Inkjet Printing of Two Dimensional Materials

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Abstract

Over the last decade, two dimensional (2D) materials have attracted considerable attention from both the scientific and engineering community due to their unique properties. One important advance of 2D materials is that they can be exfoliated into nanosheets suspended in a liquid phase and that this allows the formulation of 2D nanomaterials inks. Such inks can be deposited as functional components through low-cost inkjet printing techniques. Many 2D materials based inks have been produced over the years. This thesis investigates the use of inkjet printing to deposit 2D materials such as graphene oxide (GO) and black phosphorus (BP).GO, a derivative of graphene, has been widely used to produce graphene-based conductors via inkjet printing owing to its good stability in readily available solvents such as water. In this work, highly conductive reduced graphene oxide (rGO) films with bulk conductivity in excess of 2 × 10^4 Sm-1 have been prepared by inkjet printing a GO aqueous ink, with mean flake size 35.9 μm, through a 60 μm inkjet printing nozzle followed by a reduction step. Experimental results showed that individual GO flakes up to 200 μm diameter can be successfully printed with no instances of nozzle blocking or poor printing performance. The mechanism by which this occurs is believed to be GO sheet folding during drop formation followed by elastic unfolding during drop impact and spreading. In addition, the influence of GO flake size on rGO film conductivity has been investigated. It was found that the rGO film conductivity increased about 60% when the mean flake size of the GO flakes in the ink increases from 0.68 μm to 35.9 μm.The drying behaviour of printed GO droplets has been studied on eight GO aqueous inks in which the mean flake size of GO was varied over a range from 0.68 to 35.9 μm. It was found that the coffee ring effect (inhomogeneous drying of a droplet to leave a ring like deposit) of dried droplets of the GO ink weakened and disappeared when the flake size increasing. It was found that, with a printed deposit around 340 μm in diameter, the coffee ring effect (CRE) was suppressed with the mean flake size > 10.3 μm. The critical flake size for CRE suppression reduced to 5.97 and 3.68 μm when the substrate temperature was 40 and 50 °C, respectively. It was further found that the CRE weakened with decreasing printed drop size, with the critical flake size reducing to 1.58 μm with a printed drop diameter of 30 μm.The interaction between BP nanometre thickness flakes and humid atmospheres was investigated using an inkjet printed BP sensor. The BP sensor showed was very sensitive to changes in humidity with a response time of a few seconds and the effect is reproducible in minutes. However, long term exposure to humid air with a relative humidity (RH) > 11% leads to a significant chemical change in the BP films, with Fourier transform infra-red spectroscopy (FTIR) indicating partial hydrolysis of the BP to form phosphate and phosphonate ions. Low temperature heat treatment of BP films under dry conditions after exposure to elevated RH leads to a partial recovery of the impedance response and reversion to a chemical state similar to that before exposure to a humid environment. The recovery of BP properties is most complete after exposure to lower humidity environments (RH < 11%), although exact replication of the original impedance response and FTIR spectrum was not possible.

Keyword(s)

Black phosphorus; Coffee ring effect; Graphene oxide; Inkjet printing; Reduced graphene oxide

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