Copper (II) oxide as a giant dielectric materialSudipta Sarkar, Pradip Kumar Jana, B. K. Chaudhuri, and H. Sakata Citation: Appl. Phys. Lett. 89, 212905 (2006); doi: 10.1063/1.2393001 View online: http://dx.doi.org/10.1063/1.2393001 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v89/i21 Published by the American Institute of Physics. Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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Copper „II… oxide as a giant dielectric materialSudipta Sarkar, Pradip Kumar Jana, and B. K. Chaudhuria�

Department of Solid State Physics, Indian Association for the Cultivation of Science, Kolkata,West Bengal 700032, India

H. SakataDepartment of Applied Chemistry, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292,Japan

�Received 25 August 2006; accepted 13 October 2006; published online 22 November 2006�

Bulk copper �II� oxide �CuO�, heat treated at 1223 K, shows extraordinarily high dielectric constant��r�104�, almost independent of temperature �above 230 K� and frequency in the kilohertz region.A sudden decrease of �r is observed at lower temperature �below 150 K�. X-ray photoelectronspectroscopy and high resolution transmission electron microscopy studies confirm the presence ofa microscopic amount of Cu3+ in annealed CuO. The dielectric behavior of CuO can be explainedby Maxwell-Wagner-type polarization mechanism and thermally activated mechanism. © 2006American Institute of Physics. �DOI: 10.1063/1.2393001�

Copper �II� oxide, CuO with monoclinic structure, atroom temperature is of special importance as it has closerelationship to the problem of high-TC superconductivity.Several optical studies demonstrate CuO as a charge transferinsulator.1,2 But excess holes arising from the presence ofmicrotraces of Cu3+ in CuO make it a semiconductor.3 Elec-tron microscopic study also detects superlattice reflectionsdue to Cu3+ and charge stripes have been discovered inCuO.4 Previous measurement of dielectric constant of CuOshowed surprisingly low �r value ��25 for polycrystallinesample and 3000 for single crystal�.5 This large difference inthe value of �r for the CuO single crystal compared to that ofthe corresponding polycrystalline sample prompted us to per-form dielectric measurement on pure polycrystalline CuOsample. Interestingly, we observe a very high dielectric con-stant value even in polycrystalline CuO comparable to thoseof recently discovered non-ferroelectric CaCu3Ti4O12�CCTO� �Refs. 6 and 7� and Li and Ti doped NiO �LTNO��Ref. 8� ceramics. The large �r value with weak temperatureand frequency dependency as observed in CCTO and LTNOis very much desirable for device implementation. Investiga-tion of the dielectric properties of CuO would also be veryinteresting as it possesses excess charge in its microstructure.

In this letter, we report high dielectric permittivity in thelead-free polycrystalline CuO suitable for its probable appli-cation in microelectronics as capacitors and memory devices.Attempt has also been made to explain the origin of suchhigh �r value in simple CuO from x-ray photoelectron spec-troscopy �XPS� and electron microscopy studies.

Powder sample of high purity CuO �99%� was obtainedfrom Merck. For our measurement purpose, CuO powderwas first heated at 873 K under atmospheric pressure for 8 h.The calcined powder was then ball milled for 5 h to obtainultrafine CuO powder. The powder was then pelletized. Fi-nally the cold pressed pellets of synthesized powder weresintered at 1223 K for 10 h and allowed to cool at the rate of1 K/min down to room temperature. Electrical contacts onthe highly polished surface of the pellets were made by goldsputtering. Single phase character of CuO was confirmed

from x-ray diffraction studies. Dielectric measurements wereperformed using an HP4192A impedance analyzer. The XPS�Kratos Analytical Ltd., UK� studies were carried out em-ploying an Al K� monochromatic x-ray source. High resolu-tion transmission electron microscope �JEOL JEM 2010�with an accelerating voltage of 200 keV was used for takingelectron diffraction pattern �EDP�. Scanning electron micro-scopic image of the sample was obtained using field emis-sion scanning electron microscopy �JEOL JSM 6700F�. Tem-perature controlling for dielectric measurements was donewith an accuracy of ±0.5 K or better using a Eurotherm tem-perature controller.

The temperature dependent dielectric constant of poly-crystalline CuO shown in Fig. 1�a� exhibits a striking featureof very high dielectric constant ��2�104� comparable tothose of the recently reported high dielectric materials CCTOand LTNO.7,8 Most importantly, such extraordinarily high di-electric constant is found to hardly depend on temperature�above 230 K� and frequency �kilohertz region�. The relativepermittivity abruptly drops at low temperature ��150 K� re-sembling the behavior of CCTO or LTNO. The frequencydependent dielectric constant of CuO at various fixed tem-peratures represented in Fig. 1�a� �inset� also shows almosttemperature independent behavior of �r at lower frequenciesand a steplike decrease of �r at higher frequencies. More-over, the characteristic frequency of the step shifts to thehigher frequency side as temperature increases similar toCCTO and LTNO ceramics.7,8

The frequency dependent dielectric loss �tan �� dis-played in Fig. 1�b� demonstrates that the dissipation peakshifts to the higher frequency side with increasing tempera-ture. It indicates that thermally activated relaxation also oc-curs in CuO as observed earlier in the cases of CCTO andLTNO.7,8 Here it should be mentioned that around room tem-perature, in the low frequency region, the value of tan � inCuO is higher than that of CCTO and increases with tem-perature which is possibly due to the contribution of the dcconductivity. In Fig. 1�b� �inset� the relaxation time ��� ob-tained from the tan � curve has been plotted as a function ofinverse temperature. The experimental data �filled circles� fitwell with the Arrhenius relation, �=�0 exp�Ea /kBT�, withina�Electronic mail: sspbkc@rediffmail.com

APPLIED PHYSICS LETTERS 89, 212905 �2006�

0003-6951/2006/89�21�/212905/3/$23.00 © 2006 American Institute of Physics89, 212905-1

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the temperature range of 170–230 K. The activation energyEa obtained from the fitting curve �solid line� according tothe above relation is found to be 0.248 eV.

Collins et al.3 first suggested the presence of a micro-quantity of Cu3+ as high as 0.7% in CuO. We believe thisminute Cu3+ within CuO plays a significant contribution tothe observed high dielectric response of CuO. It is very hardto observe such a small quantity of Cu3+ from x-ray diffrac-tion studies. As copper exists in the divalent state in CuOhaving mainly d9 character,1,2 XPS is a powerful technique todetect such a feeble amount of charge coexisting with Cu2+

in CuO. The high resolution spectra corresponding to O 1sand Cu 2p are displayed, respectively, in Figs. 2�a� and 2�b�which are plotted after correction of charging effects using abinding energy of 284.4 eV as C 1s peak. The O 1s spectrashown in Fig. 2�a� indicate a peak at a binding energy of529.107 eV and the corresponding peak fit reveals a mainpeak at 529.105 eV along with a shoulder at 530.248 eV,also observed earlier by Robert et al.9 who attributed theappearance of the shoulder to chemisorption of oxygen. TheO 1s spectra also indicate that Cu2O phase does not coexistwith CuO as the main peak of Cu2O in O 1s spectra usuallyappears at 530.2 eV. The XPS detected Cu 2p3/2 and Cu2p1/2 spectra respectively at 933.307 and 953.246 eV areshown in Fig. 2�b�. The Cu 2p3/2 spectra also accompaniedwith a satellite on the higher binding energy side at about

9 eV agrees well with that reported earlier.1 In the case ofCuO, the ground state contains d holes and in a photoelec-tron emission process, electron charges are transferred fromthe surrounding to the core holes. Such a situation causes themain peak to be associated with a satellite at higher bindingenergy having d9 shell configuration as shown here in CuO.In the case of Cu2O, the satellite peak is absent as Cu2O hasa completely filled d shell �d10�. The Cu 2p3/2 peak fit revealsa large peak at 933.249 eV and a very small peak at934.74 eV. The peak at 933.249 is characteristic of Cu2+ andthe associated small peak at 934.74 eV corresponding toCu3+ state arises due to slight excess oxygen in CuO. Noevidence of Cu1+ has, however, been detected as normallyCu1+ peak is observed at lower binding energy in comparisonwith the corresponding Cu2+ peak.

Electron diffraction pattern of polycrystalline CuOshown in Fig. 3�a� exhibits superlattice reflection in additionto the fundamental reflection of CuO. The result is similarwith that reported by Zheng et al.4 who performed a similarexperiment with single crystal of CuO. The weak superlatticereflection in Fig. 3�a� is due to the presence of Cu3+, i.e.,

FIG. 1. �a� Temperature dependence of dielectric constant �r of CuO atselected frequencies. The inset displays frequency dependent dielectric con-stant of CuO at various fixed temperatures. �b� Dielectric loss tan � of CuOat different fixed temperatures. The inset shows typical Arrhenius plot of therelaxation time � with inverse temperature of CuO sample.

FIG. 2. X-ray photoelectron spectrum of CuO showing �a� O 1s at529.107 eV and �b� Cu 2p3/2 and Cu 2p1/2 at 933.307 and 953.246 eV,respectively.

FIG. 3. �a� Selected area electron diffraction pattern of CuO. �b� Scanningelectron micrograph of CuO sample.

212905-2 Sarkar et al. Appl. Phys. Lett. 89, 212905 �2006�

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excess holes in CuO which in turn form charge stripes inCuO. Scanning electron microscopic image of CuO repre-sented in Fig. 3�b� reveals distinct grain boundaries betweenCuO grains. The average grain size is around �1 �m. AsXPS and EDP confirm the presence of excess holes in CuO,the conduction mechanism in CuO is mainly governed byhole hopping between Cu2+ and Cu3+. Hopping of holes pro-duces Cu3+ around Cu2+ causing CuO grains to be enrichedwith Cu3+ �p-type conductance�, which become semicon-ducting. But the grain boundary might consist of only Cu2+

and it behaves as an insulating wall. As a result, an array ofboundary barriers and grains is formed and a giant dielectricvalue appears. Impedance spectroscopy analysis �not dis-cussed here� also supports that point. The existence of Cu3+

leads to the formation of an internal boundary layer betweengrains and such a stable dielectric value over 230 K ismainly governed by an extrinsic mechanism such asMaxwell-Wagner-type polarization also known as interfacialpolarization, similar to that observed earlier in other highdielectric materials such as CCTO,10 Bi2/3Cu3Ti4O12,

11 andLi and Al doped NiO, �Ref. 12� ceramics. As the permittivityof the CuO sample increases slightly with increase of tem-perature over the whole frequency range �shown in Fig. 1�a��thermally activated mechanism also plays its role partly inthe high dielectric response of CuO. It is further noticed thatchanging the duration of heat treatment causes variations of�r as well as dielectric loss. This implies that both grain sizeand concentration of Cu3+ in CuO depend on the duration ofheat treatments.

It is observed from Fig. 1�a� that the �r value decreasesbelow 230 K at which temperature antiferromagnetic �AF�ordering was observed earlier.13 It appears that the AF order-ing interaction acts as a “pinning” force5 for the charge su-perlattice causing decrease of �r. The sudden large decreaseof �r below 150 K over the entire frequency range is due tofreezing of excess holes �stopping the movement of thetrapped charges due to complete ordering of the spins to theAF ordering state� in CuO.

The very small �r value ��25� of the CuO observed byearlier authors5 in powder sample is, therefore, due to theabsence of an appropriate amount of Cu3+ in their sampleswhich is, however, present5 in the corresponding single crys-tal showing relatively large �r value. No Cu3+ is, however,

present in the insulating single crystal of CuO which natu-rally showed very low dielectric constant ��3–4�.5 Here itcan be mentioned that the presence of Cu3+ is also importantfor the cuprate superconducting system. Even the importanceof the presence of a minute amount of Cu3+ in CCTO cannotbe ignored which is yet to be confirmed.

In conclusion, high dielectric permittivity observed inCuO arises from the microquantity of Cu3+ confirmed byXPS and EDP studies. A simple oxide such as CuO appearsto be an interesting low cost material showing giant dielec-tric constant which merely depends on temperature and fre-quency �kilohertz region�. Such a unique property of lead-free high dielectric material �CuO� seems to be of paramountimportance in the microelectronic industry. Further studieson the conduction mechanism, impedance spectroscopy, anddifferent heat treatments of CuO are going on and will bepublished elsewhere.

One of the authors �B.K.C.� is grateful to the Council ofScientific and Industrial Research �CSIR�, Government ofIndia for financial assistance to complete the work.

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