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Effect of structure and morphology on the UV photo detection of ZnO nanostructures and microstructures
Accepted Manuscript Effect of Structure and Morphology on the UV Photo Detection of ZnO Nanostructures and Microstructures S. Acharya, S.K. Biswal, S.N. Sarangi PII: DOI: Reference:
S0301-0104(18)31441-1 https://doi.org/10.1016/j.chemphys.2019.04.014 CHEMPH 10354
To appear in:
Chemical Physics
Received Date: Revised Date: Accepted Date:
24 December 2018 13 March 2019 17 April 2019
Please cite this article as: S. Acharya, S.K. Biswal, S.N. Sarangi, Effect of Structure and Morphology on the UV Photo Detection of ZnO Nanostructures and Microstructures, Chemical Physics (2019), doi: https://doi.org/10.1016/ j.chemphys.2019.04.014
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Effect of Structure and Morphology on the UV Photo Detection of ZnO Nanostructures and Microstructures S. Acharya a, S. K. Biswala,#, S. N. Sarangib, * a
b
Centurion University of Technology and Management, Odisha, India. Institute of Physics, Sachivalaya Marg, P.O.- Sainik School, Bhubaneswar-751005, India. Corresponding author’s E-mail:* [email protected], # [email protected]
ABSTRACT We establish a simple approach to control the morphology of ZnO materials, by using chemical bath deposition (CBD) technique. The effect of variation in precursor concentration on the morphology and crystalline characteristics are investigated by using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques. Morphological effect on the current – voltage (I-V) characteristic of ZnO nanostructure and microstructure devices are investigated. Nanostructure ZnO devices show an enhancement in the forward bias current in comparison to microstructure devices. Further enhancement in photocurrent under ultraviolet (UV) light for nanostructure ZnO devices demonstrate the capability of a potential UV photo detector.
Key Words: ZnO Nanostructures, SEM, XRD, Lattice deformation, Strain, photo detector
1. Introduction The huge interest towards preparation of one-dimensional semiconductor nanostructures such as, nanorods, nanowires, nanosheets, and nanotubes have attracted much attention due to its unique optoelectronic properties and for their diverse applications [1-9]. Zinc oxide (ZnO) has been widely studied since 1935 [1]. The low-dimensional studies on ZnO are very popular because of its various applications in the field of optoelectronics [2,3], photo detector [4,5], photo catalysis [6-11], sensors [11-16], ultraviolet (UV) laser [17]and different nanotechnology [2-18] field. In addition to that, ZnO nanomaterial exhibit novel biomedical as well as environmental applications for social benefits [18,19].Moreover, ZnO nanomaterial can be tuned for the UV detection [4,5] which is very important for many applications and it is inexpensive and environmentally friendly as compared to other metal oxides. Properties of ZnO can be tuned according to the research interest, by doping with various metal atoms to suit specific needs and applications. Zinc oxide is known to be a low-cost n-type semiconductor having wide band gap of 3.4 eV and large exciton binding energy of 60 meV [6]. Basically,
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ZnO exist in three polymorphic forms such as wurtzite, zinc blende, and rock salt phase. Out of these three, the wurtzite structure seems to be the most stable one at room temperature and in ambient pressure, with Zn and O atoms forming separate hexagonal close-packed (hcp) sub lattice, where every atom has a tetragonal coordination [20].
Both physical and chemical deposition techniques like, laser ablation [21], hydrothermal methods [22,23], chemical bath [24,25], electrochemical depositions [26,27], sol–gel method [28,29], chemical vapor deposition [30], spray pyrolysis [31], thermal decomposition [32], combustion method [33,34]and solvothermal method [35]are employed for the deposition of ZnO materials on different substrates. However, most of the solutionphase methods have been used to synthesize ZnO materials because of their simplicity of solution-phase synthesis, which is carried out at ambient or slightly elevated temperature. Among many solution-phase methods, chemical bath technique is found to be a simple and cost-effective technique for preparing ZnO materials [25,26]. In this report, we have synthesized ZnO materials by chemical bath technique and establish a control on the morphology of ZnO by varying the precursor concentration. The detail study of effect of precursor concentration on the morphology and crystalline structure of the ZnO material have investigated and an UV photo-detector fabricated using ZnO nanostructures/ n-Si heterojunction.
2. Experimental n-Si wafers were used as substrate for synthesis of ZnO compound semiconductor via chemical bath deposition (CBD) technique. Prior to the deposition, the Si wafers were rinsed for 1-2 min into aqua regia in order to remove the native oxide present in it. Then the samples were ultrasonically cleaned with acetone and ethanol for 5 min respectively. Finally the wafers were rinsed with de-ionized (DI) water and then dried in ambient condition. The synthesis was carried out by varying the precursor concentrations at 10mM, 50mM, 100mM, 200mM, 500mM, keeping the other synthesis parameters constant. Equimolar concentration of Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and hexamethylenetetramine (HMT) ((CH2)6N4) salts were dissolved in aqueous DI water to form a 100 mL solution. The cleaned substrates were dipped in the solution mixture. The glass beaker containing solution precursors and substrates was covered with an aluminum foil and kept in an electric oven at 100°C for 3 hour. After the required growth, the samples were subsequently removed from the glass beaker, rinsed with DI water and dried in air at 60°C for several hours. The sample for 10 mM concentration was 2
designated as S1, 50mM concentration as S2, 100mM concentration as S3, 200mM concentration as S4 and 500mM concentration as S5 respectively. The morphology and structural properties of samples were investigated by field-emission scanning electron microscope (FESEM), and X-ray diffraction (XRD) (Philips X’pert), where X-ray diffractometer equipped with a monochromatic CuKα radiation source (1.54178Å). The current–voltage (I–V) characteristics of the fabricated n-Si/ZnO nanostructures heterojunction were measured both in dark and under illumination by a keithely electrometer using an UV lamp emitting at 325 nm with a power density of 0.2mW/cm 2.
Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) was used as a precursor and HMT act as surfactant for ZnO growth. It was observed that a white precipitate of ZnO deposited on the substrates at the bottom of the flask. The reaction mechanism proposed here, was already reported by Sarangi et al. [19]. The corresponding chemical reaction for this particular chemical bath technique is as follows
In the above reaction, Zn2+ ions are combined with OH - radicals in the aqueous solution to form a Zn(OH)2 colloid through the reaction Zn2+ + 2OH-→ Zn(OH)2. Later, during the growth process, the Zn(OH)2 is separated into Zn2+ ions and OH- radicals according to reaction 2,then, ZnO nuclei are formed. This is well known that pH has a greater influence during the synthesis of ZnO materials by using CBD route [36]. However, in our case, it has been found that pH value varies from 10 to 11 for different concentration of sample from 10mM to 500mM respectively. Since the variation in pH for different samples are not significant, emphasis has been made to see the effect of precursor concentration on morphology and structure of the samples.
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3. Results and discussion 3.1. Morphological studies The morphology of chemically grown ZnO materials were investigated by using FESEM measurement. Fig. 1.shows the FESEM images of ZnO materials and the effects of precursor concentration on the morphology of ZnO. Fig.1a. shows the morphology of sample S1 where the precursor concentration was chosen, 10 mM. The growth pattern shows ZnO nanostructures were uniformly distributed over the surface of the substrate. The length of the nanostructures found to be varied from 2.0 µm to 6.0 µm and the diameter of the ZnO nanostructures was found to be in the range of 70 to 90 nm. Fig.1b. shows the morphology of sample S2 where the precursor concentration was chosen, 50 mM. As shown in Fig. 1b.the SEM image depicts a nanorod morphology with several nanostructures were originate from one nucleation cite. The length of the nanostructures found to be varied from 4.0 µm to 6.0 µm and the diameter of the nanostructures was found to vary from 600 nm to 900 nm. In case of 100 mM precursor concentration (sample S3), the length of the nanostructures found to be reduced with respect to sample S1 and S2 and varied from 1.0 µm to 3.0 µm whereas the diameter of the ZnO nanostructures was found to be increases with respect to sample S1 and S2 and in the range of 900 nm to 1.0 µm as shown in Fig. 1c. Fig. 1d. represents the morphology of sample S4, where the precursor concentration taken as 200mM. Here the diameters of nanostructures were very large as compared to sample S1, S2 and S3 respectively. The diameter of sample S4 was found to be more than microns. This result suggests that increasing precursor concentration, increases the nucleation cites as well as the growth rate for which the diameter of the ZnO nanorod increases. The observation shows that the precursor variation has a dramatic effect on ZnO growth morphology. As the precursor concentration is increased, the nanostructures cross sections become progressively larger while their length decreases. Fig.1e. shows the FESEM micrograph of sample S5, where the precursor concentration taken as 500mM. One can observe, no nanostructures present in the sample and the morphology of this sample clearly 4
depict ZnO material were deposited in micro structural form. The widths of the nanostructures are markedly increased at higher precursor concentrations. This trend leads to microstructure, plate-like, hexagonal crystals as shown in Fig. 1e. These observations are illustrated quantitatively by plotting the aspect ratio Vs. molar concentration as shown in Fig. 1f. Similar results reported by Amin. et. al. [36] where they have shown there is an increase in aspect ratio due to increase in growth temperature. In our case, we have fixed the growth temperature and growth time to realize the effect of precursor concentration during growth.
The overall FESEM result predicts, with increasing the precursor concentration, the growth rate of ZnO nanostructures increases and due to the availability of more nucleation center on the surface of the substrate, the agglomeration of number of nanostructures evolve into microscopic structures. The effect of precursor concentration on morphology and the dimension of ZnO material are clearly visible in the above set of samples. We believe, the precursor concentration is a key factor in the CBD technique to achieve controlled ZnO nanostructure growth. A model has been proposed inorder to understand the growth of nanostructure to microstructure, which is schematically represented in Fig.2.
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Fig.1. FESEM images of ZnO material with different concentrations, (a) 10 mM (sample S1), (b) 50 mM (sample S2), (c) 100mM (sample S3), (d) 200mM (sample S4) and (e) 500mM (sample S5), (f) the aspect ratio Vs precursor concentration plot.
3.2. Mechanism for the growth of ZnO nanostructures and its transformation to microstructures HMT is a highly water soluble, non-ionic tetra dentate cyclic tertiary amine. The rate of decomposition of HMT is independent of reaction, which yields ZnO indicating HMT act as kinetic buffer. It plays a very important role in producing different morphology of ZnO nanostructures, especially vertical nanostructures form. During the growth of ZnO nanostructures, reaction of Zn(NO3)2.6H2O and HMT produces OH− anions, which react with Zn2+ ions to form ZnO as shown in the chemical equation 2 and 3. The OH – concentration can control the growth process of new [Zn(OH)4]2-ions and enhance the crystal growth along a particular crystallographic direction. 6
It is known that the hexagonal ZnO crystal has both polar and nonpolar faces. The polar faces with surface dipoles are thermodynamically less stable than nonpolar faces with surface dipoles; which often undergo a rearrangement to reduce their surface energy [37]. The preferential adsorption leads to different growth rates of planes, that is, V(0001) >V(1̄011̄) >V(1̄010) >V(1̄011) >V(0001̄) [38]. Hence, in the absence of structure modifiers under chemical growth condition, the (0001) polar plane is energetically unfavorable and has a faster growth rate than other planes. In our case, HMT, being a non-ionic tertiary amine derivative and a non-polar chelating agent, preferentially attaches to the nonpolar facets of the ZnO crystal, thereby exposing only the (0001) plane for the growth. Usually, preferential growth of ZnO occurs along the (0002) direction in case of seed deposited substrates or on lattice matched substrates. But in our case, we have taken a Si substrate without any seed layer deposition. The mechanism for the growth of ZnO nanostructures and its transformation to ZnO microstructures are shown in the schematic representation as Fig. 2. Initially the ZnO nanostructures grow on Si substrates for 3 hours duration having 10 mM concentration is represented as step-I of Fig. 2.With increasing precursor concentration from 10 mM to 500 mM, the growth of ZnO nanostructures continues and the nanostructures growth fused together to form microstructure form as shown in the different steps of Fig. 2.It is clear from the schematic Fig. 2. that with increasing the precursor concentration, the nanostructures are extend readily from the center and grow as thick nanostructures and finally, cover the Si substrate as microstructure form as shown in step-V of Fig. 2. During the 1st stage of growth (Step-I), due to availability of small nucleation center, thin ZnO nanostructures are grown randomly on the Si- substrate. As the precursor concentration slowly increases from 10mM to 500mM, i.e. 50 times of initial concentration, the competition between the nanorods growth readily and along
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the length increases because of the availability of more nucleation cites. As a result, the thickness of nanostructures increases gradually with increase in precursor concentration. The nanostructures growth overlap with each other and the average growth rate increased by many fold, thus produces microstructure rather than nanostructure morphology as shown in step V of the schematic Fig. 2.
Fig. 2. Schematic diagrams for transformation of ZnO nanostructures to microstructures.
3.3. Structural studies To investigate the effect of the precursor concentration on the crystalline structure of ZnO nanostructures/ microstructures, XRD analysis was performed on ZnO grown on a Si substrate through chemical bath techniques. It can be seen from Fig.3.that all the patterns have peaks at 31.76, 34.40, 36.24, 47.56, 56.64, 62.86, 66.87, 68.09 and 69.70 corresponding to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes/orientations of ZnO, respectively, which correspond to the typical diffraction peaks of hexagonal wurtzite ZnO (JCPDS No. 36e1451). The presence of several peaks in the XRD reveals that the nanostructures/ microstructures are polycrystalline by nature. The interesting feature of the
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XRD spectra as shown in Fig. 3. is that the ZnO (101) diffraction peak is much stronger than the ZnO (002) peak for all samples. This indicates that the formed ZnO nanostructures and microstructures have a preferential crystallographic (101) orientation. The peak broadening occurs during the growth of ZnO nanostructures and narrowing of peaks observed in case of ZnO microstructures. The XRD peak broadening is a general feature for nanostructures observed by many researchers [39]. Further, the absence of peaks other than ZnO nanostructures observed in XRD spectra indicates, no impurity phases were present in the sample. Hence, the use of Zinc nitrate hexahydrate (Zn(NO3)2.6H2O)and HMT precursors seem to be an effective way to grow ZnO nanostructures by CBD technique without inducing any kind of undesired compounds.
Fig. 3. X-ray diffraction patterns of ZnO material of different concentrations deposited on Si substrate (a) 10 mM (sample S1), (b) 50 mM (sample S2), (c) 100mM (sample S3), (d) 200mM (sample S4) and (e) 500mM (sample S5)
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This is very important to observe (0 0 2) peak for ZnO material, because it gives the c-axis parameter as well as disorder-ness in crystal lattice of the wurtzite ZnO structure. We have made the fittings of (0 0 2) peak of the samples S1, S2, S3, S4, S5respectively as shown in Fig.4 (a). As shown in Fig. 4(a), it has been observed that the peak position at nearly 34.42 degree corresponds to (002) planes, in which the peak full width half maximum (FWHM) gradually decreases from sample S1 to sample S5 and the FWHM values for the all samples were presented in Table 1.
Fig. 4.(a) Gaussian fittings of (0 0 2) peak of ZnO material of different concentrations, (b)Williamson–Hall plots of ZnO material of different concentrations.
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Moreover, XRD peak broadening also involves the self-induced strain developed during the growth. We have calculated the strain for all samples, using the Williamson–Hall (W-H) formulae [40] : 𝛽 𝐶𝑜𝑠 𝜃 =
𝑘𝜆 𝐷
4 𝜀 𝑆𝑖𝑛 𝜃
(1)
where k is the shape coefficient for the reciprocal lattice point, is the X-ray wavelength, corresponds to the peak position, is the strain and D is the effective crystallite size, respectively. Generally, (W-H) plot yields a straight line where the strain is given by the slope of the line. Fig. 4(b) shows plots of Cos versus Sin (W–H plot) for the ZnO material synthesized at different concentrations. The lattice strain calculated using (W-H) plot of XRD line profile showed a decrease with the increase in precursor concentration. All the values of the lattice strain are presented in Table 1. The other structural parameters such as Inter-planner spacing, d(Å), Lattice constants a(Å), c(Å), c/a ratio, unit cell volume(Å3) and bond length, L(Å) were calculated using the fitting parameters of the intense peaks from XRD, which are presented in Table 1 by using following equations [39, 41].
1 4 h 2 hk k 2 l 2 Interplanar Spacing 2 2 (2) 3 d a2 c 0
Lattice constant, c( A)
Sin hkl
(3)
Unit cell Volume, V (Å3) = 0.866 × a2 × c 0
Bond Length, L( A) Where u p
up
(4)
a2 1 ( u p )2 c2 3 2
(5)
Wurzite structure positional parameter, calculated as,
a2 0.25 3c 2
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From Table1, it has been cleared that the full-width-half-maximum (FWHM) values of (0 0 2) peak of ZnO nanostructure samples S1, S2 and S3 found more than that of ZnO microstructures sample S4 and S5. This result suggests that as the precursor concentration increases, the FWHM value of (0 0 2) peak decreases, confirms the improvement in the crystalline quality of ZnO in case microstructure sample. The similar result predicted by R. Jacob et. al.[41] for nanocrystalline ceramic sample. The reasonably narrow FWHM widths for ZnO microstructure (S5) sample demonstrate the high crystal quality of ZnO obtained by chemical bath technique on Si substrates.
As shown in Fig. 4. the (002) peak positions have a slight shift towards lower angles in case of nanostructure samples (S1 to S3) with respect to its microstructure sample (S4 and S5), suggest a deformation in original structure, thus there is an overall change in all the structural parameter such as inter-planner spacing, lattice constants a, c, c/a ratio, unit cell volume, bond length and strain as shown in Table1. Table 1. Structural parameters of the ZnO materials varying precursor concentrations. Sample FWHM Interplanar Lattice Constants Spacing, d(Å) a c (101) S1 0.8126 2.491 3.280 5.186 S2 0.7747 2.484 3.265 5.194 S3 0.7462 2.481 3.260 5.194 S4 0.3438 2.476 3.240 5.206 S5 0.3110 2.476 3.240 5.206
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c/a ratio
1.581 1.590 1.593 1.606 1.606
Unit Bond Strain cell Length, (ɛ x10 -3) volume, L(Å) V(Å)3 48.316 1.988 0.01204 47.949 1.983 0.01240 47.802 1.980 0.01240 47.327 1.973 0.00397 47.327 1.973 0.00389
Fig. 5. (a) Precursor Concentration Vs Inter-planner spacing, (b) Precursor Concentration Vs Strain and Precursor Concentration Vs (002) Peak Position of ZnO material.
As shown from Fig.5(a), the inter-plannar spacing (d) between crystal lattice of ZnO nanostructure increases by decreasing precursor concentration. This observation can be described on the basis of distortion of the wurtzite structure. The distortion in wurtzite structure between the atoms can develop a lattice strain due to the deviation in bond lengths and bond angles [42]. According to Bragg’s law, the Bragg’s angle and inter planner spacing (d) are correlated with each other, i.e. the strain increases the d-spacing which causes a shifting of peak towards lower 2θ values. As shown in Fig. 5b. the lattice strain for 10 mM concentration (S1) is more with respect to the sample with 500 mM concentration (S5) and the peak position for 10 mM concentration (S1) sample is shifting more towards lower 2θ values with respect to the sample with 500 mM concentration (S5).e The observed phenomena also confirm that, lattice strain increases with decreasing nanorod dimensions. Similar results observed by Andrade et al [40] for their work related to BaF2 nanoparticles.
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Fig. 6.(a) Precursor Concentration Vs Lattice Constants, (b) Precursor Concentration Vs Bond Length and Precursor Concentration Vs Unit-Cell Volume (Å3) of ZnO material.
Fig. 6(a), shows how the lattice constant varies with increasing precursor concentration during growth. As shown in Fig. 6(a), the lattice constants, “a” decrease with increasing precursor concentration, whereas the lattice constants, “c” increase with increasing precursor concentration. The role of particle size in the change of lattice parameters is theoretically and experimentally studied [42]. Deformation of lattice parameter plays an important role in the particle size is established by many reporters [43, 44]. As shown in Fig. 6b. bond length and unit cell volume decrease with increasing precursor concentration. The similar result of decrease in unit cell volume with increasing particle size is already reported by Luca et al. [45]. Initially as precursor concentration increases from 10mM to 100mM, both bond length and unit cell volume decreases rapidly up to some extent, then it saturates and at 500mM concentration (S5). The Zn–O bond length calculated for sample S1 to S5, decreases from 1.988Åto 1.973Å in our case where as the reported Zn–O bond length in the unit cell of ZnO and neighboring
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atoms is 1.976 Å. The calculated bond length fairly agrees with the Zn–O bond length as reported by Bindu et al. [39].
Fig. 7(a) shows the I−V measurements performed under dark condition for ZnO nanostructures/n-Si and ZnO microstructures/n-Si heterojunction devices. Both the device shows the better characteristics of a rectifying diode. The I-V results confirms four times enhancement in the forward bias current for ZnO nanostructures/n-Si device with respect to ZnO microstructures/n-Si heterojunction device. The increase in forward bias current could be due to the presence of a native oxide layer in the interface as reported by Sarangi et al. [46]. In case of a thin oxide layer between the nanostructures and metal junction, under forward bias condition, the carriers can cross over the device interface by tunneling and contribute to the total current flow [47].As there is a reasonable increase in forward bias current for ZnO nanostructure-based device, we have selected this device to check for the photo-detection under UV light (wavelength 325 nm) illuminations. As shown in Fig. 7(b), the I-V measurements performed under UV light illuminations for ZnO nanostructures/n-Si heterojunction device, found to generate higher currents than the dark condition, which enables the ZnO nanostructure sample useful for UV photo detector. The mechanism for enhancement in photo current is due to surface oxygen/ native oxide layer as described by many reports [47]. Similar approach may consider in our case. Under dark condition, oxygen species adsorbed on the surface of ZnO nanostructures capture free electrons thus produces a depletion layer near the nanorods surface, resulting in an upward band bending near the surface. The adsorption of oxygen molecule significantly reduces the conductivity of the nanostructures due to large surface to volume ratio of ZnO nanostructures. Exciting with photon energy (UV) higher than the energy gap of ZnO generates electron–hole pairs in the ZnO nanostructures. The holes migrate to the nanorod surface and recombine with O2– trapped electrons, thus releasing oxygen from the surface.
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Therefore, adsorbed oxygen and the photo generated hole contribute to local charge neutrality and free up the photogenerated electrons which enhance the conductivity of the detector. Fig. 7(c) shows the speed of photo detector response and the recovery time. The response time and the recovery time of the ZnO nanostructures device under UV illumination were found to be 12 and 22 s, respectively. Recently, Alenezi et. al. [48] and Paul et. al. [49] have reported photo response characteristics of ZnO nanostructures. The photo response time and the recovery time found to be 42s and 55s by Alenezi et. al. at a bias voltage of 2 volts [48] whereas the photo response time and the recovery time found to be 1.56 s and 2.34 s by Paul et. al. [49] on zero bias voltage. However, in our case the photo response time and the recovery time found to be 12 and 22 s respectively on applying 4 volt forward bias as shown in Fig. 7(c). This is to be noted that electron-hole generation is a fast process [4] for which a high photo current produced in a very short time i.e 12s in our case. Then during the re-adsorption process, it slowly reached the steady-state of photo current value. As the adsorption and photodesorption processes are slow processes [4], the photo current reaches the saturation value very slowly for which the recovery time is 22 s in our case.
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Fig. 7. I-V characteristics of (a) the ZnO nanostructures/n-Si and ZnO nanostructures/n-Si heterojunction, (b) ZnO nanostructures/n-Si heterojunction under dark and under UV illumination, (c) ZnO nanostructures/n-Siphoto detector response and the recovery time at 4V.
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4. Conclusions In summary, this work demonstrates a novel strategy to achieve controllable synthesis of ZnO material using a simple chemical technique by varying the precursor concentration. Five ZnO samples were synthesized by varying precursor concentration and their morphologies and structures were investigated. The results show that theZnO morphology can be tuned from nanostructures to microstructures by varying the precursor concentration. The structural analysis has confirmed the improvements in crystallinity in micrsostructure samples. The results obtained in this study demonstrate that the structure and morphology of ZnO can be tailored by tuning the precursor concentration. Approxmately 4 times enhancement found in forward bias current for ZnO nanostructures/n-Si heterojunction device with respect to the ZnO microstructures/n-Si heterojunction device and further enhancement in forward bias current under UV illuminations for the ZnO nanostructures/n-Si heterojunction device enables it for a suitable nano-electronic device application and the phenomenon may support for a unique platform for large scale and cost-efficient UV photo detectors. The experimetal observation suggests that the variance in the precursor concentration, results in various nucleation habits, induces the formation of ZnO with different morphologies and structures which can be used for specific nano-electronic device applications.
Acknowledgements One of the authors (S. N. Sarangi) gratefully acknowledges the late Professor S. N. Sahu then at Institute of Physics, Bhubaneswar and Professor S. Nozaki, UEC, Tokyo for their continuous support and advice.
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[27] H. T. Pham, T. D. Nguyen, M. E. Islam, D. Q. Tran, M. Akabori,Enhanced ferromagnetism of ZnO@Co/Ni hybrid core@shell nanowires grown by electrochemical deposition method, RSC Adv.8 (2018) 632-639. [28] M. Ristic, S. Mun-Sic, M. Ivanda, S. Popovic,Sol–gel synthesis and characterization of nanocrystalline ZnO powders, J. Alloys Compd. 397(2005)L1-L4. [29] H. Liu, X. Cheng, J. Yang, Y. Liu, X. Liu, M. Gao, M. Wei, X. Zhang, Y. Jiang, Structural, optical and magnetic properties of Cu and V co-doped ZnO nanoparticles, Physica E 47 (2013) 1. [30] J. J. Wu, S. C. Liu, Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition, Adv. Mater. 14 (2002) 215-218. [31] H. R. Ghaffarian, M. Saiedi, M. A. Sayyadnejad, A. Rashidi, Synthesis of ZnO Nanoparticles by Spray Pyrolysis Method, Iran. J. Chem. Chem. Eng. 30 (2011) 1-6. [32] R. C. Wang, C. C. Tsai,Efficient synthesis of ZnO nanoparticles, nanowalls, and nanowires by thermal decomposition of zinc acetate at a low temperature, Appl. Phys. A. 94 (2009) 241-245. [33] D.G. Lamas, G.E. Lascalea, N.E. Walsoc, Synthesis and characterization of nanocrystalline powders for partially stabilized zirconia ceramics, J. Eur. Ceram. Soc. 18 (1998) 1217-1221. [34] S. Badhuri, S. B. Badhuri,Enhanced low temperature toughness of Al2O3-ZrO2 nano/nano composites. Nanostrct. Mater. 8 (1997) 755-763. [35] D. Guruvammal, S. Selvaraj, S. M. Sundar,Effect of Ni-doping on the structural, optical and magnetic propertiesof ZnO nanoparticles by solvothermal method, J. Alloys Compd. 682 (2016) 850-855. [36] G. Amin, M. H. Asif, A. Zainelabdin, S. Zaman, O. Nur, M. Willander, Influence of pH, Precursor Concentration, Growth Time, and Temperature on the Morphology of ZnO Nanostructures Grown by the Hydrothermal Method, J. Nanomater., v2011, 2011. doi:10.1155/2011/269692. [37] W. Guo, T. Liu, L. Huang, H. Zhang, Q. Zhou, W. Zeng, HMT assisted hydrothermal synthesis of various ZnO nanostructures: Structure, growth and gas sensor properties, Physica E. 44 (2011) 680-685. [38] W. J. Li, E. W. Shi, W. Z. Zhong, Z. W. Yin, Growth mechanism and growth habit of oxide crystals, J. Cryst. Growth. 203 (1999) 186-196. [39] P. Bindu, S. Thomas, Estimation of lattice strain in ZnO nanoparticles: X-ray peak profile analysis, J. Theor. Appl. Phys. 8 (2014) 123-134.
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Highlightts 1. High quality of ZnO nanstructures and microstructures were grown successfully using a simple chemical route technique. 2. The morphology of the ZnO materials were tuned by varying the precursor concentration. 3. Variation in precursor concentration induces a structural change in ZnO. 4. The crystallinity of ZnO material improves by tuning the precursor concentration. 5. The I-V characteristics of the ZnO nanostructures and microstructure devices explains, the nanostructure devices are useful for potential applications in UV photo detection. 6. The procedure demonstrates a low cost and simple technique for achieving qualitative ZnO nanostructure/ microstructure via chemical route technique and its possible use in optoelectronic devices.
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S0301-0104(18)31441-1 https://doi.org/10.1016/j.chemphys.2019.04.014 CHEMPH 10354
To appear in:
Chemical Physics
Received Date: Revised Date: Accepted Date:
24 December 2018 13 March 2019 17 April 2019
Please cite this article as: S. Acharya, S.K. Biswal, S.N. Sarangi, Effect of Structure and Morphology on the UV Photo Detection of ZnO Nanostructures and Microstructures, Chemical Physics (2019), doi: https://doi.org/10.1016/ j.chemphys.2019.04.014
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Effect of Structure and Morphology on the UV Photo Detection of ZnO Nanostructures and Microstructures S. Acharya a, S. K. Biswala,#, S. N. Sarangib, * a
b
Centurion University of Technology and Management, Odisha, India. Institute of Physics, Sachivalaya Marg, P.O.- Sainik School, Bhubaneswar-751005, India. Corresponding author’s E-mail:* [email protected], # [email protected]
ABSTRACT We establish a simple approach to control the morphology of ZnO materials, by using chemical bath deposition (CBD) technique. The effect of variation in precursor concentration on the morphology and crystalline characteristics are investigated by using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques. Morphological effect on the current – voltage (I-V) characteristic of ZnO nanostructure and microstructure devices are investigated. Nanostructure ZnO devices show an enhancement in the forward bias current in comparison to microstructure devices. Further enhancement in photocurrent under ultraviolet (UV) light for nanostructure ZnO devices demonstrate the capability of a potential UV photo detector.
Key Words: ZnO Nanostructures, SEM, XRD, Lattice deformation, Strain, photo detector
1. Introduction The huge interest towards preparation of one-dimensional semiconductor nanostructures such as, nanorods, nanowires, nanosheets, and nanotubes have attracted much attention due to its unique optoelectronic properties and for their diverse applications [1-9]. Zinc oxide (ZnO) has been widely studied since 1935 [1]. The low-dimensional studies on ZnO are very popular because of its various applications in the field of optoelectronics [2,3], photo detector [4,5], photo catalysis [6-11], sensors [11-16], ultraviolet (UV) laser [17]and different nanotechnology [2-18] field. In addition to that, ZnO nanomaterial exhibit novel biomedical as well as environmental applications for social benefits [18,19].Moreover, ZnO nanomaterial can be tuned for the UV detection [4,5] which is very important for many applications and it is inexpensive and environmentally friendly as compared to other metal oxides. Properties of ZnO can be tuned according to the research interest, by doping with various metal atoms to suit specific needs and applications. Zinc oxide is known to be a low-cost n-type semiconductor having wide band gap of 3.4 eV and large exciton binding energy of 60 meV [6]. Basically,
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ZnO exist in three polymorphic forms such as wurtzite, zinc blende, and rock salt phase. Out of these three, the wurtzite structure seems to be the most stable one at room temperature and in ambient pressure, with Zn and O atoms forming separate hexagonal close-packed (hcp) sub lattice, where every atom has a tetragonal coordination [20].
Both physical and chemical deposition techniques like, laser ablation [21], hydrothermal methods [22,23], chemical bath [24,25], electrochemical depositions [26,27], sol–gel method [28,29], chemical vapor deposition [30], spray pyrolysis [31], thermal decomposition [32], combustion method [33,34]and solvothermal method [35]are employed for the deposition of ZnO materials on different substrates. However, most of the solutionphase methods have been used to synthesize ZnO materials because of their simplicity of solution-phase synthesis, which is carried out at ambient or slightly elevated temperature. Among many solution-phase methods, chemical bath technique is found to be a simple and cost-effective technique for preparing ZnO materials [25,26]. In this report, we have synthesized ZnO materials by chemical bath technique and establish a control on the morphology of ZnO by varying the precursor concentration. The detail study of effect of precursor concentration on the morphology and crystalline structure of the ZnO material have investigated and an UV photo-detector fabricated using ZnO nanostructures/ n-Si heterojunction.
2. Experimental n-Si wafers were used as substrate for synthesis of ZnO compound semiconductor via chemical bath deposition (CBD) technique. Prior to the deposition, the Si wafers were rinsed for 1-2 min into aqua regia in order to remove the native oxide present in it. Then the samples were ultrasonically cleaned with acetone and ethanol for 5 min respectively. Finally the wafers were rinsed with de-ionized (DI) water and then dried in ambient condition. The synthesis was carried out by varying the precursor concentrations at 10mM, 50mM, 100mM, 200mM, 500mM, keeping the other synthesis parameters constant. Equimolar concentration of Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and hexamethylenetetramine (HMT) ((CH2)6N4) salts were dissolved in aqueous DI water to form a 100 mL solution. The cleaned substrates were dipped in the solution mixture. The glass beaker containing solution precursors and substrates was covered with an aluminum foil and kept in an electric oven at 100°C for 3 hour. After the required growth, the samples were subsequently removed from the glass beaker, rinsed with DI water and dried in air at 60°C for several hours. The sample for 10 mM concentration was 2
designated as S1, 50mM concentration as S2, 100mM concentration as S3, 200mM concentration as S4 and 500mM concentration as S5 respectively. The morphology and structural properties of samples were investigated by field-emission scanning electron microscope (FESEM), and X-ray diffraction (XRD) (Philips X’pert), where X-ray diffractometer equipped with a monochromatic CuKα radiation source (1.54178Å). The current–voltage (I–V) characteristics of the fabricated n-Si/ZnO nanostructures heterojunction were measured both in dark and under illumination by a keithely electrometer using an UV lamp emitting at 325 nm with a power density of 0.2mW/cm 2.
Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) was used as a precursor and HMT act as surfactant for ZnO growth. It was observed that a white precipitate of ZnO deposited on the substrates at the bottom of the flask. The reaction mechanism proposed here, was already reported by Sarangi et al. [19]. The corresponding chemical reaction for this particular chemical bath technique is as follows
In the above reaction, Zn2+ ions are combined with OH - radicals in the aqueous solution to form a Zn(OH)2 colloid through the reaction Zn2+ + 2OH-→ Zn(OH)2. Later, during the growth process, the Zn(OH)2 is separated into Zn2+ ions and OH- radicals according to reaction 2,then, ZnO nuclei are formed. This is well known that pH has a greater influence during the synthesis of ZnO materials by using CBD route [36]. However, in our case, it has been found that pH value varies from 10 to 11 for different concentration of sample from 10mM to 500mM respectively. Since the variation in pH for different samples are not significant, emphasis has been made to see the effect of precursor concentration on morphology and structure of the samples.
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3. Results and discussion 3.1. Morphological studies The morphology of chemically grown ZnO materials were investigated by using FESEM measurement. Fig. 1.shows the FESEM images of ZnO materials and the effects of precursor concentration on the morphology of ZnO. Fig.1a. shows the morphology of sample S1 where the precursor concentration was chosen, 10 mM. The growth pattern shows ZnO nanostructures were uniformly distributed over the surface of the substrate. The length of the nanostructures found to be varied from 2.0 µm to 6.0 µm and the diameter of the ZnO nanostructures was found to be in the range of 70 to 90 nm. Fig.1b. shows the morphology of sample S2 where the precursor concentration was chosen, 50 mM. As shown in Fig. 1b.the SEM image depicts a nanorod morphology with several nanostructures were originate from one nucleation cite. The length of the nanostructures found to be varied from 4.0 µm to 6.0 µm and the diameter of the nanostructures was found to vary from 600 nm to 900 nm. In case of 100 mM precursor concentration (sample S3), the length of the nanostructures found to be reduced with respect to sample S1 and S2 and varied from 1.0 µm to 3.0 µm whereas the diameter of the ZnO nanostructures was found to be increases with respect to sample S1 and S2 and in the range of 900 nm to 1.0 µm as shown in Fig. 1c. Fig. 1d. represents the morphology of sample S4, where the precursor concentration taken as 200mM. Here the diameters of nanostructures were very large as compared to sample S1, S2 and S3 respectively. The diameter of sample S4 was found to be more than microns. This result suggests that increasing precursor concentration, increases the nucleation cites as well as the growth rate for which the diameter of the ZnO nanorod increases. The observation shows that the precursor variation has a dramatic effect on ZnO growth morphology. As the precursor concentration is increased, the nanostructures cross sections become progressively larger while their length decreases. Fig.1e. shows the FESEM micrograph of sample S5, where the precursor concentration taken as 500mM. One can observe, no nanostructures present in the sample and the morphology of this sample clearly 4
depict ZnO material were deposited in micro structural form. The widths of the nanostructures are markedly increased at higher precursor concentrations. This trend leads to microstructure, plate-like, hexagonal crystals as shown in Fig. 1e. These observations are illustrated quantitatively by plotting the aspect ratio Vs. molar concentration as shown in Fig. 1f. Similar results reported by Amin. et. al. [36] where they have shown there is an increase in aspect ratio due to increase in growth temperature. In our case, we have fixed the growth temperature and growth time to realize the effect of precursor concentration during growth.
The overall FESEM result predicts, with increasing the precursor concentration, the growth rate of ZnO nanostructures increases and due to the availability of more nucleation center on the surface of the substrate, the agglomeration of number of nanostructures evolve into microscopic structures. The effect of precursor concentration on morphology and the dimension of ZnO material are clearly visible in the above set of samples. We believe, the precursor concentration is a key factor in the CBD technique to achieve controlled ZnO nanostructure growth. A model has been proposed inorder to understand the growth of nanostructure to microstructure, which is schematically represented in Fig.2.
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Fig.1. FESEM images of ZnO material with different concentrations, (a) 10 mM (sample S1), (b) 50 mM (sample S2), (c) 100mM (sample S3), (d) 200mM (sample S4) and (e) 500mM (sample S5), (f) the aspect ratio Vs precursor concentration plot.
3.2. Mechanism for the growth of ZnO nanostructures and its transformation to microstructures HMT is a highly water soluble, non-ionic tetra dentate cyclic tertiary amine. The rate of decomposition of HMT is independent of reaction, which yields ZnO indicating HMT act as kinetic buffer. It plays a very important role in producing different morphology of ZnO nanostructures, especially vertical nanostructures form. During the growth of ZnO nanostructures, reaction of Zn(NO3)2.6H2O and HMT produces OH− anions, which react with Zn2+ ions to form ZnO as shown in the chemical equation 2 and 3. The OH – concentration can control the growth process of new [Zn(OH)4]2-ions and enhance the crystal growth along a particular crystallographic direction. 6
It is known that the hexagonal ZnO crystal has both polar and nonpolar faces. The polar faces with surface dipoles are thermodynamically less stable than nonpolar faces with surface dipoles; which often undergo a rearrangement to reduce their surface energy [37]. The preferential adsorption leads to different growth rates of planes, that is, V(0001) >V(1̄011̄) >V(1̄010) >V(1̄011) >V(0001̄) [38]. Hence, in the absence of structure modifiers under chemical growth condition, the (0001) polar plane is energetically unfavorable and has a faster growth rate than other planes. In our case, HMT, being a non-ionic tertiary amine derivative and a non-polar chelating agent, preferentially attaches to the nonpolar facets of the ZnO crystal, thereby exposing only the (0001) plane for the growth. Usually, preferential growth of ZnO occurs along the (0002) direction in case of seed deposited substrates or on lattice matched substrates. But in our case, we have taken a Si substrate without any seed layer deposition. The mechanism for the growth of ZnO nanostructures and its transformation to ZnO microstructures are shown in the schematic representation as Fig. 2. Initially the ZnO nanostructures grow on Si substrates for 3 hours duration having 10 mM concentration is represented as step-I of Fig. 2.With increasing precursor concentration from 10 mM to 500 mM, the growth of ZnO nanostructures continues and the nanostructures growth fused together to form microstructure form as shown in the different steps of Fig. 2.It is clear from the schematic Fig. 2. that with increasing the precursor concentration, the nanostructures are extend readily from the center and grow as thick nanostructures and finally, cover the Si substrate as microstructure form as shown in step-V of Fig. 2. During the 1st stage of growth (Step-I), due to availability of small nucleation center, thin ZnO nanostructures are grown randomly on the Si- substrate. As the precursor concentration slowly increases from 10mM to 500mM, i.e. 50 times of initial concentration, the competition between the nanorods growth readily and along
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the length increases because of the availability of more nucleation cites. As a result, the thickness of nanostructures increases gradually with increase in precursor concentration. The nanostructures growth overlap with each other and the average growth rate increased by many fold, thus produces microstructure rather than nanostructure morphology as shown in step V of the schematic Fig. 2.
Fig. 2. Schematic diagrams for transformation of ZnO nanostructures to microstructures.
3.3. Structural studies To investigate the effect of the precursor concentration on the crystalline structure of ZnO nanostructures/ microstructures, XRD analysis was performed on ZnO grown on a Si substrate through chemical bath techniques. It can be seen from Fig.3.that all the patterns have peaks at 31.76, 34.40, 36.24, 47.56, 56.64, 62.86, 66.87, 68.09 and 69.70 corresponding to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes/orientations of ZnO, respectively, which correspond to the typical diffraction peaks of hexagonal wurtzite ZnO (JCPDS No. 36e1451). The presence of several peaks in the XRD reveals that the nanostructures/ microstructures are polycrystalline by nature. The interesting feature of the
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XRD spectra as shown in Fig. 3. is that the ZnO (101) diffraction peak is much stronger than the ZnO (002) peak for all samples. This indicates that the formed ZnO nanostructures and microstructures have a preferential crystallographic (101) orientation. The peak broadening occurs during the growth of ZnO nanostructures and narrowing of peaks observed in case of ZnO microstructures. The XRD peak broadening is a general feature for nanostructures observed by many researchers [39]. Further, the absence of peaks other than ZnO nanostructures observed in XRD spectra indicates, no impurity phases were present in the sample. Hence, the use of Zinc nitrate hexahydrate (Zn(NO3)2.6H2O)and HMT precursors seem to be an effective way to grow ZnO nanostructures by CBD technique without inducing any kind of undesired compounds.
Fig. 3. X-ray diffraction patterns of ZnO material of different concentrations deposited on Si substrate (a) 10 mM (sample S1), (b) 50 mM (sample S2), (c) 100mM (sample S3), (d) 200mM (sample S4) and (e) 500mM (sample S5)
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This is very important to observe (0 0 2) peak for ZnO material, because it gives the c-axis parameter as well as disorder-ness in crystal lattice of the wurtzite ZnO structure. We have made the fittings of (0 0 2) peak of the samples S1, S2, S3, S4, S5respectively as shown in Fig.4 (a). As shown in Fig. 4(a), it has been observed that the peak position at nearly 34.42 degree corresponds to (002) planes, in which the peak full width half maximum (FWHM) gradually decreases from sample S1 to sample S5 and the FWHM values for the all samples were presented in Table 1.
Fig. 4.(a) Gaussian fittings of (0 0 2) peak of ZnO material of different concentrations, (b)Williamson–Hall plots of ZnO material of different concentrations.
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Moreover, XRD peak broadening also involves the self-induced strain developed during the growth. We have calculated the strain for all samples, using the Williamson–Hall (W-H) formulae [40] : 𝛽 𝐶𝑜𝑠 𝜃 =
𝑘𝜆 𝐷
4 𝜀 𝑆𝑖𝑛 𝜃
(1)
where k is the shape coefficient for the reciprocal lattice point, is the X-ray wavelength, corresponds to the peak position, is the strain and D is the effective crystallite size, respectively. Generally, (W-H) plot yields a straight line where the strain is given by the slope of the line. Fig. 4(b) shows plots of Cos versus Sin (W–H plot) for the ZnO material synthesized at different concentrations. The lattice strain calculated using (W-H) plot of XRD line profile showed a decrease with the increase in precursor concentration. All the values of the lattice strain are presented in Table 1. The other structural parameters such as Inter-planner spacing, d(Å), Lattice constants a(Å), c(Å), c/a ratio, unit cell volume(Å3) and bond length, L(Å) were calculated using the fitting parameters of the intense peaks from XRD, which are presented in Table 1 by using following equations [39, 41].
1 4 h 2 hk k 2 l 2 Interplanar Spacing 2 2 (2) 3 d a2 c 0
Lattice constant, c( A)
Sin hkl
(3)
Unit cell Volume, V (Å3) = 0.866 × a2 × c 0
Bond Length, L( A) Where u p
up
(4)
a2 1 ( u p )2 c2 3 2
(5)
Wurzite structure positional parameter, calculated as,
a2 0.25 3c 2
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From Table1, it has been cleared that the full-width-half-maximum (FWHM) values of (0 0 2) peak of ZnO nanostructure samples S1, S2 and S3 found more than that of ZnO microstructures sample S4 and S5. This result suggests that as the precursor concentration increases, the FWHM value of (0 0 2) peak decreases, confirms the improvement in the crystalline quality of ZnO in case microstructure sample. The similar result predicted by R. Jacob et. al.[41] for nanocrystalline ceramic sample. The reasonably narrow FWHM widths for ZnO microstructure (S5) sample demonstrate the high crystal quality of ZnO obtained by chemical bath technique on Si substrates.
As shown in Fig. 4. the (002) peak positions have a slight shift towards lower angles in case of nanostructure samples (S1 to S3) with respect to its microstructure sample (S4 and S5), suggest a deformation in original structure, thus there is an overall change in all the structural parameter such as inter-planner spacing, lattice constants a, c, c/a ratio, unit cell volume, bond length and strain as shown in Table1. Table 1. Structural parameters of the ZnO materials varying precursor concentrations. Sample FWHM Interplanar Lattice Constants Spacing, d(Å) a c (101) S1 0.8126 2.491 3.280 5.186 S2 0.7747 2.484 3.265 5.194 S3 0.7462 2.481 3.260 5.194 S4 0.3438 2.476 3.240 5.206 S5 0.3110 2.476 3.240 5.206
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c/a ratio
1.581 1.590 1.593 1.606 1.606
Unit Bond Strain cell Length, (ɛ x10 -3) volume, L(Å) V(Å)3 48.316 1.988 0.01204 47.949 1.983 0.01240 47.802 1.980 0.01240 47.327 1.973 0.00397 47.327 1.973 0.00389
Fig. 5. (a) Precursor Concentration Vs Inter-planner spacing, (b) Precursor Concentration Vs Strain and Precursor Concentration Vs (002) Peak Position of ZnO material.
As shown from Fig.5(a), the inter-plannar spacing (d) between crystal lattice of ZnO nanostructure increases by decreasing precursor concentration. This observation can be described on the basis of distortion of the wurtzite structure. The distortion in wurtzite structure between the atoms can develop a lattice strain due to the deviation in bond lengths and bond angles [42]. According to Bragg’s law, the Bragg’s angle and inter planner spacing (d) are correlated with each other, i.e. the strain increases the d-spacing which causes a shifting of peak towards lower 2θ values. As shown in Fig. 5b. the lattice strain for 10 mM concentration (S1) is more with respect to the sample with 500 mM concentration (S5) and the peak position for 10 mM concentration (S1) sample is shifting more towards lower 2θ values with respect to the sample with 500 mM concentration (S5).e The observed phenomena also confirm that, lattice strain increases with decreasing nanorod dimensions. Similar results observed by Andrade et al [40] for their work related to BaF2 nanoparticles.
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Fig. 6.(a) Precursor Concentration Vs Lattice Constants, (b) Precursor Concentration Vs Bond Length and Precursor Concentration Vs Unit-Cell Volume (Å3) of ZnO material.
Fig. 6(a), shows how the lattice constant varies with increasing precursor concentration during growth. As shown in Fig. 6(a), the lattice constants, “a” decrease with increasing precursor concentration, whereas the lattice constants, “c” increase with increasing precursor concentration. The role of particle size in the change of lattice parameters is theoretically and experimentally studied [42]. Deformation of lattice parameter plays an important role in the particle size is established by many reporters [43, 44]. As shown in Fig. 6b. bond length and unit cell volume decrease with increasing precursor concentration. The similar result of decrease in unit cell volume with increasing particle size is already reported by Luca et al. [45]. Initially as precursor concentration increases from 10mM to 100mM, both bond length and unit cell volume decreases rapidly up to some extent, then it saturates and at 500mM concentration (S5). The Zn–O bond length calculated for sample S1 to S5, decreases from 1.988Åto 1.973Å in our case where as the reported Zn–O bond length in the unit cell of ZnO and neighboring
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atoms is 1.976 Å. The calculated bond length fairly agrees with the Zn–O bond length as reported by Bindu et al. [39].
Fig. 7(a) shows the I−V measurements performed under dark condition for ZnO nanostructures/n-Si and ZnO microstructures/n-Si heterojunction devices. Both the device shows the better characteristics of a rectifying diode. The I-V results confirms four times enhancement in the forward bias current for ZnO nanostructures/n-Si device with respect to ZnO microstructures/n-Si heterojunction device. The increase in forward bias current could be due to the presence of a native oxide layer in the interface as reported by Sarangi et al. [46]. In case of a thin oxide layer between the nanostructures and metal junction, under forward bias condition, the carriers can cross over the device interface by tunneling and contribute to the total current flow [47].As there is a reasonable increase in forward bias current for ZnO nanostructure-based device, we have selected this device to check for the photo-detection under UV light (wavelength 325 nm) illuminations. As shown in Fig. 7(b), the I-V measurements performed under UV light illuminations for ZnO nanostructures/n-Si heterojunction device, found to generate higher currents than the dark condition, which enables the ZnO nanostructure sample useful for UV photo detector. The mechanism for enhancement in photo current is due to surface oxygen/ native oxide layer as described by many reports [47]. Similar approach may consider in our case. Under dark condition, oxygen species adsorbed on the surface of ZnO nanostructures capture free electrons thus produces a depletion layer near the nanorods surface, resulting in an upward band bending near the surface. The adsorption of oxygen molecule significantly reduces the conductivity of the nanostructures due to large surface to volume ratio of ZnO nanostructures. Exciting with photon energy (UV) higher than the energy gap of ZnO generates electron–hole pairs in the ZnO nanostructures. The holes migrate to the nanorod surface and recombine with O2– trapped electrons, thus releasing oxygen from the surface.
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Therefore, adsorbed oxygen and the photo generated hole contribute to local charge neutrality and free up the photogenerated electrons which enhance the conductivity of the detector. Fig. 7(c) shows the speed of photo detector response and the recovery time. The response time and the recovery time of the ZnO nanostructures device under UV illumination were found to be 12 and 22 s, respectively. Recently, Alenezi et. al. [48] and Paul et. al. [49] have reported photo response characteristics of ZnO nanostructures. The photo response time and the recovery time found to be 42s and 55s by Alenezi et. al. at a bias voltage of 2 volts [48] whereas the photo response time and the recovery time found to be 1.56 s and 2.34 s by Paul et. al. [49] on zero bias voltage. However, in our case the photo response time and the recovery time found to be 12 and 22 s respectively on applying 4 volt forward bias as shown in Fig. 7(c). This is to be noted that electron-hole generation is a fast process [4] for which a high photo current produced in a very short time i.e 12s in our case. Then during the re-adsorption process, it slowly reached the steady-state of photo current value. As the adsorption and photodesorption processes are slow processes [4], the photo current reaches the saturation value very slowly for which the recovery time is 22 s in our case.
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Fig. 7. I-V characteristics of (a) the ZnO nanostructures/n-Si and ZnO nanostructures/n-Si heterojunction, (b) ZnO nanostructures/n-Si heterojunction under dark and under UV illumination, (c) ZnO nanostructures/n-Siphoto detector response and the recovery time at 4V.
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4. Conclusions In summary, this work demonstrates a novel strategy to achieve controllable synthesis of ZnO material using a simple chemical technique by varying the precursor concentration. Five ZnO samples were synthesized by varying precursor concentration and their morphologies and structures were investigated. The results show that theZnO morphology can be tuned from nanostructures to microstructures by varying the precursor concentration. The structural analysis has confirmed the improvements in crystallinity in micrsostructure samples. The results obtained in this study demonstrate that the structure and morphology of ZnO can be tailored by tuning the precursor concentration. Approxmately 4 times enhancement found in forward bias current for ZnO nanostructures/n-Si heterojunction device with respect to the ZnO microstructures/n-Si heterojunction device and further enhancement in forward bias current under UV illuminations for the ZnO nanostructures/n-Si heterojunction device enables it for a suitable nano-electronic device application and the phenomenon may support for a unique platform for large scale and cost-efficient UV photo detectors. The experimetal observation suggests that the variance in the precursor concentration, results in various nucleation habits, induces the formation of ZnO with different morphologies and structures which can be used for specific nano-electronic device applications.
Acknowledgements One of the authors (S. N. Sarangi) gratefully acknowledges the late Professor S. N. Sahu then at Institute of Physics, Bhubaneswar and Professor S. Nozaki, UEC, Tokyo for their continuous support and advice.
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Highlightts 1. High quality of ZnO nanstructures and microstructures were grown successfully using a simple chemical route technique. 2. The morphology of the ZnO materials were tuned by varying the precursor concentration. 3. Variation in precursor concentration induces a structural change in ZnO. 4. The crystallinity of ZnO material improves by tuning the precursor concentration. 5. The I-V characteristics of the ZnO nanostructures and microstructure devices explains, the nanostructure devices are useful for potential applications in UV photo detection. 6. The procedure demonstrates a low cost and simple technique for achieving qualitative ZnO nanostructure/ microstructure via chemical route technique and its possible use in optoelectronic devices.
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