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Plasmon Induced Energy Transfer: When the Game is worth the Candle. Pavel Moroz, Natalia Razgoniaeva, Abigail Vore, Holly Eckard, Natalia Kholmicheva, Ariana McDarby, Anton O. Razgoniaev, Alexis D Ostrowski, Dmitriy Khon, and Mikhail Zamkov ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00527 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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Plasmon Induced Energy Transfer: When the Game is worth the Candle.

Pavel Moroz1,2, Natalia Razgoniaeva1,2, Abigail Vore3, Holly Eckard3, Natalia Kholmicheva1,2, Ariana McDarby4, Anton O. Razgoniaev1,3, Alexis D. Ostrowski1,3, Dmitriy Khon4, Mikhail Zamkov1,2*.

The Center for Photochemical Sciences1, Department of Physics2 and Department of Chemistry3 Bowling Green State University, Bowling Green, Ohio 43403. Department of Chemistry and Biochemistry4, St. Mary’s University, San Antonio, Texas, 78228.

* [email protected]

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required)

Abstract. The superior optical extinction characteristics of noble metal nanoparticles have long been considered for enhancing the solar energy absorption in light-harvesting devices. The energy captured through a plasmon resonance mechanism can potentially be transferred to a surrounding semiconductor matrix in form of excitons or charge carriers offering a promising light-sensitization strategy. Of a particular interest is the plasmon near-field energy conversion, which is predicted to yield substantial


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gains in the photocarrier generation. Such short-range interaction, however, is often inhibited by processes of backward electron and energy transfer, which obscure its net benefit. Here, we employ the sample-transmitted excitation photoluminescence (STEP) spectroscopy to determine the quantum efficiency for the plasmon induced energy transfer (ET) in assemblies of Au nanoparticles and CdSe nanocrystals. The present technique distinguishes the Au-to-CdSe ET contribution from metal-induced quenching processes thus enabling accurate estimates of the photon-to-exciton conversion efficiency. We show that in the case of 9.1-nm Au nanoparticles, only 1-2% of the Au absorbed radiation is converted to excitons in the surrounding CdSe nanocrystal matrix. For larger, 21.0-nm Au, the photonto-exciton conversion efficiency increases to 29.5%. The results of present measurements were used to develop an empirical model for estimating the maximum gain in plasmon-induced carriers versus the mass-fraction of Au in a film. KEYWORDS: Energy transfer, nanocrystals, FRET, crosstalk, bleed through, plasmonics.

Plasmonic nanostructures have long been considered as a possible solution to overcome the limited light absorption in thin-film solar cells.1-11 Their enhanced optical extinction arises from surface plasmon resonances of free electrons, which capture the incident radiation from the area that is considerably bigger than the particle size. As a result, the volume-adjusted absorption of the solar energy by noble metal nanoparticles surpasses those of semiconductor nanocrystals or organic sensitizers by several orders of magnitude (Fig. SF3). The energy absorbed by plasmonic nanostructures can then be transferred to a surrounding semiconductor matrix through two primary mechanisms: (i) far field scattering,12-15 and (ii) - near-field charge or resonant energy transfer.16-18 The first mechanism represents a simple geometric scattering of radiation, which increases the effective path of light within a semiconductor film through a waveguide effect.19-20 The second mechanism relies on a locally enhanced


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electromagnetic field to transfer the metal-absorbed energy via the plasmon-to-electron21-24 or plasmonto-exciton25-28 conversion processes. The difficulty in the experimental realization of the latter process lies in harvesting the near field of metal nanoparticles due to its evanescent decay character. Even small increases in the metal-semiconductor distance or a nanoparticle morphology result in significant changes in the energy transfer rate. Multiple studies have explored the possibility of harvesting the plasmon near-field energy through processes of a hot electron injection into the conduction band of a semiconductor21-24,29 or a resonant energy transfer (RET)25-27,30,31 in form of excitons. The latter mechanism has recently generated a considerable interest as it offers a zero-threshold energy exchange via a dipole-dipole coupling. In this case, the plasmon energy could be directly converted into electron-hole pairs near the semiconductor band edge. The photovoltaic benefit of RET, however, is lessened by the back flow of charges and energy to the metal surface (exciton quenching).32-34 This often leads to insignificant net increases or even damping of the photocurrent due to the presence of metal nanostructures in the film. While such back excitation flow may be manageable through the use of insulating spacers, its presence masks the forward resonant energy transfer effect, making it difficult to estimate the plasmon-related contribution into the photocurrent. Here, we report on the quantum efficiency measurements of the plasmon-to-exciton energy transfer in assemblies of CdSe semiconductor nanocrystals and Au nanoparticles (NPs). By using the Sample Transmitted Excitation Photoluminescence (STEP) spectroscopy,35 we were able to accurately determine the percentage of photons absorbed by metal nanostructures that ultimately lead to the excitation of electron-hole pairs in the semiconductor nanocrystal matrix. The reported method relies on spectral shaping of the excitation light with the solution of Au nanoparticles (or similar colloids exhibiting Au-like absorbance profile) that served the role of an excitation filter. Placing such filter


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solutions in front of the excitation beam allowed suppressing the excitation of Au nanostructures in the (Au, CdSe) sample, thus inhibiting the Au-to-CdSe energy transfer. The resulting reduction in the CdSe photoluminescence (PL) due to the absence of the near-field contribution from Au was then used to estimate the Au-to-CdSe energy transfer efficiency, EAu→CdSe. The unique advantage of this methodology in applications to studying plasmon resonances lies in its ability to exclude the metal-induced effects of backward charge transfer and photoluminescence quenching in semiconductor nanocrystals. As a result, the STEP efficiency, EAu→CdSe, represents the net gain in the exciton population, which is an important parameter for estimating the feasibility of plasmonic nanostructures in photovoltaic devices. In the present work, we explore two types of metal colloids featuring 9.1-nm Au and 21.0-nm Au NPs. The energy transfer assemblies were fabricated by incorporating metal nanoparticles within solid matrices of CdSe nanocrystals.

Figure 1. (a). Illustration of the STEP measurement strategy that employs a solution of donor nanoparticles in form of a filter to suppress the excitation of donor species in the (Au, CdSe) sample.


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The resulting modulation of the acceptor emission, fFL, with the increasing optical density of the excitation filter (plotted in the insert) is then analyzed using Eq. 2 to obtain the donor-acceptor energy transfer efficiency, ED→A. The emission of the acceptor was collected using a total internal reflection geometry in order to prevent the excitation light from entering the spectrometer. (b). Schematic illustration of the plasmon energy transfer scheme for the case of 9.1-nm Au/CdS nanoparticles within a CdSe/CdS nanocrystal matrix. (c). A dark field TEM image of Au/CdS core/shell and CdSe nanoparticles.

The present experiments demonstrate a significant difference in the amount of energy that can be harvested from small- and large-diameter Au NPs. We found that the plasmon-to-exciton energy transfer efficiency for small-diameter Au NPs (~ 9.1 nm) incorporated within CdSe nanocrystal matrices falls within a 1-2% range. Here, the observed EAu→CdSe efficiency values exemplify the maximum amount of the “transferable” energy that can be retrieved from the near-field of small-diameter Au nanostructures using a disordered (Au, CdSe) assembly. On the contrary, the quantum efficiency of the energy transfer from 21.0-nm Au to a CdSe NC matrix reached 29.5%. Owing to a larger size of Au in this case, the measured efficiency included both near- and far-field contributions into the Au-to-CdSe energy transfer. The present measurements were subsequently used to develop an empirical model for predicting the maximum allowable photovoltaic gain due to the plasmonic energy transfer as a function of Au fraction in the CdSe semiconductor film. We expect that the demonstrated STEP spectroscopy approach could lead to the development of similar predictive models for other absorber materials utilizing various metal/semiconductor combinations.


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The details of the STEP technique for measuring the energy transfer between donor and acceptor species have been described in a recent publication (see Ref. 35). This method is based on the general assumption that the number of photons emitted by an acceptor fluorophore, N APL , depends linearly on the number of excited acceptor (A) and donor (D) molecules, NA and ND, respectively:

N APL = QYA ( N A + ED→ A N D )

(1)

where E D → A is the quantum efficiency for the D→A energy transfer, and QYA is the emission quantum yield of the fluorophore A in the presence of the fluorophore D (as measured in the donor-acceptor assembly). The linearity of Eq. 1 was confirmed experimentally in this work using acceptor-only samples (see Figs. 2b, 3c) and is generally expected under low-power excitation conditions. To determine E D → A , a sample comprising a blend of donor and acceptor molecules was excited using a broad-band light source. The relative ratio of photons absorbed by each material was computed as the A B ratio of donor to acceptor absorption probabilities: N A0 N D0 = PAbs , where PAbs is derived from the PAbs

spectral profile of the excitation light, n(λ), and the donor-to-acceptor absorbance ratio (see Eq. SE4). Application of the donor-molecule solution as an excitation filter results in spectral shaping of the incident radiation causing a reduction in the amount of photons absorbed by donor molecules in a sample. The corresponding drop of the acceptor emission caused by the application of the donor-type excitation filter is then used to extract the energy transfer efficiency, ED→A. In the case, when only one type of donor molecules is present in the sample, ED→A can be obtained by calculating the ratio of the acceptor emission, N APL to the relative number of photons absorbed by the acceptor molecules in the sample, f = N APL N A . With the increasing optical density (OD) of the excitation filter, f value reaches an asymptotic value (Figs. 1a and SF2), revealing two experimentally measurable parameters: M1 and


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M2. As illustrated in Fig. SF2, the value of M1 is proportional to ED→A, while M2 = QYA. The donor-toacceptor energy transfer efficiency, ED→A, can then be determined as follows:

(

E D → A = (M 1 M 2 ) × N A0 N D0

)

(2)

where N A0 and N D0 are the relative numbers of excited acceptor and donor nanoparticles prior to the application of the excitation filter, given by the probability of the photon absorbance by each specie in the investigated sample (Eq. SE4). Application of the STEP approach to studying the plasmon-to-semiconductor energy transfer has an important advantage of excluding the contribution of back energy/charge transfer processes. Namely, the measured ED→A is equal to the percentage of photons absorbed by a plasmonic nanoparticle that ultimately induce band-gap excitons within the semiconductor domain. The backward decay of excitons due to the vicinity of metal surfaces is reflected by the reduced value of semiconductor emission quantum yield, QYA, which value cancels out of the Eq. 2 (see Fig. SF2). In the case when plasmonic and semiconductor nanoparticles are homogeneously blended into a film, the value ED→A gives the ratio of plasmon-induced excitons in all semiconductor nanocrystals to the number of photons absorbed by all plasmonic nanoparticles in a solid. According to model calculations, the donor filter composed of Au nanoparticle solution does not represent an ideal choice for suppressing the excitation of Au nanoparticles in a (Au, CdSe) film. As shown in Fig. SF4a, the ratio of excited donor to acceptor nanoparticles in the sample, f = N Au N CdSe does not drop quickly with increasing filter OD, requiring large OD values to reach an asymptotic behavior of the f-ratio. Furthermore, the emission of CdSe NCs becomes strongly damped with increasing Au filter OD (Fig. SF4b), which makes it difficult to measure the f-ratio without changing the intensity of the excitation light. To address these issues, we have employed a solution of Cy3.5 dyes in lieu of Au colloids to serve the role of the excitation filter. The absorbance profile of Cy3.5 is well


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match to that of a Au plasmon resonance (Fig. SF5b), such that the application of the Cy3.5 excitation filter allows reducing the ratio of Au to CdSe excitations in the sample more efficiently than in the case of Au filter solution (Fig. SF4a). Furthermore, with increasing filter OD, the emission of CdSe in the samples does not get quenched as strongly as in the case of Au filter solution (Fig. SF4b), which allows measuring the entire f-ratio OD-dependence using the same intensity of the excitation light. The confining geometry of metal nanoparticles plays an important role in the ensuing mechanism of the plasmon energy conversion. According to Mie theory,36 far-field scattering of light is prevalent in large-diameter NPs (d > 35 nm), which absorb less than 50% of the incident radiation. With the diminishing nanoparticle size, the light absorbance increases while scattering becomes suppressed. This situation is illustrated in Fig. SF8, showing that the ratio of scattering to absorption cross section in spherical Au NPs approaches zero when the particle size falls below 15 nm.37 In this regard, the two Au morphologies selected for the current study (9.1 and 21.0 nm in diameter) represent the two characteristic cases of absorption-only and absorption/scattering plasmon resonances, respectively. The diameter of CdSe NCs for harvesting the plasmon energy (d = 4.5 nm) was chosen to be sufficiently large to exhibit a red-shifted absorption edge (E(exciton) ≈ 2.0 eV) relative to plasmon resonance modes of Au nanoparticles (Efilm(plasmon) = 2.1-2.3 eV, depending on the surface capping material). Under these conditions, the plasmon-to-CdSe energy transfer is expected to be downhill and favorable. A thin shell of CdS was grown on the surface of CdSe NCs to provide a better passivation of dangling bonds. The total diameter of CdSe/CdS core/shell NCs (Figs. 2a and 2b) was estimated to be 6.2 nm. A relatively large size of the CdSe core domain was also preferred for inducing the localization of both carriers in the CdSe phase38 which facilitated decoupling of the semiconductor excited states from extending into Au. To further suppress the carrier exchange between metal and semiconductor surfaces, CdS passivation was applied to small-diameter Au NCs.38,39 A detailed strategy for overcoating Au NPs with a CdS shell through a non-epitaxial deposition method,40,41 is described in the experimental section. According to TEM images in Figs. 2c and 2d, a CdS layer grew evenly around the Au core


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averaging 2.5 nm in thickness. The diameter distribution for small-diameter Au/CdS colloidal samples was 9.1±1.1 nm (based on the size of the Au core). The growth of the CdS shell was accompanied by the red-shift of the plasmon resonance from λAu = 524 nm to λAu/CdS = 575 nm due to changes in the surrounding dielectric constant. The presence of the CdS shell on Au and CdSe NCs allowed placing metal and semiconductor domains within small distances of each other towards enhancing the energy transfer rate. Further reduction of interparticle gaps between Au and CdSe nanoparticles was accomplished by replacing the original surface ligands with short-chain oxalic acid (interparticle separation ≈ 0.45 nm).42

Figure 2. (a,b) TEM images of 6.2-nm CdSe/CdS core/shell nanocrystals. The average size of the CdSe core domain was estimated from the absorbance edge of CdSe colloids to be 4.5 nm. The thickness of the CdS shell is approximately 2 monolayers (0.8 nm). (c,d). TEM images of 9.1-nm Au/CdS core/shell nanoparticles featuring a 2.5-nm CdS shell. The standard deviation of Au core diameters was estimated


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at 9.1±1.1 nm (e-f). Scanning electron microscope images of a plasmonic solid featuring 9.1-nm Au/CdS and 6.2-nm CdSe/CdS nanocrystals at an approximate particle ratio of 1:60. (g). TEM images of 21.0nm Au nanocrystals. The standard deviation of Au core diameters was less than 12%.

A homogenous mixture of Au and CdSe NCs was prepared on a glass slide by spincoating from nonpolar solvents where both species exhibited high levels of solubility. Based on the scanning electron microscopy (SEM) analysis of (9.1-nm Au, CdSe) solids in Figs. 2e and 2f, we conclude that Au nanocrystals were uniformly dispersed in a semiconductor matrix, with no apparent sign of a metal/semiconductor phase separation. To perform energy transfer measurements, the glass slide was placed on top of a prism and illuminated from the back using a total internal reflection configuration. In this geometry, the excitation light was efficiently separated from the CdSe emission, as illustrated in Fig. 1a. The film photoluminescence was collected with a N.A. = 0.28 microscope objective fiber-coupled to a spectrometer (see Fig. SF1 for details of the experimental setup). In our first experiment, the Au-to-CdSe energy transfer efficiency was measured in solids of smalldiameter Au/CdS nanoparticles incorporated within CdSe nanocrystal matrices. The ratio of metal to semiconductor nanoparticles was estimated from respective extinction coefficients (6×107 for 9.1-nm Au and 5×105 for CdSe/CdS)43 to be 1:60 (by the particle count). It should be noted that the experimental STEP energy transfer efficiency, ED→A, is not affected by the nanoparticle ratio, as it expresses the number of Au-absorbed photons transferred to a semiconductor sub-lattice. It is important, however, that Au nanoparticles in a solid are fully incorporated into a semiconductor NC matrix for a maximum transfer rate, which stipulates that the Au:CdSe ratio is sufficiently low.


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Figure 3. STEP measurements of the plasmon-to-exciton energy transfer efficiency in (9.1-nm Au, CdSe) films. (a). Absorption profiles of Au/CdS (donor) and CdSe/CdS (acceptor) colloids along with the emission of CdSe/CdS (red curve). (b). Comparison of the CdSe PL intensity decay for CdSe-only and (9.1-nm Au, CdSe) films. The reduction of the PL lifetime in the latter case is attributed to metalinduced processes of the exciton dissociation and back charge transfer. (c). Evolution of the scaled CdSe emission (f-ratio) versus the optical density of the donor-type excitation filter (Cy3.5) is shown by blue circles. The experimental data for Au-CdSe assemblies is compared with two parametric model curves, corresponding to the EAu→CdSe = 1% and 2%. f-ratio measured for the acceptor-only control sample (CdSe NC film – red circles) reveals no dependence on the donor-filter optical density, consistent with the absence of the energy transfer in this case.

A control sample of CdSe-only NC films featuring no plasmonic contribution was measured in the PL N CdSe ratio first run to establish the experimental baseline. According to Fig. 3c, the resulting f = N CdSe

(red circles) revealed no dependence on the donor-filter (Cy3.5) optical density. The observed zero-


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slope trend is in apparent agreement with model calculations based on Eq. 1 for the no-ET scenario, shown as a straight red line in Fig. 3c. In the case of mixed (Au, CdSe) samples (blue circles), the observed f-ratio showed a slight deviation from the control sample, indicating a small energy transfer contribution into the CdSe emission. A model fit to the experimental fexp in Fig. 3c (blue curves corresponding to f = 1% and f = 2%) estimates such a contribution of plasmon resonances into CdSe exciton generation to be less than 2%. Indeed, the observed fexp trend indicates that when the ratio of Au to CdSe excitations in the sample is reduced by a factor of 10 corresponding to a filter OD of 4 (see Fig. SF4a, red curve), the scaled CdSe emission (f-ratio) drops by only 1-2%. This suggests that Au excitations do not contribute significantly into the CdSe exciton population. Additional evidence supporting this conclusion was provided by the STEP measurements of (9.0-nm Au, CdSe) solids featuring uncapped Au nanocrystals (see Fig. SF7a). These films were developed as a control sample for estimating the effect of the CdS capping on the surface of Au. According to Fig. SF7b, the measured energy transfer efficiency in this case was EAu→CdSe ≈ 1.1%, similar to the efficiency observed for a (Au/CdS + CdSe) system. We, theretofore, propose that the plasmon-to-exciton energy transfer in disordered assemblies of small-size Au nanoparticles (d = 9.0-9.1 nm) and semiconductor nanocrystals exhibits a relatively low efficiency (< 2%). One should not, however, exclude the possibility that more advanced photonic assemblies of aforementioned 9.1-nm Au and CdSe nanocrystals could potentially lead to higher EAu→CdSe.


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Figure 4. STEP measurements of the plasmon-to-exciton energy transfer efficiency in (21.0-nm Au, CdSe) films. (a). Absorption profiles of Au (Donor) and CdSe/CdS (Acceptor) colloids along with the emission of CdSe/CdS (grey line). (b). Comparison of the CdSe PL intensity decay for CdSe-only and (21.0-nm Au, CdSe) films. The reduction of the PL lifetime in the latter case is attributed to metal-


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induced processes of the exciton dissociation and back charge transfer. (c). Evolution of the scaled CdSe emission (f-ratio) versus the optical density of the donor-type excitation filter (Cy3.5) is shown by blue circles. The experimental data for Au-CdSe assemblies was fitted with a set of model parametric curves, indicating that EAu→CdSe = 29-30%. f-ratio measured for the acceptor-only control sample (CdSe NC film – red circles) is independent of the donor-filter optical density, consistent with the absence of the energy transfer in this case. (d). (21.0-nm Au, CdSe) nanoparticle blend in solution designed to assess the far field contribution of the plasmon scattering into the EAu→CdSe energy transfer. Evolution of the scaled CdSe emission (f-ratio) versus the optical density of the donor-type excitation filter (Cy3.5) is shown by blue circles. The experimental data for Au-CdSe assemblies was fitted with a set of model parametric curves, indicating that EAu→CdSe(solution) ≈ 4%.

An assembly of large-size ~ 21.0-nm Au NP (Fig. 2g) and CdSe NCs was investigated next. To compensate for the lack of ligand capping on large Au NPs, the surfaces of CdSe/CdS NCs were coated with longer-chain oleic acid ligands in lieu of OX ligands used for (9.1-nm Au, CdSe) assemblies. We estimate that the Au-to-CdSe gaps in the case of large-diameter Au assemblies were ~2 nm (based on the length of the OA carbon chain), which is about 1 nm shorter than in the case of small-diameter CdScapped Au assemblies and approximately the same as in the case of (9.0-nm Au, CdSe) solids featuring uncapped Au NPs (see Fig. SF7). According to the strategy used in the first experiment, we have started by measuring the f-ratio in a control sample containing only CdSe NCs. The resulting evolution of fCdSeonly

with the increasing donor-filter (Cy3.5) optical density exhibited an expected zero-slope trend (Fig.

4c) confirming the absence of the energy transfer in non-plasmonic films. Conversely, an assembly of 21.0-nm Au and CdS-capped CdSe nanoparticles showed a considerable decline of the f-ratio with increasing excitation filter OD. The comparison of the experimental data with model calculations (Eq. 1)


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was used to estimate the energy transfer efficiency to be 29.5%. Consequently, in the case of the largerdiameter Au nanostructures, 29-30% of photons absorbed/scattered by the plasmonic effect was transferred to the CdSe sub lattice in form of excitons. Two factors could potentially contribute to the increased energy transfer efficiency observed in the case of larger-dimeter Au. The first is the enhanced spatial extension of the plasmon near-field around larger Au nanoparticles. Such localized modes of the plasmon electromagnetic field exhibit an evanescent decay character with the e-r/R distance dependence, where R is roughly the particle radius. As a result, CdSe NCs located in the vicinity of large-diameter Au NPs could experience a greater amplitude of the electromagnetic field than in the case of smaller Au. The second factor is the increased far field scattering of light by larger Au NPs. It is expected that 21-nm Au spheres scatter up to 10% of the incident radiation (Fig. SF8), which is partly reabsorbed by the CdSe matrix, contributing to EAu→CdSe. The far field scattering portion of the energy transfer efficiency in (21-nm Au, CdSe) solids was estimated by running a control STEP measurement on solutions of 21-nm Au nanoparticles and CdSe/CdS NCs. At low concentrations, the Au-CdSe interparticle distances are too large to engage the near-field plasmon energy transfer, such that only the far field contribution into ED→A is measured. Furthermore, if light scattering by metal nanoparticles in solution occurs through the same optical density of CdSe/CdS colloids as in the case of (Au, CdSe) solid samples, a valid comparison of the far field scattering contribution into EAu→CdSe can be obtained. The results of STEP measurements for the solution mixture of 21-nm Au and 6.5-nm CdSe/CdS NCs (≈ 1:60, by the particle count) are summarized in Fig. 4d. For a (Au, CdSe) solution exhibiting 2.5 times the optical density of the (21-nm Au, CdSe) film, the energy transfer efficiency was estimated at 4% (Fig. 4d). However, a lower concentration of the (21-nm Au, CdSe) solution designed to match the optical density of the film


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resulted in the f-ratio that was not substantially different from the baseline measurement (red curricles), indicating a vanishing contribution of the far field scattering into the total energy transfer. Based on these results, we conclude that the far field scattering contribution into the energy transfer efficiency for large-diameter Au, CdSe solids is inferior to that of a near field. To address the possible correlation between the spatial dimensions of the near-field enhancement region in Au nanoparticles and EAu→CdSe, we have calculated the electric field intensity around 9.1-nm and 21-nm Au nanoparticles (Fig. 5b). To this end, a classical approach utilizing the T matrix linking of the outgoing and incident fields was employed.44 The resulting field enhancement, log10(|E/E0|2), was graphically superimposed onto a matrix of CdSe/CdS NCs placed at minimum expected distances from the surfaces of metal NPs, as shown in Fig. 5b. The qualitative comparison of the electric field ranges in the two cases confirms our initial premise that larger Au nanoparticles induce a stronger electric field in neighboring CdSe/CdS. This effect can be quantified by integrating the E-filed enhancement around the spherical shell with the radius of RET, as shown in Fig. 5b. This approximate approach estimates the Efield in larger Au to be 6.3 times greater than in the case of 9.1-nm Au, which is consistent with the observed differences in EAu→CdSe between the two samples.


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Figure 5. (a). The enhancement in the number of CdSe excitons due to plasmon absorption of Au nanoparticles versus the mass fraction of Au in the (Au, CdSe) film. The blue and green percentage labels in the figure denote the percentages of photons absorbed by the solid that were captured through plasmon resonances in large- and small-Au solids, respectively. The hot carrier injection into semiconductor and possible losses of photoinduced charges in CdSe due to back electron transfer to metal are not taken into account. (b). The electric field intensity amplification calculated for 21-nm and 9.1-nm Au nanoparticles. The electric field response of spherical nanoparticles was calculated classically by the T matrix linking the outgoing (Hankel) fields with the incident (Bessel) fields.44 The dielectric field of Au was assumed to be ε = -8.4953 + 1.6239i.

The error analysis in Fig. 4c suggests that the measured efficiency in the case of large-diameter Au samples was accurate within 3%. For 9.1-nm and 9.0-nm Au samples, the uncertainty was ~20% owing to small absolute values of EAu→CdSe. When considering the reported assessment of the energy transfer efficiency, it should be noted that the observed values are inferred only for a particular film morphology


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ACS Photonics

and in principle could vary if a different assembly strategy or a spacer material are employed. What the present experiments unequivocally demonstrate is that: (i) there is a forward Au-to-CdSe resonant energy transfer process, which converts the Au plasmon energy into CdSe excitons, and (ii) there is a direct correlation between the energy transfer efficiency and Au nanoparticle size. The latter phenomenon could be explained by the two factors. First, the increased fraction of the far field radiation in larger diameter Au nanoparticles (Fig. SF8) contributes to the exciton generation in CdSe via scattering mechanism that enhances the optical path in the film. Second, the near field radiation of larger-size Au nanoparticle extends further from metal surfaces making it more probable to induce excitons in CdSe (Fig. 5b). In regard to the absolute efficiently figures observed for both (Au, CdSe) samples, we expect that the measured 1-2% and 29.5% values are likely to represent the upper limit of the transferable plasmon energy within investigated morphologies. Considering that the average interparticle gaps between metal and semiconductor particles in these measurements were fairly small, there is a good chance that a fraction of semiconductor excitons could be dissociated due to the back charge transfer to metal. The phenomenon is seen in the comparison of emission lifetimes for CdSe-only and (Au, CdSe) films in Figs. 3b and 4b. The introduction of Au nanoparticles into semiconductor NC films leads to the average reduction of the CdSe exciton lifetime45 by 8% and 11% for small- and large-Au samples, respectively. Such quenching of the CdSe emission in the presence of metal nanoparticles removes photoinduced carriers from the semiconductor matrix, ultimately reducing the overall plasmon enhancement effect. We anticipate that the design of more robust photovoltaic or photocatalytic assemblies could lead to the reduction in the amount of such back charge transfer by incorporating more effective spacers. To extrapolate the results of the present measurements to systems with different plasmonic nanoparticle loading, we have estimated the net gain in the number of excitons in the CdSe nanocrystal


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matrix (due to plasmon absorption) versus the mass fraction of Au nanoparticles in the film. This simple model takes into an account the extinction coefficients of Au nanoparticles (6×107 for 9.1-nm Au and 9×108 for 21.0-nm Au) and assumes that the plasmon-enhanced exciton generation in CdSe occurs at efficiencies measured in present experiments (~1.5% and 29.5%). We then calculate the percentage of excitons in the CdSe semiconductor matrix that were gained as a result of doping semiconductor solids with a given amount of Au. The predicted enhancement in the exciton generation does not take into account the electric field interference of close lying metal nanoparticles (in cases of high concentrations of Au)46 or the loss of excitons due to the backward transfer of photoinduced charges to Au. Figure 5a shows the expected gains in the CdSe exciton generation due to the energy transfer from incorporated metal nanoparticles for the cases of large 21.0-nm (blue) and small 9.1-nm (green) Au nanocrystals. The corresponding blue and green percentage labels in the figure denote the percentages of photons absorbed through plasmon resonances in large- and small-Au solids, respectively.

In conclusion, we have employed a newly developed STEP spectroscopy to estimate the quantum efficiency for the photoinduced energy transfer from plasmon resonances of metal nanoparticles to semiconductor nanocrystal matrices. The present strategy allows distinguishing between charge and energy transfer processes that often coexist in plasmon-exciton interactions providing new insights into the feasibility of plasmonic devices. Here, the plasmon-to-exciton energy transfer efficiency was measured in assemblies of Au nanoparticles and CdSe nanocrystals, which represent a suitable model system of plasmonic antennas. We show that in the case of 9.1-nm Au nanoparticles, only 1-2% of the Au absorbed radiation is converted to excitons in the surrounding CdSe nanocrystal matrix. For larger, 21.0-nm Au, the percentage of absorbed photons that is converted to excitons in CdSe NCs increases to 29.5%. Since the measured efficiencies exclude carrier losses in CdSe induced by the presence of metal


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ACS Photonics

surfaces (estimated here at 8-11% of plasmon-generated excitons), the projected enhancement of the film photocurrent due to plasmon energy transfer would be lower than the measured efficiencies. Based on the results of present measurements, we estimate the net gain in the number of generated excitons in the CdSe nanocrystal matrix versus the mass fraction of Au nanoparticles in the film. We expect that STEP measurements in other plasmon-semiconductor systems will lead to the development of similar predictive plasmon-gain curves.

Supporting information. Experimental section, additional figures, and details of calculation. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment. We gratefully acknowledge OBOR “Material Networks” program and NSF Awards CHE-1465052 and CBET-1510503 for financial support. AM was funded by the Welch foundation grant # U-0047. References.

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