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. 2023 Jan 23;15(4):5620–5627. doi: 10.1021/acsami.2c19584

Dry Synthesis of Pure and Ultrathin Nanoporous Metallic Films

Hyunah Kwon †,‡, Hannah-Noa Barad §, Alex Ricardo Silva Olaya ∥, Mariana Alarcón-Correa †,‡, Kersten Hahn #, Gunther Richter ⊥, Gunther Wittstock ∥, Peer Fischer †,‡,*
PMCID: PMC9906609  PMID: 36690332

Abstract

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Nanoporous metals possess unique properties attributed to their high surface area and interconnected nanoscale ligaments. They are mostly fabricated by wet synthetic methods that are not universal to various metals and not free from impurities due to solution-based etching processes. Here, we demonstrate that the plasma treatment of metal nanoparticles formed by physical vapor deposition is a general route to form such films with many metals including the non-noble ones. The resultant nanoporous metallic films are free of impurities and possess highly curved ligaments and nanopores. The metal films are ultrathin, yet remarkably robust and very well connected, and thus are highly promising for various applications such as transparent conducting electrodes.

Keywords: nanoporous gold, nanoporous metal, dry synthesis, plasma processing, metallic film, transparent electrode

1. Introduction

Nanoporous metals (NPMs) exhibit unique properties distinct from bulk metals.1 The nanoscale pore architecture directly determines the optical,2 electrical,3 chemical,4 and mechanical5 properties. The high surface-area-to-volume ratios and large number density of high surface energy atoms at curved surfaces give rise to significant catalytic activities that are absent in the bulk metal.6,7 NPMs are also lightweight and can be optically transparent when they are very thin.8,9 The interconnected films of NPMs have thus attracted strong interest for applications as electrode materials for synthesis,10 energy conversion,11 sensing,12 or as plasmonic materials.13 Since NPMs do not occur naturally, they need to be synthesized.

Many different schemes to synthesize NPMs have been developed.14−17 A few studies report the deposition of metals onto pre-formed inorganic or polymeric nanoporous templates, typically by electrochemical deposition or sputtering, followed by an etching step that removes the sacrificial porous support structure.18−21 However, the requirement of a separate template and multiple fabrication steps is a disadvantage. A wet-chemical sol–gel scheme that relies on the reduction of metal salts to form metal nanoparticles followed by their aggregation has also been reported.8,15 While it affords some freedom to vary the composition, the resulting network shows few highly curved nanoporous features that can lead to interesting electrochemical properties.22 By far, the most widely used method to synthesize NPMs is dealloying as it can yield reactive metallic films with higher reactivity than the flat homologue. Dealloying involves the selective chemical etching of one or more less noble metals out of an alloy.14,23,24 For instance, nanoporous Au structures are generally formed by etching silver out of Ag–Au alloys.25 While the method is relatively simple and effective, the less noble metal, here Ag, cannot be completely removed for thermodynamic reasons. The remaining content of the less noble element is difficult to precisely control and may greatly affect the various properties of the resulting NPM.26,27 For brevity, we call the remaining content of the less noble metal in the NPM an “impurity”. In addition, the dealloying process to form NPMs with non-noble metals requires carefully chosen combinations of materials and chemistries so that the metals in the master alloy can be selectively etched. Generalizing the fabrication conditions using a wet synthesis method is thus challenging.28,29 Another issue of the dealloying method is to fabricate the free-standing ultrathin films of porous metals. These could be of use as transparent flexible electrodes and in other applications. Vapor dealloying30 and phase boundary gelation9 were reported to make 2D metal networks, but they are not general, that is, they only work for a particular metal and are not suitable for large-area fabrication. It is therefore desirable to develop a scheme that permits the controllable synthesis of NPMs that are pure, well connected but ultrathin and able to be formed from various metals including non-noble metals.

Here, we demonstrate a highly facile synthetic route to obtain impurity-free NPM films (NPMFs) with highly curved pore structures of adjustable sizes. The NPMFs are formed by the coalescence of metal nanoparticles in a low-temperature plasma. No solution processing or harsh chemicals are required, and the process can be generally applied. Our method yields pure nanoporous Au, Ag, Pt, Pd, Ni, and Fe films without any secondary metal impurities. Many of these elements could thus far not be processed to form NPMFs. Furthermore, our films are ultrathin and very well connected and thus can have high transmittance and low sheet resistance simultaneously, opening an opportunity as transparent conducting electrodes.

2. Experimental Section

2.1. Fabrication of NPMs

Poly(methyl methacrylate) (PMMA, average Mw ∼ 120,000 g/mol from Sigma-Aldrich) was dissolved (1.5 wt %) in chloroform for 12 h at room temperature by magnetic stirring. The solution was spin-coated on a Si wafer substrate [boron-doped, ⟨100⟩ orientation, native oxide covered, and cleaned in Piranha solution (a mixture of 3 volume parts of concentrated H2SO4 and 1 volume part of 30% H2O2 for 30 min)] at 1000 rpm for 1 min. Each metal (>99.99% purity) was evaporated by e-beam on a PMMA thin film at room temperature with an oblique angle of 80°, a rate of 0.05 nm/s, a target thickness of 10 nm considering the tooling factor, and a rotation speed of 0.72°/s. The deposited Au film was plasma-treated in 0.4 mbar air ambient with 200 W for 15 min. The Ag NPMF was obtained after treatment in 0.4 mbar Ar ambient with 200 W for 15 min, the Ni NPMF in 0.4 mbar W10 ambient (Ar 90%, H2 10%) with 150 W for 15 min, and other NPMFs (Pt, Pd, and Fe) in 0.4 mbar W10 ambient with 300 W for 15 min.

2.2. Compositional Analysis

A nanoporous gold film (NPGF) was fabricated on a Si substrate. After complete etching of PMMA, the film was used for combustion analysis and X-ray photoelectron spectroscopy (XPS). For the combustion analysis, a sample was burned in a flowing stream of oxygen.35 Very low concentrations of carbon can be detected [resolution below 1 ppm (wt)]. We compared the absorption spectra of three NPGFs on Si substrates and three cleaned bare Si substrates. Any CO2 was detected by absorption of infrared radiation. The measurement was repeated three times. XPS measurements were performed on a Theta Probe angle-resolved XPS system (Thermo Fisher Scientific Inc.). The base pressure was 3 × 10–10 mbar, and the excitation X-ray source was a monochromatic Al Kα radiation (100 W, hν = 1486.68 eV). Survey spectra were recorded at a pass energy of 200 eV, followed by high-resolution spectra for Au 4f, C 1s, O 1s, and Si 2p with a pass energy of 10 eV and a step size of 0.05 eV. Charge was corrected for all the binding energies by shifting the C–C (or C–H) part of the C 1s peak to 284.8 eV. Peak fitting was performed using the Avantage software (version 5.9904).

2.3. Electron Microscopy

For scanning electron microscopy (SEM), a NPMF fabricated by the method described herein on a Si substrate was used. For the transmission electron microscopy (TEM) preparation, the PMMA film was not etched entirely, and the sample was immersed in pure acetone to dissolve the PMMA film to obtain a free-standing Au film that could be lifted-off. The film was then picked up with a plasma-cleaned TEM carbon grid, followed by drying in an Ar flow for an hour. For TEM energy-dispersive X-ray (EDX) measurement, a SiO2 grid was used to avoid strong carbon signals from the grid. A 200 kV JEOL ARM200CF scanning transmission electron microscope equipped with a cold field emission electron source and a CETCOR image corrector (CEOS GmbH) was used to obtain high-resolution TEM (HR-TEM) images, and the ZEISS SESAM (sub-electronvolt-sub-angstrom-microscope) with a field emission gun (200 kV), equipped with a monochromator, a MANDOLINE-filter, and a 60 mm2 Thermo Fischer ultradry EDX detector, was used to obtain the TEM-EDX data.

2.4. Transmittance and Sheet Resistance

The NPGFs were fabricated on a double-polished sapphire substrate that was cleaned in Piranha solution. The UV–vis measurements were performed in a Varian Cary 4000 UV–vis spectrometer, with the wavelength range between 400 and 800 nm, in the scan mode. A baseline was established with a bare sapphire substrate. The Ossila four-point probe system was used to measure the sheet resistance. Probes have a separation of ∼1.3 mm between each other.

3. Results

3.1. Dry Synthesis of NPMFs

Our dry synthesis scheme is based on the plasma treatment of a dense layer of nanoparticles deposited on a sacrificial thin polymer film. The process is depicted in Figure 1a. First, a 1 μm-thick PMMA film is formed on a flat substrate, such as silicon, by spin-coating (see the Experimental Section). A dense layer of metal nanoparticles is then obtained by e-beam evaporation of the metal onto the dried PMMA film at an oblique angle, here 80°. The deposited metal atoms (e.g., Au, Ag, Pt, Pd, Ni, and Fe) form random clusters on the substrate, which give rise to shadowing under oblique angle growth. This growth results in a rough nanostructured thin film formed as a dense layer of metallic nanoparticles on top of the PMMA film. The angle of incidence affects the distribution of nanoparticles on the substrate31 and hence are also expected to affect the final morphology of NPMFs. It could be tuned to optimize the processes. The substrate is then treated in a simple laboratory plasma system. Figure 1b shows the SEM images of the resulting mesh-like NPMFs fabricated with Au, Pt, Pd, and Ag, and the non-noble metals Ni and Fe. The films have pore sizes between 20 nm and hundreds of nanometers, and the ligament sizes of the metal film ligaments range from 10 nm to 80 nm. An exemplary ligament in the network is marked by red lines in Figure 1b.

Figure 1.

Figure 1

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(a) Schematic: dry synthesis of NPMs. The deposited metal NPs on the PMMA layer transforms to NPM structures after plasma treatment. (b) SEM images of different NPMs. The scale bar is 100 nm for all panels. Red lines with arrows indicate the ligaments of the structure.

The formation of mesh-like NPMFs during plasma treatment is schematically shown in Figure 2a. During the plasma treatment, PMMA starts to be etched by plasma ions that penetrate through the gaps between the metal nanoparticles. As a result, the PMMA layer becomes rough and shrinks. The metal nanoparticles remain compact on the shrinking PMMA islands since the interfacial energy is small. At the same time, the metal nanoparticles are also bombarded with high-energy ions, which renders them mobile and causes their coalescence,32 facilitating cluster migration at low temperatures.33 The thermal energy is not high enough to liquefy the nanoparticles and cause their fusion as shown in Figure 2e but rather that the coalescence leads to polycrystalline structures with many grain boundaries (see Section 2.3).34,35 When the remaining PMMA layer has been completely etched away, the porous metal structures contact the substrate, where the metal is less mobile since the interfacial energy is lower compared to that between nanoparticles and PMMA. Coalescence then ceases and the NPMF remains robust despite further plasma treatment. Figure 2b shows the SEM images of NPGFs with dependance on the plasma time. Before plasma treatment, a dense layer of separated metal nanoparticles with a diameter of about 10 nm lies on the PMMA layer. The metal particles start to coalesce within 1 min and form thicker ligaments and larger pores with further plasma treatment. We checked by XPS and combustion analysis (see Section 2.3) and verified that the PMMA layer is completely etched after 15 min of plasma treatment. The interconnected structures of the NPGF are maintained with further plasma treatment.

Figure 2.

Figure 2

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(a) Cross-sectional schematics that show how the metal nanoparticles are thought to evolve over time (see the text for details). (b) SEM images of top view of NPGFs depending on the plasma time. (c) Layer of Au nanoparticles on a Si wafer after plasma treatment. (d) Au nanostructures fabricated by dry synthesis but without using the shadow growth technique. The Au film on PMMA was deposited under normal incidence without substrate tilting. (e) Au nanoparticles on PMMA with heating at 300 °C for 5 min. Networked Au structures are not formed without a sacrificial PMMA layer, shadow growth, and plasma treatment.

We analyzed the roles of the PMMA sacrificial layer, conditions of the shadow growth, and plasma treatment on the formation of NPGFs. First, Au nanoparticles were deposited on a bare Si substrate, followed by the plasma treatment as shown in Figure 2c. Several small (<10 nm) nanoparticles coalesce initially, but the resultant larger nanoparticles (larger than ∼20 nm) are no longer mobile enough and do not lead to networked structures on a Si wafer without a PMMA layer. Preliminary experiments indicate that the process can also be extended to other polymeric sacrificial layers such as polystyrene. Second, as a control, Au nanostructures were fabricated by dry synthesis but without using the shadow growth technique. For this, the Au film on PMMA was deposited under normal incidence without substrate tilting (thickness of 8 nm). The resultant film in Figure 1d is not continuous in contrast to those seen in Figure 1b, but it has discontinuous and much thicker ligaments. This is because a thin film has much smaller surface area than a surface covered with nanoparticles. Liquefaction occurs on the top surface area, and due to the surface energy, a discontinuous larger (than a nanoparticle) structure forms on the wafer. Therefore, shadow growth is necessary to obtain an interconnected mesh-like nanoporous film. Finally, heating the nanoparticle-decorated PMMA film without plasma treatment did not yield nanoporous metallic structures as shown in Figure 2e. Hence, the process does not simply result by thermal effects. Rather, the etching of PMMA plays an important role, in combination with surface tension effects and differences in interfacial energies between metal and the two substrates.

We assume that the NPMF morphology is mainly a function of the melting points of the metals, surface energies of the metals, and the interfacial energies between the PMMA and metals. The interfacial energies are difficult to measure or predict. Nevertheless, a trend emerges by comparing different morphologies of various metals, their respective melting points, and surface energies, as shown in Table S1 and Figure 1. For example, Ag has the lowest melting point and surface energy of the metals, and it gives rise to a NPMF that shows a small variation in the pore and ligament sizes. This may be explained by the low surface energy of Ag, which means that Ag NPs will not aggregate as much as the other metals. In contrast, Pt has the highest melting point and surface energy, and it gives rise to a non-uniform morphology with larger variations in pore and ligament sizes. This can be because Pt NPs are less mobile and tend to aggregate quickly compared to Ni or Fe NPMFs.

The ligament or pore sizes can be tuned by controlling the conditions during the plasma treatment, for example, the duration of the plasma treatment (Figure 2b). Furthermore, the nanoporous Au and Ni morphology can be controlled through the gas composition and the plasma power as shown in Figure S1. The kinetic energy of the ions in the plasma will vary according to the plasma conditions, resulting in different metal atom mobilities and different etching rates of the sacrificial PMMA layer.

3.2. Purity

Combustion analysis was carried out to check for carbon residues in the NPGF. In this method, CO2 resulting from the combustion of carbon present in the sample can be detected by infrared absorption.36 Only 5 ppm (wt) carbon residues from the plasma treatment could be detected. Hence, our NPMFs do not contain any significant carbon residues from the plasma treatment. This is also confirmed in the subsequent XPS analysis. As a note, the solubility of C in solid Au is known to reach only 50 ppm at temperatures above 1000 °C.

We also carried out XPS measurements to evaluate the surface of the NPGF and its purity. The Au 4f signals from a NPGF on a Si wafer are plotted in Figure 3. The signals indicate a typical metallic Au 4f7/2 binding energy of 84.4 eV with the Au 4f5/2 component separated by a spin–orbit splitting of 3.7 eV.37 The Au 4f7/2 component at 84.1 eV is known as the surface component of Au38 and is consistent with the large surface area of the NPGF. No peaks are seen that correspond to Au2O3.39 Comparison of the O 1s and C 1s spectra from the NPGF and a spin-coated PMMA layer is shown in Figure 3b,c, respectively. The O 1s spectrum only shows one symmetric peak with a maximum at 533.5 eV, which corresponds to SiO2 of the Si substrate.37 It is known that the O 1s peak for the metal oxide (excluding SiO2) appears below 531 eV, and it is clearly absent in our NPGF. The characteristic O 1s spectrum from a film containing only spin-coated PMMA40 disappears in the NPGF sample, indicating that PMMA is not present in the NPGF. The C 1s spectrum of PMMA in Figure 3c shows multiple peaks at around 288.9, 286.9, 285.5, and 283.5 eV that originate from O–C=O, C–O, C–C=O, and −CH3 bonds, respectively.40 However, these peaks do not appear in the NPGF sample (besides a very low intensity peak, which may relate to the C–O state), which corroborates that PMMA is completely removed by the plasma. The overall C content is extremely low in the NPGF sample and originates from unavoidable contamination during sample transfer. Our combustion analysis and XPS results indicate that PMMA is completely etched away and that no significant C impurities from PMMA remain after the plasma etching step. Compared to the conventional nanoporous Au films prepared by dealloying or solution phase synthesis, the dry synthesis reported here is a much more convenient and general route to obtain NPMFs that are also free from impurities.

Figure 3.

Figure 3

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XPS results of the NPGF sample. (a) Au 4f of the NPGF sample. (b) Comparison of the O 1s spectrum and (c) C 1s spectrum from the NPGF sample and a spin-coated PMMA sample. Black dots are from the measurement and red lines are the fitted curves.

3.3. TEM Characterization

TEM measurements were performed on the nanoporous Au and Ni films. Figure 4a shows that the diameter and length of the ligaments in the NPGF range from 10 to 30 nm and from 10 to 100 nm, respectively. Numerous pores are observed, and the surface shows a high curvature including many concave structures. A protruding section in the NPGF was also investigated by EDX mapping, as shown in Figure 4b. The image shows that Au is uniformly distributed over the NPGF structure. Moreover, C and O maps are uniform and indistinguishable from the background signals, in agreement with our previous observation that the NPGFs do not contain any significant carbon or oxygen contamination from the sacrificial PMMA. HR-TEM images of nanoporous Au (Figure 4c) and Ni (Figure 4d) films were also obtained. One can clearly identify many grain boundaries between the nanograins with a size of a few nanometers—comparable to the initial size of metal nanoparticles. They are formed when the nanoparticles approach each other and aggregate together during plasma treatment.41,42

Figure 4.

Figure 4

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TEM measurements of NPMFs. (a) Low-magnification image of the NPGF showing highly curved ligaments and pores. The ligament diameters are between 10 and 30 nm, which are even smaller than what is typically reported for nanoporous Au structures made by dealloying. (b) TEM–EDX analysis of a protruded region of the NPGF. The Au M signal is clear, but C K and O K signals are not distinguishable from background signals. High-resolution TEM images of the (c) NPGF pore region and (d) nanoporous Ni film. The NPMF has surfaces with multiple crystal directions, especially in curved regions, and many grain boundaries between grains of few nanometers in size.

3.4. Potential as Transparent Conducting Electrodes

The method described herein yields ultrathin NPM structures, which suggest further promising applications as transparent conducting electrodes. This has been one of the challenging points in the conventional dealloying method. To verify this, both sheet resistance and transmittance were characterized in NPGFs that were fabricated with different plasma times on a double-sided polished sapphire substrate (Figure S3). Figure 5a shows the transmittance of NPGFs depending on the plasma time. NPGFs have, on average, much higher transmittance in the visible range compared to as-deposited Au nanoparticles. The sheet resistance of the NPGF was determined to lie in the range of 150–300 Ω/sq depending on the film morphology, as indicated in Figure 5b. We also checked the connectivity of nanoporous networks by image processing (pixel connectivity), as is seen in the connectivity colormap (Figure S4). It confirms that >99% of ligaments are very well connected. Note that the highest transmittance and lowest sheet resistances were obtained from the NPGF of 5 min of plasma treatment (88% at 500 nm and 170 Ω/sq). The figure of merit (FoM), which was calculated by the equation in the inset of Figure 5c,43 was determined to be 16.7. As described in Section 2.1, the ligaments become thicker, while the pore size increases and the number of interconnected ligament points decreases as the plasma time increases. Large pore size can enhance the transmittance of the film, but thicker ligaments will degrade the transmittance. In a similar way, thicker and rigid ligaments will lower the sheet resistance, but a reduced number of ligaments can increase the sheet resistance. Therefore, ligament and pore sizes can be optimized by tuning the plasma treatment time.

Figure 5.

Figure 5

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Measured (a) transmittance and (b) sheet resistance of NPGF depending on the plasma treatment time. (c) Plot of the FoM values of NPGFs depending on the plasma time. FoM was calculated by the equation in the inset. (d) AFM image of the NPGF. RMS roughness was calculated to be 8.9 nm. (e,f) Free-standing NPGF. (e) PMMA that has not been completely etched during the plasma treatment is dissolved in acetone, and the NPGF was lifted off from the Si substrate. (f) 3 μm-thick PMMA film was spin-coated on the NPGF on the Si substrate. This sample was immersed in KOH solution (28.5%) for a minute and rinsed in deionized water, repeatedly, until the film completely lifts-off. Spin-coated PMMA serves as a supporting layer to stabilize the NPGF. The films were also transferred to another half-inch Si substrate.

Besides optoelectronic properties, surface roughness is also an important parameter as high surface roughness can be directly related to high leakage currents in device applications.43,44 The root mean square (RMS) of a surface roughness of our NPGF was measured to be only 8.9 nm from atomic force microscopy (AFM) as shown in Figure 5d, which is, on average, much lower than that reported for metal-based transparent electrodes.

It is also possible to lift off the film from the substrate and obtain free-standing NPMFs. When the PMMA layer is thick enough and remains under the NPMF after plasma treatment, the NPMF can be easily lifted off in acetone by dissolving the remaining PMMA. As shown in Figure 5e, the well-connected NPGF was detached from the Si wafer and floated in solution. We can also coat an additional PMMA layer on the NPMF after plasma treatment to support free-standing ultrathin films. As the Si substrate can be etched slowly in diluted KOH solution, the whole NPMF/PMMA film is lifted off. The PMMA layer can be used as a flexible and transparent electrode by itself or it can be transferred to another substrate as shown in Figure 5f.

3.5. Scalability

Dry synthesis of NPMFs is based on physical vapor deposition and the use of plasma, and thus is not limited to a specific size, and can potentially be adapted to large-area fabrication of NPMFs. The NPGF, for example, was fabricated in our current setup on a 3 in. wafer as shown in Figure 6a. Furthermore, our scheme can be extended to produce 3D NPM structures by stacking and folding the NPMFs. Figure 6b shows five-layer-stacked NPGFs fabricated by repeating the dry synthesis procedures on the NPGF sample. It has around 120 nm thickness, and the ligaments are continuous in 3D due to the repeated plasma treatments. We also succeeded in folding the film after lifting it off in a diluted KOH solution. Figure 6c shows a stacked nanoporous Au structure with a thickness approaching 1 μm. These results imply that our NPMFs can be extended to 3D-like NPM structures. The folded NPGF in a diluted KOH solution still has interconnected ligaments and well-defined nanopores as shown in Figure S5, implying that NPGFs withstand the lift-off conditions.

Figure 6.

Figure 6

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(a) NPGF fabricated on a 3 in. wafer. SEM images of nine different regions on the wafer confirm that the NPGF forms on large areas and is uniform across the entire wafer. (b) Multi-stacks of NPGFs: SEM image of a view onto a tilted structure of a five-layer-thick stack of the NPGF. The stack was obtained by repeating the growth steps: PMMA coating onto a previously obtained film → shadow (GLAD) deposition → plasma treatment. The ligament size is around 20–30 nm, which is comparable to that within the single-layer NPGF. The layers are connected, which is attributed to the repeated plasma treatment steps. (c) Larger view of the experimentally obtained structure with the view onto the structure as well as the cross-sectional view of the folded multi-stack NPGF, obtained after FIB milling. A slight “blurring effect” in the electron micrograph is observed in the cross-sectional view, which is caused by the FIB milling process.

4. Conclusions

Plasma-based dry synthesis is a facile and general method to produce NPMFs. The majority of methods have relied on dealloying that cannot avoid the presence of the less noble metal (or sacrificial metal) in the obtained nanoporous structure. Our method fully overcomes the requirement of a master alloy and a wasteful dissolution process (typical master alloys contain more than 60% of the sacrificial metal) and instead produces a pristine material, free of residues of additional compounds. Other methods including electrochemical deposition, template-assisted methods, and sol–gel synthesis do not achieve the curved structures at the nanoscale that our method provides. Since our plasma-based dry synthesis does not include wet-chemical reaction steps, it can be applied to many metals including Au, Ag, Pt, Pd, Ni, Fe, and even alloys. Considering that physical vapor deposition is used in the process, any metals that can be evaporated can be used with the synthesis we developed to form NPMFs. This opens the opportunity to employ co-deposition and hence extend the fabrication to nanoporous alloy metal films (Figure S6). The plasma conditions can be adjusted to the nature of the metal film. Non-noble metal films were obtained using a mixture of Ar and H2 to prevent the oxidation of the metal during the removal of the PMMA film. Our fabrication scheme to obtain NPMFs can also be used to form transparent electrodes. An advantage to our method is that it can be used for large-scale fabrication and that our NPMFs show very low surface roughness, both are more challenging to address with other current schemes to obtain metal-based transparent electrodes.9,44 The performance of transmittance and sheet resistance in our films is slightly lower than other metal-based transparent electrodes;43,44 however, we believe that this can be improved by further optimizing fabrication parameters and that the ease of fabrication of large, smooth films is significant. We expect that our dry synthesis method, which is robust and can reproducibly yield impurity-free films, will open up new opportunities in the application of NPMs.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.2c19584.

  • Melting points and surface energies of metals; comparison of NPMF morphologies depending on the metal elements; morphology control of NPMFs as a function of the plasma conditions; SEM image of the NPGF on a sapphire substrate; analysis of the image pixel connectivity of the NPGF; and top view of the folded NPGF (PDF)

The authors declare no competing financial interest.

Notes

Data availability: the authors declare that all experimental data supporting this study are included in the published article and its Supporting Information. The raw data are also available from the authors upon reasonable request.

Supplementary Material

am2c19584_si_001.pdf (819.7KB, pdf)

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