Since their discovery in the early 21st century, quantum 2D materials have shown vast potential for a variety of applications, being amongst the best performing nanomaterials currently the focus of research in materials science.
Study: Photoexfoliation Synthesis of 2D Materials. Image Credit: Gorodenkoff/Shutterstock.com
Current preparation methods, however, have exhibited drawbacks, which have led to the investigation of novel, innovative techniques. One of these is photoexfoliation, which is the focus of a new research paper in ACS Materials Letters.
2D Quantum Materials
The discovery of graphene revolutionized the materials science industry. With properties such as superior thermal conductivity, transparency to light, and the highest electronic mobility of any material, graphene has been explored for a slew of applications in multiple industries.
Following the discovery of graphene, other novel Xenes have been developed in laboratory studies worldwide. Materials that have been discovered include phosphorene, borophene, germanene, 2D oxides, MXenes, stanene, and transition-metal dichalcogenides. The electronics industry has made widespread use of these materials to produce electronic chips with superior performance that surpass previous generations of devices.
2D Xenes have proven to be advantageous for applications such as energy storage, renewable energy generation, molecular sensing, and laser shielding. Each material has its own unique characteristics, novel features, and properties. Due to this, various synthesis methods have been developed for each material to explore its various applications.
Synthesis Methods and Their Challenges
As stated, numerous synthesis methods have been developed for these novel 2D materials. Mechanical cleavage of graphene transfers sheets onto a substrate directly, but challenges exist in achieving a uniform deposition layer. Chemical vapor deposition, molecular beam epitaxy, and atomic layer deposition are expensive. Liquid-phase sonochemical synthesis is time-consuming. Some methods leave behind defects that are hard to remove, and others still are poorly reproducible.
Due to the limitations of various conventional 2D quantum material synthesis methods, the need for a method that can deliver bulk 2D crystals free of remnant surface functionalities and defects is of urgent concern. Additionally, the need for reproducible, scalable, and “green” methods is a central focus of current research into viable alternative methods to ones currently used.
Photoexfoliation: An Alternative Synthesis Approach
Using photons to exfoliate van der Waals crystals to synthesize 2D Xenes has garnered research interest in recent years. Lasers are the main source of photons for this purpose. The method has vast potential to provide breakthroughs in 2D materials science, promising to produce defect-free atomic layers of materials. Sample purity can be ensured by using just the parent material and solvents, and therefore no precursors or substrate materials are necessary.
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This method relies on suitable levels of exfoliation energy. The use of pulsed lasers to exfoliate graphene, boron nitride, and molybdenum disulfide has been reported in studies. However, literature has been lacking when it comes to factors such as the correlation amongst laser parameters, quality of synthesized 2D crystals, monolayer abundance, and lateral size distribution. This has facilitated the need for a more intimate understanding of the process’s finer details.
The Study
The current research paper published in ACS Materials Letters has sought to provide pertinent information on the photoexfoliation process to fill the current knowledge gap.
The authors have reported photoexfoliation of atomic-scale layers of graphene, molybdenum disulfide, and boron nitrate using KrF laser irradiation. Aqueous dispersions of the parent material powders in the solution were irradiated. It was reported that the number of atomic layers and the lateral sheet sizes decreased gradually when the duration of laser irradiation was increased. Laser fluence was reported to be the critical control parameter for atomic layer number and lateral sheet size.
At 1.5 J/cm2 the average lateral sheet size was ~400 nm. At 4 J/cm2, the sheet size was reduced to 20-30 nm. An accompanying surge of the ratio of sheets with fewer atomic layers was also observed in the study’s experiments. The authors correlated the laser processing parameters with the sample size. Molecular-molecular interactions were analyzed.
A key observation of the study was that stretching of the interlayer distance lowered the exfoliation process’s activation energy. By reducing the activation barrier, 2D crystals could be synthesized in solution.
The authors concluded, based on their numerous observations, that photoexfoliation of 2D materials, a sustainable, reproducible method, shows huge promise for the development of next-generation electronic devices for a variety of commercial and research applications in diverse fields such as renewable energy harvesting, storage, and sensors. The paper provides a significant contribution to the field of 2D material synthesis.
Further Reading
Kumar, P et al. (2022) Photoexfoliation Synthesis of 2D Materials [online] ACS Materials Lett. | pubs.acs.org. Available at: https://pubs.acs.org/doi/10.1021/acsmaterialslett.1c00651
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