www.sciencemag.org/content/352/6293/1565/suppl/DC1

Supplementary Materials for

Polyelemental nanoparticle libraries Peng-Cheng Chen, Xiaolong Liu, James L. Hedrick, Zhuang Xie, Shunzhi Wang, Qing-

Yuan Lin, Mark C. Hersam, Vinayak P. Dravid, Chad A. Mirkin*

*Corresponding author. Email: chadnano@northwestern.edu

Published 24 June 2016, Science 352, 1565 (2016) DOI: 10.1126/science.aaf8402

This PDF file includes:

Materials and Methods Figs. S1 to S26 Reference (34)

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Materials and Methods Materials

Poly(ethylene oxide)-b-poly(2-vinyl pyridine) (PEO-b-P2VP, Mn = 2.8-b-1.5 kg/mol, polydispersity index = 1.11) was purchased from Polymer Source, Inc. Metal compounds, HAuCl4·3H2O, AgNO3, Cu(NO3)2·xH2O, Co(NO3)2·6H2O, and Ni(NO3)2·6H2O were purchased from Sigma-Aldrich, Inc. and used without further purification. DPN 1D pen arrays (type M, no gold coating) were purchased from Nanoink, Inc. TEM grids with hydrophobic silicon nitride support films (Si3N4 film thickness = 15 or 50 nm, modified with 2.5 nm atomic layer-deposited alumina and fluoro-methyl-silane) were purchased from Ted Pella, Inc. Silicon wafers were purchased from Nova Electronic Materials.

Preparation of block copolymer inks

Polymer inks were prepared by dissolving PEO-b-P2VP and different metal compounds in de-ionized water in predetermined molar ratios to achieve inks with specified molar ratios of metal precursors. The ink solution had a polymer concentration of 5 mg/mL and the molar ratio of pyridyl group to total metal precursors was 64:1. We expect that the use of an excess of pyridyl group will lead to a complete complexation of metal ions from the metal precursor salt onto PEO-b-P2VP. The pH of the ink solution was adjusted to between 3-4 by addition of HNO3. The ink solution was stirred for 24 h at room temperature prior to use.

Scanning probe block copolymer lithography (SPBCL) patterning process

DPN pen arrays were dip-coated with inks and dried under ambient conditions. Following the inking step, the pen arrays were mounted onto an AFM (XE-150, Park Systems) and brought into contact with a TEM grid to deposit arrays of polymer nanoreactors. The patterning process was performed in a chamber at a controlled temperature of 25 °C and relative humidity of 90%. The size of the polymer nanoreactors was tuned by adjusting the tip-sample dwell time. In order to convert polymer features into nanoparticles, the TEM grids were put into a tube furnace and thermally annealed. The heating conditions were programmed as follows: ramp to 120 °C under Ar in 1 h, hold at 120 °C for 48 h, cool back to room temperature in 4 h, switch the atmosphere into H2, ramp to 500 °C in 2 h, thermally anneal the substrate at 500 °C for 12 h, and cool down to room temperature in 6 h.

Polymer pen arrays that are made of PDMS were used to perform SPBCL in a massively parallel fashion (24). The pen array was first treated with oxygen plasma (60 W, 4 min) to improve the conformity of ink coating. Following this step, the pen array was spin-coated with the ink solution (1000 rpm, 1 min), allowed to dry in air, and mounted onto an AFM (XE-150, Park Systems) to deposit polymer features on a Si substrate. The Si substrate was pre-treated with hexamethyldisilazane to make it hydrophobic. The patterning was carried out at a controlled temperature of 25 °C and relative humidity of 90%. The resulting polymer features were converted to nanoparticles using the aforementioned thermal annealing process.

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Characterization SEM images of the nanoparticles synthesized on Si wafers were taken with a Hitachi

S-8030 field-emission scanning electron microscope at an acceleration voltage of 5 kV and a current of 20 µA. An in-house designed dual-energy dispersive x-ray spectroscopy (EDS) detectors equipped Hitachi HD-2300 dedicated scanning transmission electron microscope (STEM) was used to image nanoparticles synthesized on TEM grids with 50 nm silicon nitride support films (34). The bright-field images were recorded using a phase contrast detector, and the dark-field images were taken with a high-angle annular dark-field (HAADF) detector at an electron acceleration voltage of 200 kV. Nanoparticle composition was studied using the dual EDS detectors (Thermo Scientific NSS 2.3) equipped on the HD-2300 STEM with a 200 kV acceleration voltage. The Lα peaks of Au and Ag, and the Kα peaks of Cu, Co, and Ni in the EDS spectra were used for composition quantification and elemental mapping. The composition measured by EDS has an inherent error of less than 5% due to x-ray absorption and fluorescence. Each EDS map is built based on 30 frames with pixel dimensions of 256×192 and pixel dwell time of 203 µs. High-resolution TEM (HR-TEM) images were taken on a JEOL 2100F transmission electron microscope at an acceleration voltage of 200 kV with nanoparticles prepared on TEM grids with 15 nm silicon nitride support films. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo Scientific ESCALAB 250Xi with samples made by drop-casting polymer ink solution, followed by the aforementioned thermal annealing process.

Supplementary Text Structural analysis of tetrametallic nanoparticles

EDS mapping in Fig. S4A-S4E shows the structures of tetrametallic nanoparticles for each quaternary combination of Au, Ag, Cu, Co, and Ni. The structures can be assigned based on the compatibility of components, as described in the binary and ternary analysis in the main text.

Fig. S4A, AuAgCuNi: the particle consists of three consecutive domains, i.e., AuAg alloy, AuCu alloy, and Ni. The components of Au, Ag, and Cu in this particle segregate into an AuAg alloy domain and an AuCu alloy domain, which is similar to the structure of AuAgCu particle in Fig. 3B. The fourth component, Ni, forms an additional domain due to its immiscibility with Au.

Fig. S4B, AgCuCoNi: the particle consists of three consecutive domains, i.e., CoNi alloy, Cu, and Ag. The components of Ag, Cu, and Co in this particle segregate into three domains, which form the primary particle. Ni is mainly present in the Co domain due to its higher affinity for Co than Cu.

Fig. S4C, AuAgCoNi: the particle has a dimer structure with one part being an AuAg alloy and the other a CoNi alloy because Au is only miscible with Ag and Co is only miscible with Ni.

Fig. S4D, AuAgCuCo: the particle consists of three consecutive domains, i.e., AuAg alloy, AuCu alloy, and Co. The components of Ag, Cu, and Co segregate into three domains, which form the primary particle. Au is present in the Ag and Cu domains since it is compatible with Ag and Cu while being immiscible with Co.

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Fig. S4E, AuCuCoNi: the particle has a dimer structure with one part being an AuCu alloy and the other a CoNi alloy. AuCoNi segregates into an Au domain and a CoNi domain, which form the primary particle. Cu is only present in the Au domain since it phase-segregates from Co and has a higher affinity for Au than Ni.

Valence state of the metal elements in nanoparticles

To confirm that the nanoparticles are mainly composed of metals after being annealed under H2 at 500 °C, we thermally annealed AuAgNi, AuAgCu, and AgCuCo nanoparticles in O2 and characterized the morphology change of nanoparticles with STEM-EDS. As shown in Fig. S7, S14 and S21, when Cu, Co or Ni are transformed into oxides, they collapse onto the substrate (alumina on a Si3N4 film is used here) and change the configuration of original nanoparticles. This suggests that particles composed of oxides are not stable enough to maintain their structures when they are subjected to long-term thermal treatment. The oxidation state of the metals that comprise the nanoparticles was further explored by XPS with drop-cast samples (Fig. S7C, S14C and S21C). The Co, Ni, and Cu components in the nanoparticles are in the 0 valent state after being annealed under H2 at 500 °C. However, once the particles are exposed to air, the Co, Ni, or Cu components begin to oxidize.

Polymer-based bulk syntheses of polyelemental nanoparticles

It is difficult to synthesize heterostructured trimetallic nanoparticles with a desired size and composition by thermally treating a bulk polymer ink solution directly. Attempts to make trimetallic particles by bulk synthesis lead to complex mixtures of ill-defined materials with no control over size and particle composition. In fact, many of the particles have only one or two of the elements, and of the particles that have all three, they do not have the desired ratio (as controlled in the SPBCL technique and the stoichiometry of the precursor in the ink). As shown in Fig. S10, bulk synthesis of AuAgNi results in ternary particles with little Ni content or particles almost composed entirely of Ni. Meanwhile, bulk synthesis of AgCuCo results in particles mainly consisting of one element (Fig. S11). This phenomenon is reasonable since different metal precursors possess different reduction kinetics (30) and exhibit element-specific nucleation (1, 5).

Fig. S23 shows histograms of the content of each element in SPBCL-synthesized quinary particles. The compositional variation of quinary particles synthesized by SPBCL is larger than that of ternary particles. About half of the particles are composed of five elements. By contrast, bulk synthesis of quinary particles by direct thermal treatment of the polymer solution generates a mixture of particles from unary to quinary while ternary ones are primarily obtained (Fig. S24). Additionally, the compositional variation of the particles is very broad (Fig. S24C). Even for the ternary and quaternary particles, the content of each element is not equal (Fig. S24B). Particles with one or two elements predominating the composition are generally obtained. In addition to synthesizing quinary particles from the polymer ink solution, we attempted to make quinary particles by an aqueous solution-based, one-pot synthesis with NaBH4 as the reducing agent (Fig. S24D). Since the Au, Ag, Cu, Co, and Ni have a large variation in reduction potentials, Au, Ag, and Cu precipitate immediately before Co and Ni. The synthesis needs delicate optimization to balance the reduction kinetics of the different metal ion precursors and to control the site-selective nucleation of each metal.

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Incomplete coalescing of metal precursors in polymer reactors

If only one nanoparticle is formed in each nanoreactor in SPBCL, the nanoparticle size will be determined by the amount of total available precursors for forming nanoparticles (20-22) and the nanoparticle composition will solely depend on the loading ratio of each constituent metal precursor in the ink solution (23). However, if more than one nanoparticle is formed in each reactor, the amount and the type of precursors that aggregate to form each nanoparticle can no longer be controlled. This results in nanoparticles with various sizes and compositions (Fig. S12).

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Fig. S1. HAADF-STEM images and EDS spectra of the unary nanoparticles in Fig. 1B. (A) Au, (B) Ag, (C) Cu, (D) Co, (E) Ni. Scale bars: 15 nm.

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Fig. S2 HAADF-STEM images, EDS spectra, and detailed EDS mapping information of the binary nanoparticles in Fig. 1C. (A) AuAg (48% Au, 52% Ag), (B) AuCo (55% Au, 45% Co), (C) AuNi (54% Au, 46% Ni), (D) AuCu (49% Au, 51% Cu), (E) AgCu (42% Ag, 58% Cu), (F) AgNi (57% Ag, 43% Ni), (G) AgCo (63% Ag, 37% Co), (H) CuNi (58% Cu, 42% Ni), (I) CuCo (41% Cu, 59% Co), (J) CoNi (60% Co, 40% Ni). Scale bars: 15 nm.

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Fig. S3 HAADF-STEM images, EDS spectra, and detailed EDS mapping information of the ternary nanoparticles in Fig. 1D. (A) AuCuNi (23% Au, 17% Cu, 60% Ni), (B) AuAgCo (25% Au, 19% Ag, 56% Co), (C) AuCuCo (21% Au, 35% Cu, 44% Co), (D) AuCoNi (32% Au, 29% Co, 39% Ni), (E) AgCoNi (48% Ag, 28% Co, 24% Ni), (F) AgCuNi (50% Ag, 25% Cu, 25% Ni), (G) CuCoNi (33% Cu, 52% Co, 15% Ni), (H) AuAgNi (24% Au, 24% Ag, 52% Ni), (I) AgCuCo (35% Ag, 38% Cu, 27% Co), (J) AuAgCu (29% Au, 37% Ag, 34% Cu). Scale bars: 15 nm. The STEM images and EDS mapping information of (H-J) are shown in Fig. 2 and Fig. 3.

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Fig. S4 HAADF-STEM images, EDS spectra, and detailed EDS mapping information of the quaternary and quinary nanoparticles in Fig. 1E and 1F. (A) AuAgCuNi (32% Au, 19% Ag, 27% Cu, 22% Ni), (B) AgCuCoNi (27% Ag, 22% Cu, 35% Co, 16% Ni), (C) AuAgCoNi (26% Au, 22% Ag, 16% Co, 36% Ni), (D) AuAgCuCo (15% Au, 29% Ag, 18% Cu, 38% Co), (E) AuCuCoNi (23% Au, 42% Cu, 16% Co, 19% Ni), (F) AuAgCuCoNi (19% Au, 24% Ag, 28% Cu, 14% Co, 15% Ni). Scale bars: 15 nm. The STEM images and EDS mapping information of (F) are shown in Fig. 4.

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Fig. S5 (A) SEM image of an array of SPBCL-generated AuAgNi nanoparticles on a Si substrate. (B) Fast Fourier transform (FFT) of the AuAg and Ni domains in Fig. 2C.

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Fig. S6 EDS elemental mapping of four neighboring AuAgNi nanoparticles in an array. All four of nanoparticles are heterodimers where one domain is AuAg alloy and the other domain is Ni. Scale bars of insets: 10 nm.

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Fig. S7 HAADF-STEM images and the corresponding EDS elemental mapping of a AuAgNi nanoparticle. (A) AuAgNi nanoparticle generated by the regular SPBCL process. (B) The same nanoparticle after being thermally annealed at 300 °C in O2 for 12h. Scale bars: 20 nm. (C) XPS spectra of AuAgNi nanoparticles made by drop-casting polymer ink solution after the thermal annealing process used in SPBCL (black line), and then the same sample after being further thermally annealed at 300 °C in O2 for 12h (red line).

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Fig. S8 Centimeter-scale AuAgNi nanoparticle arrays generated by a combination of SPBCL and Polymer Pen Lithography (PPL) (24). (A) Dark-field optical microscopy image of a section of the centimeter-scale pattern of polymer features. (B) SEM image of the polymer features printed by PPL. (C) SEM image of the arrays of AuAgNi nanoparticles synthesized from the polymer nanoreactors that were printed by PPL. Upon combining SPBCL with a large area cantilever-free scanning probe technique such as polymer pen lithography, over 10 million particles were synthesized simultaneously over a 1 cm2 area. With the development of high throughput inking strategies and increased polymer dome deposition densities, this technique will eventually allow for the synthesis of billions of particles simultaneously over square centimeter areas.

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Fig. S9 Compositional diagrams assessing the uniformity of SPBCL-generated AuAgNi nanoparticles. The metal loading ratio of the ink solution is: (A) 40% Au, 40% Ag, 20% Ni, (B) 33% Au, 33% Ag, 33% Ni, (C) 25% Au, 25% Ag, 50% Ni, (D) 17% Au, 17% Ag, 66% Ni, and (E) 10% Au, 10% Ag, 80% Ni. (sample size: 50 for each composition). The composition of the nanoparticles follows the composition of the ink solution (see Fig. 2).

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Fig. S10 (A, B) DF-STEM images of AuAgNi nanoparticles synthesized from bulk polymer ink solution with the two-step annealing procedure used in SPBCL. (C) The EDS spectra of the two nanoparticles in B, showing that bulk synthesis results in a mixture of different types of nanoparticles. (D) The compositional uniformity of the AuAgNi nanoparticles synthesized from bulk polymer ink solution (sample size: 50).

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Fig. S11 (A, B) DF-STEM images of AgCuCo nanoparticles synthesized from a bulk polymer ink solution with the two-step annealing procedure used in SPBCL. (C) EDS spectra of the two nanoparticles in B, showing that bulk synthesis results in a mixture of different types of nanoparticles. (D) The compositional uniformity (or lack thereof) of the AgCuCo nanoparticles synthesized from bulk polymer ink solution (sample size: 50).

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Fig. S12 Incomplete coalescing of metal precursors results in multiple nanoparticles forming within a single polymer dome. (A) HAADF-STEM image of multiple AuAgCoNi nanoparticles formed in one large polymer dot. The polymer dot was prepared from an ink solution with a metal loading ratio of 25% Au, 25% Ag, 25% Co, and 25% Ni. The circle highlights the contour of original polymer reactor. (B) Zoomed-in HAADF-STEM image of four representative nanoparticles. (C) EDS elemental mapping of the four nanoparticles. (D) Overlay of the element maps of Au, Ag, Co, and Ni.

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Fig. S13 (A) Compositional uniformity of SPBCL-generated AgCuCo nanoparticles (sample size: 50). The metal loading ratio of the ink solution is: 33% Ag, 33% Cu, and 33% Co. (B) EDS elemental mapping of nine neighboring AgCuCo nanoparticles in an array. All nanoparticles are heterotrimers where Ag, Cu, and Co segregate into three segments. Scale bars in EDS mapping images: 15 nm.

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Fig. S14 HAADF-STEM images and EDS elemental mapping of a AgCuCo nanoparticle. (A) AgCuCo nanoparticle generated by annealing at 500 °C in H2 for 12h. (B) The same nanoparticle after being thermally annealed at 300 °C in O2 for 12h. Scale bars: 20 nm. (C) XPS spectra of AgCuCo nanoparticles made by drop-casting polymer ink solution followed by the thermal annealing process used in SPBCL (black line) and the same sample after being further thermally annealed at 300 °C in O2 for 12h (red line).

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Fig. S15 Nonparallel phase boundaries in AgCuCo nanoparticles generated by SPBCL. (A) Dark-field STEM image and corresponding EDS elemental mapping of a AgCuCo nanoparticle with nonparallel phase boundaries. Dashed lines highlight the position of Ag-Cu phase boundary and Cu-Co phase boundary. The two interfaces are perpendicular with respect to image plane. Inset: scheme depicting the proposed structure of this nanoparticle. (B) Another representative AgCuCo nanoparticle with nonparallel phase boundaries. Here, the Cu-Co interface is perpendicular to the image plane while the Ag-Cu interface is inclined with respect to the image plane. Inset: scheme depicting the proposed structure of this nanoparticle.

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Fig. S16 (A) HR-TEM image of the interface between Ag and Cu in a AgCu nanoparticle. The observed lattice spacings near the interface are 2.34 Å and 2.08 Å (extracted from FFT), which closely match the Ag (111) and Cu (111) planes. The inset shows the particle with the zoomed-in region indicated with a dashed square. (B) HR-TEM image of the interface between AuCu and Co in a AuCuCo nanoparticle. Cu and Co have a very close lattice parameter and similar atomic number contrast in HR-TEM. It is difficult to directly identify the Cu-Co interface in a CuCo nanoparticle using HR-TEM. Here, 15% Au was added into CuCo nanoparticles to increase the contrast between Cu domain and Co domain. The observed lattice spacing around the interface are 2.21 Å and 2.03 Å, which can be assigned to the AuCu (111) and Co (111) planes. The inset shows the particle with the zoomed-in region indicated with a dashed square.

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Fig. S17 SPBCL AgCuCo nanoparticle with low Cu content and three phase boundaries. (A) Dark-field STEM image of a AgCuCo nanoparticle (43% Ag, 19% Cu, 38% Co). Inset: scheme depicting the structure of this nanoparticle. (B) EDS elemental mapping of the nanoparticle. (C) Overlay of the element maps of Ag, Cu, and Co. (D) Overlay of the element maps of Ag and Co. (E) Overlay of the element maps of Cu and Co. Dotted circles highlight the position of Ag-Co phase boundary. When the content of Cu is less than 20% in AgCuCo nanoparticles, the amount of Cu is not enough to fully block the contact between Ag and Co. As shown in Fig. S17, a AgCuCo nanoparticle with Ag-Cu, Cu-Co, and Ag-Co phase boundaries is observed.

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Fig. S18 Additional EDS mapping information of the nanoparticles in Fig. 3A. Scale bar: 20 nm. The content of Au in the nanoparticle is 10%. The overlay of Ag and Au element maps and the overlay of Cu and Au maps clearly indicate that Au is mainly concentrated in the Cu domain.

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Fig. S19 (A) Compositional uniformity of SPBCL-generated AuAgCu nanoparticles (sample size: 50). The metal loading ratio of the ink solution is: 20% Au, 40% Ag, and 40% Cu. (B) EDS elemental mapping of nine neighboring AuAgCu nanoparticles in an array. The nanoparticles are heterodimers where one domain is an AuAg alloy and the other domain is an AuCu alloy. Au is mainly concentrated in the AuCu domain. Scale bars in EDS mapping images: 10 nm.

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Fig. S20 (A) HR-TEM image of a typical AuAgCu nanoparticle made from ink containing 30% Au, 35% Ag and 35% Cu. The nanoparticle is crystalline with observed lattice spacings of 2.04 Å and 2.32 Å (extracted from FFT). (B) HR-TEM image of a typical AuAgCu nanoparticle made from ink containing 70% Au, 15% Ag and 15% Cu. The nanoparticle is crystalline with an observed lattice spacing of 2.35 Å (extracted from FFT). Dashed squares in the insets indicate the zoomed-in region of the nanoparticles.

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Fig. S21 HAADF-STEM images and corresponding EDS elemental mapping of a AuAgCu nanoparticle. (A) AuAgCu nanoparticle generated by annealing at 500 °C in H2 for 12 h. (B) The same nanoparticle after being thermally annealed at 300 °C in O2 for 12 h. Scale bars: 20 nm. (C) XPS spectra of AuAgCu nanoparticles made by drop-casting a polymer ink solution followed by the thermal annealing process used in SPBCL (black line) and the same sample after being further thermally annealed at 300 °C in O2 for 12h (red line).

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Fig. S22 EDS elemental mapping of nine neighboring AuAgCuCoNi nanoparticles in an array. Scale bars in EDS mapping images: 15 nm.

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Fig. S23 Compositional uniformity of SPBCL-generated AuAgCuCoNi nanoparticles (sample size: 50). The metal loading ratio of the ink solution is: 20% Au, 20% Ag, 20% Cu, 20% Co, and 20% Ni. Since EDS has an inherent error of less than 5% due to x-ray absorption and fluorescence, only elements with content larger than 5% are considered as an effective component when counting the number of components in particles.

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Fig. S24 (A) DF-STEM images of nanoparticles synthesized from bulk polymer solution with the two-step annealing procedure used in SPBCL. The metal loading ratio of the polymer solution is: 20% Au, 20% Ag, 20% Cu, 20% Co, and 20% Ni. (B) EDS spectra of the three nanoparticles in B, showing that bulk synthesis results in a mixture of different types of nanoparticles. (C) Compositional uniformity of the nanoparticles synthesized from a bulk polymer ink solution (sample size: 50). (D) One-pot aqueous solution based synthesis of quinary particles with NaBH4 as reducing agent (Scale bar: 30 nm). Au, Ag, and Cu are more readily reduced than Co and Ni.

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Fig. S25 EDS elemental mapping of a AuAgCuCoNi nanoparticle. Scale bar: 15 nm. The phase boundary between AuAg alloy and AuCu alloy is not parallel with the phase boundary between AuCu alloy and CoNi alloy. Dashed lines highlight the position of phase boundaries.

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Fig. S26 HR-TEM image of the interface between AuCu alloy and CoNi alloy in a AuCuCoNi nanoparticle. The observed lattice spacings around the interface are 1.96 Å and 1.77 Å (extracted from FFT), which are in agreement with the AuCu alloy (200) and alloy CoNi (200) planes. Dashed square in the inset indicates the zoomed-in region of the nanoparticle. There are two phase boundaries in a AuAgCuCoNi nanoparticle (Fig. 4 and S25), i.e., the boundary between AuAg alloy and AuCu alloy, and the boundary between AuCu alloy and CoNi alloy. HR-TEM of the AuAgCu nanoparticles (Fig. S20A) confirms the crystallinity of the domains that contain the boundary between AuAg alloy and AuCu alloy. For the other phase boundary, we performed HR-TEM on the AuCuCoNi nanoparticles. As shown in Fig. S26, the domains near this boundary are crystalline with observed lattice spacings being 1.96 Å and 1.77 Å, respectively.

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