Photoluminescence of ZnO Nanowires: A Review

Abstract

One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy-cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands.

Keywords: VLS; ZnO nanowires; emission mechanism; hydrothermal; photoluminescence.

Figure 2

(a) Basic principle of the vapor-liquid-solid (VLS) growth mechanism of ZnO NWs using gold as a metal catalyst. (b) 45° side-view SEM image of hexagonally ordered ZnO nanorod grown by VLS method on patterned Au-covered substrates. (c) Schematic representation of the growth of nanowires by the hydrothermal technique [60]. Copyright 2015 Elsevier.

Figure 3

(a) Band structure splitting of the hexagonal ZnO structure caused by the crystal field and spin-orbit splitting [63]. Copyright 2004 John Wiley and Sons (b) Refractive index (n) and extinction coefficient (k) of bulk ZnO obtained by the envelope method [84]. Copyright 2012 AIP Publishing. (c) Photoluminescence spectrum of bulk ZnO obtained with He-Cd excitation. Excitonic, donor-acceptor pair (DA_xP), and longitudinal optical phonon replicas (LO) are indicated [33]. Copyright 2004 John Wiley and Sons.

Figure 8

A summary of different defect-related energy levels in the ZnO bandgap reported in the literature by different groups: (a) [129]. (b). [145]. (c). [133]. (d). [147]. (e). [148]. (f). [138] (g) [149]. (h) [146]. and (i). [150]. A color code has been used to point out the different defect states, e.g., the Zn defect is represented by orange lines, oxygen defects are presented by a dark violet color, the Li defect is presented as a yellow bar, the hydrogen level is represented by a light-blue bar, and the complex state attributed to the combination of oxygen and zinc defect states is represented by a green color.

Figure 10

Proposed deep level transition in the different colors indicating the defect involved (a) [132], (b) [157], (c) [154], (d) [165], (e) [166], (f) [134], (g) [130], (h) [168,169], (i) [145,146,149].

Figure 1

(a) ZnO unit cell with wurtzite structure. (b) Various crystal planes of the ZnO wurtzite structure [42].

Figure 4

The exciton-bound transition energy reported in [66] at 6 K. D⁰X (denotes exciton bound to neutral donors), A⁰X (exciton bounded to a neutral acceptor), TES (two-electron satellite transition), DAP (donor-acceptor pair transitions), and DAP-LO (first-order longitudinal optical). Quenching effect of the D⁰X/I₂ emission as the temperature is increased from 6 to 180 K.

Figure 5

Low-temperature time-resolved photoluminescence (TRPL) spectra at 3.365 eV for three ZnO NWs arrays with different diameters. A diameter dependence over the exciton lifetime was observed [94]. Copyright 2010 AIP Publishing.

Figure 6

(a) Illustration of different intrinsic point defects in the ZnO lattice. Oxygen vacancy (V_O), zinc vacancy (V_Zn), oxygen interstitial (O_i), and zinc interstitial (Zn_i) in different charge configurations are presented [125]. Copyright 2006 Royal Society of Chemistry (b) Schematic band diagram of the DLE emissions in ZnO based on the full-potential linear muffin-tin orbital method and the reported data as described in references [126,127,128,129,130,131,132,133,134,135]. Also, oxygen vacancies situated 1.65 eV below the conduction band are denoted to contribute to the red emission [130].

Figure 7

Visible PL spectra of a single ZnO nanowire measured at different temperatures. Three characteristic emission bands (yellow, green, and blue) were observed [113]. Copyright 2008 Elsevier.

Figure 9

(a) Secondary electron micrograph of the ZnO thin film, (b) CL monochromatic map taken under ZnO NBE emission, (c) Green emission CL monochromatic map at ZnO; all the scale bars are 500 nm long [70]. (d) STEM image of nanowire amplitude maps of (e) near-band-edge emission, (f) surface emission, (g) defect emission 1, and (h) defect emission 2 deconvolved components. (i) Counts integrated across all energies; for further permissions related to the material (d–i), please refer to ACS https://pubs.acs.org/doi/10.1021/acs.jpclett.8b03286 [76].