Ionic liquid passivated black phosphorus for stabilized compliant electronics

Black phosphorus (BP) is an attractive material for two-dimensional (2D) electronics and optoelectronics given its direct optical band gap in the bulk. However, the vulnerability of BP to oxygen and moisture under ambient conditions has been a significant impediment for the adoption of this material towards practical applications. This vulnerability has also curtailed the development of additively manufactured, solution processed, ink-jet printed BP devices, which are cost-effective alternatives to lithographically patterned rigid electronics and optoelectronics based on silicon. In this work, we have fabricated stable BP electronic devices on flexible and compliant substrates using low-cost, additive manufacturing techniques with ink-jet printing through scalable chemical exfoliation routes. To address the stability issues with BP, ionic liquids (ILs) were used as a passivation layer on the surface of the BP to minimize oxidative degradation. The enhanced stability of BP was inferred through Raman spectroscopy and scanning probe microscopy techniques, where no observable changes in the ${\mathrm{A}}_{\mathrm{g}}^1$ and ${\mathrm{A}}_{\mathrm{g}}^2$ Raman vibrational modes were observed with time for the BP films passivated with ILs over a period of 168 h under ambient conditions. On the other hand, a blue-shift in these Raman modes was evident for unpassivated samples. Atomic probe microscopy measurements clearly revealed the difference in the surface characteristics through localized regions of degradation that intensified with time, which was clearly absent in IL/BP samples. The stability measurements were also conducted in electronic device platforms for IL coated BP devices, where the temperature T dependence of the I_ds–V_ds characteristic was measured from T ~ 5.4 to 335 K. Prototypical demonstrations of stabilized ILs/BP devices at ambient printed on flexible polyimide substrates were also made. The irradiation with broadband radiation appears to have a significant influence on the electrical behavior of BP, evident through the variation of photocurrent with temperature. Overall, our study shows that by engineering the exfoliation chemistry of BP with the use of ILs, highly concentrated BP inks can be additively manufactured, that present remarkable opportunities for the large-area integration of BP in printed and flexible electronics using low-cost, scalable approaches benefiting from the direct bandgap characteristics of BP in the bulk.