Hot Electron Generation and Cathodoluminescence Nanoscopy of

Jul 27, 2016 - Energy–Momentum Cathodoluminescence Spectroscopy of Dielectric Nanostructures. Sandro Mignuzzi , Mónica Mota , Toon Coenen , Yi Li ,...
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Letter pubs.acs.org/NanoLett

Hot Electron Generation and Cathodoluminescence Nanoscopy of Chiral Split Ring Resonators Yurui Fang,*,† Ruggero Verre,† Lei Shao,† Peter Nordlander,‡ and Mikael Kal̈ l*,† †

Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden Department of Physics and Astronomy, Rice University, 77005 Houston, United States

‡

S Supporting Information *

ABSTRACT: Three-dimensional chiral plasmonic nanostructures have been shown to be able to dramatically boost photon-spin selective light-matter interactions, potentially leading to novel photonics, molecular spectroscopy, and light-harvesting applications based on circularly polarized light. Here, we show that chiral split-ring gold nanoresonators interfaced to a wide band gap semiconductor exhibit a contrast in hot-electron transfer rate between left-handed and right-handed visible light that essentially mimics the far-field circular dichroism of the structures. We trace down the origin of this effect to the differential excitation of the thinnest part of the split-ring structures using dichroic-sensitive cathodoluminescence imaging with nanometer spatial resolution. The results highlight the intricate interplay between the near-field and farfield chiral response of a nanostructure and establishes a clear link to the emerging field of hot carrier plasmonics with numerous potential applications in photocatalysis and solar light harvesting. KEYWORDS: Chiral nanostructures, hot electron generation, surface plasmons, dichroic-sensitive cathodoluminescence, circular dichroism

S

experiments on chiral plasmonic structures have concerned farfield optical properties, like CD, and there are so far very few studies that have experimentally probed differences in optical near-fields between LCP and RCP excitation.13−15 This is despite that many of the potentially most powerful applications of plasmonics are based on near-field effects, including surfaceenhanced spectroscopies, second harmonic generation, and plasmonic molecular sensing.9,11,16−19 Indeed, much of the recent interest in chiral plasmonics stems from the still open question of using so-called superchiral near-fields for selectively probing molecular enantiomers. Plasmon-induced hot electron generation in metal nanostructures is intimately related to nearfield enhancement and has recently generated significant interest in relation to potential applications in photocatalysis, photovoltaics, and photodetection.20−25 The basic idea here is that electrons (or holes) resulting from plasmon decay can have sufficiently high energy to overcome the potential barrier formed at the interface between the metal nanostructure and a semiconductor, oxide, or molecule. The resulting electron−hole separation can then be used to drive a current or a chemical reaction. Differences in near-field responses between LCP and RCP excitation of a chiral plasmonic nanostructure can thus be expected to result in a circularly dichroic hot electron generation. Such an effect was recently observed for a quasi2D Ag-metamaterial/mirror geometry interfaced to Si and used

tructural chirality is ubiquitous in nature and involves objects ranging in size from simple organic compounds to macroscopic structures, like our hands. Many biomolecules, including essential amino acids, nucleic acids and proteins are chiral.1,2 Moreover, biological function is sometimes crucially dependent on which handedness (enantiomer) of a chiral molecule that is involved in a reaction.3 The analysis of chirality is therefore important in fields such as biochemistry, medical diagnostics, and drug development.4 The essential method for analyzing molecular chirality is circular dichroism (CD) spectroscopy, which utilizes that chiral molecules exhibit weak optical activity and small differences in absorption between lefthanded and right-handed circularly polarized light (LCP and RCP, respectively).2 Structural chirality is also the basis for many optical elements, like calcite waveplates and liquid crystal retarders, used for generating and detecting circularly polarized light. Such elements are of increasing importance in photonics, for example, in display technology, and key to the possible future realization of quantum optics applications based on photon spin.5 Over the last several years, a large research effort has been devoted to developing chiral metal nanostructures and metamaterials as a route toward controlling circular polarization and enhancing chiral light-matter interaction6,7 at deep subwavelength scales and as a basis for novel optically active elements for photonics applications. The investigated structures range from simple spherical nanoparticles assembled on chiral molecular templates, like DNA, to elaborate 3D structures manufactured through advanced lithography.8−12 Almost all © XXXX American Chemical Society

Received: May 28, 2016 Revised: July 22, 2016

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Figure 1. Chiral Au SRRs on TiO2 fabricated by hole-mask colloidal lithography. (a) Schematics of the HCL method used. Polystyrene (PS) beads are self-assembled on a resist/TiO2/ITO glass substrate. Au deposition followed by tape stripping and oxygen plasma etching results in a hole mask with a large undercut. Chiral SRRs are formed through a second Au deposition at an appropriate tilt angle while the substrate is rotating. (b) AFM image demonstrating the 3D chiral character of the SRRs. (c,d) SEM images of SRRs of opposite handedness. The right (left) parts of the images show the samples before (after) the annealing step used to create an efficient Au−TiO2 Schottky barrier.

an overview of the fabrication process (details can be found in Methods). As can be seen from the atomic force microscope image in Figure 1b, the SRR structures exhibit a broad and relatively thick “head” and a narrow and thin “tail”, resulting in a clear 3D chiral topography. The 3D character is important because it implies that space and time inversion symmetry is broken along the light propagation direction,33,34 which is a prerequisite for an intrinsic rather than an extrinsic chiral response.19,26 By changing the sample rotation direction during evaporation, both right-handed (RH) and left-handed (LH) structures can be produced, as shown in the scanning electron microscope (SEM) images in Figure 1c,d. The sample substrates used were either indium tin oxide (ITO) coated glass covered by a thin (∼130 nm) layer of sputter-deposited TiO2, used for photoconductivity experiments, or Si wafers. The latter was used for CL measurements to avoid charging effects. In order to form a good Au/TiO2 contact and Schottky barrier, which is necessary for hot electron injection, the samples were annealed before photoconductivity experiments. This resulted in an unfortunate reshaping and increased shape dispersion, as can be seen by comparing the left and right parts of Figure 1c,d. However, the annealed structures still displayed a clear chiral response. From SEM analysis, we obtained that the average outer side diameter of the annealed SRRs is about 180 nm, the thicker lobe is ∼65 nm wide and ∼35 nm thick, whereas the thinner lobe is about ∼20 nm wide and ∼7 nm thick. The samples were first characterized through standard far field transmission (T) spectroscopy in the visible range. Figure 2a shows the extinction (E = 1 − T) for right-handed (RH) and

to differentially detect infrared light with impressive RCP/LCP contrast.23 However, to the best of our knowledge, there are so far no reports on hot electron generation or subwavelength imaging of constitutionally chiral nanoparticles, that is, structures that possess true 3D chirality similar to the chirality centers of molecules. In this paper, following our previous work on plasmonic chiral and hot electron structures26,27 we report on the CD and dichroic photoconductivity of meta-surfaces composed of nanoscopic 3D chiral split ring resonators (SRRs). We find that the incident photon-to-charge conversion efficiency (IPCE) difference between RCP and LCP illumination in the visible is in remarkable agreement with standard CD measurements based on extinction measurements. Moreover, by using spectroscopic cathodoluminescence (CL) polarimetry we show that it is possible to image the chiral response of individual SRRs with less than 10 nm spatial resolution. Finally, we establish a clear link between hot electron generation efficiency and CL through near-field electrodynamics simulations.

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RESULTS

A wide range of SRR structures have been described in the literature, ranging from the classical double SRR developed for negative index THz metamaterials28 to simplified horseshoe or crescent-like shapes with stronger response in the visible wavelength range.29,30 We fabricated chiral SRRs of the latter type using a variant of hole-mask colloidal lithography (HCL)31,32 similar to the one described by Frank et al.18 HCL is a scalable cost-effective method for fabricating shortrange ordered nanostructures over several cm2. Figure 1a gives B

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compensated by a negative CD peak in another interval and vice versa. In the present case, the CD response in the visible is mostly compensated by opposing spectral changes in the nearinfrared (ref 18 and Figure S1). We used the same samples as in Figure 2 to demonstrate dichroic hot-electron transfer to TiO2. The measurements were done using an electrochemical set up similar to the one described in ref 27. More details on the measurement procedures can be found in Methods and the Supporting Information (Figure S2). We expect that the different coupling efficiencies between RCP or LCP light and the SRRs manifested in the CD spectra leads to corresponding differences in optical near fields and degree of plasmon excitation. As schematically illustrated in Figure 3a, subsequent

Figure 2. Far-field circular dichroism of chiral split-ring resonators. (a) Ensemble-averaged extinction spectra of RH and LH chiral SRRs on TiO2 in water for RCP and LCP illumination. (b) Difference in extinction between LCP and RCP light for RH (red) and LH (blue) SRRs. (c) Extinction difference in terms of circular dichroism (CD) angle for RH and LH SRRs.

left-handed (LH) samples measured with RCP and LCP light, while Figure 2b shows the respective difference (ERCP − ELCP), which defines the circular dichroism of the structures. For completeness, we also plot the CD angle θ obtained from tan θ = ( TRCP − TLCP )/( TRCP + TLCP ) in Figure 2c. The extinction spectra contain at least three overlapping plasmon resonances but they are difficult to resolve due to inhomogeneous broadening effects (c.f. shape dispersion in Figure 1c,d). Electrodynamics simulations (Figure S1 in the Supporting Information (SI)) reveals several more modes in the near-infrared, as expected from the results reported in ref 18. In a simplified picture, these SRR resonances can be thought of as multipolar surface plasmons similar to those of a Au nanowire35 but with considerable plasmon shifts and interferences due to the varying width, height, and curvature of the SRRs (see ref 18 for a thorough discussion on mode assignment). Simulated surface charge distributions (Figure S1) and CL images, described below, confirm the multipolar character of the modes. As expected for a 3D chiral system, the CD plots of the RH and LH samples are essentially mirror images of each other (the CD amplitude difference is likely due to slight variation in SRR morphology and surface density between the RH and LH samples). The dominant CD response at around 750 nm is mostly induced by a difference in amplitude and position of the modes at ∼660 nm and ∼850 nm between RCP and LCP excitation. We verified that the CD response was independent of sample orientation and confirmed the overall reproducibility of the results using several samples prepared using the same parameters as the ones in Figure 2. One may note that the fsum rule for extinction dictates that the wavelength integrated extinction must be independent of excitation polarization.36 Hence, a positive CD peak in one wavelength interval must be

Figure 3. Dichroic hot electron transfer from chiral gold SRRs to TiO2. (a) Schematic illustration of the hot electron transfer process. Right-handed or left-handed circularly polarized light (RCP or LCP) with photon energy below the TiO2 band gap (EG = EC − EV ≈ 3.2 eV) excite plasmons in the chiral SRRs with different efficiency. (b) Incident photon to charge conversion efficiency (IPCE) spectra for RH and LH samples under LCP and RCP illumination. (c) IPCE response difference between RCP and LCP light (ΔIPCE, squares with error bars) compared to the circular dichroism (ΔExtinction, full lines) measured for the same samples.

plasmon decay generate hot electrons that may transfer to the semiconductor conduction band (EC) given that their energy relative to the metal Fermi level (EF) is higher than the Schottky barrier (ϕS) and given that they have not yet thermalized. The photogenerated holes lead to water splitting at the metal/water interface, which closes the current loop (Figure 3a). The internal photoemission process is quantified through the incident photon-to-charge conversion efficiency, IPCE = ne /N, where ne is the number of collected photoelectrons and N is the number of incident photons. The IPCE can be expressed as IPCE[%] = C

Isc(A) P(W )

×

1240 λ(nm)

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Figure 4. Chiral response of SRR’s as seen through cathodoluminescence spectroscopy and imaging. (a) Schematics of the measurement configuration: the SRR is excited by a 30 keV electron beam and the emitted cathodoluminescence is passed through a quarter wave plate (QWP) and a polarizer aligned to transmit either LCP or RCP light and fiber-coupled to a spectrometer. (b) LCP and RCP CL spectra of a representative LH SRR obtained by integrating the signal over the whole structure. (c) CL excitation maps of LCP and RCP emission at λ = 820 ± 10 nm (marked by vertical lines in (b) for the LH structure. The spatial resolution of the system is of the order of a few nm. (d,e) Same as above but for a representative RH structure. SEM images of the corresponding particles are shown as insets in (a) with scale bars = 100 nm. (f) ΔCL extracted from (b,d).

induced current, P is the incident light power, and λ is the light wavelength. In Figure 3b, we plot the action spectra (IPCE vs wavelength) for the RH and LH samples and the two different polarizations (see Figure S3 in the Supporting Information for corresponding relative changes expressed through the chirooptical asymmetry factors gCD and gIPCE). Although the broad IPCE peaks roughly match the peaks in the corresponding extinction spectra, the overall spectral shape is quite different with a gradually increasing efficiency toward shorter wavelengths.27 The decrease in the IPCE at around 550 nm is due to the onset of interband transitions that weakens the plasmon induced fields. However, as can be seen in Figure 3c, the IPCE dichroism, defined as ΔIPCE = IPCERCP − IPCELCP, shows a very similar variation to the far-field CD response. The gradual increase in IPCE toward higher photon energies is qualitatively similar to what has been observed previously for nonchiral Au particles on TiO2 and indicate an underlying variation in internal quantum efficiency (IQE) with incident wavelength similar to what can be expected from the semiempirical Fowler rule.27 The far-field and hot electron chiral response described above ultimately results from differences in the optical nearfields generated by LCP and RCP light in an SRR with a specific handedness. We used a spectroscopic CL polarimeter interfaced to a scanning electron microscope (SEM) in order to

further elucidate this process. The electron beam is raster scanned across selected SRRs fabricated on Si and the emitted visible radiation generated at each position is collected with a parabolic mirror coupled to a spectrometer. To achieve polarization contrast, a circular analyzer composed of a quarter wave-plate and a linear polarizer were inserted in the light path before the spectrometer, as sketched in Figure 4a. By rotating the quarter wave-plate by ±45° with respect to the linear polarizers main axis, one can selectively measure RCP or LCP emission.37 The instrument uses 30 keV high-energy electrons as the excitation source, resulting in CL maps with a few nanometers resolution. Electrodynamics simulations showed that changing from TiO2 to Si substrates has minor influence on the optical response of the SRRs, the main effect being small peak shifts due to the higher refractive index of Si (see Figure S4 in the SI). In Figure 4b, we show CL spectra obtained by integrating the signal over an entire LH SRR. Clear differences between RCP and LCP emission can be seen and are in good overall agreement with the far-field measurements. However, as the CL spectra refer to an individual nanostructure rather than an ensemble, the multiple resonances corresponding to different plasmon modes are now clearly visible. The CL dichroism is particularly large at ∼820 nm, where LCP emission is about twice as strong as RCP emission from the nanostructure. The D

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overcome the Schottky barrier (ϕS ≈ 0.9 eV = 1400 nm for Au/ TiO227) corresponding to the smallest photon energy required to promote an electron from the metal to the semiconductor, which means that hot electrons resulting from transitions from deep levels (such as interband transitions) will be filtered out. Recently, it was shown that the probability for hot electron generation and transfer across the Schottky barrier P(λ) for Au nanoantennas on a TiO2 substrates could be modeled as38

corresponding CL excitation map, integrated within a 20 nm spectral window around 820 nm, indeed show clear differences: three distinctly confined “hot-spots” are observed for LCP emission but only two for RCP (Figure 4c, Figure S5 in the SI display CL maps for several other wavelengths). This behavior is essentially reversed for the RH structure (Figure 4d,e). Note in particular that the apex “hot spot” seen for LCP emission from the LH structure instead appears for RCP emission in the case of the RH structure because of the interference of the incident and induced electric fields along the surface of the SRR. The “chirality” of the CL response can be quantified in a similar manner as the ΔExtinction and ΔIPCE by defining a metric ΔCL = CLRCP − CLLCP. This quantity is shown in Figure 4f and is found to exhibit qualitatively similar wavelength dependence as ΔExtinction and ΔIPCE, that is, the LH (RH) difference spectra all exhibit a deep dip (peak) in the ∼800 nm region followed by smaller peaks (dips) and dips (peaks) toward shorter wavelengths (Figure S6 in the SI).

P(λ) ∝

1 VMFP

∫V

dVpG (r, λ) ∼

MFP

1 VMFP

∫V

dV |E(r, λ)|2

MFP

(1)

The MFP ≈ 40 nm for Au, which is larger than the thickest part of the SRR structures (t ≈ 35 nm). This allows us to define a spatially resolved local hot electron generation map showing the locations (x,y) on an SRR where the hot electron production is large according to 39

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PG(x , y , λ) ∝

DISCUSSION The results above show that far-field extinction, hot electron transfer, and CL from chiral split ring resonators exhibit consistent differences between left-handed and right-handed circular polarized visible light. Specifically, the LH SRRs shows higher extinction, higher IPCE, and higher CL emission for LCP than for RCP light in a rather wide spectral region in the red, while the RH SRRs shows the reverse behavior. CL maps with deep subwavelength resolution pinpoint the origin of this effect to specific subregions of the SRRs in particular the thin and narrow SRR apex. An overall agreement between the three types of chirality measurements can be expected on general grounds since they all involve the radiative plasmon modes supported by an SRR. However, the three different experiments clearly probe these modes in very different manners. The far-field chiral response of an SRR is determined by the difference in extinction cross sections, which includes contributions from both absorption and scattering (σext = σabs + σscat), between LCP and RCP plane waves E0 incident from the far-field normal to the sample plane (z-axis). The extinction is connected to the local field E(r) (and field-enhancement factors) inside an SRR with volume V via the optical theorem (σext(λ) ∝ |E0|−2 ∫ dV Im[E*0 ·(α(λ)E(r, λ))], where α is the V polarizability of a volume element), but the extinction can also be obtained by explicitly adding the integrated scattered far field electromagnetic energy to the absorbed energy ″ (λ)|E(r, λ))|2 , where εAu ∝|E0|−2 ∫ dVεAu ″ is the imaginary part V of the dielectric function of gold. We use the latter method to simulate CD spectra below. The phenomenon of plasmon induced hot carrier transfer to semiconductors is incompletely understood in details, though a consistent qualitative picture has begun to emerge.22,24,25,38 The probability for local hot electron generation pG(r, λ) at a position r inside the nanostructure can be calculated using Fermi’s golden rule with a transition matrix element that depends on the local electric field intensity |E(r, λ)|2. However, in order to transfer the hot electrons to the semiconductor, they have to be generated within a distance less than the mean free path (MFP) of the Au/TiO2 interface, meaning that the relevant volume for hot carrier generation is a reduced volume VMFP < V. Furthermore, they need to be energetic enough to

∫ dz |E(x , y , λ)|2

(2)

The current understanding of the CL response is that it is proportional to the radiative local photon density of states (rLDOS) projected along the beam path through the sample.40−42 The incident high energy electrons carry a localized broad-band electric field polarized primarily in the direction of the electron trajectory (z-direction in the present case). This field can excite the plasmon modes of the irradiated nanostructure, which in turn can emit CL to the far-field. The magnitude of the emission depend on the radiance of the mode and its spatial and polarization overlap with the excitation field. The CL emission process can then be simulated by placing unit z-polarized dipole sources at a given impact parameter (x,y) and calculating the resulting far-field emission into a specific direction and polarization state (corresponding to a differential rLDOS). Equivalently, by virtue of electromagnetic reciprocity, the response can be calculated by propagating a unit wave from the far field and integrating the z-component of the induced local electric field Eind z intensity along the electron path at (x,y) according to43,44 ΓCL(x , y , λ) ∝

2 ∫ dz |Eind z (x , y , λ )|

(3)

In Figure 5a−c, we compare the calculated extinction spectra for LCP and RCP excitation of a model LH SRR with the calculated scattering cross section and hot electron generation efficiency spectra calculated using eq 1 (with VMFP = V). The mode structures in the spectra are obviously very similar, although the amplitudes of the different resonances vary quite a lot between the three types of calculations. However, in all cases we see that the largest chiral response (difference between RCP and LCP excitation) occurs in the red part of the spectrum, which is in agreement with the experiments. In Figure 5d−g, we show spatial plots of PG(x, y, λ) and ΓCL(x, y, λ) according to eqs 2 and 3, respectively, at λ = 850 nm, where the calculated chiral response is the largest. The plots for the respective light handedness obviously correlate well to each other. The reasons for this is that the two plots involve the same plasmon modes: the total field in eq 2 is dominated by the plasmon induced local field Eind, since E = Eind + E0 ≈ Eind near resonance, and the localized fields are strongly inclined toward the high index TiO2 substrate (z-direction). More importantly, the plots are also in excellent agreement with the experimental CL maps shown in Figure 4. In particular, one sees that the E

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that varies with wavelength and spatially across a nanostructure needs to be taken into account in all experiments that attempts to use chiral nanoplasmonic structures for enantioselective molecular spectroscopy, synthesis and catalysis.

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METHODS Sample Fabrication. Sample substrates were made by sputtering 130 nm thin TiO2 films on ITO glass coated with a 1 nm Cr adhesion layer. The HCL procedure used to fabricate the SRR structures involves (a) spin coating a PMMA layer on the TiO2 film; (b) deposition of 100 nm diameter polystyrene (PS) beads; (c) evaporation of a 10 nm Au film; (d) tape stripping away the PS beads; (e) etching the exposed PMMA in an oxygen plasma (50 W, 7 min, 250 mTorr, O2 10 sccm); (f) evaporation of 120 nm Au (evaporation rate 2 Å/s) at a substrate tilt angle of 15° while rotating the substrate at a constant speed (0.13 rpm) over a total rotation angle of 270°; (g) annealing the sample in cycles with intervening cooling steps (200 °C/10 min, 200 °C/30 min, 250 °C/2 min × 5) to induce a good Au/TiO2 contact while decreasing particle reshaping effects. Optical Measurement. Transmission spectra were recorded with the samples in PBS buffer using a simple spectrophotometry setup using ITO glass as a reference and the same illumination system as used for the IPCE measurements. IPCE Measurement. White light from a fiber coupled laser driven light source (LDLS, Energetic, EQ-99FC LDLS System) was collimated to a beam with about 6 mm diameter. The light was passed through an acousto-optical tunable filter (AOTF, Brimrose, two channels, 400−1000 nm) with 10 nm bandwidth and a quarter wave Fresnel rhomb retarder (Thorlabs FR600QM) before hitting the sample. The custom-made flow cell includes a Pt wire as counter electrode and a chloridized Ag wire as reference electrode. The front side is sealed with a cover glass and the plasmonic nanostructure acts as the working electrode. The electrolyte is a standard PBS buffer (Sigma Aldrich). The flow cell is connected to a commercial potentiostat (Gamry Reference 600). The AOTF and the potentiostat are controlled by a Labview program. The AOTF was controlled to scan the wavelength from 500 to 1000 nm in 10 nm step. Measurements were performed using a small positive bias (0.3 V), which decreases recombination between the photogenerated electrons and positive ions at the PBS/ TiO2 interface. Cathodoluminescence. The chiral SRRs were fabricated by the same technique as described above but using Si substrate to avoid charging effects. Measurements were performed using a commercial CL system (SPARC, DELMIC, Holland) coupled to a SEM (FEI Quanta 300) operated at 30 kV in a low vacuum (80 Pa) environment of H2O. The emitted radiation is collected by an Al parabolic mirror and sent through a 2 in. quarter wave plate (WPQ20ME-780, Thorlabs) and a linear polarizer placed outside the SEM chamber. The light is then focused onto a 550 μm fiber connected to a grating spectrometer (Andor, U.K.). Prior to the measurements, the substrate was aligned with respect to the parabolic mirror using a charge-coupled device camera (iDus, Andor). Because of the low signal, an electron current of ∼6 nA was used during the measurements, 16 pixels of the camera were binned, and an integration time of 500 ms was used per measured pixel. The Si background was subtracted from the recorded CL raw spectra and the resulting spectra were normalized by a system response

Figure 5. Simulated extinction, hot electron generation efficiency, and CL for a model left-handed split ring resonator on TiO2. (a) Far-field extinction spectra for LCP and RCP excitation. (b) Hot electron generation efficiency calculated using eq 1 for LCP and RCP excitation and (c) scattering spectra for the same nanostructure. (d,e) Local hotelectron generation efficiency PG (eq 2) and (f,g) local CL efficiency ΓCL (eq 3 with z ∈ [−5 nm, +40 nm]) at 850 nm for LCP and RCP excitation. The simulated CL map is an average over 33 incidence directions within a cone of π sr. around the substrate normal. Note the presence of a hot spot at the SRR apex for LCP excitation, similar to the experimental CL map.

presence of a “hot spot” at the thin apex region of the structure for LCP but not for RCP excitation is the main origin behind the spectral differences seen in Figure 5a−c (see also Figure S7 in the SI, where we show separate spectra according to eq 1 for the apex and base regions of the SRR). Because the apex region is also the thinnest part of the structure, we expect that hot electrons generated there have a high probability of reaching the semiconductor even if they are reflected from the bare Au interface. Figure 5b shows a significant dichroic response also at shorter wavelengths, but this feature originates from field enhancement in the thick base of the structure, which may explain why a similar feature is only weakly seen the ΔIPCE curves. The agreement between the spatial distribution of the calculated ΓCL and PG in Figure 5 combined with the general agreement between the experimentally measured ΔCL and ΔIPCE suggests that CL imaging in principle could be used to identify which regions of a nanostructure provide the largest hot carrier generation for different wavelengths. In conclusion, we have used a robust and cost-effective approach for large area nanofabrication to produce chiral threedimensional split-ring nanoresonators on wide band gap semiconductor substrates. Their chirality is investigated using far-field circular dichroism spectroscopy and found to be in good agreement with theoretical simulations. We study plasmon-induced hot electron generation in these structures and show that the hot carrier generation exhibits similar polarization and wavelength dependence as the far-field CD response, thus reflecting the polarization sensitive light harvesting properties of chiral nanostructures. Using dichroicsensitive cathodoluminescence spectroscopy and imaging, we show that their chiral response is caused by the polarization dependence of the local field hot-spot distributions associated with the plasmon resonances. Furthermore, we show that the spatial distributions of the local CL efficiency is very similar to the spatial distribution of the calculated local plasmon-induced hot carrier distribution, suggesting that CL imaging could be a highly useful tool for the rational design of efficient plasmonic photocatalysts or photovoltaics based on hot carrier generation. The possibility of a dichroic hot electron generation efficiency F

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function obtained by measuring the emission of a calibrated light source. To ensure minimal distortion of the polarization due to the presence of the parabolic mirror,37 the sample was mounted as indicated in the inset of Figure 4a. For this orientation, most of the radiation from the nanostructure was emitted toward the regions of largest curvature of the parabolic mirror, thus preserving the handedness of the emitted radiation.37 Electrodynamics Simulations. Numerical simulations of optical properties were performed using a commercially available implementation of the finite element method (FEM, COMSOL Multiphysics 5.0) and tabulated values for the dielectric functions. The model structures were constructed according to the SEM and AFM images of annealed samples on TiO2 or Si (including a 3 nm SiO2 layer) in water or in vacuum, respectively. The structures were excited by RCP or LCP plane waves entering from the water (air) side. Scattering and absorption spectra were obtained by integrating the far-field scattering over all angles and by integrating the dissipated energy inside the particle, respectively. The simulated CL maps in Figure 5f,g were obtained using a z-integration spanning the range from 5 nm below to 5 nm above the thickest part of the structure (z ∈ [−5 nm, +40 nm]) and by averaging maps obtained for 33 different incidence directions (0, 15, 30, and 60° compared to the substrate normal and 8 equally spaced azimuthal angles) in order to simulate the solid angle subtended by the collection optics of the experimental CL setup. All other calculations were made for normal incidence.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b02154. Simulated extinction spectra for LCP and RCP excitation covering the full plasmonic resonance spectral region of a LH SRR (Figure S1). Schematics of the photocurrent measurement setup (Figure S2). Chiroptical asymmetry spectra for extinction and IPCE (Figure S3). Calculated scattering spectra and near-field response for a LH SRR on TiO2 and Si (Figure S4). CL maps at multiple wavelengths (Figure S5). ΔIPCE and ΔCL for a LH structure (Figure S6). LCP and RCP hot electron generation efficiency P according to eq 1 at 850 nm for the apex and base regions of an SRR (Figure S7) (PDF) CL map video at multiple wavelengths (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Yurui Fang). *E-mail: [email protected] (Mikael Käll). Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

This work was supported by the Knut and Alice Wallenberg Foundation. P.N. acknowledges support from the Robert A. Welch Foundation under Grant C-1222. R.V. would like to thank T. Coenen and DELMIC for advice on CL measurements. G

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DOI: 10.1021/acs.nanolett.6b02154 Nano Lett. XXXX, XXX, XXX−XXX