- Email: [email protected]
Structural, morphological and optical properties of Ag–AgO thin films with the effect of increasing film thickness and annealing temperature
Optical Materials 48 (2015) 121–132
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Structural, morphological and optical properties of Ag–AgO thin films with the effect of increasing film thickness and annealing temperature Anil Kumar Pal, D. Bharathi Mohan ⇑ Department of Physics, School of Physical, Chemical and Applied Sciences, Pondicherry University, R.V. Nagar, Kalapet, Puducherry 605014, India
a r t i c l e
i n f o
Article history: Received 23 May 2015 Received in revised form 12 July 2015 Accepted 20 July 2015
Keywords: Ultra thin to thick Ag films Thermal evaporation Morphological properties Crystal structure Raman mapping Surface plasmon resonance Dielectric constant
a b s t r a c t Ag films of thickness ranging from 5 to 60 nm were deposited by thermal evaporation technique followed by air annealing process with temperature varying from 50 to 250 °C. Morphological properties such as particle size, shape, surface roughness and number particles density were studied by atomic force microscope (AFM). The structural transition from quasi-amorphous to nanocrystalline to crystalline upon increasing film thickness and annealing temperature were studied. Ag films with smallest particle size and surface roughness were achieved up to film thickness of 7 nm. The possibility of surface oxidation of Ag on both as deposited and annealed films was studied through Raman mapping by using confocal Raman spectroscopy. Ag film was X-ray amorphous even after annealing process up to the film thickness of 7 nm and above which the crystallinity reached maximum at 250 °C. The surface plasmon resonance (SPR) with a symmetric line shape due to dipole–dipole interactions was found to be very strong for film thickness of 5 nm at 100 °C, attributed to the formation of smaller Ag NPs size of 22 nm with least size distribution and higher particles number density of 1625 lm2 in a self-organized fashion. With an increase of film thickness and annealing temperature, an asymmetric broad absorption arose due to increase in damping of collective electron oscillation on bulky NPs. Theoretical absorption spectra were simulated using extended Maxwell garnet method showing a decent agreement with experimental data. The real and imaginary parts of dielectric constants were determined and plotted for different film thicknesses of as deposited Ag films. Even though the film is oxidized at the surface level, it still can be used for plasmonic sensor applications however the film thickness should be approximately 7 nm for the enhanced result. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Silver plasmonic nanostructures have fascinated a subject of extensive research due to its unique optical property of strong resonant electromagnetic interaction with light which enables us to tune the surface Plasmon resonance (SPR) in wide optical band spectrum from 300 to 1000 nm [1,2]. Ag films hold a wide range of applications such as chemical sensors [3], biological sensors [4], surface enhanced Raman spectroscopy (SERS) [5], surface enhanced fluorescence spectroscopy (SEFS) [6] and photovoltaics [7]. Hence, it is very important to reinforce and tune the resonance in Ag nanostructure. The resonance frequency and the intensity of SPR mainly depends on several factors of which some of them following, (i) nature of the chemical element; (ii) size and shape of NPs; (iii) inter-nanoparticles distance; and (iv) surrounding ⇑ Corresponding author. E-mail address: [email protected] (D. Bharathi Mohan). http://dx.doi.org/10.1016/j.optmat.2015.07.029 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.
medium [8–10]. Ag possess the largest quality factor 10, negative real and imaginary parts of the dielectric functions across most of the spectrum when compared to any other noble metals such as Au, Cu, Al, Li, Pd and Pt [1]. Apart from the above facts, Ag is relatively cheaper than Au but has suffered from a tarnished reputation because it is more prone to oxidization in environment. Ag NPs oxidizes rapidly even at ambient condition because of the large surface to volume ratio however it is only at the surface level limiting to only a few nanometers [11]. Ag NPs completely oxidizes at higher temperature above 350 °C [12]. Recently, Yang et al. [13] demonstrated that the bimetal Ag–Au NPs far more stable than the pure Ag NPs against oxidation. Ag NPs are prepared by various wet chemistry methods [14,15], vacuum deposition techniques (VDT) which includes thermal evaporation [16], sputtering [17] and pulsed laser deposition [18] etc. Amongst, VDT is advantageous over wet chemistry routes because it can directly be used for the above mentioned applications before going for any surplus process. However, it oxidizes rapidly as soon as exposed to atmosphere
122
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
[11,19,20]. It is a great challenge to deposit un-oxidized Ag NPs with the optimized size and shape which exhibits strong SPR near UV–Vis region. Thin film of self-assembled Ag NPs with particle size 20 nm was possible to fabricate by using VDT when it was deposited in ultra-thin (film thickness from 1 to 10 nm) range otherwise bigger Ag NPs forms due to coalescence process. Closely spaced Ag NPs result in larger enhancement of electromagnetic coupling between NPs due to strong confinement of local electric field [5]. The SPR wavelength was observed in blue region for smaller size of NPs to that of bigger NPs due to an increase of surface energy resulting from an increased surface to volume ratio [21]. The tenability of SPR frequency can be achieved by varying film thickness and post deposition annealing temperature because these two parameters could alter NPs size, shape, number particles density and inter NPs distance. The formation of AgO at the surface level due to air annealing process would not be really a big concern for SPR. However, it leads to a red shift due to an increase of dielectric constant of the medium in which NPs are present [12,20]. AgO is wide bad semiconducting material having potential applications such as optical memories [22], filters [23] and plasmon photonic devices [24]. In this paper, Ag thin films of thickness ranging from 5 to 60 nm were prepared by using thermal evaporation technique followed by air annealing process with temperature varying from 50 to 250 °C. Film thickness and morphological properties involving particle size, shape, number density and surface roughness were studied by using Atomic Force Microscopy (AFM). The effect of air annealing towards oxidation of Ag films was studied by confocal Raman spectrometer. The effect of surface oxidation and its corresponding morphological changes were studied through Raman mapping and AFM. The crystal structure and optical absorption as a function of film thickness and annealing temperature were also studied. Theoretical simulation of absorption spectra of as-deposited Ag is presented by using extended Maxwell garnet method using AFM data’s. The novelty of this work was to study the behavior of SPR in a mixed phase of Ag–AgO films in a wide range of film thickness upon air annealing process.
2. Experimental details For the deposition of Ag thin films, Ag metal powder (particle size varying from 2 to 3.5 lm, purity: 99.9%) was purchased from Sigma Aldrich, USA. Prior to the deposition, borosilicate glass slides (Riviera, 100 300 ) were cleaned thoroughly by using chemicals such as soap solution (Fisher Scientific, USA), nitric acid (HNO3) (Fisher Scientific, USA) and Acetone (Himedia, India) by following the similar procedure mentioned in AKP et al. work [25]. Ag films of mass thickness ranging from 1 to 50 nm (according to digital thickness monitor (DTM)) were deposited on cleaned glass substrates by thermal evaporation technique at the base pressure of 1.5 106 mbar. The DC current supply was kept constant at 70 A throughout the deposition process. In order to increase film thickness, the deposition time was increased from 2 min to 38 min by keeping deposition rate as constant at 0.1 Å/sec. Thickness of Ag films was measured using quartz crystal thickness oscillator – an inbuilt sensor available within the thermal evaporation unit. The as-deposited (ASD) Ag films were then immediately air annealed in hot air oven at temperatures 50, 100, 150, 200 and 250 °C about an hour. The crystal structure, surface morphology, optical absorption and surface oxidation of as deposited and annealed silver films were studied by using characterization techniques such as Glancing Angle (1.5°) X-ray diffraction (GAXRD) (PANalytical-X’Pert PRO), Atomic Force Microscope (AFM) (Nanoscope – tapping mode), UV–Vis Spectrophotometer (Ocean
Optics, HR 4000) and confocal Raman spectrometer (RENISHA WinVia Raman Microscope, UK) respectively.
3. Results and discussions 3.1. Atomic force microscopy: Film thickness and morphological properties The actual thicknesses of Ag films were measured through AFM scanning over the edge of a scratch made on Ag films by a sharp blade. Figs. 1 and 2 exhibit 3D AFM images and their corresponding surface profile data’s of ASD and annealed Ag films. The morphology of Ag films obtained by AFM suggests that the Ag NPs are formed with a discontinuous film of few numbers of mono layers for ultrathin film thickness up to 7 nm and several mono layers for thicker films by island growth process so called Vollmer– Weber (VW) growth process. The occurrence of VM process is favored when the contact angle between the deposited material and the substrate is higher than zero; means the elastic strain of the film exceeds the adhesion force due to strong adatom cohesive force [26]. Here, the formation of Ag NPs occurs through nucleation sites. Once a sufficient number of Ag atoms reached the substrate, then the nucleation starts and grows into a layer of nanoparticles. DTM analysis is an in situ process and measures relative thickness referring continuous layer formation on its sensor known as quartz crystal microbalance. One of the major disadvantages of this technique was making the tooling factor very precisely. In thermal evaporator, the sensor and the substrate usually cannot be placed in the same direction from the deposition source and may not even be at the same distance from it. Therefore, the rate at which the material was deposited on the sensor was not equal to the rate at which it was deposited on glass substrate. The ratio of the two rates sometimes called the ‘‘tooling factor’’. In this experimental work, both the sensor and the substrate were almost placed at the same height however the direction was not maintained same for both of them from the deposition source. However the film formed on glass substrate follows VM growth process and therefore the thickness differs from the value measured by DTM. Due to the formation of Ag islands, voids are created on the substrate which makes the film porous with the height of NPs greater than the film thickness measured by DTM. The height of the islands is considered to be the actual film thickness measured by AFM. A comparative analysis is shown in Table 1 based on film thickness monitored from digital thickness monitor (DTM) and determined from surface profile data analysis. The % of error in thickness measurement is calculated based on the relation [(AFM–DTM)/AFM] % as resulting from AFM image and DTM thickness estimation. It is found that the % of error is decreasing systematically with increasing film thickness measured by DTM due to decrease in porosity of the films with increase in film thickness which occurs as a result of increasing surface coverage. The height of nanoparticles increases with increasing annealing temperature at 250 °C could be due to crystallization of Ag and the new phase formation of AgO structure. The particle size increases with increasing film thickness and annealing temperature. Hereafter, the entire discussion will proceed based on the film thickness measured by AFM. The fabrication of fully crystalline silver structure could not be possible from the deposition of silver film on glass substrate held at ambient temperature. In thermal evaporation, the mobility of the atoms arises due to local temperature which is less enough for the crystallization in ultra-thin films. As the film thickness increases, the mobility of silver atoms increases due to increase of local thermal energy and partially crystallizes the silver atoms near the surface leading to form highly disordered Ag phase [27].
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
123
Fig. 1. 3D AFM images and the corresponding line profile of Ag films with mass thickness of 1–10 nm (according to DTM). The scan was performed across the scratch made on the film in order to measure the film thickness. (Note: ASD means as deposited film and 250 denotes the annealing temperature).
Fig. 2. 3D AFM images and the corresponding line profile of Ag films of mass thickness from 15 to 50 nm (according to DTM). The scan was performed across the scratch made on the film in order to measure the film thickness. (Note: ASD means as deposited film and 250 denotes the annealing temperature).
124
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Table 1 Thickness determined from AFM images and its comparison with DTM. Sample ID
Thickness monitored by DTM (nm)
Ag1 Ag2 Ag5 Ag10 Ag15 Ag20 Ag25 Ag50
1 2 5 10 15 20 25 50
Thickness determined from AFM image (nm) ASD
250 °C
5±1 7±1 10 ± 1 18 ± 2 25 ± 2 31 ± 5 39 ± 7 60 ± 7
6±2 10 ± 3 17 ± 7 28 ± 6 35 ± 14 54 ± 12 80 ± 20 106 ± 25
% of error with respect to DTM concerning only ASD
80 71 50 44 40 35 35 16
In order to achieve complete crystalline phase, it is mandatory to anneal ASD film at higher temperature at least above 100 °C. The transition from disordered to ordered phase is essentially driven by external thermal energy. During the course of time, Ag films much more oxidized at the surface level which will be discussed in Raman study. As deposited (ASD) Ag film forms a discontinuous film containing non-spherical particles with large surface coverage. The
transition from quasi-amorphous to bulk crystalline is studied on Ag films when annealed at above 100 °C [25], where the discontinuous films transform into regular sized and shaped particle with decreased surface coverage. The evolution of surface morphology in Ag films with thickness ranging from 5 nm (Ag1) to 60 nm (Ag50) for ASD and annealed films (at 100 and 250 °C) were studied by AFM (Figs. 3 and 4). It is observed that, an irregular enlargement of particles occurs with an increase of film thickness due to coalescence and forms clusters. The particles are formed in oblate spheroid like shape with the length along x, y direction which is greater than the height in z-direction (c < a = b). The size of a nanoparticle is taken as the diameter of a circle along x–y direction which is higher than the height (along z-direction) of the particle. The number density of NPs was calculated for all films and found to be maximum for film thickness of 5 nm annealed at 100 °C. The higher homogeneity of Ag NPs both in size and shape was accomplished up to film thickness of 18 nm. Moreover, agglomeration was not conspicuous until film with thickness of 18 nm even with air annealing at 250 °C except the change of particle size while it was realized from thickness of 25–60 nm. A very heterogeneous size and shape distribution is observed with higher film thickness above 18 nm suggesting that these films have not achieved the minimum crystallization temperature. It is also extremely difficult
Fig. 3. 2D AFM images of Ag films with thickness of 5 (Ag1), 7 (Ag2), 10 (Ag5) and 18 (Ag10) nm recorded over 1 1 lm scan area. The insert shows the corresponding roughness profile. (ASD – as deposited film; 100 and 250 denotes annealing temperature).
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
125
Fig. 4. 2D AFM images of Ag films with thickness of 25 (Ag15), 31(Ag20), 39 (Ag25) and 60 (Ag50) nm recorded over 1 1 lm scan area. The insert shows the corresponding roughness profile. (ASD – as deposited film; 100 and 250 denotes annealing temperature).
to accomplish a monolayer with uniform size and shape distributions above the film thickness of 18 nm. The particles are bigger for thickness above 18 nm when annealed at 250 °C. The particle size, RMS surface roughness and number density per one lm2 values are calculated from particle size distribution and roughness profile are shown in Fig. 5. The particle size increases from 19 to 130 nm for ASD films, 22–256 nm and 35–474 nm for the films annealed at 100 and 250 °C respectively with increasing film thickness. There is no much variation in particle size up to 18 nm film thickness even after annealing process whereas it increases abruptly and faster with increasing film thickness and annealing temperature. The RMS surface roughness of Ag films increases from 1.44 to 6.98 nm for ASD films, 1.88–14.2 nm at 100 °C and 2.25–29.7 nm at 250 °C. The RMS roughness value increases with increasing film thickness and annealing temperature due to increase of particle size and agglomeration [12,25]. With increasing annealing temperature, surface properties such as number density, size, shape, inter-particle distance and surface coverage area were altered through coalescence followed by Ostwald ripening processes [25]. For the film thicknesses up to 18 nm, the particles are distributed disjointedly even upon annealing. However beyond 18 nm, particles grow bigger and coalesce together due to which the particle number density was not
calculated. Fig. 5(b) represents the variation of particle number density with respect to film thickness up to 18 nm. It is observed that the number density decreases with increase in film thickness and annealing temperature due to increase in particle size. But the same fashion does not happen for Ag film with thickness of 5 nm. From AFM, ASD Ag film with thickness of 5 nm shows hazy like morphology due to quasi-amorphous phase nature lead to less particle number density of 1050 lm2 as compared to the film annealed at 100 °C which shows highest number density of 1625 lm2. 3.2. X-ray diffraction: Crystal structure Fig. 6 shows an amorphous phase up to the film thickness of 7 nm (Ag1) indicating the supremacy of short range order of silver atoms in both ASD and annealed Ag films [28]. With increasing film thickness to 15 nm, the crystal planes, (1 1 1), (2 2 0) and (3 1 1) arise with a small intensity at angles 38.18, 64.43 and 77.49 (2h in degree) respectively. The intensities of all crystal planes increases with increasing film thickness to 60 nm along with a new plane (2 0 0) emerge at 44.33 (2h) suggesting the fact that the number density of silver atoms deposited on glass substrate was increased. When silver film is annealed at 250 °C, the peak
126
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Fig. 5. Variation of (a) particle size, (b) number density per one lm2 area and (c) surface roughness of Ag films as a function of film thickness. (ASD – as deposited film, 100 and 250 denotes annealing temperature).
Fig. 6. GA-XRD of silver films as-deposited and air annealed at 250 °C with film thickness of 7 (Ag2), 15 (Ag10), 31 (Ag20) and 60 (Ag50) nm.
intensity of crystal planes further enhanced in case of 18 and 31 nm film thicknesses which is attributed to increase in crystallinity of face centered cubic phase of Ag. The lattice parameter (a = 4.085 Å) is calculated from XRD pattern after comparing with JCPDS card number 04-0783 for both as deposited and annealed films, which is exactly matching with the standard value (a = 4.085 Å) corroborating the formation of fcc phase of Ag. It is indispensable to mark statement here that no peak from AgO phase is observed, suggesting the formation of minor AgO phase typically at the surface level with only few nm, may be XRD is not the accurate technique to probe such level of phase formation. 3.3. C. Raman spectra and mapping of silver oxide (AgO) Noble metals are chemically inert under ambient conditions which mean they are relatively stable and inactive towards oxidation. However, it is not true at nanoscale because of an increase of surface to volume ratio and thus oxidizes immediately [11].The surface oxidation depends mainly on oxidation rate which varies at different temperature. Annealing below 250 °C controls oxidation only to the depth level of a few nm on the surface and hence still favoring SPR of Ag which in accordance with optical absorption results. The bulk silver oxide (AgO) exhibits Raman modes at 219, 300, 375, 429, 468 and 487 cm1 and silver (I) oxide (Ag2O) exhibits a single Raman mode at 490 cm1 [26]. Thermally evaporated Ag films with thickness of 250 nm annealed at above 350 °C
showed Raman modes at 230 and 239 cm1 [12]. According to our knowledge, there is no Raman data published in thermally evaporated Ag films with less than 60 nm film thickness so far. Figs. 7a and 7b represent Raman spectra of Ag films with thickness of 5–60 nm for as deposited, 100 and 200 °C annealed films. All peaks correspond to AgO phase formed when the films are exposed to atmosphere and annealed at higher temperature. As deposited Ag films with thicknesses of 5 and 10 nm exhibit Raman modes at 226 cm1 and then at 230 cm1 for films with thickness of 18, 31 and 60 nm confirming the formation of AgO phase. Later, peak shifts to 234 cm1 when annealed at above 100 °C. Raman mode shifts towards higher wave number at 234 cm1 because of an increase of film thickness and the oxidation rate [12]. Raman modes observed at 230 and 234 cm1 correspond to the stretching modes of Ag and O [29]. XRD confirms the formation of Ag structure while Raman study proves the oxidation of Ag films. The oxidation of Ag film is limited to the surface level with the depth of about only few nanometers otherwise UV– Visible spectra would not exhibit SPR. The role of film thickness and annealing temperature on surface oxidation is discussed through Raman mapping on as-deposited and annealed (at 250 °C) Ag films with thickness of 10 and 60 nm. The Raman mapping recorded over an area of 5 5 lm with a step size of 500 nm by exciting at 488 nm Ar+ laser with an acquisition time of 30 s and power of 10 mW. As observed from Raman spectra, Ag5-ASD and Ag50-ASD films show AgO peak at
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
127
Fig. 7a. Raman spectra of Ag films with thickness of 5 (Ag1) and 10 (Ag5) nm for as deposited (ASD) and annealed (at 100 and 200 °C) films.
Fig. 7b. Raman spectra of Ag films with thickness of 18 (Ag10), 31 (Ag20) and 60 (Ag50) nm for as deposited (ASD) and annealed (at 100 and 250 °C) films.
226 cm1 whereas Ag5–250 and Ag50–250 show AgO peak at 234 cm1. Hence Raman intensity from above respective peaks showing the formation of AgO is considered for Raman mapping. Fig. 8 shows the Raman mapping of Ag films. The thickness and annealing temperature of Ag films has a significant role on the oxidation level of Ag films as proven by their large Raman intensity variation. The peak intensity value varies in the range of 210–390, 4840–7680, 11,750–20,800 and 16,200–42,000 arbitrary units for Ag5-ASD, Ag5–250, Ag50-ASD and Ag50–250 respectively. In other words, the higher value of peak intensity of Ag5-ASD, Ag5–250 and Ag50-ASD films are of 0.92, 18.28 and 49.52 percentage with respect to Ag50–250 film. This large variation in Raman intensity reveals that the surface oxidation of Ag films increases with increasing the film thickness as well as annealing temperature. 3.4. Optical absorption: The effect of annealing temperature and film thickness on tuning surface plasmon resonance Fig. 9 demonstrates the optical absorbance spectra of Ag films for different film thicknesses of 5, 7, 10 and 18 nm as a function of annealing temperature from 50 to 250 °C including as deposited film. The line shape, width and kmax of SPR depend on their morphological property which includes size, shape, distribution, inter-particles distance, particles number density and also the surrounding dielectric matrix [27,30,31]. It is clearly noticed that the as-deposited Ag film with thickness of 5 nm exhibits strong SPR at 402 nm with a symmetric line shape (Gaussian shape) corresponding to dipolar resonance of Ag NPs and then it slowly deviates from symmetric line shape with a red shift (10 nm) as the film thickness increases to 7 nm. Ag clusters with size of several
tenth of nm and large filling factor show optical absorption spectra characterized by typical redshift and broadening of plasmon resonance due to retardation effects and multipole modes of electronic oscillations [32]. Fig. 10 shows that above the film thickness of 25 nm, SPR increases initially up to 450 nm and then become constant up to 800 nm for which the dipolar resonance is absent due to increase in particle size, the deformation from spheroid shape of particles and increase in surface roughness. Moreover, there is a shoulder peak observed at 311 nm (near UV-range) for film thickness of 5 nm and appears to be less prominent as compared to the dipolar resonance which does not shift much with increasing annealing temperature is attributed to inter band edge transition [32]. The narrowing of SPR spectra is attributed to reduced radiation damping of collective electron oscillation from smaller NPs of size in the range of 20–30 nm [9,33]. The broadening and red shift of SPR spectra occurs as a result of increase in light scattering due to increase in particle volume as the film thickness increases. The radiative damping of collective electron oscillation increases with increase in particle volume which is connected to light scattering by particles. The change in the interaction between the particles and dielectric matrix can also contribute to line width of absorption spectra [34]. There is no much shift observed up to film thickness of 7 nm except a small change in the line shape of the spectra pertaining to different size distribution of Ag NPs. When the film thickness increases, the particle size increases and the charge separation on the nanoparticles also increases, leading to lower the frequency of collective oscillation of electrons as indicated by a red shift from 400 to 500 nm approximately [35]. The annealing process shows considerable change in SPR in the temperature region from 50 to 250 °C. The SPR peak observed at
128
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Fig. 8. Raman mapping images of Ag films with thickness of 10 (Ag5) and 60 (Ag50) nm for both ASD and annealed (at 250 °C) films.
402 nm for ASD film with the thickness of 5 nm becomes stronger at 100 °C due to the formation of nanocrystalline Ag for which the number particles density increases [36]. Subsequently, the intensity falls at 150 °C with a red shift of 10 nm could be due to an increase of particle size and the formation of AgO phase [20,25]. Finally, the SPR blue shifts above 150 °C. Such trend is observed up to the film thickness of 18 nm. The inter-particle distance and the contact area between Ag NPs and glass substrate have pronounced effect on SPR peak shift. The increase in surface coverage area upon increasing film thickness decreases the inter-particle gap resulting a broad SPR with red shift due to dipole–dipole interaction [9]. The increase in contact area increases the back word scattering by Ag NPs. The increase in particle size could be the reason for the initial red shift up to annealing temperature of 150 °C because there was no much change in surface coverage except transformation to crystalline form. The further annealing process decreases the surface coverage as a consequence of increased vertical height with a small increase in the average lateral size of particles leading to a blue shift of SPR. At higher annealing temperature, additional redshift and broadening observed because the clusters accentuate the non-spherical shape, with progressive modification of the geometrical factor [37]. 3.4.1. Theoretical simulation For Ag NP’s layer, the position of SPR is determined by its size, shape, material and dielectric constant of surrounding medium [30,31,38]. For the purpose of illustration, the theoretical spectra of absorption of Ag films are calculated based on thickness, particle shape, size and inter particle distance obtained from AFM analysis with the help of standard thin film formula [39]. The optical properties of Ag/glass multilayer system is described by effective complex dielectric constant (E eff ) as defined in generalized Extended Maxwell Garnett (EMG) theory by using the equation [37].
E eff ¼ E ext
FE Ag þ E ext ð1 FÞ þ qð1 FÞðE Ag E ext Þ FE Ag þ E ext ð1 FÞ qFðE Ag E ext Þ
ð1Þ
where E Ag and E ext are dielectric constants of Ag and the external medium surrounding the NPs respectively, q is the volume filling factor of Ag film which determines how much volume of the film is really covered by the particles and F is the geometrical factor of 2
NPs. The volume filling factor q is defined as q ¼ V=l h, where V is the volume of particle, h is the height of the particle and l is the average distance between NPs. The wavelength-dependent bulk dielectric constants for silver have been used from Babar et al. in all calculations [40]. In this model, Ag films are considered to be single layer of Ag NPs with the same shape, size and orientation for each film. The Ag NPs are considered as oblate spheroid (c < a = b) where a, b and c are the half axes of the spheroid. And it is obvious that the thickness of Ag film is equivalent to the height of the NPs. The geometrical factor of Ag NPs of oblate shape is defined as [37],
F¼
1 þ 2e2 ðe arctaneÞ e3
ð2Þ
where e is the eccentricity of the ellipsoid. For the matching of SPR spectra with the theoretical model, the value of q and F are varied through varying the Ag particle size and inter-particle distance. The complex dielectric constant of Ag is used from the empirical result of Baber et al. [40]. In the presence of surrounding medium such as air and glass substrate, the effective refractive index (neff ) and extinction coefficient (keff ) of Ag NPs can be obtained from MG theory and given by [37,41],
neff ¼
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi uqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 t E effreal þ E effreal þ E effreal 2
ð3Þ
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
129
Fig. 9. Optical absorption spectra of Ag films with thickness of 5 (Ag1), 7 (Ag2), 10 (Ag5) and 18 (Ag10) nm with increasing annealing temperature ranging from 50 to 250 °C. SPR is very significant in case of 5 nm while it becomes weaker with increasing film thickness.
keff ¼
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi uqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 t E effreal þ E effreal E effreal 2
A ¼ log ð4Þ
The transmission spectra of a single Ag layer on glass substrate can be calculated with the standard thin film formula as follows [42],
T¼
jt 123 j2 jt 31 j2 eð2ImðdÞÞ 1 jr321 j2 jr 31 j2 eð4ImðdÞÞ
ð5Þ
where the transmission t 123 , t 31 and the reflection, r321 , r 31 are typical Fresnel coefficients at normal incidence for the three indexed mediums such as air, Ag film and glass substrate subscripted by 1, 2 and 3 respectively. The phase change, d experienced in a medium, i at normal incidence is presented by,
d¼
2pni di k
ð6Þ
where ni and di are the refractive index and thickness of the corresponding ith layer respectively. Finally the absorbance A is calculated by using the standard relation given by
1 T
ð7Þ
Fig. 11 compares the result of measured and calculated optical absorbance of as-deposited Ag films which shows good agreement of calculated SPR spectra with experimental data over entire region of absorption spectra. However, a slight mismatch was observed for some Ag films could be due to the effect of surface oxidation which was not taken into the consideration for simulation. The surface oxidation initially low for as-deposited Ag films which then increased for annealed films as described in Raman analysis. The complex refractive index and the dielectric function describes the optical properties of any solid material. The real part of dielectric constant (er) is related to polarization and anomalous dispersion of Ag NPs, while the imaginary part (ei) is associated with the dissipation of energy into the surrounding medium. The dielectric dispersion is a significant factor in optical communication and in designing devices for spectral dispersion [41]. The changes in er directs the change in polarization of Ag NPs and similarly, change in ei controls the dissipation of energy absorbed by plasmons which result in shifting of SPR peak. The effective complex dielectric constant of Ag thin films laying on glass substrates
130
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Fig. 10. Optical absorption spectra of Ag films with thickness of 25 (Ag15), 31 (Ag20), 39 (Ag25) and 60 (Ag50) nm with increasing annealing temperature raging from 50 to 250 °C.
Fig. 11. Measured and simulated optical absorption spectra of as-deposited Ag films of various film thicknesses (a): 5 (Ag1), 7 (Ag2), 10 (Ag5), 18 (Ag10) nm and (b): 25 (Ag15), 31 (Ag20), 39 (Ag25), 60 (Ag50) nm.
are calculated based on Eq. (1). The variation of er and ei of as-deposited Ag films of different thickness from 5 to 60 nm with respect to wavelength is shown in Fig. 12. The complex dielectric constants of Ag films are compared with the bulk Ag as determined
by Babar et al. [40]. It can be noticed that the change in er and ei of Ag films occur in a linear fashion with change in film thickness. As shown in Fig. 12(a), er value of bulk Ag is decreasing linearly from 1.09 to 31.41 along the wavelength region from 300 to
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
131
Fig. 12. Calculated (a) Real and (b) imaginary part of dielectric constants of as-deposited Ag films of various film thicknesses such as 5 (Ag1), 7 (Ag2), 10 (Ag5), 18 (Ag10), 25 (Ag15), 31 (Ag20), 39 (Ag25) and 60 (Ag50) nm. The presented real and imaginary curves of dielectric constant of bulk Ag film drawn from a published article by Babr et al. [40].
800 nm respectively [40]. er observed in the negative scale for Ag thin films which is still higher than the value of bulk Ag. Ag film of 5 nm thickness (Ag1) shows higher value with close to zero and varies from 0.42 to 2.09 in the wavelength range of 300– 800 nm. With increase in Ag film thickness from 5 to 60 nm, er decreases from 2.096 to 10.94 at absorption wavelength of 800 nm. On the other hand, ei of Ag films observed in the positive scale close to zero and then increases with increase in film thickness (Fig. 12(b)), however lies far below as compared to bulk Ag film. The surrounding medium such as glass and air have approximately constant dielectric values of 1.5 and 1 respectively in the wave length range of 300–800 nm. As-deposited Ag constituting of small nanoparticles in the range of 19–130 nm in the stack of air and glass substrates of higher dielectric constant value and showed lower dielectric constant value as compared to bulk Ag. The dielectric constant is deeply dependent on the surface scattering by the metal nanoparticles [43,44,41]. For small size nanoparticles, the absorption and scattering bands are strong and coincident, and the extinction spectrum more strongly contributed by absorption than the scattering [1]. When the particle size increases beyond 20 nm, the scattering becomes predominant over absorption [45]. Hence the scattering increases for Ag films of thickness ranging from 10 to 60 nm due to an increase in average particle size above 20 nm. This increase in scattering results in decrease of dielectric constant of Ag films beyond the film thickness of 7 nm which goes far below than zero value. The condition for a plasmonic material to support strong SPR is when the value of dielectric constant is close to zero [1]. Hence the lower value of effective dielectric constant (er and ei) observed in case of Ag films of thickness 5 and 7 nm showed a strong SPR as conformed from Fig. 9. 4. Conclusions Ag films of various thicknesses of 5, 7, 10, 18, 25, 31, 39 and 60 nm were deposited by thermal evaporation method followed by air annealing process at temperatures 50, 100, 150, 200 and 250 °C. The film thickness analysis showed higher experimental error (in %) as the thickness goes down to ultra-thin measured by quartz crystal thickness monitor. Unagglomerated and spheroid shaped of Ag NPs were formed up to film thickness of 18 nm
beyond which non-spherical and bigger particles were formed due to coalescence and Ostwald ripening processes with the effect of post deposition air annealing process. Ag NPs size of 19 nm with a trivial surface oxidation and a least surface roughness of 1.28 nm were accomplished on 5 nm thick Ag film. The presence of stretching mode at 234 cm1 (stretching) in Raman spectra and hkl planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1) in XRD confirmed the formation of mixed phase of Ag–AgO in Ag films. Raman mapping showed the level of oxidation in Ag films which increases with increasing film thickness and annealing temperature. The SPR due to dipole– dipole interactions was very strong for film thickness of 5 nm annealed at 100 °C due to smaller Ag NPs size of 22 nm with least size distribution and highest particles number density 1625 lm2. With increasing film thickness and annealing temperature, a broad asymmetric absorption was observed due to bulky size of NPs leading to increase in damping of collective electron oscillation. Teoretically simulated absorption curves using extended Maxwell garnet method are matching very well with their experimental data’s. The real and imaginary parts of dielectric constants was determined for as deposited Ag films increases with increasing film thickness. The least value of dielectric constants was observed for Ag film with thickness of 5 nm bearing a very strong SPR. It is understood from absorption results that the film thickness approximately 7 nm could be the critical limit to obtain a strong SPR. The tuning of SPR in ultrathin Ag films was realized despite of the formation of AgO phase at surface level. Acknowledgement The work is supported by UGC Major Project (F. No. 4151868/2012(Sr)). The author AKP gratefully acknowledges UGC, India for the research fellowship. Authors thank Hima Annapoorna for contribution towards material preparation. Authors also thank Dr. R. Venkatesan and Mr. Rajesh, Department of Chemistry for using UV–Visible spectrophotometer. Central Instrumentation Facility of Pondicherry University provided the facilities for characterization studies. References [1] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Controlling the synthesis and assembly of silver nanostructures for plasmonic applications, Chem. Rev. 111 (2011) 3669–3712.
132
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
[2] L.M. Liz-Marzán, Tailoring surface plasmons through the morphology and assembly of metal nanoparticles, Langmuir 22 (2005) 32–41. [3] H. Zhou, D. Yang, N.P. Ivleva, N.E. Mircescu, R. Niessner, C. Haisch, SERS detection of bacteria in water by in situ coating with Ag nanoparticles, Anal. Chem. 86 (2014) 1525–1533. [4] R. Gradess, R. Abargues, A. Habbou, J. Canet-Ferrer, E. Pedrueza, A. Russell, J.L. Valdes, J.P. Martinez-Pastor, Localized surface plasmon resonance sensor based on Ag-PVA nanocomposite thin films, J. Mater. Chem. 19 (2009) 9233–9240. [5] C. Wang, W. Ruan, N. Ji, W. Ji, S. Lv, C. Zhao, B. Zhao, Preparation of nanoscale Ag semishell array with tunable interparticle distance and its application in surface-enhanced Raman scattering, J. Phys. Chem. C 114 (2010) 2886–2890. [6] M.-G. Kang, T. Xu, H.J. Park, X. Luo, L.J. Guo, Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrodes, Adv. Mater. 22 (2010) 4378–4383. [7] A. Sánchez-González, S. Corni, B. Mennucci, Surface-enhanced fluorescence within a metal nanoparticle array: the role of solvent and plasmon couplings, J. Phys. Chem. C 115 (2011) 5450–5460. [8] L.A. Lyon, D.J. Peña, M.J. Natan, Surface plasmon resonance of au colloidmodified au films: particle size dependence, J. Phys. Chem. B 103 (1999) 5826– 5831. [9] E. Thouti, N. Chander, V. Dutta, V.K. Komarala, Optical properties of Ag nanoparticle layers deposited on silicon substrates, J. Opt. 15 (2013) 035005. [10] A. Tao, P. Sinsermsuksakul, P. Yang, Tunable plasmonic lattices of silver nanocrystals, Nat. Nanotechnol. 2 (2007) 435–440. [11] W. Cai, H. Zhong, L. Zhang, Optical measurements of oxidation behavior of silver nanometer particle within pores of silica host, J. Appl. Phys. 83 (1998) 1705–1710. [12] J. Lv, F. Lai, L. Lin, Y. Lin, Z. Huang, R. Chen, Thermal stability of Ag films in air prepared by thermal evaporation, Appl. Sur. Sci. 253 (2007) 7036–7040. [13] H. Yang, Y. Wang, H. Huang, L. Gell, L. Lehtovaara, S. Malola, H. Häkkinen, N. Zheng, All-thiol-stabilized Ag44 and Au12Ag32 nanoparticles with singlecrystal structures, Nat. Commun. 4 (2013). [14] Q.H. Tran, V.Q. Nguyen, A.-T. Le, Silver nanoparticles: synthesis, properties, toxicology, applications and perspectives, Adv. Nat. Sci. Nanosci. Nanotechnol. 4 (2013) 033001. [15] S.E. Skrabalak, L. Au, X. Li, Y. Xia, Facile synthesis of Ag nanocubes and Au nanocages, Nat. Protoc. 2 (2007) 2182–2190. [16] D. Gaspar, A.C. Pimentel, T. Mateus, J.P. Leitão, J. Soares, B.P. Falcão, A. Araújo, A. Vicente, S.A. Filonovich, H. Águas, R. Martins, I. Ferreira, Influence of the layer thickness in plasmonic gold nanoparticles produced by thermal evaporation, Sci. Rep. 3 (2013) 1469. [17] S. Kundu, S. Hazra, S. Banerjee, M.K. Sanyal, S.K. Mandal, S. Chaudhuri, A.K. Pal, Morphology of thin silver film grown by dc sputtering on Si(0 0 1), J. Phys. D Appl. Phys. 31 (1998) L73. [18] K. Koppole, K.C. Sekhar, A. Almeida, J.A. Moreira, M.J.M. Gomes, Tuning the surface plasmon resonance and surface-enhanced Raman scattering of pulsed laser deposited silver nanoparticle films by ambience and deposition temperature, J. Opt. 16 (2014) 055002. [19] C.I. Carlisle, T. Fujimoto, W.S. Sim, D.A. King, Atomic imaging of the transition between oxygen chemisorption and oxide film growth on Ag {1 1 1}, Sur. Sci. 470 (2000) 15–31. [20] A. Kuzma, M. Weis, S. Flickyngerova, J. Jakabovic, A. Satka, E. Dobrocka, J. Chlpik, J. Cirak, M. Donoval, P. Telek, F. Uherek, D. Donoval, Influence of surface oxidation on plasmon resonance in monolayer of gold and silver nanoparticles, J. Appl. Phys. 112 (2012) 103531. [21] Y. Lu, G. Xue, J. Dong, HNO3 etched silver foil as an effective substrate for surface-enhanced Raman scattering (SERS) analysis, Appl. Sur. Sci. 68 (1993) 485–489. [22] Y. Chiu, U. Rambabu, M.-H. Hsu, H.-P.D. Shieh, C.-Y. Chen, H.-H. Lin, Fabrication and nonlinear optical properties of nanoparticle silver oxide films, J. Appl. Phys. 94 (2003) 1996–2001.
[23] C.M. Wang, D.P. Tsai, Plasmonic infrared bandstop reflective filter, IEEE J. Sel. Top. Quantum Electron. 19 (2013) 4601005. [24] J. Tominaga, The application of silver oxide thin films to plasmon photonic devices, J. Phys.: Condens. Matter 15 (2003) R1101–R1122. [25] A.K. Pal, D.B. Mohan, Fabrication of partially oxidized ultra-thin nanocrystalline silver films: effect of surface plasmon resonance on fluorescence quenching and surface enhanced Raman scattering, Mater. Res. Exp. 1 (2014) 025014. [26] H. Luth, Solid surfaces and interfaces thin films, in: W.T. Rhodes, H.E. Stanley, R. Needs (Eds.), Morphology and Structure of Surfaces, Interfaces and Thin films, Springer, New York, 2010, pp. 67–131. [27] D.B. Mohan, C.S. Sunandana, Iodization of RF sputter induced disordered Ag thin films reveals volume plasmon-exciton ‘‘transition’’, J. Appl. Phys. 100 (2006) 064314. [28] D.B. Mohan, K. Sreejith, C.S. Sunandana, Surface plasmon–exciton transition in ultra-thin silver and silver iodide films, Appl. Phys. B 89 (2007) 59–63. [29] Y. Zhao, Y. Xie, Y.Y. Hui, L. Tang, W. Jie, Y. Jiang, L. Xu, S.P. Lau, Y. Chai, Highly impermeable and transparent graphene as an ultra-thin protection barrier for Ag thin films, J. Mater. Chem. C 1 (2013) 4956–4961. [30] M. Gnanavel, D.B. Mohan, C.S. Sunandana, Optics of quasi-particle phase transitions in nanostructured Ag thin films, Thin Solid Films 517 (2008) 1058– 1062. [31] E.A. Coronado, E.R. Encina, F.D. Stefani, Optical properties of metallic nanoparticles: manipulating light, heat and forces at the nanoscale, Nanoscale 3 (2011) 4042–4059. [32] C. Sonnichsen, T. Franzl, T. Wilk, G. Plessen, J. Feldmann, O. Wilson, P. Mulvaney, Drastic damping of Plasmon damping in gold nanorods, Phys. Rev. Lett. 88 (2002) 077402. [33] P. Royer, J.P. Goudonnet, R.J. Warmack, T.L. Ferrell, Substrate effect on surfacePlasmon spectra in metal island films, Phys. Rev. B 35 (1987) 3753–3759. [34] H.R. Stuart, D.G. Hall, Enhanced dipole-dipole interaction between elementary radiators near a surface, Phys. Rev. Lett. 80 (1998) 5663–5666. [35] Y. Yu, Y. Jiang, Z. Tang, Q. Guo, J. Jia, Q. Xue, K. Wu, E. Wang, Thickness dependence of surface plasmon damping and dispersion in ultrathin Ag films, Phys. Rev. B 72 (2005) 205405. [36] M.A.K. Abdelhalim, M.M. Mady, M.M. Ghannam, Physical properties of different gold nanoparticles: ultraviolet-visible and fluorescence measurements, J. Nanomed. Nanotechol. 3 (2012) 5. [37] N. Venugopal, A. Mitra, Optical transparency of ZnO thin film using localized surface plasmons of Ag islands, Opt. Mater. 35 (2013) 1467–1476. [38] S. Zhu, C. Du, Y. Fu, Localized surface plasmon resonance-based hybrid Au–Ag nanoparticles for detection of Staphylococcus aureus enterotoxin B, Opt. Mater. 31 (2009) 1608–1613. [39] O.S. Heavens, Optical Properties of Thin Solid Films, Butterworths Scientific Publication, London, 1955. [40] S. Babar, J.H. Weaver, Optical constants of Cu, Ag, and Au revisited, Appl. Opt. 54 (2015) 477–481. [41] P. Zhao, W. Su, R. Wang, X. Xu, F. Zhang, Properties of thin silver films with different thickness, Physica E 41 (2009) 387–390. [42] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [43] S.N. Kasarova, N.G. Sultanova, C.D. Ivanov, I.D. Nikolov, Analysis of the dispersion of optical plastic materials, Opt. Mater. 29 (2007) 1481–1490. [44] E.J. Zeman, G.C. Schatz, An accurate electromagnetic theory study of surface enhancement factors for Ag, Au, Cu, Li, Na, AI, Ga, In, Zn, and Cd, J. Phys. Chem. 91 (1987) 634–643. [45] V.A.G. Rivera, S.P.A. Osorio, D. Manzani, Y. Messaddeq, L.A.O. Nunes, E.M. Jr, Growth of silver nano-particle embedded in tellurite glass: interaction between localized surface plasmon resonance and Er3+ ions, Opt. Mater. 32 (2011) 888–892.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Structural, morphological and optical properties of Ag–AgO thin films with the effect of increasing film thickness and annealing temperature Anil Kumar Pal, D. Bharathi Mohan ⇑ Department of Physics, School of Physical, Chemical and Applied Sciences, Pondicherry University, R.V. Nagar, Kalapet, Puducherry 605014, India
a r t i c l e
i n f o
Article history: Received 23 May 2015 Received in revised form 12 July 2015 Accepted 20 July 2015
Keywords: Ultra thin to thick Ag films Thermal evaporation Morphological properties Crystal structure Raman mapping Surface plasmon resonance Dielectric constant
a b s t r a c t Ag films of thickness ranging from 5 to 60 nm were deposited by thermal evaporation technique followed by air annealing process with temperature varying from 50 to 250 °C. Morphological properties such as particle size, shape, surface roughness and number particles density were studied by atomic force microscope (AFM). The structural transition from quasi-amorphous to nanocrystalline to crystalline upon increasing film thickness and annealing temperature were studied. Ag films with smallest particle size and surface roughness were achieved up to film thickness of 7 nm. The possibility of surface oxidation of Ag on both as deposited and annealed films was studied through Raman mapping by using confocal Raman spectroscopy. Ag film was X-ray amorphous even after annealing process up to the film thickness of 7 nm and above which the crystallinity reached maximum at 250 °C. The surface plasmon resonance (SPR) with a symmetric line shape due to dipole–dipole interactions was found to be very strong for film thickness of 5 nm at 100 °C, attributed to the formation of smaller Ag NPs size of 22 nm with least size distribution and higher particles number density of 1625 lm2 in a self-organized fashion. With an increase of film thickness and annealing temperature, an asymmetric broad absorption arose due to increase in damping of collective electron oscillation on bulky NPs. Theoretical absorption spectra were simulated using extended Maxwell garnet method showing a decent agreement with experimental data. The real and imaginary parts of dielectric constants were determined and plotted for different film thicknesses of as deposited Ag films. Even though the film is oxidized at the surface level, it still can be used for plasmonic sensor applications however the film thickness should be approximately 7 nm for the enhanced result. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Silver plasmonic nanostructures have fascinated a subject of extensive research due to its unique optical property of strong resonant electromagnetic interaction with light which enables us to tune the surface Plasmon resonance (SPR) in wide optical band spectrum from 300 to 1000 nm [1,2]. Ag films hold a wide range of applications such as chemical sensors [3], biological sensors [4], surface enhanced Raman spectroscopy (SERS) [5], surface enhanced fluorescence spectroscopy (SEFS) [6] and photovoltaics [7]. Hence, it is very important to reinforce and tune the resonance in Ag nanostructure. The resonance frequency and the intensity of SPR mainly depends on several factors of which some of them following, (i) nature of the chemical element; (ii) size and shape of NPs; (iii) inter-nanoparticles distance; and (iv) surrounding ⇑ Corresponding author. E-mail address: [email protected] (D. Bharathi Mohan). http://dx.doi.org/10.1016/j.optmat.2015.07.029 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.
medium [8–10]. Ag possess the largest quality factor 10, negative real and imaginary parts of the dielectric functions across most of the spectrum when compared to any other noble metals such as Au, Cu, Al, Li, Pd and Pt [1]. Apart from the above facts, Ag is relatively cheaper than Au but has suffered from a tarnished reputation because it is more prone to oxidization in environment. Ag NPs oxidizes rapidly even at ambient condition because of the large surface to volume ratio however it is only at the surface level limiting to only a few nanometers [11]. Ag NPs completely oxidizes at higher temperature above 350 °C [12]. Recently, Yang et al. [13] demonstrated that the bimetal Ag–Au NPs far more stable than the pure Ag NPs against oxidation. Ag NPs are prepared by various wet chemistry methods [14,15], vacuum deposition techniques (VDT) which includes thermal evaporation [16], sputtering [17] and pulsed laser deposition [18] etc. Amongst, VDT is advantageous over wet chemistry routes because it can directly be used for the above mentioned applications before going for any surplus process. However, it oxidizes rapidly as soon as exposed to atmosphere
122
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
[11,19,20]. It is a great challenge to deposit un-oxidized Ag NPs with the optimized size and shape which exhibits strong SPR near UV–Vis region. Thin film of self-assembled Ag NPs with particle size 20 nm was possible to fabricate by using VDT when it was deposited in ultra-thin (film thickness from 1 to 10 nm) range otherwise bigger Ag NPs forms due to coalescence process. Closely spaced Ag NPs result in larger enhancement of electromagnetic coupling between NPs due to strong confinement of local electric field [5]. The SPR wavelength was observed in blue region for smaller size of NPs to that of bigger NPs due to an increase of surface energy resulting from an increased surface to volume ratio [21]. The tenability of SPR frequency can be achieved by varying film thickness and post deposition annealing temperature because these two parameters could alter NPs size, shape, number particles density and inter NPs distance. The formation of AgO at the surface level due to air annealing process would not be really a big concern for SPR. However, it leads to a red shift due to an increase of dielectric constant of the medium in which NPs are present [12,20]. AgO is wide bad semiconducting material having potential applications such as optical memories [22], filters [23] and plasmon photonic devices [24]. In this paper, Ag thin films of thickness ranging from 5 to 60 nm were prepared by using thermal evaporation technique followed by air annealing process with temperature varying from 50 to 250 °C. Film thickness and morphological properties involving particle size, shape, number density and surface roughness were studied by using Atomic Force Microscopy (AFM). The effect of air annealing towards oxidation of Ag films was studied by confocal Raman spectrometer. The effect of surface oxidation and its corresponding morphological changes were studied through Raman mapping and AFM. The crystal structure and optical absorption as a function of film thickness and annealing temperature were also studied. Theoretical simulation of absorption spectra of as-deposited Ag is presented by using extended Maxwell garnet method using AFM data’s. The novelty of this work was to study the behavior of SPR in a mixed phase of Ag–AgO films in a wide range of film thickness upon air annealing process.
2. Experimental details For the deposition of Ag thin films, Ag metal powder (particle size varying from 2 to 3.5 lm, purity: 99.9%) was purchased from Sigma Aldrich, USA. Prior to the deposition, borosilicate glass slides (Riviera, 100 300 ) were cleaned thoroughly by using chemicals such as soap solution (Fisher Scientific, USA), nitric acid (HNO3) (Fisher Scientific, USA) and Acetone (Himedia, India) by following the similar procedure mentioned in AKP et al. work [25]. Ag films of mass thickness ranging from 1 to 50 nm (according to digital thickness monitor (DTM)) were deposited on cleaned glass substrates by thermal evaporation technique at the base pressure of 1.5 106 mbar. The DC current supply was kept constant at 70 A throughout the deposition process. In order to increase film thickness, the deposition time was increased from 2 min to 38 min by keeping deposition rate as constant at 0.1 Å/sec. Thickness of Ag films was measured using quartz crystal thickness oscillator – an inbuilt sensor available within the thermal evaporation unit. The as-deposited (ASD) Ag films were then immediately air annealed in hot air oven at temperatures 50, 100, 150, 200 and 250 °C about an hour. The crystal structure, surface morphology, optical absorption and surface oxidation of as deposited and annealed silver films were studied by using characterization techniques such as Glancing Angle (1.5°) X-ray diffraction (GAXRD) (PANalytical-X’Pert PRO), Atomic Force Microscope (AFM) (Nanoscope – tapping mode), UV–Vis Spectrophotometer (Ocean
Optics, HR 4000) and confocal Raman spectrometer (RENISHA WinVia Raman Microscope, UK) respectively.
3. Results and discussions 3.1. Atomic force microscopy: Film thickness and morphological properties The actual thicknesses of Ag films were measured through AFM scanning over the edge of a scratch made on Ag films by a sharp blade. Figs. 1 and 2 exhibit 3D AFM images and their corresponding surface profile data’s of ASD and annealed Ag films. The morphology of Ag films obtained by AFM suggests that the Ag NPs are formed with a discontinuous film of few numbers of mono layers for ultrathin film thickness up to 7 nm and several mono layers for thicker films by island growth process so called Vollmer– Weber (VW) growth process. The occurrence of VM process is favored when the contact angle between the deposited material and the substrate is higher than zero; means the elastic strain of the film exceeds the adhesion force due to strong adatom cohesive force [26]. Here, the formation of Ag NPs occurs through nucleation sites. Once a sufficient number of Ag atoms reached the substrate, then the nucleation starts and grows into a layer of nanoparticles. DTM analysis is an in situ process and measures relative thickness referring continuous layer formation on its sensor known as quartz crystal microbalance. One of the major disadvantages of this technique was making the tooling factor very precisely. In thermal evaporator, the sensor and the substrate usually cannot be placed in the same direction from the deposition source and may not even be at the same distance from it. Therefore, the rate at which the material was deposited on the sensor was not equal to the rate at which it was deposited on glass substrate. The ratio of the two rates sometimes called the ‘‘tooling factor’’. In this experimental work, both the sensor and the substrate were almost placed at the same height however the direction was not maintained same for both of them from the deposition source. However the film formed on glass substrate follows VM growth process and therefore the thickness differs from the value measured by DTM. Due to the formation of Ag islands, voids are created on the substrate which makes the film porous with the height of NPs greater than the film thickness measured by DTM. The height of the islands is considered to be the actual film thickness measured by AFM. A comparative analysis is shown in Table 1 based on film thickness monitored from digital thickness monitor (DTM) and determined from surface profile data analysis. The % of error in thickness measurement is calculated based on the relation [(AFM–DTM)/AFM] % as resulting from AFM image and DTM thickness estimation. It is found that the % of error is decreasing systematically with increasing film thickness measured by DTM due to decrease in porosity of the films with increase in film thickness which occurs as a result of increasing surface coverage. The height of nanoparticles increases with increasing annealing temperature at 250 °C could be due to crystallization of Ag and the new phase formation of AgO structure. The particle size increases with increasing film thickness and annealing temperature. Hereafter, the entire discussion will proceed based on the film thickness measured by AFM. The fabrication of fully crystalline silver structure could not be possible from the deposition of silver film on glass substrate held at ambient temperature. In thermal evaporation, the mobility of the atoms arises due to local temperature which is less enough for the crystallization in ultra-thin films. As the film thickness increases, the mobility of silver atoms increases due to increase of local thermal energy and partially crystallizes the silver atoms near the surface leading to form highly disordered Ag phase [27].
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
123
Fig. 1. 3D AFM images and the corresponding line profile of Ag films with mass thickness of 1–10 nm (according to DTM). The scan was performed across the scratch made on the film in order to measure the film thickness. (Note: ASD means as deposited film and 250 denotes the annealing temperature).
Fig. 2. 3D AFM images and the corresponding line profile of Ag films of mass thickness from 15 to 50 nm (according to DTM). The scan was performed across the scratch made on the film in order to measure the film thickness. (Note: ASD means as deposited film and 250 denotes the annealing temperature).
124
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Table 1 Thickness determined from AFM images and its comparison with DTM. Sample ID
Thickness monitored by DTM (nm)
Ag1 Ag2 Ag5 Ag10 Ag15 Ag20 Ag25 Ag50
1 2 5 10 15 20 25 50
Thickness determined from AFM image (nm) ASD
250 °C
5±1 7±1 10 ± 1 18 ± 2 25 ± 2 31 ± 5 39 ± 7 60 ± 7
6±2 10 ± 3 17 ± 7 28 ± 6 35 ± 14 54 ± 12 80 ± 20 106 ± 25
% of error with respect to DTM concerning only ASD
80 71 50 44 40 35 35 16
In order to achieve complete crystalline phase, it is mandatory to anneal ASD film at higher temperature at least above 100 °C. The transition from disordered to ordered phase is essentially driven by external thermal energy. During the course of time, Ag films much more oxidized at the surface level which will be discussed in Raman study. As deposited (ASD) Ag film forms a discontinuous film containing non-spherical particles with large surface coverage. The
transition from quasi-amorphous to bulk crystalline is studied on Ag films when annealed at above 100 °C [25], where the discontinuous films transform into regular sized and shaped particle with decreased surface coverage. The evolution of surface morphology in Ag films with thickness ranging from 5 nm (Ag1) to 60 nm (Ag50) for ASD and annealed films (at 100 and 250 °C) were studied by AFM (Figs. 3 and 4). It is observed that, an irregular enlargement of particles occurs with an increase of film thickness due to coalescence and forms clusters. The particles are formed in oblate spheroid like shape with the length along x, y direction which is greater than the height in z-direction (c < a = b). The size of a nanoparticle is taken as the diameter of a circle along x–y direction which is higher than the height (along z-direction) of the particle. The number density of NPs was calculated for all films and found to be maximum for film thickness of 5 nm annealed at 100 °C. The higher homogeneity of Ag NPs both in size and shape was accomplished up to film thickness of 18 nm. Moreover, agglomeration was not conspicuous until film with thickness of 18 nm even with air annealing at 250 °C except the change of particle size while it was realized from thickness of 25–60 nm. A very heterogeneous size and shape distribution is observed with higher film thickness above 18 nm suggesting that these films have not achieved the minimum crystallization temperature. It is also extremely difficult
Fig. 3. 2D AFM images of Ag films with thickness of 5 (Ag1), 7 (Ag2), 10 (Ag5) and 18 (Ag10) nm recorded over 1 1 lm scan area. The insert shows the corresponding roughness profile. (ASD – as deposited film; 100 and 250 denotes annealing temperature).
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
125
Fig. 4. 2D AFM images of Ag films with thickness of 25 (Ag15), 31(Ag20), 39 (Ag25) and 60 (Ag50) nm recorded over 1 1 lm scan area. The insert shows the corresponding roughness profile. (ASD – as deposited film; 100 and 250 denotes annealing temperature).
to accomplish a monolayer with uniform size and shape distributions above the film thickness of 18 nm. The particles are bigger for thickness above 18 nm when annealed at 250 °C. The particle size, RMS surface roughness and number density per one lm2 values are calculated from particle size distribution and roughness profile are shown in Fig. 5. The particle size increases from 19 to 130 nm for ASD films, 22–256 nm and 35–474 nm for the films annealed at 100 and 250 °C respectively with increasing film thickness. There is no much variation in particle size up to 18 nm film thickness even after annealing process whereas it increases abruptly and faster with increasing film thickness and annealing temperature. The RMS surface roughness of Ag films increases from 1.44 to 6.98 nm for ASD films, 1.88–14.2 nm at 100 °C and 2.25–29.7 nm at 250 °C. The RMS roughness value increases with increasing film thickness and annealing temperature due to increase of particle size and agglomeration [12,25]. With increasing annealing temperature, surface properties such as number density, size, shape, inter-particle distance and surface coverage area were altered through coalescence followed by Ostwald ripening processes [25]. For the film thicknesses up to 18 nm, the particles are distributed disjointedly even upon annealing. However beyond 18 nm, particles grow bigger and coalesce together due to which the particle number density was not
calculated. Fig. 5(b) represents the variation of particle number density with respect to film thickness up to 18 nm. It is observed that the number density decreases with increase in film thickness and annealing temperature due to increase in particle size. But the same fashion does not happen for Ag film with thickness of 5 nm. From AFM, ASD Ag film with thickness of 5 nm shows hazy like morphology due to quasi-amorphous phase nature lead to less particle number density of 1050 lm2 as compared to the film annealed at 100 °C which shows highest number density of 1625 lm2. 3.2. X-ray diffraction: Crystal structure Fig. 6 shows an amorphous phase up to the film thickness of 7 nm (Ag1) indicating the supremacy of short range order of silver atoms in both ASD and annealed Ag films [28]. With increasing film thickness to 15 nm, the crystal planes, (1 1 1), (2 2 0) and (3 1 1) arise with a small intensity at angles 38.18, 64.43 and 77.49 (2h in degree) respectively. The intensities of all crystal planes increases with increasing film thickness to 60 nm along with a new plane (2 0 0) emerge at 44.33 (2h) suggesting the fact that the number density of silver atoms deposited on glass substrate was increased. When silver film is annealed at 250 °C, the peak
126
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Fig. 5. Variation of (a) particle size, (b) number density per one lm2 area and (c) surface roughness of Ag films as a function of film thickness. (ASD – as deposited film, 100 and 250 denotes annealing temperature).
Fig. 6. GA-XRD of silver films as-deposited and air annealed at 250 °C with film thickness of 7 (Ag2), 15 (Ag10), 31 (Ag20) and 60 (Ag50) nm.
intensity of crystal planes further enhanced in case of 18 and 31 nm film thicknesses which is attributed to increase in crystallinity of face centered cubic phase of Ag. The lattice parameter (a = 4.085 Å) is calculated from XRD pattern after comparing with JCPDS card number 04-0783 for both as deposited and annealed films, which is exactly matching with the standard value (a = 4.085 Å) corroborating the formation of fcc phase of Ag. It is indispensable to mark statement here that no peak from AgO phase is observed, suggesting the formation of minor AgO phase typically at the surface level with only few nm, may be XRD is not the accurate technique to probe such level of phase formation. 3.3. C. Raman spectra and mapping of silver oxide (AgO) Noble metals are chemically inert under ambient conditions which mean they are relatively stable and inactive towards oxidation. However, it is not true at nanoscale because of an increase of surface to volume ratio and thus oxidizes immediately [11].The surface oxidation depends mainly on oxidation rate which varies at different temperature. Annealing below 250 °C controls oxidation only to the depth level of a few nm on the surface and hence still favoring SPR of Ag which in accordance with optical absorption results. The bulk silver oxide (AgO) exhibits Raman modes at 219, 300, 375, 429, 468 and 487 cm1 and silver (I) oxide (Ag2O) exhibits a single Raman mode at 490 cm1 [26]. Thermally evaporated Ag films with thickness of 250 nm annealed at above 350 °C
showed Raman modes at 230 and 239 cm1 [12]. According to our knowledge, there is no Raman data published in thermally evaporated Ag films with less than 60 nm film thickness so far. Figs. 7a and 7b represent Raman spectra of Ag films with thickness of 5–60 nm for as deposited, 100 and 200 °C annealed films. All peaks correspond to AgO phase formed when the films are exposed to atmosphere and annealed at higher temperature. As deposited Ag films with thicknesses of 5 and 10 nm exhibit Raman modes at 226 cm1 and then at 230 cm1 for films with thickness of 18, 31 and 60 nm confirming the formation of AgO phase. Later, peak shifts to 234 cm1 when annealed at above 100 °C. Raman mode shifts towards higher wave number at 234 cm1 because of an increase of film thickness and the oxidation rate [12]. Raman modes observed at 230 and 234 cm1 correspond to the stretching modes of Ag and O [29]. XRD confirms the formation of Ag structure while Raman study proves the oxidation of Ag films. The oxidation of Ag film is limited to the surface level with the depth of about only few nanometers otherwise UV– Visible spectra would not exhibit SPR. The role of film thickness and annealing temperature on surface oxidation is discussed through Raman mapping on as-deposited and annealed (at 250 °C) Ag films with thickness of 10 and 60 nm. The Raman mapping recorded over an area of 5 5 lm with a step size of 500 nm by exciting at 488 nm Ar+ laser with an acquisition time of 30 s and power of 10 mW. As observed from Raman spectra, Ag5-ASD and Ag50-ASD films show AgO peak at
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
127
Fig. 7a. Raman spectra of Ag films with thickness of 5 (Ag1) and 10 (Ag5) nm for as deposited (ASD) and annealed (at 100 and 200 °C) films.
Fig. 7b. Raman spectra of Ag films with thickness of 18 (Ag10), 31 (Ag20) and 60 (Ag50) nm for as deposited (ASD) and annealed (at 100 and 250 °C) films.
226 cm1 whereas Ag5–250 and Ag50–250 show AgO peak at 234 cm1. Hence Raman intensity from above respective peaks showing the formation of AgO is considered for Raman mapping. Fig. 8 shows the Raman mapping of Ag films. The thickness and annealing temperature of Ag films has a significant role on the oxidation level of Ag films as proven by their large Raman intensity variation. The peak intensity value varies in the range of 210–390, 4840–7680, 11,750–20,800 and 16,200–42,000 arbitrary units for Ag5-ASD, Ag5–250, Ag50-ASD and Ag50–250 respectively. In other words, the higher value of peak intensity of Ag5-ASD, Ag5–250 and Ag50-ASD films are of 0.92, 18.28 and 49.52 percentage with respect to Ag50–250 film. This large variation in Raman intensity reveals that the surface oxidation of Ag films increases with increasing the film thickness as well as annealing temperature. 3.4. Optical absorption: The effect of annealing temperature and film thickness on tuning surface plasmon resonance Fig. 9 demonstrates the optical absorbance spectra of Ag films for different film thicknesses of 5, 7, 10 and 18 nm as a function of annealing temperature from 50 to 250 °C including as deposited film. The line shape, width and kmax of SPR depend on their morphological property which includes size, shape, distribution, inter-particles distance, particles number density and also the surrounding dielectric matrix [27,30,31]. It is clearly noticed that the as-deposited Ag film with thickness of 5 nm exhibits strong SPR at 402 nm with a symmetric line shape (Gaussian shape) corresponding to dipolar resonance of Ag NPs and then it slowly deviates from symmetric line shape with a red shift (10 nm) as the film thickness increases to 7 nm. Ag clusters with size of several
tenth of nm and large filling factor show optical absorption spectra characterized by typical redshift and broadening of plasmon resonance due to retardation effects and multipole modes of electronic oscillations [32]. Fig. 10 shows that above the film thickness of 25 nm, SPR increases initially up to 450 nm and then become constant up to 800 nm for which the dipolar resonance is absent due to increase in particle size, the deformation from spheroid shape of particles and increase in surface roughness. Moreover, there is a shoulder peak observed at 311 nm (near UV-range) for film thickness of 5 nm and appears to be less prominent as compared to the dipolar resonance which does not shift much with increasing annealing temperature is attributed to inter band edge transition [32]. The narrowing of SPR spectra is attributed to reduced radiation damping of collective electron oscillation from smaller NPs of size in the range of 20–30 nm [9,33]. The broadening and red shift of SPR spectra occurs as a result of increase in light scattering due to increase in particle volume as the film thickness increases. The radiative damping of collective electron oscillation increases with increase in particle volume which is connected to light scattering by particles. The change in the interaction between the particles and dielectric matrix can also contribute to line width of absorption spectra [34]. There is no much shift observed up to film thickness of 7 nm except a small change in the line shape of the spectra pertaining to different size distribution of Ag NPs. When the film thickness increases, the particle size increases and the charge separation on the nanoparticles also increases, leading to lower the frequency of collective oscillation of electrons as indicated by a red shift from 400 to 500 nm approximately [35]. The annealing process shows considerable change in SPR in the temperature region from 50 to 250 °C. The SPR peak observed at
128
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Fig. 8. Raman mapping images of Ag films with thickness of 10 (Ag5) and 60 (Ag50) nm for both ASD and annealed (at 250 °C) films.
402 nm for ASD film with the thickness of 5 nm becomes stronger at 100 °C due to the formation of nanocrystalline Ag for which the number particles density increases [36]. Subsequently, the intensity falls at 150 °C with a red shift of 10 nm could be due to an increase of particle size and the formation of AgO phase [20,25]. Finally, the SPR blue shifts above 150 °C. Such trend is observed up to the film thickness of 18 nm. The inter-particle distance and the contact area between Ag NPs and glass substrate have pronounced effect on SPR peak shift. The increase in surface coverage area upon increasing film thickness decreases the inter-particle gap resulting a broad SPR with red shift due to dipole–dipole interaction [9]. The increase in contact area increases the back word scattering by Ag NPs. The increase in particle size could be the reason for the initial red shift up to annealing temperature of 150 °C because there was no much change in surface coverage except transformation to crystalline form. The further annealing process decreases the surface coverage as a consequence of increased vertical height with a small increase in the average lateral size of particles leading to a blue shift of SPR. At higher annealing temperature, additional redshift and broadening observed because the clusters accentuate the non-spherical shape, with progressive modification of the geometrical factor [37]. 3.4.1. Theoretical simulation For Ag NP’s layer, the position of SPR is determined by its size, shape, material and dielectric constant of surrounding medium [30,31,38]. For the purpose of illustration, the theoretical spectra of absorption of Ag films are calculated based on thickness, particle shape, size and inter particle distance obtained from AFM analysis with the help of standard thin film formula [39]. The optical properties of Ag/glass multilayer system is described by effective complex dielectric constant (E eff ) as defined in generalized Extended Maxwell Garnett (EMG) theory by using the equation [37].
E eff ¼ E ext
FE Ag þ E ext ð1 FÞ þ qð1 FÞðE Ag E ext Þ FE Ag þ E ext ð1 FÞ qFðE Ag E ext Þ
ð1Þ
where E Ag and E ext are dielectric constants of Ag and the external medium surrounding the NPs respectively, q is the volume filling factor of Ag film which determines how much volume of the film is really covered by the particles and F is the geometrical factor of 2
NPs. The volume filling factor q is defined as q ¼ V=l h, where V is the volume of particle, h is the height of the particle and l is the average distance between NPs. The wavelength-dependent bulk dielectric constants for silver have been used from Babar et al. in all calculations [40]. In this model, Ag films are considered to be single layer of Ag NPs with the same shape, size and orientation for each film. The Ag NPs are considered as oblate spheroid (c < a = b) where a, b and c are the half axes of the spheroid. And it is obvious that the thickness of Ag film is equivalent to the height of the NPs. The geometrical factor of Ag NPs of oblate shape is defined as [37],
F¼
1 þ 2e2 ðe arctaneÞ e3
ð2Þ
where e is the eccentricity of the ellipsoid. For the matching of SPR spectra with the theoretical model, the value of q and F are varied through varying the Ag particle size and inter-particle distance. The complex dielectric constant of Ag is used from the empirical result of Baber et al. [40]. In the presence of surrounding medium such as air and glass substrate, the effective refractive index (neff ) and extinction coefficient (keff ) of Ag NPs can be obtained from MG theory and given by [37,41],
neff ¼
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi uqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 t E effreal þ E effreal þ E effreal 2
ð3Þ
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
129
Fig. 9. Optical absorption spectra of Ag films with thickness of 5 (Ag1), 7 (Ag2), 10 (Ag5) and 18 (Ag10) nm with increasing annealing temperature ranging from 50 to 250 °C. SPR is very significant in case of 5 nm while it becomes weaker with increasing film thickness.
keff ¼
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi uqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 t E effreal þ E effreal E effreal 2
A ¼ log ð4Þ
The transmission spectra of a single Ag layer on glass substrate can be calculated with the standard thin film formula as follows [42],
T¼
jt 123 j2 jt 31 j2 eð2ImðdÞÞ 1 jr321 j2 jr 31 j2 eð4ImðdÞÞ
ð5Þ
where the transmission t 123 , t 31 and the reflection, r321 , r 31 are typical Fresnel coefficients at normal incidence for the three indexed mediums such as air, Ag film and glass substrate subscripted by 1, 2 and 3 respectively. The phase change, d experienced in a medium, i at normal incidence is presented by,
d¼
2pni di k
ð6Þ
where ni and di are the refractive index and thickness of the corresponding ith layer respectively. Finally the absorbance A is calculated by using the standard relation given by
1 T
ð7Þ
Fig. 11 compares the result of measured and calculated optical absorbance of as-deposited Ag films which shows good agreement of calculated SPR spectra with experimental data over entire region of absorption spectra. However, a slight mismatch was observed for some Ag films could be due to the effect of surface oxidation which was not taken into the consideration for simulation. The surface oxidation initially low for as-deposited Ag films which then increased for annealed films as described in Raman analysis. The complex refractive index and the dielectric function describes the optical properties of any solid material. The real part of dielectric constant (er) is related to polarization and anomalous dispersion of Ag NPs, while the imaginary part (ei) is associated with the dissipation of energy into the surrounding medium. The dielectric dispersion is a significant factor in optical communication and in designing devices for spectral dispersion [41]. The changes in er directs the change in polarization of Ag NPs and similarly, change in ei controls the dissipation of energy absorbed by plasmons which result in shifting of SPR peak. The effective complex dielectric constant of Ag thin films laying on glass substrates
130
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
Fig. 10. Optical absorption spectra of Ag films with thickness of 25 (Ag15), 31 (Ag20), 39 (Ag25) and 60 (Ag50) nm with increasing annealing temperature raging from 50 to 250 °C.
Fig. 11. Measured and simulated optical absorption spectra of as-deposited Ag films of various film thicknesses (a): 5 (Ag1), 7 (Ag2), 10 (Ag5), 18 (Ag10) nm and (b): 25 (Ag15), 31 (Ag20), 39 (Ag25), 60 (Ag50) nm.
are calculated based on Eq. (1). The variation of er and ei of as-deposited Ag films of different thickness from 5 to 60 nm with respect to wavelength is shown in Fig. 12. The complex dielectric constants of Ag films are compared with the bulk Ag as determined
by Babar et al. [40]. It can be noticed that the change in er and ei of Ag films occur in a linear fashion with change in film thickness. As shown in Fig. 12(a), er value of bulk Ag is decreasing linearly from 1.09 to 31.41 along the wavelength region from 300 to
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
131
Fig. 12. Calculated (a) Real and (b) imaginary part of dielectric constants of as-deposited Ag films of various film thicknesses such as 5 (Ag1), 7 (Ag2), 10 (Ag5), 18 (Ag10), 25 (Ag15), 31 (Ag20), 39 (Ag25) and 60 (Ag50) nm. The presented real and imaginary curves of dielectric constant of bulk Ag film drawn from a published article by Babr et al. [40].
800 nm respectively [40]. er observed in the negative scale for Ag thin films which is still higher than the value of bulk Ag. Ag film of 5 nm thickness (Ag1) shows higher value with close to zero and varies from 0.42 to 2.09 in the wavelength range of 300– 800 nm. With increase in Ag film thickness from 5 to 60 nm, er decreases from 2.096 to 10.94 at absorption wavelength of 800 nm. On the other hand, ei of Ag films observed in the positive scale close to zero and then increases with increase in film thickness (Fig. 12(b)), however lies far below as compared to bulk Ag film. The surrounding medium such as glass and air have approximately constant dielectric values of 1.5 and 1 respectively in the wave length range of 300–800 nm. As-deposited Ag constituting of small nanoparticles in the range of 19–130 nm in the stack of air and glass substrates of higher dielectric constant value and showed lower dielectric constant value as compared to bulk Ag. The dielectric constant is deeply dependent on the surface scattering by the metal nanoparticles [43,44,41]. For small size nanoparticles, the absorption and scattering bands are strong and coincident, and the extinction spectrum more strongly contributed by absorption than the scattering [1]. When the particle size increases beyond 20 nm, the scattering becomes predominant over absorption [45]. Hence the scattering increases for Ag films of thickness ranging from 10 to 60 nm due to an increase in average particle size above 20 nm. This increase in scattering results in decrease of dielectric constant of Ag films beyond the film thickness of 7 nm which goes far below than zero value. The condition for a plasmonic material to support strong SPR is when the value of dielectric constant is close to zero [1]. Hence the lower value of effective dielectric constant (er and ei) observed in case of Ag films of thickness 5 and 7 nm showed a strong SPR as conformed from Fig. 9. 4. Conclusions Ag films of various thicknesses of 5, 7, 10, 18, 25, 31, 39 and 60 nm were deposited by thermal evaporation method followed by air annealing process at temperatures 50, 100, 150, 200 and 250 °C. The film thickness analysis showed higher experimental error (in %) as the thickness goes down to ultra-thin measured by quartz crystal thickness monitor. Unagglomerated and spheroid shaped of Ag NPs were formed up to film thickness of 18 nm
beyond which non-spherical and bigger particles were formed due to coalescence and Ostwald ripening processes with the effect of post deposition air annealing process. Ag NPs size of 19 nm with a trivial surface oxidation and a least surface roughness of 1.28 nm were accomplished on 5 nm thick Ag film. The presence of stretching mode at 234 cm1 (stretching) in Raman spectra and hkl planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1) in XRD confirmed the formation of mixed phase of Ag–AgO in Ag films. Raman mapping showed the level of oxidation in Ag films which increases with increasing film thickness and annealing temperature. The SPR due to dipole– dipole interactions was very strong for film thickness of 5 nm annealed at 100 °C due to smaller Ag NPs size of 22 nm with least size distribution and highest particles number density 1625 lm2. With increasing film thickness and annealing temperature, a broad asymmetric absorption was observed due to bulky size of NPs leading to increase in damping of collective electron oscillation. Teoretically simulated absorption curves using extended Maxwell garnet method are matching very well with their experimental data’s. The real and imaginary parts of dielectric constants was determined for as deposited Ag films increases with increasing film thickness. The least value of dielectric constants was observed for Ag film with thickness of 5 nm bearing a very strong SPR. It is understood from absorption results that the film thickness approximately 7 nm could be the critical limit to obtain a strong SPR. The tuning of SPR in ultrathin Ag films was realized despite of the formation of AgO phase at surface level. Acknowledgement The work is supported by UGC Major Project (F. No. 4151868/2012(Sr)). The author AKP gratefully acknowledges UGC, India for the research fellowship. Authors thank Hima Annapoorna for contribution towards material preparation. Authors also thank Dr. R. Venkatesan and Mr. Rajesh, Department of Chemistry for using UV–Visible spectrophotometer. Central Instrumentation Facility of Pondicherry University provided the facilities for characterization studies. References [1] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Controlling the synthesis and assembly of silver nanostructures for plasmonic applications, Chem. Rev. 111 (2011) 3669–3712.
132
A.K. Pal, D. Bharathi Mohan / Optical Materials 48 (2015) 121–132
[2] L.M. Liz-Marzán, Tailoring surface plasmons through the morphology and assembly of metal nanoparticles, Langmuir 22 (2005) 32–41. [3] H. Zhou, D. Yang, N.P. Ivleva, N.E. Mircescu, R. Niessner, C. Haisch, SERS detection of bacteria in water by in situ coating with Ag nanoparticles, Anal. Chem. 86 (2014) 1525–1533. [4] R. Gradess, R. Abargues, A. Habbou, J. Canet-Ferrer, E. Pedrueza, A. Russell, J.L. Valdes, J.P. Martinez-Pastor, Localized surface plasmon resonance sensor based on Ag-PVA nanocomposite thin films, J. Mater. Chem. 19 (2009) 9233–9240. [5] C. Wang, W. Ruan, N. Ji, W. Ji, S. Lv, C. Zhao, B. Zhao, Preparation of nanoscale Ag semishell array with tunable interparticle distance and its application in surface-enhanced Raman scattering, J. Phys. Chem. C 114 (2010) 2886–2890. [6] M.-G. Kang, T. Xu, H.J. Park, X. Luo, L.J. Guo, Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrodes, Adv. Mater. 22 (2010) 4378–4383. [7] A. Sánchez-González, S. Corni, B. Mennucci, Surface-enhanced fluorescence within a metal nanoparticle array: the role of solvent and plasmon couplings, J. Phys. Chem. C 115 (2011) 5450–5460. [8] L.A. Lyon, D.J. Peña, M.J. Natan, Surface plasmon resonance of au colloidmodified au films: particle size dependence, J. Phys. Chem. B 103 (1999) 5826– 5831. [9] E. Thouti, N. Chander, V. Dutta, V.K. Komarala, Optical properties of Ag nanoparticle layers deposited on silicon substrates, J. Opt. 15 (2013) 035005. [10] A. Tao, P. Sinsermsuksakul, P. Yang, Tunable plasmonic lattices of silver nanocrystals, Nat. Nanotechnol. 2 (2007) 435–440. [11] W. Cai, H. Zhong, L. Zhang, Optical measurements of oxidation behavior of silver nanometer particle within pores of silica host, J. Appl. Phys. 83 (1998) 1705–1710. [12] J. Lv, F. Lai, L. Lin, Y. Lin, Z. Huang, R. Chen, Thermal stability of Ag films in air prepared by thermal evaporation, Appl. Sur. Sci. 253 (2007) 7036–7040. [13] H. Yang, Y. Wang, H. Huang, L. Gell, L. Lehtovaara, S. Malola, H. Häkkinen, N. Zheng, All-thiol-stabilized Ag44 and Au12Ag32 nanoparticles with singlecrystal structures, Nat. Commun. 4 (2013). [14] Q.H. Tran, V.Q. Nguyen, A.-T. Le, Silver nanoparticles: synthesis, properties, toxicology, applications and perspectives, Adv. Nat. Sci. Nanosci. Nanotechnol. 4 (2013) 033001. [15] S.E. Skrabalak, L. Au, X. Li, Y. Xia, Facile synthesis of Ag nanocubes and Au nanocages, Nat. Protoc. 2 (2007) 2182–2190. [16] D. Gaspar, A.C. Pimentel, T. Mateus, J.P. Leitão, J. Soares, B.P. Falcão, A. Araújo, A. Vicente, S.A. Filonovich, H. Águas, R. Martins, I. Ferreira, Influence of the layer thickness in plasmonic gold nanoparticles produced by thermal evaporation, Sci. Rep. 3 (2013) 1469. [17] S. Kundu, S. Hazra, S. Banerjee, M.K. Sanyal, S.K. Mandal, S. Chaudhuri, A.K. Pal, Morphology of thin silver film grown by dc sputtering on Si(0 0 1), J. Phys. D Appl. Phys. 31 (1998) L73. [18] K. Koppole, K.C. Sekhar, A. Almeida, J.A. Moreira, M.J.M. Gomes, Tuning the surface plasmon resonance and surface-enhanced Raman scattering of pulsed laser deposited silver nanoparticle films by ambience and deposition temperature, J. Opt. 16 (2014) 055002. [19] C.I. Carlisle, T. Fujimoto, W.S. Sim, D.A. King, Atomic imaging of the transition between oxygen chemisorption and oxide film growth on Ag {1 1 1}, Sur. Sci. 470 (2000) 15–31. [20] A. Kuzma, M. Weis, S. Flickyngerova, J. Jakabovic, A. Satka, E. Dobrocka, J. Chlpik, J. Cirak, M. Donoval, P. Telek, F. Uherek, D. Donoval, Influence of surface oxidation on plasmon resonance in monolayer of gold and silver nanoparticles, J. Appl. Phys. 112 (2012) 103531. [21] Y. Lu, G. Xue, J. Dong, HNO3 etched silver foil as an effective substrate for surface-enhanced Raman scattering (SERS) analysis, Appl. Sur. Sci. 68 (1993) 485–489. [22] Y. Chiu, U. Rambabu, M.-H. Hsu, H.-P.D. Shieh, C.-Y. Chen, H.-H. Lin, Fabrication and nonlinear optical properties of nanoparticle silver oxide films, J. Appl. Phys. 94 (2003) 1996–2001.
[23] C.M. Wang, D.P. Tsai, Plasmonic infrared bandstop reflective filter, IEEE J. Sel. Top. Quantum Electron. 19 (2013) 4601005. [24] J. Tominaga, The application of silver oxide thin films to plasmon photonic devices, J. Phys.: Condens. Matter 15 (2003) R1101–R1122. [25] A.K. Pal, D.B. Mohan, Fabrication of partially oxidized ultra-thin nanocrystalline silver films: effect of surface plasmon resonance on fluorescence quenching and surface enhanced Raman scattering, Mater. Res. Exp. 1 (2014) 025014. [26] H. Luth, Solid surfaces and interfaces thin films, in: W.T. Rhodes, H.E. Stanley, R. Needs (Eds.), Morphology and Structure of Surfaces, Interfaces and Thin films, Springer, New York, 2010, pp. 67–131. [27] D.B. Mohan, C.S. Sunandana, Iodization of RF sputter induced disordered Ag thin films reveals volume plasmon-exciton ‘‘transition’’, J. Appl. Phys. 100 (2006) 064314. [28] D.B. Mohan, K. Sreejith, C.S. Sunandana, Surface plasmon–exciton transition in ultra-thin silver and silver iodide films, Appl. Phys. B 89 (2007) 59–63. [29] Y. Zhao, Y. Xie, Y.Y. Hui, L. Tang, W. Jie, Y. Jiang, L. Xu, S.P. Lau, Y. Chai, Highly impermeable and transparent graphene as an ultra-thin protection barrier for Ag thin films, J. Mater. Chem. C 1 (2013) 4956–4961. [30] M. Gnanavel, D.B. Mohan, C.S. Sunandana, Optics of quasi-particle phase transitions in nanostructured Ag thin films, Thin Solid Films 517 (2008) 1058– 1062. [31] E.A. Coronado, E.R. Encina, F.D. Stefani, Optical properties of metallic nanoparticles: manipulating light, heat and forces at the nanoscale, Nanoscale 3 (2011) 4042–4059. [32] C. Sonnichsen, T. Franzl, T. Wilk, G. Plessen, J. Feldmann, O. Wilson, P. Mulvaney, Drastic damping of Plasmon damping in gold nanorods, Phys. Rev. Lett. 88 (2002) 077402. [33] P. Royer, J.P. Goudonnet, R.J. Warmack, T.L. Ferrell, Substrate effect on surfacePlasmon spectra in metal island films, Phys. Rev. B 35 (1987) 3753–3759. [34] H.R. Stuart, D.G. Hall, Enhanced dipole-dipole interaction between elementary radiators near a surface, Phys. Rev. Lett. 80 (1998) 5663–5666. [35] Y. Yu, Y. Jiang, Z. Tang, Q. Guo, J. Jia, Q. Xue, K. Wu, E. Wang, Thickness dependence of surface plasmon damping and dispersion in ultrathin Ag films, Phys. Rev. B 72 (2005) 205405. [36] M.A.K. Abdelhalim, M.M. Mady, M.M. Ghannam, Physical properties of different gold nanoparticles: ultraviolet-visible and fluorescence measurements, J. Nanomed. Nanotechol. 3 (2012) 5. [37] N. Venugopal, A. Mitra, Optical transparency of ZnO thin film using localized surface plasmons of Ag islands, Opt. Mater. 35 (2013) 1467–1476. [38] S. Zhu, C. Du, Y. Fu, Localized surface plasmon resonance-based hybrid Au–Ag nanoparticles for detection of Staphylococcus aureus enterotoxin B, Opt. Mater. 31 (2009) 1608–1613. [39] O.S. Heavens, Optical Properties of Thin Solid Films, Butterworths Scientific Publication, London, 1955. [40] S. Babar, J.H. Weaver, Optical constants of Cu, Ag, and Au revisited, Appl. Opt. 54 (2015) 477–481. [41] P. Zhao, W. Su, R. Wang, X. Xu, F. Zhang, Properties of thin silver films with different thickness, Physica E 41 (2009) 387–390. [42] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [43] S.N. Kasarova, N.G. Sultanova, C.D. Ivanov, I.D. Nikolov, Analysis of the dispersion of optical plastic materials, Opt. Mater. 29 (2007) 1481–1490. [44] E.J. Zeman, G.C. Schatz, An accurate electromagnetic theory study of surface enhancement factors for Ag, Au, Cu, Li, Na, AI, Ga, In, Zn, and Cd, J. Phys. Chem. 91 (1987) 634–643. [45] V.A.G. Rivera, S.P.A. Osorio, D. Manzani, Y. Messaddeq, L.A.O. Nunes, E.M. Jr, Growth of silver nano-particle embedded in tellurite glass: interaction between localized surface plasmon resonance and Er3+ ions, Opt. Mater. 32 (2011) 888–892.