Electron-beam irradiation induced phase transformation, optical absorption and surface-enhanced Raman scattering of Indium tin alloy thin films

Accepted Manuscript Electron-beam irradiation induced phase transformation, optical absorption and surface-enhanced Raman scattering of Indium tin alloy thin films Wenzuo Wei, Ruijin Hong, Yan Meng, Chunxian Tao, Dawei Zhang PII:

S0749-6036(17)30504-9

DOI:

10.1016/j.spmi.2017.03.045

Reference:

YSPMI 4912

To appear in:

Superlattices and Microstructures

Received Date: 28 February 2017 Revised Date:

27 March 2017

Accepted Date: 27 March 2017

Please cite this article as: W. Wei, R. Hong, Y. Meng, C. Tao, D. Zhang, Electron-beam irradiation induced phase transformation, optical absorption and surface-enhanced Raman scattering of Indium tin alloy thin films, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2017.03.045. 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.

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Electron-beam Irradiation Induced Phase Transformation, Optical

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Absorption and Surface-Enhanced Raman Scattering of Indium Tin

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Alloy Thin Films Wenzuo Wei1, Ruijin Hong1, Yan Meng1, Chunxian Tao1, Dawei Zhang1*

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Education and Shanghai Key Lab of Modern Optical System, University of Shanghai

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for Science and Technology, No.516 Jungong Road, Shanghai 200093, People’s

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Republic of China

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* Corresponding author.

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Engineering Research Center of Optical Instrument and System, Ministry of

E-mail address: [email protected] (D.W. Zhang).

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Abstract

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Electron beam (EB) irradiation experiments on Indium-tin (In-Sn) alloy thin films

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are reported. The structure and the optical properties of the samples were investigated

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by atomic force microscopy, X-ray diffraction, UV-vis-NIR double beam

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spectrometer and Raman system, respectively. Those results show that EB irradiation

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has the effects of changing the preferred orientation, improving the crystalline,

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enhancing the absorption, and improving the surface-enhanced Raman scattering

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(SERS) of samples. In addition, Finite-Difference Time-Domain (FDTD) was

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performed for the surface plasmon resonance properties of the as-irradiated samples,

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and the results are in good agreement with the experiments.

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Keywords: EB irradiation, In-Sn alloy thin films, Optical properties, SERS, FDTD

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1.Introduction Localized surface plasmon resonance (LSPR) is referred to the collective electron

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oscillation which is excited by incident light in metal nanoparticles (NPs) [1,2]. Due

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to its strong confinement and enhancement of electric fields near the vicinity of metal

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NPs, strong LSPR has been applied in many areas, such as photoluminescence,

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photocatalysis [3,4], Raman spectroscopy [5,6], fluorescence spectroscopy [7], optical

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sensing [8], solar cells [9,10], and other fields. Since the efficiency of these

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applications is depended on the plasmon resonance wavelength, it is greatly important

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to tune a desired wavelength. However, not only the frequency and intensity of LSPR

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absorption band lie on the size, the shape of the metal NPs and the dielectric constant

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of the surrounding media, but also on what metal material is used.

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Silver and gold have been focused for years due to their excellent LSPR tunability

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and good performance in SERS. Interestingly, a number of other metals (i.e., Li, Na,

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Al, Cu, and In) also meet this criterion and may possibly support LSPR for at least

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part of the UV-vis-near-IR region, but there has been far less experimental work with

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these metals. Among these metals, Indium tin (In-Sn) alloy, as a kind of indispensable

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material to prepare Indium tin oxide (ITO), has been extensively applied in aviation,

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sensors and many other fields in these decades [12-14]. Owing to its low-melting

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point [13], In-Sn alloy has also become an irreplaceable material in electronic and

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high-vacuum fields. However, currently there is a few reports on the In-Sn alloy thin

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film concerning its preparation methods, structure and especially the LSPR property,

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which is demonstrated to have good performance on SERS in the present study. In this study, In-Sn alloy films were fabricated by EB evaporation and we showed

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for the first time that both the optical-electrical and micro-structural properties of the

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samples can be influenced by EB irradiation. The effects of EB irradiation on the

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surface morphology and SERS of the samples were also discussed in this paper.

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Furthermore, the finite-difference time domain (FDTD) method was employed to

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calculate the electronic-field distribution of In-Sn thin films.

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2.Experimental details

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In-Sn alloy thin films were grown on fused quartz substrates by EB evaporation

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from In-Sn coating materials (99.99%). Prior to deposition, the substrates were

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ultrasonically cleaned in acetone, ethanol and deionized water for 20 min respectively,

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and subsequently dried with a flow of nitrogen. The chamber was evacuated to a base

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pressure of less than 5×10-4 Pa. Baking temperature was set at 100 . The thicknesses

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of In-Sn films were set as approx. 20nm. The thickness of the film was monitored by

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an in situ quartz crystal microbalance. To guarantee the uniformity of the film

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thickness, all substrates were set on the fixture with the same radius of the circle.

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After deposition, EB irradiation with a power of 100W was applied in this experiment

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to change the optical and electrical properties of the samples. The irradiated area was

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the whole substrate (diameter =30 mm and thickness =1.37 mm); the irradiation time

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were 5, 10, 15 and 20 min, respectively. For comparison, film without being irradiated

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deposited on bare substrate with the same thickness was also used in this experiment.

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As-deposited film and as-irradiated films were denoted as S0, S1, S2, S3 and S4

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respectively, with the irradiation time increasing (0-20min). The structural properties and the crystallinity of the samples were analyzed by

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X-ray diffraction (XRD) using a Bruker AXS/D8 Advance system, with Cu kα

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radiation(λ=0.15408nm). The optical constants of the samples were measured with an

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UV-vis-NIR double beam spectrophotometer (Lambda 1050, Perkins Elmer, USA).

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The surface morphology and roughness were characterized by atomic force

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microscopy (AFM) (XE-100, Park System). Raman scattering spectra were obtained

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by using a confocal microprobe Raman system (LabRAM Aramis, France) operated

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with a He-Na laser ( λ = 473nm ). All the measurements were carried out at room

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temperature.

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3.Results and discussion

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(1) X-ray diffraction studies and texture coefficient values The representative XRD patterns as shown in Fig.1 reveal the influences of EB

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irradiation on the In-Sn thin films. Apparently, there are 3 diffraction peaks in the

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spectra, namely the (101), (002) and (110) peaks of In, as confirmed by the standard

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card (JCPDS:89-1409). For the as-deposited sample (S0), the preferred orientation at

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about 32.9° is corresponded to In (101) plane. However, after EB irradiation, the

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as-irradiated samples (S1-S4) show the strongest intensity peak at approx. 36.3°, with

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C-axis preferred orientation, no diffraction from randomly oriented grains or impurity

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phases can be observed from the X-ray pattern [14]. The surface energy density of the

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ACCEPTED MANUSCRIPT (002) orientation is the lowest in the In crystal. Grains with the lower surface energy

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will become larger as the film grows. Then, the growth orientation develops into one

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crystallographic direction of the lowest surface energy. This means that the (002)

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textured film must form in an effective equilibrium state where enough surface

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mobility is given to impinging atoms under a certain deposition condition [15]. In

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addition, the phase transformation of the as-irradiated samples is possibly due to the

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increased thermal energy, which is transferred from EB gun to the samples. During

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EB irradiation, the In charge are melted, some atoms are evaporated and some of them

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migrate on the substrate from the high surface energy planes to the low surface energy

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planes due to the free energy minimization [16]. Thus we can see the preferred

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orientation is changed to (002) plane and intensity is decreased after EB irradiation.

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This may also increase the carrier concentration of the In-Sn film by substituting Sn

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into In site in the sample [17-18].

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Compared with the In-Sn alloy powders, all the films formed in our experiment

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exhibit discrepancy in d-value (d is interplanar spacing) (Table.1), which is due to the

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variation of residual stress in the films. The full-width at half-maximum (FWHM) of

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S0, S1, S2, S3 and S4 have the values of 0.215°, 0.314°, 0.328°, 0.331° and 0.283°,

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respectively. The diffraction peaks are 36.580°, 36.320°, 36.279°, 36.260°, and

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36.163° (2θ), respectively. The EB irradiation has the effect of narrowing the

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diffraction peak, indicating that grain growth has occurred, and shifting the (002) peak

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to lower 2θ angles, as a result of the partial relief of residual stresses within the

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as-irradiated samples. From the integral width and peak position of the (002) peak, the

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ACCEPTED MANUSCRIPT grain size is calculated. It shows that the grain size is related to the deposited energy,

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namely EB irradiation time. With increasing EB irradiation time, the grain sizes

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decrease while the absolute peak intensities of the (002) peaks of as-irradiated

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samples increase when compared to the as-deposited samples. The grain sizes in the

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films can be estimated by Scherrer Formula, using FWHM values of the XRD

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diffraction peaks as follows [19] : D =

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λ (=1.5406Å) is the wavelength of X-ray radiation, β is the full width at half

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maximum (FWHM) [20] and θ the diffraction angle. The average gain sizes of S0,

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S1, S2, S3 and S4 are 43.62, 33.66, 31.96, 32.81 and 39.05nm, respectively. The

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quality of samples is degraded with the irradiation time increasing to 10min, then

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improved with increasing the irradiation time to 15min and degraded again with time

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further increasing. It is possibly because EB irradiation changes the energy inside the

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films, leading to the preferred (002) orientation and forming recrystallization of the

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films.

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0.9λ , where D is the grain size of crystallite, βcosθ

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Fig.1 XRD pattern of as-deposited and as-irradiated In-Sn alloy films

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In order to quantitatively investigate the influences of EB irradiation time on the

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preferred orientation of In-Sn films, their texture coefficients were analyzed by

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evaluating the texture coefficient TC(hkl) [21]. This factor can be calculated by using

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the following formula:TC (hkl) =

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, where TC(hkl) is the texture

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coefficient of (hkl) plane, I(hkl) is the measured relative intensity of a plane (hkl), I0(hkl)

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is the standard intensity of the plane (hkl) corresponding to the JCPDS (85-1409) and

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N the number of preferred growth directions. It is well known that the value of

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TC(hkl)=1 represents films with randomly oriented crystallites, while higher values

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indicate the abundance of grains oriented in a given (hkl) direction [22]. As shown in

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Fig.2, the TC value of (002) peak is the highest among the three polylines. This value

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significantly declines from 2.84 to 0.88 as the EB irradiation time increasing. By

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contrast, both TC (101) and TC (110) keep a slight change with the EB irradiation

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time increasing. The results clearly indicate that the samples irradiated with certain

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time possess the highly preferred (002) orientation, while the as-deposited one tends

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Fig.2 Texture Coefficient of In-Sn films as a function of EB irradiation time (2) Micro-structural and surface characterization

Fig.3 shows the AFM images with scanning area 8×8µm of the samples before

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and after EB irradiation. Compared with as-deposited In-Sn film (Fig.3(e)), it is clear

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that very good textured In-Sn nanospheres can be obtained by EB irradiation through

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controlling irradiation time. Before EB irradiation, the as-deposited sample is

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featureless. After EB irradiation, however, the as-irradiated samples show resemble

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surface morphology. Similar-size nanospheres can be observed in each separated film

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(Fig.3(a)-(d)). During EB irradiation, energy is transferred to In-Sn film, creating

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point defects. Then these defects will recombine with opposite sign defects or same

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sign defects. It is general that interstitial motion with high mobility determines defect

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annihilation and agglomeration which results in recrystallization [23]. The

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recrystallization can be interpreted from both XRD and AFM results. Fig.3(f) clearly

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shows that the root-mean-square (RMS) surface roughness values of samples are

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ACCEPTED MANUSCRIPT increased with increasing EB irradiation time. They are 10.81, 16.44, 18.64, 20.63 and

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21.37 nm, respectively. It is indicated that S4 films have much rugged surface

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morphology with the maximum value of 21.37 nm. The increase of RMS is attributed

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to the grain boundary between In-Sn nanospheres.

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(a)5min

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(c)15min

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(e)as-deposited In-Sn film

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Fig.3 (a)-(e) AFM images of In-Sn alloy films before and after EB irradiation and (f)

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RMS values of In-Sn film irradiated with increasing time.

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(3) Absorption constants Fig.4 shows the optical absorption spectra obtained from In-Sn thin films before

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and after EB irradiation. Before EB irradiation, an absorption peak at approx. 330 nm

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is observed for In-Sn thin film corresponding to the plasma edge of as-deposited

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In-Sn. However, after EB irradiation, the absorption intensities are changed. Its

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absorption peaks at longer wavelength (600-800nm) with a broad shoulder for 10

ACCEPTED MANUSCRIPT different irradiation time. The position, width and intensity of the LSPR peak of

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as-irradiated samples are different from that of the as-deposited samples. Obviously,

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with increasing irradiation time, the absorptions of as-irradiated films are decreased

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slowly to the bottom at irradiation time of 15 min and then rise to the top at irradiation

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time of 20 min.

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For the as-deposited sample, the free-electronic density is well distributed on the

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surface. The surface plasmons energy dissipates on In-Sn film surfaces due to internal

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absorption. It is known that the LSPR well exists in structured film [24], however,

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LSPR of the as-deposited sample is very weak because of the non-structured and

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consecutive surface. When the EB irradiation is applied to bombard the as-deposited

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In-Sn film, its flat surface is transformed into a nanospheres structure (Fig.3 (a)-(e)),

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which enables stronger LSPR in In-Sn films. Then the density of conductive electrons

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distributed on its surface are increased and electron oscillations frequency improved

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because thermal energy from EB gun is converted into kinetic energy of atoms. Thus,

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the incident light, being absorbed by In-Sn film, can cause the localized

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electronmagnetic field coupling inside the film. Plasmonic coupling takes place when

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film surface is irradiated with different energy or time, and therefore the plasmon

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resonance of In-Sn films is different. The strength of plasmonic coupling depends on

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both the surface energy and the incident light wavelength. Different irradiation time

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will cause a shift in the electric field density on the surface, resulting in a change of

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the electron oscillation frequency, thereby generating different cross-sections for its

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optical properties including absorption, as shown in Fig.4. We know that plasmons in

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another, leading to further shifts of the resonance. When the distance between NPs is

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the most situated, the coupling strength is strong. The coupling strength varies with

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different distance between NPs. As a result, the resonance of In-Sn film shows a

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red-shift. The shift is apparently due to the coalescence between In-Sn islands on the

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surface with the increase of irradiation time, which is well explained by

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Maxwell-Garnett (MG) theory [25,26].

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Fig.4 Absorption of In-Sn films with increasing e-beam irradiation time

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(4) SERS performance As a demonstration of the potential application of the tunable LSPR absorption,

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the as-deposited and as-irradiated samples were used as SERS substrates for probing

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Rhodamine B (Rh B) molecules (concentration=1×10-4 M), a typical artificial dye

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chosen as a test compound for studying the application of as-irradiated In-Sn thin film.

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Three strongest Raman peaks at about 1648, 1361 and 1504 cm-1, corresponding to 12

ACCEPTED MANUSCRIPT C=C stretching modes of aromatic rings. These peaks can be observed in the samples

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shown in Fig.5. Apparently, Raman signal intensities of as-irradiated samples (S1-S4)

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are increased gradually (S1-S4) with increasing EB irradiation time, while that of

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as-deposited ones is weak.

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Based on the electromagnetic enhancement theory, the as-deposited In-Sn thin

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film has the lowest SERS enhancement due to the non-structured and continuous film

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surface, which possessed less tips or “hot spots”. After introducing EB irradiation, its

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smooth surface becomes rougher and structured. Namely, if the surfaces of

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as-irradiated samples are rougher or well-structured, there will be more “tips” and

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“hot-spots”, and therefore much stronger local electrical fields will be excited [11].

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This would enhance Raman signal intensities of these material molecules absorbed on

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the surface of substrates, as shown in Fig.5. Meanwhile, the Raman signal

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enhancement is also in good agreement with the growth trend of RMS, as shown in

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Fig.3(f).

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Fig.5 Raman scattering spectra of Rh B on In-Sn alloy films onto a silicon substrate

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before and after EB irradiation

To further verify the conclusions above, we use Finite-difference time domain

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(FDTD), a well-known theoretical simulation, to calculate the electric field

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distribution for all the irradiated samples. In the calculation, a 470-nm laser irradiates

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perpendicularly to the

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y-axis direction. Fig.6 (a)-(e) illustrates the electric field density in In-Sn thin films

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with various degrees of surface roughness. According to Fig. 6(a), the electric field

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distribution is uniform, and the intensity is weak when the In-Sn thin film is smooth

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one. With the surface roughness increasing, the intensity of the electric field increases

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and the distribution uniformity of the free electron density shows a trend toward

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degradation. The “hot spots” are obviously formed in the surface (as shown in Fig.

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6b-e). FDTD simulation results demonstrate that the effects of surface roughness on

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x-y plane of the In-Sn film, with the polarization along the

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ACCEPTED MANUSCRIPT In-Sn thin film get more strengthened with the increase of EB irradiation time, which

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are in good agreement with the experimental results.

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e Fig.6 FDTD simulated electric field amplitude patterns for as-irradiated In-Sn thin 15

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4.Conclusions In general, we investigated the influences of EB irradiation on the optical

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properties and structure of In-Sn alloy thin films. By applying the EB irradiation, the

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preferred orientation of In-Sn film is transformed from (101) to (002) plane and its

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optical absorption is increased. The Raman scattering are also increased due to the

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variation of surface plasmon in the In-Sn thin films, which is in good agreement with

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the results of FDTD simulation.

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Acknowledgments

This work was partially supported by the National key research and development

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program of China (2016YFB1102303), the National Basic Research Program of

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China (973Program) (2015CB352001), and National Natural Science Foundation of

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China (61378060).

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electron beam, Nano Lett. 1 (6) (2001) 329-332

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[24] S.A. Maier, Plasmonics: Fundamentals and applications, Springer, 52 (2007)

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49-74

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[25] G. Xu, M. Tazawa, P. Jin, S. Nakao, Surface plasmon resonance of sputtered Ag

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films: substrate and mass thickness dependence, Appl. Phys. A 80 (2005) 1535-1540

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[26] T.C. Choy, Effective Medium Theory: Principles and Applications, Clarendon

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Press, Oxford, 1999

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Table 1 The interplanar spacing (d), FWHMs and average grain size evaluated from XRD θ-2θ scans for the samples before and after EB irradiation Interplanar spacing d (nm)

FWHM (°)

Average grain Size (nm)

S0

0.2455

0.215

43.62

S1 S2 S3 S4

0.2475 0.2474 0.2480 0.2482

0.314 0.328 0.331 0.283

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Sample

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33.66 31.96 32.81 39.05

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Highlights: 1. We show for the first time that the properties of In-Sn alloy thin films can be influenced by Electron-beam irradiation. 2. In this paper, it is demonstrated that as-irradiated In-Sn films have

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good performance on surface-enhanced Raman scattering (SERS). 3. FDTD simulation results are in good agreement with the SERS

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performance.