Band Gap Tuning in ZnO Through Ni Doping via Spray Pyrolysis - The

Article pubs.acs.org/JPCC

Band Gap Tuning in ZnO Through Ni Doping via Spray Pyrolysis Sanjoy C. Das,† Robert J. Green,‡ Jiban Podder,*,† Tom Z. Regier,§ Gap Soo Chang,‡ and Alexander Moewes‡ †

Department of Physics, Bangladesh University of Engineering & Technology, Dhaka-1000, Bangladesh Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan, Canada, S7N 5E2 § Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 0X4 ‡

ABSTRACT: The tuning of semiconductor band gaps can often provide significant performance increases and new applications for electronic, optoelectronic, and photocatalytic devices. Here, we study the band gaps of pure and nickeldoped zinc oxide thin films synthesized using the low-cost spray pyrolysis deposition method. Nickel concentration is varied from 0 to 15%, and the effects that this doping has on the electronic structure are analyzed. Using optical and synchrotron X-ray techniques, two regimes of band gap reduction via Ni doping are uncovered. For doping up to 4% Ni, there is a strong reduction in the gap, while continued doping up to 15% further reduces the gap, but to a lesser extent. The results are explained using X-ray spectroscopy and an Anderson impurity model. These tools show that the low doping case is driven by the interaction of the Ni 3d and O 2p states in both the valence and conduction bands. At high doping, the removal of Zn 3d states from the valence band and the change in Ni coordination from Td to Oh both contribute to counteract the gap reduction. These results show how Ni can be used to tune the ZnO band gap over a large range useful for many applications.

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INTRODUCTION Zinc oxide (ZnO) is a popular semiconductor, exhibiting attractive properties such as nontoxicity and good electrical, optical, and piezoelectric behaviors.1−3 Further, the wide band gap of ZnO (3.4 eV) makes it appealing for the development of light-emitting diodes,4 photocatalysists,5,6 gas sensors,7 solar cells,8 etc. Recently, ZnO has become prominent in the field of magnetic semiconductorsa class of materials intensively studied for applications to spintronics.9,10 Though ZnO itself does not exhibit ferromagnetic properties, the introduction of transition-metal dopants into ZnO has been shown to yield ferromagnetism, resulting in materials belonging to a class called diluted magnetic semiconductors (DMS).11−18 These DMS materials exhibit simultaneously ferromagnetic and semiconducting properties and are thus targeted toward spintronics applications. Previous studies of Ni-doped ZnO have uncovered ferromagnetic order at room temperature.19 Small amounts of Ni can introduce useful changes to the structural, optical, and electrical properties of ZnO, but the doping can be challenging due to the propensity for phase segregation into NiO and ZnO.20 Many different techniques can be used when doping via thin film deposition. Among the host of possibilities, spray pyrolysis deposition is a simple, economical, viable approach and capable of producing good quality films without the use of high-vacuum systems. In the work reported here, we synthesize Ni-doped ZnO films using spray pyrolysis. For varying Ni concentrations © XXXX American Chemical Society

we employ various characterization techniques to study the physical properties of the films. We uncover a nanofibrous structure and a varied reduction of the electronic band gap that is dependent on the doping level. Using soft X-ray spectroscopy, we show that the occupied Ni 3d states are responsible for raising the valence band maximum and lowering the conduction band minimum, thus reducing the band gap. Further, we show clearly that Ni is a substitutional dopant (for Zn) at low Ni concentrations, but an octahedrally coordinated Ni phase appears at higher doping levels. Our results provide insight into both film synthesis using spray pyrolysis and the applicability of Ni-doped ZnO to optical and electronic applications.

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EXPERIMENTAL AND THEORETICAL METHODS Ni-doped ZnO thin films were deposited onto chemically cleaned glass substrates using the spray pyrolysis technique. The deposition was carried out in a simple and low-cost fume chamber. As shown in Figure 1, the setup includes the precursor solution, carrier gas assembly, heater, substrate holder, and temperature measurement arrangement. The carrier air pressure was kept at 0.5 bar and the spray time was 5 min for each sample. Zinc acetate [Zn(COOCH3)·2H2O] and Received: December 21, 2012 Revised: May 24, 2013

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dx.doi.org/10.1021/jp3126329 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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source. Optical transmittance measurements were performed to measure the band gap, and temperature dependent electrical resistivity was studied from 300 to 440 K via the Vander Pauw method. To study the electronic structure of the films, soft X-ray emission spectra (XES) and resonant inelastic X-ray scattering (RIXS) spectra were measured using Beamline 8.0.1 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory.21 The beamline uses a Rowland circle-type diffraction grating spectrometer with a 90° scattering angle, and the incident radiation is linearly polarized within the scattering plane. Soft X-ray absorption spectra (XAS) were measured using the spherical grating monochromator (SGM) beamline of the Canadian Light Source (CLS) at the University of Saskatchewan.22 Oxygen K-edge spectra were collected using partial fluorescence yield (PFY), while the Ni L-edge spectra were obtained using inverse partial fluorescence yield (IPFY),23 both with horizontal plane-polarized incident radiation. The IPFY technique uses the nonresonant fluorescence from bystander (oxygen) atoms as a bulk-sensitive, saturation-free probe of the X-ray attenuation when measuring the Ni L-edge absorption. This technique was implemented in order to avoid sample charging distortions found in electron yield spectra and saturation effects found in fluorescence yield spectra, even for Ni concentration as low as 10%. For these IPFY measurements, two to four scans were measured per sample with three separate silicon drift detectors (SDDs) simultaneously recording, in order to obtain sufficient statistics from the dilute atoms. Calibration of the O K-edge spectra was performed using a bismuth germanate reference sample, calibrating the main XES peak to 526.4 eV and the first XAS peak to 532.7 eV. The resulting calibration of our pure ZnO spectra agrees well with the literature.24 For the Ni L-edge spectra, NiO was used as a reference, and the main L3 XES and XAS peaks were calibrated to 850.9 and 853.1 eV, respectively. Using this calibration the L3 RIXS spectrum for NiO, when plotted on an energy loss scale, agrees well with the literature.25

Figure 1. Experimental setup of the spray pyrolysis system.

nickel acetate [Ni(COOH3)3·4H2O] were used as the precursor sources of Zn and Ni, and deionized water was the solvent. Zn1−xNixO thin films were prepared using 2, 4, 10, and 15 mol % of nickel. The concentration of the solution was kept at 0.1 mol/L. The substrate temperature was kept near 300 °C and was monitored using a copper−constantan thermocouple. Given the acetate precursors, the reaction that takes place on the heated substrate is Zn(COOCH3) ·2H 2O + Ni(COOH3)3 ·4H 2O 300 ° C

⎯⎯⎯⎯⎯⎯→ Zn1 − xNixO + CO2 + CH4 + steam

The relative stoichiometries of the films were verified using energy dispersive X-ray (EDX) spectroscopy and were found to be within 10% of the expected values according to the precursor mole ratios. Scanning electron microscopy (SEM) micrographs were obtained to study the structural morphology. The crystal structure properties of the films were investigated using X-ray diffraction (XRD) with a Co Kα (λ = 1.79 Å)

Figure 2. SEM micrographs of Zn1−xNixO films with doping fractions x of (a) 0.00, (b) 0.02, (c) 0.04, (d) 0.10, and (e) 0.15. The white scale bar in the lower right of each image is 10 μm. B



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The analysis of the Ni L2,3-edge XAS and RIXS was aided by the use of multiplet ligand field theory (MLFT)26 simulations. The simulations include multiplet effects, crystal field splittings, and hybridization with ligands. Here, we use effectively a single impurity Anderson model (SIAM), where hybridization with the broad O 2p band is included, rather than just the nearest neighbors as in the cluster approach. While the cluster approach is usually very successful for XAS simulations, the impurity method is essential for reproducing the broad charge transfer features of RIXS spectra, as shown below. The SIAM as employed here is a model Hamiltonian approach, where adjustable parameters are fit to the experiment. The parameter set includes typical crystal field splitting paramers (10Dq, Dσ, Dτ)26,27 and symmetry-dependent hopping integrals (Ve, Vt2).26 Also included are the charge transfer energy (Δ), the difference between the onsite Coulomb repulsion and the core hole potential (U − Q), and the ligand bandwidth (W) and shape. The RIXS spectra were simulated with the same scattering geometry as used in experiment. Due to the polycrystalline nature of the samples (discussed below), randomly oriented local structures were assumed for both XAS and RIXS spectra. Separate calculations were done with the incident linear polarization vector along the x, y, and z axes, and the resulting spectra were summed together for comparison with experiment. Both the simulated XAS and RIXS spectra were convolved with Gaussian and Lorentzian line shapes to account for instrumental and lifetime broadening, respectively.

Figure 3. XRD patterns for Zn1−xNixO films. The inset shows the calculated grain sizes based on the XRD data.

Table 1. Crystallite Size of Zn1−xNixO Films Deposited on Glass Substrates

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RESULTS Structural and Compositional Properties. Figure 2 shows the surface morphologies of the deposited films taken by SEM at 10 000 × magnification. The film surfaces were found to cover the substrates uniformly. The SEM images revealed that the deposition led to clusters with well-defined nucleation centers and the clusters consist of highly dense ganglia-like fibers over a large area around the nucleation centers. From the similar structures seen in each case, it is clear that the crystallization behavior of ZnO is not much influenced by the level of Ni doping used. The fibers are randomly oriented with various lengths, typically around 2−2.5 μm. Similar nanofiber structures were observed for sol−gel deposited Ni-doped ZnO.28 Figure 3 shows XRD patterns of the synthesized films. The prominent diffraction peaks can be identified as a ZnO phase with hexagonal wurtzite crystal structure. From the broad backgrounds in the patterns, it is evident that the films are somewhat amorphous but with a mainly polycrystalline nature in all cases. The characteristic peaks at 2θ = 37.45°, 40.55°, 42.60°, 52.70°, 55.85°, and 63.10° correspond to (hkl) values of (100), (002), (101), (102), (110), and (103), respectively.29 From these peaks, the interplanar spacing was calculated from 1/2 ⎡ 4 ⎛ h2 + hk + k 2 ⎞ 1 l2 ⎤ =⎢ ⎜ ⎟ + 2⎥ dhkl ⎠ c ⎦ a2 ⎣3⎝

a (Å)

c (Å)

c/a ratio

grain size (nm)

3.219 3.227 3.227 3.231 3.227

5.166 5.172 5.178 5.184 5.178

1.6046 1.6023 1.6043 1.6041 1.6043

20 16 11 13 14

the doping changes, with slight increases for increasing Ni up to 10% and then a decrease as the doping is increased to 15%. Also evident in the higher doped samples is a peak at 2θ = 44.5°, indicative of a segregated NiO-like phase being formed. The appearance of a secondary phase at this doping is reasonable, as earlier studies have suggested that maintaining a single phase beyond this level is difficult.30 The crystallite sizes of the films were calculated from the XRD results using31 D = 0.94

λ B cos θ

(3)

where D is the crystallite size, λ is the wavelength of the X-rays used, θ is the diffraction angle, and B is the full width at halfmaximum (fwhm) of the XRD peak. The (100) plane XRD peak was used for the calculations, and the results are shown in Table 1 and the inset of Figure 3. We see a minimum crystallite size at x = 0.04, similar to a study on Zn1−xNixO prepared via a solid-state reaction technique.32 This increased grain size above ∼4% doping could be related to the increased formation of an NiO-like phase, as described above. Electrical Properties. The resistivity of the pure ZnO and Ni-doped ZnO thin films was measured by the Vander Pauws method. Resistivity measurements were made on as-deposited films from room temperature to 440 K in air. During each measurement, the temperature was increased slowly to ensure that the whole film had a uniform temperature. The variation of resistivity with temperature is shown in Figure 4. One can see that the resistivity gradually decreases with increasing temperature, indicative of the semiconducting nature of the materials. The figure also shows that the resistivity decreases with increasing concentration of Ni, as has been noted in previous

(1)

where dhkl, the inter planar spacing, is related to the diffraction angle by 2 sin θhkl 1 = dhkl λ

x 0.00 0.02 0.04 0.10 0.15

(2)

The calculated lattice parameters are reported in Table 1. It is evident that the lattice parameters remain almost constant as C



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where A is a constant, hν is the photon energy, and α is the absorption coefficient. The values of (αhν)2 are plotted against hν in Figure 5b. The linear nature of the plots at the absorption edge confirm that the films are semiconductors with direct band gaps. The optical band gap decreases from 3.47 eV for the undoped film to 2.87 eV for 15% Ni doping. The variation of band gap with Ni concentration is shown in Table 2 and Table 2. Band Gap Energies Determined from Optical and X-ray Spectra Eg (eV)

Figure 4. Temperature-dependent resistivity measurements for the Zn1−xNixO films.

Zn1−xNixO studies.33 The room temperature resistivity of the films decreases with increasing Ni and is found to be 14.48 × 10−3, 10.52 × 10−3, 10.12 × 10−3, 7.95 × 10−3, and 6.50 × 10−3 Ω·m for doping levels of x = 0.00, 0.02, 0.04, 0.10, and 0.15, respectively. At a temperature of 440 K, the resistivity of the films drops to 2.99 × 10−3, 1.80 × 10−3, 0.92 × 10−3, 0.51 × 10−3, and 0.22 × 10−3 Ω·m for the same respective doping levels. This observed reduction in resistivity upon doping is understandable, as some Zn atoms with full 3d shells (providing only semicore valence band states) are replaced with Ni, where the 3d shell is not full (3d8) and has states near the bottom of the conduction band. This addition of conduction band states will accommodate more conduction electrons and therefore lower the resistivity. Optical Properties. Optical transmission spectra for the films over the 300−1100 nm wavelength range are shown in Figure 5a. Absorbance measurements were performed to determine the band gaps of the films. The optical band gap (Eg) was determined using34 (αhν)2 = A(hν − Eg )

x

optical (±0.10)

X-ray (±0.15)

0.00 0.02 0.04 0.10 0.15

3.47 3.15 3.03 2.94 2.87

3.32 3.13 2.98 2.89 2.93

(4)

Figure 6. Oxygen K-edge XAS and XES spectra. The complete spectra are shown in the upper panel (a). Panel b in the lower left shows a close up of the near gap region and marks the valence band maxima (VBmax) and conduction band minima (CBmin) determined from the second derivatives, which are also shown. Panel c plots the VBmax and CBmin against the Ni concentration. Panel d in the lower right compares the band gaps derived from the X-ray spectra to the optical gaps described earlier.

plotted in the bottom right panel of Figure 6. It is observed that a small amount of Ni present in the films greatly reduces the optical band gap of ZnO, while increased doping levels lead to a less drastic but continued reduction. The error associated with these measured gaps is extracted from the linear regression used to fit the straight lines and from the limited number data points within the straight line region of the data. Note that the band gap of pure NiO is in the range of ∼3.7−4.0 eV,35,36 and therefore, the doping is not tuning the gap of ZnO in the direction of that of NiO. This is perhaps not surprising, since the gap modification upon doping is a consequence of the interaction of the Ni states and host ZnO states, which is not

Figure 5. Optical properties of the films. Panel a shows the transmittance of the films in the optical region, while panel b displays the band gap extraction. D



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the XRD results above, and will also be discussed in the Discussion section. To obtain more specific details about the Ni dopants and their interaction with the ZnO, Ni L2,3 XAS spectra were measured, as presented in Figure 7a. The spectra show a

necessarily related to the band gap mechanism in pure NiO. Finally, this band gap reduction with increased Ni concentration could be attributed to the introduction of Ni states at the top of the valence band9 and suggests that the band gap could be tuned for applications in various electronic and optical applications. Electronic Structure. X-ray spectroscopy provides an alternative, complementary way to probe the electronic band gap, as well as projected components of the density of states (DOS). Using O K-edge XES and XAS, one can probe the O 2p-projected states in the valence and conduction bands, respectively. For oxides and related materials, this is an effective way to determine the electronic band gap by measuring the relative energies of the valence band maximum (VBmax) and conduction band minimum (CBmin).37 Considering first the O K-edge XAS results in Figure 6a, we see that the spectra generally share a similar shape typical of ZnO.38 This similarity is expected since the spectra are probes of the O atoms, and not directly the Ni dopants. Additionally, we see that the features in the spectra of the doped samples appear washed out compared to the undoped sample. Such an observation is understandable, since the Ni doping may introduce a different chemical potential than Zn, which alters the electronic structure. Similar observations can be made about the XES spectra, which are excited off-resonance at ∼570 eV. The XES spectra of the samples again share a common ZnO-like shape, but the spectra of the doped samples are broadened slightly compared to that of the undoped sample. Figure 6b shows an enlarged view of the near-gap region for the XAS and XES. Here the positions of VBmax and CBmin are marked (using the same color scheme as the upper panel), as determined using the second derivative method.37 For reference, the second derivatives of the spectra are shown at the bottom of the panel. The extracted energy values of CBmin and VBmax are plotted against the Ni doping concentration in Figure 6c. We see that the overall band gap reduction is due to a continuous raising of the VBmax through doping and an initial lowering of the CBmin, which reverses when the doping becomes too high. From these values we can extract estimates of the band gaps, which are plotted in comparison with the optical gaps in Figure 6d. The error values for the gaps determined from the X-ray spectra are estimated on the basis of the point spacing of the data (0.1 eV for XAS and 0.05 eV for XES). We see good agreement between the two techniques, with the XAS/XES technique generally providing slightly reduced gaps. This is to be expected, to a certain extent, as the 1s core hole present in the X-ray absorption process often pulls the conduction band states down by 0.1 eV or more, depending on the system. Returning to Figure 6c, some interesting notes can be made. First, we see that the value of VBmax increases strongly for doping up to x = 0.04 and then increases at a reduced rate as doping is increased to 15%. The increase in VBmax is further evidence of the presence of Ni 3d states at the top of the VB, as alluded to above when discussing the optical results and suggested in an earlier study.9 The diminishing nature of the increase in VBmax is perhaps intuitively understandable, but it will be explained below in the Discussion section. Also in Figure 6c, we see the value of CBmin is reduced strongly for low doping but actually starts to increase again as doping is raised above 10%. This is an indication of strong changes in the material, likely related to the NiO-like phase that was noted in

Figure 7. Nickel L2,3-edge XAS spectra. Panel a shows the experimental spectra. Panel b shows an enlarged view of the L3 region along with SIAM calculations, while panel c shows the same for the L2 region.

general shape typical of Ni2+ (3d8) compounds,26,39 evident here from their similarity to a NiO spectrum also shown in the figure. However, while the higher-concentration (x = 0.15) sample shows a very similar spectrum to NiO, the lowconcentration samples (with x = 0.02 and 0.04) show distinctly different features. For example, a shoulder is evident on the low-energy side of the main L3 peak (∼852.5 eV) in these spectra. Additionally, the splitting of the L2 region into two peaks is less distinguished for lower doping. These marked differences indicate distinct changes in the bonding environments of the Ni ions as the doping level changes. To determine the origin of these differences, MLFT calculations26 were performed. The calculated spectra, shown in Figure 7b,c, are SIAM simulations, taking into account all multiplet and crystal field effects as well as hybridization between the Ni atoms and the broad O 2p band. From Figure 7b,c, we see that the spectrum of the x = 0.15 sample is best modeled by a Ni2+ ion in octahedral (Oh) coordination. For this calculation, model parameters very close to those determined for NiO using an ab initio approach40 were used, and the explicit shape of the O 2p band for NiO was input from experiment (approximated from O K-edge XES37). The calculation parameters used here are shown in Table 3. Note that for all calculations Slater integrals were scaled to 0.80 and 0.95 of Hartree−Fock values for the 2p63d8 and 2p53d9 configurations, respectively.26 For the lower-concentration (x = 0.04) sample, the spectrum is best modeled by a Ni2+ ion in a tetrahedral Td environment, with a compression along the C3 axis of the tetrahedron (this means the ion is formally in C3v symmetry, but we refer to it as Td for clarity to the reader). In the L3 region, shown in the lower left panel of Figure 7, we see E



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Table 3. Parameters for the SIAM Simulationsa symmetry

10Dq

Dσ

Dτ

Ve

Vt2

Δ

U−Q

W

Oh Td (C3v)b

0.65 −0.60

0.20

0.17

2.05 0.90

−1.20 −1.50

4.0 4.0

−1.0 −1.0

5.4 3.0

a

All values have units of eV. bThe symmetry for the Td case is formally C3v, as a distortion along the C3 axis of the tetrahedron was necessary to reproduce the spectra.

that the Td calculation successfully reproduces the shoulder at 852.5 eV. Additionally, as shown in the panel on the right, each calculation reproduces the corresponding experimentally observed amount of L2 peak splitting. The identification of T d coordination shows that, at the low (x ≤ 0.04) concentration, the Ni atoms substitute into the Zn sites of the wurtzite lattice. The presence of the compression along the C3 axis is also to be expected, as this is a native feature of the wurtzite ZnO lattice (the C axis Zn−O bond length is about 1.80 Å, while the other bonds are 2.04 Å41). These calculation results are in agreement with the XRD shown above, indicating that a pure ZnO structure was retained for low doping, but for higher doping, peaks appeared indicating a NiO-like phase. Here, we can also identify the NiO-like phase via the Oh coordination for higher doping. To gain further insight into the interaction of the Ni dopants with the host lattice, RIXS spectra were measured with an excitation energy tuned to the XAS maximum at the Ni L3edge. RIXS experiments have previously been shown to be useful for studying dopant atoms in a variety of DMS materials.42 This technique involves the resonant excitation of a 2p electron into an empty 3d orbital, followed by the decay of a 3d electron to fill the 2p core hole. The whole process is a single-step scattering phenomenon, and as such the energy lost by the scattered X-rays corresponds to the energy of elementary excitations, such as dd or charge transfer excitations.26,43 The measured RIXS spectra are shown in Figure 8a. The spectral intensities are plotted against the energy of the scattered X-rays relative to the incident X-rays. The spectra can be divided into

two slightly overlapping regions: a region consisting of primarily dd excitations at lower energy transfers, and a region consisting of mainly charge transfer excitations (where an O 2p electron has transferred to an Ni 3d orbital) at higher energy transfers. Similar to the XAS results, the spectra show a clear trend among samples, with the x = 0.15 sample exhibiting a spectrum similar to NiO, while the x = 0.02 and 0.04 samples have spectra that are somewhat different, and the x = 0.10 spectrum is intermediate. While the overall experimental energy resolution of ∼1.2 eV is not adequate to clearly resolve the numerous multiplet features, the main dd peak corresponding to the (t26eg2 → t2g5eg3) excitation in NiO is visible at ∼1.2 eV energy loss. The energy of this peak corresponds roughly to the t2g−eg energy splitting.25 While the energy resolution is not quite sufficient to perform a thorough analysis of the dd excitations, there is a shift of this main peak from the NiO value to lower energies as the doping level decreases. This again is consistent with a Td coordination at lower doping and Oh at higher doping, as t2g−eg splitting values of the former are generally lower in energy than for the latter (recall, however, that e and t2 energies, as well as the sign of 10Dq, are flipped for Td compared to Oh). Using the models developed for the XAS calculations above, the RIXS spectra were simulated as well. These simulations are shown in Figure 8b and agree well with the corresponding experimental results in both the dd and charge transfer regions. Note that due to the limited energy resolution, the crystal field parameters were not adjusted further from the XAS fits; only the charge transfer energies and bandwidths were analyzed using the RIXS. The implications of these experimental and calculated RIXS results are discussed in detail below.

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DISCUSSION The changes in the VBmax and CBmin and the corresponding changes in the band gap can be understood via the spectroscopic and computational results. First, considering the changes to VBmax, we note that for pure ZnO, the VB consists primarily of deep Zn 3d states (the XES peak at ∼520 eV in Figure 6a) and O 2p states (dominating the region between 523 and 527 eV in the same figure). Significant repulsion exists between these states, which stabilizes the VB.44 Ni doping leads to the introduction of the shallower, occupied Ni 3d states, which (unlike the Zn 3d states) mix strongly with the O 2p states, widening the overall VB and pushing VBmax up in energy, thus reducing the band gap. This process leads to the strong increase in VBmax between 0% and 4% doping. Increasing the doping level further continues this process, but the act of replacing Zn with Ni gradually reduces the contribution of Zn 3d states. This can be seen as the XES peak at 520 eV (characteristic of the Zn 3d states) begins to wash into the rest of the spectrum, especially at x = 0.15. This depletion of Zn reduces the amount of Zn 3d−O 2p repulsion, allowing the overall VB to relax to lower energies, as has been studied recently for ZnTiO3.44 Such a process slightly

Figure 8. Nickel L3-edge RIXS spectra. The experimental spectra are shown in panel a. Panel b shows spectra calculated using the same models as fitted for the Ni XAS spectra. The charge transfer (CT) and dd regions of the spectra are indicated. The vertical lines are present to compare energies of experimental and calculated features. The inset (c) shows the O K-edge XES of ZnO and NiO to show the different valence band widths that enter into the calculations. F



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allows the VB to relax down in energy, counteracting the effects of Ni doping. (4) The net effect is a strong reduction in the band gap for low doping, tapering off to a weaker reduction at higher doping levels.

counteracts the effect of the Ni 3d states pushing VBmax up in energy, thus leading to the less aggressive increase between x = 0.04 and x = 0.15. Additionally, the change in Ni coordination from substitutional Td to Oh likely also has an effect on the energy of VBmax. Recall that for the Td case, the Ni 3d occupation is t24e4 while for Oh coordination we have t2g6eg2. The electrons of different symmetry covalently mix with the O 2p band to varying degrees (see hopping parameters in Table 3), indicating that a change in coordination affects the overall interaction strength between the Ni 3d states and the O 2p band and, therefore, the overall effect on VBmax. Considering now the dependence of CB min on Ni concentration, it was shown in Figure 6 that a strong lowering of CBmin took place for doping from 0 to 4%, but further doping up to 15% actually began to raise CBmin in energy. This change in behavior again is possibly correlated to the change in Ni coordination that takes place between x = 0.04 and 0.10 doping. Note that for the Td coordination the only unoccupied Ni 3d orbitals are t2 orbitals, while for the Oh coordination the only unoccupied 3d orbitals are eg orbitals. As with the VB, the different degrees of mixing for these two orbital symmetries are possibly the cause of the varying behavior of CBmin. Further, the change in symmetry changes the energies of the Ni d states and would therefore directly affect their interaction with the O 2p states at the bottom of the conduction band. Finally, the RIXS results provide additional information about the interaction between the Ni and O states. Again, while the experimental resolution does not permit a detailed analysis of the dd excitations, the inherently broad nature of the charge transfer (CT) excitations allows for some conclusions to be drawn. As is evident in Figure 8a, CT excitations cover a significantly wider energy range in the higher-doped samples (and in NiO), while for lower doping, the CT band is more localized. This was reproduced in the calculations by including hybridization with a wider, NiO-like O 2p band for the Oh calculation. For the Td calculation, a narrower band was used, indicative of the O 2p band of ZnO, which is shaped in part by the deep Zn 3d states. Note that these different band widths are evident simply in the O K XES spectra as well, where the spectra become broader and more washed out as doping increases. However, the effect can be seen more clearly in the Ni CT excitations of the RIXS spectra, which relate directly to the Ni interaction with the VB. Thus, the RIXS results are a clear example of the Ni atoms acting as an impurity for low doping, causing small changes to the host electronic structure. Conversely, at high doping a new phase begins to form where the combination of an Ni abundance and Zn depletion leads to a strongly modified VB. For clarity, the results discussed above can be summarized: (1) For low (4% or less) Ni doping, the dominating effect is that the hybridization between the occupied O 2p states and shallow Ni 3d states acts to widen the valence band, pushing VBmax up in energy and narrowing the band gap. A similar effect is seen in the conduction band, where the unoccupied Ni t2 orbitals act to lower the conduction band minimum. (2) As the doping is increased past 4%, the Ni atoms are no longer Td substitutional dopants and a NiO-like phase with O h coordination begins to form. This change in coordination leads to changes in the VB and CB hybridization, causing the VBmax to increase by a reduced amount and causing the CBmin to increase rather than decrease. (3) Also at higher doping, the removal of Zn 3d states also starts to have a more prominent effect, as the corresponding reduction in Zn 3d−O 2p repulsion

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CONCLUSIONS ZnO and Ni-doped ZnO thin films were prepared using the spray pyrolysis method. The homogeneous films made up of nanofibrous structures have the hexagonal wurtzite structure. For concentrations at or under 4% Ni, the Ni substitutes into the Zn sites of the lattice, while higher concentrations led to formation of a NiO-like phase. As determined from optical and X-ray techniques, for the low-doping regime, the band gap of Zn1−xNixO is strongly reduced, while for the higher-doping regime the gap reduction is still present, but weaker. Comprehensive X-ray spectroscopy studies and a single impurity Anderson model show that the band gap behavior at low doping can be explained by Ni 3d−O 2p hybridization acting to raise the valence band maximum and lower the conduction band minimum. At high doping, the removal of Zn 3d states acts to relax the valence band, and the change in Ni coordination counteracts the lowering of the conduction band. These results demonstrate the ability of Ni doping for tuning the ZnO band gap between ∼2.8 and 3.4 eV.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the Materials Science Division, Atomic Energy Commission, Dhaka, Bangladesh, for SEM and UV−visible spectroscopy measurement as well as the Ministry of Science and Technology, the People’s Republic of Bangladesh, for providing the “NSICT Fellowship”. This work was also supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and the Canada Research Chair Program. The Canadian Light Source is supported by NSERC, the National Research Council (NSC) Canada, the Canadian Institute of Health Research (CIHR), the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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