Effect of Ce doping on the optoelectronic and sensing properties of electrospun ZnO nanofibers

Abstract

Pure n-type ZnO nanofibers and p-type Ce-doped ZnO nanofibers were prepared by electrospinning followed by calcination. Their surface morphology, elemental composition, crystal structure, and optical and electronic properties were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and photoluminescence and UV-visible spectroscopy techniques and by their current–voltage (I–V) curves. The energy-dispersive X-ray spectroscopy and X-ray diffraction spectra showed that Ce was successfully incorporated into the ZnO crystal lattice and that the atomic percentage of Ce to Zn was 1.46%. The photoluminescence integrated intensity ratio of the UV emission to the deep-level green emission for Ce-doped ZnO nanofibers was over twice than that of pure ZnO. The UV-visible absorption edge of the Ce-doped ZnO nanofibers red-shifted by 2.6 nm compared with the pure ZnO nanofibers. The ZnO nanofibers had a good response to ultraviolet radiation. The sensitivity (I_max/I₀) of the Ce-doped ZnO nanofibers was 10², which was one order higher in magnitude than that of pure ZnO nanofibers. The field-effect curve suggested that the synthesized Ce-doped ZnO nanofibers were p-type semiconductors. A p–n homojunction device was prepared using the ZnO nanofibers and showed good rectifying behavior. The turn-on voltage reduced by about 10 V under UV irradiation. Both the ZnO nanofibers and the ZnO p–n homojunction had excellent UV sensibilities. These results suggest that Ce-doped ZnO nanofibers may have widespread applications in optical and electronic devices.

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1. Introduction

Ultraviolet (UV) photodetectors are used in military and civil applications, including missile detection, as probes to detect burning and in chemical analysis.¹ Historically, the most common UV detectors have been photomultipliers and silicon-based UV photodiodes; however, these detectors require expensive filters and are also sensitive to visible light. GaN and ZnO are promising candidates for use in UV photodetectors because of their excellent UV photosensitivity;² ZnO can be used to prevent UV damage and is cheaper than GaN.³ ZnO nanowires have a simple structure, diverse methods of growth and the nanostructure of ZnO can be controlled.⁴ ZnO has exceptional properties, such as a wide, direct band gap and large photoresponse, which makes it an ideal candidate for use in UV photodetectors. Shinde and Rajpure⁵ reported the synthesis of a Ga-doped ZnO UV detector on an alumina substrate by spray pyrolysis. The fabrication of UV detectors based on Ce-doped ZnO nanofibers via electrospinning has not previously been reported.

Pure ZnO is an n-type semiconductor as a result of its intrinsic defects.⁶ New techniques to produce high-quality p-type ZnO thin films will facilitate their application. However, it is difficult to prepare p-type ZnO by doping, mainly due to the low solubility of the acceptor dopants, the deep acceptor level and the self-compensation effect of the intrinsic defects.⁷ Several approaches have been used to prepare p-type ZnO using groups I and V elements as alternative dopants for Zn and O, respectively,⁸ but the p-type conduction was unstable.⁹ p-Type ZnO has also been prepared by introducing different dopants via pulsed laser deposition,¹⁰ sputtering,¹¹ metal–organic chemical vapor deposition¹² and spray pyrolysis techniques.¹³ Chang et al.¹⁴ prepared Ce-doped ZnO nanorods at 90 °C by a hydrothermal method and tested their visible light emission. However, these approaches can only fabricate ZnO membranes and short nanowires so their applications to different devices is limited.^15,16 Electrospinning is a good technique for the preparation of ZnO fibers and is simple, versatile and cheap. It can produce both ultrathin and long continuous fibers. García-Méndez et al.¹⁷ reported the preparation of Ce-doped ZnO thin films with excellent optical properties by RF-reactive magnetron sputtering technology. p-Type ZnO is difficult to prepare by electrospinning, although ZnO nanofibers have been produced by this technique.¹⁸ Lang et al.¹⁹ reported the synthesis of Ce-doped ZnO nanorods under mild hydrothermal conditions. The preparation of p-type Ce-doped ZnO fibers and rectifying devices via electrospinning–calcination technology has been reported by our group, but there have only been a few reports of the optoelectronic properties of Ce-doped ZnO nanofibers and ZnO p–n junctions. p–n junctions have wide applications in optical devices such as semiconductor laser diodes and UV photodetectors.²⁰ There has been no previous report of the photosensitivity of pure ZnO/Ce-doped ZnO homojunctions prepared by electrospinning.

We synthesized n-type pure ZnO nanofibers and p-type Ce-doped nanofibers via electrospinning and calcination. ZnO nanofibrous homojunctions were prepared by the same technique. The surface topography, crystal structure, electrical and optical properties and the basic theory of p-type ZnO formation are reported here.

2. Experimental

2.1 Fabrication of ZnO nanofibers and homojunctions

Undoped ZnO nanofibers and Ce-doped ZnO nanofibers were synthesized by electrospinning and calcination. The raw materials included 1.2 g of polyvinyl pyrrolidone (molecular weight 1 300 000 g mol⁻¹, Aldrich), 1.2 g of zinc acetate (Zn(CH₃COO)₂·2H₂O, Aldrich), 0.02 g of cerium nitrate (Ce(NO₃)₃·6H₂O, Aldrich), 10.5 g of ethanol and 0.45 g of deionized water. Fig. 1 shows a schematic diagram of the preparation technique. The only difference in the precursor solutions for Ce–ZnO and pure ZnO was the need for additional cerium nitrate (Ce(NO₃)₃·6H₂O, Aldrich). The experiments were carried out at room temperature.

Fig. 1 Synthetic route for the ZnO nanofibers.

The ZnO p–n homojunction nanofibers were fabricated as follows. The pure ZnO precursor solution was electrospun on the Si wafer. The Ce–ZnO precursor solution was then electrospun as the second fiber membrane under the same conditions of 20 °C, 50% RH, an electrospinning voltage of 20 kV and an electrospinning distance of 13.5 cm. The p–n homojunction nanofibers were formed after the fibrous membrane was calcined in a muffle furnace at 900 °C for 2 h.

2.2 Fabrication of ZnO nanofiber device

A ZnO device was synthesized to investigate the UV properties of the ZnO nanofibers. ZnO nanofibers were placed on a glass sheet. Two copper wires were then connected to the power supply and the ZnO fibrous membrane via a silver paste. The ZnO micro-/nanofibrous thin films acted as ohmic contacts. Fig. 2 shows a schematic diagram of the ZnO nanofiber device.

Fig. 2 Schematic diagram of ZnO nanofiber device.

The ZnO p–n homojunction device was prepared as follows. The p-type side (lower side) of the ZnO homojunction was connected to aluminum foil and the n-type side (upper side) of the ZnO homojunction was connected to the ITO electrode. The upper and lower sides were then connected to the negative and positive electrodes, respectively, of the power supply using copper conductors.

2.3 Characterization

The morphology of the ZnO nanofibers was characterized by scanning electron microscopy (SEM; Hitachi TM-1000). The composition of the ZnO fibers was investigated by energy-dispersive X-ray spectroscopy (EDS; Hitachi S4800). Crystal structural characterization of the ZnO nanofibers was performed by X-ray diffraction (XRD; Rigaku D/max-2400). The photoluminescence (PL) spectra were analyzed using a spectrofluorimeter (F-4600, Hitachi). The UV-visible absorption spectrum was measured with a UV/vis/NIR spectrophotometer (UV-4100, Hitachi). The gate-effect curves of an individual ZnO nanofiber were examined using a two-metal microprobe testing platform (HP 4156A). The current–voltage (I–V) characteristic curves of the ZnO fibers and p–n homojunction under illumination by a xenon lamp were realized using a source meter (Keithley 6487). All the measurements were made at room temperature.

3. Results and discussion

3.1 Morphology and structure

The surface morphologies of the pure ZnO precursor nanofibers prepared by electrospinning were observed by SEM (Fig. 3a). The randomly oriented nanofibers had a smooth surface morphology with an average diameter of about 550 nm and a fiber length of several centimeters. Fig. 3b shows that the nanofibers maintained a sequential nanostructure after annealing at 900 °C in air. The surface morphology shows nanofiber structures consisting of crystalline grains. The surface of the nanofibers became ragged and the average diameter decreased to about 250 nm as a result of the disappearance of the organic components and the disintegration of the metallic precursors. The structural morphology of the ZnO nanofibers became more complex. Ce has similar lattice constants to Zn, which means that Ce can be incorporated into the ZnO nanofibers. An SEM image of the as-grown Ce-doped ZnO precursor nanofibers is shown in Fig. 3c and an SEM image of Ce-doped ZnO nanofibers after calcination is shown in Fig. 3d. The crystalline grain diameter of Ce-doped ZnO is larger than that of pure ZnO and the distribution of the fiber diameters becomes more uniform (Fig. 3c and d). The average diameter of the Ce-doped ZnO nanofibers pre- and post-calcination are about 650 and 350 nm, respectively. The grain diameter of Ce-doped ZnO is larger than that of pure ZnO, which is consistent with previously reported results.¹⁸ Further investigations are required to better understand the formation of the ZnO nanofibers.

Fig. 3 SEM images of the ZnO nanofibers. (a) As-spun pure ZnO precursor nanofibers; (b) pure ZnO nanofibers after calcination; (c) Ce-doped ZnO precursor nanofibers; and (d) Ce-doped ZnO nanofibers after sintering at 900 °C for 2 h. Insets show magnified views of the respective ZnO nanofibers.

Fig. 4a shows the elemental analyses of pure ZnO nanofibers analyzed by EDS. The EDS spectrum of pure ZnO nanofibers only contains Zn and O, which demonstrates that the resultant nanofibers are pure ZnO. The EDS result for Ce-doped ZnO nanofibers (Fig. 4b) shows that Ce is also present in the nanofibers. No other peak associated with impurities was observed, indicating that the fabricated nanofibers were Ce-doped ZnO nanofibers. The experimental atomic percentage of Ce to Zn (obtained from the EDS analysis) was 1.46%, which is consistent with the theoretical value. The theoretical value was calculated by the molar ratio of Ce to Zn (0.02 g Ce(NO₃)₃·6H₂O and 1.2 g Zn(CH₃COO)₂·2H₂O). The mass percentage of Ce that can be incorporated into the ZnO host was therefore about 0.891%.

Fig. 4 EDS spectra of (a) pure ZnO nanofibers and (b) Ce-doped ZnO nanofibers.

Fig. 5 shows the XRD patterns of the pure ZnO nanofibers and Ce-doped ZnO nanofibers, demonstrating the crystallographic characteristics of the pure ZnO and Ce-doped ZnO nanofibers. The peaks of the diffraction pattern were high and the full-width half-maximum was narrow, which indicated that the pure ZnO and Ce-doped ZnO both had good crystallinity. The XRD spectrum proved that the pure ZnO nanofibers had a hexagonal wurtzite structure (JCPDS 36-1451) with the diffraction peaks at 2θ values of 31.77, 34.42 and 36.26° corresponding to the (100), (002) and (101) lattice planes, respectively. In contrast with the undoped ZnO, a new peak located at 2θ = 28.55°, corresponding to the (111) diffraction peak of CeO₂ (JCPDS 34-0394), appears in Ce-doped ZnO nanofibers as a result of doping with Ce ions. Comparison of the XRD profiles for the pure ZnO nanofibers and the Ce-doped ZnO nanofibers showed that the diffraction peaks of the Ce-doped ZnO fibers had a small shift of about 0.31° to a lower angle. The atomic radius of Ce⁴⁺ (87 pm) is larger than that of Zn²⁺ (74 pm), leading to a larger ZnO lattice. According to the Bragg equation, the lattice constant is in proportion to d (the interplanar distance) and the diffraction peaks will shift to smaller angle with increasing atom radius, indicating the incorporation of Ce ions into the ZnO lattice. The XRD results also demonstrated that the synthesized nanofibers were a compound phase with a hexagonal wurtzite structure for ZnO and a cubic structure for CeO₂. There was no impurity peak in the XRD patterns, so the samples were of high purity.

Fig. 5 XRD patterns of pure ZnO and Ce-doped ZnO nanofibers after calcination; inset shows an enlarged view of the (101) diffraction.

3.2 Photoelectric properties

Fig. 6a–c shows the PL properties of the ZnO hexagonal nanofibers excited by 325, 350 and 365 nm irradiation in a backscattering geometry at room temperature. The PL spectra of the pure ZnO nanofibers contained a strong UV emission at about 397 nm and a weak shoulder of deep-level emission near 471 nm. A relatively wide and strong green band was seen in the visible region centered at about 498 nm. The CIE color coordinates of pure ZnO and Ce-doped ZnO were plotted in Fig. 6d–f, respectively. These are: (0.208, 0.384) and (0.198, 0.318) under 325 nm excitation; (0.209, 0.387) and (0.199, 0.327) under 350 nm excitation; and (0.210, 0.381) and (0.201, 0.320) under 365 nm excitation. The UV emission was ascribed to the recombination of free excitons after an exciton–exciton collision, which corresponds to the near band-edge emission of ZnO.²¹ Several different mechanisms have been proposed for the visible light emission of ZnO. It is well-known that the green emission peak originates from deep-level or trap-state emission.²²

Fig. 6 PL emission spectra at different excitation wavelengths of (a) 325, (b) 350 and (c) 365 nm for pure ZnO and Ce-doped ZnO. Standard CIE coordinate color graphs of pure ZnO and Ce-doped ZnO nanofibers under UV irradiation at (d) 325, (e) 350 and (f) 365 nm.

The broad green emission band with a peak near 500 nm is attributed to electron–hole recombination process, in which electron can transfer from the shallow donor level from the intrinsic defect centers, such as interstitial zinc (Zn_i) and oxygen vacancy (V_O), to the valence band⁶ or conduction band.²³ The presence of impurities or excess oxygen²⁴ may also be responsible for the green emission. A weak peak is also observed at about 471 nm, which may be a result of the existence of defects or impurities linked to local levels in the ZnO gap.¹⁷ García-Méndez et al.¹⁷ reported that the peak of RF-sputtered Ce-doped ZnO membranes appeared at 467 nm because the electric dipole allowed the transition of the Ce³⁺ ions. These peaks originated from the lower 5d (²D) excited state to the split ²F_5/2 and ²F_7/2 energy levels.¹⁷

When elemental Ce is doped into ZnO nanofibers, the PL intensity of ZnO decreases. The difference in the emission intensity may be associated with their different nanostructural morphology and crystallinity. The doping of Ce may increase lattice defects and lead to a change in the crystal field and band gap, thus decreasing the PL intensity of Ce-doped ZnO nanofibers (Fig. 6a–c). With Ce⁴⁺ ions as donors in the ZnO matrix, a small red shift may appear in the UV emission because of a slight modulation of the ZnO band gap and the formation of new emission centers.¹⁹ In this case, the luminescent process is changed. Li et al.²⁵ reported that the incorporation of Ce⁴⁺ ions resulted in transitions between the 5d and 4f orbits, which may bring about multi-emission peaks in the blue-green region. It is generally considered that the intensity ratio between the UV emission band and the visible emission band is an indicator of ZnO crystalline materials. The intense UV and weak green band correspond to a good crystal surface.²⁶ In this investigation, the PL ratios of the integrated intensity between the UV emission and the deep-level green emission (I_UV/I_DLE) were 0.2553, 0.3667 and 0.6067 on excitation at 325, 350 and 365 nm for ZnO and 0.5968, 0.7762 and 1.2851 on excitation at 325, 350 and 365 nm for the Ce-doped ZnO nanofibers, respectively. A larger ratio indicates that the Ce-doped ZnO nanofibers have good crystallinity. These results show that Ce-doped ZnO nanofibers have promising applications in optoelectronic devices. The PL intensity is larger at shorter wavelengths because more electrons are excited with increasing energy. I_UV/I_DLE is larger at longer wavelengths of excitation.

As the energy of UV light is close to the band gap of semiconductors, it is absorbed by many metal oxide semiconductors. Bulk ZnO has a direct band gap of 3.37 eV.¹⁶ The optical absorption patterns of the synthesized pure ZnO and Ce-doped ZnO nanofibers were investigated by UV-visible spectrometry (Fig. 7). Fig. 7a shows the absorption of ZnO at different wavelengths. The intersection of the absorbance and photon energy (hν) axes corresponds to the band gap of ZnO nanofibers (Fig. 7b). The synthesized Ce–ZnO nanofibers had a higher absorption coefficient than pure ZnO nanofibers in the UV region (250–400 nm). The absorption edge revealed that the ZnO nanofibers had a direct band gap that could be obtained by extending the linear portion of the absorption pattern to the photon energy of the horizontal axis. For pure ZnO, the point of intersection was 3.10 eV, corresponding to an absorption edge of 398 nm, which was smaller than the cited value of 3.37 eV corresponding to the band-to-band transition of bulk ZnO.²⁷ By contrast, the band gap of Ce–ZnO was about 3.08 eV (at a wavelength of 402 nm). The absorption results revealed that the peak of Ce-doped ZnO nanofibers shifted to longer wavelengths, which can be attributed to quantum confinement and small size effects.²⁸ The ZnO nanofibers effectively absorbed the photon energy when exposed to 254 nm irradiation, which has an energy of 4.9 eV. This is enough energy to excite electrons diametrically from the valence band to the conduction band. The absorption ability may be different in the same material under different irradiation conditions. According to previous research,²⁹ the absorption coefficient of ZnO nanofibers decreases with increasing UV wavelength. Thus the sensitivity of ZnO nanofibers under 365 nm UV irradiation, which has an energy of 3.40 eV, is lower than under 254 nm irradiation. The reduced band gap indicated that the optical properties of Ce-doped ZnO nanofibers was improved.

Fig. 7 (a) UV-visible absorption spectra of the pure ZnO and Ce-doped ZnO nanofibers and (b) plot of absorbance versus photon energy for ZnO nanofibers.

Fig. 8a and b show the photoresponse and I–V characteristic curves of pure ZnO and Ce-doped ZnO nanofibers under no illumination and under 254 and 365 nm UV irradiation, respectively. The photocurrents of the pure ZnO nanofibers increased from the background current (under a bias of 30.0 V) of I₀ = 6.31 nA to I_max = 2.43 μA (254 nm UV irradiation) and 1.55 μA (365 nm UV irradiation). The photocurrents of Ce-doped ZnO nanofibers increased from a background current of I₀ = 1.69 nA to I_max = 1.21 μA (254 nm UV irradiation) and 0.365 μA (365 nm UV irradiation). It is evident from Fig. 8a and b that the photocurrent decreased with increasing UV wavelength. Under UV irradiation of 254 and 365 nm, the photocurrent of the Ce–ZnO nanofibers decreased compared with that of the pure ZnO nanofibers because pure ZnO is intrinsically an n-type semiconductor and is difficult to turn into a p-type semiconductor. The Ce acts as a doping impurity, which reduces the electron concentration and increases the hole concentration in the ZnO nanofibers. Doping of Ce may also increase the lattice defects and decrease the photocurrent (I₀ and I_max).

Fig. 8 I–V images of (a) pure ZnO and (b) Ce-doped ZnO nanofibers exposed to UV irradiation at different wavelengths. Time-dependent photocurrent curves of (c) pure ZnO nanofibers and (d) Ce-doped ZnO nanofibers exposed to UV irradiation.

Compared with the initial current I₀ (background current), the photocurrent increased enormously. The linear behavior of the I–V characteristics of the ZnO membranes suggests ohmic contacts. When the ZnO nanofibers were periodically exposed to UV irradiation, the current curves changed (Fig. 8c and d). The photocurrent increased and decreased periodically when the UV lamp was switched on and off. Fig. 8c and d also show the good reversibility after repeated exposure to UV irradiation. The response and recovery time were both <10 s. The calculated sensitivities (I_max/I₀) were 42.81 and 26.84 for pure ZnO under 254 and 365 nm UV irradiation, respectively. The calculated sensitivity was 208.46 and 90.69 for Ce-doped ZnO under 254 and 365 nm UV irradiation, respectively. The Ce-doped ZnO had smaller values of I_max and I₀, but the sensitivity was much more greater than that of pure ZnO. The sensitivity decreased with increasing UV wavelength – that is, the photocurrent increased with decreasing UV wavelength because the electronic transitions from the valence band to the conduction band to produce electron–hole pairs were easier. The sensitivity of Ce-doped ZnO was greater than that of pure ZnO under the same UV illumination, which is an advantage of doping with Ce. These results suggest promising applications for ZnO optical devices.

To investigate whether the ZnO nanofibers were n-type or p-type semiconductors, the field-effect curves were obtained through a two-metal microprobe testing platform. A single ZnO nanofiber was cast across two Au electrodes (Fig. 9a). The gate-effect curves of individual ZnO nanofibers are shown in Fig. 9b. The source–drain current of undoped ZnO nanofibers under a positive gate voltage was higher than the current under reversed voltage. This showed that pure ZnO nanofibers were n-type semiconductors. Conversely, the source–drain currents of Ce-doped ZnO nanofibers were unimpeded under a negative voltage and it was difficult to pass positive currents through. The typical gate effect indicated that the Ce-doped ZnO nanofibers were p-type semiconductors. There was no obvious change when the FET curves of ZnO nanofibers were measured after 6 months, indicating that the ZnO nanofibers were stable. However, only a limited number of samples were investigated in this work and the p-type stability of Ce-doped ZnO nanofibers needs more systematic experiments and theoretical work.

Fig. 9 (a) Schematic illustration of a single pure ZnO nanofiber and Ce-doped ZnO nanofiber tested using a two-metal-microprobe. (b) FET curves of an individual ZnO nanofiber prepared via electrospinning; the red (triangles) and black (squares) curves represent pure ZnO and Ce-doped ZnO nanofibers, respectively.

3.3 Rectification effect of ZnO p–n homojunction

As the Ce-doped ZnO is a p-type semiconductor, we produced a ZnO-based p–n homojunction via electrospinning followed by calcination. Fig. 10a is a schematic diagram of the ZnO homojunction device. Rectifying behavior is an important function of p–n junctions. The I–V curves of the pure ZnO/Ce-doped ZnO homojunction were therefore investigated under illumination at different wavelengths (Fig. 10b). The ZnO homojunction fibers showed fairly good rectifying behavior. The inset of Fig. 10b shows the characteristic I–V curve of the ZnO homojunction device with no illumination. The turn-on voltage was about 15 V applied by a forward-biased voltage under no illumination; the leakage current was very small when the reverse-biased voltage was applied and can be ignored. The current was significantly increased and the threshold voltage reduced under UV illumination. The current decreased and the threshold voltage increased as the wavelength increased. The threshold voltages were 3.5 and 5.5 eV under the forward-biased voltage when the ZnO homojunction was exposed to 254 and 365 nm UV irradiation, indicating that the ZnO homojunction is easier to turn on with increasing light energy. The leakage current was small and could be ignored. The current rectification ratio for the synthesized p–n homojunction under a voltage of 30 V was about 18.05 with no illumination, whereas the rectification ratios were 55.74 and 21.48 under UV irradiation at 254 and 365 nm, respectively. The reverse current was about 0.42, 7.89 and 4.99 nA at −30 V with no illumination and under UV irradiation at 254 and 365 nm, respectively. The rectification characteristics of the ZnO p–n homojunction confirmed that Ce-doped ZnO nanofibers have a p-type conductivity as the intrinsic ZnO is a n-type semiconductor. The incorporation of Ce into ZnO may relate to a variety of models of complex defects.³⁰ Substitutional cerium (Ce_Zn) and interstitial oxygen (O_i) may form because the Zn position is occupied by the extrinsic Ce atom. The substituted cerium (Ce_Zn) is a donor with the lowest formation energy, whereas the cerium–oxygen complex defect at the Zn site (Ce_Zn + 2O_i) is an acceptor with a relatively high formation energy. Self-compensation exists in Ce-doped ZnO nanofibers due to competition between the two defects. The possible extrinsic donor (Ce_Zn) is suppressed by the potential acceptor (Ce_Zn + 2O_i) if the Fermi level is close to the bottom of the conduction band. The p-type conduction of Ce-doped ZnO nanofibers is stable only under the condition where possible donors (Ce_Zn) are suppressed under the non-equilibrium process of thermal annealing. Therefore Ce_Zn and 2O_i were amalgamated into the cerium–oxygen complex defect (Ce_Zn + 2O_i), which is an acceptor, leading to the conclusion that Ce-doped ZnO is a p-type semiconductor. Fig. 11 is a schematic illustration of the Ce doping mechanism. These results suggest that it is feasible to fabricate p-type ZnO-based optical/electric devices, for instance, UV laser diodes (LDs) and UV photodetectors (PDs).

Fig. 10 (a) Schematic diagram of the homojunction device and (b) characteristic I–V curves of pure ZnO fiber/Ce-doped ZnO fiber homojunction device under illumination with different wavelengths of UV irradiation; inset shows the I–V curve of the ZnO homojunction device with no illumination.

Fig. 11 Schematic diagram of Ce doping mechanism.

4. Conclusions

ZnO nanofibers were fabricated by electrospinning and calcination and their morphological, structural, optical and electronic properties were measured. The UV-visible absorption wavelength of the Ce-doped ZnO nanofibers was larger than that of pure ZnO. The absorption edge of the Ce-doped ZnO nanofibers was red-shifted, mainly as a result of quantum confinement and small size effects. The reduced band gap indicated that the optical properties of the Ce-doped ZnO nanofibers was improved. The ZnO fibers showed an apparent photoresponse to UV irradiation of various wavelengths and the sensitivity (I_max/I₀) was as high as 42 and 208 for the pure ZnO and Ce-doped ZnO nanofibers, respectively. The gate-effect curves suggested that the Ce-doped ZnO nanofibers were p-type semiconductors and the pure ZnO fibers were n-type semiconductors. A device based on ZnO nanofibrous p–n homojunctions was fabricated. The I–V curves of the homojunction showed good rectifying behavior, with rectification ratios of 55.74 (254 nm) and 21.48 (365 nm). The turn-on voltage for the p–n homojunction decreased under UV illumination. These results suggest a convenient method of fabricating ZnO nanofiber p–n homojunctions for applications in optoelectronic devices.

Acknowledgements

This work was financially supported by the China Postdoctoral Science Foundation (2016M592128), the National Natural Science Foundation of China (51373082, 11304173, 11404181 and 51673103) and the Taishan Scholars Program of Shandong Province, China (ts20120528).

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Footnote

† These authors contributed to this work equally.

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