Like many other two-dimensional (2D) materials, black phosphorous has gathered interest of late and is mainly due to its high carrier mobility and thickness dependent direct bandgap.
However, despite materialistic advances, Researchers have found it to be inherently difficult to deposit black phosphorus onto other materials because of its instability under ambient conditions. A global team of Researchers from China, Finland and the UK have created a binder-free black phosphorus ink that can be inkjet printed onto optoelectronic and photonic devices in a scalable fashion under ambient conditions.
Despite its great properties, it is notoriously difficult to affix black phosphorous to a given substrate. Black phosphorus has the potential to be used across photonic and optoelectronic applications in a range of devices that include transistors, light emitting diodes, photodetectors, solar cells and all-optical switches for ultrafast lasers, but its commercial viability has been held back by the instability issue.
Away from other failed methods which have attempted to deposit black phosphorus on to a substrate, the team of Researchers have utilized a different angle to solve the problem by producing a binder-free black phosphorous inkjet ink composed of black phosphorous and a binary solvent carrier of isopropyl alcohol and 2-butanol.
The inkjet ink was formulated through a series of steps which included centrifugal, ultrasonic irradiation and drop casting methods. The inkjet printer used was a Fujifilm Dimatix DMP-2831 with a Dimatix DMC-11610 cartridge, which produced a droplet size of roughly 10 picolitres (pL). The substrates of choice were silicon, silicon dioxide, glass and PET, all of which were untreated bar cleaning with acetone and water prior to deposition.
The inks were characterized using a range of techniques, including Raman Spectroscopy, optical absorption measurements, light scattering measurements, pendant droplet and parallel plate rheometer measurements and contact angle measurements.
The formulation allowed for stable jetting, a recirculation of the Marangoni flow and the wetting of the untreated substrates. The low boiling point of the alcohols with the two component formulation allowed the ink to be rapidly dried, as the solvent evaporated quickly (within 10 seconds at temperatures below 60 °C). These properties also helped to suppress the ‘coffee ring’ formation seen by many inks.
The Researchers also raised the ink concentration and reduced the number of printing repetitions to optimize the amount of black phosphorus deposited during the fabrication of a given device. The combination of the quick drying time and the reduced printing time (as a result of fewer repetitions) allowed the deposition to occur with an unlikelihood of oxidation occurring, even under ambient conditions.
By optimizing the printing process (and the associated printing characteristics), the Researchers produced an ink with a high printing consistency that showed less than 2% variation across printing repetitions and a spatial uniformity with less than 3.4% variation across the printed patterns.
The Researchers also encapsulated the ink with a well-known conformal coating called Parylene-C. The encapsulation of the ink allowed the black phosphorous to remain stable for longer than 30 days under ambient conditions.
The combination of the printing consistency, uniformity and long-term stability prompted the Researchers to develop a series of photonic and optoelectronic devices using the black phosphorous ink, including a saturable absorber- which produced a stable generation of ultrashort pulses at an intense irradiation of 32.7 MW cm-2 for greater than 30 days; and a broadband photodetector for visible and near-IR wavelengths, which possessed a responsivity of up to 164 mA W-1.
The photodetector also showed an enhancement in detection performance by up to 10 times at 450 nm wavelengths and extended the detection range to include 1550 nm whilst using the black phosphorous ink.
The Researchers have a produced a scalable process in which the black phosphorous ink is stable throughout the fabrication process of the device(s), and thereafter. The process has demonstrated a high suitability towards large-scale commercialization of these inks for use in future photonic and optoelectronic technologies that require long-term stability.
“Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics”- Hu. G., et al, Nature Communications, 2017, DOI: 10.1038/s41467-017-00358-1