nanostructures can be enhanced by accompanying molecular Raman scattering during surface enhanced Raman scattering (SERS). The interaction between the nanostructure and the molecule was modeled based on the concept of quantized optical cavity. We found that the plasmonic nanostructures can elastically scatter the energy coupling from the molecules, resulting in significantly enhanced molecule Raman scattering. Meanwhile part of the coupling energy was inelastically radiated through plasmon emission resulting in a slight enhancement in background emission. By comparing a single nanoparticle experiment and theoretical calculations, we reveal that the light emission enhancement from the nanostructures is ascribed mainly to the increase in local field felt by the nanostructures. Since the “hot spot” resulting from the nanostructures can strongly polarize the molecules, the induced field of the polarized molecules influences the local field in turn. These findings contribute to the deeper understanding of surface enhanced Raman scattering and suggest that plasmonic nanostructures and molecules are considered as a hybrid entity to analyze and optimize surface enhanced spectroscopy.


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Introduction Surface enhanced Raman scattering (SERS) has been widely investigated since the first report 40 years ago, due to the possibility of applying vibrational spectroscopy to structurally sensitive molecule and nanoscale probes.1 A broad consensus within the SERS community is that the SERS mechanism is mainly a consequence of local electromagnetic enhancement of localized surface plasmon resonances (LSPs) supported in metallic nanostructures, supplemented by a chemical contribution.2-6 While a broad continuum emission called “background” is always observed in SERS spectra, such accompanying background has often been ignored using background subtraction methods, since the background was understood to be a stable continuum.7-8 Much of the discussion regarding SERS background was confined to the initial years following the discovery of SERS.9-10 (It should be noted that SERS backgrounds, which can be ascribed to other factors such as carbon contaminants or glass substrates, are not discussed here.) The origin of this background and its understanding and mechanism are not always reported.11-12 For example, the origin of background in SERS or tip enhanced Raman scattering is subject to various interpretations, such as luminescence from the metal, carbon contamination, or molecular fluorescence. While several reports have demonstrated that there is still intrinsic generation of background emission on metallic nanostructures without the presence of molecules.12-13 Currently the consensus is that plasmonic nanostructures can emit light at their LSP bands under excitation of electrons (e.g. high energy beam or low energy tunneling electrons) or photons (e.g. continuous wave (CW) light or pulsed laser),14-15 although the



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mechanism is still the subject of debate. Photoluminescence (PL) from metal thin films under CW laser excitation was first reported in 1969.16 Later, it was found that rough surfaces increase PL efficiencies through the enhancement of local optical fields.13 Recently, PL from metallic nanostructures has been observed with considerably higher efficiencies.17-19 The spectral shape of the SERS background emission can be related to the size of the metal particles.20 And phenomenological models have been developed to explain the correlation between the line shape of the background and the mean diameter of the metallic nanoparticles.11, 21 Hence, the PL of metal nanostructures can be correlated with the SERS continuum background.20, 22-25 It is well known that the PL or scattering intensity of plasmonic nanostructures is non-bleaching and non-blinking. The influence of adsorbed molecules on the localized field has often been presumed to be negligible, and the SERS background continuum has often been supposed to be constant. SERS background emission due to metal nanostructures has also been overlooked in conventional electromagnetic theory.26-27 The influence of the molecules on light emission from metallic nanostructures was rarely reported. While some studies showed that the SERS broad background fluctuated with the vibrational Raman peaks of the molecule.28-29 In fact, the understanding of SERS background and its correlation to the light emission of metal has been discussed detailedly in previous report.12,28 Even a model based on the images approach technique has been developed to explain the broad SERS background.11 In brief, the light emission from the nano-metal is changeable during SERS due to the interaction between the nano-metal and the molecules, and previous assumptions relating to the SERS background are incorrect.


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In this study, we develop a phenomenological theory model based on a quantized optical cavity concept to simulate the interaction of the molecules and the plasmonic nanostructures.

The

plasmonic

nanostructure

was

modeled

as

an

optical

nano-resonator.31-32 The nano-resonator not only elastically scatters the incident light resonantly at the excitation wavelength (i.e. resonant scattering), but also radiates inelastically the coupling incident light through the plasmon decay (i.e. the observed PL emission).30,

33

The Raman scattering process of the molecules was modeled as a

three-level atom system in the Λ configuration whose excited state was considered to be overlaid with the nano-resonator LSP resonance.34 We found that the presence of the molecules enables an enhanced plasmon emission from the plasmonic nano-resonator. Furthermore, a SERS blinking experiment based on a single gold nano-flower (GNF) demonstrated the fluctuations of SERS background emission. The theoretical and experimental results agree qualitatively, i.e. the SERS background is always present and can always be enhanced by accompanying it with the SERS vibrational Raman peaks of molecules when their interaction in the “hot spot” is strong. The induced field of the molecules dominates the enhancement of light emission from the GNF rather than the plasmonic coupling effect.

Theory and Discussion Fig. 1 shows a schematic diagram of the model. The metal nanoparticle (MNP) is considered a nano-cavity with a resonance frequency and a total decay rate . The molecule is considered to be a three-level atom with the vibrational energy levels |1> and



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|2>, and virtual level |3>. For simplicity, we focus on the Stokes Raman emission as an example to demonstrate the interaction between the molecule and the nano-cavity. A quasi-continuous-wave laser beam with frequency couples the ground state |1> and the virtual state |3>, where the energy difference between |1> and |3> is = Ω − Ω . We assume that = . Subsequently, the excited electron at |3> drops into |2> and emits a photon with frequency = = Ω − Ω as Raman scattering. Therefore, = − = − or Δω = − = . Thus, we obtain the frequency relationship between the excitation and emitted photons. Regarding the MNP plasmonic resonator, the free energy of the LSP mode with the resonance frequency , and Hamiltonian is described as = . For the free atom, the Hamiltonian is written as = ∑ Ω , where (i, j = 1, 2, 3) are operators in the atom subspace. Specifically, ( ≠ ) are the transition operators from state | j > to state | i >. Therefore, the free Hamiltonian of the atom-cavity system without any interaction is written as: = + = + ∑ Ω

(1)

In addition, the interaction Hamiltonian is described as = + +

!

" # $%&' ( + " # %&' ( ) +

" # %&' (

" #

$%&' (

+ (2)

where the first term describes that the LSP mode couples with the atom states |3> and |2>, and is the coupling constant. The second and third terms describe the processes that the excitation field couples with the LSP mode and states |1> and |3>, respectively, and

and

are the respective coupling constants. " and " are the respective


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localized field amplitudes that the MNP and the atom feel. Hence, the Hamiltonian of this system is given by = +

(3)

The dynamics of these modes can be solved by the equations: + = ,, . − = − − − −

" #

$%&' (

+ = ,, . − / = − − / + 0 −

(4) " #

$%&' (

(5)

where and / are the total decay of the MNP and the atom respectively, and 0 = − . The equations can be solved rigorously, and the forms of the solutions are: = 1 # $%2 $32( + 1 # $%4 $34 ( + 1 # $%&' (

(6)

= 5 # $%2 $32 ( + 5 # $%4 $34 ( + 5 # $%&' (

(7)

where , and , are given as follows: , = − 678 ∓ 8 , and

, = − :#8 ∓ 8 ,

where

we

define

8 = − − / − −

and

8 = ;, − / + − . + 4 0 . The coefficients 1 and 1 are complex amplitudes of modes , for the MNP LSP operator . 5 and 5 are complex amplitudes of modes , for the atom operator . 1 and 5 are complex amplitudes of scattering modes for and , respectively (we do not discuss 1 and 5 terms here since they are related to elastic scattering of incident light). For a weak coupling system, we assume that ≪ − /, thus the eigen frequencies become , ≈ , , and the decay rates become , ≈ , /, respectively. In the case of weak coupling, we ignore all of the items and obtain these coefficients: 1 = %

?2 @A

A $%&'

+ ,% B3

C?4 @2D E42 ; A $%&' B3.,%A $%&F B3$G.



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1 = − ,% 5 = ,%

C?4 @2D E42 BG.H,% &F $%&' A $%&F B3$G.I

C?2 @A EJD4 A $%&' B3.,%A $%&F B3$G. ?4 @2D E42 &F $%&' BG

5 = − %

− ,%

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;

; C?2 @A EJD4

&F $%&' BG.H,%A $%&F B3$G.I

;

Using the input-output relation 〈LM( 〉 = ;2P 〈〉, where 〈QR 〉 is the quantum average of the operator QR and P is the outgoing coupling rate of the MNP, the detected intensity of light emission from the nano-cavity can be evaluated by [

W + XLM( X〉# $%Y ZWI = 6SMTT$P = ℜHV 〈LM( [

]

ℜ \V \] V LM( W + XLM( XZX^ # $%Y ZW^

(8)

where ℜ,Q. stands for the real part of Q. 6SMTT$P includes PL 6à$P and scattering 6bc$P . After filtering the input laser field and using the quantum regression theorem, we obtain the PL intensity of the nano-cavity from 6SMTT$P as 6à$P = 2P \|1 | e

$ f4g2 h 32 ]

3

i %$% 24 B34 + |1 | e 2

$ f4g4 h

2

34 ]

3

i %$% 44 B34^ 4

4

(9)

where j is the effective interaction time between the electrons and light wave packet. Similarly, by using the relation 〈LM( 〉 = ;2k 〈 〉, where k is the outgoing coupling rate of the atom, the detected intensity of light emission from the atom can be evaluated by [

W + XLM( X〉 # $%Y ZWI = 6SMTT$E = ℜHV 〈LM( [

]

ℜ \V \] V LM( W + XLM( XZX^ # $%Y ZW^

(10)

then the PL intensity of the atom from 6SMTT$E as 6à$E = 2k \|5 | e

$ f4g2 h 32 ]

3

i %$% 24 B34 + |5 | e 2

2

$ f4g4 h 34 ]


3

i %$% 44 B34 ^ 4

4

(11)

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Hence, the total detected emission spectrum would be 6à = 6à$P + 6à$E

(12)

So far, we have deduced the light emission spectra from both the MNP and the atom with the coupling coefficient . For an extreme case ( = 0), which suggests that there is no interaction between MNP and the atom, and the coefficients would be given by: 1 = %

?2 @A

A $%&' B3

?4 @2D E42 . &F $%&' BG

, 1 = 0, 5 = 0, 5 = − %

The modes and decays are

regressed to the original form: , = , , , = , / . Thus, the light emission spectrum from each operator (, ) presents a single Lorentz line shape: 6à$P C = 2P |1 | e

$ f4gh

6à$E C = 2k |5 | e

3]

$ f4mh G]

3 4 4 A B3

i %

%$i %

%$(13)

G

(14)

4 4 D4 BG

which corresponds to the emission from the single emitter (resonator or atom, " = " = " ). Representative simulation spectra from eqn. 12-14 are plotted in Fig. 2 based on conditions

that

give

= 2π × 1.776 #o,

"

= 1 #o ,

"

= 0.02 #o,

= 2π × 1.849 #o (Fig.

2a,

= 2π × 1.964 #o, Fig.

2c),

= 2π ×

1.703 #o (Fig. 2b, Fig. 2d), = 0.4791 #o , / = 0.0086 #o , P = 1 #o , k =

0.02 #o, T = 10 ns, 0 = − { , = − (see Supplementary Material). The black and red curves are the free single resonator and atom emission spectra (interaction with incident light only), respectively (according to eqn. 13 and 14). We show that the emission spectrum exhibits a Lorentz line as a narrow Raman line. The broad background is light emission by LSP mode radiative decay of the nano-resonator.



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For ≠ 0, i.e. there is interaction between the MNP and the atom (Fig. 2), the light emission from the molecule and plasmonic nanostructure both increase simultaneously. The coefficient 1 for operator is the key parameter related to the SERS background emission. We define the background enhancement factor for the nano-resonator:

3 4 ~ C

3 4 C

3

| = 4 }~2 C} + &' 4 }~2 C} = 3 3 3 2

2

3 4 ?2 324

}

?

+

&'

2

2

C?4 @2D E42 } ? @A ,%A $%&F B3$G.

+

CEJD4 } . ? %A $%&F B3$G

3&' 3 4 ?2

3&' 324

}

Meanwhile, we can also define the Raman enhancement factor for the molecule: 3

G4 ~4 C } &' 4 4 C

| = 3&' 34 } 3&' G4 ?4 3&' 344

}?

C

%A $%&F

G4 4 C } 4 4 C

+ 34 }

G4 ?

C?2 @A EJD4 } @2D E42 ,%A $%&F B3$G.

= 34 }?4 + ? 4

+

} . B3$G

The blue, green and purple curves in Fig. 2 are the emission spectra of the coupled system (eqn. 12) with different coupling coefficients . The local field which the atom feels is set as " = 10" . For these curves, the local field " that the MNP feels is set as " = " in Fig. 2 a and b corresponding to = 1.849 #o and = 1.703 #o. Here, we assumed at first that the induced field of the molecule does not influence the field felt by the MNP; while the atom feels a large enhancement due to the plasmonic near field effect of the MNP. Additionally, the molecule Raman enhancement factors | are

calculated

as

| = 97.5a/102.8b blue,

95.4a/105.4b green,

93.6a/109.0b purple . In particular, | = 100 not shown corresponds to = 0, which indicates that in this case (no interaction) the origin of the enhancement of ?

the Raman signal is the enhancement of the near field }?4 } = 100. For the MNP, we


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obtained an increased plasmon emission in Fig. 2a (insert) and a decreased one in Fig. 2b (insert), and the change was small (less than 1%) for both cases. This is reasonable because the dipole moment of the molecule is much smaller that of the MNP resulting in little energy transfer, and the inelastic radiation efficiency of the MNP is also very low. Hence the plasmon emission intensity due to the energy coupling from the molecules should be low. This does not sufficiently explain the intensity change of the plasmon emission in the experiment shown in Fig. 3. Thus, we proposed that the local field felt by the nanostructure changed due to the polarized molecules. We set " = 1.1" and obtained considerable enhancement of the plasmon emission as shown in Fig. 2c and d. The intensity of the plasmon emission is proportional to |" | . Therefore, the main parameter influencing | is " , but not the coupling coefficient . That implies that the induced field produced by the polarization of the molecules cannot be overlooked. Regarding the local field " felt by the MNP, it is widely accepted in the research field of strong coupling that the induced field of a large quantum dot can become comparable to, or higher than, the external excitation field.35 In the present SERS system, we propose that a giant localized electromagnetic field within the “hot spot” polarizes the molecules, and then the polarized molecules in turn produce an induced field greatly influencing the local field felt by the MNP. To estimate the local field felt by the MNP, we take into account of the induced field of the molecule. Here, the molecule is regarded as a dielectric particle (DP), giving:

"` = " + {

`

D &f

,

"` = " + {

`

D &f

where, R is the distance between the MNP and the DP, and E is the incident electric field. E` is the electric field felt by the MNP, which consists of the external field E and the



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induced field produced by the polarization of the DP. Similarly, E` is the electric field felt by the DP, which consists of the external field E and the induced field produced by the MNP. The effective dielectric constants of the MNP and the DP are SS$ = B %

and SS$ =

B %

, respectively. Where and are the

dielectric constants of the MNP and the DP, respectively. ¡¢ = 2−1 when the applied field is parallel (perpendicular) to the major axis of the system. The polarization of the MNP is £` = 4πε γ¦ § E + %$ , %

where, γ¦ = B

£` = 4πε γ¨ § E + %$

where, γ¨ = B

1 ¡¢ £` 4πε SS$ : § is the radius of the MNP. The polarization of the DP is

1 ¡¢ £` 4πε SS$ : , § is the radius of the DP. Through iterations, we estimate that

%

the total electric field felt by the MNP is E` = " + Here,

©2D

D

− § ¡¢ § 3 ¡¢ 1 ¡¢ £` γ E + E + ¨ : SS$ : 2 + 2 + 4πε SS$ :

~10$ , which is small if § = 1 «7 and : = 10 «7. However, the term

containing

4

%$

B % B %

can be very large when the frequency of the incident

light matches the LSP mode of the MNP, i.e. 2 + ≈ 0. Therefore, the latter term in E` will be comparable to the incident field E. Thus, the induced field of the molecules becomes detectable and the enhancement of the field " felt by the MNP is reasonable. Therefore, the polarized molecules in the so called “hot spot” could alter the


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local field, which would increase the absorption of the MNP in the SERS process resulting in enhanced background emission.

Experimental Section To demonstrate that the plasmonic nanostructure and molecule mutually enhance their light emission in the SERS process, we employed a single gold nano-flower (GNF) to perform a single nanoparticle based SERS blinking experiment. The advantage of the GNFs is that “hot spots” are readily available and Raman blinking happens easily. Fig. 3a shows a schematic diagram of the system and the Raman spectra blinking. We speculate that the molecule migration or rotation at the “hot spot” site leads to the SERS spectra temporal blinking, i.e. converting between the states of = 0 and ≠ 0.36-39 In other words, when the spectrum is at background level (with no molecule Raman signal), the coupling coefficient was assumed to be ≈ 0, and the background spectrum is attributed to the light emission of the GNF. In the case of a spectrum burst, presenting strong Raman peak signal of the molecule, the coupling coefficient was assumed to be ≠ 0, which shows the light emission spectrum of the coupling system. A microspectroscopy system based on an inverted optical microscope was developed to combine multifunctional optical measurements. The setup allowed us to identify single isolated nanoparticles and measure the luminescence and Raman spectra in situ near a single nanoparticle. The experimental configuration was described in detail in a previous study.29 We used an oil-immersion objective lens for the measurements. A continuous



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Page 14 of 22

wave laser at a wavelength of 633 nm was used as the excitation light source with excitation power of ~160 µW. The GNFs used had a diameter of 70 ± 10 nm and were synthesized using a wet chemical method. The nanoparticles were immobilized onto silane functionalized glass coverslips with an average interparticle spacing of several micrometers. Subsequently the chip with immobilized GNFs was treated with 1 µM 4-mercaptopyridine solution for over 3 hours and dried in a stream of N2. 50 consecutive representative spectra recorded from a single GNF over an acquirement time of 0.1 s are plotted in Fig. 3c. We observed Raman blinking phenomena, and both the background due to the GNF and the Raman peaks of the molecule fluctuated. Three representative spectra were selected for comparison as shown in Fig. 3d. According to the above conclusion in Fig. 2, when is larger, the Raman blinking peaks have greater intensity. Therefore, the red and blue curves in Fig. 3d represent strong coupling and the black line represents the weakest coupling ( ≈ 0). As previously stated, the SERS background results from the LSP mode and presents a Lorentz line shape the same as the uncoupled LSP mode. In fact, by correlating scattering, PL and SERS spectra in previous experiments, the SERS background can be ascribed to the PL from the plasmonic nanostructure. Hence, we can assume that the black line approximately represents the PL from the GNF. We can obtain a Raman spectrum without broad background by subtracting the PL spectrum of the GNF multiplied by an enhancement factor, from the SERS blinking spectra. Fig. 3e shows the resulting curves from the spectra in Fig. 3d. Corresponding multiples for background enhancement factor are 1.24 and 1.53 for the blue and red curves, respectively.

This

means the induced field of the molecule increases the effective local field intensity "


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felt by the GNF by ~11% and 24%, respectively. And the subtracted spectra do not represent the background, which implies that the PL from the GNF mainly contributes the SERS background. Our calculations and experimental results agree qualitatively, and provide a self-consistent understanding of the background emission.

Conclusion In conclusion, we investigated the fluctuations in the background continuum for the SERS process based on the concept of quantized optical cavity. We found that the inevitable presence of background emission resulting from the metallic nanostructure plasmon emission is not a constant as ordinarily speculated, and simultaneously increased when the Raman scattering of the molecules was enhanced. The plasmonic nanostructures not only scatter the coupling energy from the excited states of the molecules directly, but can also convert it into surface plasmon that decays partly radiatively. The model enables us to understand qualitatively the background fluctuations of the SERS blinking spectra based on a single GNF. The background fluctuations in the experiment were mainly due to the induced field of the molecules, which increased the local field felt by the nanostructures. The SERS background is changeable and it can be an indicator for the interaction strength between the plasmonic nanostructures and the molecules. These findings support the consideration of SERS as an entity system to analyze and optimize the interaction signal. The concept would also be effective for other surface enhanced spectroscopy such as surface enhanced fluorescence.40



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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest. Supporting Information The following files are available free of charge: ¬ and ®®®®®®®®®® |¬ | . Estimation and assumption of the values of ®®®®®®® Acknowledgment This work was supported by the National Key Basic Research Program of China (grant no. 2013CB328703) and the National Natural Science Foundation of China (grant nos. 61422502, 11374026, 61521004, 11527901).


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FIGURE CAPTIONS Figure 1. (Color online) Schematic diagram of the model describing the interaction between the MNP and molecule which are excited by the incident electromagnetic field with frequency . The coupling coefficient is . The energy level diagram presents the Stokes Raman process.



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Figure 2.

(Color online) Simulated spectra of single MNP (black), single atom (red)

and coupled system (blue, green, and purple) for different coupling coefficients . The insets represent enlargements of the background and Raman peaks. " is set as " = " for (a) and (b), and " = 1.1" for (c) and (d).


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Figure 3.

(Color online) (a) Schematic diagram of SERS system. (b) Representative

SEM image of the GNFs with scale bar = 70 nm. (c) SERS blinking of the GNF and 4-mercaptopyridine molecule system. (d) SERS spectra selected from Fig. 3c. (e) Difference from (d), blue and red lines represent 6( − 1.24 × 6( and 6( − 1.53 × 6( respectively.



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"Table of Contents"


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Enhanced Light Emission from Plasmonic Nanostructures by Molecules

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