Plasmonic sensing using metallic nano-sculptured thin films.

Thin Films

Plasmonic Sensing Using Metallic Nano-Sculptured Thin Films Ibrahim Abdulhalim*

From the Contents 1. Introduction ..............................................2 2. Nano-Sculptured Thin Films .......................2 3. Extended SPR Sensing Based on Metallic nSTFs.......................................3 4. Localised SPR from metallic nSTFs .............7 5. SERS from metallic nSTFs ...........................9 6. SEF from Metallic nSTFs ...........................12 7. Conclusions, New Challenges and Future Perspective ............................13

small 2014, DOI: 10.1002/smll.201303181

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ano-sculptured thin films (nSTFs) is a group of meterials prepared by the oblique or the glancing angle deposition technique. They take the form of rods having different shapes such as nanocolumns, nanoscrews, nanozigzags and many other nanoshapes. Their potential for biosensing is highlighted in this review particularly the metallic ones due to their remarkable plasmonic properties. The techniques that have been shown so far to be of high potential are: extended surface plasmon resonance (SPR), localised SPR, surface enhanced flourescence (SEF) and Raman scattering (SERS). The use of metal nSTFs in SPR biosensors with Kretschmann-Raether configuration enhances both the angular and the spectral sensitivities due to the porosity and adds more degrees of freedom in designing evanescent waves based techniques. The metallic nSTFs, exhibit remarkable localised plasmonic properties that make them a promising substrate for enhanced spectroscopies. Their long term stability in water environment makes them suitable candidates for biosensing in water as it is already demonstrated for several water pollutants. The influences of the nanostructures’ size, topology, the substrate features, and the preparation conditions on the enhancement of SEF and SERS are highlighted with emphases on the unresolved issues and future trends.

© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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1. Introduction The two key world resources: energy and water are considered as limited valuable commodities for which their sustainability has been an essential issue until the present time. At their forefront is the use of nanometer scale technologies which are exhibiting tremendous opportunities. Nanomaterials for water and energy management is a wide multidisciplinary subject that may include variety of materials and structures to improve water quality and efficiency of energy related devices, components and systems. One of the important classes of nanomaterials are metallic nanostructures which exhibit remarkable plasmonic properties at the nanoscale. Hence the field of nanoplasmonics which is newly emerging deals with surface plasmon resonance (SPR) related phenomena in metallic structures at the nanoscale such as nanoparticles, nanoholes, nanowires and other combinations of different geometrical nanoscale shapes. One of the most important prerequisites to improve water quality is to be able to monitor the concentrations of critical pollutants in water. In this regard biosensors in thin film form based on SPR related technology have been developed in various configurations and formats for sensing a variety of target samples in water, including pesticides, chemicals, biological pathogens, toxins, and diseased tissue.[1,2] The sensitivity and detection limit of SPR-based sensors continues to improve so that samples of ever smaller volumes can be detected with enhanced reliability and specificity in particular when combined with spectroscopic probes such as surface enhanced Raman scattering (SERS), fluorescence (SEF) and infrared absorption spectroscopies (SEIRA). Platforms of nanofeatures in the form of thin film on substrate are preferable for biochip applications in which a 2D array of small areas (0.1 mm × 0.1 mm) on substrate are used to sense a large number of analytes in parallel. Nanostructured thin films come in variety of forms including perforated metal films for enhanced optical transmission and other periodic arrays of nanoparticles. There was a great interest during the last decade to develop plasmonic biosensors for water quality control with better precision and reliability. As a result improved biosensors were developed based on nanostructured plasmonic and photonic thin films in variety of forms as well as using SPR in Kretschmann configuration by combining dense metallic films with additional nanolayers.[1,2] The signal to noise ratio was improved significantly as well using several reading methodologies such as phase detection, polarimetric and imaging methods.[2] An important class of nanostructured thin films is called nano-sculptured thin films (nSTFs) which are prepared in large areas easily using the oblique or glancing angle deposition (GLAD).[3] During the last decade there was a great interest in exploring the potential of nSTFs for sensing particularly the metallic ones due their intriguing plasmonic properties. The purpose of this review is to highlight the potential of metallic nSTFs for optical biosensing in water based on SPR, localized SPR (LSPR), SEF and SERS. These films are shown to have a great potential for pollutants sensing in water as large area thin films, easy to fabricate, exhibiting high sensitivity in SPR due to the porosity and highly sensitive spectroscopic detection using SERS, SEF and

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SEIRA. The plasmonic field enhancement is also important for improving the efficiency of photovoltaics, photodetectors, light emitting diodes and other optoelectronic devices. Hence metallic nSTFs can have a potential in energy related devices when made of, or mixed with semiconducting materials. Following a description of the structure of nSTFs and their preparation conditions in section 2, the SPR properties are reviewed in section 3 and 4 while in sections 5 and 6 the works done so far on surface enhanced spectroscopies particularly SERS and SEF are reviewed. In the last section future perspective and unresolved issues are highlighted.

2. Nano-Sculptured Thin Films The GLAD technique is a sophisticated method to create 3D nanostructures in thin film forms with a tailored geometry such as columnar, twisted, zigzag and other periodic forms. These films are nanostructured inorganic or even organic materials with columnar structure exhibiting anisotropic optical properties. They can be designed in a controlled manner using physical vapor deposition (PVD) methods in which the particles flux is incident on the substrate at a glancing angle (GLAD) for potentially multifunctional materials and products. The self - organized nanostructure growth is based on concurrent growth mechanism due to geometrical shadowing in combination with kinetic limitation for surface adatoms. The morphology can be controlled by varying the substrate orientation and other deposition conditions such as the temperature (Figure 1).[3] The technique requires a particle flux reaching the substrate under an extremely oblique angle of incidence (typically > 85 deg, see Figure 2a). The growth conditions support a columnar growth, and the samples consist of 3D needles, which are slanted into the direction of the particle flux (Figures 2b and 2c). The instantaneous change of the growth direction due to a simple variation of the incident vapour flux (by substrate rotation) allows for the fabrication of 3D nanostructures with manifold morphologies. Since the growth process is mainly determined by the shadowing length and the surface diffusion length, materials ranging from insulators, metals and semiconductors can be grown with 3D morphologies like spirals, chevrons, screws, pillars, needles etc. (Figures 2b– 2f). Nanostructures with n-fold shaped symmetry result from a step-wise substrate rotation by the angle

Prof. I. Abdulhalim Department of Electro optic Engineering and The Ilse Katz Institute for Nanoscale Sciences and Technology Ben Gurion Unevirsity Beer Sheva 84105, Israel E-mail: [email protected] Prof. I. Abdulhalim School of Materials Science and Engineering Nanyang Technological University Singapore 637722 DOI: 10.1002/smll.201303181


small 2014, DOI: 10.1002/smll.201303181

Plasmonic Sensing Using Metallic Nano-Sculptured Thin Films

Figure 1. (a) Schematic drawing to illustrate the deposition conditions of nSTFs; (b) the shadowing effect between neighbouring structures results in a self-organized formation of inclined nanocolumns; (c) and (d) are SEM images of nSTFs prepared under different deposition temperatures and rotation conditions of the substrate. Reproduced with permission.[4], Institute of Physics.

360/n deg. The application of GLAD to fabricate metallic (Ag, Al, Co, Cr, Cu, Fe, Mg, Nb, Ni, Ru, Ta, Ti, W), semiconductor (Ge, Si), and compound nanostructures such as oxides (SiO2, TiO2, ZnO) or chalcogenides (GeSbSe) has been reported.[5,6] Furthermore, as long as chemical interactions can be neglected the deposition is independent of the used substrate. Therefore also cheap lime glass or flexible polyimide foils may serve as appropriate substrate material for GLAD. To improve the arrangement and the uniformity of GLAD-deposited nanostructures patterned substrates with symmetrically ordered surface mounds were successfully applied (Figure 3). The artificial definition of pre-patterns can be performed by electron beam lithography or nanosphere lithography. [7] Based on the latter honeycomb and hexagonal patterns of extremely uniform Si and Ge nanostructures of different shape were demonstrated. The promising option of preparing the nano-sculptured thin films (nSTFs) using the GLAD technique with a suitable substrate rotation, gives the advantageous possibility to grow nearly arbitrarily shaped, separated nano-nSTFs in a single deposition step without any pre- or post-deposition patterning of the film. Nanostructure and porosity both cause enhancement of the optical signature from biomolecules attached or in close proximity to the nano-features. small 2014, DOI: 10.1002/smll.201303181

Periodicity and birefringence yields the polarization-dependence of the optical response, a fact that can be used for multi-sensing action. The wide variety of material possibilities for engineering nSTFs helps to match special needs. As compared to existing methodologies of preparing metallic nanostructures and porous materials, nSTFs exhibit a wider range of possibilities, yet they cover the benefits that one expects from such biosensing systems. The porosity can be engineered to be within the 10–90% range. Being porous, a nSTF can function as a nano-reactor; a capability that can be harnessed for variety of biosensing applications.

3. Extended SPR Sensing Based on Metallic nSTFs Without any rotations during the deposition the nSTFs have mostly a columnar structure so sometimes called nano columnar thin films (nCTFs). The prism coupling of surface plasmons (SPs) in the Otto geometry was used at the infrared wavelength of 3.391 µm to determine the surface optical anisotropy of several obliquely deposited nickel films.[8] The SPR response from the nCTFs of nickel was measured and estimated by the Bruggeman effective medium theory for the optical properties and the geometrical factors


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Figure 2. Deposition of materials under oblique angle incidence. a) Basic principle: The shadowing between neighbouring structures results in a self-organized formation of inclined nanocolumns. b) Si and c) Ag sculptured thin films. (d)–(f): Si nanostructures fabricated by GLAD with continuous substrate rotation. With respect to the angular velocity different morphologies were obtained: d) ω > 0.175 rev/min: vertical posts, e) 0.03 < ω < 0.175 rev/min: screws, and f) ω < 0.03 rev/min: spirals. Pictures produced at the IOM, Leipzig, Germany provided kindly by Prof. Bernd Rauschenbach.

of the metallic films. Nano porous gold films in the context of SPR were also investigated recently.[9] However for SPR sensing applications the Kretschmann-Raether configuration is the most used both in the angular and in the spectral modes. In our pioneering studies,[10] we demonstrated that thin films of porous metals can be used as biosensors. It was shown that using metallic nCTFs facilitates the development of sensors with higher sensitivity. For Ag and Au nCTFs, the

SPR dip sensitivity increases by about a factor of 2 with 30% porosity as compared to nonporous films. It was demonstrated theoretically and experimentally that the sensitivity of nCTFs based SPR sensors is higher than the conventional SPR sensor, and furthermore the sensitivity can be tuned by varying the porosity of the nCTF. For the theoretical modeling, the metallic nCTF was divided into two porous layers with different thicknesses and the porosity was varied to obtain optimal fit with the experiments results. Short description of the calculation procedure of SPR from nSTFs is given in the appendix. The best fit to the experimental data was found when the columns were treated as ellipsoids and not as cylinders.[10] Figures 4 and 5 show SPR signals in the angular mode for different Ag and Au nCTFs which meant to represent different porosities. The parameters that give the best fit to the experimental results from the different samples are summarized in the papers by Shalabney et al.[10,11] The general behavior is that as the porosity increases, the SPR dip widens and becomes asymmetric because of increasing scattering losses in the nCTF that are due to the non-homogeneous distribution of matter therein. As the porosity increases beyond 0.75, the SPR dip almost disappears, with a vestigial peak near the onset to the total internal reflection (TIR) regime which can be also used as a peak sensor. [12] The appearance of a peak at the onset of the TIR regime when the SPR broadens can also be seen when the broadening is due to absorption loss rather than scattering loss inside a thin metal film. This can be seen from the fact that the propaga2 tion length Lx = λ ⋅ ε mr 2π ε mi

3/2

⎡ ε s + ε mr ⎤ of the SP wave depends ⋅⎢ ⎥ ⎣ ε s ⋅ ε mr ⎦ on the real εmr and imaginary εmi parts of the metal dielectric constant with εs being that of the sample. For example for Al and Cr, the ratio εmr/εmi is relatively small at wavelengths in the visible region; therefore, Lx is small which means the absorption loss is strong in these metals and the peak can

Figure 3. Fabrication of ordered Au pre-patterns on Si (100) substrate. a,b) HCP and square arrangement by electron beam lithography, c) honeycomb arrangement by nanosphere lithography; d) columnar GLAD nanostructures in honeycomb arrangement (left: top view; right: crosssection). Images are from the IOM, Leipzig, Germany provided kindly by Prof. Bernd Rauschenbach.



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Figure 4. Measured and calculated SPR curves from Ag nCTFs prepared to be with increasing porosity from a-c. Reproduced with permission.[10] Elsevier.

be seen even with dense films because the width of the SPR dip becomes larger when Lx is smaller.[13] All this assumes that the metal film thickness is at the optimum for the SPR

to occur because if it is thinner, then radiation losses start to take place. On the other hand for Ag the ratio εmr/εmi is much larger both in the visible and in the infra red ranges, thus making Ag an excellent candidate for obtaining sharper SP resonances. The same conclusion can be drawn for Au at the wavlengths starting from the near infrared and above. The appearance of the peak in conventional SPR sensor with dense metallic film indicates that peak sensor can be produced also due to absorption losses. One can notice from Figures 4c, 5c and 5d that the disagreement between the experimental and the theoretical curves increases as the porosity increases and the SPR dip becomes wider. The collapse of the SPR dip above certain porosity may be correlated with the percolation type transition of the conductivity observed by Maaroof et al.[14] as shown in Figure 6a. In order to verify this correlation we have calculated the SPR curves of Ag nCTF at different film porosities as shown in figure 6b. One can easily see that nearly above 40% the SPR dip becomes very wide and almost disappears in agreement with the conductivity percolation threshold. This behavior is expected as the conductivity is directly related to the density of the free electrons in the metal and their mean free path. When the porosity increases although one can still consider the free electrons density to remain the same within the nanorod volume, the mean free path is reduced which basically defines the plasmon propagation length. The plasmons become more localised than extended and in fact the Bruggeman homogenization formalism becomes less valid, although it is able to show the percolation behavior which is one of the advantages of the Bruggeman homogenization formalism over the MaxwellGarnett formalism. The Bruggeman homogenization formalism applied to nSTFs particularly metallic ones is still an active field of research.[15,16] Neverthless for the sensing applications we can conclude that metallic nCTFs have a well

Figure 5. Measured and calculated SPR curves from Au nCTFs prepared to be with increasing porosity from a-d. Reproduced with permission.[10], Elsevier. small 2014, DOI: 10.1002/smll.201303181



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all analyte RIs. Porous films response is characterized by wider and red-shifted dips which may apparently indicate larger sensitivities compared to their counterpart dense films. The dips locations of the experimental results were determined using a parabolic fit method in the minimum region of the dip. One can see from Figure 8d that the resonance wavelength is red shifted as the porosity increases. These larger resonance wavelengths can be considered as a guarantee for higher sensitivity in the case of porous metallic films. The enhanced sensitivity of the nCTF layer versus the dense film is meaningful only if the experimental conditions are similar, namely the same incidence angles and thicknesses. The SPR sensitivity enhancement was correlated by Abdulhalim with the the field overlap integral. According to Abdulhalim,[17] the shift in the wave vector is proportional to the overlap integral which in turn is proportional to the interaction volumeVin and the field distribution: k δk ≈ i 2

Figure 6. (a) Resistance of porous Ag films versus the porosity showing percolation threshold around 45%, taken from Maaroof et al.[14]; (b) Calculated SPR cruves for TM polarized light from 47 nm nanorods of silver with aspect ration of 1:1:5 at different porosities using the Bruggeman formalism showing the correlation between the widening of the SPR dip and the percolation threshold. Reproduced with permission.[14] 2013, Institute of Physics.

defined SPR dip useful for sensing uptill porosities as high as 35% with the benefit of increased sensitivity. In SPR sensors with dense metal film, the shift in the resonance angle is solely due to the change in the analyte refractive index (RI). On the other hand, in nCTF-based SPR sensor, variation in the sample RI causes a modification to the metal effective layer as well as a change in the analyte RI. Basically in the nCTF layer the porosity increases the surface-to-volume ratio and thus the sensitivity is expected to increase. Numerical calculations (Figure 7c) demonstrate that the angular sensitivity increases from 62 deg/RIU for non-porous Ag film to 177 deg/RIU for nCTF-based SPR sensor when a porosity around 0.34 (34%). This result is obtained when the thickness of the films is adjusted to satisfy the best coupling condition and the prism is made from SF11 glass. The spectral interrugation mode using nCTFs was investigated both theoretically and experimentally by Shalabney et al.[11] In this study the porosity effect on broadening the reflectance dip can be seen from the reflected spectrum for


* i ⋅ E f ⋅ dr * i ⋅ E i ⋅ dr

∫ δε ⋅ E ∫ ε⋅ E Vin

V

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Where Ei, ki are the electrical field and its wave vector before the variation in the analyte refractive index took place, while Ef is the field after the index perturbation and δk is the associated shift in the wave vector due to a change from ε to ε + δε in the analyte dielectric constant. The denominator integral in Equation (1) is over the whole volume V. Since δk expresses the change in the incidence angle or alternatively the change in the wavelength, then δk/δε represents the sensitivity of the sensor, which is proportional to the overlap integral in the numerator of Equation (1) normalized to the total energy. Hence to maximize the sensitivity one needs to maximize this integral which can be accomplished by increasing the interaction volume, that is the evanescence depth, the SP propagation length along the surface or by increasing the field intensity in the analyte region. This correlation was proved by Shalabney and Abdulhalim[18] based on which the SPR sensitivity enhancement was explained in several cases. In order to verify that this is indeed the case even with metallic nCTFs the field distribution was calculated for Ag films at different porosities. Figure 9 shows that the field is indeed enhanced as the porosity increases which explains why the sensitivity increases. For the field calculation algorithm, see reference 11. In addition to these experimental works, it should be mentioned that extended SPR on the surface of anisotropic films had recently caught the attention of several theoretical researchers exhibiting several novel phenomena. Excitation of TE and TM surface plasmon waves at the interface between thin anisotropic film and metal was shown[19] to be possible in the Otto configuration with the anisotropic layer being thicker than the evanescent region. Total absorption occurs at specific wavelengths or incidence angles both for TE and TM waves. Polarization conversion at the resonance was also shown to occur when both modes are excited in the anisotropic layer. Tuning the orientation of the dielectric tensor ellipsoid or its principal values was shown to allow tuning


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of the resonance location. It was shown recently[20,21] that it is possible to excite SP waves directly on the interface of a uniaxial medium and metal. Both TE and TM SP waves were shown to be possible with large sensitivity to the orientation of the optic axis. Morphological effects for the excitation of SPP waves at the interface with structurally chiral films were recently reported[22,23] and multiple SPR excitations were observed numerically[24,25]. Recently an experimental verification was reported on the multiple SPR resonances both TM and TE polarizations using MgF2 chiral nSTF on gold film[26]. However these resonances are known even when uniform isotropic dielectric is deposited on top of the metal film with high enough thickness to support guided modes known as coupled wave SPR[27]. Part of these resonances are simply guided modes which express themselves as dips in the reflectivity function. Because they are guided their field distribution is mainly within the wavguide layer and therefore their sensitivity to the analyte RI is smaller even though they are narrower. Although in the case of reference[26] the top dielectric is a twisted structure, a fact that adds some additional features to the reflection spectrum, care should be taken in the interpretation of these resonances as to whether they are SP waves or waveguide modes. small 2014, DOI: 10.1002/smll.201303181

4. Localised SPR from metallic nSTFs As ellipsoidal nanorods, metallic nCTFs are expected to exhibit two or three different localised SPR (LSPR) excitations along the principal axis of the ellipsoid depending on the incident light polarization. One of the most general descriptions of a smooth and regular shape is an ellipsoid with three axes a, b and c, (a ≥ b ≥ c). If b = c, the ellipsoid becomes a prolate spheroid (cigar shape), and if a = b the ellipsoid becomes oblate spheroid (pancake shape). In a similar manner we can define polarizability for the structure in each direction of the major axes of the ellipsoid. Namely the polarizability that an applied electric field produces depends on the polarization direction of the external applied field, and consequently the resonance wavelength is determined for each axis when the electric field is polarized along that particular axis.[28,29] ⎛ ⎞ εm − εd α i = 4π abc ⎜ , ⎝ 3Li ( ε m − ε d ) + 3ε d ⎟⎠

(2)

where here εm,d are the dielectric constants of the metal nanorod and the dielectric medium surrounding it, Li, is a form factor of axis i when the electric field is polarized along this axis, αi is the polarizability corresponding to the same



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Figure 8. (a) SEM image of silver columnar thin film illustrating the existence of two layers forming the films, (b) Illustration of the multilayered structure used in the modeling consisting of Ti layer and the two nCTF layers bounded by two semi-infinite, homogenous and dielectric media of the prism and the sample materials in the Kretschmann configuration. (c) Angular sensitivity versus porosity for single nCTF layer. The lower curve corresponds to nCTF layer with fixed thickness of 47 nm, vertical nanostructures to the substrate with aspect ratios of γ2 = 1, γ3 = 5, and the analyte is water with na = 1.33. The upper curve corresponds to the same nCTF layer data as the lower curve but the thickness is adjusted to fulfill the critical coupling condition. The calculations were done for 637 nm laser wavelength and SF11 prism. (d) Spectral sensitivity and resonance wavelength as a function of the porosity of the film. Single nCTF layer was considered with fixed thickness of 47 nm and fixed incidence angle of 54.74 deg. It should be mentioned that the silver film thickness here is around 50 nm which is why the columns are not as clear as in the case of the thick films shown in figure 1. Reproduced with permission.[11], Elsevier.

axis. The geometrical factors Li obey the following rules 0 ≤ Li ≤ 1 and La + Lb + Lc = 1 when considering the axes a, b, and c. 1 For a sphere La = Lb = Lc = due to spherical symmetry. 3 The absorption cross section when the applied field is polarized parallel to the i-axis according to Mie scattering theory[28,29] will be: Cabs = kIm(α i )

(3) [30]

when Now, there are peaks in the absorption spectra, εm = εd (1-1/Li). Since 0 ≤ Li ≤ 1, the term (1/Li) yields a wide range of frequencies (see Figure 10). Similar behavior for nanodiscs and nanorods can be obtained because they are the limits of oblate and prolate spheroids respectively. Furthermore, one can describe the response of structures in terms of ellipsoids with various shapes. One can design the resonance frequency by tailoring the shape and the size of


the nanoparticles. In Figure 10, the absorption cross section was calculated for silver nano spheroids with various shapes and sizes. The peak can be red-shifted or blue-shifted compared with the peak of a sphere by controlling the size and the shape of the spheroid. The geometrical factors Li can be analytically calculated for standard smooth particles such as spheres, spheroids, ellipsoids, and cylinders.[31] In metallic nCTFs however the nanorods are not ordered, they have some distribution of shapes and dimensions and they are close to each other. The result is a broadening of the LSPR absorption peak. This broadening has the disadvantage that it becomes less suitable for biosensing because it is difficult to determine the peak location accurately. Also as the resonance is not well defined it means the field enhancement near the nanorods is reduced. On the other hand it has the advantage of relaxing the design tolerance of the LSPR


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Figure 9. Electric field intensity distribution for three different porous Ag nCTFs and the dense film case in the near infrared wavelength 1550 nm showing the field enhancement with the porosity. 3

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frequency required for surface enhanced spectroscopies. An example of such broadened LSPR absorption spectra are shown in figure 11 following Suzuki et al.[32] for the two polarization orientations demonstrating that for the long axis LSPR the frequency is shorter than for the short axis LSPR. The design of the structure was to obtain LSPR at the Raman excitation wavelength of 785nm.

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Figure 10. Calculated absorption cross sections for various sizes of isolated spheroidal Ag nanoparticles: (a) Absorption cross section for prolate spheroid with different axes lengths. L- Indicates the long axis of the spheroid and S- indicates the short axis of the spheroid. (b) Absorption cross section calculated for oblate spheroids. For each spheroid, the absorption was calculated for both the polarization parallel to the long axis (solid curves-right vertical axis) and parallel to the short axis (dashed curves-left vertical axis). Although the short and long axis sizes of the ellipsoids are shown, the resonances depend on the ratio between these sizes. small 2014, DOI: 10.1002/smll.201303181

Plasmonic interaction between light and localised SPs causes further optical signals to arise and be enhanced such as Surface Enhanced Raman Scattering (SERS) and Surface Enhanced Fluorescence (SEF). SERS is a very sensitive spectroscopy that allows the detection of organic molecules adsorbed on noble metal substrates (silver, gold, copper) at sub-micro-molar concentrations. Since SERS depends on the local field to the fourth power (|E|4) in the vicinity of an adsorbate molecule.[33,34] one can significantly enhance the SERS intensity by enhancing the field near the scattering molecules. By engineering the nanoparticle structure size and shape to yield resonance wavelength that is suitably located between the exciting laser and the Raman frequency, one can obtain large enhancement factors of Raman scattering signals.[35] The effects of the nanostructures features (substrate material, nanoparticles material, nanoparticles size, nanoparticles shape, etc.) on the SERS enhancement were intensively investigated during the last few years.[4,36–38] A significant scattered wave is detected if a relative relationship between, the particles size, arrangement period, Raman frequency and the surface plasmon wave is satisfied. With this in mind several works investigated SERS from metallic nSTFs



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was predicted. Greenler model was modified, improved, and developed recently by Liu and Zhao[51] to the case of SERS from tilted nanorods deposited on flat substrates. The reflectivity of the underlayer was investigated[52] and found to increase linearly with the reflectivity following Greenler model. Although silver nanorods were and still widely used by many researchers as an attractive substrate in investigating SERS,[53–55] only few investigators examined the most optimal nanostructures for larger enhancement in terms of optimizing the size, shape, separation and substrate material. In Figure 12 an example of SERS signal is shown in which the probe molecule 4-Aminothiophenol was diluted in ethanol at 1 wt%. After immersing the nSTFs in the solution for 24 hours, the ethanol was evaporated by exposing the samples to air for about 20 minutes. The specific vibrational modes of the adsorbed molecule were considered in order to evaluate the enhancement factors of SERS from the different nSTFs.[56] The SERS measurements were performed using Raman spectroscopy system equipped with a laser diode emitting at 785 nm as the excitation wavelength and with about 50 mW power on the sample (∼80 W/cm2). The excitation is coupled to the sample using a 200 µm core fiber and the back scattered radiation was collected using 400 µm core fiber. For each sample, a dense film of the same material and with the same thickness deposited on similar substrate was taken as a reference. The enhancement was defined as the ratio between the SERS intensity of the nSTF and the corresponding intensity from the reference film. To determine the limit of detection the 4-Aminothiophenol (4-ATP) concentration in ethanol varies from 0.5% wt. (4 g/l) down to 9.6 × 10−10% wt. (7 × 10−7g/l) until the intensity of the peak at 1495 cm−1 vanished. One can see from the work of Shalabney et al.[57] that a concentration of 0.7 µg/l of 4-ATP can be easily detected by some of the nSTFs substrates. From Figure 12, one can see that, nanostructures deposited on Si(100) give large enhancement that estimated to be of the order of 106. Figure 12 shows only the results of nSTFs made of about 90 4-ATP molecule 350 nm height vertical Ag columns depos80 ited on different substrates. Although the 350nm Ag CTF results of the slanted columns with the 100nm Ag slanted 70 on Si(100) same sizes are not shown here, they exhibcolumns on substrate ited the same dependence with the subSi(100) substrate 60 strate material. This behavior of the nSTFs 50 versus the substrate type was repeatable and independent of the columns height 40 for almost all the size categories in this group of samples. Extensive report of this 30 study can be found in Shalabney et al.[57] A summary of the enhancement factors 20 for the different samples is shown in the Reference 10 table of Figure 13 showing that they are comparable to the best enhancement fac0 tors obtained from other different metal 250 750 1250 1750 2250 nanoparticles existing in the literature -1) such as nano rings, nano cubes, nano wires Raman shift (cm (1/cm) and nano holes. Figure 12. Raman signals from Ag nSTFs with vertical columns of height nearly 350 nm that The dependence of the SERS are deposited on silicon substrate and silicon covered with 15 nm Ti as reference. enhancement on the depoistion angle was

Counts (subtract background) × 10 3

at different preparation conditions and obtained enhancement factors of few orders of magnitude and used it even in fiber from.[39] Following the fundamental invetigations of SERS from metallic nSTFs during the last decade, biosensing applications started to appear as well. Shanmukh et al.[40] demonstrated the detection of different viruses in small volumes of the order of 1 µl. SERS from metallic nSTF was in fact reported in some early works.[41–44] Localized Surface Plasmon (LSP) from an array of particles deposited on a substrate differ from the LSP on the surface of a single particle because the interaction with the substrate and the particles around[45,46]. Coupling between surface plasmons of adjacent particles produces “hot spots” with further electromagnetic enhancement. In this direction, Zhou et al.[4] reported that the detection limit of Rhodamine 6G (R6G) can be 1 × 10−14 mol/l from well-separated silver nanorods deposited on Si substrate. Furthermore, SERS spectroscopy was used to distinguish between LSPs from an array of oblate golden spheroids and propagating surface plasmon from thin gold film which was deposited beneath.[47] Nanowires prepared by the Langmuir-Blodgett technique (LB) with length on the order of 2–3 microns and 50 nm diameter with pentagonal cross section exhibited a large enhancement of SERS.[48] According to the expressions of the enhancement factor of SERS due to LSP excitation, the larger gain is attainable when the excitation wavelength is close to the LSP wavelength of the particle adjacent to the probe molecule. Since the enhancement comes from both the incident (exciting) field and the scattered field with shifted frequency, it is adequate to require that the LSP frequency located between the exciting and the shifted frequency. Felidj and collaborators made attempts to confirm this fact experimentally.[49] The effect of the substrate type on the Raman scattering intensity was formally treated, for the first time, by Greenler and Slager[50] before even the SERS phenomenon



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Figure 13. Table of estimated SERS enhancement factors from 4-ATP on different Ag nCTF substrates to show that they are comparable to the best enhancement factors obtained from other different metal nanoparticles existing in the literature. Pictures reproduced with permission. From left to right:[58a] Optical Society of America,[58b] Elsevier,[58c] American Chemical Society,[58d] The Owners Societies.

investigated by Song et al.[59] as shown in Figure 14 showing that only above certain deposition angle (∼85o) the enhancement starts. This behavior is most likely correlated with the same percolation threshold behavior of the conductivity and the extended SPR dip width with the porosity presented in figure 6. Above the percolation threshold the plamons

excited become localized causing the electromagnetic field enhancement. Recently we have found that actually 30% porosity is also about the optimum for SERS enahncement and the results will be published shortly.[60] Another interesting morphological effect is when zigzag type nSTF morphology is used. Zhou et al.[61] have shown that

Figure 14. SERS from Ag nCTF at different deposition angles with respect to the normal to the substrate. The proble molecule is trans-1,2-bis(4pyridyl) ethene (BPE) and the excitation wavelength is 633 nm. Reproduced with permission.[59], Institute of Physics. small 2014, DOI: 10.1002/smll.201303181



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Figure 15. SEM micrographs of Ag nSTFs having different number of zigzags. On the right side the Raman signal for the different samples with different zigzag numbers. Reproduced with permission.[61], American Institute of Physics.

enhancement increases as the number of zigzags N increases up to N = 4 then starts to decrease as shown in Figure 15. The reason for the enhancement may be explained as a result of the increase of the number of hot spots. However the optimum at N = 4 and the decrease above that was not explained satisfactorly. Their explanation was that the aspect ratio of the arms of zigzags become smaller so that the LSPR frequency shifts further away from the Raman excitation frequency. The polarization dependence from Ag nCTFs was measured by Suzuki et al.[32] as shown in Figure 16a for the two polarization orientations demonstrating that for the long axis (s-polarization) the enhancement factor is larger than the case of excitation along the short axis (p-polarization). This is in correlation with the LSPR absorption spectra presented in figure 11. The design of the structure was to obtain LSPR at the Raman excitation wavelength of 785 nm when the polarization is along the long axis of the nanorod. Note that the enhancement factor and the absorption at the resonance frequency are correlated and vary as Malus law with the polarization angle where the maxima correspond to the s-polarization. Similar behavior was also reported by Wu and Cunningham.[62] The explanation may be understood based on the following expression for the Raman enhancement factor: 2

η=

E local (ω L ,θ ) E max (ω R ) E 0 (ω L ) E 0 (ω R ) 2

2

2

E max (ω L ) E max (ω R ) ≈ cos2 θ E 0 (ω L ) E 0 (ω R )


(4)

Where we used E local (ω L ) ≈ E max (ω L )cosθ according to Wei et al.[63] as the local electric field at the laser frequency ωL which has some polarization dependence and Emax(ωL,R) is the maximum local field at the laser ωL or Raman frequency ωR (at θ = 0) respectivelt. When ωL ≈ ωR then we get the dependence |Emax|4 cos2θ which is similar to Malus law.

6. SEF from Metallic nSTFs In addition to SERS, the SEF from nCTFs was also investigated in.[64–67] The potential of SEF was rediscovered during the past decade due to the emerging developments in the optics of metallic nanostructures.[68] It is a very useful phenomenon with significant applications in biotechnology and life sciences.[69] Localization of the electromagnetic field near nanotips, corners, holes, needles, etc. has been shown to produce large SEF by factors of up to a few hundreds in what is known as the lightning nanoantenna effect.[70] Recent review[71] articles summarized the fluorophore molecule interactions with the metal surface under SP waves excitation. Enhancement factors larger than 70 were observed using nSTFs made of silver and aluminum, however less than that from gold, and copper with respect to their dense film counterparts.[64–67] The effect of the substrate material, constituent rods material, porosity, and the rods tilt angle on the enhancement factor of SEF were all examined. Enhancement


small 2014, DOI: 10.1002/smll.201303181


interaction. This observation makes nSTFs potential candidates as SEF nano-beds for biosensing and bio-imaging. In Figure 17, one can see the advantage of SEF from sculptured films and from their counterpart dense and flat films. Figure 17 demonstrates SEF from luminisent bacteria in aqeueous solution obtained both as a misrospot image (Figures 17a and 17b) and as a spectrum measured with a spectrometer attached to the eyepiece of the fluorescent microscope (Figure 17c). Comparing the images in Figures 17a and 17b it is very clear that using the nSTF substrate it is possible to image the luminiscent bacteria because of the SEF enhancement but not on dense films. Varieties of nSTFs were investigated to find the optimum structure for biosensing based on the surface enhanced fluourescence (SEF). A comparative study was performed from nSTFs containing variety of nano structures different in their shape, height (h), and tilt angle with respect to the surface normal (α), thickness (d) and arrangement. The highest enhancement of the fluorescent signal was found for Ag based nSTFs on Si(100) giving an enhancement factor of x71, where h = 400 nm, d = 75 nm, α = 67o relative to Ag dense film using fluorescent dye Rhodamine 123. Immobilization of a fluorescent receptor to the thiol self assembly monolayer on Ag based nSTF and Ag dense film was also shown to demonstrate the applications of nSTFs for specific biosensing.[66,67] Upon excitation of the fluorophore by Hg light source, a CCD camera with a controlled exposure time detected the pattern of fluorescent receptor Anti-Rabbit IgG on the surfaces as shown in Figure 18. Figure 16. (a) Raman signal for s and p polarizationg from SiO2 nCTFs coated with Ag nano cabs corresponding to the 1041cm−1 peak of 4,4′-bipyridine (BiPy) in water at 1 mmole/l concentration; (b) Variation of the Raman signal and LSPR absorption with the polarizer angle. Reproduced with permission.[32], American Institute of Physics.

7. Conclusions, New Challenges and Future Perspective Metallic nano-sculptured thin films investigated during the last decade by SPR, SEF and SERS exhibit a strong potential as thin film platforms for optical biosensing in water. In the extended SPR sensing they exhibit higher sensitivity

of SEF from porous, metallic nSTFs was applied to biosensing in water. The main SEF mechanisms, are believed to be the lightning nanoantenna effect and the dipole-dipole (a)

(c)

(b)

Figure 17. SEF images from (a) Ag-nanorod nSTF and (b) a dense Ag film immersed, in an aqueous solution of luminescent E. coli. (c) Typical SEF spectra from an Ag-nanorod nSTF and an Al-nanorod nSTF and from the corresponding reference films obtained by covering the films with nano-layer of Rhodamine 123. Inset: SEM micrograph of an Ag-nanorod nSTF showing the highest enhancement factor. Reproduced with permission.[64], American Institute of Physics. small 2014, DOI: 10.1002/smll.201303181



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(a)

Emission LSPR

Receptors SAM

Y Y Y

Y

11-MUA

Ag STF

Substrate

Ag

(b)

Ag nSTF

Ag dense film

Figure 18. (a) Schematic drawing of the functionalized sample utilizing SEF due to LSPR field enhancement and the interfacial molecular architecture for the detection of receptors; (b) Fluorescence images from: dense Ag film and Ag nCTF. Both slides were immersed in diluted 1:100 (antibody to PBS) Anti-Rabbit IgG (whole molecule). The Ag nSTF used is from the same batch of samples as those shown in figure 17c. Reproduced with permission.[67], SPIE.

in virtue of the porosity which may then be combined with additional nanolayers[72] and using detection methodologies with enhanced precision such as the diverging beam approach,[73] it will be possible to create a highly sensitive and reliable water sensor. In SEF and SERS the localized SPR in the nanorods enhances the electromagnetic near their surface which in turn enhances the Raman and fluorescence intensities. SERS enhancement factor of 6–8 orders of magnitude are achievable while in SEF enhancement factors of few tens are easily achievable. Using SEF we have demonstrated bacteria sensing in water[64–67] with enhancement factor of 71. The fact that their production is relatively cheap, can be produced in large areas, can be patterned so that biochips can be produced makes nSTFs true candidates as biosensing platforms. The stability of Ag nSTFs against environmental conditions was also found[11] to be much better than dense silver films which together with the fact that silver is cheaper to deposit and exhibits excellent plasmonic properties makes Ag nSTFs strong candidates as plasmonic substrates. Due to the large electromagnetic field enhancement metallic nSTFs have a great potential for improving the efficiency of energy related devices such as photovoltaics and photodetectors if mixed with semiconducting materials.[74] Thermochromic nSTFs of VO2 were made successfully and found recently to improve the solar modulation ability over the regular dense thin films.[75] The ability to control the porosity and deposit nSTFs from different materials such as oxides and in multilayers opens another possibility for producing novel transparent electrodes for photovoltaic solar cells.[76] As antireflection coatings TiO2 nSTFs were demonstrated to improve the efficiency of hydrogenated amorphous Si solar cell.[77] The fact that it is possible to produce nSTFs


in a periodic form such as helical or zigzag or even 3D periodic patterns widens the possibilities of field enhancement mechanisms as a photonic crystal and plasmonic material simultaneously. As a one dimensional helical anisotropic structures they were already shown to reveal unique polarization features when made from light emitting material such as Alq3.[78] Structurally chiral materials exhibit the circular Bragg phenomenon[3]: most significantly, a structurally right/ left-handed chiral nSTF almost completely reflects normally incident right/left circularly polarized (RCP/LCP) plane waves, whereas normally incident LCP/RCP plane waves are reflected very little, within a spectral regime called the circular Bragg regime.[79,80] Thus, a chiral nSTF functions as a wideband-rejection filter[81] for circularly polarized radiation of the same handedness as that of its own helical columns, similar to helical liquid crystals. Once an appropriate bulk material has been chosen, the spectral regime of operation can be engineered quite simply through the structural period (i.e., the period of the helical columns) and the angle of rise of the helical columns. The center wavelength of the circular Bragg regime can be expressed as λpeak = Λna, where Λ is the structural period and the dimensionless quantity na depends on the three principal refractive indices and the angle of rise. A chiral STF with a central phase defect can function as a narrowband spectral-hole filter for circularly polarized radiation of the same handedness as that of its own helical columns. This photonic crystal type nSTF structure was shown to act as a biosensor for water as well as for enhancing the efficiency of luminescence.[82,83] When filled with electrooptic material such as liquid crystal it is even possible to tune their reflection peak.[84] In the future more biosensing applications are expected to emerge, a fact which requires collaboration between physicists, biologists, biochemists and engineers to work on the surface functionalization problems, improve the specificity of detection and build true functioning biosensing devices. Biofunctionalization of the nSTF surface is very important for specific and reliable sensing. Although there are many protocols for functionalizing dense metal film surfaces, there is almost nothing for the case of metallic nSTFs. Although there is one published work[53] on surface enhanced infrared absorption (SEIRA) spectroscopy, there is a place for further deep investigations in this regard both from fundamental and from practical points of view. The multiple SPR dips observed when a top dielectric nSTF layer is deposited on the metal is a subject that needs additional experimental investigation to confirm the theoretical prediction highlighted in references.[22–25] Homogenization models for the case of metallic nSTFs need to be improved when the porosities become larger than 35%. As lithography and patterning of nSTFs is possible then three dimensional structures are possible to produce as exemplified in figure 3. Enhanced optical transmission through nanoslits and nanoholes in metallic nSTFs is another subject not investigated yet and is expected to reveal biosensors with larger sensitivity than in the case of dense films.[85–88] The investigation of such 3D nSTF structures is yet to be done. It requires more rigorous electromagnetic simulations important for designing biosensors. The rigorous simulations are also required for modeling the plasmonic


small 2014, DOI: 10.1002/smll.201303181


field enhancement calculations to explain the SERS and SEF enhancement factors and their dependence on the material and geometry as well as for the SEIRA and the connection between the three techniques. SERS substrates based on metallic nSTFs are already commercially available so one expects the same for SPR and SEF to start appearing in the market.[89]

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Acknowledgements [31]

This research is conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technology Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minster's Office, Singapore. I am grateful to my students Alina Karabchevsky, Atef Shalabney and Sachin Kumar Srivastava and for my collaborators Prof. Bernd Rauschenbach, Prof. Akhlesh Lakhtakia and Prof. Robert Marks for the productive work we performed together during the last few years upon which large part of this review is

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Received: October 5, 2013 Revised: November 21, 2013 Published online:

small 2014, DOI: 10.1002/smll.201303181

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