# Laser Irradiation-Hindered Growth of Small-Diameter Single-Walled Carbon Nanotubes by Chemical Vapor Deposition

Authors: Yunlei Fu; Xiuyun Zhang; Chunfeng Lao; Danhong Shang; Maoshuai He
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

SWNTs are synthesized on a Co/MgO catalyst using “laser-disturbed” CVD with CO as the carbon source. Compared with SWNTs grown by thermal CVD without laser irradiation (normal CVD), SWNTs synthesized under laser irradiation demonstrate the suppression of small-diameter SWNT growth, as indicated by Raman spectroscopy. Such a phenomenon is also observed for other supported catalysts, such as Co/SiO2 and Fe/MgO. Controlled experiments were carried out to clarify the effects of lasers. On the one hand, laser irradiation increases the reaction temperature locally, favoring the growth of SWNTs at a set temperature as low as 350°C. On the other hand, laser irradiation inhibits the nucleation of small SWNT caps, leading to the growth of large-diameter SWNT species. This work opens a new avenue for growing SWNTs with controlled diameters.


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## Body

## 1. Introduction

As one of the new carbon nanomaterials, carbon nanotubes have been the focus of intensive research because of their fascinating structures and properties [1]. Particularly, single-walled carbon nanotubes (SWNTs) possess extraordinary electrical and optical properties, rendering them exciting candidates for nanoelectronics and optoelectronics [2, 3]. To realize the proposed applications of SWNTs, it is necessary to synthesize SWNTs with controlled orientation, density, diameter, and even chirality. In the past two decades, chemical vapor deposition (CVD) arises as the most promising technique for synthesizing SWNTs with controlled configurations or in large quantities. In the CVD process, especially in support CVD, the presence of a catalyst-support interaction could regulate the morphology and size of a catalyst, which ultimately determine the structure and diameter of nucleated SWNTs [4–7]. Some recent reviews have been dedicated to addressing the importance of a catalyst in governing both nucleation thermodynamics and growth kinetics [7, 8]. Indeed, breakthroughs have been made in the chirality-specific growth of SWNTs using a well-designed solid catalyst [9, 10]. For example, the predominant growth of (12, 6) SWNTs was achieved on a MoC catalyst formed by carbonization of Mo nanoparticles preembedded in a sapphire substrate [10]. Recently, Wu et al. [11] proposed to grow SWNTs with a selectivity of the (12, 6) species on an iron silicate catalyst.Besides a stable CVD process where SWNTs are grown under a constant gas environment (called as normal CVD), “disturbed” CVD during which the growth environment is changing has demonstrated many interesting growth results [12–17]. The “disturbance” a can be temperature variation [12], a carbon source change [13], the introduction of laser irradiation [14–16], or an acoustic wave [17]. For example, Yao et al. [12] reported the temperature-mediated growth of SWNT intramolecular junctions by changing the growth temperature. In the CVD process, isolated catalyst particles are floating in the gas phase. The carbon concentration inside a catalyst increases with the increase of the reaction temperature, which could be related to the decreased SWNT diameter. Similarly, intramolecular junctions of SWNTs were recently synthesized by modulating the growth mode of SWNTs [13]. The SWNT growth mode is governed by the carbon concentration inside an Fe catalyst particle, which is sensitive to the supplied carbon source. By alternating the carbon source from CH4 to CO, the SWNT growth mode varies from a tangential mode to a perpendicular one, responsible for the formation SWNT junctions with striking diameter differences between the thick and thin segments. In addition to the generation of SWNT junctions, chirality modification was reported by Zhao et al. [18] who named it the “tandem template CVD” method allowing periodic change of the reaction temperature. Such a periodic change varies SWNT chirality multiple times to build an energetically favorable SWNT-catalyst interface. Consequently, SWNTs with lower interfacial energies and small chiral angles are enriched.In addition to changing the growth parameters during CVD, it is also applicable to introduce an “external” source to disturb SWNT growth. Zhu et al. [17] recently developed an acoustic-assisted growth. In this approach, acoustic waves are introduced during a SWNT elongation process. The introduced acoustic waves interfere with the gas flow, leading to an easy entanglement of ultralong SWNTs. The entangled decimeter-long SWNTs demonstrate chiral consistency and structural perfection, which are promising in fabricating SWNT-based transistors with a high on-state current. Besides acoustic waves, laser irradiation is also compatible with the CVD setup and can modify carbon nanotube growth results. Pioneer work by Rohmund et al. [15] reported the growth of multiwalled carbon nanotubes by laser-assisted CVD. The key role of the laser is to locally heat and activate the catalyst for nucleating carbon nanotubes. Since then, several groups have extended the application of laser irradiation in the position-specific growth of carbon nanotubes [14, 16]. The heating effect has been considered as one contribution of the laser irradiation. However, some ex situ work suggests that laser irradiation could preferentially destroy the metallic SWNTs or SWNTs which are resonant with the laser wavelength in a different atmosphere [19]. So far, whether the laser irradiation selectively removes SWNTs with specific structures or not during CVD remains controversial.To fill this gap, in the work reported here, we performed the growth of SWNTs in the presence of a He-Ne laser (632.8 nm) irradiation on a Co/MgO catalyst and characterized SWNTs using Raman spectroscopy. Compared with SWNTs synthesized without laser irradiation, SWNTs synthesized under laser irradiation demonstrate relatively large diameters. Such a phenomenon was also observed on other supported catalysts, including the Co/SiO2 and Fe/MgO catalysts. Besides the heating effect, the laser irradiation is supposed to hinder the nucleation of SWNTs with relatively small diameters. Further experiment suggests that the laser irradiation would not destroy as-formed SWNTs but inhibit the nucleation of small-diameter SWNT species.


## 2. Experimental

The Co/MgO catalyst was prepared by atomic layer deposition (ALD), as has been described elsewhere [20]. MgO was prepared by the thermal decomposition of magnesium carbonate hydroxide hydrate and has BET areas of 50 m2/g [21]. A cobalt (III) acetylacetonate precursor was evaporated at 190°C and was passed through the porous MgO at a temperature of 400°C. After a 6 h deposition process, the system was purged with N2 and finally annealed in an air flow to remove residual organic ligands. The Co/SiO2 catalyst [22] and Fe/MgO [23] were also prepared by ALD using a similar procedure.The growth of carbon nanotubes was carried out in a CCR1000 microreactor (Linkam Scientific Instruments:http://www.linkam.co.uk/ccr1000-features) [24, 25]. The powder catalyst was loaded into a resistive heating crucible. The reactor was heated to the desired temperature in a flow of 50 sccm Ar (99.9%). After being stabilized, CO (99.9%) with a flow rate of 50 sccm was introduced to initiate the synthesis of carbon nanotubes. The growth run lasted 15 min. During the laser-disturbed CVD growth process, laser excitation is turned on with an intensity of 3.0 mW/μm2 (laser power: 6 mW; spot size: 2 μm). The samples were finally cooled in Ar flow and characterized by Raman spectroscopy (JY LabRam HR 300) with two excitation wavelengths: 633 nm and 514 nm. The excitation laser power was kept at ~10 μW for all the Raman measurements unless otherwise specified.To evaluate the heating effect of the laser irradiation, CVD-grown SWNTs were first loaded in the CCR1000 reactor. The G mode of the SWNTs was captured by Raman spectroscopy, and the temperature-dependent frequency of G mode (dω/dT) was thus obtained. Scanning electron microscopy (SEM, JEOL JSM-7500F) was applied to characterize the morphologies of the catalyst and SWNTs.


## 3. Results and Discussion

Figures1(a) and 1(b) respectively show the SEM image of the Co/MgO and as-produced SWNTs by normal CVD. The porous structure of the catalyst and the production of carbon nanotubes are clearly seen. Figures 1(c) and 1(d) present the Raman spectra of SWNTs grown on the Co/MgO catalyst using CO as the carbon precursor at 850°C. The excitation wavelengths for acquiring Raman spectra are 632.8 nm (Figure 1(c)) and 514 nm (Figure 1(d)), respectively. In all the spectra, the presence of radial breathing modes (RBMs) indicates the formation of SWNTs, as reported previously [20]. For the SWNTs grown without laser irradiation, the G/D intensity ratios acquired with the 633 nm and 514 nm laser excitation wavelengths are 39 and 28, respectively, indicating that the quality of the SWNTs is relatively high. However, striking differences are seen from the Raman spectra acquired at different spots of the catalyst surface. In Figure 1(c), Raman spectrum taken from areas without laser irradiation demonstrates RBMs with two intense peaks centered at 190 cm-1 and 280 cm-1, respectively. The appearance of these RBMs indicates that the product contains SWNTs with diameters in the range of 0.8-1.3 nm. For SWNTs grown under laser irradiation, RBM peaks corresponding with small-diameter SWNTs diminish and only low-frequency RBMs are preserved. The disappearance of small-diameter SWNTs is also verified by the Raman spectra obtained with the 514 nm laser excitation wavelength (Figure 1(d)). In the product synthesized in the absence of laser irradiation, a strong RBM centered at about 263 cm-1 correlating with the metallic SWNT species is observed [26]. The resonant metallic species is also reflected by the wide and soft Breit-Wigner-Fano (BWF) mode in the G mode. Under laser irradiation, the intensities of both the 263 cm-1 RBM and the BWF line shape decrease dramatically, suggesting the lack of small-diameter SWNTs in the materials. In the transition area, the intensity and frequency of RBMs and G mode in the Raman spectra are in between.Figure 1
(a) An SEM image of the as-prepared Co/MgO catalyst and (b) the catalyst after CVD growth. Raman spectra of SWNTs acquired with different excitation wavelengths of (c) 633 nm and (d) 514 nm from different positions on the Co/MgO catalyst after CVD growth at 850°C. Top: laser center; middle: transition zone; and bottom: without laser irradiation.
(a)
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(d)It is noted that Raman scattering is a resonance process, and only SWNTs which are resonant with the excitation lasers can be detected by Raman spectroscopy. Although the assignment of SWNT chiralities is possible by the Kataura plot [26], the overall chirality distribution of SWNTs should be cross-checked by other techniques, like UV-vis-NIR absorption and photoluminescence. Indeed, previous studies have revealed that near armchair species, like (6, 5) and (7, 5) SWNTs, were preferentially synthesized on the Co/MgO catalyst by normal CVD [20]. Unfortunately, the small amount of SWNTs synthesized under laser irradiation (a spot size of 2 μm) is insufficient for further characterizations. Herein, we mainly used Raman spectroscopy to characterize the synthesized SWNTs in this work. 633 nm and 514 nm lasers, which have been widely used to evaluate the diameter distribution and metallic/semiconducting selectivity of produced SWNTs [27, 28], were adopted to excite the synthesized SWNTs.In order to confirm the abovementioned phenomena, laser-assisted CVD growth was also performed with other catalysts, including Co/SiO2 and Fe/MgO (Figure 2). Compared with SWNTs grown without laser irradiation, SWNTs synthesized under laser irradiation demonstrate the diminishing of small-diameter species, especially for those with diameters smaller than 1.0 nm. The roles of the laser irradiation should be considered when explaining the results. Heating effect [14, 16] was widely applied to promote the growth of SWNTs at low temperatures. By locally heating the catalyst, site-specific growth of multiwalled carbon nanotubes and SWNTs were previously demonstrated [14, 16]. In addition, when characterizing carbon nanotubes by Raman spectroscopy, the laser heating not only helped eliminate the possible impurities adsorbed on the carbon nanotube surface but also would shift the position of characteristic peaks due to the heating effect [29].Figure 2
Raman spectra of SWNTs grown from different catalysts: (a) Co/SiO2 and (b) Fe/MgO with or without laser irradiation. The excitation laser wavelength is 633 nm.
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(b)With the aim of studying the heating effect brought by the laser irradiation, the G mode frequency was plotted as a function of the Raman characterization temperature using minimal laser power. Figure3(a) presents the G mode of SWNTs acquired at different temperatures. By linearly fitting the temperature-dependent G mode frequency (Figure 3(b)), the slope of the line (dω/dT) was determined to be -0.02441 cm-1/K. The laser irradiation causes a G mode frequency downshift of 7.6 cm-1 (Figure 3(c)) corresponding to a temperature increase of 311 K.Figure 3
(a) Raman spectra of SWNTs grown on the Co/MgO catalyst acquired at different temperatures. (b) The G mode frequency plots against temperature. (c) The G mode of SWNTs under 3 mW/μm2 demonstrating an ~7.6 cm-1 redshift compared with Raman spectra acquired at room temperature.
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(c)Combined with laser irradiation, the presence of other active molecules would react with certain SWNT species, resulting in the selective removal of SWNTs. For SWNTs dispersed in H2O2, their oxidative-degradation rate is greatly enhanced with laser irradiation [30]. At a suitable irradiation period, the RBMs of SWNTs which have gap energies matching the laser wavelength selectively disappeared, suggesting that the SWNTs resonant with the excitation laser wavelength are selectively removed. Huang et al. [19] investigated the stabilities of SWNTs with different conductivities under laser irradiation in air. The results revealed that metallic SWNTs in a thin film tube were destroyed in preference to their semiconducting counterparts. The different photolysis-assisted oxidation rates account for the preferential destruction of metallic SWNTs.In our CVD system, CO is introduced as a carbon source. During the growth process, the disproportionation of CO leads to the formation of CO2, which could act as an etching agent and selectively etch small-diameter SWNTs [31]. Under normal growth conditions, the concentration of CO2 is not high enough to etch SWNTs. As shown in Figure 1, small-diameter SWNTs with a large portion can be synthesized on the Co/MgO catalyst even at a growth temperature of 850°C. This result is consistent with our previous work that SWNTs grown on the catalyst exhibit a relatively narrow chirality distribution at high temperature [20]. At such a growth temperature, cobalt oxide would be reduced and form active Co nanoparticles, catalyzing the growth of SWNTs by a perpendicular mode [20]. As a result, small-diameter SWNTs are generated.Under laser irradiation, the local temperature of the catalyst is well above 850°C. The effect of temperature has been well studied in many previous researches [5, 32]. Generally, with the increase of temperature, catalyst particles in a bulk catalyst might favor the growth of SWNTs with relatively large diameters [22, 32]. So far, two reasons are raised to explain such a result. One is the coalescence of catalyst nanoparticles at high temperatures [33], causing the nucleation and growth of SWNTs with large diameters. The other is the recently proposed tube configurational entropy [32], which is related to temperature and defines the chirality distribution of SWNTs. Obviously, the laser irradiation-induced temperature increase partly accounts for the elimination of the small-diameter species in the product. Indeed, in the Raman spectra of SWNTs grown at higher temperatures (Figure 4(a)), the intensities of RBMs with higher frequencies are relatively weak, resembling the Raman spectra of SWNTs grown under laser irradiation at 850°C (Figure 1(a)).Figure 4
(a) Raman spectra of SWNTs grown on Co/MgO at 900°C and 1000°C. (b) Raman spectrum of SWNTs grown on Co/MgO at 650°C and under laser irradiation at 350°C. (c) Raman spectrum of SWNTs grown under laser irradiation after 2 min growth at 850°C (top); Raman spectrum of SWNTs grown under normal CVD (bottom).
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(c)The laser heating effect also facilitates the growth of SWNTs at low temperature. The lowest temperature for possible SWNT growth on the Co/MgO catalyst is 550°C [34]. However, with the assistance of laser irradiation, SWNTs were successfully grown at a set temperature of 350°C (Figure 4(b)), as evidenced by the presence of RBMs. Further decreasing the reaction temperature would not produce carbon nanotubes. Taking the laser heating effect shown in Figure 2 into account, the local reaction temperature under laser irradiation is estimated to be ~650°C. From the Raman spectrum shown in Figure 4(b), it can be seen that RBMs with higher frequencies are also weak. Such a character of the Raman spectrum is different from those of SWNTs grown by normal CVD at 650°C. In another word, the low-temperature SWNT growth result under laser irradiation is different from those grown at higher temperatures and the presence of laser irradiation is unfavorable for small-diameter synthesis.With the aim of clarifying whether laser irradiation inhibits the nucleation of small-diameter SWNTs or destroys growing small-diameter SWNTs, a controlled experiment is performed. After increasing the reaction temperature to 850°C, CO was introduced to initiate the growth of SWNTs, and the growth lasted 2 min. Laser was then turned on and the growth was processed for another 13 min. The Raman spectrum on the product is compared with that captured from SWNTs grown by normal CVD (Figure4(c)). The presence of high-frequency RBMs indicates that after the nucleation of small-diameter SWNTs, laser irradiation cannot destroy them. In addition, the heating effect of laser could favor the formation of large-diameter SWNTs, as indicated by the slightly high intensity of low-frequency RBMs (centered at ~190 cm-1). Laser irradiation during CVD prevents the synthesis of small-diameter SWNTs at the nucleation stage, probably with the help of CO2 presence, like in the presence of other weak oxidation species, such as O2 [19] and H2O2 [30]. A schematic illustration of laser-assisted CVD growth of SWNTs is presented in Figure 5. In the reduced Co/MgO catalyst, active particles of different sizes are produced, resulting in the growth of SWNTs with diameters in the range of 0.8-1.3 nm. In contrast, for an active Co particle under laser irradiation, only large nucleated SWNT caps survive, leading to the growth of SWNTs with relatively large diameters. The small SWNT cap is unstable because of its highly curved carbon surfaces and low stability. Consequently, only SWNTs with large diameters and low RBM frequencies are observed. Further work on controlling SWNT diameters could be realized by regulating the SWNT nucleation stage.Figure 5
Schematic illustration of SWNT nucleation and growth with or without laser irradiation.


## 4. Conclusion

To conclude, we have shown the “laser-disturbed” CVD growth of SWNTs on Co/MgO, Co/SiO2, and Fe/MgO catalysts. Based on Raman characterizations, SWNTs grown under laser irradiation demonstrate the inhibited synthesis of small-diameter species. The roles of laser irradiation can be divided into two parts: one is the heating effect, which facilitates the growth of SWNTs even at a temperature of 350°C, and the other is the inhibition of the nucleation of small-diameter SWNT caps, which have relatively low thermal stabilities. This work not only extends the understanding on the effect of laser irradiation during CVD process, but also sheds more light on the growth of SWNTs with controlled diameters.



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