Department of Physics, Loughborough University, Loughborough, LE11 3TU, UK.
Nanoscale. 2017 Aug 17;9(32):11463-11474. doi: 10.1039/c6nr08999b.
We report the discovery of remarkable photo-physical phenomena with characteristics unique to epitaxial graphene grown on 6H-SiC (000-1). Surprisingly, the electrical resistance of graphene increases under light illumination in contrast to conventional materials where it normally decreases. The resistance shows logarithmic temperature dependences which may be attributed to an Altshuler-Aronov effect. We show that the photoresistance depends on the frequency of the irradiating light, with three lasers (red, green, and violet) used to demonstrate the phenomenon. The counterintuitive rise of the positive photoresistance may be attributed to a creation of trapped charges upon irradiation. We argue that the origin of the photoresistance is related to the texture formed by the graphene flakes. Photovoltage also exists and increases with light intensity. However, its value saturates quickly with irradiation and does not change with time. The saturation of the photovoltage may be associated with the formation of a quasi-equilibrium state of the excited electrons and holes associated with a charge redistribution between the graphene and SiC substrate. The obtained physical picture is in agreement with the photoresistance measurements: X-ray photoelectron spectrometry "XPS", atomic force microscopy "AFM", Raman spectroscopy and the magnetic dependence of photoresistance decay measurements. We also observed non-decaying photoresistance and linear magnetoresistance in magnetic fields up to 1 T. We argue that this is due to topological phases spontaneously induced by persistent current formation within the graphene flake edges by magnetic fields.
我们报告了在 6H-SiC(000-1)上生长的外延石墨烯所具有的独特光电物理现象的发现。令人惊讶的是，与通常情况下电阻随光照减小相反，石墨烯的电阻在光照下会增加。电阻呈现对数温度依赖性，这可能归因于 Altshuler-Aronov 效应。我们表明，光电阻取决于辐照光的频率，使用了三个激光器（红、绿和紫）来演示该现象。正光电阻的反常增加可能归因于辐照时产生的被俘获电荷。我们认为，光电阻的起源与石墨烯薄片形成的纹理有关。还存在光电压，并且随着光强度的增加而增加。然而，其值随辐照迅速饱和，并且不随时间变化。光电压的饱和可能与与石墨烯和 SiC 衬底之间的电荷重新分布相关的受激电子和空穴的准平衡态的形成有关。所得到的物理图像与光电阻测量结果一致：X 射线光电子能谱“XPS”、原子力显微镜“AFM”、拉曼光谱和光电阻衰减测量的磁场依赖性。我们还观察到在高达 1 T 的磁场中存在非衰减的光电阻和线性磁电阻。我们认为，这是由于磁场在石墨烯薄片边缘内形成持续电流而自发诱导的拓扑相所致。
