Structural Defects in Graphene Quantum Dots: A Review

Graphene quantum dots (GQDs) are known for their low toxicity, strong fluorescence, high surface area, large solubility and tunable band gaps. However, the change in their properties depends on the preparation processes of GQDs. Thus, certain types of preparation lead to certain defects, such as surface defect, edge defects, Stone-Wales defect. These structural defects are responsible for hindering GQDs to possess their regular shape that affects the morphological properties of GQDs. Thus, the optical and electrical properties get affected. The GQDs, which are synthesized via acidic methods are generally more vulnerable to defects compared to those synthesized using eco-friendly methods. Thereby, the aim of this review is to discuss the causes of structural defects. Moreover, it focuses on how they affect the properties of GQDs and to what extent they affect them. The processes of regulating defects have been elucidated so that more efficient applications can be designed using GQDs with controlled amounts of defects.

Effects of Oxygenated Acids on Graphene Oxide: The Source of Oxygen-Containing Functional Group

Graphene oxide is an important member of the graphene family which has a wide range of applications. The chemical method, especially the liquid phase method, is one of the most common and important methods for its preparation. However, the complex solution environment not only gives them rich structure, but also brings great challenges for its large-scale industrial synthesis. In order to better realize its industrial application, it is important to understand its structure, such as the source of oxygen-containing functional groups. Here we studied the contribution of four oxygenated acids to oxygen-containing functional groups in Hummers’ method using first principles. We found that the permanganic acid molecules that exist instantaneously due to energy fluctuations can be the source of oxygen-containing functional group. In addition, Stone-Wales defect have a certain effect on the formation of oxygen-containing functional groups, but this effect is not as good as that of solvation effect. This work provides a guide for exploring the source of oxygen-containing functional groups on graphene oxide.

Tight–Binding Analysis of Electronic Structure of Germanene Sheet and Nanoribbons Including Stone-Wales Defect

In this paper, we investigate the electronic characteristics of germanene using the tight binding approximation. Germanene as the germanium-based analogue of graphene has attracted much research interest in recent years. Our analysis is focused on the pristine sheet of germanene as well as defective monolayer. The Stone-Wales defect, which is one of the most common topological defects in such structures, is considered in this work. Not only the infinite sheet of germanene but also the germanene nanoribbons in different orientations are analyzed. The obtained results show that applying the Stone–Wales defect into the germanene monolayer changes the energy band structure; the E-k curves around the Dirac point are no longer linear, a band gap is opened, and the Fermi velocity is reduced to half of that of defect-free germanene. In the case of nanoribbon structures, the armchair germanene nanoribbons with nanoribbon widths of 3p and 3p+1 reveal the semiconductor behaviour. However, armchair germanene nanoribbon with width of 3p+2 is semi-metal. After applying the Stone–Wales defect, the band gap of armchair germanene nanoribbons with widths of 3p and 3p+1 is reduced and it is increased for the width of 3p+2. Furthermore, there is no band gap in the energy band structure of zigzag germanene nanoribbon and the metallic behaviour is obvious.

Theoretical Study on the Structures of Single-Atom M (M=Fe, Co and Ni) Adsorption Outside and Inside the Defect Carbon Nanotubes

Single-atom confinement inside carbon nanotubes has attracted much attention in many fields. This class of materials may not only serve as a catalyst but also as a support material for certain reactions. In this paper, we have studied the single-walled carbon nanotubes (SWCNT), single vacancy defect (SV) and Stone-Wales defect (SW) carbon nanotubes with Fe, Co and Ni atom by both inside and outside adsorption structures in density function theory (DFT). Our results reveal that the binding abilities of atomic Fe, Co, Ni onto the internal and external surfaces of the SWCNT, SV and SW are in following orders by metals: Ni>Co>Fe. The adsorption energies of SV toward Fe, Co and Ni are more stable than those of SWCNT and SW, which can be attributed to the three active carbon sites created by a C atom removing, while the SWCNT and SW demonstrate similar adsorption energy due to the similar structure. Generally, the stability of external adsorption structures is stronger than those of internal adsorption structures, but as for the SW, the stability of internal and external adsorption structures is close, which means that the defects have improved the confinement of carbon nanotubes to M (M=Fe, Co Ni).

Stone–Wales defects in hexagonal boron nitride as ultraviolet emitters

Many quantum emitters have been measured close or near the grain boundaries of the two-dimensional hexagonal boron nitride where various Stone–Wales defects appear. We show by means of first principles density functional theory calculations that the pentagon–heptagon Stone–Wales defect is an ultraviolet emitter and its optical properties closely follow the characteristics of a 4.08-eV quantum emitter, often observed in polycrystalline hexagonal boron nitride. We also show that the square–octagon Stone–Wales line defects are optically active in the ultraviolet region with varying gaps depending on their density in hexagonal boron nitride. Our results may introduce a paradigm shift in the identification of fluorescent centres in this material.