Space Charge Doping Induced Band Modulation in Mono- and Bi-layer Graphene: a nano-ARPES study

Controlled modulation of electronic band structure in two-dimensional (2D) materials via doping is crucial for devices fabrication. For instance doped graphene has been envisaged for various applications like sensors, super-capacitors, transistors, p-n junctions, photo-detectors, etc. Many different techniques have been developed to achieve desired doping in 2D materials, like chemical doping, electrostatic doping, substrate doping, etc. Here, we have combined space charge doping with space and angle resolved photoemission (nano-ARPES), in order to directly observe the Fermi level modulation on micron-sized flakes of monolayer and bilayer graphene. The doping level can be tuned in a controlled manner, which allows us to directly observe the Fermi level tuning. In our experiment we successfully doped the graphene with p- and n-type carriers (holes/electrons) which are directly observed through band shift in ARPES measurements. The observed band shift is $\sim$250 meV for bilayer and $\sim$500 meV for monolayer graphene. The results from our experiment promote the space charge doping technique and nano-ARPES into other materials such as 2D semiconductors and superconductors, in order to directly observe the physical phenomena such as band gap transition and phase transition as function of carrier doping.