# Plasma-Driven Solid-Liquid Dynamics in Cu-Sn Catalysts and Nucleation of Silicon Nanowires Revealed by Environmental TEM

Weixi Wang, Eric Ngo, Pavel Bulkin, Martin Foldyna, Pere Roca i Cabarrocas, Erik V Johnson, Didier Pribat, Zhengyu Zhang, Jean-Luc Maurice

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Weixi Wang, Eric Ngo, Pavel Bulkin, Martin Foldyna, Pere Roca i Cabarrocas, et al.. Plasma-Driven Solid-Liquid Dynamics in Cu-Sn Catalysts and Nucleation of Silicon Nanowires Revealed by Environmental TEM. Nano Letters, 2025, 25 (44), pp.15832-15839. 10.1021/acs.nanolett.5c03612. hal-05363494

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Distributed under a Creative Commons Attribution 4.0 International License
Plasma-Driven Solid–Liquid Dynamics in Cu–Sn Catalysts and Nucleation of Silicon

Nanowires Revealed by Environmental TEM

Weixi Wang,*,† Eric Ngo,† Pavel Bulkin,† Martin Foldyna,† Pere Roca i Cabarrocas,† Erik V. Johnson,†
Didier Pribat,† Zhengyu Zhang,‡ and Jean-Luc Maurice*,**

† Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91128 Palaiseau, France

‡ Laboratoire SPMS, CentraleSupélec, CNRS, Université Paris-Saclay, Gif-sur-Yvette 91190, France

Email: weixi.wang@polytechnique.edu

Email: jean-luc.maurice@polytechnique.edu
**Abstract**

We employ environmental TEM to observe phase transitions driven by plasma radicals, in Cu–Sn catalysts for silicon nanowire (SiNW) growth. Previously unrecognized solid–liquid dynamics enables the stable growth of ultrathin (~7 nm) SiNWs with alternating cubic and hexagonal segments. At 250 °C, H radicals induce a solid–liquid Cu₆Sn₅–Sn nanostructure, defining small nucleation sites. At 400 °C, SiH₃ radicals drive SiNW nucleation simultaneously with catalyst phase separation. A Sn-rich liquid supplies atomic steps that propagate into the solid catalyst. A solid Cu₃Si phase remains epitaxially aligned with the SiNW, anchoring the liquid phase. The solid–liquid catalyst actively reorients, causing the Si step propagation direction to reverse periodically. These dynamic behaviors are reproduced using a Cu–In system, underscoring the versatility of combining high- and low-melting-point catalysts. Our findings demonstrate that plasma-radical-driven nonequilibrium chemistry can be harnessed to control nanowire growth at the atomic scale.

**Key words:** Plasma-radical-driven reactions; Solid–liquid catalyst dynamics; Nucleation of Si nanowire; *In situ* TEM; Cu–Sn bimetallic catalyst
In the Chemical Vapor Deposition (CVD) growth of Silicon Nanowires (SiNWs), silicon radicals are generated by thermal decomposition of precursor gases, typically silane and disilane [1]. In contrast, cold plasma provides sufficient energy to dissociate gases, enabling Plasma-Enhanced CVD (PECVD) of SiNWs at temperatures insufficient for thermal decomposition of precursors [2]. Liquid metal catalysts promote favorable growth kinetics by providing intermediate pathways that lower the energy barrier between gaseous precursors and solid Si, as illustrated in Scheme 1a. Although crystalline Si is thermodynamically more stable than silane, the direct reaction is hindered by a substantial activation barrier (curve 1, red). Catalysts offer intermediate pathways that lower the energy barrier, thereby facilitating the reaction kinetically (curve 2, blue). At temperatures insufficient for precursor thermal decomposition, cold plasma generates Si radicals (illustrated as SiH₃), thereby enabling the reaction (curve 3, green). This is known as the Vapor-Liquid-Solid (VLS) growth mechanism [3].

High-surface-tension catalysts, such as Au [4], typically form droplets that catalyze the advancing growth front of SiNWs, whereas low-surface-tension catalysts tend to spread as wetting layers around SiNWs [5]. However, plasma-generated atomic hydrogen markedly lowers the surface energy of SiNWs [6], thereby enabling droplet formation even with low-surface-tension catalysts (Scheme 1b), such as Sn [7], In [8], and Ga [9]. Single-metal catalysts have been extensively studied for SiNW growth with diameters above ~10 nm [10-12]. More recently, bimetallic systems have been explored, including Au-Ag for Si-Ge nanowire heterojunctions [13], Au-Ni for embedded quantum dots within SiNWs [14], and Cu-Sn for ultrathin SiNWs containing the hexagonal phase [15]. We previously reported the first plasma Environmental Transmission Electron Microscopy (ETEM) observations of Cu-Sn-catalyzed SiNW growth [16]. However, the mechanisms underlying catalyst structural evolution and their influence on SiNW growth remain to be elucidated. In the current work, we uncover the dynamics of plasma-radical-driven reactions in Cu-Sn catalysts.
<figure><img src="image_2.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_5.png" /></figure>

Scheme 1. Thermodynamic perspectives on the Vapor–Liquid–Solid (VLS) growth mechanism of SiNWs in PECVD and experimental procedures. (a) Comparison of three hypothetical reaction pathways: (1) $SiH_4 \rightarrow crystalline Si$; (2) $SiH_4 \rightarrow catalyst \rightarrow crystalline Si$; (3) $Si$ radical $\rightarrow$ catalyst $\rightarrow$ SiNW. (b) Schematic illustrating how hydrogen plasma stabilizes low-surface-tension catalysts, where $\sigma_S$, $\sigma_L$ and $\sigma_{SL}$ represent the solid, liquid, and solid–liquid surface tensions, respectively. Low-surface-tension catalysts typically spread as a wetting layer around SiNWs. Plasma-generated atomic hydrogen passivates the SiNW sidewall, effectively reducing $\sigma_S$ and stabilizing the catalyst droplet at the growth front. (c) Thermal evaporation of Cu–Sn catalysts. (d) Plasma ETEM configuration and *in situ* observation of catalyst evolution. $SiH_4$ gas and plasma-generated atomic hydrogen were introduced separately into the TEM column to create a reactive plasma-radical atmosphere. Experimental details are provided in the Methods section. Catalyst compositions and remote-plasma parameters for each experiment are listed in the Supporting Information (SI).

Scheme 1c–d outline the experimental workflow. Cu–Sn catalysts were deposited onto substrates via thermal evaporation (Scheme 1c), forming nanoparticles (NPs). Scheme 1d depicts the plasma ETEM configuration. An Electron Cyclotron Resonance (ECR) plasma source was coupled to the $H_2$ gas line,
external to the TEM column to avoid interference with the electron beam. Simulations indicate that predominantly neutral hydrogen atoms, rather than ionized species, reach the sample [17]. The substrate temperature was maintained by the *in situ* TEM chips. Structural evolution was monitored in two stages. (i) During a 2-min pretreatment at 250 °C, plasma-generated atomic hydrogen activated Cu–Sn NPs by reducing native oxides while minimizing agglomeration. (ii) In the subsequent growth stage at 400 °C, SiH₃ radicals [18] were produced by introducing silane (SiH₄) into the TEM column under remote hydrogen plasma. The growth stage lasted 1–2 h for continuous *in situ* observation. Although the plasma atmosphere above the sample lacks the high-frequency electromagnetic excitation present for PECVD processes, it nonetheless delivers the reactive radicals essential for mechanistic investigation. The plasma atmosphere drives the Si concentration in catalyst droplets far from equilibrium, leading to SiNW nucleation simultaneous with catalyst solid–liquid phase separation. The dynamic solid–liquid catalysts cocatalyze SiNW growth: the liquid phase supplies atomic steps, and the solid phase in turn anchors the catalyst droplet at the growth front. Formation of alternating segments of cubic and hexagonal Si with diameters of ~ 7 nm is observed. These findings open new avenues for catalyst design and improved nanoscale control in bottom-up SiNW synthesis.

## Solid–liquid Cu<sub>6</sub>Sn<sub>5</sub>–Sn nanostructure during pretreatment

Figure 1 (extracted from Video 1) summarizes the structural evolution of the Cu–Sn NPs at 250 °C under a hydrogen flux, before and after remote plasma ignition. Energy-Dispersive X-ray spectroscopy (EDX) reveals that the as-deposited catalyst NPs consist of an amorphous Cu-rich core encased by a Sn-rich shell with native oxide (Figure S1, SI). Catalyst NPs with different compositions are provided for reference in Figure S2, SI. Under ambient conditions, Sn forms a native oxide layer 2–3 nm thick [19], comparable to the 2–4 nm radius of the Cu–Sn NPs. When annealed under molecular H₂ flux without plasma (Figure 1a), the NPs remain immobile and unreactive, despite the substrate temperature exceeding the melting point of Sn, 232 °C [20]. The native oxide substantially raises the effective melting point—1630 °C for bulk SnO₂ [21]—thereby hindering NP mobility [22]. At the bulk scale, interdiffusion between liquid
Sn and solid Cu produces a 3 µm thick Cu₆Sn₅ layer within 5 min at 250 °C [23]. However, the native oxide severely limits atomic diffusivity and reactivity between Sn and Cu. Upon plasma ignition, atomic hydrogen rapidly reduces the native oxide, triggering dynamic restructuring of the Cu–Sn NPs. Dark features immediately appear and fluctuate within the NPs (Figure 1b). The NPs wobble about their equilibrium positions, with adjacent ones prone to coalescence. Within 65 s, crystalline facets emerge (Figure 1c) and coexist with an amorphous part. The crystalline part is identified as intermetallic Cu₆Sn₅ (η phase) [24] via Fast Fourier Transform (FFT) (Figure 1c and Figure S3). The coexisting amorphous phase is attributed to liquid Sn, based on the Cu–Sn phase diagram at 250 °C [20]. Figure 1d compares the coalescence kinetics of Cu–Sn under the hydrogen plasma atmosphere with those of single-metallic Sn NPs. Within 20 s, the average size of the Sn NPs increases, accompanied by a broadening size distribution. In contrast, solid Cu₆Sn₅ anchors liquid Sn, restricting NP mobility and coalescence; consequently, the NPs remain smaller with narrower size distributions (Figure S4, SI), which, in turn, determine the SiNW diameter during subsequent growth [25].
<figure><img src="image_6.png" /><figcaption>Figure 1. Evolution of Cu–Sn catalyst NPs during hydrogen plasma pretreatment at 250 °C, extracted from High-Resolution TEM (HRTEM) Video 1. (a) Minimal structural evolution is observed under a molecular hydrogen flux before plasma ignition. The as-deposited Cu–Sn NPs exhibit a core-shell structure. EDX analysis (Figure S1, SI) confirms a Cu-rich core encased by a Sn-rich shell containing native oxide. (b) Dark objects appear and fluctuate within the NPs upon plasma ignition. (c) Formation of a solid–liquid Cu₆Sn₅–Sn structure. FFT analysis identifies the crystalline part as Cu₆Sn₅, in the [0-1-2] zone axis. (d) Kinetics of Cu–Sn and Sn NPs size evolution under the hydrogen plasma atmosphere. Solid diamonds denote average diameters; hollow diamonds represent individual sizes, fitted to normal distributions. The accompanying schematic shows that solid Cu₆Sn₅ anchors liquid Sn, suppressing coalescence and yielding smaller NPs with narrower size distributions.</figcaption></figure>

SiNW nucleation simultaneous with catalyst phase separation

Figure 2 presents the nucleation of crystalline Si via the Cu–Sn nanodroplet at 400 °C under a hydrogen–silane plasma atmosphere. In Figure 2a, the solid–liquid Cu₆Sn₅–Sn NPs transform into droplets within 50 s of heating. Although bulk Cu₆Sn₅ melts at 408 °C [20], nanocrystals are thermodynamically less stable [26] and more prone to melting. However, this temperature difference likely falls within the
uncertainty of the temperature control. Figure 2b (Video 2) captures SiNW nucleation occurring simultaneously with catalyst phase separation. Upon exposure to Si radicals, a nucleus emerges at the droplet edge and later develops into a SiNW. Within 0.5 s of nucleation, crystalline facets take shape, and FFT confirms the cubic Si structure. Simultaneously, the catalyst segregates into a faceted domain and a darker amorphous part, both remaining in contact with the Si nucleus. FFT identifies the faceted domain as Cu₃Si [27], epitaxially aligned with the Si nucleus, with crystal plane (0001)Cu₃Si parallel to (111)Si. EDX analysis attributes the amorphous part to a Sn-rich liquid containing ~80 at% Sn (Figure S5, SI). Figure 2c highlights this three-phase coexistence defining the active growth front of SiNWs. The nucleation induction time is approximately 70 s after silane introduction. The schematic summarizes the observed nucleation mechanism. The plasma atmosphere drives Si atoms into Cu–Sn droplets, leading to supersaturation and nucleation of crystalline Si. This observation is in accordance with the Cu–Sn–Si ternary phase diagram at 600 °C [28], where liquid Cu–Sn containing ~47 at.% Sn dissolves only ~5 at% Si. Continued influx of plasma-generated Si radicals further enriches the Si concentration, resulting in the coexistence of a Sn-rich liquid, Cu₃Si, and crystalline Si—conditions rarely attainable with conventional VLS growth. Here, a sustained flux of plasma-generated Si radicals is essential to establish the solid–liquid Cu₃Si–Sn catalyst configuration that co-catalyzes SiNW growth.
<figure><img src="image_7.png" /><figcaption>Figure 2. Nucleation of crystalline Si from Cu–Sn nanodroplets, simultaneous with catalyst solid–liquid phase separation into Cu₃Si–Sn, at 400 °C under the hydrogen–silane plasma atmosphere. (a) HRTEM images showing the transformation of Cu₆Sn₅–Sn NPs into nanodroplets. (b) HRTEM images (Video 2) capturing the nucleation of cubic Si and simultaneous catalyst phase separation into Cu₃Si and a Sn-rich liquid, both in contact with crystalline Si. FFT analysis identifies Cu₃Si [27] epitaxially aligned with the Si nucleus, with crystal plane (0001)Cu₃Si parallel to (111)Si. (c) HRTEM images showing the three-phase coexistence—Cu₃Si, Sn-rich liquid, and crystalline Si—defines the active growth front of SiNWs. The schematic summarizes the observed structural evolution. To avoid visual complexity, catalyst phases are grouped into Cu-rich and Sn-rich categories and visualized using a two-color scheme (orange and gray, respectively).</figcaption></figure>
Anchoring effect of solid–liquid catalyst structure during growth

Figure 3 illustrates the anchoring effect of the solid–liquid Cu₃Si–Sn catalyst structure during SiNW growth at 400 °C under a hydrogen–silane plasma atmosphere. The effectiveness of a pure Sn catalyst relies on plasma-generated atomic hydrogen, which lowers the surface energy of SiNW sidewalls [6], thereby enabling Sn to form droplets [22]. In the absence of hydrogen plasma, liquid Sn spreads into a wetting layer around SiNWs (Figure S6, SI) because of its low surface tension. Although the total flux of atomic hydrogen produced by the ECR source is continuous and stable, the flux of radicals will fluctuate over an area on the order of the nanoscale. Consequently, the Sn droplet intermittently creeps down the SiNW sidewall in response to fluctuations in hydrogen flux (Figure 3a), leading to catalyst loss and a tapered SiNW morphology (Figure 3c, left). In contrast, the Cu–Sn catalyst exhibits an anchoring effect that markedly improves its stability (Figure 3b, Video 3). Like the pure Sn catalyst, the Sn-rich liquid in the solid–liquid configuration also transiently wets the SiNW sidewall. However, unlike the single-Sn case, the Sn-rich liquid in the solid–liquid configuration rapidly retracts and re-establishes a balanced geometry within 1.5 s, restoring the SiNW tip as the equilibrium position (yellow arrows in Figure 3b). As illustrated in the schematic, when the Sn-rich liquid contacts both surfaces of Cu₃Si and Si, these two solids compete for downward wetting. The contact angle of the Sn-rich liquid on the Cu₃Si (θ₂ ≈ 10°) is significantly smaller than that on the SiNW sidewall (θ₁ ≈ 60°). This preferential wetting towards Cu₃Si draws the liquid back and anchors it at the SiNW tip during growth. As a result, the SiNW maintains a uniform diameter along its entire length (Figure 3c, right).
<figure><img src="image_8.png" /><figcaption>Figure 3. Anchoring effect of the solid-liquid Cu₃Si-Sn nanostructure that stabilizes the catalyst at the SiNW tip during growth at 400 °C under the hydrogen-silane plasma atmosphere. (a) HRTEM image showing intermittent leakage of liquid Sn catalyst along SiNW sidewall. (b) HRTEM images (Video 3) showing, preferential wetting of the Sn-rich liquid toward the solid Cu₃Si phase in the Cu-Sn catalyst, evidenced by contact angles, which retracts the leakage within 2.5 s and restores the droplet at the SiNW tip (yellow arrows). (c) *In situ* TEM images comparing the uniform-diameter SiNW grown with Cu-Sn and the tapered SiNW morphology grown with Sn. (d) *Ex situ* SEM images of SiNWs grown with Sn and Cu-Sn catalysts in a PECVD reactor for 3 min. (e) Statistical analysis of SiNW taper, $\Delta D = D_{\text{base}} - D_{\text{tip}}$, where $D_{\text{base}}$ and $D_{\text{tip}}$ are the diameters at the bottom and tip, respectively. Open diamonds denote individual measurements, fitted to normal distributions; solid diamonds denote mean values, with error bars representing one standard deviation.</figcaption></figure>

Scanning Electron Microscopy (SEM) images (Figure 3d) show that the Cu-Sn-catalyzed SiNWs are thinner (even with a thicker catalyst deposition) and reach a density of $5 \times 10^{10} \text{ cm}^{-2}$, nearly two orders of magnitude higher than the $6 \times 10^{8} \text{ cm}^{-2}$ obtained with Sn. The tapered morphology is quantified by the base and tip diameters, $D_{\text{base}}$ and $D_{\text{tip}}$, and their difference $\Delta D$ (Figure 3e). The mean $D_{\text{base}}$ of the Cu-Sn-catalyzed SiNWs is 7.1 ± 1.2 nm, roughly half that of the Sn-catalyzed counterparts (15.2 ± 2.2 nm), and
exhibits a narrower distribution. The higher density and smaller diameter arise from the anchoring effect of the solid–liquid Cu₆Sn₅–Sn nanostructure active during pretreatment, which immobilizes NPs and suppresses coalescence. After 3 min of growth, the Cu–Sn catalyst produces an average of 15% of diameter reduction in SiNWs, versus a 40% decrease for Sn catalyst. This greater uniformity reflects the anchoring effect during growth, which prevents catalyst loss. Collectively, the solid–liquid catalyst configurations—active both before and during growth—offer a clear advantage for fabricating SiNWs with small and uniform diameters.

## Dynamics of solid–liquid catalyst reorientation during growth

Figure 4 presents the dynamics of the solid–liquid–solid coexistence driven by plasma-generated radicals during SiNW growth. As the plasma continuously supplies Si radicals, the Sn-rich liquid phase and the Cu₃Si solid phase co-catalyze SiNW growth (Figure 4a, Video 4). A new (111) Si half-plane rapidly completes stacking at the [liquid Sn]–[solid Si] interface in less than 0.25 s (time resolution limited by the 4 fps acquisition rate), followed by a significantly slower 14–31 s propagation along the [solid Cu₃Si]–[solid Si] boundary. The growth front is indicated by the yellow arrows. Nucleation intervals of Si layers range between 8–32 seconds, broader than the monolayer stacking time of 14–31 s. Consequently, simultaneous stacking of two Si layers is frequently observed (Figure 4b, Video 5). This behavior—where the liquid catalyst supplies atomic steps propagating into the solid catalyst—is named the liquid-assisted VLS growth mechanism [16]. In the current work, we observe that the solid–liquid catalyst interface actively rearranges over time (Figure 4c, Video 6). The solid Cu₃Si phase migrates within the droplet while remaining epitaxially aligned with the growing SiNW, and the Sn-rich liquid stays in contact with both the Cu₃Si and the SiNW. The two phases switch sides every 1–2 min (based on 6 observations), reorienting the Si step propagation direction periodically (reversing directions every 4–8 layers, as in Figure 4a and b), while consistently proceeding from liquid to solid. The SiNW growth rate is about 0.015 nm·s⁻¹. Figure 4d summarizes the characteristic timescales of SiNW growth catalyzed by the dynamic solid–liquid Cu₃Si–Sn catalyst under a plasma-radical environment.
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

Figure 4. Dynamics of solid–liquid Cu₃Si–Sn catalyzing during SiNW growth at 400 °C driven by plasma-generated Si radicals. (a–c) share a common scale bar. (a) Single (111) Si plane step propagates from the liquid catalyst to the solid catalyst (Video 4), with the growth front marked by yellow arrows. (b) Two Si layers stack simultaneously (Video 5). The step propagation direction reorients with the evolving catalyst phases, consistently proceeding from liquid to solid. (c) Dynamic rearrangement of the solid–liquid Cu₃Si–Sn interface (Video 6). (d) Characteristic timescales of the SiNW growth, derived from 4–8
in situ TEM observations. (e) Alternating segments of metastable hexagonal (2H) phase, twinned cubic (3C) phase, and stacking fault (SF) in a 7 nm diameter SiNW. FFT indicates growth in direction [111] for the 3C phase and [0001] for the 2H phase. The interplanar spacings are $d_{(111)}$ = 0.312 nm for 3C, and $d_{(0002)}$ = 0.312 nm and $d_{(10\bar{1}0)}$ = 0.333 nm for 2H (with errors of ± 0.005 nm), with a 90° angle between the two 2H planes. The ratio of measured interplanar spacings, $d_{(10\bar{1}0)}/d_{(0002)}$, is 1.067, which is consistent with the theoretical value of 1.068 of the 2H phase [29].

This dynamic solid–liquid–solid coexistence is driven by the plasma-radical environment. At 400 °C, a Cu₃Si–Sn catalyst is unable to sustain SiNW growth via CVD without plasma assistance. When the ECR plasma source is deactivated, SiNW growth ceases immediately (Figure S7, SI), despite the continued presence of the Cu₃Si–Sn catalyst and silane molecules. The plasma continuously supplies Si radicals, maintaining the supersaturation and driving both the growth of new Si layers and the dynamic coexistence of solid–liquid Cu₃Si–Sn. The catalyst actively participates in the growth: the Sn-rich liquid facilitates Si incorporation and diffusion, with new Si layers consistently nucleating on its side; simultaneously, Cu reacts with incoming silicon to form solid Cu₃Si, exchanging atoms with both the liquid phase and the growing SiNW. In a control experiment without liquid Sn (Figure S8, SI), solid Cu alone does not initiate SiNW growth under identical plasma conditions. The reorientation of the solid–liquid Cu₃Si–Sn interface randomizes the Si step propagation direction, thereby maintaining straight growth. The solid Cu₃Si phase remains epitaxially aligned with the SiNW, anchoring liquid Sn at the growth front and ensuring diameter uniformity. This cooperative configuration—combining a reactive solid with a catalytic liquid—balances catalytic activity and morphological stability during SiNW growth in PECVD.

Finally, Figure 4e (see also Figure S9 for *ex situ* PECVD result) provides a glimpse of alternating segments of twinned cubic (3C) and metastable hexagonal (2H) phases catalyzed by the solid–liquid catalyst. The 2H phase—predicted by density-functional theory to have a direct bandgap [30] and to become stable for SiNW diameters below 13 nm [6]—has been observed in ultra-thin SiNWs (< 8 nm) synthesized via PECVD [15, 31]. The dynamic solid–liquid catalyst structure is particularly effective for fabricating ultra-thin SiNWs and thereby facilitates the formation of the 2H phase. The periodic appearance of stacking
fault layers (noted as SF in Figure 4e) suggests that the formation of the 2H phase may be related to the evolution of multilayer stacking faults (Figure S10, SI). In III–V NWs, nucleation occurs at the triple junction, and phase switching between zincblende and wurtzite is governed by the droplet contact angle, which can be tuned by adjusting the group III and V fluxes [32, 33]. By contrast, in SiNWs we have previously observed that each new Si (111) plane nucleates at the triple line where the solid–liquid interface of the Sn–Cu₃Si catalyst meets the NW growth front [16]. After the rapid stacking of a half Si plane within the liquid region of the catalyst, propagation continues in the solid region, beginning at the solid–liquid boundary. Thus, the growth of 2H phase is not directly linked to the catalyst contact angle. The formation of 2H phase may be related to the dynamic restructuring of this solid–liquid interface. However, further investigation is required to fully understand the direct synthesis and stabilization of the 2H phase.

In this system, the coexistence of solid and liquid catalyst phases arises from the combination of a high-melting-point metal (1084.9 °C, Cu) with a low-melting-point metal (232.0 °C, Sn) [20]. To explore the broader applicability of this approach, we replaced Sn with In (melting point 156.6 °C [34]), another effective catalyst for SiNW [35]. Using Cu–In catalyst under the same *in situ* ETEM conditions, we successfully reproduced the same solid–liquid dynamics, with a migrating solid phase embedded in a nanodroplet, co-catalyzing SiNW growth (Figure S11, Video 7). The obtained SiNW also exhibits sub-10 nm diameters and contain the metastable 2H phase [36]. These findings further underscore the potential of solid–liquid bimetallic catalysts for controlling nanowire diameter and crystal phase under plasma-radical conditions.

We employed environmental TEM to directly observe the evolution of Cu–Sn catalyst and the nucleation of Si nanowires, both driven by plasma-generated radicals. Atomic hydrogen and SiH₃ radicals, generated by a remote plasma source, were introduced into the TEM column to probe the underlying mechanisms. The combination of high- and low-melting-point catalysts enables enhanced control over nanowire diameter and crystalline structure. We uncovered previously unrecognized Solid-Anchored Vapor–Liquid–Solid (SA-VLS) dynamics.
At 250 °C, atomic hydrogen reduces native oxides in the as-deposited Cu–Sn nanoparticles, forming a solid Cu₆Sn₅ core that anchors Sn-rich liquid, thereby creating dense, small nucleation sites. Upon heating to 400 °C, the catalyst transforms into nanodroplets. As plasma-generated SiH₃ radicals introduce high supersaturation into the catalyst nanodroplets, Si nucleation occurs simultaneously with catalyst phase separation into Cu₃Si solid and Sn-rich liquid. The liquid phase supplies atomic steps within 0.25 s that propagate into the solid catalyst over 14–31 s, while the solid Cu₃Si remains epitaxial to Si and anchors the liquid phase at the growth front. The solid–liquid catalyst interface undergoes continuous rearrangement every 1–2 min, causing the Si step propagation direction to reverse every 4–8 layers—a previously unreported growth pattern.

The dynamic SA-VLS growth balances catalytic activity and morphological stability, conferring two advantages over conventional VLS growth with single-metal catalysts. First, the anchoring effect offers small nucleation sites and prevents droplet leakage, yielding ultrathin (~7 nm diameter) nanowires with minimal tapering (15 % diameter reduction after 3 min, compared to 40 % for Sn). Second, the dual-phase catalyst imposes the direction of motion of each new silicon atomic step. The NWs grown with a rotating catalyst (i.e., with rotation of the direction of step motion) contain a high density of stacking faults, twins and 2H domains. Exploring the relationships between the step motion direction and the Si stacking order offers an interesting prospect towards the deposition of pure 3C or pure 2H segments, with potential relevance to direct-bandgap applications and quantum devices.

These findings open new avenues for catalyst design and enhanced nanoscale control in bottom-up nanowire growth. Notably, control experiments confirm that nanowire growth does not occur in the absence of plasma. Plasma-generated radicals are essential for driving this far-from-equilibrium chemistry. Replacing Sn with In reproduces the solid–liquid catalyst dynamics, highlighting the broader applicability of combining high- and low-melting-point metals to engineer solid–liquid catalysts. Future studies should extend this strategy to other semiconductor systems and elucidate the thermodynamic and kinetic factors related to the 2H phase selection.
# Methods

## Catalyst preparation and nanowire growth

Cu and Sn were sequentially deposited onto substrates using a BOC Edwards Auto 306 FL 400 thermal evaporator. Deposited thicknesses were monitored by a quartz microbalance in the thermal evaporator. *In situ* growth was conducted on TEM chips in ETEM (NanoMAX), and *ex situ* growth on silicon wafer substrates in a capacitively-coupled radio frequency (13.56 MHz) PECVD reactor (Plasfil), both following the two-stage procedure as described in the main text. Catalyst compositions and plasma parameters for each experiment are enlisted in the SI.

### *In situ* TEM substrates

A Fusion TEM chip (Protochips) with a SiC membrane was used to provide uniform active temperature control. The chip was resistively heated, and the current-temperature relationship was calibrated by the manufacturer with a reported 95 % accuracy [37]. However, the resistance of the heating SiC membrane decreased by approximately 5–10 % from the start to the end of the growth process [38], potentially introducing deviations in the indicated temperature.

A monocrystalline Si <110> cantilever substrate, fabricated on silicon-on-insulator chips and provided by the Mirsaidov Lab of the National University of Singapore, was used for epitaxial growth of SiNWs [39]. SiNWs grew on the (111) Si lateral facets of the cantilever substrate, aligning along the [110] zone axis for high-resolution imaging. The substrate was resistively heated, and its temperature was calibrated with SiNW growth rate [40].

### Plasma ETEM setup

NanoMAX is a Cs (spherical aberration)-corrected Thermo Fisher Titan ETEM customized for CVD and Molecular Beam Epitaxy (MBE) growth. An Aura-Wave ECR plasma source (SAIREM) was integrated 40 cm upstream in the H₂ gas line of NanoMAX [22]. The remote positioning of the plasma source avoids interaction between the electron beam and plasma, which compromises imaging resolution.
Plasma-generated species diffuse downstream into the TEM column. Simulations show that primarily neutral hydrogen atoms reach the sample [17]. SiH₄ was introduced into the TEM column through a separate gas line. The appearance of silicon deposits on the plasma chamber window confirmed the back-diffusion of SiH₄ and the generation of Si radicals near the plasma source. The dominant Si radicals reached the sample were SiH₃ [41]. The pressure in the ETEM column was at an order of 10⁻² mbar (compared with 3×10⁻⁷ mbar without gas).

## Imaging and characterization

Atomic-resolved TEM videos were acquired with two Gatan CCD cameras: a US1000 camera operating at 4 fps and a K2IS camera at 40 fps. FFT of HRTEM images was performed using DigitalMicrograph 3.5 [42]. Diffraction patterns were analyzed using JEMS software [43]. Elemental analysis was conducted using an Oxford-INCA EDX system. SiNW morphology and density were assessed from images acquired with a Hitachi S-4800 SEM.
**Supporting Information**

Supporting Figures and Experimental parameters (PDF)

Video 1 (Figure 1): Cu–Sn nanoparticles during hydrogen plasma pretreatment (AVI)

Video 2 (Figure 2b): Nucleation of a Si nanowire and catalyst solid–liquid separation (AVI)

Video 3 (Figure 3c): Solid Cu<sub>3</sub>Si stabilizing Sn-rich liquid at Si nanowire growth front (AVI)

Video 4 (Figure 4a): Single Si layer stacking, with step propagation proceeding from liquid catalyst to solid catalyst (AVI)

Video 5 (Figure 4b): Simultaneous stacking of two Si layers, with step propagation direction reoriented by the catalyst phases (AVI)

Video 6 (Figure 4c): Dynamic solid–liquid Cu<sub>3</sub>Si–Sn catalyst during Si nanowire growth (AVI)

Video 7 (Figure S11): Dynamic two-phase Cu–In catalyst (AVI)

**Author Information**

**Corresponding Authors**

**Weixi Wang** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France; ORCID: 0009-0004-9869-0149; Email: weixi.wang@polytechnique.edu

**Jean-Luc Maurice** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France; ORCID: 0000-0002-5005-7174; Email: jean-luc.maurice@polytechnique.edu

**Authors**

**Eric Ngo** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France

**Pavel Bulkin** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France; ORCID: 0000-0001-5591-3470; Email: pavel.bulkin@polytechnique.edu

**Martin Foldyna** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France; ORCID: 0000-0001-8413-0504; Email: martin.foldyna@polytechnique.edu

**Pere Roca i Cabarrocas** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France; ORCID: 0000-0003-2241-2762; Email: pere.roca@polytechnique.edu

**Erik V. Johnson** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France; ORCID: 0000-0003-0920-2111; Email: erik.johnson@polytechnique.edu

**Didier Pribat** - Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France; ORCID: 0000-0002-5539-2051; Email: pribat2@yahoo.fr

**Zhengyu Zhang** - Laboratoire SPMS, CentraleSupélec, CNRS, Université Paris-Saclay, Gif-sur-Yvette 91190, France; ORCID: 0000-0002-1254-6225; Email: zhengyu.zhang@centralesupelec.fr

**Conflicts of Interest**
The authors declare no competing financial interest.

Acknowledgements

This work was supported until 2021 by the ANR grant "HexaNW" (No. ANR-17-CE09-0011) and subsequently by the Total-LPICM ANR Industrial Chair "PISTOL" (No. ANR-17-CHIN-0002-01). ETEM experiments were performed using the NanoMAX instrument at the "Centre Interdisciplinaire de Microscopie Electronique de l'X" (CIMEX). We thank Federico Panciera (C2N, CNRS, University Paris-Saclay) and the Mirsaidov Lab at the National University of Singapore for providing the cantilever substrates. We are also grateful to C. Jadaud and J-C. Vanel (LPICM) for developing and maintaining the PECVD experiments in the PLASFIL reactor and the NanoMAX TEM.
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Plasma-Driven Solid–Liquid Dynamics in Cu–Sn Catalysts and Nucleation of Silicon

Nanowires Revealed by Environmental TEM

Weixi Wang<fnref n="1" />*, Eric Ngo<fnref n="1" />, Pavel Bulkin<fnref n="1" />, Martin Foldyna<fnref n="1" />, Pere Roca i Cabarrocas<fnref n="1" />, Erik V. Johnson<fnref n="1" />,
Didier Pribat<fnref n="1" />, Zhengyu Zhang<fnref n="2" />, and Jean-Luc Maurice<fnref n="1" />*

Email: weixi.wang@polytechnique.edu; jean-luc.maurice@polytechnique.edu

<footnote n="1">¹ Laboratoire de physique des interfaces et couches minces, École polytechnique, CNRS, IP Paris, 91120 Palaiseau, France</footnote>

<footnote n="2">² Laboratoire SPMS, CentraleSupélec, CNRS, Université Paris-Saclay, Gif-sur-Yvette 91190, France</footnote>
## TOC Graphic

<figure><img src="image_14.png" /></figure>

## Synopsis

Environmental TEM reveals plasma-radical-driven dynamics of Si nanowire nucleation simultaneous with catalyst solid-liquid phase separation in Cu–Sn nanodroplets.