Enhanced photoluminescence of Si nanocrystals-doped cellulose nanofibers byplasmonic light scattering
Hiroshi Sugimoto, Ran Zhang, Björn M. Reinhard, Minoru Fujii, Giovanni Perotto, Benedetto Marelli, Fiorenzo G.
Omenetto, and Luca Dal Negro
Citation: Applied Physics Letters 107, 041111 (2015); doi: 10.1063/1.4927672
View online: http://dx.doi.org/10.1063/1.4927672
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/107/4?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Enhanced emission efficiency in electrospun polyfluorene copolymer fibers
Appl. Phys. Lett. 102, 211911 (2013); 10.1063/1.4807894
Light scattering assisted surface plasmon resonance at electrospun nanofiber-coated gold surfaces
Appl. Phys. Lett. 98, 241109 (2011); 10.1063/1.3601465
Oxygen defects-modulated green photoluminescence of Tb-doped ZrO 2 nanofibers
Appl. Phys. Lett. 97, 141916 (2010); 10.1063/1.3496471
Enhanced exciton migration in electrospun poly[2-methoxy-5- ( 2 ′ -ethylhexyloxy)-1,4phenylenevinylene]/poly(vinyl pyrrolidone) nanofibers
Appl. Phys. Lett. 96, 133309 (2010); 10.1063/1.3374336
Photoluminescence of rare earth 3 + doped uniaxially aligned HfO 2 nanotubes prepared by sputtering withelectrospun polyvinylpyrolidone nanofibers as templates
J. Appl. Phys. 107, 024309 (2010); 10.1063/1.3290974

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
133.30.106.15 On: Thu, 06 Aug 2015 23:13:04

APPLIED PHYSICS LETTERS 107, 041111 (2015)

Enhanced photoluminescence of Si nanocrystals-doped cellulose
nanofibers by plasmonic light scattering€rn M. Reinhard,4 Minoru Fujii,2 Giovanni Perotto,5Hiroshi Sugimoto,1,2,a) Ran Zhang,3,a) Bjo
5
Benedetto Marelli, Fiorenzo G. Omenetto,5 and Luca Dal Negro1,3,b)

1
Department of Electrical and Computer Engineering and Photonics Center, Boston University,
8 Saint Mary Street, Boston, Massachusetts 02215, USA
2
Department of Electrical and Electronic Engineering, Graduate School of Engineering,
Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan
3
Division of Materials Science and Engineering, Boston University, 15 Saint Mary’s Street,
Brookline, Massachusetts 02446, USA
4
Department of Chemistry and Photonics Center, Boston University, Boston, Massachusetts 02215, USA
5
Department of Biomedical Engineering and Department of Physics, Tufts University, 4 Colby Street,
Medford, Massachusetts 02155, USA(Received 28 May 2015; accepted 20 July 2015; published online 28 July 2015)
We report the development of bio-compatible cellulose nanofibers doped with light emitting silicon
nanocrystals and Au nanoparticles via facile electrospinning. By performing photoluminescence(PL) spectroscopy as a function of excitation wavelength, we demonstrate plasmon-enhanced PL
by a factor of 2.2 with negligible non-radiative quenching due to plasmon-enhanced scattering of
excitation light from Au nanoparticles to silicon nanocrystals inside the nanofibers. These findings
provide an alternative approach for the development of plasmon-enhanced active systems integrated within the compact nanofiber geometry. Furthermore, bio-compatible light-emitting nanofibers prepared by a cost-effective solution-based processing are very promising platforms for
C 2015 AIP Publishing LLC.
biophotonic applications such as fluorescence sensing and imaging. V[http://dx.doi.org/10.1063/1.4927672]

Polymer nanofibers prepared by electrospinning arecheap, flexible, and bio-compatible platforms for manipulating light at the nanoscale in nanophotonic and biophotonic
applications.1,2 The elegance of electrospinning techniquerelies on its simplicity and solution-based process in the absence of additional harsh chemicals and precisely controlledenvironment (pressure, temperature, etc.) that are traditionally required for most nanofabrication techniques. Lightemitting active nanofibers doped with semiconductor nanocrystals (NCs)3,4 or organic dyes5 have particularly attracted
much attention due to their wide range of applications including waveguides,6–8 light-sources,9–11 optical sensors.3,12,13
It is well-known that plasmonic nanostructures such as
Au and Ag nanoparticles (NPs) and nanorods can modify theemission properties of emitters located in their close proximity.14,15 By carefully designing the material and structure,enhanced emission of NCs coupled to localized surface plasmon resonance (LSPR) of metal NPs has been demonstrated.16,17 This is also applicable to NCs-doped electrospun
nanofibers.18 However, in order to suppress the non-radiative
quenching of emitters coupled to metal surfaces or NPs,19,20
complex fabrication techniques must be developed capable
to accurately control the distance between metal NPs andemitters, which is a very challenging task.
In this work, using the cost-effective electrospinning technique, we demonstrate plasmon-enhanced emission in biocompatible light-emitting silicon nanocrystals (Si-NCs) doped
nanofibers. The combination of water-based hydroxypropyla)H. Sugimoto and R. Zhang contributed equally to this work.
Author to whom correspondence should be addressed. Electronic mail:dalnegro@bu.edu.

b)0003-6951/2015/107(4)/041111/4/$30.00

cellulose (HPC) polymer precursor and nontoxic Si-NCs emitting in the biological window (650–1300 nm)21,22 enables the
fabrication of all-biocompatible active nanofiber systems forsensing23 and imaging.24 Furthermore, we engineer the lightemission properties of the nanofibers by Au-doping them withplasmonic NPs. By performing photoluminescence (PL) spectroscopy of Au-doped nanofibers with and without metal NPs,
we demonstrate plasmon-driven PL enhancement by a factorof 2.2 in the absence of non-radiative quenching due to thestrong scattering of excitation light by Au NPs in side
nanofibers.
Colloidal dispersions of Si-NCs are prepared by themethod described elsewhere.25,26 Si-rich borophosphosilicate
glasses (BPSG) were first deposited by co-sputtering Si,
SiO2, B2O3, and P2O5 in an RF-sputtering apparatus. The
films were peeled off from the plates and annealed at1075  C in a N2 gas atmosphere for 30 min to grow Si-NCs
in BPSG matrices. The Si-NCs were isolated from matrices
by dissolving in HF solution (46 wt. %). Isolated Si-NCs
were then transferred to water. As reported in our previous
work,27 co-doping B and P enables us to obtain the waterdispersible Si-QDs without organic ligand passivation. In
Figure 1(a), we show a photograph of Si-NC dispersion anda transmission electron microscope (TEM) (Tecnai Osiris,
FEI) image. Si-NCs are well-dispersed in water, and no
agglomerates were observed in the image. The average diameter of Si-NCs estimated by TEM was 3.3 6 0.9 nm.
To prepare the precursor of electrospun nanofibers, wefirst dissolved HPC in DI water and then added Si-NC solution to the polymer solution, followed by stirring for about
1 h to obtain a homogeneous mixture solution containing

107, 041111-1

C 2015 AIP Publishing LLC
VThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
133.30.106.15 On: Thu, 06 Aug 2015 23:13:04

041111-2

Sugimoto et al.

FIG. 1. (a) Photograph of colloidal dispersion and TEM image of Si-NCs(Scale bar: 10 nm). (b) Absorbance (black) and normalized PL spectra of SiNCs in water (red) and planar HPC film (blue). The excitation wavelength is
488 nm. Inset: Photograph of HPC solution (45 wt. %) containing Si-NCs
under UV excitation. The color of the emission appears pink due to the contribution of the weak blue emission from the HPC polymer under UV excitation. (c) Extinction spectra of single- (black) and Au-doped (red) HPC films.

45 wt. % HPC and 0.8 mg/ml Si NCs. Photoluminescence
spectra were measured using a monochromator (Oriel
Cornerstone 260, Newport) and photomultiplier tubes (Oriel
77348, Newport). The excitation source was 488 nm line of
an Ar ion laser (Spectra Physics, 177–602). Figure 1(b)shows absorbance and normalized PL spectra of Si-NCs inwater (red) and in the HPC solution (blue). In the inset, weshow a photograph of HPC solution containing Si-NCs under
UV light excitation, exhibiting bright red luminescence. The
Si-NCs in HPC solution exhibit a broad PL peaked at
710 nm, which is identical to that observed for Si-NCs inwater.
Plasmonic NPs and Si-NCs co-doped (Au-doped) polymer solution was prepared by directly adding concentrated
Au NPs (70 nm diameter) synthesized by a seeded growthmethod to the mixture solution. The concentration of the
final mixture solution was 45 wt. % HPC with 0.8 mg/ml SiNCs and 1.2 mg/ml Au NPs. In the following, we refer to thesample doped with both Si-NCs and Au NPs as Au-doped
sample, and to that doped with Si-NCs alone as single-doped
sample. In Figure 1(c), we plot the extinction spectra of single- (black) and Au-doped (red) HPC films prepared by spincoating mixture solutions. In the Au-doped HPC film, theextinction peak originating from LSPR of Au NPs is clearlyobserved around 550 nm. The slight increase of extinction ata wavelength shorter than 400 nm observed in both spectra isdue to the absorption of Si-NCs. We notice that in thisAppl. Phys. Lett. 107, 041111 (2015)

wavelength range, the absorption (and scattering) of the
HPC host film is negligibly small compared to that of Au
NPs.
In order to create active nanofibers by electrospinning,
the HPC mixture solution was poured into 10 ml Syringe (diameter 18.94 mm) fitted with a 20 gauge stainless steel needle. The syringe was then transferred to an infusion syringed
pump (Braintree scientific) with the needle connected to thepositive terminal of a high power voltage source (Gamma
high voltage). Controlled by the syringe pump, the precursor(45 wt. % HPC solution) was pushed continuously to the needle tip with a flow rate of 0.4 ml/h. Driven by the high voltage (18 kV) established in the Taylor cone between thenozzle and metal plate collector, the precursor was ejectedfrom the nozzle creating HPC nanofibers after solvent evaporation. The charged HPC nanofibers are neutralized at thecollector (kept 20 cm from the nozzle), where they form random networks of nanofibers. Figure 2(a) shows a representative scanning electron microscope (SEM) image of theelectrospun HPC nanofibers Au-doped with Si-NCs and with
Au NPs. The average diameter of the nanofibers is 249 nmwith the standard deviation of 31 nm. The nanofibers form a
relatively uniform film with a thickness of 7.6 lm measured
by cross-section SEM analysis. We confirmed the same morphology (i.e., thickness and diameter) of both single- andAu-doped nanofibers films. In Figure 2(b), we show a TEM
image of the Au-doped system, which demonstrates dopingwith 70 nm Au NPs within the nanofiber. The TEM image
demonstrates that the Au NPs are incorporated into the nanofibers without forming aggregates at this doping concentration. The density of Au NPs (i.e., number of NPs per unit
volume) is estimated from TEM analysis to be 1013 cm3in good agreement with the concentration of the precursor
solution. The high-resolution TEM image of the nanofiber in

FIG. 2. (a) SEM image of electrospun HPC nanofibers doped with Si NCs
and Au NPs. (b) TEM image of Au-doped nanofibers. (c) High-resolution
TEM image showing a Si NC doped in a nanofiber. (d) Microscope PL
image of Au-doped nanofibers (excited at 430 nm).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
133.30.106.15 On: Thu, 06 Aug 2015 23:13:04

041111-3

Sugimoto et al.

Figure 2(c) shows an incorporated Si-NC with lattice fringes
corresponding to {111} planes of the Si crystal. Figure 2(d)shows a PL image of Si-NCs Au-doped nanofibers obtainedby an optical microscope with excitation at 430 nm. Since
only the PL signal from Si-NCs (red to near-IR range) is collected by filtering the excitation light, the image clearly demonstrates that Si-NCs are homogeneously incorporated
within the nanofiber network.
To investigate the effect of Au NP-doping on the lightemission of Si-NCs, we performed PL excitation (PLE) spectroscopy for single- and Au-doped nanofiber samples using atunable (340 nm–2200 nm) optical parametric oscillator(SpectraPhysics Inspire) pumped by a Ti:sapphire laser(MaiTai HP SpectraPhysics, 82 MHz repetition rate) using
an average power of 2.5 mW. The samples are directly irradiated by a focused excitation beam without waveguiding
excitation.7 In Figures 3(a) and 3(b), we show the PL spectraof single- and Au-doped nanofibers excited at 490 and
545 nm, respectively. We measured the PL spectra at 5 different excitation spots, and calculated the average PL intensity and error bars. When excited at 490 nm, which is
detuned from LSPR of Au NPs, both the single- and Audoped nanofibers samples show comparable PL intensity(within the error bars). Figure 3(c) shows the measured PL
decay traces of single- (black) and Au-doped (red) nanofiber
detected at 720 nm using the excitation by the Ar laser beam
(488 nm), which has been modulated with an acousto-optic
modulator at a frequency of 1 kHz. The decay curves are

FIG. 3. PL spectra of single- (black) and Au-doped (red) nanofibers excitedat (a) 490 and (b) 545 nm. PL peak wavelength of single- (black) and Audoped (red) nanofibers as a function of excitation wavelength. (c) PL decay
curves of single- (black) and Au-doped (red) nanofibers detected at 720 nmwith the fitting curve (blue). (d) PL enhancement factors (black) as a function of excitation wavelength. Red dashed curve shows the scattering spectrum of Au-doped nanofibers. Inset: PL peak wavelength of single- (black)and Au-doped (red) nanofibers excited at different wavelengths.

Appl. Phys. Lett. 107, 041111 (2015)

well-fitted by a stretched exponential decay function,
I ¼ I0 exp[(t/s)]b, where s is the effective lifetime and b is
the stretching parameter. The same s (32 ls) and b (0.74) are
obtained for both samples, and almost identical to that of SiNCs doped in a HPC film (i.e., s ¼ 31 lsec and b ¼ 0.77),
which indicates that there is no significant change in the
decay time of Si-NCs by electrospinning. Moreover, thecomparison between single- and Au-doped nanofibers demonstrate that the spontaneous decay rate of Si-NCs is notaffected by the Au-doping with Au NPs, consistent with thelarge average distance between Si-NCs and Au NP (i.e., precluding near-field interactions).
On the contrary, when we excite the systems at 545 nm,
the PL intensity of the Au-doped nanofibers is enhanced byapproximately a factor of 2 compared to the one of singledoped nanofibers. To better understand the origin of themeasured PL enhancement, we study the PL enhancement
spectrum (the PL intensity ratio of Au-doped to single-doped
nanofibers). Figure 3(d) shows the enhancement factors of
the PL of Si-NCs and the scattering spectrum of Au NPs
doped nanofibers. The scattering spectrum of Au-doped nanofibers (red curve in Figure 3(d)) is obtained from the darkfield scattering measurements with the optical microscopeusing 100 W tungsten halogen lamp. We find that the PL
enhancement factor strongly depends on the excitation wavelength and that its trend follows the scattering spectrum of
Au NPs in the nanofibers. The PL enhancement in a genericsystem of emitters (of identical density) can originate fromseveral factors: the modifications of (a) excitation efficiency;(b) extraction efficiency of emitted light; and (c) quantum efficiency (i.e., emission rate) of the emitters. The separation
between the extinction peak of Au NPs (at 550 nm shown in
Figure 1(c)) and the emission peak of Si-NCs (at 720 nm)
allows us to exclude significant enhancement of the extraction efficiency at the emission wavelength. Moreover, due tothe large average distance between Si-NCs and Au NPs, we
did not observe sizable modifications of the PL decay timesof Si-NCs in both samples. Therefore, we can deduce that themeasured PL enhancement originates from the plasmonenhanced light scattering, which increases the effective excitation efficiency of Si-NCs due to the redistribution and trapping of light inside the Au NPs doped nanofibers. The
presence of strongly scattering Au NPs in the nanofibers leads
to an increase of the absorption rate of the pump inside the
Si-NCs doped samples. Our data demonstrate that the Audoping of Au NPs in active nanofibers significantly enhances
the emission of Si-NCs in the absence of non-radiative
quenching, which limits the performance of alternative active
systems coupled to plasmonic NPs in the near-field.19
In the inset of Figure 3(d), we show the PL peak wavelength of single- and Au-doped nanofibers as a function of
excitation wavelengths, demonstrating the red-shift withincreasing excitation wavelength. This behavior is commonly observed in an ensemble of Si-NCs.28,29 Due to thesize distribution of Si-NCs, only larger Si-NCs with smaller
optical bandgap energies can be excited when the excitation
wavelength is close to the PL wavelength.28,29 In addition,
we observe a small red-shift between single- and Au-doped
samples across the overall excitation wavelength range. Thiscan be explained by photon re-absorption in the presence ofThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
133.30.106.15 On: Thu, 06 Aug 2015 23:13:04

041111-4

Sugimoto et al.

an inhomogeneous population of Si-NCs dispersed inside
nanofibers (i.e., the emission from small Si-NCs is reabsorbed by the larger Si-NCs). Since the scattering spectrum of Au NPs overlaps with the shorter wavelength side of
the PL spectrum, the emission from the smaller Si-NCs isstrongly scattered by the Au NPs and efficiently re-absorbed
at lower energy by the larger Si-NCs in the nanofibers. This
leads to the slight red-shift of the PL observed in the Audoped samples.
In conclusion, we developed active light-emitting HPC
nanofibers doped with Si-NCs and Au NPs. By performing
time-resolved PL and PLE measurements, we demonstratethat Au-doped nanofibers give rise to enhanced PL by a factor of 2.2 in the absence of non-radiative quenching of the
Si-NCs emission due to plasmon-enhanced scattering ofpump radiation from Au NPs to Si-NCs inside the nanofibers. Our findings provide not only an alternative concept forthe plasmon-enhanced PL but also a unique opportunity forbio-compatible active platforms that leverage cost-effective
and largely scalable processing with Si-NCs light emission
for biophotonics applications.
This work was partly supported by the AFOSR program“Emitters for High Density Information Processing using
Photonic-Plasmonic Coupling in Coaxial Nanopillars” underAward No. FA9550-13-1-0011. H.S.’s work acknowledges
support from Grant-in-Aid for JSPS Fellows (No. 26-3120).
F.G.O. acknowledges support from the Office of Naval
Research (N00014-13-1-0596). B.M.R. acknowledges
support from the National Science Foundation under Award
No. CBET-1159552.
1

P. Wang, Y. Wang, and L. Tong, Light Sci. Appl. 2, e102 (2013).
F. Di Benedetto, A. Camposeo, S. Pagliara, E. Mele, L. Persano, R.
Stabile, R. Cingolani, and D. Pisignano, Nat. Nanotechnol. 3, 614 (2008).
3
C. Meng, Y. Xiao, P. Wang, L. Zhang, Y. Liu, and L. Tong, Adv. Mater.
23, 3770 (2011).
4
M. Li, J. Zhang, H. Zhang, Y. Liu, C. Wang, X. Xu, Y. Tang, and B.
Yang, Adv. Funct. Mater. 17, 3650 (2007).
5
A. Camposeo, F. Di Benedetto, R. Stabile, R. Cingolani, and D. Pisignano,Appl. Phys. Lett. 90, 143115 (2007).
2

Appl. Phys. Lett. 107, 041111 (2015)
6

V. Fasano, A. Polini, G. Morello, M. Moffa, A. Camposeo, and D.
Pisignano, Macromolecules 46, 5935 (2013).
7
F. Gu, H. Yu, P. Wang, Z. Yang, and L. Tong, ACS Nano 4, 5332
(2010).
8
H. Liu, J. B. Edel, L. M. Bellan, and H. G. Craighead, Small 2, 495(2006).
9
A. Camposeo, F. Di Benedetto, R. Stabile, A. A. R. Neves, R. Cingolani,
and D. Pisignano, Small 5, 562 (2009).
10
D. Di Camillo, V. Fasano, F. Ruggieri, S. Santucci, L. Lozzi, A.
Camposeo, and D. Pisignano, Nanoscale 5, 11637 (2013).
11
J. M. Moran-Mirabal, J. D. Slinker, J. A. DeFranco, S. S. Verbridge, R.
Ilic, S. Flores-Torres, H. Abru~
na, G. G. Malliaras, and H. G. Craighead,
Nano Lett. 7, 458 (2007).
12
F. Gu, L. Zhang, X. Yin, and L. Tong, Nano Lett. 8, 2757 (2008).
13
P. Wang, L. Zhang, Y. Xia, L. Tong, X. Xu, and Y. Ying, Nano Lett. 12,3145 (2012).
14
K. Aslan, I. Gryczynski, J. Malicka, E. Matveeva, J. R. Lakowicz, and C.
D. Geddes, Curr. Opin. Biotechnol. 16, 55 (2005).
15
J. R. Lakowicz, Plasmonics 1, 5 (2006).
16
P. P. Pompa, L. Martiradonna, A. D. Torre, F. D. Sala, L. Manna, M. De
Vittorio, F. Calabi, R. Cingolani, and R. Rinaldi, Nat. Nanotechnol. 1, 126(2006).
17
A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. M€
ullen, and W. E.
Moerner, Nat. Photonics 3, 654 (2009).
18
X. Yang, R. Xu, D. Bao, and B. Li, ACS Appl. Mater. Interfaces 6, 11846(2014).
19
P. Viste, J. Plain, R. Jaffiol, A. Vial, P. M. Adam, and P. Royer, ACS
Nano 4, 759 (2010).
20
E. Dulkeith, A. C. Morteani, T. Niedereichholz, T. A. Klar, J. Feldmann,
S. A. Levi, F. C. J. M. van Veggel, D. N. Reinhoudt, M. M€
oller, and D. I.
Gittins, Phys. Rev. Lett. 89, 203002 (2002).
21
F. Erogbogbo, K. Yong, I. Roy, G. Xu, P. N. Prasad, and M. T. Swihart,
ACS Nano 2, 873 (2008).
22
L. Shi, T. Yu, L. Sun, X. Pi, and X. Peng, Phys. Chem. Chem. Phys. 15,
20140 (2013).
23
Y. Yang, X. Fan, Y. Long, K. Su, D. Zou, N. Li, J. Zhou, K. Li, and F.
Liu, J. Mater. Chem. 19, 7290 (2009).
24
W. Liu, J. Wei, Y. Chen, P. Huo, and Y. Wei, ACS Appl. Mater.
Interfaces 5, 680 (2013).
25
H. Sugimoto, M. Fujii, K. Imakita, S. Hayashi, and K. Akamatsu, J. Phys.
Chem. C 116, 17969 (2012).
26
H. Sugimoto, M. Fujii, K. Imakita, S. Hayashi, and K. Akamatsu, J. Phys.
Chem. C 117, 11850 (2013).
27
H. Sugimoto, M. Fujii, Y. Fukuda, K. Imakita, and K. Akamatsu,
Nanoscale 6, 122 (2014).
28
S. T. Li, S. J. Silvers, and M. S. ElShall, J. Phys. Chem. B 101, 1794
(1997).
29
L. E. Brus, P. F. Szajowski, W. L. Wilson, T. D. Harris, S. Schuppler, andP. H. Citrin, J. Am. Chem. Soc. 117, 2915 (1995).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
133.30.106.15 On: Thu, 06 Aug 2015 23:13:04