S-H Yang et al Current Applied Physics 9(2009)e180-e184 In this study, Zno nanostructures were grown on Ag-film From experiments, we found that no Zno nanostructures were deposited Si substrate by using a single tube experimental setup grown if the synthesis time was short. Even when the synthesis y vapor-phase transport synthesis at low-temperature. The crys- time was increased to 30 min, only a small quantity of Zno nano- tallinity and PL characteristics of the Zno nanostructures were structures was grown. However, when the synthesis time was in measured, and the field emission characteristics were investigated creased to 60 min, an abundance of Zno nanostructures was produced. Therefore, in this study the synthesis time was set at 60 min for nanostructure growth. From SEM analyses, we found that nanowhiskers, nanotips and well-aligned nanorods were obtained when the substrate temper A4 cm2 p-type(100)silicon wafer with a resistivity of 10 Q2 per atures were 620, 650 and 680C, respectively. Fig. 1 shows XRD square was used as substrate for the growth of Zno nanost of the nanowhiskers, nanotips and well-aligned nanorods Zno(99.995%)and graphite( 99.999%)powders were Diffraction peaks of the Al element were not observed in the XRD source materials. Silver (Ag. 99.995%) was adopted as a patterns, and the Zno nanostructures had a wurtzite structure with it has a lower melting point than au and cu of the most commonly lattice constants a=b=3.26A and c=5.22 A. For the nanowhis used catalysts for nanostructure growth. The vapor-phase trans- kers, the dominant growth plane was the(101) plane located at port process was used for Zno nanostructure synthesis. First, a 20=36. 2. The preferred growth plane was the(002)plane for 7 nm-thick Ag film was evaporated on Si substrate under a cham- the nanotips. Furthermore the xrd pattern of the well-aligned ber pressure of 10- Torr at an evaporation rate of 0.02 nm/s Sub- nanorods showed a sharp(002) diffraction peak, indicating that sequently, Zno and graphite powders mixed with weight ratio 1: 1 the Zno nanorods were grown with c-axis orientation in a direc- were put in an alumina boat and then placed at the center of a tube tion perpendicular to the substrate surface. Under this thermal furnace. The Ag-film-deposited Si substrate was placed 19 cm deposition condition, the enhancement in the growth rate in the nstream of the source powders simultaneously. The flow ratio vertical direction rather than the lateral direction was attributed of N2/O2 gas used for nanostructure growth was set at 7/2. The to the lowest surface energy density of the(002) plane in the temperatures of the source powders were set at 1050, 1100 and Zno crystal under rapid flow of the Zn vapor [18]. The values of full 1150C, and the temperatures of the substrates were fixed at width at half maximum of the(002)peak were 0. 26, 0.21 and 20, 650 and 680C. The synthesis times of the Zno nanostructures 0.16 for nanowhiskers, nanotips and well-aligned nanorods were 10, 30 and 60 min. After the synthesis was finished, dark gray respectively. The relative intensity of the(002)peak in the XRD nanostructures were grown on the substrate surface. patterns showed that the c-axis growth of the well-aligned nano he surface morphologies of the Zno nanostructures were ob rods was superior to that of the nanowhiskers and nanotips: con- served by scanning electron microscopy(SEM, Philips XL40 FE- sequently, the well-aligned nanorods were expected to show SEM). The crystal structure of the nanostructure was investigated better field emission characteristics. by X-ray diffraction(XRD, Siemens D5000)using Cu Ko radiation Fig 2a-f shows SEM photos of the Zno nanostructures grown nd a nickel filter. The PL spectrum of the Zno nanostructures at substrate temperature of 620-680C. The inclined nanowhis was measured using a Hitachi F-4500 fluorescence spectrophotom- kers were grown at 620C( Fig. 2a and b )As the substrate tem- eter with a 150 W Xe lamp at room temperature; the excitation perature increased to 650C, the growth rate in the vertical wavelength was set at 325 nm. The field emission characteristics direction relatively decreased and a nanostructure-merging phe- of the Zno nanostructures were evaluated in a vacuum chamber nomenon was developed Consequently the nanowhiskers would nder a pressure of 5 x 10- Torr at room temperature. Indium be buried, and tip-like nanostructures were obtained as shown in tin oxide(itoy-coated glass with a resistivity of 10 Q2 per square Fig. 2c and d [17, 19]. The Zno nanostructures with vertically was used as an anode. The distance between anode and emitter aligned morphology together with preferred growth orientation cathode(zno nanostructures)was 130 um. The field emission along [001 were grown at a substrate temperature of 680C rent-voltage characteristics were evaluated with a Keithley 2410 (Fig. 2e and f). From XRD and SEM analyses, it was shown that the Zno with better crystallinity resulted in a homogenous nano- rod array. Evidently, the morphology of the Zno nanostructures 3. Results and discussion was related to the source and substrate temperatures, which af ected the flow of Zn and Zno vapors and influenced both nu- The Zno nanostructures were grown by the vapor-phase trans- cleus density and stoichiometry of the Zno nanostructures. The ort mechanism using Zno and graphite powders as source mate- diameters of the emitter tips were approximately 40 and rials. During nanostructure growth, Zno powder was reduced by carbon and carbon monoxide (co) to Zn and ZnOx suboxide (x<1) with low melting point of approximately 419C[17]. Zn and ZnO vapors were then transferred by the processing gas of N2/0, to the low-temperature region to be condensed and formed nanodroplets. These nanodroplets recombined with oxygen to form nano-Zno as nuclei on the Ag-deposited Si substrate. Growth of the nanostructures began and continued as long as the reactant flow was maintained. Absorption-desorption on the droplet sur- ace, transportation of Zn atoms, condensation, and oxidation of Zn atoms at the growth front were the major processes in the growth of the Zno nanostructures. When the substrate tempera ture was increased the Ag nanograins on the substrate surface vere condensed into a larger cluster that led to the growth of thick and strong ZnO nanostructures, whereas a small Ag grain resulted in the growth of thin and slender zno nanostructures. The growth of the Zno nanostructures continued until the supply of Zn and Fig 1. XRD patterns of (a)nanowhiskers, (b)nanotips and(c)well-aligned ZnOx vapors was stopped.In this study, ZnO nanostructures were grown on Ag-film￾deposited Si substrate by using a single tube experimental setup by vapor-phase transport synthesis at low-temperature. The crys￾tallinity and PL characteristics of the ZnO nanostructures were measured, and the field emission characteristics were investigated as well. 2. Experimental A 4 cm2 p-type (1 0 0) silicon wafer with a resistivity of 10 X per square was used as substrate for the growth of ZnO nanostructures. ZnO (99.995%) and graphite (99.999%) powders were used as source materials. Silver (Ag, 99.995%) was adopted as a catalyst; it has a lower melting point than Au and Cu of the most commonly used catalysts for nanostructure growth. The vapor-phase trans￾port process was used for ZnO nanostructure synthesis. First, a 7 nm-thick Ag film was evaporated on Si substrate under a cham￾ber pressure of 107 Torr at an evaporation rate of 0.02 nm/s. Sub￾sequently, ZnO and graphite powders mixed with weight ratio 1:1 were put in an alumina boat and then placed at the center of a tube furnace. The Ag-film-deposited Si substrate was placed 19 cm downstream of the source powders simultaneously. The flow ratio of N2/O2 gas used for nanostructure growth was set at 7/2. The temperatures of the source powders were set at 1050, 1100 and 1150 C, and the temperatures of the substrates were fixed at 620, 650 and 680 C. The synthesis times of the ZnO nanostructures were 10, 30 and 60 min. After the synthesis was finished, dark gray nanostructures were grown on the substrate surface. The surface morphologies of the ZnO nanostructures were ob￾served by scanning electron microscopy (SEM, Philips XL40 FE￾SEM). The crystal structure of the nanostructure was investigated by X-ray diffraction (XRD, Siemens D5000) using Cu Ka radiation and a nickel filter. The PL spectrum of the ZnO nanostructures was measured using a Hitachi F-4500 fluorescence spectrophotom￾eter with a 150 W Xe lamp at room temperature; the excitation wavelength was set at 325 nm. The field emission characteristics of the ZnO nanostructures were evaluated in a vacuum chamber under a pressure of 5 106 Torr at room temperature. Indium tin oxide (ITO)-coated glass with a resistivity of 10 X per square was used as an anode. The distance between anode and emitter cathode (ZnO nanostructures) was 130 lm. The field emission cur￾rent–voltage characteristics were evaluated with a Keithley 2410 programmable power source. 3. Results and discussion The ZnO nanostructures were grown by the vapor-phase trans￾port mechanism using ZnO and graphite powders as source mate￾rials. During nanostructure growth, ZnO powder was reduced by carbon and carbon monoxide (CO) to Zn and ZnOx suboxide (x < 1) with low melting point of approximately 419 C [17]. Zn and ZnOx vapors were then transferred by the processing gas of N2/O2 to the low-temperature region to be condensed and formed nanodroplets. These nanodroplets recombined with oxygen to form nano-ZnO as nuclei on the Ag-deposited Si substrate. Growth of the nanostructures began and continued as long as the reactant flow was maintained. Absorption–desorption on the droplet sur￾face, transportation of Zn atoms, condensation, and oxidation of Zn atoms at the growth front were the major processes in the growth of the ZnO nanostructures. When the substrate tempera￾ture was increased, the Ag nanograins on the substrate surface were condensed into a larger cluster that led to the growth of thick and strong ZnO nanostructures, whereas a small Ag grain resulted in the growth of thin and slender ZnO nanostructures. The growth of the ZnO nanostructures continued until the supply of Zn and ZnOx vapors was stopped. From experiments, we found that no ZnO nanostructures were grown if the synthesis time was short. Even when the synthesis time was increased to 30 min, only a small quantity of ZnO nano￾structures was grown. However, when the synthesis time was in￾creased to 60 min, an abundance of ZnO nanostructures was produced. Therefore, in this study the synthesis time was set at 60 min for nanostructure growth. From SEM analyses, we found that nanowhiskers, nanotips and well-aligned nanorods were obtained when the substrate temper￾atures were 620, 650 and 680 C, respectively. Fig. 1 shows XRD patterns of the nanowhiskers, nanotips and well-aligned nanorods. Diffraction peaks of the Al element were not observed in the XRD patterns, and the ZnO nanostructures had a wurtzite structure with lattice constants a = b = 3.26 Å and c = 5.22 Å. For the nanowhis￾kers, the dominant growth plane was the (1 01) plane located at 2h = 36.2. The preferred growth plane was the (00 2) plane for the nanotips. Furthermore, the XRD pattern of the well-aligned nanorods showed a sharp (00 2) diffraction peak, indicating that the ZnO nanorods were grown with c-axis orientation in a direc￾tion perpendicular to the substrate surface. Under this thermal deposition condition, the enhancement in the growth rate in the vertical direction rather than the lateral direction was attributed to the lowest surface energy density of the (00 2) plane in the ZnO crystal under rapid flow of the Zn vapor [18]. The values of full width at half maximum of the (00 2) peak were 0.26, 0.21 and 0.16 for nanowhiskers, nanotips and well-aligned nanorods, respectively. The relative intensity of the (00 2) peak in the XRD patterns showed that the c-axis growth of the well-aligned nano￾rods was superior to that of the nanowhiskers and nanotips; con￾sequently, the well-aligned nanorods were expected to show better field emission characteristics. Fig. 2a–f shows SEM photos of the ZnO nanostructures grown at substrate temperature of 620–680 C. The inclined nanowhis￾kers were grown at 620 C (Fig. 2a and b.) As the substrate tem￾perature increased to 650 C, the growth rate in the vertical direction relatively decreased and a nanostructure-merging phe￾nomenon was developed. Consequently, the nanowhiskers would be buried, and tip-like nanostructures were obtained, as shown in Fig. 2c and d [17,19]. The ZnO nanostructures with vertically aligned morphology together with preferred growth orientation along [0 01] were grown at a substrate temperature of 680 C (Fig. 2e and f). From XRD and SEM analyses, it was shown that the ZnO with better crystallinity resulted in a homogenous nano￾rod array. Evidently, the morphology of the ZnO nanostructures was related to the source and substrate temperatures, which af￾fected the flow of Zn and ZnOx vapors and influenced both nu￾cleus density and stoichiometry of the ZnO nanostructures. The diameters of the emitter tips were approximately 40 and 30 40 50 60 (103) (110) (101) (102) (002) (100) Intensity (a.u.) c b 2θ (degree) a Fig. 1. XRD patterns of (a) nanowhiskers, (b) nanotips and (c) well-aligned nanorods. S.-H. Yang et al. / Current Applied Physics 9 (2009) e180–e184 e181