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Plasmon enhanced fluorescence from quaternary CuInZnS quantum dots
Accepted Manuscript Title: Plasmon Enhanced Fluorescence from Quaternary Cu−In−Zn−S Quantum Dots Author: Weiguang Kong Bingpo Zhang Ruifeng Li Feifei Wu Tianning Xu Huizhen Wu PII: DOI: Reference:
S0169-4332(14)02437-4 http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.171 APSUSC 29036
To appear in:
APSUSC
Received date: Revised date: Accepted date:
9-5-2014 21-10-2014 28-10-2014
Please cite this article as: W. Kong, B. Zhang, R. Li, F. Wu, T. Xu, H. Wu, Plasmon Enhanced Fluorescence from Quaternary CuminusInminusZnminusS Quantum Dots, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.171 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Plasmon Enhanced Fluorescence from Quaternary Cu−In−Zn−S Quantum Dots Weiguang Kong1, Bingpo Zhang1, Ruifeng Li1, Feifei Wu1, Tianning Xu2, Huizhen
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Wu1*
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1 Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China
2 Department of Science, Zhijiang College of Zhejiang University of Technology,
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Hangzhou, Zhejiang 310024, P. R. China
E-mail: [email protected] Corresponding author: Huizhen Wu1
Fax number: +086-0571-87953885
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E-mail: [email protected]
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Other co-authors: Bingpo Zhang
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Telephone number: +086-0571-87953885;
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First author: Weiguang Kong
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E-mail: [email protected] Ruifeng Li
E-mail: [email protected] FeiFei Wu
E-mail: [email protected] Tianning Xu
E-mail: [email protected]
*
Corresponding Author. E-mail address: [email protected]. 1
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Abstract: Plasmon enhanced fluorescence of heavy-metal-free Cu−In−Zn−S quantum dots was explored. Cu−In−Zn−S were chemically synthesized with different sizes via variation of the growth time of the Cu-In-S while plasmonic silver films (PSFs) were also fabricated through physical deposition. The QD/PSF coupling structure gives
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significant rise to fluorescence intensity and stability of the QDs. As high as 45 folds
enhancement in emission intensity and 41.23% decrease in fluorescence lifetimes are
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observed. The metal-enhanced fluorescence offers promise for a range of applications,
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including LEDs, sensor technology, microarrays and single-molecule studies.
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Keyword: CuInZnS; quantum dots; surface plasmons; silver film; fluorescence
1. Introduction
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Colloidal semiconductor quantum dots (QDs) are considered extremely promising for applications in many different areas, such as biomedical labeling, solar cells, and
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photo-electronic devices. [1-3]. To tune the band gap and improve the quantum yield (QY) of QDs, previous researches were mainly focused on the synthesis stage of the
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QDs by changing relative stoichiometries and reactivity of the various chemical
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species in QDs and growing a passivated shell over the QDs cores [4-6]. Other schemes such as QDs being excited by interaction with localized surface plasmons (LSPs) were also performed [7-9]. LSPs are charge density oscillations confined to metallic nanostructures, which is thought to lead to plasmonic fluorescence enhancement. Metallic nanostructures have long been studied due to their ability to
manipulate incident light. It becomes more appealing when applying the QDs to random distributions of metallic nanoparticles or nanoscale roughness in metallic films. However, up to now, most of these methods were realized based on the binary QDs [7-9]. These QDs generally contain elements that are thought to be detrimental to health and the environment, such as cadmium, lead, etc. Few articles are about the ternary I−III−VI or quaternary systems. Herein, Cu−In−Zn−S quaternary QDs according to the previous reports were fabricated [3-5, 10]. The plasmonic silver films (PSFs) on planar substrates with in-homogenous shapes and dimensions were also 2
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fabricated through vapor deposition with a post thermal treatment. SP coupling was realized by spin-coating the CIZS QDs to the PSFs without any spacer. Upon proper optimization, such an easy and reproducible method increases the excitation or emission rates of the QDs, thus resulting in a strong enhancement of their
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fluorescence, as high as 45 folds enhancement in emission intensity was achieved. Compared to other SP coupling strategies, this one is cheaper and easier to be
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fabricated. The metal-enhanced fluorescence (MEF) offers promise for a range of
applications, including LEDs, sensor technology, microarrays and single-molecule
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studies. 2. Materials and Experiment
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2.1. Materials and synthesis of QDs.
Octadecene (ODE, technical grade, 90%), indium (III) acetate (In(Ac)3, 99.99%),
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copper(I) iodide (CuI, purum, ≥99.5%), 1-dodecanethiol (DDT, ≥98%), and zinc stearate (technical grade) were purchased from Sigma-Aldrich. Tri-noctylphosphine
without further purification.
d
(TOP, min 97%) was purchased from Strem Chemicals. All the chemicals were used
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Due to the promising properties of CIZS, here we explored controlled synthesis of
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CIZS-alloyed QDs using a heat-up and solvent-free method following previous works [3-5, 10], and tuning their PL emission wavelength via variation of growth time of CIS. In a typical synthesis, a solution of CuI (0.4 mmol), In(Ac)3 (0.4 mmol), and 5
mL of DDT was loaded into a three-neck flask and was heated to 120 °C and maintained at this temperature under argon flow until a clear solution was obtained. The temperature was then raised to 230 °C for a certain time (10, 40 and 90 min) to let the particles grow. Then, the reaction was quenched by cooling the solution to room temperature. The crude solution obtained from the above steps was heated at 120 °C (under argon atmosphere), and then a solution of zinc stearate (0.4 mmol) dissolved in 5 mL of ODE and 0.5 mL of TOP was added dropwise. The temperature was further raised to 210 °C for 90 min, after which the flask was cooled down to room temperature and the QDs were separated by addition of ethanol followed by centrifugation. The QDs were washed three times by repeated dissolution in toluene 3
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and reprecipitation in ethanol. 2.2. Preparation of SP-QDs coupling. Plasmonic silver films were fabricated with a vapor deposition system. Crystalline silicon wafers was used as substrates. The cleaning procedure includes washing the
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wafers with tetrachloride, successive sonication in acetone and ethanol, followed by DI water rinse. The wafers were dried with nitrogen blowing before introducing into
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the vapor deposition chamber. Half of a Si wafer was coated with an Ag film of a
film were measured by the quartz monitor crystals.
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certain thickness by thermal evaporation. The deposition rate and the thickness of the
The as-deposited silver films on silicon substrates were transferred to an oven for
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post-annealing treatment at 200ºC for 30 min. After annealing treatment, the CIZS QD/toluene solution was spin-coated on the Si wafer.
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2.4 Test Equipment and Methods
The PL and lifetimes of the fluorescence were measured by an Edinburgh FLS920
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system with the excitation of a 405 nm laser. The fluorescence quantum yield (QY) is measured by the Edinburgh FLS920 system equipped with an integrating sphere, and
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the excitation source is a wavelength tunable xenon lamp. To conduct the high
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resolution transmission electron microscopy (HRTEM), QDs were deposited from toluene solutions onto ultrathin carbon membrane by spraying the solvent in air at room temperature. HRTEM image was obtained via a Tecnai G2 F20 S-TWIN TEM operating at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) characterization was carried out with a Hitachi S4800. The absorption and extinction spectra were recorded at room temperature using a Shimadzu UV-2450/2550 spectrophotometer. X ray diffraction (XRD) patterns of the samples deposited on Si (100) substrates were obtained on a Panalytical X'Pert PRO diffractometer.
3. Results and Discussion 3.1 Characteristics of the synthesized QDs. Fig. 1a shows the XRD patterns for the structural characterization of the representative CIS and CIZS QDs. There is an obvious shift to larger angles for the 4
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CIZS peaks compared to the CIS phase, indicating a smaller lattice constant for CIZS than that of CIS. The X-ray diffraction peaks for CIZS QDs at 28.119°, 46.739° and 55.127° correspond to (112), (204) and (312) planes. The crystal lattice constants are determined to be a=b=5.505Å, c=10.935Å. The crystallite size of 3.0 nm estimated
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from the full width at half maximum (FWHM) of the (110) peak and the d-spacing of 3.171Å corresponding to (1 1 2) planes calculated from the XRD pattern are both
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compatible with those obtained from the HRTEM results as shown Fig.1b.
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The CIS QDs used in this article were grown with the same growth time of 10 min. The QY of the CIZS samples dispersed in toluene is determined to be ~15%. It is noted that there is an obvious redshift for the emission peak of CIZS QDs in
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solid-state, with respect to the CIZS samples dispersed in toluene. This is possibly due to coalescence of the adjacent QDs, as shown in TEM image in the inset of Fig.1a, i.e.
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two or more QDs approach or contacts each other, leading to electron coupling between these QDs thus an enlargement in QDs’ effective sizes. Fig. 1c shows the
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absorption and photoluminescence (PL) spectra of the CIZS QDs dispersed in toluene
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with different synthesis time. All the samples show obvious Stokes shifts. As the growth time of CIS increases, the absorption and emission edges of the CIZS
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gradually shift to longer wavelengths.
(a)
XRD Intensity (a.u.)
(112)
20
CIZS CIS
(204) (312)
30
40
50
60
2θ (Degree)
5
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Absorption
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(c)
d te
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Intensity (a.u.)
PL
400
500
600
700
800
Wavelength (nm)
Fig. 1 (a) XRD patterns of CIZS and CIS QDs; (b) HRTEM image of the CIZS QDs with 10 min growth time on an ultrathin carbon membrane. The average size is about 2.6 nm. Inset: the HRTEM for a representative CIZS QD with d-spacing of 3.168Å. (c) The evolutions of absorption and emission of CIZS QDs with different growth time of CIS: 5 min (blue), 60 min (red) and 90 min (black).
3.2 Effects of the surface morphology of the PSFs on SP coupling
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(b)
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(a)
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XRD Intensity (a.u.)
as-prepared 150°C 200°C
(111)
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(200)
20
25
30
35
40
45
50
(220)
55
60
65
2θ
(d)
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(c)
Fig.2 Annealing temperature on the morphology of the Ag films: (a) as-prepared Ag film, (b) being
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annealed under 150ºC, and (c) 200ºC for 30 min. (d) XRD patterns of the PSFs with different
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annealing treatments. The thickness of the Ag film is 120 nm.
It is known that the optical properties of Ag films depend on their mass thickness, the
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rate of Ag deposition, the temperature of substrates and the procedure of post-deposition annealing of the films [11]. The as-deposited Ag film of 120 nm is smooth and continuous as shown by SEM image in Fig.3a. After 150ºC annealing, the morphology of the Ag film remains continuous but with granular structures (Fig.3b). After 200ºC annealing, the granular structures grow larger, lead to a local discontinuity for the Ag film (Fig. 3c). Good crystallinity of Ag films is a vital factor for the SP coupling [7]. Fig.3d shows the XRD of the Ag film annealed under different temperatures. The as-deposited Ag film has a low crystallinity. As the annealing temperature increases, the crystallinity of the Ag film is gradually promoted. Our study shows that the Ag film thermally treated at 200°C for 30 min results in a good crystallinity.
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(a) QDs on Si QDs on PSF 30 nm
IPSF/ISi=5.2
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6000
4000
0 500
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2000
550
600
650
700
an 60 nm
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0
500
QDs on Si QDs on PSF
IPSF/ISi=9.88
d
9000
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12000
3000
800
(b)
15000
6000
750
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Emission Intensity (Counts)
Wavelength (nm))
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Emission Intensity (Counts)
8000
550
600
650
700
750
800
Wavelength (nm)
8
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(c) QDs on Silicon QDs on PSF
60000 120 nm
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IPSF/ISi=45.3
45000
30000
cr
×10
15000
0 500
550
600
650
700
750
800
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Wavelength (nm)
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Emission Intensity (Counts)
75000
50
(d)
M d
30
20
10
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0
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Enhancement Factor
40
50
100
150
200
Thickness of the Ag (nm)
Fig. 3 Emission comparison of QDs on PSFs with different thicknesses (red), and Si (black): (a) 30
nm, (b) 60 nm, and (c)120nm; (d) Variations of the enhancement factor with the thickness of the PSFs; Insets (above): CCD images of QDs on PSFs with different thicknesses (left), and Si (right) photographed under identical sampling conditions. Insets (below): SEM images of the Ag films with different thicknesses; the annealing temperature is fixed at 200°C.
Fig.3 a, b and c shows significant difference of the PL intensity between the application of the QDs to PSF and non-plasmon coupling substrate (Si). As the thickness of the PSFs increases, the magnification quickly increases and then slowly decreases. More than 40 folds fluorescence intensity increase is realized when the 9
Page 9 of 18
PSF is ~120 nm. The representative CCD images shown in the insets of Fig.3 also demonstrate the significant difference of brightness between the QDs on PSF/Si (left side) and Si substrate only (right side). Notice that there is an obvious blue shift in the PL band for the QDs on PSF/Si; this is possibly because the resonance absorption
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peak of the QD/PSFs couplers locates at ~630 nm according to our previous report. [7]
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The SEM images in the insets in Fig.3a, b and c illustrate the effects of the thickness
on the morphology of the Ag films. In this case, the annealing temperature was fixed
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at 200°C. As the thickness of the Ag film increases, the continuous Ag film breaks and self-assembled superficial clusters and grains in ellipsoidal islets are formed.
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Thereafter, the ellipsoidal grains gradually grow and eventually form a rather rough film. The changes of PL intensity can be attributed to the interaction of LSP with QDs.
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As the LSP is confined in the regions near the metal particles, the coupled LSP energy can be scattered to far-field only if the interface of metal/semiconductor is sufficiently
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rough.
Fig. 4 shows the absorption spectra of the solid QDs film on PSF with 120 nm
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thickness which are all deposited on the quartz substrates. A difference spectrum (the
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intensity of absorption spectrum for QD/PSF/quartz minus that for QD/quartz) is also plotted. The difference spectrum reflects, to a certain extent, the interaction between QDs and PSF through LSP coupling. As plasmonic resonance (PR) frequency depends on the size and morphology of the metallic nanostructures, PSFs with asymmetrical size distributions (as shown in the insets of Fig.3) exhibit multiple SPR modes with a broad resonance absorption shoulder. The broad absorption peaks ranging from 570 to 730 nm is possibly a superposition effect of different PR modes generated by larger Ag nanostructures. A larger nanostructure has a lower electronic confining potential, thus the PR wavelength shifts toward longer one according to Mie resonance theory. Besides, our PL excitation (PLE) spectra show a remarkable change for the QD/PSF/Si coupler compared to the counterpart without PSF, as shown in Fig. 4. A broad band at ~420 nm in the PLE spectrum is assigned to the interaction between the QDs and the PSF. The highest QY of the QD/PSF coupling samples is 10
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32% (Fig.3c) at 405 nm wavelength excitation obtained by the following equation: [1] where I, F stand for the integrated PL intensity and the absorbance at the excitation
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wavelength, respectively. The subscript 0 refers to the QD/Si reference sample, which
is measured to have the QY of 3.6% under excitation of a 405 nm laser. Compared to
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homogenous nanostructures mainly realized via either electron beam lithography or self-assembled QDs [13], PSFs give rise to a remarkable increase in intensity and
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Extinction Efficiency
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fabrication.
Spectra Difference CIZS/PSF CIZS PLE on Si PLE on PSF
Ac ce p
te
×10
300
400
500
600
700
PLE Intensity (Counts)
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stability of the fluorescence with low manufacturing cost and easy up-scaling
800
Wavelength (nm)
Fig. 4 Absorption spectra of the QDs on PSF with 120 nm thickness and solid QDs film which are all deposited on the quartz substrates. The difference spectrum (QD/PSF minus QDs spectra) is also plotted out. The PLE spectra of QDs on PSF and Si are plotted with the scattered circles and squares,
respectively.
3.3 Effects of PMMA on SP coupling. Due to the high surface-to-volume ratio in nanocrystals, CIZS QDs exposed to the air are easy to be oxidized, leading to fluorescence quenching [14]. Thus, QDs/PSF emitters should be stored in vacuum or protected via a simple capsulation. Here, CIZS QDs are firstly blended with Polymethylmethacrylate (PMMA) in toluene, and then 11
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spin coated on the PSF substrates. PMMA can greatly suppress the aggregation of the QDs and avoid the contact of the QDs with air. After a proper annealing to extract the residual solvent, the emission property of the QDs:PMMA/PSF emitters is tested in air. The result shows that the PSF works as the substrate of the composite of QDs and
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PMMA also induces an obvious increase in intensity of the emission compared to the
QDs on Si substrates (shown in Fig.5a). However, the enhancement factor is only ~15,
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almost 3 times less than that of QDs/PSF emitters. This may be assigned to the fact
that the electromagnetic field of the LSPs is confined in the region near the interface
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between QDs and PSF parallel to the x-y plane as shown in Fig.5a. Fig.5b shows the distribution of the electric field along z axis. When coated on the PSF, CIZS QDs in
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PMMA may be randomly distributed in z direction, thus portions of the QDs are too far from the surface of the PSF for the LSP coupling, leading to a lower coupling
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efficiency. Accordingly, increasing the thickness of the QDs layer on the PSF also results in a lower enhancement factor [7]. The results show that a spacer on a rough
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surface of PSF for LSP coupling is not necessary. The blue shift in the enhanced PL for the QDs in the PMMA can be due to the smaller dielectric constant of the PMMA
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higher energy side.
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compared to the bare QDs which results in the SP resonance energy shifting toward
(a)
(b)
12
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PL Intensity (a.u.)
on Si on PSF
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IPSF/ISi=15.07
×5
550
600
650
700
(c)
750
800
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Wavelength (nm)
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500
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Ralated with PMMA
Fig. 5 (a) Cross-section of the QDs:PMMA/PSF emitters; (b) Variation of the electric field
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intensity along z axis; (c) PL comparison of QDs:PMMA on PSFs and Si wafer; inset: CCD images
d
of QDs:PMMA blend on a patterned PSFs under a 405 nm lamp.
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3.4 Lifetimes of fluorescence of CIZS/PSF emitters Fig. 6 shows PL intensity decays for samples of CIZS QDs dispersed in toluene and
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coated on a Si wafer and PSFs. It has been reported that the size dependent band gap, intrinsic defects and surface defects all influence on the fluorescence of QDs [15, 16]. Different electron-hole recombination mechanisms cause different fluorescence decay lifetimes. Herein, the PL decay curves of the CIZS QDs can be well fitted by a tri-exponential function:
I(t)=A1exp(-t/τ1)+A2exp(-t/τ2)+A3exp(-t/τ3)
[2]
where A1, A2 and A3 represent the fractional contributions of PL decay lifetime τ1, τ2 and τ3 respectively. The average decay time is given by: τave=( A1τ12+A2τ22+A3τ32)/ (A1τ1+A2τ2+A3τ3)
[3]
The fast decay component (τ1), on the time scale of several nanoseconds, is assigned 13
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to the initially populated core-state recombination or free to bound transition, while the intermediate lifetime (τ2) with tens of nanoseconds is due to the surface-related radiative recombination of carriers. The longest decay component (>100 nanoseconds) is attributed to DAP transitions [5, 16, 18]. All the shorter and longer lifetimes are
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widely distributed at the entire PL spectra. Fig. 6a shows the PL decay spectrum of
the CIZS QDs dispersed in toluene with τ1=8.12 ns, τ2 =44.75, τ3=207.31 ns at 630 nm.
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When QDs were spin-coated on the substrate to form a solid film, the emission of the CIZS QDs in the solid form decays faster than the QDs dispersed in toluene (τave), and
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the corresponding parameter values are listed in Table 1. Solvent molecules can partially passivate the surface of the QDs on one hand; on the other hand they can
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prevent the aggregation of the QDs [14, 17], effectively suppressing the non-radiative recombination and leading to a longer lifetime. After SP coupling, all the three
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lifetime components τ1, τ2, and τ3 exhibit a significant decrease, from 10.17, 38.83 and 130.27 to 3.91, 20.53 and 81.17, by 61.55, 47.13 and 37.69%, respectively. The
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average lifetime is reduced by 41.23% from 82.61 to 48.52 ns. A reduced lifetime of the fluorescence means the electrons relax faster by radiative recombination routes
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from the excited states while the probability for photo-bleaching is reduced, thus the
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quantum efficiency increases [13, 19, 20].
2
χ = 0.99925
Counts (a.u.)
Decay Fitting τ1=8.12 ns τ2=44.57 ns τ3= 207.31 ns
55.76%
27.24% 17%
0
100
200
300
400
Time (ns)
14
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0
100
200
300
400
500
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Time (ns)
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Counts (a.u.)
CIZS QDs QDs on 50 nm PSF QDs on 80 nm PSF QDs on 120 nm PSF
Fig. 6 (a) PL intensity decays (at 620 nm) and exponential decay components of the CIZS QDs dispersed in toluene. (b) PL intensity decays (at 630 nm) of the CIZS QDs coated on a Si wafer and
d
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PSFs.
τ1/ns
A1/%
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Samples
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Table 1. Fitting parameters deriving from I(t)=A1exp(-t/τ1)+A2exp(-t/τ2)+A3exp(-t/τ3)
QDs in toluene (620 nm)
QDs on Silicon (640 nm)
QDs on 120 nm PSF (630 nm)
τ2/ns
A2/%
τ3/ns
A3/%
Average Lifetime/ns
8.12
17.00
44.57
27.24
207.31
55.76
190
10.17
34.68
38.83
48.053
130.27
17.27
82.61
3.91
35.60
20.53
51.00
81.17
13.40
48.52
4. Conclusions Cu−In−Zn−S quaternary QDs were fabricated in non-coordinating solvent ODE in the presence of the DDT by a two-step synthesis route. The size of the QDs is tuned via variation of the growth time of the CIS in the first step. The PSFs on silicon substrates 15
Page 15 of 18
with in-homogenous shapes and dimensions were also fabricated through vapor deposition with a post thermal treatment. Through direct spin-coating the CIZS QDs to the PSFs to form a SP coupling system, the QD/PSF hybrid emitter was realized. Such an easy and reproducible method gives rise to a great enhancement in intensity
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and stability of the emissions of the QDs. As high as 45 folds enhancement in emission intensity and 41.23% decrease in fluorescence lifetime are realized. The
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plasma enhanced fluorescence of eco-friendly quaternary CIZS QDs offers great
potential for bio detection and other applications, such as LEDs, sensor technology,
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microarrays and single-molecule studies.
ACKNOWLEDGMENTS
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This work was supported by National Key Basic Research Program of China (Grant
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Highlights Plasmon enhanced fluorescence of heavy-metal-free CuInZnS quantum dots was explored, since the study of the interaction between quaternary CuInZnS quantum dots and the plasmonic metal films has not yet been covered. As high as 45 folds enhancement in emission intensity and 41.23% decrease in fluorescence lifetimes were observed. The QDs capped with PMMA as a protect layer was explored, and also the possible physical mechanism.
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S0169-4332(14)02437-4 http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.171 APSUSC 29036
To appear in:
APSUSC
Received date: Revised date: Accepted date:
9-5-2014 21-10-2014 28-10-2014
Please cite this article as: W. Kong, B. Zhang, R. Li, F. Wu, T. Xu, H. Wu, Plasmon Enhanced Fluorescence from Quaternary CuminusInminusZnminusS Quantum Dots, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.171 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Plasmon Enhanced Fluorescence from Quaternary Cu−In−Zn−S Quantum Dots Weiguang Kong1, Bingpo Zhang1, Ruifeng Li1, Feifei Wu1, Tianning Xu2, Huizhen
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Wu1*
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1 Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China
2 Department of Science, Zhijiang College of Zhejiang University of Technology,
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Hangzhou, Zhejiang 310024, P. R. China
E-mail: [email protected] Corresponding author: Huizhen Wu1
Fax number: +086-0571-87953885
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E-mail: [email protected]
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Other co-authors: Bingpo Zhang
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Telephone number: +086-0571-87953885;
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First author: Weiguang Kong
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E-mail: [email protected] Ruifeng Li
E-mail: [email protected] FeiFei Wu
E-mail: [email protected] Tianning Xu
E-mail: [email protected]
*
Corresponding Author. E-mail address: [email protected]. 1
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Abstract: Plasmon enhanced fluorescence of heavy-metal-free Cu−In−Zn−S quantum dots was explored. Cu−In−Zn−S were chemically synthesized with different sizes via variation of the growth time of the Cu-In-S while plasmonic silver films (PSFs) were also fabricated through physical deposition. The QD/PSF coupling structure gives
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significant rise to fluorescence intensity and stability of the QDs. As high as 45 folds
enhancement in emission intensity and 41.23% decrease in fluorescence lifetimes are
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observed. The metal-enhanced fluorescence offers promise for a range of applications,
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including LEDs, sensor technology, microarrays and single-molecule studies.
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Keyword: CuInZnS; quantum dots; surface plasmons; silver film; fluorescence
1. Introduction
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Colloidal semiconductor quantum dots (QDs) are considered extremely promising for applications in many different areas, such as biomedical labeling, solar cells, and
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photo-electronic devices. [1-3]. To tune the band gap and improve the quantum yield (QY) of QDs, previous researches were mainly focused on the synthesis stage of the
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QDs by changing relative stoichiometries and reactivity of the various chemical
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species in QDs and growing a passivated shell over the QDs cores [4-6]. Other schemes such as QDs being excited by interaction with localized surface plasmons (LSPs) were also performed [7-9]. LSPs are charge density oscillations confined to metallic nanostructures, which is thought to lead to plasmonic fluorescence enhancement. Metallic nanostructures have long been studied due to their ability to
manipulate incident light. It becomes more appealing when applying the QDs to random distributions of metallic nanoparticles or nanoscale roughness in metallic films. However, up to now, most of these methods were realized based on the binary QDs [7-9]. These QDs generally contain elements that are thought to be detrimental to health and the environment, such as cadmium, lead, etc. Few articles are about the ternary I−III−VI or quaternary systems. Herein, Cu−In−Zn−S quaternary QDs according to the previous reports were fabricated [3-5, 10]. The plasmonic silver films (PSFs) on planar substrates with in-homogenous shapes and dimensions were also 2
Page 2 of 18
fabricated through vapor deposition with a post thermal treatment. SP coupling was realized by spin-coating the CIZS QDs to the PSFs without any spacer. Upon proper optimization, such an easy and reproducible method increases the excitation or emission rates of the QDs, thus resulting in a strong enhancement of their
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fluorescence, as high as 45 folds enhancement in emission intensity was achieved. Compared to other SP coupling strategies, this one is cheaper and easier to be
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fabricated. The metal-enhanced fluorescence (MEF) offers promise for a range of
applications, including LEDs, sensor technology, microarrays and single-molecule
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studies. 2. Materials and Experiment
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2.1. Materials and synthesis of QDs.
Octadecene (ODE, technical grade, 90%), indium (III) acetate (In(Ac)3, 99.99%),
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copper(I) iodide (CuI, purum, ≥99.5%), 1-dodecanethiol (DDT, ≥98%), and zinc stearate (technical grade) were purchased from Sigma-Aldrich. Tri-noctylphosphine
without further purification.
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(TOP, min 97%) was purchased from Strem Chemicals. All the chemicals were used
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Due to the promising properties of CIZS, here we explored controlled synthesis of
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CIZS-alloyed QDs using a heat-up and solvent-free method following previous works [3-5, 10], and tuning their PL emission wavelength via variation of growth time of CIS. In a typical synthesis, a solution of CuI (0.4 mmol), In(Ac)3 (0.4 mmol), and 5
mL of DDT was loaded into a three-neck flask and was heated to 120 °C and maintained at this temperature under argon flow until a clear solution was obtained. The temperature was then raised to 230 °C for a certain time (10, 40 and 90 min) to let the particles grow. Then, the reaction was quenched by cooling the solution to room temperature. The crude solution obtained from the above steps was heated at 120 °C (under argon atmosphere), and then a solution of zinc stearate (0.4 mmol) dissolved in 5 mL of ODE and 0.5 mL of TOP was added dropwise. The temperature was further raised to 210 °C for 90 min, after which the flask was cooled down to room temperature and the QDs were separated by addition of ethanol followed by centrifugation. The QDs were washed three times by repeated dissolution in toluene 3
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and reprecipitation in ethanol. 2.2. Preparation of SP-QDs coupling. Plasmonic silver films were fabricated with a vapor deposition system. Crystalline silicon wafers was used as substrates. The cleaning procedure includes washing the
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wafers with tetrachloride, successive sonication in acetone and ethanol, followed by DI water rinse. The wafers were dried with nitrogen blowing before introducing into
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the vapor deposition chamber. Half of a Si wafer was coated with an Ag film of a
film were measured by the quartz monitor crystals.
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certain thickness by thermal evaporation. The deposition rate and the thickness of the
The as-deposited silver films on silicon substrates were transferred to an oven for
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post-annealing treatment at 200ºC for 30 min. After annealing treatment, the CIZS QD/toluene solution was spin-coated on the Si wafer.
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2.4 Test Equipment and Methods
The PL and lifetimes of the fluorescence were measured by an Edinburgh FLS920
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system with the excitation of a 405 nm laser. The fluorescence quantum yield (QY) is measured by the Edinburgh FLS920 system equipped with an integrating sphere, and
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the excitation source is a wavelength tunable xenon lamp. To conduct the high
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resolution transmission electron microscopy (HRTEM), QDs were deposited from toluene solutions onto ultrathin carbon membrane by spraying the solvent in air at room temperature. HRTEM image was obtained via a Tecnai G2 F20 S-TWIN TEM operating at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) characterization was carried out with a Hitachi S4800. The absorption and extinction spectra were recorded at room temperature using a Shimadzu UV-2450/2550 spectrophotometer. X ray diffraction (XRD) patterns of the samples deposited on Si (100) substrates were obtained on a Panalytical X'Pert PRO diffractometer.
3. Results and Discussion 3.1 Characteristics of the synthesized QDs. Fig. 1a shows the XRD patterns for the structural characterization of the representative CIS and CIZS QDs. There is an obvious shift to larger angles for the 4
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CIZS peaks compared to the CIS phase, indicating a smaller lattice constant for CIZS than that of CIS. The X-ray diffraction peaks for CIZS QDs at 28.119°, 46.739° and 55.127° correspond to (112), (204) and (312) planes. The crystal lattice constants are determined to be a=b=5.505Å, c=10.935Å. The crystallite size of 3.0 nm estimated
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from the full width at half maximum (FWHM) of the (110) peak and the d-spacing of 3.171Å corresponding to (1 1 2) planes calculated from the XRD pattern are both
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compatible with those obtained from the HRTEM results as shown Fig.1b.
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The CIS QDs used in this article were grown with the same growth time of 10 min. The QY of the CIZS samples dispersed in toluene is determined to be ~15%. It is noted that there is an obvious redshift for the emission peak of CIZS QDs in
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solid-state, with respect to the CIZS samples dispersed in toluene. This is possibly due to coalescence of the adjacent QDs, as shown in TEM image in the inset of Fig.1a, i.e.
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two or more QDs approach or contacts each other, leading to electron coupling between these QDs thus an enlargement in QDs’ effective sizes. Fig. 1c shows the
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absorption and photoluminescence (PL) spectra of the CIZS QDs dispersed in toluene
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with different synthesis time. All the samples show obvious Stokes shifts. As the growth time of CIS increases, the absorption and emission edges of the CIZS
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gradually shift to longer wavelengths.
(a)
XRD Intensity (a.u.)
(112)
20
CIZS CIS
(204) (312)
30
40
50
60
2θ (Degree)
5
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Absorption
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(c)
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Intensity (a.u.)
PL
400
500
600
700
800
Wavelength (nm)
Fig. 1 (a) XRD patterns of CIZS and CIS QDs; (b) HRTEM image of the CIZS QDs with 10 min growth time on an ultrathin carbon membrane. The average size is about 2.6 nm. Inset: the HRTEM for a representative CIZS QD with d-spacing of 3.168Å. (c) The evolutions of absorption and emission of CIZS QDs with different growth time of CIS: 5 min (blue), 60 min (red) and 90 min (black).
3.2 Effects of the surface morphology of the PSFs on SP coupling
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(b)
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(a)
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XRD Intensity (a.u.)
as-prepared 150°C 200°C
(111)
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(200)
20
25
30
35
40
45
50
(220)
55
60
65
2θ
(d)
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(c)
Fig.2 Annealing temperature on the morphology of the Ag films: (a) as-prepared Ag film, (b) being
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annealed under 150ºC, and (c) 200ºC for 30 min. (d) XRD patterns of the PSFs with different
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annealing treatments. The thickness of the Ag film is 120 nm.
It is known that the optical properties of Ag films depend on their mass thickness, the
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rate of Ag deposition, the temperature of substrates and the procedure of post-deposition annealing of the films [11]. The as-deposited Ag film of 120 nm is smooth and continuous as shown by SEM image in Fig.3a. After 150ºC annealing, the morphology of the Ag film remains continuous but with granular structures (Fig.3b). After 200ºC annealing, the granular structures grow larger, lead to a local discontinuity for the Ag film (Fig. 3c). Good crystallinity of Ag films is a vital factor for the SP coupling [7]. Fig.3d shows the XRD of the Ag film annealed under different temperatures. The as-deposited Ag film has a low crystallinity. As the annealing temperature increases, the crystallinity of the Ag film is gradually promoted. Our study shows that the Ag film thermally treated at 200°C for 30 min results in a good crystallinity.
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(a) QDs on Si QDs on PSF 30 nm
IPSF/ISi=5.2
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6000
4000
0 500
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2000
550
600
650
700
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0
500
QDs on Si QDs on PSF
IPSF/ISi=9.88
d
9000
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12000
3000
800
(b)
15000
6000
750
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Emission Intensity (Counts)
Wavelength (nm))
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Emission Intensity (Counts)
8000
550
600
650
700
750
800
Wavelength (nm)
8
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(c) QDs on Silicon QDs on PSF
60000 120 nm
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IPSF/ISi=45.3
45000
30000
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×10
15000
0 500
550
600
650
700
750
800
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Wavelength (nm)
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Emission Intensity (Counts)
75000
50
(d)
M d
30
20
10
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0
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Enhancement Factor
40
50
100
150
200
Thickness of the Ag (nm)
Fig. 3 Emission comparison of QDs on PSFs with different thicknesses (red), and Si (black): (a) 30
nm, (b) 60 nm, and (c)120nm; (d) Variations of the enhancement factor with the thickness of the PSFs; Insets (above): CCD images of QDs on PSFs with different thicknesses (left), and Si (right) photographed under identical sampling conditions. Insets (below): SEM images of the Ag films with different thicknesses; the annealing temperature is fixed at 200°C.
Fig.3 a, b and c shows significant difference of the PL intensity between the application of the QDs to PSF and non-plasmon coupling substrate (Si). As the thickness of the PSFs increases, the magnification quickly increases and then slowly decreases. More than 40 folds fluorescence intensity increase is realized when the 9
Page 9 of 18
PSF is ~120 nm. The representative CCD images shown in the insets of Fig.3 also demonstrate the significant difference of brightness between the QDs on PSF/Si (left side) and Si substrate only (right side). Notice that there is an obvious blue shift in the PL band for the QDs on PSF/Si; this is possibly because the resonance absorption
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peak of the QD/PSFs couplers locates at ~630 nm according to our previous report. [7]
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The SEM images in the insets in Fig.3a, b and c illustrate the effects of the thickness
on the morphology of the Ag films. In this case, the annealing temperature was fixed
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at 200°C. As the thickness of the Ag film increases, the continuous Ag film breaks and self-assembled superficial clusters and grains in ellipsoidal islets are formed.
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Thereafter, the ellipsoidal grains gradually grow and eventually form a rather rough film. The changes of PL intensity can be attributed to the interaction of LSP with QDs.
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As the LSP is confined in the regions near the metal particles, the coupled LSP energy can be scattered to far-field only if the interface of metal/semiconductor is sufficiently
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rough.
Fig. 4 shows the absorption spectra of the solid QDs film on PSF with 120 nm
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thickness which are all deposited on the quartz substrates. A difference spectrum (the
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intensity of absorption spectrum for QD/PSF/quartz minus that for QD/quartz) is also plotted. The difference spectrum reflects, to a certain extent, the interaction between QDs and PSF through LSP coupling. As plasmonic resonance (PR) frequency depends on the size and morphology of the metallic nanostructures, PSFs with asymmetrical size distributions (as shown in the insets of Fig.3) exhibit multiple SPR modes with a broad resonance absorption shoulder. The broad absorption peaks ranging from 570 to 730 nm is possibly a superposition effect of different PR modes generated by larger Ag nanostructures. A larger nanostructure has a lower electronic confining potential, thus the PR wavelength shifts toward longer one according to Mie resonance theory. Besides, our PL excitation (PLE) spectra show a remarkable change for the QD/PSF/Si coupler compared to the counterpart without PSF, as shown in Fig. 4. A broad band at ~420 nm in the PLE spectrum is assigned to the interaction between the QDs and the PSF. The highest QY of the QD/PSF coupling samples is 10
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32% (Fig.3c) at 405 nm wavelength excitation obtained by the following equation: [1] where I, F stand for the integrated PL intensity and the absorbance at the excitation
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wavelength, respectively. The subscript 0 refers to the QD/Si reference sample, which
is measured to have the QY of 3.6% under excitation of a 405 nm laser. Compared to
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homogenous nanostructures mainly realized via either electron beam lithography or self-assembled QDs [13], PSFs give rise to a remarkable increase in intensity and
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Extinction Efficiency
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fabrication.
Spectra Difference CIZS/PSF CIZS PLE on Si PLE on PSF
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×10
300
400
500
600
700
PLE Intensity (Counts)
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stability of the fluorescence with low manufacturing cost and easy up-scaling
800
Wavelength (nm)
Fig. 4 Absorption spectra of the QDs on PSF with 120 nm thickness and solid QDs film which are all deposited on the quartz substrates. The difference spectrum (QD/PSF minus QDs spectra) is also plotted out. The PLE spectra of QDs on PSF and Si are plotted with the scattered circles and squares,
respectively.
3.3 Effects of PMMA on SP coupling. Due to the high surface-to-volume ratio in nanocrystals, CIZS QDs exposed to the air are easy to be oxidized, leading to fluorescence quenching [14]. Thus, QDs/PSF emitters should be stored in vacuum or protected via a simple capsulation. Here, CIZS QDs are firstly blended with Polymethylmethacrylate (PMMA) in toluene, and then 11
Page 11 of 18
spin coated on the PSF substrates. PMMA can greatly suppress the aggregation of the QDs and avoid the contact of the QDs with air. After a proper annealing to extract the residual solvent, the emission property of the QDs:PMMA/PSF emitters is tested in air. The result shows that the PSF works as the substrate of the composite of QDs and
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PMMA also induces an obvious increase in intensity of the emission compared to the
QDs on Si substrates (shown in Fig.5a). However, the enhancement factor is only ~15,
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almost 3 times less than that of QDs/PSF emitters. This may be assigned to the fact
that the electromagnetic field of the LSPs is confined in the region near the interface
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between QDs and PSF parallel to the x-y plane as shown in Fig.5a. Fig.5b shows the distribution of the electric field along z axis. When coated on the PSF, CIZS QDs in
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PMMA may be randomly distributed in z direction, thus portions of the QDs are too far from the surface of the PSF for the LSP coupling, leading to a lower coupling
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efficiency. Accordingly, increasing the thickness of the QDs layer on the PSF also results in a lower enhancement factor [7]. The results show that a spacer on a rough
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surface of PSF for LSP coupling is not necessary. The blue shift in the enhanced PL for the QDs in the PMMA can be due to the smaller dielectric constant of the PMMA
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higher energy side.
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compared to the bare QDs which results in the SP resonance energy shifting toward
(a)
(b)
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PL Intensity (a.u.)
on Si on PSF
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IPSF/ISi=15.07
×5
550
600
650
700
(c)
750
800
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Wavelength (nm)
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500
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Ralated with PMMA
Fig. 5 (a) Cross-section of the QDs:PMMA/PSF emitters; (b) Variation of the electric field
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intensity along z axis; (c) PL comparison of QDs:PMMA on PSFs and Si wafer; inset: CCD images
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of QDs:PMMA blend on a patterned PSFs under a 405 nm lamp.
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3.4 Lifetimes of fluorescence of CIZS/PSF emitters Fig. 6 shows PL intensity decays for samples of CIZS QDs dispersed in toluene and
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coated on a Si wafer and PSFs. It has been reported that the size dependent band gap, intrinsic defects and surface defects all influence on the fluorescence of QDs [15, 16]. Different electron-hole recombination mechanisms cause different fluorescence decay lifetimes. Herein, the PL decay curves of the CIZS QDs can be well fitted by a tri-exponential function:
I(t)=A1exp(-t/τ1)+A2exp(-t/τ2)+A3exp(-t/τ3)
[2]
where A1, A2 and A3 represent the fractional contributions of PL decay lifetime τ1, τ2 and τ3 respectively. The average decay time is given by: τave=( A1τ12+A2τ22+A3τ32)/ (A1τ1+A2τ2+A3τ3)
[3]
The fast decay component (τ1), on the time scale of several nanoseconds, is assigned 13
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to the initially populated core-state recombination or free to bound transition, while the intermediate lifetime (τ2) with tens of nanoseconds is due to the surface-related radiative recombination of carriers. The longest decay component (>100 nanoseconds) is attributed to DAP transitions [5, 16, 18]. All the shorter and longer lifetimes are
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widely distributed at the entire PL spectra. Fig. 6a shows the PL decay spectrum of
the CIZS QDs dispersed in toluene with τ1=8.12 ns, τ2 =44.75, τ3=207.31 ns at 630 nm.
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When QDs were spin-coated on the substrate to form a solid film, the emission of the CIZS QDs in the solid form decays faster than the QDs dispersed in toluene (τave), and
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the corresponding parameter values are listed in Table 1. Solvent molecules can partially passivate the surface of the QDs on one hand; on the other hand they can
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prevent the aggregation of the QDs [14, 17], effectively suppressing the non-radiative recombination and leading to a longer lifetime. After SP coupling, all the three
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lifetime components τ1, τ2, and τ3 exhibit a significant decrease, from 10.17, 38.83 and 130.27 to 3.91, 20.53 and 81.17, by 61.55, 47.13 and 37.69%, respectively. The
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average lifetime is reduced by 41.23% from 82.61 to 48.52 ns. A reduced lifetime of the fluorescence means the electrons relax faster by radiative recombination routes
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from the excited states while the probability for photo-bleaching is reduced, thus the
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quantum efficiency increases [13, 19, 20].
2
χ = 0.99925
Counts (a.u.)
Decay Fitting τ1=8.12 ns τ2=44.57 ns τ3= 207.31 ns
55.76%
27.24% 17%
0
100
200
300
400
Time (ns)
14
Page 14 of 18
0
100
200
300
400
500
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Time (ns)
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Counts (a.u.)
CIZS QDs QDs on 50 nm PSF QDs on 80 nm PSF QDs on 120 nm PSF
Fig. 6 (a) PL intensity decays (at 620 nm) and exponential decay components of the CIZS QDs dispersed in toluene. (b) PL intensity decays (at 630 nm) of the CIZS QDs coated on a Si wafer and
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PSFs.
τ1/ns
A1/%
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Samples
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Table 1. Fitting parameters deriving from I(t)=A1exp(-t/τ1)+A2exp(-t/τ2)+A3exp(-t/τ3)
QDs in toluene (620 nm)
QDs on Silicon (640 nm)
QDs on 120 nm PSF (630 nm)
τ2/ns
A2/%
τ3/ns
A3/%
Average Lifetime/ns
8.12
17.00
44.57
27.24
207.31
55.76
190
10.17
34.68
38.83
48.053
130.27
17.27
82.61
3.91
35.60
20.53
51.00
81.17
13.40
48.52
4. Conclusions Cu−In−Zn−S quaternary QDs were fabricated in non-coordinating solvent ODE in the presence of the DDT by a two-step synthesis route. The size of the QDs is tuned via variation of the growth time of the CIS in the first step. The PSFs on silicon substrates 15
Page 15 of 18
with in-homogenous shapes and dimensions were also fabricated through vapor deposition with a post thermal treatment. Through direct spin-coating the CIZS QDs to the PSFs to form a SP coupling system, the QD/PSF hybrid emitter was realized. Such an easy and reproducible method gives rise to a great enhancement in intensity
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and stability of the emissions of the QDs. As high as 45 folds enhancement in emission intensity and 41.23% decrease in fluorescence lifetime are realized. The
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plasma enhanced fluorescence of eco-friendly quaternary CIZS QDs offers great
potential for bio detection and other applications, such as LEDs, sensor technology,
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microarrays and single-molecule studies.
ACKNOWLEDGMENTS
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This work was supported by National Key Basic Research Program of China (Grant
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Highlights Plasmon enhanced fluorescence of heavy-metal-free CuInZnS quantum dots was explored, since the study of the interaction between quaternary CuInZnS quantum dots and the plasmonic metal films has not yet been covered. As high as 45 folds enhancement in emission intensity and 41.23% decrease in fluorescence lifetimes were observed. The QDs capped with PMMA as a protect layer was explored, and also the possible physical mechanism.
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