Semiconductor Plasmon Induced Up-Conversion Enhancement in

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Semiconductor plasmon induced upconversion enhancement in mCu S@SiO@YO:Yb ,Er core-shell nanocomposites 2-x

2

2

3

3+

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Donglei Zhou, Dongyu Li, Xiangyu Zhou, Wen Xu, Xu Chen, Dali Liu, Yongsheng Zhu, and Hongwei Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09850 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Applied Materials & Interfaces

Semiconductor plasmon induced upconversion enhancement in mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell nanocomposites Donglei Zhou†, Dongyu Li†, Xiangyu Zhou†, Wen Xu*†, Xu Chen†, Dali Liu†, Yongsheng Zhu‡, Hongwei Song*† †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science

and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China. ‡

Department of Physics, Nanyang Normal University, Nanyang 473061, China

ACS Paragon Plus Environment


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ABSTRACT The ability to modulate the intensity of electromagnetic field by semiconductor plasmon nanoparticles is becoming attractive owing to its unique doping induced local surface plasmon resonance (LSPR) effect different from metals. Herein, we synthesized

the

mCu2−xS@SiO2@Y2O3:Yb3+,Er3+

core-shell

composites

and

experimentally and theoretically studied the semiconductor plasmon induced upconversion enhancement, and obtained 30 folds upconversion enhancement compared to that of SiO2@Y2O3:Yb3+, Er3+ composites. The upconversion enhancement was induced by the synthetic effect: amplification of excitation field and the increase of resonance energy transfer (ET) rate from Yb3+ ions to Er3+ ions. The experimental results were analyzed in the light of FDTD calculations confirming the effect of amplification of excitation field. In addition, UCL spectra, upconversion enhancement and dynamics dependent on concentration (Yb3+/Er3+ ions) were investigated and found that the resonance energy transfer (ET) rate from Yb3+ ions to Er3+ ions increased ~25% in the effect of LSPR waves. Finally, power dependence of fingerprint

identification

was

successfully

performed

based

on

the

mCu2−xS@SiO2@Y2O3:Yb3+, Er3+ core-shell composites, the color of which can change from green to orange with excitation power increasing. Our work opens up a new concept to design and fabricate the upconversion core-shell structure based on semiconductor plasmon nanoparticles (NPs) and provides applications for upconversion nanocrystals (UCNPs) and semiconductor plasmon NPs in photonics. Keywords:

Local

Surface

Plasmon

Resonance;

Semiconductor

Plasmon;

Upconversion Nanoparticles; Upconversion Enhancement; Fingerprint Identification


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INTRODUCTION The emergence of localized surface plasmon resonances (LSPRs) in semiconductors has attracted significant attention in the past several decades owing to its unique double characters of semiconductor and metal1-4. It is commonly considered that the LSPR behavior of semiconductor nanoparticles (NPs) arises from collective oscillations of excess free carriers associated with constitutional vacancies or ionized dopant impurities in the lattice5-7. Different from noble metals, LSPR in semiconductor NPs can be tuned by changing the doping proportion or stoichiometry8, 9

, providing an additional means of tuning the optical properties. Meanwhile,

compared with noble metals, semiconductor nanoparticles are very cheap and result in less thermal effect when irradiated by excitation light.10,

11

Furthermore,

semiconductor nanoparticles can absorb one photon or two photon jumping from valence band to conduction band, which may induce the energy transfer to other luminescence materials.12,

13

Semiconductor NPs allow realization of LSPR and

quantum-confined excitons within the same structure, opening up the possibility of strong coupling of photonic to electric modes, with implications for light harvesting, nonlinear optics and quantum information processing4, 5. Among them, Cu2-xS, a kind of heavily p-doped semiconductor nanoparticle, exhibits a broad band LSPRs in the near-infrared region, which has attracted considerable interests owing to its ability to fundamentally alter light-matter interactions and its potential applications in enhanced spectroscopies14, sensing15, photocatalysis16, and optical devices17. However, the optical interaction between Cu2−xS NPs and luminescent materials (such as dye



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molecules, quantum dots, rare earth etc.) have been rarely studied, which may exhibit many different optical properties and physical mechanisms. Lanthanides (Ln) doped upconversion nanoparticles (UCNPs) are attracting extensive current interest owing to their application potential in solar cells18, infrared detection19, bio-labels20, bio-sensors21 etc. Particularly, Y2O3 has been a promising host material owing to better chemical durability, thermal stability, lower phonon energy as well ease of synthesis in nanometer region. However, the low luminescent efficiency and the small absorption cross section for UCNPs have seriously limited their applications. The LSPRs based on noble metals has been used to modulate the upconversion of Y2O3:Yb3+, Er3+/Tm3+ (excitation enhancement and Purcell effect), especially for core-shell design and applied to various photoelectric devices and bio-applications22. To realize the effective coupling between UCNPs and metals, the complex surfactant-assisted preparation of noble metals were performed to tune their LSPR peak, which remains challenges for their future developments. Compared with metallic nanoparticles, one of advantages for semiconductor nanocrystals is their facile tunability for optical response. Until now, the modulation of UCL in core-shell structure by LSPRs in semiconductor NPs has been poorly investigated, in which the interaction distance between Cu2−xS NPs and UCNPs can be controlled. Therefore, the fabrication of the Cu2−xS-UCNPs core-shell structure and investigation of the optical and electrical interaction between them are significant. In this work, we fabricated the mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ (“mCu2−xS” means “multi Cu2−xS”) core-shell composites and systematically studied the


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upconversion luminescence properties depending on illumination power and doping concentration of Yb3+ and Er3+ ions in Y2O3:Yb3+,Er3+under the excitation of 980-nm laser diode. About 30-fold upconversion enhancement was obtained in the core-shell structure composites compared to the SiO2@Y2O3:Yb3+, Er3+ composites, which was analyzed in the light of FDTD calculations. The mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites was successfully used for the identification of high-resolution fingerprint. RESSULTS AND DISCUSSION Morphology,

structure

and

optical

properties

of

mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites

Figure 1 (a) Schematic diagram of mCu2-xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites. (b)



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TEM image of Cu2−xS nanoparticles. (c) TEM image of mCu2−xS@SiO2 composites. (d) TEM image of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites. (e) Energy dispersive X-ray (EDX) mapping of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites. (f) HR-TEM images of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites. (g) XRD patterns of Cu2−xS, Y2O3 and mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites. (h) Extinction spectra of Cu2−xS, Y2O3 and mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites.

Figure 1a illustrates the core-shell structure of the mCu2-xS@SiO2@Y2O3:Yb3+, Er3+, where the Cu2−xS nanoparticles (NPs) were randomly distributed in a SiO2 sphere, and further covered by the Y2O3 shell. Firstly, the mono-dispersed and homogeneous Cu2−xS NPs with an average size of ~6 nm were synthesized through the hot-injection method23(Figure 1 b).The mCu2−xS@SiO2 composites with average diameter of ~50 nm were prepared by capsulating the Cu2−xS nanoparticles into a SiO2 sphere via the modified stober method24(Figure 1c and S1). Then, the mCu2−xS@SiO2@Y(OH)CO3 composites were synthesized using the urea co-precipitation method. After that, the composites were annealed at 500 ℃ for 2 hours, forming the mCu2−xS@SiO2 @Y2O3:Yb3+, Er3+ core-shell composites (Figure 1d and S1). It can be seen that the spherical composites were randomly inlayed with several Cu2−xS NPs with an 8-nm thickness of Y2O3:Yb3+, Er3+ outer shell. The influence of SiO2 thickness on the luminescence was widely investigated in the literatures, and the optimum thickness is around 8-12 nm,25, 26 so we did not discuss it in this paper. In the Elemental Analysis of EDX mapping of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites, all of the elements (copper, sulphur, silicon, yttrium and ytterbium elements) were observed


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in the sphere, and their diameter processes the rule of d(Cu,S)＜d(Si)＜d(Y, Yb), which confirms the formation of mCu2−xS@SiO2@Y2O3:Yb (10%), Er (2%) core-shell structure (Figure 1e). As the proportion of Yb3+ was 10%, the pixel points of mapping image distributed sparsely compared to that of other elements. In addition, the high resolution transmission electron microscopy (HR-TEM) images (Figure 1f) show that two different fringe spacings are determined, to be ~0.307 nm and ~0.321 nm, which correspond with the spacing of the (2,2,2) plane of cubic Y2O3 (0.307 nm) (JCPDS No. 43-1036) and the (0,0,15) plane of rhombohedral Cu2−xS (0.321 nm) (JCPDS No. 47-1748), respectively. To ascertain the crystallinity of core-shell structure, X-ray diffraction (XRD) patterns of mCu2−xS@SiO2@Y2O3:Yb (10%), Er (2%) composites annealed at 500 ℃ was further measured, as shown in Figure 1g. The red, purple and green curves are the experimental results. The histograms at the bottom are the corresponding standard JCPDS cards. Four main diffraction peaks of pure Cu2−xS NPs can be identified, locating at 27.8, 32.2, 46.2, and 54.7°, which are indexed to the (0,0,15), (1,0,10), (0,1,20), and (1,1,15) planes of rhombohedral Cu2−xS (digenite) with space group R3̅m (JCPDS No. 47-1748), respectively. Similarly, diffraction peaks of pure Y2O3: Yb3+, Er3+ NPs are indexed to (2,2,2), (4,0,0), (4,4,0) and (6,2,2) planes of Y2O3 (JCPDS No. 43-1036). In the patterns of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ composites, peaks of Cu2−xS and Y2O3 can be identified. No peaks of SiO2 are observed owing to its amorphous phase. Based on the above facts, mCu2−xS@SiO2@Y2O3:Yb3+, Er3+ core-shell composites have been successfully fabricated.



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Figure 1h shows the UV-Vis-NIR absorption spectra of Cu2−xS, Y2O3 and mCu2−xS@SiO2@Y2O3 film. The film was used for measurement to serve for the fingerprint identification in the following text, which was prepared by spin-coating on the glass substrate (see Supporting Information). A broad LSPR absorption band expanding from visible to near-infrared range centering at ~1200 nm can be identified in the Cu2−xS film, which can cover the excitation wavelength (980 nm). It is known that the LSPR of noble metals, such as Au nanoparticles, can be adjusted to 980 nm by changing the morphology. However, synthesis of Au nanoparticles is a complex and time-consuming process, and requires strict conditions, which is hard to control exactly. The LSPR band of Cu2−xS NPs can be tuned by just changing the Cu to S ratio, which is easy to control. As known, LSPR in the Cu2−xS NPs arises from the Cu vacancies induced free carriers. Based on the Mie−Drude theory by Luther et al.4, the density of charge carriers was estimated to be 3.1×1021 cm−3. In the mCu2−xS@SiO2@Y2O3 film, the LSPR absorption band still exists but weakens a little, which could be attributed to oxidation of surface ligands during the annealing process. No obvious peaks can be identified in the spectrum of SiO2@Y2O3 film. Meanwhile, it decreases with the increase of wavelength, which should be attributed to the scattering of the SiO2@Y2O3 nanoparticles. UC enhancement in mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites


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Figure 2 (a) Optical test path diagram. (b) Emission spectra of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites with illumination of power density of 980 nm laser of 30 mW/mm2. (c) Schematic of the mechanism of upconversion emission of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell

composites.

(d)

UCL

dynamics

of

2

H11/2–4I15/2

transitions

in

mCu2−xS@SiO2@Y2O3:Yb3+, Er3+ core-shell composites and SiO2@Y2O3:Yb3+, Er3+ NPs. (e) Simulated electric field intensity distribution of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+ core-shell composites. The incident light wavelengths are 980 nm, 650 nm and 545 nm, respectively. The |E|2/|E0 |2 (shell) represents the average electric field enhancement factor at the shell of core-shell structure. (f) Simulated electric field intensity distribution of mCu2−xS@SiO2@Y2O3:Yb3+,Er3+



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core-shell film. The incident light wavelengths are 980 nm, 650 nm and 545 nm, respectively. The |E|2/|E0|2

represents the average electric field enhancement factor in the whole mesh region.

To elucidate the UCL properties of mCu2−xS@SiO2@Y2O3:Yb3+, Er3+ core-shell composites, we measured the UCL spectra under excitation of 980 nm laser. The reflection measurement was selected in this work, and the optical path was fixed in all the measurements, as shown in Figure 2a. Figure 2b shows the UCL spectra of mCu2−xS@SiO2@Y2O3:Yb3+(10%), Er3+ (2%) film and SiO2@Y2O3:Yb3+(10%), Er3+ (2%) film irradiated by 980 nm light. The green 2H11/2/4S3/2-4I9/2 and red 4F9/2-4I15/2 transitions of Er3+ ion can be clearly distinguished. Firstly, Yb3+ ions were excited under 980 nm illumination, and the electrons on 2F7/2 were excited to 2F5/2, followed by the energy transfer (ET) to multiple energy states of minority Er3+ ions. After non-radiative relaxation, two main emission peaks, the two-photons green 2

H11/2/4S3/2-4I9/2 and red 4F9/2-4I15/2 transitions of Er3+ were realized27, 28 (Figure 2c).

Interestingly, the emission intensity of mCu2−xS@SiO2@Y2O3:Yb3+ (10%), Er3+ (2%) composites enhanced about 30 fold compared to that of SiO2@Y2O3:Yb3+ (10%), Er3+ (2%) NPs. Figure 2d shows the UCL dynamics of

2

H11/2–4I15/2 transitions in

mCu2−xS@SiO2@Y2O3:Yb3+, Er3+ core-shell composites and SiO2@Y2O3:Yb3+, Er3+ NPs. The decay time constants for Er3+ ions in core-shell composites and SiO2@Y2O3:Yb3+, Er3+ NPs are 21µs and 24µs, respectively. The results indicate that in the core-shell composites, the total spontaneous transition rates, including radiative and non-radiative ones for these transitions increase only about 10 %. This indicates


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that in the core-shell composites, the UCL enhancement mainly results from the interaction of LSPR with excitation electromagnetic field, rather than that with the emission electromagnetic field.29 The excitation electromagnetic field was amplified induced by the coincidence of excitation light and LSPR wavelength, which resulted in the plasmon-enhanced enhancement. The upconversion internal quantum efficiencies were 0.091% and 0.085% for mCu2−xS@SiO2@Y2O3: Yb3+, Er3+ core-shell composites and SiO2@Y2O3:Yb3+, Er3+ NPs respectively, which were nearly the same. The upconversion enhancement was induced by the amplification of excitation field, which increased the absorption to excitation light. That is, the absorption of mCu2−xS@SiO2@Y2O3: Yb3+, Er3+ sample increased as well along with the emission enhancement. As the radiative transition rate changed little, the upconversion internal quantum efficiency kept unchanged. The simulation of electric field using the finite-difference time-domain (FDTD) solution can prove this conclusion as well (Figure 2e, 2f and Note S1). It is clear that the |E|2 of Cu2−xS is concentrated around the mCu2−xS@SiO2@Y2O3:Yb3+, Er3+ core-shell composites with a maximum intensity enhancement of 85 times at the hot spots under the excitation of 980 nm light. The enhancement decreases to 15 and 10 when changing the wavelength to 650 nm and 545 nm (central emission wavelength of the green 2

H11/2/4S3/2-4I9/2 and red 4F9/2-4I15/2 emissions) respectively. Because these wavelengths

are far away from the LSPR central wavelength (1200 nm) of Cu2−xS nanoparticles. The average electric field enhancement factors near the shell is obtained by calculating the average value of the electric field intensity enhancement in the shell



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range, which shows 14 times enhancement at the excitation of 980 nm. While, only 3.2 times and 3.1 times enhancement are obtained excited by 650 nm and 545 nm respectively. The simulations of electric field of mCu2−xS@SiO2@Y2O3:Yb3+, Er3+ core-shell film in Figure 2f show similar results with core-shell particles, indicating the interaction between particles in the film has little influence on the electric field. The average electric field enhancement in the film reached about 17 times excited by 980 nm light. Considering a two-photon upconversion process, the enhancement of UCL induced by the excitation field amplification is calculated to be 160 times at most (EF is proportional to ( |E|2/|E0|2 )2 in the film. The experimental results are lower than the theoretical calculation, which may be attributed to saturation effect and/or thermal effect under the excitation of strong power density. For two-photon upconversion luminescence, the upconversion luminescence intensity is proportional to the square of excitation power density, however, if saturation effect or local thermal effect happens, the slope n deviates from 2 in power law (n

Semiconductor Plasmon Induced Up-Conversion Enhancement in

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