Tuning the work function of randomly oriented ZnO nanostructures by capping with faceted Au nanostructure and oxygen defects: enhanced field emission experiments and DFT studies

Abstract

The lowering of the work function (Φ) can lead to a better field emission (FE) behavior at lower threshold fields. We report on enhanced FE from randomly oriented and faceted Au-capped ZnO hetero-nanostructures (HNs) having more oxygen defects. Large-area arrays of non-aligned, faceted Au-capped ZnO HNs, such as nanowires (NWs) and triangular nanoflakes (TNFs) are grown using the chemical vapor deposition (CVD) method. Enhanced FE properties from the TNF sample resulted in a turn-on field as low as 0.52 V μm(-1) at a current density of 0.1 mA cm(-2) and a field enhancement factor (β) as high as ≈5.16 × 10(5). Under similar experimental conditions, drawing the same current density from an NW specimen needs a higher turn-on field (0.86 V μm(-1)) and to exhibit nearly four times less field enhancement factor compared to the TNFs samples. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements confirm the presence of more oxygen defects in the TNF samples compared to the NW samples. Kelvin probe force microscopy (KPFM) measurements show the average local work function to be 4.70 ± 0.1 eV for the TNF sample, which is ≈ 0.34 eV lower than the NW sample. Using density functional theory (DFT) calculations, the estimated Φ values are found to be 4.98 eV for ZnO(0001), 4.17 eV for Au(001)/ZnO(0001) and 3.91 eV for Au(001)/Ovac-ZnO(0001) surfaces. The DFT results are qualitatively in agreement with our experimental results. The presence of Au nanostructures on top of O-deficient and sharp-tipped TNFs results in enhanced FE performance following their reduced tunneling barrier via pinning of effective Φ.

Controllable Ag nanoparticle coated ZnO nanorod arrays on an alloy substrate with enhanced field emission performance

“…These multistage slope characteristics are also observed in other eld emitters, which might be attributed to the space charge effect, localized state and adsorbates. 14,[32][33][34][35][36][37][38] Generally, the eld enhancement factor exhibits a small value at a low applied eld and a large value at a high applied eld. However, in this work, FE phenomenon of the slope feature is just opposite, probably due to the surface states or shallow levels resulted from the intrinsic defects of the ZNAs.…”

High performance field emission of silicon carbide nanowires and their applications in flexible field emission displays

“…In all the ways of electron emission, this technique has attracted much attention due to its fundamental and technological requirements. 6 Generally, the field emission process is a theoretical model proposed by Fowler and Nordheim, which is based on quantum mechanical tunneling. 7,8 In order to make highly performance field emission emitters having large emission current, low turn-on and threshold fields, there are two challenges to enhance field emission properties of emitters like (i) lowered work function, (ii) increased field enhancement factor of the emitters.…”

Controlled Hetero‐Architectures of Au‐Nanoparticles‐Decorated ZnO Nanowires for Enhanced Field Electron Emission Displays

“…Where hω is the energy of the source used (≡ 23 eV), E sec is the onset of the secondary emission, and E FE is the Fermi edge. The sharply pointed tip appearance of ZnO nanowires resulted in the work‐function of 4.3 eV (i. e., Φ Au@ZnO ) after decorating Au nanoparticles for 24 hr, which is remarkably lower than reported for Al‐doped ZnO nanowires, Ag decorated ZnO nanorods, and Au faceted oxygen‐deficient ZnO nanostructures . This reduced value of work‐function might be one of the drives for enhancement in the FE performance of Au decorated ZnO nanowires.…”

“…The unoptimized coating of Au nanoparticles along the wire body has resulted in the E on of 2 V/μm, which was defined at a very low current density of 1 μA/cm 2 . Ghosh et al . have reported enhanced FE performance even after the larger work function values (i. e., 5.04‐4.7 eV) for randomly oriented and highly oxygen defective ZnO nanostructures capped at its tip with Au nanoparticles.…”