As a zero bandgap material[
Infrared and Laser Engineering, Volume. 49, Issue 9, 20200298(2020)
Laser-induced transformation of carbon nanotubes into graphene nanoribbons and their conductive properties
GNRs can be readily produced by unzipping the nanotubes because CNT structure can be analogically considered as graphene sheets rolled up. This is a special 2D graphitic structure performing the exceptional properties. Due to the unique structure and the outstanding properties, GNRs have been used in a vast range of applications, including transistors, optical and microwave communication devices, biosensors, chemical sensors, electronic memory and processing devices, nano electromechanical systems, and composites. The morphology of the fiilms was observed by scanning electron microscopy (SEM), and the properties of graphene were characterized by Raman spectroscopy. The conductivity of the fiilms was measured by a semiconductor parameter measurement system. Raman spectroscopy showed that the Raman characteristics of graphene characterized by optimized process were enhanced. Laser energy and irradiation time were two important parameters for the preparation of graphene from carbon nanotubes. In this study, to open carbon nanotubes by laser, graphene nanoribbons were produced by excimer laser irradiation of carbon nanotubes thin films. The experimental results show that, with the laser energy 150 mJ, the carbon nanotubes are not opened while the connection is observed. With the energy 450 mJ, the carbon nanotubes can be effectively destroyed, and graphene strips can be partially opened to form. At this time, the conductivity of the fiilm reaches the maximum value. Due to the thermal accumulation effect, a large number of porous structures appear on the wall of carbon nanotubes.
0 Introduction
As a zero bandgap material[
At present, there are many synthesis methods of GNRS, including graphene photolithography[
Here, a single-step method to unravel multi-walled carbon nanotubes by laser irradiation is reported. This method can produce GNRs efficiently and without pollution. The excimer laser irradiation of multi-walled carbon nanotubes (MWCNTs) was used to dissociate MWCNTs. The surface morphology of MWCNTs was investigated by changing laser energy and irradiation time. The graphene properties were characterized by Raman spectroscopy. Finally, the conductivity of MWCNTs was measured by a semiconductor parameter measurement system.
1 Experiment
The experiment scheme is shown in Fig.1. The excimer laser used in the experiment is Lambda-Physik (LPX 305iF, Germany). The laser buffer gas is Ne with working gas of KrF. The emission wavelength is 248 nm with the pulse width 30 ns and repetition rate in the range of 1-50 Hz. The single pulse energy of the laser is adjustable with a maximum energy of 800 mJ. The spot size applied is about 35.4 mm × 16.4 mm.
Figure 1.Schematic diagram of laser irradiation
The sample graphite target was prepared by the deposition of MWNTs suspension on a polished silicon wafer. Firstly, the silicon substrates were thoroughly cleaned by deionized water-isopropanol-alcohol-acetone-alcohol. The 1.5 mL of water dispersion (0.2% of dispersant concentration) of 1 mg/mL multi-walled carbon nanotubes were added to 98.5 mL isopropanol, and then 10 mg Mg(NO3)2·6H2O was added to ultrasonic vibration for 10 minutes to obtain uniform and dispersed electrophoretic solution[
Oxygen and nitrogen in the air will react with carbon in laser irradiation, causing adverse effects[
2 Results and discussions
The macro-morphology of carbon nanotube films was measured by the optical profiler. As shown in Fig.2(a), the film thickness is uniform, and the shape is regular. The SEM of MWNTs without laser irradiation shows that the wall of MWNTs is smooth and uniform, and no obvious defects and damage are observed. Its diameter is about 20-30 nm and has a large aspect ratio.
Figure 2.Deposited multiwalled carbon nanotube films
The SEM images of multi-walled carbon nanotube films irradiated by excimer laser (Lambda-Physik LPX 305iF/248 nm/30 ns/50 pulses) are shown in Fig.3. The laser energy is adjusted in the range of 150-550 mJ to test its role in opening nanotubes. As shown in Fig.3 (a), carbon nanotubes were not destroyed when irradiated with 150 mJ. The diameter of carbon nanotubes was 20-30 nm. Figure 3(b) is an SEM image of GNRS obtained from carbon nanotube films irradiated by 450 mJ laser. The width of GNRs is about 70-90 nm. It can be seen that the excimer laser irradiation can well unravel carbon nanotubes to form graphene nanoribbons. As can be seen from Fig.3(c): The wall morphology of 150 mJ irradiated tube is similar to that of MWNTs without laser irradiation, and no dissociation occurs. (2) The Outer wall of MWNTs fused and combined with each other, and there is a connection at the overlap. Figure 3(d) shows that for undissociated MWNTs at 550 mJ, the wall of MWNTs is severely damaged due to thermal accumul, and the aspect ratio of MWNTs is severely reduced, resulting in aggregation of carbon nanotubes.
Figure 3.SEM after laser irradiation with energy of (a) 150 mJ, (b) 450 mJ, (c) 250 mJ, and (d) 550 mJ
Raman spectroscopy is one of the important means to characterize the quality and defects of carbon nanotubes and graphene[
Figure 4.Raman spectra of carbon nanotube films
Raman spectrum G band is sensitive to the displacement of Fermi level[
Figure 5.Raman spectra of GNRs produced by laser irradiation
By testing the I-V characteristic curve of the film, it can be seen that the conductivity of the film is greatly enhanced after laser irradiation, especially after 450 mJ laser energy irradiation (see Fig.6). This is because excimer laser irradiation can effectively open carbon nanotubes to form GNRS and improve the conductivity of the films. However, when the laser energy increases to 550 mJ, the conductivity of the films decreases sharply due to the damage of thermal accumulation.
Figure 6.
3 Conclusion
In summary, our study shows that the excimer laser irradiation of carbon nanotubes thin films is a simple method for preparing graphene nanoribbons without surface pollution. When MWNTs are irradiated, the defect location is easy to reach a high temperature that leads to longitudinal dissociation. With the pulse energy of 450 mJ, the film disassembled at the best result and the conductivity is improved significantly. In addition, when the laser energy is 150 mJ, no carbon nanotubes are opened, and the connection of carbon nanotubes can be observed. At 550 mJ, the thermal accumulation of the film was seriously damaged, and the conductivity of the film decreased sharply.The result shows a certain significance for the efficient preparation of 2D graphene ribbon structure materials.
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Zhi Liu, Jimin Chen, Dongfang Li, Chenyu Zhang. Laser-induced transformation of carbon nanotubes into graphene nanoribbons and their conductive properties[J]. Infrared and Laser Engineering, 2020, 49(9): 20200298
Category: Lasers & Laser optics
Received: Apr. 6, 2020
Accepted: --
Published Online: Jan. 4, 2021
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