Semiconductor and Dimensional Metrology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
Nano Lett. 2013 Jan 9;13(1):131-6. doi: 10.1021/nl303669w. Epub 2012 Dec 20.
We report the direct measurement of the Dirac point, the Fermi level, and the work function of graphene by performing internal photoemission measurements on a graphene/SiO(2)/Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO(2)/Si interfaces is accurately established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO(2) cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV ± 0.05 eV below the bottom of the SiO(2) conduction band. When combined with the shift of the Fermi level from the Dirac point, we are able to ascertain the position of the Dirac point at 3.6 eV ± 0.05 eV with respect to the bottom of the SiO(2) conduction band edge, yielding a work function of 4.5 eV ± 0.05 eV which is in an excellent agreement with theory. The accurate determination of the work function of graphene is of significant importance to the engineering of graphene-based devices, and the measurement technique we have advanced in this Letter will have significant impact on numerous applications for emerging graphene-like 2-dimensional material systems.
我们通过对具有独特光学腔增强测试结构的石墨烯/SiO2/Si 结构进行内光电发射测量，直接测量了狄拉克点、费米能级和石墨烯的功函数。准确建立了石墨烯/SiO2/Si 界面处完整的电子能带排列。利用 SiO2 腔中的建设性光干涉，有可能观察到单层石墨烯的增强光电发射。在狄拉克点附近，光电发射产率被发现遵循众所周知的线性态密度色散。在平带条件下，提取出费米能级并发现其位于 SiO2 导带底部以下 3.3 eV ± 0.05 eV。当结合费米能级从狄拉克点的位移时，我们能够确定狄拉克点的位置在 3.6 eV ± 0.05 eV 相对于 SiO2 导带边缘的底部，得出功函数为 4.5 eV ± 0.05 eV，与理论非常吻合。准确确定石墨烯的功函数对于基于石墨烯的器件的工程设计具有重要意义，我们在本信中提出的测量技术将对新兴的类似石墨烯的二维材料系统的众多应用产生重大影响。
