Tunable photoluminescence effect from ZnO films of Ag-decorated localized surface plasmon resonance by varying positions of Ag nanoparticles

Accepted Manuscript Title: Tunable Photoluminescence Effect from ZnO Films of Ag-Decorated Localized Surface Plasmon Resonance by Varying Positions of Ag Nanoparticles Authors: Chunqing Huo, Hua Jiang, Youming Lu, Shun Han, Fang Jia, Yuxiang Zeng, Peijiang Cao, Wenjun Liu, Wangying Xu, Xinke Liu, Deliang Zhu PII: DOI: Reference:

S0025-5408(18)32342-0 https://doi.org/10.1016/j.materresbull.2018.10.037 MRB 10249

To appear in:

MRB

Received date: Revised date: Accepted date:

24-7-2018 22-10-2018 23-10-2018

Please cite this article as: Huo C, Jiang H, Lu Y, Han S, Jia F, Zeng Y, Cao P, Liu W, Wangying X, Liu X, Zhu D, Tunable Photoluminescence Effect from ZnO Films of Ag-Decorated Localized Surface Plasmon Resonance by Varying Positions of Ag Nanoparticles, Materials Research Bulletin (2018), https://doi.org/10.1016/j.materresbull.2018.10.037 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.

Tunable Photoluminescence Effect from ZnO Films of Ag-Decorated Localized Surface Plasmon Resonance by Varying Positions of Ag Nanoparticles Chunqing Huo1,2, Hua Jiang1, Youming Lu*1, Shun Han1, Fang Jia1, Yuxiang Zeng1, Peijiang Cao1,

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Wenjun Liu1, Wangying Xu1, Xinke Liu1, and Deliang Zhu1

College of Materials Science and Engineering, Shenzhen Key Laboratory of Special Functional

Materials, Shenzhen Engineering Laboratory for Advanced Technology of Ceramics, Guangdong Research Center for Interfacial Engineering of Functional Materials, Shenzhen University, Shenzhen 518060, China 2

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College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China

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* [email protected], 0755-86713979

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Graphical abstract

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Highlights

For the ZnO films/Ag NPs (II) structure, the oscillating electrons can be excited to

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a higher energy level by LSP and subsequently transfer to the CB of ZnO, causing 

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an enhanced PL intensity of ZnO NBE emission. For the Ag NPs (I)/ZnO films structure, the electron transition to Fermi level of the

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metal occurs at the MS interface due to the bending energy band that excites electrons to be close to the Fermi level of the metal, leading to a reduced PL intensity of ZnO NBE emission. The enhancement or reduction of luminescence intensity depends on the transport

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probability of the electrons to the LSP level or to the Fermi level of the metal.

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The defect emission subsequently remained almost the same with weak LSP resonance coupling and energy mismatch in the structure.

Abstract

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By employing localized surface plasmon resonance (LSPR), the near band edge (NBE) emission intensity of ZnO films were greatly varied, while defect emission remained almost the same. This photoluminescence (PL) intensity enhancement or reduction is

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tunable by changing the position of Ag nanoparticles (NPs) relative to the ZnO films. The remarkable variation of the NBE emission was investigated deeply. These experimental

results reveal that the Ag NPs play a key role in tuning the PL performance of the semiconductor material, where LSPR occurs.

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Keywords

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photoluminescence; ZnO films; Ag nanoparticles; surface plasmon resonance;

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1. Introduction

Znic oxide (ZnO), the II-VI group compound semiconductor, is regarded as a promising

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candidate for ultraviolet (UV) light emitting diodes (LEDs) and laser diodes (LDs) because

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of its wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV [1, 2].

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However, the luminescence efficiency of ZnO-based LEDs on either homojunctions or heterojunctions is not as high as expected due to the considerable amounts of interface

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defects [3] or the lack of high-quality and stable p-type doping of ZnO, since the strong self-compensation effect of native point defects such as zinc interstitial or oxygen vacancy,

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still remained [4]. To improve the luminescence efficiency, localized surface plasmon (LSP) has been introduced into the LEDs design [5, 6].

Different metals like Ag, Au, Al, and Pt etc. have been used to enhance the luminescence efficiency due to the resonant coupling between spontaneous emission in ZnO and LSP 3

from the metal nanoparticles (NPs) [7-9], improving both the light-extraction-efficiency (LEE) and the internal-quantum-efficiency (IQE) of the LEDs [3]. For example, Yan et al. prepared Ag nanowires (NWs)/aluminum-doped zinc oxide composite transparent

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conducting electrodes for the n-ZnO/p-GaN heterojunction LEDs, and found that the composite electrode has been demonstrated to enhance the electroluminescence (EL) intensity of the LEDs due to the improvement of electrons injection efficiency and the

resonant coupling between the ZnO excitons and the AgNWs’ LSP [10]. Liu et al. has

introduced Ag NPs into the ZnO/SiO2 core/shell nanorod array (NRA)/p-GaN

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heterostructures, and found that a 3.5-fold photoluminescence (PL) enhancement and a 7-

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fold EL enhancement was achieved [3]. They also proved that the higher-ratio EL

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enhancement stems from both the IQE improvement induced by exciton-LSP coupling and

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the LEE improvement caused by photon-LSP coupling.

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In this paper, we demonstrated the enhanced or reduced PL emission intensity from the

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ZnO films induced by the random Ag NPs’ LSP. More interestingly, this UV emission enhancement or reduction is largely dependent on the position of the Ag NPs, and the

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mechanism is deeply investigated.

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2. Experimental

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The growth of ZnO films on the c-Al2O3 substrate was carried out in a pulsed laser deposition (PLD) system, which consisted by two parts. One laser part using COMPexPro205 KrF excimer laser (Lambda Physics Company) with laser average power at 300 W, laser wavelength at 248 nm, and pulse width at 20 ns. The other vacuum deposition part using PLD-450 deposition system (SKY Technology Development 4

Company), and the base pressure was first maintained around 10 Pa with a mechanical pump and then about 6×10-4 Pa with a turbo molecular pump. Oxygen (O2) was introduced into the chamber through a mass flow controller. A high purity ZnO (99.999 %) target with

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a diameter of 30 mm was equipped, and is 60 mm away from the substrate. Ag films were first evaporated onto the substrate under a base pressure of about 5×10-3 Pa using an optical multilayer coating machine DMDE450 at room temperature, and a crystal oscillator was used to monitor the thickness of the Ag films. Subsequently, the Ag films

were annealed to form metal nanostructures. Since the Ag NPs were decorated at two

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different positions relative to the ZnO films, we mark the ones on the top as Ag NPs (I)

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and the ones at the bottom as Ag NPs (II) to distinguish them. The Ag NPs (I) was

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fabricated by annealing the Ag films under a vacuum of about 5×10-3 Pa at around 400 ℃. The Ag NPs (II) was formed by annealing the Ag films using the PLD system under a

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vacuum of about 6×10-4 Pa at around 500 ℃.

The surface morphology of the Ag nanostructures and the ZnO films were characterized

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by a SU-70 thermal field emission scanning electron microscope (SEM). The optical properties were obtained through PL spectra at room temperature (RT) with a 30 mW He-

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Cd 325 nm laser and a confocal Raman Spectroscopy. The extinction spectra were

carried

out

by

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UV-2450

ultraviolet-visible

scanning

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measurements

spectrophotometer. The cross-sectional electron microscopic image and energy dispersive spectrometer (EDS) was taken by scanning transmission electron microscopy (STEM) from FEI of the U.S.A. 3. Results and discussion 5

At first, the Ag thin films with different thicknesses were evaporated on the c-Al2O3 substrates, and then annealed by the first method to form Ag NPs (I) with different morphologies. As shown in Fig. 1 (a, b, c, d), the Ag NPs (I) with different shapes and

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sizes are randomly dispersed onto the substrates. The 20 nm thick Ag film after annealing in Fig. 1 (a) are still discontinuous metal layer with a few triangular rod shaped Ag NPs

(Ia) distributed sparsely. The 15 nm thick Ag film after annealing in Fig. 1 (b) are conglutinated Ag islands with average diameter of ~ 400 nm and Ag NPs (Ib) ellipsoids of

~ 100 nm. This reveals that the thickness of the Ag films is critical to the formation of the

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Ag NPs (I), and too thick Ag films are adverse to the conglobation of the Ag NPs (I). The

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Ag NPs (Ic) annealed from the 10 nm Ag films in Fig. 1 (c) are globoid and ellipsoid in

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regular shapes with average diameter of ~ 150 nm. The Ag NPs (Id) annealed from the 5

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nm Ag films in Fig. 1 (d) are cylinder and ellipsoid in irregular shapes with average diameter of ~ 80 nm. The Ag NPs (Ic) and (Id) are clearly conglobated better, and the NPs

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(Id) distributed more intensively than (Ic) with smaller average diameter.

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Fig. 1 The SEM images of the Ag NPs (I) annealed from different Ag film thicknesses, (a) 20 nm, (b) 15 nm, (c) 10 nm, and (d) 5 nm. The SEM images of the Ag NPs (I) annealed

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at different annealing time length, (e) 60 min, (f) 45 min, (g) 30 min, and (h) 15 min.

Then, the Ag thin films with 10 nm thickness were evaporated on the c-Al2O3 substrates, and later annealed by the first way at different time length to form Ag NPs (I). The different morphologies are shown in Fig. 1 (e, f, g, h). The Ag NPs (Ie, Ig, Ih) annealed at 60 min,

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30 min, and 15 min time length in Fig. 1 (e, g, h) are conglutinated droplets in irregular shape with average diameter of ~ 300 nm. Only at 45 min, the Ag NPs (If) in Fig. 1 (f) are globoid and ellipsoid in regular shapes with average diameter of ~ 100 nm. This means that

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the annealing time is also critical to the formation of the Ag NPs (I), since only at proper time length could the Ag NPs (I) conglobated better. In a word, results in Fig. 1 indicate that the morphology of the Ag NPs is closely related to the fabrication process.

Two different LSP modes were associated with the two axes of the Ag NPs as shown in Fig. 2 (a). Here, the narrow quadrupole resonance peak (high energy band) in the short

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wavelength region comes from the oscillation of the normal mode (out-of-plane axes).

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While the broad dipole resonance peak (low energy band) in the long wavelength regime

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is due to the dipole oscillation parallel to the substrate plane (in-plane axes). The out-of-

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plane resonance band for the Ag NPs could only vary in a narrow range but the in-plane

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resonance band could shift in a wide spectrum [11]. This is reflected in Fig. 2 (b, c) which

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showed the extinction spectra of the Ag NPs (Ia, Ib, Ic, Id) and Ag NPs (Ie, If, Ig, Ih). In the visible spectrum range, the broad dipole resonance mode is significantly red-shifted

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with the increasing average diameter of the Ag NPs (I). While in the UV wavelength regime,

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the narrow quadrupole resonance peak (shaded area) could be tailored more minutely.

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Fig. 2 (a) The defined directions relative to the substrate (up-right). The corresponding extinction spectra of (b) the Ag NPs (Ia, Ib, Ic, Id), and (c) the Ag NPs (Ie, If, Ig, Ih).

Moreover, the growth environment is another factor that should be considered, such as if

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the Ag NPs were grown at the surface of the semiconductor (Ag NPs (I)) or at the interface between the semiconductor and the substrate (Ag NPs (II)). Here, the Ag NPs (II) was

fabricated first by depositing 10 nm thickness Ag films on the substrate and then annealed

through the second method during the ZnO film deposition process. In order to investigate the effect of Ag NPs on the optical property of the ZnO films, the morphology and the

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extinction spectra of the Ag NPs decorated at the top and at the bottom of the films should

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be the same. However, the substrate would be heated to around 500 ℃during the deposition

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process of the ZnO films. Thus, for any Ag films or Ag NPs prepared beforehand on the substrate would anyhow gone through an annealing process during the ZnO films

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deposition process, so that this annealing process can be conveniently used as the second

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annealing method to form the Ag NPs (II) at the bottom of the ZnO films. This would result in the difference of the fabrication process between the Ag NPs (I) and (II), and inevitably

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differences in their morphology and extinction spectra. To decrease or eliminate the effect

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of the different fabrication process, the Ag NPs (I) annealed from the 10 nm Ag films with 15 min time length is selected to decorate at the top of the ZnO films after comparing the

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morphology of Ag NPs (II) decorated at the bottom of ZnO films. The morphology of the Ag NPs (II) shown in Fig. 3 (e) was obtained through in-situ preparation and then followed by the corrosion of the top ZnO film with dilute hydrochloric acid. There exist both regular globoid and ellipsoid shapes with average diameter of ~ 200 nm and irregular shapes with

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average diameter of ~ 400 nm, and the morphology of Fig. 3 (e) is more closer to that of

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Fig. 1 (h) after comprehensive comparison of the Ag NPs (I) in Fig. 1.

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Fig. 3 (a) The schematic structures of Ag NPs/ZnO film/c-Al2O3. The SEM images of (b) the ZnO film and (c) the Ag NPs (I)/ZnO film. (d) The schematic structures of ZnO film/Ag

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NPs/c-Al2O3. The SEM images of (e) the Ag NPs (II) and (f) the ZnO film/Ag NPs (II).

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All the ZnO films were deposited under the same experimental condition, which allows us to compare the PL efficiency purely induced by the Ag nanostructures. Fig. 3 (a) shows

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the schematic structure of Ag NPs/ZnO film/c-Al2O3. Here, the ZnO film as shown in Fig. 3 (b) was deposited on the substrate first, and the Ag NPs (I) annealed by the first method were decorated on the surface of the film as shown in Fig. 3 (c). Fig. 3 (d) shows the other schematic structure of ZnO film/Ag NPs/c-Al2O3. Here, the Ag film was evaporated on the substrate first to form the Ag NPs (II) during the ZnO film deposition process so that the 10

ZnO film/Ag NPs (II)/c-Al2O3structure can be prepared. The SEM images of the prepared Ag NPs (II) and ZnO films were shown in Fig. 3 (e) and (f) respectively. The ZnO films are both continuous and compact as can be seen in Fig. 3 (b) and (f). Ag NPs (II) in Fig. 3

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(e) is globoid and ellipsoid in regular shapes and droplet in irregular shapes, which clearly shows that the morphology is very close to Ag NPs (I) in Fig. 1 (h). Not only that, the Ag

NPs (I) from Fig. 1 (h) and Ag NPs (II) from Fig. 3 (e) also exhibit very similar extinction

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spectra.

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Fig. 4 (a) STEM image of the cross-sectional Ag NPs/ZnO film/c-Al2O3 structure at the Ag NPs/ZnO film interface. EDS of (b) Ag, (c) Zn, and (d) O element distributions at the Ag NPs/ZnO film interface. (e) STEM image of the cross-sectional ZnO film/Ag NPs/c-

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Al2O3 structure at the ZnO film/Ag NPs interface. EDS of (f) Ag, (g) Zn, and (h) O element distributions at the ZnO film/Ag NPs interface.

The morphology and distribution of the Ag NPs at the interface of the above two structures are shown in Fig. 4 (a, e). From the cross-sectional STEM images, we can see that both NPs droplets have similar average diameter, height and gaps between NPs. The EDS of

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element distributions in Fig. 4 (b, c, d, f, g, h) show that the Ag element mainly focus at

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the interfaces of Ag NPs/ZnO film and ZnO film/Ag NPs respectively, the Zn element

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films and the Al2O3 substrates.

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primarily distributes in the ZnO films, and the O element chiefly distributes in the ZnO

Fig. 5 (a) The PL intensity of bare ZnO (black) and Ag NPs (I)/ZnO film (red), and the extinction spectra of Ag NPs (I) (blue). (b) The PL intensity of bare ZnO (black) and ZnO film/Ag NPs (II) (red), and the extinction spectra of Ag NPs (II) (blue).

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The extinction spectra of the Ag NPs in Fig. 5 (a) and (b) are plotted with blue curves, in which a narrow hybridized quadrupole peak in the UV range and a broad hybridized dipole mode in the visible regime are observed. However, the PL efficiency of the two different

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schematic structures relative to the above Ag NPs is quite different due to the different Ag NPs positions. The PL intensity of the bare ZnO film and the Ag NPs (I)/ZnO film are shown in Fig. 5 (a). For the bare ZnO [12], there is a narrow near-band-edge (NBE) UV emission around 370 nm and a broad visible emission peak at 575 nm. The hybrid Ag NPs

(I)/ZnO sample shows a weaker NBE emission which is induced by the Ag NPs (I), while

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the defect emission changes a little. Fig. 5 (b) shows the PL intensity of the bare ZnO film

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and the ZnO film/Ag NPs (II). For the bare ZnO, there is a NBE emission around 375 nm

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and a visible emission peak at 575 nm. The hybrid ZnO/Ag NPs (II) sample shows an

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obvious NBE emission enhancement, which is induced by the Ag NPs (II) while the

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intensity of the defect emission remains almost the same.

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Most of the reports currently are about the enhancement of the luminescence intensity for ZnO films due to the metal NPs decoration [3, 7, 10], while less work is related to the

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weakening of the luminescence intensity [13]. From Fig. 5, we noticed that the PL

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enhancement or reduction largely depends on the position of the Ag NPs. Usually the Ag NPs on the top or at the bottom of the ZnO films could influence the direction of the

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scattered light which will reduce or enhance the PL intensity. However, this influence factor can be eliminated here first because it should impact both the UV and defect emission instead of only the UV emission as in this case. Besides, the Rayleigh scattering could also be eliminated. This is because the size of the NPs has to be much smaller than the incident wavelength to cause the Rayleigh scattering, however, the size of the Ag NPs 13

(~ hundreds of nm) here is in the same order of the incident wavelength where met scattering usually happens. In order to investigate the mechanism for the UV emission variation, it is necessary to understand the different energy band structures for different Ag

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NPs and ZnO films schematic structures, as shown in Fig. 6.

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Fig. 6 Energy band diagram for (a) the Ag NPs (I)/ZnO films structure and (b) the ZnO

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films/Ag NPs (II) structure.

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When metal NPs and semiconductor materials contact with each other, carriers transition occurs between their interfaces due to the different work function until the two systems

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achieve the uniform Fermi level. It is known that the work function of Ag is about 4.26 eV

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versus absolute vacuum scale (E0), while the work function of ZnO is about 4.3~5.2 eV (varies with carrier density) versus E0 and its first electron affinity is about 4.3 eV versus

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E0. The conduction band (CB) of ZnO bends downwards (from the inside to the surface of ZnO) because of the existing electric field in the space charge region. This enables the electrons gathered at the interface to transfer from Ag to ZnO to achieve a thermal balance. However, this balance will be broken once incident light emits to form unbalanced carriers that diffuse. As can be seen in Fig. 6 (a), the incident light (ℎ𝜈) first passes through the Ag 14

NPs (I) to emit on the surface of the ZnO films, meaning that the light absorption occurs at the interface of the metal/semiconductor (MS). On this occasion, the energy band at the surface of the semiconductor bends downwards, so that the energy at the surface is lower

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than the inside. In Fig. 6 (b), on the opposite, the incident light (ℎ𝜈) emit on the surface of the ZnO films directly and the light absorption occurs at the surface instead of the MS interface. Hence, the energy at the surface on this occasion is higher relevant to the MS

interface occasion in Fig. 6 (a). As a result, the electron transition to Fermi level of the

metal occurs at the MS interface in Fig. 6 (a) due to the bending energy band that excites

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electrons to be close to the Fermi level of the metal [14]. While in Fig. 6 (b), higher electron

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energy at the semiconductor surface makes it difficult for electrons to transfer to the Fermi

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level of the metal. These oscillating electrons can be excited to a higher energy level by

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LSP and subsequently transfer to the CB of ZnO, giving rise to an enhanced transition where the electron from the CB combine radiatively with the holes in the valence band

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(VB) and causing an enhanced PL intensity of ZnO NBE emission [13].

This means that the enhancement or reduction of luminescence intensity depends on the

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transport probability of the electrons to the LSP level or to the Fermi level of the metal. In

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case of the Ag NPs decorated at the bottom of ZnO film, the energy transfer can be greatly enhanced through the resonant coupling because the emitted photon energy from the ZnO

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is equal to the LSP energy of the Ag NPs. When the Ag NPs is on the top of ZnO thin films, the surface electrons in ZnO CB can easily transfer to the Fermi level of Ag due to the lower energy barrier, leading to the reduction of the luminescence intensity. However, the luminescence properties of the Ag NPs decorated at the top of ZnO film are not always attenuated. Actually, Cheng et al. [11] reported that by sputtering Ag islands onto the 15

surface of ZnO films, the NBE emission enhanced. This clearly contradicts our results here, and it may be determined by the bending extent of the energy band which is related to the position of the semiconductor Fermi level (i.e. the density of carriers). The closer is the

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two Fermi level between the semiconductor and the metal, the easier for the electrons to transfer and the less electrons left to combine with the holes to emit photons.

In Ref. [13], it has been reported that the suppression of the defect emission from ZnO is due to the surface modification by the Pt NPs. The broad defect emission is known to arise

from the defect level to the VB, i.e. the transition where the electrons from the defect state

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combine with the holes in the VB. The carriers transfer easily from the defect level to the

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Pt Fermi level due to the closeness of the two levels. This is very similar to the results

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shown in this paper, except what we observed is that the NBE emission is weakened and

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the defect emission is not altered by the modification of the Ag NPs. More specifically, our

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result shows that no matter where the site of Ag NPs is, the modification of Ag NPs only

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significantly changes the UV emission but has little effect on the defect emission. This indicates that the effect of Ag NPs modification on electron transport in the defect states is

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minimal due to the energy mismatch in our structure. On the other hand, it also further

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indicates that the exciton-LSP coupling largely depends on the wavelength of the spectrum, i.e., the exciton and LSP strongly coupled around 370 nm but weakly coupled around 575

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nm. With the strong coupling, surface plasmons can increase the density of states and the spontaneous emission rate in the ZnO films [13], and lead to the enhancement of the radiative UV emission. This fact indicates the existence of some non-radiative processes competing with the radiative process in the ZnO films decorated by Ag NPs at different positions, which will need further study. 16

4. Conclusions

ZnO films were prepared on the c-Al2O3 substrate by a PLD technique. The metal (Ag) films were first evaporated onto the substrate and then annealed to form metal

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nanostructures to decorate the ZnO films. The PL intensity of the ZnO NBE emission

largely tunable by the position of the Ag NPs, and the mechanisms were deeply

investigated. The enhancement or reduction of luminescence intensity depends on the transport probability of the electrons to the LSP level or to the Fermi level of the metal. An enhanced PL intensity of ZnO NBE emission can be caused by subsequently transferring

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the electrons to the CB of ZnO, giving rise to an enhanced transition where the electron

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from the CB combine radiatively with the holes in the VB. The decreased PL intensity of

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ZnO NBE emission is resulted from the electron transition to Fermi level of the metal at

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the MS interface from the CB of ZnO. The defect emission subsequently remained almost

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the same with weak LSP resonance coupling and energy mismatch in the structure, since

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the position of the Ag NPs has little effect here.

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Acknowledgments

This work is supported by Hainan Provincial Natural Science Foundation of China [grant

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number 518QN211], the Scientific Research Project Funding of Hainan Higher Education Institution [grant number Hnky2018-4], the National Natural Science Foundation of China

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[grant numbers 51872187, 60976036, 11774241, and 61704111], National Key Research and Development Program of China [grant number 2017YFB0400304], the Natural Science Foundation of Guangdong Province [grant numbers 2016A030313060, 2017A030310524],

the

Fundamental

Research

Project

of

Shenzhen

[JCYJ20180206162132006], and the Science and Technology Foundation of Shenzhen. References 17

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