Engineering Shape and Light‑Interaction of PdAuAg and PdAg Alloy Nanoparticles on Sapphire via Solid‑State Dewetting

Background

Recent advances in nanodevice fabrication emphasize the importance of engineering multi‑metallic nanostructures, semiconducting polymers, and metal/semiconductor nanomembranes [1–10]. These heterogeneous particles are key to applications in sensing, photovoltaics, biomedicine, and catalysis because they combine electronic diversity, site‑specific reactivity, and multifunctionality—attributes unattainable with monometallic counterparts [11–14]. For instance, bimetallic Ag–Au clusters broaden the LSPR bandwidth, boosting photovoltaic power conversion compared to monometallic Au or Ag nanoclusters [15,16]. Similarly, NiAuPt trimetallic nanoparticles exhibit superior electro‑catalytic activity for ethanol oxidation due to synergistic Pt‑mediated dehydrogenation and Ni/Ag‑mediated removal of intermediates [17]. Among metals, Au and Ag are prized for plasmonics, while Pd offers catalytic strength and chemical stability [18–20]. Yet, controlled synthesis of PdAg and PdAuAg alloy structures via physical deposition remains unexplored. Here, we systematically produce these alloys on sapphire (0001) by solid‑state dewetting, exploring how deposition scheme, thickness, and annealing parameters dictate morphology, composition, and optical response.

Methods/Experimental

Materials Preparation and Thin‑Film Deposition

Sapphire (0001) wafers (430 µm thick, ±0.1° off‑axis) were diced into 6×6 mm² chips. They were degassed at 600 °C for 30 min under 1×10⁻⁴ Torr in a pulsed‑laser deposition (PLD) chamber, yielding atomically smooth surfaces (see Additional File 1: Fig. S1(b)). We fabricated three deposition sequences (Fig. 1a–c). Tri‑layers consisted of 5 nm Pd, 5 nm Au, and 5 nm Ag (15 nm total). Multi‑layers repeated a 1 nm Pd/Au/Ag stack five times (15 layers, 15 nm total). Bi‑layers were Pd_150 nm/Ag_80 nm. Films were sputtered at 0.1 nm s⁻¹, 3 mA ionization current, 1×10⁻¹ Torr, room temperature.

Schematic of deposition schemes: a 15 nm Pd/Au/Ag (5 nm each) (tri‑layers). b 15 nm Pd/Au/Ag (1 nm each, 15 layers) (multi‑layers). c Pd_150 nm/Ag_80 nm (bi‑layers). d–i Evolution of tri‑metallic PdAuAg nanoparticles between 400 and 900 °C (450 s).

Fabrication of PdAuAg and PdAg Alloy Nanostructures

After deposition, tri‑ and multi‑layers displayed smooth surfaces (Additional File 1: Figs. S1(c)–(d)). They were annealed from 400 to 900 °C in a PLD chamber under 1×10⁻⁴ Torr, reaching each target temperature at 4 °C s⁻¹. For bi‑layers, annealing time ranged from 0 to 3600 s at 850 °C to probe time‑dependent evolution.

Characterization

Atomic force microscopy (AFM, XE‑70, Park Systems) provided top‑ and side‑views in non‑contact mode at ambient conditions. Scanning electron microscopy (SEM, COXEM CX‑200) imaged large‑scale morphology at 20 kV. Energy‑dispersive X‑ray spectroscopy (EDS, Thermo Fisher Noran System 7) mapped elemental composition. UV–VIS–NIR reflectance was measured with an UNIRAM II system (UniNanoTech).

Results and Discussion

Figure 1d–i depict the morphological evolution of PdAuAg nanoparticles derived from the 15 nm tri‑layer at 450 s annealing. AFM top‑ and side‑views reveal distinct morphologies with temperature: over‑grown alloy NPs, voids, wiggly nanostructures, and isolated particles. Solid‑state dewetting (SSD) converts a uniform thin film into isolated particles well below the melting point, driven by surface diffusion and energy minimization. In multi‑metallic systems, SSD is further influenced by inter‑diffusion and alloying. The tri‑layer’s Pd, Au, and Ag—all fcc with slightly different lattice constants—exhibit surface energies of 1808, 1363, and 1065 mJ m⁻² [24] and diffusivities of Ag > Au > Pd. This hierarchy accelerates Ag and Au inter‑diffusion, especially at Au–Ag interfaces. The Arrhenius relation D = D₀ exp(−E_a / kT) predicts that higher annealing temperatures enhance inter‑diffusion, promoting PdAuAg alloy formation. Subsequent dewetting initiates at low‑energy sites, forming pinholes and voids that grow via capillary forces and become wiggly structures as temperature increases. At the highest temperatures, Rayleigh‑like instability fragments wiggly structures into isolated alloy nanoparticles.

At 400 °C (Fig. 1d), over‑grown NPs (~200 nm wide, 30 nm tall) appear, likely due to rapid Au–Ag accumulation at the upper interface. Raising the temperature to 500 °C enhances inter‑diffusion across all interfaces, creating voids (~17 nm deep) that partially engulf the over‑grown NPs. Further heating to 600 °C enlarges voids, while 700 °C drives wiggly nanostructures that fragment into isolated particles. Between 800 and 900 °C, isolated PdAuAg NPs dominate, their regularity reflecting surface‑energy minimization. RMS roughness (Rq) and surface area ratio (SAR) increase steadily with temperature (Rq: 5.7 → 25.3 nm; SAR: 1.13 → 11.36 %) (Fig. 2d). EDS confirms the presence of Pd, Au, and Ag at all temperatures; Au counts decline above 800 °C, and Ag counts fall between 600 and 900 °C, consistent with sublimation thresholds (~500 °C for Ag, ~800 °C for Au) [26–28].

a–c SEM images of tri‑metallic PdAuAg NPs annealed 700–900 °C. d Rq and SAR versus temperature. e Full‑spectrum EDS (400 °C). f Pd, Au, and Ag EDS counts.

Reflectance spectra (Fig. 3) reveal a UV quadrupolar peak (~380 nm), a visible dip (~500 nm), and a NIR shoulder (900–1000 nm). Normalizing at 300 nm shows that the visible dip blue‑shifts from ~510 nm to ~475 nm with higher temperatures, while the NIR shoulder also shifts and broadens—attributable to reduced particle size, increased spacing, and Ag loss. The UV peak remains stable, underscoring its quadrupolar nature.

Reflectance spectra of PdAuAg nanostructures annealed 400–900 °C (tri‑layers). a Raw spectra. b Average reflectance vs. temperature. c Normalized spectra. c‑1–c‑2 Zoomed regions showing dip and shoulder evolution.

Switching to multi‑layer deposition (15 layers of 1 nm each) dramatically accelerates dewetting (Fig. 4). Voids form already at 400 °C (depth ~15 nm, width ~100 nm). As temperature rises, voids coalesce, and isolated, regularly spaced PdAuAg NPs emerge by 900 °C. Compared to tri‑layers, the multi‑layer system yields larger voids and nanoparticles, reflected in higher Rq and SAR values. Reflectance behavior mirrors that of tri‑layers but with slightly lower average reflectance due to reduced surface coverage. The absorption dip shifts from ~520 nm to ~465 nm, again indicating size reduction and enhanced spacing.

Evolution of PdAuAg nanostructures from 15 nm multi‑layer Pd/Au/Ag (15 layers). a–f AFM top‑views (3×3 µm²), a‑1–f‑1 side‑views, a‑2–f‑2 line profiles.

Multi‑layer PdAuAg analysis: a–b Rq and SAR, c Pd, Au, Ag counts, d Reflectance spectra, e Average reflectance, f Normalized spectra (with zooms).

Time‑dependent annealing of the Pd_150 nm/Ag_80 nm bi‑layer at 850 °C (Fig. 6) demonstrates the role of high‑diffusivity Ag in accelerating dewetting of low‑diffusivity Pd. Immediately (0 s) wiggly, connected structures (~300 nm tall) appear. After 240 s, they transform into isolated NPs (~500 nm tall). Longer anneals (1800–3600 s) reduce NP size, indicating Ag sublimation. Rq peaks at 240 s then declines, while SAR decreases monotonically with time.

Bi‑layer PdAg evolution at 850 °C: a–d SEM, a‑1–d‑1 AFM side‑views, a‑2–d‑2 line profiles, e–f Rq and SAR.

Elemental mapping (Fig. 7) confirms alloy formation: Pd and Ag co‑localize within the wiggly structures. EDS line profiles show Pd dominating due to Ag loss. Reflectance spectra evolve with time: initial 0 s spectra display a ~380 nm peak, a deep visible dip (500–600 nm), and an NIR peak. As isolated NPs form (240–3600 s), the dipolar resonance blue‑shifts from ~990 nm to <850 nm, and average reflectance stabilizes around 3 %—consistent with the strongly absorbing, widely spaced PdAg nanoparticles.

Bi‑layer PdAg analysis: a SEM, b 3D map, c–e Pd, Ag, and overlap maps, f–g EDS line profiles and spectrum, h Reflectance spectra, i Normalized spectra, j Average reflectance.

Conclusions

We have demonstrated that solid‑state dewetting of multi‑metallic thin films on sapphire (0001) yields controllable PdAg and PdAuAg alloy nanoparticles. By adjusting annealing temperature, time, and deposition architecture (bi‑, tri‑, multi‑layers), we can steer the transition from over‑grown particles to voids, wiggly structures, and finally isolated, well‑spaced alloy nanoparticles. Multi‑layer sequences enhance dewetting, creating voids at lower temperatures and regular nanoparticles at higher temperatures due to increased inter‑diffusion in thinner layers. Time‑controlled annealing of Pd/Ag bi‑layers further demonstrates the conversion from wiggly to isolated particles, driven by enhanced diffusivity of the Ag component. Optical measurements reveal dynamic LSPR features: quadrupolar peaks (~380 nm) that remain stable, and dipolar peaks that blue‑shift with size reduction, while the visible absorption dip also shifts. These findings establish a versatile, physically deposited platform for tailoring the morphology and optical response of multi‑metallic nanoparticles for applications in plasmonics, catalysis, and beyond.

Abbreviations

AFM

Atomic force microscope

EDS

Energy‑dispersive X‑ray spectroscope

LSPR

Localized surface plasmon resonance

NIR

Near‑infrared

NPs

Nanoparticles

PLD

Pulsed laser deposition

Rq

RMS roughness

SAR

Surface area ratio

SEM

Scanning electron microscope

SSD

Solid‑state dewetting

UV

Ultra‑violet

VIS

Visible