J. Phys. Chem. C 2008, 112, 10707–10714
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Light-Absorption Enhancement by Nanospaced Trilayer Structure with Highly Reflective Surface Plasmon Scatterer Mitsuo Kawasaki* and Ritsu Tsubaki Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Katsura, Kyoto 615-8510, Japan ReceiVed: March 16, 2008; ReVised Manuscript ReceiVed: May 1, 2008
A fully conductive bulk Ag film, when sputter-grown on a substrate directly exposed to the glow discharge plasma, developed unique nanoscale surface roughness features that caused distinct single-nanoparticle-like surface plasmon (SP) scattering. This highly reflective SP scatterer, capable to give substantial light scattering with minor surface roughness, functioned as a superior reflector in a novel nanospaced trilayer structure for high-efficiency light-absorption enhancement. The small base (intrinsic) absorptance less than ∼3% of a thin light absorber layer atop was thus easily enhanced by more than 10 times in a wide range of the visible region. The results illuminate an important role of the part of the scatter-reflected light that undergoes internal multiple total reflection and thereby allows the absorber layer an efficient absorptive interaction with the evanescent field. The results also suggest that the incoming and scatter-reflected lights are favorably phasecorrelated in the optimally nanospaced trilayer structure so as to yield substantially more enhanced scattering and evanescent fields to interact with the absorber layer. Introduction Light absorption is the essential requisite for whatever photochemical processes to be initiated. In many of photochemical systems, however, it is not necessarily a very efficient step. In particular, for systems that rely on a molecularly thin absorber layer for the light harvesting role, the net light absorptance rarely exceeds 10% even at the relevant absorption peak, unless the available absorption band can be extremely sharp. For example, two-dimensional J-aggregates of cyanine dyes have played a major role for the spectral sensitization in conventional silver halide imaging.1 The J-aggregate is a unique class of dye aggregate that gives rise to an extraordinary sharp absorption band,1–3 but in its two-dimensional form as adsorbed on silver halides, even the peak light absorptance is still limited to ∼10%. Dye-sensitized solar cells also rely on a similar spectral sensitization process involving dyes adsorbed on TiO2 semiconductors.4,5 There, it has been the common practice that the inefficient light absorption by essentially monolayer dye adsorbates is compensated by some huge increase of the effective surface area with nanocrystalline TiO2 matrix made into a highly porous structure. In view of the carrier transportation efficiency,6,7 however, this format may not necessarily represent an ideal cell construction. In absorption- or fluorescencebased analytical situations, the weak light absorption of a minor amount of light capturing analytes bound to a solid surface would also limit the overall sensitivity. If the net light absorptance well below 10% of such a thin and weak absorber layer could be enhanced by more than 10 times, it seems to be highly encouraging for many photochemical applications. In this paper, we address a novel method to bring this idea into practice by using a nanospaced trilayer structure that features a highly reflective surface plasmon (SP) scatterer. What we refer to as a trilayer structure is one that consists of a highly reflective mirror at the bottom, an * To whom correspondence should be addressed. E-mail: Kawasaki@ ap6.mbox.media.kyoto-u.ac.jp. Tel, FAX: (+81)-75-383-2574.
intermediate transparent spacer layer, and a thin light absorber layer atop. In its standard form, the bottom reflector is nonscattering, and with a spacer thickness approximately λ/4 (λ specifies wavelength in the spacer material) the incoming and reflected lights are brought to be in-phase in the absorber layer.8 The resultant constructive interference gives rise to a standing wave with about 2-fold field enhancement, and hence approximately 4-fold enhancement in the net light absorptance. This concept is not new, however.9 The present idea is to replace the bottom reflector by a scatter-reflector while preserving the in-phase correlation between the incoming and, in this case, scatter-reflected lights. This extended trilayer structure is also featured with a qualitatively different mechanism of lightabsorption enhancement that involves internal multiple total reflection (IMTR). It is not an easy task, however, to make a system in which these mechanisms really operate, because the required spacer thickness of ∼λ/4 is less than ∼100 nm in general. The surface roughness of the scatter-reflector needs to be suppressed significantly smaller than this for it to be flattened by such a nanospacer (this is the condition for making IMTR active), and yet it has to maintain sufficient lightscattering capability. We have found that a fully conductive bulk Ag film, when sputter-grown on a substrate directly exposed to the glow discharge plasma, exhibits distinct light scattering via SP excitation involving a nanoscale (and hence minor) surface roughness. The film thus serves as a superior scatter-reflector that fits in the proposed trilayer device. It is known that the interaction of light with metal nanoparticles and/or roughened metal surfaces leads to a range of physical and chemical processes via the SP excitation, such as surface enhanced Raman scattering,10,11 metal-surface enhanced fluorescence,12–15 and plasmonic waveguide.16 The nanospaced trilayer structure introduced in this paper adds a new and simple application of the SP excitation (SP-induced far fields) by the interaction of light with nanostructured bulk Ag surface.
10.1021/jp802290c CCC: $40.75 2008 American Chemical Society Published on Web 06/26/2008
10708 J. Phys. Chem. C, Vol. 112, No. 29, 2008 Experimental Section Preparation of Ag Films. Sputtered Ag films were grown on a plain slide glass by direct-current Ar+-ion sputtering method in a mechanically pumped small vacuum chamber ∼15 cm in inner diameter and 3 dm3 in volume. A circular silver (99.9%) plate 50 mm in diameter as the cathode target was located laterally at the bottom position, and the substrate was fixed to a heating unit right above the target with a face-to-face distance of ∼35 mm. The substrate was heated in advance to a desired deposition temperature in the range of 100-280 °C in pure Ar atmosphere maintained at the pressure of ∼60 Pa under continuous flow of Ar gas (>99.9999%) at the rate of ∼20 sccm. The glow discharge sputtering was allowed under reduced Ar pressure and flow rate of ∼20 Pa and ∼5 sccm, respectively, for typically 10-15 min at the total discharge current of ∼16 mA and the cathode (target) negative voltage of 1.2-1.4 kV. In these conditions, the gap between the substrate and the target was filled with glow discharge plasma, so that the growing film surface was subjected to some significant level of ion and/or electron bombardment. To gain additional control over the nanoscale surface roughness features of the sputtered Ag films, we also employed a two-step deposition, where the initial and the follow-up depositions were made at considerably different substrate temperatures (e.g., at ∼250 °C first and then at ∼100 °C with intermission for temperature readjustment). The typical film thickness was 250-300 nm, as measured by a surface profilometer scanned across a step formed by partial masking of the substrate. The sputtered Ag films grown in this manner were fully conductive and achieved the highest possible integrated total reflectivity (see below) near 100% in a wide range of the visible region. Their surface roughness and texture were inspected by atomic force microscope (AFM, type VN80000, Keyence Co.). Optically flat Ag films, as a reference nonscattering Ag substrate, were prepared also on a plain slide glass (heated to ∼200 °C) by conventional vacuum (base pressure of 1.3 × 10-3 Pa) evaporation with a resistive heating. We chose a considerably high film deposition rate, so that a film typically ∼300 nm thick could be grown in less than 5 min. Preparation of Trilayer Structure. For preparation of the nanospaced trilayer structures, we have chosen commercial spinon-glass (SOG, type 103AS from Honeywell) as the spacer material. The original solution was diluted with ethanol to the concentration for its spin coating at 3000 rpm and subsequent curing at 200 °C for 30 min to yield the required thickness ranging from ∼10 to ∼190 nm. The film thickness and refractive index (1.33) were determined by combination of ellipsometry, X-ray photoelectron spectroscopy (XPS), and AFM. For preparation of microspaced trilayer structures for comparative purpose, we employed polymethylmethachrylate (PMMA, obtained from Wako Pure Chemical Industries, Ltd.) as the spacer material. We casted a 1.5 mL portion of 4 wt % PMMA solution in cyclohexanone upon a sputtered Ag film or onto any other reference substrates, ∼15 × 15 mm2 in area, and allowed it to dry on a hot plate (∼60 °C). The same procedure was repeated once more to gain the final spacer thickness of roughly ∼30 µm. We chose several organic dyes for preparation of the thin light absorber layer placed atop the trilayer structure. The dyes obtained from Exciton Inc. include RhB (chloride), R-700 (perchlorate), and Coumarin 307. They were dissolved in either water or mixed water and ethanol (with 3:7 volume ratio) solvent containing 0.1-0.2 wt % polyvinylalcohol (PVA, polymerization degree of ∼500, from Wako Pure Chemical Industries,
Kawasaki and Tsubaki Ltd.). In the case of R-700 and Coumarin 307, we also used their ethanolic solutions mixed with a small amount of SOG solution (type 111 from Honeywell) at 44:1 volume ratio. The dye concentration ranged broadly from 0.03 to 5 mM. The resultant solution was spun onto the spacer-coated substrate at 2000-3000 rpm, to yield a thin (3-5 nm) light absorber layer of dyes in a polymer matrix. The total amount of the dye incorporated in the absorber layer per unit projective area was determined by spectrophotometric quantification of the dyes extracted from each sample of known area into plain solvent. Spectroscopy Measurements. The specular transmission and reflection spectra (at normal incidence) of all kinds of nonscattering samples were acquired by using a home-assembled microscopic spectrometer system. For scattering samples, the reflection spectrum taken in this system with a 50× objective lens (0.8 in numerical aperture) had contributions of all lights scatter-reflected into a cone of 53° in polar angle (half of the vertex angle). The integrated total reflectivity spectrum could be obtained by combination of this information with the angleresolved scattering spectra (see below) integrated over the remaining polar angles from 53 to 90°. A series of angle-resolved scattering spectra were measured by using the same apparatus as described in detail elsewhere14 that allows quantitative measurement of angle-resolved fluorescence and scattering. Specifically, the scattering spectra were taken at every 10 degrees from 30 to 90°, with an MgO pellet as the reference standard scatterer that obeys the Lambert’s law.17 Since the spectra taken in this way showed no anomalies in angle dependence, the scattering signals for intermediate angles necessary for the integration purpose were determined by the interpolation method. In the present study, the fluorescence spectra were taken at a fixed detection angle of 40°. A 150 W Xe lamp combined with a monochromator allowed us to select any wavelength in the visible region for fluorescence excitation at normal incidence, where the excitation intensity was adjusted by a series of neutral density filters. Results and Discussion Surface Structures and SP Scattering of Sputtered Ag Films. Figure 1 shows a couple of typical AFM images taken for sputtered Ag films grown in our standard deposition conditions. The films developed unique nanoscale surface roughness as if it consisted of a dense, random, two-dimensional array of oblate Ag nanoparticles, 20-50 nm in average height and 50-200 nm across. However, the films are never comparable to a discontinuous metal island film, since they were made so thick to be as highly reflective and conductive as a bulk Ag film. The surface roughness depended in particular on the glow discharge current and voltage that controlled both film deposition rate and plasma interaction with the film surface, as well as on the deposition temperature. As such, the sputtered Ag films exhibited distinct light scattering via nanoparticle-like SP excitation as exemplified by a series of angle-resolved scattering spectra presented in Figure 2, where we also included the spectra separated into s- and p-polarized components. There is a considerably more ppolarized character, particularly for large scattering angles. Furthermore, a common spectral shape singly peaked at ∼420 nm in this example resembles the SP scattering of isolated spherical Ag nanoparticles, ∼100 nm in diameter according to the Mie theory.18,19 This is rather surprising considering the closely spaced nanoscale roughness features as imaged in Figure 1. For reference, when we analyzed the light scattering by
Light-Absorption Enhancement by Nanospaced Structure
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Figure 1. Examples of 4 × 5 µm2 AFM images of sputtered Ag films with two different levels of surface roughness, along with typical crosssectional height profiles demonstrating nanoscale features, 20-50 nm in average height and 50-200 nm in lateral dimension.
Figure 2. Typical series of angle-resolved scattering spectra measured for as-grown sputtered Ag film, demonstrating single-nanoparticle-like surface plasmon (SP) scattering. Numbers refer to scattering angles in degrees. (a) Polarization-averaged spectra. (b,c) Spectra separated into s- and p-polarized components.
Figure 3. Series of 4 × 5 µm2 AFM images showing how the original surface roughness associated with a sputtered Ag film (image on top) is smoothed out by coating of SOG spacer with increasing thickness (three bottom images).
nonconducting thin films of discreet Ag islands 50-100 nm across that formed in the early stage of the present film growth, the scattering spectra were likewise more p-polarized, but as expected, largely red-shifted (broadly peaked at around ∼600 nm) due to the interparticle SP interactions. These observations suggest that the SP scattering by the sputtered Ag films also stems from a localized dipolar SP excitation20 involving the individual nanoparticle-like features, but it is speculated that the presence of the underlying bulk Ag phase somehow made it more localized to cause a single-nanoparticle-like SP scattering. Enhanced SP Scattering in Nanospaced Trilayer Structure. We found that the unique SP scattering associated with the sputtered Ag films, as shown above, was strongly modified
once the Ag films were covered by the SOG nanospacer. We next analyze this significant effect of the nanospacer, which has an important relevance to the enhanced light absorption. Figure 3 shows a series of AFM images that first verify the expected capability of the SOG nanospacer to smooth out the relatively minor surface roughness associated with the sputtered Ag films. In the case examined in Figure 3, ∼40 nm thick SOG spacer was just about enough to reduce the surface roughness to our satisfaction. Here, the spacer thickness refers to that measured for a film prepared in the identical condition on a flat substrate. We found that even for the roughest sputtered Ag film used in the present work, ∼80 nm thick spacer sufficed the required roughness relaxation.
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Figure 5. Effects of SOG spacer of optimum thickness (∼80 nm) on scattering spectra and their polarization character for nanospaced trilayer structure. Reference spectra taken for sputtered Ag film alone are given in dashed lines. Figure 4. Series of scattering spectra taken at a fixed angle of 30° for nanospaced trilayer structures with various SOG spacer thicknesses indicated by numbers. RhB coverage is ∼5 × 1013/cm2. The reference spectrum (in dotted line) represents SP scattering of sputtered Ag film alone.
Figure 4 shows a series of scattering spectra taken at a fixed angle (30°) for nanospaced trilayer structures with the spacer thickness ranging from 12 to 190 nm. In general, the SP scattering of metal nanoparticles is significantly influenced when the scattering centers are embedded in a refractive medium.18,19 Figure 4 not only demonstrates this large effect of the refractive environment (SOG spacer in this case) but also indicates that the effect was strongly sensitive to the nanospacer thickness. It can be seen that a particularly large enhancement in the scattering signal as a whole occurred with a spacer thickness in the 60-80 nm range, which we refer to as the optimum spacer thickness. Considering that the original surface roughness was sufficiently smoothed out by a ∼40 nm thick nanospacer already (Figure 3), the large changes in the scattering spectra observed for the range of spacer thickness up to 190 nm may not be due to the change of surface roughness. The systematic changes of the spectra caused by the change of spacer thickness rather suggests a strong phase correlation between the incoming and scatter-reflected lights at the air/medium interface. Under the optimum spacer thickness, they probably become in-phase or approximately in-phase for some widest range of wavelength, thereby yielding an enhanced scattering field at the air/medium interface. The enhanced scattering signals that we detected in the front air space must have originated from such an enhanced scattering field. This enhanced field, right in the position of the absorber layer, naturally makes major contribution to the enhanced light absorption. Careful inspection of Figure 4 indeed lets us know that the scattering spectra for the optimally spaced samples are accompanied by noticeably deeper dips in the region of the RhB absorption band. This dip represents the direct absorptive interactions between the absorber layer and the enhanced scattering fields. Another striking evidence for the in-phase correlation between the incoming and scatter-reflected lights in the optimally nanospaced structure comes from a strongly s-polarized character of the enhanced scattering signals, as shown in Figure 5. This is diametrically different from the more p-polarized tendency of the SP scattering originally associated with the sputtered Ag films. The reversed polarization can be easily justified, however, considering that the scatter-reflected light that is s-polarized has the electric vector always parallel to the
surface. The incoming light that we used at normal incidence also gives only surface-parallel electric fields. Thus, the in-phase constructive interference to yield the enhanced scattering field at the medium/air interface becomes naturally stronger with s-polarized light, particularly for larger scattering angles where the p-polarized component of the scatter-reflected light gives minor surface-parallel electric field. Figure 5 indeed suggests that the larger the scattering angle, the more strongly the scattering signal is s-polarized. An important consequence of such a phase correlation is the possibility that the part of the scatter-reflected light that undergoes internal total reflection can also be in-phase (or approximately in-phase) with the incoming light. This potentially results in a stronger evanescent field to interact with the absorber layer (see related discussion given later). The strong phase correlation between the incoming and scatter-reflected lights in the nanospaced trilayer structures was corroborated further by the scattering spectra comparatively measured for a microspaced version of trilayer structure with an optically thick PMMA spacer. The PMMA spacer, as we prepared, had significant inhomogeneity in thickness with probable maximum difference easily exceeding λ in the visible region. Besides, the spacer thickness is so large that an even minor difference in the scattering angle results in the difference of the order of wavelength with respect to the distance over which the scatter-reflected light travels in the spacer to meet the absorber layer. In these conditions, one cannot expect any significant phase correlation between the incoming and scatterreflected lights in the thin absorber layer atop. The corresponding scattering spectra shown in Figure 6 exhibit a common effect of the refractive environment to significantly broaden the spectra, but the considerably more p-polarized character of the SP scattering was not influenced at all, as expected. Nevertheless, there still remains a question of how such a strong in-phase correlation between the incoming and scatterreflected lights comes about in the system involving the minor but yet substantial nanoscale surface roughness associated with the sputtered Ag films. According to Figure 1, the surface roughness (in terms of hollow-to-top height difference) of the present Ag films was typically 20-50 nm, except that it occasionally reached ∼100 nm in some limited portions for the roughest films. When this original roughness is smoothed out by a nanospacer in the optimum range (e.g., ∼70 nm thick), the local spacer thickness (distance between the Ag surface and the absorber layer) cannot be uniform. We crudely estimate it to vary in the range, 40-100 nm; that is average spacer
Light-Absorption Enhancement by Nanospaced Structure
Figure 6. Comparison of scattering spectra between as-grown sputtered Ag film (dashed curves) and microspaced trilayer structure (solid curves) with an intermediate thick (∼30 µm) PMMA spacer. RhB coverage atop is ∼1.2 × 1014/cm2.
thickness plus (minus) average original roughness. Furthermore, the lights scatter-reflected by the sputtered Ag film in various directions travel in the refractive medium for varied distances for them to reach the absorber layer. Thus the exact phasematching condition with the incoming light depends also on the angle of scattering. However, as a compensation for these complexities in the present system, the condition of phase difference for two lights to positively interfere is not necessarily very strict. When two lights of intensities I1 and I2 with phase difference δ interfere, the interference term in the overall intensity is given by 2I1I2 cos δ. Thus two lights with a substantial phase difference up to ( π/3 can still lead to at least half the maximum positive interference. This means that if the incoming and vertically reflected lights are brought to be exactly in-phase in the absorber layer with spacer thickness λ/4, then the substantial positive interference can still be expected for spacer thickness ranging from λ/6 to λ/3. In the short- and long-wavelength limits of 400 and 800 nm (wavelength in vacuum) in the visible region, this acceptable range of spacer thickness falls in 50-100 nm and 100-220 nm, respectively, for the spacer material with n ) 1.33. In the case of scatter-reflected lights that travel for longer distances in the spacer, the lower limit of the acceptable spacer thickness further decreases accordingly; e.g., by a factor of 1.2 for light scattered at 45° near the critical angle. The experimental optimum spacer thickness of ∼70 nm, possibly ranging locally from 40 to 100 nm in the presence of the underlying nanoscale roughness features, reasonably overlaps with the above-estimated acceptable ranges of spacer thickness for some wide range of wavelength in the visible region. This explains why the optimally nanospaced trilayer structures gave rise to the strongly enhanced scattering in a widest range of the visible region in Figure 4. The numerical estimates described above also suggest that an approximate phase matching between the incoming and scatter-reflected lights may be possible also with spacers thicker than 100 nm insofar as the spectral region near the longwavelength limit is concerned. The corresponding scattering spectra presented in Figure 4 in fact exhibit even stronger scattering signals in this tail region as compared to that with the optimum spacers. The spacers thinner than the optimum range may likewise cause a partial phase matching in the region near the short wavelength limit. The corresponding scattering spectra in Figure 4 again exhibit reasonably strong enhanced signals in this region. However, the scattering signals in this
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Figure 7. Example of integrated total reflectivity spectrum (solid line) of optimally (by ∼80 nm) spaced trilayer structure made of sputtered Ag film that allows relatively large contribution of IMTR, as manifested by significant drop of total reflectivity in the whole spectral region from that measured for sputtered Ag film alone (dashed line).
case may be influenced also by the residual surface roughness not yet smoothed out with such thin spacers. Light Absorption Enhancement by Nanospaced Trilayer Structure. The enhanced light absorption in the optimally nanospaced trilayer structure, under the favorable phase correlation between the incoming and scatter-reflected lights, as discussed above, is reflected in part in the series of integrated total reflectivity spectra presented in Figure 7. The spectrum measured for a sputtered Ag film alone (dashed line) showed high total reflectivity near 100% in a wide spectral region except for the blue (
