LETTER pubs.acs.org/NanoLett
Plasmonic Photosensitization of a Wide Band Gap Semiconductor: Converting Plasmons to Charge Carriers Syed Mubeen,† Gerardo Hernandez-Sosa,‡ Daniel Moses,‡ Joun Lee,† and Martin Moskovits*,† †
Department of Chemistry and Biochemistry and ‡Center for Polymers and Organic Solids, University of California, Santa Barbara, California 93106, United States
bS Supporting Information ABSTRACT: A fruitful paradigm in the development of lowcost and efficient photovoltaics is to dope or otherwise photosensitize wide band gap semiconductors in order to improve their light harvesting ability for light with sub-band-gap photon energies.1 8 Here, we report significant photosensitization of TiO2 due to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor (TiO2). Surface plasmon decay produces electron hole pairs in the gold.9 15 We propose that a significant fraction of these electrons tunnel into the semiconductor’s conduction band resulting in a significant electron current in the TiO2 even when the device is illuminated with light with photon energies well below the semiconductor’s band gap. Devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film produced over 1000-fold increase in photoconductance when illuminated at 600 nm over what TiO2 films devoid of AuNPs produced. The overall current resulting from illumination with visible light is ∼50% of the device current measured with UV (pω > Eg band gap) illumination. The above observations suggest that plasmonic nanostructures (which can be fabricated with absorption properties that cover the full solar spectrum) can function as a viable alternative to organic photosensitizers for photovoltaic and photodetection applications. KEYWORDS: Plasmonics, photoconductance, gold nanoparticles, titania, impedance spectroscopy
P
lasmonic concentration and propagation have recently been proposed as strategies for enhancing photovoltaic and photocatalytic performance.16 20 In this Letter we describe a device, fabricated entirely using foundry processes with which a wide band gap semiconductor is photosensitized by embedding plasmonic nanoparticles within it, thereby significantly broadening its photoconversion ability beyond the ultraviolet region. The active element of the device is a composite solid film consisting of multiple, dense two-dimensional planar arrays of gold nanoparticles with each layer well separated by TiO2. Ohmic Ti/Au metal interdigitating electrical contacts were fashioned on the upper surface of the film using photolithography (Figure 1a, and the methods section in the Supporting Information). The ultraviolet/visible absorption/extinction spectrum of gold nanoparticles produced on a quartz substrate shows a localized surface plasmon resonance (LSPR) maxima at 520 nm, indicative of well-separated gold nanoparticles (Figure 1b). When capped by a TiO2 film of 200 nm (mass thickness), the LSPR red shifts by 100 nm and becomes more intense, primarily due to the increase in the dielectric constant of the surrounding medium (ranges from 55 to 130 for polycrystalline anatase TiO2) over that of air.21 24 Surrounding the gold nanoparticles with titania also creates a Schottky junction at the metal semiconductor interface, which (in this case) results in r 2011 American Chemical Society
charge transfer from the TiO2 to the gold nanoparticles (AuNPs), charging the gold negatively and the TiO2 positively in the vicinity of the NPs, and producing a potential barrier ∼0.9 eV (Figure 1c).25,26 TiO2 (Eg = 3.3 eV) was chosen on account of its excellent electron-accepting capability due to the high density of states in the conduction band and the absence of photoresponse in the visible, suggesting a low density of appropriately located defect states.27,28 The major goal of the present study is to determine to what extent the plasmonic energy resident in optically excited gold nanoparticles can be transferred to electrons that either overcome the metal semiconductor energy barrier or tunnel through it to become conduction electrons in the semiconductor, which can be probed as changes in the conductance of the TiO2/Au composite. Prima facie evidence that those TiO2 conduction electrons began life as plasmonic excitation of the AuNPs would be the degree to which the device’s photoconductance tracks the plasmonic extinction spectrum of the composite material keeping in mind that the gold nanoparticles are spaced significantly below the conductance percolation threshold of the AuNPs. Received: October 3, 2011 Revised: October 28, 2011 Published: October 31, 2011 5548
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Nano Letters
Figure 1. Plasmonic photosensitization of TiO2. (a) Cross section scanning electron micrograph of the device used in this work, consisting of multistack layers of AuNPs of diameter ∼14 nm embedded in a TiO2 layer of total thickness ∼200 nm. The various components of the device were artificially colored for clarity. Scale bar, 100 nm. (b) UV/visible spectra of TiO2 film, AuNPs and AuNPs TiO2 composite on a quartz substrate at normal incidence. Absorption spectra of AuNPs were multiplied by a factor of 3 for clarity. The absorbance corresponding to the bare quartz substrate was subtracted from each absorption spectrum. (c) Schematic illustrating the band-bending effect of the Schottky junction between Au NP and the TiO2 layer surrounding it. Band bending creates an energy barrier which allows an electron excited by an incoming photon of energy pω from a filled to an empty level of the metal’s conduction band to tunnel directly into the conduction band (ECB) of the TiO2.
Hence, the only material through which continuous conductance can occur is the TiO2. Very recently, evidence that such metal to semiconductor tunneling could occur was reported from gold nanoantennas fabricated on silicon to the silicon.29 The wavelength-dependent photocurrent response measured between interdigitating electrodes patterned on AuNP TiO2 samples with various levels of gold doping are shown in Figure 2, together with the corresponding absorbance/extinction spectrum. The results show incontrovertibly that the visible-light response of the device is due to the AuNP loading and that the effect is significant, in aggregate on par with the current component resulting from direct band gap excitation of the TiO2. The photoinduced conductances and the quantum efficiency (see supplementary Figure S1 in the Supporting Information) faithfully track the (plasmonic) absorbance/extinction spectra of the corresponding AuNP TiO2 film, with the largest responsivities occurring for wavelengths in the 500 700 nm range and maximizing at ∼620 nm, implying that the plasmonic excitation is directly responsible for the increased numbers of carriers in the conduction band of the TiO2. The efficiency (electron per incident photon) at 620 nm is ∼0.2%. In the absence of gold, the device shows negligible photoconductance when illuminated with wavelengths longer than ∼370 nm. The large currents observed with wavelengths shorter than 370 nm are due to direct interband carrier photoexcitation in TiO2. The conductance changes were found to increase monotonically with increasing numbers of nanoparticle layers in the
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Figure 2. Photoconductance of TiO2 and AuNP TiO2 films. Measured UV visible (solid lines) and photocurrent spectra (closed symbols) of TiO2 and AuNPs in TiO2 for varying nanoparticle densities. The photocurrent, normalized for the intensity of the monochromatized light source as a function of wavelength, is shown in units of microamperes per watt. Right side: the corresponding cross sectional SEM image of the devices measured (artificially colored for clarity). All scale bars correspond to 100 nm.
Figure 3. Effect of photon fluence. Photocurrent responsivity of two AuNPs in TiO2 devices with varying Au nanoparticle loadings (TiO2 with four layers of gold nanoparticles (solid red squares) and TiO2 with three layers of gold nanoparticles (solid blue circles)) as a function of the irradiating light intensity at 600 nm. The measurements were carried out with 500 W xenon lamp sources. Inset shows that at low intensities the photocurrent is linear with fluence.
AuNP TiO2 composite layer (and with its corresponding optical absorbance). For the device with four layers of gold nanoparticles, over 30% of its integrated photoresponse is due to 5549
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Nano Letters the contribution from the visible region of the spectrum. However, gold loading reduced the device’s photoresponse in the UV region likely due to the Schottky depletion of TiO2 carriers at the metal/semiconductor junction, and possibly also due to the increased extinction by the gold nanoparticles in the UV region, and the (somewhat) reduced volume fraction of TiO2. The large photoconductance observed for the AuNP TiO2 device when illuminated with visible light might result from one or more of the following processes: (1) photoexcitation of filled midgap donor states associated with TiO2 defects; (2) formation of excitons in the TiO2 by a two-photon process excited by resonant energy transfer from the Au plasmon; and (3) the production of electron hole pairs in the gold as one of the decay channels of the surface plasmon excitation followed by electron transfer to the TiO2. The negligible currents produced by visible light illumination of TiO2 (devoid of Au) indicate that the numbers of filled midgap states are very few, so the first process cannot be a major contributor. Measurements of the dependence of the photocurrent intensity on the fluence of the incident illumination (Figure 3) indicate a largely linear dependence of the photocurrent on light intensity in the low intensity regime, ruling out a two photon absorption mechanism. Some saturation is found to occur at the higher photon flux values. However, the photon flux threshold at which photocurrent saturation occurs is found to be independent of AuNP loading (Figure 3). Referring to Figure 1c, the process begins with the production of electron hole pairs in the conduction band of the metal by the decaying SP, with the excited electrons occupying normally empty states in the metal’s conduction band. A (significant) fraction of these excited electrons is transferred (by quantum tunneling) to the TiO2. This process reduces the negative charge already present on the gold as a result of Schottky junction formation and restores to the TiO2 some (or all) of the depleted negative charge. In contrast, direct band gap excitation of the TiO2 or the AuNP TiO2 composite medium produces electron hole pairs wholly resident on the TiO2, which would leave the overall charge states of the TiO2 and the Au unchanged. Saturation may result from the fact that a sufficient number of carriers (hot electrons) are generated following surface plasmon decay to neutralize many of the surface states which initially caused the band bending. These differences in charge disposition resulting from alternately illuminating of the AuNP TiO2 device by visible and UV light should have distinguishable effects on the low frequency (i.e., dc) dielectric constants of the materials when measured under red (600 nm) as opposed to UV (330 nm) illumination. This proposition was explored by performing impedance measurements on the devices in the dark and under UV and red light illumination in the range 400 Hz to 100 kHz. Representative results are shown in Figure 4 as plots of the imaginary versus the real part of the measured impedances. The overall trends in the behavior of measured impedance values as a function of frequency are not unlike what had been previously reported for polycrystalline TiO2.30 Such films which can have complex micro- and nanostructures due to defects and various structural and compositional inhomogeneities are often modeled as constant phase elements.31,32 The resistive and capacitive contributions to a constant phase element can be evaluated using expressions such as those reported in refs 32 and 33. This is summarized briefly in the Supporting Information. Indeed, the equivalent circuit shown in Figure 4c was found to fit our measured
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Figure 4. Impedance spectroscopy. Spectral dependence on the complex impedance plots (Nyquist plots) obtained for (a) TiO2 and (b) AuNPs-TiO2 films. Both plots: open symbols experimental data and solid lines simulated data using the equivalent circuit shown. Several frequencies are labeled in the Nyquist plot. (c) The equivalent circuit used to simulate the impedance measurements as a function of frequency. Briefly it consists of a single resistor that accounts for all contact resistances in series with a constant phase device in parallel with a shunt resistor which represents the AuNP TiO2 composite material (with or without Au loading), and finally, a series capacitor represents the effect of the silica layer that insulates the device from the bottom ITO electrode.
Table 1. Calculated Dielectric Constants of TiO2 and AuNPs TiO2 Thin Films Derived from the Impedance Measurements TiO2 dark
330 nm
AuNPs + TiO2 600 nm
dark
330 nm
600 nm 97.1
RContact (Ω)
87.3
83.7
87.0
96.6
97.0
R1 (Ω)
11500
5115
11047
18500
14000
17101
εSiO2
3.92
3.98
3.90
3.98
4.02
3.86
effective εTiO2
90.3
315
89.3
82.2
84.7
59
impedance values very satisfactorily over the entire frequency range used. The electrical materials properties derived from the impedance measurements on the most highly AuNP loaded device, are summarized in Table 1 (see supplementary Table 1 in the Supporting Information for more details). It is noteworthy that despite the parametric freedom of the fit, the values of the isolation capacitance CSiO2 returned are almost constant for all of the devices studied, and under all illumination conditions. Moreover, the value of the dielectric constant calculated from this capacitance (averaging ∼3.9) corresponds closely to the reported dc dielectric constant of silica.34 Gratifyingly, the contact resistance is found to be rather small (∼90 Ω) and also fairly constant for all measurements. By contrast the resistance values reflecting the conductance of the TiO2 and AuNP-loaded TiO2 depend markedly on the presence or absence of the gold nanoparticles and on the wavelength of illumination. For the pristine TiO2 the dark resistance is measured to be ∼11.5 kΩ. This resistance value is almost unchanged (∼11.0 kΩ) upon 600 nm illumination, 5550
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Nano Letters indicating (not unexpectedly) that TiO2 shows no photoconductive response to visible light. Illuminating the device with 330 nm light, however, reduces the device’s resistance to 5.1 kΩ— a consequence of the fact that UV light induces direct band gap transitions. The increased conductance reflects the increased density of the newly formed electrons and holes. The behavior of the AuNP-loaded TiO2 is markedly different. First, the dark resistance of the device is some 60% larger (18.5 kΩ) than for the device fabricated with Au-free TiO2—due almost certainly to the charge depletion at the Au/TiO2 Schottky junction. Second, the decrease in this resistance value induced by illumination with 330 nm light is only ∼24%—half what it was for the Au-free material. This also follows from the premise that Schottky depletion significantly reduces the density of accessible electrons in the valence band of the TiO2. With 600 nm illumination this device behaves radically differently from its Au-free counterpart. The device resistance drops by 7.5% to 17.1 kΩ, corresponding to ∼30% of the drop produced by UV (330 nm) illumination. More significantly, the photoconductance change integrated between 500 nm and 750 nm is ∼50% of the integrated response in the UV. In other words, with AuNP loading the device is only ∼50% less conductive with red light illumination as it is in the UV. The capacitance results are even more distinctive. The dielectric constant for the (gold-free) TiO2 calculated from capacitance measurements carried out in the dark εTiO2,dark ≈ 90, corresponds well with what is expected for polycrystalline anatase TiO2. The value measured with 600 nm illumination (εTiO2,600 ≈ 89) is almost identical to the dark value. However, with 330 nm illumination the value of the dielectric constant increases dramatically (εTiO2,330 ≈ 315) reflecting the significant increase in electron population in the material’s conduction band and hole population in the valence band, rendering it significantly more polarizable.35 Once again the AuNP-loaded TiO2 behaves dramatically differently. The dark value of the dielectric constant is reduced to 82. This reflects two opposing contributions. Schottky depletion: which reduces the value of the material’s dielectric constant, and metal loading: which increases its value due to the higher dc polarizability of the metal over that of the semiconductor. Illumination with 330 nm light increases the material’s dielectric constant only very slightly (εAuNP‑TiO2,330 ≈ 84). By contrast, exposure of the device to 600 nm light produces a dramatic effect. The calculated dielectric constant decreases to ∼72% of its dark value (from 82 to 59). This large decrease is consistent with the tentative proposition that at 600 nm, a significant fraction of electrons excited as electron hole pairs in the metal, cross from the metal into the TiO2 thereby reducing the Au NP’s polarizability (and also the effective dc dielectric constant of the metal). As a result the decrease in the composite medium’s dielectric constant induced by Schottky depletion is no longer offset by the polarizability of the metal to the same extent as it was in the absence of illumination, and the composite’s dielectric constant should markedly decrease, as observed. Current voltage measurements (see supplementary Figure S2 in the Supporting Information) are consistent with the above interpretation. The I(V) curves show almost Ohmic behavior when the device is in the dark or illuminated with 600 nm light. But when illuminated with UV (330 nm) it shows the I(V) characteristics of back-to-back diodes reflecting the changes in the effective location of the band edges with respect to the Fermi energies of the Ti/Au contact pads.
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In summary, devices fabricated by embedding Au nanoparticles in TiO2 show significant additional photoconductances (∼30%) when illuminated by light with photon energies well below the band gap. The photoconductance is found to track the plasmonic absorption/extinction spectrum of the AuNPs faithfully. This impressive change in photoconductance is ascribed both to quantum tunneling of hot electrons from the metal directly into the conduction band of the TiO2 for those electrons with energies lower than the 0.9 eV needed to overcome the barrier and to energetic electrons going over the barrier transport. All of these electrons originate as electron hole pairs in the gold NPs produced by plasmonic excitation and decay. Device impedance measurements carried out in the dark and under illumination with UV and red wavelengths reinforce this mechanism. Substantial improvement in performance can be expected by further improving the light harvesting design. This could include (i) increased gold nanoparticle loading (it is particularly intriguing to consider what would happen if the nanoparticle density is high enough to produce electromagnetic hot spots); (ii) engineering a device architecture that increases the interfacial area between plasmonic nanostructure and a wide band gap semiconductor, for example, by using arrays of gold nanorods (absorption of which can be tuned to cover the entire solar spectrum); (iii) improving the crystallinity of the semiconductor; and (iv) selecting oxides with better electron mobility.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed experimental methods, calculation of quantum efficiency, capacitance and dielectric constants, detailed impedance measurements results, XRD diffraction patterns, schematic of the photocurrent measurement apparatus, and transient photocurrent measurement results. This material is available free of charge via the Internet at http://pubs. acs.org
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank Ashok Ramu for photolithography support, Namhoon Kim for technical support, and Peter Allen for graphic support. This work was supported by the Institute for Collaborative Biotechnologies through Grant DAAD19-03-D-0004 from the U.S. Army Research Office and made extensive use of the MRL Central Facilities at UCSB supported by the National Science Foundation under award nos. DMR-0080034 and DMR0216466 for the HRTEM/STEM microscopy. We also gratefully acknowledge research support from the Institute for Energy Efficiency, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Award Number DE-SC0001009. ’ REFERENCES (1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737–740. (2) Bach, U.; Lupo., D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Graetzel, M. Nature 1998, 395, 583–585. (3) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. 5551
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