Out-diffused silver island films for surface-enhanced Raman scattering protected with TiO2 films using atomic layer deposition
© Chervinskii et al.; licensee Springer. 2014
Received: 18 July 2014
Accepted: 5 August 2014
Published: 15 August 2014
We fabricated self-assembled silver nanoisland films using a recently developed technique based on out-diffusion of silver from an ion-exchanged glass substrate in reducing atmosphere. We demonstrate that the position of the surface plasmon resonance of the films depends on the conditions of the film growth. The resonance can be gradually shifted up to 100 nm towards longer wavelengths by using atomic layer deposition of titania, from 3 to 100 nm in thickness, upon the film. Examination of the nanoisland films in surface-enhanced Raman spectrometry showed that, in spite of a drop of the surface-enhanced Raman spectroscopy (SERS) signal after the titania spacer deposition, the Raman signal can be observed with spacers up to 7 nm in thickness. Denser nanoisland films show slower decay of the SERS signal with the increase in spacer thickness.
78.67.Sc (nanoaggregates; nanocomposites); 81.16.Dn (self-assembly); 74.25.nd (Raman and optical spectroscopy)
KeywordsSurface plasmon resonance Nanoparticle Nanoisland Film Self-assembly Dielectric spacer
Metal island films (MIFs) have attracted significant attention due to the strong surface plasmon resonance (SPR) effect in these nanoislands. The spectral position of the SPR is influenced and can be tuned by the MIF density as well as the substrate and cover materials used [1–3]. Surface-enhanced Raman spectroscopy (SERS) in biological and chemical sensing  can be regarded as one of the most intriguing applications of MIFs. It can provide at least 1010- to 1012-fold intensity enhancement compared to the normal Raman scattering . The main reason for this intensity enhancement is the electromagnetic (EM) enhancement mechanism prevailing over the chemical enhancement (CHEM) by several orders of magnitude . This is because the EM enhancement is proportional to about the forth power of the SPR-increased local electric field input in Raman scattering, i.e., in the analyzed media adsorbed on the MIF (an adsorbate), while the reported CHEM enhancement factors, due to metal island-adsorbate interaction, are approximately 102. It is essential to decrease the distance between separate metal islands in a MIF, which results in the increase of the local electric field intensity and, consequently, in a larger SERS signal . Other prospective applications of MIFs include catalysis [6, 7], photovoltaics , and fluorescence enhancement . For many practical uses, MIFs should be protected with a dielectric cover, which influences not only the CHEM but also the EM enhancement of SERS through the change of local electric field in adsorbates. At the same time, cover-induced shifts of the SPR spectral position can be used to tune SERS measurements for a specific wavelength, which is of high importance for surface-enhanced resonance Raman scattering . The influence of MIF dielectric covers (spacers between the MIF and an analyte) on SERS intensity has been studied for more than two decades . However, only the recent use of a very precise atomic layer deposition (ALD) technique has allowed obtaining quantitative results related to the SERS influence by alumina spacers deposited on metal microspheres , MIFs , and metal nanowires . However, due to the difference in metal nanoislands and nanoparticles used in the experiment, these results can hardly be compared, and they contradict the data obtained in SERS experiments using MIFs covered with non-ALD spacers . It is worth to note that the key issues in this comparison are the SPR shift and the local electric field decay vs the spacer thickness. It is worth to note that dielectric-capped isolated metal nanospheres have already demonstrated their effective applicability in photovoltaics  and SERS .
Here we present our studies on the influence of a high-index TiO2 ALD spacer on the SPR position and SERS intensity in the case of silver island films grown on soda-lime glass substrates using our recently developed silver out-diffusion (SOD) technique . It is important to note that MIFs are highly fragile and, therefore, they must be protected for any practical use. The use of conformally grown ALD films is ideal for protecting MIFs with a cover layer, since the layer thickness can be controlled at an atomic level and the initial surface relief structure can be maintained with thin cover layer thicknesses . In the experiments, we varied the thickness of the ALD TiO2 spacer and the MIF structure. The interest in TiO2 spacers is twofold: (1) the high catalytic abilities of TiO2[19–21] allowing the use of SERS with a titanium dioxide spacer in nanoscale organic and biochemistry studies and (2) the high refractive index of TiO2 providing stronger control of the ALD-coated MIF structure, which results in wider spectral tunability of the system.
MIF formation and characterization
We fabricated silver nanoisland films using SOD from glass in the course of the ion-exchanged glass substrate annealing in a reducing hydrogen atmosphere. In the experiments, we used soda-lime glass microscope slides produced by Menzel . The silver-sodium ion exchange was performed at 325°C in an ion-exchange bath containing 5 wt.% of silver nitrate and 95 wt.% of sodium nitrate as was reported elsewhere . One-millimeter-thick slides with a size of 20 × 30 mm2 were immersed in the melt for 20 min, which provided a few microns of silver penetration depth in the glass. Optical absorption spectroscopy of the ion-exchanged slides did not show any absorption peaks in the spectral range corresponding to the surface plasmon resonance, which indicated the absence of silver nanoparticles both in the bulk and on the surface of the slides. The ion-exchanged slides were annealed in hydrogen for 10 min to reduce silver ions to atoms and get a supersaturated solid solution of neutral silver in the glass matrix. According to the proposed mechanism , this results in the formation of both silver nanoparticles within the glass and a silver island film on the glass surface (MIF) due to the out-diffusion of silver atoms.
After the MIF formation, we measured the optical absorption spectra of the samples using a Specord 50 spectrophotometer (Analytik Jena AG, Jena, Germany). To distinguish the MIF optical absorption and absorption of light by silver nanoparticles formed in the bulk of the glass slides, we subtracted the spectra of the samples after the surface film removal from the spectra measured after the processing of the glass slides in hydrogen atmosphere. The fragility of the MIFs allowed cleaning the glass surface from the nanoislands using just cotton with acetone. The topography of the MIFs was characterized with a Veeco Dimension 3100 atomic force microscope (AFM; Veeco Instruments Inc., Plainview, NY, USA), which allowed studying both the shape of separate silver islands and their size and distribution corresponding to different SOD regimes.
Atomic layer deposition and characterization
ALD was used to coat the MIF samples with thin layers of titanium dioxide. TiO2 was chosen for its high refractive index (n = 2.27) strongly influencing the SPR wavelength and because of its applicability for photocatalysis. Films were deposited at 120°C with Beneq TFS-200 reactor (Beneq, Espoo, Finland) using titanium tetrachloride (TiCl4) and water (H2O) as precursors, and between each deposition cycle, a nitrogen purge was used to remove extra precursor materials from the reactor chamber.
The samples covered with TiO2 film of different thicknesses were also characterized with a Specord 50 spectrophotometer and a Veeco Dimension 3100 atomic force microscope.
Surface-enhanced Raman scattering measurements
Signal enhancement properties of the MIF samples were examined using rhodamine 6G as a target molecule. Five-microliter droplets of 1 μM rhodamine (diluted in water) were deposited on all samples and allowed to dry forming an analyte-covered circular area of 4 to 5 mm in diameter. Raman scattering was measured using an inVia Raman microscope system (Renishaw, Gloucestershire, UK) with a 514-nm excitation laser. The beam was focused into an approximately 5-μm spot, and for each sample, nine measurements were performed from an area of 50 × 50 μm2 and the spectra were collected using an optical power of 50 μW and exposure times of 10 and 20 s for the uncoated and coated samples, respectively. The collected spectra were averaged and the background fluorescence was subtracted using an asymmetric least squares smoothing.
Results and discussion
Structure and optical absorption of initial MIF
Optical absorption and structure of MIF with TiO2 cover
SERS studies: TiO2 spacer and MIF structure influence
The spectral shift of the SPR saturates when the electric field E generated by a nanoisland under probing electromagnetic wave is completely localized within the covering film and the glass substrate as shown in Figure 9 (inset c). For thinner TiO2 films, the tail of the SPR electric field penetrates through the covering layer, that is, the electric field is partly localized in the air (see Figure 9, inset b). In other words, the effective dielectric permittivity of the nanoisland surrounding is less for thinner covers than for thicker covers. This results in weaker dielectric loading of the SPR and corresponds to its unsaturated spectral shift, which tends to saturate with the TiO2 film thickness increase. Thus, the saturated SPR shift indicates that the thickness of the cover exceeds the length of the SPR electric field penetration into the cover (Figure 9, inset c).
As measured with absorption spectroscopy, the spectral shift of the SPR in TiO2-covered MIF saturates at about 40- to 50-nm cover thickness. We can suppose that the SPR electric field intensity decays in TiO2 film at about the same length. Unfortunately, comparing the dependences of the SPR spectral shift in Figure 5, one can hardly conclude whether there is a difference in the SPR decay length for differently prepared MIFs. The measured Raman scattering signal IRaman should decay much faster. If the glass surface is covered with silver nanospheres,  for separate molecules and  for a monolayer of an analyte, where r is the radius of silver microsphere and d is the distance from the microsphere to the analyte. Definitely, one can see a very fast Raman signal decay in Figure 6 where the decrease in the signal relative to the uncovered MIF is presented. The decay is due to the spacer thickness influence and due to the absence of CHEM input (if any in the present case). At the same time, the spacer protects the MIF providing its longer time stability.
The increase in MIF density, that is, in size and in surface concentration of nanoislands, should result in a higher SERS signal (Figure 6). This is because of (a) the increase of the cross section of the nanoisland-analyte interaction due to a geometrical factor, that is, the increase of the effective area of the MIF, and (b) the surface concentration of ‘hot spots’ which are supposed to be the main origin of extremely high SERS signals [30, 31]. This can be easily seen in Figure 6a where a denser film provides higher IRaman. At the same time, the increase in the size of nanoislands, indicated by the redshift of the SPR (Figure 4), and their coagulation definitely result in the slowing of the spatial decay of the SPR electric field with the spacer thickness. Figures 7 and 8, where one can see that the Raman signal decay with the spacer thickness is slower for the denser film, clearly illustrate this. This phenomenon can be very roughly explained through the increase in the effective size of nanoislands d, but its detailed description will definitely require accounting for peculiarities related to the redistribution of local SPR fields in the partly aggregated MIF . It is worth to note that thicker TiO2 films, corresponding to full decay of the local electric field within the spacer, exclude SERS-related applications of the MIFs. However, they can be effectively used in applications which do not require the use of the tail of the electric field outside the film. Examples of such applications include tuning of optical absorption spectra, enhancement of resonant luminescence of emitters embedded into the film, and tuning the wavelength range of optical nonlinearity.
The performed studies demonstrate that silver nanoisland films formed using out-diffusion of silver from glass substrates during thermal processing in hydrogen atmosphere can be effectively used in SERS measurements. The enhancement of the Raman signal increases with the density of the nanoisland film. The surface profile of dielectrics deposited upon the MIF using the ALD technique replicates the profile of the initial MIF, and the smoothing of the dielectric surface profile with the deposited thickness is rather slow except for the smallest gaps between the nanoislands. The deposition of a titanium dioxide film results in a redshift of the SPR wavelength relative to the SPR wavelength of the initial film. This shift is up to hundred nanometers allowing the tuning of the central wavelength of the SPR. The shift saturates at a titania film thickness of 40 to 50 nm. SERS experiments performed with a R6G probe show that the SPR field spatial decay is less for denser MIFs, that is, for these MIFs, the titania spacer can be thicker. Finally, catalytically active ALD-TiO2 films can be effectively used to protect very fragile silver nanoislands from sulfidation, oxidation, pollutions, etc., allowing the maintenance of the SERS properties of the MIF. Additionally, this allows the fine-tuning of the SPR position and, respectively, conditions for surface-enhanced resonant Raman scattering (SERRS).
atomic force microscopy
atomic layer deposition
metal island film
surface-enhanced Raman scattering
surface plasmon resonance.
This study was supported by the FP7 project NANOCOM, ERA.Net RUS project AN2, Russian Foundation for Basic Research, Ministry of Education and Science of Russian Federation project 16.1233.2014/K, and Academy of Finland project #267270. The AFM studies were performed using the equipment of the Joint Research Centre ‘Material science and characterization in advanced technology’ (Ioffe Institute, St. Petersburg, Russia).
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