Synthesis of Fe-Doped ZnO Nanorods by Rapid Mixing Hydrothermal Method and Its Application for High Performance UV Photodetector

We have successfully synthesized Fe-doped ZnO nanorods by a new and simple method in which the adopted approach is by using ammonia as a continuous source of OH for hydrolysis instead of hexamethylenetetramine (HMT). The energy dispersive X-ray (EDX) spectra revealed that the Fe peakswere presented in the grownFe-dopedZnOnanorods samples and theX-ray photoelectron spectroscopy (XPS) results suggested that Fe is incorporated into the ZnO lattice. Structural characterization indicated that the Fe-doped ZnO nanorods grow along the c-axis with a hexagonal wurtzite structure and have single crystalline nature without any secondary phases or clusters of FeO or Fe


Introduction
Diluted transition metals (TMs) doped ZnO nanomaterials result in changing of the structural, electrical, magnetic, and optical properties of ZnO nanostructures.Transition metal doped ZnO especially is promising material as a room temperature ferromagnetic diluted magnetic semiconductors.Therefore, TMs doped ZnO nanomaterials are of interest in many current and future applications such as nanoelectronics, optoelectronics, photonic devices, spin electronics applications, and sensor devices, for example, spin-based lightemitting diodes, UV sensors, spin transistors, nonvolatile memory, and ultrafast optical switches [1][2][3][4][5][6][7].Among the TMs doped ZnO nanomaterials, Fe-doped ZnO nanorods are of great potential in many applications due to the excellent electronic, magnetic, and optical properties [8].The doping of semiconductor materials prepared by different methods or by the same method but different preparation processes usually shows different properties.New device applications of Fedoped ZnO nanomaterials have attracted many researchers to synthetize this material using many different physical and chemical methods.Many methods have been used to synthesize Fe-doped ZnO nanomaterial with different morphologies which has been published in the literature.To mention some, Fe-doped ZnO nanoparticles were prepared by the coprecipitation method [9][10][11], while Fe-doped ZnO powders and Fe-doped ZnO nanorods array have been synthesized via other high temperature methods [12,13]; Fe-doped ZnO thin films were deposited by sputtering and spin coated methods [14][15][16][17][18][19][20][21][22][23], and Fe-doped ZnO nanorods were grown by the hydrothermal methods [24][25][26][27].Among the different growth methods, the hydrothermal methods are favorable, friendly, and attractive due to simplicity, low cost, being less hazardous, scale-up possibility, and they are performed at low temperature (lower than 100 ∘ C).Furthermore, the latest property is advantageous since it can be used to grow ZnO nanostructures on flexible and foldable substrates.Moreover, the morphology and properties of ZnO nanostructures can be controlled by varying the growth conditions such as the temperature, growth time, precursor concentration, and the pH of the growth solution [28,29].
In this work, Fe-doped ZnO nanorods were synthesized by a modified preparation procedure using the low temperature hydrothermal approach.The Fe-doped ZnO nanorods growth developed here represents a new and simple method which adopted approach by using ammonia as a continuous source of OH − for hydrolysis during the growth instead of hexamethylenetetramine (HMT).Morphology, chemical composition, and structural and room temperature magnetic properties of the Fe-doped ZnO have been investigated.Finally, a simple Au/Fe-doped ZnO Schottky diode based UV photodetector was fabricated and IV characteristic and timedependent photoresponse have been conducted under on/off UV illuminations.The performance of the UV photodetector based Au/Fe-doped ZnO nanorods Schottky diode with large detection area (1 cm 2 ) has been studied.Then the growth solution was subsequently stirred with a magnetic stirrer at room temperature for one hour and then ammonia solution is added dropwise to the growth solution at room temperature, resulting in an orange solution with a pH = 9.3.This solution was kept under magnetic stirring for one hour at room temperature.The substrates were cleaned with isopropanol in an ultrasonic bath and then spin coated three times with a seed solution containing zinc acetate at 2500 rpm for 30 s; then the samples were annealed at 120 ∘ C for 10 minutes.Finally, the substrates were placed horizontally in the growth solution and kept in a preheated oven at 90 ∘ C for 6 hours.After the growth duration is completed, the samples were collected and washed with deionized water and dried at room temperature for further characterization.We believe that synthesis of high quality of Fe-doped ZnO nanorods can be useful for nanotechnology applications.In this adopted approach, ammonia was used to tailor pH of the growth solution and to facilitate ZnO nanocrystals growth.Ammonia reacts with water to provide continuous source of OH − required for hydrolysis and aid precipitation of final products.Furthermore, ammonia can generate a large amount of zinc ammine complexes immediately in the solution and these complexes are absorbed on the six side planes of ZnO nanorods, which can facilitate the growth of ZnO nanorods structure by slowing down the growth velocity of the side surfaces [30].By increasing the ammonia content in the growth solution, the nuclei of ZnO nanocrystals can rapidly form on the substrates, which produces dense and long ZnO nanorods over a large area [31,32].Therefore, high ammonia contents as additive to hydrolyze in the growth solution provide Fe ions doped in ZnO nanorods without morphology deformation.

Characterization Process.
The field emission scanning electron microscope (FESEM), EDX, XPS, and XRD are used to characterize the surface morphology, chemical composition, and crystal structural of the grown samples, while the room temperature ferromagnetic properties were investigated by superconducting quantum interference device (SQUID) measurements.

Device Fabrication Process.
For the fabrication of the Au/Fe-doped ZnO nanorods UV photodetector, transparent FTO film was taped on the top of the Fe-doped ZnO nanorods grown on gold coated glass substrate.The schematic diagram of the simple UV photodetector based on large area Au/ZnO nanorods/FTO Schottky diode is shown in Figure 1.In a UV photodetector, large Schottky barrier height at metal semiconductor interface results in improved responsivity and improved photocurrent to dark current ratio [33,34].Therefore, Au with high work function was used to form a large Schottky barrier height on Fe-doped ZnO nanorods.The conducting FTO film was used as the ohmic contact due to its transparency and it also provides almost ideal ohmic contact with n-ZnO [35].The current-voltage (I-V) curves of the fabricated diodes under dark and under UV illumination were measured by Agilent 4155B Semiconductor Parameter Analyzer.In this experiment, a constant UV illumination of 2 mW/cm 2 emitting at 365 nm was used as the excitation source.

Morphology and Chemicals Composition.
The surface morphology of the grown Fe-doped ZnO nanorods under different doping concentration has been performed by using SEM.Figures 2(a) and 2(b) show the SEM images of the 1% and 5.0% Fe-doped ZnO nanorods, respectively.The Fedoped ZnO nanorods have hexagonal shapes with diameters varying between 100 and 300 nm.The chemical composition of the grown Zn 0.99 Fe 0.01 O was measured by using EDX which is shown in Figure 2(c).The EDX data revealed that the Fe's peaks were at 0.705 keV, 6.404 keV, and 7.058 keV.This indicates the Fe ions were presented in the Fe-doped ZnO nanorods samples.
In order to verify the substitutions of the Fe ions in the Zn 1− Fe  O nanorods, XPS measurements were performed.Figure 3(a) shows the XPS spectra of O 1s peaks of both ZnO nanorods and Fe-doped ZnO nanorods.For ZnO nanorods, the O 1s spectrum centered at 530.9 which belongs to O 2− in the wurtzite structure of a ZnO monocrystal and at 532.3 eV is attributed to the presence of loosely bound oxygen on the surface [15,17].In Fe-doped ZnO nanorods, the XPS spectra of the O 2− were slightly shifted to the higher binding energy value and the shoulder peaks are broader in comparison to the pure ZnO nanorods.This result indicates that the Fe ions indeed influence the optical properties of the ZnO nanorods.The chemical shift of the O 1s of the doped ZnO has also been revealed in the previous works [22,27].As we know, the binding energy of Fe 2p signals was between 700 and 740 eV and some of Zn Auger peaks are also presented in this region.Therefore, the XPS measurements were conducted for both ZnO nanorods and Zn 1− Fe  O nanorods.As it is known, the FeO has a peak position of Fe 2p 1/2 at 722.3 eV and Fe 2p 3/2 at 709.3 eV and Fe 2 O 3 at 724.9 eV and at 710.5 eV, respectively.In this study, the XPS signals at binding energy from 695 eV to 735 eV for pure ZnO and Fe-doped ZnO are shown in Figure 3(b).From this figure, the Fe related signal in the Zn 0.99 Fe 0.01 O nanorods is not resolved from the Zn Auger because it is relatively small.This is a very similar case for the Fe-doped ZnO in the previous works [20,26].However, the Fe 2p core level photoemission spectrum for Zn 0.95 Fe 0.05 O nanorods is clearly observed, from which Fe 2p 1/2 and Fe 2p 3/2 peaks located at 725.47 and 711.7 eV can be found.These Fe 2p peak positions are almost the same as values which have been reported in many previous works [15, 17, 19-24, 26, 27].These obtained binding energies are larger than Fe 3+ ; it is suggested that Fe is incorporated into the ZnO lattice in a state close to Fe 3+ .Furthermore, the investigated spectra of Fe 2p showed that the spin-orbit split energy difference between Fe 2p 1/2 and Fe 2p  there is no possibility of existence of Fe 2+ or Fe 0 in the samples because the energy difference of metallic ion and the FeO should be 13.10 eV and 13.4 eV, respectively [20].diffraction angle with the increasing of the Fe concentration and their full width at half maximum (FWHM) were also becomes larger while increasing the Fe concentration.The shift of the peaks positions and the relatively larger FWHM clearly indicated that the Fe ions replaced the Zn sites in the ZnO nanorods crystal matrix.These observations were also evident in similar samples grown by other techniques reported elsewhere [10, 11, 14, 17-20, 24, 25].

Magnetic Property. Superconducting quantum interference device (SQUID) measurements have been performed
to investigate the room temperature ferromagnetic behavior of our Fe-doped ZnO nanorods samples.Figure 5 shows the magnetic hysteresis (M-H) curves measured from −10  to 10 kOe at 300 K of Fe-doped ZnO samples.From these M-H curves, the room temperature ferromagnetic hysteresis loops are clearly observed.According to theory, Fe-doped ZnO possesses ferromagnetic property at room temperature and the magnetic moments observed are due to the Fe 3d orbitals and the observed magnetization value increases with the increase of the Fe concentration [15-17, 20, 22, 24, 25].This supports our experimental results since we observed magnetic hysteresis at room temperature and the magnetization values observed increase from 2.5 emu to 9.1 emu for Zn 0.99 Fe 0.01 O and Zn 0.95 Fe 0.05 O, respectively.This M-H loop is higher than the results reported in [21,36].As we see from the EDX spectra, XPS spectra, and XRD patterns of our samples, it is clearly shown that there was not any other secondary phase of Fe or Fe oxides that has been observed.Therefore, the observed room temperature ferromagnetism in our Fe-doped ZnO nanorods originates from the Fe ions substituting the Zn ions in the ZnO nanorods matrix.

UV Sensor
Based Au/Fe-Doped ZnO Schottky Diode.The I-V characteristics for both the undoped ZnO and the Fedoped ZnO Schottky diodes were investigated under dark and UV illumination.diode has smaller leakage current, smaller turn on voltage with higher current at forward bias voltage than Au/ZnO Schottky diode.Without UV illumination, the observed dark current was approximately 1.62 mA and 3.56 mA at bias of 5 V for the Au/ZnO Schottky and the Au/Zn 0.95 Fe 0.05 O, respectively.This high dark current indicated that the Fe-doped ZnO nanorods have intrinsic donor defects which generates many free electrons and enhanced the dark conductance [37].
where  ph =  illuminated −  dark is the photocurrent and  inc is the incident optical power at a given wavelength ().The responsivity values calculated at 5 V forward bias are 0.60 A/W for Au/ZnO Schottky diode and 2.33 A/W for Au/Zn 0.95 Fe 0.05 O Schottky diode.The responsivity value of Au/Zn 0.95 Fe 0.05 O is also higher than the commercial GaN UV detector (0.1 A/W) and other photodetectors reported in [38][39][40][41][42][43] and it is comparable to the UV photodetector reported [44].The responsivity ratio between Au/Zn 0.95 Fe 0.05 O Schottky diode and Au/ZnO Schottky diode is equal to 3.9 and it is given by where  doped and  undoped are the responsivity of the Au/Zn

Conclusion
In summary, a series of high quality single crystalline of Fe-doped ZnO nanorods has been successfully synthesized using a modified hydrothermal method.A systematic study was performed to investigate the morphology and structural and magnetic properties of the Fe-doped ZnO nanorods.Finally, the grown Fe-doped ZnO was used to fabricate high performance UV photodetector.SEM results show that the Fe-doped ZnO nanorods have hexagonal shapes and the EDX data revealed that the Fe peaks were presented in the Fe-doped ZnO nanorods samples and the XPS results suggested that Fe 3+ is incorporated into the ZnO lattice.The XRD analysis showed that by increasing the concentration of the Fe in the growth solution the 002 peak position and the FWHM were shifted to higher angle and become relatively larger, respectively.It is also shown that the Fe ions replaced Zn sites and were incorporated into the ZnO matrix with no secondary phases or clusters of FeO or Fe 3 O 4 observed in the grown samples.The substitution of the Fe ions in the ZnO nanorods matrix significantly was manifested in a clear ferromagnetic behavior at room temperature (300 K) and the magnetization magnitude was observed to increase from 2.5 emu to 9.

Figure 1 :
Figure 1: Schematic diagram of simple photodetector based on large area Au/Fe-doped ZnO nanorods/FTO Schottky diode.

Figure 3 :
Figure 3: XPS spectra of (a) O 1s for ZnO and Fe-doped ZnO and (b) Zn Auger from ZnO and Fe 2p Fe-doped ZnO.

3. 2 .
Structural Characterization.The XRD patterns of the undoped ZnO nanorods, Zn 0.99 Fe 0.01 O and Zn 0.95 Fe 0.05 O nanorods, are shown in Figure 4(a).The high diffraction peaks at 002 direction are an indication of the hexagonal wurtzite structure with single crystalline nature and they indicate that the Fe-doped ZnO nanorods grow along the caxis of the ZnO consistent with the JCPDS number 36-1451 file.No evidence of any other secondary phase such as FeO or Fe 3 O 4 has been observed.In Figure 4(b), we observed that the peaks position at 002 direction was shifted towards higher 2

Figure 4 :
Figure 4: (a) The XRD patterns of undoped ZnO and Fe-doped ZnO nanorods; (b) the XRD patterns of Fe-doped ZnO at 002 peaks shifted to the higher 2 values.

Figure 6 :
Figure 6: (a) I-V curves of Au/ZnO and Au/Zn 0.95 Fe 0.05 O Schottky diodes under dark; (b) I-V characteristics of the Au/Zn 0.95 Fe 0.05 O Schottky diode under dark and under UV illumination; (c) I-V characteristics of the Au/ZnO Schottky diode under dark and under UV illumination; (d) time response of Au/Zn 0.95 Fe 0.05 O Schottky diode; (e) time response of Au/ZnO Schottky diode.
3/2 is 13.77 eV.These results suggested that 0.95 Fe 0.05 O Schottky diode and Au/ZnO Schottky diode, respectively.The device based Au/Zn 0.95 Fe 0.05 O gives higher responsivity because when Fe ions incorporated into the ZnO latticeit acts as a donor which contributes to carriers and consequently improve its optical property, which possesses more electron-hole pairs generated under UV excitation.Therefore, the incremental mobility of the Zn 0.95 Fe 0.
05 O nanorods exhibits higher values compared to ZnO nanorods.Figures6(d) and 6(e) show 5 s and 7 s of rising time response for Au/Zn 0.95 Fe 0.05 O Schottky and Au/ZnO Schottky diodes, respectively.While the decaying time is approximately 29 s for both devices, the rising time response is defined as the times required for the photocurrent reaching 63.3% of its saturated photocurrent and the decaying time response is refereed to 36.7% of its saturated photocurrent.
1 emu for Zn 0.99 Fe 0.01 O and Zn 0.95 Fe 0.05 O, respectively.Moreover, the fabricated Au/Fedoped ZnO nanorods UV photodetector device achieved both high photoresponse and fast time response.Compared