Synthesis and Field Emission Properties of Hierarchical ZnONanostructures

Three novel kinds of hierarchical ZnO nanostructures: nanocombs nanoscrewdrivers and nanonails, have been synthesized in large quantities via a simple thermal evaporation process in the same run of growth in different regions of the quartz tube. The morphologies could be well controlled by adjusting the distances between the source materials and the substrates. These ZnO products were investigated by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The field emission properties of the ZnO nanostructures were investigated. These hierarchical ZnO nanostructures may be attractive building blocks for field emission microelectronic devices and other devices.


Introduction
Nanoscale one-dimensional semiconducting materials have attracted much attention due to their importance in understanding the fundamental properties at low dimensionality, as well as for their applications in nanodevices.As an important II-VI semiconductor, zinc oxide (ZnO) is of considerable interest due to its distinguished performance in electronics, optics, and mechanics.ZnO with a band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature is a key technological material [1,2].In particular, it is expected that they possess a good electron field emission (FE) property due to their high aspect ratio and small tip radius of curvature.Intensive investigation has been carried out over the last several decades on field emission due to numerous applications such as flat-panel displays, microwave amplifiers, and vacuum microelectronic devices.Today, field-emission characteristics of carbon nanotubes (CNTs) have been extensively investigated, and there have been a large number of laboratory work reported [3][4][5].However, thermal stability and ambient insensitivity are as important as the geometric factors to the operation of field emitters [4].ZnO exhibits a high melting point, excellent chemical stability, and negative electron affinity.Therefore, a ZnO-based 1D structure could be an appropriate alternative to CNTs for field emission displays.The field-emission properties for ZnO nanowire arrays [5], tetrapod-like ZnO nanostructures [6], ZnO nanoneedles [7,8], and cuboid zinc oxide nanorods [9] have been attempted recently.In this paper, the field-emission properties for three kinds of novel ZnO nanostructures: nanocombs, nanoscrewdrivers, and nanonails were reported.It must be noted that the structure determines the properties of a given nanomaterial with respect to its specific application, so the study on fieldemission behaviors of them is of great interest both from scientific and technological perspectives.
In this paper, using a simple physical evaporation method, we synthesized some ZnO nanostructures: ZnO nanocombs, nanoscrewdrivers, and ZnO nanonails highly aligned as flowers with single-crystal peculiar hexagon nanonails.The morphologies could be well controlled by adjusting the deposition position.

Experimental Section
The hierarchical ZnO nanostructures were grown at the atmospheric pressure by thermal evaporation of the mixture of ZnO powders and graphite powder (both 99.9%) in a twoheating-zone furnace system.The distance between the twoheating-zone was about 50 cm.three n-type (111) Si wafers were used as the substrates for deposition.The furnace was set to the desired temperature (750 • C-1100 • C) and a closedend small quartz tube (60 cm in length, 2 cm in diameter), containing the source material (graphite and ZnO powders with Moore ratio 1 : 1) and Si substrates, were inserted.The source materials were placed in the high temperature heating zone (upstream), and the n-type (111) Si wafers were placed ordinal in the second heating zone (downstream) to collect the products.After the system was purged with N 2 for 10 min, the system was heated to the desired temperature at a rate of 10 • C/min and kept at that temperature with the carrier gas of nitrogen flow rate of 3 L/min.When the substrates were taken out from the quartz tube after an hour, we could see gray-white wool-like products on substrates.The resulting samples were collected for characterization and measurements.
The morphologies were analyzed using scanning electron microscopy (SEM), which looks like nanocombs, nanoscrewdrivers, and nanonails, respectively.Their crystal structure features were characterized using X-ray diffraction (XRD).Furthermore, Raman spectra and field emission properties of them were investigated.Field emission measurements were performed with diode structure in a vacuum chamber under a pressure of 5 × 10 −5 Pa.The synthesized nanocombs nanoscrewdrivers and nanonails (as a cathode) were separated from a phosphor/ITO/glass anode by two Teflon spacers, respectively.Through a window of the vacuum chamber, the distribution of the field emission sites on fluorescent anode was recorded with a camera.Meanwhile, the emission current versus voltage curve was measured with standard electronic instruments after the bias voltage sweeps were conducted several times for the emitter to reach a stable emission for each given applied field.

Results and Discussion
After thermal evaporation, several self-organized structures were obtained.Figures show typical SEM images with different magnifications of the ZnO products obtained at different regions of the quartz tube.3.1.Nanocombs of ZnO. Figure 1(a) shows the SEM images of the ribbon/comb structures deposited on Si substrates 1.For each comb, the nanorod branches resembling comb teeth are distributed on one side of the belt-like stem.The diameters of the comb teeth range from 80 nm to 200 nm and their lengths are evenly 10 μm.Some of the neighboring teeth have been merged into larger ones, as shown in high magnified Figure 1(b).Noneven teeth are observed in the premature combs, which are possibly due to differences in growth temperature.Comb-like structure is believed to be the result of rapid crystallization at large super saturation, analogous to the appearance of dendrites crystal in bulk crystal growth [10].The formation of our single-sided ZnO nanocombs may be reasonably explained by the surface polarity-induced asymmetric growth proposed by Wang et al. [11].

Nanonails of ZnO.
In our experiments, it was found that the substrate 3 is covered with a large number of nanonails.These hierarchical structures have been found as shown in Figure 3(a), which look like flowers in which each coaxial rod radically grows from one center.The nanonails are welldistributed as shown in Figure 3   The Raman spectra of the three different ZnO nanostructures are shown in Figure 5. Raman properties of ZnO crystals measurements were carried out at room temperature in a backscattering geometry.In results, the three spectra have similar shapes drawn in Figure 5.The peaks of ZnO at ∼331, ∼440, and ∼1000 cm −1 were clearly observed in Figure 5, with the A1 (TO), E2 (high), and E1 (2LO) vibration modes of ZnO, respectively [12,13].The formation of hierarchical ZnO nanostructures indicates that complicated reactions and self-assembled mechanism occurred in the present experiments.Though it is still not completely clear what exactly happened during the growth, undoubtedly deposition temperature plays an important role for these complicated structures as in the above results.
In our experiments, field emission properties of these products were investigated.The turn-on and threshold field, defined as the electric field required to produce a current density of 1 μA/cm 2 and 0.1 mA/cm 2 , respectively.Current density versus electric field (J-E) for them are shown in Figure 6 with the same turn-on field of 8 V/μm, but the screwdriver-like hexagonal coaxial rods with more keenedged nanotips have the lowest threshold field of 12.6 V/μm, which is attributed to high aspect ratio of the tip of the screwdriver-like ZnO.Other experimental factors being equal, the nanostructures with sharper tips are easier to emit electrons.
Fluorescence screen images of the field emission of the hierarchical ZnO nanostructures were shown Figure 7.

Conclusion
In summary, we fabricated ZnO nanocombs, nanoscrewdrivers, and ZnO nanonails structures by a simple method of carbon reduction of ZnO in a tube furnace at atmospheric pressure.Their morphologies are affected by the position of substrate, but these ZnO nanostructures have similar XRD pattern and Raman spectra properties.

2 )Figure 6 :
Figure 6: FE current density versus electric field (J-E) for the three kinds of hierarchical ZnO nanostructures: nanoscrewdrivers, nanocombs, and nanonails (from up to down).

Figure 2
(a) on substrate 2 is an interesting configuration, in which the hexagonal coaxial rods with two segments of different diameters stacking each other.The bigger rods look like screwdrivers with the diameter about 2 μm and the diameter of the smaller rod is about 200 nm.The length of each pod is about 20 μm.From the hexagonal symmetry of the coaxial rod, its growth direction is [0001].A high-magnified SEM image is shown in Figure2(b).
(a).The high-magnified SEM images are shown in Figure3(b) corresponding.And

Figure 7 :
Figure 7: Fluorescence screen images of the field emission of the hierarchical ZnO Nanostructures.(a) is for ZnO nanonails; (b) is for ZnO nanocombs; (c) is for ZnO nanoscrewdrivers.

Figure 3 (
Figure 3(b) shows the high-magnification SEM images like nail morphologies, which indicates clear that the nanostructures are composed of perfect hexagonal prism caps and prism shafts connected with smaller diameter bottom.From the hexagonal symmetry of the nanonail, its growth direction is [0001].Each nanonail structure is several tens of nanometers in tail diameter with a perfect hexagonalshaped cap about 0.5-1 μm in diameter and 10-20 μm in length.Further studies are necessary to understand the morphologies.These nanostructures of ZnO have similar XRD pattern as shown in Figure 4.All of the diffraction peaks can be indexed to the Wurtzite-structured (hexagonal) ZnO (JCPDS no.80-0075).No diffraction peaks from other impurities have been detected.The strong (002) peak of ZnO wurtzite structure and much weaker (100) and (101) peaks are due to the imperfect vertical growth of the nanostructures.The exact growth mechanism undoubtedly needs further studies.The Raman spectra of the three different ZnO nanostructures are shown in Figure5.Raman properties of ZnO crystals measurements were carried out at room temperature in a backscattering geometry.In results, the three spectra have similar shapes drawn in Figure5.The peaks of ZnO at ∼331, ∼440, and ∼1000 cm −1 were clearly observed in Figure5, with the A1 (TO), E2 (high), and E1 (2LO) vibration modes of ZnO, respectively[12,13].The formation of hierarchical ZnO nanostructures indicates that complicated reactions and self-assembled mechanism occurred in the present experiments.Though it is still not completely clear what exactly happened during the growth, undoubtedly deposition temperature plays an important role for these complicated structures as in the above results.In our experiments, field emission properties of these products were investigated.The turn-on and threshold field, defined as the electric field required to produce a current density of 1 μA/cm 2 and 0.1 mA/cm 2 , respectively.Current density versus electric field (J-E) for them are shown in Figure6with the same turn-on field of 8 V/μm, but the screwdriver-like hexagonal coaxial rods with more keenedged nanotips have the lowest threshold field of 12.6 V/μm, which is attributed to high aspect ratio of the tip of

Figure 7 (
a) is for ZnO nanonails, Figure 7(b) is for ZnO nanocombs, and Figure 7(c) is for ZnO nanoscrewdrivers.The homogeneous emission and high electron emission spot number per unit area of the nanoscrewdrivers may be attributed to a large number of emitters on the surface of the nanoscrewdrivers contributing to emission.