Bipolar Photoresponse of a Graphene Field-Effect Transistor Induced by Photochemical Reactions

Graphene is an air-friendly material that can be easily p-doped by oxygen; therefore, a stable, defect-free, and efficient graphene n-doping technique should be developed for achieving high performance in electronic and optoelectronic devices. In this study, we present a unique method for n-type chemical doping of monolayer graphene grown through chemical vapor deposition. The doping process is thoroughly examined using X-ray photoelectron spectroscopy, Raman spectroscopy, and ultraviolet photoelectron spectroscopy. The findings demonstrate that the use of KBr solution is highly effective in achieving n-type doping in monolayer graphene, offering promising prospects for its practical application. Also, we fabricated graphene field-effect transistors and studied their electrical properties before (pristine) and after doping the graphene channel with different KBr concentrations (0, 0.05, 0.15, 0.20, and 0.25 M) in dark and under deep-ultraviolet (DUV) light conditions. During graphene doping in the dark environment, the charge neutrality point (CNP) shifted toward negative back gate voltages and then saturated at 0.25 M. After photochemical doping under DUV light, CNP further shifted toward negative gate voltages with improved carrier mobility at the same molar concentration of 0.25 M. Additionally, the photodetectors are fabricated from pristine and doped graphene which demonstrated bipolar photoresponse, thereby, a transition of negative photocurrent to a positive photocurrent when the concentration of the KBr solution reached 0.20 M. Moreover, their response time decreased from 8 to 3.5 s with increasing KBr concentration from 0 to 0.30 M. Finally, the gate voltage-dependent broadband photoresponsivity of doped graphene (0.25 M) was investigated at different wavelengths (220, 365, 530, and 850 nm). Thus, the controlled doping-induced bidirectional photoresponse can provide a facile route for logic gate applications.