Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc

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Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc Oxide Nanorods Buddha Deka Boruah, and Abha Misra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11387 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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ACS Applied Materials & Interfaces

Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc Oxide Nanorods Buddha Deka Boruah and Abha Misra* Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, Karnataka, India 560012

ABSTRACT Cobalt integrated zinc oxide nanorod (Co-ZnO NR) array is presented as novel heterostructure for UV photodetector (PD). Defect states in Co-ZnO NRs surface induces an enhancement in photocurrent as compared to pristine ZnO NRs PD. Presented Co-ZnO NRs PD are highly sensitive to external magnetic field that demonstrated 185.7% enhancement in response current. It is concluded that due to the opposite polarization of electron and holes in the presence of external fields contribute in effective separation of electron-hole pair that are drifted upon UV illumination. Moreover, Co-ZnO NRs PD shows a faster photodetection speed (1.2 s response time and 7.4 s recovery time) as compared to the pristine ZnO NRs where the response and recovery times are observed as 38 s and 195 s, respectively.

KEYWORDS: Zinc oxide, nanorods, cobalt, hybrid heterostructure, photodetector.

*

Corresponding Author: Abha Misra, Email: [email protected]

1 ACS Paragon Plus Environment


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1. INTRODUCTION Compound semiconductors with large energy band gap are broadly used in optoelectronic devices due to their unique optical properties. 1-3 Among these, zinc oxide (ZnO) is a direct band gap compound semiconductor with large exciton binding energy of 60 meV, which is much larger than the thermal energy (26 meV) and exhibits an excitonic emission at room temperature.4 Hence one-dimensional (1D) ZnO nanostructures are most preferable in ultraviolet (UV) photodetectors (PDs) due to large surface-to-volume ratio.5 It should be noted that due to high electrical resistivity (less charge carriers mobility) of ZnO, it displays longer response and recovery times (defined later) in UV PDs.6 It has been proposed that the heterostructure based on 1D ZnO nanostructures can improve the performance of UV photoresponse as compared to pristine 1D ZnO nanostructures.7 Different approaches are employed for the fabrication of heterostructure based on ZnO for the enhancement of photoresponse along with the improvement in both response and recovery times. Recently, Fang et al. reported ZnO/zinc sulfide (ZnS) heterostructure based UV PD, where 40 times enhancement in photocurrent was observed as compared to pristine ZnO nanostructure. 8 It observed that the generation of built-in electric field at the interfaces between ZnO and ZnS plays an important role in photoresponse by enhancing the exciton separation for longer lifetime of photogenerated free charge carriers by reducing recombination.8 Likewise, Liu et al. reported an enhancement of photoresponse in ZnO nanowire by introducing surface plasmons of gold nanoparticles to increase the photocurrent to dark current ratio.9 Similarly, the transition metals are preferable for surface modification of ZnO for the generation of surface trapping sites on nanostructure which allow to tune the optical properties. Various studies have been emphasized on the surface modification of ZnO through transitional metals and it has been realized that the transition metal cobalt is an effective material 2 ACS Paragon Plus Environment

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for the surface modification of ZnO. 10, 11 Cobalt (Co) doped or decorated ZnO nanostructures display good electrical, optical as well magnetic properties where cobalt ions successfully substitute zinc ions in the lattice. Due to the difference of less ionic radius of Co ion (Co2+) (0.056 nm) from zinc ion (Zn2+) (0.060 nm) allow only small change of the actual lattice parameters by maintaining hexagonal wurtzite crystal structure of ZnO. Jiang and co-workers reported Co-doped ZnO nanorods (NRs) for photocatalyst application and significant improvement of the performance was observed.10 Likewise, Gibson and co-worker observed the enhancement of stable ferromagnetic properties along with the optical properties in Co dopedZnO nanowires system.11 Therefore, these studies realize the fabrication of Co decorated ZnO NRs for the high performance UV photodetection which allows for tuning the photoresponses by introducing the surface defects along with the external driving force which open the new photoresponse phenomenon. In this report, we have fabricated highly sensitive, Co coated ZnO (Co-ZnO) NR array novel structure for UV PD that revealed a high sensitivity to the external magnetic field, where response current enhanced by 186%. Moreover, the presented Co-ZnO NRs PD shows highly tunable, photoelastic response that shows 2.15 times enhancement in response current as compared to pristine ZnO NRs along with significant improvement in photodetection speed.

2. RESULTS AND DISCUSSION The microstructure of pristine ZnO NRs is shown in figure 1(a) that reveals highly dense well-aligned ZnO NRs. The top inset in figure 1(a) shows a magnified image where the diameter of ZnO NRs ranges from 80 to 150 nm and bottom inset reveals the well-vertically aligned ZnO NRs of length around 1.8 μm. Figure 1(b) shows the microstructure of Co-ZnO NRs, where Co



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is coated uniformly on ZnO NRs and top inset depicts the magnified image of Co-ZnO NRs whereas the length of Co-ZnO NRs is observed around 1.9 μm (bottom inset in figure 1(b)), respectively. The EDS spectra of both pristine ZnO NRs and Co-ZnO NRs are shown in figures 1(c) and 1(d), respectively. The spectrum of pristine ZnO NRs reveals presence of only zinc and oxygen whereas spectrum for Co-ZnO NRs confirms the presence of Co along with zinc, oxygen without any impurities. Figure 1(e) and 1(f) reveal the transmission electron microscopy (TEM) images of Co-ZnO NRs at lower and higher magnifications. It is clear that Co layer is uniformly coated on the ZnO NRs surfaces and the average diameter of Co-ZnO NRs is around 120 nm. The crystal structures of both pristine ZnO NRs and Co-ZnO NRs are shown in figure 2(a). Pristine ZnO NRs shows the diffraction intensity peaks appear at around 2θ = 34.440, 36.180, 47.490 and 62.850 representing (002), (101), (102) and (103) crystal diffraction planes of hexagonal wurtzite crystal structure of ZnO. A high intensity diffraction peak at 2θ = 34.440 for (002) diffraction plane demonstrates that most of ZnO NRs have c-axis orientation.12 However, the diffraction peaks for (002), (101) and (102) planes shifted to 2θ = 34.50, 36.260 and 47.560, respectively in Co-ZnO NRs towards the higher diffraction angle as compared to pristine ZnO NRs and inset shows the magnified image. Likewise, Jiang and co-workers observed the shifting of XRD diffraction peaks towards the higher angle in Co-doped ZnO system as compared to pristine ZnO system.10 Also, the lattice parameters of ZnO NRs after Co decorated changed from a = 0.326 nm and c = 0.521 nm to a = 0.325 nm and c = 0.519 nm. The shifting of the diffraction peaks toward the higher angle can be related to the difference of ionic radius of Co 2+ (0.056 nm) from Zn2+ (0.060 nm) which is smaller than zinc ion and thus allows to induce the small lattice mismatch in ZnO structure. The XRD pattern of Co-ZnO NRs suggests that Co2+ ions are


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successfully doped on ZnO NRs surface by maintaining the original hexagonal wurtzite crystal structure of ZnO.13

(a)

(b)

200 nm

200 nm

1 µm

2 µm

1 µm

2 µm

(d)

Intensity (a.u.)

(c)

Intensity (a.u.)

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Energy (KeV)

Energy (KeV) (e)

200 nm

(f)

100 nm

Figure 1: SEM images of (a) pristine ZnO NRs and (b) Co-ZnO NRs. Bottom and top insets in (a) and (b) are the high resolution and cross sectional images. EDS spectra of (c) pristine ZnO NRs and (d) Co-ZnO NRs. (e) And (f) are TEM images of Co-ZnO NRs.



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(a)

(b)

(c)

(d)

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Figure 2: (a) XRD patterns and (b) Raman spectra of both pristine ZnO NRs and Co-ZnO NRs. Room temperature magnetic hysteresis loop of (c) pristine ZnO NRs and (d) Co-ZnO NRs. Figure 2(b) shows the Raman analysis of both the pristine ZnO NRs and Co-ZnO NRs. In pristine ZnO NRs, the Raman peaks appear at 437.5 cm-1, 578 cm-1 and 332 cm-1 representing the nonpolar optical high-frequency E2 mode (E2 (High)), longitudinal optical A1 symmetry mode (A1(LO)) and the second order low-frequency E2 phonon mode (E2 (Low)) of hexagonal wurtzite crystal structure of ZnO.13 It is observed that the intensity of second order lowfrequency E2 phonon mode almost disappears in Co-ZnO NRs (as shown in figure 2(b)), which could mainly be attributed to the reduction of lattice symmetry along with the formation of structural defects induced by Co coating on ZnO NRs interface. Also a shift in longitudinal


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optical A1 symmetric mode of ZnO to lower side could also be due to the reduction in lattice symmetry at the interfaces.13, 14

(a)

(b)

(c)

(d)

Figure 3: (a) Absorption and (b) photoluminescence spectra of ZnO NRs and Co-ZnO NRs. Analysis of photoluminescence spectra of (c) ZnO NRs and (d) Co-ZnO NRs. The effect of magnetic field on pristine ZnO NRs and Co-ZnO NRs are shown in figures 2(c) and 2(d), respectively, where variation in magnetization is observed with respect to the applied magnetic field and demonstrated the formation of hysteresis loop. The Co coated ZnO NRs exhibits better ferromagnetic ordering as compared to pristine ZnO NRs at same applied magnetic fields. In pristine ZnO NRs, the saturation magnetization of 2 memu/gm is observed (figure 2(c)), where it increased further to 1280 memu/gm in Co-ZnO NRs (figure 2(d)). In 7 ACS Paragon Plus Environment


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general, transition magnetic metals doped or coated ZnO exhibit superior ferromagnetic magnetic behavior at room temperature.15, 16 Most interestingly the week ferromagnetic behavior in undoped ZnO is also observed at room temperature. Numerous studies have been employed for the investigation of the origin of ferromagnetic behavior of undoped ZnO. It has been proposed that the intrinsic defects which include oxygen vacancy, oxygen vacancy cluster, zinc interstitial, zinc vacancy, and oxygen interstitial induced the ferromagnetism behavior in undoped ZnO.17-20 Among these, zinc vacancies were majorly originated the ferromagnetic nature of pristine ZnO.21-23 Thus, the origin of ferromagnetic nature of pristine ZnO NRs (figure 2(c)) could be due to the presents of zinc related defects in the sample and inset in figure 2(d) clearly shows the enhancement of saturation magnetization in Co-ZnO NRs by 640 times as compared to ZnO NRs that reveals the enhancement of ferromagnetic properties. 24 Hence, Co coating on ZnO NRs surface successfully enhanced the ferromagnetic behavior of pristine ZnO NRs. The room temperature optical absorption spectra of both pristine ZnO NRs and Co-ZnO NRs with respect to the wavelength are shown in figure 3(a). The optical absorption edge for both the NRs appears in the wavelength range of 360 to 370 nm that is attributed mainly due to transition of electrons from valance to the conduction band of ZnO. The inset represents the variation of first derivative of optical absorption intensity with respect to the illuminated photon energy, where the maximum derivative appears at around 3.37 eV for both ZnO NRs and CoZnO NRs. This indicates the optical bang gap of both ZnO NRs and Co-ZnO NRs and shows the sensitivity towards UV radiation only. 25-27 Similarly, the photoluminescence spectra of both ZnO NRs and Co-ZnO NRs are shown in figure 3(b) at the excitation wavelength of 330 nm. The major emission peaks appear at around 378 nm and 383 nm in ZnO NRs and Co-ZnO NRs due to


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the intrinsic recombination of free excitons, hence representing the UV emission. The details of photoluminescence analysis on both ZnO NRs and Co-ZnO NRs are shown in figure 3(c) and 3(d), respectively. The fitted spectrum in figure 3(c) shows the emission peak at around 377 nm and a broad peak at 382 nm, these denote intrinsic band gap emissions and near-band-edge emission of ZnO by the direct recombination of excited conduction band electrons with the photogenerated valance band holes.28, 29 On the other hand, the additional emission peaks appear at around 428 nm and 470 nm, in Co-ZnO NRs. These peaks could be attributed to the surface related interstitial and structural defects, where some of photoexcited electrons from valance to the conduction band of ZnO transitioned into defect energy levels induced by zinc interstitial as well as surface structural defects induced by Co in Co-ZnO NRs and then recombine with the photogenerated valance band holes by illuminating the respective emissions. 13, 14, 30, 31

(b)

(a) Al

Magnets

Sample

Al

Sample 100 µm

Co-ZnO NRs

A

Figure 4: (a) Optical photograph of experimental setup and inset shows the optical image of CoZnO NRs PD and (b) the schematic of the respective PD device. The optical photograph of the experimental setup is shown in figure 4(a) and inset depicts the optical image of Co-ZnO NRs, where the separation between the electrodes is 90 μm. Figure 4(b) depicts the schematic of Co-ZnO NRs PD for photoresponse measurements.



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The current-voltage (I-V) characteristic of Co-ZnO NRs PD is shown in figure 5(a) in presence and absence of both UV illumination and external magnetic field. It is observed that, the photocurrent (under UV illumination of 365 nm wavelength) in both absence and presence of external magnetic field is enhanced as compared to the dark current (in absence of UV illumination).

(a)

(b)

(c)

(d)

Figure 5: Photoresponse measurement of Co-ZnO NRs in absence and presence of external magnetic fields; (a) I-V analysis and (b) differential conductivity (dI/dV) measurements with and without UV illumination. (c) Saturation photoresponses with external magnetic fields and (d) variation of saturation response currents with external magnetic field strength at a constant bias voltage of 5 V.


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The enhancement in photocurrents is mainly due to the generation of electron-hole pairs during UV illumination. Under an external magnetic field of 2400 Gauss (G) higher photocurrent is observed during UV illumination as compared to in absence of magnetic field (figure 5(a)). The symmetric variation in I-V for Co-ZnO NRs PD as shown in figure 5(a) indicates Ohmic contact formation in the PD7, 32 and the inset shows the variation of dark current with the bias voltage in presence (2400 G) and absence of external magnetic fields, where the decrease of dark current was observed in presence of external magnetic field. Similarly, figure 5(b) shows the differential conductivity (dI/dV) analysis, which is estimated from the I-V analysis both in presence and absence of external magnetic field. An enhancement of 34 times in photo differential conductivity (dI/dV)ph is observed as compared to dark differential conductivity (dI/dV)d in absence of external magnetic field, which is mainly due to the radiation induced higher free charge carrier concentration at zero bias voltage. Since the drift velocity of free charge carriers increases with the bias voltage and hence (dI/dV)ph also increases with bias voltage.5,

33, 34

In

presence of external magnetic field of 2400 G, a drastic enhancement of 118 times in (dI/dV)ph is observed as compared to (dI/dV)d. The variation of response current (I = IUV – Idark) in Co-ZnO NWs PD, where IUV and Idark represent the current in presence and absence of UV illumination is measured at different external magnetic field intensities, respectively, is plotted with UV illumination cycle for 30 s at a fixed 5 V bias voltage as shown in figure 5(c). It is observed that the response current increases with the intensity of the external magnetic field. Likewise, figure 5(d) shows the variation of saturation response current with the strength of external magnetic field (2400 G) at 5 V bias voltage, where, the experiments were repeated for three times under each magnetic field intensity to estimate the error bar for saturation response currents. Only a small variation in saturation



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response current at all the strength of external magnetic fields demonstrated the stability of photoresponse. Also, almost three times enhancement in response current is observed as compared to absence of magnetic field. Generally, the UV photoresponse in ZnO nanostructure based PD is mainly dominated by oxygen adsorption and desorption processes in air. In dark, the oxygen molecules adsorb from air by

capturing

the

free

electrons

from

the

n-type

ZnO

NRs

surface

[ O2 ( gas)  e  O2 (adsorption) ] and forms low conductivity depletion region on the surface of NRs. The formation of low conductive depletion region on ZnO NRs surface introduced the upward energy band-bending due to the surface potential barrier. 5,

35

Figure 6(a) shows the

schematic of adsorption of oxygen on the pristine ZnO NR surface in dark, where EV, EC and E F represent the conduction band, valance band and Fermi energy level, respectively and the bottom schematic depicts the upward energy band-bending during adsorption of oxygen. However, Co decorated ZnO NRs exhibit more defect states on the surface as was observed in photoluminescence analysis (figure 3(d)) and hence exhibit higher defect trapping density of states.13,

14, 36

More defect trapping states in Co-ZnO NRs could drastically increase the

concentration of adsorbed oxygen in dark and results in an increase in the width of depletion region and induce more upward energy band-bending (Figure 6(b)).36 Under UV illumination, once photon energy is equal or greater than band gap of ZnO, electron-hole pairs are photogenerated [ hv  e  h ]. The photogenerated holes migrate to ZnO NR surfaces due to the potential slope (induced by energy band-bending) and desorb the oxygen from the NRs surface [ h  O2 (adsorption)  O2 ( gas) ]. In Co-ZnO NRs system, during UV illumination more oxygen desorption takes place due to the presence of large adsorbed oxygen on Co-ZnO NRs surface introduced by the surface trapping states in dark. 36 The photogenerated large 12 ACS Paragon Plus Environment

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concentration of unpaired free electrons contribute in enhancing the electrical conductivity in the form of photocurrent by drastically reducing the surface depletion region (figure 6(c)).5, 35 (a)

(c)

(b)

: O2 -

e : electron h : hole NR : neutral region DR : depletion region

e

e

e

e

e

e

e

e ee e e e e e e

h

DR

h

h

NR

Figure 6: Schematics represent the interaction of chemisorbed oxygen molecules on (a) pristine ZnO NR and (b) Co-ZnO NR in dark with electrons present in respective energy bands. (c) Desorption of oxygen during UV illumination on Co-ZnO NR with the respective energy band diagram. However, a significant enhancement in photocurrent is observed in presence of external magnetic field (figure 5) and that can be explained as follows: the external magnetic field induces the spin polarized free charges due to the presence of magnetic Co on Co-ZnO NRs surface that further helps in migration of existing free electrons towards the Co-ZnO NRs surface. This effect resulted in significant increase in the concentration of free electrons on Co-



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ZnO NRs surface to adsorb more oxygen in dark. Inset in figure 5(a) demonstrated a decrease in dark current in presence of external magnetic field that supports an increased oxygen adsorption on Co-ZnO NRs surface. However, UV illumination generates more unpaired photogenerated free electrons by desorbing adsorbed oxygen form the surface that contribute in enhancing the photocurrent.36,

37

Moreover, the spin polarization of charges help in the electron-hole pair

separation process during UV illumination due to opposite polarization of photogenerated electrons and holes which are quickly drift by reduced recombination rate.37 In addition, the Hall measurement of Co-ZnO NRs at room-temperature displays -8.7 V sum of hall voltage at external magnetic field of 2500 G. This result reveals n-type semiconducting nature Co-ZnO NRs. The cyclic photoresponse of PD signifies the stability as well as reproducibility of the photoresponse. Figure 7(a) shows the cyclic photoresponse of Co-ZnO NRs at a fixed 5 V bias voltage where response current is plotted with UV illumination cycle for 15 s. The cyclic photoresponse was measured for 8 cycles of UV exposure in both absence and presence of external magnetic field intensities. It is observed that the maximum response current under all UV illumination cycles remains same and highly reproducible. Moreover, the variation of response current with bias voltage ranging from 1 to 5 V in absence and presence of external magnetic field is shown in figure 7(b). The increase of response current with bias voltage in both absence and presence of external magnetic field is mainly due to the increase in drift velocity of photogenerated free charge carriers with the bias voltage. 5, 33, 34 It is also observed that response current enhances by ~186% in presence of magnetic field (2400 G) as compared to without magnetic field at a bias voltage of 5 V, because the magnetic spin polarization of charges is directly proportional to magnetic flux. Hence, these results reveal that the photoresponse of Co-


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ZnO NRs PD presents high stability, reproducibility and sensitivity to the external magnetic field as well as bias voltage. (a)

(b)

(c)

(d)

Figure 7: (a) Cyclic photoresponse of Co-ZnO NRs at 5 V bias voltage and (b) variation of saturation response current with bias voltage in absence and presence of external magnetic fields. Determination of response and recovery times of (c) Co-ZnO NRs PD and (d) pristine ZnO NRs PD. The detection speed of a PD is defined from the PD parameters namely response time (tr) defined as the time required to rise the response current from 10 to 90% of its saturation value and recovery time (td), the time required to fall the response current from 90 to 10% from its maximum value.7, 38 Figure 7(c) shows the estimation of tr and td for Co-ZnO NRs PD are 1.2 s and 7.4 s, respectively, where recovery time is longer than the response time. In the recovery 15 ACS Paragon Plus Environment


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process, two different decay mechanisms take place where initially the response current decay is very fast and then slows as shown in figure 7(c). The faster decay process is contributed mainly to the direct band-to-band recombination of photogenerated charge carriers. The slower decay could be related to the adsorption of oxygen molecules in NRs surface by trapping free electrons from n-type NRs as mentioned earlier. Therefore this process suggested the indirect recombination of charge carriers during decay of response current. However, the direct recombination (band-to-band) process is quite faster than that of indirect recombination (chemisorption of oxygen molecule)and hence result shows two different decay process during photoresponse.39-42 Similarly, the saturation photoresponse of pristine ZnO NRs PD (same device area as that of Co-ZnO NRs PD) is also evaluated by estimating tr and td. The observed values of tr and td are 38 s and 195 s, respectively, (as shown in figure 7(d)) which are much longer than the Co-ZnO NRs PD. The faster UV photoresponse observed in Co-ZnO NRs system as compared to ZnO NRs could be explained as follows: photogenerated electrons quickly transfer from ZnO conduction band to the Fermi level of Co and then quickly re-adsorbed oxygen molecules and some of directly recombined with the photogenerated valance band holes of ZnO.30, 31 The smaller values of tr and td demonstrate the fast responsive UV PD based on CoZnO NRs as compared to most of previously reported bulk ZnO nanostructure based UV PDs.7, 39, 43-45

Also, response current enhanced by 2.15 times in Co-ZnO NRs PD as compared to

pristine ZnO NRs PD in absence of external magnetic field this could be due to the charge trapping states generated on Co-ZnO NRs surface as discussed earlier. We have also performed the dependence of photoresponse analysis with external magnetic fields of pristine ZnO NRs PD, but no significant change in response current was observed. Table 1 demonstrates the detail comparison of photoresponse parameters namely response time, recovery time and response


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current at different strength of external magnetic fields of both the PDs at illumination intensity of 1.3 mW/cm2. It clearly shows the enhancement of photoresponse parameters in Co-ZnO NRs PD at presence and absence of magnetic fields. Table 1: Comparison of photoresponse parameters of both the pristine ZnO NRs and Co-ZnO NRs PDs. λ (nm)

PD

Voltage (V)

tr (s)

td (s)

Response current (μA) at external magnetic field Gauss (G) 0G

800 G

1600 G

2400 G

ZnO NRs

365

5

38

195

13

13.5

14

15

Co-ZnO NRs

365

5

1.2

7.4

28

48

60

80

The study of spectral photoresponses of both pristine ZnO NRs and Co-ZnO NRs with the wavelength range from 340 to 800 nm at a bias voltage of 5 V is shown in figure 8(a). The maximum peak in both the photodetectors observed around 355 nm that is mainly due to generation of the maximum number of electron-hole pairs where photon energy is larger than the bang gap of ZnO. The spectral photoresponses also reveal that both the pristine ZnO NRs and Co-ZnO NRs based PDs are only sensitive to the UV illumination (visible blind). Moreover, the additional parameters responsivity (Rs) defined as the generation of response current per unit UV illumination power, numerically, RS  I P0 A ; where Po is the illumination intensity and A is the effective device area of UV illumination. Gain (G) of the PD is defined as the ratio of the number of charge carriers generated per unit incident photons on the



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PD and is given by, G  RS hv q ; where, q is the elementary charge, υ is the frequency of absorbed photon and h is Planck’s constant which are also responsible for the performance of PDs.46, 47 The variation of responsivity and gain with external magnetic fields is shown in figure 8(b).

(a)

(c)

(b)

(d)

Figure 8: (a) Spectral photoresponses of both pristine ZnO NRs and Co-ZnO NRs. Variation of responsivity and gain of Co-ZnO NRs PD with (b) external magnetic fields and (c) and (d) with voltage at presence and absence of external magnetic fields. A linear dependence of responsivity and gain with the strength of external magnetic field is observed with the slope of 20 AW-1 Gauss-1 and 67 Gauss-1, respectively. The values of responsivity and gain are highly comparable with the previously reported UV PDs and results 18 ACS Paragon Plus Environment

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reveal that the presented device exhibit high performance and excellent sensitivity to the external magnetic field.45,

48-50

Likewise, the linear variations of responsivity and gain with the bias

voltage are shown in figure 8(c) and 8(d) in presence and absence of external magnetic fields. Again, the variation in responsivity with the slope of 54 AW-1 V-1 at zero magnetic field and 158 AW-1 V-1 at 2400 Gauss along with the gain of slope 184 V-1 and 539 V-1 at zero magnetic field and 2400 Gauss external magnetic field represents the linear increment with the bias voltage. Similarly the linear increment is mainly attributed to the increase in drift velocity of photogenerated charge carriers with the external electric field during UV illumination.5, 33, 34

3. CONCLUSION High performance, external magnetic field sensitive novel structure of Co integrated ZnO NRs based UV PDs is fabricated. Generation of defects states on the Co-ZnO NRs surface caused by decorated Co induces an enhancement of photocurrent as compared to pristine ZnO NRs based PD. Furthermore, migration of free charge carriers towards the Co-ZnO NRs surface induced more accumulation of free electrons due to the induced spin polarization of charges in presence of external magnetic fields in dark which results in inducing additional unpaired free electrons during UV illumination and amplify the photocurrent. Moreover, presence of external magnetic field separates electron-hole pair and reduces the recombination by inducing the opposite spin polarization of photogenerated electrons and holes. Overall, the presented novel Co-ZnO NRs PD demonstrates the faster photodetection ability (1.2 s response time and 7.4 s recovery time), excellent reproducibility photoresponse as well as highly sensitive to the external magnetic fields.



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4. EXPERIMENTAL SECTION 4.1 Material preparation Glass substrate was cleaned with acetone, isopropyl alcohol and de-ionized water followed by drying with nitrogen gas. First, ZnO seed layer was deposited on cleaned glass substrate using 10 mM zinc acetate dihydrate solution dissolved in ethanol and spin coated on top of cleaned glass substrate at 1000 rpm for 1 minute and heated at 150 0C for 5 minutes in air. The process of spin coating was performed for five times to get a uniform ZnO seed layer and finally heated at 350 0C for 1 hour in air.51 Thereafter, the thermally treated zinc acetate dihydrate transformed into uniform ZnO seed layer. For the growth of ZnO NRs, ZnO seed layer coated on glass substrate was dipped into 50 ml solution of 25 mM zinc nitrate hexahydrate and 25 mM hexamethyleneteramine. The growth of ZnO NRs was performed at constant temperature of 90 0C for 12 hours.51 The sample was then properly cleaned by using ethanol and rinsed with de-ionized water for 4 to 5 times and then dried in air at 150 0C for 1 hour. For the synthesis of Co coated ZnO NRs, 10 mM cobalt nitrate hexahydrate dissolved into 50 ml de-ionized water and stirred for 1 hour at room temperature. Afterward, ZnO NRs sample was cut into two pieces and one piece was then dipped in the cobalt nitrate hexahydrate solution in such a way that ZnO NRs coated surface should face downward direction and maintained at 95 0C for 12 hour. Finally, the Co coated ZnO NRs (Co-ZnO) sample was then rinsed using de-ionized water and allowed to dry at 150 0C for 1 hour. 4.2 Characterization The microstructure of pristine ZnO NRs and Co-ZnO NRs was investigated using scanning electron microscope (SEM) (FEI Sirion XL30 FEG SEM) and energy dispersive 20 ACS Paragon Plus Environment

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spectroscopy (EDS) technique used for elemental compositional analysis. Similarly, the surface morphology of Co-ZnO

NRs

was

performed

using

TEM

(FEI

Tecnai

F30

S-

TWIN).Furthermore, the crystal structure of pristine ZnO NRs and Co-ZnO NRs was analyzed by X-ray diffraction (XRD) (Rigaku Smart Lab). In addition, UV-Vis-NIR spectroscopy (Perkin Elmer Lambda 750) and photoluminescence analysis (RF-5301pc spectrofluorophotometer) were performed for the optical studies of pristine ZnO NRs and Co-ZnO NRs, respectively. Magnetic properties of pristine ZnO NRs and Co-ZnO NRs were evaluated in vibrating sample magnetometer (VSM, LakeShore). Also, the fabricated device was evaluated under optical microscope (Nikon ECLIPSE LV100). 4.3 Device fabrication and photoresponse measurements To study of UV photoresponses of both pristine ZnO NRs and Co-ZnO NRs PDs, aluminum electrodes of thickness around 300 nm were deposited on the top surface of NRs using resistive thermal evaporation technique by maintaining 90 μm separation using a shadow mask (figure 4(a)). Photoresponses were monitored with an UV radiation source of 1.3 mW/cm2 power density at 365 nm wavelength at constant 5 V and different bias voltages range from 1 to 5 V under an external magnetic field applied using bar magnets where magnetic field direction was perpendicular to the axis of NRs. The UV photoresponses were recorded by Keithley 2611B source-meter system in both presence and absence of external fields. In addition Hall measurement was performed using van der Pauw technique at 2500 G external magnetic field.

5. REFERENCES 1. Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter 2004, 16, R829-R858. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

2. Alaie, Z.; Nejad, S. M.; Yousefi, M. H. Recent Advances in Ultraviolet Photodetectors. Mater. Sci. Semicond. Process. 2015, 29, 16-55. 3. Peng, L.; Hu, L.; Fang, X. Low-Dimensional Nanostructure Ultraviolet Photodetectors. Adv. Mater. 2013, 25, 5321-5328. 4. Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Room-Temperature Ultraviolet Nanowire Nanolasers. Science 2001, 292, 18971899. 5. Soci, C.; Zhang, A.; Xiang, B.; Dayes, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. ZnO Nanowire UV Photodetectors with High Internal Gain. Nano Lett. 2007, 7, 1003-1009. 6. Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301-041403. 7. Tian, W.; Zhai, T.; Zhang, C.; Li, S. –L.; Wang, X.; Liu, F.; Liu, D.; Cai, X.; Tsukagoshi, K.; Golberg, D.; Bando, Y. Low-Cost Fully Transparent Ultraviolet Photodetectors Based on Electrospun ZnO-SnO2 Heterojunction Nanofibers. Adv. Mater. 2013, 25, 4625-4630. 8. Fang, X.; Wei, Z.; Chen, R.; Tang, J.; Zhao, H.; Zhang, L.; Zhao, D.; Fang, D.; Li, J.; Fang, F.; Chu. X.; Wang, X. Influence of Exciton Localization on the Emission and Ultraviolet Photoresponse of ZnO/ZnS Core−Shell Nanowires. ACS Appl. Mater. Interfaces 2015, 7, 10331-10336.


Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9. Liu, K.; Sakurai, M.; Liao, M.; Aono, M. Giant Improvement of the Performance of ZnO Nanowire Photodetectors by Au Nanoparticles. J. Phys. Chem. C 2010, 114, 1983519839. 10. Lu, Y.; Lin, Y.; Wang, D.; Wang, L.; Xie, T.; Jiang, T. A High Performance CobaltDoped ZnO Visible Light Photocatalyst and Its Photogenerated Charge Transfer Properties. Nano Res. 2011, 4, 1144-1152. 11. Cui, J.; Gibson, U. Thermal Modification of Magnetism in Cobalt-Doped ZnO Nanowires Grown at Low Temperatures. Phys. Rev. B 2006, 74, 045416-045423. 12. Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. –J. Controlled Growth of ZnO Nanowires and Their Optical Properties. Adv. Funct. Mater. 2002, 12, 323-331. 13. Panigrahy, B.; Aslam, M.; Bahadur, D. Aqueous Synthesis of Mn- and Co-Doped ZnO Nanorods. J. Phys. Chem. C 2010, 114, 11758-11763. 14. Shi, S.; Yang, Y.; Xu, J.; Li, L.; Zhang, X.; Hue, G. –H.; Dang, Z. –M. Structural, Optical and Magnetic Properties of Co-doped ZnO Nanorods Prepared by Hydrothermal Method. J. Alloys Compd. 2013, 576, 59-65. 15. Vijayaprasath, G.; Murugan, R.; Mahalingam, T.; Ravi, G. Comparative Study of Structural and Magnetic Properties of Transition Metal (Co, Ni) Doped ZnO Nanoparticles. J. Mater. Sci. Mater. Electron. 2015, 26, 7205-7213.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

16. Pazhanivelu, V.; Selvadurai, A. P. B.; Zhao, Y.; Thiyagarajan, R.; Murugaraj, R. Room Temperature Ferromagnetism in Mn Doped ZnO:Co Nanoparticles by Co-Pecipitation Method. Physica B 2016, 481, 91-96. 17. Li, Z.; Zhong, W.; Li, X.; Zeng, H.; Wang, G.; Wang, W.; Yang, Z.; Zhang, Y. Strong Room-Temperature Ferromagnetism of Pure ZnO Nanostructure Arrays via Colloidal Template. J. Mater. Chem. C 2013, 1, 6807-6812. 18. Peng, C.; Liang, Y.; Wang, K.; Zhang, Y.; Zhao, G.; Wang, Y. Possible Origin of Ferromagnetism in an Undoped ZnO d0 Semiconductor. J. Phys. Chem. C 2012, 116, 9709-9715. 19. Xu, Q.; Wen, Z.; Xu, L.; Gao, J.; Wu, D.; Shen, K.; Qiu, T.; Tang, S.; Xu, M. Room Temperature Ferromagnetic Pure ZnO. Physica B 2011, 406, 19-23. 20. Phan, T. –L.; Zhang, Y. D.; Yang, D. S.; Nghia, N. X.; Thanh, T. D.; Yu, S. C. DefectInduced Ferromagnetism in ZnO Nanoparticles Prepared by Mechanical Milling. Appl. Phys. Lett. 2013, 102, 072408- 072412. 21. Xu, Q.; Schmidt, H.; Zhou, S.; Potzger, K.; Helm, M.; Hochmuth, H.; Lorenz, M.; Setzer, A.; Esquinazi, P.; Meinecke, C.; Grundmann, M. Room Temperature Ferromagnetism in ZnO Films due to Defects. Appl. Phys. Lett. 2008, 92, 082508-082510. 22. Yi, J. B.; Pan, H.; Lin, J. Y.; Ding, J.; Feng, Y. P.; Thongmee, S.; Liu, T.; Gong, H.; Wang, L. Ferromagnetism in ZnO Nanowires Derived from Electro-deposition on AAO Template and Subsequent Oxidation. Adv. Mater. 2008, 20, 1170-1174.


Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Wang, Q.; Sun, Q.; Chen, G.; Kawazoe, Y.; Jena, P. Vacancy-Induced Magnetism in ZnO Thin Films and Nanowires. Phys. Rev. B 2008, 77, 205411-205417. 24. Zou, C.W.; Shao, L. X.; Guo, L. P.; Fu, D. J.; Kang, T. W. Ferromagnetism and Ferroelectric Properties of (Mn, Li) Co-Doped ZnO Nanorods Arrays Deposited by Electrodeposition. J. Cryst. Growth 2011, 331, 44-48. 25. Chen, Y.; Zeng, D.; Zhang, K.; Lu, A.; Wang, L.; Peng, D. -L. Au–ZnO Hybrid Nanoflowers, Nanomultipods and Nanopyramids: One-Pot Reaction Synthesis and Photocatalytic Properties. Nanoscale 2014, 6, 874-881. 26. Zak, A. K.; Razali, R.; Majid, W. H. A.; Darroudi, M. Synthesis and Characterization of a Narrow Size Distribution of Zinc Oxide Nanoparticles. Int. J. Nanomedicine 2011, 6, 1399-1403. 27. Boruah, B. D.; Misra, A. ZnO Quantum Dots and Graphene Based Heterostructure for Excellent Photoelastic and Highly Sensitive Ultraviolet Photodetector. RSC Adv. 2015, 5, 90838-90846. 28. Liu, J.; Motta, N.; Lee, S. Ultraviolet Photodetection of Flexible ZnO Nanowire Sheets in Polydimethylsiloxane Polymer. Beilstein J. Nanotechnol. 2012, 3, 353-359. 29. Li, G. P.; Chen, R.; Guo, D. L.; Wong, L. M.; Wang, S. J.; Suna, H. D.; Wu, T. Nanoscale Semiconductor–Insulator–Metal Core/Shell Heterostructures: Facile Synthesis and Light Emission. Nanoscale 2011, 3, 3170-3177.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

30. Chang, Y.; Xu, J.; Zhang, Y.; Ma, S.; Xin, L.; Zhu, L.; Xu, C. Optical Properties and Photocatalytic Performances of Pd Modified ZnO Samples. J. Phys. Chem. C 2009, 113, 18761-18767. 31. Bera,

A.;

Basak,

D.

Pd-Nanoparticle-Decorated

ZnO

Nanowires:

Ultraviolet

Photosensitivity and Photoluminescence Properties. Nanotechnology 2011, 22, 265501265507. 32. Brillson, L. J.; Lu, Y. ZnO Schottky barriers and Ohmic contacts. J. Appl. Phys. 2011, 109, 121301-121333. 33. Boruah, B. D.; Ferry, D. B.; Mukherjee, A.; Misra, A. Few-Layer Graphene/ZnO Nanowires Based High Performance UV Photodetector. Nanotechnology 2015, 26, 235703-235709. 34. Ok, J. G.; Lee, J. Y.; Baac, H. W.; Tawfick, S. H.; Guo, L. J.; Hart, A. J. Rapid Anisotropic Photoconductive Response of ZnO-Coated Aligned Carbon Nanotube Sheets. ACS Appl. Mater. Interfaces 2014, 6, 874-881. 35. Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. Nanowire Ultraviolet Photodetectors and Optical Switches. Adv. Mater. 2002, 14, 158-160. 36. Hsu, C. –L.; Chang, S. –J. Doped ZnO 1D Nanostructures: Synthesis, Properties, and Photodetector Application. Small 2014, 10, 4562-4585. 37. Lu, M. –L.; Weng, T. –M.; Chen, J. –Y.; Chen, Y. –F. Ultrahigh-Gain Single SnO2 Nanowire Photodetectors Made with Ferromagnetic Nickel Electrodes. NPG Asia Mater. 2012, 4, e26. 26 ACS Paragon Plus Environment

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


38. Boruah, B. D.; Mukherjee, A.; Sridhar, S.; Misra, A. Highly Dense ZnO Nanowires Grown on Graphene Foam for Ultraviolet Photodetection. ACS Appl. Mater. Interfaces 2015, 7, 10606-10611. 39. Liu, N.; Fang, G.; Zeng, W.; Zhou, H.; Cheng, F.; Zheng, Q.; Yuan, L.; Zou, X.; Zhao, X. Direct Growth of Lateral ZnO Nanorod UV Photodetectors with Schottky Contact by a Single-Step Hydrothermal Reaction. ACS Appl. Mater. Interfaces 2010, 2, 1973-1979. 40. Dai, W.; Pan, X.; Chen, C.; Chen, S.; Chen, W.; Zhang, H.; Ye, Z. Enhanced UV Detection Performance Using a Cu Doped ZnO Nanorod Array Film. RSC Adv. 2014, 4, 31969-31972. 41. Ni, P. –N.; Shan, C. –X.; Wang, S. –P.; Liu, X. –Y.; Shen, D. –Z. Self-Powered Spectrum-Selective Photodetectors Fabricated from n-ZnO/p-NiO Core–Shell Nanowire Arrays. J. Mater. Chem. C 2013, 1, 4445-4449. 42. Chiu, H. –M.; Wu, J. –M. Opto-Electrical Properties and Chemisorption Reactivity of Ga-Doped ZnO Nanopagodas. J. Mater. Chem. A 2013, 1, 5524-5534. 43. Hu, L.; Yan, J.; Liao, M.; Xiang, H.; Gong, X.; Zhang, L.; Fang, X. An Optimized Ultraviolet-A Light Photodetector with Wide-Range Photoresponse Based on ZnS/ZnO Biaxial Nanobelt. Adv. Mater. 2012, 24, 2305-2309. 44. Manekkathodi, A.; Lu, M. –Y.; Wang, C. W.; Chen, L. –J. Direct Growth of Aligned Zinc Oxide Nanorods on Paper Substrates for Low‐Cost Flexible Electronics. Adv. Mater. 2010, 22, 4059-4063.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

45. Hu, L.; Chen, M.; Shan, W.; Zhan, T.; Liao, M.; Fang, X.; Hu, X.; Wu, L. Stacking‐Order‐Dependent Optoelectronic Properties of Bilayer Nanofilm Photodetectors Made From Hollow ZnS and ZnO Microspheres. Adv. Mater. 2012, 24, 5872-5877. 46. Wu, J. M.; Chang, W. E. Ultrahigh Responsivity and External Quantum Efficiency of an Ultraviolet-Light Photodetector Based on a Single VO2 Microwire. ACS Appl. Mater. Interfaces 2014, 6, 14286-14292. 47. Sun, Z.; Chang, H. Graphene and Graphene-Like Two-Dimensional Materials in Photodetection: Mechanisms and Methodology. ACS Nano 2014, 8, 4133-4156. 48. Qin, L. Q.; Shing, C.; Sawyer, S. Metal–Semiconductor–Metal Ultraviolet Photodetectors Based on Zinc-Oxide Colloidal Nanoparticles. IEEE Electron Device Lett. 2011, 32, 5153. 49. Shao, D.; Yu, M.; Sun, H.; Hu, T.; Lian, J.; Sawyer, S. High Responsivity, Fast Ultraviolet Photodetector Fabricated from ZnO Nanoparticle–Graphene Core–Shell Structures. Nanoscale 2013, 5, 3664-3667. 50. Chang, H.; Sun, Z.; Ho, K. Y. –F.; Tao, X.; Yan, F.; Kwok, W. –M.; Zheng, Z. A Highly Sensitive Ultraviolet Sensor Based on a Facile in Situ Solution-Grown ZnO Nanorod/Graphene Heterostructure. Nanoscale 2011, 3, 258-264. 51. Rao, A. D.; Karalatti, S.; Thomas, T.; Ramamurthy, P. C. Self-Assembled, Aligned ZnO Nanorod Buffer Layers for High-Current-Density, Inverted Organic Photovoltaics. ACS Appl. Mater. Interfaces 2014, 6, 16792-16799.


Page 29 of 29

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Table of content:

Co-ZnO NRs

A


Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc

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