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Black phosphorus (BP) is an emerging two-dimensional material that has gained an increasing amount of interest for various applications, especially within the electronics sector. The largely unstable nature of BP that causes it to be rapidly oxidized has inspired a group of Researchers from Aalto University in Finland to develop a BP ink that is compatible with conventional inkjet printing for large scale production of BP.
With its nanosheets that are layered together by van der Waals interactions, similar to the way in which graphene nanosheets form graphite when stacked together, BP is advantageous over graphene as a result of its direct bandgap in mono-few-layer and its bulk forms1.
BP’s bandgap ranges from 0.3 to 2.0 electron volts, which bridges the gap between graphene, whose bandgap is 0, and that of transition-metal dichalcogenides (TMDCs), whose bandgap can range from 1.0 to 2.52. This thickness-dependent bandgap, along with its high carrier mobility, allots BP to be an ideal material for various optoelectronic and photonic devices including light emitting dioides, solar cells, photodetectors, transistors and much more.
BP is typically produced by liquid exfoliation, however, as a result of its instability when present with water and oxygen, this synthesis method is unfavorable for the production of BP on a large scale. Several studies have looked towards minimizing the oxidation of BP during its fabrication by encapsulating BP or changing its solvent for the liquid phase1.
While certain issues associated with difficulty during exfoliation processes has been alleviated by utilizing polymer binders to the dispersions for certain 2D materials, the requirement to remove binders through high temperature annealing is impractical for BP purposes, as it does not avoid the exposure of the material to ambient conditions that encourage oxidation reactions.
Despite these efforts, little progress has been done on achieving the adequate production of BP nanosheets in large quantities. While inkjet printing techniques have been applied to the production of other 2D materials, particularly graphene and molybdenum disulphide (MoS2), to date, there has been no report of the inkjet printing of BP until the Aalto study.
In their method, the Researchers employed a binder-free inkjet ink that was composed of BP made by ultrasound-assisted liquid phase exfoliation (UALPE), as well as a binary solvent of isopropyl alcohol (IPA) and 2-butanol.
The formulation of this inkjet ink begins with the dispersion of thin BP flakes produced by UALPE in varying concentrations of IPA and 2-butanol solvents. The low boiling points of both IPA, whose boiling point in 82.6 °C, and n-butanol of 117.7 °C , provide an ideal liquid phase for the exfoliation of BP as it largely limits the ability of the material to oxidize during the inkjet printing process.
Once this was optimized, the Researchers designed the ink so that it would jet in a stable manner during printing, while also maintaining the wet environment of the substrate to ensure the adequate and precisely flat deposition of the BP ink onto the material3.
The stability of the jetting process is optimal in determining how the material is deposited into areas of interest, as compared to being placed onto untargeted areas, therefore, the viscosity of the ink, surface tension and density all play a determining role in consistent printing.
The rapid ink drying, which is promoted by the presence of both alcohols IPA and 2-butanol, prevents the unfavorable oxidation reactions from being a problem in this fabrication method of BP. Similarly, the BP ink favors a high printing consistency, which was found to exhibit a negligible variation of 2% between printing repetitions, as well as an impressive spatial uniformity of approximately 3.4% across printing patterns3.
The rapid inkjet printing of BP that was successfully performed in this study allows for a scalable, reproducible, uniform, highly stable and novel way to integrate BP material with future optoelectronic and photonic technologies.
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References:
- “Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics” D. Hanlon, C. Backes, et al. Nature Communications. (2015). DOI: 10.1038/ncomms9563.
- “Is Black Phosphorus the New Graphene?” – IEEE Spectrum
- “Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics” G. Hu, T. Albrow-Owen, et al. Nature Communications. (2017). DOI: 10.1038/s41467-017-00358-1.
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