Graphene is a monolayer of graphite, and its atoms are arranged in hexagonal lattices. With its excellent electrical, optical, mechanical and thermal properties, graphene has attracted much attention. Thus, the mechanism of graphene size changing under irradiation has become one of the hotspots of current research.
The change in size of graphene under single-photon excitation has been studied in many experiments. It has been found that, under irradiation of femtosecond laser, the area of graphene would somehow shrink, even to nanometer scale. Among them, the most outstanding result is the size of a graphene nanoribbon that is only 1nm wide. Meanwhile, the spacing between carbon layers is about 0.25- 0.3nm, far less than the usual spacing between layers of a graphite crystal that is about 0.354nm.
Although the size of graphene has been reduced under laser irradiation, some researchers are not satisfied with just confirming the size changing effect. They would like to find out the mechanism involved and figure out the limiting factor of reducing its size.
From a theoretical point of view, the mechanism of the graphene size reducing can be simplified as follows: In graphite, π electrons around each carbon atom formed a conjugate bond and connected the entire lattice structure. Under continuous photon excitation, the orbit around carbon atoms shrinks and the conjugate bond between them is weakened or destroyed. Without enough electrons to form the next bond, the structure of graphene becomes loose, resulting in an overall shrinkage of the graphene size.
Besides theoretical mechanism, experimental explanation of the size changing of graphene under laser radiation was also proposed. By using spectrometer and Raman microscope, researchers can analyze the spectrum of the scattered photons. By obtaining the spectrum data and then analyzing their frequency and intensity, we can further get the energy and direction of the laser beam. From that we can deduce the detail of the energy transfer and the direction of movement of electrons. Thus, the electron orbit around carbon atom can be shrunk and the bond between them can be destroyed.
So far, no matter from the theoretical or from the experimental point of view, the underlying mechanism of the graphene size reduction is believed to be the collapse of π electron orbit of carbon atoms.
Additionally, by analyzing the area of reduced graphene, some researchers found out that the size-reducing power of the laser beam has a clear limitation. Smaller than a certain limit, it will be difficult to break the π electrons’ orbit and some of them in the edge of graphene are difficult to be destroyed. Thus, reducing the size of graphene has obvious limitations and is difficult to go beyond a certain size.
In conclusion, in the applications of graphene or other two-dimensional materials, the size changing of them should be taken as one of the important factors. Researchers need to know the theory and mechanism of the size-reducing effects before they can better utilize these materials.
有机石墨是一种单层石墨，其原子排列成六角格栅。由于其优异的电学、光学、力学和热性能，石墨烯引起了人们的极大兴趣。因此，石墨烯在辐照下尺寸变化的机制已成为当前研究的热点之一。
单光子激发下石墨烯的尺寸变化已在许多实验中得到研究。研究发现，在飞秒激光照射下，石墨烯的面积会以某种方式收缩，甚至收缩到纳米尺度。其中最突出的结果是石墨烯纳米条宽度只有1nm。同时，碳层之间的间距约为0.25-0.3nm，远低于石墨晶体中碳层之间通常的间距0.354nm。
虽然石墨烯在激光照射下的尺寸发生了变化，但一些研究人员并不满足于仅仅确认其尺寸变化的效果，他们想要找出其中涉及的机理，搞清楚减少尺寸的极限因素。
从理论的角度来看，石墨烯尺寸减少的机制可简化为：在石墨中，每个碳原子周围的π电子形成共轭键，将整个点阵结构连接起来。在持续的光子激发下，碳原子周围的轨道收缩，碳原子之间的共轭键被削弱或破坏。没有足够的电子形成下一对键，石墨烯的结构变松，导致石墨烯尺寸整体缩小。
除了理论机理外，实验上也提出了石墨烯在激光辐射下尺寸变化的解释。通过分光仪和拉曼显微镜，研究人员可以分析散射光的光谱。通过获得光谱数据，然后分析其频率和强度，我们可以进一步得到激光束的能量和方向。从而可以推断出能量转移的细节以及电子的运动方向。从而使得碳原子周围的电子轨道收缩，碳原子之间的键被破坏。
到目前为止，无论是从理论上还是从实验上来看，石墨烯尺寸减小的潜在机制都被认为是π电子轨道的崩溃。
此外，通过分析减小的石墨烯面积，一些研究人员发现激光束的尺寸减少的功率有明确的限制 。小
