Six-Fold-Symmetrical Hierarchical ZnO Nanostructure Arrays: Synthesis, Characterization, and Field Emission Properties

Abstract

Herein we present the successful synthesis of aligned arrays of single-crystalline, 6-fold-symmetrical, hierarchical ZnO nanostructures via a two-step vapor-phase transport method. The X-ray diffraction and transmission electron microscopy results illustrate that the backbones of the as-grown product have excellent orientation along the [001] direction, and the branches epitaxially grow on the backbone along the AE100ae direction. At room temperature, the 6-fold-symmetrical hierarchical ZnO nanostructure arrays exhibit a very strong green luminescence centered at 515 nm. The field emission measurements show that they have a turn-on field of 5.6 V/μm and field-enhancement factor of 1351, which indicate their potential application in field emission nanodevices.Since the discovery of carbon nanotubes, 1 enormous efforts have been devoted to researching one-dimensional (1D) nanostructures due to their exciting physical properties and wide applications in electronic, optoelectronic, and electrochemical nanodevices. [2][3][4][5][6][7][8] Recently, the self-assembly of 1D nanostructures into hierarchical nanoarchitectures as well as nanoheterostructures has become the focus of intensive attention because they may exhibit some novel properties resulting from their unique multidimensional shape and the combination of both micrometer-scale and nanometer-scale building blocks. 9-19 Furthermore, they can be used as building units in nanodevices, which is crucial for the development of future nanodevices and nanotechnology. Up to now, the design and synthesis of hierarchical nanostructures with a controlled crystallographic direction and assembling them into arrays remain one of the main challenges to realize functional nanodevices.As one of the most important multifunctional semiconductors, ZnO with a wide band gap energy of 3.37 eV and a large exciton binding energy of 60 meV at room temperature has a wide range of applications in electronic, optoelectronic, electrochemical, and electromechanical devices, such as ultraviolet (UV) lasers, 6,8 light-emitting diodes, 20 field emission devices, 21 solar cells, 22 and piezo-nanogenerators. 23 In the past few years, hierarchical ZnO micro-and nanostructures with different morphologies were synthesized via gas-phase and solution-phase approaches. 10,[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] Nevertheless, how to assemble the as-prepared hierarchical ZnO nanostructures into arrays with excellent orientation has remained a challenge. In this study, we report the successful fabrication of aligned arrays of hierarchical ZnO nanostructures via a two-step vapor-phase transport method. The as-synthesized product has essential features of nanoscaled, 6-fold-symmetrical hierarchy and single crystallinity. More important, the backbones of the product have excellent orientation along the [001] direction, and the branches epitaxially grow on the backbone along the AE100ae direction. At room temperature, the photoluminescence (PL) results show that the 6-fold-symmetri...

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

“…Only a minute increase in the green to UV peak intensity ratio was observed due to the volume expansion of the ZnO NPs by increasing the ZnO growth time from 0.5 to 1 h. The great increase in the surface area of ZnO by the hierarchical growth of ZnO NRs from the core-shell NWs resulted in the development of the green emission. Similar observation was reported by Wang et al [52] in the comparison of PL properties of hierarchical grown ZnO NWs with ZnO NWs. Furthermore, our initial growth of ZnO NRs shows significant amount of kinking and bending structures.…”

“…Although the origin of the green emission remains questionable, it is generally attributed to the transition of donor-acceptor pair related to the oxygen vacancies [14-16,50-52]. A number of studies suggested that the green emission is a surface-related process, where the radiative recombination of electrons with photogenerated holes trapped in singly ionized oxygen vacancies takes place in the surface structures of the ZnO [51,52]. In other words, this defect emission can be enhanced due to the large surface area of ZnO nanostructures under oxygen deficient conditions.…”

Controllable Low Temperature Vapor-Solid Growth and Hexagonal Disk Enhanced Field Emission Property of ZnO Nanorod Arrays and Hexagonal Nanodisk Networks

“…This material can display excellent field emission (FE) properties. 34 The FE property of nanorod/nanowire arrays, 35−39 tetrapods, 40 nanocone arrays, 41 nanoneedles, 42 and hierarchical nanostructures constructed with nanowires 43 of ZnO has been investigated. However, the investigation on FE property of two-dimensional ZnO nanostructures is quite rare.…”

Synthesis and field-emission properties of novel hierarchical ZnO hexagonal towers

“…[3]. Many reports were focused on the development of nanostructures with different morphology or shape, such as ZnS branched architectures [4], hierarchical single-crystalline ˇ-SiC nanoarchitectures [5], ZnS tetrapod tree-like heterostructures [6], carbon-in-Al 4 C 3 nanowires [7] and sixfold-symmetrical hierarchical ZnO [8].…”

“…Focus more on FE properties from ZnO has been increasing due to a wide range of morphologies and its chemical stability good enough. Many studies have already been performed regarding the synthesis and FE characteristics of ZnO nanostructures [8][9][10][11][12][13], concluding that one-dimensional hierarchical structure is one of the ideal morphologies for achieving high FE current density at a low electric field [11]. However, understanding of hierarchical ZnO micro-/nano-structures and their associated FE properties is limited, although such problem is very important to their potential applications.…”