Journal of Nano Research Vol. 5 (2009) pp 231-237 online at http://www.scientific.net © (2009) Trans Tech Publications, Switzerland Online available since 2009/Feb/06
Field Emission Properties of Al-doped ZnO Nanostructures Shalaka C. Navale1,2, Farid Jamali Sheini2, Sandip S. Patil2, Imtiaz S. Mulla1,a, Dilip S. Joag2, Mahendra A. More2, and Suresh W. Gosavi2,b 1
Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India 2 Center for Advanced Studies in Material Science and Condensed Matter Physics Department of Physics, University of Pune, Pune – 411007, India a
[email protected],
[email protected] (corresponding author) rd
nd
rd
Received: September 3 , 2007, revised: February 2 , 2008, accepted: February 23 , 2008
Keywords: ZnO, Field Emission, Nanostructures, Fowler-Nordheim plots
Abstract. Field emission from Al-doped ZnO nanostrcutures has been investigated in planar diode configuration under ultra high vacuum conditions. The Al-doped ZnO nanostructures were synthesized by co-precipitation method with varying aluminium concentrations. The as- synthesized product was characterized by x-ray diffraction, scanning electron microscope and energy dispersive x-ray analysis. The threshold field required to draw a current density of ~ 1 μA/cm2 was observed to be ~ 2.0 V/μm and ~ 2.3 V/μm for Al-doped ZnO nanostructures synthesized with aluminium concentrations of 1% and 3%, respectively. The Fowler- Nordheim (F-N) plots for both the specimens exhibit non-linear behaviour, which is observed to be specimen dependent. The nonlinearity observed in the F-N plots has been interpreted on the basis of the theory of electron emission from semiconductor emitters. The field enhancement factors, estimated from the slope of the F-N plots, are found to be ~ 9.3 x 103 and 3.9 x 103 for 1% and 3% Al-doped ZnO emitters, respectively. The high values of the field enhancement factor suggest that the emission is from the nanostructures. The emission current stability measured at the preset value of ~ 2 μA over a period of more than three hours is found to be fairly stable. The results indicate use of Al-doped ZnO nanostructures as promising emitters for field emission based devices.
Introduction With the advent of nanotechnology, one-dimensional nanostructures of metals and semiconductors having different morphologies such as wires, belts, rods and tubes have attracted considerable attention due to their unique physical and chemical properties leading to diverse technological applications. Owing to their nano-dimensions and anisotropic structures offering high aspect ratio, it is expected that the electric field required to observe the field emission could be achieved at relatively low values of applied voltage as compared to the conventional micro-tip metallic emitters. In addition to this, the capability of delivering very high current densities via configuring these nanostructures in the form of an ‘array’, makes them most suitable for practical applications in field emission displays. It is also easier to generate nearly monoenergetic electron beam by accurately controlling the density and geometry of the nanostructured field emitters. This has resulted in field emission studies on various nanostructures like carbon nanotubes (CNTs), AlN, GaN, MoO3, SnO2 and ZnO [1-8]. Amongst these, ZnO nanostructures have been intensively studied as they have high mechanical stability, low electron affinity and high aspect ratio [9,10]. The influence of morphology of the ZnO nanostructures, their shapes, size and areal density, on the field emission behavior has All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 196.1.114.240-16/02/09,14:28:41)
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also been investigated [8b,10,11]. It is well known that the morphology of the ZnO nanostructures is strongly influenced by the synthesis route [12-14]. U. Pal et al. have carried out synthesis of 1-D ZnO nanostructures having controlled morphologies using a simple low temperature hydrothermal process [12]. A novel aqueous relatively low temperature synthesis technique for ZnO nanostructured thin film growth has been reported by S. Shishiyanu et al. [13]. In most of the field emission studies, pure ZnO nanostructures having various sizes, shapes and geometries have been employed. It is well known that addition of impurities in the semiconducting lattice alters its electrical properties significantly. As a result the dopedsemiconducting nanostructures have great application potential. Recently, we have reported gas sensing characteristics of Al-doped ZnO nanostructures for detection of NOx [15]. In this paper we report the investigation on field emission characteristics of the Al-doped ZnO nanostructures deposited on silicon substrates. The results show that the Fowler-Nordheim plots exhibit non-linear behaviour and interestingly the non-linear character is found to be specimen dependent. The nonlinearity has been explained on the basis of field emission from semiconducting emitters.
Experimental The Al-doped ZnO nanostructures were synthesized by co-precipitation method. The reaction mixture was prepared by dissolving appropriate quantity of zinc nitrate (Zn(NO3)2●6H2O) in 200 ml distilled water. Sodium dodecyl sulphate (SDS) was added to the reaction mixture, which acted as a surfactant and has a beneficial effect of preventing of agglomeration of particles to a certain extent. A foamy solution obtained was then kept at the ice temperature with stirring. To this, a standard NH3 solution (30%) was added drop wise till the pH reached 8. After completion of the precipitation the solution was filtered and the precipitate was then washed several times with double distilled water till it became neutral. This was followed by drying of the precipitate overnight in an oven at 100 ◦C and further calcinating at ~ 300 ◦C for 6 hours. Al-doping was achieved by adding requisite amount of Al(NO3)3●9H2O salt to the solution containing zinc nitrate and SDS. The as-synthesized product was characterized by x-ray diffraction (XRD), employing Cu-Kα radiation performed on Rigaku Miniflex Diffractometer. The specimens for field emission studies were prepared by a simple solution-cast method. The as-synthesized Al-doped ZnO nanostructured powder was dispersed in methanol by ultrasonicating for 10 minutes. A drop of this solution was put on to a pre-cleaned silicon substrate and left for drying under infrared lamp for 10 minutes. The surface morphology of the specimens thus prepared was investigated employing scanning electron microscope (SEM) by JEOL, JSM – 6360. The microscope was operated at an accelerating voltage of 20 kV and filament current 60 A. An energy dispersive X-ray analyzer (EDAX) attached to the SEM was used to determine the elemental composition of the specimens. The field emission measurements were carried out in an all metal chamber evacuated at a pressure ~1×10−8 mbar. The vacuum processing was carried out using ultra high vacuum system consisting of a turbomolecular pump, a sputter ion pump and a titanium sublimation pump. A typical ‘diode’ configuration, comprising a phosphor coated tin oxide glass plate (circular disc of diameter ~ 50 mm) as anode and the Al-ZnO nanostructures deposited on silicon substrate as a cathode, was used. The specimens were mounted, one at a time, on a stainless steel stub (diameter
