Research & Development, Tata Steel Limited, Jamshedpur, 831001, IndiaMST Division, CSIR-National Metallurgical Laboratory, Jamshedpur, 831007, India
r t i c l e i n f o
rticle history:eceived 18 September 2013eceived in revised form 30 October 2013ccepted 31 October 2013vailable online 8 November 2013
eywords:-ray photoelectron spectroscopy (XPS)
a b s t r a c t
An economic successive ionic layer adsorption and reaction (SILAR) method has been investigated fordepositing thin Cu2O layers on steel. The mole ratios of the Cu+ ions to OH− ions in the alkali bathwere varied and the changes in the properties were characterised by X-ray photoelectron spectroscopy(XPS), photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM) and Ultraviolet–visiblespectroscopy techniques. The increase in the binding energy values of the Cu 2p peaks in XPS establishedthat an optimum copper vacancy concentration can be obtained for a Cu+/OH− mole ratio between 1:10and 1:15; SEM studies confirmed a dense, uniform microstructure for Cu2O thin films coated with these
compositions. The strongest absolute peak intensity counts in PL for the peak at 580 nm along with lowenergy peaks (1.2–1.4 eV) due to Cu vacancy was found to be most prominent for thin film made withCu+/OH− mole ratio 1:15. The role of alkali concentration has been explained in relation to create a stableCu2−ıO structure with optimum copper vacancy. This is an easy way to modulate surface reactivity of theCu2O thin layers and the concept can be utilised for large area device integrations for various electricaland mechanical applications.
The Cu–O system has two stable oxides: cupric oxide (CuO)nd cuprous oxide (Cu2O). Copper and copper oxide (metal-emiconductor) is one of the first photovoltaic cells invented [1].upric oxide (CuO) is a non-stoichiometric semiconductor hav-
ng a monoclinic crystal structure and a band gap of 1.9–2.1 eV2]. Cuprous oxide (Cu2O) is an attractive semiconductor mate-ial that could be used as anode material in thin film lithiumatteries [3] as well as in solar cells [4]. Cu2O is a reddish p-ype semiconductor with a direct band gap of 2.17 eV [5]. Foru2O, it is known that the material usually includes considerablemounts of non-stoichiometry such as copper vacancies, VCu, andxygen vacancies, VO [5]. The Cu2O p-type conductivity is usuallyinked to the Cu vacancies (VCu), however the existence of oxy-en vacancies (VO) has also been considered to explain the general
ehaviour of their physicochemical properties [5,6]. Cuprous oxideraws significant interest because of its high optical absorption
∗ Corresponding author at: Materials Science & Engineering, Indian Institute ofechnology Patna, Patliputra Colony, Patna, BR 800013, India. Tel.: +91 612 2260232;ax: +91 612 2277383.
coefficient in the visible range with reasonably good electrical prop-erties [7].
Oxide layers on metal surface play a pivotal role by influenc-ing their properties for various technological applications owingto their nature, thickness and stoichiometry. Some of these casesinclude adhesion of elastomers to metal surfaces [8–11], sealing ofelectronic packaging [12–16], lowering of friction during wet draw-ing [17,18], enhancing solar cell efficiency [19–21], and extent ofbiocompatibility of biomaterials [22–24]. Cu2O has been proven tobe one such candidate that can improve metal–rubber adhesion inpneumatic cycle-tire beads [8,9] and as an adhesion promoter inthe area of electronic packaging [15,16].
Properties of bead wire have been investigated for past severaldecades since the introduction of coating on the bead wire, andadhesion is one of the big concerns for bead wire. Traditionally, itis assumed that adhesion is established by the chemical reactionbetween copper in the coating of a bead wire and active sulphurfrom rubber during vulcanisation. The properties of the oxide layerthat forms on most of metals play a vital part in the formation ofbonding during vulcanisation. In the case of copper, the ionic statecould be either monovalent of copper Cu+ or divalent of copper,
Cu2+, due to its dual oxide phases; Cu2O and CuO. It is thereforeproposed that copper sulphide is formed by the reaction betweenactive state of sulphur and the ionic state of copper, and this reac-tion requires electron transfer during the process. The particular
xide, which is the key ingredient for higher pull out force or bet-er adhesion, was found to be cuprous oxide [8]. Su et al. [9] haslso described in situ, continuous monitoring system designed toetect the rate of adhesive bond formation between rubber andteel reinforcement during the vulcanisation process. Parametersuch as oxide chemical structure and thickness were investigatedo assess the rate of bond formation during vulcanisation and theirffect on adhesion. The study also points towards the need of aethod for obtaining a defect controlled oxide surface.In this work, we have shown a simple method for developing a
u2O coated surface with modulated defect concentrations whichill finally change the surface reactivity. Cu2O thin films were
haracterised by X-ray photoelectron spectroscopy (XPS), photolu-inescence (PL) spectroscopy, scanning electron microscopy (SEM)
nd ultraviolet–visible spectroscopy techniques. The changes in thetructural properties of the thin Cu2O coatings have been suitablyharacterised and a sound correlation was obtained with respecto the varying defect concentrations of the coatings. The concen-ration of defects is expected to play a pivotal role during anyhemical reaction(s) thereafter, where, this oxide layer plays theole of an interface, e.g., between metal–rubber interface duringulcanisation. Moreover, these Cu2O coatings are made on steely an economic chemical process which can be readily used forny large area applications and will also be suitable for any devicentegration. The same Cu2O with modulated surface defects is alsoxpected to play an important role in other electronic and opto-lectronic applications, e.g., in solar cell and printed circuit boardsor electronic packaging.
. Experimental
We have used the parent recipe first reported by Ristov et al.25]. The recipe is based on a type of successive ionic layer absorp-ion and reaction (SILAR) process which was primarily popularisedor making sulphide thin films [26]. The core idea of the recipes to find a stable Cu+ complex which the authors [25] had madey mixing sodium thiosulphate and copper(II) sulphate solutions.he process then leads to the successive deposition of the complexolution and in a hot sodium hydroxide solution at 70–80 ◦C.
This reported recipe [25] used a particular molar ratio of Cu ionas in the complex) to the hydroxide solution concentration. Latern, many other researchers [27–31] have also followed the sameecipe with the same molar ratio for different applications. How-ver, there was no reason explained behind such a selection in anyf these reports. We, therefore, have tried to vary this particularolar ratio to investigate the effects that it could bring in terms of
he structural and other related properties of the films. The moleatios of the Cu+ ions to OH− ions were varied in the followingay: 1:4, 1:7, 1:10, 1:15 and 1:20. 1 M CuSO4·5H2O solution wasixed with a freshly prepared 1 M Na2S2O3·5H2O solution to obtain
final colourless solution; only demineralized water was used forhe whole synthesis process. 1 M hot solution of NaOH was madeeparately and was kept at 70 ◦C. For all the coatings, the wholerocess was carried out for two times with five different concentra-ions of NaOH bath. For each case, thin Cu2O layers were depositedn steel and sometimes on cleaned glass substrates. For the ease ofnderstanding, the respective samples will be designated as sam-les A (1:4), B (1:7), C (1:10), D (1:15) and E (1:20) throughout thisaper.
X-ray photoelectron spectroscopy (XPS) analyses were carriedut with SPECS, Germany spectrometer using Mg K� X-ray source
h� = 1253.6 eV). The binding energy of each XPS peak was cali-rated with the C 1s peak of energy 285.0 eV.
Photoluminescence (PL) spectroscopy measurements were car-ied out at room temperature using He–Cd laser operating at
Fig. 1. Cu 2p core-level XPS spectra of samples A–E; the inset figure (a) shows theextent of peak shifts (i.e. variation in binding energy) in all these samples.
325 nm. The PL signals were analysed using a TRIAX 320 monochro-mator and detected by a Hamamatsu R928 photomultipliertube.
The microstructure and surface morphology of the thin Cu2Ocoated steel samples were observed by a scanning electron micro-scope (SEM, SUPRA 25, ZEISS FEG). The voltage used for analysis was15 kV. Grain size measurements were carried out using an imageanalysis system and the mean linear intercept grain size was con-sidered. Measurements were carried out for minimum 400 grainsfor the selected samples. Film thickness was measured by Olym-pus (LEXT OLS4000) confocal laser scanning microscope. A step wasmade by etching the film with aqueous NH4Cl solution.
UV–visible diffuse reflectance spectra of Cu2O thin films on steelwere obtained using an UV–vis spectrometer (Shimadzu UV-2550)by using BaSO4 as a reference and the optical band gap values wereobtained using reflection data by the Kubelka–Munk method.
3. Results and discussion
XPS is a powerful tool to determine the chemical states of Cu.Therefore, this would be an ideal instrument to know the type(s)of compositional phases formed in the coated film. Furthermore,the binding energy of peaks depends on the chemical bonding ofCu atom with the neighbouring atoms. In fact, creation of vacancyor doping of any other elements around the Cu atom alters theelectron density on it, which ultimately affects the binding energy.Therefore, the chemical shift of XPS would also provide informationabout the vacancy formed either by the Cu ion or oxygen.
Figs. 1–2 show the Cu 2p and Auger XPS spectra of the samplesA–E prepared by varying Cu+ ions and alkali at the ratio of 1:4, 1:7,1:10, 1:15 and 1:20. The Cu 2p exhibits a doublet peak (Fig. 1) due tothe strong spin–orbit coupling. The peaks at 932.37 and 952.19 eVof the sample A (1:4) are assigned to the Cu 2p3/2 and Cu 2p1/2 peak,respectively. According to the literature data [32–38], the observedpeak at 932.6 eV is due to the presence of Cu2O. This also rules outthe presence of impurity phase of CuO, which shows the main peakand satellite peak at 933.8 eV and between 940–945 eV respectively[34,35]. However, due to the proximity between the binding energy
values [35–37] of Cu and Cu2O, we decided to analyse the Cu LMMAuger peaks, Fig. 2. The individual energy values are illustrated inTable 1. In general, the Auger parameter depends on the dielec-tric and electronic properties of the photoexcited atom. Since the
432 A. Chowdhury et al. / Applied Surface Science 289 (2014) 430– 436
Fig. 2. Cu(LMM) Auger spectra for samples A–E with varying mole ratios of the Cu+
ions to OH− ions.
Table 1XPS data table showing binding energy for the Cu 2p peak and the kinetic energyvalues for the Cu LMM Auger peaks for Cu2O.
Cu(2p3/2) Cu LMM (KE) Reference
932.37 (1:4) 917.02 (1:4) Present work; the ratiosmentioned in the bracketrepresent mole ratios of theCu+ ions to OH− ions as usedfor synthesis
lectronic structure of Cu2O is different than Cu, the Auger peakf Cu2O appears at 916.5 eV [36] while the Cu LMM appears at18.4 eV [36]. We conclude from the binding energy of the Cu 2peak in association with the Auger peak that our coated samplesn steel predominantly contains Cu2O phase. Besides, it is notedrom the spectra that the binding energy values of the Cu 2p peakncreases with the alkali ratio up to 1:10 (sample C) and then,t decreases. We have confirmed this trend in our repeated trials
ith different thickness values for the Cu2O films. Since it rules outhe presence of impurity phase, the shifting of binding energy cane explained related to the vacancy concentrations in these Cu2Olms. The copper or oxygen vacancy formation in Cu2O could haveappened either by missing of Cu atom or oxygen atom. It is knownhat the missing of Cu or oxygen atoms alters the electron densityf its neighbouring atoms, which leads to chemical shift. Therefore,he analysis of chemical shift yields important information abouthe origin of defect. In a stochiometric copper oxide compound,hemical shift is dominated by electronegativity. Since O atom hasore electronegativity value than the Cu atom, a partial positive
harge develops on the Cu atom when the Cu atom is bonded withhe O atom. Such electronegative effect on Cu atom does not existn the case of the missing of its neighbouring O atom [39–41]. As
result, based on the electron density on Cu atom, it is expectedhat the binding energy should decrease in the case of vacancy dueo oxygen atom, whereas it should increase due to the Cu vacancy.
Fig. 3. PL spectra for samples B, C, D and E with normalised intensity counts; theinset plot (a) showing the same with absolute intensity counts.
Therefore, the shifting of binding energy towards higher energyvalues indicates that the concentration of the Cu vacancies createdin the film also increases as the alkali (NaOH) ratio is increasedup to the optimum ratio of 1:10. Due to closeness of the bindingenergy values between C (1:10) and D (1:15), it can be concludedfrom the XPS analyses that the changing of alkali ratio in the rangebetween 1:10 and 1:15 optimises Cu vacancy concentrations in thefilms. It should also be noteworthy that the evidence Cu(OH)2 is notobserved in the XPS spectrum.
Photoluminescence (PL) spectroscopy is an effective tool tostudy the defects and opto-electronic properties of the samples.Generally, the peaks relating to the different electronic transitionscan be categorised into two main groups: the band-to-band tran-sition (free exciton emissions) and the impurity transition (boundexciton emission from impurities). The excitation wavelength of325 nm is used as the excitation source with photon energy of3.8 eV, which was higher than the band gap of Cu2O. PL spectraof the Cu2O thin films coated on steel substrates are shown inFig. 3. Steel substrate will be PL inactive and hence we believethat all the signals are coming from the thin layers of Cu2O. Theabsolute intensity PL plot (inset Fig. 3a) clearly depicts that the sig-nal intensity is highest for sample D (1:15) for both 500–700 nmand 900–1050 nm regions. The PL plot has been shown separatelyfor sample D (1:15) with peak fitting, Fig. 4. For the ease of dis-cussion, we have split the PL spectra in two wavelength regions:900–1050 nm and 500–700 nm.
The higher wavelength region of 900–1050 nm is primarilydue to different vacancy related defects. For sample D, the majorpeaks and/or shoulders are noticed at energies of 1.33 eV (933 nm),1.31 eV (950 nm), 1.27 eV (980 nm) and 1.24 eV (1000 nm). The nor-malised spectra also clearly reveal that this region (900–1050 nm)is most prominent and intense for sample D (1:15), hence we haveselected sample D for designating the peaks for the energy transi-tions. The peaks at emitting energy around 1.27 eV (980 nm) and1.31 eV (950 nm) are due to the emission from excitons boundedby copper vacancies [42,43]. The peak at 1.24 eV (1000 nm) is com-ing from transitions from the trap states created by the copper andoxygen vacancies [44]. Park et al. also have attributed the peaks
below 2 eV to the defect peaks [45].
The next section is for direct band-to-band transition (freeexciton emissions) in the wavelength range 500–700 nm. The
A. Chowdhury et al. / Applied Surface Science 289 (2014) 430– 436 433
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ig. 4. PL spectra for sample D (1:15) showing peaks at 554 nm, 580 nm and 605 nmfter Gaussian multipeak fitting.
ajor peaks located here are as follows: 2.24 eV (554 nm), 2.14 eV580 nm), 2.05 eV (605 nm) and 1.91 eV (650 nm). Quite a few smallut sharp peaks appeared between 660 and 890 nm region areelieved to be due to plasmonic resonance and hence not includedor our discussion. The emission at 1.91 eV (650 nm) is due to theecombination of excitons bounded to oxygen vacancies (VO) [43].his peak has often been correlated with an intrinsic property of therystal due to the formation of exciton-defect complexes; whichre analogous to �-centres in the alkali halides [44,46]. The highestntensity peak was noticed for 580 nm (2.14 eV) peak for transitionrom 3d shell; this is the strongest peak observed in all samples.esearchers have reported strongest intensity PL peak at 570 nm
or Cu2O nanostructures and have correlated this to quantum con-nement effect [47–49].
Because the band gap of Cu2O is about 2.17 eV, the emitting at.05 eV (605 nm) can be referred as near band emission. Also, themission peak of 2.24 eV (554 nm) is attributed to the excitonicransitions from the different sublevels of conduction band to theu d-shells of the valence band [43]. Noteworthy is the change inhe ratio of their relative intensities of the peaks at 554 nm and80 nm as observed in the normalised intensity plot (Fig. 3). This
554/I580 ratio is minimum (0.66) for sample E (1:20) and graduallyncreases to 0.69, 0.78 and 0.78 for samples B (1:7), C (1:10) and
(1:15), respectively. The peak intensity, in any PL experimentill be proportional to the number of photons released, which is
gain directly related to the density of carriers. Cu vacancy is theredominant carriers in all these cases and therefore, we concludehat this concentration of vacancy is highest for these samples (Cnd D). From XPS analyses also, we found that the binding energyor the Cu 2p is maximum and very close for samples C (1:10) and D1:15). Although, it must be noted that, the absolute peak intensityounts for the peak at 580 nm was maximum for sample D and alsohe low energy peaks (900–1050 nm) due to Cu vacancy was foundo be most prominent for sample D (1:15).
The proposed band diagram has been shown in Fig. 5 and ourL results obtained room temperature matches reasonably well
ith previous works [44,46]. The types of defects mostly discussed
or Cu2O lattice involve: (a) neutral or ionised copper and oxygenacancies, (b) associations and/or correlations of multipolar char-cters formed by copper and oxygen vacancies. The concentrations
Fig. 5. Schematic energy band diagram (based on photoluminescence results)showing the defect levels in Cu2O.
and the ionisation states of these defects vary upon annealing andcooling conditions [44]. Presumably, a lot of these defect associa-tions are found at low temperatures as reported previously [44,46].Our PL data at room temperature, however, mostly found tran-sitions involving neutral copper and oxygen vacancies. The newaddition is due to the peak at 2.24 eV (554 nm) which we corre-lated with the transitions from the higher sub-levels to the 3d state[43,45].
The film morphology of sample A (1:4) is typical of a combina-tion of different sized spheres accumulated on a surface with anaverage size of 130 ± 40 nm, Fig. 6a. Presence of sharp ribbon-likestructures was also noted in abundance. These structures are notedmore at the interface of the coated and uncoated part of the steelsubstrate. We are unsure about their origin. Similar long ribbon-likenanostructures with sharp boundary has been previously reportedfor Cu(OH)2 [50]. Researchers have also reported these types ofrod/needle shaped structures for Cu-oxide systems [51–54] .Thisevidence has been correlated with the Raman data and discussedin supplementary Fig. s1.
Presence of these ribbons is also noted also in B (1:7) but in smallnumbers, Fig. 6b. No ribbon-like flakes was observed (Fig. 6c–d) forsamples C (1:10), D (1:15). Their grain sizes were also similar forsamples B (70 ± 20 nm), C (70 ± 20 nm) and D (60 ± 20 nm). Sam-ple B (1:7) showed less topography compared to A (1:4) and thegrain size distribution is more uniform here. In terms of film con-tinuity and density the film samples B (1:7), C (1:10) and D (1:15)appears to be similar. The thickness of the films for C (1:10) and D(1:15) appears to be similar as 70 ± 15 nm. Unlike sample C and D,film E (1:20) lacked the continuity and the thickness was around85 ± 30 nm.
Fig. 7 represents the spectral variation of the (F(R)h�)2 of sam-ple B and C cuprous oxide thin films deposited on steel substrates;reflectance (R) versus wavelength graph of same samples is shownin inset. The diffuse reflectance R can be used to define a functionF(R), Kubelka–Munk function [55,56]. Cuprous oxide is a well-known direct band gap semiconductor [20,57] and the energy gap(Eg) can thus be estimated by assuming direct transition betweenconduction band and valance bands.
The value of the direct band gap has been found to be 2.64 eVand 2.87 eV for B (1:7) and C (1:10) cuprous oxide films respectively,Fig. 7. This value is slightly higher [31,58] than those reported for
Cu2O thin films. We attribute these changes to film thickness, grainsize, annealing conditions and vacancy/defect concentrations. Nairet al. [31] reported an optical band gap of 2.4 eV for Cu2O film of
434 A. Chowdhury et al. / Applied Surface Science 289 (2014) 430– 436
Fig. 6. SEM micrographs of the selected Cu2O thin films coated on steel substrate
Fss
71a
dv
ig. 7. Determination of band gap of selected Cu2O coatings (samples B and C) onteel substrates by drawing a tangent line at[F(R)h�]2 = 0. The inset figure in (a)hows the diffuse reflectance (%R) spectra of those Cu2O coatings.
0 nm thickness to approximately 2.0 eV in the case of a film of50 nm thickness. Annealing of such films further resulted in minor
lteration of the band gap values.
Cu2O has often been termed as a self-compensating semicon-uctor [59] where the concentration of the copper and oxygenacancies are competing each other. Even without any doping, a
for samples (a) A (1:4), (b) B (1:7), (c) C (1:10) and (d) D (1:15) respectively.
small synthesis parameter alteration, e.g., pH [60], surfactant [61]changes its type of conductivity by changing the concentrations ofthe major carrier. It has been widely reported that p-type Cu2O isformed in alkaline condition (pH 7–12) while strong acidic condi-tions (<pH 5) facilitate the growth of n-type Cu2O [60,62].
Also, it has been previously reported that band gap shift has adirect connection with NaOH concentration for other oxide sys-tems, e.g. ZnO [63,64] where the authors correlated the decrease ofoptical band gap of ZnO with the decrease of NaOH concentrationduring synthesis. In our case for the Cu2O thin films, we observe asimilar trend in the band gap value for C (1:10) compared to B (1:7)as the NaOH concentration is decreased. Noteworthy is also thechange in slope in the lower h� region (2.49–2.67 eV). Researchershave correlated such definite changes in slope with different exist-ing trap levels in other systems [65]. However, we believe thatthis needs a further in-depth analysis of the existing spectroscopyresults and hence keep this open for a scope of future work.
The thiosulfatocuprate(I) complex is a stable complex for Cu+
ions. In our synthesis process the main reason for varying the moleratios of the Cu+ ions to OH− ions was to find a suitable conditionsfor film synthesis process with tailor-made defect concentrations,if any. From the SEM study it was revealed that the mole ratio ofthe NaOH bath plays a pivotal role in the final film compositionand continuity. Phase evolution study by XRD showed only Cu2O
phase in all these films (supplementary Fig. s2). Combined resultsfrom PL and XPS studies have also shown that the mole ratio of theNaOH bath is also very important for optimising the defect/vacancyconcentrations of these films.
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The presence of flaky ribbon like nanostructures is in abundanceor sample A (1:4) and in reduced quantity for B (1:7) probablyndicates the formation of Cu(OH)2. The reaction mechanism forhe formation of the Cu2O thin films can be summarised as:
Cu (S2O3)]− ↔ Cu+ + S2O32− (1)
u+ + OH− → CuOH (2)
CuOH → Cu2O + H2O (3)
The concentration of the alkali bath here plays an important rolen the supply of OH− ions in the solution which in turn controls theate of reactions. In SILAR methods, all reaction precedes via selec-ive adsorption method. In other words, when the supply of OH−
ons is low, the free Cu+ ions react with water during washing of thelms and while drying by hot air blower they get converted to Cu-ydroxide, i.e., the reaction sequence (Eqs. (1)–(3)) is not followed.s the alkali concentrations increased (for samples C, D and E), all
he Cu+ ions get combined with enough number of OH− ions toscertain the formation of Cu2O only (according to Eq. (3)). How-ver, this higher concentration of OH− ions for sample C (1:10)nd D (1:15) also plays a crucial role in changing the defect con-entration in those films. More number OH− ions is also expectedo increase the supply of more oxygen atoms in the Cu2O latticend thereby changing the effective Cu:O ratio of the parent struc-ure. Finally, this leads to an increase in the Cu vacancies along withhe possible formation of oxygen interstitials and thereby changinghe stoichiometry of these films as Cu2-�O. Apart from the changen alkali concentration, other processing parameters were sameor all the films. Also from SEM study, we do not see any drastichanges in microstructure between samples B (1:7), C (1:10) and
(1:15) and hence correlate all the changes in the properties tohe alteration of the Cu vacancy concentrations. Researchers havelso investigated this issue to understand this non-stoichiometrynd structural defects present in cuprous oxide system [66–68]. Weried to find out this value of ‘ı’ by XPS but it was not possible dueo the involvement of the carbon tapes which was also interferingith the oxygen atoms. In other words, it would have been erro-eous to correlate the oxygen contribution completely to the Cu
ons for the XPS experiment.Beyond a certain point, as observed in sample E (1:20), this con-
ept fails. The concentration of defects is decreased and the SEMicrograph also depicts a porous and non-uniform film (supple-entary Fig. s3). This is probably due to severe lattice distortions
ccurred during the synthesis of the Cu2O thin films due to veryigh alkali concentrations. Therefore, the whole study confirms thathe mole ratios of the Cu+ ions to OH− ions is ideal between 1:10 and:15 for optimised film properties. These defects are found to be sta-le, i.e., the properties were reproducible when measured betweenifferent time periods in weeks. This gives a unique opportunity forhese Cu2O thin films to be integrated directly with steel and a rangef other substrates for advanced technological applications.
. Conclusions
A simple economic method has been derived for synthesisingu2O thin films with modulated surface reactivity by altering theefect concentrations in the films. The XPS analyses showed an
ncreasing trend in binding energy with the increase in the Cu+ iono alkali bath concentration below 1:15. The PL results depictedarious possible transitions due to enhancement of copper vacan-ies and illustrated most intense response for sample D (1:15). A
ense microstructure with uniform grains were observed for sam-le C (1:10) and D (1:15). The gradual fall of the curves in UV-visiblepectra indicates that the band gap of Cu2O films is mostly associ-ted with trap states. The role of alkali bath has been explained
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Science 289 (2014) 430– 436 435
in relation to optimum copper vacancies against severe lattice dis-tortions which degrade film properties at very high ratios (beyond1:15). These films coated on steel can be readily integrated for alltypes of large area electronic and mechanical applications whererole of Cu2O can be utilised as an interface.
Acknowledgement
AC is grateful to Prof. Anushree Roy and Ms Barnita Paul for theirhelp in carrying out the Raman experiments at their laboratory atIIT Kharagpur. AC and PKB also extend their thankfulness to Prof.Samit Kumar Ray and Ms Tamita Rakshit for necessary help in con-ducting the room temperature photoluminescence spectroscopyexperiments at IIT Kharagpur.
Appendix A. Supplementary data
Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2013.10.183.
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