Table Of ContentChia-ChingandCheng-FuNanoscaleResearchLetters2013,8:33
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NANO EXPRESS Open Access
Investigation of the properties of nanostructured
Li-doped NiO films using the modified spray
pyrolysis method
Wu Chia-Ching1 and Yang Cheng-Fu2*
Abstract
The lithium-doped nickel oxide (L-NiO)films were synthetizedusing the modified spray pyrolysis method with a
two-step grown process. By observing the spectra of X-ray photoemissionspectroscopy ofL-NiO films, the intensity
ofNi 2p peak of Ni3+ bonding state increases withincreasing Li concentration that causes thedecrease of
3/2
transparency and resistivity. The L-NiO films with optimum characteristics were obtained atLi =8 at%, where a
p-type resistivity of4.1 × 10−1 Ω cm and optical transparency above 76% inthe visible region are achieved.
Keywords: Modified spray pyrolysis method, Nickel oxide,Lithium, Conductivity,Transparency
Background NiO films is about 1 Ω cm [7], which is almost 1 order
N-type transparent conductive oxide (TCO) films, such of magnitude higher than that of sputter-deposited NiO
as indium tin oxide, aluminum zinc oxide, indium gal- thinfilms.
liumzincoxide,etc., arewidelyusedastransparent elec- Undoped NiO has a wide E value and exhibits low
g
trodes, solar cells, and touch panels. However, not many p-type conductivity. The conduction mechanism of NiO
TCO films have the p-type properties, and they are also films is primarily determined by holes generated from
required in other applications. Nickel oxide (NiO) films nickel vacancies, oxygen interstitial atoms, and used
are a promising candidate for p-type semi-TCO in the dopant. The resistivity of NiO-based films can be
visiblelight with thebandgap (E )values from 3.6to4.0 decreased by doping with lithium (Li) [8]. In 2003, Ohta
g
eV. NiO films have a wide range of applications, such as et al. fabricated an ultraviolet detector based on lithium-
(1) transparent conductive films [1], (2) electrochromic doped NiO (L-NiO) and ZnO films [9]. However, only
display devices [2], (3) anode material in organic light few efforts have been made to systematically investigate
emitting diodes [3], and (4) functional layer material for the effects of deposition parameters and Li concentra-
chemicalsensors[4]. tion on the electrical and physical properties of SPM
In the past, NiO films were prepared by various meth- deposited NiO films. In this research, a modified SPM
ods, including electron beam evaporation, chemical de- method was used to develop the L-NiO films with
position, atomic layer deposition, sol–gel, and spray higher electrical conductivity. We would investigate the
pyrolysis method (SPM) [5]. Sputtering is one of the effects of Li concentration on the physical, optical, and
most popular methods to deposit NiO films with low electricalproperties ofNiOthinfilms.
resistivity of 1.4 × 10−1 Ω cm [6]. The SPM is a very
important non-vacuum deposition method to fabricate
Methods
TCO films because it is a relatively simple and inexpen-
Lithium-doped nickel oxide films were prepared by SPM
sive non-vacuum deposition method for large-area coat-
with 1 M solution. The nickel nitrate (Alfa Aesar, MA,
ing. However, the resistivity of SPM deposited doped
USA) and lithium nitrate (J. T. Baker, NJ, USA) were
mixed with deionized water to form the 2 to 10 at%
*Correspondence:[email protected] L-NiO solutions. The isopropyl alcohol was added in
2DepartmentofChemicalandMaterialsEngineering,NationalUniversityof
L-NiO solutiontoreducethesurfacetensiononglasssub-
Kaohsiung,Kaohsiung81148,Taiwan
Fulllistofauthorinformationisavailableattheendofthearticle strate; then, the solution was deposited on the Corning
©2013Chia-ChingandCheng-Fu;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreative
CommonsAttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,and
reproductioninanymedium,providedtheoriginalworkisproperlycited.
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Eagle XG glass substrates (Corning Incorporated, NY, ofL-NiOfilmsdecreasesfrom11.96to1.25cm2/V/sasthe
USA). The L-NiO films were then backed at 140°C and Liconcentrationincreasesfrom2to10at%.Forthecarrier
annealed at 600°C for densification and crystallization. The mobility, dopant materials asthe scattering center, the car-
L-NiO films were formed according to the following rier mobility will encounter more hindered concentration
reaction: withincreasingLiamount,whichleadsthedecreaseofmo-
bility. The mobility of L-NiO films decreases with Li con-
1
NiðNO3Þ2⋅6H2OH⇒eatedNiOþ2NO2þ2O2þ6H2O; centration; two reasons will cause this result: (1) As Li
concentration increases, the number of Li atoms substitut-
ð1Þ
ing the Ni atoms increases; thus, the carrier concentration
andthereactionofLi Ois increases from 1.91 × 1017 to 3.12 × 1018 cm−3. (2) As the
2
Li concentration increases, more Li ions substitute Ni2+
2LiNO3H⇒eatedLi2Oþ2NO2þ1O2 ð2Þ in the normal crystal sites and create holes, as shown in
2 Equation 4. Therefore, the resistivity of Li-doped NiO film
with 2 at% doping amount is 1.98 Ω cm, and it decreases
The surface morphology and crystalline phase of with Li concentration and reaches a minimum value of
L-NiO films were examined using the field-emission 1.2×10−1ΩcmattheLiconcentrationof10at%.
scanning electron microscope (FE-SEM) and X-ray dif-
fraction(XRD)pattern,respectively.Theatomicbonding (cid:3)1 Oð2gÞþLi2O⇔2OOx þ2LiNi′þ2h˙ ð4Þ
state of L-NiO films was analyzed using the X-ray 2
photoemission spectroscopy (XPS). The electrical resist- Figure 2 shows the surface FE-SEM images of L-NiO
ivityandtheHalleffectcoefficientsweremeasuredusing films. As Li = 2 at%, the L-NiO films have smooth but
a Bio-Rad Hall set-up (Bio-Rad Laboratories, Inc., CA, not compact surface morphology, and an average grain
USA). To determine the optical transmission and Eg of size of about 25 nm. The grain size of L-NiO films
L-NiO thin films, the transmittance spectrum was car- increases,andthe pores decrease with increasing Licon-
ried out from 230 to 1,100 nm using a Hitachi 330 spec- centration. The improved grain growth can be attributed
trophotometer (Hitachi, Ltd., Tokyo, Japan). The Eg to the small radius, low activation energy, and high ionic
value of L-NiO films was obtained from the extrapola- mobility of the Li ions. During the crystal growth
tion oflinear part ofthe(αhv)2 curves versusphoton en- process, it is easier for these ions with low activation en-
ergy(hv)usingthefollowingequation: ergy to escape from trap sites and transfer to nucleation
αhv¼A(cid:2)ðhv(cid:3)EgÞn; ð3Þ sites, leading to larger grain size [11]. Therefore, the
crystallization of the modified SPM deposited L-NiO
where α is the absorption coefficient, hv is the photon films is better than that of traditionally SPM deposited
energy, A is a constant, E is the energy band gap (eV), films [7] and similar to that of sputter-deposited films
g
and n is the type of energy band gap. The NiO films are [12].Thetraditionalmethodistospraythenickelnitrate
anindirecttransitionmaterial,andnissetto2[10]. solution onto the preheated glass substrates (>300°C),
which undergoes evaporation, solute precipitation, and
Results and discussion pyrolytic decomposition. However, as the substrates are
Figure 1 shows resistivity (ρ), carrier mobility (μ), and heated at higher temperatures, the evaporation ratio of
carrier concentration (n) of L-NiO films as a function of solutions on glass substrate is too swift, resulting in the
Liconcentration.AsshowninFigure1,thecarriermobility formation inferior to NiO films. In this study, using the
modified SPM, the water and solvent in L-NiO solution
were evaporated at 140°C, and the crystal growth of
L-NiO films was formed at 600°C. Therefore, the better
crystallization of L-NiO films is obtained using the
modifiedSPMmethod.
The XRD patterns of L-NiO films as a function of Li
concentrationareshowninFigure3.AlltheL-NiOfilms
have the polycrystalline structure and include the (111),
(200), and (220) diffraction peaks. The diffraction inten-
sity of (111), (200), and (220) peaks increases with
Li concentration, which leads to the increase of
crystallization.ThegrazingincidenceangleX-raydiffrac-
Figure1Resistivity,mobility,andcarrierconcentrationof tion (GIAXRD) patterns of L-NiO films in the 2θ range
L-NiOfilmsasafunctionofLiconcentration.
of 36° to 45° are also shown in the right side of Figure 3.
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Figure2SurfaceFE-EMimagesofL-NiOfilmswithdifferentLiconcentrations.(a)2,(b)4(c)6(d)8,and(e)10at%.
Using the texture coefficient (TC) equation, each peak from0.394to0.357asLiconcentrationincreasesfrom2to
areaintheGIAXRDspectra can bedefinedas: 10at%.Conversely,theTC valuechangesfrom0.602to
(200)
0.641, while the TC value decreases from 0.393 to
(220)
TCðhklÞ ¼XIðhklÞ (cid:2)100; ð5Þ 0.360.Itiswellknownthatthe(200)planeofionicrocksalt
IðhklÞ materialsisconsideredasanon-polarcleavageplaneandis
thermodynamicallystable,andthemoststableNiOtermin-
where h, k, and l are the Miller indices,TC(hkl) is theTC ation has a surface energy of 1.74 Jm−2. In contrast, the
value of specific (hkl) plane, I(hkl) is the measured peak (111)planeispolar andunstable.Therefore,the(200)pre-
intensity, and ΣI(hkl) is the summation of all intensities ferred orientation of L-NiO films can take on the better
for the peaks of L-NiO films. The TC value decreases conductive properties and can resist electrical aging. In
(111)
addition, the 2θ value of (111) diffraction peak is shifted
from 37.22° to 37.38° as Li content increases from 2 to 10
at%.ItimpliesthattheLi+(0.6Å)ionssubstitutetheNi2+
(0.69 Å) ions, and the smaller radius of Li+ ions would re-
sultinadecreaseoflatticeconstant.
The Ni 2p and O 1s XPS spectra of L-NiO films are
3/2
shown in Figure 4 as a function of Li concentration. The
deconvolution of Ni 2p electron binding energy to
3/2
Gaussian fit for NiO, Ni O , and Ni(OH) peaks is
2 3 2
854.0, 855.8, and 856.5 eV, respectively [12,13]. For
Ni 2p electron binding energy, the intensities of Ni2+
3/2
and Ni3+ bonding states increase with Li concentration
and lead to the decrease of resistivity for the L-NiO
Figure3XRDandGIXRDpatternsofL-NiOfilmsasafunction
films. The Ni(OH) bonding state is caused by the
ofLiconcentration. 2
adsorption of H O, and its intensity increases with
2
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Figure4DeconvolutionofNi2p andO1sXPSspectraofL-NiOfilms.Ni2p XPSspectraofL-NiOfilmswith(a)2,(b)6,and(c)10at%
3/2 3/2
ofLi.TheO1sXPSspectraofL-NiOfilmswith(d)2,(e)6,and(f)10at%ofLi.
Liconcentration.ThetendencyofNi2p peak suggests and the scattering effect occurs in higher Li-doped con-
3/2
that the Ni3+ bonding state increases with Li concentra- centration.(2)TheexistenceofNi3+ionsmeasuredfrom
tion, as shown in Figure 4a,b,c. The O 1s XPS spectrum XPS gives rise to the brown or black colorations [18].
ofL-NiO films is shown inFigure4d,e,f.Theintensity of The inset of Figure 5 presents the plots of (αhν)1/2 ver-
O 1s peak increases as Li concentration increases, and sus hν (photon energy) for L-NiO films. The optical
the deconvolution of electron binding energy of Li O band gap has been calculated by extrapolating the linear
2
(528.5 eV), NiO (529.9 eV), LiOH (531.1 eV), Ni O part of the curves. The optical band gap of L-NiO films
2 3
(531.9 eV), Ni(OH) (531.9 eV), and adsorbed O or H O graduallydecreasesfrom 3.08 to2.75 eV with Liconcen-
2 2
(532.5 eV) exists in the L-NiO films [13-17]. The inten- trationbecause ofthe decreasein carrier mobility. These
sity of LiOH bonding state, which is caused by the com- results are caused by the dopant Li ions which act as the
bining Li+ and the OH− bonds of H O, slightly increases scatteringcenter andhinder the carriertomove.
2
with Li concentration. Compared with other electron
binding energy, the binding energies for the Ni 2p of
3/2
Ni(OH) (856.2 eV) andthe O 1sofLiOH (531.1 eV) are
2
weaker in the modified SPM deposited L-NiO films.
This result demonstrates that the non-polar (200) phase
of L-NiO films increases with Li concentration (as
shown in Figure 3) because the non-polar (200) phase
exists with fewer dangling bonds, which cause the less
binding probability to exist between in L-NiO films and
water molecules.
The optical transmittance spectra of L-NiO films in
the wavelength range from 200 to 1,100 nm are shown
in Figure 5. The transparency of L-NiO films decreases
from approximately 89% to approximately 57% as Li
concentration increases from 2 to 10 at%. Two reasons
will cause this result: (1) Observing from the surface
morphology (FE-SEM images), the crystallization and Figure5TransmittancespectraofL-NiOfilmsdepositedwith
differentLiconcentrations.
grain size of L-NiO films increase with Li concentration,
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Conclusions 8. JosephDP,SaravananM,MuthuraamanB,RenugambalP,SambasivamS,
Non-vacuum SPMmethodwasusedtodeposithighqual- RajaSP,MaruthamuthuP,VenkateswaranC:Spraydepositionand
characterizationofnanostructuredLidopedNiOthinfilmsfor
ity p-type L-NiO films. The (200) preferred orientation of
applicationindye-sensitizedsolarcells.Nanotechnology2008,19:485707.
L-NiO films increases over (111) as the Li concentration 9. OhtaH,KamiyaM,KamiyaT,HiranoM,HosonoH:UV-detectorbasedon
increases,whichwouldcausethebetterconductiveproper- pn-heterojunctiondiodecomposedoftransparentoxide
semiconductors,p-NiO/n-ZnO.ThinSolidFilms2003,445:317–321.
ties and resist electrical aging in the L-NiO films. In this
10. MattheissLF:Electronicstructureofthe3Dtransition-metalmonoxides.I.
study,thecharacteristicsofmodifiedSPMdepositedL-NiO Energy-bandresults.PhysRev1972,B5:209.
films were comparable to the sputter-deposited ones, and 11. ChenX,ZhaoL,NiuQ:Electricalandopticalpropertiesofp-typeLi,
Cu-codopedNiOthinfilms.JElectroMater2012,41:3382–3386.
theoptimumLidopingamountissetat8at%.
12. JangWL,LuYM,HwangWS,ChenWC:ElectricalpropertiesofLi-doped
NiOfilms.JEurCeramSoc2010,30:503–508.
Competinginterests
13. YuGH,ZhuFW,ChaiCL:X-rayphotoelectronspectroscopystudyof
Theauthorsdeclarethattheyhavenocompetinginterests. magneticfilms.ApplPhysA2003,76:45–47.
14. OswaldS,BrucknerW:XPSdepthprofileanalysisofnon-stoichiometric
Authors’contributions NiOfilms.SurfInterfaceAnal2004,36:17–22.
C-CWparticipatedinthefabricationofLidopedNiOfilms,SEM,XRDand
15. TanakaS,TaniguchiM,TanigawaH:XPSandUPSstudiesonelectronic
XPSanalysis.C-FYparticipatedintheHallmeasurementandcalculatedthe structureofLiO.NuclJMater2000,283–287:1405–1408.
opticalbandgapofL-NiO.Allauthorsreadandapprovedthefinal 2
16. DedryvèreR,LaruelleS,GrugeonS,PoizotP,GonbeauD,TarasconJM:
manuscript.
ContributionofX-rayphotoelectronspectroscopytothestudyofthe
electrochemicalreactivityofCoOtowardlithium.ChemMater2004,
Authors’information 16:1056–1061.
C-CWwasborninTaiwan,in1979.HereceivedthePh.D.degreeinelectrical
17. WuQH,ThissenA,JaegermannW:Photoelectronspectroscopicstudyof
engineeringfromtheNationalSunYat-senUniversity,Kaohsiung,Taiwan,in
LioxidesonLiover-depositedVO thinfilmsurfaces.ApplSurfSci2005,
2009.In2009,hejoineddepartmentofelectronicengineering,KaoYuan 250:57–62. 2 5
University,whereheinvestigatedonorganic/inorganicnanocomposites
18. LuYM,HwangWS,YangJS:Effectofsubstratetemperatureonthe
materials,integratedpassivedevices(IPDs),transparentconductiveoxide resistivityofnon-stoichiometricsputteredNiOxfilms.SurfCoatTechnol
(TCO)films,electronceramicsandcarbonnanotubesandgraphene. 2002,155:231–235.
C-FYwasborninTaiwan,in1964.HereceivedtheBS,MS,andPh.Ddegree
inelectricalengineeringfromtheNationalChengKungUniversity,Tainan,
doi:10.1186/1556-276X-8-33
Taiwan,in1986,1988,and1993.In2014,hejoineddepartmentofChemical
Citethisarticleas:Chia-ChingandCheng-Fu:Investigationofthe
andMaterialsEngineering,NationalUniversityofKaohsiung,wherehe
propertiesofnanostructuredLi-dopedNiOfilmsusingthemodified
investigatedonferroelectricceramicsandthinfilms,applicationferroelectric spraypyrolysismethod.NanoscaleResearchLetters20138:33.
materialsinmemorydevices,organic/nanotubesnanocomposites,organic/
inorganicnanocomposites,YZOthinfilms,transparentconductionoxidethin
filmsandtheirapplicationsinsolarcells,microwaveantennas,and
microwavefilters.
Acknowledgement
TheauthorsacknowledgethefinancialsupportoftheNationalScience
CounciloftheRepublicofChina(NSC101-2221-E-244-006
and101-3113-S-244-001).
Authordetails
1DepartmentofElectronicEngineering,KaoYuanUniversity,Kaohsiung
82151,Taiwan.2DepartmentofChemicalandMaterialsEngineering,National
UniversityofKaohsiung,Kaohsiung81148,Taiwan.
Received:14November2012Accepted:22December2012
Published:18January2013
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