Selective growth and enhanced field emission properties of micropatterned iron phthalocyanine nanofiber arrays

In this study, we developed a simple method for the micropatterned growth of iron phthalocyanine (FePc) nanofiber arrays using a thermal evaporation process. By controlling the surface energy and the temperature of the substrate (Tsub), we obtained FePc films featuring a grain-like (in-plane) morphology on Si surfaces (higher surface energy) and a fiber-like (out-of-plane) morphology on Ag surfaces (lower surface energy) within a certain range of values of Tsub. On the Ag surfaces, these temperature-induced FePc nanofibers featured a high aspect ratio (AR) of 30.3 ± 3.6, with a mean length of 699 ± 216 nm and a mean radius of 22.2 ± 4.3 nm, as-prepared at a value of Tsub of 240 °C. The FePc films obtained at values of Tsub of 25, 120, 180, and 240 °C all possessed α-phase crystalline structures. Because the growth structures of the FePc molecules on the Si and Ag substrates were quite different, we could control the growth of micropatterned 1D FePc nanofiber arrays on previously patterned Ag/Si substrates. From the comparison of the field emission (FE) properties in different ARs of patterned devices, higher AR (30.3 ± 3.6) of devices (FE-240-P; Tsub of 240 °C) exhibited better FE performance than lower AR (6.0 ± 2.6) of devices (FE-180-P; Tsub of 180 °C). The FE current density of devices (Tsub of 240 °C) increased from 0.13 mA/cm2 for the unpatterned device (FE-240-N) to 6.77 mA/cm2 for the patterned device (FE-240-P) at an applied electric field of 12 V/μm. The turn-on electric fields required to produce a current density of 10 μA/cm2 were 7.7 and 10.3 V/μm for the patterned and unpatterned FePc emitters, respectively. From the slopes of Fowler–Nordheim plots, we estimated the field enhancement factors (β) of FE-240-P and FE-240-N to be 314 and 329, respectively. Studies of the emission current stability revealed that the FePc nanofibers possessed outstanding anti-degrading capability. During stability tests, the micropatterned FePc emitter (FE-240-P) displayed an efficient emission current with fluctuations of less than 20%. Because this facile platform allows control over the morphologies of films of small organic molecules merely by tuning the surface energy of the substrates, such micropatterned-FePc nanofibers might have great applicability in practical field emitters.

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► FePc films featured a grain-like morphology on Si surfaces and a fiber-like morphology on Ag surfaces.
► These well-aligned 1D FePc nanofibers exhibited FE characteristics and follow FN behavior.
► We can control the growth of micropatterned FePc nanofibers on previously patterned Ag/Si substrates.
► All FePc emitters revealed their stable and superior FE properties during stability tests.