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

Abstract

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 (T(sub)), 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 T(sub). 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 T(sub) of 240 degrees C. The FePc films obtained at values of T(sub) of 25, 120, 180, and 240 degrees C all possessed alpha-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; T(sub) of 240 degrees C) exhibited better FE performance than lower AR (6.0 +/- 2.6) of devices (FE-180-P; T(sub) of 180 degrees C). The FE current density of devices (T(sub) of 240 degrees C) increased from 0.13 mA/cm(2) for the unpatterned device (FE-240-N) to 6.77 mA/cm(2) for the patterned device (FE240- P) at an applied electric field of 12 V/mu m. The turn-on electric fields required to produce a current density of 10 mu A/cm(2) were 7.7 and 10.3 V/mu m for the patterned and unpatterned FePc emitters, respectively. From the slopes of Fowler-Nordheim plots, we estimated the field enhancement factors (beta) 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. (C) 2011 Elsevier B.V. All rights reserved.