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Fabrication, characterization and some applications of graded chiral zigzag shaped nano-sculptured silver thin films
Applied Surface Science 257 (2011) 9425–9434
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Fabrication, characterization and some applications of graded chiral zigzag shaped nano-sculptured silver thin films Hadi Savaloni ∗ , Ali Esfandiar Department of Physics, University of Tehran, North-Kargar Street, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 20 February 2011 Received in revised form 3 May 2011 Accepted 6 June 2011 Available online 14 June 2011 Keywords: Graded chiral zig–zag Silver Oblique angle deposition Sculptured thin films Hydrophobicity Field ionization gas sensors
a b s t r a c t Graded chiral zig–zag shaped nano-sculptured silver thin films (GCZSSTF) were produced in two stages using oblique deposition technique together with rotation of substrate about its surface normal while a shadowing block was also fixed at the center of the substrate holder. Chrystallographic and morphological structure of these films were obtained using X-ray diffraction (XRD) and atomic force microscopy (AFM). Spectrophotometry was used to obtain their optical behavior while their application in both hydrophobicity and gas sensing was also investigated. XRD results showed a dominant (1 1 1) orientation growth on the zig arm of the structure while by addition of the second arm (zag) the crystallographical growth orientation changed to (2 2 0). The anisotropic nano-structure of these films was also distinguished through (1 − R) spectra. A common peak at about 350 nm related to the TM mode of plasmon resonances and a broad shoulder at about 420 nm for the s-polarized light and at 620 nm for the p-polarized light corresponding to the LM mode of plasmon resonances are observed. These peaks are directly related to the nano-columns topography. The film system used here proved to act as a physical method for producing layer-by-layer structure for obtaining enhanced hydrophobic surfaces rather than the usual chemical methods reported in the literature. In addition, the GCZSSTF also acted as good as reported results for nano-tubes when applied as cathode in the field ionization gas sensing setup. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nano-structure of noble metals (gold, silver and copper) depending on their size and geometrical shape has attracted a great deal of research interest in different fields, such as optics [1,2], electronic [3,4], biomedicine [5–7] and quantum size domain [8]. This group of metallic nano-structures because of their ability in exhibiting localized surface plasmon resonances (LSPR) have found many applications in biodetection [9]. LSPR excitation can considerably enhance the local electric field which is the main reported mechanism of surface enhanced Raman spectroscopy (SERS) [10]. Hot spots in noble metal nanostructures, which yield large amount of field enhancement, occur at given interstices (e.g., surface or tips) in between nano-structures, hence enhancing the Raman scattering of molecules adsorbed at the vicinity [11]. Many simple shaped nano-structures, such as spheres, spheroids, pyramids, cubes, and cylinders have been studied for shape dependence of LSPR [12]. In recent years, oblique angle deposition (OAD) of thin films as a physical vapor deposition method has provided facilities for production
∗ Corresponding author. Tel.: +98 21 88635776; fax: +98 21 88004781. E-mail address: [email protected] (H. Savaloni). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.06.023
of variety of nano-structures with structural anisotropy which can be controlled by pre-design of the structure [13,14]. OAD and glancing angle deposition (GLAD) methods together with the rotation of substrate about its surface normal can be used to produce differently shaped nanostructures such as chirals [15], zigzags [16], S-shaped and other shapes [17]. Sculptured nano-structures fabricated by these methods have many applications such as optical filters [18], photonic crystals [19], catalysts [20], magnetic storages [21], micro-batteries [22] bioscaffolds [23] and microchannels [24]. Oblique silver nano-columns grown using OAD method without rotation of substrate have shown surface enhanced Raman spectra (SERS) [25] and surface enhanced fluorescence (SEF) [26] which can give useful and desired results in detection of minute amount of chemicals and viruses. Also appropriate thicknesses of noble metals depositions on dielectric nano-structures produced using GLAD technique, provides anisotropic nano-structures for surface enhanced Raman spectroscopy (SERS) [27]. Recently by using a shadowing block in the GLAD technique TiO2 chiral slanted sculptured thin films with nanostructures that their structural parameters vary with distance from the edge of the shadowing block are reported [28]. In our earlier work [29] we studied optical behavior of these structures theoretically. In
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this work we have used a two staged oblique incident deposition technique together with rotation of the substrate holder and a shadowing block which is fixed at the center of the substrate holder to produce graded chiral zigzag silver sculptured thin films (GCZSSTF), while the nanostructural characteristics of these films are obtained using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), atomic force microscope (AFM) and optical spectroscopy. Their field ionization gas sensor property is also investigated.
2. Experimental details The graded chiral zigzag nano-sculptured silver thin films (GCZSSTF) were deposited on glass (microscope slide) substrates by resistive evaporation from tungsten boat at room temperature. The purity of silver was 99.99%. An Edwards (Edwards E19 A3) coating plant with a base pressure of 2 × 10−7 mbar was used. The deposition angle was fixed at 80◦ and a substrate azimuthal rotation speed of 0.025 rpm was chosen. Fig. 1 shows schematic of the evaporation system showing substrate position and rotation for sculptured chiral thin film growth. The movement of the stepper motor and its speed of revolution as well as facility for dividing each revolution to different sectors are controlled through interface to a computer in which the related software is written and installed. All these are domestic made. The substrate holder was a stainless steel disc of 100 mm diameter. At the center of this disc a cylindrical block (20 mm in diameter and 18 mm in height) was fixed as a shadowing block. The substrates were fixed at 10, 20 and 30 mm distances from this shadowing block along the radius of the substrate holder disc. The deposition of GCZSSTF was carried out in two steps. First the substrate holder was rotated anti-clockwise and after completion of the zig part of the zigzag structure, deposition was stopped and the substrate holder was removed from the chamber, then the substrates were rotated on the substrate holder by 180◦ and finally the substrate holder was fixed on the shaft of the stepping motor in the deposition chamber and the system was evacuated for the second step of deposition to be carried out. The second step of deposition was carried out with the clockwise rotation of the substrate holder. Each zig and zag of the structure consists of four pitches of the chiral structure. The deposition rate for both deposition steps was fixed at 0.12 nm/s. In order to provide a point source for geometrical considerations, a plate of tungsten with a 6 mm diameter hole in the middle was used as a mask on top of the evaporation boat. The distance between the center of this point source and the center of the substrate holder disc was 28 cm (Fig. 1). Prior to deposition, all glass substrates were ultrasonically cleaned in heated acetone then ethanol. The surface roughness of the glass substrates was measured by a Talysurf profilometer and AFM and the rms substrate roughness Rq obtained using these methods was 0.3 nm and 0.9 nm, respectively. The deposition process was repeated a few times and the reproducibility of the results was confirmed (i.e., optical reflection from the reproduced samples agreed within 5%). The deposition rate was measured by a quartz crystal deposition rate controller (Sigma Instruments, SQM-160, USA) positioned close to the substrate holder and at almost the same azimuthal angle as that of the substrate. This was corrected after obtaining the film thickness using field emission electron microscope. The film thicknesses and column shapes and sizes were analysed by field emission electron microscope (FESEM) (Hitachi S-4100 SEM, Japan). The FESEM samples were coated with a very thin layer of gold to prevent the charging effect. Nanostructure of these films was obtained using a Siemens D500 X-ray Diffractometer (CuK␣ radiation; 40 kV, 30 mA) with a step
Fig. 1. (a) Schematic of oblique deposition with rotation of substrate holder about its surface normal and a shadowing block fixed at the center of the substrate holder; (b) stage 1, production of zig arm; (c) stage 2, the zag arm of zigzag is grown on the zig arm.
size of 0.02◦ and count time of 1 s per step, while the surface physical morphology and roughness was obtained by means of AFM (NT-MDT, SOLVER) analysis with a Si tip of 10 nm in diameter and in non-contact mode. The transmittance spectra of the samples were obtained using a single beam spectrophotometer (Aquila nkd8000) in the spectral range of (350–1050 nm) using both s and p polarization measurements in steps of 5 nm wavelength. 3. Results and discussions 3.1. Topography and surface morphology Fig. 2 shows the FESEM images of the surfaces and cross-sections of the GCZSSTFs produced at three different distances of 10, 20 and
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Fig. 2. SEM images of cross-sections and surfaces of graded chiral zigzag nano-sculptured silver thin films (GCZSSTF) at different distances from the edge of the shadowing block. (a) and (a ) 10 mm; (b) and (b ) 20 mm; (c) and (c ) 30 mm.
30 mm from the edge of the shadowing block. These figures clearly show the chiral columnar nanostructures with a zigzag shape. It can also be observed that by increasing the distance from the shadowing block the angle between the zig and the zag of these nanosculptured thin films increases and at the farthest distance from the edge of the shadowing block it almost becomes vertical (zig is almost aligned with the zag). These angles are measured and are given in Table 1. The shadowing block is responsible for the variation in growth of these slanted chiral nano-sculptured structures (i.e., in zig or zag arms of the zig–zag structure) and it can be observed that the tilt angle (˛t ) decreases and the diameter of the chiral structures increases with increasing distance from the edge of the shadowing block. The reason for this is that the shadow of the block does not let the vapour flux reaches the area near and
behind the shadowing block (shadowed region) during half of the substrate rotation. Hence the growing structure only receives vapour flux from one side. Therefore, in this region the nanocolumns are inclined towards the higher flux (towards the outer edge of the substrate holder). However, at distances far from the edge of the shadowing block (outside of the shadowed region) the arriving vapour flux is more isotropic. This area receives higher vapour flux during one complete rotation of the substrate holder. Hence, the chiral structures are less inclined. It is also worthwhile to mention that the farthest areas from the edge of the shadowing block are subject to highest variation of the incidence angle during one complete rotation of the substrate holder (i.e., the incidence angle for the half circle nearer to the deposition source is smaller than for the half circle farthest away from the deposition source). The difference between the incident angle at the center of
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Table 1 Results of characterization measurements. D (mm)
Thickness (nm)
(◦ )
(2 2 0)/(1 1 1)
Dave (nm)
DXRD d2 2 0 (nm)
Fv (%)
Rq (nm)
10 20 30
1920 ± 10 2050 ± 10 2120 ± 10
114.5 128.7 180
1.82 1.48 1.14
472 514 583
81.7 112.2 128.5
18.2 17.4 14.1
97.5 82.6 76.9
D, distance from shadowing block; , angle between two arms (◦ ); Dave , average grain size; DXRD , coherently diffracting domain size; Fv , surface void fraction; Rq , root mean square roughness.
the shadowing block and the outer edge of the substrate holder for two positions of up (highest vertical distance from the source) and down (lowest vertical distance from the source) of the substrates was calculated to be ±5◦ . From the results reported for the size of a shadowed area by a lithographical seed [30], one may estimate the size of the shadowed area by the shadowing block used in this work. In fact we may suggest that the macroscopic shadowing block produces a shadow similar to the seed and forms a conical shadow. The projection of this area on the substrate can be fitted by its resemblance to a triangle (Fig. 3). Length of this triangle (length of the shadow) (lsh ) depends on the height of the shadowing block (h) and vapour incident angle (˛) and its width depends on the radius of this block (rb ). Depending on the distance from the edge of the shadowing block (di ) an arc (fraction of half a circle) (fi ) is located in the shadow. Using the above mentioned geometrical parameters, the inclination of the columns from normal to the substrate surface (˛t ) can be calculated, as follows (Fig. 3): lsh =
h tan(90◦ − ˛)
y=−
r b
lsh
i = tan−1 fi =
x + rb
y x
2i 180◦
˛t = tan−1 (2fi )
(1) (2)
where x and y are intersect points of the arc with arms of the triangles. Relation (5) predicts the inclination of chiral-columns from normal to the substrate with relatively good accuracy (±2◦ ) for both this work and Brett et al.’s results [28]. Fig. 4 shows a schematic of two chiral structures that are grown in two different regions of the substrate, one (region 1) is the chiral structure grown near the edge of the shadowing block and the other (region 2) is the chiral structure grown far from the edge of the shadowing block. As mentioned above (fi ) is the arc (fraction of half a circle) located in the shadow of the block. Hence (1 − fi ) indicates the amount of the vapor flux at distance i from the edge of the shadowing block. Therefore, even inside the shadow of the shadowing block due to oblique deposition the rate of the evaporant flow is graded. Hence during each revolution of the substrate holder the area inside the shadow only receives a limited amount of evaporant when it is not in the shadow of the shadowing block (almost at half cycle of the substrate holder rotation) and it is directly facing the incident flow of evaporant, while the area outside the shadow receives the flow of evaporant at all times. Hence the areas further to the shadowing block almost grow in the similar way as ordinary chiral sculptured thin films while those in the shadowing block tend to tilt with respect to the surface normal. Also since the chirals rotate with the same angular speed the structural pitch along the chiral axis at all regions should be the
(3) (4) (5)
Fig. 3. Schematic of shadowing block and shadowed region. (a) Cross-section and (b) top view. 2 1 and 2 2 are the arc fractions at d1 and d2 distances from the edge of the shadowing block (explained in the text).
Fig. 4. Schematic of chiral structures fabricated at two different regions; region 1: near the edge of the shadowing block; region 2: far from the edge of the shadowing block.
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Fig. 5. 2 and 3D AFM images of graded chiral zigzag nano-sculptured silver thin films (GCZSSTF) at different distances from the edge of the shadowing block. (a) 10 mm; (b) 20 mm; (c) 30 mm.
same. With respect to Fig. 4 one may write; 2˝1 D1 1 − f1 = 1 − f2 2˝2 D2
(6)
Since (f1 > f2 ), hence vapor flow at region 1 is less than region 2 and the right side of Eq. (6) is smaller than unity. In addition as explained above, ˝1 = ˝2 , therefore D1 < D2 . Of course, one may extend this
argument to three dimensions which does not make a difference in the concluding result. This simple geometrical relation, in addition to the description given above shows that the chiral structures at near distance to the shadowing block should be narrower. This is also in agreement with Bret et al. observation (see Fig. 2 in Ref. [28]). However, due to high surface diffusion of silver adatoms and the initial temperature of the substrate (room temperature) and the heat accumulation during the long time deposition of each arm of
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3.2. Crystal structure XRD patterns of the samples produced in this work for three distances from the edge of the shadowing block are given in Fig. 6(b). Crystalline structure of the samples is consistent with the JCPDS card No.: 04-0783. Zhou et al. [36] reported crystalline structure for their silver nanorod samples even when they reduced the substrate temperature to −40 ◦ C. However we observed the interesting feature in our results for the GCZSSTF that is the unusual high intensity for Ag(2 2 0) diffraction line relative to Ag(1 1 1) peak. It is well known that silver has a fcc structure with lowest surface energy for the (1 1 1) facet, hence it should grow with higher abundance in this direction. The JCPDS card for powder sample also give the 100% intensity for Ag(1 1 1) peak, while the intensity for Ag(2 2 0) is 25%. Our observation of high intensity for Ag(2 2 0), to the best of our
(a)
250
200
Intensity(arb.u)
the zigzag (∼4.0 h), the pitches of the chiral shapes are not clearly distinguishable. The formation of a dense bottom layer in addition to the nanosculptured thin films is reported by Alouach et al. [31] and Savaloni et al. [32]. In the FESEM images in Fig. 3 this bottom layer can also be distinguished. In addition, it can be seen that the width of the columns (diameter of chirals) is increased with film thickness and the columns have branched at the film surface becoming facetted. This intriguing behavior of the nano-sculptured thin films is also being reported for copper (another noble metal) thin film [33,34]. The above observations may be due to few processes as follows; the bottom layer surface due to the shadowing effect in oblique incidence tends to be rough, hence providing peaks that can shadow the lower areas and the following film will grow predominantly on these peaks and the lower areas do not receive any (receive less) flow of evaporant. In addition, owing to high diffusion of silver adadtoms the film growing on these peaks (columns) become broader by thickness. The branching of the columns by growth is due to competitive growth between the peaks and the lower (shadowed) areas [35]. In Fig. 2(c) and the AFM results (Fig. 5) the crystal structure of the growing columns can be clearly distinguished as also reported by Kesapregada for chiral sculptured copper thin films (another noble metal) [35]. In Fig. 5 it can be observed that the size of the grains on the film surface (diameter of columns/chirals) increases and the surface void fraction decreases with increasing the distance from the edge of the shadowing block. The quantitative data obtained from the AFM images are given in Table 1. Diameter of the grains and the void fraction were measured using the J-Microvision software and the surface roughness of the films was obtained from the AFM measurements. Fig. 5 clearly shows the variation of the grain size and void fractions by increasing the distance from the edge of the shadowing block. At far distances from the edge of the shadowing block the adatoms after incidence on the surface will have higher linear velocity (due to the rotation of the substrate) which results in higher mobility for these adatoms, while these areas also receive higher flow of evaporant, hence all these effects together will enhance the diffusion effect which in turn results in larger diameter columns/chirals, therefore reducing the void fraction. This process is also the cause of reduction of the film surface roughness. Since the columns/chirals during their growth do not reach each other one may conclude that they follow a competitive growth procedure, as discussed by Zhou and Gall [35]. Therefore in this section by describing the physical features of these films we have shown that by using a macroscopic block and some further manipulation of the deposition conditions, such as distance from the edge of the shadowing block, we can change the structural parameters of the nano-sculptured thin films.
150
100
50 30
40
50
60 70 2θ ( °)
(b) 1000
80
90
100
10 mm from edge of block 20 mm from edge of block 30 mm from edge of block
800
Intensity (arb.u)
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600
400
200
0 30
40
50
60
70 2θ ( ° )
80
90
0 0.1 0.2
Fig. 6. XRD spectra of graded chiral zigzag nano-sculptured silver thin films (GCZSSTF). (a) XRD from the stage 1 (zig arm of the zigzag structure produced at 20 mm from the shadowing block); (b) XRD from the zigzag structure at different distances from the shadowing block.
knowledge, is not seen or reported in any of the physical synthesis techniques while similar results are reported for chemical methods [37,38] by introducing some limitations in the growth process. As mentioned above the dominant crystallinity in Ag structure is related to Ag(1 1 1) as we observed for the films produced at the first step of our deposition (i.e., only zig arm of the zigzag). As a typical example this is shown in Fig. 6(a) for the film produced at 20 mm distance from the edge of the shadowing block after the first step of deposition was completed (only the zig section of zigzag structure is formed). However after deposition of the second step (the zag section) on this film (Fig. 6(b)) the Ag(2 2 0) has become the dominant peak. This preferred growth direction may be explained in the following way; after the completion of the first step of the deposition which shows the usual dominant peak of Ag(1 1 1), the surface of this film consists of ledged and kinked form sites which act as irreversible growth sites for the adatoms in the second step of deposition [39]. According to the periodic bond chain theory (PBC) [39], {1 1 0} and {1 1 1} faces of fcc crystals form sites with lowest reversibility relative to other faces, hence the adatoms become adsorbed on these faces and will have the highest growth rate. But since the deposition is taking place at oblique angle and in the presence of the shadowing block, the vapour flux should arrive on the (1 1 1) face of the zig arm (formed in the first step of the film growth) while it is also confined by both oblique angle and the shadowing block. These effects may cause the film to grow in the second stage in the (1 1 0) orientation, as observed in Fig. 6(b). (2 2 0)/(1 1 1) ratio for these samples is given in Table 1, which shows reduction by dis-
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Recently Zhang and Zhao [41] have reported on the theoretical study (i.e., discrete dipole approximation (DDA)) of light scattering from silver chiral shaped sculptured structures and multi-ring structures as well as aligned silver nanorod arrays. In both works they have also designated the shorter wavelength peaks (i.e., ∼350 and 400 nm) to the TM mode (transverse dipole mode (TDM) and transverse quadruple mode (TQM)) and the longer wavelength peak to the LM mode of plasmon excitation. Fig. 8(a) shows the (1 − R) spectra for both s- and p-polarizations for the sample positioned at 10 mm distance from the edge of the
(a) 1 P polarization S polarization
Fig. 7. The schematic for the chiral nano-sculptured column and the definition of the incident polarization directions as well as decomposition of the p-polarized field by changing the incident light direction.
1-Relectance (arb.u)
0.9
0.8
0.7
0.6
0.5
tancing from the edge of the shadowing block which again shows the effect of the tilting angle of the GCZSSTF on the crystallographic structure of these films.
500
600
700
800
900
1000
(b) 1 P polarization S polarization 0.9 1-Reflectance (arb.u)
3.3. Optical characterization
0.8
0.7
0.6
0.5
400
500
600
700
800
900
1000
(c) 1 P polarization S polarization 0.9 1-Reflectance (arb.u)
Both reflection and transmission spectra were obtained at 30◦ incidence angle and the opposite direction of that of the second arm of the zigzag film produced at different distances from the edge of the shadowing block, using both s- and p-polarized incident light. We define the p-polarization at the zigzag plane and the s-polarization at normal to that. In obtaining the absorption/extinction spectra from our optical measurements the following effects/phenomenon were considered: (a) the transmittance spectrum from our silver thin films was negligible due to the high film thickness, (b) usually the nonspecular reflection is very low for Ag films, (c) it is also well known that by increasing the particle size the absorption maximum shifts towards lower energies and the band/peak becomes broader, and it is also possible that higher order modes become excited, and (d) in case of particles whose size is smaller than the wavelength of the incident light, the electromagnetic wave cannot distinguish their structural details. However, the absorption spectrum may be affected by their shape. When features with different shapes are distributed in the structure of thin films, they cause broadening of the absorption peaks. Therefore, it may be a good approximation to consider (1 − R) as the film extinction which may show the surface plasmon resonances in the Ag nano-structures. The incident angle is an important parameter in plasmon excitation [40]. The 30◦ incidence angle was chosen to make sure that the p-polarized light interacts with the Ag column with almost its maximum electric field. In Fig. 7 it is shown that when the incident light interacts with the chiral structure at an angle the p-polarization field, Ep decomposes into two components: a field parallel EpII to, and a field perpendicular Ep⊥ to the long axis of the Ag rod. Therefore, the larger i results in larger EpII component. Hence EpII excites the longitudinal mode (LM) and Ep⊥ excites the transverse mode (TM) [41].
400
0.8
0.7
0.6
0.5
400
500
600 700 800 Wavelength (nm)
900
1000
Fig. 8. Optical spectra (s- and p-polarizations) of graded chiral zigzag nanosculptured silver thin films (GCZSSTF) at different distances from the shadowing block. (a) 10 mm; (b) 20 mm; (c) 30 mm.
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shadowing block which reflect the anisotropic nano-structure of the film too. A common peak at about 350 nm related to the TM mode (i.e., TDM) of plasmon resonances [42] can be distinguished in both spectra though our range of wave length measurement started from 350 nm. In addition a broad shoulder at about 420 nm for the s-polarized light (TQM) and at 620 nm for the p-polarized light corresponding to the LM mode of plasmon resonances can be distinguished with reference to the preceding discussion about decomposition of the Ep into two components of EpII and Ep⊥ . These peaks are directly related to the nano-columns topography. The peak at short wave length for s-polarized incidence light is indicative of the smaller aspect ratio of the nano-structures in the resonance plane of s-polarization (i.e., plane normal to the zigzag plane). On the other hand, we may consider the second arm of the zigzag as an assembly of thinner nano-columns/clusters which are aligned in the direction of the axis of the column. This scheme is consistent with the XRD results discussed in Section 3.2. In Fig. 8(b) and (c) the (1 − R) spectra for both s- and ppolarizations for the samples positioned at 20 mm and 30 mm from the edge of the shadowing block are given. It can be seen that the anisotropy is decreased relative to Fig. 8(a), while the intensity of (1 − R) spectra is increased by increasing the distance from the edge of the shadowing block. This can be explained in the following way: as the distance from the edge of the shadowing block increases the angle between two arms of the zigzag structure increases and the two arms almost become aligned (helical-like structure is formed) for the sample positioned at 30 mm from the edge of the shadowing block. Hence not only anisotropy is decreased but also this structure results in reduced reflection due to increased penetration path of light inside the film thickness (i.e., somewhat like channeling effect) which in turn increases 1 − R spectra. It is worthwhile to point out here the reasons for not observing the circular Bragg Phenomenon [43] in our reflection (1 − R) spectrum of the sample deposited at 30 mm from the edge of the shadowing block (Fig. 8(c)), as helical-like structures of silver thin film are obtained. The absorption transitions from dielectric to metal in metal–dielectric composite are described by percolation phenomenon: the percolation threshold (i.e., the insulator to conductor transition) occurs sharply in the permittivity of a homogenized composite material when the volumetric fraction of metallic particles, fv increases continuously from zero [44]. Circular Bragg phenomenon in metallic films occurs when the volumetric fraction of metal inclusion is low [29,43,45]. This is because at this volumetric fraction the composite layer (metal inclusion + voids) has not reached the percolation threshold. In Table 1 (column 7) the calculated volume fraction of void for this film is given as 14.1% and so ∼86% for metal which is a high volume fraction considering the results given in Ref. [45] (e.g., 20%). In addition in order to observe the circular Bragg phenomenon for such a sample in which the columns are grown almost normal to the substrate surface the light should incidence on the sample at normal angle. We used 30◦ incidence angle in our optical measurements. As the p-polarized light oscillates along the axis of the column, the effective plasmon oscillation length is longer and consequently its peak appears at longer wavelengths. Of course, due to the fact that all columns are not in the same direction (due to branching and size distribution of columns resulting from competitive growth) these peaks appear with a considerable breadth/width. By controlling the growth parameters of the nano-sculptured thin films and considering their nano-structural anisotropy one may control the surface plasmon resonance peak wavelength, using different polarized lights and angles of incident. This feature of silver nanostructures also provides an opportunity for their application in chemical detectors and virus sensors [46].
3.4. Hydrophobicity It is well established that superhydrophobic surface can be achieved through combination of high surface roughness and low surface free energy [47–49]. A large number of physical techniques such as lithographic patterning and etching, molding, and imprinting have been used to obtain rough surfaces in order to produce superhydrophobic surfaces [50–52]. On the other hand, direct deposition of micro/nanostructures of different materials such as metals [53], polymers [54], oxides [55] or carbon nanotubes [56] on substrates through chemical reactions and or assembly processes also have proved to show superhydrophobicity. Layerby-layer (LbL) assembly was first introduced by Decher and Hong [57] in order to achieve superhydrophbic surface, as a versatile method to assemble layered nanostructures with tailored composition and architecture [58]. In this work, we have used sculptured silver thin films production technique and engineered the sample surface to produce layer-by-layer structures, in order to both enhance the surface roughness and achieve crystallographic facet with possible low surface free energy (i.e., graded chiral zigzag nano-sculptured silver thin films (GCZSSTF)), so that we can provide one of the intriguing applications of GCZSSTF. AFM and FESEM results discussed in Section 3.1, illustrated that the top of these films are formed in pyramid shape. Since silver has a fcc structure, these facets considering their orientations and the XRD results may be related to (1 1 1) planes, which have the lowest surface free energy in fcc structure. AFM results (Section 3.1) also show that the surface roughness is increased when the zag arm of the zigzag structure is formed on the zig arm. Hence one expects to observe enhanced hydrophobicity property in these films. In Fig. 9, results of hydrophobicity test on a silver film of 300 nm thickness deposited under conventional condition of normal vapour incidence, graded chiral sculptured silver film produced after the first step of deposition in this work (i.e., only the zig arm produced at 10 mm distance from the shadowing block), and the graded chiral sculptured zigzag silver film produced after the second step of deposition at 10 mm distance from the edge of the shadowing block in this work, are compared. It can be observed that the contact angle for the latter sample is considerably increased relative to the two former samples (i.e., 125◦ compared to 106◦ and 74◦ ). In second step, the morphology (length and shape) of nano columns as well as interspaces and air diffusion between nano columns change. These variations of structural parameters are the main factors for the increase of contact angle [59]. Hence in this work we have achieved enhanced hydrophobic surface by engineering the sample surface via sculptured thin film deposition to produce layer-by-layer nanostructure. 3.5. Field ionization gas sensing In field ionization gas sensor the gas molecules/atoms become ionized at a certain voltage applied between anode and cathode, under an appropriate pressure. Potential difference at locations of small curvatures (sharp points) are responsible for this phenomenon, where strong fields are produced at relatively low voltages [60] which is based on the Townsend discharge before the electrical breakdown occurs at the tips (sharp points) by creation of strong fields. When a strong electrical field occurs at the sharp points, gas molecules are ionized and ions and electrons are collected by the cathode and the anode that are positioned at a certain distance from each other. The higher the aspect ratio for a conductive nano-structure, the stronger field lines forms around their features that may be produced of different shapes with controlled void fraction by glancing/oblique angle deposition and rotation of the substrate as reported in this work. This procedure can provide
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Fig. 9. Contact angles for water droplets on (a) 300 nm silver thin film deposited at normal angle to the substrate without rotation of substrate holder; (b) graded chiral zig arm of the zigzag nano-sculptured silver thin film; (c) graded chiral zigzag shape nano-sculptured silver thin film.
new approaches in making a new generation of sensors. Current discharge and breakdown voltage depend on the type of the gas, working pressure and the distance between the two electrodes [61]. Sharp points, keen edges and branched walls which can be engineered and produced in sculptured thin films are suitable for enhancement of local fields that affect the performance of the field ionization sensors.
Fig. 10(a) shows our gas sensing setup in which the GCZZSSTF sample grown at close distance to the edge of the shadowing block is used as cathode and an Al plate is used as anode. The distance between the two electrodes was fixed at 100 m. Air, argon, hydrogen, acetylene and oxygen gases were used. Results of breakdown voltage versus gas pressure obtained for different gases are given in Fig. 10(b), while current versus breakdown voltage for different gases at 10 Torr is given in Fig. 10(c). Apart from the results obtained for oxygen which shows a high breakdown voltage, due to surface oxidation of silver, the results for other gases are comparable to that usually obtained from sensors fabricated by carbon nano-tubes (e.g., see Figs. 2 and 3 in Ref. [55]) and metal oxide nanowires (e.g., see Figs. 3 and 4 in Ref. [56]). This is achieved because of high porosity and conductivity of the nanostructure used as well as presence of sharp points (keen edges) on the columnar structure. It is worthwhile to mention that both carbon nano-tubes and metal oxide nano-wires (e.g., ZnO) [56] have also shown high breakdown voltages for Oxygen, comparable to our results. Although our field ionization gas sensor results for GCZSSTF are comparable to those of carbon nano-tubes, but one may also point out the cumbersome procedure for the production of such sculptured thin films. This may be compensated by better design of the thin film nanostructure and the structure of the setup for gas sensing which can provide even better results. We are currently working on this issue and will report our results and the setup arrangement in the near future. 4. Conclusions
Fig. 10. (a) Schematic of field ionization gas sensor setup; (b) break down voltage versus pressure for different gases; (c) current versus breakdown voltage for different gases.
In this work, by using the oblique deposition technique together with rotation of substrate holder about its surface normal while a shadowing block was fixed at the center of the substrate holder in two separate stages, we produced graded chiral zigzag nanosculptured silver thin films (GCZSSTF). Our characterization of the structure of film showed variation of the structural parameters (i.e., tilt angle, void fraction and the diameter of the columns) at different distances from the shadowing block. XRD results showed that the crystallographic structure changes from (1 1 1) orientation at the first stage of growth (zig arm of the nano-structure) to (2 2 0) in the second stage of growth (zag arm of the nano-structure). (1 − R) spectra for both s- and p-polarizations showed that nanostructure of the films are anisotropic. A common peak at about 350 nm related to the TM mode of plasmon resonances and a broad shoulder at about 420 nm for the s-polarized light and at 620 nm for the p-polarized light corresponding to the LM mode of plasmon resonances are observed. These peaks are directly related to the nano-columns topography. The chiral columnar zigzag structure produced in this work also showed that it can act as a physical method in producing layer-bylayer structure for obtaining enhanced hydrophobic surfaces rather than the usual chemical methods reported in the literature. In addition, these structures also showed that can be used successfully in field ionization gas sensing and the results obtained are comparable
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to those of nano-tubes, though production of these sculptured thin films is both more difficult and costly. Acknowledgements This work was carried out with the support of the University of Tehran. H.S. is grateful to the Institute for Research and Planning in Higher Education for the partial support of this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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[24] K.D. Harris, M.J. Brett, T.J. Smy, C. Backhouse, J. Electrochem. Soc. 147 (2000) 2002. [25] M. Suzuki, W. Maekita, Y. Wada, K. Nagai, K. Nakajima, K. Kimura, T. Fukuoka, Y. Mori, Nanotechnology 19 (2008) 265304. [26] I. Abdulhalim, A. Karabchevsky, C. Patzig, B. Rauschenbach, B. Fuhrmann, E. Eltzov, R. Marks, J. Xu, F. Zhang, A. Lakhtakia, Appl. Phys. Lett. 94 (2009) 063106. [27] J.G. Fan, Y.P. Zhao, Langmuir 24 (2008) 14172. [28] K.M. Krause, M.J. Brett, Adv. Funct. Mater. 18 (2008) 3111. [29] F. Babaei, A. Esfandiar, H. Savaloni, Opt. Commun. 283 (2010) 2849. [30] M.A. Summers, M.J. Brett, Nanotechnology 19 (2008) 415203. [31] H. Alouach, G.J. Mankey, J. Vac. Sci. Technol. A 22 (2004) 1379. [32] H. Savaloni, F. Babaei, S. Song, F. Placido, Appl. Surf. Sci. 255 (2009) 8041. [33] S.V. Kesapragada, D. Gall, Appl. Phys. Lett. 89 (2006) 203121. [34] J. Wang, H. Huang, S.V. Kesapragada, D. Gall, Nano Lett. 5 (2005) 2505. [35] C.M. Zhou, D. Gall, Thin Solid Films 515 (2006) 1223. [36] Q. Zhou, Z. Li, Y. Yang, Z. Zhang, J. Phys. D: Appl. Phys. 41 (2008) 152007. [37] X.Y. Sun, F.Q. Xu, Z.M. Li, W.H. Zhang, Mater. Chem. Phys. 90 (2005) 69. [38] R.L. Zong, J. Zhou, Q. Li, B. Du, B. Li, M. Fu, X.W. Qi, L.T. Li, J. Phys. Chem. B 108 (2004) 16713. [39] P. Hartman, W.G. Perdok, Acta Crystallogr. 8 (1955) 49. [40] S. Kim, Y.J. Jung, G.H. Gu, J.S. Suh, S.M. Park, S. Ryu, J. Phys. Chem. C 113 (2009) 16321. [41] P. Zhao, S.B. Chaney, Z.Y. Zhang, J. Appl. Phys. 100 (2006) 063527. [42] Y.P.P. Zhao, S.B. Chaney, Z.Y. Zhang, J. Appl. Phys. 100 (2006) 063527. [43] A. Lakhtakia, Microw. Opt. Technol. Lett. 24 (2000) 239. [44] T.G. Mackay, A. Lakhtakia, Opt. Commun. 259 (2006) 727. [45] F. Babaei, H. Savaloni, Opt. Commun. 278 (2007) 221. [46] C. Ni, P.A. Hassan, E.W. Kaler, Langmuir 21 (2005) 3334. [47] R.N. Wenzel, J. Ind. Eng. Chem. 28 (1936) 988. [48] B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 546. [49] N. Zhao, F. Shi, Z. Wang, X. Zhang, Langmuir 21 (2005) 4713. [50] M. Nosonovsky, B. Bhushan, Nano Lett. 7 (2007) 2633. [51] W.K. Cho, I.S. Choi, Adv. Funct. Mater. 18 (2008) 1089. [52] A. Winkleman, G. Gotesman, A. Yoffe, R. Naaman, Nano Lett. 8 (2008) 1241. [53] I.A. Larmour, S.E.J. Bell, C. Saunders, Angew. Chem. Int. Ed. 46 (2007) 1710. [54] B. Balu, V. Breedveld, D.W. Hess, Langmuir 24 (2008) 4785. [55] F. Namavar, C.L. Cheung, R. Sabirianov, W.N. Mei, X.C. Zeng, G. Wang, H. Haidar, K.L. Garvin, Nano Lett. 8 (2008) 988. [56] S.J. Pastine, D. Pkawa, B. Kessler, M. Rolandi, M. Llorente, A. Zettl, J.M. Frechet, J. Am. Chem. Soc. 130 (2008) 4238. [57] G. Decher, J.D. Hong, Makromol. Chem. Macromol. Symp. 46 (1991) 321. [58] G. Decher, J.B. Schlenoff, Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials, Wiley-VCH, Weinheim, Germany, 2002. [59] Y.H. Cheng, C.K. Chou, C. Chen, S.Y. Cheng, Chem. Phys. Lett. 397 (2004) 17. [60] A. Modi, N. Koratkar, E. Lass, B. Wei, P. Ajayan, Nature 424 (2003) 171. [61] L. Liao, H.B. Lu, M. Shuai, J.C. Li, Y.L. Liu, Nanotechnology 19 (2008) 175501.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication, characterization and some applications of graded chiral zigzag shaped nano-sculptured silver thin films Hadi Savaloni ∗ , Ali Esfandiar Department of Physics, University of Tehran, North-Kargar Street, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 20 February 2011 Received in revised form 3 May 2011 Accepted 6 June 2011 Available online 14 June 2011 Keywords: Graded chiral zig–zag Silver Oblique angle deposition Sculptured thin films Hydrophobicity Field ionization gas sensors
a b s t r a c t Graded chiral zig–zag shaped nano-sculptured silver thin films (GCZSSTF) were produced in two stages using oblique deposition technique together with rotation of substrate about its surface normal while a shadowing block was also fixed at the center of the substrate holder. Chrystallographic and morphological structure of these films were obtained using X-ray diffraction (XRD) and atomic force microscopy (AFM). Spectrophotometry was used to obtain their optical behavior while their application in both hydrophobicity and gas sensing was also investigated. XRD results showed a dominant (1 1 1) orientation growth on the zig arm of the structure while by addition of the second arm (zag) the crystallographical growth orientation changed to (2 2 0). The anisotropic nano-structure of these films was also distinguished through (1 − R) spectra. A common peak at about 350 nm related to the TM mode of plasmon resonances and a broad shoulder at about 420 nm for the s-polarized light and at 620 nm for the p-polarized light corresponding to the LM mode of plasmon resonances are observed. These peaks are directly related to the nano-columns topography. The film system used here proved to act as a physical method for producing layer-by-layer structure for obtaining enhanced hydrophobic surfaces rather than the usual chemical methods reported in the literature. In addition, the GCZSSTF also acted as good as reported results for nano-tubes when applied as cathode in the field ionization gas sensing setup. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nano-structure of noble metals (gold, silver and copper) depending on their size and geometrical shape has attracted a great deal of research interest in different fields, such as optics [1,2], electronic [3,4], biomedicine [5–7] and quantum size domain [8]. This group of metallic nano-structures because of their ability in exhibiting localized surface plasmon resonances (LSPR) have found many applications in biodetection [9]. LSPR excitation can considerably enhance the local electric field which is the main reported mechanism of surface enhanced Raman spectroscopy (SERS) [10]. Hot spots in noble metal nanostructures, which yield large amount of field enhancement, occur at given interstices (e.g., surface or tips) in between nano-structures, hence enhancing the Raman scattering of molecules adsorbed at the vicinity [11]. Many simple shaped nano-structures, such as spheres, spheroids, pyramids, cubes, and cylinders have been studied for shape dependence of LSPR [12]. In recent years, oblique angle deposition (OAD) of thin films as a physical vapor deposition method has provided facilities for production
∗ Corresponding author. Tel.: +98 21 88635776; fax: +98 21 88004781. E-mail address: [email protected] (H. Savaloni). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.06.023
of variety of nano-structures with structural anisotropy which can be controlled by pre-design of the structure [13,14]. OAD and glancing angle deposition (GLAD) methods together with the rotation of substrate about its surface normal can be used to produce differently shaped nanostructures such as chirals [15], zigzags [16], S-shaped and other shapes [17]. Sculptured nano-structures fabricated by these methods have many applications such as optical filters [18], photonic crystals [19], catalysts [20], magnetic storages [21], micro-batteries [22] bioscaffolds [23] and microchannels [24]. Oblique silver nano-columns grown using OAD method without rotation of substrate have shown surface enhanced Raman spectra (SERS) [25] and surface enhanced fluorescence (SEF) [26] which can give useful and desired results in detection of minute amount of chemicals and viruses. Also appropriate thicknesses of noble metals depositions on dielectric nano-structures produced using GLAD technique, provides anisotropic nano-structures for surface enhanced Raman spectroscopy (SERS) [27]. Recently by using a shadowing block in the GLAD technique TiO2 chiral slanted sculptured thin films with nanostructures that their structural parameters vary with distance from the edge of the shadowing block are reported [28]. In our earlier work [29] we studied optical behavior of these structures theoretically. In
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this work we have used a two staged oblique incident deposition technique together with rotation of the substrate holder and a shadowing block which is fixed at the center of the substrate holder to produce graded chiral zigzag silver sculptured thin films (GCZSSTF), while the nanostructural characteristics of these films are obtained using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), atomic force microscope (AFM) and optical spectroscopy. Their field ionization gas sensor property is also investigated.
2. Experimental details The graded chiral zigzag nano-sculptured silver thin films (GCZSSTF) were deposited on glass (microscope slide) substrates by resistive evaporation from tungsten boat at room temperature. The purity of silver was 99.99%. An Edwards (Edwards E19 A3) coating plant with a base pressure of 2 × 10−7 mbar was used. The deposition angle was fixed at 80◦ and a substrate azimuthal rotation speed of 0.025 rpm was chosen. Fig. 1 shows schematic of the evaporation system showing substrate position and rotation for sculptured chiral thin film growth. The movement of the stepper motor and its speed of revolution as well as facility for dividing each revolution to different sectors are controlled through interface to a computer in which the related software is written and installed. All these are domestic made. The substrate holder was a stainless steel disc of 100 mm diameter. At the center of this disc a cylindrical block (20 mm in diameter and 18 mm in height) was fixed as a shadowing block. The substrates were fixed at 10, 20 and 30 mm distances from this shadowing block along the radius of the substrate holder disc. The deposition of GCZSSTF was carried out in two steps. First the substrate holder was rotated anti-clockwise and after completion of the zig part of the zigzag structure, deposition was stopped and the substrate holder was removed from the chamber, then the substrates were rotated on the substrate holder by 180◦ and finally the substrate holder was fixed on the shaft of the stepping motor in the deposition chamber and the system was evacuated for the second step of deposition to be carried out. The second step of deposition was carried out with the clockwise rotation of the substrate holder. Each zig and zag of the structure consists of four pitches of the chiral structure. The deposition rate for both deposition steps was fixed at 0.12 nm/s. In order to provide a point source for geometrical considerations, a plate of tungsten with a 6 mm diameter hole in the middle was used as a mask on top of the evaporation boat. The distance between the center of this point source and the center of the substrate holder disc was 28 cm (Fig. 1). Prior to deposition, all glass substrates were ultrasonically cleaned in heated acetone then ethanol. The surface roughness of the glass substrates was measured by a Talysurf profilometer and AFM and the rms substrate roughness Rq obtained using these methods was 0.3 nm and 0.9 nm, respectively. The deposition process was repeated a few times and the reproducibility of the results was confirmed (i.e., optical reflection from the reproduced samples agreed within 5%). The deposition rate was measured by a quartz crystal deposition rate controller (Sigma Instruments, SQM-160, USA) positioned close to the substrate holder and at almost the same azimuthal angle as that of the substrate. This was corrected after obtaining the film thickness using field emission electron microscope. The film thicknesses and column shapes and sizes were analysed by field emission electron microscope (FESEM) (Hitachi S-4100 SEM, Japan). The FESEM samples were coated with a very thin layer of gold to prevent the charging effect. Nanostructure of these films was obtained using a Siemens D500 X-ray Diffractometer (CuK␣ radiation; 40 kV, 30 mA) with a step
Fig. 1. (a) Schematic of oblique deposition with rotation of substrate holder about its surface normal and a shadowing block fixed at the center of the substrate holder; (b) stage 1, production of zig arm; (c) stage 2, the zag arm of zigzag is grown on the zig arm.
size of 0.02◦ and count time of 1 s per step, while the surface physical morphology and roughness was obtained by means of AFM (NT-MDT, SOLVER) analysis with a Si tip of 10 nm in diameter and in non-contact mode. The transmittance spectra of the samples were obtained using a single beam spectrophotometer (Aquila nkd8000) in the spectral range of (350–1050 nm) using both s and p polarization measurements in steps of 5 nm wavelength. 3. Results and discussions 3.1. Topography and surface morphology Fig. 2 shows the FESEM images of the surfaces and cross-sections of the GCZSSTFs produced at three different distances of 10, 20 and
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Fig. 2. SEM images of cross-sections and surfaces of graded chiral zigzag nano-sculptured silver thin films (GCZSSTF) at different distances from the edge of the shadowing block. (a) and (a ) 10 mm; (b) and (b ) 20 mm; (c) and (c ) 30 mm.
30 mm from the edge of the shadowing block. These figures clearly show the chiral columnar nanostructures with a zigzag shape. It can also be observed that by increasing the distance from the shadowing block the angle between the zig and the zag of these nanosculptured thin films increases and at the farthest distance from the edge of the shadowing block it almost becomes vertical (zig is almost aligned with the zag). These angles are measured and are given in Table 1. The shadowing block is responsible for the variation in growth of these slanted chiral nano-sculptured structures (i.e., in zig or zag arms of the zig–zag structure) and it can be observed that the tilt angle (˛t ) decreases and the diameter of the chiral structures increases with increasing distance from the edge of the shadowing block. The reason for this is that the shadow of the block does not let the vapour flux reaches the area near and
behind the shadowing block (shadowed region) during half of the substrate rotation. Hence the growing structure only receives vapour flux from one side. Therefore, in this region the nanocolumns are inclined towards the higher flux (towards the outer edge of the substrate holder). However, at distances far from the edge of the shadowing block (outside of the shadowed region) the arriving vapour flux is more isotropic. This area receives higher vapour flux during one complete rotation of the substrate holder. Hence, the chiral structures are less inclined. It is also worthwhile to mention that the farthest areas from the edge of the shadowing block are subject to highest variation of the incidence angle during one complete rotation of the substrate holder (i.e., the incidence angle for the half circle nearer to the deposition source is smaller than for the half circle farthest away from the deposition source). The difference between the incident angle at the center of
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Table 1 Results of characterization measurements. D (mm)
Thickness (nm)
(◦ )
(2 2 0)/(1 1 1)
Dave (nm)
DXRD d2 2 0 (nm)
Fv (%)
Rq (nm)
10 20 30
1920 ± 10 2050 ± 10 2120 ± 10
114.5 128.7 180
1.82 1.48 1.14
472 514 583
81.7 112.2 128.5
18.2 17.4 14.1
97.5 82.6 76.9
D, distance from shadowing block; , angle between two arms (◦ ); Dave , average grain size; DXRD , coherently diffracting domain size; Fv , surface void fraction; Rq , root mean square roughness.
the shadowing block and the outer edge of the substrate holder for two positions of up (highest vertical distance from the source) and down (lowest vertical distance from the source) of the substrates was calculated to be ±5◦ . From the results reported for the size of a shadowed area by a lithographical seed [30], one may estimate the size of the shadowed area by the shadowing block used in this work. In fact we may suggest that the macroscopic shadowing block produces a shadow similar to the seed and forms a conical shadow. The projection of this area on the substrate can be fitted by its resemblance to a triangle (Fig. 3). Length of this triangle (length of the shadow) (lsh ) depends on the height of the shadowing block (h) and vapour incident angle (˛) and its width depends on the radius of this block (rb ). Depending on the distance from the edge of the shadowing block (di ) an arc (fraction of half a circle) (fi ) is located in the shadow. Using the above mentioned geometrical parameters, the inclination of the columns from normal to the substrate surface (˛t ) can be calculated, as follows (Fig. 3): lsh =
h tan(90◦ − ˛)
y=−
r b
lsh
i = tan−1 fi =
x + rb
y x
2i 180◦
˛t = tan−1 (2fi )
(1) (2)
where x and y are intersect points of the arc with arms of the triangles. Relation (5) predicts the inclination of chiral-columns from normal to the substrate with relatively good accuracy (±2◦ ) for both this work and Brett et al.’s results [28]. Fig. 4 shows a schematic of two chiral structures that are grown in two different regions of the substrate, one (region 1) is the chiral structure grown near the edge of the shadowing block and the other (region 2) is the chiral structure grown far from the edge of the shadowing block. As mentioned above (fi ) is the arc (fraction of half a circle) located in the shadow of the block. Hence (1 − fi ) indicates the amount of the vapor flux at distance i from the edge of the shadowing block. Therefore, even inside the shadow of the shadowing block due to oblique deposition the rate of the evaporant flow is graded. Hence during each revolution of the substrate holder the area inside the shadow only receives a limited amount of evaporant when it is not in the shadow of the shadowing block (almost at half cycle of the substrate holder rotation) and it is directly facing the incident flow of evaporant, while the area outside the shadow receives the flow of evaporant at all times. Hence the areas further to the shadowing block almost grow in the similar way as ordinary chiral sculptured thin films while those in the shadowing block tend to tilt with respect to the surface normal. Also since the chirals rotate with the same angular speed the structural pitch along the chiral axis at all regions should be the
(3) (4) (5)
Fig. 3. Schematic of shadowing block and shadowed region. (a) Cross-section and (b) top view. 2 1 and 2 2 are the arc fractions at d1 and d2 distances from the edge of the shadowing block (explained in the text).
Fig. 4. Schematic of chiral structures fabricated at two different regions; region 1: near the edge of the shadowing block; region 2: far from the edge of the shadowing block.
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Fig. 5. 2 and 3D AFM images of graded chiral zigzag nano-sculptured silver thin films (GCZSSTF) at different distances from the edge of the shadowing block. (a) 10 mm; (b) 20 mm; (c) 30 mm.
same. With respect to Fig. 4 one may write; 2˝1 D1 1 − f1 = 1 − f2 2˝2 D2
(6)
Since (f1 > f2 ), hence vapor flow at region 1 is less than region 2 and the right side of Eq. (6) is smaller than unity. In addition as explained above, ˝1 = ˝2 , therefore D1 < D2 . Of course, one may extend this
argument to three dimensions which does not make a difference in the concluding result. This simple geometrical relation, in addition to the description given above shows that the chiral structures at near distance to the shadowing block should be narrower. This is also in agreement with Bret et al. observation (see Fig. 2 in Ref. [28]). However, due to high surface diffusion of silver adatoms and the initial temperature of the substrate (room temperature) and the heat accumulation during the long time deposition of each arm of
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3.2. Crystal structure XRD patterns of the samples produced in this work for three distances from the edge of the shadowing block are given in Fig. 6(b). Crystalline structure of the samples is consistent with the JCPDS card No.: 04-0783. Zhou et al. [36] reported crystalline structure for their silver nanorod samples even when they reduced the substrate temperature to −40 ◦ C. However we observed the interesting feature in our results for the GCZSSTF that is the unusual high intensity for Ag(2 2 0) diffraction line relative to Ag(1 1 1) peak. It is well known that silver has a fcc structure with lowest surface energy for the (1 1 1) facet, hence it should grow with higher abundance in this direction. The JCPDS card for powder sample also give the 100% intensity for Ag(1 1 1) peak, while the intensity for Ag(2 2 0) is 25%. Our observation of high intensity for Ag(2 2 0), to the best of our
(a)
250
200
Intensity(arb.u)
the zigzag (∼4.0 h), the pitches of the chiral shapes are not clearly distinguishable. The formation of a dense bottom layer in addition to the nanosculptured thin films is reported by Alouach et al. [31] and Savaloni et al. [32]. In the FESEM images in Fig. 3 this bottom layer can also be distinguished. In addition, it can be seen that the width of the columns (diameter of chirals) is increased with film thickness and the columns have branched at the film surface becoming facetted. This intriguing behavior of the nano-sculptured thin films is also being reported for copper (another noble metal) thin film [33,34]. The above observations may be due to few processes as follows; the bottom layer surface due to the shadowing effect in oblique incidence tends to be rough, hence providing peaks that can shadow the lower areas and the following film will grow predominantly on these peaks and the lower areas do not receive any (receive less) flow of evaporant. In addition, owing to high diffusion of silver adadtoms the film growing on these peaks (columns) become broader by thickness. The branching of the columns by growth is due to competitive growth between the peaks and the lower (shadowed) areas [35]. In Fig. 2(c) and the AFM results (Fig. 5) the crystal structure of the growing columns can be clearly distinguished as also reported by Kesapregada for chiral sculptured copper thin films (another noble metal) [35]. In Fig. 5 it can be observed that the size of the grains on the film surface (diameter of columns/chirals) increases and the surface void fraction decreases with increasing the distance from the edge of the shadowing block. The quantitative data obtained from the AFM images are given in Table 1. Diameter of the grains and the void fraction were measured using the J-Microvision software and the surface roughness of the films was obtained from the AFM measurements. Fig. 5 clearly shows the variation of the grain size and void fractions by increasing the distance from the edge of the shadowing block. At far distances from the edge of the shadowing block the adatoms after incidence on the surface will have higher linear velocity (due to the rotation of the substrate) which results in higher mobility for these adatoms, while these areas also receive higher flow of evaporant, hence all these effects together will enhance the diffusion effect which in turn results in larger diameter columns/chirals, therefore reducing the void fraction. This process is also the cause of reduction of the film surface roughness. Since the columns/chirals during their growth do not reach each other one may conclude that they follow a competitive growth procedure, as discussed by Zhou and Gall [35]. Therefore in this section by describing the physical features of these films we have shown that by using a macroscopic block and some further manipulation of the deposition conditions, such as distance from the edge of the shadowing block, we can change the structural parameters of the nano-sculptured thin films.
150
100
50 30
40
50
60 70 2θ ( °)
(b) 1000
80
90
100
10 mm from edge of block 20 mm from edge of block 30 mm from edge of block
800
Intensity (arb.u)
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600
400
200
0 30
40
50
60
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Fig. 6. XRD spectra of graded chiral zigzag nano-sculptured silver thin films (GCZSSTF). (a) XRD from the stage 1 (zig arm of the zigzag structure produced at 20 mm from the shadowing block); (b) XRD from the zigzag structure at different distances from the shadowing block.
knowledge, is not seen or reported in any of the physical synthesis techniques while similar results are reported for chemical methods [37,38] by introducing some limitations in the growth process. As mentioned above the dominant crystallinity in Ag structure is related to Ag(1 1 1) as we observed for the films produced at the first step of our deposition (i.e., only zig arm of the zigzag). As a typical example this is shown in Fig. 6(a) for the film produced at 20 mm distance from the edge of the shadowing block after the first step of deposition was completed (only the zig section of zigzag structure is formed). However after deposition of the second step (the zag section) on this film (Fig. 6(b)) the Ag(2 2 0) has become the dominant peak. This preferred growth direction may be explained in the following way; after the completion of the first step of the deposition which shows the usual dominant peak of Ag(1 1 1), the surface of this film consists of ledged and kinked form sites which act as irreversible growth sites for the adatoms in the second step of deposition [39]. According to the periodic bond chain theory (PBC) [39], {1 1 0} and {1 1 1} faces of fcc crystals form sites with lowest reversibility relative to other faces, hence the adatoms become adsorbed on these faces and will have the highest growth rate. But since the deposition is taking place at oblique angle and in the presence of the shadowing block, the vapour flux should arrive on the (1 1 1) face of the zig arm (formed in the first step of the film growth) while it is also confined by both oblique angle and the shadowing block. These effects may cause the film to grow in the second stage in the (1 1 0) orientation, as observed in Fig. 6(b). (2 2 0)/(1 1 1) ratio for these samples is given in Table 1, which shows reduction by dis-
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Recently Zhang and Zhao [41] have reported on the theoretical study (i.e., discrete dipole approximation (DDA)) of light scattering from silver chiral shaped sculptured structures and multi-ring structures as well as aligned silver nanorod arrays. In both works they have also designated the shorter wavelength peaks (i.e., ∼350 and 400 nm) to the TM mode (transverse dipole mode (TDM) and transverse quadruple mode (TQM)) and the longer wavelength peak to the LM mode of plasmon excitation. Fig. 8(a) shows the (1 − R) spectra for both s- and p-polarizations for the sample positioned at 10 mm distance from the edge of the
(a) 1 P polarization S polarization
Fig. 7. The schematic for the chiral nano-sculptured column and the definition of the incident polarization directions as well as decomposition of the p-polarized field by changing the incident light direction.
1-Relectance (arb.u)
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tancing from the edge of the shadowing block which again shows the effect of the tilting angle of the GCZSSTF on the crystallographic structure of these films.
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Both reflection and transmission spectra were obtained at 30◦ incidence angle and the opposite direction of that of the second arm of the zigzag film produced at different distances from the edge of the shadowing block, using both s- and p-polarized incident light. We define the p-polarization at the zigzag plane and the s-polarization at normal to that. In obtaining the absorption/extinction spectra from our optical measurements the following effects/phenomenon were considered: (a) the transmittance spectrum from our silver thin films was negligible due to the high film thickness, (b) usually the nonspecular reflection is very low for Ag films, (c) it is also well known that by increasing the particle size the absorption maximum shifts towards lower energies and the band/peak becomes broader, and it is also possible that higher order modes become excited, and (d) in case of particles whose size is smaller than the wavelength of the incident light, the electromagnetic wave cannot distinguish their structural details. However, the absorption spectrum may be affected by their shape. When features with different shapes are distributed in the structure of thin films, they cause broadening of the absorption peaks. Therefore, it may be a good approximation to consider (1 − R) as the film extinction which may show the surface plasmon resonances in the Ag nano-structures. The incident angle is an important parameter in plasmon excitation [40]. The 30◦ incidence angle was chosen to make sure that the p-polarized light interacts with the Ag column with almost its maximum electric field. In Fig. 7 it is shown that when the incident light interacts with the chiral structure at an angle the p-polarization field, Ep decomposes into two components: a field parallel EpII to, and a field perpendicular Ep⊥ to the long axis of the Ag rod. Therefore, the larger i results in larger EpII component. Hence EpII excites the longitudinal mode (LM) and Ep⊥ excites the transverse mode (TM) [41].
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Fig. 8. Optical spectra (s- and p-polarizations) of graded chiral zigzag nanosculptured silver thin films (GCZSSTF) at different distances from the shadowing block. (a) 10 mm; (b) 20 mm; (c) 30 mm.
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shadowing block which reflect the anisotropic nano-structure of the film too. A common peak at about 350 nm related to the TM mode (i.e., TDM) of plasmon resonances [42] can be distinguished in both spectra though our range of wave length measurement started from 350 nm. In addition a broad shoulder at about 420 nm for the s-polarized light (TQM) and at 620 nm for the p-polarized light corresponding to the LM mode of plasmon resonances can be distinguished with reference to the preceding discussion about decomposition of the Ep into two components of EpII and Ep⊥ . These peaks are directly related to the nano-columns topography. The peak at short wave length for s-polarized incidence light is indicative of the smaller aspect ratio of the nano-structures in the resonance plane of s-polarization (i.e., plane normal to the zigzag plane). On the other hand, we may consider the second arm of the zigzag as an assembly of thinner nano-columns/clusters which are aligned in the direction of the axis of the column. This scheme is consistent with the XRD results discussed in Section 3.2. In Fig. 8(b) and (c) the (1 − R) spectra for both s- and ppolarizations for the samples positioned at 20 mm and 30 mm from the edge of the shadowing block are given. It can be seen that the anisotropy is decreased relative to Fig. 8(a), while the intensity of (1 − R) spectra is increased by increasing the distance from the edge of the shadowing block. This can be explained in the following way: as the distance from the edge of the shadowing block increases the angle between two arms of the zigzag structure increases and the two arms almost become aligned (helical-like structure is formed) for the sample positioned at 30 mm from the edge of the shadowing block. Hence not only anisotropy is decreased but also this structure results in reduced reflection due to increased penetration path of light inside the film thickness (i.e., somewhat like channeling effect) which in turn increases 1 − R spectra. It is worthwhile to point out here the reasons for not observing the circular Bragg Phenomenon [43] in our reflection (1 − R) spectrum of the sample deposited at 30 mm from the edge of the shadowing block (Fig. 8(c)), as helical-like structures of silver thin film are obtained. The absorption transitions from dielectric to metal in metal–dielectric composite are described by percolation phenomenon: the percolation threshold (i.e., the insulator to conductor transition) occurs sharply in the permittivity of a homogenized composite material when the volumetric fraction of metallic particles, fv increases continuously from zero [44]. Circular Bragg phenomenon in metallic films occurs when the volumetric fraction of metal inclusion is low [29,43,45]. This is because at this volumetric fraction the composite layer (metal inclusion + voids) has not reached the percolation threshold. In Table 1 (column 7) the calculated volume fraction of void for this film is given as 14.1% and so ∼86% for metal which is a high volume fraction considering the results given in Ref. [45] (e.g., 20%). In addition in order to observe the circular Bragg phenomenon for such a sample in which the columns are grown almost normal to the substrate surface the light should incidence on the sample at normal angle. We used 30◦ incidence angle in our optical measurements. As the p-polarized light oscillates along the axis of the column, the effective plasmon oscillation length is longer and consequently its peak appears at longer wavelengths. Of course, due to the fact that all columns are not in the same direction (due to branching and size distribution of columns resulting from competitive growth) these peaks appear with a considerable breadth/width. By controlling the growth parameters of the nano-sculptured thin films and considering their nano-structural anisotropy one may control the surface plasmon resonance peak wavelength, using different polarized lights and angles of incident. This feature of silver nanostructures also provides an opportunity for their application in chemical detectors and virus sensors [46].
3.4. Hydrophobicity It is well established that superhydrophobic surface can be achieved through combination of high surface roughness and low surface free energy [47–49]. A large number of physical techniques such as lithographic patterning and etching, molding, and imprinting have been used to obtain rough surfaces in order to produce superhydrophobic surfaces [50–52]. On the other hand, direct deposition of micro/nanostructures of different materials such as metals [53], polymers [54], oxides [55] or carbon nanotubes [56] on substrates through chemical reactions and or assembly processes also have proved to show superhydrophobicity. Layerby-layer (LbL) assembly was first introduced by Decher and Hong [57] in order to achieve superhydrophbic surface, as a versatile method to assemble layered nanostructures with tailored composition and architecture [58]. In this work, we have used sculptured silver thin films production technique and engineered the sample surface to produce layer-by-layer structures, in order to both enhance the surface roughness and achieve crystallographic facet with possible low surface free energy (i.e., graded chiral zigzag nano-sculptured silver thin films (GCZSSTF)), so that we can provide one of the intriguing applications of GCZSSTF. AFM and FESEM results discussed in Section 3.1, illustrated that the top of these films are formed in pyramid shape. Since silver has a fcc structure, these facets considering their orientations and the XRD results may be related to (1 1 1) planes, which have the lowest surface free energy in fcc structure. AFM results (Section 3.1) also show that the surface roughness is increased when the zag arm of the zigzag structure is formed on the zig arm. Hence one expects to observe enhanced hydrophobicity property in these films. In Fig. 9, results of hydrophobicity test on a silver film of 300 nm thickness deposited under conventional condition of normal vapour incidence, graded chiral sculptured silver film produced after the first step of deposition in this work (i.e., only the zig arm produced at 10 mm distance from the shadowing block), and the graded chiral sculptured zigzag silver film produced after the second step of deposition at 10 mm distance from the edge of the shadowing block in this work, are compared. It can be observed that the contact angle for the latter sample is considerably increased relative to the two former samples (i.e., 125◦ compared to 106◦ and 74◦ ). In second step, the morphology (length and shape) of nano columns as well as interspaces and air diffusion between nano columns change. These variations of structural parameters are the main factors for the increase of contact angle [59]. Hence in this work we have achieved enhanced hydrophobic surface by engineering the sample surface via sculptured thin film deposition to produce layer-by-layer nanostructure. 3.5. Field ionization gas sensing In field ionization gas sensor the gas molecules/atoms become ionized at a certain voltage applied between anode and cathode, under an appropriate pressure. Potential difference at locations of small curvatures (sharp points) are responsible for this phenomenon, where strong fields are produced at relatively low voltages [60] which is based on the Townsend discharge before the electrical breakdown occurs at the tips (sharp points) by creation of strong fields. When a strong electrical field occurs at the sharp points, gas molecules are ionized and ions and electrons are collected by the cathode and the anode that are positioned at a certain distance from each other. The higher the aspect ratio for a conductive nano-structure, the stronger field lines forms around their features that may be produced of different shapes with controlled void fraction by glancing/oblique angle deposition and rotation of the substrate as reported in this work. This procedure can provide
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Fig. 9. Contact angles for water droplets on (a) 300 nm silver thin film deposited at normal angle to the substrate without rotation of substrate holder; (b) graded chiral zig arm of the zigzag nano-sculptured silver thin film; (c) graded chiral zigzag shape nano-sculptured silver thin film.
new approaches in making a new generation of sensors. Current discharge and breakdown voltage depend on the type of the gas, working pressure and the distance between the two electrodes [61]. Sharp points, keen edges and branched walls which can be engineered and produced in sculptured thin films are suitable for enhancement of local fields that affect the performance of the field ionization sensors.
Fig. 10(a) shows our gas sensing setup in which the GCZZSSTF sample grown at close distance to the edge of the shadowing block is used as cathode and an Al plate is used as anode. The distance between the two electrodes was fixed at 100 m. Air, argon, hydrogen, acetylene and oxygen gases were used. Results of breakdown voltage versus gas pressure obtained for different gases are given in Fig. 10(b), while current versus breakdown voltage for different gases at 10 Torr is given in Fig. 10(c). Apart from the results obtained for oxygen which shows a high breakdown voltage, due to surface oxidation of silver, the results for other gases are comparable to that usually obtained from sensors fabricated by carbon nano-tubes (e.g., see Figs. 2 and 3 in Ref. [55]) and metal oxide nanowires (e.g., see Figs. 3 and 4 in Ref. [56]). This is achieved because of high porosity and conductivity of the nanostructure used as well as presence of sharp points (keen edges) on the columnar structure. It is worthwhile to mention that both carbon nano-tubes and metal oxide nano-wires (e.g., ZnO) [56] have also shown high breakdown voltages for Oxygen, comparable to our results. Although our field ionization gas sensor results for GCZSSTF are comparable to those of carbon nano-tubes, but one may also point out the cumbersome procedure for the production of such sculptured thin films. This may be compensated by better design of the thin film nanostructure and the structure of the setup for gas sensing which can provide even better results. We are currently working on this issue and will report our results and the setup arrangement in the near future. 4. Conclusions
Fig. 10. (a) Schematic of field ionization gas sensor setup; (b) break down voltage versus pressure for different gases; (c) current versus breakdown voltage for different gases.
In this work, by using the oblique deposition technique together with rotation of substrate holder about its surface normal while a shadowing block was fixed at the center of the substrate holder in two separate stages, we produced graded chiral zigzag nanosculptured silver thin films (GCZSSTF). Our characterization of the structure of film showed variation of the structural parameters (i.e., tilt angle, void fraction and the diameter of the columns) at different distances from the shadowing block. XRD results showed that the crystallographic structure changes from (1 1 1) orientation at the first stage of growth (zig arm of the nano-structure) to (2 2 0) in the second stage of growth (zag arm of the nano-structure). (1 − R) spectra for both s- and p-polarizations showed that nanostructure of the films are anisotropic. A common peak at about 350 nm related to the TM mode of plasmon resonances and a broad shoulder at about 420 nm for the s-polarized light and at 620 nm for the p-polarized light corresponding to the LM mode of plasmon resonances are observed. These peaks are directly related to the nano-columns topography. The chiral columnar zigzag structure produced in this work also showed that it can act as a physical method in producing layer-bylayer structure for obtaining enhanced hydrophobic surfaces rather than the usual chemical methods reported in the literature. In addition, these structures also showed that can be used successfully in field ionization gas sensing and the results obtained are comparable
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to those of nano-tubes, though production of these sculptured thin films is both more difficult and costly. Acknowledgements This work was carried out with the support of the University of Tehran. H.S. is grateful to the Institute for Research and Planning in Higher Education for the partial support of this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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