Exploring the Relationship between Plasmon Damping and

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Exploring the Relationship between Plasmon Damping and Luminescence in Lithographically Prepared Gold Nanorods Lawrence J. Tauzin, Yi-Yu Cai, Kyle Warren Smith, Seyyed Ali Hosseini Jebeli, Ujjal Bhattacharjee, Wei-Shun Chang, and Stephan Link ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00258 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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Exploring the Relationship between Plasmon Damping and Luminescence in Lithographically Prepared Gold Nanorods Authors: Lawrence J. Tauzin†, Yi-yu Cai†, Kyle W. Smith†, Seyyed Ali Hosseini Jebeli†, Ujjal Bhattacharjee†, Wei-Shun Chang†, Stephan Link†‡┴ ⃰ † Department of Chemistry, Rice University, Houston, TX 77251 United States ‡ Department of Electrical and Computer Engineering, Rice University, Houston, TX 77251 United States ┴ Laboratory for Nanophotonics, Rice University, Houston, TX 77251 United States

Abstract In order to engineer plasmonic structures for specific applications, the energy decay pathways upon photon absorption must be understood. One of the decay pathways is the emission of light. In this work, we explore the effects of plasmon damping on the photonic density of states and resulting Purcell enhancement factor for gold nanorods and their relationship to the luminescence quantum yield. We compare the correlated scattering, photoluminescence, and quantum yield of different sizes of lithographically prepared nanorods. We recover a similar aspect ratio dependence for lithographically prepared nanorods as has been previously observed for chemical rods. We change the damping experienced by the nanorods by removing the metal adhesion layer and compare to chemically synthesized nanorods of similar size. We also develop a gradual annealing method to decrease the damping experienced by our lithography nanorods by removing internal scattering defects. In all cases, we find a strong positive correlation between the degree of damping, expressed quantitatively through the resonance Quality Factor, and the luminescence quantum yield: as the Quality Factor increases the quantum yield follows in a roughly linear relationship. Simulations illustrate a corresponding increase in the photonic density of states as the Q-Factor increases. 1 ACS Paragon Plus Environment

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Keywords: gold nanoparticle; one-photon photoluminescence; quantum yield; single-particle spectroscopy; surface plasmon resonance.

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Introduction The plasmon resonance of a noble metal nanoparticle, defined as the collective oscillation of conduction band electrons, can be precisely tuned to suit specific optical requirements.1,2 In addition to scattering, photons are absorbed by these nanoparticles generating hot carriers that may be subsequently harvested to support catalysis or electricity generation.3-5 To optimize plasmonic materials for such applications, the excitation decay pathways following the absorption of light (specifically the decay of hot carriers) must be precisely understood. The photoluminescence (PL) of a nanoparticle, independent of material, offers a unique window into the radiative subset of energy decay pathways of hot carriers.6,7 The detection of PL is relatively simple and offers single particle specificity, removing effects from ensemble averaging and allowing for correlation with other measurements.8 Using PL as a probe for energy decay pathways in plasmonic nanoparticles requires that its mechanism be well understood. Despite the plethora of observations on Au nanoparticle PL, the precise mechanism is a contested issue. The PL spectrum of Au nanorods (AuNRs) has two defining features. The main feature is a primary peak that mostly overlaps with the longitudinal plasmon resonance of the AuNR except for a slight blue-shift.9-12 The other main feature is an excitation wavelength dependent side peak on the blue edge of the spectrum.13 This peak was originally assumed to correspond to the transverse plasmon mode,9 but the recently demonstrated wavelength dependence and lack of a strong polarization dependence suggest that this peak is primarily the result of interband transitions.12,14 Initial hypotheses on the origin of AuNR PL sought to explain the similarity to the plasmon resonance peak by positing direct radiative relaxation of the plasmon itself.15 It is thought that hot carriers act as an intermediate step in the electronic relaxation pathways either through direct interband excitation or plasmon decay.9,10,16-18 Either 3 ACS Paragon Plus Environment

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way, an antenna effect has been invoked to explain the enhanced emission of plasmonic nanoparticles.15,19-21 There is, however, also research suggesting that the PL might be the result of an electronic inelastic light scattering mechanism.22-25 Although many of these mechanisms are qualitatively similar and all agree that the plasmon enhances emission, it is also clear that the precise quantitative mechanism is still a matter under active investigation. Furthermore, phenomena likely to be encountered in real-world applications such as plasmon damping will need to be accounted for. We recently showed that AuNR PL can be described quantitatively as the result of Purcell enhanced recombination of hot carriers.14 Typically, the Purcell effect describes the enhancement of the spontaneous emission rate of an emitter in an environment like a resonator cavity.26,27 We considered hot carriers to be analogous to a spontaneous emitter and the nanoparticle with its plasmon resonance as analogous to a resonator cavity. The resonance within the cavity creates an increased density of states and because Fermi’s golden rule states that the emission rate is proportional to the final density of states of the transition, the emission is enhanced.28 The plasmon resonance determines the photonic density of states (PDOS) resulting in the main peak of the PL spectrum appearing at a similar spectral position to the plasmon peak. The intensity of a plasmon may be reduced with a corresponding increase in its linewidth through a variety of different decay pathways that we collectively refer to as damping. For instance, when nanoparticles are smaller than the mean free path of an electron or grain boundaries create smaller domains within a nanostructure, electron surface scattering occurs and dampens the plasmon.29 For larger nanostructures such as the ones used in this study, radiative damping which scales with the volume of the nanoparticle also plays a dominant role.29 Bulk damping is present in both the bulk metal and its nanoparticles and arises from electrons 4 ACS Paragon Plus Environment

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interacting with phonons.30 Chemical interface damping describes the interaction of surface electrons with the interface of the nanoparticle metal and either another metal or a chemical environment including surface ligands and solvent.31,32 Plasmon damping broadens the dark-field scattering (DFS) spectrum and can be quantified using the Quality Factor (Q-Factor), defined as resonance energy divided by the homogeneous linewidth where the latter can only be obtained via single particle measurements. We expect any damping of the plasmon to correspondingly decrease the Q-Factor, reduce the PDOS, and thus affect the Purcell enhancement factor which should lower the radiative decay rate of hot carriers. The relationship should manifest itself as a decrease in PL quantum yield (QY). A small number of studies have investigated the effects of grain size on PL, but these have focused on small (~20 nm) nanoparticles with resonances close to the interband transition and did not examine the potential changes to the PDOS.33-35 Our previous study focused on manipulating the post-excitation hot carrier distribution and its effects on PL, while the effect of manipulating the PDOS remains unexplored.14 Additionally, previous studies have focused almost exclusively on the QY of chemically prepared AuNRs.9,10,12 In this paper we examine the QYs of chemically prepared AuNRs in comparison with AuNRs of varying sizes prepared via electron-beam lithography (EBL) to determine the effects of damping of the resonator cavity and thus changing the PDOS and its influence on the Purcell enhancement factor. The Q-Factor is used as a measure of the quality of the resonator cavity. We used EBL AuNRs because they are known to have poor crystallinity compared to their chemically synthesized counterparts resulting in a damped plasmon due to electron surface/defect scattering.36-39 Plasmons in EBL nanostructures are often further damped through chemical interface damping because of the use of metal adhesion layers, an effect that we explore here as well.40 Finally, we developed a method of gradual annealing to increase the 5 ACS Paragon Plus Environment

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crystallinity and therefore decrease the plasmon damping in EBL AuNRs and found indeed an increase in the PL intensity as result of crystallinity and not shape, confirming the relationship between plasmon damping, Q-factor, PDOS, Purcell enhancement, and PL.

Materials and Methods Sample Preparation Arrays of 40 x 60, 40 x 80, and 40 x 100 nm AuNRs were patterned on cleaned fused quartz microscope slides (Advalue Technologies) using EBL. The slides were first spin-coated with a high-resolution electron-beam resist consisting of PMMA 950 A4 . The slides were then baked at 180 °C for 90 s. Patterning of the substrates was carried out in a scanning electron microscope (FEI Quanta 650) equipped with Nabity NPGS software and a beam blanker. The patterned substrates were developed for 30 s using a 1:3 v/v mixture of 4-Methyl-2-pentanone:isopropyl alcohol. 2 nm Ti and 40 nm Au were sequentially evaporated using electron-beam evaporation at a pressure of ~2 x 10-7 Torr and an evaporation rate of < 1 Å/s. For Ti free samples the Ti deposition step was omitted. Finally, the slides were soaked in acetone for 15 h to lift off the resist followed by gentle rinsing with acetone and drying under a flow of nitrogen. The AuNR arrays were characterized with the same SEM operated in low-vacuum mode. SEM determined size distributions for all lithography AuNRs analyzed in this work are shown in Figures S3 and S4. For the purposes of this work, the AuNR arrays will be referred to by their designed specifications. Colloidal AuNR Synthesis Sodium oleate was purchased from TCI and all other chemicals were purchased from Sigma Aldrich. Chemically synthesized 40 x 90 nm AuNRs (actual dimensions 43 ± 3 x 94 ± 6 6 ACS Paragon Plus Environment

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nm) were synthesized using the binary surfactant mixture seed-mediated growth method developed by the Murray group.41 The seed solution was prepared by mixing 5 mL 0.1 M cetyltrimethylammonium bromide (CTAB) and 5 mL 0.5 mM HAuCl4. This mixture was then reduced with 1 mL 6 mM NaBH4 solution under stirring for 2 minutes at 1200 rpm. The seed solution was aged for 30 minutes and then used to prepare the AuNRs. To grow the AuNRs from the seed solution, a growth solution was prepared by adding 1.234 g sodium oleate and 7 g CTAB to 250 mL of water at 80 °C. This solution was cooled to 30 °C after which 18 mL 4 mM AgNO3 was injected. This solution was allowed to rest at 30 °C for 15 minutes. After resting, 250 mL of 1 mM HAuCl4 was added and the resulting solution was stirred at 700 rpm for 90 minutes. 1.5 mL 37 wt. % HCl was then added and the solution was slowly stirred for 15 minutes. Finally, 1.25 mL 64 mM ascorbic acid and 0.4 mL seed solution were injected into the growth solution with 30 s stirring after each injection. The AuNRs were allowed to grow for 12 h at 30 °C. The resulting AuNRs were centrifuged at 8000 rpm for 30 minutes in order to concentrate the AuNRs for storage. Dark Field Scattering and Photoluminescence Measurements DFS and PL measurements were conducted on a home built inverted microscope. For DFS, white light from a halogen lamp was directed into an oil immersion condenser set to dark field mode. Scattered light from the sample was collected by an air space objective (NA = 0.8, EC Epiplan-Neofluar 50x/0.8 HD M27, Zeiss). For imaging, the collected light was directed onto an avalanche photodiode (SPCM-AQRH-15, PerkinElmer) while the sample was raster-scanned using a three-axis piezo stage (P-517.3CL, Physik Instrument). A 50 µm pinhole placed in the confocal plane was used as a spatial filter. Spectra were collected using a CCD camera (iDus 420 BEX2-DD, Andor) coupled to a spectrometer (Shamrock 193i, Andor). A depolarizer (Edmund 7 ACS Paragon Plus Environment

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Optics) was used directly before the spectrometer to account for the polarization sensitivity of the grating. For PL measurements, excitation light was provided by a continuous wave 488 nm diode laser (OBIS, Coherent). The laser light was directed to the sample by a 50/50 beam splitter (Chroma). A 488 nm notch filter and a 496 nm long pass filter (Semrock) were used to remove background laser light from the detected signal. For all PL measurements an excitation power of 0.5 mW (1.1 x105 W/cm2) was used. A calibrated lamp was used to correct the PL spectra based on the detection efficiency of the CCD camera. A schematic of the instrument can be found in the supporting information (SI, Figure S1). All DFS and PL spectra were background corrected by measuring the spectrum of a blank sample area near each particle. Spectral properties such as the resonance energy and the linewidth were extracted from the spectra by fitting to a Lorentzian function using custom analysis software written in MATLAB R2014a. Quantum Yield Calculations The QY was calculated according to previously published procedures.42 Briefly, the QY is defined as the ratio of the number of photons absorbed (Nabs) to the number of photons emitted (Nem).

QY =

N (1) N

Nabs can be calculated using the following relationship: Nabs=

(2)

where Iexc is the excitation power density, σabs is the absorption cross section of the AuNR (determined from FDTD simulations), and hυexc is the photon energy. Nem was obtained by correcting the raw photon counts Nraw emitted by a AuNR and measured by scanning an image 8 ACS Paragon Plus Environment

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using an avalanche photodiode (APD) with the wavelength dependent efficiencies of the optics and filters used Π. Nem =

∙. !!

(3)

Det.Eff is the efficiency of the detector and was determined by a calibration procedure using Rhodamine 6G. To perform this calibration, Rhodamine 6G immobilized on a glass substrate was imaged using the same method as for the AuNR PL. The measured florescence intensity and the known absorption cross section and quantum yield of Rhodamine 6G43 were used to determine that Det.Eff was 1.6% on our setup. In order to estimate the error in our QY measurements we measured the QY of the same 20 AuNRs three times and determined the error to be ~15%. This uncertainty is consistent with previous QY measurements of AuNRs.12 Finite Difference Time Domain (FDTD) Simulations In order to determine the absorption cross section for each AuNR to use in the corresponding QY calculation, FDTD simulations were employed. The size of each AuNR was determined from SEM measurements and applied to a simulated right rectangular cuboid on a quartz substrate for EBL AuNRs using the dielectric function determined by Johnson and Christy.44 For samples with an adhesion layer, 2 nm Ti was also included in the simulations. For chemically synthesized AuNRs a hemisphere capped cylinder was used. The scattering and absorption cross sections were determined by running the Lumerical FDTD solver with default convergence parameters. Slight tuning of the length and width of the AuNR within the standard deviation of SEM measurements of ~ 1.5 nm was employed so that the simulated and experimental scattering spectra for each AuNR overlapped. To validate this method of estimating the absorption cross section we compared the distributions of the simulated absorption cross 9 ACS Paragon Plus Environment

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sections and measured photothermal absorption signals of two samples of ~40 x 60 nm EBL AuNRs with and without Ti adhesion layer. As shown in Figure S2, we found that theory and experiments gave similar results. Results and Discussion Correlated DFS and PL spectra, along with PL intensity images for QY calculations were acquired for 40 x 60, 40 x 80, and 40 x 100 nm lithographically prepared AuNRs with a 2 nm Ti adhesion layer. Figure 1 shows representative DFS and PL spectra along with correlated SEM of a 40 x 60 and a 40 x 100 nm AuNR. The SEM images reveal the rough structure of the EBL AuNRs. Our selection of aspect ratios gives us access to resonance energies between ~600 ~800 nm (1.55 – 2.07 eV). Representative 40 x 80 nm AuNR data can be found in the SI (Figure S3). The length, width, and aspect ratio of all EBL AuNR samples measured for this study are characterized in Figure S4 and Figure S5. The PL of EBL AuNRs has the same overall properties as the PL from chemically synthesized AuNRs. The PL spectra resemble the corresponding DFS spectra but are also distinct in two ways. First, the main peak is blue-shifted slightly; consistent with previous studies of PL.10-12,14 This blue-shift of the PL maximum from the DFS maximum for all of our measured EBL AuNRs is quantified in Figure S6 and found to be ~50 meV for colloidal and EBL AuNRs with 2 nm of Ti and ~90 meV for EBL AuNRs without Ti. The blueshift has been assigned to result from different charge carrier distributions created by the excitation photon energy and intensity as well as the resulting contributions from inter- and intraband transitions to the PL.14 The polarization dependence of the scattering and main PL peaks are also characterized in Figures S8 and S9. Second, there is a side peak on the blue edge of the spectrum. This side peak is located at the same energy regardless of the resonance energy of the AuNR (all spectra were acquired with 488 nm excitation). This peak has been shown to 10 ACS Paragon Plus Environment

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depend on the excitation energy and is attributed to interband transitions because it does not show a polarization dependence.9,12 The AuNR shape and size affect its scattering spectrum in a variety of ways, and in particular, the resonance wavelength is strongly dependent on the aspect ratio: longer and skinnier AuNRs have lower energy resonances.45-47 The linewidth of the resonance is a measure of the plasmon decay rate.48-51 Increased damping results in broader spectra. Because of the relatively large size of our EBL AuNRs radiative damping is expected to contribute significantly to the measured linewidth.52,53 For high energy plasmon resonances, the plasmon is also likely to decay into interband excitations yielding an increase in spectral width that decreases for AuNRs with lower resonance energies. This effect is known as interband damping.53 In addition to intrinsic bulk damping, we furthermore expect electron scattering at grain boundaries to manifest itself in increased plasmon linewidths.54 In order to use the DFS spectra to quantify damping, we use the Q-Factor.53,55 The Q-Factor is defined as the resonance energy Eres divided by the homogeneous linewidth Γ. "=

#$%& '

(4)

Figure S10 plots the Q-Factor against the resonance wavelengths for all AuNRs we studied. As expected, AuNRs with high resonance energies have lower Q-Factors due to the increased linewidth from interband damping. AuNRs with resonance energies lower than 2 eV have roughly the same Q-Factor regardless of the plasmon peak position, resulting from a narrowing of the spectra as the resonance energy decreases.48,53 In order to probe differences in PL for the nanostructures investigated here we will use the Q-Factor obtained experimentally through DFS


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spectra to relate to measured QYs and via simulations establish a relationship between damping, Q-Factor, and PDOS that determine the Purcell enhancement.

Figure 1 – Representative normalized DFS and PL spectra of a 40 x 60 nm and 40 x 100 nm EBL AuNR. Insets show correlated SEM images.

For EBL AuNRs with a 2 nm Ti adhesion layer, we find that the QY remains constant for AuNRs below a resonance energy of about 2 eV and starts decreasing when further increasing the resonance energy for shorter AuNRs (Figure 2). Intensity images acquired with an APD, along with the PL spectrum and calibration measurements, as described in the Methods section, were used to calculate the QY of each AuNR. Previous QY measurements on chemically synthesized AuNRs of various sizes found that the QY decreases for AuNRs with resonance wavelengths less than 650 nm (greater than 1.9 eV).12 Interband damping was suggested to cause this decrease in PL QY. This explanation is consistent with the view that plasmon decay into interband transitions broadens the plasmon linewidth and decreases the Q-Factor and therefore makes the AuNR a worse resonator for amplifying PL.14 Figure 2a presents a scatter plot of the PL QY for the 3 EBL AuNR samples as a function of resonance energy determined by DFS. Each color represents a different size. At wavelengths less than ~650 nm the QY indeed 12 ACS Paragon Plus Environment

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decreases as a function of resonance energy. This value is the same cutoff wavelength observed in previous experiments, despite the fact that we are using polycrystalline EBL AuNRs.12 Figure 2b shows the corresponding cumulative distributions of the QY for each of the 3 different AuNR sizes. This representation confirms that for 40 x 80 and 40 x 100 nm EBL AuNRs the QY is mostly the same (~1.5 x 10-6), while for the 40 x 60 nm AuNRs with a plasmon resonances near 600 nm the QY is decreased to ~0.7 x 10-6.


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Figure 2 – Correlated resonance wavelength and QY of EBL AuNRs of various sizes. A) Scatter plot of resonance wavelength vs. QY for three different sizes of EBL AuNRs. B) Complementary cumulative distributions of the QY for each AuNR sample. The error bar represents the estimated error of ~15%.

Damping in lithographically fabricated nanostructures does not arise solely from their polycrystallinity, but also from any metal or chemical adhesion layer via chemical interface damping.40,56 Our AuNRs were fabricated with a 2 nm Ti layer and removing this layer is expected to decrease damping and therefore increase the QY. Indeed, we observe experimentally that removing the Ti layer increases the quality of the resonator cavity and results in an increase in QY. The same lithography procedure was used to prepare 40 x 60, 40 x 80, and 40 x 100 nm AuNRs without a Ti adhesion layer. Figure 3a shows comparative DFS spectra of a 14 ACS Paragon Plus Environment

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representative 40 x 80 nm AuNR with and without a Ti layer (spectra for the other sizes can be found in the SI, Figure S7). Removal of the Ti layer results in spectra that are blue-shifted (from 685 to 652 nm) and narrower (175 compared to 169 meV) due to the removal of damping by the Ti layer.40 Thus, the Q-factor is higher for Ti free AuNRs, 11.2 compared to 10.3 for the example spectra in Figure 3. As discussed above, we find that for the same type of sample the Q-Factor exhibits a transition to lower values at resonance wavelengths shorter than ~ 650 nm (Figure S10) and, based on just the definition of the Q-Factor, counterintuitively decreases for even shorter resonance wavelengths (larger resonance energies). Therefore, we can conclude that the increase in Q-Factor in Figure 3a is not expected to result from the blue-shift of the resonance energy, but is dominated by the decrease in plasmon linewidth in the absence of chemical interface damping by the Ti adhesion layer.40 We note that while chemical interface damping is the primary candidate for the change in damping, there may also be contributions from other sources such as slight differences in the crystallinity between samples caused by the presence vs. absence of an adhesion layer.


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Figure 3 – Improving the quality of the resonator cavity improves the QY. A) Representative DFS spectra of a 40 x 80 nm EBL AuNR with and without a 2 nm Ti adhesion layer. B) Resonance wavelength vs. QY for EBL AuNRs with and without a 2 nm Ti adhesion layer. The error bar represents the estimated error of ~15%.

Consistent with the increase in Q-Factor, AuNRs without a Ti adhesion layer exhibit higher QYs compared to similar AuNRs with a Ti layer. Figure 3b shows a scatter plot of the QYs of all EBL AuNRs fabricated with and without the Ti adhesion layer. For both types of samples, the QY decreases for wavelengths less than ~650 nm following the trend already discussed in Figure 2a. However, in the case of the Ti free AuNRs, the overall QY is higher by a factor of 1.5 – 2. The higher QY is the result of less damping due to the lack of the Ti layer 16 ACS Paragon Plus Environment

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resulting in the AuNR acting as a higher quality resonator cavity amplifying the radiative recombination of excited electron and holes. If the quality of the resonator is the key factor for similarly sized and shaped nanostructures with comparable plasmon resonance wavelengths, then we expect that the QY from chemically synthesized AuNRs should be higher still.

Figure 4 – Improving the quality of the resonator cavity improves the QY by switching to chemically prepared AuNRs. A) Representative DFS spectra of a 40 x 100 nm EBL AuNR (no Ti) and a 40 x 90 nm chemically synthesized AuNR. B) Complementary cumulative distributions of the 40 x 100 nm EBL AuNR QY and the 40 x 90 nm colloidal AuNR QY. The inset shows a representative SEM image of a 40 x 90 nm chemically synthesized AuNR.


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Chemically synthesized AuNRs act as an even better resonator cavity due to their higher crystallinity and thus have a higher QY than similarly sized EBL AuNRs. We measured the DFS and PL spectra along with QYs of 40 x 90 nm chemically synthesized AuNRs (Figure 4). The resonance energy vs. QY for all AuNRs including colloidal AuNRs is shown in Figure S13. The size and aspect ratio distributions of all colloidal AuNRs measured are reported in Figure S11 while representative DFS and PL spectra are provided in Figure S12. These colloidal AuNRs have a blue-shifted and narrower DFS peak compared to similarly sized EBL AuNRs without a Ti adhesion layer (Figure 4a). The blue-shift is due to the AuNR shape and not a change in size (Figure S15). The dominating decrease in linewidth again leads to larger Q-Factors (Figure S10).38,39,57 Based on our previous arguments, the increase in Q-Factor correlates with an increase in PL QY of these colloidal AuNRs compared to EBL AuNRs without a Ti adhesion layer (Figure 4b). Cumulative distributions for the QY of 40 x 100 nm Ti free EBL AuNRs and the QY of 40 x 90 chemically synthesized AuNRs illustrate an increase by a factor of 1.5 – 2 (Figure 4b). These distributions do not overlap confirming statistically significant separation. We attribute this increase in PL QY to the crystalline nature of chemically prepared AuNRs leading to reduced damping, a larger Q-Factor, and hence stronger emission enhancement. To further verify our arguments, we hypothesize that annealing our EBL AuNRs (without significant size/shape changes) results in larger Q-Factors and QYs compared to the same AuNR prior to annealing. In order to selectively anneal single AuNRs and perform correlated measurements on the same AuNR before and after annealing, we developed a gradual photothermal annealing method using the excitation laser on our microscope setup. Annealing of EBL nanostructures is commonly used to improve their crystallinity and minimize damping.55,58 It is also well known 18 ACS Paragon Plus Environment

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that the direct application of high power laser irradiation can melt and completely reshape small nanoparticles such as AuNRs.59,60 Recently, the Yang group reported a method of encapsulated annealing in which nanostructures are encapsulated in a polymer prior to annealing which prevents large shape changes during the annealing process.55 However, the deposition of a polymer can itself damp the plasmon and we, therefore, sought an alternative method in which our EBL AuNRs were exposed to gradually increasing laser power. Individual 40 x 100 nm EBL AuNRs without Ti were characterized prior to and after annealing. Annealing was conducted by exposing the AuNRs to progressively higher laser powers (3 mW (6.6 x 105 W/cm2) for 1 hr, 4 mW (8.8 x 105 W/cm2) for 1 hr, and finally 6 mW (1.3 x 106 W/cm2) for 1 hr) using the same 488 nm diode laser used for PL measurements. We found that the AuNRs melt after less than a second of exposure to 6 mW of laser intensity if annealing at lower powers was not conducted first (Figure S16). With the sequential annealing steps, the AuNRs were stable under 6 mW excitation and the size of the AuNRs after the annealing process closely resembled the size of un-annealed AuNRs in the same array (Figure 5a). On average, annealed AuNRs were 1- 2 nm shorter than un-annealed AuNRs and 1 - 2 nm wider. Such a small shape change alone is not expected to have dramatic effects on the optical properties of the AuNRs. We suspect that the gradual annealing process anneals the outer layers of gold on the particle surface at the initial lower laser powers which act as a more rigid shell allowing the particle to withstand higher laser powers without dramatic shape changes, similar to the mechanism of encapsulated annealing.55 The gradual photothermal annealing process greatly increases the Q-Factor of the AuNRs and results in an increase in PL intensity. The resonance energy is blue-shifted nearly 100 nm from 694 to 586 nm and the DFS peak narrows. The resulting Q-Factor is 12.1 compared to 9.9 19 ACS Paragon Plus Environment

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for the AuNR shown in Figure 5b. Additionally, we find that annealed AuNRs have an increased scattering intensity (Figure S14) despite the resonance blue-shift.45 These results are consistent with an increase in the crystallinity of the AuNRs and, based on the results in Figure 4, we therefore expect the PL intensity to increase as well. Figure 5c tracks the relative intensity of the AuNRs during the annealing process. During the first hour of annealing a laser power of 3 mW was used, during hour 2, 4 mW was used, and during hour 3, 6 mW was used. To probe the intensity, the laser power was reduced to 0.5 mW and an APD image was acquired every 30 minutes. In order to account for variations in laser power and focus between measurements, the intensity of each annealed AuNR (Ia) was normalized by the intensity of a nearby un-annealed AuNR (Iu). At t = 0, no annealing has occurred and thus the normalized intensity of each AuNR is equal to 1. As annealing progresses, the AuNRs gradually become brighter relative to the unannealed AuNRs, up to a value 1.8 times brighter. The increase in PL intensity along with the increased Q-Factor suggests that we are improving the quality of the resonator cavity and thus increasing the QY of the AuNRs. Slight shape changes are unlikely the reason for the PL intensity increase, as shifting the resonance energy to less than 650 nm through the geometry in the absence of other changes results in a decrease of the PL QY, as shown above. It is important to note that we do not directly calculate the QY of the annealed AuNRs but instead just compare PL intensities in images that contain both annealed and un-annealed AuNRs. It is therefore also possible that the AuNRs may appear brighter due to an increase in the absorption cross section resulting from slight shape changes, but we estimate this effect to have at best only a minor contribution. In order to confirm that the observed brightness changes are not the result of small shape changes such as the rounding of the NR edges we performed FDTD simulations comparing a 40 20 ACS Paragon Plus Environment

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x 100 nm EBL AuNR with a 40 x 100 nm colloidal AuNR (a nanorod with all edges rounded) to approximate the most extreme scenario (Figure S15). These simulations show that the two nanorod shapes have roughly the same absorption cross section at 488nm. We predict the absorption cross section at 488 nm for a 40 x 100 nm EBL AuNR to be 4.7 x 10-15 cm2 while that of a 40 x 100 nm colloidal shaped AuNR to be 3.2 x 10-15 cm2 which would actually result in less absorption and thus does not explain our results. Therefore, we conclude that the change in relative brightness is the result of something other than the nanorod shape, most likely due to the improved crystallinity of the annealed nanoparticles resulting in less damping. Depending on how laser irradiation is applied, the PL intensity of the AuNRs can be decreased as well. AuNRs that are exposed to 6 mW laser irradiation without slow annealing at lower powers first melted within 1 s resulting in a dramatic shape change into sphere-like nanostructures (Figure S16). These particles can be visibly distinguished after annealing in a simple DFS color image taken with a digital single-lens reflex (SLR) camera. The inset in Figure 5c shows an SLR camera image of a section of the EBL array prior to annealing (top half ) and the same section post-annealing (bottom half). The colored arrows indicate the annealed AuNRs whose intensity is plotted in Figure 5c. White circles indicate AuNRs exposed to 6 mW laser excitation without low power gradual annealing. All of these particles appear green confirming their reduced aspect ratio and suggest a sphere-like shape. The relative Pl intensity of these melted particles decreased by more than 50% compared to un-annealed AuNRs (Figure S16B).


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Figure 5 – Photothermal annealing of EBL AuNRs. A) SEM images showing a 40 x 100 nm AuNR that has not and one that has gone through the photothermal annealing process. B) DFS spectra of a AuNR before and after photothermal annealing. C) Relative PL intensity measured at 30 minute intervals during the photothermal annealing process for the 5 AuNRs marked with the correspondingly colored arrows in the image (inset). The laser power was increased every hour. The top axis shows the estimated temperature increase of the AuNR for each laser excitation power. The PL intensity was normalized by the emission from a AuNR that was located in the same field of view but that did not undergo the annealing process. The inset shows images of the EBL AuNR array before (top) and after (bottom) the annealing experiments. White circles indicate AuNRs that were not gradually


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annealed but instead exposed directly to the highest laser power causing them to reshape into spheres.

AuNRs with higher Q-Factor have higher QY. Figure 6A plots the Q-Factor and correlated QY for all AuNRs studied here. With the exception of some very low Q-Factor Ti free AuNRs (Q-Factor > 7), there is an increasing, almost linear trend with increasing Q-Factor. A change in Q-Factor from ~6 to ~13 results in an increase in QY from ~0.5 x 10-6 to ~2.5 x 10-6. This trend clearly demonstrates the link between damping and the luminescence QY particularly for AuNRs with a Q-Factor greater than 7. The deviation for AuNRs with a Q-Factor less than 7 occurs for AuNRs with larger widths and higher resonance energies (wavelengths less than 650 nm, see discussion above). It is also possible that there were larger fitting errors due to the larger spectral linewidths of these sphere-like nanostructures. Other nonradiative decay mechanisms can furthermore contribute to differences among these samples. For example, electrons might be lost through injection into the Ti layer, while electron-phonon coupling is reduced for monocrystalline chemically prepared samples.61 In Figure S18 we plot the QY vs. Q-Factor for EBL AuNRs with and without a Ti adhesion layer separately and note that an increasing linear trend still exists, although the slopes differ, suggesting that factors other than the Q-Factor also contribute to the QY. The overall relationship between QY and Q-Factor is not therefore exactly proportional; the exact scaling relationship between the Q-Factor and QY over a broad range of resonance energies certainly warrants further study. Decreasing the plasmon damping and thus increasing the Q-Factor results in a higher PDOS which drives a higher QY. This relationship is clearly demonstrated in Figure 6b and 6c. The two driving factors for PL are the generation of hot carriers upon the absorption of a photon, and the PDOS. In previous work, we used different excitation wavelengths to change the 23 ACS Paragon Plus Environment

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distribution of hot carriers upon excitation.14 Here we change PDOS by changing the damping. To illustrate the effects of damping on the PDOS we used the Drude-Lorentz model to simulate the dielectric function of gold. Values were calculated for 3 different degrees of damping term (see SI for details). The resulting dielectric functions were used in boundary element method (BEM) simulations of a 40 x 100 nm AuNR to extract the scattering cross section (Figure 6b) and the corresponding PDOS (Figure 6c) using the MNPBEM toolbox for MATLAB.62 The damping term in the Drude-Lorentz model effectively determines the width and intensity of the scattering spectrum (Figure 6b). We established in Figure 3 that the resonance wavelength does not have a large effect on QY except for more sphere-like AuNRs; therefore, the spectral linewidth as the only other factor used in calculating Q-Factor should be most important. The values for the damping term were adjusted to yield similar Q-Factors to the experimental data reported in this work (7.8, 9, and 11.3). Figure 6c clearly demonstrates the decrease of the PDOS in response to decreasing Q-Factor and this decrease is roughly linear with the integrated area of the PDOS (Figure S17). The PDOS is the specific component that links the plasmon to the Purcell emission enhancement factor and the PDOS has previously been used to probe plasmon damping through electron energy loss spectroscopy.63 Therefore; we expect a decreased PDOS to yield a decrease in QY. It needs to be noted though that other emission mechanisms, such as electronic Raman scattering, would also be enhanced by the Purcell effect and hence the plasmon resonance and the resulting PDOS. Our results presented here cannot rule out electronic Raman scattering but are consistent with all aspect of the suggested mechanism of radiative recombination enhanced by the Purcell effect.14


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Figure 6 – Dependence of PDOS and QY on Q-Factor. A) QY as a function of Q-Factor for EBL AuNRs with and without a Ti adhesion layer and 40 x 90 nm chemically synthesized colloidal AuNRs. The error bar represents the estimated error of ~15%. B) Simulated scattering spectra for a 40 x 100 nm colloidal AuNR with 3 different Q-Factors, modeled by varying the damping factor in the Drude-Lorentz model. C) Corresponding PDOS for the simulated AuNRs in B.

Conclusions 25 ACS Paragon Plus Environment

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We have shown that the damping resulting from poor crystallinity in lithographic nanostructures and adhesion layers commonly used in lithography can be quantitatively linked to the decrease in PL QY compared to chemical AuNRs of a similar size. We also show that through a gradual annealing process, lithographically prepared AuNRs can be individually hardened against high power laser irradiation with an accompanying increase in emission intensity. We interpret these phenomena in the context of our hypothesis that photoluminescence is the result of the Purcell Effect modified recombination of hot carriers and link the change in QY to the damping induced changes to the PDOS of the AuNRs finding a roughly linear trend in Q-Factor vs. QY.

Supporting Information Instrument configuration, 40 x 80 nm EBL AuNR spectra, length, width, and aspect ratio histograms for all AuNR used in this study, PL blue-shift, Ti free EBL AuNR spectra, DFS and PL polarization dependence, Q-Factor vs. QY plot, DFS and PL of chemically prepared AuNR, QY vs. resonance wavelength, non-normalized pre and post-annealing DFS spectra, FDTD simulations of different shaped AuNRs, photothermal EBL annealing procedure, gradual vs. direct annealing, and damping simulations. Comparison of photothermal measurements and simulated absorption cross sections, QY vs. Q-Factor for individual samples. This material is available free of charge via the Internet at http://pubs.acs.org

Acknowledgements


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S.L. acknowledges financial support from the Robert A. Welch Foundation (C-1664) and the Air Force Office of Scientific Research (MURI FA9550-15-1-0022). K.W.S. acknowledges that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program (1450681).


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For Table of Contents Use Only Exploring the Relationship between Plasmon Damping and Luminescence in Lithographically Prepared Gold Nanorods Authors: Lawrence J. Tauzin, Yi-yu Cai, Kyle W. Smith, Seyyed Ali Hosseini Jebeli, Ujjal Bhattacharjee, Wei-Shun Chang, Stephan Link

In this paper, we relate the increase of the photonic density of states as the Q-Factor of a nanorod increases to a corresponding increase in the rods quantum yield. This paper represents a step in understanding the energy decay pathways of plasmonic nanomaterials, which have applications in sustainable energy harvesting and storage.


Exploring the Relationship between Plasmon Damping and

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