shape controlled sn doped zno nanostructures for tunable optical emission and transport properties

Shape controlled Sn doped ZnO nanostructures for tunable optical emission and transport properties , , T Rakshit, I Manna , and S K Ray Citation: AIP Advances 3, 112112 (2013); doi: 10.1063/1.4832219 View online: http://dx.doi.org/10.1063/1.4832219 View Table of Contents: http://aip.scitation.org/toc/adv/3/11 Published by the American Institute of Physics AIP ADVANCES 3, 112112 (2013) Shape controlled Sn doped ZnO nanostructures for tunable optical emission and transport properties T Rakshit,1 I Manna,2,a and S K Ray3,b Advanced Technology Development Centre, IIT Kharagpur, 721 302, India Department of Metallurgical and Materials Engineering, IIT Kharagpur, 721 302, India Department of Physics & Meteorology, IIT Kharagpur, 721 302, India (Received July 2013; accepted November 2013; published online 13 November 2013) Pure and Sn doped ZnO nanostructures have been grown on SiO2 /Si substrates by vapor-solid technique without using any catalysts It has been found that the morphology of the nanostructures depend strongly on the growth temperature and doping concentration By proper tuning of the growth temperature, morphology of pure ZnO can be changed from tetrapods to multipods On the other hand, by varying the doping concentration of Sn in ZnO, the morphology can be tuned from tetrapods to flowerlike multipods to nanowires X-ray diffraction pattern reveals that the nanostructures have a preferred (0002) growth orientation, and they are tensile strained with the increase of Sn doping in ZnO Temperature-dependent photoluminescence characteristics of these nanostructures have been investigated in the range from 10 to 300 K Pure ZnO tetrapods exhibited less defect state emissions than that of pure ZnO multipods The defect emission is reduced with low concentration of Sn doping, but again increases at higher concentration of doping because of increased defects Transport properties of pure and Sn doped ZnO tetrapods have been studied using complexplane impedance spectroscopy The contribution from the arms and junctions of a tetrapod could be distinguished Sn doped ZnO samples showed lower conductivity but higher relaxation time than that of pure ZnO tetrapods C 2013 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4832219] I INTRODUCTION Zinc oxide (ZnO), with direct band gap of 3.37 eV and high excitonic binding energy of 60 meV at room temperature, has been used in fabricating field effect transistors,1 field emission devices,2 sensors,3 room temperature hydrogen storage,4 nanolasers,5 etc ZnO has the ability to form many configurations and exhibited wide range of morphologies such as nanowires,4 nanorods,6 nanotubes,7 nanorings,8 etc., which play an important part in governing the properties and applications of nanomaterials Doping with selective elements can play an active role in tuning the basic physical properties and shape transformation of ZnO.9–12 Doping ZnO with different elements such as Li, Na, etc in different concentrations resulted in varying morphologies.13 Shen et al showed that by varying the concentration of sulphur in S-doped ZnO nanostructures, the morphology can be changed from nanonails to nanowires.14 This change in morphology affects the optical properties of the nanostructures.13, 14 The ionic radius of Zn2+ (0.74 Å) is nearly equal to that of Sn4+ (0.69 Å), so Zn can be easily substituted by Sn without resulting in much lattice distortion Gao et al.15 and Ding et al.16 studied the growth mechanism of Sn doped ZnO nanobelts, and found that the crystalline orientation of Sn particles determined the growth direction and side faces of the nanobelts a Present address: IIT Kanpur, 208 016, India b Corresponding author’s e-mail address: physkr@phy.iitkgp.ernet.in 2158-3226/2013/3(11)/112112/12 3, 112112-1 C Author(s) 2013 112112-2 Rakshit, Manna, and Ray AIP Advances 3, 112112 (2013) Besides, doping ZnO with In, Ga, Sn, etc can increase its conductivity.17 SnO2 has a larger band gap (3.62 eV) and higher excitonic binding energy (130 meV) at room temperature as compared to ZnO Thus, Sn doped ZnO can be used for shorter wavelength optoelectronic devices and room temperature UV laser applications However, a systematic study on morphology, optical emission, and transport properties of Sn doped ZnO nanostructures have not been reported till date In this paper, we report on the growth of ZnO nanostructures, doped with different concentrations of Sn, by a simple vapor-solid technique The effect of doping on the structural properties of ZnO has been investigated by electron microscopy and X-ray diffraction analysis The temperature dependent behavior of different excitonic peaks in the near band edge region has been studied in details The transport properties of pure and Sn doped ZnO tetrapods have been studied by impedance analysis, and to the best of our knowledge are being reported for the first time II EXPERIMENTAL Pure and Sn doped ZnO nanostructures were grown on SiO2 /Si substrates by catalyst-free vaporsolid (VS) technique Zinc powder of high purity (99.99%) was used as the source material for the growth of pure ZnO nanostructures For the growth of Sn doped ZnO nanostructures, a mixture of zinc and tin powder (both of purity 99.99%), mixed in different weight ratios was used The powder was kept in an alumina crucible with the oxidized Si(100) substrate placed on the top of it The thickness of SiO2 layer was about 50 nm The crucible was then inserted in the centre of a horizontal tube furnace For the growth of pure ZnO, the deposition was carried out at temperatures of 800 ◦ C and 900 ◦ C for 30 under a constant flow of 50 SCCM (represents cubic centimeter per minute at STP) of Ar gas Sn doped ZnO nanostructures were grown at a constant temperature of 800 ◦ C and keeping other parameters same, but varying the concentration of tin powder in the mixture The morphology of the nanostructures was characterized using a field emission scanning electron microscope (FESEM, ZEISS), equipped with energy dispersive X-ray (EDX) analysis EDX analysis was used to determine the average atomic concentration of tin in the Sn doped ZnO samples The phase of the samples was studied by X-ray diffractometer (XRD) (Philips X-Pert MRD) using Cu Kα radiation (45 kV, 40 mA) of wavelength 0.15418 nm at a grazing angle of 2.0◦ The optical characteristics was studied through photoluminescence measurements in the temperature range 10– 300 K, using a He-Cd laser (325 nm) as an excitation source, and an output power of 45 mW, along with a cooled Hamamatsu R928 photomultiplier detector fitted TRIAX 320 monochromator Electrical properties of doped ZnO were studied using impedance spectroscopy data recorded with an Agilent 4294A impedance analyzer III RESULTS AND DISCUSSION Figs 1(a) and 1(b) show the FESEM images of pure ZnO nanostructures synthesized at a growth temperature of 800 ◦ C and 900 ◦ C, respectively At a growth temperature of 800 ◦ C, ZnO tetrapods are formed As shown in the inset of Fig 1(a), the arms are smooth with hexagonal cross-section with length varying between 1.5-2 μm and uniform diameter of 500–700 nm When the growth temperature is increased to 900 ◦ C, multipods are formed The arms of the multipods have length of 1-3 μm and diameter of 300-500 nm At a higher growth temperature, ZnO species with higher surface migration accumulate to form larger clusters Several molecules in the tip of the cluster act as nucleation centres Due to the presence of multiple nuclei, the growth of arms of the multipods occurs in different directions from the nucleus It may be noted that our previous study on ZnO showed the growth of mainly nanorods at 650 ◦ C; and tripods were formed when the growth temperature was increased to 750 ◦ C.18 That’s why the present study aimed on growing ZnO tetrapods and multipods at higher growth temperatures The grazing incidence X-ray diffraction (XRD) pattern of the above nanostructures is shown in Fig 2(a) All the diffraction peaks correspond to the wurtzite ZnO structure, and no peak from zinc or suboxide phases are found, confirming the formation of pure ZnO The growth exhibited a preferred (0002) orientation, with tetrapods showing stronger intensity than the multipods The photoluminescence (PL) spectra at 10 K of the ZnO tetrapods and multipods, recorded with an 112112-3 Rakshit, Manna, and Ray AIP Advances 3, 112112 (2013) (a) (b) FIG FESEM micrographs of pure ZnO nanostructures grown for 30 at different growth temperature of (a) 800 ◦ C and (b) 900 ◦ C Inset shows the magnified view of the micrographs excitation wavelength of 325 nm are shown in Fig 2(b) The broad peak around 2.471 eV in the blue-green region is related to complex defect species, which is usually attributed to zinc interstitial or oxygen vacancies.19 The peak in the ultraviolet region is related to excitonic emission It can be seen that the tetrapods exhibited much lower defect state emissions than the multipods This may be due to the enhanced surface-to-volume ratio with increasing number of arms, and this may induce more oxygen vacancies in the multipods.20 Since better growth orientation and lower defect state emissions were observed for the nanostructures grown at 800 ◦ C (tetrapods), we doped these nanostructures with tin to see its effect on the structural, optical, and electrical properties of ZnO Samples with Sn content of 3, 5, 10, and 16 at.% in ZnO were synthesized, which are designated as SZO3, SZO5, SZO10, and SZO16, respectively, hereafter The morphologies of the grown nanostructures have been studied using FESEM images Figs 3(a)–3(d) illustrate the FESEM micrographs of the grown SZO3, SZO5, SZO10, and SZO16, respectively As observed, SZO3 and SZO5 have tetrapod structure with diameter and length of arms similar to those of pure ZnO tetrapods The yields of the samples are also almost same The arms of Rakshit, Manna, and Ray (0002) (2022) (1122) (2021) (1013) (A) (1120) (A) Pure ZnO tetrapods (B) Pure ZnO multipods (1012) (1010) Intensity (arb unit) (a) AIP Advances 3, 112112 (2013) (1011) 112112-4 (B) 30 40 50 60 (degree) 70 80 (b) Normalized Intensity (arb unit) Pure ZnO Tetrapods Pure ZnO Multipods 2.0 2.2 2.4 2.6 2.8 3.0 Energy (eV) 3.2 3.4 FIG (a) Grazing incidence XRD patterns, and (b) photoluminescence spectra at 10 K, of pure ZnO nanostructures SZO3 tetrapods are smooth with hexagonal cross-section, which is clearly seen in the inset of the Fig 3(a) But the arms of SZO5 tetrapods are rough, mainly at the end As a result, the hexagonal cross-section of the arms is not clearly visible (shown in the inset of Fig 3(b)) This may be due to the induced strain as a result of higher concentration of Sn in ZnO Interestingly, any further increase of Sn content in ZnO completely changes the morphology SZO10 shows a flower-like multipod structure (Fig 3(c)) Thus, in Sn doped ZnO, multipods of different morphology can be grown at a much lower temperature than that of pure ZnO The arms of these Sn doped ZnO multipods are about 2-3 μm long and of uniform diameter of μm upto a certain length, which then decreases gradually towards the tip The size of the multipods is not same at all places over the substrate and the yield is also low compared to that of pure ZnO When Sn concentration is further increased to 16 at.%, nanowires of several micrometers long and diameter of about 50-190 nm are formed (Fig 3(d)) Inset of the figure shows the magnified view of a single nanowire The grazing incidence XRD pattern of the above nanostructures is shown in Fig 4(a) The XRD pattern of pure ZnO tetrapods has also been presented for comparison All the samples 112112-5 Rakshit, Manna, and Ray AIP Advances 3, 112112 (2013) (a) (b) (c) (d) FIG FESEM micrographs of Sn doped ZnO nanostructures grown at 800 ◦ C for 30 min., for Sn doping concentrations of (a) at.%, (b) at.%, (c) 10 at.%, and (d) 16 at.% have a preferred (0002) growth orientation, with the intensity of the peak gradually decreasing with doping The diffraction peaks of SZO3 tetrapods, SZO5 tetrapods, and SZO10 flower-like multipods correspond to the wurtzite ZnO structure and no peak from SnO2 was observed For SZO16 nanowires, in addition to the peaks of ZnO, a peak is observed at 26.53◦ corresponding to the tetragonal (110) SnO2 structure Fig 4(b) presents the high resolution XRD data showing (0002) peak of pure ZnO tetrapods and Sn doped ZnO nanostructures It can be seen that with increasing Sn doping, the (0002) peak shifts towards the higher diffraction angle side This is attributed to the smaller ionic radius of Sn4+ (0.69 Å) compared to that of Zn2+ (0.74 Å), resulting in in-plane tensile strain The XRD result clearly suggests that Sn (upto 10 at.%) is easily incorporated in ZnO lattice without changing the crystal structure But at high doping concentration of Sn (16 at.%), in addition to ZnO, separate SnO2 phase is formed The full-width-at-half-maximum (FWHM) of the (0002) peak as a function of Sn content in ZnO is shown in Fig 4(c) The variation of FWHM is not much pronounced upto Sn doping concentration of at.%, but increases significantly thereafter, which may be due to increased lattice strain for high Sn concentration Low temperature (10 K) photoluminescence spectra of the nanostructures, recorded with an excitation wavelength of 325 nm, are shown in Fig 5(a) A typical spectrum reveals two major peaks: one in the ultraviolet region (due to excitonic emissions) and another in the visible region (due to defect states), same as that of pure ZnO nanostructures It is found that the ratio of the intensity of the defect state emission to excitonic emissions gradually decreases with increase of Sn doping upto at.% But the defect state emission again gradually increases with further doping This is because at higher doping concentration of tin, when all the zinc lattice sites are occupied, the former begin to occupy the interstitial sites resulting in increased concentration of defects Fig 5(b) shows the PL spectra at 10 K of the excitonic emission bands of the grown nanostructures For all the samples, the peak at 3.375 eV is assigned to free excitonic emissions, denoted by FX The peaks at 3.360 eV and 3.364 eV are attributed to excitons bound to neutral donors, i.e D0 X and D0 X, respectively.21, 22 A peak at 3.311 eV is observed, which is not related to Sn doping, since it is also observed in pure ZnO The binding energy of this peak has been found to be (3.375-3.311 eV) Rakshit, Manna, and Ray (b) Normalized Intensity (arb unit) ZnO (1122) (2021) (1120) (1013) (1011) SnO2 (1012) Intensity (arb unit) (1010) (0002) (a) AIP Advances 3, 112112 (2013) (2022) 112112-6 (A) (B) (C) (110) (D) (E) 30 40 50 60 (degree) 70 Pure ZnO tetrapods SZO3 tetrapods SZO5 tetrapods SZO10 flower-like multipods SZO16 nanowires 1.2 0.8 0.4 0.0 80 34.0 34.5 35.0 (degree) 35.5 (c) 1.2 FWHM (degree) 1.0 0.8 0.6 0.4 0.2 10 12 14 16 18 Sn content in ZnO (at.%) FIG (a) Grazing incidence XRD patterns of (A) pure ZnO tetrapods, (B) SZO3 tetrapods, (C) SZO5 tetrapods, (D) SZO10 flower-like multipods, and (E) SZO16 nanowires (b) Showing only the (0002) peak of the XRD pattern (c) Variation of full-width-at-half-maximum of the (0002) peak with Sn content in ZnO i.e 64 meV, suggesting the peak to be excitonic in nature We have assigned this emission due to excitons bound to defect states (SX).23, 24 The peaks at 3.238 eV, 3.166 eV, and 3.094 eV are close to the integral multiple of longitudinal optical (LO) phonon replica (72 meV)25 of SX, and attributed to SX-1LO, SX-2LO, and SX-3LO, respectively For Sn doped samples, another peak is observed at 3.355 eV, which is also attributed to excitons bound to neutral donors, i.e D0 X Doping of Sn in ZnO may have induced a deep level energy state in ZnO, which may give rise to this emission Fig 5(c) shows clearly the presence of the D0 X peaks The intensity of these peaks increases with Sn doping in tetrapods, but decreases for SZO10 flower-like multipods and SZO16 nanowires At higher doping concentration, Sn occupies the interstitial sites and contributes to defect states as discussed before, and thus the contribution of excitonic emission decreases For SZO5 tetrapods, some additional peaks are observed The peaks at 3.288 eV, 3.216 eV, and 3.144 eV are close to the integral multiple of LO phonon (72 meV) replica of D0 X, and assigned to D0 X-1LO, D0 X-2LO, and D0 X-3LO, respectively Figs 6(a)–6(e) show the temperature dependent PL spectra of the excitonic emission bands of pure ZnO tetrapods, SZO3 tetrapods, SZO5 tetrapods, SZO10 flower-like multipods, and SZO16 nanowires, respectively For pure ZnO, SZO3, and SZO5 tetrapods, the D0 X peaks can be distinguished separately upto 40 K, 25 K, and 70 K, respectively, and thereafter the peaks merge The D0 X emission disappears at T>125 K for these samples For SZO10 flower-like multipods and SZO16 nanowires, the D0 X peak cannot be distinguished separately after 25 K, but the D0 X and D0 X peaks can be distinguished upto 70 K and 55 K, respectively The temperatures above which the D0 X peak disappears are 85 K and 100 K, respectively, for flower-like multipods and nanowires With 112112-7 1 1 AIP Advances 3, 112112 (2013) (b) (E) SX SX-1LO (D) Intensity (arb unit) Normalized Intensity (arb unit) (a) Rakshit, Manna, and Ray (C) (B) (A) FX SX-2LO SX-3LO (E) D 1X-1LO D 1X-2LO D 1X-3LO (D) (C) (B) (A) 2.0 D 1X D 3X D 2X 2.2 2.4 2.6 2.8 3.0 Energy (eV) 3.2 (c) Intensity (arb unit) D 3X 3.08 3.4 D 1X 3.15 3.22 3.29 Energy (eV) 3.36 3.43 D 2X FX (E) (D) (C) (B) (A) 3.34 3.35 3.36 3.37 3.38 Energy (eV) 3.39 FIG Photoluminescence spectra at 10 K of (A) pure ZnO tetrapods, (B) SZO3 tetrapods, (C) SZO5 tetrapods, (D) SZO10 flower-like multipods, and (E) SZO16 nanowires, showing (a) in full range of 2-3.55 eV, (b) only the excitonic emission bands, and (c) only the free excitonic and excitons bound to neutral donor peaks increase in temperature for all the samples, D0 X peak intensity gradually decreases as it thermally dissociates to FX The PL spectrum at 300 K has been fitted using Lorentzian function It is found that the spectrum of all the samples is best fitted with three curves, which corresponds to emission peak due to FX, SX, and SX-1LO Since the binding energy of SX is near to the excitonic binding energy, SX and its phonon replica plays an important role in emission upto 300 K The variation of FX, SX, and SX-1LO peaks with temperature of all the samples has been investigated Fig 6(f) shows the results for pure ZnO tetrapods as a representative plot The FX peak of all the samples can be well described by Bose-Einstein-type expression:26 E g (T ) = E g (0) − K θE exp T , (1) −1 where E g (T ) is the band gap energy, K is the electron-phonon coupling strength, and θ E is the Einstein temperature The extracted Einstein temperatures obtained from the fitted plots are summarized in Table I The values agree quite well with that reported in reference 27 For a detailed investigation of the electrical properties, frequency dependent impedance measurement was carried out at room temperature The impedance spectrum provides an interconnection between the structural and electrical properties of a material The resistive and capacitive properties of different regions of the material become well separable when the individual components are related to different relaxation times, τ This relaxation time (τ = RC) is an intrinsic and unique property of the material, which is independent of any sample geometry factors Therefore, the 112112-8 Rakshit, Manna, and Ray AIP Advances 3, 112112 (2013) TABLE I Einstein temperature of pure and Sn doped ZnO nanostructures Sample Einstein Temperature (in K) 259 ± 240 ± 179 ± 197 ± 242 ± Pure ZnO tetrapods SZO3 tetrapods SZO5 tetrapods SZO10 flower-like mulitpods SZO16 nanowires (a) D01X SX (b) D 2X FX SX D01X D 2X SX-1LO Intensity (arb unit) Intensity (arb unit) SX-1LO D 3X SX-2LO SX-3LO 10 K 25 K 40 K 55 K 70 K 85 K 100 K 125 K 150 K 175 K FX SX-2LO SX-3LO 10 K 25 K 40 K 55 K 70 K 85 K 100 K 125 K 150 K 175 K 200 K 225 K 200 K 225 K 250 K 275 K 300 K 3.15 3.22 3.29 Energy (eV) D03X SX-1LO D 1X-1LO SX-2LO D X-2LO SX-3LO D 1X-3LO 3.08 SX 3.15 3.22 3.29 Energy (eV) (e) SX SX 3.36 D03X SX-1LO 3.36 3.43 D01X D 2X FX SX-2LO SX-3LO 10 K 25 K 40 K 55 K 70 K 85 K 100 K 125 K 150 K 175 K 200 K 225 K 225 K 250 K 250 K 275 K 275 K 300 K 300 K 3.43 3.08 3.15 3.22 3.29 3.36 3.43 Energy (eV) D03X (f) D 1X D02X FX 3.39 FX SX SX-1LO 3.36 SX-2LO 3.33 10 K 25 K 40 K 55 K 70 K 85 K 100 K 125 K 150 K 175 K 200 K 225 K 250 K 275 K 300 K 3.08 3.22 3.29 Energy (eV) FX SX-1LO SX-3LO 3.15 (d) D01X D02X 10 K 25 K 40 K 55 K 70 K 85 K 100 K 125 K 150 K 175 K 200 K 3.08 Intensity (arb unit) 3.43 3.15 3.22 3.29 Energy (eV) 3.36 3.43 Energy (eV) Intensity (arb unit) (c) 3.36 Intensity (arb unit) 3.08 250 K 275 K 300 K 3.30 3.27 3.24 3.21 3.18 50 100 150 200 Temperature (K) 250 300 FIG Temperature dependent photoluminescence spectra of (a) pure ZnO tetrapods, (b) SZO3 tetrapods, (c) SZO5 tetrapods, (d) SZO10 flower-like multipods, and (e) SZO16 nanowires 300 K graph (solid line) has been fitted with Lorentzian curves (dash lines) The sums of the three Lorentzian curves are indicated by open circles (f) Variation in free excitonic peak, peak due to excitons bound to defect states and its LO phonon replica, with temperature of pure ZnO tetrapods Free excitonic peak have been fitted with Bose-Einstein-type expression (solid line) 112112-9 Rakshit, Manna, and Ray AIP Advances 3, 112112 (2013) (a) Sn doped or undoped ZnO tetrapods Al electrode SiO2 Si (b) (c) (d) FIG (a) Schematic diagram of the fabricated device with Al contact on the top Magnified view of the FESEM micrographs of (b) pure ZnO tetrapods, (c) SZO3 tetrapods, and (d) SZO5 tetrapods relaxation processes occurring within a material can be obtained from the analysis of the impedance data Impedance measurements were carried out with pure and Sn doped ZnO tetrapods only, since they have almost the same size and yield Fig 7(a) shows the schematic diagram of the fabricated structure that has been used for the impedance measurement The possible conduction path through tetrapods between the electrodes is shown by the red line Since there is a layer of SiO2 between the tetrapods and Si, the contribution of Si substrate in the impedance measurement is eliminated Figs 7(b)–7(d) are magnified view of the FESEM images of pure ZnO, SZO3, and SZO5 tetrapods, respectively As observed, the arm of a tetrapod is connected to an arm or junction of another one (shown by red arrows), thus making an interconnection between the tetrapods This interconnection makes conduction possible through the tetrapods between the two electrodes Figs 8(a)–8(c) show the complex plane plot of imaginary Z versus real Z by applying different dc bias voltages for pure ZnO, SZO3, and SZO5 tetrapods, respectively Within the measured range, a typical plot shows two overlapping semicircles, a larger semicircle in the low frequency region (say, semicircle-1) and another one in the high frequency region (say, semicircle-2) Inset of each figure clearly shows the presence of semicircle-2 A tetrapod consists of arm and junction, which are two structurally and electrically different regions It is proposed that the origin of semicircles is attributed one from the arm and the other from the tetrapod junction The proposed equivalent 112112-10 Rakshit, Manna, and Ray AIP Advances 3, 112112 (2013) (b) (a) (A) (B) (C) (D) 5.0X103 (A) 4.0X103 (A) (B) (C) (D) 2.5X104 0V 0.5 V 1V 1.5 V 2.0X104 (A) (C) 3.0X103 Semicircle-1 -Im Z ( ) -Im Z ( ) (B) (D) 2.0X103 1.5X104 (B) (C) 1.0X104 Semicircle-1 4.0X102 1.5X102 3.0X102 -Im Z ( ) -Im Z ( ) 5.0X103 1.0X102 5.0X101 Semicircle-2 2.0X102 1.0X102 3.0X102 0 3.0X103 4.0X102 Re Z ( ) 5.0X102 6.0X103 9.0X103 0 1.2X104 Semicircle-2 2.5X102 5.0X102 Re Z ( ) 7.5X102 2.0X104 Re Z ( ) 4.0X104 Re Z ( ) 6.0X104 8.0X104 5.0X104 (A) (B) (C) (D) 4.0X104 0V 0.5 V 1V 1.5 V (d) (A) -Im Z ( ) (D) 2.0X102 1.0X103 (c) 0V 0.5 V 1V 1.5 V 3.0X104 Ca Cj Ra Rj Rs 2.0X104 (B) 6.0X102 1.0X104 (C) (D) -Im Z ( ) Semicircle-1 4.0X102 2.0X102 Semicircle-2 0 4.0X102 3.0X104 6.0X104 8.0X102 Re Z ( ) 1.2X103 9.0X104 1.2X105 Re Z ( ) FIG Complex-plane impedance spectra for (a) pure ZnO tetrapods, (b) SZO3 tetrapods, and (c) SZO5 tetrapods Inset shows the semicircle in the higher frequency region Open symbols are experimental data and solid curves are the corresponding fit (d) Proposed equivalent circuit circuit (shown in Fig 8(d)) consists of a series combination of two sets of parallel RC elements along with a resistor RS in series in order to account for the shift in the high frequency range along the real Z-axis Ra , Ca and Rj , Cj are the possible resistance and capacitance values due to the arms and junctions of the tetrapods, respectively The impedance obtained from the equivalent circuit can be represented as: Z = Rs + + iωCa Ra −1 + + iωC j Rj −1 (2) The solid curves in the Figs 8(a)–8(c) are obtained by best fit of the complex plane plots by using Eq (2) Fitted curves show good agreement between the proposed equivalent model and the complex plane plot Figs 9(a) and 9(b) show the extracted resistance of semicircle-1 and semicircle-2, respectively, plotted as a function of the dc bias voltage The resistance decreases with increasing dc bias voltage for both the cases, but the change of resistance of semicircle-1 is much more than that of the semicircle-2 In semicircle-1, the rate of decrease of resistance with dc bias voltage enhances with the increase in Sn doping Also, for both cases, the resistance is higher for the Sn doped samples than pure ZnO For semicircle-1, the resistance value increases uniformly with Sn doping at V, but the resistance of SZO5 tetrapods is found to be lower than that of SZO3 tetrapods from 0.5 V onwards However, for semicircle-2, the resistance increases gradually with increase in Sn doping From these observations it appears that semicircle-1 is originated due to the junction and semicircle-2 due to the tetrapod arms The arms and junctions contain some defects (oxygen vacancy), so the carriers injected due to the applied bias find a conducting path because of these defects, resulting in a decrease of resistance Since the junction is the meeting point of the four arms, the defect density is likely to be more 112112-11 Rakshit, Manna, and Ray (a) (b) 1.2X10 Pure ZnO tetrapods SZO3 tetrapods SZO5 tetrapods 4 6.0X10 4.0X10 6.0X10 4.0X10 2.0X10 2.0X10 0.0 0.5 1.0 Voltage (V) 1.5 (c) 0.0 2.0X10 -4 1.6X10 -4 1.2X10 -5 8.0X10 Realaxation time (sec) (d) Pure ZnO tetrapods SZO3 tetrapods SZO5 tetrapods -4 Realaxation time (sec) 8.0X10 ) 8.0X10 Pure ZnO tetrapods SZO3 tetrapods SZO5 tetrapods 1.0X10 Resistance ( ) 1.0X10 Resistance ( AIP Advances 3, 112112 (2013) 0.5 1.0 Voltage (V) 1.5 -8 7.0X10 Pure ZnO tetrapods SZO3 tetrapods SZO5 tetrapods -8 6.0X10 -8 5.0X10 -8 -5 4.0X10 4.0X10 0.0 0.5 1.0 Voltage (V) 1.5 0.0 0.5 1.0 Voltage (V) 1.5 FIG Variations of (a) resistance of semicircle-1, (b) resistance of semicircle-2, (c) relaxation time of the junction, and (d) relaxation time of the arm, with the dc bias voltage in junctions than the solid arms As a result, the resistance of the arms changes slowly compared to that of the junction It may be noted, ZnO shows n-type conductivity due to the presence of native defects.28, 29 As has been observed from the PL spectra of Fig 5(a), the defect state emission decreases with increase of Sn doping in tetrapods This means that the conductivity must decrease with Sn doping But when Sn is doped in ZnO, Sn4+ substitutes Zn2+ in the ZnO crystal, thus resulting in two more free electrons, which may also contribute to the electrical conduction When the doping concentration is low (3 at.%), the contribution of excess electrons is less compared to that of defect state emissions But with further increases of doping to at.%, the contribution from excess electrons increases With increase of dc bias voltage, more carriers are injected, so the contribution from excess electrons overpowers the defect state emissions, resulting in a decrease of resistance in the junction of SZO5 tetrapods than SZO3 tetrapods But on the other hand, the defect density is much lower for the arms, thereby the effect of injected carriers on the conductivity is expected to be less pronounced So, the resistance increases with Sn doping at all voltages in the case of arms The above assumption appears to be supported by the work done by Huh et al.,30 where impedance measurement on a single tetrapod revealed that the semicircle in the higher frequency range corresponds to wurtzite arm, and that in the lower frequency range corresponds to junction of ZnO tetrapod Figs 9(c) and 9(d) show the calculated relaxation time of the tetrapod junction and arm, respectively, plotted as a function of the dc bias voltage The relaxation time of the arm and junction largely follows the resistance curve of Figs 9(a) and 9(b) It is found that the relaxation time decreases with increase of dc bias voltage, except for SZO3 where the relaxation time first increases slightly with dc bias voltage and then decreases The relaxation time is found to be higher for the Sn doped samples than pure ZnO Therefore, the conductivity and relaxation time of pure ZnO tetrapods can be tuned by varying the doping concentration of Sn 112112-12 Rakshit, Manna, and Ray AIP Advances 3, 112112 (2013) IV CONCLUSIONS Pure and Sn doped ZnO nanostructures were grown on SiO2 /Si substrates at varying growth temperature and Sn concentration, by a simple vapor-solid technique By properly tuning the growth temperature, the morphology of pure ZnO were changed from tetrapods to multipods; and that from tetrapods to flower-like multipods to nanowires by changing the Sn doping concentrations in ZnO XRD pattern revealed that the nanostructures have a preferred (0002) growth orientation PL spectrum showed that with increase in the number of arms (i.e from tetrapods to multipods) of pure ZnO, the emission from the defect states also increases With increase of Sn doping in ZnO, emissions from defect states gradually decreases; but again increases at much higher doping The transport properties of pure and Sn doped ZnO tetrapods were studied using complex-plane impedance measurements The complex-plane impedance spectrum showed two semicircles, which have been attributed to arms and junctions of tetrapods The Sn doped samples exhibited lower conductivity and higher relaxation time than that of pure ZnO tetrapods ACKNOWLEDGMENTS This work was supported by DST SERI sponsored NSH project grant Z Fan, D Wang, P.-C Chang, W.-Y Tseng, and J G Lu, Appl Phys Lett 85, 5923 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