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Asymmetric Light Absorption and Radiation of Ag-Cu Hybrid Nanoparticles Carl Wadell, Akira Yasuhara, and Takumi Sannomiya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09926 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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The Journal of Physical Chemistry

Asymmetric Light Absorption and Radiation of AgCu Hybrid Nanoparticles Carl Wadell*,1, Akira Yasuhara2, and Takumi Sannomiya*,1

1

Department of Materials Science and Engineering, School of Materials and Chemical

Technologies, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama, 226-8503 Japan. 2

JEOL Ltd. 3-1-2 Musashino, Akishima, Tokyo, 196-8558, Japan.

ACS Paragon Plus Environment

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ABSTRACT

In this article we study the plasmonic properties of phase separated Ag-Cu nanoparticles. The particles are fabricated using simple thin film evaporation followed by particle formation by vacuum annealing. The formed particles, feature a two-faced Janus structure. Characterization is carried out at the single particle level utilizing transmission electron microscopy in combination with electron energy loss spectroscopy and cathodoluminescence, and modeled by finite element method simulations. We find that these particle sustain two kinds of resonances: resonances localized to the Ag half of the particle, and resonances involving the entire particle. This is due to the difference in onset energy for interband transitions for the two metals. We find that as the resonances are excited in Ag, large enhancements of energy absorption can be achieved in the Cu half of the particle. We also find a slight asymmetry to the emission of the particles in cathodoluminescence, with a preferential emission towards the silver side of the particle. Enhanced energy absorption into Cu means an increased number of generated hot electrons. This together with the ease of fabrication of the particles makes these structures interesting candidates for plasmon enhanced photocatalysis. Furthermore, because of the inherent phase separation of the materials, stability even at elevated temperatures is enhanced. Not being limited to the Ag-Cu system, a similar approach should work equally well for other phase separated systems.


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TEXT

Introduction The resonant optical properties of plasmonic nanoparticles are useful in a wide variety of fields, e.g. high-performance bio/chemosensors, emission enhancers, or energy convertors.1-4 Therefore, in the field of nanoparticle plasmonics, one is interested in tailoring the plasmonic response of metal nanopartcicle systems. This can be, for instance, to have a resonance at certain wavelength, control the scattering direction of the system, or its light absorption. In order to achieve these goals many different avenues have been explored, most commonly through the size, shape and material of the constituent nanoparticles.5-9 In some cases several different materials are used to achieve the desired effect, and such hybrid nanostructures is the topic of the work presented here. In this article we look at plasmonic properties of phase separated Ag-Cu nanoparticles. This system consists of two metals which both feature strong plasmonic responses. However, the responses differ significantly, mainly due to the difference in onset energy for interband transitions in the two metals (4 and 2.2 eV for Ag and Cu respectively).10 Another feature of this system is that it is inherently phase separated. This ensures that Ag and Cu does not intermingle in the particle, and therefore produce a system which is more stable compared to for instance the Ag-Au system which forms isomorphous alloys and therefore interdiffuse even at a room temperature. The phase separation of the two metals also allows for easy fabrication, as we will demonstrate here. We use the Ag-Cu system as a model for other phase separated metal systems, but note that a similar approach should work also for other systems, such as Ag-Ni, Ag-Rh, AgPt, Au-Co, Au-Ni, Au-Rh, and Au-Pt.11, 12


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The formation of Ag-Cu nanoparticles has previously been studied theoretically,13-15 and have been experimentally fabricated through wet-chemisty,16-19 and pulsed laser deposition.20, 21 The previous studies of plasmonic properties of Ag-Cu particles have dealt with smaller particles (about 5 nm), or wet-synthesized particles, both with core-shell structures.16, 19 The particles under scrutiny in this study on the other hand have a Janus structure, which is the predominantly formed structure for the larger particles (above 50 nm) from our fabrication method. This type of “two-faced” structure creates opportunities to further tailor the plasmonic response of the system, as shown previously for Au-Ag and Au-Pd particles.22 To study the resonances of these particles in detail we use electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) in a scanning transmission electron microscope (STEM). These methods allow for nanoscale mapping of the plasmonic resonances.23-25 We find that, because of the very different onset energies of interband transitions in Ag and Cu, combining the two can be used to greatly enhance absorption in Cu. Absorption enhancement in plasmonic materials have previously been studied with the main interest being for applications in plasmon enhanced catalysis.9, 26-29 Since enhanced absorption leads to an increased number of hot electrons in the metal nanoparticles, it is being investigated if these hot electrons can be used to drive or enhance chemical reactions running on the surface of the nanoparticles.30-36 This in combination with the thermal stability of the phase separated particles, and the catalytic properties of Cu makes them interesting candidates for plasmon enhanced catalysis.37

Methods Nanoparticle fabrication


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Nanoparticles were fabricated by thermal evaporation of a thin metal film onto the desired substrate. A total nominal metal film thickness of 5 nm was deposited. In the case of Ag-Cu particles the metals were co-evaporated with a volume ratio of 1:1. The substrates used were borosilicate glass slides for optical characterization, as well as SiO2 membrane TEM windows (20 nm thick, EM Japan) for STEM measurements. Following metal film evaporation, the samples were inserted into a vacuum furnace (base pressure of 10-4 Pa) and annealed for 4 hours at 400 °C to form particles.

Optical extinction Optical extinction spectra were collected in a transmission configuration utilizing a fiber coupled halogen lamp (Ocean Optics LS-1 tungsten halogen lamp) together with a fiber coupled spectrometer (Hamamatsu MiniSpectrometer TM-VIS/NIR).

STEM EDS/EELS Energy dispersive X-ray spectroscopy (EDS) and EELS maps were carried out in a JEOL JEMF200 with a cold field emission gun. The microscope was equipped with two JEOL SDD EDS detectors and a Gatan GIF QuantumER EELS detector. Measurements were carried out at an acceleration voltage of 200 kV. The EDS tomography was extracted from a tilt series (-72 to +72 degrees in 4 degree steps) using TEMography software.

Cathodoluminesence mapping and angle resolved measurements CL mapping was carried out in a customized JEOL STEM 2100F equipped with an optical window in the column. A parabolic mirror around the sample position allows for the collection


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of the CL signal. The mirror collimates light emitted by the sample through the window and on to a spectrophotometer (Shamrock, Andor). By recording the spectra of the emitted light while scanning the electron beam across the sample, mapping of the plasmon excitations can be performed. By placing a mask in the light path allows for selectively measuring the signal emitted in a certain direction from the sample, see Supporting Information. Angle resolved measurements were carried out by scanning the mask across the emission angles. To calibrate the intensity of the angle resolved measurement, emission from a 900×900 nm area was collected. The emission from this area, containing particles of random orientation is assumed to have no discernible directionality and is used as reference for the intensity at each wavelength, assuming only in-plane dipolar emission. We only consider in-plane dipoles since the emission from outof-plane dipoles is greatly reduced by shadowing from the substrate. This is also evident from that only in-plane dipoles are observed in CL-mappings carried out without a mask present. All measurements were carried out at 80 kV.

FEM model FEM modeling was carried out in COMSOL Multiphysics utilizing the wave optics module. The Ag-Cu particle was modeled as a prolate spheroid (long axes radius = 40 nm, shorter axes radii 30 nm). The spheroid was enclosed inside a PML spherical shell with an inner radius that is λ/2 larger than the long axes radius of the spheroid. The thickness of the PLM layer was also set to λ/2. The scattered field upon plane wave excitation in the wavelength range 300 – 800 nm in steps of 10 nm was calculated. This was done for both s and p polarization directions. The dielectric constants for Ag and Cu were taken from Johnson and Christy.38 The surrounding media was set as air (εr = 1).


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Results and Discussion Ag-Cu nanoparticles were fabricated by co-evaporation of Ag and Cu (5 nm nominal thickness, volume ratio 1:1) onto SiO2 membrane TEM windows, as well as on borosilicate glass substrates, followed by vacuum annealing. Due to poor wetting between the SiO2 substrates and the metal film, nanoparticles are formed (similar to the process studied by Karakouz et al.).39 Figure 1a shows a high angle annular dark field (HAADF) STEM image of the sample following the annealing procedure. A large number of the particles show regions with two clear contrasts. Assuming phase separation of the Ag and Cu has occurred, brighter regions should correspond to Ag rich regions, and darker to Cu rich ones. This is due to stronger electron scattering by the heavier Ag atoms. The optical extinction spectrum of the sample was recorded through a simple transmission measurement using a sample with a glass substrate. The resulting spectrum is displayed in Figure 1b. The spectrum is dominated by a strong resonance peak around 450 nm, together with a smaller peak on the shoulder of the other at around 570 nm. For reference pure Ag and Cu particles were fabricated using the same procedure. 5 nm thick Ag or Cu films were evaporated onto glass slides followed by vacuum annealing. The optical spectra of these samples are also shown in Figure 1b. These spectra feature single resonance peaks at 425 nm and 600 nm for Ag and Cu particles respectively. The spectrum of the Cu particles also feature absorption as brought on by interband transitions which start to occur below 560 nm.


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Figure 1. a) HAADF STEM image of Ag-Cu particles after vacuum annealing. b) Optical extinction spectrum of the Ag-Cu particles, together with spectra for pure Ag and Cu particles fabricated using the same process.

To verify that the contrast seen in Figure 1a is the result of Ag and Cu regions, as well as to confirm complete phase separation, EDS mapping was carried out. Figure 2a shows a HAADF STEM image of some of the formed particles, and Figure 2b and 2c show EDS maps of Ag and Cu content in the same region. As can be seen, a clear phase separation has taken place. There


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appears to be some Cu present in a thin shell around the entire particles, also covering the Ag regions. We believe this is due to the formation of copper oxide, as also observed by Pellarin et al. in their study of smaller Ag-Cu particles.16 High resolution TEM images also reveal regions of amorphous contrast around the particles, further indicating the formation of an oxide shell, see Supporting Information Figure S2. From 3D EDS tomography mapping, see Supporting Information, we observe that the border between Ag and Cu appear to form predominantly perpendicular to the substrate surface. From a simple consideration of minimizing the total energy stemming from surface and interfaces, see Supporting Information, one can show that slightly elongated prolate spheroids are the most stable configuration for the Ag-Cu particles. This when comparing to core-shell structures, or complete separation of Ag and Cu into different particles. The instrument used for the EDS mapping was also equipped with an EELS detector, and feature the possibility to carry out simultaneous EDS and EELS mapping. We used this possibility to see if we can shed some light on the two different resonances seen in the optical extinction spectrum of the particles. Figure 2d shows the EELS spectrum from scanning the sample area shown in Figure 1a. Thanks to the narrow energy spread of the cold cathode, energy losses in the LSPR range can be resolved. EELS mapping was carried out by integrating the EELS intensity in two energy ranges (1.7 – 2.1 eV and 2.8 – 3.1 eV) corresponding to the two resonances observed in the optical spectrum. This was done after removal of the intensity from the tail of the zero loss peak. Figure 2e and f show combined EDS and EELS maps of the sample area shown in Figure 1a for the two energy loss ranges. We find that in the high energy range (2.8 – 3.1 eV), corresponding to the main peak in the extinction spectrum, losses are occurring predominantly on the Ag side of the particles. The lower energy range (1.7 – 2.1 eV),


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corresponding to the weaker peak in the extinction spectrum, losses covering the entire particles were found.

Figure 2. HAADF STEM image of Ag-Cu nanoparticles (a) together with EDS maps of Ag (b) and Cu (c) content. d) EELS spectra collected from the sample area shown in Figure 1a. e-f) Combined EDS and EELS maps of the same sample area for two energy loss ranges as indicated in the EELS spectrum in (d). All measurements were carried out at an acceleration voltage of 200 kV.

To gain further insight into the resonances of the Ag-Cu phase separated nanoparticles we performed STEM-CL mapping. In CL, light emitted by the sample upon excitation by an electron beam is measured. We performed these measurements in a customized JEOL STEM 2100F equipped with an optical window in the column. This together with a parabolic mirror


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around the sample position allows for the collection of the CL signal. The mirror collimates light emitted by the sample through the window and on to a spectrophotometer. By recording the spectra of the emitted light while scanning the electron beam across the sample, maps of the plasmon excitations can be performed. All CL measurements in this work was carried out at an acceleration voltage of 80 kV. For details on the STEM-CL system we refer the reader to the works by Yamamoto and Sannomiya et al.40, 41 From the STEM-CL maps of a single Ag-Cu particle we could identify three apparent resonances, as shown in Figure 3 (CL spectrum of the particle can be found in Figure S4 in the Supporting Information). Two of the resonances occur at shorter wavelengths (385 nm and 450 nm) and the third more towards the red part of the spectrum (630 nm). The two resonances at shorter wavelengths appear to be more localized towards the Ag half of the particle, whereas the resonance at 630 nm has a more dipolar appearance covering the entire particle. This is in agreement with the observed behavior from the EELS measurement. We performed finite element method (FEM) modeling of light scattering by a Ag-Cu particle with similar dimensions, see Figure 3f-h. For details on the modeling procedure, see the Methods section. Since CL signal corresponds to the z-component (electronbeam path direction) of the radiative electromagnetic local density of state (EMLDOS), we approximate the signal as the z-electric field intensity.42 Comparing the experimentally mapped resonances in Figure 3 to the modelled ones, a good agreement, although qualitative, is found. We find two resonances at shorter wavelengths with field intensities localized to the Ag half of the particle (Figure 3b and f, and Figure 3c and g). For the silver-localized mode with long axis polarization (Figure 3c and g), the z-field on the right edge is not clearly confined. This is because the flux penetrates through Cu which is a lossy and high index dielectric at this wavelength. This issue related to the energy loss will be discussed later. A resonance including


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the entire particle is also found at longer wavelengths. We note however, that the modeled resonances are slightly shifted towards shorter wavelengths. This is most likely due to the effect of the substrate in the measurement as well as differences in shape, size and used dielectric constant. The small red shift induced by a low index substrate, such as the one used here, is discussed more in detail in Figure S12 in the Supporting Information. The measured field distribution is slightly rotated compared to the simulation, which probably originates from the non-symmetric detection by the parabolic mirror covering the solid angle of 3π std. (see Figure S3 in the Supporting Information). CL mapping results of more particles and simulated z-field spectrum are found in the Supporting Information.

Figure 3. a) STEM HAADF image of a Ag-Cu phase separated nanoparticle. b-d) CL maps of the particle in (a) at three different wavelengths. The maps are collected from emission in all directions. e-g) FEM model of a comparable particle to the one in (a). e) Sketch of the geometry used in the calculation, a prolate spheroid with a long axes of 80 nm and two shorter axes of 60


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nm. f-g) Integrated electric z-field intensity (|Ez|2) from plane wave illumination at resonant wavelengths. The plots show the sum of field intensity from plane wave illumination of both s and p polarization. Bottom left are sketches of the resonances.

From these measurements and the modeling we conclude that the main resonance peak seen in the optical extinction spectrum in Figure 1 is the result of resonances localized to the silver half of the nanoparticles. At these wavelengths the Cu half of the particles is subject to interband transitions and its behavior is mainly absorbing (not metallic). It is therefore not contributing in the resonances at these wavelengths. However, the presence of the Cu does cause the Ag resonances to redshift and dampen (wider peak) as evident when comparing to the spectrum of the pure Ag particles to that of the Ag-Cu ones in Figure 1. At the longer wavelength resonance (630 nm) Cu is no longer affected by interband transitions, and hence a dipolar resonance including both the Cu and Ag parts of the particle can occur. Comparing to the resonance of pure Cu particles this resonance occur at a shorter wavelength, see Figure 1. This is due to the change in the effective dielectric function of the Ag-Cu particle compared to a pure Cu particle.

From the FEM model we can look more into detail about what is happening inside the particle. We estimate absorption by looking at resistive heat losses. Figure 4 shows the total resistive heat losses in a Ag-Cu particle, as well as in the Ag and Cu part of the particle, upon plane wave illumination (sum of both polarization while illuminating from top). Also shown are the losses occurring in Ag and Cu particles of the same shape as the parts of the Ag-Cu particle. These were calculated by simply replacing the material of Ag or Cu with air and running the model.


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From the results it is clear that the majority of the losses occur in the Cu part of the particle. Only the resonance at 350 nm show predominantly losses in Ag. Furthermore, comparing Cu in the Ag-Cu particle to a particle without the Ag, a strong enhancement in losses is observed for Cu in the Janus structure. This enhancement mainly occurs for the Ag resonance at 380 nm. For the Ag part of the particle the opposite is true, less losses occur in Ag in the Ag-Cu particle compared to a particle without Cu. Figure 4 b-c show the nearfield intensity and resistive losses upon excitation of the 380 nm silver resonance. A strong enhancement of the nearfield is seen at the Ag side of the particle, whereas for the lossy Cu side the field is much weaker and predominantly resistive losses are seen. This is a process similar to what has been previously reported for absorption enhancement in Pd for Au-Pd particle systems.9, 26 That is, Ag acts as an antenna collecting energy which is funneled into the Cu where it is absorbed.


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Figure 4 a) Calculated resistive losses upon plane wave illumination from top (sum of both polarizations). Solid lines shows the losses in the entire particle (black), and in Ag (gray) and Cu (yellow) parts respectively. For comparison, dotted lines show losses in particles where either Ag or Cu has been replaced by air. b) Electric field intensity (|E|2) upon excitation of the 380 nm Ag resonance. c) Resistive losses upon excitation of the 380 nm Ag resonance.

We now turn to look at the directionality of the scattering/emission from the three resonances of the Ag-Cu particles. Figure 5 shows angle resolved CL measurements of single Ag-Cu particles. In our STEM-CL setup it is possible to measure angle resolved emission by inserting a mask in the beam path, for details see the Supporting Information. The detection angle was scanned from -40° to 40° with respect to the electron beam incidence, where 0° correspond to the direction where the electron beam is coming from, see Figure 5a. To calibrate the intensity of the angle resolved measurement, emission from a 900×900 nm area was collected. The emission from this area, containing particles of random orientation is assumed to have no discernible directionality and is used as reference for the intensity at each wavelength, assuming only in-plane dipolar emission. We only consider in-plane dipoles since the emission from out-of-plane dipoles is greatly reduced by shadowing from the substrate. This is also evident from the CL-mappings in Figure 3 where only in-plane dipoles are observed. Figure 5b and 5c show angle resolved CL spectra for two Ag-Cu particles, one oriented with the Ag half to the left and one oriented in the opposite direction. We show the two different orientations to ensure that the calibration procedure described above is working, and that the observed directionality is not stemming from the measurement setup. For the short wavelength resonances we observe emission preferentially in the direction of the Ag half of the particles. Measurements for several more Ag-Cu particles


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can be found in the Supporting Information. In general in the wavelength range 400 – 550 nm a clear preferential emission towards the Ag side of the particles can be seen. This range corresponds to the Ag localized resonances, where Cu is mainly absorptive. At longer wavelengths, above 600 nm where the dipolar resonance of the whole particle is located, the picture becomes less clear and no trend in the directionality of the particles is observed. We believe that the directionality at this wavelength range is more related to the exact shape of the studied particles.

Figure 5. Measured angle resolved STEM-CL spectra, collected by the use of a mask in the light path while scanning the electron beam over the entire particle, see Supporting Information Figure S3. a) Sketch of the measurement configuration and angle definition. b-c) Measured angle-resolved CL emission spectra for two Ag-Cu particles with opposite orientation. Scale bars in inset HAADF STEM images correspond to 50 nm.

To calculate the directionality of the CL signal in the FEM model used above, the illumination angle was varied between -90° and 90°, see Figure 6a. Upon this illumination the intensity of the


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z-component of the electric field was integrated inside a spherical volume with a 100 nm radius, to mimic the scanning of the electron beam across the entire particle. The incident plane wave direction is set so that the wave vector matches the CL detection. Since the CL measurements were carried out without polarizer, both s and p polarizations were calculated and summed up. The resulting angle dependence at the resonant wavelengths is shown in Figure 6b. The resonance at 380 nm shows a slight preference towards the Ag side of the particle, whereas the resonance at 590 nm is centered around 0°. This is in agreement with the observed CL signal. The resonance at 350 nm increases as the angle moves away from zero. This is due to the out-ofplane mode of this resonance which is increasingly excited as the angle is increased. We note that in the experiment the perpendicular mode is not as efficiently collected as the in-plane mode because of the presence of the sample support. This can also be seen in the maps in Figure 3, where only in-plane resonances are observed. The normalization process carried out (assuming in-plane dipoles) also suppresses such high angle emission. Figure 6c shows a similar plot as Figure 6b, but instead displays resistive losses in the Cu part of the particle. Here the opposite is seen, i.e. at 380 nm the losses are slightly diverted towards the Cu direction. We conclude that this asymmetric absorption is the reason for the observed directionality of the emission.


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Figure 6. Model of CL directional intensity. An Ag-Cu particle was illuminated at an angle with plane waves of two different polarizations, as indicated in (a). The intensity of the z-component of the electric field (|Ez|2), corresponding to the CL signal, was integrated inside a spherical volume with a radius of 100 nm (indicated by black circle). b) Normalized Ez-field intensity


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angle dependence at resonant wavelengths. c) Normalized angle dependent resistive losses in Cu at resonant wavelengths.

Conclusions We study, in detail, the plasmonic properties of Ag-Cu phase separated Janus nanostructures. Such structures can easily be fabricated through co-evaporation followed by an annealing treatment. Here we demonstrate such fabrication using Ag and Cu as a model system, however, the same process should be applicable also for other phase separated metal combinations. For the Ag-Cu system under investigation here, Janus structure particles were formed. These particles, due to different onset energy of interband transition, feature additional plasmonic resonances in a single particle. At longer wavelengths the expected dipolar resonance is found, but at shorter wavelengths, where Cu is mainly absorptive due to interband transitions, resonances can still occur in the Ag part of the particles. Apart from giving rise to interesting ways to tailor resonances in particles, it also allows for engineering absorption in the system, as we show for the enhanced absorption in Cu in our particles. This is of interest for photocatalysis where the increased number of hot electrons generated in the system can be utilized. We note that this occurs at wavelengths normally not reachable by resonances in pure Cu particles, giving the possibility to have more energetic hot electrons. The Cu oxide shell we observe on the particles could also be beneficial for photocatalysis as demonstrated in the work by Zhang et al.43 Alternatively, the plasmon enhancement might help reduce the oxide making it more active for other reactions, e.g. propylene epoxidation, as demonstrated for pure Cu particle by Marimuthu et al.44 The lossy nature of the Cu also gives rise to a Ag resonance with a slight directionality of


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its emission. The interface between Cu and Ag in the fabricated particles is predominantly perpendicular to the substrate surface. However, the in-plane orientation on the surface of the formed particles is still random. We envision that by employing a slightly more advanced fabrication process, such as the process presented by Nugroho et al.,45 and seeding one side of the particles with either Cu or Ag it might be possible to control the direction of the particles.


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ASSOCIATED CONTENT Supporting Information. 3D EDS maps (including video file), high resolution TEM images, description of the STEM-CL setup, CL spectrum of a single Ag-Cu particle, overview CL map images, surface free-energy calculations, and additional measured angle resolved STEM-CL spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Carl Wadell (Email: [email protected]) * Takumi Sannomiya (Email: [email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by Japanese Society for the Promotion of Science #15F15744 and #17K19025, SEI Group CSR foundation and JGC-S Scholarship Foundation.


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