Applied Science and Convergence Technology 2019; 28(5): 148-154
Published online September 30, 2019
Copyright © The Korean Vacuum Society.
Seok-Ki Hyeonga,c, Kwang-Hun Choib,c, Seoung-Woong Parkc, Sukang Baec, Min Parkd, Seongwoo Ryub,*, Jae-Hyun Leea,*, and Seoung-Ki Leec,*
aDepartment of Energy Systems Research and Department of Materials Science and Engineering, Ajou University, Suwon 16499, Republic of Korea
bDepartment of Polymer Engineering, College of Engineering, Suwon University, Suwon 18323, Republic of Korea
cFunctional Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeollabuk-do 55324, Republic of Korea
dPhotoelectronic Hybrid Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Since the breakthrough in fabricating graphene by mechanical exfoliation in 2004, numerous methods have been developed to synthesize high-quality graphene materials on a large scale, including chemical exfoliation, thermolysis, and chemical vapor deposition. Recently, laser thermal treatments have emerged as facile methods for the direct synthesis of functionalized graphene materials, which show potential for use in a wide range of applications. The graphene materials produced by laser-based syntheses are classified by the fabrication method as either laser-reduced graphene or laser-induced graphene (LIG). The former is obtained through the chemical reduction of graphene oxide, while the latter utilizes the carbonization of a polymer precursor. In this review, we summarize the mechanisms of laser-assisted graphene syntheses, the structural and chemical functionalization of laser-scribed graphene, and various practical demonstrations of graphene-based materials in the field of mechanical and electrochemical sensors.
Keywords: Laser ablation, Graphene, Functionalization, Electromechanical sensor, Electrochemical sensor
With the advent of the Internet of Things (IoT), the importance of sensors that can communicate data by digitizing analog signals in real time has been increasing. Sensor technology is widely used in diverse areas. Two key applications of next-generation sensors are in healthcare, in which the signals produced by the human body are monitored ; and in safety inspections, whereby hazardous materials and environmental contaminants are detected, including nitrogen oxide (NO
The fabrication of graphene materials has been achieved through mechanical/chemical exfoliation , chemical vapor deposition , and epitaxial growth . However, these methods require precise synthesis conditions, high temperatures, and complex post-treatments such as transferring or patterning, making them cost-intensive and time-consuming. Recently, a facile method for the synthesis of selectively patterned graphene has been reported by laser scribing [14,15], which largely resolves the above-mentioned drawbacks. Laser-based syntheses allow the rapid realization of graphene within minutes, with good control over the structure and functional groups. As such, laser-based syntheses have been actively studied in various areas. This review paper summarizes recent research trends regarding the synthesis of graphene using laser ablation methods, and the application of the resultant devices as sensors. Depending on the synthesis method and mechanism, such graphene is classified as either laser-reduced graphene (LRG) or laser-induced graphene (LIG). Herein, we first discuss the synthesis of LRG from the reduction of graphene oxide (GO), and its functionalization. Subsequently, we summarize the synthesis and functionalization of LIG from the direct carbonization of a polymer substrate. Finally, we explore the application of these materials as different types of mechanical and electrochemical sensors.
LRG is synthesized
The mechanism of LRG synthesis is thought to involve both photochemical and photothermal processes, depending on the wavelength of the irradiating laser. The reduction of GO to rGO reportedly occurs
The electrical and mechanical properties of LRG can be improved by doping with metal NPs, metal oxide, and chemical functional groups. For example, Strong
Another method of improving the performance of LRGs involves chemical functionalization. The fluorination of graphene can be achieved by irradiating a fluoropolymer-coated graphene substrate with a laser [Fig. 3(c)]. Fluorine radicals are locally generated upon laser scribing, which induces C–F bonding on the graphene sheets. The kinetics of C–F bond formation are affected by the laser power and fluoropolymer thickness . Notably, selective synthesis using this method facilitates the structural functionalization of graphene. Cheng
While LRG technology facilitates the selective synthesis of graphene, the preparation is lengthy on account of the formation of the GO film that is used as source material. In contrast, LIG is synthesized directly from a polymer precursor using functionalization, reducing the overall processing time. Figure 4(a) shows a representative process for the synthesis of LIG. Carbonization of the source polymer occurs owing to local heating upon laser irradiation, allowing graphene to be formed selectively. During the carbonization process, the oxygen and nitrogen groups in the substrate are decomposed; simultaneously, the carbon atoms in sp3 bonds reform as sp2 bonds . Typical polymers that can be used as raw materials include polyimide (PI), phenolic resin, polyethyleneimine, and lignocellulose [15,33–35].
Notably, when LIG is synthesized on a PI film, a 3D porous structure can be easily formed by using the gas produced during carbonization. The resultant LIG material has an increased specific surface area in comparison to graphene with a planar structure, which improves the capacitance and performance. SEM images of LIG [Fig. 4(b)] reveal a well-defined pattern of porous graphene with clear edges. Similarly to LRG, the characteristics of LIG can be modulated by changing the laser power. Figure 4(c) shows the Raman spectra depending on the laser power. The crystallite size, which can be calculated from the intensity ratio of the
Multifunctional graphene can be fabricated by doping LIG materials with metal or metal oxide NPs, as well as by controlling the macro- and/or microstructure of the material. Figure 5(a) shows the fabrication process of metal NP-embedded LIG. By mixing a poly (acrylic acid) solution with a metal precursor, a metal NP/LIG composite can be directly created through a laser-based photothermal process . Transmission electron microscopy (TEM) images confirm that the LIG contains a catalytic metal of spherical Fe3O4 or Co3O4 with a diameter of 50 nm; nevertheless, the crystalline structure is maintained [Fig. 5(b)]. It has been reported that doping with metal NPs can improve the performance of LIG-based oxygen reduction reaction electrodes through electrocatalysis .
The functionalization of LIG has been shown to facilitate biorecognition. Fenzl
LIG is suitable for both sound-emitting and sound-detecting applications owing to its thermoacoustic properties, high thermal conductivity, and low thermal capacity . Figure 6(d) illustrates the soundwaves emitted from a LIG device, which are generated when an alternating current (AC) is applied to the sensor, because periodic Joule heating in the LIG device induces air expansion. Interestingly, the vibration of vocal cords can be monitored by measuring the change in the resistance of the device. Accordingly, sound emission and detection can be performed simultaneously, facilitating applications in artificial throat and sound sensors [Fig 6(e)]. Figure 6(f) shows how the resistance of an artificial LIG throat device changes with different sound types (coughing, humming, screaming, swallowing, and nodding). When the same type of sound was repeated, very similar resistance changes were observed. In the cases of swallowing and nodding, the resistance changes induced by the movements of muscles around the neck could be clearly seen .
LIG synthesized from PI has a 3D porous structure and a large surface area compared with conventional LIG. The expanded surface area leads to an extreme increase in the number of adsorption sites for gas molecules. Therefore, it is considered a suitable material for improving the sensitivity for gas sensing. Figures 7(a) and 7(b) show how the resistance of a LIG-based gas sensor changes based on the interaction with a target gas (NH3 and NO2, respectively) . LIG has p-type characteristics in air. When exposed to electron-donating and electron-accepting gases (NH3 and NO2, respectively), the resistance across the graphene sample increased and decreased, respectively. Nevertheless, in both cases, the sensitivity decreased as the concentration of the gas decreased. When a LIG gas sensor was subjected to bending tests for more than 1000 cycles with a curvature radius of 7 mm, the variation in resistance was uniform and repeatable [Fig. 7(c)].
Therefore, LIG gas sensors simultaneously reflect the excellent electrical properties and mechanical flexibility of graphene, implying that they can be used for bendable and stretchable gas sensor applications. The porous characteristics of LIG facilitate the formation of functional composites with other materials. Figure 7(d) shows the fabrication process and an optical image of a LIG/cement composite sensor. First, cement slurry is intercalated into the pore structure and anchored with LIG. Subsequently, the PI substrate is removed by thermal treatment, forming a carbon electronic structure embedded in the concrete. The sensor embedded in the composite system is free from corrosion owing to the surrounding environment .
Combining LIG gas sensors with a solid-state ionic liquid and metal particles can improve the sensitivity and detection limit. Figure 7(e) illustrates a gas sensor fabricated by depositing a mixture of porous polyvinylidene difluoride (PVDF), Pd NPs, and a room-temperature ionic liquid (RTIL) upon a LIG electrode. The Pd NPs attach to the LIG, increasing its sensitivity to the target gas. In addition, the porous PVDF/RTIL layer increases the porosity of the sensor structure, which creates a larger contact area and flow path, leading to a faster response time. The formation of such composites is of great importance for the development of improved gas sensors .
Humidity sensors are widely applied in indoor humidity measurements, weather observations, and industrial applications [49–51]. Cai
In this review, a quick, inexpensive, and simple synthesis method for porous graphene with a 3D structure is presented, along with the sensor applications of such materials. The facile laser-based thermal annealing approach allows the large-area synthesis of graphene devices with diverse microscale architectures within a few minutes. Notably, this method overcomes the drawbacks of conventional graphene-based sensor fabrication, which requires high temperatures, vacuum conditions, and multiple process steps. Laser scribing can be used to readily fabricate electromechanical and electrochemical sensors, including piezoresistive-based strain sensors, pressure sensors, artificial throat and sound sensors, hazardous gas sensors, and humidity sensors. The function of graphene can be optimized by controlling the laser wavelength, working distance, and structure of the source materials. This attractive laser-based method of synthesizing functionalized graphene is emerging as a key method owing to its advantages such as diverse material property control, high processing speed, and low production cost; it is expected to be adopted in the industry beyond the laboratory scale in the near future.
This research was supported by the Korea Institute of Science and Technology (KIST) Institutional Program and Jeong-in Engineering Program.