ZnO/Al₂O₃ Core-shell Nanorod Arrays: Processing, Structural Characterization, and Luminescent Property

Abstract

We reported the aqueous chemical method to fabricate the well-aligned ZnO/Al 2 O 3 coreshell nanorod arrays (NRAs). The shell is composed of α-Al 2 O 3 nanocrystals in amorphous Al 2 O 3 layers. The photoluminescence (PL) measurements showed that the enhancement of nearband-edge emission in ZnO NRAs arrays due to the addition of Al 2 O 3 shell was observed. The Al 2 O 3 shell layer resulting in flatband effect near ZnO surface leads to a stronger overlap of the wavefunctions of electrons and holes in the ZnO core, further enhancing the NBE emission.

Probing Surface Band Bending of Surface-Engineered Metal Oxide Nanowires

“…A comparison of the SBB in different conditions is shown in Figure . It is noteworthy that the measured value is higher than the ideally predicted value based on the electron affinity rule (ϕ B = Φ M – χ S = 1.2 eV; Φ M is the work function of Au; χ S is the electron affinity of ZnO), ,, and the reported value (ϕ B = 0.7–1.2 eV) measured from typical IV and CV measurements. − Several concepts are invoked to the unusually high band bending for Au NP-decorated ZnO surfaces: (1) according to Schottky–Mott theory, electrons flow from the ZnO NWs to the Au NPs until both the Fermi levels line up, leading to the open-circuit nano-Schottky junctions; , (2) due to the ∼40% coverage rate of randomly distributed Au NPs, there are still oxygen molecules on the NW surfaces capturing electrons inside the NWs and altering the surface Fermi level of the NWs; ,− (3) spillover effect: Au NPs catalytically activate the formation of charged oxygen molecules, greatly increasing the quantity of oxygen adsorbates at the surface of ZnO NWs. ,, Due to the combination of the above effects, a greater amount of electrons drawn from the core of the Au NP-decorated NWs than the pristine NWs can be obtained, giving rise to the severe band bending. , Au NPs at ZnO surfaces creating Schottky junctions and enhancing the formation of the oxygen adsorbates amplify the separation of electron–hole pairs and further enhance the UV photogain of ZnO NW PDs . Additionally, we do note that the enhanced photogain by Au decoration could cause from either (1) the increased oxygen absorption on the surface due to the presence of Au , or (2) the surface plasmon-enhanced light absorption by the decorated Au nanopartices .…”

“…As a result, the adsorbates acting as acceptors reduce the free carrier density and thus deplete the surface electron states, leading to the formation of the space charge region and the band bending near the NW surface. It has been speculated that the SBB of NWs is related to NW diameter, doping level, surface roughness, and molecule adsorption , /metal decoration − on NW surfaces. As considering the diameter and the doping only, a qualitative description of the energy difference, ϕ, produced by SBB obeys the following equations: and where N d is the donor concentration (cm –3 ), d is the NW diameter, d crit is the critical diameter of the NWs, ε r is the dielectric constant of the NWs, and ε 0 is the permittivity of free space.…”

“…Therefore, the SBB of the oxygen-adsorbed ZnO NWs is estimated to be ∼1.54 ± 0.15 eV. Physically speaking, as oxygen molecules adsorb on the NW surfaces and form negative ions by capturing electrons inside the NWs [O 2 (g) + e – → O 2 – (ad)], a low conductivity depletion layer is formed near the surface, leading to more upward SBB. ,− The adsorbed oxygen molecules change upwardly the band bending at the nonpolar surfaces ( i.e. , m -plane or a -plane) of the c -axis-oriented ZnO NWs by 0.8 eV.…”

“…Nanowires (NWs) have attracted extensive attention due to their importance in fundamental research and potential applications in nanoscale electronics and optoelectronics. − With large surface-to-volume ratios and Debye lengths comparable to their diameters, the electronic and the optoelectronic properties of NWs are strongly affected by the surface effect via chemisorption/photodesorption ,,− and native surface defects/states . Accordingly, one-dimensional (1-D) nanostructures exhibit a superior sensitivity to light/chemical molecules, as compared to their thin film counterpart. − Moreover, the surface modification of NWs can enhance the sensitivity to light/chemical molecules due to an enhanced surface effect and can be achieved by surface functionalization, polymer coating, metal doping, and nanoparticle (NP) decoration. ,, …”

Concurrent Improvement in Photogain and Speed of a Metal Oxide Nanowire Photodetector through Enhancing Surface Band Bending via Incorporating a Nanoscale Heterojunction

“…An effective recovery time (τ eff = A 1 τ 1 + A 2 τ 2 ) of 133.5 s for the NiO NP-decorated NW is nearly 5 times shorter than that of the pristine one (657.4 s). The results reveal that the formation of NHJs at ZnO surfaces by employing NiO NPs improves the relaxation dynamics of ZnO NW by minimizing the interaction of photocarriers with oxygen molecules due to surface passivation. ,, …”