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Measurement of optical anisotropy in ultrathin films using surface plasmon resonance
Journal of Molecular Structure 1103 (2016) 281e285
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Measurement of optical anisotropy in ultrathin films using surface plasmon resonance V.P. Devanarayanan a, V. Manjuladevi a, Monika Poonia a, R.K. Gupta a, *, Sanjeev K. Gupta b, Jamil Akhtar c a b c
Department of Physics, Birla Institute of Technology and Science, Pilani, Rajasthan, 333031, India Department of Electronics and Information Technology (DeitY), New Delhi, 110003, India CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan, 333031, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 April 2015 Received in revised form 31 August 2015 Accepted 29 September 2015 Available online 8 October 2015
The optical phenomenon, surface plasmon resonance (SPR) is employed for the measurement of optical anisotropy in the ultrathin films fabricated through LangmuireBlodgett (LB) and self-assembled monolayer (SAM) techniques onto 50 nm gold film supported on BK7 glass substrates. The resonance angle (RA) is measured using a home built setup in Kretschmann configuration. The LB films and SAM can provide a single layer of highly ordered and organized molecules on the two dimensional surface. If the film forming molecules are anisotropic, their organization in the LB films and SAM can yield an anisotropic film due to tilt of the molecules with respect to the surface normal. The SPR spectra are recorded for the two orthogonal directions of the film with respect to the plane of incidence. The spectra are simulated by modeling the Fresnel's reflection from 4-layers viz., prism, gold, ultrathin films and air; and the real and imaginary parts of refractive index are estimated. Our study shows the metallic and dielectric nature of the LB films of bundles of single walled carbon nanotubes (SWCNTs) when the long axis of SWCNTs are aligned parallel and perpendicular to plane of incidence, respectively. The optical anisotropy was estimated from the change in real part of refractive index (Dnr) of the ultrathin films measured in the orthogonal directions. In addition, we have also studied such optical anisotropy in the LB film of cadmium-stearate and self-assembled monolayer of octadecanethiol. © 2015 Elsevier B.V. All rights reserved.
Keywords: Surface plasmon resonance LangmuireBlodgett films Single-walled carbon nanotubes
1. Introduction Surface plasmon resonance (SPR) is an optical phenomenon which is being potentially used for the detection of molecular specific interaction [1]. Such phenomenon resulted in the development of biological, chemical and gas sensors [2]. SPR technique is label free and non-destructive and hence exhibits numerous advantages over the conventional sensing technology. In the Kretschmann configuration [3], the surface plasmon is excited in 50 nm gold film deposited onto glass substrate which is coupled to a prism with a refractive index (RI) matching fluid. For an incident p-polarized light of fixed wavelength, the SPR occurs at a unique angle of incidence also known as resonance angle (RA). The RA is extremely sensitive to the dielectric layer adsorbed onto the gold surface. Any change in the dielectric layer causes a shift in the RA
* Corresponding author. E-mail address: [email protected] (R.K. Gupta). http://dx.doi.org/10.1016/j.molstruc.2015.09.018 0022-2860/© 2015 Elsevier B.V. All rights reserved.
with respect to the reference. The dielectric (hence optical) properties of the adsorbed material can be estimated by modeling the reflection from different interfaces involved during the process by employing the Fresnel's equation [4]. Wang has demonstrated the condition for the SPR can be altered by altering the dielectrics of a liquid crystal on the metal surface due to the application of electric field [5]. The optical anisotropy in a stable ultrathin film at the airewater interface (known as Langmuir monolayer) has been observed and studied extensively using Brewster angle microscope (BAM) [6e8]. Such anisotropy in Langmuir monolayer arises due to tilt of the molecules with respect to the surface normal [9e12]. In Brewster angle microscopy, the reflected intensity from the airewater interface is extremely sensitive to a change in refractive index at the interface. The refractive index changes due to change in the surface density or tilt variation of the molecules. Therefore, any change in the RI due to tilt variation of the molecules in a given phase of the Langmuir monolayer, changes the reflectivity significantly resulting in variation in grayshades in the visible region of
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the BAM images [10,13,14]. In this paper, we report the measurement of optical parameters (refractive index) in the orthogonal directions of the ultrathin films of well organized bundles of single-walled carbon nanotubes (SWCNTs), octadecanethiol (ODT) and cadmium stearate (CdSA) using a SPR instrument. The ultrathin films of SWCNTs and CdSA are fabricated through LangmuireBlodgett (LB) technique whereas that of ODT through self assembly on the gold surface. Such ultrathin films offer a well organized and orientationally ordered single layer of the molecules [15,16], which exhibits anisotropy due to the alignment of the molecules (or SWCNTs) in a preferential direction on the substrate. The anisotropy in the films depends on the shape anisotropy of the molecules and their tilt orientation on the substrate with respect to the surface normal [6]. The SPR spectra of the fabricated ultrathin films are recorded in the orthogonal directions of the film i.e., parallel (0 ) and perpendicular (90 ) with respect to the plane of incidence. We aligned the long axis of SWCNTs along the direction of dipping during the LB film fabrication process [17]. The SPR spectra of LB film of SWCNTs show very interesting result. The film of SWCNTs behaves metallike and insulating dielectric layer when the long axis of SWCNTs is aligned parallel and perpendicular to the plane of incidence, respectively. The ultrathin films of structurally similar molecules viz., ODT and stearic acid on the gold surface differ with respect to the orientational tilt of the molecules in the single layer. ODT in SAM and CdSA in LB are known to tilt ~30 and ~10 with respect to the surface normal, respectively [18,19]. Therefore, there is a different degree of anisotropy in SAM of ODT as compared to that of LB film of CdSA. We found the optical anisotropy as a change in the real part of refractive index (Dnr) of SAM of ODT and LB film of CdSA to be 0.24 and 0.10, respectively. 2. Experimental section The SPR setup in Kretschmann configuration was built in the laboratory (Fig. 1).The material of semi-cylindrical prism was BK7 having the refractive index of 1.51. The 50 nm gold film was deposited onto BK7 substrates by electron beam deposition technique. The thickness of deposited gold layer was monitored in-situ during evaporation process by employing quartz crystal based digital thickness monitor. The substrate and the prism are optically coupled by using refractive index matching fluid (Cargille). The ultrathin films of CdSA and SWCNT were fabricated by LB technique using a teflon trough (Apex Instruments). The stearic acid (SA) was obtained from SigmaeAldrich. A chloroform solution of the SA having a concentration of 3.5 mM was spread onto an aqueous subphase of 1 105 M of cadmium chloride (CdCl2) in ultrapure ion-free water (MilliQ, DQ5). A single layer of LB film of CdSA is deposited on the gold substrate at a target surface pressure of
30 mN/m. The tilt of the chains of the molecules in the LB films was reported to be around 10 with respect the surface normal [19]. An isotropic thin film of SA is deposited onto the gold substrate by spin coating technique. A 5 ml of the chloroform solution of SA is spread onto gold substrate rotated at a speed of 7000 rotations per minute. In order to obtain isotropy and homogeneity in the film, it is annealed at a temperature of about 80 C for 15 min. A uniformly dispersed solution of 9 103 mg/ml of SWCNTs (Carbon Solutions, P2-SWNT) was obtained by dissolving the nanotubes in dimethylformamide solvent and ultrasonicating the dispersion for about 1 h. The Langmuir film of SWCNTs is found to be stable with a collapse surface pressure of 11 mN/m. The LB films of SWCNTs are deposited onto solid substrates at 2 mN/m and the surface morphology were studied using atomic force microscope (SolverPro, NTMDT). We found that the long axis of SWCNTs oriented along the direction of deposition of the film [17]. Such morphology can yield exceptionally high anisotropy in the electrical and optical properties when measured in orthogonal directions. The SAM of ODT was prepared by immersing the gold substrates into the 1 mM solution of ODT in absolute ethanol for about 12 h. The ODT deposited gold substrates were rinsed thoroughly by absolute ethanol followed by HPLC grade chloroform before mounting on the scanning stage of the SPR instrument. The SPR spectra were collected by changing the angle of incidence at a step of 21.2 103 and recording the reflected intensity simultaneously. The reported SPR spectra of the ultrathin films are little broader at the minimum because of low resolution of the data acquisition card (12-bit, National Instruments). In order to check the reproducibility of the data, the SPR spectra were collected from different locations of the films, and from the films deposited in different batches. The RA for the gold film (goldeair interface) was found to be 44 . The average shift in RA (Dq) for the ultrathin films of different materials were estimated with reference to that of RA for the gold film (i.e. 44 ). The standard deviation of the RA was found to be in the range of 0.01 e0.02 .
3. Modeling The SPR spectra are simulated by modeling reflection of light from a 4-layer system viz., prism, gold, dielectric layer, air. In the 4layer system, the final Fresnel's equation is obtained by considering the reflection from three interfaces viz., prism-gold, gold-dielectric film and dielectric film-air [4]. The final Fresnel's equation of reflectance Rp for the p-polarized light is given by
Rp ¼ rp2
(1)
where rp is the reflection coefficient for p-polarized wave.
rp ¼
ðM11 þ M12 q4 Þq1 ðM21 þ M22 q4 Þ ðM11 þ M12 q4 Þq1 þ ðM21 þ M22 q4 Þ 3 Y
Mij ¼
k¼2
(2)
! ; i; j ¼ 1; 2
Mk
(3)
ij
For the kth layer,
Mk ¼ Fig. 1. The schematic of the SPR instrument setup. (a) laser, 635 nm, Newport, (b) semi-cylindrical prism of material BK7, (c) photodiode, OSI-Optoelectronics, (d)Data acquisition card, 12-bit, National Instruments, and (e) computer.
Here,
cosbk iqk sinbk
isinbk =qk cosbk
(4)
V.P. Devanarayanan et al. / Journal of Molecular Structure 1103 (2016) 281e285
1=2
qk ¼ ðmk =~εk Þ
bk ¼ dk
1=2 ~εk n21 sin2 q1 cosqk y ~εk
1=2 2p ~εk n21 sin2 q1 l
(5)
(6)
where, mk y 1, q1 and qk are the angle of incidences at the interface of media 1 2 and (k 1)k, n1 is the real part of refractive index of the medium 1, ~εk and dk are the complex dielectric constant and thickness of kth layer, respectively. For a given thickness of the ultrathin film deposited either by LB or self assembly, the reflected intensity (Rp) is simulated and reflected intensity as a function of incident angle (q) is obtained. The RA is dependent on the both ~εk and dk of the dielectric film deposited on the gold surface. Since the thickness of LB films and SAM are known, the values of complex dielectric constant (~εk ) and hence the real and imaginary part of the refractive index of the dielectric films are estimated from the simulation. The thickness of the LB films of SWCNTs and CdSA were 25 nm [17] and 2.4 nm [19] and that of SAM of ODT was 2.2 nm [18].
4. Results and discussion The SWCNTs are highly anisotropic material exhibiting extraordinary electrical, optical and mechanical properties [20]. We aligned the bundles of SWCNTs onto the substrates by LB technique. During the LB film fabrication process, in a very simple mechanism the SWCNTs can be aligned either parallel or perpendicular to the dipping direction by aligning the substrate-normal perpendicular or parallel to the film compression direction [21], respectively. The surface topography of the LB film of SWCNTs obtained using the atomic force microscope is shown in the inset of Fig. 2. The bundles of SWCNTs are found to orient with its longer axis parallel to the dipping direction of the substrate during the LB film fabrication process. The LB films of SWCNTs were deposited onto patterned interdigitated electrodes (IDE) and the currentevoltage (IeV) measurement was carried out. The IeV curves for the parallel and perpendicular orientation of SWCNTs in the LB films with respect to the applied voltage V are shown in Fig. 2. The IeV curves obtained
Fig. 2. CurrenteVoltage curves of the LB films when SWCNTs are aligned parallel and perpendicular to the electric field (E). The inset in the top figure shows the AFM image of the LB film of SWCNTs. The bundles are aligned in the direction of dipping the substrate during LB film fabrication. The right-bottom of each figure shows schematic for the alignment of SWCNTs on interdigitated electrodes.
283
for parallel orientation of the SWCNTs shows the normal ohmic behavior, and the estimated film resistance is found to be merely 7.8 U.Therefore, such orientation of the SWCNTs with respect to the applied electric field (E) indicates metallic nature of the film. However, the IeV curve for the film with SWCNTs align orthogonal to the applied electric field shows non-metallic behavior. Hence such orientation of the SWCNTs with respect to the electric field can be considered as insulating dielectric film. A single layer of SWCNTs is deposited onto gold substrate at a target surface pressure of 2 mN/m. The SPR spectra of the LB films were obtained for two different orientations of the long axis of SWCNTs with respect to the plane of incidence viz. parallel (0 ) and perpendicular (90 ) (Fig. 3).The spectra are simulated using the Eq. (1) and the optical parameters like real part of RI (nr) and imaginary part of RI (nim) are estimated. The values are tabulated in Table 1.The average shift in RA with respect to the gold film (Dq) for SWCNTs aligned parallel to the plane of incidence (i.e. 0 ) is found to be merely 0.07 whereas the Dq is found to be 0.28 for the film where SWCNTs aligned perpendicular (i.e. 90 ) with respect to the plane of incidence. Since, the Dq is not significant, the LB film with zero degree alignment of SWCNTs can be treated as a metallic film deposited over the 50 nm gold film. The SPR spectrum for such film is simulated using Eq. (1) and the values of nr and nim are estimated to be 0.2 and 3.3, respectively. Since the Dq for such film is very low, the values of nr and nim are comparable to that of gold. The Dq for the film with 90 orientation of SWCNTs with respect to the plane of incidence is found to be 0.28 . Such orientation of SWCNTs in the LB film yields a dielectric insulating layer. Therefore, the SPR spectrum shows a large shift in RA as compared to that of film with parallel alignment of SWCNTs. We have also studied the SPR spectra of the ultrathin films composed of structurally similar rod shaped molecules viz. ODT and SA. Such film forming molecules possess lower length-to-width ratio and hence exhibit much less anisotropy in properties as compared to that of SWCNTs. The amount of anisotropy in the ultrathin films of the rod shaped molecules depends on the amount of molecular tilt with respect to the surface normal [22,5]. Therefore, we studied the SPR behavior of self assembled monolayer of ODT and LB films of CdSA on gold substrates. The molecular tilts of ODT molecules in SAM on gold is reported to be around 30 [18] and CdSA in the LB films deposited at 30 mN/m is around 10 [19]. The SAM of ODT may offer more anisotropy as compared to that of LB films of CdSA. The recorded SPR spectra of the SAM of ODT on the gold substrate in the orthogonal directions are shown in Fig. 4.The Dq of the SAM of ODT measured in the orthogonal directions are 0.2 and
Fig. 3. Surface plasmon resonance spectra of gold and one layer of LB film of SWCNTs where the long axis of SWCNTs are aligned parallel (0 ) and perpendicular (90 ) to the plane of incidence. The curves are shifted vertically for visual clarity.
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Table 1 Estimated values of refractive index (RI) of LB film of SWCNTs through simulation. nr and nim are the real and imaginary part of RI. The average shift in RA is Dq. The refractive index for the gold was chosen to be 0.172 (nr) and 3.421 (nim). LB film of SWCNTs
Parallel (0 ) Perpendicular (90 )
Dq
0.07 0.28
nr
nim
0.2 1.04
3.3 0.0045
Fig. 4. Surface plasmon resonance spectra of gold and self assembled monolayer (SAM) of octadecanethiol for the orthogonal directions: parallel (0 ) and perpendicular (90 ) to the plane of incidence. The curves are shifted vertically for visual clarity.
0.31. The spectra are simulated using Eq. (1) and nr and nim are estimated. The refractive index is tabulated in Table 2. The Dnr is found to be 0.24. The LB films of CdSA deposited at a target surface pressure of 30 mN/m yield an ordered film with molecular orientation ~10 with respect to the surface normal [19]. The SPR spectra of one layer of LB film of CdSA is shown in Fig. 5(a).The Dq corresponding to the orientations 0+ and 90+ are 0.195+ and 0.10+, respectively. The spectra are simulated and the refractive indices of the dielectric films are estimated. The simulated values of refractive indices for the different films are tabulated in Table 3. The Dnr is found to be 0.10. The SPR spectra of the spin-coated thin film of SA on gold substrate are shown in 5(b). In the spin-coated thin films, the molecules orient isotropically onto the surface. The average shift in the RA for the two orthogonal directions are found to be constant (¼0.2+). This clearly indicates that the difference in shift in SPR angle during the orthogonal scanning is essentially due to difference in refractive index in the film which normally arises due to film anisotropy. The estimated nr for the ultrathin films of different organized materials in the orthogonal directions are shown as bar diagram in Fig. 6.
Fig. 5. Surface plasmon resonance spectra of gold and (a) one layer of LB film of CdSA where the dipping direction is oriented parallel (0 ) and perpendicular (90+) to the plane of incidence (b) spin-coated film of stearic acid (isotropically oriented molecules) for the orthogonal directions. The curves are shifted vertically for visual clarity.
Table 3 Estimated values of refractive index (RI) of LB film of CdSA through simulation. nr and nim are the real and imaginary part of RI. The average shift in resonance angle is Dq. The refractive index for the gold was chosen to be 0.172 (nr) and 3.421 (nim). LB film of CdSA +
Parallel (0 ) Perpendicular (90+)
Dq 0.195 0.1+
+
nr
nim
1.21 1.11
0.0045 0.0045
even few of molecular specific interaction at the given metaldielectric interface. The change in the optical (or dielectric) properties due to the anisotropy in thin films can be detected using SPR.
5. Conclusion SPR is a very powerful technique to identify molecular specific interaction. The technique is label free and very sensitive to detect
Table 2 Estimated values of refractive index (RI) of SAM of ODT through simulation. nr and nim are the real and imaginary part of RI. The average shift in resonance angle is Dq. The refractive index for the gold was chosen to be 0.172 (nr) and 3.421 (nim). SAM of ODT
Parallel (0 ) Perpendicular (90 )
Dq
0.2 0.31
nr
nim
1.24 1.48
0.0045 0.0045
Fig. 6. The bar diagram showing the nr for the ultrathin films of different materials measured in the orthogonal directions. The two bars for each of the film represent the nr in the orthogonal directions.
V.P. Devanarayanan et al. / Journal of Molecular Structure 1103 (2016) 281e285
In this paper, we have demonstrated the capability SPR for the measurement of optical anisotropy of the ultrathin films grown using the conventional LB and self assembly. The LB film of SWCNTs shows interesting results. The LB film of SWCNTs shows both the metallic and dielectric behavior when scanned in 0+ and 90+ alignment of SWCNTs with respect to the plane of polarization, respectively. The SAM of ODT with 30+ tilted molecules shows anisotropy in the real part of refractive index of about 0.24 whereas the LB films of CdSA with 10+ tilted molecules shows anisotropy in the real part of refractive index of about 0.10. Future work includes study of SPR of ultrathin films of different shape anisotropic liquid crystal molecules. Acknowledgments The authors (BITS Pilani) are thankful to Department of Science and Technology,India for funding the project under Instrument Development Programme(IDP/IND/12/2010) and University Grant Commission,India for DRS-SAP.The authors thank Prof V.Lakshminarayanan of RRI for his valuable suggestions. References [1] J. Hamola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Act. B 54 (1999) 3e15. [2] J. Hamola, Surface plasmon resonance sensors for detection of chemical and biological species, Chem. Rev. 108 (2008) 462e493. [3] E. Kretschmann, The determination of the optical constants of metals by excitation of surface plasmons, Z. Phys. 241 (1971) 313e324. [4] R.M.A. Azam, N.M. Bashara, Ellipsometry and Polarized Light, NH Publishing Co., Amsterdam, 1977. [5] Y. Wang, Voltage-induced color-selective absorption with surface plasmons, Appl. Phys. Lett. 67 (1995) 2759. [6] J. Meunier, Why a Brewster angle microscope, Col. Surf. A 171 (2000) 33e40.
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[7] P. Dynarowicz-Ła ̧tka, A. Dhanabalan, O.N. Oliveira Jr., Modern physicochemical research on Langmuir monolayers, Adv. Col. Inter. Sci. 91 (2001) 221e293. €hwald, P. Dutta, Structure and phase transitions in [8] V.M. Kaganer, H. Mo Langmuir monolayers, Rev. Mod. Phys. 71 (1999) 779e819. s-Mullol, D.K. Schwartz, Molecular orientation in Langmuir monolayers [9] J. Igne under shear, Langmuir 17 (2001) 3017e3029. s-Mullol, D.K. Schwartz, Shear-induced molecular precession in a hex[10] J. Igne atic Langmuir monolayer, Nature 410 (2001) 348e351. [11] D. Vollhardt, Effect of unsaturation in fatty acids on the main characteristics of Langmuir monolayers, J. Phys. Chem. C 111 (2007) 6805e6812. [12] J. Sanchez, A. Badia, Spatial variation in the molecular tilt orientational order within the solid domains of phase-separated, mixed dialkylphosphatidylcholine monolayers, Chem. Phys. Lipids 152 (2008) 24e37. s-Mullol, J. Claret, R. Reigada, F. Sagues, Spread monolayers: structure, [13] J. Igne flows and dynamic self-organization phenomena, Phys. Rep. 448 (2007) 163e179. non, R. Mercier, G. Tissot, P. Fournet, J. Meunier, A new [14] C. Lheveder, S. He Brewster angle microscope, Rev. Sci. Instr. 69 (1998) 1446e1450. [15] A. Ulman, An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self Assembly, Academic Press, New York, 1991. [16] M.C. Petty, Langmuir-Blodgett films: an introduction, Cambridge University Press, New York, 1996. [17] M. Poonia, V. Manjuladevi, R.K. Gupta, S.K. Gupta, J. Singh, P.B. Agarwal, J. Akhtar, Ultrathin films of single-walled carbon nanotubes: a potential methane gas sensor, Sci. Adv. Maters. 7 (2015) 455e462. [18] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Self-assembled monolayers of thiolates on metals as a form of nanotechnology, Chem. Rev. 105 (2005) 1103e1170. [19] D.L. Allara, R.G. Nuzzo, Spontaneously organized molecular assemblies. 2. Quantitative infrared spectroscopic determination of equilibrium structures of solution-adsorbed n-alkanoic acids on an oxidized aluminum surface, Langmuir 1 (1985) 52e66. [20] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 2007. [21] M.K. Massey, C. Pearson, D.A. Zeze, B.G. Mendis, M.C. Petty, The electrical and optical properties of oriented Langmuir-Blodgett films of single-walled carbon nanotubes, Carbon 49 (2011) 2424e2430. [22] J. Umemura, T. Kamata, T. Kawai, T. Takenaka, Quantitative evaluation of molecular orientation in thin Langmuir-Blodgett films by FT-IR transmission and reflection-absorption spectroscopy, J. Phys. Chem. 94 (1990) 62e67.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Measurement of optical anisotropy in ultrathin films using surface plasmon resonance V.P. Devanarayanan a, V. Manjuladevi a, Monika Poonia a, R.K. Gupta a, *, Sanjeev K. Gupta b, Jamil Akhtar c a b c
Department of Physics, Birla Institute of Technology and Science, Pilani, Rajasthan, 333031, India Department of Electronics and Information Technology (DeitY), New Delhi, 110003, India CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan, 333031, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 April 2015 Received in revised form 31 August 2015 Accepted 29 September 2015 Available online 8 October 2015
The optical phenomenon, surface plasmon resonance (SPR) is employed for the measurement of optical anisotropy in the ultrathin films fabricated through LangmuireBlodgett (LB) and self-assembled monolayer (SAM) techniques onto 50 nm gold film supported on BK7 glass substrates. The resonance angle (RA) is measured using a home built setup in Kretschmann configuration. The LB films and SAM can provide a single layer of highly ordered and organized molecules on the two dimensional surface. If the film forming molecules are anisotropic, their organization in the LB films and SAM can yield an anisotropic film due to tilt of the molecules with respect to the surface normal. The SPR spectra are recorded for the two orthogonal directions of the film with respect to the plane of incidence. The spectra are simulated by modeling the Fresnel's reflection from 4-layers viz., prism, gold, ultrathin films and air; and the real and imaginary parts of refractive index are estimated. Our study shows the metallic and dielectric nature of the LB films of bundles of single walled carbon nanotubes (SWCNTs) when the long axis of SWCNTs are aligned parallel and perpendicular to plane of incidence, respectively. The optical anisotropy was estimated from the change in real part of refractive index (Dnr) of the ultrathin films measured in the orthogonal directions. In addition, we have also studied such optical anisotropy in the LB film of cadmium-stearate and self-assembled monolayer of octadecanethiol. © 2015 Elsevier B.V. All rights reserved.
Keywords: Surface plasmon resonance LangmuireBlodgett films Single-walled carbon nanotubes
1. Introduction Surface plasmon resonance (SPR) is an optical phenomenon which is being potentially used for the detection of molecular specific interaction [1]. Such phenomenon resulted in the development of biological, chemical and gas sensors [2]. SPR technique is label free and non-destructive and hence exhibits numerous advantages over the conventional sensing technology. In the Kretschmann configuration [3], the surface plasmon is excited in 50 nm gold film deposited onto glass substrate which is coupled to a prism with a refractive index (RI) matching fluid. For an incident p-polarized light of fixed wavelength, the SPR occurs at a unique angle of incidence also known as resonance angle (RA). The RA is extremely sensitive to the dielectric layer adsorbed onto the gold surface. Any change in the dielectric layer causes a shift in the RA
* Corresponding author. E-mail address: [email protected] (R.K. Gupta). http://dx.doi.org/10.1016/j.molstruc.2015.09.018 0022-2860/© 2015 Elsevier B.V. All rights reserved.
with respect to the reference. The dielectric (hence optical) properties of the adsorbed material can be estimated by modeling the reflection from different interfaces involved during the process by employing the Fresnel's equation [4]. Wang has demonstrated the condition for the SPR can be altered by altering the dielectrics of a liquid crystal on the metal surface due to the application of electric field [5]. The optical anisotropy in a stable ultrathin film at the airewater interface (known as Langmuir monolayer) has been observed and studied extensively using Brewster angle microscope (BAM) [6e8]. Such anisotropy in Langmuir monolayer arises due to tilt of the molecules with respect to the surface normal [9e12]. In Brewster angle microscopy, the reflected intensity from the airewater interface is extremely sensitive to a change in refractive index at the interface. The refractive index changes due to change in the surface density or tilt variation of the molecules. Therefore, any change in the RI due to tilt variation of the molecules in a given phase of the Langmuir monolayer, changes the reflectivity significantly resulting in variation in grayshades in the visible region of
282
V.P. Devanarayanan et al. / Journal of Molecular Structure 1103 (2016) 281e285
the BAM images [10,13,14]. In this paper, we report the measurement of optical parameters (refractive index) in the orthogonal directions of the ultrathin films of well organized bundles of single-walled carbon nanotubes (SWCNTs), octadecanethiol (ODT) and cadmium stearate (CdSA) using a SPR instrument. The ultrathin films of SWCNTs and CdSA are fabricated through LangmuireBlodgett (LB) technique whereas that of ODT through self assembly on the gold surface. Such ultrathin films offer a well organized and orientationally ordered single layer of the molecules [15,16], which exhibits anisotropy due to the alignment of the molecules (or SWCNTs) in a preferential direction on the substrate. The anisotropy in the films depends on the shape anisotropy of the molecules and their tilt orientation on the substrate with respect to the surface normal [6]. The SPR spectra of the fabricated ultrathin films are recorded in the orthogonal directions of the film i.e., parallel (0 ) and perpendicular (90 ) with respect to the plane of incidence. We aligned the long axis of SWCNTs along the direction of dipping during the LB film fabrication process [17]. The SPR spectra of LB film of SWCNTs show very interesting result. The film of SWCNTs behaves metallike and insulating dielectric layer when the long axis of SWCNTs is aligned parallel and perpendicular to the plane of incidence, respectively. The ultrathin films of structurally similar molecules viz., ODT and stearic acid on the gold surface differ with respect to the orientational tilt of the molecules in the single layer. ODT in SAM and CdSA in LB are known to tilt ~30 and ~10 with respect to the surface normal, respectively [18,19]. Therefore, there is a different degree of anisotropy in SAM of ODT as compared to that of LB film of CdSA. We found the optical anisotropy as a change in the real part of refractive index (Dnr) of SAM of ODT and LB film of CdSA to be 0.24 and 0.10, respectively. 2. Experimental section The SPR setup in Kretschmann configuration was built in the laboratory (Fig. 1).The material of semi-cylindrical prism was BK7 having the refractive index of 1.51. The 50 nm gold film was deposited onto BK7 substrates by electron beam deposition technique. The thickness of deposited gold layer was monitored in-situ during evaporation process by employing quartz crystal based digital thickness monitor. The substrate and the prism are optically coupled by using refractive index matching fluid (Cargille). The ultrathin films of CdSA and SWCNT were fabricated by LB technique using a teflon trough (Apex Instruments). The stearic acid (SA) was obtained from SigmaeAldrich. A chloroform solution of the SA having a concentration of 3.5 mM was spread onto an aqueous subphase of 1 105 M of cadmium chloride (CdCl2) in ultrapure ion-free water (MilliQ, DQ5). A single layer of LB film of CdSA is deposited on the gold substrate at a target surface pressure of
30 mN/m. The tilt of the chains of the molecules in the LB films was reported to be around 10 with respect the surface normal [19]. An isotropic thin film of SA is deposited onto the gold substrate by spin coating technique. A 5 ml of the chloroform solution of SA is spread onto gold substrate rotated at a speed of 7000 rotations per minute. In order to obtain isotropy and homogeneity in the film, it is annealed at a temperature of about 80 C for 15 min. A uniformly dispersed solution of 9 103 mg/ml of SWCNTs (Carbon Solutions, P2-SWNT) was obtained by dissolving the nanotubes in dimethylformamide solvent and ultrasonicating the dispersion for about 1 h. The Langmuir film of SWCNTs is found to be stable with a collapse surface pressure of 11 mN/m. The LB films of SWCNTs are deposited onto solid substrates at 2 mN/m and the surface morphology were studied using atomic force microscope (SolverPro, NTMDT). We found that the long axis of SWCNTs oriented along the direction of deposition of the film [17]. Such morphology can yield exceptionally high anisotropy in the electrical and optical properties when measured in orthogonal directions. The SAM of ODT was prepared by immersing the gold substrates into the 1 mM solution of ODT in absolute ethanol for about 12 h. The ODT deposited gold substrates were rinsed thoroughly by absolute ethanol followed by HPLC grade chloroform before mounting on the scanning stage of the SPR instrument. The SPR spectra were collected by changing the angle of incidence at a step of 21.2 103 and recording the reflected intensity simultaneously. The reported SPR spectra of the ultrathin films are little broader at the minimum because of low resolution of the data acquisition card (12-bit, National Instruments). In order to check the reproducibility of the data, the SPR spectra were collected from different locations of the films, and from the films deposited in different batches. The RA for the gold film (goldeair interface) was found to be 44 . The average shift in RA (Dq) for the ultrathin films of different materials were estimated with reference to that of RA for the gold film (i.e. 44 ). The standard deviation of the RA was found to be in the range of 0.01 e0.02 .
3. Modeling The SPR spectra are simulated by modeling reflection of light from a 4-layer system viz., prism, gold, dielectric layer, air. In the 4layer system, the final Fresnel's equation is obtained by considering the reflection from three interfaces viz., prism-gold, gold-dielectric film and dielectric film-air [4]. The final Fresnel's equation of reflectance Rp for the p-polarized light is given by
Rp ¼ rp2
(1)
where rp is the reflection coefficient for p-polarized wave.
rp ¼
ðM11 þ M12 q4 Þq1 ðM21 þ M22 q4 Þ ðM11 þ M12 q4 Þq1 þ ðM21 þ M22 q4 Þ 3 Y
Mij ¼
k¼2
(2)
! ; i; j ¼ 1; 2
Mk
(3)
ij
For the kth layer,
Mk ¼ Fig. 1. The schematic of the SPR instrument setup. (a) laser, 635 nm, Newport, (b) semi-cylindrical prism of material BK7, (c) photodiode, OSI-Optoelectronics, (d)Data acquisition card, 12-bit, National Instruments, and (e) computer.
Here,
cosbk iqk sinbk
isinbk =qk cosbk
(4)
V.P. Devanarayanan et al. / Journal of Molecular Structure 1103 (2016) 281e285
1=2
qk ¼ ðmk =~εk Þ
bk ¼ dk
1=2 ~εk n21 sin2 q1 cosqk y ~εk
1=2 2p ~εk n21 sin2 q1 l
(5)
(6)
where, mk y 1, q1 and qk are the angle of incidences at the interface of media 1 2 and (k 1)k, n1 is the real part of refractive index of the medium 1, ~εk and dk are the complex dielectric constant and thickness of kth layer, respectively. For a given thickness of the ultrathin film deposited either by LB or self assembly, the reflected intensity (Rp) is simulated and reflected intensity as a function of incident angle (q) is obtained. The RA is dependent on the both ~εk and dk of the dielectric film deposited on the gold surface. Since the thickness of LB films and SAM are known, the values of complex dielectric constant (~εk ) and hence the real and imaginary part of the refractive index of the dielectric films are estimated from the simulation. The thickness of the LB films of SWCNTs and CdSA were 25 nm [17] and 2.4 nm [19] and that of SAM of ODT was 2.2 nm [18].
4. Results and discussion The SWCNTs are highly anisotropic material exhibiting extraordinary electrical, optical and mechanical properties [20]. We aligned the bundles of SWCNTs onto the substrates by LB technique. During the LB film fabrication process, in a very simple mechanism the SWCNTs can be aligned either parallel or perpendicular to the dipping direction by aligning the substrate-normal perpendicular or parallel to the film compression direction [21], respectively. The surface topography of the LB film of SWCNTs obtained using the atomic force microscope is shown in the inset of Fig. 2. The bundles of SWCNTs are found to orient with its longer axis parallel to the dipping direction of the substrate during the LB film fabrication process. The LB films of SWCNTs were deposited onto patterned interdigitated electrodes (IDE) and the currentevoltage (IeV) measurement was carried out. The IeV curves for the parallel and perpendicular orientation of SWCNTs in the LB films with respect to the applied voltage V are shown in Fig. 2. The IeV curves obtained
Fig. 2. CurrenteVoltage curves of the LB films when SWCNTs are aligned parallel and perpendicular to the electric field (E). The inset in the top figure shows the AFM image of the LB film of SWCNTs. The bundles are aligned in the direction of dipping the substrate during LB film fabrication. The right-bottom of each figure shows schematic for the alignment of SWCNTs on interdigitated electrodes.
283
for parallel orientation of the SWCNTs shows the normal ohmic behavior, and the estimated film resistance is found to be merely 7.8 U.Therefore, such orientation of the SWCNTs with respect to the applied electric field (E) indicates metallic nature of the film. However, the IeV curve for the film with SWCNTs align orthogonal to the applied electric field shows non-metallic behavior. Hence such orientation of the SWCNTs with respect to the electric field can be considered as insulating dielectric film. A single layer of SWCNTs is deposited onto gold substrate at a target surface pressure of 2 mN/m. The SPR spectra of the LB films were obtained for two different orientations of the long axis of SWCNTs with respect to the plane of incidence viz. parallel (0 ) and perpendicular (90 ) (Fig. 3).The spectra are simulated using the Eq. (1) and the optical parameters like real part of RI (nr) and imaginary part of RI (nim) are estimated. The values are tabulated in Table 1.The average shift in RA with respect to the gold film (Dq) for SWCNTs aligned parallel to the plane of incidence (i.e. 0 ) is found to be merely 0.07 whereas the Dq is found to be 0.28 for the film where SWCNTs aligned perpendicular (i.e. 90 ) with respect to the plane of incidence. Since, the Dq is not significant, the LB film with zero degree alignment of SWCNTs can be treated as a metallic film deposited over the 50 nm gold film. The SPR spectrum for such film is simulated using Eq. (1) and the values of nr and nim are estimated to be 0.2 and 3.3, respectively. Since the Dq for such film is very low, the values of nr and nim are comparable to that of gold. The Dq for the film with 90 orientation of SWCNTs with respect to the plane of incidence is found to be 0.28 . Such orientation of SWCNTs in the LB film yields a dielectric insulating layer. Therefore, the SPR spectrum shows a large shift in RA as compared to that of film with parallel alignment of SWCNTs. We have also studied the SPR spectra of the ultrathin films composed of structurally similar rod shaped molecules viz. ODT and SA. Such film forming molecules possess lower length-to-width ratio and hence exhibit much less anisotropy in properties as compared to that of SWCNTs. The amount of anisotropy in the ultrathin films of the rod shaped molecules depends on the amount of molecular tilt with respect to the surface normal [22,5]. Therefore, we studied the SPR behavior of self assembled monolayer of ODT and LB films of CdSA on gold substrates. The molecular tilts of ODT molecules in SAM on gold is reported to be around 30 [18] and CdSA in the LB films deposited at 30 mN/m is around 10 [19]. The SAM of ODT may offer more anisotropy as compared to that of LB films of CdSA. The recorded SPR spectra of the SAM of ODT on the gold substrate in the orthogonal directions are shown in Fig. 4.The Dq of the SAM of ODT measured in the orthogonal directions are 0.2 and
Fig. 3. Surface plasmon resonance spectra of gold and one layer of LB film of SWCNTs where the long axis of SWCNTs are aligned parallel (0 ) and perpendicular (90 ) to the plane of incidence. The curves are shifted vertically for visual clarity.
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Table 1 Estimated values of refractive index (RI) of LB film of SWCNTs through simulation. nr and nim are the real and imaginary part of RI. The average shift in RA is Dq. The refractive index for the gold was chosen to be 0.172 (nr) and 3.421 (nim). LB film of SWCNTs
Parallel (0 ) Perpendicular (90 )
Dq
0.07 0.28
nr
nim
0.2 1.04
3.3 0.0045
Fig. 4. Surface plasmon resonance spectra of gold and self assembled monolayer (SAM) of octadecanethiol for the orthogonal directions: parallel (0 ) and perpendicular (90 ) to the plane of incidence. The curves are shifted vertically for visual clarity.
0.31. The spectra are simulated using Eq. (1) and nr and nim are estimated. The refractive index is tabulated in Table 2. The Dnr is found to be 0.24. The LB films of CdSA deposited at a target surface pressure of 30 mN/m yield an ordered film with molecular orientation ~10 with respect to the surface normal [19]. The SPR spectra of one layer of LB film of CdSA is shown in Fig. 5(a).The Dq corresponding to the orientations 0+ and 90+ are 0.195+ and 0.10+, respectively. The spectra are simulated and the refractive indices of the dielectric films are estimated. The simulated values of refractive indices for the different films are tabulated in Table 3. The Dnr is found to be 0.10. The SPR spectra of the spin-coated thin film of SA on gold substrate are shown in 5(b). In the spin-coated thin films, the molecules orient isotropically onto the surface. The average shift in the RA for the two orthogonal directions are found to be constant (¼0.2+). This clearly indicates that the difference in shift in SPR angle during the orthogonal scanning is essentially due to difference in refractive index in the film which normally arises due to film anisotropy. The estimated nr for the ultrathin films of different organized materials in the orthogonal directions are shown as bar diagram in Fig. 6.
Fig. 5. Surface plasmon resonance spectra of gold and (a) one layer of LB film of CdSA where the dipping direction is oriented parallel (0 ) and perpendicular (90+) to the plane of incidence (b) spin-coated film of stearic acid (isotropically oriented molecules) for the orthogonal directions. The curves are shifted vertically for visual clarity.
Table 3 Estimated values of refractive index (RI) of LB film of CdSA through simulation. nr and nim are the real and imaginary part of RI. The average shift in resonance angle is Dq. The refractive index for the gold was chosen to be 0.172 (nr) and 3.421 (nim). LB film of CdSA +
Parallel (0 ) Perpendicular (90+)
Dq 0.195 0.1+
+
nr
nim
1.21 1.11
0.0045 0.0045
even few of molecular specific interaction at the given metaldielectric interface. The change in the optical (or dielectric) properties due to the anisotropy in thin films can be detected using SPR.
5. Conclusion SPR is a very powerful technique to identify molecular specific interaction. The technique is label free and very sensitive to detect
Table 2 Estimated values of refractive index (RI) of SAM of ODT through simulation. nr and nim are the real and imaginary part of RI. The average shift in resonance angle is Dq. The refractive index for the gold was chosen to be 0.172 (nr) and 3.421 (nim). SAM of ODT
Parallel (0 ) Perpendicular (90 )
Dq
0.2 0.31
nr
nim
1.24 1.48
0.0045 0.0045
Fig. 6. The bar diagram showing the nr for the ultrathin films of different materials measured in the orthogonal directions. The two bars for each of the film represent the nr in the orthogonal directions.
V.P. Devanarayanan et al. / Journal of Molecular Structure 1103 (2016) 281e285
In this paper, we have demonstrated the capability SPR for the measurement of optical anisotropy of the ultrathin films grown using the conventional LB and self assembly. The LB film of SWCNTs shows interesting results. The LB film of SWCNTs shows both the metallic and dielectric behavior when scanned in 0+ and 90+ alignment of SWCNTs with respect to the plane of polarization, respectively. The SAM of ODT with 30+ tilted molecules shows anisotropy in the real part of refractive index of about 0.24 whereas the LB films of CdSA with 10+ tilted molecules shows anisotropy in the real part of refractive index of about 0.10. Future work includes study of SPR of ultrathin films of different shape anisotropic liquid crystal molecules. Acknowledgments The authors (BITS Pilani) are thankful to Department of Science and Technology,India for funding the project under Instrument Development Programme(IDP/IND/12/2010) and University Grant Commission,India for DRS-SAP.The authors thank Prof V.Lakshminarayanan of RRI for his valuable suggestions. References [1] J. Hamola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Act. B 54 (1999) 3e15. [2] J. Hamola, Surface plasmon resonance sensors for detection of chemical and biological species, Chem. Rev. 108 (2008) 462e493. [3] E. Kretschmann, The determination of the optical constants of metals by excitation of surface plasmons, Z. Phys. 241 (1971) 313e324. [4] R.M.A. Azam, N.M. Bashara, Ellipsometry and Polarized Light, NH Publishing Co., Amsterdam, 1977. [5] Y. Wang, Voltage-induced color-selective absorption with surface plasmons, Appl. Phys. Lett. 67 (1995) 2759. [6] J. Meunier, Why a Brewster angle microscope, Col. Surf. A 171 (2000) 33e40.
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