# Investigation of substrate and hydrogen pretreatment time to modulate SWCNT diameter and growth yield

Haifa Taoum, Mariam Ezzedine, Costel-Sorin Cojocaru

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Haifa Taoum, Mariam Ezzedine, Costel-Sorin Cojocaru. Investigation of substrate and hydrogen pretreatment time to modulate SWCNT diameter and growth yield. Applied Surface Science, 2025, 691, pp.162664. 10.1016/j.apsusc.2025.162664. hal-05381953

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# Investigation of substrate and hydrogen pretreatment time to modulate SWCNT diameter and growth yield

Haifa Taoum, Mariam Ezzedine, Costel-Sorin Cojocaru*

Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), CNRS, École Polytechnique, IP Paris 91128, Palaiseau Cedex, France

**Keywords:**

Single-walled carbon
nanotube Chemical
vapor deposition
Catalyst-support
Pretreatment time

## ABSTRACT

Synthesizing high-quality single-walled carbon nanotubes (SWCNT) requires innovative approaches that surpass traditional catalysts supported on amorphous alumina or silica. This study investigates SWCNT growth on various substrates, including SiO₂/Si, quartz, C-sapphire, and R-sapphire, with and without a 5 nm amorphous alumina buffer layer, using different pretreatment times via hot-filament catalytic chemical vapor deposition (HF-CCVD). A 30-seconds atomic hydrogen pretreatment effectively promotes SWCNT growth on all plain substrates. The addition of the thin alumina buffer layer further enhances yield, resulting in a dense network of SWCNT. However, extending the pretreatment to 120 s leads to catalyst particle coarsening via Ostwald ripening, significantly reducing SWCNT yield and favoring multi-walled carbon nanotubes (MWCNT) formation, regard-less of the presence of a buffer layer. Our results demonstrate that Fe catalyst activity and the resulting CNT structure is tightly linked to substrate characteristics, including crystallinity, porosity, and surface roughness. Importantly, high-yield SWCNT growth is achievable directly on plain substrates without the need for an alumina buffer layer. This work highlights the critical role of substrate-catalyst interactions in CNT synthesis and high-lights the potential for fine-tuning SWCNT growth under short atomic hydrogen pretreatment conditions.

## 1. Introduction

Single-walled carbon nanotubes (SWCNT) are ideal materials for electronic and optoelectronics applications due to their extremely high carrier mobility, structure-tunable bandgap, and nanoscale dimensions [1]. They are widely used in nanoelectronics, including transistors, sensors, and flexible circuits [2–4]. Based on the atomic arrangement structure, SWCNT can be either semiconducting or metallic [5]. Metallic SWCNT (M-SWCNT) can withstand ultrahigh current densities due to ballistic electron transport, while semiconducting SWCNT (S-SWCNT) exhibit high intrinsic carrier mobility [6]. Among the various methods for SWCNT growth, hot-filament catalytic chemical vapor deposition (HF-CCVD) has gained prominence due to its simplicity and ease of process control [7,8]. Optimized HF-CCVD conditions can produce different forms of CNT, including vertically or horizontally aligned multi-walled and single-walled nanotubes [8–10]. Highly activated/ atomic hydrogen (H<sub>at</sub>) exposure is required to effectively form nano- particles from catalyst thin films, to reduce the catalyst (which is commonly oxidized due to air exposure), and, most importantly, to create surface defects with high trapping energy to pin the forming

metal clusters [11]. For example, while alumina (Al₂O₃) is a stable oxide with a surface terminated by oxygen atoms, its exposure to highly reactive H<sub>at</sub> reduces surface oxide by reaction with the oxygen atoms, creating vacancies or defects on the surface. These oxygen vacancies act as local defects and can serve as "pinning sites" that trap metal atoms or clusters, preventing their diffusion across the surface [11]. Previous studies have demonstrated that HF-CCVD enables the synthesis of high- quality SWCNT with improved structural integrity. For instance, Raman spectroscopy analysis has shown a decrease in the I<sub>D</sub>/I<sub>G</sub> ratio when the hot filament is used (0.09 vs. 0.15 for samples grown with and without hot filament, respectively), indicating improved crystalline quality. Furthermore, confirming the presence of SWCNT when the hot filament is used, the radial breathing mode (RBM) peaks exhibit increased in- tensity and number of occurrences [12]. In addition, studies of atomic hydrogen-driven catalyst size control indicate that catalyst nano- particles remain smaller when exposed to atomic hydrogen rather than molecular hydrogen [11]. A key advantage of HF-CCVD is its ability to pre-decompose hydrocarbon feedstock as it passes through the hot filament, resulting in a higher concentration of reactive carbon species at the catalyst surface. This mechanism enhances CNT yield, accelerates

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: costel-sorin.cojocaru@polytechnique.edu (C.-S. Cojocaru).</footnote>
growth rates, and reduces structural defects compared to thermal CVD, where the density of active carbon species remains lower, resulting in slower CNT growth [13].

Efficient CNT growth typically requires the catalysts to be supported by insulating materials, such as amorphous alumina or silica [14]. However, these insulator materials are usually not compatible with a direct integration into applications requiring conductive substrates, such as energy storage [10,15] and sensing devices [16,17]. There are few reports on the growth of SWCNT on non-alumina supports [18–20]. A significant challenge with such substrates is typically their high sur-face energy, which inhibits the formation of catalyst nanoparticles during annealing. For alumina-supported catalysts, the as-deposited thin catalyst film undergoes a transformation into high surface area nano-particles and becomes catalytically active. This transformation is influ-enced by the interaction between the catalyst and the support, which also impacts the final size distribution of the catalyst particles. During annealing, the catalyst film tends to dewet from the support layer due to differences in surface energy between the metal catalyst and the support [7,11]. Non-alumina substrates, however, are less effective at stabilizing the catalyst and preventing catalyst poisoning, which generally impedes high-quality SWCNT growth.

In this study, we investigated SWCNT growth using H<sub>at</sub> pretreatment in HF-CCVD, on plain substrates, including SiO<sub>2</sub>/Si, quartz, C-sapphire, and R-sapphire, and compared it to their growth on 5 nm Al<sub>2</sub>O<sub>3</sub>-sup- ported iron (Fe) catalysts. Such relatively thin alumina layer is generally considered to not impede drastically the electronic transport across the barrier [10,21]. A parametric study was conducted by varying the pre-treatment time under H<sub>at</sub>. This study systematically explored catalyst-support interactions for both the plain substrates and the Al<sub>2</sub>O<sub>3</sub>-sup- ported Fe films, focusing on how they influence catalyst mobility and as-formed catalyst size distribution during annealing, as well as their impact on the subsequent SWCNT growth.

## 2. Experimental

### 2.1. Synthesis of the SWCNT

Substrates used in the experiment include SiO<sub>2</sub>/Si (300 nm thermal oxide), α-ST-cut quartz, C-plane sapphire, and R-plane sapphire. SiO<sub>2</sub>-coated Si was chosen to prevent silicide formation during the SWCNT growth. Furthermore, since SiO<sub>2</sub> is chemically identical to quartz but differs in surface properties such as surface energy and crystallinity, it provides an ideal platform for studying and comparing the SWCNT growth on these two surfaces [14]. Substrates were thoroughly cleaned with an isopropanol-acetone mixture. A controlled 0.8 nm ± 0.04 nm Fe catalyst was deposited on the various substrates via molecular beam evaporation (MBE) under high vacuum (10<sup>-9</sup> mbar). It should be noted that the catalyst thickness was chosen based on prior well-established effectiveness for growing SWCNT with high efficiency on the amor-phous alumina layer. The catalyst-deposited substrates were then transferred to a custom-built six hot-filament CVD (6HF-CVD) reactor for the synthesis of SWCNT. The six high temperature heated Tantalum (Ta) filaments enable the growth of SWCNT with increased uniformity in both diameter and length as well as importantly, their synthesis over large, up to 6" substrate areas. Catalyst pretreatment was conducted at 900 °C under activated hydrogen (H<sub>2</sub>) for 30, 60, 90, and 120 s. The activated hydrogen passed through the six Ta filaments, which were heated to over 2500 °C (900 W applied power), with a flow rate of 30 sccm at a reactor pressure of 2 mbar. SWCNT growth was then per-formed by exposing the pretreated substrate to a gas mixture of methane (CH<sub>4</sub>) and H<sub>2</sub>, with flow rates of 10 and 20 sccm, respectively, and at a reactor pressure of 2 mbar. The growth time was 20 min with the fil-aments maintained at the same power as for the pretreatment step for an effective predissociation of the CH<sub>4</sub> precursor. The generation of atomic hydrogen during this stage served to prevent excessive deposition of amorphous carbon and to maintain the catalytic activity of

nanoparticles.

### 2.2. Characterization of the SWCNT

Scanning electron microscopy (SEM, HITACHI S 4800 FEG) oper-ating at 1 kV was used to characterize the morphology and density of the as-grown CNT on the various substrates. Transmission electron micro-scopy (TEM) analysis was carried out in conventional TEM mode using a TITAN G2 electron microscope operating at 300 kV and equipped with Oxford on-side detector. TEM is a highly effective technique for struc-tural analysis, providing detailed insights into both crystalline and amorphous materials at the atomic level. However, TEM is a localized technique, primarily examining material collected from the surface, which limits its ability to provide information across a larger sample area. Hence, in this study, TEM has been used as a complementary technique to validate a specific hypothesis regarding the structure and quality of as-grown SWCNT synthesized with extended pretreatment times. Raman spectroscopy was performed using a HORIBA spectrom-eter with a ×100 objective lens, at two excitation wavelengths (473 nm and 633 nm) in order to assess the diameter of the as-grown SWCNT. For each sample, and at both excitation wavelengths, several mappings were recorded at random locations on the substrate for ensuring statistically meaningful analysis. The mappings were performed over a 20 µm by 20 µm area, with a step size of 0.1 µm. In-situ XPS measurements were conducted under 10<sup>-10</sup> mbar using an AlKα energy source. Spectra were acquired at an emission angle of 45° with respect to the analyzer and an incident light beam at ∞55°. Such setup arrangement minimizes the analysis depth to a maximum 5 nm and maximizes the analyzed spot to typically 0.5 cm<sup>2</sup> area. Electron energy detection was performed with a double hemispherical analyzer Alpha 110 Thermo Fisher™.

## 3. Results

### 3.1. SWCNT growth on SiO<sub>2</sub>/Si and quartz substrates

To investigate the effect of pretreatment time on SWCNT growth, a series of experiments was conducted with pretreatment times ranging from 30 s to 120 s on two substrates. Fig. 1 summarizes the SEM mi-crographs comparing growth results of SWCNT nucleated from sub-nanometer Fe films (0.8 nm) deposited on SiO<sub>2</sub>/Si-supported (Fig. 1a–d) and quartz-supported (Fig. 1e–h) at four different pretreatment times. An increase in pretreatment time to 90 s resulted in a significant decrease in the density of CNT growth for both types of substrate. The substrate influence indicating changes in catalyst behavior is notable, no SWCNT was observed on the SiO<sub>2</sub>/Si substrate after 120 s of pretreatment, but rather a small aggregate of carbon materials suggestive of multi-walled CNT (Fig. 1d).

Raman spectroscopy (summarized Fig. 2) was carried out on all substrates in order to retrieve global information regarding the crys-tallinity and diameter of the as-grown SWCNT. The resulting spectra were averaged and fitted. Typically, SWCNT peaks exhibit Lorentzian line shapes, except for the ω<sub>G</sub>- component of the G-band in metallic SWCNT, which follows a broad Breit-Wigner-Fano line shape. However, for simplicity, all spectra were fitted using Lorentzian line shapes [22] in Origin software. Fig. 2a displays the typical Raman spectra of SWCNT grown on SiO<sub>2</sub>/Si substrate. The spectra exhibit the radial breathing modes (RBMs) between 300 and 100 cm<sup>-1</sup>, a sharp G-band at ~1589 cm<sup>-1</sup>, with a shoulder occurring at low energies 1550 cm<sup>-1</sup> corre-sponding to G' peak, a weak D-band at ~1322 cm<sup>-1</sup>, and a 2D band at 2602 cm<sup>-1</sup>. The sharp G-band is associated with the sp<sup>2</sup> hybridized carbon atoms in the nanotube walls, while the weak D-band indicates the presence of structural defects on the surfaces of SWCNT. The reso-nant 2D-band is considered an overtone of the D-band. These peaks are representative of typical SWCNT Raman spectra on SiO<sub>2</sub>/Si substrate [22]. The peaks marked with asterisk at 303, 521, and 963 cm<sup>-1</sup> are peaks related to SiO<sub>2</sub>/Si substrate [23].
<figure><img src="image_2.png" /><figcaption>Fig. 1. SEM images of SWCNT synthesized on (a-d) SiO₂/Si substrate and (e-h) quartz substrate, showing the effect of different pretreatment time.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2. Raman spectra of CNT grown on (a) SiO₂/Si substrate and (b) quartz substrate, measured using an excitation wavelength of 633 nm at different pretreatment times.</figcaption></figure>

Fig. 2b presents the Raman spectra of SWCNT grown on quartz substrate. Several characteristic peaks from the quartz substrate, such as at 126 cm⁻¹ and 205 cm⁻¹, overlap with the RBM region, making it difficult to unambiguously identify the RBM peaks and consequently estimate the SWCNT's diameter. A prominent quartz peak is also observed at 464 cm⁻¹ [24]. To address this, several Raman spectra of bare quartz were measured and normalized to the main 464 cm⁻¹ peak

intensity. After normalizing the SWCNT's Raman spectra to the respect of this same peak, the quartz fingerprint could be subtracted. Subsequently, the substrate-corrected (subtracted) SWCNT spectra were further normalized to the respect of the G-band intensity for further accurate peak fitting and diameter calculations. The Raman spectra for SWCNT on quartz display RBMs features between 300 and 100 cm⁻¹, a sharp G-band at ~1592 cm⁻¹, with a broader G- peak at lower energies
of 1552 cm⁻¹, a weak D-band at ~1320 cm⁻¹, and a small 2D-band at 2608 cm⁻¹. As the pretreatment time increases, a slight decrease in RBM intensity is observed. After 120 s of pretreatment, the spectra exhibit a peak around 110 cm⁻¹, indicating the presence of relatively larger inner diameter CNT as compared to those grown with shorter pretreatment times. The I₀/I₉ ratio is commonly used to evaluate the crystallinity of SWCNT structures. For both types of substrates, the I₀/I₉ ratio increases quasi-linearly as the pretreatment time increases, implying a higher concentration of defects potentially due to the increased probability of forming catalyst particles of non-uniform volume at higher pretreatment times.

To determine the average diameter size of the SWCNT, the empirical formula $\omega_{RBM} = (A/d_i) + B$, where A = 234 cm⁻¹ and B = 10 cm⁻¹, can be used, taking into account the influence of neighboring tubes in SWCNT bundles [25]. Two laser excitations were employed to estimate the diameters of the SWCNT. The RBM frequency ($\omega_{RBM}$) for each SWCNT was determined by fitting the relevant regions of the spectrum using a Lorentzian function. The full width at half maximum (FWHM) for each peak is found to range between 3 and 4 cm⁻¹, which corresponds to the expected natural linewidth for isolated SWCNT. For SWCNT bundles, the FWHM reflects the resonance of multiple tubes with the laser [26], highlighting the importance of using multiple laser excitations to obtain accurate diameter estimates. Comparing the I₉ to the main intensity of the substrate, Iₛₐb, the Iₛₐb/I₉ ratio increases with longer pretreatment times, indicating a lower SWCNT yield at higher pretreatment times. Table 1 summarizes the Iₛₐb/I₉ ratios for SiO₂/Si and quartz substrates, illustrating how increased pretreatment time reduces the yield of the grown SWCNT. This method is, in fact, effective and straightforward for evaluating the synthesis yield, as the intensity of the G-band is directly related to the concentration of sp²-bonded carbon in the sample [22].

Only the peaks that are 10% above the baseline were further taken into consideration. Fig. 3 presents the diameter distribution of SWCNT grown on SiO₂/Si and quartz substrates as a function of different pretreatment times. Although the RBM distribution measured with only two excitation wavelengths is not sufficient for a full analysis, it allows for a qualitative comparison of the relative diameter distribution. We can notice that the shape of the diameter histogram changes with pretreatment time. For the growth performed subsequent to 30 s of pretreatment, both types of substrates exhibit a narrow Gaussian distribution, with the most frequent diameters ranging from 1.2 to 1.5 nm with an average diameter of 1.41 ± 0.08 nm for the SiO₂/Si substrate and respectively 1.39 ± 0.08 nm for the quartz substrate. The distributions also exhibit sparse occurrences between 0.8–1.2 and 1.6–2.2 nm cumulatively totaling nearly 11% of the counted SWCNT. As the pretreatment time increases, the corresponding distribution is shifted towards larger diameters and it appears less well-defined. For the SiO₂/Si substrate, extended pretreatment (up to 90 s) results in an increase in the average SWCNT diameter (1.94 ± 0.09 nm), with only a few SWCNT remaining within the detection range. No SWCNT are detectable after 120 s of pretreatment.

On the counterpart, on the quartz substrate, after 90 s, the diameter histogram becomes bimodal, showing two predominant populations: one between 1.2 and 1.5 nm, and another between 1.8 and 2.1 nm. After 120 s of pretreatment, the overall observed diameter continues to increase, exhibiting a non-uniform size distribution of the SWCNT, with a

<table><caption>**Table 1**
The ratio of Iₛₐb/I₉ for SiO₂/Si and quartz substrates with a 0 nm Al₂O₃ layer after pretreatment times of 30, 60, 90, and 120 s.</caption><thead><tr><td>Time</td><td>SiO<sub>2</sub>/Si</td><td>Quartz</td></tr></thead><tbody><tr><td>30 s</td><td>6.8</td><td>0.3</td></tr><tr><td>60 s</td><td>34.5</td><td>1.02</td></tr><tr><td>90 s</td><td>65</td><td>1.55</td></tr><tr><td>120 s</td><td>128</td><td>1.89</td></tr></tbody></table>

predominant diameter of ~2.1 nm. This further confirms the low yield of small-diameter nanotubes growth upon longer pretreatment times, indicating that such extended pretreatment is detrimental to a diameter selective growth of SWCNT. Various factors may influence the size and yield of the as-grown SWCNT aside from the atomic hydrogen-assisted pretreatment used in this study for the CNT growth. The pretreatment has a role in reducing the oxidation state of the catalyst, often oxidized due to air exposure, which can impact the nanoparticles forming and dewetting from the catalyst's thin films at a certain temperature [27]. The activated hydrogen pretreatment can thus significantly affect the diameter, density, and uniformity of the catalyst nanoparticles formed for subsequent SWCNT growth [7,11]. On the other hand, the activated hydrogen can create trapping sites on the substrate surface that contribute to reducing the diffusion and drive the formation of smaller particle sizes than originally expected from the thickness of the pre-deposited catalyst film alone. According to the SEM micrographs observations and the Raman characterization, only few-walled, relatively large CNT were detected in the case of 120 s of pretreatment for the SiO₂/Si substrate, and a larger bimodal population started to form after extending the pretreatment time for the quartz substrate. Thus, the prolonged hydrogen exposure tends to result in larger catalyst particles due to coarsening via Ostwald ripening [7]. These observations are consistent with previous studies that have highlighted the importance of controlling and tuning the pretreatment time to maximize the yield of high-quality SWCNT [28].

The size difference between the SiO₂/Si and quartz substrates can be explained by the surface roughness, another critical factor to be considered for influencing SWCNT growth [29]. The SiO₂/Si substrate surface is amorphous with a microelectronic grade smooth and polished surface and low surface energy [30]. The low surface energy will increase the in-surface diffusion coefficient of the catalyst promoting the coalescence phenomenon and the formation of larger-size catalyst particles [14]. In contrast, quartz is a monocrystalline substrate with a well-defined, unidimensional lattice structure which, via the presence of surface atomic steps, limits diffusion and restricts particle mobility, thus effectively selecting for the formation of relatively smaller particles. As a result, quartz substrates favor the formation of SWCNT, as opposed to the larger agglomerates observed on SiO₂/Si substrate, because the ridges on the quartz substrate act as traps, and limit the diffusion of the catalyst during the pretreatment step, thereby preventing the formation of larger particles.

The bimodal distribution of SWCNT diameters observed exclusively on quartz substrate can be partly attributed to the ability of the substrate to constrain particle sizes within an optimal range. In addition, the continuing activated hydrogen flow during the growth step is likely to reactivate agglomerated catalyst particles (re-nucleation rather than ripening), allowing for the growth of larger-diameter SWCNT [29] by re-pumping carbon atoms from the newly activated particles. Thus, CNT diameter is strongly correlated with the catalyst particle size distribution [31], and the bimodal SWCNT diameter distribution reflects the underlying bimodal catalyst particle size distribution. This observation highlights the need for further investigation of this phenomenon.

In order to gain a deeper insight into the particle formation on the SiO₂/Si substrate, TEM analysis was performed, by scratching the substrate surface and subsequently dry transferring the collected powder onto a TEM grid, for samples after 30 s of pretreatment time (Fig. 4a) and after 120 s pretreatment time (Fig. 4b and c). Fig. 4a shows agglomerated CNT (presumably due to the specimen preparation method) which are mostly single-walled with some multi-walled. The diameter of the SWCNT detected is in the range of 1 to 2.4 nm, confirming the Raman analysis. In contrast, Fig. 4b shows crystalline Fe particles with diameters ranging from 10 to 20 nm, together with MWCNT (indicated by white arrows) and some occurrences of onion-like carbons. These onion-like carbons consist of spherical concentric layers of graphitic carbon surrounding a central 'core', which may be either hollow or containing a nanoparticle catalyst. It appears that the catalyst nanoparticles with
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Diameter distribution histogram based on RBM peaks, obtained from the contribution of the two excitation wavelengths for SiO₂/Si and quartz substrates for (a) 30, (b) 60, (c) 90, and (d) 120 s pretreatment times.</figcaption></figure>

diameters smaller than an optimal size are quickly overfed, leading to poisoning and the formation of a full surrounding graphitic shell that inhibits further tube nucleation. The nanoparticles with larger diameters appear inactive most probably due to underfeeding upon our experimental growth conditions. High-resolution TEM (HRTEM) analysis of several observed catalytic particles, along with fast Fourier transform (FFT) analysis of the corresponding images, confirmed the presence of encapsulated cementite (Fe₃C) (Fig. 4c) [32] with measured interplanar spacings ranging from 0.20 to 0.22 nm. Identifying the active phase in Fe-catalyzed CNT growth is challenging due to the variety of Fe phases that may form in carbon-rich environments. Previous studies have demonstrated that CNT can grow from crystalline cementite [32,33]. However, other reports suggest that cementite nanoparticles are inactive for CNT synthesis [34,35].

To identify the active phase in the SWCNT growth process, XPS analysis was performed on SWCNT with a 30-seconds pretreatment time. Fig. 5 shows the XPS core-level spectra for C 1s and Fe 2p of this sample. In the C 1s spectrum, Fig. 5a, a single narrow peak appears at 284.6 eV, corresponding to the distinct C–C (sp²) bonding characteristic of CNT [36]. Notably, no Fe–C bonding peaks, typically observed at around 283 eV, were detected. The Fe 2p (Fig. 5b) core-level spectrum has been fitted according to Biesinger et al., where the peak at 707 eV corresponds to Fe 2p₃/2 of the metallic phase of the iron, and the other fitted peaks to Fe 2p₃/2 and Fe 2p₁/₂ correspond to the Fe(III)-O oxide phases of Fe₂O₃ [37]. This confirms the absence of Fe-C bonds in the sample. Thus, during the initial 30 s of pretreatment, Fe particles interact with carbon radicals until reaching a saturation limit, forming a carbon cap on the Fe particles. Carbon atoms then precipitate from the surface of Fe₃C particles, promoting SWCNT growth as Fe₃C is converted back to metallic Fe. In contrast, with a 120 s pretreatment, Fe₃C particles tend to coarsen, which can lead to the growth of graphitic-like material

or MWCNT, depending on the particle size and carbon saturation level. This suggests that shorter pretreatment times favor SWCNT formation, while extended pretreatment leads to coarsening of Fe₃C and the growth of thicker graphitic structures.

### 3.2. SWCNT growth on C-sapphire and R-sapphire substrates

Sapphire wafers, as monocrystalline forms of Al₂O₃, are commonly used as substrates for thin film and epitaxial layers' growth in semiconductor devices. These wafers can be prepared with different crystallographic orientations, or 'cuts,' such as the C-plane (0001) and R-plane (1-102) (Fig. 6). The C-plane has no preferential in-plane orientation and exhibits relatively low surface energy compared to the R-plane, which has higher surface energy and promotes stronger adhesion [38,39] and we infer that the surface orientation of the R-plane can significantly influence the shape of the grown SWCNT.

The comparative effect of the pretreatment time on SWCNT growth on C- and R-plane sapphire substrates is presented in Fig. 7. The SEM images reveal that after 30 s of pretreatment, only a few SWCNT were able to grow on the R-plane sapphire substrate, with an estimated density of 3 SWCNT/µm, along with the formation of MWCNT and large catalyst particles. Similar MWCNT were observed on the C-plane sapphire substrate after 30 s of pretreatment. When the pretreatment time was extended to 60 s, large catalyst particles were clearly formed on both substrates. These particles appear uniformly distributed and ori-ented along the surface steps of the R-plane sapphire, while on the C-plane sapphire, the particles were homogeneously distributed over the entire substrate surface.

Previous reported studies have suggested that the surface steps of R-plane sapphire can induce the alignment of catalyst particles, leading to aligned CNT growth [2,40,41]. However, in our case, no particular
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** TEM analysis of CNT growth on SiO₂/Si substrate: (a) after 30 s of pretreatment, (b) after 120 s of pretreatment, and (c) HRTEM image of Fe₃C particles with the corresponding FFT after 120 s of pretreatment.</figcaption></figure>

SWCNT alignment could be observed. We hypothesize that a too high initial thickness of the Fe film led to the formation of larger catalyst particles, which may have been subsequently poisoned. The alignment and growth of SWCNT on sapphire substrates are influenced by many factors, including crystallographic orientation, catalyst type, catalyst film thickness, and growth conditions [38,40].

Raman spectra recorded for the as-grown CNT on the C-sapphire cut and R-sapphire cut substrates are summarized in Fig. 8. The main peaks at 1370 cm⁻¹ and 1400 cm⁻¹ and the peaks at lower wavenumber (750 and 421 cm⁻¹) are substrate characteristic peaks that have been taken into consideration during peak fitting [42]. We can notice in Fig. 8a that the spectra are largely similar to an expected typical multiwall spectra with the D-band at 1340 cm⁻¹ and the G-band at 1591 cm⁻¹ and no RBM bands (100–300 cm⁻¹) were observed, indicating the absence of SWCNT in these samples. Interestingly, RBM peaks were detected in the case of

the R-sapphire cut substrate only after 30 s of pretreatment. The C-plane, with no specific crystallographic orientation, has relatively low surface energy compared to the R-plane, which has higher surface energy and promotes stronger adhesion. This difference in surface energy influences surface diffusion length, which is inversely related to the surface energy. As a result, larger particle sizes with lower density are expected on the C-plane [43] that are in good agreement with our observations of larger catalyst particles, which tend to produce MWCNT, formed on the C-plane.

Fig. 9a and b present TEM analysis of samples grown on R-cut sapphire substrates after 30 and 120 s of pretreatment, respectively. The sample subjected to 30 s of pretreatment shows single- to multi-walled CNT with diameters around 2.2 nm, larger than those observed on SiO₂/Si substrates, consistent with Raman analysis. After 120 s of pretreatment, the sample predominantly exhibits larger graphitic materials,
<figure><img src="image_6.png" /><figcaption>Fig. 5. XPS core-level spectra of (a) C1s and (b) Fe2p for SWCNT grown on a SiO₂/Si substrate after 30 s pretreatment time.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. Schematic of the crystallographic faces of sapphire [39].</figcaption></figure>

similar to those observed on SiO₂/Si substrates under identical conditions. Large catalyst particles with an average diameter of 15 nm were observed. These particles correspond to the Fe₃C structure, characterized by an interplanar spacing of 0.2 nm and an interplanar angle of 43.6°, as measured across multiple particles.

### 3.3. SWCNT growth on different substrates with a 5 nm Al₂O₃ buffer layer

Al₂O₃ is the most commonly used barrier layer for the growth of SWCNT, MWCNT, or aligned CNT on various substrates due to its ability to effectively support different types of metal catalysts, and reduce subsurface diffusion [14]. An evaporated very thin layer of Al₂O₃ using an MBE method can also yield a relatively porous nature which is prone to facilitate efficient permeation of the feedstock gas into the catalyst, in the end resulting in more carbon atoms dissolving into the catalyst particles. As might be expected from such consideration, CNT grown on Al₂O₃ exhibit higher density and yield, as can be observed in Fig. 10

which comparatively summarizes the growth results for previously discussed types of substrate covered with a 5 nm thin Al₂O₃ buffer layer. For samples grown at 30 and 60 s, the SEM micrographs show a dense, randomly oriented distribution of SWCNT covering the surface of all type of substrates. However, when the pretreatment time extends to 120 s, SWCNT growth tend to become sparse, with fewer observed nanotubes and the formation of larger particles. In fact, larger catalyst particles, which hinder SWCNT growth, can easily be formed upon higher pre-treatment time due to Ostwald ripening. Compared to the results discussed in the previous sections for the SWCNT growth directly on the substrate, the obtained SEM images confirm that the presence of the Al₂O₃ buffer layer significantly improves the CNT yield across all sub-strates, and the effect is particularly strong at shorter pretreatment times.

The corresponding Raman spectra series are presented in Fig. 11. Overall, the observed D band appears weaker as compared to corresponding growth on substrates without a buffer layer. The general trend of increased intensity of the D band with the increase of the pretreatment time is still observed, however, the Raman spectroscopy analysis did not reveal significant differences in the I_D/I_G ratio for the same pretreatment time. This may be attributed to the fact that the CNT growth conditions, which are known to be one of the primary influences of the crystallinity of the SWCNT, remained constant through the all series except for the pretreatment time. Nevertheless, significant shifts and splitting variations of the G bands around 1600 cm⁻¹ can clearly be observed and by correlation with the corresponding RBM peaks at low frequencies they are providing valuable information on the SWCNT diameter distribution.

As earlier discussed, as the RBM peaks are resonant modes, the intensity of the peak is related to the number of SWCNT that are in resonance with an incident wavelength. With this in mind, a very narrow RBM area of SWCNT and sharp intense peaks are indirectly indicative of a CNT uniformity that is close to an optimum. Under this framework, for all substrates presented here, the shorter pretreatment time can clearly be identified as the optimum condition in our study for obtaining well-distributed SWCNT with small diameters. As the pretreatment time increases, in case of the growth on substrates without buffer layer, we remarked that instead of a well-defined distribution featuring a main component with an identifiable main frequency, the spreading of diameters tend to yield numerous components that can be hardly distinguished.

The distribution of the SWCNT diameter for the growth on substrates with the buffer layer is presented in Fig. 12. The observed nanotube diameter for all substrates is in average between 1.25 nm and 1.38 nm and exhibits a general trend of mean diameter increase with the pre-treatment time. By contrast to the growth on corresponding substrates without buffer layer, for the SiO₂/Si and the quartz ones, the distribution remains monomodal (See Fig. 3) even at long pretreatment time. The 5
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** SEM images of SWCNT synthesized on (a-d) C-sapphire substrate and (e-h) R-sapphire substrate, showing the effect of different pretreatment times with a 0 nm Al₂O₃ layer.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Raman spectra of CNT grown on (a) C-sapphire substrate and (b) R-sapphire substrate, measured using an excitation wavelength of 633 nm at different pretreatment times with a 0 nm Al₂O₃ layer.</figcaption></figure>

nm thickness buffer layer appears, thus, to shield this type of substrate influence. As for the C-sapphire and R-sapphire substrates, for the corresponding ones with a buffer layer, we noticed SWCNT with an overall mean diameter centered around 1.3 nm and in case of the C-cut sapphire substrate a wider diameter distribution.

The overall growth yield of the SWCNT can be indicatively estimated

by the $I_{sub}/I_G$ ratio and clearly, a higher yield can be identified for all types of substrates at 30 s of pretreatment, with a clear trend of decreasing yield with the increase of the pretreatment time (Table 2).

Fig. 13 represents the TEM micrographs for Al₂O₃ coated samples in the case of SiO₂/Si substrate grown after (a) 30 s pretreatment time and (b) 120 s pretreatment time. At lower pretreatment time, bundles-like
<figure><img src="image_10.png" /><figcaption>Fig. 9. TEM analysis of CNT growth on R-sapphire: (a) after 30 s of pretreatment, (b) after 120 s of pretreatment, with inset showing the corresponding FFT of Fe<sub>3</sub>C particles.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10. SEM images of SWCNT synthesized on SiO<sub>2</sub>/Si substrate, quartz substrate, C-plane sapphire substrate, and R-plane sapphire substrate at different pretreatment times with a 5 nm Al<sub>2</sub>O<sub>3</sub> layer.</figcaption></figure>

SWCNT are observed, with a small diameter range of around 1.6 nm similar to the diameter distribution obtained from the Raman analysis. At higher pretreatment time, more agglomerated SWCNT are presented where the Fe particles are also presented in the sample (Fig. 13b). Considering that SWCNT have diameters ranging from 0.8 to 2.2 nm, they are highly sensitive to irradiation, which can lead to structural modifications or amorphization, making it challenging to determine the diameter distribution from TEM images [44,45].

## 4. Discussion

### 4.1. Amorphous alumina layer

In a typical substrate-supported SWCNT growth process, after the deposition of a continuous thin film catalyst, i.e. Fe, the substrate is exposed to air prior to being transferred to the CVD reactor. A hydrogen reduction step is therefore essential before SWCNT growth. This pretreatment helps in fragmenting the continuous Fe thin film into small
<figure><img src="image_12.png" /><figcaption>Fig. 11. Raman spectra of CNT grown on (a) SiO₂/Si substrate, (b) quartz substrate, (c) C-plane sapphire substrate, and (d) R-plane sapphire substrate, measured using an excitation wavelength of 633 nm at different pretreatment times with a 5 nm Al₂O₃ layer.</figcaption></figure>

clusters, which are necessary for subsequent CNT nucleation and growth. The formation of such small clusters is driven by a reduction in the surface energy of the Fe film. At room temperature during catalyst deposition, there is insufficient thermal energy to promote surface diffusion, resulting in Fe atoms adhering to the amorphous alumina surface and forming a quasi-continuous layer. However, at higher temperatures, surface diffusion occurs, allowing the system to minimize the

Fe surface area while maximizing the exposed Al₂O₃ surface, which has lower energy. As described by Jeong et al. [11], the Fe surface contains

numerous dangling bonds and exhibits a high surface defect density, ≈10¹⁴ defects/cm² or higher. These defects are passivated by oxygen when exposed to air, thereby reducing the surface energy. During annealing in a reducing atmosphere (H₂), the oxygen is removed, restoring the Fe surface to a higher energy state. In contrast, the Al₂O₃ surface has a lower defect density, around 10¹² defects/cm². This energy difference prompts the Fe film to fragment into small islands, facilitating the nucleation of Fe nanoparticles on the surface defects of Al₂O₃. To prevent excessive growth of Fe clusters, it's crucial to control surface diffusion over short distances (a few nm), thereby maintaining small cluster sizes that are effective for SWCNT growth. In our reactor configuration, the use of highly activates and reactive Hₐt creates and stabilizes defects with high trapping energy on the Al₂O₃ surface, effectively hindering surface diffusion and enhancing the pinning of catalyst clusters. Assuming a uniform catalyst cluster distribution with a diameter of 1.3 nm (≈50 Fe atoms per cluster), and that each cluster is pinned to a surface defect, the surface defect density can be estimated. If Fe atom surface density (assuming uniform distribution) is approximately 10¹⁵ atoms/cm², which is typical for atomic layers, the number of clusters per unit area is determined by dividing the total number of Fe

atoms per unit area by the number of atoms per cluster. This yields a surface defect of ≈2 × 10¹³ defects/cm² (with each defect corresponding to a pinned cluster). This density is about 10 times higher than that of the as-deposited Al₂O₃. If the clusters are arranged on a square lattice, the distance between defects can be estimated. Given a surface defect density of 2 × 10¹³ defects/cm², the area covered by each defect is:

1

$$ \text{Area per defect} = \frac{1}{2 \times 10^{13}} = 5 \times 10^{-14} \text{defects/cm}^2 $$

Since the defects are evenly spaced in a square pattern, the distance between is the square root of the area:

$$ \sqrt{5 \times 10^{-14}} = 2.23 \text{nm} $$

This indicates that the defects are spaced approximately 2.23 nm apart on average. Given that the catalyst clusters have a diameter of about 1.3 nm, there would be 0.93 nm of space between the clusters, which could lead to some degree of aggregation. The Hₐt treatment significantly increases the Al₂O₃ surface defect density, leading to more pinning sites for catalyst clusters and reducing the surface diffusion of Fe. Increasing the pretreatment time to 120 s slightly shifted the SWCNT size distribution toward larger diameters, likely due to slow rate catalyst aggregation, even in the presence of atomic hydrogen. This can be explained by coalescence when the average distance between defects becomes smaller than the average diameter of the catalyst nanoparticles.

## 4.2. Amorphous alumina layer vs sapphire substrates

Various factors influence need to be accounted for the growth of SWCNT during the CVD process. These factors include the type of
<figure><img src="image_13.png" /></figure>

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

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>**Fig. 12.** Diameter distribution histograms based on RBM peaks, obtained from the contributions of two excitation wavelengths on SiO₂/Si and quartz substrates (a-d), and on R-sapphire and C-sapphire substrates (e-h) with a 5 nm Al₂O₃ layer.</figcaption></figure>
<table><caption>**Table 2**
The ratio of $I_{sub}/I_{G}$ for $SiO_{2}/Si$ and quartz substrates with a 5 nm $Al_{2}O_{3}$ layer after pretreatment times of 30, 60, 90, and 120 s.</caption><thead><tr><th>Time</th><th>SiO<sub>2</sub>/Si</th><th>Quartz</th></tr></thead><tbody><tr><td>30 s</td><td>0.52</td><td>0.004</td></tr><tr><td>60 s</td><td>1</td><td>0.1</td></tr><tr><td>90 s</td><td>3.4</td><td>0.45</td></tr><tr><td>120 s</td><td>11.82</td><td>0.72</td></tr></tbody></table>

precursor gas, choice of metal catalyst, catalytic particle size, growth temperature and time, gas flow, pretreatment time, and the properties of the supporting substrate, such as crystallinity, roughness, and porosity. In our case, we focus primarily on the supporting substrate surface, as the other factors are kept constant. The substrate's crystallinity, whether amorphous or monocrystalline, significantly impacts the type and density of CNT formed. A comparison between amorphous alumina and monocrystalline sapphire substrates reveals that the alumina substrate supports appreciable SWCNT growth, while the sapphire substrate primarily yields growth of MWCNT under the same conditions (i.e., catalyst thickness, temperature, and pressure). This suggests that an amorphous layer promotes SWCNT nucleation and growth, likely due to variations in the particle size formed on the substrate. As previously discussed, the use of atomic hydrogen and the high number of defects on amorphous alumina lead to the formation of nanoparticles between 1.0 and 3.0 nm in diameter distribution. The monocrystalline substrate, which mainly yielded MWCNT, likely has a broader particle size distribution, with some particles <3 nm in size. The exception to this trend is observed for the R-plane sapphire substrate, in which case after 30 s of pretreatment, a mix of MWCNT and SWCNT has been obtained.

One of the key factors influencing SWCNT growth is the roughness and porosity of the underlying substrate. In the case of quartz substrate and a thin buffer layer of MBE deposited amorphous $Al_{2}O_{3}$ produces smaller diameter nanotubes compared to $SiO_{2}/Si$ and sapphire substrates. Most nanotube growth occurs at the steps and edges present on the rough surface. The higher amount of edges and ridges on the substrates can be viewed as triggering the deposited Fe thin film to conform to these high surface energy sites during the deposition process and form local layer thickness variations that in turn can lead, upon pretreatment, to a less dispersed distribution of particle sizes compared to the others substrate, which contained a variety of small and large diameter nanotubes. We must bear in mind that such edge sites will also act as local 'sinks' for further diffusion of the Fe during the CVD growth thus preventing the formation of larger particles. The buffer layer of amorphous alumina is a structurally very defective layer with many nanoscale and subnanometer scale pores, pinholes, dangling bonds, and defects. Such locally heterogeneous substrate surfaces favor high dispersion of the Fe

catalyst into and along the surface of the substrate, allowing for a very fine dispersion and higher surface area of the catalyst.

### 4.3. Amorphous alumina layer vs silica layer

The nucleation density and the overall CNT yield are significantly lower on $SiO_{2}$-supported layers than on $Al_{2}O_{3}$-supported layers during growth at 900 °C. As reported in previous studies, the CNT yield for Fe on $SiO_{2}$ is much lower than for Fe on $Al_{2}O_{3}$ [46]. The absence of SWCNT on the $SiO_{2}/Si$ substrate can be attributed to the formation of larger catalyst particles during prolonged pretreatment. Mattevi et al. characterized the weak interface interactions between Fe and $SiO_{2}$, which prevent the formation of appropriately sized catalyst particles for CNT growth [47]. Consequently, Fe nanoislands coarsen through cluster migration or Ostwald ripening, with coalescence driven by the minimization of surface free energy and the interface free energy between the support and metal.

## 5. Conclusion

The type of CNT formed is closely correlated to the activity of the Fe catalyst, which is strongly influenced by substrate properties. Key factors such as crystallinity, porosity, and, most importantly, substrate roughness play a significant role in determining whether SWCNT or MWCNT will nucleate and grow. SWCNT were successfully grown on various plain substrates, as well as on substrates with a 5 nm thin amorphous alumina buffer layer, using an HF-CCVD process with different pretreatment times. Growth comparisons across these substrates and the pretreatment conditions revealed that the use of a very thin alumina buffer layer induces a higher growth yield of SWCNT than the chemically and structurally corresponding substrates with no buffer layer. Prolonged atomic hydrogen exposure can lead to catalyst particle coarsening through Ostwald ripening, resulting in the formation of larger clusters and promoting the growth of MWCNT instead of SWCNT. However, we demonstrated that is also possible to grow SWCNT directly on plain substrates without the use of a buffer layer. Further investigation of different substrate surfaces is needed to gain a better understanding of the substrate-catalyst interactions that govern nanotube growth.

### CRediT authorship contribution statement

**Haifa Taoum:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Mariam Ezzedine:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Funding acquisition, Formal analysis,

<figure><img src="image_21.png" /><figcaption>**Fig. 13.** TEM observations of SWCNT synthesized on $SiO_{2}/Si$ substrates with a 5 nm $Al_{2}O_{3}$ layer after (a) 30 s and (b) 120 s of pretreatment.</figcaption></figure>
Conceptualization, Costel-Sorin Cojocaru: Writing - review & editing, Validation, Supervision, Resources, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge financial support from: The French state managed by the National Research Agency under the grant ANR-22-CE42-0030 (SPACESENSE) and ANR-22-CE09-0005 (SOLITUBE). The work was partially supported by Agence de l'Innovation de Défense-AID - via Centre Interdisciplinaire d'Etudes pour la Défense et la Securité - CIEDS - (project 2023 -HiPALIs). We would like to acknowledge the Centre Interdisciplinaire de Microscopie Electronique de l'X (CIMEX). This work is part of the NanoMaDe-3E Initiative.

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