Fabrication techniques are used in establishing nanostructures in chemical materials for a number of applications. The reason why graphene is employed in most applications lies in its great potential electronics, renewable energy, and, optoelectronics (“High-Throughput Solution Processing of Large Scale Graphene and Device Applications” 105). As much as graphene nanostructures are still under research, it is believed that a great progress has been brought about in the past few decades in a sense that it is now possible to achieve atomically well-defined structures (Yu et al. 555). Additionally, nowadays it is also possible to make graphene nanostructures over a centimeter scale and in the process, make devices based on the application of numerous substrates especially due to the development of transfer techniques and syntheses. Since the first fabrication of graphene in the year 2004, it has been acknowledged that tremendous progress can be witnessed as far as fabrication of graphene is concerned including the application of different graphene nanostructures.
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General Overview of Fabrication and Graphene
Carbon is listed among the most-studied elements in the Periodic Table. In this case, the versatility of chemical bonds found in carbon elements makes them the potential target for the formation of numerous carbon allotropes. Thus, it is acknowledged that in three dimensional carbon forms, carbon can exist as graphite and diamonds which can be made of sp2 and sp3 covalent bonds (“High-Throughput Solution Processing of Large Scale Graphene and Device Applications” 105). Additionally, in the 1990s and the preceding 1980s, another important carbon allotropes under the name of the zero-dimensional fullerene as well as one-dimensional carbon nanotubes were discovered (Yu et al. 555). Upon their discovery, it was established that they are nano materials with amazing chemical and physical properties and in the process many researches were performed on them. Nevertheless, the two-dimensional counterpart of carbon allotrope is an element that was still missing until the year 2004 when graphene or rather a single graphite layer was successfully isolated on substrate. Carbon nanotubes (CNT) are made by rolling a number of graphene sheets into a cylinder. In this case, these nano structures are established with length to length diameter ratio that can be assumed as being up to (1.32. 10^8):1 which makes it significantly more extensive than any other material. CNTs are among the promising candidates when it comes to the application fields of nanoelectronics taking into consideration the issue of interconnect applications. In other words, metallic CNTs have caused major concerns among researchers with the primary interest revolving around their applicability because of their high demand, high thermal stability, and high thermal conductivity in the field of VLSI interconnects, as well as their ability to carry extensive current.
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CNT Fabrication
Ancient process in regard to carbon nanotubes synthesis revolves around laser ablation, arc discharge, and chemical vapor decomposition (CVD). If to compare all these methods, it can be acknowledged that Chemical Vapor decomposition is the most prominent since it is widely employed due to its advantages including cost, high throughput, and controllable synthesis (Yu et al. 555). In this case, by controlling the parameters of the source of carbon, catalyst parameter, reaction temperature, and the gas pressure, it will be possible for researchers to control such areas as orientation, diameter, as well as location of the CNTS based on specific applications. CNTS first came into existence in the field of science during the fullerene arc-evaporation in 1991. Additionally, laser ablation as a technique was first employed by Guo et al to create SWCNT around the year 1995 (“High-Throughput Solution Processing of Large Scale Graphene and Device Applications” 105).Moreover, arc discharge as a method was first employed in carbon nanotubes synthesis in extensive scale by Ebbesen and Ajayan in the year 1992. It is also important to note that some of these traditional methods such as arc and laser ablation require extensive temperatures such as 1200 degrees Celsius in order to effectively fabricate CNTs since the alignment and location of CNTs are very challenging to control when using these methods. Carbon nanotubes for transistors that are of a thin film nature are in most cases synthesized by PECVD (Plasma-Enhanced Chemical Vapor or Thermal CVD). FCCVD-Floating Chemical Vapor Deposition is another alternative technique that can potentially be used in growing CNTs (Yu et al. 555)
Thermal Chemical Vapor Deposition is an ancient technique mostly preferable in the CNT growth. In other words, many individuals and groups have employed this technique in their experiments. In this process, CNTS grown on Fe or CNTs are salt catalysts at intensive temperatures of about 900 degrees Celsius or even more. In some experiments such as the one carried out by Huzcko, the temperatures were lowered from 500 to 750 degrees Celsius, and the tangible results were still realized. In this case, it is important to note that the thickness of the catalyst is the primary factor in establishing the diameter and density of the CNTs. In other words, the thinner the catalyst, the higher the density, and in the process, it makes the CNT’s diameter smaller.
Plasma enhanced Chemical Composition has been employed in synthesizing CNTS since the year 1992. Some scholars such as Li et al. realized a high ratio of CNTS as semiconductors using this technique (Peng et al. 2772). Additionally, carbon nanotubes were created on SiO2 at a temperature of approximately 600 degrees Celsius in Ar together with monodispersed ferritin particles using such catalyst as Fe thin film. In this case, the diameters of CNTs ranged from 0.8nm with a mean diameter of no less than 1.2 nm (“High-Throughput Solution Processing of Large Scale Graphene and Device Applications” 105). It is for this reason that PECVD was deemed advantageous especially due to the fact that its products are in most cases semiconducting nanotubes that boast a ration as high as 90%. In this case, it is important to note that the semiconducting ratio of CNT is also crucial when it comes to the fabrication of thin film transistors. Liu et al. made a proposal to employ FCCVD in the fabrication of CNTs. In so doing, they have demonstrated that it is possible for an individual to make high quality CNTs even when they are grown at low temperatures. It is advantageous because it is very cheap being suitable for making CNTs in mass production with different CNTs diameters. However, as much as some of the products that were employed in Liu et al.’s experiment are multiwall carbon nanotubes, it was proven further by the scientists that SWCNTs are created from carbon monoxide and ferrocene in the temperature ranging from 891-928 degrees Celsius through the employment of FCCVD (Peng et al. 2772).
Separation of Metallic semiconducting CNTs
Semiconducting and metallic nanotubes coexist in carbon nanotubes that are synthesized all the time. Apparently, this is a fatal obstacle that hinders the development and application of SWCNT-based electronics since the metallic CNTs are known to lack gate control and in the process, they degrade the ON/OFF device ratio (“High-Throughput Solution Processing of Large Scale Graphene and Device Applications” 109).In other words, separating the semiconducting and metallic nanotubes is very crucial for technology since it is one of the few issues that have attracted researchers. As a result, a number of different innovative approaches have been established to deal with these challenges which also include electrical breakdown, gel-based separation, density gradient ultracentrifugation, diaelectrophoresis, as well as the DNA sequence separation (“High-Throughput Solution Processing of Large Scale Graphene and Device Applications” 109). Apparently, each of the proposed sequences and alternative techniques has their own advantages. In this case, for instance, the utilization of electrical breakdown to curb or rather separate CNTs does not necessarily need any extra-process steps in the course of device separation. Apparently, when an individual employs the density gradient ultracentrifugation to perform the CNTS separation, it is possible that they can achieve approximately 98% purity in regard to semiconducting carbon nanotubes (Yu et al. 555).
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Potential application of Graphene fabrication
Some of the industrial applications of graphene include the woven textile, light weight, durable screen technology, solar cells, and various medical applications. It is suggested that the exfoliation produced graphene is among the most expensive materials around the globe since a sample that is close to a human hair costs around $1000 (Das and Smita, 319).As a result, exfoliation procedures have been expanded and since the year 2008, many companies have been in a position to sell graphene in extensive quantities. Another important application of fabricated graphene is the tissue engineering. In this case, it has been largely proven that graphene is a reinforcing agent when it comes to the improvement of the mechanical properties of biodegradable materials or rather polymeric nanocomposites materials that are biodegradable being used in bone tissue engineering application (Yu et al. 557). In other words, it is argued that the adding of fabricated graphene to the polymer matrix results into tremendous improvement when it comes to the cross linking density of the material and in the process, it helps improve the load transfer from the matrix of the polymer to the underlying nanomaterial hence increasing the mechanical properties.
Fabricated graphene can also be used as a contrast agent in bioimaging. In this case, surfactant and functionalized dispersed graphene solutions are acknowledged to have been designed as blood pool contrast agents for MRI (Das and Smita, 319). Additionally, manganese and iodine incorporating graphene nanoparticles are also universally acknowledged to have served multimodal MRI. In fact, some of the fabricated graphene has also been proved to have effectively taken up the cancerous cells hence helping the medical scientists to establish effective techniques for delivering cancer therapy (Das and Smita, 319).
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One particular area that the world will soon consider in terms of the employment of fabricated graphene on a commercial scale is basically the optoelectronics, especially in liquid crystal displays, touch screens and organic light emitting diodes (Zeng et al. 5310. In this case, for any carbon material to be employed in optoelectronic, it must be able to transmit approximately 90% of light while at the same time, it must also possess electrical conducive properties that are more than 1 multiplied by 106 ohms1m1 and as such, it must also have a lower level of electrical resistance. Graphene as a material is also transparent being able to carry out electrical transmission of approximately 97.7% of light (Yu et al. 555). Moreover, it is also highly conducive hence making it one of the best materials that should be employed in optoelectronics such as LCD touch screens, smartphones, desktop computers and tablets.
Conclusion
It is acknowledged that since the first fabrication of graphene in the year 2004, tremendous progress has been witnessed as far as the fabrication of graphene is concerned as well as the application of different graphene nanostructures. As much as numerous scholars concentrate on studying electronic devices, it is acknowledged that graphene nanostructures are also crucial for optical devices such as phototransistors and plasmodic devices. However, it is also argued that there are a number of devices that are still under research since the fabrication of graphene nanostructures is something that still continues to evolve.