Fe doped LaGaO₃: good white light emitters

Abstract

A polycrystalline sample of LaGa_1−xFe_xO₃ has been prepared by solid state reaction. The structural, optical and electronic structure of Fe doped LaGaO₃ have been investigated. It is observed that with Fe doping the lattice parameter of the prepared samples systematically increases whereas the optical band gap decreases. First-principles density functional theory (DFT) calculations were carried out to understand the effect of Fe doping on the electronic structure and to understand the origin of systematic decrease in optical band gap. DFT studies suggests that there exists an indirect band gap to direct band gap transition with Fe doping which is further confirmed using photoluminescence measurements. Photoluminescence studies suggest that Fe doped LaGaO₃ are potential candidates as white light emitting materials.

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Introduction

The search for new materials for various electronic applications has dominated research worldwide particularly after the development of silicon based tiny transistors/integrated circuits. During the last five decades the nature of electronic devices has witnessed huge changes. Tiny as well as energy efficient electronic devices are now dominating research worldwide and replacing conventional electronic circuit elements. White light emitting diodes, spin valves and signal transmission using optical cables are examples.¹ The design and development of direct band gap materials for light emitting diodes (LED), field emission display and florescent displays etc. are of great scientific and technological interest. In this connection the indium gallium arsenide (InGaAs) is one of the most successful candidates.² One of the main problems associated with the InGaAs based structure is degradation due to oxidation (ageing effect).^3–6 In this connection the direct band gap oxides have attracted much attention.^7–11 The LaGaO₃ is well known wide band gap perovskite oxide material having band gap close to 4.0 eV in its bulk form;^12,13 further doped LaGaO₃ has been widely explored for the phosphor/luminescence/field emission properties by doping of Yb, Eu, Ce, Cr, Sm, Tb, Tm etc. and found to be a potential candidate for the above mentioned luminescence applications.^7,14–16 However systematic studies such as effect of doping concentration and type of dopants on structural properties, electronic band gap and photoluminescent properties of doped LaGaO₃ may add new information in this regard. Despite its luminescence applications, LaGaO₃ is one of the most widely studied materials for solid state fuel cells.¹⁷ The effect of Fe, Ni, Co, Mg, Sr etc. doping on the ionic conductivity of this material has been investigated.^18–20 Recently Rai et al.^21,22 have demonstrated magneto-dielectric effect and relatively high dielectric constant in Mn doped LaGaO₃ at room temperature; whereas Natile et al.²³ studied the gas sensing properties of Fe doped LaGaO₃. Thus it appears that the ABO₃ type perovskite gallates are potential candidates for various electronic applications. As discussed above the doped LaGaO₃ is widely explored for the luminescence applications and it is observed that with different dopants it is possible to tune the emission properties of doped LaGaO₃. At this juncture it is important to note that the end compound LaGaO₃ and LaFeO₃ and solid solution i.e. intermediate compound possesses octahedral symmetry around transition metal ion²⁴ with space group Pnma.^25–27 Thus the Fe doping in LaGaO₃ expected to create minimum structural disorder and may be ideal candidate to study the effect of Fe doping on various properties. Further due to difference in the ionic radius of Ga³⁺ and Fe³⁺ it is expected that with Fe doping in LaGaO₃ may lead to systematic variation in lattice parameters as observed in similar perovskites oxides²⁸ and consequently variation in band gap.¹⁰ As LaGaO₃ is already know to show the excellent luminescence properties^7,14–16 hence it may be beneficial to investigate the photoluminescence properties of Fe doped LaGaO₃. Keeping above all in view, here we aim to study the effect of Fe doping on the structural, optical, density functional theory (DFT) based first principles electronic and photoluminescence, properties of LaGa_1−xFe_xO₃.

It is observed that with Fe doping the volume of the unit cell systematically increases whereas the band gap decreases from 3.50 eV to 2.10 eV. DFT studies confirmed that Fe doing creates some impurity states (with respect to pure un-doped LaGaO₃) in the band gap, which in turn reduces the band gap of the system. The spin polarized DFT calculations suggests that the antiferromagnetic is the ground state for Fe doped LaGaO₃ samples, whereas ferromagnetic state is the ground state for LaFeO₃, these calculations further suggests that there exist indirect band gap to direct band gap transition with Fe doping which is further confirmed using photoluminescence measurements. The photoluminescence studies suggest that the Fe doped LaGaO₃ are the potential candidates as a white light emitting material.

Experimental

(i) Sample preparation

The polycrystalline samples of LaGa_1−xFe_xO₃ (0 < x < 1) were prepared by conventional solid-state reaction route²⁸ with starting materials; La₂O₃ (99.99%), Ga₂O₃ (99.99%), and Fe₂O₃ (99.98%). These starting materials were mixed in stoichiometric amount and homogeneously mixed with propanol as a mixing medium. The resulting homogenous mixture was calcined in air ambient at 1000 °C, 1110 °C, and 1200 °C each time for 24 hours and final sintering was carried out at 1350 °C in air for 24 hours.

(ii) Structural characterizations

In order to examine the structural phase purity of the prepared samples the powder X-ray diffraction (XRD) experiments were carried out on Bruker D8 diffractometer equipped with Cu target. The obtain X-ray diffraction data is analyze using Fullprof-2k Rietveld refinement program.²⁹

(iii) Optical band gap/diffuse reflectance measurements

The optical band gap of prepared samples has been measured using diffuse reflectivity measurements. These measurements have been performed in the 190 nm to 800 nm wavelength range using Cary-60 UV-VIS-NIR spectrophotometer having Harrick Video-Barrelino diffuse reflectance probe. The beam spot size on the sample was around 1.5 mm in diameter and an integral sphere detector is used for diffuse signal detection.

Electronic structure calculations. We have investigated the structural and electronic properties of pure and Fe-doped LaGaO₃ systems by employing first-principles calculations to elucidate the effect of Fe doping in LaGaO₃. We have used spin-polarized density functional theory (DFT) calculations as implemented in the Vienna Ab initio Simulation Package (VASP).³⁰ Projected augmented wave (PAW) method^31,32 is employed using an energy cut-off of 470 eV to describe the electronic wave functions. The local density approximation (LDA) + U method³³ is used to account for the strong on-site correlated electrons in the partially filled d orbitals. We have employed correlation energy (U) of 4 eV and exchange energy (J) of 0 eV for Fe d-orbitals. These U and J values have been well tested and used in similar systems.^34,35 40 (4 × 2 × 1) and 20 atoms (2 × 2 × 1) supercells are used to study pure and doped LaGaO₃ systems to achieve different Fe-doping concentrations. Our optimized structural parameters are very much in agreement with the respective crystal structures. The Monkhorst–Pack³⁶ generated set of 5 × 5 × 5 (for 40 atoms supercell) and 7 × 7 × 7 (for 20 atoms supercell) K-points are used to optimize the structures. The convergence criteria for energy and force are set at 10⁻⁶ eV and 10⁻³ eV Å⁻¹, respectively. 7 × 7 × 7 (for 40 atoms supercell) and 15 × 15 × 15 (for 20 atoms supercell) Monkhorst–Pack³⁶ generated K-points are used for the spin-polarized density of state calculations (DOS). Band structures of pure and Fe-doped LaGaO₃ systems are calculated along the M → X → T → Γ → M path in Brillouin zone.

Photoluminescence measurements. Photoluminescence (PL) spectra were recorded using Dongwoo Optron DM 500i, having He–Cd laser as an excitation source consisting of a continuous wave of wavelength, 325 nm and a PMT detector. At the sample surface the laser makes circular spot having diameter of ∼0.5 mm with total power at ∼15 mW.

Results and discussions

(I) Structural analysis

Fig. 1a shows the representative refined powder XRD LaGa_0.8Fe_0.2O₃ refined considering orthorhombic structure with Pnma space group.^25–27 The diffraction data for all the prepared samples and the details of refinements such as; goodness of fitting, Bragg R-factor, RF-factor and χ² which defines the quality of fitting are provided in the ESI.† In order to get the clear view about the Ga/Fe–O bond length and Ga/Fe–O–Ga/Fe bond angle the representative structure is provided in the inset of Fig. 1. The variation of lattice parameters due to Fe doping is shown in Fig. 2. The systematic increase in the value of lattice constant is due to the difference in the ionic radius of Ga³⁺ (0.62) and Fe³⁺ i.e. (0.645).³⁷

Fig. 1 Representative powder X-ray diffraction pattern for the prepared samples the inset shows the view of the refined crystal structure. Various band angles and bond length are visible in the given crystal structure.

Fig. 2 Variation of lattice parameter as a function of Fe doping. The errors in the values of lattice parameters are comparable to that of size of the symbols used.

The values of the structural parameters obtained through Rietveld refinement are given in Table 1.

Table 1 Refined values of Wyckoff positions and lattice parameters for LaGa_1−xFe_xO₃

Fe% (X)	La (4c) (x, y, z)	Ga/Fe (4a) (x, y, z)	O1 (8d) (x, y, z)	O2 (4c) (x, y, z)	Lattice parameter a (Å)	Lattice parameter b (Å)	Lattice parameter c (Å)
0	0.01260, 1/4, −0.00426	0.5, 0, 0	0.25909, 0.05259, 0.28016	0.50364, 1/4, −0.05581	5.4894(1)	7.7686(2)	5.5209(1)
0.5	0.01733, 1/4, 0.00355	0.5, 0, 0	0.27746, 0.03310, 0.27289	0.49746, 1/4, −0.07123	5.4922(3)	7.7741(3)	5.5239(8)
1	0.01720, 1/4, 0.00331	0.5, 0, 0	0.28145, 0.03427, 0.27252	0.50171, 1/4, −0.06808	5.4928(4)	7.7749(3)	5.5244(1)
1.5	0.01729, 1/4, 0.00363	0.5, 0, 0	0.27634, 0.03329, 0.27380	0.49945, 1/4, −0.06779	5.4932(7)	7.7757(3)	5.5246(3)
2	0.01683, 1/4, 0.00311	0.5, 0, 0	0.28270, 0.03062, 0.26220	0.50113, 1/4, −0.07907	5.4942(6)	7.7769(8)	5.5261(1)
4	0.01750, 1/4, 0.00345	0.5, 0, 0	0.28105, 0.03232, 0.27463	0.50328, 1/4, −0.07037	5.4948(7)	7.7777(8)	5.5256(3)
6	0.01792, 1/4, 0.00433	0.5, 0, 0	0.28290, 0.03237, 0.27114	0.49938, 1/4, −0.07137	5.4959(1)	7.7794(1)	5.5259(4)
8	0.01795, 1/4, 0.00396	0.5, 0, 0	0.28317, 0.03139, 0.27135	0.49878, 1/4, −0.07578	5.4970(5)	7.7811(1)	5.5264(7)
10	0.01821, 1/4, 0.00336	0.5, 0, 0	0.27811, 0.03201, 0.27163	0.49945, 1/4, −0.07327	5.4987(3)	7.7834(2)	5.5274(5)
15	0.01948, 1/4, 0.00403	0.5, 0, 0	0.28191, 0.03442, 0.27018	0.49857, 1/4, −0.07106	5.5020(8)	7.7882(3)	5.5291(9)
20	0.01998, 1/4, 0.00516	0.5, 0, 0	0.28181, 0.03271, 0.27479	0.49825, 1/4, −0.06943	5.5059(7)	7.7933(8)	5.5311(7)
25	0.02073, 1/4, 0.00428	0.5, 0, 0	0.27816, 0.03315, 0.27234	0.50019, 1/4, −0.07590	5.5093(1)	7.7977(1)	5.5325(2)
30	0.02134, 1/4, 0.00501	0.5, 0, 0	0.28441, 0.02984, 0.26947	0.50265, 1/4, −0.07969	5.5135(4)	7.8028(7)	5.5346(5)
40	0.02275, 1/4, 0.00406	0.5, 0, 0	0.28307, 0.02864, 0.26709	0.50048, 1/4, −0.08929	5.5216(7)	7.8121(8)	5.5375(7)
50	0.02411, 1/4, 0.00522	0.5, 0, 0	0.27331, 0.03153, 0.28382	0.49878, 1/4, −0.08100	5.5296(8)	7.8204(5)	5.5411(6)
60	0.02545, 1/4, 0.00574	0.5, 0, 0	0.27356, 0.03368, 0.28610	0.49421, 1/4, −0.08003	5.5377(9)	7.8282(7)	5.5450(1)
70	0.02613, 1/4, 0.00565	0.5, 0, 0	0.28026, 0.03300, 0.28390	0.49315, 1/4, −0.07947	5.5442(2)	7.8347(7)	5.5485(5)
80	0.02719, 1/4, 0.00549	0.5, 0, 0	0.27847, 0.03412, 0.28326	0.48996, 1/4, −0.07383	5.5522(7)	7.8424(3)	5.5513(8)
90	0.02826, 1/4, 0.00612	0.5, 0, 0	0.28078, 0.03504, 0.28816	0.49206, 1/4, −0.07442	5.5584(8)	7.8482(8)	5.5531(2)
100	0.02963, 1/4, 0.00492	0.5, 0, 0	0.27253, 0.04029, 0.28596	0.49386, 1/4, −0.07152	5.5654(8)	7.8539(3)	5.5545(3)

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From the Table 1 it is clear that with Fe doping the x, y, z coordinates i.e. atomic position of oxygen and lanthanum atoms significantly changes and this lead to change in the orbital overlap (bond length) between oxygen–lanthanum and oxygen–Ga/Fe orbitals. The variation of Ga–O/Fe–O bond length and Ga/Fe–O–Ga/Fe bond angle as a function of Fe doping is shown in Fig. 3.

Fig. 3 (a) Variation of Ga–O/Fe–O bond length as a function of Fe doping. (b) Variation of Ga/Fe–O–Ga/Fe bond angle as a function of Fe.

From the Fig. 3a and b it is clear that this bond length initially increases with doping and then saturates at x = 0.4%, whereas the bond angle shows the dip at x = 0.4. It should be noted that as the scattering factor for oxygen being low Z element is small hence the Ga/Fe–O bond length and Ga/Fe–O–Ga/Fe bond angle may have errors slightly higher than that of obtain from refinements. Neutron diffraction measurements will be more useful in this regard.

(II) Optical band gap calculation and its relation with structure

In order to estimate the optical band gap the diffuse reflectance spectra has been converted to equivalent absorption spectra using Kubelka–Munk equation.¹⁰R_∞ = R_sample/R_standard. R_sample is the diffuse reflectance of the sample and R_standard is that of the standard (BaSO₄ in present case). K and S are the Kubelka–Munk absorption and scattering functions, respectively. As we have used powder samples for diffuse reflectance measurements hence the assumption of reflected light scatters in a perfectly diffuse manner holds true, for such case the scattering function S is nearly constant with wavelength¹⁰ and the Kubelka–Munk function can be related/proportional to the absorption coefficient (α) ashere n has the value of 2 for direct bandgap transitions, while n is equal to 1/2 for an indirect transition. Thus, a plot between [F(R_∞) × hν]ⁿ versus hν yields a straight line and the intercept on the energy axis gives the value of the bandgap. We have used the values of n = 2 to determine the optical gap of LaGa_1−xFe_xO₃ as the doped LaGaO₃ is known to demonstrate excellent photoluminescence properties (which is a possible signature of direct band gap material)^38–45 which is further supported by our DFT studies. Fig. 4a shows a plot between [F(R_∞) × hν]² and hν for various LaGa_1−xFe_xO₃. It is clear that with increase in the doping of Fe in LaGaO₃ the intensity feature at E ∼ 3.30 eV and E ∼ 2.65 eV become more prominent. It should be noted that the X-ray diffraction data confirms the phase purity of the prepared samples, hence; the observed extra intensity feature appears to be intrinsic property of the Fe doped LaGaO₃. The variations of direct band gap for the prepared samples as function of Fe doping (experimentally observed and obtain from DFT calculations) are shown in Fig. 5.

Fig. 4 A plot between [F(R_∞) × hν]² and hν for various LaGa_1−xFe_xO₃ the emergence of new peaks at lower energy level are indicated by arrows.

Fig. 5 Variation of direct band gap as function of Fe doping.

In the case of perovskite oxides it is well known that the effective geometrical band width (W) critically controls the energy band gap⁴⁶ given by E_g = U−W; ; , Δ(O–Ga/Fe–O) is the band angle and U is charge transfer parameter whose value almost remain constant for given series.⁴⁶ We have estimated band width and plotted the same in Fig. 6.

Fig. 6 Variation of W (band width) as a function of Fe.

It is important to note that the variation of optical band gap (with Fe doping) and that of calculated band width (W) is very similar both decreases up to 0.4 and almost saturates. Thus it appears that the Ga/Fe–O bond length and O–Ga/Fe–O bond angle effectively controls the optical band gap in the prepared samples.

(III) spin polarized DFT calculations

Fig. 7a shows the structural, electronic and magnetic properties of pure LaGaO₃. We find that the pure LaGaO₃ has a wide band gap of 3.59 eV, which is quite in agreement with the experimental report.^47,48 Our band structure shows that it has an indirect band gap of 3.59 eV along the M → X symmetry points in Brillouin zone.⁴⁹ Further, we have studied 12.5%, 25%, 50% and 100% Fe-doped LaGaO₃ systems to compare our data with experimental findings. In all these cases, Fe-doping is done at the Ga site and the most stable structure is considered for electronic and magnetic studies.

Fig. 7 The structural, electronic and magnetic properties of (a) LaGaO₃ (b) 12.5% Fe-doped LaGaO₃, (c) 25% Fe-doped LaGaO₃, (d) 50% Fe-doped LaGaO₃, and (e) LaFeO₃. Here, blue, green, yellow and red color balls denote La, Ga, Fe and O atoms, respectively. The Fermi level is set to zero.

Interestingly, our electronic structures (Fig. 7) show that the band gap of LaGaO₃ reduces as we increase the Fe-doping concentration and the band gap is lowest (2.10 eV) in pure LaFeO₃, this is very much consistence with experimental findings.⁵⁰ Further, our band structure plots show that all these Fe-doped systems have a direct band gap, which is exactly opposite to pure LaGaO₃. The partial density of states (PDOS) plots show that Fe impurity states appearing in the band gap and thus the band gap reduces with Fe-doping concentrations. This could be more due to higher energy of Fe-3d states than Ga 3d states as Ga is more electronegative than Fe. Further, we have investigated the magnetic properties of these systems using spin polarized calculations. We find that the pure LaGaO₃ system is a non-magnetic one, whereas Fe-doped LaGaO₃ systems are magnetic. The total magnetic moment calculated to be 5 μB per Fe which is due to the +3 oxidation state of Fe (3d₅). We have also investigated the magnetic ground state⁵¹ for all these systems and we find that the anti-ferromagnetic (AFM) state is the magnetic ground state for Fe-doped systems, whereas ferromagnetic is the ground state for LaFeO₃ this is very much consistent with experimental findings.¹⁸

(IV) Analysis of photoluminescence spectra

As clear from optical absorption and DFT studies the Fe doped LaGaO₃ are direct band gap material and the band gap of these samples varies systematically with Fe doping (up to x = 0.4). Hence in order to investigate the possible applicability of the prepared samples for light emitting application; the photoluminescence studies were carried out. At this juncture it is important to note that the ideal high quality indirect band gap materials do not produce strong photoluminescence⁵² hence; the occurrence of strong photoluminescence may be treated as a possible experimental proof of direct band gap material and the same is experimentally confirmed earlier by various research groups.^38–45 At this juncture it is important to note that the indirect band gap materials know to produce PL due to presence of defect states,⁵³ hence the occurrence of PL may be only the possible signature but not be the ultimate tool to comment on the nature of the band gap. The angle resolved photoemission spectroscopy and inverse photoemission spectroscopic studies⁵⁴ on single crystalline samples may be terms as direct experimental tool to comment on the nature of the band gap. The mapping of band-structure using angle resolved photoemission spectroscopy and inverse photoemission spectroscopic is beyond the scope for present set of samples (due to polycrystalline nature of these samples). Fig. 8a shows the photoluminescence spectra of the studied samples, the inset of each graph shows the corresponding luminescence due to ultraviolet He–Cd laser. It is observed that the pure LaGaO₃ do not show any luminescence/photoluminescence in the wavelength region corresponding to its band gap (this may be due to the indirect nature of band gap for pure LaGaO₃ as clear form DFT studies and present PL measurements), whereas for Fe doped samples; we could detect the PL spectra, the observation of PL form Fe doped samples may be the possible signature of the direct band gap nature of the Fe doped LaGaO₃ samples, which is also evident from the first principle calculation studies. These samples also show the broadening similar to that of observed in polycrystalline as prepared ZnO samples^55,56 and the observation of this type of broadening in PL spectra is attributed to the deep level emission from defects states.^55,56 It is very important here to notice that all the samples reported here are prepared by solid state reaction route (same procedure is followed to prepare all these samples) and hence may have similar level of defect density. Hence the luminescence observed in Fe doped samples may not be due to defects but due to the direct nature of its band gap (as the same is also evident form first principle studies shown in Fig. 7). The presence of such defects if any may contribute to the broadening of the PL spectra.^52,55,56 The observation of white light luminescence in other words broadening of PL spectra for the present set of samples may also be understood as follows; in the case of transition metal doped materials the splitting of 3d energy states due to the electrical field induced by neighboring ions (crystal field splitting) results in extra emission lines this is well known Tanabe–Sugano effect^57–63 (Fig. 8b). It is important to note that for transition metal doped samples such broadening is well known^64–74 and explained on the basis of above mentioned Tanabe–Sugano effect.^57–63 We believe that in present case the observed broadening is due to Tanabe–Sugano effect (as Fe³⁺ ions is surrounded by six oxygen anions in octahedral cage) plus possible broadening due to deep level emission by defect levels within the band gap.^52,54,55 Further it is observed that the area under the curve shows increasing trend with Fe doping; but as the photoluminescence intensity is very sensitive to surface roughness and the present measurements were carried out on as prepared polycrystalline samples (the sample surface is very rough with respect to wave length of laser and further different samples are having different surface roughness) hence we are not showing the variation of area under the curve as function of Fe concentration in the present manuscript. We are of the opinion that the experiments on high quality single crystalline samples or epitaxial thin film (low defect density low surface roughness) will be more useful for these samples, further the single crystalline samples will be useful to map the experimental band structure of these samples using angle resolved photoemission spectroscopy.⁵⁴ Here we would like to convey that the present studies clearly demonstrate the possible application of the prepared polycrystalline samples of Fe doped LaGaO₃ as a graded violet to pure white light emitting material.

Fig. 8 (a) Photoluminescence spectra for Fe doped LaGaO₃ samples. The corresponding color luminescence due to He–Cd laser is shown in the inset. (b) Proposed schematic of the photoluminescence spectra for Fe doped LaGaO₃. The 4G state is split by the crystal field into the ground state 6A₁(S) and the excited states 4T₁(G), 4T₂(G) and 4E(G), the transitions between these states as indicated by the arrows describes the observed PL spectra.

Conclusion

Effect of Fe doping on Ga site in LaGaO₃ is investigated on the structural, optical and electronic properties. It is observed that the optical band gap of the prepared samples is controlled by the orbital overlap between Ga/Fe–O overlap i.e. effective band width (W). The first principle studies suggest that with Fe doping the extra states (with respect to pure LaGaO₃) appears which reduces the effective band gap. Further the spin polarized DFT analysis suggests that the low value of Fe doping the sample favours antiferromagnetic states where as pure LaFeO₃ favours ferromagnetic state. Further there exist indirect band gap to direct band gap transition with Fe doping which is further confirmed using photoluminescence measurements. Further the experiments on high quality single crystalline samples or epitaxial thin film having low defect density and smooth surface will be useful to understand the origin and deeper insight on the photoluminescence properties of these samples.

Acknowledgements

Authors sincerely thank Prof. P. Mathur Director IIT Indore for his encouragement. SIC IIT Indore is acknowledged for providing various experimental facilities. Dr Vipul Singh is acknowledged for kindly extending the access to photoluminescence facilities. CSIR New Delhi is acknowledged for funding the high temperature furnace under the project 03(1274)/13/EMR-II. Mr Preetam Singh is thankful to Mr Shailendra K. Saxena, Mr Kamal Warshi, Mr Tajendra Dixit and Mrs Pryanka of IIT Indore for their help at various stages of this work. Indrani, Hari Mohan, Vikash would like to thank MHRD Govt. of India, for financial support for their Ph.D. fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21693e

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