Plasmons in Photocharged ZnO Nanocrystals Revealing the Nature of

Letter pubs.acs.org/JPCL

Plasmons in Photocharged ZnO Nanocrystals Revealing the Nature of Charge Dynamics Jacob A. Faucheaux† and Prashant K. Jain*,†,‡,§ †

Department of Chemistry, ‡Materials Research Lab, and §Department of Physics, University of Illinois Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: Localized surface plasmon resonances (LSPRs), known for their fascinating optical properties, have thus far been limited to nanostructures of materials with high steady-state charge carrier densities. Here, we show that even a nonequilibrium charge population can support a LSPR mode. Photocharged zinc oxide (ZnO) nanocrystals show an infrared LSPR, which can be dynamically turned off by discharging via redox activity. It is deduced that the photoinduced LSPR is a collective mode of as few as four conduction band electrons, the least observed thus far. The sustenance of a free-electron plasma in charged ZnO, supported by the LSPR observation, leads us to propose the existence of a many-body excitonic state and suggest a mechanism for previously unresolved charge trapping dynamics in ZnO. The LSPR, which serves as an optical signature of a charged state of the nanoparticle, is also demonstrated as a useful probe of surface redox reactions. SECTION: Plasmonics, Optical Materials, and Hard Matter

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by a fewer number of charge carriers than conventionally thought. Using the LSPR, we also propose the existence of an exotic many-body excitonic state and attempt to address the long-standing question of why hole trapping may be quenched in charged ZnO nanocrystals. Extensive work has been done on the optical properties of charged ZnO nanoparticles, including the observation of intraband absorption, exciton bleaching, and quenching of defect emission.19,20 A consistent and comprehensive explanation of all of these effects has, however, been lacking. The novelty of this work lies in showing that many of these optical properties are consistent with the sustenance of a free-electron plasma in charged ZnO, which we accomplish via the LSPR mode assignment. The model of a free-electron plasma in charged ZnO allows us to suggest mechanistic answers to other long-standing questions in the field. The accumulation of electrons in response to above band gap radiation is illustrated in Scheme 1. When excited with an above band gap photon, an electron and hole are created. Hole scavenging by an alcohol or hydroxyl groups quickly occurs, forming protons in solution.15 In the absence of oxygen and other electron scavengers, the electrons accumulate in the conduction band stabilized by protons in the solution. The conduction band electrons in this transiently photocharged state of ZnO nanocrystals give rise to a LSPR mode in the IR. We support the assignment of this IR absorption band to a LSPR by means of several tests, (a) the mode frequency and intensity increase (decrease) with increasing (decreasing) charging level, as expected for LSPRs, (b) internanoparticle electromagnetic coupling resulting in a red shift of the

ocalized surface plasmon resonances (LSPRs), which have received much attention due to their ability to fundamentally alter light−matter interactions,1−4 were thought to be limited to noble metal nanoparticles. Recently, this notion was revised when LSPRs were demonstrated in heavily doped semiconductor and oxide nanoparticles.5−7 High levels of doping can increase the charge carrier concentration in the conduction band of these nanoparticles to a level that can sustain an infrared (IR) LSPR.8−11 However, in both the metal and doped semiconductor cases, the LSPR arises from an equilibrium steady-state charge carrier population within the nanoparticle. We show here that even nonequilibrium charge carrier populations can display LSPRs. The LSPR can thus serve as an optical signature of a nonequilibrium charge state, from which previously unknown carrier dynamics can be revealed. We chose zinc oxide (ZnO) as a model system for achieving nonequilibrium charge densities and dynamic LSPRs. While ntype doped ZnO films with high steady-state charge carrier densities have shown metal-like behavior and a thin film surface plasmon similar to other doped semiconductors,12,13 the ability of ZnO to be charged in a nonequilibrium manner has not been exploited for plasmonics. Electrochemical14 and photochemical15,16 charging are known to cause accumulation of conduction band17,18 electrons in ZnO nanocrystals. Here, we demonstrate that charging of ZnO nanocrystals via photoexcitation of electrons results in the formation of LSPRs in the IR region. These LSPRs are dynamic and can be turned on/off by photocharging/redox discharging, which we exploit for sensing a reduction reaction involving excited ZnO electrons. We also find that this LSPR mode is sustained by as few as four electrons, the least observed thus far, from which we conclude that a collective plasmon mode can be supported © XXXX American Chemical Society

Received: August 12, 2013 Accepted: August 26, 2013

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The Journal of Physical Chemistry Letters

Letter

with a progressive increase in the carrier density (Figure 1B). Once a saturation level of carriers is reached, the LSPR band undergoes no further change. Concomitant with the emergence of a LSPR, the accumulation of electrons in the conduction band results in a bleach of the exciton, which has also been seen in past studies. Essentially, we believe that the electron−hole Coulombic attraction characteristic of the excitonic state is completely screened, as would be consistent with the formation of a conduction-electron plasma. A Moss−Burstein shift of the absorption band edge (Figure 1C), consistent with a rise in the Fermi level within the conduction band, is also seen. Figure 1D demonstrates the dynamic nature of the generated LSPR mode. The LSPR mode can be switched off by slow exposure of the solution to oxygen. Electrons are scavenged by oxygen, resulting in a gradual shift to lower energies (implying a reduction in net carrier density) and suppression of the LSPR. Concomitantly, the exciton is recovered as the material returns to the equilibrium semiconducting state. A defining signature of a LSPR is the sensitivity of its resonance frequency to coupling with adjacent plasmonic nanoparticles.22−24 This coupling is manifested in aggregated nanospheres by a drastic red shift of the LSPR. The LSPR of photocharged ZnO nanocrystals, aggregated by drying into a thin film, demonstrates such a shift, as shown in Figure 2. ZnO nanocrystals in the film show a LSPR at energies below the range of our detector (1020 cm−3) are independent of the density of unpassivated surface defects. In past work, an IR absorbance band in charged ZnO nanoparticles similar to the one seen here was attributed to single-electron transitions between discrete electronic levels.19,37 However, if this were the case, the observed band would shift to lower energies upon an increasing amount of

determines the oscillation frequency. Thus, as long as the electrons occupy the conduction band (i.e., have free character) and are significantly coupled via Coulomb repulsion, a plasma should be sustainable.31 The LSPR is simply the manifestation of collective intraband free carrier transitions of a plasma confined to a nanoscale geometry. Small numbers of electrons sustaining LSPRs have been demonstrated in metallic clusters, for instance, in Hg6+ clusters where n = 11.32 However, at cluster sizes (Hg5 and smaller) needed to access even smaller n, there is a change in the electronic structure to one with well-separated electronic states, which does not permit a collective free-electron resonance. Only single-electron excitations between these states (in particular, filled 6s and empty 6p states) are possible. The photocharged ZnO system has an advantage in this regard due to its much lower carrier density (1/10× that of Hg and other such metals). In ZnO, one is able to achieve a very low number of carriers without the need to go to the cluster size regime, where free-electron bands are no longer possible. The photoinduced LSPR also helps elucidate the nature of charge trapping in ZnO. In ZnO nanocrystals, charge-trapassisted recombination of electrons with deep-hole traps is known to lead to a bright green defect photoluminescence 3027



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Figure 5. Chemical reduction by the accumulated photoelectrons can be monitored by their LSPR signature. (a) The LSPR band of photocharged ZnO nanocrystals red shifts and decreases in intensity upon addition of 1 mM resazurin. (b) Resazurin utilizes the accumulated electrons of photocharged ZnO nanocrystals and gets reduced to resorufin. Absorption of resorufin (characterized by its the band at 572 nm) increases upon progressive additions of resazurin, concomitant with the suppression of the LSPR band. The absorption spectrum of resazurin (dashed line) with its characteristic 602 nm band is shown for reference at the final concentration (14 μM). The noise in the absorption spectrum at 0.5−0.6 eV is due to infrared absorption by the solvent, specifically overtones of stretching vibrations in methanol-d4.

trapping but also the increased lifetime and intensity of the band edge PL in photocharged ZnO. The dynamic nature of the LSPR gives it the ability to serve as an optical probe of charge dynamics and redox processes (Figure 5A). We demonstrate this with resazurin, a well-known redox and catalysis indicator.44,45 Under reducing conditions, resazurin can be converted to resorufin, which has a strong absorption band at 572 nm. Resazurin added to a solution of charged ZnO QDs is quickly reduced to resorufin, indicated by the emergence of an absorption band at 572 nm (Figure 5B). The utilization of electrons in this process can be optically monitored by the red shift and suppression of the LSPR. The chemical reduction in this scenario occurs after irradiation occurs; the electrons are effectively stored until the introduction of a reducible species, as demonstrated by the work of the Kamat group.41 The response of the LSPR is consistent with this picture. Carrier densities deducible from the LSPR frequency may be exploitable in the future for quantitatively monitoring the rates of surface redox reactions on charged ZnO and other semiconductors nanocrystals. In conclusion, we reported the presence of LSPRs in ZnO nanocrystals that can be switched on by light and turned off by redox reactions. Such unique control of optical response at the nanoscale can lead to interesting device applications in digital logic and solar harvesting. These LSPRs are fundamentally interesting because they demonstrate the ability of extremely small numbers of electrons to sustain a plasmon resonance and signify the close connection between intraband single-carrier transitions and plasmon resonance in nanostructures. The LSPR also serves as an optical signature of nonequilibrium charge states and dynamics. By means of the LSPR, we provided insight into the nature of charge traps in ZnO and explored the possibility of exotic many-body states. Finally, it was demonstrated that dynamic LSPRs can be used to probe redox processes involving the excited charge carriers.

charging because the electrons would populate increasingly higher electronic levels that are more closely spaced with respect to each other. Rather, we observe a shift of the band to higher energies upon an increasing amount of charging, consistent with a free-electron plasma, at least at the level of charging (4−5 electrons/NC) in the current study. It must however be acknowledged that there is likely a close connection between intraband single-carrier transitions and a collective LSPR mode; the former is expected to transition into the latter at a high-enough carrier density, which is probably related to the Mott density. This transition merits further experimental and theoretical investigation. In small ZnO nanocrystals, even one electron is equivalent to a carrier density much higher than the value of 6 × 1018 cm−3, the Mott density in ZnO.38 Above this carrier density, the screening of electron−hole attraction would not allow regular hole-bound one-electron states (such as 1Se, 1Pe, etc.), as also evidenced by the bleach of the excitonic absorption band. Rather, a degenerate electron plasma is expected to be sustained within the lowest conduction band states. For degenerately doped semiconductors (carrier densities above the Mott transition), exotic states, known as Mahan or continuum excitons, have been predicted, which involve the interaction of an electron−hole pair with the free-electron plasma. We propose the existence of Mahan excitons in our charged nanoparticles.39,40 One signature of such an exciton would be the strong attraction of a photogenerated hole to the free-electron plasma. In this state, there would be significant energy stabilization of the hole due to strong solvation by the electron plasma (see Figure 4). In such a scenario, the rate of hole trapping is expected to be significantly reduced or completely shut off. In fact, we observed, consistent with some past observations,15,16,41 that in the charged state, the green defect PL from our ZnO nanoparticles is completely quenched (Figure 4), which indicates the shutting off of hole trapping. Others have suggested that the green PL quenching is the result of fast Auger recombination;42 however, the latter mechanism alone fails to explain the concomitant enhancement in the band edge PL observed here (Figure 4) and by others.34,43 Similarly, increased band edge PL lifetimes have been measured for charged ZnO.15,34 Stabilization of the hole in the Mahan excitonic state explains not only the reduced degree of hole

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of ZnO nancrystals, absorption and PL spectra measurement methods, reduction sensing measurements, and transmission electron micrographs. This material is available free of charge via the Internet at http://pubs.acs.org. 3028



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Jeremy Smith for assistance with TEM images and Will Shaw for helpful discussions. J.F. acknowledges support from a National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144245. P.K.J. acknowledges support from the DuPont Young Professor Award and the Frederick Seitz Material Research Lab.

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Plasmons in Photocharged ZnO Nanocrystals Revealing the Nature of

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