Current Applied Physics 9(2009)e180-e184 Contents lists available at Science Direct Current Applied physics ELSEVIER journalhomepagewww.elsevier.com/locate/cap Characterizations of Ag-catalyzed Zno nanostructures prepared by vapor-solid mechanism Su-Hua Yang, Pao-Chih Chen, Sheng-Yu Hong Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Kaohsiung 807, Taiwan, ROC ARTICLE INFO A BSTRACT (Zno) nanostructures eceived 11 July 2008 grown on Ag-deposited Si substrate by using the vapor-solid(vs)mechanism. The morphology of the ceived in revised form 21 October 2008 Zno nanostructures was related to source and substrate temperatures. Nanowhiskers, nanotips and Available online 13 march 2009 well-aligned nanorods were grown at substrate temperatures of 620, 650 and 680C, respectively. A trong bluish-green photoluminescence(PL)indicated that the nanostructures had many surface defects correlated with oxygen vacancies. Field emission analyses showed that the turn-on fields of Zno nano- hikers, nanotips and well-aligned nanorods were 4.5, 5.8 and 6.1 V/um, respectively. The high emitter density and high aspect ratio of the well-aligned nanorods resulted in an enhancement in emission cu rent density. Furthermore, the field emission stability of the well-aligned nanorods was better than that of the nanowhiskers and nanotips. The emission current densities of the Zno nanowhiskers, nanotips and well-aligned nanorods, measured after continuous operation for 6 h, were approximately 2.05, 4.68 and photoluminescence 20.5 HA/cm, respectively Field emission e 2009 Elsevier B.V. All rights reserved. Aspect ratio 1 Introduction FEDs. As regards the emitter materials, although carbon is usually used in the synthesis of nanotubes, zinc oxide(zno) is another Field emission displays( FEDs)are a new and promising tech- alternative Zno is a wide-bandgap(3.37 ev) semiconductor, with nology for flat panel displays FEDs use less power consumption an exciton binding energy of approximately 60 mev. It offers the than plasma display panels(PDP)and liquid crystal displays possibility to fabricate nanostructures by self-organization with a (LCD). A FED usually comprises a cathode plane(field emitter ar- high degree of c-axis orientation. One-dimensional Zno nanostruc ray, FEA)at the rear, a narrow vacuum gap in the middle, and an tures have been synthesized successfully by high-temperature va- anode plane (phosphor screen) that forms the faceplate on which por-solid (VS) growth [10-13, electrochemical deposition [14, he image is formed [ 1, 2). The FEA configuration is arguably the hydrothermal synthesis [15. and chemical vapor deposition most complicated structure in the FED, where electron conducting (CVD)[16 Nanowhiskers were grown by Wagner [12]on Au-cat- tracks are first arranged on a substrate. this structure must have lyzed Si substrate at 950C via a vapor-liquid-solid (vls )mech- the capabilities of emitting electrons at low electric fields and anism. He presented the concept of the VLs mechanism and offering sufficient current densities to generate bright fluorescence referred the growth of a small globule at the tip of a nanowhisker from the associated phosphor screen. Conventional FEDs are based to the VLs mechanism. This concept was adopted by Chen [10, 11] n microtip technology: however, the drawback of these FEDs is and Li [13] to demonstrate the growth mechanisms of nanostruc- that their dimensions cannot be increased to obtain a widescreen tures which were synthesized at high-temperature by using a hor- display. izontal double-tube experimental setup. They found that the For the fea, carbon nanotubes were first fabricated in 1991 3. nanostructures grown with a high-temperature VS or VLS process ince then, nanotip technology has revolutionized the develop- depended on the growth temperature, source vapor concentration, ment of FEDs. One-dimensional nanostructures, such as nanotubes, and preparation procedure no matter whether a metal catalyst or a nanowires and nanorods[4-9], have been studied actively because film was pre-deposited on the substrate before nanostructure of their exclusive physical properties and potential application in growth. To date, nanostructures have been synthesized and their photoluminescence( PL)characteristic has been measured by many Corresponding author. Tel: +8867 381 4526: fax: +8867 381 1182 researchers. However. their field emission characteristics have rarely been reported 1567-1739S-see front matter e 2009 Elsevier B V. All rights reserved
Characterizations of Ag-catalyzed ZnO nanostructures prepared by vapor–solid mechanism Su-Hua Yang *, Pao-Chih Chen, Sheng-Yu Hong Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Kaohsiung 807, Taiwan, ROC article info Article history: Received 11 July 2008 Received in revised form 21 October 2008 Accepted 1 December 2008 Available online 13 March 2009 PACS: 61.46.Km 78.67.Bf Keywords: Nanostructure Photoluminescence Field emission Aspect ratio abstract This paper presents the synthesis and field emission characteristics of zinc oxide (ZnO) nanostructures grown on Ag-deposited Si substrate by using the vapor–solid (VS) mechanism. The morphology of the ZnO nanostructures was related to source and substrate temperatures. Nanowhiskers, nanotips and well-aligned nanorods were grown at substrate temperatures of 620, 650 and 680 C, respectively. A strong bluish-green photoluminescence (PL) indicated that the nanostructures had many surface defects correlated with oxygen vacancies. Field emission analyses showed that the turn-on fields of ZnO nanowhiskers, nanotips and well-aligned nanorods were 4.5, 5.8 and 6.1 V/lm, respectively. The high emitter density and high aspect ratio of the well-aligned nanorods resulted in an enhancement in emission current density. Furthermore, the field emission stability of the well-aligned nanorods was better than that of the nanowhiskers and nanotips. The emission current densities of the ZnO nanowhiskers, nanotips and well-aligned nanorods, measured after continuous operation for 6 h, were approximately 2.05, 4.68 and 20.5 lA/cm2 , respectively. 2009 Elsevier B.V. All rights reserved. 1. Introduction Field emission displays (FEDs) are a new and promising technology for flat panel displays. FEDs use less power consumption than plasma display panels (PDP) and liquid crystal displays (LCD). A FED usually comprises a cathode plane (field emitter array, FEA) at the rear, a narrow vacuum gap in the middle, and an anode plane (phosphor screen) that forms the faceplate on which the image is formed [1,2]. The FEA configuration is arguably the most complicated structure in the FED, where electron conducting tracks are first arranged on a substrate. This structure must have the capabilities of emitting electrons at low electric fields and offering sufficient current densities to generate bright fluorescence from the associated phosphor screen. Conventional FEDs are based on microtip technology; however, the drawback of these FEDs is that their dimensions cannot be increased to obtain a widescreen display. For the FEA, carbon nanotubes were first fabricated in 1991 [3]. Since then, nanotip technology has revolutionized the development of FEDs. One-dimensional nanostructures, such as nanotubes, nanowires and nanorods [4–9], have been studied actively because of their exclusive physical properties and potential application in FEDs. As regards the emitter materials, although carbon is usually used in the synthesis of nanotubes, zinc oxide (ZnO) is another alternative. ZnO is a wide-bandgap (3.37 eV) semiconductor, with an exciton binding energy of approximately 60 meV. It offers the possibility to fabricate nanostructures by self-organization with a high degree of c-axis orientation. One-dimensional ZnO nanostructures have been synthesized successfully by high-temperature vapor–solid (VS) growth [10–13], electrochemical deposition [14], hydrothermal synthesis [15], and chemical vapor deposition (CVD) [16]. Nanowhiskers were grown by Wagner [12] on Au-catalyzed Si substrate at 950 C via a vapor–liquid–solid (VLS) mechanism. He presented the concept of the VLS mechanism and referred the growth of a small globule at the tip of a nanowhisker to the VLS mechanism. This concept was adopted by Chen [10,11] and Li [13] to demonstrate the growth mechanisms of nanostructures which were synthesized at high-temperature by using a horizontal double-tube experimental setup. They found that the nanostructures grown with a high-temperature VS or VLS process depended on the growth temperature, source vapor concentration, and preparation procedure no matter whether a metal catalyst or a film was pre-deposited on the substrate before nanostructure growth. To date, nanostructures have been synthesized and their photoluminescence (PL) characteristic has been measured by many researchers. However, their field emission characteristics have rarely been reported. 1567-1739/$ - see front matter 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2008.12.056 * Corresponding author. Tel.: +886 7 381 4526; fax: +886 7 381 1182. E-mail address: shya@cc.kuas.edu.tw (S.-H. Yang). Current Applied Physics 9 (2009) e180–e184 Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap
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-filmdeposited Si substrate by using a single tube experimental setup by vapor-phase transport synthesis at low-temperature. The crystallinity 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 transport process was used for ZnO nanostructure synthesis. First, a 7 nm-thick Ag film was evaporated on Si substrate under a chamber pressure of 107 Torr at an evaporation rate of 0.02 nm/s. Subsequently, 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 observed by scanning electron microscopy (SEM, Philips XL40 FESEM). 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 spectrophotometer 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 current–voltage characteristics were evaluated with a Keithley 2410 programmable power source. 3. Results and discussion The ZnO nanostructures were grown by the vapor-phase transport mechanism using ZnO and graphite powders as source materials. 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 surface, 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 temperature 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 nanostructures was grown. However, when the synthesis time was increased 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 temperatures 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 nanowhiskers, 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 direction 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 nanorods was superior to that of the nanowhiskers and nanotips; consequently, 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 nanowhiskers were grown at 620 C (Fig. 2a and b.) As the substrate temperature increased to 650 C, the growth rate in the vertical direction relatively decreased and a nanostructure-merging phenomenon 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 nanorod array. Evidently, the morphology of the ZnO nanostructures was related to the source and substrate temperatures, which affected the flow of Zn and ZnOx vapors and influenced both nucleus 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
S-H. Yang et aL/Current Applied Physics 9(2009)e180-e184 a b d e Fig. 2. SEM photographs of nanowhiskers, nanotips and well-aligned nanorods. (a, b)nanowhiskers grown at 620C:(c, d) nanotips grown at 650";(e, f) well-aligned nanorods 150 nm for the nanowhiskers and well-aligned nanorods, respec- X-ray fluorescence(EDX) analysis showed that the nanostructures with Ag film. This shows that Ag nanograins are crucial to the consisted of only Zn ando(Fig 3). Furthermore, we found that growth process of the Zno nanostructures. At low substrate te the melting point and boiling point of Ag metal were 961.78 peratures, the Ag nanograin formed many small solid islands on nd 2162C. In the meantime, the eutectic temperature for the the Si surface, which obstructed the flow of Zn and ZnO vapors. Ag-Si system was 840C [20]. Hence at low substrate tempera- This promoted Zn/ZnOx self catalyzing to form Zno nanoparticles tures, 620-680C, it is difficult to form Ag droplets or Ag-Si li- on the Si surface. Thus, the Ag-film-deposited on Si acted like a uid alloy on Si substrate. In addition, no Ag element was catalyst which enhanced the growth of Zno nanostructures. neasured from EDX analyses, and no Ag particle at the tips was he optical properties of the Zno nanostructures were charac observed. This confirmed that the Zno nanowhiskers, nanotips terized by PL measurements at room temperature. Fig. 4 shows not only that a relatively weak Uv band with a peak at 376 nm was observed but also that a strong broad bluish-green band with a peak at 493 nm was measured for all the Zno nanostructures. The UV emission band was assigned to the near-band-edge(nbe)free exciton transition from the localized level below the conduction band to the valance band of Zno, while the bluish-green band emission was attributed to the radial recombination of a photogen erated hole with an electron that belonged to a singly ionized oxy gen vacancy(Vo)in the surface and subsurface lattices of Zno [21 he existence of surface and subsurface oxygen vacancies in the nanostructures was correlated to a higher surface area to volume ratio of the nanostructures. It is generally accepted that the surface 112.13.14.15.16.17.1 states significantly influence the PL spectrum of Zno nanostruc Energy (k tures. The strong bluish-green emission in Fig. 4 indicated that the as-prepared Zno nanostructures contained many surface de- of the zno nanostructures fects related to oxygen vacancies
150 nm for the nanowhiskers and well-aligned nanorods, respectively; their lengths were approximately 2 lm. Energy dispersive X-ray fluorescence (EDX) analysis showed that the nanostructures consisted of only Zn and O (Fig. 3). Furthermore, we found that the melting point and boiling point of Ag metal were 961.78 and 2162 C. In the meantime, the eutectic temperature for the Ag–Si system was 840 C [20]. Hence, at low substrate temperatures, 620680 C, it is difficult to form Ag droplets or Ag–Si liquid alloy on Si substrate. In addition, no Ag element was measured from EDX analyses, and no Ag particle at the tips was observed. This confirmed that the ZnO nanowhiskers, nanotips and nanorods were grown by the VS mechanism. However, no nanostructures were grown if the substrate was not deposited with Ag film. This shows that Ag nanograins are crucial to the growth process of the ZnO nanostructures. At low substrate temperatures, the Ag nanograin formed many small solid islands on the Si surface, which obstructed the flow of Zn and ZnOx vapors. This promoted Zn/ZnOx self catalyzing to form ZnO nanoparticles on the Si surface. Thus, the Ag-film-deposited on Si acted like a catalyst which enhanced the growth of ZnO nanostructures. The optical properties of the ZnO nanostructures were characterized by PL measurements at room temperature. Fig. 4 shows not only that a relatively weak UV band with a peak at 376 nm was observed but also that a strong broad bluish-green band with a peak at 493 nm was measured for all the ZnO nanostructures. The UV emission band was assigned to the near-band-edge (NBE) free exciton transition from the localized level below the conduction band to the valance band of ZnO, while the bluish-green band emission was attributed to the radial recombination of a photogenerated hole with an electron that belonged to a singly ionized oxygen vacancy (V O) in the surface and subsurface lattices of ZnO [21]. The existence of surface and subsurface oxygen vacancies in the nanostructures was correlated to a higher surface area to volume ratio of the nanostructures. It is generally accepted that the surface states significantly influence the PL spectrum of ZnO nanostructures. The strong bluish-green emission in Fig. 4 indicated that the as-prepared ZnO nanostructures contained many surface defects related to oxygen vacancies. Fig. 2. SEM photographs of nanowhiskers, nanotips and well-aligned nanorods. (a, b) nanowhiskers grown at 620 C; (c, d) nanotips grown at 650 C; (e, f) well-aligned nanorods grown at 680 C. Fig. 3. EDX analysis of the ZnO nanostructures. e182 S.-H. Yang et al. / Current Applied Physics 9 (2009) e180–e184
S-H Yang et al Current Applied Physics 9(2009)e180-e184 D Nanowhiskers 3≥2 091.01.11.21.3141.5 Wavelength(nm) 1000/V(V Fig 4. PL spectrum of the Zno nanostructures. The field emission characteristics of the zno nanostructures (field emitters) were measured and the Fowler-Nordheim (FN) equation, describing electrons passing across an energy barrier under the effect of an electric field e at the emitter tip, was adopted [2223: 154×106E2 exp|-687×10 where t(y)and t() are the correction factors related to the electric field, and y can be approximated as y=3.79 x 10E/]. In most y. The electric field the emitter tip can be represented as E= BV; B denotes the field enhancement factor and v represents the voltage applied to the an- ode and emitter electrodes. In this experiment, the turn-on field Time(min) was defined as the electric field required for achieving a current density () of 0.1 HA/cm2. Fig. 5 shows the J-E curves of the ZnO Fig. 7. Variation of emission current as a function of operation time for nanowhis- nanostructures. It was found that the turn-on fields of the zno nanotips and well-aligned nanorod nanowhiskers, nanotips and well-aligned nanorods were 4.5, 5.8 and 6.1 V/um, respectively. The turn-on fields of the nanowhiskers nd nanotips were lower than that of the well-aligned nanorods; tion zmo=5.ev, was 1115, 1149 and 1167 cm for the Zno this was due to the small radius of the emitter tips, as shown in nano nanowhiskers nanotips and well-aligned nanorods, respectively Fig. 2. However, the high emitter density and high aspect ratio It was found that the value of p was related to the geometry, struc (height/width) of the well-aligned nanorods resulted in an enhance- ture and density of the nanostructures. The well-aligned nanorods ment in the emission current density the current densities at an exhibited a high crystalline quality together with an appropriate electric field of 8.46 V/um of the nanowhiskers, nanotips and emitter density and a preferable aspect ratio, resulting in a high well-aligned nanorods were 3.7, 4.0 and 20.5 HA/cm, respectively. B value and superior field emission characteristics FN plots of the Zno nanostructures are shown in Fig. 6. The The variation in emission current of the Zno nanostructures as a straight line indicates that the emitting electrons were function of operation time is shown in Fig. 7, where the duced by FN field emission From the averaged slope of plot, plied between the Ito anode and the Zno emitter cathode was the estimated field enhancement factor B, assuming the work func- fixed at 11 The maximum and minimum emission current densities of the nanostructures were correspondingly 3 and 1.1 HA cm for the nanowhiskers(63% variation): 9 and 0.36 cm for the nanotips(96% variation); and 25 and 15 HA/cm- for the well-aligned nanorods(40% variation). This revealed that the well-aligned nanorods exhibited considerably higher stability dur g field emission than the nanowhiskers and nanotips. The emis sion current densities of the ZnO nanowhiskers, nanotips and well aligned nanorods measured after continuous operation for 6 h were approximately 2.05, 4.68 and 20.5 A/cm, respectively 4. Conclusions Zno nanostructures were grown on an Ag-deposited Si sub strate by the vs mechanism using Zno and graphite powders Electric Field (v/r source materials at different temperatures. No diffraction peaks of the Al element were observed in the XRD patterns, and the Fig. 5. Relationship between current density and electric field of the Zno Zno had a wurtzite structure. The morphology of the Zno nano- anowhiskers, nanotips and well-aligned nanorods. structures was related to the source and substrate temperatures
The field emission characteristics of the ZnO nanostructures (field emitters) were measured and the Fowler–Nordheim (FN) equation, describing electrons passing across an energy barrier / under the effect of an electric field E at the emitter tip, was adopted [22,23]: J ¼ 1:54 106 E2 / t 2ðyÞ exp 6:87 107 /3=2 vðyÞ E " #A=cm2 ð1Þ where t(y) and v(y) are the correction factors related to the electric field, and y can be approximated as y ¼ 3:79 104 E1=2 =/. In most applications, t 2ðyÞ ffi 1 and vðyÞ ¼ 0:95 y2: The electric field at the emitter tip can be represented as E ¼ bV;b denotes the field enhancement factor and V represents the voltage applied to the anode and emitter electrodes. In this experiment, the turn-on field was defined as the electric field required for achieving a current density (J) of 0.1 lA/cm2 . Fig. 5 shows the J–E curves of the ZnO nanostructures. It was found that the turn-on fields of the ZnO nanowhiskers, nanotips and well-aligned nanorods were 4.5, 5.8 and 6.1 V/lm, respectively. The turn-on fields of the nanowhiskers and nanotips were lower than that of the well-aligned nanorods; this was due to the small radius of the emitter tips, as shown in Fig. 2. However, the high emitter density and high aspect ratio (height/width) of the well-aligned nanorods resulted in an enhancement in the emission current density: the current densities at an electric field of 8.46 V/lm of the nanowhiskers, nanotips and well-aligned nanorods were 3.7, 4.0 and 20.5 lA/cm2 , respectively. FN plots of the ZnO nanostructures are shown in Fig. 6. The straight line indicates that the emitting electrons were mainly produced by FN field emission. From the averaged slope of the FN plot, the estimated field enhancement factor b, assuming the work function /ZnO = 5.3 eV, was 1115, 1149 and 1167 cm1 for the ZnO nanowhiskers nanotips and well-aligned nanorods, respectively. It was found that the value of b was related to the geometry, structure and density of the nanostructures. The well-aligned nanorods exhibited a high crystalline quality together with an appropriate emitter density and a preferable aspect ratio, resulting in a high b value and superior field emission characteristics. The variation in emission current of the ZnO nanostructures as a function of operation time is shown in Fig. 7, where the voltage applied between the ITO anode and the ZnO emitter cathode was fixed at 1100 V. The maximum and minimum emission current densities of the nanostructures were correspondingly 3 and 1.1 lA/cm2 for the nanowhiskers (63% variation); 9 and 0.36 lA/ cm2 for the nanotips (96% variation); and 25 and 15 lA/cm2 for the well-aligned nanorods (40% variation). This revealed that the well-aligned nanorods exhibited considerably higher stability during field emission than the nanowhiskers and nanotips. The emission current densities of the ZnO nanowhiskers, nanotips and wellaligned nanorods measured after continuous operation for 6 h were approximately 2.05, 4.68 and 20.5 lA/cm2 , respectively. 4. Conclusions ZnO nanostructures were grown on an Ag-deposited Si substrate by the VS mechanism using ZnO and graphite powders as source materials at different temperatures. No diffraction peaks of the Al element were observed in the XRD patterns, and the ZnO had a wurtzite structure. The morphology of the ZnO nanostructures was related to the source and substrate temperatures 350 400 450 500 550 600 650 PL Intensity (a.u.) Wavelength (nm) Fig. 4. PL spectrum of the ZnO nanostructures. 56789 0 5 10 15 20 Current Density ( μA/cm2 ) Electric Field (V/μm) Nanowhiskers Nanotips Nanorods Fig. 5. Relationship between current density and electric field of the ZnO nanowhiskers, nanotips and well-aligned nanorods. 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 -29 -28 -27 -26 -25 -24 Ln(I/V2) (AV-2 ) 1000/V (V-1) Nanorods Nanotips Nanowhiskers Fig. 6. F–N plots for the ZnO nanowhiskers, nanotips and well-aligned nanorods. 0 100 200 300 400 0 10 20 30 40 Current (μA) Time (min) Nanorods Nanotips Nanowhiskers Fig. 7. Variation of emission current as a function of operation time for nanowhiskers, nanotips and well-aligned nanorods. S.-H. Yang et al. / Current Applied Physics 9 (2009) e180–e184 e183
that affected the flow of Zn vapor and influenced both nucleus den- [21 H. Kim, B.K. Ju, Y-H. Lee, J.Jang. M -H. Oh, J. Electrochem. Soc. 147(2000) sity and stoichiometry of the Zno nanostructures. A relatively 4705. weak UV band and a strong broad bluish-green band were ob Y Cui, C.M. Lieber, Science 291(2001)85 served in the PL spectrum, indicating that the as-prepared Zno [5] w l Park, G -C Yi, M. Kim, s.J. Pen Adv. Mater.15(2003)526 anostructures had many surface defects related to oxygen vacan-[6X. Xu, G.R. Brandes, Appl. Phys. Lett. 74(1999)2549 ies. The turn-on fields of the Zno nanowhiskers, nanotips and L. Zhu, Xu, Y xiu, D W. Hess, C.P. Wong, Electron Mater. 35(2006)195 well-aligned nanorods were 4.5, 5.8 and 6.1 V/um, respectively Electrochem. Soc. The well-aligned nanorods exhibited a high stability in field emis- [9 M.K. Patra, K Manzoor, M. Manoth, S.R. Vadera, N Kumar. J Lumin 128(2008) sion and achieved current density of 20.5 HA/cm at an 110)Yx Chen, M. Lewis, WL Zhou, I Cryst. Growth 282(2005)85 electric field of 8.46 V/um after cont us operation for 6 h [111 Y. Chen, ]. Li, Y. Han, x Yang. Dai, ]. Cryst. Growth 245(2002)163. [12]RS. Wagner, W.C. Ellis, AppL. [13 C Li, G Fang. Q Fu, F Su, G. Li, x Wu, x Zhao, Cryst. Growth 292(2006)19 The authors thank the National Science Council of the Republic [16] S.P. Turano, J. Ready, J. Electron. Mater. 35(2006)192 of China for financially supporting this work under Contract No. 101 B.D. Yao, Y. Chan,N Wang. Appl. Phys. Lett 81 2002)757 SsC96-2221E-151-021 19 M-T. Chen, 1-M. Ti Zhao, M. Zhang P. R. Liu, XY. Zhang, LM. Cao, D.Y. Dai, H. Chen, Y.F. Xu, References [1] H.S. Uh, S.S. Park, J Electrochem. Soc. 150(2003)H12. bertson, Carbon 37(1999)759 [23]S.-H. Yang, M. Yokoyama, Jpn. AppL Phys, Part 1 36(1997)5275
that affected the flow of Zn vapor and influenced both nucleus density and stoichiometry of the ZnO nanostructures. A relatively weak UV band and a strong broad bluish-green band were observed in the PL spectrum, indicating that the as-prepared ZnO nanostructures had many surface defects related to oxygen vacancies. The turn-on fields of the ZnO nanowhiskers, nanotips and well-aligned nanorods were 4.5, 5.8 and 6.1 V/lm, respectively. The well-aligned nanorods exhibited a high stability in field emission and achieved an emission current density of 20.5 lA/cm2 at an electric field of 8.46 V/lm after continuous operation for 6 h. Acknowledgements The authors thank the National Science Council of the Republic of China for financially supporting this work under Contract No. NSC 96-2221-E-151-021. References [1] H.S. Uh, S.S. Park, J. Electrochem. Soc. 150 (2003) H12. [2] H. Kim, B.-K. Ju, Y.-H. Lee, J. Jang, M.-H. Oh, J. Electrochem. Soc. 147 (2000) 4705. [3] S. Iijima, Nature 354 (1991) 56. [4] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [5] W.I. Park, G.-C. Yi, M. Kim, S.J. Pennycook, Adv. Mater. 15 (2003) 526. [6] X. Xu, G.R. Brandes, Appl. Phys. Lett. 74 (1999) 2549. [7] L. Zhu, J. Xu, Y. Xiu, D.W. Hess, C.P. Wong, J. Electron. Mater. 35 (2006) 195. [8] F.M. Kolb, H. Hofmeister, R. Scholz, M. Zacharias, U. Gösele, D.D. Ma, S.-T. Lee, J. Electrochem. Soc. 151 (2004) G472. [9] M.K. Patra, K. Manzoor, M. Manoth, S.R. Vadera, N. Kumar, J. Lumin. 128 (2008) 267. [10] Y.X. Chen, M. Lewis, W.L. Zhou, J. Cryst. Growth 282 (2005) 85. [11] Y. Chen, J. Li, Y. Han, X. Yang, J. Dai, J. Cryst. Growth 245 (2002) 163. [12] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [13] C. Li, G. Fang, Q. Fu, F. Su, G. Li, X. Wu, X. Zhao, J. Cryst. Growth 292 (2006) 19. [14] H. Guo, J. Zhou, Z. Lin, Electrochem. Commun. 10 (2008) 146. [15] M. Guo, P. Diao, S. Cai, J. Solid State Chem. 178 (2005) 1864. [16] S.P. Turano, J. Ready, J. Electron. Mater. 35 (2006) 192. [17] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757. [18] B.H. Kong, H.K. Cho, J. Cryst. Growth 289 (2006) 370. [19] M.-T. Chen, J.-M. Ting, Thin Solid Films 494 (2006) 250. [20] J.H. Zhao, M. Zhang, P.R. Liu, X.Y. Zhang, L.M. Cao, D.Y. Dai, H. Chen, Y.F. Xu, W.K. Wang, J. Mater. Res. 14 (1999) 2888. [21] A. Khan, M.E. Kordesch, Physica E 30 (2005) 51. [22] J. Robertson, Carbon 37 (1999) 759. [23] S.-H. Yang, M. Yokoyama, Jpn. J. Appl. Phys., Part 1 36 (1997) 5275. e184 S.-H. Yang et al. / Current Applied Physics 9 (2009) e180–e184