Abstract Four kinds of new one-dimensional nanostruc-tures, celery-shaped nanorods, needle-shaped nanorods,twist fold-shaped nanorods, and awl-shaped nanorods ofZnO, have been grown on single silicon substrates by anAu catalyst assisted thermal evaporation of ZnO and activecarbon powders. The morphology and structure of the pre-pared nanorods are determined on the basis of field-emissionscanning electron microscopy (FESEM) and x-ray diffrac-tion (XRD). The photoluminescence spectra (PL) analysisnoted that UV emission band is the band-to-band emissionpeak and the emission bands in the visible range are at-tributed to the oxygen vacancies, Zn interstitials, or impu-rities. The field-emission properties of four kinds of ZnOnanorods have been invested and the awl-shaped nanorodsof ZnO have preferable characteristics due to the smallestemitter radius on the nanoscale in the tip in comparisonwith other nanorods. The growth mechanism of the ZnOnanorods can be explained on the basis of the vapor–liquid–solid (VLS) processes.
1 Introduction
One-dimensional nanostructures such as wires, rods, andtubes have become the focus of intensive research owing to
B. Wang (�) · X. Jin · Z.B. Ouyang · P. XuShenzhen Key Lab of Micro-nano Photonic InformationTechnology, School of Electronic Science and Technology,Shenzhen University, Shenzhen, 518060, Chinae-mail: [email protected]: +86-755-26534624
B. WangShenzhen Key Laboratory of Sensor Technology, ShenzhenUniversity, Shenzhen, 518060, China
their unique applications in mesoscopic physics and fabri-cation of nanoscale devices [1]. ZnO is an important mem-ber in II–VI group semiconductors. It has profound appli-cations in optics, optoelectronics, sensors, and actuators dueto its semiconducting, piezoelectric, and pyroelectric prop-erties [2]. 1D ZnO nanostructures have attracted great inter-est in the past few years [2–4]. Extensive efforts have beenmade to synthesize ZnO nanostructures, such as nanowires[3], nanobelts [5], nanotubes [6], nanorings [7], etc., becausenanostructural morphology is one of the important factorsdetermining the properties [2]. Field emission is one of themost fascinating properties of semiconductor 1D nanoma-terials and has been extensively studied due to its impor-tance in view of the field emission flat display, x-ray sources,and microwave devices [8]. Field-emission properties of1D ZnO nanostructures have been reported, such as well-aligned ZnO nanowires grown at low temperature [9], ZnOnanowires on a tungsten substrate [10], ZnO nanoneedle ar-rays [11, 12], tetrapodlike ZnO nanostructures [13], gallium-doped ZnO nanofiber arrays [14], and ZnO nanowires grownon carbon cloth [15]. These works revealed excellent field-emission properties of the ZnO nanostructures and shedlight on potential applications in the near future [16].
In this paper, four new one-dimensional nanostruc-tures, celery-shaped nanorods, needle-shaped nanorods,twist fold-shaped nanorods and awl-shaped nanorods ofZnO, have been grown on single silicon substrates byan Au catalyst assisted carbothermal evaporation of ZnOand active carbon powders. The morphology and structureof the prepared nanorods are determined on the basis offield-emission scanning electron microscopy (FESEM) andx-ray diffraction (XRD). The photoluminescence spectra(PL) analysis noted that UV emission band is the band-to-band emission peak and the emission bands in the visiblerange are attributed to the oxygen vacancies, Zn intersti-
Fig. 1 (a), (b) The lowmagnified FESEM image ofZnO celery-shaped nanorods.(c), (d) The high magnifiedFESEM image of ZnOcelery-shaped nanorods
tials, or impurities. The field-emission properties of fourkinds of ZnO nanorods have been invested and the awl-shaped nanorods of ZnO have preferable characteristics dueto smallest emitter radius on the nanoscale in the tip in com-parison with other nanorods. The growth mechanism of theZnO nanorods can be explained on the basis of the vapor–liquid–solid (VLS) processes.
2 Experimental
The synthesis of these ZnO nanorods is minutely depictedas follows. The Au layer (about 10 nm in thickness) is de-posited on single silicon (001) substrates with an area of 5mm2 by sputtering. Note that we do not find that silver playsany catalyst role in the synthesis of ZnO nanorods in ourcase. The active carbon and ZnO powders (both 99.99 %)are mixed in a 1:1 weight ratio and placed into a small quartztube. The four Si substrates covered by Au are put near themixture of carbon and ZnO inside the small quartz tube.Then the small quartz tube is pulled into a large quartz tube,and together they are inserted in a horizontal tube electricfurnace. The whole system is evacuated by a vacuum pumpfor 20 minutes, then the Argon gas is guided into the systemat 50 sccm, and the pressure is kept at 350 Torr. Afterward,the system is rapidly heated up to 1030 °C from the roomtemperature and kept at the temperature for 1 hour. Finally,
the system is cooled down to the room temperature in sev-eral hours. When four substrates are taken out, we can seegray production on substrates. Field emission scanning elec-tron microscopy (FESEM) and x-ray diffraction (XRD) areemployed to identify the morphology and structure of thesynthesized productions. Note that we can easily repeat theexperimental results, suggesting that our method is flexibleand reproducible.
3 Results and discussion
Morphologies of the synthesized ZnO nanorods in four sub-strates are shown as Figs. 1–4. The celery-shaped nanorodsof ZnO are shown in Fig. 1. Figure 1(a), (b) is a low-magnified FESEM image. Figure 1(c), (d) is a high mag-nified FESEM image, from which the morphologies of thecelery-shaped nanorods of ZnO are clearly displayed andthe size of the celery-shaped nanorods of ZnO is about 300–400 nm. The needle-shaped nanorods of ZnO are shown inFig. 2. Figure 2(a), (b) is a low-magnified FESEM image.Figure 2(c), (d) is a high magnified FESEM image, fromwhich the morphologies of the needle-shaped nanorods ofZnO are clearly displayed. The average size of the needle-shaped nanorods of ZnO is about 150–200 nm, and the tipof the needle-shaped nanorods of ZnO is about 50 nm. Thetwist fold-shaped nanorods of ZnO are shown in Fig. 3. Fig-ure 3(a), (b) is a low-magnified FESEM image. Figure 3(c),
Photoluminescence and field emission of 1D ZnO nanorods fabricated by thermal evaporation 197
Fig. 2 (a), (b) The lowmagnified FESEM image ofZnO needle-shaped nanorods.(c), (d) The high magnifiedFESEM image of ZnOneedle-shaped nanorods
(d) is a high magnified FESEM image, from which the mor-phologies of the twist fold-shaped nanorods of ZnO areclearly displayed. The average size of the twist fold-shaped
nanorods of ZnO is about 100 nm, and the tip of the twistfold-shaped nanorods of ZnO is about 25 nm. The awl-shaped nanorods of ZnO are shown in Fig. 4. Figure 4(a), (b)
198 B. Wang et al.
Fig. 4 (a), (b) The lowmagnified FESEM image ofZnO awl-shaped nanorods. (c),(d) The high magnified FESEMimage of ZnO awl-shapednanorods
Fig. 5 The XRD pattern of four kinds of ZnO nanorods
is a low-magnified FESEM image. Figure 4(c), (d) is a highmagnified FESEM image, from which the morphologies ofthe awl-shaped nanorods of ZnO are clearly displayed. Theaverage size of the awl-shaped nanorods of ZnO is about80–100 nm, and the tip of the awl-shaped nanorods of ZnOis about 10 nm.
The x-ray diffraction (XRD) measurement is carried outto identify the crystalline structure of the four kinds of ZnOnanorods and Fig. 5 shows the XRD patterns. Four ZnOsamples show a hierarchical ZnO phase with (100), (002),(101), (102), (110), and (103) diffraction peaks. The XRD
Fig. 6 Room PL spectrums of four kinds of ZnO nanorods
patterns of the samples indicate the crystalline structural pa-rameters can be indexed to the hexagonal wurtzite struc-ture of ZnO with lattice constants of a = 0.325 nm andc = 0.521 nm (JCPDS 36-1451).
Photoluminescence of the obtained ZnO nanorods in twosamples is investigated at room temperature and the result isshown in Fig. 6. The PL spectrum of the ZnO celery-shapednanorods contains UV emission band centered at 375 nm.The PL spectrum of the ZnO needle-shaped nanorods, Twistfold-shaped nanorods and awl-shaped nanorods, respec-tively, contain relatively weak UV emission band centered
Photoluminescence and field emission of 1D ZnO nanorods fabricated by thermal evaporation 199
Fig. 7 (a) Field emission current density of the samples as a functionof the electronic field. (b) The corresponding Fowler–Nordheim plotof the field emission current densities
at 375 nm, 379 nm, and 380 nm as well as relatively strongand wide visible emission band. The UV emission is orig-inated from excitonic recombination corresponding to thenear band-gap emission of ZnO, while the emission bandsin the visible range are due to the recombination of photo-generated holes with singly ionized charge states in intrinsicdefects such as oxygen vacancies, Zn interstitials, or impu-rities [17].
The field emission (FE) measurements of the ZnOnanorods are carried out in an ultrahigh vacuum chamberat a pressure of 10−9 Torr at room temperature with the dis-tance between the anode and cathode about 300 µm. FromFig. 7(a), we can see that the turn-on electric field (Eto) ofZnO celery-shaped nanorods, needle-shaped nanorods, twistfold-shaped nanorods, and awl-shaped nanorods, which isdefined as the field required to produce a current den-sity of 10 µA/cm2, is respectively 7.6 V/um, 7.2 V/um,6.9 V/um, and 5.2 V/um. All of the applied electric field ofthe ZnO celery-shaped nanorods, needle-shaped nanorods,twist fold-shaped nanorods, and awl-shaped nanorods is
12 V/µm when the current density of them respectivelyreaches 0.31 mA/cm2, 0.4 mA/cm2, 0.8 mA/cm2, and1.03 mA/cm2. According to the Fowler–Nordheim (FN) the-ory [18], the relationship between the current density J andthe applied field strength (E = V/d) can be depicted as
J = (Aβ2E2/Φ
)exp
(−BΦ3/2/βE)
(1)
The formula can be changed:
ln(J/E2) = ln
(Aβ2/Φ
) − BΦ3/2/βE (2)
where A = 1.54 × 10−6 A eV V−2, B = 6.83 ×103 eV−3/2 V µm−1, β is the field-enhancement factor, andΦ is the work function of an emitting material. The Fowler–Nordheim (FN) plot of the samples in Fig. 7(b) is linearby and large, which indicates that the FN theory well con-forms to the field-emission behavior of the samples. As-suming the work function of the ZnO is 5.3 eV [19], β ofthe ZnO celery-shaped nanorods, needle-shaped nanorods,twist fold-shaped nanorods, and awl-shaped nanorods is re-spectively estimated to be 1395, 1436, 2500, and 3333. Ac-cording to the FE mechanism, the field-emission current ismainly produced from the tip of the materials so as to de-duce that the field-emission current is mainly produced fromthe tip of the nanorods in this paper. Comparing with otherZnO nanorods, the ZnO awl-shaped nanorods have the low-est turn-on field, highest β , and largest emission efficiencyowing to the smallest emitter radius on the nanoscale in thetip.
In experimental, Au as catalysts is usually used to assistthe ZnO powders synthesis upon carbothermal evaporationof ZnO and active carbon according to VLS processes [20].ZnO powders first react with active carbon, which results inthe production of Zn and CO2 in the high temperature regionin the furnace. Subsequently, Zn reacts with relatively littleO2 so as to produce ZnO vapor. Zn falls on the substratewith relatively low temperature and forms Zn–Au alloyeddroplets by reacting with the Au particles [21, 22]. Simul-taneously, these alloying droplets can provide the energeti-cally favored sites for the adsorption of the ZnO vapor [23].The continuous dissolution of ZnO leads to a supersaturatedsolution [24]. Finally, ZnO nanorods grow by the precipita-tion of ZnO from the supersaturated droplets. Additionally,the different distances of the samples from the source couldlead to the different substrate temperatures, which is one ofthe main reasons causing different synthesized morpholo-gies of ZnO nanorods. Naturally, the different degrees of su-persaturation of the alloying droplets plays an important rolein the formation of different nanostructures due to differentsubstrate temperatures [20].
4 Conclusion
In summary, four kinds of new one-dimensional nanos-tructures, celery-shaped nanorods, needle-shaped nanorods,
200 B. Wang et al.
twist fold-shaped nanorods, and awl-shaped nanorods ofZnO, have been grown on single silicon substrates by anAu catalyst assisted carbothermal evaporation of ZnO andactive carbon powders. The morphology and structure ofthe prepared nanorods are determined on the basis of field-emission scanning electron microscopy (FESEM) and x-ray diffraction (XRD). The photoluminescence spectra (PL)analysis noted that UV emission band is the band-to-bandemission peak and the emission bands in the visible rangeare attributed to the oxygen vacancies, Zn interstitials, or im-purities. The field-emission properties of four kinds of ZnOnanorods have been invested and the awl-shaped nanorodsof ZnO have preferable characteristics due to the smallestemitter radius on the nanoscale in the tip in comparisonwith other nanorods. The growth mechanism of the ZnOnanorods can be explained on the basis of the vapor–liquid–solid (VLS) processes.
Acknowledgements Project 50902097 is supported by the Na-tional Natural Science Foundation of China, Shenzhen Key Labora-tory of Micro-nano Photonic Information Technology Open Project(MN201107), Shenzhen Key Laboratory of Sensor Technology OpenProject (SST201102), and the Shenzhen Basic Research EmphasisProject of Three Industry (JC201104210013A).
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