Multifunctional Silicon Optoelectronics Integrated with Plasmonic Scattering Color Long Wen, Qin Chen,* Xin Hu, Huacun Wang, Lin Jin, and Qiang Su Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, People’s Republic of China S Supporting Information *
ABSTRACT: Plasmonic scattering from metallic nanoparticles has been used for centuries to create the colorful appearance of stained glass. Besides their use as passive spectral filtering components, multifunctional optoelectronic applications can be achieved by integrating the nanoscatters with semiconductors that generate electricity using the complementary spectral components of plasmonic colors. To suppress the usual degradation of both efficiency and the gamut of plasmonic scattering coloration in highly asymmetric index configurations like a silicon host, aluminum nanodisks on indium tin oxide (ITO) coated silicon were experimentally studied and demonstrated color sorting in the full visible range along with photocurrent generation. Interestingly, the photocurrents were found to be comparable to the reference devices with only antireflection coatings in spite of the power loss for coloration. Detailed investigation shows that ITO serves as both the impedance matching layer for promoting the backward scattering and schottky contact with silicon, and moreover, plasmonic nanoscatters efficiently harvest the complement spectrum components for charge generation. The present approach combines the capacities of nanoscale color sorting and photoelectric converting at a negligible cost of efficiency, thus providing a broad flexibility of being utilized in various optoelectronic applications including self-powered display, filter-free imaging, and colorful photovoltaics. KEYWORDS: plasmonic, scattering, silicon, color filtering, light trapping term stability. In previous studies, the plasmonic filtering elements have been attached or fabricated on CMOS image sensors and demonstrated their full functionality as efficient passive optical components.19,20 More recently, integration of the structural colors with the photovoltaic (PV) effects represents an attractive direction for self-power display, filterfree imaging, and aesthetically pleasing solar cells.21−25 Among these cutting-edge researches, Si-based or Si-compatible strategies are, in particular, interesting for the PV-integrated filters used in energy-harvesting reflective displays or integrated electro-optical devices, which feature a compact form factor by merging of the color filter and sensor into one.21,22 The striking light-trapping characteristics arising from the plasmonic scattering,26 near-field localization,27,28 or light guiding29,30 can substantially enhance the PV conversion. Thus, bridging structural coloration and light trapping in optoelectronic devices with plasmonic elements renders the aforementioned multifunctional applications holding high quantum efficiency.
T
he use of surface plasmon excitation in metal nanostructures for efficient light manipulation and concentration at the nanoscale is a rapidly emerging field of nanophotonics that has demonstrated great potential for sensing, color filtering, imaging, anticounterfeiting, and light harvesting.1−7 In particular, the application of plasmonic nanoparticles in structural coloration can be traced back for centuries, famously exemplified by the Lycurgus Cup. It is wellestablished that the optical properties of metal nanoparticles are governed by collective oscillations of free electrons, which give rise to a localized surface plasmon resonance (LSPR) at characteristic frequencies in the visible region.8 By simply adjusting the shape and the size of those isolated nanostructures, LSPR can be spectrally tuned throughout the entire visible range with more attractive features,9−13 rendering them as color filters,14,15 holograms,16 and polarimeters.17 Moreover, an ultrahigh spatial resolution of 100000 dpi was demonstrated using plasmonic coloration.18 As compared with the pigmentbased color schemes, the incorporation of the single layered subwavelength plasmonic elements not only allows the colors to be tailored in more efficient and compact forms but also gains the advantages of low cost, ease of fabrication, and long© 2016 American Chemical Society
Received: September 3, 2016 Accepted: November 29, 2016 Published: November 29, 2016 11076
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Figure 1. Calculated (a) backward and (b) forward scattering cross sections of the Al nanodisks (diameter D = 120 nm, thickness t = 30 nm) in various configurations including free-standing in air, sitting directly on the substrates (nsub = 1.5, 2.0, 2.5, and 3.0) and the impedance matching layer (n = 2.0, thickness = 70 nm) coated silicon. The inset in (a) illustrates the numerical model used in the total-field scatteredfield (TFSF) source method. (c) Normalized magnetic field distributions in various configurations.
RESULTS AND DISCUSSION Theoretical analysis of the scattering behavior of the isolated metallic nanoscatters was performed by focusing on the impacts of the underlying substrate. To achieve spectral control across the whole visible region, plasmonic nanodisks made from aluminum were employed because of its higher plasma frequency as compared with noble plasmonic metals and its the attractive features including low cost and self-limiting oxidation induced stability.15,32 Figure 1a,b shows the calculated forward/backward scattering cross sections (Qscat) of the isolated Al nanodisks free-standing in air and on substrates with varied refractive indices (the simulation methods and parameters are provided in the Supporting Information). In general, there exists a dominant peak in the scattering spectra for all dielectric configurations, corresponding to the LSPR of the Al nanodisks. For the case of free-standing Al nanodisks, the incident wave is scattered strongly and symmetrically to the backward and forward directions. This symmetric scattering behavior is broken when an underlying substrate appears beneath the Al nanodisks. With the increasing refractive index of the substrate, the scattering peaks red-shift and broaden in line width because of the retardation effects. Moreover, it is observed that the intensity difference between the backward and forward scattering becomes significant, especially for the high index substrate. The backward scattering cross sections are greatly diminished for the cases where the refraction index of substrate nsub is over 2.0. In contrast, the forward scattering on resonance maintains relatively high values and becomes considerably broader.
The structural colors relying on the scattering characteristics of nanostructures are highly dependent on the dielectric environment. In previous approaches, plasmonic nanoscatters were fabricated on a low-index substrate for the purpose of obtaining the scattered additive reflection colors or subtractive transmission colors.14,15,17,31−34 However, for the case of plasmonic nanoscatters sitting on high index optoelectronic semiconductors like silicon, the reflective color extraction efficiency can be significantly diminished because of the narrowed escape cone in the optically dense medium according to Snell’s law.35 In addition, the resonance peak becomes redshifted and broadened, primarily owing to the increased retardation effect,36 resulting in poor chromaticity control. The above limitations impede the progress of the plasmonic scattering coloration for broader application in optoelectronics. In this paper, both theoretical and experimental studies on integrated plasmonic coloration and light harvesting toward the silicon optoelectronic platform were performed by incorporating of the metallic nanodisks on top of ITO coated silicon. The proposed devices display distinct additive colors across the visible regime and harvest the complement spectrum component for efficient photocurrent extraction. Further, at the nonresonant wavelengths the better light coupling and angular spread of the forward scattered waves in the substrates allow the colored devices sustaining comparable power conversion efficiency against the antireflective reference devices. Meanwhile, the proposed approach is also capable of polarization-switchable coloration by utilizing elliptical-shaped nanodisks. 11077
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Figure 2. (a) Normalized driving field intensity of the homogeneous substrate with index of 2 and the dielectric (nd = 2, td = 20, 70, and 130 nm) coated silicon. (b) Backward Qscat of the same substrate configurations as (a) but including the Al nanodisks (D = 120 nm). (c) (Top) Typical electric field distributions on resonance illustrating the driving field enhancement of the layered substrate over the homogeneous structure. (Bottom) MLWA and numerical calculated backward Qscat for the cases of the homogeneous substrate and the dielectric (td =70 nm) separated silicon.
above analysis. The first is spectrally reshaping the resonance to match the situation of low index substrate and enhancing the backward scattering extraction by breaking the limitations arising from the small escape cone of silicon. The second is the further increased backscattering over the homogeneous substrate with low index due to the interference effect of the layered plasmonic-dielectric systems. To confirm the physical origin of these two facts, quantitative studies relying on the modified long wavelength approximation (MLWA) spheroid theory were carried out37−39 (see the Supporting Information for details). As shown in Figure 2c, the backward scattering cross-section obtained from the MLWA (blue hollow circle) is found to be consistent with the numerical calculations (red solid circle) very well in both the resonant peak position and the line-width for the case of homogeneous substrate (nSub = 2) supported nanodisks. When plasmonic nanodisks are separated from a substrate (silicon) with high refractive index by a dielectric film with low refractive index, the local driving field at the position of nanoscatters for LSPR is determined by the interference between the incident and reflected field, which results in strong modified amplitude of the scattering efficiency.40,41 Figure 2a shows the calculated driving field (Ed2 = |E/Ei|2, normalized electric field intensity at the interface of air and dielectric film) of the semi-infinite dielectric substrate and the layered dielectric/silicon substrate configurations in the absence of nanodisks. As compared with the featureless and flat spectra of the semi-infinite dielectric structure, the layered structures with varied thickness of the dielectric films show obvious spectral peaks and valleys due to the constructive or destructive interference of the incident and reflected fields. In Figure 2b, the backward scattering cross sections of the plasmonic nanodisks were calculated numerically by mounting the
The prominent and broad features of the forward scattering are highly preferred to the light-trapping feature in photovoltaic cells.26,35 However, to obtain the reflective coloration, the efficiency of backward scattering should be improved. The decrease in backward scattering on high index substrate is mainly ascribed to the narrowed escape cone for light in the optically dense medium. Considering the refractive indices of silicon (n ≈ 4), the critical angle for the light escape cone is about 15°. As a result, only a small fraction of the scattering light can be radiated back into air. To this end, the impedance matching layer with moderate index was inserted between the nanodisks and the substrate, aiming at directional shaping of the scattered light. As shown in Figure 1a,b, the backward scattering efficiency from the impedance-matched structure is significantly improved as compared to the case of the nanodisks directly sitting on silicon. Meanwhile, it is observed that the impedance-matched structure exhibits a back-scattering peak located at the same resonant wavelength but at least 2-fold enhanced over the case of that homogeneous substrate with the same refractive index as the impedance matching layer. Further insight into the scattering behavior for the dielectric medium separated plasmonic nanoscatters can be obtained by plotting the electromagnetic field distributions as depicted in Figure 1c. Consistent with the calculated Qscat, the near-field patterns around the free-standing nanodisks clearly show the symmetrically scattering features in which the incident wave is radiated efficiently for both forward and backward directions, while for the high index substrate supported nanodisks it prefers to guide the scattering wave into the forward direction as witnessed by the dramatically altered strength and angular distribution of the field patterns in air. There are two main consequences of the addition impedance matching layer on scattering properties can be addressed from 11078
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Figure 3. (a) Schematic drawings of the proposed plasmonic Si optoelectronics. The Al nanodisks with fixed thickness of 30 nm are arranged in a square lattice and characterized by their diameter D, period P. The supporting layered structures consist of a 64 nm thick ITO transparent anode, n-type Si substrate, and high reflective Al cathode. (b) Contour plot of the reflectance as a function of the diameter of nanodisks. D varies from 0 to 200 nm (where D = 0 corresponding to the bare ITO/Si in absence of nanodisks). The fill factor F defined by D/P is fixed to 0.5. (c) CIE 1931xy chromaticity coordinates represent for the predicted colors of the multicolored devices with reflectance corresponding to the spectra calculated in (b). (See the Supporting Information.)
ITO film as the impedance matching layer, and its thickness is fixed at 64 nm for the best trade-off between the suppression of the blue and red-end background colors. The thin layer of ITO has excellent conductivity and possesses high transparency throughout the visible and near-infrared regimes. Meanwhile, it typically forms a schottky-like contact with n-type silicon and thus can be considered as an efficient electrode herein for the collection of the photogenerated carriers in silicon. The proposed plasmonic nanoscatters integrated optoelectronic devices combing the capacities of color sorting and photocurrent generation are schematically illustrated in Figure 3a. By tailoring the geometric parameters including the diameter and packing density of the nanodisk arrays, the devices fulfill their coloration capability by output distinct reflective colors with desired luminosity. Unlike the previous filtering schemes,1,3,13,14,16,42 where the coloration is always accompanied by energy loss and a significant amount of light is rejected or parasitically absorbed by the filters, our approach attempts to harvest the complementary portions of light for the usage of photocurrent output. For the purpose of ensuring high purity coloration and PV harvesting, the thickness of nanodisks is fixed at 30 nm, as it is found to be optimal for narrowing the resonant scattering and concomitantly suppressing parasitic metallic absorption. Figure 3b shows a contour plot of the calculated reflection spectra as a function of the nanodisk diameter. The well-defined narrow band light reflection arising
nanoscatters on the corresponding substrate configurations. There is a clear spectral dependence of the scattering behaviors of the nanodisks on the local driving field as depicted in Figure 2a,b. When the constructive or destructive driving fields occur at a specific spectral position which coincides with the LSPR, the resulting Qscat can be significantly enhanced or diminished, as confirmed by the cases of the separated dielectric films with thickness of 70 and 130 nm. The contour plots in Figure 2c show that the impedance matched structure has high electric field strengths inside the dielectric spacer and clear amplified hot spots around the bottom corners of the nanodisks, illustrating the enhanced driving field and thus the resonant scattering over the homogeneous substrate. On account of the driving field effect, it is also possible to verify this speculation qualitatively. By multiplying the enhancement ratio of driving field intensity for the impedance matched structures over the homogeneous substrate on the Qscat, one can see that the modified MLWA scattering efficiency of the impedance matched plasmonic nanoscatters shown in Figure 2c is well consistent with the numerical results, except the slightly blueshifted resonance. It should be noted that the total reflection from the structures comprises not only the scattered portion but also the specular reflection. For the sake of ensuring high color saturation and wide gamut, it is crucial to suppress the specular reflection across the whole visible regime. Here, we chose the 11079
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Figure 4. (a) SEM image of the Al nanodisk array fabricated by EBL. D and P are estimated to be 165 and 392 nm, respectively. The scale bar is 2 μm. (b) Microscope image of the fabricated plasmonic color pixels (20 × 20 μm2). The illumination source of the microscope is a tungsten halogen lamp. From left to right, the pixels contain arrays of nanodisks with diameters of 80, 95, 115, 135, and 165 nm and periods of 264, 296, 328, 360, and 392 nm, respectively. (c) The calculated and (d) measured reflection spectra corresponding to the pixels shown in (b).
from the backward scattering of the nanodisks shifts its spectral positions continuously (from 400 to 700 nm) with the increasing diameter, suggesting an efficient visible color sorting capability of the proposed devices. To provide a quantitative measure of the saturation of the resulting reflective colors, the chromaticity coordinates in accordance with the spectral reflection were calculated and superimposed in the CIE 1931 xy color space diagram, as depicted in Figure 3c. The black solid circle represents the predicted color for the bare reference devices in absence of Al nanodisks, which tends to be a pinkish red. The incorporation of Al nanodisks atop the layered substrates will significantly alter the color perceptions of the devices. It is found that the proposed devices permit the coloration to encompass all the primary colors by simply adjusting the diameter of the nanodisks. Experimental realization of the proposed plasmonic nanoscatters integrated optoelectronic devices are conducted combing the functionalities of color sorting and photocurrent generation. The starting substrate is n-type (1−10 Ω·cm) and double-polished Si with a thickness of 300 μm. The high conductive and transparent ITO layer was deposited on the RCA-cleaned Si substrate by the magnetron sputtering and subsequently annealed at 300 °C for 1 h. As a critical parameter for suppression of the background specular reflection, the thicknesses of the prepared ITO films were carefully examined by spectroscopic ellipsometer. For the rear reflective electrode, 300 nm thick Al was deposited on Si and then thermally treated below the Al−Si eutectic temperature. Plasmonic nanodisk arrays were fabricated on top of the ITO layer by means of
electron beam lithography (EBL), Al evaporation, and liftoff. A scanning electron microscopy (SEM) image of one nanodisk array is shown in Figure 4a. The fabricated nanodisks are round shaped with a uniform size distribution for the 20 × 20 μm2 plasmonic pixels. Bright-field optical microscope images of the fabricated color pixels are presented in Figure 4b, demonstrating a high degree of color homogeneity and saturation for the five primary colors. The optical spectral responses of the fabricated plasmonic pixels are characterized in the visible range with a microscopebased fiber spectrometer measurement setup. As shown in Figure 4c, the bare ITO/Si devices in the absence of plasmonic nanoscatters show a typical antireflection feature of the onequarter coating, in which a reflection minima appears at λ = 480 nm. For the integrated devices containing an array of Al nanodisks with small diameters and ultralow surface coverage (e.g., in the cases of D = 80, 95 nm, the surface coverage of nanodisks is only around 7−8%), the resonant backward scattering of nanoscatters results in a dominate reflection peak at short wavelengths. The reflection peaks of these sparsely patterned nanodisk arrays have small peak amplitudes but provide the dominant contribution for color perception (refers to Figure 4b) due to the greatly suppressed red-end portion of reflection. With an increase of the diameter and surface coverage of the Al nanodisks, the resonant reflection peaks tend to red-shift and possess higher peak amplitudes, thus leading to varied scattering colors shown in Figure 4b. The measured reflection spectra are verified with numerical simulations using the FDTD method as shown in Figure 4d. By adopting the 11080
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Figure 5. (a) (Left) Microscope images for plasmonic pixels with varying diameters (D = 80−160 nm) and fill factor (F = 0.33−0.5) of the nanodisks. (Right) Measured reflection spectra of four selected pixel rows. (b) CIE 1931xy chromaticity coordinates of the plasmonic pixels with the experimental reflectance shown in (a). The impact of the increasing period on the color chromaticities is indicted by arrows for each diameter. (c) Predicted RGB colors of the plasmonic pixels under standard D65 source illumination by using the experimental spectra shown in (a).
Figure 6. (a, b) Spectral reflection spectra and microscopy images of two plasmonic color pixels under varied polarization angles. The color pixels (20 μm × 20 μm) consist of elliptical nanodisk arrays with different geometry parameters: (left) short axis DS = 70 nm, long axis DL = 130 nm; (right) DS = 70 nm, long axis DL = 170 nm. The nanodisks were assembled in a rectangle lattice which has a same period of PS = 240 nm in the direction parallel to short axis of the nanodisks but different periods (PL = 360 and 400 nm for the left and right pixels) in the perpendicular direction. (c) Microscopic plasmonic patterns of a cat cartoon display different image information at different polarization states. The numbers marked in the left image represent four color pixels containing different arrays of elliptical nanodisks. The size of the pattern is 60 μm × 60 μm.
nanodisks. For moderate packing densities, i.e., the nanodisks interact weakly with each other, increasing the packing densities can improve the luminosity of the plasmonic pixels without significantly altering their color perceptions. The microscope
actual experimental geometry of the nanodisks, the numerical results show good agreement with the experimental data. Because of their localized resonance origins, the scattering colors are mainly sensitive to the profile and the size of Al 11081
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Figure 7. (a) Absorption enhancement of three selected plasmonic nanoscatter integrated Si optoelectronic devices over the bare reference cell. (b) Normalized magnetic field distributions of the colored device at the on-resonance and off-resonance wavelengths. The white lines in the right plot were deduced by diffraction equations. (c) Typical current−voltage curves for the ITO coated Si devices with different experimental treatments. The insert plot is the experimental results of the current−voltage and efficiency voltage curves measured under 1 sun illumination. (d) Predicted EQE and parasitic absorption in ITO and nanodisks of the plasmonic devices. Experimental EQE of the reference device is also plotted.
where two selected plasmonic pixels containing arrays of asymmetric plasmonic nanoscatters are characterized in terms of the spectral response and color perceptions at different polarization angles θ. The polarization angles were controlled by inserting an optical polarizer in the optical path of the microscope. In both cases, the reflectance varied almost linearly with respect to the continuous rotation of the polarization angle. Under illumination with a polarized angle of θ = 0°, these two color pixels have different resonant reflection peaks and thus display distinctive colors due to their different sizes in the long axis of the elliptical nanodisks. For θ = 90°, the polarization parallels the short axis of the elliptical nanodisks, resulting in similar spectral reflection and color perception as shown in Figure 6a,b. Based upon the polarization-switchable coloration features of the elliptical nanodisks, two sets of optical information including the shapes and colors can be stored in one picture with predesigned color pixels. As shown in the first plot of Figure 6c, the plasmonic pattern consists of four color pixels (marked by the numbers), where different Al nanodisks are arranged in periodic lattices. The pixel 1 containing symmetric configuration of nanodisks represents the violet-blue colored background of the cartoons, which is irrelevant to the polarization states. In the remaining three pixels, different elliptical nanodisks are arranged in a rectangle lattice. The main axes of the elliptical nanodisks in these pixels are orientated in x or y directions. To achieve favorable image shift between two orthogonal polarization states, simulations were performed to calibrate the anisotropic coloration response in terms of the reflectance amplitudes and profiles (see the Supporting
images along with the measured reflection spectra of the fabricated plasmonic color pixels with different diameters and filling ratios are plotted in Figure 5a. It is obvious that the amplitudes of the scattering intensities and thus the luminosities of the pixels increase with the increasing filling ratio. In addition, for a given diameter, slight color variations are also observed in the chromaticity space (Figure 5b) with increasing period as a result of the blue-shifted localized resonance that is attributed to the radiative dipolar coupling between the nanodisks in pixel array.43 Figure 5c shows the predicted RGB colors of the plasmonic pixels with varying diameter and filling ratio by using the experimental reflection spectra plotted in Figure 5a. The predicted RGB colors provide direct visual perception of each color pixels, illustrating the full visible coloration capacity of the plasmonic pixels under the standard CIE-D65 white light illumination. We anticipate that by properly tailoring the sizes and packing densities of the Al nanodisks, plasmonic pixels with desired primary colors and luminosities can be produced. In above sections, the proposed devices containing plasmonic nanodisks with circular shapes are polarization independent under normal incidence, owing to their 4-fold rotational symmetry and square lattice arrangement. However, in some specific applications such as polarization-resolved imaging techniques for remote sensing and surveillance, the coloration along with the polarization-dependent character is also highly preferred.44 This attempt can be straightforwardly achieved based upon elliptical shaped nanodisks by means of tailoring their long and short axes. The polarization-switchable colorations are successfully demonstrated in Figure 6a,b, 11082
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front surface and the back surface field (BSF) scheme at the rear of the Si, but this substantially raises the device’s cost and complexity. As an important device metric for optoelectronics, the external quantum efficiency (EQE) of the plasmonic devices should be evaluated. The result of external quantum efficiency (EQE) measurement on reference cell is shown in Figure 7d (black diamond). The corresponding internal quantum efficiency (IQE) can be extracted by using the simple relationship: IQE = EQE/(1 − R), where R represents the experimental reflectance of the reference cell (see the Supporting Information). Assuming that the plasmonic devices have the same internal carrier extraction efficiency, the EQE of the plasmonic devices therefore can be calculated by weighting the above IQE data with the numerically determined optical absorption of Si substrates (ASi = 1 − R − Ap, where Ap is the sum of parasitic absorption in ITO and nanodisk layer, as shown in Figure 7d). Since the diffraction enhancements on IQE are neglected, this approach is regarded to give conservative but reasonably accurate predictions of EQE for plasmonic devices. To better mimic the realistic optical response, the simulations of the complete plasmonic devices including 300 μm thick Si substrates were carried out. The validation of the numerical methodological is confirmed by comparing the reflectance with the measured one. As shown in Figure 7d, the EQE curves of the plasmonic colored devices are comparable or higher than the reference, except for the resonance regime where plasmonic coloration and parasitic absorption take place. Considering white light illuminations (e.g., AM1.5G), these three selected plasmonic devices can provide photocurrents that are around 96−99% of the antireflective reference device (see the Supporting Information), i.e. achieving coloration at a negligible cost of the power conversion efficiency. Spectral filtering, polarizing, and light harvesting via plasmonic nanostructures are flourishing fields that hold great importance in their respective device applications.2−5,7,14 The present work aims to find an elegant combination of these merits for construction of the plasmonic optoelectronics relying on a Si platform with multifunctional capacities. The integrated devices selectively scatter light only over the desired ranges of wavelengths to access the s-RGB colors while efficiently converting their complementary spectrum portions for electricity generation. Thus, we foresee that such an energyefficient strategy can be employed in the reflective displays without resorting to the use of pigments filters, meanwhile generating power to charge the devices under bright ambient light illumination. In contrast, the color reflective display based on interferometric absorption reported recently relies on the electrostatic actuation but has no power generation.44 Similarly, the proposed devices can be considered as the compact, CMOS-compatible filter integrated photodetector, therefore applicable in the imaging sensors. The advantages over the image sensors19,20 with plasmonic filters fabricated atop include (1) less spatial optical crosstalk, (2) light harvesting for improved photoresponsitivity, (3) improved integrity, and (4) simplified fabrication eliminating alignment. Besides possible interest in the above two functionalities, the demonstrated polarization controlled coloration is also attractive. For instance, this feature can be used in polarization-resolved imaging for remote-sensing or surveillance.45 It should also be noted that some existing limitations need to be addressed for practical applications in the above-mentioned
Information for details). Under nonpolarized illumination, these four color pixels can be easily recognized from the microscope plasmonic pattern (the left plot), but their colors are the results of mixing two primary scattering colors associated with the short- and long-axis polarization components. In contrast, with an illumination polarized linearly along the x or y direction, some color pixels are immersed in the background, and thus, only one of the cat cartoons with specific designed color can be clearly resolved as demonstrated in the middle and right plots of Figure 6c. Since our plasmonic nanoscatters were designed to output specific on-resonance reflective colors in the visible region, substantial portions of incident light can transmit into the underlying silicon and contribute to the photocurrents. Meanwhile, several gain mechanisms for light trapping can take place at the off-resonance regimes of the nanoscatters, as confirmed in various literature citations.7,26 Therefore, from a perspective of photon harvesting, we calculate the enhancement of absorption in the plasmonic nanoscatter integrated devices over the bare reference. As shown in Figure 7a, obvious dips located at wavelengths near LSPRs are observed for all of the plasmonic devices, owing to the construction of the reflective coloration. However, absorption enhancements are observed at rest wavelengths for the plasmonic devices. The sharp enhancement peaks appearing at short wavelengths are ascribed to the excitations of the surface plasmon polariton (SPP) modes around the interface of Al and air. This can be easily confirmed by checking their phase-matching conditions. At long wavelengths, plasmonic devices show spectrally broad enhancement over the reference structure, implying an efficient nonresonant coupling of the incidence. Apart from the above plasmonic in-coupling mechanisms, the angular distribution of the propagation waves in Si lengthens the optical path and thus promotes the extraction efficiency of the photogenerated carriers. As depicted in Figure 7b, the field distributions at the on-resonance and off-resonance wavelengths show typical periodic diffraction patterns. Due to the presence of the nanodisks atop the silicon, the transmitted or scattered waves are coupled into various diffraction modes, allowing high absorption within relative shallow depths. The white lines superposed in the right plot are deduced from the diffraction equation by taking into account the first-order diffraction. Considering the finite diffusion length of minority carriers in n-type Si used herein, the angular distribution of diffracted beam in Si can substantially increase the internal quantum efficiency (IQE) of the devices. Figure 7c shows the typical dark current−voltage curves of the reference devices. The rectifying performance of ITO/Si junction is crucial for achieving efficient photocurrent conversion. According to our experiments, we found that the thermally annealed ITO layer forms a more reliable schottkylike contact with Si as compared with the as-deposited films. Further, incorporation of an ultrathin oxide (2−4 nm) layer by a chemical oxidation process between ITO and Si greatly improves the barrier height and suppresses reverse saturation current effectively. The light characteristics of the reference cells with such an optimized MIS configuration were experimentally evaluated under the simulated AM1.5 G illumination. As shown in the inset, the short-circuit current is 29.7 mA/cm2, open-circuit voltage around 0.39 V, and the conversion efficiency is found to be 8.3%. The conversion efficiency of the devices can be significantly improved by establishing the commonly employed P−N junction at the 11083
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Figure 8. (a) SEM image (45° titled view, scale bar is 1 μm) shows the plasmonic nanodisks embedded in a 40 nm SiO2 capping layer. (b) Measured reflectance and microscope images of the embedded plasmonic nanodisks configurations. The black line represents the reflection spectra of the bare SiO2/ITO/Si. The results for five nanodisks integrated pixels with varying diameters and periods (D = 80, 100, 120, 140, and 160 nm, P = 240, 260, 290, 320, and 360 nm) are plotted in the colored lines (red, blue, dark cyan, magenta, and dark yellow). (c) CIE 1931xy chromaticity coordinates represents for the predicted colors of the pixels with the experimental spectra corresponding to (b).
Figure 9. Simulated (a) and measured (b) reflection spectra of the plasmonic color pixel at different incident angles.
fields. (1) Low chromaticity of plasmonic colors toward the red end: As shown in Figure 4 and 5, the line width of plasmonic scattering color of large nanodisks becomes broad in the long wavelength region. It results in reduced chromaticities hindering applications in full visible range. Line width narrowing strategies should be explored on the presented scattering framework for future studies. For example, improved chromaticities can be observed in Figure 8c, where a thin lowindex capping layer is coated on the pixels as shown in Figure 8a. It not only provides protection for the plasmonic nanoscatters but also improves both the efficiency and the gamut of the scattering coloration. More vivid colors in red can be observed as shown in Figure 8b compared to those in Figure 5a. (2) Angular dependence of the scattering colorations: Both theoretical and experimental studies on the impact of incident angle to the spectral reflectance of the plasmonic pixels were performed, and the results are shown in Figure 9. The theoretical results are calculated by the FDTD method and averaged by S- and P-polarized reflectance for difference incident angles. The reflection measurements on the fabricated color pixel (Al nanodisks: D = 110 nm, P = 240 nm; pixel size is 20 × 20 μm2) were carried out by microscopic angle resolved photonic spectroscopy (MARPS, Ideaoptics, Inc.). In general, increasing the oblique angle of the incidence, the amplitudes of the scattering peaks decrease, and the spectral positions of the peaks causes a slight red-shift. The locations of the reflection
peaks shift less than 10 nm when the incident angle varies from 10° to 30°. Generally, the f-number of the common camera ranges from 1.8 to 16, and the corresponding incident angle is less than 17°.20 Therefore, the proposed pixels can sustain their color chromaticity with an oblique incident angle up to 30° owing to their localized resonance features, which is reasonably good for imaging. However, the decrease in amplitude is more obvious, and further optimization is required for display applications. For example, nonperiodic or quasiperiodic patterning of the nanoscatters can enable excellent angleinsensitive (up to 60°) performance of the scattering colorations.46
CONCLUSIONS In conclusion, we have proposed and successfully demonstrated multiple functional optoelectronics on the silicon platform by integrating plasmonic nanoscatters onto a schottky diode. Reflective coloration across the full visible spectrum was achieved together with a power conversion efficiency of 8.3% that is 96−99% of the reference device without nanoscatters, i.e., realizing coloration at a negligible cost of the power conversion efficiency. The integration offers both color-sorting and light-harvesting capabilities with superiorities such as reduced spatial optical crosstalk, improved photoresponsitivity, self-charging, high integrity, and so on in potential applications including advanced display, imaging, and photovoltaics. 11084
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ACS Nano
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Modified long wavelength approximation spheroid theory and numerical simulation reveal the underlying physics of the proposed structures, where the incorporation of an impedance matching layer provides spectral and spatial controls on the scattering behaviors of the plasmonic nanodisks and enlarges the local driving force that increases the scattering efficiency. Further, based upon the asymmetric nanodisks, we show two sets of color image information can be stored or shifted by controlling the polarization state. The presented concepts are generalizable to other optoelectronic material platforms and, thus, hold great potentials for development of integrated display and sensing optoelectronics.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05960. Additional information as noted in the text (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Long Wen: 0000-0002-8796-4376 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We are thankful for the technical support from Nano-X of SINANO, CAS. This work was supported by the grants from the National Natural Science Foundation of China (Nos. 11274344, 11604367, and 61574158), the National Key Research and Development Program (No. 2016YFB0402501), the Cutting-edge Key Research Program of Chinese Academy of Sciences (No. QYZDB-SSW-JSC014), the Natural Science Foundation of Jiangsu Province (NO.BK20150369), and the Suzhou Science and Technology Development Program Foundation (No. SYG201529). REFERENCES (1) Tittl, A.; Mai, P.; Taubert, R.; Dregely, D.; Liu, N.; Giessen, H. Palladium-Based Plasmonic Perfect Absorber in the Visible Wavelength Range and Its Application to Hydrogen Sensing. Nano Lett. 2011, 11, 4366−4369. (2) Yokogawa, S.; Burgos, S. P.; Atwater, H. A. Plasmonic Color Filters for CMOS Image Sensor Applications. Nano Lett. 2012, 12, 4349−4354. (3) Chen, Q.; Hu, X.; Wen, L.; Yu, Y.; Cumming, D. R. S. Nanophotonic Image Sensors. Small 2016, 12, 4922−4935. (4) Xu, T.; Wu, Y. K.; Luo, X.; Guo, L. J. Plasmonic Nanoresonators for High-Resolution Colour Filtering and Spectral Imaging. Nat. Commun. 2010, 1, 59. (5) Walls, K.; Chen, Q.; Grant, J.; Collins, S.; Cumming, D. R.; Drysdale, T. D. Narrow Band Multispectral Filter Set for Visible Band. Opt. Express 2012, 20, 21917−21923. (6) Cui, Y.; Phang, I. Y.; Lee, Y. H.; Lee, M. R.; Zhang, Q.; Ling, X. Y. Multiplex Plasmonic Anti-Counterfeiting Security Labels Based on Surface-Enhanced Raman Scattering. Chem. Commun. 2015, 51, 5363−5366. (7) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (8) Fan, X.; Zheng, W.; Singh, D. J. Light Scattering and Surface Plasmons on Small Spherical Particles. Light: Sci. Appl. 2014, 3, e179. 11085
DOI: 10.1021/acsnano.6b05960 ACS Nano 2016, 10, 11076−11086
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DOI: 10.1021/acsnano.6b05960 ACS Nano 2016, 10, 11076−11086
