Reducing sheet resistance of self-assembled transparent graphene films by defect patching and doping with UV/ozone treatment

Abstract

Highlights Photochemical oxidation (UV/ozone treatment) of self-assembled solution-processed graphene films is demonstrated for the first time  The effect that photochemical oxidation has on self-assembled graphene films is a reduction of sheet resistance, the opposite of the earlier reported effect on CVD or mechanically exfoliated graphene  Edges as the dominant defect type in self-assembled graphene films play a crucial role in the presented defect density decrease  Photochemical oxidation is proposed as a method of increasing the carrier mean free path, doping, and lowering sheet resistance of solution-processed graphene films Abstract Liquid phase exfoliation followed by Langmuir-Blodgett self-assembly (LBSA) is a promising method for scalable production of thin graphene films for transparent conductor applications.However, monolayer assembly into thin films often induces a high density of defects, resulting in a large sheet resistance that hinders practical use. We introduce UV/ozone as a novel photochemical treatment that reduces sheet resistance of LBSA graphene threefold, while preserving the high optical transparency. The effect of such treatment on our films is opposite to the effect it has on mechanically exfoliated or CVD films, where UV/ozone creates additional defects in the graphene plane, increasing sheet resistance. Raman scattering shows that exposure to UV/ozone reduces the defect density in LBSA graphene, where edges are the dominant defect type. FTIR spectroscopy indicates binding of oxygen to the graphene lattice during exposure to ozone. In addition, work function measurements reveal that the treatment dopes the LBSA film, making it more conductive. Such defect patching paired with doping leads to an accessible way of improving the transparent conductor performance of LBSA graphene, making solution-processed thin films a candidate for industrial use.

Carbon Dioxide Sensing with Langmuir–Blodgett Graphene Films

“…similar morphology to the material presented here, contains reactive edge defect sites that are responsible for sensitivity to various analytes [52]. After each purging step, and before subsequent exposure to CO2, the measured resistance returned to approximately the same baseline value, which indicates recovery of the sensing surface, without the need for any treatment, such as heating.…”

“…A negative TCR is commonly observed in single-layer graphene [10] and in multilayer graphene materials that possess a large number of defect sites, such as purposely introduced mechanical defects [49] or exfoliation-induced defects [50,51]. We previously demonstrated that liquid-phase exfoliated graphene, which is of similar morphology to the material presented here, contains reactive edge defect sites that are responsible for sensitivity to various analytes [52]. After each purging step, and before subsequent exposure to CO 2 , the measured resistance returned to approximately the same baseline value, which indicates recovery of the sensing surface, without the need for any treatment, such as heating.…”

Ultrafast humidity sensor based on liquid phase exfoliated graphene

“…The film area between contacts has dimensions of ~1500 µm x 190 µm. Taking into account film geometry yields sheet resistance of 3-7 kΩ sq -1 for our films, the smallest values reported for post-processing-free single-deposition Langmuir-Blodgett graphene films to date [16,18,20,22]. Such small sheet resistance is a result of fabrication process streamlining and careful four-terminal resistance measurements.…”

“…The thin film is deposited on a pre-immersed substrate of choice following the Langmuir-Blodgett method [16,[18][19][20].…”

“…Here, we demonstrate fast humidity sensors made with an inexpensive and facile production method that is compatible with large-area sensor production. The active sensing area is made from liquid-phase exfoliated graphene [16] that requires only a single ultrasonic processing step. The humidity response times of our sensors are as low as ~30 ms, which allows us to show real-time breath monitoring and finger proximity detection as exemplary applications of ultrafast humidity sensing.…”