Periodic TiO₂ Nanorod Arrays with Hexagonal Nonclose‐Packed Arrangements: Excellent Field Emitters by Parameter Optimization

Abstract

Periodic TiO2 nanorod arrays with hexagonal nonclose‐packed (hncp) arrangements are synthesized by pulsed laser deposition (PLD) using polystyrene colloidal monolayers as templates and with subsequent annealing in air. The hncp‐array formation is governed by in situ volume shrinkage of amorphous TiO2 nanorods in the crystallizing process during annealing. The array periodicity can easily be tuned by different sphere sizes of the colloidal template, whereas the distance between neighboring nanorods can be controlled by altering the background gas pressure during the PLD process, at a given periodicity for the nanorod array. Parameter‐controlled growth is helpful for investigating and optimizing the parameter‐dependent field‐emission properties. The hncp nanorod array exhibits an enhanced field‐emission (FE) performance compared to both particle films and nanorod arrays with top aggregation. With an increase in periodicity of a hncp nanorod array, the field‐enhancement factor decreases and the turn‐on FE field increases. FE characteristics can be further enhanced by increasing the distance between adjacent nanorods while maintaining the same periodicity. The parameter‐optimized results suggest that the arrays with a smaller periodicity and a larger distance display the best FE performance and could be highly valuable for designing field‐emission devices based on these periodic nanorod arrays.

Surface Patterning with Colloidal Monolayers

“…Thus, nanoscale discs, [127] rings, [128] ellipsoids [11] and crescent shaped particles [129][130] were successfully prepared as well as hole-films, [74,131] nanobowls, [132] nanoneedles, [14] membranes [132] and arrays of pillars of different materials. [133][134][135] …”

Physical Deposition Assisted Colloidal Lithography: A Technique to Ordered Micro/Nanostructured Arrays

“…Periodic TiO 2 micro/nano-rod arrays with hncp arrangement can be synthesized by combining a colloidal monolayer template with pulsed laser deposition (PLD) followed by annealing in ambient air, as described before. 86 By this route, the periodicity of such special nanorods can be easily tuned by changing the colloidal sphere size in the colloidal monolayer template. While a distance between neighboring nanorods can be controlled by varying the background gas pressure during the PLD process if periodicity is fixed for a nanorod array.…”

“…86 Figure 19 presents FE-SEM images of the hierarchical micro/nano rod array obtained by PLD assisted colloidal lithography (PS sphere size: 350 nm; background O 2 pressure: 6.7 Pa; longer deposition time: 60 min) after annealed at 650 o C for 2 h in air. Figure 19 a and b indicate that a periodic nanorod array takes an hncp arrangement and that each nanorod consists of many nanoparticles.…”

“…Such PLD assisted colloidal lithography can be extended to prepare hexagonal non-close-packed (hncp) micro/nano-particle or nanorod arrays by further annealing process. [85][86][87][88] PS colloidal monolayer was first fabricated on Si substrates by a self-assembly process. It was then placed in a PLD chamber for deposition at room temperature.…”

“…The well tunable periodicity and distance between neighboring nanorods are very useful for investigating and optimizing their FE performance. 86 The periodic hncp TiO 2 nanorod array was fabricated by PLD using a colloidal monolayer template with 350nm PS spheres at 6.7 Pa O 2 for 60 min and subsequent annealing at 650 o C for 2 h in air ( Figure 19). It demonstrated a low turn-on field of about 5.6 V μm -1 (here the turn-on field was defined as the value of electric field when an emission current density was 4.5 nA cm -2 ) according to the FE current density-applied electric field curve (J-E) at a working distance of 60 μm from the anode to the nanorod array serving as the cathode (Figure 33).…”