While exfoliation has historical roots dating back centuries, significant advances and widespread research gained momentum after Novoselov and Geim's discovery of graphene using Scotch tape in 2004.
[1] However, during exfoliation, the high energy input leads to an extreme bond-breaking process that irreversibly separates the layers into single sheets.
Arguably, the first research that delved into the mechanism of the process rather than its usage was Brodie's work, which revealed that certain acids produced lamellar carbon structures in 1855.
[6] This discovery laid the groundwork for a more solid theoretical framework, enabling scientists to apply the method in their production processes.
[7] The development of electrochemical exfoliation piqued the interest of more researchers and more people started regarding the process as a production technique.
One of the most notable examples of the success of the method as a mass production technique was the invention of the first commercial lithium carbon fluoride batteries in 1973.
This discovery demonstrated that exfoliation could occur without relying on weak bonds, which opened up new and promising applications in the industry.
Despite the high number of methods it is possible to classify them into three distinct categories based on the source of energy used in the process: mechanical, chemical, and thermal exfoliation.
Sometimes, a solvent is introduced to the material to facilitate complete breakdown, as liquid environments significantly reduce bond strength compared to vacuum conditions.
When these bubbles reach a critical size, they collapse with an instantaneous temperature of 5000 K, a local pressure of 20MPa, and a heating/cooling rate up to 109 K s−1.These sudden physical differences create shock waves that can act on lamellar materials and break the weak forces in between the layers.
[17] This innovative procedure has been adapted for household kitchen mixers, significantly reducing the costs and complexity of the exfoliation methods, thereby sparking another wave of research in layered structures.
[1] For example, in the case of van der Waals forces, which are common in chemical exfoliation, positive and negative regions are induced, attracting ions.
[9] Typically, these stronger bonds lead to the creation of functional groups that significantly reduce interlayer attractions.
At this stage, the interlayer attraction becomes low, and thanks to the ability of the functional groups to decompose with further processing, the layers can be easily separated.
In addition to its scalability, the variety of chemicals available plays an important role in encouraging researchers to explore various production methods.
This method utilizes a transition metal film as a base layer and exposes it to hydrocarbons at high temperatures(900-1000°C) and ambient pressure.
During the process, hydrocarbon decomposes, and carbon atoms form one to ten layers of graphene flakes over the metal film.
It involves introducing oxide functional groups into the lamellar structure, which doubles the distance between graphite layers and reduces van der Waals attractions.
[9] These functional groups are then removed using reductants, resulting in single graphene layers from the graphite, which can now be easily exfoliated due to reduced van der Waals attractions.
The presence of a large number of holes and defects made the produced graphene unsuitable for electronics, and the chemicals used were hazardous.
[15] In 2014, a research group succeeded in isolating graphene layers without the use of oxidants, significantly increasing the purity of the samples and eliminating the need for further processing of the products.
Therefore, many researchers aim to implement the method into the industry for the mass production of carbon nanomaterials and transition metal dichalcogenide monolayers.
For this purpose, low-temperature thermal exfoliation employs relatively lower temperatures of 200°C-550°C to decompose the functional groups.
Additionally, low-temperature thermal exfoliation allows for fine-tuning of the bandgap properties of materials, making it an ideal method for electronic applications.
These local changes trigger significant physical and chemical phenomena that result in complete exfoliation of the lamellar material.
Currently, graphene is projected to play a crucial role in the production of low-cost solar cells, energy storage systems, and sensors.
The challenging processing of graphene and its lack of an obvious band structure have led many researchers to explore new uses of the exfoliation methods.
This shift has recently increased research into efficient production methods for transition metal dichalcogenide (TMD) monolayers significantly.
[13] Currently, TMD monolayers find applications in electronic devices such as solar cells, photodetectors, light-emitting diodes, and phototransistors.
One notable non-van der Waals material is the Hematane which is a single sheet of hematite, the most abundant form of iron ore.[2] Hematane is known to have interesting photocatalytic properties due to its modified bandgap properties, offering potential applications in energy storage, optoelectronics, and biomedicine.