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Mysterious amorphous carbon: Why is its structure so unique?
In materials science and modern chemistry, amorphous carbon, as a special form of carbon, has attracted the attention of countless researchers. This type of carbon is unique in that it does not have any crystalline structure, which makes it a very flexible and changeable material. Amorphous carbon is often referred to simply as aC, and when combined with hydrogen, it is called aC:H or hydrogenated amorphous carbon (HAC); while tetrahedral amorphous carbon is called ta-C, also known as quasi-C:H or hydrogenated amorphous carbon (HAC). Diamond carbon. In the field of physical science, the study of amorphous carbon has revealed a variety of potential applications, from electronic devices to biomedicine. The unique characteristics of amorphous carbon make it a material worth exploring in depth.
Amorphous carbon materials may eliminate the π bonds at the corners by combining with hydrogen, thereby stabilizing their structure.
In mineralogy, the term amorphous carbon is used to describe coal, carbide-derived carbon, and other impure forms of carbon. These substances are not typical graphite or diamond. While these materials are not completely amorphous crystallographically, they are often polycrystalline materials with graphite or diamond. In commercial applications, amorphous carbon often also contains other elements that may form significant crystalline impurities, further complicating the properties of amorphous carbon.
With the development of modern thin film deposition and growth technologies in the second half of the 20th century, such as chemical vapor deposition, sputter deposition, and cathodic arc deposition, truly amorphous carbon materials were created. These materials possess localized pi electrons, which are not formed at consistent lengths with other allotropes of carbon, compared to graphite's aromatic pi bonds. Amorphous carbon also contains relatively high dangling bonds, which can cause deviations in interatomic distances of more than 5%, and significant changes in bond angles can also be observed.
The properties of amorphous carbon films vary depending on the parameters used during deposition.
The main characterization method for amorphous carbon is to measure the ratio of sp2 and sp3 mixed bonds in the material. Graphite is composed entirely of sp2 mixed bonds, while diamond is composed entirely of sp3 mixed bonds. When the proportion of sp3 mixed bonds in the material is high, this type of amorphous carbon is also called tetrahedral amorphous carbon or diamond-like carbon. This is because the four-sided shape formed by sp3 mixed bonds makes this type of material have many physical properties similar to diamonds. Experimentally, the ratio of sp2 to sp3 can be determined by comparing the relative intensities of different spectral peaks, including EELS, XPS, and Raman spectra.
Interestingly, although the one-dimensional property change of amorphous carbon materials between graphite and diamond can be shown based on the ratio of sp2 to sp3, this statement is not actually true. Current research is providing insights into the properties and potential applications of amorphous carbon materials. It cannot be ignored that PAHs, tar, are present in large amounts in hydrogenated carbon entities found in everyday life (e.g. smoke, chimney dust, mined coals such as bitumen and anthracite) and are therefore almost all carcinogenic.
In addition, research in recent years has also introduced a new amorphous carbon material called Q-carbon. Q-carbon, referred to as annealed carbon, is claimed to be ferromagnetic, conductive, even harder than diamond, and capable of demonstrating high-temperature superconductivity. In 2015, a professor named Jagdish Narayan and his research team first announced the discovery of Q-carbon. They published many papers on the synthesis and characterization of Q-carbon, but several years later, the properties of this substance have not yet been verified by independent experiments.
According to the researchers, Q-carbon exhibits a random amorphous structure and is intertwined in sp2 and sp3 bonding.
Their team used nanosecond laser pulses to melt the carbon and then quickly cooled it to form Q-carbon or a mixture of Q-carbon and diamond. The material can take many forms, from nanoneedle-like structures to large diamond films. They also reported making materials such as nitrogen-vacancy nanodiamonds and Q-boron nitride, and creating technology to convert carbon into diamond at ambient temperature and pressure. Although in 2018, a group of researchers at the University of Texas at Austin used simulations to propose a theoretical explanation for Q-carbon's high-temperature superconductivity, ferromagnetism, and hardness, these results have not been confirmed by others.
In any case, research on amorphous carbon continues to be in-depth, and this special form of carbon material has great potential. How will future development affect our lives and technology? Perhaps only time can give us the answer?
