Oxygen Induced Enhanced Photoanodic Response of ZnTe:O Thin

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Oxygen Induced Enhanced Photoanodic Response of ZnTe:O Thin Films: Modifications in Optical and Electronic Properties Intu Sharma, Aadesh Pratap Singh, Neha Tyagi, Nishant Saini, Sushil Auluck, and Bodh Raj Mehta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11034 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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The Journal of Physical Chemistry

Oxygen Induced Enhanced Photoanodic Response of ZnTe:O Thin Films: Modifications in Optical and Electronic Properties Intu Sharma, Aadesh P. Singh, Neha Tyagi, Nishant Saini, Sushil Auluck and Bodh. R. Mehta* Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

ABSTRACT In the present work, photoanodic response of ZnTe thin films is enhanced by incorporating oxygen which is explained by analyzing oxygen induced modifications in structural, optical and electronic behaviour of ZnTe thin films, using detailed experimental characterizations and density functional theory (DFT) based calculations. On incorporating oxygen, nanocrystalline character of ZnTe is increased with change in optical properties due to absorption through sub band states and increase in fundamental absorption edge. From DFT analysis, origin of these sub band gap states is attributed to oxygen incorporation induced electronic states and Te vacancies. Photoelectrochemical (PEC) performances of ZnTe with and without oxygen have been investigated where a change over from photocathodic response for ZnTe to enhanced photoanodic response for ZnTe:O thin films along with increased response for low energy photons is observed. These findings are explained in terms of oxygen induced modification in visible light absorption, enhanced surface area due to increased nanocrystalline character and modified electronic properties of ZnTe:O thin films. Modifications in optical properties and enhancement in PEC performance by oxygen 1 ACS Paragon Plus Environment


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incorporation shown in the present study may be useful for developing ZnTe based photovoltaic devices. *Electronic mail of corresponding author: [email protected]

1. INTRODUCTION Zinc telluride (ZnTe) is an II-VI direct band gap (Eg=2.24 eV) semiconductor, which usually exists in cubic zinc blende crystal structure. ZnTe is intrinsically a p-type semiconductor; however, n-type conductive ZnTe is also observed with suitable dopants such as Cl, Al and B 1-6

. The possibility of having p-ZnTe and/or n-ZnTe makes it a favorable candidate for

optoelectronic applications. Accordingly, ZnTe has been utilized in various devices such as green and red light emitting diodes (LEDs)

7-11

, homo and hetero junction solar cells

12-15

,

photo-detectors 16-17 and transparent conductors 18-19. In ZnTe, tuning of band gap to harvest a wide portion of sun light is demonstrated using its nanostructures 20-22 and alloys 23-25. Among ZnTe based alloys, ZnTe1-xOx, a highly mismatched alloy of ZnTe (a=6.104Å) and ZnO (a=4.565Å) which exist in cubic zinc blende and wurtzite crystal structure, respectively, is of potential interest due to enhanced optical absorption of wide portion of sun light. These alloys are mainly prepared by molecular beam epitaxial (MBE) technique and employed as intermediate band (IB) semiconductor in solar cell

26-33

. Existing literature on the growth of

such alloys by techniques other than MBE is limited. To the best of our knowledge, there is only couple of reports where radio frequency (RF) magnetron sputtering

34

and pulsed laser

depositions 35 techniques have been used to synthesis these alloys. As low cost techniques are more suitable for large area applications, it is essential to synthesis ZnTe1-xOx alloys by these techniques and then carry out a detailed investigation of oxygen induced modification in structural, optical and electronic behaviour of this highly mismatched alloy system.

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Recently, ZnTe has shown potential as a photocathode for photoelectrochemical (PEC) water splitting applications and photocatalytic activities to generate CO and H2 from water and CO2 36-40

. For instance, Jang et al. 36 reported the use of ZnTe films as photocathode for PEC water

splitting experiment. In their study, improved PEC performance was observed for ZnTe modified with MoS2 and carbon in comparison to bare ZnTe due to protection of ZnTe surface by carbon against photo corrosion and catalytic behaviour of MoS2. In another study by Won et al. 37, improved photocatalytic activity was observed for polypyrrole coated ZnTe in comparison to pristine ZnTe due to enhanced electron-hole pair and charge separation. ZnTe is selected for these studies due to its p type nature with most negative CB minimum (CBM) (-1.63 VRHE) that allows transfer of photoexcited electrons from ZnTe to acceptors in electrolyte. Also band gap of ZnTe i. e. 2.24 eV lying in the visible part of solar spectrum makes it a suitable choice for these applications41-43. As enhanced absorption of sun light has been demonstrated in oxygen incorporated ZnTe thin films therefore their PEC performance is expected to be dependent on oxygen content and needs to be investigated for development of ZnTe based PEC applications. To the best of our knowledge this aspect i.e. PEC performance of ZnTe:O thin film is not investigated so far and is the motivation of present work. Present study reports modifications in the structural, optical and electronic behaviour of ZnTe thin films with change in oxygen concentration in terms of increased nano-crystallinity and increased absorption due to sub band states. Densities functional theory (DFT) based calculations have been carried out to investigate origin of these sub band states. The effect of oxygen incorporation is further studied by comparing the PEC performance of ZnTe with and without oxygen.



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2. EXPERIMENTAL METHODS ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films have been prepared on glass and indium tin oxide (ITO) substrates from ZnTe (99.95%) target. Prior to any deposition, the sputtering chamber is evacuated down to a pressure of 3×10-6 Torr, followed by introduction of Ar gas at rate of 90 SCCM (Standard cubic centimeter) for deposition of ZnTe films. For the deposition of ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films, Ar and 2% O2 mixed with Ar gases are used and oxygen percentages 0.02% and 0.2% are maintained by Ar flow rate at 90 and 2% O2 mixed with Ar flow rate at 0.9 and 10 SCCM, respectively. Depositions have been carried out for 30 minutes with RF power of 50 W at room temperature (RT) and 300ºC. The thicknesses of the prepared films are approximately 500 nm as measured with stylus profilometry. For high resolution transmission electron microscopy (HRTEM) studies, depositions are carried out for 30 s on carbon coated Cu grids. Structural characterizations have been performed with high resolution X-ray diffraction (HRXRD) technique using Phillips PANalytical’s X' Pert PRO material research diffractometer. Micro-structural properties of the prepared films are investigated using high resolution transmission electron microscope (HRTEM) (FEI-Technai-G20 with LiB6 filament). Optical studies have been carried out in the wavelength (λ) range of 200 to 1000 nm by using variable angle spectroscopic ellipsometer (VASE) (M-2000 F ellipsometer from J. A. Woollam Co. Inc., Lin Coln, NE, 68508, USA). Data is acquired at incident angles of 55o, 65o and 75o and fitting is performed in WVASE32 software for all samples. Cauchy and Gauss oscillator models are used to fit SE data in optically transparent and absorbing region, respectively. Refractive index (n), extinction coefficient (k), dielectric constant (ɛ1, ɛ2), thickness and roughness are extracted out by modeling the layers structure and fitting the SE data. The photoluminescence (PL) measurements have been carried out with He-Cd laser (λ=325 nm) in the range of 413 to 1000 nm with Horiba Scientific LabRAM HR Evolution


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Raman spectrometer. Theoretical calculations for the band structure and density of states (DOS) spectra for ZnTe, ZnTe doped with oxygen (ZnTe:O) and ZnTe with Te vacancy (ZnTe:VTe) are performed by using DFT. Calculations are performed using the full-potential linear augmented plane wave (FP-LAPW) method based on DFT as implemented in the WIEN2k code44. In present study, exchange correlation potentials based on the generalized gradient approximations with the Perdew-Burke-Ernzerhof parameterization (GGA-PBE)

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have been used. For PEC investigations, ZnTe and ZnTe:O thin films have been converted to the photoelectrodes with an active surface area of 1x1 cm2. Measurements are carried out in a three electrode PEC cell using ZnTe or ZnTe:O thin films as working electrode, Pt mesh as the counter electrode and Ag/AgCl saturated as reference electrode, respectively. 0.1 M Na2SO4 solution is employed as electrolyte and PEC cell is controlled by AutoLab PEC work station with nova software. Linear sweep voltammetry scans are obtained under dark and light conditions where 150 W Xenon lamp fitted with AM1.5 filter (output illumination intensity =100 mW/cm2) is used as light source. Electrochemical impedance spectroscopy studies (EIS) are carried out in frequency range of 200 kHz to 100 mHz at an applied potential (V) of 10 mV vs. Ag/AgCl reference electrode. Mott-Schottky (MS) analyses have been performed at an AC frequency of 1 kHz. Experimental potentials vs. Ag/AgCl (EAg/AgCl ) are converted to the reversible hydrogen electrode (RHE, theoretical redox potential to produce H2 from H+) scale via the Nernst equation as ERHE = EAg/AgCl + 0.059 pH + EoAg/AgCl where EoAg/AgCl is the standard potential of Ag/AgCl at 25ºC (0.1976 V) 46. 3. RESULTS AND DISCUSSION 3.1 Modification in optical properties of ZnTe thin film on oxygen incorporation: Spectroscopic ellipsometry studies are carried out to investigate modification in optical behaviour of ZnTe thin film on oxygen incorporation. The obtained spectra for real (ɛ1) and



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imaginary (ɛ2) parts of dielectric constants for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) samples deposited at RT and substrate temperature of 300oC are shown in Fig. 1(a, b) and (c, d), respectively. For crystalline materials, energy dependence of dielectric constant spectra i.e. ε(E) is an exact replica of the Brillouin Zone (BZ) associated with crystal structure 20, 47.

Fig. 1(a) and (b) represent real (ɛ1) and imaginary (ɛ2) parts of dielectric constant for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films prepared at RT. Similar images for samples prepared at substrate temperature of 300oC are shown in (c) and (d). Sharp features in ɛ(E) spectra of samples prepared at 300oC show better crystallinity. Non-zero values of ɛ2 for ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films below the band edge of ZnTe show absorption through sub band gap states. Features observed in ɛ(E) spectra of ZnTe are due to the edge excitons and interband critical points (CPs), and are denoted by Eo, Eo+∆o, E1, E1+∆1 and E2 as shown in Fig. 1(c) and (d). 6 ACS Paragon Plus Environment

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As substrate temperature increases from RT to 300oC, features observed in ɛ(E) spectra get sharpened and enhanced which indicate better crystallinity of samples deposited at substrate temperature of 300°C which is also observed in earlier studies

48-49

Transition close to 2.24

eV labelled as Eo (attributed to the fundamental direct band gap of ZnTe) occurs at Γ point (k=0) of BZ and is from highest occupied valence band (VB) to the conduction band (CB). Spin orbit coupling in zinc blende phase splits the VB into two bands causing transitions at Γ point to be doubly degenerate which are separated from each other by spin orbit coupling energy (∆o ~ 0.92 eV). Further transition at energy values ~3.78, 4.34, 5.23 eV are denoted by E1, E1+∆1 and E2, respectively and occur at L4,5 , L6 and X points of the BZ, respectively. The values of dielectric constants and features in ɛ(E) spectra of ZnTe nanocrystalline films in the present study are in agreement with the existing literature 20, 50. Non-zero values of ɛ2 for ZnTe:O (0.2%, 0.02%) thin films below the band edge of ZnTe ( 3%)

52

. The absorption

spectra of ZnTe and ZnTe:O (0.2%, 0.02%) obtained from SE studies is in agreement with the above theoretical findings. In ZnTe:O (0.2%) samples, two separate features, 1.2 to 2.0 eV due to IB and 2.4 to 2.5 eV due to the absorption edge are clearly observed. ZnTe:O (0.2%) samples deposited at 300oC and RT show a hump and long tail, respectively on the lower energy side of absorption spectra. These are attributed to structural defects in ZnTe samples arising from low substrate temperatures and nanocrystalline nature of films. Experimental observations of shift in the band gap to higher values and emergence of IB in case of ZnTe (0.2%) is also consistent with the DFT calculations. Therefore, experimentally observed sub band gap PL emissions in the energy range (1.2-2.0) are combined effect of the oxygen incorporation and Te vacancies. PL peaks at 1.6-2.0 eV are attributed to oxygen incorporation while peaks in the energy range 1.2-1.5 eV are assigned to Te vacancies as theoretically predicted by DFT calculations. The sum of the PL intensities of lower energy



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peaks for ZnTe:O (0.02%) as well as ZnTe:O (0.2%) nanocrystalline films in the range 1.41.6 eV is found to increase with increase in substrate temperature. An increase in substrate temperature results in Te vacancies due to higher vapor pressure of Te

48

. Correlation

between theoretical and experimental findings shows that energy states which are introduced due to Te vacancies, structural defects and oxygen incorporations results in the modification of structural, optical and electronic behaviour of ZnTe thin film. 3.3 Photoelectrochemical measurements From experimental and theoretical findings as discussed in the earlier sections, oxygen incorporation resulted in absorption of below band gap light and increase in nanocrystallinity character. This may have a direct impact on PEC device performance, thus, PEC properties of pristine ZnTe and ZnTe:O nanocrystalline films deposited at RT have been investigated. Current density (J) vs. RHE plot under dark and light conditions for ZnTe thin film is shown in Fig. 6(a) while similar plots for ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline thin films are shown in Fig. 6(b). The pristine ZnTe sample exhibits a photocurrent density (Jph) of -0.05 mA/cm2 at 0 V vs. RHE on cathodic side and JPh onset potential is found to be ~0.05 V vs. RHE (see Fig. 6(a)). Obtained photocathodic current is due to the p-type conductivity of ZnTe which is consistent with existing reports

36-37

. On the other hand, ZnTe:O (0.02%)

and ZnTe:O (0.2%) nanocrystalline films show enhanced photocurrent on the anodic side as compared to cathodic behaviour in ZnTe thin film which indicates change in the interfacial electronic nature of ZnTe:O nanocrystalline film. Due to large surface/volume ratio, electronic properties of semiconductor nanocrystalline film/electrolyte are governed by interface trap density and defect concentration. Any change in them may modify electronic transport at the semiconductor/electrolyte interface

53-55

. It is also reported that oxygen

incorporation in ZnTe decreases Zn vacancies which results in turn decreases the holes concentration 31, 56-57.


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Fig. 6(a) Current density versus RHE curves under dark and light conditions for ZnTe. (b) Similar plots for ZnTe:O (0.02%) and ZnTe:O (0.2%) samples. ZnTe:O (0.02%) sample exhibits a JPh of ~3.05 mA/cm2 at 1.23 V vs. RHE, with photocurrent onset potential of 0.40 V vs. RHE while ZnTe:O (0.2%) photoanode shows enhanced value of JPh and negatively shifted onset potential. ZnTe:O (0.2%) sample Jph of ~7.80 mA/cm-2 at 1.23 V vs. RHE. It is observed that JPh first increases to 1.19 mA/cm-2 at 0.50 V and afterward decreases as shown in Fig. 6(b). This behaviour of JPh in ZnTe:O (0.2%) photoanode is attributed to the sub band states and defect introduced on oxygen incorporation as supported by the PL, SE and DFT calculations as discussed earlier in section 3.1 and 3.2. ZnTe:O (0.2%) photoanode shows 2.5 times higher Jph as compared to ZnTe:O (0.02%) sample i. e. from 3.05 to 7.80 mA/cm2 at a bias voltage of 1.23 V vs. RHE. In addition, a shift of the 17 ACS Paragon Plus Environment


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onset potential by 0.25 V (0.40 to 0.15 V) is observed for ZnTe:O (0.2%) photoanode with respect to ZnTe:O (0.02%). It may be mentioned that different oxygen partial pressure lead to slightly different thickness of the ZnTe, ZnTe:O (0.2%) and ZnTe:O (0.02%) samples. ZnTe films having larger thickness expected to show better PEC response due to the absorption of more photons. However, in our experiment, ZnTe:O (0.2%) sample having least thickness shows better response. If one normalizes PEC response with thickness, the improvement in PEC response on increasing oxygen content will be even more which is consistent with our experimental observation of enhanced absorption in ZnTe:O (0.2%) alloys. Transmittance values (%T) for ZnTe, ZnTe:O (0.02%) and ZnTe (0.2%) samples at wavelength 500 nm (2.4 eV) are 2.51%, 2.41% and 1.24%, respectively which are very small (1-2%). Values for ZnTe, ZnTe:O (0.02%) and ZnTe (0.2%) samples at wavelength 700 nm (1.8 eV) are 66%, 58% and 14%, respectively. Observed %T values show that absorption of low energy photons increases for ZnTe:O samples with higher O content. In previous section, enhanced photocurrent in ZnTe:O thin films is attributed to increased absorption of the low energy photon due to oxygen incorporated electronic states formed with in the band gap. This is further verified by investigating PEC performance of ZnTe, ZnTe:O(0.02%) and ZnTe:O(0.2%) thin films under low (1.8 eV) and high energy (2.4 eV) illuminations and corresponding J-V characteristics are shown in Figs. 7(a), (b) and (c), respectively.


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Fig. 7(a), (b) and (c) are photocurrent density versus RHE plots for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) thin films, respectively under 2.4 and 1.8 eV energy. Table I summarizes the ratio of current at energy of 1.8 to 2.4 eV at a particular voltage for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films. This ratio is least for ZnTe



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and increases from ZnTe:O (0.02%) to ZnTe:O (0.2%) sample which indicates enhanced contribution of low energy photons in samples having higher oxygen content. Table 1: Photocurrent ratios of 2.4 to 1.8 eV for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) samples. Sample

Voltage(V)

ZnTe ZnTe:O (0.02%) ZnTe:O (0.2%)

- 0.30 1.23 1.23

I1.8 I2.4 I2.4/I1.8 (mA) (mA) -0.01 -0.05 5.00 0.05 0.11 2.20 1.26 1.93 1.53

To further investigate the change in electronic behaviour of ZnTe thin film on oxygen incorporation, Mott-Schottky analysis have been performed and obtained plots are shown in Figs. 8(a), (b) and (c) for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) films, respectively. ZnTe thin film shows a negative slope in the MS plot with VFB of 1.54 V vs. RHE. However, ZnTe:O photoelectrodes exhibit positive slopes (see Fig. 8(b) and (c) ) indicating opposite behaviour of ZnTe:O (0.2%) and ZnTe:O (0.02%) samples. Negative and positive slopes in MS plots are consistent with the observed current direction in ZnTe and ZnTe:O nanocrystalline film.


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Fig. 8 MS plots for (a) ZnTe , (b) ZnTe:O (0.02%) and ZnTe:O (0.2%) thin films under dark condition with a scan rate of 20 mV/s as a function of applied potential (V vs. RHE). Electrochemical impedance spectroscopy measurements are also carried out on the three samples under visible light illumination as shown in Fig. 9. The magnitude of the EIS



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obtained for ZnTe:O (0.2%) thin film sample indicates its lower charge transfer resistance as compared to ZnTe and ZnTe:O (0.02%). In EIS experiments, smaller radius of semicircle in high frequency region represent electrons transfer resistance at the surface arising from the diffusion of electrons. Much smaller radius in EIS plots for ZnTe:O (0.2%) as compared to both ZnTe and ZnTe:O (0.02%) thin films indicate that former outperforms other two in term of charge transport properties.

Fig.9 EIS Nyquist plots for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) photoelectrodes. 4. CONCLUSIONS In summary, an effort is made to enhance photoanodic behaviour of ZnTe thin film by incorporating oxygen which opens up the possibility of using this material for PEC water splitting applications. Effect of oxygen incorporation on structural, optical and electronic behaviour of ZnTe thin films has been investigated experimentally using HRXRD, HRTEM, PL and SE characterizations where oxygen incorporation resulted in increased nanocrystallnity and introduction of sub band gap states which are ascribed to stoichiometric defects. Both ZnTe and ZnTe:O nanocrystalline films show a broad band edge PL emissions in the range of 2.00-2.50 eV. In addition, dominant sub band PL emissions in energy range of 22 ACS Paragon Plus Environment

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1.60-2.00 eV along with a number of small sharp peaks lying in the range of 1.20-1.50 eV are observed for ZnTe:O nanocrystalline films. From DFT calculations, PL emissions in ZnTe:O in the range of 1.60-2.00 eV are attributed to oxygen incorporation while the sharp peaks in the range of 1.20-1.50 eV are assigned to Te vacancies. Effect of oxygen incorporation on PEC performance is investigated by analyzing J-V characteristics, Mott Schottky plots and EIS measurements of ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) thin films. ZnTe:O thin films exhibit improved PEC performance with reduced over potential, enhanced photocurrent, and improved photostability under 1 Sun illumination. The comparative analysis of ZnTe nanocrystalline films with and without oxygen incorporation in present work may be useful for development and advancement of intermediate band semiconductor through low cost deposition techniques such as sputtering for PEC and other applications. ASSOCIATED CONTENT Supporting Information: HRXRD and HRTEM results of ZnTe and ZnTe:O thin films. AUTHOR INFORMATION *E-mail: [email protected] NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS Intu Sharma acknowledges Council of Scientific and Industrial Research (CSIR) India for providing Senior Research Fellowship. B. R. Mehta acknowledges the support of the Schlumberger Chair Professorship. We also acknowledge the Nanoscale Research Facility (NRF) at IIT Delhi. Sushil Auluck would like to thank Centre for Mathematical Modelling and Computer Simulation (CMMACS) in Bangaluru and Inter University Accelerator Centre (IUAC) in New Delhi for the use of the HPC facilities.



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TOC GRAPHIC



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Fig. 1(a) and (b) represent real (ɛ1) and imaginary (ɛ2) parts of dielectric constant for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films prepared at RT. Similar images for samples prepared at substrate temperature of 300oC are shown in (c) and (d). Sharp features in ɛ(E) spectra of samples prepared at 300oC show better crystallinity. Non-zero values of ɛ2 for ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films below the band edge of ZnTe show absorption through sub band gap states. 181x141mm (300 x 300 DPI)

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Fig. 2(a) and (b) show absorption coefficients (α(E)) spectra for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) nanocrystalline films prepared at RT and 300°C, respectively. Non-zero values of α at energy below the band gap of ZnTe are observed in ZnTe:O. (c) and (d) show Tauc’s plots for ZnTe and ZnTe:O prepared at RT and 300°C. A long tail and a hump in Tauc’s plot of ZnTe:O (0.2%) nanocrystalline films prepared at RT and 300°C, respectively can be seen along with an increase in the fundamental absorption edge. 180x144mm (300 x 300 DPI)



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Fig. 3(a) to (f) show PL spectra for ZnTe, ZnTe:O (0.02%) and ZnT:O (0.2%) nanocrystalline films prepared at RT and 300oC. ZnTe nanocrystalline films show emissions corresponding to band edge only while ZnTe:O show sub band PL emissions. Dominate emissions in range 1.60-2.00 eV are due to oxygen and number of small sharp peaks in range 1.20-1.50 eV are attributed to Te vacancies as verified from DFT calculations. 152x93mm (300 x 300 DPI)


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Fig. 4(a), (b) and (c) show calculated band structures for ZnTe, ZnTe:O and ZnTe:VTe, respectively. The horizontal lines through reference zero represent Fermi energy positions. Arrows indicate the band gap values and separation of IB from the bottom of CB. 170x146mm (300 x 300 DPI)



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Fig. 5(a) DOS spectra for ZnTe, ZnTe:O and ZnTe:VTe systems near the band gap region. Valence band maxima are represented by the energy position 0 eV. (b) Magnified view of DOS spectra for ZnTe and ZnTe:O near the CB edge. 181x278mm (300 x 300 DPI)


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Fig. 6(a) Current density versus RHE curves under dark and light conditions for ZnTe. (b) Similar plots for ZnTe:O (0.02%) and ZnTe:O (0.2%) samples. 181x268mm (300 x 300 DPI)



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Fig. 7(a), (b) and (c) are photocurrent density versus RHE plots for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) thin films, respectively under 2.4 and 1.8 eV energy. 188x403mm (300 x 300 DPI)


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Fig. 8 MS plots for (a) ZnTe , (b) ZnTe:O (0.02%) and ZnTe:O (0.2%) thin films under dark condition with a scan rate of 20 mV/s as a function of applied potential (V vs. RHE). 188x409mm (300 x 300 DPI)



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Fig.9 EIS Nyquist plots for ZnTe, ZnTe:O (0.02%) and ZnTe:O (0.2%) photoelectrodes. 177x136mm (300 x 300 DPI)


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Oxygen Induced Enhanced Photoanodic Response of ZnTe:O Thin

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