Probing Purcell enhancement in plasmonic nanoantennas by broadband luminescent
Si quantum dots
Hiroshi Sugimoto, Shiho Yashima, Kenta Furuta, Asuka Inoue, and Minoru Fujii
Citation: Applied Physics Letters 108, 241103 (2016); doi: 10.1063/1.4953829
View online: http://dx.doi.org/10.1063/1.4953829
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APPLIED PHYSICS LETTERS 108, 241103 (2016)

Probing Purcell enhancement in plasmonic nanoantennas by broadband
luminescent Si quantum dots
Hiroshi Sugimoto, Shiho Yashima, Kenta Furuta, Asuka Inoue, and Minoru Fujiia)
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,Rokkodai, Nada, Kobe 657-8501, Japan(Received 27 April 2016; accepted 1 June 2016; published online 14 June 2016)
Colloidal silicon quantum dots (Si QDs) with a very broad photoluminescence (PL) band areproposed as a probe to monitor the Purcell enhancement in a plasmonic nanostructure. Si QDs
placed on an arbitrary plasmonic nanostructure enable us to determine the Purcell enhancement
factors in a broad spectral range (600–900 nm). As a proof-of-concept experiment, a layer of Si
QDs is spin-coated on gold film-over nanosphere structures, and the Purcell enhancement is quantitatively determined from the analyses of the PL spectra and the decay rates. The method proposedin this work provides a facile approach to quantitatively measure the performance of plasmonic
substrates for PL and Raman enhancements. Published by AIP Publishing.
[http://dx.doi.org/10.1063/1.4953829]

Nanoscale plasmonic structures with a variety of shapes
such as spheres, rods, disks and their arrays have been tailored for enhanced light-matter interactions.1,2 The plasmonic nanostructures work as antennas, which enhance theexcitation and emission rates of an emitter nearby. The latteris known as the Purcell effect,3 which corresponds to themodification of the local density of photonic states (LDOS)
at the position of an emitter. By carefully designing the plasmonic structure to maximize the excitation efficiency andthe radiative decay rate, surface plasmon-enhanced photoluminescence (PL) has been demonstrated for various kinds oflight emitting materials.4–9 To optimize the PL enhancement,
rigorous determination of the electromagnetic fields aroundthe plasmonic structures and quantification of the Purcell
factors are indispensable. This has been achieved by using
electromagnetic calculations such as a finite element method,
finite-difference time-domain, and boundary integral equations for well-defined plasmonic structures fabricated byelectron-beam lithography.2,10–12 On the other hand, most of
the plasmonic structures ideal for large-scale and flexible
applications do not have well-defined structures.13,14 For
example, metal island films,13 which are often used as a facile plasmonic substrate, have large distributions in the sizeand shape of metal nanoparticles and gaps between them.
The large distribution of the structural parameters makes
modeling of the structure very difficult. Without accurate
modeling of the structure, numerical calculations do not givecorrect electromagnetic field distributions. As a result, preparation parameters of large-scale plasmonic structures areusually determined in an empirical manner.
The purpose of this work is to develop a facile method
to experimentally determine the Purcell enhancement factorof a plasmonic substrate in a wide wavelength range. An
idea is to place nano-emitters onto a plasmonic substrate andstudy the emission dynamics. For this purpose, a plasmonic
substrate has to be covered by a very thin layer of an emitter
uniformly in a wide area by a low-temperature damage-freea)

Corresponding author e-mail: fujii@eedept.kobe-u.ac.jp0003-6951/2016/108(24)/241103/4/$30.00

process. Furthermore, the emission band should be broad
enough to cover the whole spectral range of the surface plasmon resonance of a plasmonic substrate, which is typicallybroader than 100 nm, and the emission should be efficientand stable in air. For the biological applications, the emission
should be efficient and stable also in water-based solution,and the wavelength is preferably in the red to near-IRregions. An emitter having broad absorption bands is alsofavorable for the larger choice of the excitation wavelength.
In this paper, we propose all-inorganic colloidal siliconquantum dots (Si QDs) as an ideal material satisfying allthese criteria.15 Advantages of the all-inorganic Si QDs compared to commercially available II-VI or IV-VI QDs are theenvironmental friendliness and the higher stability of the luminescence in air and water. As a proof-of-concept experiment, we employ a metal-film over nanosphere (metal FON)
structure plasmonic substrate, which has been widely used as
the substrate for surface enhanced Raman scattering andPL.16–18 FON substrates can be produced by a facile process
and usually exhibit very high enhancement factors.16–18 The
structure is composed of interconnected metal half shells.
Because of the large thickness gradience in the half shells
and the variation of the shape and size of gaps between thehalf shells, accurate modeling of the structure is very difficult.16–18 In this work, we cover the surface of gold (Au)
FON substrates with a very thin layer of Si QDs by spincoating and determine the Purcell factor in a wide wavelength
range by the analyses of the PL dynamics. We demonstratethat the methodology presented in this work provides an alternative approach to determine the Purcell factor of a plasmonic substrate without numerical calculations.
In all-inorganic Si QDs used in this work, boron (B) andphosphorus (P) are very heavily doped, and a high B and P
concentration shell is formed on the surface. The shell induces negative surface potential and prevents agglomeration ofthe QDs in polar solvents. Therefore, the codoped Si QDs
are dispersible in alcohol without organic ligands. The
detailed preparation procedure of the all-inorganic Si QDs is
described in Refs. 15 and 19 and briefly summarized in the108, 241103-1

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241103-2

Sugimoto et al.

Appl. Phys. Lett. 108, 241103 (2016)

FIG. 1. (a) Photograph of colloidal dispersion of Si QDs (methanol solution).
(b) SEM image of a spin-cast film of
Si QDs on a Si substrate. (c) PL spectrum of the Si QD film excited at
405 nm.

supplementary material. In this work, we employ Si QDs
with the average diameter of about 2.8 nm. Figure 1(a) showsa photograph of the colloidal dispersion of Si QDs, demonstrating the absence of scattering induced by agglomerates.
Figure 1(b) shows a SEM image of a Si QD thin film on a Si
wafer prepared by spin-coating the colloidal dispersion(2000 rpm, 15 ll of 0.9 mg/ml). A dense and smooth Si QD
film is obtained. Figure 1(c) is the PL spectrum excited withmonochromatized 405 nm light from a Xe lamp (Fluorolog-3,Horiba Jovin Yvon). The PL peak is at 750 nm with the fullwidth at half maximum (FWHM) of 200 nm (440 meV).
Figure 2(a) shows a preparation procedure of a AuFON
structure with a Si QD film. First, a 2D array of polystyrene(PS) spheres 350 nm in diameter is formed on a fused quartz
substrate. The SEM image is shown in Figure 2(b). A 10 nmthick titanium layer and a 200 nm-thick Au film are then deposited on the array by vacuum evaporation (Figure 2(c)). The Au
film surface is treated by 3-methacryloxypropyltrimethoxysilane(3-MPTS). Finally, Si QD films with 30 and 60 nm in thicknesses are deposited by spin-coating (2000 rpm) the colloidal
dispersion (1.4 mg/ml) one and two times, respectively.
Hereafter, we denote Si QD films with the thicknesses of 30 and60 nm as QD30 and QD60, respectively, and AuFON samples
with these Si QD films as AuFON-QD30 and AuFON-QD60,respectively.
Figure 2(d) shows the absorptance (A) spectra definedby 1-R-T, where R and T are measured reflectance and transmittance, respectively, of the AuFON structures with andwithout Si QD films. The AuFON substrate exhibits a strong
extinction at 620 nm arising from the surface plasmon

resonance,17,18,20,21 although the detailed origin of the resonance is still controversial due to the participation of boththe localized and propagating surface plasmon resonances.17,18,20,21 The spin-coating of the Si QD films results in a
red-shift of the resonance peak due to the increase in thelocal refractive index (refractive index of Si QD film 1.6);
the degree of the shift depends on the thickness.
Figures 3(a) and 3(b) show the room temperature PL
spectra of AuFON-QD30 and AuFON-QD60, respectively.
PL spectra of 30 and 60 nm thick Si-QD films on flat Au substrates (AuF-QD30 and AuF-QD60, respectively) are also
shown as references. The error bars in the spectra represent
the standard deviations of the PL intensities obtained at 50
different points in each sample. In AuFON-QD30, the PL intensity is enhanced, and the peak exhibits a 20 nm blue-shift
with respect to that of AuF-QD30. On the other hand, the PL
peak shifts to the opposite direction in AuFON-QD60. The
modification of the PL spectra is accompanied by significant
shortening of the lifetimes. The representative decay curves
detected at 750 nm using a gated intensified CCD (PI-Max,Princeton Instrument) are shown in the insets of Figs. 3(c)and 3(d). The excitation source is modulated 405 nm light.
For the analysis of the decay curves, a stretched exponentialfunction I ¼ I0 exp(-t/s)b is used,22 where s is the decay constant and b is the stretching parameter. The average PL lifetime is defined as save ¼ sb1CE(b1), where CE is the Euler
gamma function.22 Figures 3(c) and 3(d) plot the PL decay
rates (1/save) of AuFON-QD30 and AuFON-QD60, respectively, as a function of detection wavelength. The decay rates
are enhanced in the whole wavelength range in both samples.

FIG. 2. (a) Fabrication procedure of a
AuFON substrate with a Si QD film.
SEM images of a 2D PS nanospherearray (b) before and (c) after Au deposition. (d) Absorptance spectra of
AuFON structures with different thickness Si QD films.

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241103-3

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Appl. Phys. Lett. 108, 241103 (2016)

FIG. 3. (a) and (b) PL spectra and (c) and (d) decay rates of AuF-QD30 and60 (black) and AuFON-QD30 and 60 (red). Insets of (c) and (d) show representative PL decay curves of AuF-QD30 and 60 (black) and AuFON-QD30and 60 (red) detected at 750 nm with the fitting curves.

It should be noted here that the decay rates of Si QDs on
a flat Au film are also modified from those of colloidal SiQDs. The enhancement factors of the radiative and nonradiative decay rates of dipoles placed on a flat Au film withrespect to those in vacuum can be analytically calculated by
the method developed by Chance et al.23,24 The calculated
normalized decay rates for AuF-QD30 and AuF-QD60 are
shown in the supplementary material (Figure S1).25 In thecalculation, emitters are treated as isotropic dipoles uniformly positioned within the regions of 30 and 60 nm fromthe Au surface for AuF-QD30 and AuF-QD60, respectively.
The normalized radiative rate is below unity in the whole
wavelength range due to the interference between the incident and reflected fields, while the nonradiative decay rate isvery large, especially in the short wavelength region. This isdue to the coupling of an emitter with the surface plasmon
polariton and the lossy surface wave modes in the metal. We
will take into account these results for the determination of
the Purcell enhancement in the AuFON structure.
Figure 4(a) shows the ratio of the decay rates between
AuFON (CFON) and AuF (CF) for QD30 (black) and QD60(red) as a function of the emission wavelength. The reddashed lines are the extinction spectra of the corresponding
AuFON structures. The enhancement factors (CFON/CF) havea peak at the wavelength of the extinction peak. The maximum enhancement factors reach approximately 4.2 at
760 nm for QD30 and 1.9 at 800 nm for QD60. Figure 4(b)shows the ratio of the PL intensity ðIFONIF Þ between AuFON(IFON) and AuF (IF) for QD30 (black) and QD60 (red). Apeak can be seen near the extinction peak.
The measured PL intensity ratio ðIFONIF Þ is expressed as
the product of the ratios of the excitation cross-section
Þ, the quantum efficiency ðQQFONÞ, and the light collectionðrrFONFF
QFON rFON gFON 14,26IFON
Among them, rrFONefficiency ðgFONgF Þ; IF ¼ QF
r F gF .
F
can be approximated to be 1, because the excitation wavelength (405 nm) is far from the surface plasmon resonance

FIG. 4. (a) Decay rate (CFON/CF) and (b) PL (IFON/IF) enhancements of
AuFON-QD30 (black) and AuFON-QD60 (red) with respect to those of
AuF-QD30 and AuF-QD60, respectively, as a function of emission wavelength. (c) Purcell enhancement spectra (PFON) of AuFON-QD30 (black)and AuFON-QD60 (red).

wavelengths of the AuFON. The collection efficiency also
does not change largely between AuFON and AuF.
Therefore, the measured PL enhancement originates mainly
from the quantum efficiency enhancement.
In the AuFON and AuF structures, the radiative decay
rates are modified to PFON cr and PF cr, where PFON and PF
are the Purcell factors, respectively, and cr is the intrinsic
radiative rate of Si QDs in vacuum. By using the measured
PL decay rates (CFON and CF), QFON and QF can be
expressed as PFON cr/CFON and PF cr/CF, respectively. The
PL intensity ratio is thus
IFON QFON PFON CF¼¼:IFQFPF CFON(1)

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241103-4

Sugimoto et al.

Since IFON/IF and CF/CFON are experimentally determined
and PF can be calculated by an analytical method, PFON is
obtained from Eq. (1). Figure 4(c) shows PFON obtained for
AuFON-QD30 and QD60. In AuFON-QD30, PFON has apeak around 750 nm, while in AuFON-QD60, it has a peakaround 800 nm. In both cases, the shape of the PFON spectra
coincides with the extinction spectra very well. This proves
the validity of our analysis to determine the Purcell factor ofmetal nanostructures. The Purcell enhancement of AuFONQD30 exceeds 6 at the maximum, while it is around 2 in
AuFON-QD60. The different values of the maximum PFON
between the two samples can be explained by strong dependence of the Purcell factor of plasmonic nanostructures on thedistance from the surface.27
In conclusion, we have proposed a method to quantitatively determine the Purcell enhancement factor of metalnanostructures utilizing the broadband luminescence fromcolloidal Si QDs. As a test structure, we employed AuFONstructures and determined the Purcell enhancement factors
quantitatively in a wide wavelength range from the analysesof the PL spectra and the decay dynamics of Si QDs placedon the AuFON structures. Since a thin layer of Si QDs canbe placed on an arbitrary shape metal nanostructure by spincoating, screen printing, etc., the present method is very versatile and can be applied to plasmonic nanostructures thatare hard to be modeled accurately for numerical calculations.
This work was partly supported by 2014 JSPS Bilateral
Joint Research Projects (Japan-Czech Republic), 2015 JST
Visegrad Group (V4)-Japan Joint Research Project on
Advanced Materials and KAKENHI (16H03828). H.S.
acknowledges the support from the Grant-in-Aid for JSPSFellows (No. 26-3120).
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