- Email: [email protected]
Oblique angle deposited silver islands on Ge20Se70Te10 film substrate for surface-enhanced infrared spectroscopy
Accepted Manuscript Title: Oblique Angle Deposited Silver islands on Ge20 Se70 Te10 film substrate for Surface-Enhanced Infrared Spectroscopy Authors: Ajina Cheruvalath, V.P.N. Nampoori, Sheenu Thomas PII: DOI: Reference:
S0925-4005(19)30259-X https://doi.org/10.1016/j.snb.2019.02.045 SNB 26140
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
14 June 2018 8 February 2019 11 February 2019
Please cite this article as: Cheruvalath A, Nampoori VPN, Thomas S, Oblique Angle Deposited Silver islands on Ge20 Se70 Te10 film substrate for SurfaceEnhanced Infrared Spectroscopy, Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.02.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Oblique Angle Deposited Silver islands on Ge20Se70Te10 film substrate for SurfaceEnhanced Infrared Spectroscopy
*
International School of Photonics, CUSAT *
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Email: [email protected]
Highlights
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An extensive study on Silver - Ge20Se70Te10 film system as a substrate for SEIRA spectroscopy is carried out. SEIRA response of the system was analyzed by recording FTIR transmission spectra of analyte Hexa Decane Thiol (HDT). Silver island structures were developed on Ge20Se70Te10 film via oblique angle deposition and the optimum silver thickness which provides maximum enhancement in IR absorption is determined. The response of the system to variations in the analyte concentration is also explored
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Ajina Cheruvalath, V.P.N. Nampoori and Sheenu Thomas
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Abstract
Surface-enhanced infrared absorption (SEIRA) spectroscopy is a powerful tool for characterizing and identifying chemical/biological species. Though SEIRA technique has
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enabled detection up to few molecule levels with sophisticated device structures, there is still room for improvement (can extend to detect down to a single molecule). Superior sensor
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platforms can be developed by incorporating advanced substrate material, so as to be actively employed in trace analysis, forensic sciences etc. In this work, chalcogenide glass of composition Ge20Se70Te10 is characterized to probe its usability as a SEIRA substrate. Silver
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island structures were developed on Ge20Se70Te10 film via oblique angle deposition and its SEIRA response was recorded using FTIR spectrometer with Hexa Decane Thiol (HDT) as the analyte. Investigations reveal an optimum silver thickness at which maximum absorption enhancement is obtained. The response of the system to variations in the analyte concentration is also explored. Studies prove them to be a promising candidate as an IR substrate which can be used to develop integrated optical waveguide for highly sensitive SEIRA spectroscopy.
Keywords: GeSeTe glass, Chalcogenide glass, Silver Islands, SEIRA, IR spectroscopy, OAD Introduction Infrared spectroscopy is a widely accepted technique for the identification of molecular species and characterizing chemical reactions. This technique has the potential to replace other destructive invasive composition analysis techniques like EDS, XPS, secondary ion mass spectroscopy etc [1]. But applications of IR spectroscopy in its conventional form are
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restricted by its high detection limit. Since it is a linear spectroscopic method it obeys BeerLambert law according to which absorbance A, given by 𝛼 𝑐𝑙 ln 10
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𝐴=
can be improved by either increasing the concentration (𝑐) of the analyte or by increasing the path length of light (𝑙) which is not a solution in practical applications [2]. Another technique
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to improve absorption is field enhancement near the analyte which is achievable by
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introducing metal nanostructures above infrared transparent substrates. Enhancement in the
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infrared absorption of molecules placed very close to metal nanostructure were first observed
infrared absorption (SEIRA) [3].
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by Hartstein and coworkers in 1980 which later came to be known as surface enhanced
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The local field enhancement is due to the surface plasmon resonance (SPR) character of the metal nanostructures. As SPR wavelength is tunable with size, shape and metal type, SEIRA studies were conducted on different noble metals (Au, Ag, Cu, Pt and Fe) with various
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morphologies [4, 5]. In the beginning, SEIRA was carried out with very thin metal island films developed by coating procedures like electron beam evaporation, vacuum thermal
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evaporation, magnetic sputtering, etc. which provides a broad SPR peak and tailing in absorption covering almost entire mid-IR region [2, 6,7]. This broad nature of SPR supports the enhancement of any vibrational wavelengths of analyte so that it can be employed even with unknown molecular species. The main drawback of this method is its low field
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enhancement property [6]. Later to overcome this, metal island structures were replaced by well-shaped IR antennas which provide higher field enhancement compared to the metal island [8-12]. Field enhancement offered by IR antenna is very high (107 in ref 12) that it can be employed to detect down to few molecule levels with FTIR spectrometer [12]. The SPR peaks so obtained is not sharp and to cover the entire IR region with IR antenna so as to be employed for unknown species identification, a number of antennas with different aspect
ratios are needed. Fabrication of such well-shaped antennas requires highly sophisticated instruments [12] The IR transparent substrate material onto which the metal nanostructures are deposited should be having high infrared transparency as well as good affinity to the metal that is being used. Usually, crystals of Ge, ZnSe, CaF2, BaF2, Si etc., are used for this purpose [1,2,3,5,6]. Though reports suggest a very strong dependence of SEIRA response on the substrate
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material, very few studies are carried out to expand the available substrate choices [13,14]. Chalcogenide glasses are very promising candidates in SEIRA as they are highly transparent in the IR region (up to 20 μm depending upon composition [15]) and shows better affinity to
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metals [16-18]. In addition to this, they also provide flexibility in waveguide fabrication.
In this paper, we report studies on chalcogenide glass of composition Ge20 Se70 Te10 as an IR transparent substrate for SEIRA. Silver island structures were deposited on chalcogenide
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films via oblique angle deposition method using a conventional thermal evaporator [19].
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Hexa Decane Thiol (HDT) was used as the analyte and SEIRA was recorded with an FTIR
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spectrometer in its transmission mode.
1. Chemicals and Substrates
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Materials and methods
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5N purity elemental form Ge, Se and Te were used as purchased from Sigma Aldrich and 99.9% pure grade silver plate was purchased from local vendors. Hexa Decane Thiol (HDT)
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purchased from Sigma Aldrich was kept under refrigeration to reduce degradation. Premium grade ethanol purchased from Hayman was used as a solvent for the analyte.
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2. Bulk glass preparation Each constituent element of the glass composition was weighed according to their weight percentage and loaded in a 10cm long and 1cm diameter quartz ampoule and sealed under
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vacuum (10-5 mbar). The sealed ampoule was then kept at 1323 K for 24 hours in a rocking and rotation tube furnace. Continuous rocking and rotation are provided to the sample during this time to ensure thorough mixing and homogenization of the melt. The hot ampoule is quenched in ice cold water after 24 hours. The outer quartz ampoule is etched out using HF and the sample in the bulk form is collected. 3. Ge Se Te film preparation
Thin films were prepared using vacuum thermal evaporation unit (IHVP model: 12A4-D) for which bulk glass piece was placed in a molybdenum boat and heated via resistive heating at a pressure of 3.7 ×10-5 mbar. Films were fabricated on microscopic glass slides. Glass slides were thoroughly cleaned via sonication in a soap solution, acetone and distilled water prior to placing it for coating. 4. Silver layer fabrication
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Silver layers were fabricated on the plane glass as well as on chalcogenide glass films via oblique angle deposition method using thermal evaporation (IHVP model: 12A4-D). A silver
piece of 1gm is loaded in a tungsten boat (MP: 3695K) and is heated by applying 10A current
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in the primary. The substrate was kept 16 cm above the sample boat using a post and
substrate holder as shown in Figure 1 with θ ≈ 700 (θ is the angle between the surface normal and Ag vapour direction).
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The rate of deposition and thickness were monitored using a quartz thickness monitor associated with the thermal coating system (Model-CTM-200). The input parameters for the
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same were acoustic impedance and density of silver which were 10.5 g cm-3 and 37.8 ×105 g
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cm -2 s respectively. Each coating was carried under a pressure of ~ 3.7 ×10-5 mbar with a
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5. HDT deposition
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coating rate of ~ 0.01nms-1.
The analyte HDT (MW: 258.51 g mol-1 and density 0.84 g ml-1) was dissolved in premium grade ethanol by sonicating it for 10 minutes and a fixed volume of the same was pipetted out
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and drop casted on the SEIRA substrates. The solvent was then allowed to evaporate at room
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temperature leaving a layer on the substrate. 6. Characterizations
The UV-Vis- NIR absorption spectra were recorded with Jasco V-570 spectrophotometer and the spectra in IR (400-4000 cm-1) region was recorded with Thermo-Nicolet, Avatar 370
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model Fourier transform infrared (FTIR) spectrometer. All the FTIR measurements were carried out in transmission mode. The recorded FTIR spectra were of 4cm-1 resolution and for each spectrum, 32 interferograms were co-added. The morphology of the film was analyzed with a scanning electron microscope (JEOL Model JSM - 6390LV) and FESEM. Results and discussions
The absorption of bulk glass powder obtained from UV- VIS -NIR and FTIR spectrometer is provided in figure 2a and 2b respectively. It is observed that the lower and upper end of the transmission spectrum (which is determined by the electronic band gap and phonon vibrations respectively) lies between 1 to 12 μm. Since the fingerprints of almost all the biomolecules fall in this region, this particular glass composition can be employed in IR detection applications [20].
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For surface enhanced infrared absorption studies bulk glass was made into a thin film form by thermal evaporation and silver nanolayer was deposited above these films via oblique
angle deposition. Oblique angle deposition was selected due to the better control of surface
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topology which avoids early percolation of the surface [2] and by varying the coating
parameters complex structures [19] with a higher aspect ratio (a desirable condition to move SPR peak to IR region [6]) can be obtained. The absorption spectra recorded for
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chalcogenide film before and after silver deposition (9.8 nm) is given in figure 3 a with plane
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glass slide as the reference.
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Due to the homogeneity of the films, well resolved Fabry- Perot fringes were observed in both silver coated and bare chalcogenide films which arise due to the interference of light
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waves reflected by substrate surface and film-air interface. Due to the presence of these fringes and reduction in the SPR intensity with increased substrate refractive index the effect
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of SPR is not prominent in this spectrum [13-14]. But there is an observable shift in the fringe position for silver coated sample compared to plain chalcogenide film which can be attributed
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to the modulation in phase matching condition provided by the silver layer due to the refractive index change of silver layer around the surface plasmon resonance (SPR)
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wavelengths [21]. These fringes are used for determining the dispersion nature of the film in the Near IR region (low to medium absorption region) and also to determine the chalcogenide film thickness. Swanepoel’s envelope method was employed to find these parameters [22]. Refractive index dispersion in the absorbing region is recorded with Ellipsometer and is
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provided in figure 3 b along with that in the NIR region. The thickness of the substrate films used was of 850 ±25 nm. In order to find SPR variation with Ag thickness, silver films were deposited on bare glass slides and chalcogenide films simultaneously. The UV/Vis/NIR spectra of a plane glass slide with varying Ag thickness is given in figure 4.
Figure 4 shows the variation in SPR peak wavelength as a function of silver layer thickness. For small thickness, there is a sharp SPR peak around 450 nm with an extended tail up to NIR. The SPR peak gets broader and red-shifted with an increase in thickness. Well separated small island structures are obtained during the initial stages of the deposition when nucleation of the sample on the substrate occurs [2, 19]. The island formation is observable in SEM and
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FESEM images of the 5nm thick silver film given in figure 5a and 5b.
These islands act as metallic nanostructures and provide sharp SPR peaks. The IR tailing and broadening is due to the dipole interaction between islands. As the thickness increases the
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island density is increased reducing the space between islands which enhances the interaction between islands, broadening the SPR peak [6]. The peak wavelength shift with thickness is given in the inset of figure 4. A maximum SPR peak is observed for 7.9nm and from the
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FESEM image C, we can see that the islands are of considerable size but are separated. The
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wavelength shift increases up to a point whereby it starts decreasing which could be due to the percolation of silver on the substrate while coating for longer exposure period which
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increases the substrate temperature [23]. References also suggest that as the silver film
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thickness reaches 10 nm the island nature is lost and a uniform thin films are formed. This is observable in figure 5C. The tilt in substrate holder supports the growth of metal cluster in
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the direction of the particle beam and prevent metal deposition on the shadowed regions. There the metal clusters have the shape of needles inclined to the surface of the substrate [2]. Results confirm that the coating parameters are providing sufficient SPR effects for using
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substrate for SEIRA analysis.
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To assess the performance of the film as a SEIRA substrate, HDT is used as the analyte. The standard reference spectrum of pure HDT is obtained by taking FTIR spectra of the same dispersed in KBr pellets (figure 6a) [18]. There are two dominant peaks at 2855 cm-1 and 2924 cm-1 due to CH2 symmetric and CH2 antisymmetric stretching respectively [24]. These
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two peaks are taken as reference for further comparison as other two weak peaks are in the higher absorption region of the base glass slide. The IR transmittance spectrum of the bare glass slide is given in figure 6b. The correlation between silver island thickness and extent of SEIRA enhancement is observable from figure 7. For all measurement 14μl of the 0.001 M solution is drop casted
analogously on GeSeTe- Ag substrate. The spectra are baseline corrected in origin using reference spectra of the film recorded without HDT coating.
The FTIR response shows a similar trend to that of SPR spectra ie., peak intensity increase with silver island thickness up to an optimum point and then decreases [6]. The highest enhancement is obtained for the 7.9 nm thick film. There is a 4cm-1 shift in peak wavelength
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which can be due to the chemisorption of HDT on silver.
In order to find out the enhancement factor three samples s1,s2 and s3 with silver film
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thicknesses as 5.1, 7.9, and 10.1 nm respectively along with a plane chalcogenide film without
silver islands were immersed in a 0.001M HDT solution for 12 hours and a uniform HDT coating is obtained over the silver islands. The film is allowed to dry in air and the excess
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nonbinding HDT is rinsed out.
To fix the detection area a constant a non-transmitting blade aperture of 6mm diameter is
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introduced into FTIR spectrometer in the wave path. The IR absorbance spectra of the samples
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are provided in figure 8, where the increase in absorbance is observed with silver thickness up to s2 and it decreases thereafter. The enhancement coefficients are determined as the ratios of
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integral intensities of the absorbance band of HDT film recorded with silver islands to that without and it found to be 12.88 [25]. ]. The effect of different surface areas of the substrate on
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the absorbance is nullified by dividing the absorbance intensity with effective surface area obtained from the AFM analysis.
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The influence of chalcogenide film thickness on SEIRA was carried out by coating 9.8 nm silver film on substrates with three different chalcogenide film thicknesses. The transmission spectra of the uncoated chalcogenide film and their FTIR spectra when coated with silver and
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HDT are provided in figure 8 b. HDT is coated as mentioned in HDT deposition section. To check the sensitivity of the system to the amount of HDT content, four different molarity (10 -3, 10 -4, 10 -5, 10 -6 M) of the sample was prepared and coated on7 nm thick silver film. FTIR transmittance spectra of all the concentrations are given in figure 9 a. In the figure transmittance variation with analyte concentration is clearly visible up to 10-5 M concentration. For better visibility of the concentration effect, variation in the transmittance
dip with respect to reference spectra (ΔT) for both the wavenumbers are plotted separately (figure 9b) which shows a linear variation confirming the system can act as a concentration sensor.
Conclusions Ge20Se70Te10 glass film is characterized and its usability as an IR transparent substrate for
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SEIRA spectroscopy is demonstrated by incorporating silver island structures on the film via oblique angle deposition. For comparison, silver films were deposited on both bare and
Ge20Se70Te10 coated glass slides simultaneously. Silver island formation is confirmed via
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SEM and SPR peaks in UV-Vis-NIR spectra observed for plane glass slides. The SEIRA response is recorded with FTIR spectrometer using analyte HDT. The absorption peak
intensity is found to change with silver film thickness and the optimum Ag thickness obtained
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was 7.9 nm with an enhancement factor 13. The peak intensity is found to vary linearly with the concentration of the sample confirming that the system can act as a concentration sensor.
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The absorption enhancement and detection limit obtained with this combination can be
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improved by replacing the substrate of microscopic glass slide (with chalcogenide film) with
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pure chalcogenide glass plate and also by incorporating advanced metal structures like IR antennas on to the substrate. The enhanced IR absorption from silver coated Ge20Se70Te10 film along with its better affinity to silver proves it to be a promising candidate as an IR
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substrate for SEIRA which can be used to develop integrated optical waveguide for highly sensitive SEIRA spectroscopy.
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Acknowledgement
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Authors wish to acknowledge KSCSTE, Government of Kerala and DST for financial assistance and SAIF-STIC for analytical facilities. References
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19. K. M. A. Sobahan, Yong Jun Park, JinJoo Kim, You Suk Shin, Ji Bum Kim and Chang Kwon Hwangbo, Nanostructured optical thin films fabricated by oblique angle
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24. Crissy L. Rhodes, Scott H. Brewer, JaapFolmer, Stefan Franzen, Investigation of hexa decanethiol self-assembled monolayers on cadmium tin oxide thin films, Thin Solid Films 516 (2008) 1838–1842 25. G.I. Dovbeshko1, O.M. Fesenko, Yu.M. Shirshov, V.I. Chegel, Semiconductor
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Figure Caption
Fig 1. Schematics of oblique angle deposition set up
Fig 2. Absorption spectra of bulk Ge20 Se70 Te10 in (a) UV Vis NIR region and (b) in IR region
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Fig 3: a) UV/Vis/NIR absorption spectra of GeSeTe films with and without silver coating and b) refractive index dispersion of Chalcogenide film obtained from 1) Elipsometry (in highly absorbing region) and 2)
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Swanepoel method (low or medium absorption)
Fig 4: UV/Vis/NIR spectra of plane glass slide with Ag coating of varying thickness. Inset showing shift in SPR
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peak wavelength with film thickness.
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Fig 5: a) SEM image of Silver Island of thickness 5 nm on plane glass slide and b,cC and d are the FESEM images of Silver Island with film thickness 5.1nm, 7.9 nm and 10.1 nm respectively
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Fig 6: a) FTIR absorbance spectrum of HDT dispersed in KBr pellets without dilution and b) FTIR transmittance spectrum of bare glass slide.
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Fig 7. FTIR absorption spectra of analyte HDT deposited on silver coated chalcogenide glass film as a function of silver film thickness.
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Fig 8. a) FTIR spectra of HDT coated on chalcogenide glass film with and without silver layer and b) UV-Vis-NIR transmission spectra of chalcogenide glass films of various thickness (top) and FTIR transmission spectra of the films when silver layer of 9.8nm and HDT is deposited over it.
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Fig 9 a): FTIR transmittance spectrum of HDT deposited on ChG-Ag system with varying analyte concentration. And b) transmittance change at two wavenumbers of analyte compared to the reference spectra (without analyte) with HDT concentration.
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S0925-4005(19)30259-X https://doi.org/10.1016/j.snb.2019.02.045 SNB 26140
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
14 June 2018 8 February 2019 11 February 2019
Please cite this article as: Cheruvalath A, Nampoori VPN, Thomas S, Oblique Angle Deposited Silver islands on Ge20 Se70 Te10 film substrate for SurfaceEnhanced Infrared Spectroscopy, Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.02.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Oblique Angle Deposited Silver islands on Ge20Se70Te10 film substrate for SurfaceEnhanced Infrared Spectroscopy
*
International School of Photonics, CUSAT *
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Email: [email protected]
Highlights
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An extensive study on Silver - Ge20Se70Te10 film system as a substrate for SEIRA spectroscopy is carried out. SEIRA response of the system was analyzed by recording FTIR transmission spectra of analyte Hexa Decane Thiol (HDT). Silver island structures were developed on Ge20Se70Te10 film via oblique angle deposition and the optimum silver thickness which provides maximum enhancement in IR absorption is determined. The response of the system to variations in the analyte concentration is also explored
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Ajina Cheruvalath, V.P.N. Nampoori and Sheenu Thomas
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Abstract
Surface-enhanced infrared absorption (SEIRA) spectroscopy is a powerful tool for characterizing and identifying chemical/biological species. Though SEIRA technique has
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enabled detection up to few molecule levels with sophisticated device structures, there is still room for improvement (can extend to detect down to a single molecule). Superior sensor
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platforms can be developed by incorporating advanced substrate material, so as to be actively employed in trace analysis, forensic sciences etc. In this work, chalcogenide glass of composition Ge20Se70Te10 is characterized to probe its usability as a SEIRA substrate. Silver
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island structures were developed on Ge20Se70Te10 film via oblique angle deposition and its SEIRA response was recorded using FTIR spectrometer with Hexa Decane Thiol (HDT) as the analyte. Investigations reveal an optimum silver thickness at which maximum absorption enhancement is obtained. The response of the system to variations in the analyte concentration is also explored. Studies prove them to be a promising candidate as an IR substrate which can be used to develop integrated optical waveguide for highly sensitive SEIRA spectroscopy.
Keywords: GeSeTe glass, Chalcogenide glass, Silver Islands, SEIRA, IR spectroscopy, OAD Introduction Infrared spectroscopy is a widely accepted technique for the identification of molecular species and characterizing chemical reactions. This technique has the potential to replace other destructive invasive composition analysis techniques like EDS, XPS, secondary ion mass spectroscopy etc [1]. But applications of IR spectroscopy in its conventional form are
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restricted by its high detection limit. Since it is a linear spectroscopic method it obeys BeerLambert law according to which absorbance A, given by 𝛼 𝑐𝑙 ln 10
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𝐴=
can be improved by either increasing the concentration (𝑐) of the analyte or by increasing the path length of light (𝑙) which is not a solution in practical applications [2]. Another technique
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to improve absorption is field enhancement near the analyte which is achievable by
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introducing metal nanostructures above infrared transparent substrates. Enhancement in the
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infrared absorption of molecules placed very close to metal nanostructure were first observed
infrared absorption (SEIRA) [3].
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by Hartstein and coworkers in 1980 which later came to be known as surface enhanced
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The local field enhancement is due to the surface plasmon resonance (SPR) character of the metal nanostructures. As SPR wavelength is tunable with size, shape and metal type, SEIRA studies were conducted on different noble metals (Au, Ag, Cu, Pt and Fe) with various
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morphologies [4, 5]. In the beginning, SEIRA was carried out with very thin metal island films developed by coating procedures like electron beam evaporation, vacuum thermal
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evaporation, magnetic sputtering, etc. which provides a broad SPR peak and tailing in absorption covering almost entire mid-IR region [2, 6,7]. This broad nature of SPR supports the enhancement of any vibrational wavelengths of analyte so that it can be employed even with unknown molecular species. The main drawback of this method is its low field
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enhancement property [6]. Later to overcome this, metal island structures were replaced by well-shaped IR antennas which provide higher field enhancement compared to the metal island [8-12]. Field enhancement offered by IR antenna is very high (107 in ref 12) that it can be employed to detect down to few molecule levels with FTIR spectrometer [12]. The SPR peaks so obtained is not sharp and to cover the entire IR region with IR antenna so as to be employed for unknown species identification, a number of antennas with different aspect
ratios are needed. Fabrication of such well-shaped antennas requires highly sophisticated instruments [12] The IR transparent substrate material onto which the metal nanostructures are deposited should be having high infrared transparency as well as good affinity to the metal that is being used. Usually, crystals of Ge, ZnSe, CaF2, BaF2, Si etc., are used for this purpose [1,2,3,5,6]. Though reports suggest a very strong dependence of SEIRA response on the substrate
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material, very few studies are carried out to expand the available substrate choices [13,14]. Chalcogenide glasses are very promising candidates in SEIRA as they are highly transparent in the IR region (up to 20 μm depending upon composition [15]) and shows better affinity to
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metals [16-18]. In addition to this, they also provide flexibility in waveguide fabrication.
In this paper, we report studies on chalcogenide glass of composition Ge20 Se70 Te10 as an IR transparent substrate for SEIRA. Silver island structures were deposited on chalcogenide
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films via oblique angle deposition method using a conventional thermal evaporator [19].
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Hexa Decane Thiol (HDT) was used as the analyte and SEIRA was recorded with an FTIR
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spectrometer in its transmission mode.
1. Chemicals and Substrates
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Materials and methods
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5N purity elemental form Ge, Se and Te were used as purchased from Sigma Aldrich and 99.9% pure grade silver plate was purchased from local vendors. Hexa Decane Thiol (HDT)
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purchased from Sigma Aldrich was kept under refrigeration to reduce degradation. Premium grade ethanol purchased from Hayman was used as a solvent for the analyte.
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2. Bulk glass preparation Each constituent element of the glass composition was weighed according to their weight percentage and loaded in a 10cm long and 1cm diameter quartz ampoule and sealed under
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vacuum (10-5 mbar). The sealed ampoule was then kept at 1323 K for 24 hours in a rocking and rotation tube furnace. Continuous rocking and rotation are provided to the sample during this time to ensure thorough mixing and homogenization of the melt. The hot ampoule is quenched in ice cold water after 24 hours. The outer quartz ampoule is etched out using HF and the sample in the bulk form is collected. 3. Ge Se Te film preparation
Thin films were prepared using vacuum thermal evaporation unit (IHVP model: 12A4-D) for which bulk glass piece was placed in a molybdenum boat and heated via resistive heating at a pressure of 3.7 ×10-5 mbar. Films were fabricated on microscopic glass slides. Glass slides were thoroughly cleaned via sonication in a soap solution, acetone and distilled water prior to placing it for coating. 4. Silver layer fabrication
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Silver layers were fabricated on the plane glass as well as on chalcogenide glass films via oblique angle deposition method using thermal evaporation (IHVP model: 12A4-D). A silver
piece of 1gm is loaded in a tungsten boat (MP: 3695K) and is heated by applying 10A current
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in the primary. The substrate was kept 16 cm above the sample boat using a post and
substrate holder as shown in Figure 1 with θ ≈ 700 (θ is the angle between the surface normal and Ag vapour direction).
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The rate of deposition and thickness were monitored using a quartz thickness monitor associated with the thermal coating system (Model-CTM-200). The input parameters for the
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same were acoustic impedance and density of silver which were 10.5 g cm-3 and 37.8 ×105 g
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cm -2 s respectively. Each coating was carried under a pressure of ~ 3.7 ×10-5 mbar with a
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5. HDT deposition
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coating rate of ~ 0.01nms-1.
The analyte HDT (MW: 258.51 g mol-1 and density 0.84 g ml-1) was dissolved in premium grade ethanol by sonicating it for 10 minutes and a fixed volume of the same was pipetted out
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and drop casted on the SEIRA substrates. The solvent was then allowed to evaporate at room
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temperature leaving a layer on the substrate. 6. Characterizations
The UV-Vis- NIR absorption spectra were recorded with Jasco V-570 spectrophotometer and the spectra in IR (400-4000 cm-1) region was recorded with Thermo-Nicolet, Avatar 370
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model Fourier transform infrared (FTIR) spectrometer. All the FTIR measurements were carried out in transmission mode. The recorded FTIR spectra were of 4cm-1 resolution and for each spectrum, 32 interferograms were co-added. The morphology of the film was analyzed with a scanning electron microscope (JEOL Model JSM - 6390LV) and FESEM. Results and discussions
The absorption of bulk glass powder obtained from UV- VIS -NIR and FTIR spectrometer is provided in figure 2a and 2b respectively. It is observed that the lower and upper end of the transmission spectrum (which is determined by the electronic band gap and phonon vibrations respectively) lies between 1 to 12 μm. Since the fingerprints of almost all the biomolecules fall in this region, this particular glass composition can be employed in IR detection applications [20].
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For surface enhanced infrared absorption studies bulk glass was made into a thin film form by thermal evaporation and silver nanolayer was deposited above these films via oblique
angle deposition. Oblique angle deposition was selected due to the better control of surface
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topology which avoids early percolation of the surface [2] and by varying the coating
parameters complex structures [19] with a higher aspect ratio (a desirable condition to move SPR peak to IR region [6]) can be obtained. The absorption spectra recorded for
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chalcogenide film before and after silver deposition (9.8 nm) is given in figure 3 a with plane
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glass slide as the reference.
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Due to the homogeneity of the films, well resolved Fabry- Perot fringes were observed in both silver coated and bare chalcogenide films which arise due to the interference of light
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waves reflected by substrate surface and film-air interface. Due to the presence of these fringes and reduction in the SPR intensity with increased substrate refractive index the effect
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of SPR is not prominent in this spectrum [13-14]. But there is an observable shift in the fringe position for silver coated sample compared to plain chalcogenide film which can be attributed
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to the modulation in phase matching condition provided by the silver layer due to the refractive index change of silver layer around the surface plasmon resonance (SPR)
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wavelengths [21]. These fringes are used for determining the dispersion nature of the film in the Near IR region (low to medium absorption region) and also to determine the chalcogenide film thickness. Swanepoel’s envelope method was employed to find these parameters [22]. Refractive index dispersion in the absorbing region is recorded with Ellipsometer and is
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provided in figure 3 b along with that in the NIR region. The thickness of the substrate films used was of 850 ±25 nm. In order to find SPR variation with Ag thickness, silver films were deposited on bare glass slides and chalcogenide films simultaneously. The UV/Vis/NIR spectra of a plane glass slide with varying Ag thickness is given in figure 4.
Figure 4 shows the variation in SPR peak wavelength as a function of silver layer thickness. For small thickness, there is a sharp SPR peak around 450 nm with an extended tail up to NIR. The SPR peak gets broader and red-shifted with an increase in thickness. Well separated small island structures are obtained during the initial stages of the deposition when nucleation of the sample on the substrate occurs [2, 19]. The island formation is observable in SEM and
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FESEM images of the 5nm thick silver film given in figure 5a and 5b.
These islands act as metallic nanostructures and provide sharp SPR peaks. The IR tailing and broadening is due to the dipole interaction between islands. As the thickness increases the
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island density is increased reducing the space between islands which enhances the interaction between islands, broadening the SPR peak [6]. The peak wavelength shift with thickness is given in the inset of figure 4. A maximum SPR peak is observed for 7.9nm and from the
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FESEM image C, we can see that the islands are of considerable size but are separated. The
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wavelength shift increases up to a point whereby it starts decreasing which could be due to the percolation of silver on the substrate while coating for longer exposure period which
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increases the substrate temperature [23]. References also suggest that as the silver film
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thickness reaches 10 nm the island nature is lost and a uniform thin films are formed. This is observable in figure 5C. The tilt in substrate holder supports the growth of metal cluster in
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the direction of the particle beam and prevent metal deposition on the shadowed regions. There the metal clusters have the shape of needles inclined to the surface of the substrate [2]. Results confirm that the coating parameters are providing sufficient SPR effects for using
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substrate for SEIRA analysis.
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To assess the performance of the film as a SEIRA substrate, HDT is used as the analyte. The standard reference spectrum of pure HDT is obtained by taking FTIR spectra of the same dispersed in KBr pellets (figure 6a) [18]. There are two dominant peaks at 2855 cm-1 and 2924 cm-1 due to CH2 symmetric and CH2 antisymmetric stretching respectively [24]. These
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two peaks are taken as reference for further comparison as other two weak peaks are in the higher absorption region of the base glass slide. The IR transmittance spectrum of the bare glass slide is given in figure 6b. The correlation between silver island thickness and extent of SEIRA enhancement is observable from figure 7. For all measurement 14μl of the 0.001 M solution is drop casted
analogously on GeSeTe- Ag substrate. The spectra are baseline corrected in origin using reference spectra of the film recorded without HDT coating.
The FTIR response shows a similar trend to that of SPR spectra ie., peak intensity increase with silver island thickness up to an optimum point and then decreases [6]. The highest enhancement is obtained for the 7.9 nm thick film. There is a 4cm-1 shift in peak wavelength
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which can be due to the chemisorption of HDT on silver.
In order to find out the enhancement factor three samples s1,s2 and s3 with silver film
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thicknesses as 5.1, 7.9, and 10.1 nm respectively along with a plane chalcogenide film without
silver islands were immersed in a 0.001M HDT solution for 12 hours and a uniform HDT coating is obtained over the silver islands. The film is allowed to dry in air and the excess
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nonbinding HDT is rinsed out.
To fix the detection area a constant a non-transmitting blade aperture of 6mm diameter is
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introduced into FTIR spectrometer in the wave path. The IR absorbance spectra of the samples
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are provided in figure 8, where the increase in absorbance is observed with silver thickness up to s2 and it decreases thereafter. The enhancement coefficients are determined as the ratios of
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integral intensities of the absorbance band of HDT film recorded with silver islands to that without and it found to be 12.88 [25]. ]. The effect of different surface areas of the substrate on
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the absorbance is nullified by dividing the absorbance intensity with effective surface area obtained from the AFM analysis.
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The influence of chalcogenide film thickness on SEIRA was carried out by coating 9.8 nm silver film on substrates with three different chalcogenide film thicknesses. The transmission spectra of the uncoated chalcogenide film and their FTIR spectra when coated with silver and
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HDT are provided in figure 8 b. HDT is coated as mentioned in HDT deposition section. To check the sensitivity of the system to the amount of HDT content, four different molarity (10 -3, 10 -4, 10 -5, 10 -6 M) of the sample was prepared and coated on7 nm thick silver film. FTIR transmittance spectra of all the concentrations are given in figure 9 a. In the figure transmittance variation with analyte concentration is clearly visible up to 10-5 M concentration. For better visibility of the concentration effect, variation in the transmittance
dip with respect to reference spectra (ΔT) for both the wavenumbers are plotted separately (figure 9b) which shows a linear variation confirming the system can act as a concentration sensor.
Conclusions Ge20Se70Te10 glass film is characterized and its usability as an IR transparent substrate for
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SEIRA spectroscopy is demonstrated by incorporating silver island structures on the film via oblique angle deposition. For comparison, silver films were deposited on both bare and
Ge20Se70Te10 coated glass slides simultaneously. Silver island formation is confirmed via
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SEM and SPR peaks in UV-Vis-NIR spectra observed for plane glass slides. The SEIRA response is recorded with FTIR spectrometer using analyte HDT. The absorption peak
intensity is found to change with silver film thickness and the optimum Ag thickness obtained
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was 7.9 nm with an enhancement factor 13. The peak intensity is found to vary linearly with the concentration of the sample confirming that the system can act as a concentration sensor.
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The absorption enhancement and detection limit obtained with this combination can be
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improved by replacing the substrate of microscopic glass slide (with chalcogenide film) with
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pure chalcogenide glass plate and also by incorporating advanced metal structures like IR antennas on to the substrate. The enhanced IR absorption from silver coated Ge20Se70Te10 film along with its better affinity to silver proves it to be a promising candidate as an IR
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substrate for SEIRA which can be used to develop integrated optical waveguide for highly sensitive SEIRA spectroscopy.
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Acknowledgement
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Authors wish to acknowledge KSCSTE, Government of Kerala and DST for financial assistance and SAIF-STIC for analytical facilities. References
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Figure Caption
Fig 1. Schematics of oblique angle deposition set up
Fig 2. Absorption spectra of bulk Ge20 Se70 Te10 in (a) UV Vis NIR region and (b) in IR region
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Fig 3: a) UV/Vis/NIR absorption spectra of GeSeTe films with and without silver coating and b) refractive index dispersion of Chalcogenide film obtained from 1) Elipsometry (in highly absorbing region) and 2)
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Swanepoel method (low or medium absorption)
Fig 4: UV/Vis/NIR spectra of plane glass slide with Ag coating of varying thickness. Inset showing shift in SPR
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peak wavelength with film thickness.
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Fig 5: a) SEM image of Silver Island of thickness 5 nm on plane glass slide and b,cC and d are the FESEM images of Silver Island with film thickness 5.1nm, 7.9 nm and 10.1 nm respectively
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Fig 6: a) FTIR absorbance spectrum of HDT dispersed in KBr pellets without dilution and b) FTIR transmittance spectrum of bare glass slide.
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Fig 7. FTIR absorption spectra of analyte HDT deposited on silver coated chalcogenide glass film as a function of silver film thickness.
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Fig 8. a) FTIR spectra of HDT coated on chalcogenide glass film with and without silver layer and b) UV-Vis-NIR transmission spectra of chalcogenide glass films of various thickness (top) and FTIR transmission spectra of the films when silver layer of 9.8nm and HDT is deposited over it.
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Fig 9 a): FTIR transmittance spectrum of HDT deposited on ChG-Ag system with varying analyte concentration. And b) transmittance change at two wavenumbers of analyte compared to the reference spectra (without analyte) with HDT concentration.
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