Enhanced Ultraviolet Emission from Mg Doped SnO2 Nanocrystals at

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Enhanced Ultraviolet Emission from Mg Doped SnO2 Nanocrystals at Room Temperature and Its Modulation upon H2 Annealing Nilesh Mazumder, Dipayan Sen, Subhajit Saha, Uttam Kumar Ghorai, Nirmalya Sankar Das, and Kalyan Kumar Chattopadhyay* Thin Film and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: Photoluminescence (PL) and absorption features of high-quality nanocrystalline Sn1‑xMgxO2 series (0 ≤ x ≤ 0.03) were investigated at room temperature. Although particle sizes of as-synthesized samples are larger than Bohr radius (aB), Fermi edge absorption is observed both in undoped and doped samples. Incorporation of Mg 2p states near valence band maxima (VBM) widens the band gap (Eg) consistently and stable, high-yield room temperature free exciton (RTFE) emission (3.81 eV) results. Deconvolution of each UV emission reveals band tail emission (3.54 eV) along with excitonic recombination. A feeble green emission (2.34 eV) is also observed and its intensity strongly depends on tensile strain (η) and formation energy (Ef) of oxygen vacancy (VO) and hydrogen at VO (HO). H2 annealing at 300 °C introduces interesting doping dependent modulation in UV emission intensity. Elemental projective density of states (DOS) exhibit Sn 5s−H 1s hybridization in undoped passivated supercell and Mg 2p−H 1s hybridization in doped passivated supercells. Band gap widening and doping dependent emission modulation in both passivated and unpassivated samples are explained by first principles investigation. This study points to the scope of FE emission enhancement technique at room temperature for optoelectronic applications based on alkaline earth metal doping in direct gap metal oxide semiconductors. according to first-principles calculations performed by Zhang et al., Mg 2p DOS is found to be situated near the O 2p states with effective p−p coupling.14 Besides Mg2+ incorporation at Sn4+ would create charge imbalance and as a result Fermi level (EF) position dependent VO formation would also be obvious. It is worth investigating the effect of VO on PL intensity from a dipole allowed background. Optical,15−17 photocatalytic,18,19 and electrical20 properties of Mg doped ZnO have been studied to a generous extent recently. Band gap widening and enhancement of UV emission were the prominent observations upon Mg doping. Recently, Frenkel exciton mediated UV emission has also been observed in substoichiometric WO3 at room temperature.21 SnO2 is a promising candidate in this prospect as its bulk band gap and exciton binding energy are greater than WO3 and ZnO which can ensure the desired UV range for practical LED applications upon suitable band gap engineering. Recent theories also have invigorated interest on hydrogen multiple coordination inside metal oxide semiconductors22 as a viable source of electrical conductivity.23−26 There are two sources of H-related shallow donors according to recent firstprinciples study. The first one is hydrogen interstitial (Hi), which is formed upon hydrogen atom incorporation on an interstitial bond centered lattice site.23 The second one is

I. INTRODUCTION As a direct gap semiconductor, SnO2 is a promising host material for realization of next generation UV light-emitting diodes (LED) and laser diodes (LD)1,2 due to its large bulk band gap (3.6 eV) and high exciton binding energy of 130 meV at room temperature.2 This large value of exciton binding energy envisages efficient exciton emission at room temperature and even at higher temperature. However, the dipole-forbidden nature of the energy band structure stands as a barrier for obtaining efficient excitonic or band to band recombination.3 Less efficient UV emission of SnO2 is assumed to originate from the recombination between band and defects or unknown impurities, such as donor−acceptor pair and band to acceptor at low temperature (∼50 K).4−7 Literature shows few reports regarding SnO2 excitonic recombination only in quantum structures, colloidal suspensions or for high quality epitaxial films,8−12 although observed RTFE emission intensity is quite poor in undoped SnO2 for UV LED application. Modification of the energy band structure by suitable dopant13 and charge balance induced VO generation are the keys to break dipoleforbiddances and thereby obtaining efficient excitonic emission at room temperature, which is the principal step toward UV LED. SnO2 valence band (VB) consists of highly electronegative O 2p states, for which attachment of holes is very strong. If we dope a suitable electropositive element whose dominant orbital states nearly superpose with O 2p, effective electronegativity of VB can be reduced, prompting the holes to recombine with conduction band (CB) electrons. Interestingly, © 2013 American Chemical Society

Received: January 2, 2013 Revised: March 6, 2013 Published: March 7, 2013 6454

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III. RESULTS AND DISCUSSION Crystallographic data (Figure S1) indicates that as-synthesized samples possess tetragonal rutile structure belonging to P42/ mnm space group.35 With the minimum of η, stannic oxide with 0.5% Mg concentration exhibits better crystallinity than all other samples. For higher Mg concentrations, η is found to be gradually increasing. This variation of η cannot be due to ionic radius difference of dopant cation from the host one as they have almost identical ionic radius.28 So, it can be concluded that charge imbalance induced VO formation within crystal structure is responsible for observed η variation. Our first-principles results indicate to the similar conclusion which would be discussed later in this section. For very low Mg%, introduction of VO inside the lattice has compensating effect on η; as stoichiometric undoped SnO2 has greater supercell formation energy than the substoichiometric counterpart by 5.88 eV. The value of c/a is found to be increasing nearly consistently with Mg% up to experimental limit (Figure 1 right inset). This is

formed by the occupation of an oxygen vacancy (VO) by hydrogen atom (HO).22 Between two types of well identified Hrelated donors, Hi is more mobile at room temperature which is also stable up to few weeks’ time scale and HO is more thermally stable (up to 500 °C).27 According to the calculation by Singh et al.,25 it is very unlikely for Hi to be the reason for electrical and optical property modulation at room temperature as it is highly mobile even at relatively low temperature with migration energy barrier of 0.57 eV. So, we decided to concentrate our first-principles study on HO in Mg doped SnO2 environment and the aim was to investigate the modulation of Mg p states upon H-annealing and its effect on PL profiles.

II. EXPERIMENTAL SECTION Details of the synthesis procedure along with phase and structural characterizations have been reported elsewhere.28 Fourier transformed infrared (FTIR) spectra of the samples were acquired by a Shimadzu-8400S spectrophotometer in the 450−4500 cm−1 range. The instrument used for Raman study is a confocal Raman imaging system (WITec GmbH) alpha300RS. The UHTS 300 spectrograph is connected with a Peltier-cooled back-illuminated CCD camera with better than 90% QE in the visible excitation. For volumetric PL measurement, 0.05 g of powder sample was mixed with 3 mL of absolute ethanol in a test tube followed by 10 min stirring. Two days of sedimentation time was allowed to make the dispersion uniform. Heavier particles precipitate and 100 μL of supernatant dispersion was taken and added again with 3 mL of ethanol. After that, PL measurements were carried out within the optical range of interest by a ELICO SL 174 spectrofluorometer. Same procedures were repeated again after all of the powder samples were annealed at 300 °C under controlled hydrogen flow (250 sccm) for 1 h at the base pressure of 10−2 mbar. Optical absorption features (UV−vis) were investigated by Perkin-Elmer equipment. Our first-principles calculations were performed by CASTEP code29 which implements a supercell approach to density functional theory. Perdew−Burke−Ernzerhof (PBE) functional30 within the generalized gradient approximation (GGA) was used to deal with exchange and correlation term. Vanderbilt ultrasoft pseudopotential31 was used to represent the Sn, O, Mg, and H atoms and plane waves up to energy cut off 500 eV was used in the calculation. Brillouin zone integrations were performed within the Monkhorst Pack scheme32 using 1 × 1 × 1 k-point mesh. For geometrical optimization, the system was allowed to fully relax using BFGS (Broyden−Fletcher−Goldfarb−Shanno) scheme33 until the total energy converged to less than 2 × 10−5 eV/atom, the maximum force converged to less than 0.05 eV/Å and the maximum displacement was 0.002 Ǻ . A fully relaxed 2 × 2 × 4 SnO2 supercell (a = b = 4.892 Å, and c = 3.283 Å) containing 32 Sn atoms and 64 O atoms was used to model the SnO2 structure. 3.125% and 6.250% Mg doped SnO2 were built by substituting one (Sn31MgO64) and two (Sn30Mg2O64) Sn atoms by Mg atom(s), respectively, and geometrically optimizing the resultant structures. Now, SnO2 shows a pronounced nonstoichiometry with VO even under equilibrium growth conditions, at high temperature.34 For that both undoped and doped SnO2 structures, 3.125% O vacancy was modeled by removal of two (Sn32O62, Sn31MgO62, and Sn30Mg2O62) oxygen atoms and H passivation was tested by introducing one (Sn32O62H, Sn31MgO62H, and Sn30Mg2O62H) H atom in O vacant positions.

Figure 1. Calculated Ef variation of VO and HO as a function of Mg% with top right inset showing corresponding c/a variation of experimental, sub (VO) and p-sub (VO + HO) samples. Bottom left inset is representing three parts of supercells displaying VO, HO, and Mg positions respectively from top to bottom. Gray, red, white, and green spheres are Sn, O, H, and Mg atoms, respectively.

also consistent with first principle finding. To investigate the formation energy (Ef) variation of VO and HO with increasing Mg content, we have used the equation Ef = Edefect − Epure + n0μ0 − nHμH

(1)

where Edefect represent the total energy of substoitiometric (hereafter denoted as sub) or passivated substoitiometric (hereafter denoted as p-sub) supercells and Epure represent total energy of their stoichiometric unpassivated counterparts. μO and μH are chemical potentials of O and H, respectively, where nO and nH represent the number of VO and HO within the supercell. Figure 1 depicts the calculated variation of Ef of both VO and HO as a function of Mg%. We were unable to track the effects of small amount of Mg doping ( 0.5. Besides, some additional peaks situated at 350, 443, and 544 cm−1 are observed. In an infinite perfect crystal, only the phonons near the center of Brillouin zone contribute to the Raman active modes due to the momentum conservation rule between phonons and incident light. For nanoparticles, the vibration is limited to the size of the crystal, which gives rise to the violation of phonon momentum selection rule q0 ≈ 0, allowing phonons with q ≠ 0 to contribute to the Raman spectrum. So, those observed additional peaks can be attributed to different kinds of 6456

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Figure 4. (a) Comparison between calculated and experimental absorbance spectra of undoped and 3% Mg doped SnO2 samples; (b) RT excitation (Ex) and emission (Em) spectra of 0.5% Mg doped sample; and (c) variation of experimental and calculated (sub, p-sub, and stoichiometric) value of band-gaps with Mg%.

Figure 5. (a) Projective DOS for H s, Mg p, and Sn s of p-sub 3.125% and 6.25% Mg doped SnO2. (b) Projective DOS corresponding to a Sn atom situated at regular SnO2 lattice site, neighboring Sn of VO and HO and H s, respectively.

nm. This above band gap absorption edge is possible due to Pauli blocking which is known as Burstein−Moss shift44 in the case of degenerate semiconductors. At the lower energy end, observed absorbance is higher for the doped sample in contrast to the calculated one. In our calculations, VO concentration in all of the samples is kept constant. However, practically, the VO concentration should increase with increasing Mg% for charge balance and lower Ef. As the optical behavior of SnO2 is VO dominated, this extra O-deficiency should be considered to explain the observed spectra. Figure 4b shows the RT excitation and emission spectra of 0.5% Mg doped SnO2 recorded with concentration half compared to the samples used in the PL measurement. The excitation spectrum has a spread from 220 to 260 nm which is in close proximity with experimentally observed absorption edge (213 nm) and the mean position of the excitation spectra. The excitation peaks do not undergo any mentionable energy

shift due to Mg doping. It is to be noted that low energy end of excitation spectrum (260 nm) matches perfectly with the absorption onset (260 nm). Determination of absolute fluorescence quantum yield (Φf) of 0.5% Mg doped SnO2 was based upon the following scheme:45 λ

Φf =

∫λ em2 em1

[Ix(λem) − Ib(λem)] λem s(λem)

λex +Δλ [Ib(λex ) − Ix(λex )] λex s(λex ) λex −Δλ

∫

dλem = dλex

Nem Nabs

(3)

where the total number of emitted photons (Nem) is obtained upon integration of blank-corrected (Ib(λem)) and spectrally corrected (Ix(λem)) emission spectrum of the sample. The number of absorbed photons (Nabs) follows from the integrated difference between the excitation light resulting from measurements with the blank (Ib(λex)) and the sample (Ix(λex)). s(λem) and s(λex) are spectral responsivities of the emission and 6457

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Figure 6. Deconvoluted (green) UV emission of (a) undoped and (b) 0.5%, (c) 1%, (d) 2%, and (e) 3% Mg doped SnO2 powder samples with (f) their relative intensities. Black and orange lines represent the experimental and fit spectra, respectively.

excitation channel, respectively. Φf is obtained to be equal to 0.76 considering all possible measurement related errors. This value indicates a very high fluorescence yield for 0.5% Mg doped SnO2 sample. Band gap widening upon Mg doping is evident from Figure 4c. Calculated and observed absorption spectra were used to obtain the value of band gap for all of the samples. The absorbance coefficient (α) was calculated from the raw absorbance data. Then Eg values were determined by extrapolation of linear portion of (αhν)2 curve versus the photon energy hν to (αhν) 2 = 0. Interestingly, for stoichiometric supercells, Eg does not change upon Mg doping. The widening is only observed in the sub and p-sub supercells and is verified both theoretically and experimentally up to 3% Mg concentration. From Figure 5a, it can be seen that with the increase in Mg content, Ef lowering causes apparent energy blueshift of Sn 5s states. The position of H 1s and Mg 2p states also shift toward higher energy with the increase in Mg%. However, from Figure 5a we can see that introduction of Mg in the supercell suppresses the DOS of Sn 5s and H 1s states significantly, and this is proportional to the Mg%. Besides, charge compensation induced VO act as electron donor, making the lower vicinity of the conduction band degenerate (Figure 5b). This explains the origin of Fermi edge absorption and emission upon Mg doping along with band gap widening. Incorporation of HO introduces stable s states near VBM and reduces the optical gap. In passivated supercells, Sn 5s DOS also suppresses in comparison with its VO induced counterpart. We would notice the effect of passivation in the following sections. RTFE emission spectra of all of the samples are shown in Figure 6 upon 240 nm excitation. Green and orange lines denote the deconvoluted peaks and their combined gauss envelope respectively for Figure 6a−e of the as mentioned samples. Figure 6f shows the relative emission intensities of all of the samples. Few important features to notice are as follows: there is no apparent shift in the positions of three deconvoluted peaks upon Mg doping; PL emission intensity enhances almost 20 times for 0.5% Mg doped SnO2 than its undoped

counterpart and further increase in the Mg concentration results in gradual decrease of emission intensity and tends to achieve a stable state. It is also noteworthy that we have not observed a lattice strain dependent blueshift in emission spectra. The emission edge is situated at ∼4 eV which coincides with the experimental value of the band gap of the undoped sample. In this report, we ascribe the near band edge emissions centered at 3.81 and 3.70 eV to the FE decay for undoped and all of the doped samples which are in excellent agreement with the previous reported values.8−12 Pan et al.12 also had demonstrated 3.82 eV emission for undoped SnO2 nanocrystals film having an average particle size of 38 nm. It is known that no band to band transition emission is observed for SnO2 due to the presence of point defects, such as oxygen vacancies.46,47 Besides, first order exchange energy due to many body effects of free electrons and holes using the Thomas-Fermi screening potential is given by48 ΔEgexch = ( −2e 2kF/πε)[1 + (πκ /2kF) − (κ /kF) tan−1(kF/κ )]

(4)

κ = 2.73 × 104(me /mo)1/2 (n1/6 /ε1/2) cm−1

(5)

kF = 3.094n1/3 cm−1

(6)

where κ is the reciprocal screening length, kF is the Fermi wave vector, me and m0 are electron effective and free mass, respectively, e is the electronic charge, ε is the static dielectric constant, and n is the free carrier concentration. When κ/kF ≪ 1 (for relatively large n values, which is the case for a degenerate semiconductor like SnO2), we have ΔEgexch = ( −2e 2kF/πε) ∝ −kF ∝ −n1/3

(7)

So, for the band to band transition, the emission peak should be red-shifted with the increase in free carrier concentration n due to the enhanced many body and screening effects of free carriers. Here emission energy peaks at 6458

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Figure 7. Deconvoluted (green) UV emission of hydrogen annealed (a) undoped and (b) 0.5%, (c) 1%, (d) 2%, and (e) 3% Mg doped SnO2 powder samples with (f) their relative intensities.

E = Eg + ΔEgexch = Eg − αn1/3

comparison with its unpassivated counterpart. To clarify the contrast of PL intensities before and after H-passivation, relative UV peak intensities are plotted as a function of Mg% in Figure 8. It is observed that, for p-sub samples, the saturation

(8)

where α is a proportionality constant. From eq 7, we can see that the emission peak position has a cube root dependence on the free carrier concentration for band to band transition. However, in our study, peak positions do not show relative shift in spite of having a differences in n. From conductivity measurement,28 it is evident that n0/n0.5 ≈ 105, n0/n1 ≈ 104, n0/ n2 and n0/n3 ≈ 106, respectively; the numerical suffix denotes the Mg doping percentage in tin oxide. So the origin of the emission cannot be the band to band transition. The subsequent emission peaked at 3.54 eV can be assigned to the radiative recombination of localized exciton (LE) from the band tail states of the nanocrystalline sample. The variation of tensile strain with Mg concentration does not change either the positions of the FE or LE emissions in any samples. Although, it is well-known that tensile strain variation does affect the PL intensity, i.e., greater tensile strain diminishes the PL emission intensity for nanocrystalline samples, 3% Mg doped SnO2 (η = 24.33) exhibits 8.3 times more intense FE emission than the undoped counterpart (η = 20.46). So, it can be concluded that PL intensity variation is not a reflection of η variation within the samples. Figure 7a−e exhibits PL emission spectra of the samples recorded under the same set of conditions after H2 annealing. The emission intensity of the undoped sample increases upon passivation, but the 0.5% Mg doped sample suffers a dramatic loss in PL intensity with the least intense emission. From Figure 7b it can be observed that emission corresponding to the FE and higher energy LE components have been suppressed completely. An interesting feature displayed in Figure 5a and b can provide an explanation for this emission suppression. In Figure 5b, a prominent hybridization between Sn 5s and H 1s can be clearly observed at ∼−1 eV, although for the Mg doped sample (Figure 5a), the above-mentioned hybridization cannot be observed. Instead, hybridizations between Mg 2p and H 1s are observed at ∼−1.5 to −2 eV. These features refer to this explanation that, for Mg doped samples, absence of H 1s−Sn 5s hybridization results in a dramatically quenched emission in

Figure 8. Relative changes in the UV emission intensities (a) before and (b) after H-passivation as a function of Mg%.

intensity is more than that for the sub samples. For p-sub samples with higher Mg%, the modified valence states contain hybridized H 1s and Mg 2p states. That is why better carrier recombination probability results in greater emission intensity. According to the calculations performed regarding the changes in the DOS of the model supercells, it is seen that, for Mg doped sub SnO2, VBM arises just above the EF position although the energy landscape remains same (Figure 9a). The EF is chosen to be at the zero energy scale. VBM of the Mg doped supercells tend to degenerate and form a stable acceptor state near EF. Not much energy shift of this acceptor level is observed with increasing Mg content for this O deficient set. Relative energy gap between VBM and CBM does not change significantly. EF lowering with the availability of a stable 6459

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IV. CONCLUSION We report a method to enhance RTFE emission by introducing stable Mg p states near VBM in the energy band of high quality SnO2 nanoparticles. These states provide a suitable chance for donor−acceptor recombination canceling the dipole forbiddances. Strong emissions at 3.81, 3.70, and 3.54 eV have been observed at room temperature with different intensities for sub and p-sub samples. No energy shift in excitation and emission spectra have been observed in any sample. Although the emission intensity is feeble for low Mg concentration under passivation, higher Mg% enhances the emission intensity both before and after passivation. Calculated DOS clearly exhibits hybridization between H s and Sn s upon passivation, whereas for doped samples Mg p and H s hybridization is observed. Experimentally observed enhanced PL intensity for undoped passivated SnO2 can be related to H s−Sn s hybridization which becomes absent upon Mg incorporation. The result of this study is important regarding future research on scarcely reported novel room temperature enhanced ultraviolet emission based on lightly alkaline earth metal doped TCOs.

Figure 9. (a) DOS of sub and (b) p-sub pure (undoped) 3.125% and 6.25% Mg doped SnO2 supercells. (c) Relative green emission intensity before and after H-passivation for all of the samples.

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ASSOCIATED CONTENT

S Supporting Information *

acceptor state facilitates better probability of charge carrier recombination. However, DOS suppression is evident for doped samples near EF and at acceptor states from Figure 9a, and this is the reason, as we understand, for intensity suppression of RTFE emission with gradual increase in Mg concentration. Figure 9b represents the relative changes of DOS for p-sub supercells. Unlike sub supercells, energy saturation of VBM is not observed here, and this is in accordance with the experimental emission intensity shift observed. For p-sub samples, DOS suppression is not dominant as with the gradual increase of Mg content, valence band edge appears closer and closer to EF. Prominent hybridization is observed between Mg 2p and H 1s which introduces modification in the acceptor states and paves the way for better carrier recombination for Mg% > 0.5. This results in increased emission intensity with Mg%. However, more detailed investigation is necessary regarding the small amount of Mg doping as it is observed to be optically very sensitive under passivation. This property can be exploited in sensing various reducing atmosphere which also includes hazardous gases. It is well known that visible emission of SnO2 nanocrystals is a distinctive signature of VO related states within energy band representation.49,50 So the narrow visible emission (510−550 nm) in green region (Figure 9c) can be used to comment on VO present in the samples. Our observation indicates that sub PL profiles in the green region are shaped by η, Ef, VO, and nO within the sample. At lower Mg concentrations, η is more dominant, and at higher Mg concentrations, the effect of VO becomes more dominant with higher nO due to charge balancing and lower Ef. The increased emission is probably due to allowed transition from ionized deep donor states (VO+, VO++) to stable acceptor level (Mg 2p) formed near the valence band maxima (VBM). Upon H2 annealing, the effect of HO is evident from the modulation of emission profiles. It is also observed that the favorability of HO formation increases with the increase in Mg doping concentration monotonically as green emission increases. Our first principle finding is also the same as is evident from Figure 1.

Additional material as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 91 33 2114 6584. Phone: 91 33 24138917. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Science and Technology (DST), the Government of India for providing financial support through a project and also the UGC, the Government of India for “University with Potential for Excellence Scheme (UPE-II)”.

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REFERENCES

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dx.doi.org/10.1021/jp4000329 | J. Phys. Chem. C 2013, 117, 6454−6461