Abstract
We demonstrate the morphological control method of ZnO nanostructures by atomic layer deposition (ALD) on an Al2O3/ZnO seed layer surface and the application of a hierarchical ZnO nanostructure for a photodetector. Two layers of ZnO and Al2O3 prepared using ALD with different pH values in solution coexisted on the alloy film surface, leading to deactivation of the surface hydroxyl groups. This surface complex decreased the ZnO nucleation on the seed layer surface, and thereby effectively screened the inherent surface polarity of ZnO. As a result, a 2-D zinc hydroxyl compound nanosheet was produced. With increasing ALD cycles of ZnO in the seed layer, the nanostructure morphology changes from 2-D nanosheet to 1-D nanorod due to the recovery of the natural crystallinity and polarity of ZnO. The thin ALD ZnO seed layer conformally covers the complex nanosheet structure to produce a nanorod, then a 3-D, hierarchical ZnO nanostructure was synthesized using a combined hydrothermal and ALD method. During the deposition of the ALD ZnO seed layer, the zinc hydroxyl compound nanosheets underwent a self-annealing process at 150 °C, resulting in structural transformation to pure ZnO 3-D nanosheets without collapse of the intrinsic morphology. The investigation on band electronic properties of ZnO 2-D nanosheet and 3-D hierarchical structure revealed noticeable variations depending on the richness of Zn-OH in each morphology. The improved visible and ultraviolet photocurrent characteristics of a photodetector with the active region using 3-D hierarchical structure against those of 2-D nanosheet structure were achieved.
Highlights
Nanostructured materials, which are defined as materials with structural elements, such as clusters, crystallites or molecules, with dimensions in the 1 to 100-nm range, have been the interest of both academic and industrial fields over the past few decades because nanosize scaling allows materials to exhibit novel and significantly improved physical, chemical, and biological properties [1,2,3]
As the thickness of the atomic layer deposition (ALD) Zinc oxide (ZnO) film in the seed layer increased, the 2-D nanosheet coexisted with a 1-D nanorod
The nanostructure morphology change from a 2-D nanosheet to 1-D nanorod was controlled by changing the seed layer surface
Summary
Nanostructured materials, which are defined as materials with structural elements, such as clusters, crystallites or molecules, with dimensions in the 1 to 100-nm range, have been the interest of both academic and industrial fields over the past few decades because nanosize scaling allows materials to exhibit novel and significantly improved physical, chemical, and biological properties [1,2,3]. Previous results have demonstrated methods for successful control of the ZnO nanostructure shape, there are additional challenging factors in ZnO nanostructure growth such as (a) preparation of a pure ZnO chemical composition without undesired element incorporation from a seed layer and (b) production of preferential structural growth over large areas [17,18,19]. The latter factor is crucial to maximize reactive sites of ZnO in specific orientations for surface chemical applications (e.g., gas sensor and heterogeneous catalysis supports). Achievement of large areas with a (100) ZnO surface orientation is very useful as it has reactive O- and Zn-polar sites [20,21,22]; this part remains a technical challenge in ZnO nanostructure
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