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Large range localized surface plasmon resonance of Ag nanoparticles films dependent of surface morphology
Accepted Manuscript Title: Large range localized surface plasmon resonance of Ag nanoparticles films dependent of surface morphology Author: Lijuan Yan Yaning Yan Leilei Xu Rongrong Ma Fengxian Jiang Xiaohong Xu PII: DOI: Reference:
S0169-4332(16)30099-X http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.238 APSUSC 32470
To appear in:
APSUSC
Received date: Revised date: Accepted date:
22-11-2015 22-1-2016 25-1-2016
Please cite this article as: L. Yan, Y. Yan, L. Xu, R. Ma, F. Jiang, X. Xu, Large range localized surface plasmon resonance of Ag nanoparticles films dependent of surface morphology, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.238 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.
Large range localized surface plasmon resonance of Ag nanoparticles films dependent of surface morphology
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Lijuan Yan, Yaning Yan, Leilei Xu, Rongrong Ma, Fengxian Jiang, Xiaohong Xu*
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School of Chemistry and Materials Science, Key Laboratory of Magnetic Molecules and Magnetic Information Materials, Ministry of Education, Shanxi
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Normal University, Linfen 041004, China
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Abstract
Noble metal nanoparticles (NPs) have received enormous attention since it displays
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uniquely optical and electronic properties. In this work, we study localized surface plasmon resonances (LSPR) at different thicknesses and substrate temperatures of Ag
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NPs films grown by Laser Molecule Beam Epitaxy (LMBE). The LSPR wavelength can be largely tuned in the visible light range of 470 nm to 770 nm. The surface morphology is characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The average size of Ag NPs increased with the thickness increased which leading to the LSPR band broaden and wavelength red-shift. As the substrate temperature is increased from RT to 200°C, the Ag NPs size distribution becomes homogeneous and particle shape changes from oblate spheroid to sphere, the LSPR band displays sharp, blue-shift and significantly symmetric. Obviously, the morphology of Ag NPs films is important for tuning absorption 1
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position. We obtain the cubic crystal structure of Ag NPs with a (111) main diffraction peak from the X-ray diffraction (XRD) spectra. The high resolution TEM (HR-TEM) and selected area electron diffraction (SAED) prove that Ag NPs is polycrystal
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structure. The Ag NPs films with large range absorption in visible light region can
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composite with semiconductor to apply in various optical or photoelectric devices.
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[Keywords] Localized surface plasmon resonance; Ag nanoparticles; Morphology
1. Introduction
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Localized surface plasmon resonance of metallic NPs is collective oscillation of the surface charge that is excited the incident light [1,2]. The electromagnetic field
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on the surface of the metallic nanostructures has also great enhancement, and it localize in a limited the surface area range which is not attenuation as light
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propagating on dielectric interface. So metal NPs displays distinctively optical
properties. Strong LSPR can enhance the intensity of spectroscopy and improve the efficiency of photoelectric devices, such as photoluminescence [3], fluorescence spectroscopy [4], Raman spectroscopy [5,6], photocatalysis [7,8], optical sensing [9], solar cells [10,11], and other fields. There are many kinds of chemical and physical methods to fabricate metal NPs.
Chemical methods are easy for preparing mono-disperse NPs and good for controlling the NPs’ size and shape, such as sol-gel method, solution method, hydrothermal synthesis method and so on. If NPs synthesized using chemical methods are prepared 2
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films on substrates by spin-coating process, the NPs tend to aggregate, and the interaction between NPs and substrates should be van der Waals force and easy exfoliation. Therefore it is difficult for device application. However, physical methods
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can overcome these problems, such as the thermal evaporation [1], pulsed laser
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deposition [2], implantation [12], direct current sputtering [13], ion beam lithography
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[14], laser molecular beam epitaxy, etc. The Ag NPs are directly grown at substrate or functional under layer, and form chemical bond which is very stable and advantages
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for potential applications in optical or photoelectric device.
Gold, silver, platinum of noble metal NPs have strong LSPR effect, they show
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strong resonance absorption in visible light range. Other metal NPs such as Cu [15], Al [16] were studied also, the LSPR of these metal NPs are unstable, relatively weak
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and easily oxidized. The noble metal Ag is cheaper than other noble metal, and it has the strongest resonance absorption and low optical loss because of less dielectric
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constant imaginary part. So the metallic Ag plays a role of LSPR in various researches.
Theoretical and experimental studies indicate that the absorption wavelength and
intensity of LSPR not only depend on the properties of the metal, also it is related to the size, the shape, the dielectric constant of metal and the surrounding medium [17-20]. The influence of particles size was investigated, and the NPs films were applied in surface enhancement Raman spectroscopy (SERS), photovoltaic device [1,21]. Van Duyne and co-workers varied shape and aspect ratio to tune LSPR, such as nanospheroid, triangle and cylindrical [22-25]. Besides, they also demonstrated 3
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resonance absorption of Ag NPs in different substrates and surrounding medium respectively. Baraldi [26] and Chervinskii [27] investigated the influence of the thickness of dielectric protection layer using Al2O3 and TiO2 films, and effectively
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prevent the degradation Ag NPs respectively. Therefore we deposit Ag NPs films
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using LMBE, change the size of the NPs to tune optical properties, and realize a wide
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range the optical response of Ag NPs films.
In this work, we mainly study the effect of different thicknesses and substrate
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temperatures on resonance absorption of Ag NPs films deposited by LMBE. We observe that the optical response of Ag NPs films is correlated with their morphology
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characteristic, and further illustrate the growth process of Ag NPs films under different temperatures and thicknesses. This study displays that it can largely tune
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optical absorption of Ag NPs films in visible light range which provide a potential
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application in high efficiency optical or photoelectric devices.
2. Experiment
2.1 The Ag NPs films fabrication Ag NPs films were deposited on glass substrates which were thoroughly clean with
deionized water, ethanol by LMBE. A KrF excimer laser of 248 nm wavelength and 20 ns pulse duration was operated at 250 mJ pulse energy and 10 Hz pulse repetition rate. The base pressure in the vacuum chamber was
～
5×10-5 Pa. The Ag target
(purity: 99.99%) was placed 7 cm away from substrate. The films were grown at different substrate temperatures including RT, 100°C and 200°C. Different thicknesses 4
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(3-25 nm) of Ag NPs films were fabricated on the glass substrates by controlling sputtering time. 2.2 The Ag NPs films characterization
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The optical absorption was analyzed by UV-Vis spectrophotometer (U-3310) in the
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300 to 1000 nm wavelength where is the LSPR behavior of Ag NPs. The glass
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substrates were regarded as baselines at the absorption spectra measuring process. The surface morphology of Ag NPs films with no less than 10 nm thickness was
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characterized by SEM (JSM，7500F), and the morphology of Ag NPs films with less than 10 nm thickness was measured by TEM. And the crystal structure of Ag NPs film
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with 3 nm layer was analyzed using HR-TEM and SAED. The structure and composite of NPs films were investigated by XRD (Ultima IV) with 2Theta range of
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20° to 80° and the energy dispersive spectroscopy (EDS).
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3. Results and Discussions
3.1 Optical characterizations
The absorption spectra of Ag NPs films deposited on different substrate
temperatures and thicknesses are shown in Fig.1. The optical response of Ag NPs films deposited on glass substrate at RT is shown in Fig.1. (a). With the thickness increased the resonance absorption red-shift and broaden. The intensity also increased with the thickness increased which can be attributed by the stronger LSPR from the particles volume. The LSPR of Ag NPs films could be observed in visible light range until the thickness increased to 17 nm. With the film thickness increasing from 3 nm 5
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to 17 nm, the resonance absorption peak shift from 470 nm to 770 nm which is a relatively large range. When thickness increased up to 20 nm and 25 nm, resonance absorption is not observed in visible light range. Fig.1. (b) shows that the Ag NPs
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films grown at 100°C substrate temperature have similar optical response to those
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grown at RT. However, it appears slight red-shift with the thickness increased, and the
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absorption band become narrow. The resonance absorption peak with the thickness increased from 3 nm to 10 nm is found to be in the range of about 440 nm to 510 nm.
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When substrate temperature increased to 200°C, sharp and significantly symmetrical of the absorption band are shown in Fig.1. (c). Similarly, the resonance absorption
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shift from 435 nm to 470 nm when the film thickness increased from 3 nm to 10 nm. Therefore, the thickness is a key parameter for turning the resonance wavelength. In
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order to further comparing the absorption of 10 nm Ag NPs films grown at RT, 100°C, 200°C, are shown in Fig.1. (d). It can be clearly seen that the LSPR wavelength shows
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a blue-shift with increasing the substrate temperature. It means the substrate temperature is also a key parameter for turning the resonance wavelength. 3.2 Structural and surface morphological characterizations 3.2.1 Structural characterizations using the XRD spectra The XRD spectra of Ag NPs films are displayed in Fig.2. The dominating peak is
the (111) diffraction of the cubic crystal structure of metallic Ag. The intensity of Ag (111) peak slightly increased with the thickness of Ag NPs films because of the increasing of grain size and mass. These are no other diffraction peaks related to Ag oxide, such as AgO or Ag2O. 6
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3.2.2 Surface morphology with different substrate temperatures Here we choose 10 nm NPs films as a typical sample, and investigate the influence of substrate temperatures on LSPR. The SEM images of Ag NPs films deposited on
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substrate temperature of RT, 100°C, and 200°C with 10 nm thickness are shown in
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Fig.3. Fig.3. (a) shows the oblate spherical shape and the inhomogeneous size
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distribution of Ag NPs films deposited on RT. With substrate temperature increased, the diffusion coefficient and the atomic mobility increased which have advantage for
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the particle shapes changing from oblate spheroid to sphere and result in the uniformity of size distribution increased, as clearly seen in Fig.3. (b) and (c).
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In order to explain influence of the morphology on NPs LSPR, we carries on the analysis using the Mie theory [28] that has been used to model for the absorption
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spectrum of metallic Ag. There are some limitations that the size of NPs is much smaller than the wavelength of incident light and resonance mode is dipole limit.
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Then, it can be expressed as [29]
where
(1)
is extinction coefficient, where V =
particle, and
and
are the dielectric functions of the surrounding
medium and the material itself, respectively, where incident light, where
is the volume of the spherical
is the wavelength of the
is an added geometrical factor whose value
= 1/3 or less
than 1/3 for spherical or oblate (depending on the aspect ratio) particles, respectively. 7
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According to equation (1), the resonance absorption wavelength is inversely proportional to the geometrical factor which very clearly indicates that the particle shapes changing from oblate spheroid to sphere, the SPR wavelength blue-shift which
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is agreement with the corresponding resonance absorption band of Ag NPs films with
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substrate temperature change, as seen in Fig.1 (d). Besides, the inhomogeneous size
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distribution of Ag NPs films leads to broader resonance absorption band [1,25], showing in Fig.1 (d).
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Although we cannot find the Ag XRD peak in the 10 nm Ag NPs film, the EDS spectra clearly indicate the absorption of metal Ag, showing in Fig. 3.(d). There is
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faint volume absorption peak accordance with Lα line of Ag at 2.9844ev, which provides evidence that proves the existence of metallic Ag in samples.
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3.2.3 Surface morphology with different thicknesses Secondly, we study the influence the NPs films thickness on LSPR. The SEM
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images of Ag NPs films with thickness ≥13 nm deposited on RT glass substrate are shown in Fig.4. (a)-(d), respectively. At 13 nm thickness film, the Ag NPs appear to aggregation to from islands structure. With the film thickness increased the size of islands increased, and the islands contact each other with strong coupling which result in resonance wavelength red-shift. Continue increased to 20 nm or more than 20 nm thickness, forming a continuous film which the resonance absorption has not observed in visible light range, showing in Fig.1.(a). The equation (1) can well explain the optical properties of the small NPs, but for larger NPs will no longer valid [30,31]. For the larger NPs, the oscillation is more 8
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primary the higher order modes because the light can no longer polarize the NPs homogeneously. As a consequence, retardation effects of the electromagnetic field across the particle can cause huge shifts and broadening of the surface plasmon
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resonances [31-34]. Therefore, with increasing particle size, the resonance absorption
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band red-shift and broaden.
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The TEM images of Ag NPs films with thickness ≤7 nm grown at RT glass substrate are shown in Fig.5. (a)-(c), respectively. And the corresponding size
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distribution graphs are shown in Fig.5. (d)-(f). In Fig.5. (a), Ag NPs are isolated and dispersion spheroids with a small mean interparticle distance and inhomogeneous size
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distribution. The average size of Ag NPs is calculated 9 nm by Nano Measurer 1.2, as seen in Fig.5. (d). In Fig.5. (b), when the thickness increased to 5 nm, the average size
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of Ag NPs increased to 14 nm is shown in Fig.5. (e), the mean interparticle distance decreased, and the dimers appear because of NPs coalescence. The size distribution of
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Ag NPs is also inhomogeneous. In Fig.5. (c), the average size of Ag NPs is 16 nm is displayed in Fig.5.(f), and the number of dimers increased. By equation (1), we can know that the NPs size increased can cause resonance
absorption peaks red-shift and strong intensity with the thickness increased. And small interparticle distance and the dimers can also result in resonance absorption peaks red-shift due to the coupling increased. It is agreement with the corresponding resonance absorption of Ag NPs films with increasing thickness, as seen in Fig.1. (a). The variation of surface morphology with thickness is concerned with the growth process as well as substrate temperature. In the initial stage, forming dispersion and 9
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isolated particles on the surface since mobility exceeds the adhesion force. With the sputtering process, the particles size increased, and the particles appear aggregation. Until the number of monolayers is enough to overcome the free enthalpy and the
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supersaturation parameter, which results in form a continuous film [1,13].
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3.2.4 Structural characterizations using the TEM
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We also investigate the nanostructure of Ag NPs by the HR-TEM and the SAED. The HR-TEM image of Ag NPs film with 3 nm thickness is shown in Fig.6. (a). We
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clearly observe that the polycrystalline grains with different crystallographic orientations. In Fig.6. (b), the diffraction pattern also proves to be polycrystalline of
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crystal Ag, and we observe dominating diffraction peak (111) of Ag NPs film from the
4. Conclusions
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SAED image.
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The LMBE is a powerful technique for growing metal NPs films with
controllable particle size so that it can tune the resonance absorption of NPs films. The thickness and substrate temperature are found to be the important parameters for turning optical response of Ag NPs films. With the thickness increased the average size of Ag NPs increased and the interparticles distance decreased, which lead to the LSPR of Ag NPs films shows red-shift and broaden. Comparison with the films grown at RT, the resonance absorption of Ag NPs films grown at 200°C displays blue-shift and sharpen. By changing the thickness at RT, the LSPR wavelength varied in the ranges of 470 nm-770 nm. These results provide a deep understand of Ag NPs 10
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films LSPR properties. And it can be improved application of the visible light from the solar through this tunable resonance absorption of Ag NPs films in the visible light range. This structure can be directly applied in surface enhance Raman and
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fluorescence spectrum as a test substrate, also can be combined with function under
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photovoltaic devices and photoelectronic detectors etc.
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layer for the high efficiency application in solar cells, photocatalysis, SERS,
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*Corresponding Author
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E-mail address: [email protected] (X. Xu).
Acknowledgments
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The work is supported by 863 Program (No.2014AA032904), NSFC (Nos.51025101, and 61434002), the Special Funds of Shanxi Scholars Program No.[2012]12,
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and the Youth Science Foundation of Shanxi Province (No. 2015021064).
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highlights:
Large range tuned localized surface plasmon resonance of Ag nanoparticles films.
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The noble metal Ag has the strongest localized surface plasmon resonance and low optical loss. Besides, it is the cheaper than other noble metal.
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The nanoparticles films fabricated using physical methods have the stronger interaction with substrates than chemical methods, which are not easy exfoliation.
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*Graphical Abstract (for review)
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Figure
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Figures and Captions
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Fig.1. (a) The UV-Vis absorption spectra of Ag NPs films with various mass thicknesses deposited on glass substrate at RT. (b), (c) The absorption spectra of Ag
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NPs films deposited on glass substrate temperature at 100℃, 200℃. (d) The absorption spectra of Ag NPs films deposited at different substrate temperatures with
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10 nm thickness.
Fig.2. The XRD spectra of Ag NPs films deposited on RT glass substrate with 10, 13,
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15, 17, 20 and 25 nm thickness.
Fig.3. The SEM images of Ag NPs films with 10 nm thickness deposited on the
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ce pt
ed
NPs film deposited on RT substrate.
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substrate temperature of (a) RT (b) 100℃ and (c) 200℃. (d) The EDS spectra of Ag
Fig.4. The SEM images of Ag NPs films deposited on glass substrate of RT with (a) 13 nm (b) 15 nm (c) 17 nm (d) 20 nm thickness.
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Fig.5. Upper row: The Plan-view TEM images of Ag NPs films deposited on RT glass
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substrate with (a) 3 nm (b) 5 nm (c) 7 nm thickness, and the insets are high magnification TEM images. Lower row: The distribution of the in plan-view long axis
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ce pt
(f) 7 nm thickness.
ed
length (L) of the Ag NPs films deposited on RT glass substrate with (d) 3 nm (e) 5 nm
Fig.6. The HR-TEM (a) and the SAED (b) images of Ag NPs film with 3 nm thickness deposited on RT substrate.
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S0169-4332(16)30099-X http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.238 APSUSC 32470
To appear in:
APSUSC
Received date: Revised date: Accepted date:
22-11-2015 22-1-2016 25-1-2016
Please cite this article as: L. Yan, Y. Yan, L. Xu, R. Ma, F. Jiang, X. Xu, Large range localized surface plasmon resonance of Ag nanoparticles films dependent of surface morphology, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.238 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.
Large range localized surface plasmon resonance of Ag nanoparticles films dependent of surface morphology
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Lijuan Yan, Yaning Yan, Leilei Xu, Rongrong Ma, Fengxian Jiang, Xiaohong Xu*
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School of Chemistry and Materials Science, Key Laboratory of Magnetic Molecules and Magnetic Information Materials, Ministry of Education, Shanxi
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Normal University, Linfen 041004, China
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Abstract
Noble metal nanoparticles (NPs) have received enormous attention since it displays
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d
uniquely optical and electronic properties. In this work, we study localized surface plasmon resonances (LSPR) at different thicknesses and substrate temperatures of Ag
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NPs films grown by Laser Molecule Beam Epitaxy (LMBE). The LSPR wavelength can be largely tuned in the visible light range of 470 nm to 770 nm. The surface morphology is characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The average size of Ag NPs increased with the thickness increased which leading to the LSPR band broaden and wavelength red-shift. As the substrate temperature is increased from RT to 200°C, the Ag NPs size distribution becomes homogeneous and particle shape changes from oblate spheroid to sphere, the LSPR band displays sharp, blue-shift and significantly symmetric. Obviously, the morphology of Ag NPs films is important for tuning absorption 1
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position. We obtain the cubic crystal structure of Ag NPs with a (111) main diffraction peak from the X-ray diffraction (XRD) spectra. The high resolution TEM (HR-TEM) and selected area electron diffraction (SAED) prove that Ag NPs is polycrystal
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structure. The Ag NPs films with large range absorption in visible light region can
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composite with semiconductor to apply in various optical or photoelectric devices.
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[Keywords] Localized surface plasmon resonance; Ag nanoparticles; Morphology
1. Introduction
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Localized surface plasmon resonance of metallic NPs is collective oscillation of the surface charge that is excited the incident light [1,2]. The electromagnetic field
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on the surface of the metallic nanostructures has also great enhancement, and it localize in a limited the surface area range which is not attenuation as light
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propagating on dielectric interface. So metal NPs displays distinctively optical
properties. Strong LSPR can enhance the intensity of spectroscopy and improve the efficiency of photoelectric devices, such as photoluminescence [3], fluorescence spectroscopy [4], Raman spectroscopy [5,6], photocatalysis [7,8], optical sensing [9], solar cells [10,11], and other fields. There are many kinds of chemical and physical methods to fabricate metal NPs.
Chemical methods are easy for preparing mono-disperse NPs and good for controlling the NPs’ size and shape, such as sol-gel method, solution method, hydrothermal synthesis method and so on. If NPs synthesized using chemical methods are prepared 2
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films on substrates by spin-coating process, the NPs tend to aggregate, and the interaction between NPs and substrates should be van der Waals force and easy exfoliation. Therefore it is difficult for device application. However, physical methods
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can overcome these problems, such as the thermal evaporation [1], pulsed laser
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deposition [2], implantation [12], direct current sputtering [13], ion beam lithography
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[14], laser molecular beam epitaxy, etc. The Ag NPs are directly grown at substrate or functional under layer, and form chemical bond which is very stable and advantages
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for potential applications in optical or photoelectric device.
Gold, silver, platinum of noble metal NPs have strong LSPR effect, they show
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strong resonance absorption in visible light range. Other metal NPs such as Cu [15], Al [16] were studied also, the LSPR of these metal NPs are unstable, relatively weak
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and easily oxidized. The noble metal Ag is cheaper than other noble metal, and it has the strongest resonance absorption and low optical loss because of less dielectric
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constant imaginary part. So the metallic Ag plays a role of LSPR in various researches.
Theoretical and experimental studies indicate that the absorption wavelength and
intensity of LSPR not only depend on the properties of the metal, also it is related to the size, the shape, the dielectric constant of metal and the surrounding medium [17-20]. The influence of particles size was investigated, and the NPs films were applied in surface enhancement Raman spectroscopy (SERS), photovoltaic device [1,21]. Van Duyne and co-workers varied shape and aspect ratio to tune LSPR, such as nanospheroid, triangle and cylindrical [22-25]. Besides, they also demonstrated 3
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resonance absorption of Ag NPs in different substrates and surrounding medium respectively. Baraldi [26] and Chervinskii [27] investigated the influence of the thickness of dielectric protection layer using Al2O3 and TiO2 films, and effectively
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prevent the degradation Ag NPs respectively. Therefore we deposit Ag NPs films
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using LMBE, change the size of the NPs to tune optical properties, and realize a wide
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range the optical response of Ag NPs films.
In this work, we mainly study the effect of different thicknesses and substrate
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temperatures on resonance absorption of Ag NPs films deposited by LMBE. We observe that the optical response of Ag NPs films is correlated with their morphology
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characteristic, and further illustrate the growth process of Ag NPs films under different temperatures and thicknesses. This study displays that it can largely tune
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optical absorption of Ag NPs films in visible light range which provide a potential
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application in high efficiency optical or photoelectric devices.
2. Experiment
2.1 The Ag NPs films fabrication Ag NPs films were deposited on glass substrates which were thoroughly clean with
deionized water, ethanol by LMBE. A KrF excimer laser of 248 nm wavelength and 20 ns pulse duration was operated at 250 mJ pulse energy and 10 Hz pulse repetition rate. The base pressure in the vacuum chamber was
～
5×10-5 Pa. The Ag target
(purity: 99.99%) was placed 7 cm away from substrate. The films were grown at different substrate temperatures including RT, 100°C and 200°C. Different thicknesses 4
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(3-25 nm) of Ag NPs films were fabricated on the glass substrates by controlling sputtering time. 2.2 The Ag NPs films characterization
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The optical absorption was analyzed by UV-Vis spectrophotometer (U-3310) in the
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300 to 1000 nm wavelength where is the LSPR behavior of Ag NPs. The glass
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substrates were regarded as baselines at the absorption spectra measuring process. The surface morphology of Ag NPs films with no less than 10 nm thickness was
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characterized by SEM (JSM，7500F), and the morphology of Ag NPs films with less than 10 nm thickness was measured by TEM. And the crystal structure of Ag NPs film
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with 3 nm layer was analyzed using HR-TEM and SAED. The structure and composite of NPs films were investigated by XRD (Ultima IV) with 2Theta range of
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20° to 80° and the energy dispersive spectroscopy (EDS).
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3. Results and Discussions
3.1 Optical characterizations
The absorption spectra of Ag NPs films deposited on different substrate
temperatures and thicknesses are shown in Fig.1. The optical response of Ag NPs films deposited on glass substrate at RT is shown in Fig.1. (a). With the thickness increased the resonance absorption red-shift and broaden. The intensity also increased with the thickness increased which can be attributed by the stronger LSPR from the particles volume. The LSPR of Ag NPs films could be observed in visible light range until the thickness increased to 17 nm. With the film thickness increasing from 3 nm 5
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to 17 nm, the resonance absorption peak shift from 470 nm to 770 nm which is a relatively large range. When thickness increased up to 20 nm and 25 nm, resonance absorption is not observed in visible light range. Fig.1. (b) shows that the Ag NPs
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films grown at 100°C substrate temperature have similar optical response to those
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grown at RT. However, it appears slight red-shift with the thickness increased, and the
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absorption band become narrow. The resonance absorption peak with the thickness increased from 3 nm to 10 nm is found to be in the range of about 440 nm to 510 nm.
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When substrate temperature increased to 200°C, sharp and significantly symmetrical of the absorption band are shown in Fig.1. (c). Similarly, the resonance absorption
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shift from 435 nm to 470 nm when the film thickness increased from 3 nm to 10 nm. Therefore, the thickness is a key parameter for turning the resonance wavelength. In
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order to further comparing the absorption of 10 nm Ag NPs films grown at RT, 100°C, 200°C, are shown in Fig.1. (d). It can be clearly seen that the LSPR wavelength shows
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a blue-shift with increasing the substrate temperature. It means the substrate temperature is also a key parameter for turning the resonance wavelength. 3.2 Structural and surface morphological characterizations 3.2.1 Structural characterizations using the XRD spectra The XRD spectra of Ag NPs films are displayed in Fig.2. The dominating peak is
the (111) diffraction of the cubic crystal structure of metallic Ag. The intensity of Ag (111) peak slightly increased with the thickness of Ag NPs films because of the increasing of grain size and mass. These are no other diffraction peaks related to Ag oxide, such as AgO or Ag2O. 6
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3.2.2 Surface morphology with different substrate temperatures Here we choose 10 nm NPs films as a typical sample, and investigate the influence of substrate temperatures on LSPR. The SEM images of Ag NPs films deposited on
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substrate temperature of RT, 100°C, and 200°C with 10 nm thickness are shown in
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Fig.3. Fig.3. (a) shows the oblate spherical shape and the inhomogeneous size
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distribution of Ag NPs films deposited on RT. With substrate temperature increased, the diffusion coefficient and the atomic mobility increased which have advantage for
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the particle shapes changing from oblate spheroid to sphere and result in the uniformity of size distribution increased, as clearly seen in Fig.3. (b) and (c).
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In order to explain influence of the morphology on NPs LSPR, we carries on the analysis using the Mie theory [28] that has been used to model for the absorption
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spectrum of metallic Ag. There are some limitations that the size of NPs is much smaller than the wavelength of incident light and resonance mode is dipole limit.
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Then, it can be expressed as [29]
where
(1)
is extinction coefficient, where V =
particle, and
and
are the dielectric functions of the surrounding
medium and the material itself, respectively, where incident light, where
is the volume of the spherical
is the wavelength of the
is an added geometrical factor whose value
= 1/3 or less
than 1/3 for spherical or oblate (depending on the aspect ratio) particles, respectively. 7
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According to equation (1), the resonance absorption wavelength is inversely proportional to the geometrical factor which very clearly indicates that the particle shapes changing from oblate spheroid to sphere, the SPR wavelength blue-shift which
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is agreement with the corresponding resonance absorption band of Ag NPs films with
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substrate temperature change, as seen in Fig.1 (d). Besides, the inhomogeneous size
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distribution of Ag NPs films leads to broader resonance absorption band [1,25], showing in Fig.1 (d).
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Although we cannot find the Ag XRD peak in the 10 nm Ag NPs film, the EDS spectra clearly indicate the absorption of metal Ag, showing in Fig. 3.(d). There is
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faint volume absorption peak accordance with Lα line of Ag at 2.9844ev, which provides evidence that proves the existence of metallic Ag in samples.
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3.2.3 Surface morphology with different thicknesses Secondly, we study the influence the NPs films thickness on LSPR. The SEM
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images of Ag NPs films with thickness ≥13 nm deposited on RT glass substrate are shown in Fig.4. (a)-(d), respectively. At 13 nm thickness film, the Ag NPs appear to aggregation to from islands structure. With the film thickness increased the size of islands increased, and the islands contact each other with strong coupling which result in resonance wavelength red-shift. Continue increased to 20 nm or more than 20 nm thickness, forming a continuous film which the resonance absorption has not observed in visible light range, showing in Fig.1.(a). The equation (1) can well explain the optical properties of the small NPs, but for larger NPs will no longer valid [30,31]. For the larger NPs, the oscillation is more 8
Page 8 of 20
primary the higher order modes because the light can no longer polarize the NPs homogeneously. As a consequence, retardation effects of the electromagnetic field across the particle can cause huge shifts and broadening of the surface plasmon
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resonances [31-34]. Therefore, with increasing particle size, the resonance absorption
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band red-shift and broaden.
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The TEM images of Ag NPs films with thickness ≤7 nm grown at RT glass substrate are shown in Fig.5. (a)-(c), respectively. And the corresponding size
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distribution graphs are shown in Fig.5. (d)-(f). In Fig.5. (a), Ag NPs are isolated and dispersion spheroids with a small mean interparticle distance and inhomogeneous size
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distribution. The average size of Ag NPs is calculated 9 nm by Nano Measurer 1.2, as seen in Fig.5. (d). In Fig.5. (b), when the thickness increased to 5 nm, the average size
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of Ag NPs increased to 14 nm is shown in Fig.5. (e), the mean interparticle distance decreased, and the dimers appear because of NPs coalescence. The size distribution of
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Ag NPs is also inhomogeneous. In Fig.5. (c), the average size of Ag NPs is 16 nm is displayed in Fig.5.(f), and the number of dimers increased. By equation (1), we can know that the NPs size increased can cause resonance
absorption peaks red-shift and strong intensity with the thickness increased. And small interparticle distance and the dimers can also result in resonance absorption peaks red-shift due to the coupling increased. It is agreement with the corresponding resonance absorption of Ag NPs films with increasing thickness, as seen in Fig.1. (a). The variation of surface morphology with thickness is concerned with the growth process as well as substrate temperature. In the initial stage, forming dispersion and 9
Page 9 of 20
isolated particles on the surface since mobility exceeds the adhesion force. With the sputtering process, the particles size increased, and the particles appear aggregation. Until the number of monolayers is enough to overcome the free enthalpy and the
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supersaturation parameter, which results in form a continuous film [1,13].
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3.2.4 Structural characterizations using the TEM
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We also investigate the nanostructure of Ag NPs by the HR-TEM and the SAED. The HR-TEM image of Ag NPs film with 3 nm thickness is shown in Fig.6. (a). We
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clearly observe that the polycrystalline grains with different crystallographic orientations. In Fig.6. (b), the diffraction pattern also proves to be polycrystalline of
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crystal Ag, and we observe dominating diffraction peak (111) of Ag NPs film from the
4. Conclusions
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SAED image.
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The LMBE is a powerful technique for growing metal NPs films with
controllable particle size so that it can tune the resonance absorption of NPs films. The thickness and substrate temperature are found to be the important parameters for turning optical response of Ag NPs films. With the thickness increased the average size of Ag NPs increased and the interparticles distance decreased, which lead to the LSPR of Ag NPs films shows red-shift and broaden. Comparison with the films grown at RT, the resonance absorption of Ag NPs films grown at 200°C displays blue-shift and sharpen. By changing the thickness at RT, the LSPR wavelength varied in the ranges of 470 nm-770 nm. These results provide a deep understand of Ag NPs 10
Page 10 of 20
films LSPR properties. And it can be improved application of the visible light from the solar through this tunable resonance absorption of Ag NPs films in the visible light range. This structure can be directly applied in surface enhance Raman and
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fluorescence spectrum as a test substrate, also can be combined with function under
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photovoltaic devices and photoelectronic detectors etc.
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layer for the high efficiency application in solar cells, photocatalysis, SERS,
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*Corresponding Author
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E-mail address: [email protected] (X. Xu).
Acknowledgments
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The work is supported by 863 Program (No.2014AA032904), NSFC (Nos.51025101, and 61434002), the Special Funds of Shanxi Scholars Program No.[2012]12,
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and the Youth Science Foundation of Shanxi Province (No. 2015021064).
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34. G. C. Papavassiliou, Optical properties of small inorganic and organic metal
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highlights:
Large range tuned localized surface plasmon resonance of Ag nanoparticles films.
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The noble metal Ag has the strongest localized surface plasmon resonance and low optical loss. Besides, it is the cheaper than other noble metal.
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The nanoparticles films fabricated using physical methods have the stronger interaction with substrates than chemical methods, which are not easy exfoliation.
16
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ed
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*Graphical Abstract (for review)
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Figure
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Figures and Captions
ed
Fig.1. (a) The UV-Vis absorption spectra of Ag NPs films with various mass thicknesses deposited on glass substrate at RT. (b), (c) The absorption spectra of Ag
ce pt
NPs films deposited on glass substrate temperature at 100℃, 200℃. (d) The absorption spectra of Ag NPs films deposited at different substrate temperatures with
Ac
10 nm thickness.
Fig.2. The XRD spectra of Ag NPs films deposited on RT glass substrate with 10, 13,
Page 18 of 20
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cr
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15, 17, 20 and 25 nm thickness.
Fig.3. The SEM images of Ag NPs films with 10 nm thickness deposited on the
Ac
ce pt
ed
NPs film deposited on RT substrate.
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substrate temperature of (a) RT (b) 100℃ and (c) 200℃. (d) The EDS spectra of Ag
Fig.4. The SEM images of Ag NPs films deposited on glass substrate of RT with (a) 13 nm (b) 15 nm (c) 17 nm (d) 20 nm thickness.
Page 19 of 20
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Fig.5. Upper row: The Plan-view TEM images of Ag NPs films deposited on RT glass
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substrate with (a) 3 nm (b) 5 nm (c) 7 nm thickness, and the insets are high magnification TEM images. Lower row: The distribution of the in plan-view long axis
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ce pt
(f) 7 nm thickness.
ed
length (L) of the Ag NPs films deposited on RT glass substrate with (d) 3 nm (e) 5 nm
Fig.6. The HR-TEM (a) and the SAED (b) images of Ag NPs film with 3 nm thickness deposited on RT substrate.
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