Plasma-driven formation of vertically aligned silver–phosphorus core–shell nanostructures

Abstract

Vertically aligned phosphorus–silver core–shell nanostructures (PSCS) have been successfully grown on silicon substrates using a sequential plasma processing approach. Phosphorus incorporation is achieved through evaporation from a red phosphorus tablet, while the silicon substrate is initially coated with a thin silver layer. The formation of catalyst grains on the substrate surface is essential for nanostructure development. Hydrogen plasma treatment is employed to granulate the silver layer, enabling effective interaction with incoming phosphorus atoms. Subsequent sequential H₂/He plasma treatments initiate and sustain the growth of the core–shell structures. Initial observations indicate that PSCS growth begins with formation of a crystalline silver–phosphorus core, which is subsequently encapsulated by a phosphorus shell, resulting in a core–shell heterostructure. SEM and TEM are extensively used to examine PSCS growth and identify key parameters influencing the process. In addition, XRD and XPS were employed to investigate the crystallinity and surface chemical composition of these structures, respectively.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1038/s41598-026-61655-8.

Introduction

One-dimensional (1D) nanostructures, with their unique shape and special transport properties, have become important building blocks for the next-generation electronics. Carbon nanotubes (CNTs) represented the first major breakthrough in this direction, supporting applications like flexible transistors and strain sensors^1,2. This nanostructure relies on the sp² hybridization of carbon atoms, where delocalized π-electrons over the conjugated lattice provide an efficient pathway for electron transport³. Also, the strong covalent bonding within their graphitic lattice results in outstanding thermal conductivity, allowing efficient heat dissipation in nanoscale devices⁴. However, controlling CNT chirality remains a key challenge, restricting reproducibility and large-scale device integration⁵.

The development of one-dimensional (1D) materials has expanded beyond carbon nanotubes to a broad range of material systems with diverse applications. For example, Silicon nanowires (SiNWs) exhibit excellent electronic tunability and high carrier mobility due to quantum-confinement-induced band-structure modulation, their single-crystalline nature, and strong surface/electrostatic coupling enabled by their high surface-to-volume ratio. These structural features give SiNWs superior gate control, low leakage, and high sensitivity, making them ideal for nanoscale transistors and sensors⁶. ZnO nanowires show strong piezoelectric and optoelectronic behavior arising from their non-centrosymmetric wurtzite lattice and direct wide bandgap (~ 3.37 eV), with anisotropic single-crystalline geometry enhancing confinement and piezopotential output^7,8.

Beyond single-material nanowires, core-shell nanowires exploit radial heterostructure geometry, in which a functional core is uniformly coated with a shell of a different material. Unlike axial heterostructures, this radial design enables precise carrier confinement, uniform passivation, and the formation of radial junctions, improving charge transport and device performance⁹. A notable example is Ge/Si core–shell nanowires for high-speed transistors, where a high-mobility Ge core and doped Si shell suppress impurity scattering and enhance carrier transport¹⁰.

Phosphorus has emerged as a versatile element for advanced nanostructures owing to its diverse allotropes and multifunctional properties. Phosphorene possesses a direct bandgap at all thicknesses and layer numbers^11,12. Its electron mobility can rise to 1000 cm²/Vs, which is among the highest reported values of semiconductor 2D materials^13,14.Phosphorene also exhibits a high degree of crystallinity¹⁵. Owing to its strong reactivity, it is considered a possible candidate for biological applications and drug-delivery devices^16,17. Its well-controlled structure and strong electrical performance make it promising for use in advanced field-effect transistors¹⁸. However, despite these intriguing properties, phosphorene suffers from a major drawback: it is highly unstable and does not survive in aqueous solutions or air environments¹⁹. This instability severely limits its application in electronic devices, sensors, and biological media^20,21.

Theoretical studies propose phosphorus nanotubes (PNTs) formed via phosphorene nanoribbon self-assembly or chemical cleavage of bulk allotropes; however, experimental realization remains elusive^22,23. Several studies have reported the fabrication of phosphorus-containing nanotube structures through alternative strategies. These approaches often result in composite materials or phosphorus-doped carbon nanotubes, rather than isolated an 1D phosphorus structure. Gao et al. synthesized phosphorus-incorporated multi-walled carbon nanotubes (MWCNTs) via the pyrolysis of a sucrose and phosphoric acid mixture, where phosphorus was successfully integrated into the carbon matrix²⁴. Similarly, Jin et al. developed phosphorus-nitrogen co-doped carbon nanotubes using a floating catalyst chemical vapor deposition (CVD) method²⁵. In another approach, aerosol-assisted CVD enabled the growth of single-walled carbon nanotubes (SWCNTs) with phosphorus doping^26,27.

In addition to phosphorus-doped nanotube systems, silver-phosphorus compounds offer useful evidence that Ag can help stabilize phosphorus at the nanoscale. Nanostructured Ag₃PO₄ made through simple precipitation methods is known to be highly crystalline, with X-ray diffraction showing sharp, well-defined peaks, and XPS analysis confirming the expected Ag-P-O bonding, indicating well-organized and stable nanostructures. These materials are also photocatalytically active, showing that Ag-P compounds can form stable and functional architectures^28,29. Ag₃PO₄ and other Ag-P composites have been widely studied for antibacterial and environmental-remediation uses, where they show strong reactivity in water and good compatibility with biological environments³⁰. These reports indicate that silver can form stable phosphorus-containing nanostructures, supporting the idea that Ag-P phases may appear as intermediates during growth and may assist in forming more complex 1D phosphorus architectures.

In this work, we report a scalable method for the synthesis of Phosphorus-Silver Core-Shell nanostructures with solid red phosphorus (RP) as the precursor and silver (Ag) as the catalyst. Among available fabrication methods, direct current plasma-enhanced chemical vapor deposition (DCPECVD) is a controllable and scalable plasma-assisted approach that has been widely used for the growth of vertically aligned one-dimensional nanostructures, particularly carbon nanotubes^31,32. In this work, the growth process is described as quasi-Solid–Liquid–Solid-like (quasi-SLS-like), meaning that it is inspired by SLS-type catalyst-assisted growth, where a catalyst facilitates precursor incorporation and directional nanostructure formation under controlled plasma conditions. This term is used only as a mechanistic analogy to catalyst-assisted growth and does not imply direct proof of a classical SLS process, since the liquid state of the catalyst was not directly observed during growth. The entire fabrication process is designed to enable precise control over process parameters, which facilitates the systematic investigation of PSCS morphology and crystalline quality. The grown nanostructures are characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) to explore their structural evolution, growth mechanism, and phase composition. This study not only introduces a reliable method for the fabrication of PSCS nanostructures but also provides insight into the underlying mechanisms governing their formation, thereby laying the groundwork for further development of Phosphorus-Silver-based nanodevices.

Experimental section

Vertically aligned phosphorus-silver core-shell (PSCS) nanostructures grow on silicon via a catalyst-driven process, where the initial silver layer is essential. Figure 1a illustrates the overall growth setup. The process begins with the deposition of the silver catalyst layer onto the silicon substrate in a sputtering chamber, after which the samples are transferred to the PECVD reactor for thermal granularization and subsequent growth. The reactor contains two independently controlled heating zones. Zone 1 (Z1) corresponds to the substrate-heating region, while Zone 2 (Z2) corresponds to the phosphorus-source heating region. Heating in both zones was achieved using externally wrapped resistive wire heaters with independent temperature control.

Fig. 1.

(a) Schematic diagram displaying the overall growth setup. (b) The electrode configuration. SEM images corresponding to each stage of the process are also included: (c) Seed Formation, (d) Seed Activation, and (e-g) core growth and shell formation. (h) Detailed growth process. (SEM images are taken at 45° tilted).

As seen in Fig. 1a-b, a DC voltage is applied to the electrodes. Inside the reactor, two red phosphorus tablets were used to maintain a stable phosphorus flux during the growth process. The first tablet was positioned in Z1, close to the silicon substrate, while the second tablet was placed in Z2 as an additional phosphorus reservoir. During heating, the phosphorus flux from a single tablet may gradually decrease because of precursor consumption and changes in the local evaporation conditions. Therefore, the second tablet was used to compensate for this decrease and to sustain a high and stable phosphorus supply throughout the growth process. The distance between the two tablets was selected to minimize thermal interference between the two heating zones, allowing the temperature of each phosphorus source to be controlled more independently.

The sample preparation begins with cleaning < 100 > silicon substrates using Acetone, Isopropyl Alcohol (IPA) and deionized water, and placed in a sputtering stage to deposit a continuous silver with a thickness of around 10–20 nm under an RF power of 200 W for a duration of 2 min.

At the start of the process, the PECVD chamber is evacuated to a base pressure of 10–20 mTorr. Z1 is then heated toward the target temperature of 400 °C; once Z1 reaches approximately 200 °C, a hydrogen plasma is ignited and applied for 5 min at 20 W to initiate granulation of the silver film and activate the catalyst sites. Heating continues thereafter until Z1 reaches 400 °C, completing the initialization of the silver catalyst layer (Fig. 1c).

After the completion of first step, the hydrogen plasma and H₂ flow are turned off. Z1 is first maintained at 400 °C and then further heated to 500 °C. Once Z1 reaches 500 °C, the second step begins. To initiate this step, a gas flow of 10 SCCM H₂ and 10 SCCM He is introduced, along with 20 W plasma power for 10 min. During this process, Z2 is heated to 500 °C, and by the end of this step, both Z1 and Z2 are maintained at 500 °C, preparing the system for the next growth phase. This is the second initialization step, where the mixture of helium and hydrogen gases bombards the surface of the silver grains, which are exposed to phosphorus atoms and clusters, activating the grains for the final growth step. A mixed H₂/He plasma was employed during the second catalyst-activation step to obtain a more stable and reproducible low-pressure discharge while introducing an additional physical bombardment component from He species. In this mixture, hydrogen maintains a reducing/cleaning environment at the Ag surface, whereas helium acts mainly as an inert plasma-supporting gas. The higher momentum transfer associated with He species can assist surface restructuring and activation of the Ag catalyst particles, facilitating their subsequent interaction with incoming phosphorus species^33,34. By the end of this step the activated silver nanoparticles are formed, serving as nucleation sites for the subsequent formation of these nanostructures. (Fig. 1d).

To continue the process, the plasma is briefly turned off, and the H₂ flow is increased to 20 SCCM, while the He flow is switched off. After a brief interval, the plasma is reignited at 20 W, with both Z1 and Z2 maintained at 500 °C. During this phase, the core begins to grow as silver-phosphorus composition formation takes place, with a gradient of silver concentration in the core. As the plasma continues to apply energy, the core evolves into a crystalline Ag-P structure. The core growth eventually halts when the silver is fully consumed, marking the completion of the core formation phase. In this step, the remaining silver is fully integrated into the core structure, completing its crystalline Ag-P composition formation. The silver gradient stabilizes, and the core reaches its final shape. Both Z1 and Z2 remain at 500 °C to ensure the uniformity of the crystalline structure (Fig. 1e-f).

Once the core formation is complete, the process transitions to shell formation. With the core fully formed, the phosphorus deposited from the evaporated phosphorus tablet in Z2 begins to accumulate on the core. The H₂ flow is maintained at 20 SCCM, and the plasma continues at 20 W, with the He flow remaining off. The phosphorus forms a poorly crystalline phosphorus-rich shell around the crystalline Ag-P core. This step continues until the desired shell thickness is achieved, completing the core–shell nanostructure (Fig. 1g).

During the shell growth, the plasma environment provides an effectively isotropic flux of phosphorus species toward the nanostructure surface. This condition corresponds to the high-angle (θ ≈ 90°)(θ represents the angle of the incoming flux relative to the nanostructure sidewall) impingement regime described in the diffusion-based growth model of Han et al., where the arriving adatoms exhibit short diffusion lengths and incorporate uniformly along the nanowire sidewalls⁹. Although the model was developed for two-component alloy systems, the same fundamental framework applies when only a single adatom species (phosphorus) participates in shell formation. In this case, the four adatom processes they identify (adsorption, desorption, surface diffusion, and incorporation) remain operative, but simplify in the absence of competing species. Phosphorus atoms adsorb onto the sidewall surface, and while some may desorb depending on local bonding and temperature, their limited surface mobility at the employed growth temperature restricts diffusion to short distances before direct incorporation into the shell. When surface diffusion is minimal and incorporation is kinetically favored, radial deposition becomes conformal and exhibits minimal axial variation. Consequently, the phosphorus shell grows with a nearly constant thickness along the nanostructure length, while the overall taper is dictated by the geometry and composition of the underlying Ag-P core rather than by variations in shell-growth kinetics. This theoretical framework provides strong support for uniform, diffusion-limited shell formation in plasma-assisted processes.

Results and discussion

In this section, we present the results of the growth and characterization of vertically aligned Phosphorus-Silver nanostructures with a core-shell shape. The morphology, chemical composition, and structural properties of the nanostructures were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The results highlight the tapered morphology of the nanostructures, a consistent core-shell structure, and the effects of plasma conditions on the growth of them. In particular, the discussion will focus on the relationship between the core gradient, uniform shell growth, and the formation mechanisms under varying deposition conditions. We also explore the degradation behavior of these nanostructures, providing insight into their stability and potential applications. The SEM and TEM images presented here show representative PSCS morphologies. Because the nanostructure dimensions vary across the Si substrate, particularly between central and edge regions, a single average size distribution or growth rate would not accurately describe the full sample. A systematic position-dependent quantitative analysis will be required in future optimization studies.

The SEM images (Fig. 2) display vertically aligned nanostructures on silicon substrates using silver as the catalyst grains for different samples, shown at various magnifications, offering insight into their growth and morphological characteristics. The activation of the silver layer is a critical step in achieving the vertically aligned features, accomplished through the use of hydrogen and subsequent H₂-He plasma treatment. Figure 2a-d shows a well-aligned array of nanostructures, indicating the uniformity of the plasma-assisted deposition process. The ordered arrangement suggests that the nanostructures grow in a controlled manner, likely due to the influence of the electric field in the plasma chamber, which helps align the growing nanostructures along the field lines. The inset in Fig. 2a shows the root-side of the grown PSCS nanostructures, representing the initial part of the growth before the completion of core growth and deposition of the phosphorus shell around the inner silver core. To improve the growth of the nanostructures, a modified approach was used, where photolithography and reactive ion etching (RIE) were applied to the substrate to create etched features before the deposition process. The silicon substrate was coated with a thin layer of chromium (Cr), which allowed for etching in specific windows to create wells. This modification resulted in taller, thinner, and more uniform nanostructures with a higher growth density with same growth process. It can also be observed that the growth occurred only inside the wells and not on the Cr layer, indicating that the etched windows acted as localized sites for nanostructure formation. The etched features on the Si substrates are clearly visible in Fig. 2e and f.

Fig. 2.

A collection of SEM images of various samples of PSCS nanostructures grown on (a-d) a bare silicon substrate and grown on (e-f) a patterned structure. The inset of (a) shows the root and (c) is the cross section of PSCS nanostructures.

For further investigation of the PSCS nanostructures, transmission electron microscopy (TEM) was performed at an accelerating voltage of 200 kV. The PSCS nanostructures were mechanically detached from the silicon substrate using a razor blade, dispersed in ethanol, and drop-cast onto a copper TEM grid. Figure 3a and b show PSCS nanostructures distributed on the TEM grid, including both clustered and isolated individual structures. The contrast variation along their length reveals a distinct core-shell structure, where the inner region appears significantly darker, indicating a dense core with reduced electron transmission, while the surrounding lighter region corresponds to the shell in the isolated nanostructure (Fig. 3b).

Fig. 3.

A collection images TEM images of PSCS nanostructures: (a, b) TEM images of PSCS nanostructures drop-casted on a copper grid, (c, d) Magnified TEM images revealing the internal core-shell architecture along the nanostructure length and the inset of (c) shows the diffraction pattern of these nanostructures. (e) HR-TEM image displaying lattice fringes, (f) showing bending of the core accompanied by cracking of the shell and the inset is the lower magnification image, (g) HR-TEM image spanning the grid, shell, and core regions, clearly resolving atomic fringes within the crystalline core.

Magnified TEM images (Fig. 3c, d) provide a clearer view of the internal structure and geometry of the PSCS nanostructures. These images highlight the continuity of the structure along the nanostructure length and the well-defined separation between the inner and outer regions. The inset in Fig. 3c shows a selected-area electron diffraction (SAED) pattern exhibiting diffraction features consistent with reported Ag-P crystalline structures, confirming the formation of an Ag-P phase within the nanostructure³⁵.

High-resolution TEM images (Fig. 3e, g), providing atomic-scale insight into the internal structure of the PSCS nanostructures. In Fig. 3e, well-resolved lattice fringes with an interplanar spacing of approximately 2.3 Å are observed, indicating the formation of a crystalline phase within the nanostructure. Figure 3g presents a high-resolution image spanning the TEM grid, shell region, and inner core, where clear atomic fringes are visible within the core. The presence of these fringes confirms that crystallization occurs predominantly in the core region, consistent with a growth mechanism governed by silver-driven core formation under plasma conditions. In contrast, the surrounding phosphorus-rich shell does not show clear lattice fringes under the same HR-TEM imaging conditions. This suggests that the shell is amorphous-like or poorly crystalline rather than highly crystalline.

As silver is progressively consumed during growth, the core narrows toward the tip, while the surrounding phosphorus layer forms through isotropic plasma-assisted deposition. This decoupling between core evolution and shell deposition explains the persistence of crystalline order in the core despite the tapered morphology of the overall structure. Furthermore, the lattice fringes are preferentially oriented with respect to the nanostructure growth direction, which is aligned with the plasma sheath electric field, indicating a plasma-assisted and directionally controlled crystallization process. Figure 3f shows an individual PSCS nanostructure in which the core is visibly bent, while the outer shell exhibits cracking. The bending of the core suggests a mechanical flexibility, consistent with a metal-phosphorus crystalline phase that can accommodate strain through lattice deformation. In contrast, the cracking of the shell indicates a more brittle behavior, which is typical of amorphous or weakly bonded phosphorus layers.

Elemental distribution within the PSCS nanostructures was analyzed using EDS elemental mapping (Fig. 4a-c) and were obtained using the TEM-EDS system operated at an accelerating voltage of 200 kV, with the corresponding dark-field TEM image (Fig. 4d) indicating the mapped region. The phosphorus map (Fig. 4a) shows a continuous distribution along the entire nanostructure, confirming the presence of phosphorus throughout both the core and shell regions. In contrast, the silver map (Fig. 4b) reveals that Ag is confined to the inner region, with a noticeably higher concentration near the root and a gradual decrease toward the tip. This axial Ag gradient is consistent with a silver-driven core growth mechanism, where Ag is progressively consumed during growth. These results indicate that the growth process is closer to a base-catalyzed mechanism rather than a tip-growth mechanism. Since the Ag catalyst originates from the thin Ag layer deposited on the Si substrate, the catalyst particles remain mainly at the substrate/root side after plasma-induced granulation. During exposure to phosphorus species, these Ag-rich particles initiate Ag–P core formation from the substrate side. As growth proceeds, the available Ag is progressively consumed and redistributed along the growing core, producing an axial Ag concentration gradient. This explains the cone-shaped morphology of the core, with a wider Ag-rich root and a narrower tip. The overlay image (Fig. 4c) clearly visualizes the spatial separation between the core and the phosphorus-rich outer region, directly confirming the core-shell structure.

Fig. 4.

EDS mapping in TEM to find the distribution of elements in PSCS nanostructures: (a) Phosphorus (b) Silver (c) Overlay (d) Dark field image of a PSCS. (e) EDS spectrum of a PSCS shows the picks assigned to phosphorus and Ag (f) XRD pattern of PSCS (Experimental), AgP₂, Phosphorene and Ag₃PO₄ to investigate the composition and crystalline structure. XPS analysis of PSCS nanostructures. (g)Survey spectrum confirming the presence of phosphorus, silver, and oxygen, together with high-resolution spectra of (h) O 1 s, (i) P 2p, and (j) Ag 3 d regions.

The EDS spectrum in Fig. 4e further supports the elemental analysis by confirming the presence of phosphorus and silver in the analyzed PSCS nanostructure through their characteristic peaks. As discussed above, the spatial distribution of these elements, including the phosphorus-rich outer region and the preferential localization of silver within the Ag–P core, is determined from the EDS elemental maps shown in Fig. 4a–c. Therefore, the EDS spectrum, together with the elemental mapping results, provides compositional evidence for a phosphorus-rich shell encapsulating an Ag–P core and supports the proposed growth mechanism.”

The crystalline structure of the PSCS nanostructures was examined using X-ray diffraction (XRD), as shown in Fig. 4f. The experimental diffraction pattern exhibits several well-defined peaks, indicating the presence of crystalline domains within the nanostructures. To clarify the possible phase contributions, the PSCS diffraction pattern was compared with reference patterns of AgP₂, phosphorene and Ag₃PO₄ reference patterns. The comparison between the PSCS peaks and the AgP₂ reflections is summarized in Table S2. The PSCS peaks at approximately 2θ ≈ 29°, 38°, and 44.5° show good correspondence with the AgP₂ reference pattern, suggesting the formation of Ag–P-related crystalline domains. Additional reflections near 25.5° and 37° partially overlap with the phosphorene reference, which is consistent with the phosphorus-rich component of the nanostructures. In contrast, the main PSCS reflections do not show dominant agreement with the Ag₃PO₄ pattern, indicating that crystalline Ag₃PO₄ is not the primary crystalline phase. The peak observed near 56.37° is likely associated with the Si substrate, since the PSCS nanostructures were grown directly on silicon.

Although several experimental peaks correlate with the AgP₂ reference pattern, slight shifts are observed between the experimental and reference peak positions. These deviations can be attributed to strain generated by the round geometry of the nanostructures, which may locally distort the lattice spacing within the crystalline core. Therefore, the crystalline phase is more appropriately described as an AgₓP_γ structure rather than a fixed AgP₂ stoichiometry. Overall, the combined XRD, reference-pattern comparison, Table S2, and HR-TEM results support the formation of AgₓP_γ crystalline ordering within the PSCS nanostructures, consistent with the proposed growth mechanism. X-ray photoelectron spectroscopy (XPS) was performed to investigate the surface chemical composition and bonding states of the PSCS nanostructures (Fig. 4g-j). All spectra were charge-corrected using the adventitious carbon C 1 s peak at 284.8 eV as a reference, and the same correction was applied to the survey and high-resolution spectra. The survey spectrum (Fig. 4g) confirms the presence of phosphorus, silver, and oxygen, while the high-resolution Ag 3 d, P 2p, and O 1 s spectra provide further insight into the chemical environments of the Ag–P-containing structure and the oxidized phosphorus-rich surface.

The high-resolution O 1 s XPS spectrum (Fig. 4h) exhibits a dominant peak centered at approximately 532 eV, which is assigned to oxygen-containing phosphorus species, such as P–O/P = O bonding, formed by partial oxidation of the phosphorus-rich surface after exposure to ambient conditions. This result is consistent with the oxidized phosphorus component observed in the P 2p spectrum. The high-resolution P 2p XPS spectrum (Fig. 4i) of the PSCS nanostructures exhibits two main components centered at approximately 134 eV and 129 eV, assigned to P 2p_1/2 and P 2p_3/2, respectively. The higher-binding-energy component is attributed to oxidized phosphorus species, indicating the formation of surface P–O bonding after exposure to ambient conditions. In contrast, the lower-binding-energy component is assigned to reduced phosphorus species, which may originate from P–P bonding or Ag–P bonding within the AgₓP_γ phase. The coexistence of oxidized and reduced phosphorus states is consistent with a phosphorus-rich outer surface and an underlying Ag–P-containing structure, in agreement with the TEM, EDS, and XRD results.

The high-resolution Ag 3 d XPS spectrum (Fig. 4j) of the PSCS nanostructures exhibits two characteristic peaks centered at approximately 368 eV and 374 eV, assigned to Ag 3d₅/₂ and Ag 3d₃/₂, respectively. The spin–orbit separation of about 6 eV is characteristic of Ag 3 d splitting and confirms the presence of silver in the nanostructures. The Ag 3d₅/₂ peak near 368 eV indicates that silver is present in a reduced Ag-containing environment rather than as a strongly oxidized silver species. Together with the EDS, TEM, and XRD results, this signal supports the incorporation of silver within the AgₓP_γ crystalline core of the PSCS nanostructures.

An important aspect of the PSCS nanostructures is their environmental stability, as illustrated in Fig. 5a-d. After exposure to ambient air for approximately eight months, the nanostructures largely retain their overall morphology. SEM images in Fig. 5a and b show that while the outer phosphorus shell exhibits visible signs of degradation, the vertical geometry of the nanostructures remains preserved. TEM images in Fig. 5c and d further confirm that the inner core structure remains continuous and intact, despite surface roughening and partial deterioration of the shell. This degradation can be explained by the chemical sensitivity of the phosphorus-rich outer shell. Phosphorus-based nanomaterials, particularly black phosphorus and phosphorene, are known to degrade in ambient environments through oxygen- and moisture-assisted oxidation. Adsorbed oxygen can react with phosphorus surface sites to form oxidized P–O/P = O species, while water molecules can further promote hydration of these oxidized regions, leading to surface roughening and progressive deterioration^36,37. A similar process is likely responsible for the partial degradation observed in the phosphorus-rich shell after prolonged air exposure. However, the overall vertical morphology of the PSCS nanostructures remains largely preserved, suggesting that the inner crystalline Ag–P core is comparatively more stable and acts as a structural scaffold. This behavior differs from fully phosphorus-based nanostructures, where ambient oxidation can lead to more severe structural degradation.

Fig. 5.

(a, b) SEM images showing PSCS nanostructures before and after prolonged exposure to ambient conditions. (c, d) TEM images show shell degradation. (e-g) SEM images before and after selective wet etching. (h) SEM image after extended etching time and the inset shows that the shell begins to deform. (i-k) Interaction of breast cancer cells with PSCS nanostructures. (SEM image taken at 45° tilt).

The chemical stability and compositional separation of the PSCS nanostructures were further examined through selective wet etching using ammonium cerium(IV) nitrate (ACN). SEM images acquired before and after etching are shown in Fig. 5e-h. After initial exposure to the ACN solution, the nanostructures largely preserve their external morphology. Higher-magnification SEM images in Fig. 5g reveal localized contrast changes within the nanostructures, highlighted by dashed circles, consistent with etching of the inner Ag-P core. This selective behavior is attributed to the oxidizing nature of ACN, which readily converts Ag into soluble ionic species, thereby destabilizing and dissolving the Ag-P crystalline phase, while the phosphorus-rich shell remains comparatively resistant. Upon extended etching durations (an hour), as shown in Fig. 5h, structural changes in the shell become apparent. In this case, the shell begins to open from the top region, suggesting a gradual loss of mechanical support following complete or near-complete removal of the inner core as the inset shows. Overall, these results provide compelling chemical and structural evidence for a core-shell structure with strong compositional separation, where the Ag-P core can be selectively removed while the phosphorus shell remains largely intact.

To explore potential applications of the PSCS nanostructures, their interaction with MDA-MB-231 breast cancer cells was investigated (Fig. 5i-k). The National Cell Bank of the Iranian Pasteur Institute is where the human breast cancer cell line MDA-MB-231 was purchased. These cells were cultured in Dulbecco’s Modified Eagle Medium (DMEM, Sigma), 10% Fetal Bovine Serum (FBS), and 1% penicillin/streptomycin (Gibco), at 37 °C in a humidified incubator with 5% CO₂. Cells were cultured on silicon substrates containing PSCS nanostructures grown selectively within patterned silicon wells. After 24 h of incubation, SEM imaging revealed clear interactions between the cells and the nanostructured surface. As shown in Figure.5i-k at increasing magnifications, cells were observed to adhere to the substrate and extend across regions populated with nanostructures, with portions of the cell body and membrane interfacing with the PSCS arrays. In several cases, cells appear to bridge across the wells and interact directly with the nanostructures, suggesting that the PSCS surface supports cell attachment. Importantly, no obvious signs of membrane rupture or morphological damage were observed, indicating that the nanostructures do not adversely affect cell integrity over the examined time period. These observations suggest good initial biocompatibility of the PSCS nanostructures and highlight their potential for applications involving cell-nanostructure interfaces, such as biosensing or biomedical platforms.

Conclusion

In conclusion, we have demonstrated the successful growth of vertically aligned silver–phosphorus core–shell nanostructures (PSCS) on silicon substrates using a sequential plasma-assisted process. Phosphorus species generated by evaporation from a red phosphorus tablet serve as the growth precursor, while a thin silver layer deposited on silicon acts as the catalytic and alloying component. Controlled plasma treatments play a critical role in this process: hydrogen plasma activates and granulates the silver layer, while subsequent hydrogen–helium plasma exposure promotes interaction between silver and phosphorus, leading to the formation of a crystalline Ag–P core followed by the deposition of a phosphorus-rich shell.

Comprehensive structural and compositional characterization using SEM, TEM, HR-TEM, EDS, XRD and XPS reveals that the nanostructures exhibit a core–shell architecture with a tapered morphology, where the axial shape is governed by the silver-containing core and the shell forms through conformal, plasma-assisted deposition. Elemental mapping and diffraction analyses confirm the preferential localization of silver within the core and the crystalline nature of the Ag–P phase. Degradation and selective wet-etching experiments using ammonium cerium (IV) nitrate further validate the chemical and mechanical decoupling between the core and shell, providing strong evidence for a sequential growth mechanism. Finally, preliminary cell-interaction studies indicate that the PSCS nanostructures support cell adhesion without inducing apparent morphological damage, suggesting good initial biocompatibility. Together, these results establish a robust plasma-based approach for fabricating well-defined phosphorus-based core–shell nanostructures and highlight their potential for future applications in nanoelectronics, sensing, and biointerfaces.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Author ContributionM. Marzoughi: Has conducted the work, prepared the original manuscript and completed the work. She has reviewed and edited the manuscript.O. Babaee: Has initiated the work, prepared the data and figures for the manuscript. He has also prepared the first draft of the paper and edited the final manuscript.S. Mohajerzadeh: Has supervised the work, prepared the original manuscript and completed the manuscript.All authors have approved the final version of the manuscript.

Data availability

All the data reported in this manuscript are authentic and the results of authors’ investigation. All the data, including raw materials and fabrication details are available upon request.

Declarations

Competing interests

The authors declare no competing interests.