Research on carbon nanotubes (CNTs) and graphene is very mature, and has led to substantive breakthroughs in electronics; however, some problems remain. For example, the experimental process for obtaining high-purity, semiconducting CNTs is complex, and the ability to adjust the bandgap of graphene is restricted. However, both these materials encounter limitations when used in electronic devices, among which, Young’s modulus and stiffness are the key criteria. Theoretical research has shown that polyynes exhibit better Young’s modulus and stiffness than graphene, CNTs, and diamond. In particular, polyynes have very high tensile strength, and their performance is unaffected by bending deformation, which cannot be achieved using CNTs and graphene. Thus, the limitations of single-walled CNTs (SWCNTs) and graphene can be overcome. Moreover, polyynes demonstrate excellent electrical properties (such as π-electronic system) and good hardness (i.e., 40 times that of diamond), providing potential applications in aerospace and nanotechnology. However, polyynes suffer from instability, high preparation costs, and low concentrations. In this study, based on femtosecond laser ablation technology, polyynes were prepared using a Ti ∶sapphire femtosecond laser with the repetition rate of 1 kHz to ablate SWCNTs suspended in methanol. The optimal laser power and processing time for preparing polyynes were explored, and the mechanism of polyyne synthesis was detailed, which will play an important role in the large-scale preparation of polyynes.
Various methods have been adopted to synthesize polyynes, which can be roughly divided into electrochemical (e.g., oxidation couples and electric arcs) and laser ablation (e.g., gas and liquid phases) techniques. Oxidation coupling is complex and impurities can be easily introduced. Arc discharge is commonly used to generate carbon chains, with the arc discharge in water. However, it cannot control the groups capped at both ends, which limits the stability and electronic properties of polyynes. For laser ablation, gas-phase facilities are complex, and typically require high airtightness and temperature control. Nevertheless, owing to its comparatively low cost, variety of target materials, and ablation surroundings, liquid-phase laser ablation has been widely employed to prepare carbon nanostructures at room temperature. Nanosecond and femtosecond lasers are the most commonly used to achieve this. However, nanosecond lasers have a significant thermal effect, which is not conducive to the formation of polyynes at room temperature. In contrast, femtosecond lasers have little thermal effect and high photon energy, which is beneficial for breaking the chemical bonds of SWCNTs for polyyne preparation. Accordingly, liquid-phase laser ablation with a femtosecond laser was selected for this study.
In this work, for SERS characterization, the sample solution was mixed with a silver colloid at the volume ratio of 1∶5 and then spin-coated onto a silicon substrate for Raman spectrum measurement. The characteristic peak intensity of single-pulse laser energy increases with the increasing laser energy from 0.04 to 0.52 mJ, and decreases with the increasing laser energy from 0.52 to 1.48 mJ. When the single-pulse laser energy is 0.52 mJ, the highest intensity is obtained with the highest yield of polyynes. Because there is considerable overlap between the characteristic peaks of the short and long polyyne chains, further validation could be realized using the UV absorption spectrum. The corresponding absorption peaks of C8H2, C10H2, C12H2, C14H2, and C16H2 are approximately 225, 250, 275, 298, and 326 nm, respectively. Thus, long-chain polyynes containing more than 12 carbon atoms are generated. The characteristic peak of the C8H2 molecule was the most evident. In particular, the strongest peaks are realized for the ablation case with the pulse energy of 0.52 mJ, which is consistent with the SERS analysis. A pulse energy that is too low or too high deteriorates the generation of polyynes. Therefore, 0.52 mJ can be determined as the best single-pulse laser energy for preparing polyynes. The corresponding peak areas for sp and sp2 are further calculated. The peak area of sp carbon chain in the spectral range represents the polyyne concentration. The sp carbon peak area is the largest with the femtosecond laser ablation time of 1.5 h. This indicates that the optimal processing time is approximately 1.5 h. For the femtosecond laser with the repetition rate of 1 kHz, a continuous strong ionization region can be formed, where the laser pulse energy exceeds the saturation threshold (1.33×1015 W/cm2) and affects the connections between C2 radicals. Further increasing the laser power enhances the extent of fragmentation, but does not appreciably change the number of ionized molecules. Subsequently, the C2 radical is destroyed and cannot form sp orbital hybridization. Thus, the polyyne yield initially increases and then decreases. Moreover, with an increase in the ablation time, the bond would also be destroyed, and thus, the yield would decrease.
Laser ablation of SWCNTs suspended in methanol was performed using a Ti∶sapphire femtosecond laser. From characterization via Raman spectroscopy, high-performance liquid chromatography, and ultraviolet absorption spectroscopy, linear hydrogen-capped polyynes (CnH2: n=6, 8,10, 12, 14, and 16) were confirmed, and the main product was C8H2.
The highest yield of polyynes was realized at 1.5 h ablation with the laser power density of 1.33×1015 W/cm2. This saturation threshold may correspond to the extent of C2 radical fragmentation. The presented optimal laser ablation conditions for polyynes will facilitate the preparation of polyyne-based films and favor other practical applications.
