Photoluminescence enhancement of silicon quantum dot monolayer by plasmonic
substrate fabricated by nano-imprint lithography
Hiroto Yanagawa, Asuka Inoue, Hiroshi Sugimoto, Masahiko Shioi, and Minoru Fujii

Citation: Journal of Applied Physics 122, 223101 (2017);
View online: https://doi.org/10.1063/1.5001106
View Table of Contents: http://aip.scitation.org/toc/jap/122/22
Published by the American Institute of Physics

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JOURNAL OF APPLIED PHYSICS 122, 223101 (2017)

Photoluminescence enhancement of silicon quantum dot monolayer
by plasmonic substrate fabricated by nano-imprint lithography
Hiroto Yanagawa,1,2 Asuka Inoue,2 Hiroshi Sugimoto,2 Masahiko Shioi,1,2 and Minoru Fujii2
1

Advanced Research Division, Panasonic Corporation, Seika, Soraku, Kyoto 619-0237, Japan
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan
2(Received 22 August 2017; accepted 15 November 2017; published online 8 December 2017)
Near-field coupling between a silicon quantum dot (Si-QD) monolayer and a plasmonic substratefabricated by nano-imprint lithography and having broad multiple resonances in the near-infrared(NIR) window of biological substances was studied by precisely controlling the QDs-substratedistance. A strong enhancement of the NIR photoluminescence (PL) of Si-QDs was observed.
Detailed analyses of the PL and PL excitation spectra, the PL decay dynamics, and the reflectancespectra revealed that both the excitation cross-sections and the emission rates are enhanced by thesurface plasmon resonances, thanks to the broad multiple resonances of the plasmonic substrate,
and that the relative contribution of the two enhancement processes depends strongly on the
excitation wavelength. Under excitation by short wavelength photons (405 nm), where enhancement of the excitation cross-section is not expected, the maximum enhancement was obtained
when the QDs-substrate distance was around 30 nm. On the other hand, under long wavelength
excitation (641 nm), where strong excitation cross-section enhancement is expected, the largest
enhancement was obtained when the distance was minimum (around 1 nm). The achievement ofefficient excitation of NIR luminescence of Si-QDs by long wavelength photons paves the way forthe development of Si-QD-based fluorescence bio-sensing devices with a high bound-to-free ratio.
Published by AIP Publishing. https://doi.org/10.1063/1.5001106
I. INTRODUCTION

Semiconductor quantum dots (QDs) have several superior properties as near infrared (NIR) phosphors for biosensing devices compared to commonly used organic fluorescent dyes,1 which often suffer from fast fluorescence degradation under light illumination. Fast degradation is aserious obstacle to improve the detection sensitivity of thedevices because it limits the excitation power and the accumulation time for detection. Semiconductor QDs, especially
Cd and Pb chalcogenide QDs, usually have much more stable luminescence than organic fluorescent dyes, and due tosome additional advantages such as the narrow luminescenceband, the large tunability of the luminescence wavelength,
the broad excitation band, etc., they are considered to bepromising candidates for replacing organic fluorescentdyes.2–6 However, heavy-metal elements in these QDs arealways a concern for biomedical applications.7 As a QD
potentially suitable for biomedical applications, Si-QDs haverecently been attracting significant attention. For the past
decade, the quality of Si-QDs, in particular colloidal solutionof Si-QDs, has been improved, and now it is rapidly catching
up to that of compound semiconductor QDs.8–10
In the application of Si-QDs to bio-sensing devices, adrawback is a small absorption cross-section in the visible
and near infrared (NIR) ranges due to the indirect nature of
the energy band structure.11 Since the direct absorption of
Si-QDs starts around 400 nm, the efficient excitation requires
ultraviolet (UV) light, which degrades biomaterials andalso the device sensitivity due to the autofluorescence of biomaterials. To overcome this problem and excite Si-QDs
0021-8979/2017/122(22)/223101/6/$30.00

efficiently by long wavelength photons, coupling with surface plasmons supported by metal nanostructures has been
proposed.12–14 A strong enhancement of efficient excitation
cross-section of Si-QDs by placing it on a plasmonic substrate has been demonstrated.15 Furthermore, by tuning thesurface plasmon resonance wavelength to the emission
wavelength of Si-QDs, Purcell enhancement of the emission
rate has been reported.16–19
In previous studies on surface plasmon enhanced emission of QDs, metal nanostructures have been produced byseveral different methods.20–24 The most widely used process
is electron-beam lithography,12,13,25 which is suitable for the
proof-of-concept experiments due to the high accuracy of thefabrication process, although it is very costly. Another often
used process is nanosphere lithography,14,15,18 which uses a
two-dimensional array of spheres as a template for the formation of metal nanostructures. Although the process is simple and does not require expensive setups for the formationof metal nanostructures, the freedom of the design, and thus
the tunability of the surface plasmon resonance wavelength,
is limited. Furthermore, the process usually has a problem inthe uniformity and the reproducibility, and is not suitable forthe production of commercial devices. A metal film over ananosphere plasmonic substrate, which uses a spherical array
to produce a corrugated metal surface, has the same problem
as nanosphere lithography.
In this work, we employ nano-imprint lithography forthe fabrication of a plasmonic substrate. The nano-imprint
plasmonic substrate has high uniformity in its large area, andthe fabrication throughput is very high. Furthermore, the122, 223101-1

Published by AIP Publishing.

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J. Appl. Phys. 122, 223101 (2017)

surface plasmon resonance wavelengths can be controlledeasily by the design of the structure. The purpose of thiswork is to achieve simultaneous enhancement of the excitation cross-section and the emission rate of Si-QDs by placing
the monolayer on a properly designed nano-imprint plasmonic
substrate, especially under excitation by long wavelength photons suitable for bio-sensing applications. We control the
QDs-substrate distance very precisely by using an organic
polymer fabricated by a layer-by-layer (LbL) assembly as aspacer. We first characterize the surface plasmon resonanceproperty of the substrate by reflectance spectroscopy. We thenstudy the photoluminescence (PL) and photoluminescence
excitation (PLE) spectra and the PL decay dynamics to analyze the relative contributions of the excitation cross-section
enhancement and the emission rate enhancement. We also
extract the contribution of Purcell enhancement to the total
PL enhancement. We show that by the simultaneous enhancement of the excitation cross-section and the emission rate, thePL of a Si-QD monolayer is strongly enhanced under longwavelength excitation exceeding 600 nm.
II. EXPERIMENTAL

We used a colloidal solution of all-inorganic Si-QDs forthe formation of a Si-QD monolayer.26–29 The all-inorganic Si
QDs have a heavily B- and P-doped surface layer,27,28,30,31
which induces negative potential on the surface and prevents
the agglomeration in polar solvents by electrostatic repulsions.
The Si-QDs can thus be dispersed in water without any
organic ligands.26,27 The Si-QDs exhibit size-tunable and stable luminescence in the visible to NIR ranges in an aqueous
solution in a wide pH range.26,28 Therefore, the all-inorganic
Si-QDs are very suitable for bio-photonic applications. The
preparation procedure of the Si-QDs can be found in theliterature.26–28,32
Figure 1(a) shows a photograph of the colloidal dispersion of all-inorganic Si-QDs in methanol. Because of the perfect dispersion of the QDs, the solution is very clear and thecharacteristic behind the 1 cm2 cubicle can clearly been read.
Figure 1(b) shows the transmission electron microscopy(TEM) (JEOL, JEM-2100 F) image of a Si-QD layer prepared by drop-casting the diluted methanol solution on acarbon-coated Cu mesh. A densely packed monolayer of SiQDs is formed and no three-dimensional agglomerates areobserved. The lattice fringe in the high-resolution image[inset in Fig. 1(b)] corresponds to {1 1 1} planes of the Si
crystal. The average size and the standard deviation of thediameter estimated from TEM images are 3.9 nm and
0.8 nm, respectively.26 Figure 1(c) shows the PL spectra
excited at 405 nm and the PLE spectra detected at 850 nm ofa Si-QD monolayer on a silica substrate. The Si-QDs exhibita broad PL around 800 nm. The PL quantum yield (QY) is
about 3.9% (see supplementary material for the procedure to
determine the QY).26 The PLE spectrum reveals that theexcitation efficiency is very small in the visible range, especially above 600 nm, which is a serious problem for biophotonic applications.
Figure 1(d) shows a schematic illustration of the structure studied in this work. First, dome-shaped structures were

FIG. 1. (a) Photograph of colloidal dispersion of Si-QDs (methanol solution). (b) TEM image of a Si-QD monolayer. The inset shows a high resolution TEM image of a Si-QD. (c) PLE spectrum detected at 850 nm (black)and the PL spectrum excited at 405 nm (red) of a Si-QD monolayer on a silica substrate. (d) Schematic illustration of the cross-section of a nanoimprint plasmonic substrate with a Si-QD monolayer on the surface. SEM
image of a polyolefin substrate prepared by nano-imprinting (e) before and(f) after Au deposition. (g) Thickness of the polymer layer on a silica substrate measured by ellipsometry versus the number of polymer layers. (h)Thickness of the Si-QD layer on a silica substrate measured by ellipsometryby assuming the filling factor to be 0.25 and 0.3 versus the number of spincoating times.

produced on a polyolefin substrate by nano-imprinting. The
scanning electron microscopy (SEM) image is shown in Fig.
1(e). The width, the pitch and the height of the dome-shaped
structures are 230, 460 and 200 nm, respectively. On the
structure, a Au thin film of 300 nm in thickness was deposited by electron-beam evaporation [Fig. 1(f)]. The surface
becomes bumpy due to the island growth of Au. For theformation of a Si-QD monolayer on the plasmonic substrate,
it was first immersed in an ethanolic solution (1 mM) of
6-amino-1-hexanethiol hydrochloride (6-AHT) for 12 h forthe formation of a self-assembled monolayer (SAM) with apositively charged amino group on the surface. A polymer

223101-3

Yanagawa et al.

spacer to control the QDs-substrate distance was then fabricated by the LbL assembly process, i.e., negatively charged[poly(sodium 4-styrenesulfonate) (PSS)] and positively
charged [poly(diallyldimethylammonium chloride) (PDDA)]
polymers are deposited alternatively by immersing the substrate in the solutions (supplementary material).33–35 Figure
1(g) shows the thickness of the polymer layers estimated byspectroscopic ellipsometry (HORIBA, Auto-SE) as a function of the layer number. The refractive indices of PDDA
and PSS layers are 1.41 and 1.50, respectively.36 The thickness is controlled from 1 nm to 84 nm by changing the layer
number from 0 to 14. Finally, a monolayer of Si-QDs wasformed on the positively charged surface by spin coating adiluted colloidal solution of Si-QDs (0.2 mg/ml) (500 rpm
for 5 s followed by 1500 rpm for 30 s). The formation of amonolayer was confirmed by measuring the thickness of thelayer deposited on a flat substrate by spectroscopic ellipsometry (supplementary material). In Fig. 1(h), the thickness of
Si-QD layers on a flat Au film is plotted as a function of thenumber of spin coats (supplementary material). For the estimation of the thickness, an effective medium approximation
was used as the dielectric permittivity of a Si-QD layer byassuming the filling factor to be 0.25 or 0.3. The thickness
increases linearly with the slopes of 3.8 nm/layer and 4.7 nm/
layer for the filling factors of 0.3 and 0.25, respectively.
These values are very close to the average diameter of SiQDs (3.9 nm in diameter), and thus nearly a monolayer of
Si-QDs is formed by one spin coating process.
The fabricated structures are characterized by reflectance spectroscopy. The reflectance spectra were measured
in a wide wavelength range (180 nm–1600 nm) with the incident angle of 45 or 60 by a UV-Vis-NIR spectrophotometer (Shimadzu, SolidSpec-3700). PL and PLE spectra were
measured by a stand-alone spectrophotometer equipped witha Xe lamp and a double monochromator as an excitation
source (HORIBA, Fluorolog-3). PL spectra were also excitedby 405 nm or 641 nm semiconductor lasers (COHERENT,
CUBE). PL decay curves were obtained by using a single
spectrometer equipped with an image intensified CCD(Roper Scientific, PI-Max1024HB). The excitation source
was modulated with 405 nm light. The measurement geometries are shown in the supplementary material.
III. RESULTS AND DISCUSSION

Figure 2 shows the specular reflectance spectra of plasmonic substrates with different thicknesses of polymer layers
and a Si-QD monolayer. The incident angle is 45 . The dataobtained with the incident angle of 60 are shown in the supplementary material (Fig. S2). The number of polymer layers(L(n)) is changed from 0 to 14. In the sample without a polymer layer (L(0)), i.e., a plasmonic substrate with a Si-QD
monolayer directly on it, the reflectance dips are observed inthree wavelength ranges. The reflection loss below 500 nm isdue to the interband transitions of Au, and does not contribute to the luminescence enhancement of Si-QDs. The narrow
dip around 650 nm and the broad one around 800 nm are dueto the excitation of surface plasmons. The 800 nm band overlaps well with the emission band of Si-QDs in Fig. 1(c), andJ. Appl. Phys. 122, 223101 (2017)

FIG. 2. Reflectance spectra of Si-QD monolayer/plasmonic substrate structures. The number of spacer layers is changed from 0 to 14. The incident
angle is 45 .

thus strong enhancement of the emission rate is expected bycoupling with the mode. On the other hand, the 650 nm band
can be used for the enhancement of the excitation crosssection, if we chose the wavelength for the excitation of SiQDs. The reflectance spectrum depends strongly on the number of polymer layers. The dip around 650 nm slightly shifts
to longer wavelengths with increasing polymer thickness. On
the other hand, the broad dip around 800 nm shifts significantly to longer wavelengths with increasing thickness andreaches around 1300 nm in L(14). The long wavelength shift
is explained by the increased permittivity around Au nanostructures. The high sensitivity of the long wavelength modeto the dielectric environment suggests that the mode is adipolar mode. On the other hand, the mode around 650 nm
can be assigned to a quadrupolar mode.
Figure 3(a) shows the PLE spectra detected at 850 nm.
The bandwidth of the detection is about 15 nm. As references,
the PLE spectrum of a Si-QD monolayer on a silica substrate
and that on a Au flat film are also shown. Note that the ordinate is a logarithmic scale. On a silica substrate, the intensity
decreases monotonously to longer wavelengths and the data
become noisy above 500 nm. The intensity becomes muchsmall on a Au flat film and no reliable data are obtained
above 500 nm. This is due to non-radiative relaxation of
excited Si-QDs to lossy surface modes and surface plasmon
polaritons of a Au flat film. On plasmonic substrates, theshape of the PLE spectrum is strongly modified and the intensity increases compared to that on a silica substrate, especially above 500 nm. In order to quantitatively analyze thechange in the PLE spectral shape, we divided the PLE spectraof a Si-QD monolayer on plasmonic substrates by that on asilica substrate and obtained the PLE enhancement factor
spectra [Fig. 3(b)] (see supplementary material for the procedure). In Fig. 3(b), we can see that the PL excitation efficiency is strongly enhanced, especially above 500 nm.
In Fig. 3(c), the PLE enhancement factor spectrum ofL(0) is compared with the reflectance spectrum. The strong
enhancement around 650 nm coincides very well with the223101-4

Yanagawa et al.

J. Appl. Phys. 122, 223101 (2017)reflectance spectra. The peak of the enhancement factor spectra coincides fairly well with the dip in the reflectance spectra. Therefore, the strong modification of the spectral shape iscaused by the modification of the emission rate of Si-QDs
due to the coupling with surface plasmons in the plasmonic
substrates.37 Similar data obtained for 641 nm excitations are
summarized in the supplementary material (Fig. S4).
In Fig. 4(g), the PL enhancement factor at 850 nm isplotted as a function of the polymer layer number for 405
and 641 nm excitations. Under 405 nm excitation, the PL
enhancement factor increases with increasing layer number,reaches a maximum at L(8) (9-fold) and then decreases. On
the other hand, under 641 nm excitation, the enhancement
factor is the largest (22-fold) at L(0) and monotonously

FIG. 3. (a) PLE spectra of Si-QD monolayer/plasmonic substrate structuresdetected at 850 nm. The number of spacer layers is changed from 0 to 14.
The data for a Si-QD monolayer on a silica substrate (Silica) and on a Au
flat film (AuFlat) are also shown. (b) PLE enhancement factor spectra
obtained by dividing the PLE spectra of Si-QD monolayer/plasmonic substrate structures by that of a Si-QD monolayer on a silica substrate. (c)Comparison of the enhancement factor spectra for L(0) and the reflectancespectrum (line) measured with the incident angle of 60 .

reflection loss designated by an arrow. This indicates that the
PL enhancement in the excitation wavelength range is dueto the enhancement of the excitation cross-section induced byenhanced electric fields of surface plasmons supported by Au
nanostructures. Similar data for other polymer thicknesses are
summarized in the supplementary material (Fig. S3).
Figure 4(a) shows the PL spectra excited at 405 nm. As
references, the spectra on a silica substrate and on a Au flat
film are shown. On plasmonic substrates, the PL is strongly
enhanced. The maximum enhancement factor reaches over
10 in L(8). Furthermore, the spectral shape is strongly modified, especially in L(14). Figure 4(b) shows the PL enhancement factor obtained by dividing the PL spectra of thesamples with that on a silica substrate. The enhancement factor spectrum depends strongly on the polymer thickness. In
Figs. 4(c)–4(f), the PL enhancement factor spectra of L(0),L(4), L(8) and L(14), respectively, are compared with the

FIG. 4. (a) PL spectra of Si-QD monolayer/plasmonic substrate structures
excited at 405 nm. The number of spacer layers is changed from 0 to 14. The
data for a Si-QD monolayer on a silica substrate and on a Au flat film arealso shown. (b) PL enhancement factor spectra obtained by dividing the PL
spectra of Si-QD monolayer/plasmonic substrate structures by that of a SiQD monolayer on a silica substrate. Comparison of PL enhancement factor
spectra (red) and reflectance spectra (black) for (c) L(0), (d) L(4), (e) L(8)and (f) L(14). (g) PL enhancement factor at 850 nm versus the number ofpolymer layers under excitation at 405 nm (black) and 641 nm (red).

223101-5

Yanagawa et al.

decreases with increasing layer number. The different behavior between the two excitation wavelengths indicates that theenhancement mechanism is different. Under 405 nm excitation, the enhancement of the excitation process is notexpected because of the large imaginary part of the dielectric
constant. The PL enhancement is thus solely due to theenhancement of the quantum efficiency arising from thelarge antenna efficiency. It is well known that the antenna
efficiency has an optimal distance due to the competition
between the radiative rate enhancement and the dissipation
to lossy modes.38 In the present structure, the optimum distance seems to be around 30 nm. On the other hand, under
641 nm excitation, in addition to the quantum efficiency
enhancement, the enhancement of the excitation efficiency
plays an important role. The excitation efficiency enhancement due to the enhanced incident electric field is the largest
on the surface of metal nanostructures. The monotonous
decrease of the enhancement factor with increasing polymer
thickness indicates that the excitation efficiency enhancement is dominant under 641 nm excitation.
Figure 5(a) shows PL decay curves detected in the rangeof 800 nm to 850 nm. The excitation wavelength is 405 nm.
The decay curves are non-exponential as has been commonly
observed in Si-QD ensembles. In Fig. 5(b), the decay rate,
C ¼ 1=save , where save is the average PL lifetime, is plotted
as a function of a detection wavelength. To estimate theaverage lifetime, the decay curves are fitted by a stretchedexponential function, I ¼ I0 exp fðt=sÞb g, where s is the
decay constant and b is the stretching parameter. The average PL lifetime (save ) is defined as save ¼ sb1 CE b1 ,where CE is the Euler gamma function. On silica, the decayrate is 0.1–0.4 ls1 in the wavelength range. The larger
decay rate at shorter wavelengths is usually explained by thebetter overlap of electron and hole wavefunctions in the
momentum space in smaller QDs. The decay rate isenhanced both on a Au flat film and on plasmonic substrates.39 Within the data on the plasmonic substrates, thedecay rate is the largest in L(0) and becomes small withincreasing distance. This can be seen more clearly in theenhancement factor of the decay rates, which is defined by
the ratio of the decay rates to that of a Si-QD monolayer on asilica substrate [Fig. 5(c)].
In Fig. 5(c), the decay rate enhancement arises both fromthe Purcell enhancement (radiative rate enhancement), which
contributes to the enhancement of the quantum efficiency andthe dissipation to lossy modes.40 We extract the Purcell
enhancement factor (P=PSilica ), where P is the Purcell factor ofa sample and PSilica is that of a Si-QD monolayer on a silica
substrate, from the enhancement factors of the decay rate
(CSilica =C) and the intensity (I=ISilica ) by the following procedure. The enhancement factor of the PL intensity is expressed
as I=ISilica ¼ Q=QSilica  r=rSilica  g=gSilica , where Q is thequantum efficiency, r is the excitation cross-section and g is
the light collection efficiency. The quantum efficiency enhancement factor ðQ=QSilica Þ is Q=QSilica ¼ P=PSilica  CSilica =C.
Among these parameters, the excitation cross-section ratio(r=rSilica Þ can be approximated to be unity under 405 nm excitation. On the other hand, the collection efficiency is largely

J. Appl. Phys. 122, 223101 (2017)

different, because substantial amounts of photons are emitted
onto the substrate side and cannot be collected in a Si-QD
monolayer on a silica substrate. In the case of a dipole emitter
placed on a silica substrate with the refractive index of 1.45,

FIG. 5. (a) PL decay curves of Si-QD monolayer/plasmonic substrate structures
excited at 405 nm. The number of spacer layers is changed from 0 to 14. The
detection wavelength is in the range of 800 nm to 850 nm. (b) PL decay rates
obtained from the decay curves as a function of detection wavelength. (c) Decay
rate enhancement factor obtained by dividing the decay rates of Si-QD monolayer/plasmonic substrate structures by that of a Si-QD monolayer on a silica
substrate. (d) Enhancement factor of Purcell factors as a function of detection
wavelength.

223101-6

Yanagawa et al.

about 24% of photons are emitted upward and others onto thesubstrate. Therefore, g=gSilica becomes about 1/0.24, and
I=ISilica is approximated to P=PSilica  CSilica =C  1=0:24. Figure5(d) shows the Purcell factor enhancement as a function of thedetection wavelength. As is expected, on a Au flat film, noPurcell enhancement can be seen. On plasmonic substrates, thePurcell factor is strongly enhanced. The Purcell factor enhancements of L(0) and L(4) have similar wavelength dependence
and are larger at longer wavelengths. This suggests that theenhancement is due to the coupling with the longer wavelengthsurface plasmon mode in Fig. 2. The wavelength dependence
becomes small in L(8) and the dependence is reversed in L(14).
This can be explained by the fact that the longer wavelength
mode shifts beyond the emission wavelength and the enhancement is mainly from the coupling with the shorter wavelengthsurface plasmon mode in L(14).
IV. CONCLUSION

We have prepared a plasmonic substrate suitable for theexcitation of NIR luminescence of Si-QDs by long wavelength photons for bio-sensing applications by using nanoimprint lithography. The reflectance spectra revealed that theplasmonic substrates have two surface plasmon bands, which
can be used for excitation and emission enhancements. We
placed a monolayer of Si-QDs on the plasmonic substrate bycontrolling the QDs-substrate distance precisely by polymer
spacers. PL and PLE spectra and the decay characteristics incombination with the reflectance spectra revealed that boththe excitation and emission processes of Si-QDs are strongly
modified by the plasmonic substrate. In particular, under
641 nm excitation, the excitation efficiency was largely
enhanced. The efficient excitation of NIR PL of Si-QDs bylong wavelength photons is an important achievement for theapplication of Si-QDs in bio-sensing devices, because autofluorescence of bio-materials can be suppressed significantly
compared to the case of UV excitation.
SUPPLEMENTARY MATERIAL

See supplementary material for the procedure to determine
the QY of Si-QDs, the formation method of a polymer spacer
by the LbL assembly process, the estimation of the thickness of
the Si-QD monolayer by ellipsometry, the measurement geometry of PL and PLE, reflectance spectra of Si-QDs on a plasmonic
substrate at an incident angle of 60 , the PLE enhancement factor spectra of a Si-QD/L(n)/plasmonic substrate and the reflectance spectra, the PL spectra and PL enhancement factor spectraof a Si-QD monolayer excited at 405 and 641 nm, and the PL
decay curves of a Si-QD monolayer.
ACKNOWLEDGMENTS

This work was partly supported by the 2015 JST Visegrad
Group (V4)-Japan Joint Research Project on Advanced
Materials and a JSPS KAKENHI Grant No. 16H03828.
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