Phosphorus-Doped p–n Homojunction ZnO Nanowires: Growth

Article pubs.acs.org/cm

Phosphorus-Doped p−n Homojunction ZnO Nanowires: Growth Kinetics in Liquid and Their Optoelectronic Properties Wei-Che Lee, Jui-Yuan Chen, Chun-Wei Huang, Chung-Hua Chiu, Ting-Yi Lin, and Wen-Wei Wu* National Chiao Tung University, Department of Materials Science and Engineering, Hsinchu 300, Taiwan S Supporting Information *

ABSTRACT: For wide-ranging applications in nanoscale electronic devices, durable and reproducible p-type nanostructures are essential. In this work, simple ZnO nanowire (NW) p−n homojunctions were grown using a two-step hydrothermal synthesis method. P2O5 served as a doping source to obtain p-type ZnO NWs. The morphology of the ZnO NW arrays was examined using field emission scanning electron microscopy. The high-resolution transmission electron microscopy (HRTEM) image indicated that the ZnO NW p−n homojunction is single-crystalline with a ⟨0001⟩ growth direction. The distribution of P element was analyzed using energy-dispersive spectroscopy. The dynamic growth observation was conducted using liquid in situ TEM to investigate the ZnO nucleation and growth mechanism. We divided the ZnO nanocrystal precipitation into three processes. Whether two adjacent particles grow stably or not was found to be related to the distance. Moreover, the temperaturedependent photoluminescence spectra revealed that two extra emission peaks located at 416 and 435 nm were emitted from the ZnO NW p−n homojunction, which resulted from donor−acceptor recombination. In addition, the electron transport properties confirmed the rectification behavior of the multi ZnO NW p−n homojunctions. The turn-on voltage and the current were approximately 2.8 V and 10−4 to 10−5 A, respectively, under forward bias. The results indicate the potential application of ZnO NW p−n homojunctions as nanoscale light-emitting diodes.

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ZnO nanostructures. The easily achieved conditions of the hydrothermal method also provide the opportunity to conduct dynamic observations of the growth in solution. To observe the actual route of nucleation and growth process at the nanoscale, the use of the in situ TEM technique is a good approach.11−16 Over the past years, only a few research studies of the dynamic TEM observation technique using the special liquid holder have been reported. Zheng et al. investigated the colloidal platinum nanoparticle growth trajectories,17 showing that in situ TEM enables the visualization of single nanoparticles in solution. Liu et al. characterized the nucleation and growth kinetics of zinc oxide (ZnO) precipitated from aqueous hexamethylenetetramine (HMTA) zinc nitrate (Zn-(NO3)2) solutions, observed by in situ and ex situ transmission electron microscopy.6

mong the numbers of 1-D nanostructured materials, ZnO has attracted extensive attention due to its unique properties, such as high electron mobility, wide and direct band gap (3.37 eV), and large exciton binding energy (60 meV), which lead to the good luminous efficiency and potential semiconductor-based applications of ZnO.1−3 Many reports exist of the application of ZnO NWs as field-effect transistors; light-emitting diodes; optical, chemical, and biological sensors;4,5 and solar cells; the asymmetric hexagonal wurtzite structure provides ZnO NWs with piezoelectric and thermoelectric properties,6,7 which can be applied to nanogenerators.8 ZnO nanowires have been produced by different methods, including a sol−gel process, an electrochemical reaction, thermal evaporation, laser ablation, and a hydrothermal growth process. Among these methods, the hydrothermal method is a simple, economical, and low-temperature procedure for the growth of ZnO nanostructures.9,10 In addition, an understanding of the nucleation and growth mechanisms could enable the control of the morphology and performance of ZnO nanostructures, which would benefit the further application of © XXXX American Chemical Society

Received: December 2, 2014 Revised: May 29, 2015

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DOI: 10.1021/acs.chemmater.5b01377 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

NW and a single ZnO nanoparticle. In addition, X-ray diffraction (Bruker D2phaser) was used for structural analysis, and electron energy loss spectroscopy (EELS) was performed to provide evidence for the existence of phosphorus in ZnO NWs. To study the optical property of ZnO NWs, temperaturedependent PL with a 325 nm He−Cd laser as the excitation source was used. The electrical properties were measured using semiconductor analyzers at room temperature in ambient conditions. The morphologies of well aligned ZnO NW homojunctions were examined using FESEM (field emission scanning electron microscopy). Figure 1a shows the SEM image of the first step

Many researchers have put much effort into producing ZnO with p-type electronic characteristics via doping,18−22 which can enable the development of p-type ZnO-based LEDs (lightemitting diodes) prepared by various techniques.23,24 Fang et al. reported phosphorus-doped p-type ZnO nanorods and ZnO nanorod p−n homojunction LEDs fabricated using the hydrothermal method.25 Chen et al. reported the near UV emission from p−n homojunction ZnO nanowire array LEDs fabricated using the thermal vapor deposition method without growth catalysts.26 In this work, p−n homojunctions were grown using a twostep synthesis based on the hydrothermal method. The dynamic evolution of the nanostructured ZnO nucleation and growth were observed via liquid in situ TEM. The relationship between two adjacent particles affecting the nanocrystal precipitation was discussed. We also investigated the optical properties of ZnO NWs via temperature-dependent PL using a 325 nm He−Cd laser as the excitation source; we expected that the phosphorus-doped ZnO NWs may exhibit a different energy band gap than undoped ZnO NWs. In addition, electrical devices were fabricated to confirm that phosphorusdoped ZnO NWs were p-type and to confirm the rectification behavior of the multi ZnO NW p−n homojunctions. The ZnO NWs were synthesized on a Si(100) substrate using the hydrothermal method. A 100 nm thick ZnO film that serves as the seed layer was deposited onto the Si substrate via RF sputtering. The growth precursors consisted of a 5 mM solution of zinc acetate and HMTA in DI water. The face-down substrate sputtered with ZnO thin film was suspended in solution. Subsequently, the solution containing the suspended substrate was placed into an oven at 92 °C for 12 h. After growth, the sample was removed from the solution and annealed at 800 °C for 5 min under ambient Ar. To grow the ZnO NW p−n homojunctions, 2 mM P2O5 as a dopant source was used to obtain P-doped segments, which are grown following the as-grown pure ZnO NWs. After completing the undoped ZnO nanowire synthesis, the substrate was directly transferred to new growth solution containing the P2O5 dopant source. The solution was also maintained at 92 °C for another 12 h. After completion of the growth, the sample was removed from the solution and annealed at 800 °C for 5 min under ambient Ar. The procedures of hydrothermal synthesis are shown in the Supporting Information (Figure S1). The ZnO NWs were sonicated in alcohol and then dispersed onto the prepared electrical sample. The electrical sample fabrication process is shown in the Supporting Information (Figure S2a−e). Suitable metals were deposited as the contact electrodes of the ZnO single NW electric device via a series of techniques: methyl methacrylate (bottom layer) and poly(methyl methacrylate) (top layer) were coated as positive tone photoresists (PR), followed by 180 °C baking for 1 min and ebeam lithography. After developing the exposed area by photolithography or e-beam lithography, a 30 nm thick Ni layer and a 120 nm thick Au layer were deposited as the electrodes using the electron gun deposition system. After deposition, the PR was removed by the lift-off process in acetone (Supporting Information, Figure S2f−j). The morphology of the ZnO NW array was investigated using a scanning electron microscope (JEOL JSM-6500F). A transmission electron microscope (JEOL JEM-2100F) equipped with the in situ Poseidon liquid flow TEM holder and Poseidon E-chips (Supporting Information, Figure S3) were utilized to observe the dynamic precipitation of a single ZnO

Figure 1. FESEM images of well aligned (a) pure ZnO NWs and (b) phosphorus-doped ZnO NWs grown on pure ZnO NWs. (c) Lowmagnification TEM image of a single ZnO NW homojunction. (d) EDS line scan results showing the relative concentration of P element along the NW. Panels (e) and (f) are the corresponding HRTEM images and SAED patterns of pure and phosphorus-doped segments of a single ZnO NW homojunction.

of the two-step synthesis and the undoped ZnO NWs with diameters of approximately 80−120 nm and lengths of approximately 3.1 μm. Figure 1b shows the SEM image of the two-step synthesis of ZnO NW p−n homojunctions. The P-doped ZnO NW p−n homojunctions were grown with diameters in the range of 100−180 nm and lengths of up to 3.9 μm. From the XRD analysis, the relatively strong (0001) peak demonstrated the preferred orientation of undoped ZnO NWs and P-doped ZnO NWs, which was consistent with the result of TEM. In addition, the peak at (101̅0) was detected only from P-doped ZnO. The lateral growth was more obvious at the top side due to the defects produced after adding P2O5 as a dopant source. The lateral growth at the P-doped side could be confirmed via the XRD spectrum (Supporting Information, Figure S4). B

DOI: 10.1021/acs.chemmater.5b01377 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials HRTEM and selected area electron diffraction (SAED) were utilized to obtain more details regarding the crystal structure of the single ZnO NW homojunction. The low-magnification TEM image of a single ZnO NW homojunction is shown in Figure 1c. The P-doped segment was larger in diameter and had a rough surface compared to the undoped segment. In Figure 1d, TEM line scan provided the relative concentration of the P element along the NW. An obvious step at the relative length of 6 μm indicated that the amount of phosphorus drastically increases at the p−n interface. This result was confirmed in the EDS (energy dispersive spectroscopy) profile, as shown in the Supporting Information (Figure S5). It is difficult to determine the existence of P element in the undoped segment; nevertheless, the concentration of P drastically increased up to 2.81% in the P-doped segment. For detection of the relatively light atomic weight element, EELS measurements were performed to further identify the appearance of the phosphorus. The extracted signal revealed a P L-edge peak at 131.8 eV, which is shown in the Supporting Information (Figure S5). The structural analysis of the ZnO NW p−n homojunction was investigated using a high-resolution transmission electron microscope. Panels (e) and (f) in Figure 1 are HRTEM images of undoped and P-doped segments, respectively, which indicated that the entire single NW homojunction is of wurzite structure and single-crystalline ZnO with ⟨0001⟩ as the growth direction. However, some differences exist between the two segments. First, the d-spacing corresponding to the (0001) plane of the P-doped segment (0.510 nm) was smaller than that of the undoped segment (0.514 nm). The interplanar spacing was obtained by determining the total length of 20 layers and dividing the total length by 20. Although the third decimal place was not the accurate value of the real spacing due to the system deviation, it is meaningful because we are comparing the difference between the P-doped segment and the undoped segment. The reason for the difference in interplanar spacing was that phosphorus atoms occupying zinc sites would introduce a lattice relaxation due to the shorter bonding length of PZn−O (0.168 nm) than that of Zn−O (0.193 nm).27,28 In addition to the lattice spacing being reduced in the P-doped segment, defects, such as oxygen vacancies, increased at the same time. The influence of lattice relaxation and defects in the P-doped segment resulted in the distortion of the diffraction points and contributed to the extra spot in the SAED pattern inserted in Figure 1f. The mechanism of the hydrothermal method has been discussed for a long time. However, few studies have provided direct evidence for the evolution of nucleation and images of ZnO nanoparticle growth from a 5 mM zinc acetate and 5 mM HMTA solution in a liquid cell (also see Video S1, Supporting Information). We broadly divided the ZnO nanocrystal (Video S1). We broadly divided the ZnO nanocrystal precipitation into three processes: (1) solute fluctuation, (2) crystal precipitation, and (3) growth of the nanocrystal. To investigate the nucleation and growth mechanism, we used a TEM instrument equipped with the in situ Poseidon liquid flow TEM holder and Poseidon E-chips to conduct the observation of ZnO nanocrystal precipitation from solution. Figure 2 shows a series of in situ TEM images. Panels (a) and (b) in Figure 2 are determined to be the solute fluctuation process. The contrast variation indicates that the solute is not uniformly distributed in the solution. Before nucleation, we observed that the partial regions started to

Figure 2. A series of in situ TEM images of ZnO nanoparticles grown from a 5 mM zinc acetate and 5 mM HMTA solution in a liquid cell. The ZnO nanocrystal precipitation is broadly divided into three processes: (a, b) solute fluctuation, (c) crystal precipitation, and (d−f) crystal growth. The number labels on the top-right-hand corners of the figures are timestamps (minute:second).

fluctuate in the solution, thereby causing the contrast variation observed on the screen. The electron beam played an important role in supplying the thermal energy, while the solute in the region exposed to the beam would be active. Next was the crystal precipitation process, in which the fluctuating situation was aggravated and then suddenly stopped while the ZnO crystals began to precipitate. In Figure 2c, the sizes of the precipitates were all smaller than 40 nm. We considered these crystals as stable nuclei. After the formation of nuclei, fluctuation occurred again around the nuclei; as a result, the ZnO nanocrystals grew from 40 nm to the maximum size of approximately 150−160 nm. We refer to this situation as the crystal growth process, as shown in Figure 2d−f. The solution concentration is a restriction on the maximum particle size. In the late stage of the crystal growth process, the fluctuation around the nuclei decreases, resulting in the solutes near the grown ZnO nanocrystal being insufficient to enable further growth. The ZnO nanocrystal growth in a liquid cell is a diffusion controlled process. The bright (white) contrast regions in Figure 2 are bubbles, which may be generated via exposure or heating due to the electron beam that cause the solution to dry out. C

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Chemistry of Materials The reaction under electron beam irradiation was complex.29 Although electron beam effects are pronounced in many reports, we consider that the electron beam provided energy to heat the solution, whereas the increasing temperature resulted in the hydrothermal reaction. We consider the electron beam as a minor factor in the reaction for the following reasons: (1) The electron beam radiation was local and continuous. However, we can find the ZnO crystals generated elsewhere in the nonirradiated region, and the reaction was discontinuous, unlike the electron beam exposure. (2) The supply of electrons assists the reduction and hinders the hydrothermal reaction, which suppresses the reactions of Zn(OH)2 according to Le Chatelier’s principle.

seems that whether two adjacent particles grow stably or not was related to the distance between them. By comparing Figures 2 and 3, we found that the critical length of two adjacent completely grown ZnO nanoparticles is approximately 0.3 μm. Figure 3e plots the change in the ZnO nanoparticle size as a function of time. According to the growth rate, we divided the crystal growth process into two stages. The slope at stage 1 was greater than that at stage 2. This result confirmed that stage 2, which lacked solute, would influence the growth rate, leading to the diffusion controlled process. In addition, we also found that ZnO nanoparticles prefer axial growth in some specific situations. Figure 4 shows a series of in situ TEM images of ZnO nanowire growth trajectories (also see Video S3, Supporting

Zn(OH)2 + 2e− ⇌ Zn + 2OH−

Zn(OH)2 ⇌ ZnO + H 2O

Figure 3a−d shows a series of in situ TEM images of ZnO nanoparticles grown from a 5 mM zinc acetate and 5 mM

Figure 4. (a−d) A series of in situ TEM images of ZnO nanowire growth trajectories. The number labels on the top-right-hand corners of the figures are timestamps (minute:second).

Information). A ZnO nanoparticle grew to a nanowire initially 20 nm in length. This anisotropic growth phenomenon can occur under the appropriate growth conditions. The TEM line scan and SAED patterns demonstrated that phosphorus was actually doped into the ZnO structure; we also investigated the optical properties of the ZnO nanostructures using temperature-dependent PL (photoluminescence) with a 325 nm He−Cd laser as the excitation source. We expected that the phosphorus-doped ZnO NWs may exhibit a different energy band gap than the undoped ZnO NWs. The results of the temperature-dependent PL spectra indicated that both phosphorus-doped and pure ZnO NWs exhibited a UV light emission at 370−380 nm and a defect-related emission at 400− 750 nm. The intensity of the defect-related emission peak was stronger in the phosphorus-doped ZnO NWs, which was caused by the phosphorus doping introducing more oxygen vacancies into the ZnO structure. This phenomenon is more obvious at low temperature due to the reduced interference from thermal vibrations. Moreover, a slight blue-shift of the UV light emission in the P-doped ZnO NW was observed (Supporting Information, Figure S6). Figure 5c shows the PL spectra of ZnO NW p−n homojunctions. Remarkably, two extra peaks located at 416 and 435 nm were discovered from the ZnO NW p−n homojunctions. The extra peaks were attributed to the difference in the Fermi level between the n-type ZnO and the p-type ZnO. After

Figure 3. (a−d) A series of in situ TEM images of ZnO nanoparticles grown from a 5 mM zinc acetate and 5 mM HMTA solution in a liquid cell. Two ZnO nanoparticles stably grow at the same time. (e) The change in particle size as a function of time. The number labels on the top-right-hand corners of the figures are timestamps (minute:second).

HMTA solution in a liquid cell (see Video S2, Supporting Information). Some smaller particles were clearly dissolved completely and formed stable nuclei. The difference from the growth situation in Figure 2 was that there are two ZnO nanocrystals that stably grew in the same window. In Figure 3b, a few particles were located adjacent to particle x, but they did not grow larger, whereas particles x and y did grow larger. It D

DOI: 10.1021/acs.chemmater.5b01377 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 6. (a) Schematic of the single P-doped ZnO NW electrical device. (b) The electron transport property of a P-doped ZnO NW FET confirmed that the NW was p-type with low resistivity (0.0132 Ω· cm). (c) Schematic of the electrical device based on multi ZnO NW p−n homojunctions. (d) The I−V curve shows the rectification characteristics of the multi ZnO NW p−n homojunctions.

In summary, ZnO NW p−n homojunctions were successfully synthesized via a two-step hydrothermal method. P2O5 served as a dopant source to obtain p-type ZnO NWs. TEM analysis indicated that the entire single ZnO NW homojunction is wurtzite and single-crystalline ZnO with growth occurring in the ⟨0001⟩ direction, while the line scan revealed that the relative concentration of P element along the NW drastically increased at the interface. We also utilized liquid in situ TEM along with the in situ Poseidon liquid flow TEM holder and Poseidon E-chips to dynamically observe precipitation and growth of the ZnO nanostructures, which could broadly be divided into three processes: (1) solute fluctuation, (2) crystal precipitation, and (3) crystal growth. In the in situ TEM liquid cell, the ZnO particles finally grew to approximately 150−160 nm; in addition, the critical length of two adjacent stably grown ZnO nanoparticles was found to be approximately 0.3 μm. The plot of the change in particle size as a function of time exhibited a decrease in the growth rate, which suggested that the reaction was diffusion-limited. Moreover, analysis of the temperaturedependent photoluminescence (PL) spectra revealed not only UV light emission (370−380 nm) and defect-related emission (400−750 nm) but also two extra peaks located at 416 and 435 nm; these extra peaks were observed from ZnO NW p−n homojunctions and are due to donor−acceptor recombination. Finally, the electron transport properties of a single phosphorus-doped ZnO NW confirmed p-type conductivity with low resistivity (0.0132 Ω·cm). Furthermore, the I−V curve of field effect transistors (FETs) revealed the rectification behavior in the multi ZnO NW p−n homojunctions. The turnon voltage and the current are approximately 2.8 V and 10−4 to 10−5 A, respectively, under forward bias; the current is nearly identical to the operating current of a commercial LED, highlighting the significant potential of ZnO NWs in LED applications.

Figure 5. PL properties of (a) pure, (b) P-doped, and (c) p−n homojunction ZnO NWs. Panel (c) reveals that there are two extra peaks located at 416 and 435 nm due to donor−acceptor recombination at the twisted energy band gap at the interface.

contacting both p-type and n-type ZnO NWs, the energy band gap at the interface should be twisted to satisfy the same Fermi level to reach equilibrium. These two extra peaks were due to donor−acceptor recombination from the twisted energy band gap at the interface of the p−n homojunction.25,30 In addition to donor−acceptor recombination, some research revealed that the emission at 435 nm is related to the impurity-induced defects due to dopant incorporation in the p-type ZnO NWs.31 To obtain the electrical performance of P-doped ZnO NWs, single-NW based field-effect transistors (NWFETs) were fabricated to study the electrical transport properties. Figure 6a shows the schematic of the single P-doped ZnO NW electrical device. In Figure 6b, the electron transport property of the P-doped ZnO NW FET confirmed that the NW was ptype with a low resistivity of approximately 0.0132 Ω·cm, which is lower than that of a pure ZnO NW.4 In addition to the extra peaks observed in the PL measurement, a successful ZnO NW p−n homojunction should exhibit rectification characteristics. The electrical device based on p−n homojunctions was fabricated, as schematically shown in Figure 6c, to determine the electrical performance. In Figure 6d, the I−V curve shows the rectification behavior of the multi ZnO NW p−n homojunctions. The turn-on voltage and the current are approximately 2.8 V and 10−4 to 10−5 A, respectively, under forward bias, which was nearly identical to the operating current of a commercial LED.

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ASSOCIATED CONTENT

S Supporting Information *

The method of synthesizing ZnO nanowires, electrical sample fabrication, schematic of a liquid cell, XRD spectrum, EDS profile, and UV emission. In addition, three in situ TEM videos E

DOI: 10.1021/acs.chemmater.5b01377 Chem. Mater. XXXX, XXX, XXX−XXX

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(AVI) are included. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01377.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.-W.W.). Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS We acknowledge the support by the Ministry of Science and Technology through grants 103-2221-E-009-056-MY2 and 1032221-E-009-222-MY3.

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DOI: 10.1021/acs.chemmater.5b01377 Chem. Mater. XXXX, XXX, XXX−XXX