Efficient field emission from well-oriented Cu2O film
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Efficient field emission from well-oriented Cu 2O filmQun Tang a ,Ting Li a ,Xihong Chen b ,Dapeng Yu b ,Yitai Qian a,*aDepartment of Chemistry,Structure Research Laboratory,University of Science and Technology of China,Hefei,Anhui 230026,People’s Republic of ChinabSchool of Physics,Peking University,Beijing 100871,People’s Republic of ChinaReceived 10June 2004;accepted 20September 2004by T.T.M.PalstraAvailable online 19October 2004AbstractWell-oriented Cu 2O films comprising of octahedral-shaped crystals were grown directly on copper foil via an hydrothermal treatment.The well-oriented films were characterized by scanning electron microscopy (SEM)and X-ray diffraction (XRD).Field emission from the film showed good emission properties,and,the electron emission turn-on field (E to )and threshold field (E thr )are about 9.6and 13.4V/m m respectively,which is similar to the values reported for CuO nanofiber,although the latter has a much larger size.The corresponding Fowler–Nordheim (F–N)plots showed a linear behavior.The sharp corners of the tips are considered as main electron emitters and account for its good performance.q 2004Elsevier Ltd.All rights reserved.PACS:73.50.K h;79.70.C qKeywords:A.Semiconductors;A.Thin film;B.Chemical synthesis;D.Field emissionThe effect of dimensionality and geometry on special materials properties has been extensively investigated to meet the demands of miniaturization and better performance of electronic devices.Field emission (FE)based on both wide-band-gap and narrow-band-gap materials with low-dimensional and novel geometrical structures,such as C nanotubes [1],SiC nanowires [2],SiCN nanorods [3],Cu 2S nanowires [4],ZnO nanowires [5],In 2O 3prisms [6],and so forth,has attracted prime interest due to potential appli-cation in large-area flat panel displays.In addition,the advantages of using semiconductors includes well-con-trolled electronic properties and low electron affinity.Furthermore,the application of FE can be exploited by utilizing the unique properties of semiconductor novel structures.In this context,their use in thermoelectric conversion and photovoltaic devices based on field electronemission can be explored.In recent work,field emission from cupric oxide on copper foil (CuO/Cu)with various nanostructures synthesized by different routes was systemi-cally investigated [7,8].As another cupric oxide,Cu 2O,is a p-type semiconductor with a direct band gap of 2eV,and excitons can propagate coherently through its monocrystal-line samples,which make it a promising material for the reversible conversion between solar energy and electrical or chemical energy [9].However,there is no report as yet on a FE emitter using a Cu 2O/Cu layered structure.In this communication,we report the FE properties of aligned Cu 2O well-oriented films prepared by a novel hydrothermal oxidation route.The well-oriented Cu 2O films were synthesized on copper foils using a designed hydrothermal oxidation route.The details of the growth technique have been described elsewhere [10],only a brief description relevant to the present work is given here.First,an aqueous solution was prepared in a 50ml autoclave by mixing 35ml 1M NaOH solution and 0.5ml 30%H 2O 2.Second,a piece of copper foil (2.5!2.0!0.25mm 3),which had been0038-1098/$-see front matter q 2004Elsevier Ltd.All rights reserved.doi:10.1016/j.ssc.2004.09.063*Corresponding author.Tel.:C 865513603204;fax:C 865513607402.E-mail address:ytqian@ (Y.Qian).ultrasonically cleaned in acetone and then in deionized water,was immersed into the solution,and then the autoclave was sealed and put into an oven at 2008C for 24h,The foil was then collected from solution,rinsed with deionized water and dried in air at 408C.The morphology and orientation of the as-prepared Cu 2O film was charac-terized using scanning electron microscopy (SEM)and X-ray diffraction (XRD).Fig.1showed the XRD pattern of the Cu 2O film on copper substrate,the intensity of the (110)and (220)peak exceeded the value for the JCPDS X-ray card file for a power of Cu 2O (5-667),indicating that [110]was a predominant preferred orientation.The general morphology of the polycrystalline Cu 2O film was shown in Fig.2.It can be seen that the [110]oriented film is comprised of crystalline regular octahedra with a uniform size of 1.5–2.0m m on the copper substrate.Their surface is quite smooth.According to the XRD pattern the octahedra are surrounded by eight [110]faces.SEM images showed that the surface was fully covered with crystalline Cu 2O which also had good adherence to the substrate.The as-synthesized Cu 2O film posesses smooth facets and extremely small radius of curvature at the tips (less than 50nm),so it is expected to exibit a stable and efficient field emission.FE measurement was carried out by a two-parallel-plate configuration in a vacuum chamber with pressure !8!10K 7Pa at room temperature.The Cu 2O/Cu foil was attached to a stainless-steel plate using conducting glue as cathode with the other stainless-steel plate as anode.The distance between the emitting surface and the plate was determined by first lowing the plate to the sample until electric contact was observed and then lifting the plate to a certain value.A variable positive voltage up to 5kV with a sweep step of 50V was applied to the anode and the emission current measured by a Keithley 485electrometer.The electric field (E )was estimated by dividing the applied voltage by the anode–cathode separation (V /d ).Theemission current density (J )was calculated from the obtained emission current and the cathode surface area.The plot of the emission current densities versus electric field (J –E )for the Cu 2O octahedra with an emitting surface area of 5.0mm 2is shown in Fig.3(a).The data were collected with the effective distance of 250m m.The electron emission turn-on field (E to )and threshold field (E thr ),defined as the macroscopic fields required producing a current density of 10m A/cm 2and 10mA/cm 2are about 9.6and 13.4V/m m respectively.Generally,fairly high emission current density,i.e.,10mA/cm 2,is desirable to light a phosphor display.In addition,all current density electric field (J –E )curves exhibit a sharp increase in emission current (about 50times)within a narrow voltage ranges close to the corresponding threshold field.The as-synthesized Cu 2O film holds the highest current density among those reported produced by a solution route (e.g.Cu 2S [4],CuO [7]).It is known that at room temperature,the emission current mostly originates from the tunnelling of electrons through the surface barrier,which is described by the Fowler–Nordheim (F–N)theory.The emission current can be expressed in terms of the experimental parameters,in the following equationJ Z A ðb 2E 2=F 2Þexp ðK B F 3=2=b E Þwhere J is the current density,E is the applied field,A and B are constants,and F is the work function,while b is the field enhancement factor which is associated with the magnitude of electric field at the emitting tip surface by E local Z b E ,E local is the local electric field at the emitting tip surface.The F–N plot shows a linear relationship,which implies that the field emission from a Cu 2O prism follows the FN theory and the emitted current is indeed caused by quantum tunnelling.In the previous study,the aligned CuO nanobeltfilms,Fig.1.FESEM image of the well-oriented Cu 2O film grown on the copper foil.The [110]oriented film is comprised of regular octahedral particles with a size of 1.5–2mm.which were synthesized on copper foils under ambient conditions showed a low current density(10m A/cm2under 11V/m m)[7].However,a great increase of the emission current level was observed as the temperature was enhanced,which implies that the cupric oxide nanobelt is a promising candidate for cathode material in a thermo-electric conversion device based onfield emission.In a later report,three types of CuO nanostructures(the nanorod, nanofiber,and nanoparticle)[8],which were grown by a self-catalytic growth process at400,500,and6008C, respectively showed different emission property.That is,the FE current was significantly affected by the morphology of the CuO samples.The CuO nanofiber with a high aspect ratio possesses a better FE effect(9mA/cm2at11V/um).In this report,the as-synthesized Cu2Ofilm exibits a similar property as the CuO nanofiber,although the size of the Cu2O crystallite is in the range of millimeters and much larger than CuO nanofiber,the sharp corners of the tips is suggested to serve as main electron emitters.In fact, Koenigsfeld et al.have reported a similar result that the removal of surface asperities resulted in an increase in thresholdfield of a diamondfilm[11].X-ray photoelectron spectroscopy(XPS)was employed for further analyze the Cu2Ofilm before and after FE stability tests.However,no obvious change in chemical composition of the orientedfilm was detected(not shown here).To conclude,octahedral-shaped Cu2O crystallites were prepared on copper foil by a novel hydrothermal oxidation treatment.Thefilm shows strong h110i texture.Field emission measurements showed that the emission threshold field was obtained at about13.4V/m m,which exhibits a similar current density as for CuO nanofibers although the latter have a smaller size.In addition,a sharp increase in emission current density emerged at the thresholdfield,which is beneficial to its fast response as a good emitter. 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