Low-Temperature Facile Synthesis of Sb-Doped p-Type ZnO

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Low-Temperature Facile Synthesis of Sb-doped p-type ZnO Nanodisks and its Application in Homojunction Light-Emitting Diode Sung-Doo Baek, Pranab Biswas, Jong Woo Kim, Yun Cheol Kim, Tae Il Lee, and Jae-Min Myoung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03258 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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ACS Applied Materials & Interfaces

Low-Temperature Facile Synthesis of Sb-doped ptype ZnO Nanodisks and its Application in Homojunction Light-Emitting Diode Sung-Doo Baek,a Pranab Biswas,a Jong-Woo Kim,a Yun Cheol Kim,a Tae Il Lee,b and Jae-Min Myoung*a a

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro,

Seodaemun-gu, Seoul 120-749, Republic of Korea b

Department of BioNano Technology, Gachon University, 1342 Seongnam Daero, Seongnam,

Republic of Korea *E-mail: [email protected] KEYWORDS: Sb-doped ZnO, p-type ZnO, ZnO homojunction, low-temperature solution process, light emitting diode.

ABSTRACT This study explores low-temperature solution-process-based seed-layer-free ZnO p-n homojunction light-emitting diode (LED). In order to obtain p-type ZnO nanodisks (NDs), antimony (Sb) was doped into ZnO by using a facile chemical route at 120 °C. The x-ray

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photoelectron spectra indicated the presence of (SbZn–2VZn) acceptor complex in the Sb-doped ZnO NDs. Using these NDs as freestanding templates, undoped n-type ZnO nanorods (NRs) were epitaxially grown at 95 °C to form ZnO p-n homojunction. The homojunction with a turnon voltage of 2.5 V was found to be significantly stable up to 100 s under a constant voltage stress of 5 V. A strong orange-red emission was observed by the naked eye under a forward bias of 5 V. The electroluminescence spectra revealed three major peaks at 400, 612, and 742 nm which were attributed to the transitions from Zni to VBM, Zni to Oi, and VO to VBM, respectively. Presence of these deep level defects was confirmed by the photoluminescence of ZnO NRs. This study paves the way for future applications of ZnO homojunction LEDs using low-temperature and low-cost solution processes with the controlled use of native defects.

INTRODUCTION Recently, solid-state lighting has revolutionized and replaced conventional incandescent lamps and fluorescent tubes; therefore various direct-bandgap semiconductors have been considered as promising candidates for light-emitting diode (LED) applications.1–4 Among those, ZnO has attracted significant attention owing to its versatile advantages such as a direct and wide bandgap (3.37 eV) with a large exciton binding energy (~60 meV), morphological diversity, and low-cost production.5–7 However, because of its native donor defects, it is challenging to achieve p-type conductivity and to develop a p-n homojunction.8 Many studies have been carried out on ZnO heterojunction-based LEDs by coupling them with other p-type materials, e.g., p-GaN, p-Si, and p-type organic materials.9–11 However, a homojunction LED with an epitaxial interface is always preferred to achieve better light-emitting efficiency and power consumption. Homojunction devices, comprising the same


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material with p- and n-type conductivity and having perfectly matched crystal lattice, can preserve the lattice periodicity at the interface with minimum trap centers. In contrast, heterojunction devices induce a lattice mismatch at the interface between the component materials, producing strains that give rise to interfacial defects and deteriorate device performance. A considerable number of studies have reported on realizing ZnO homojunction LEDs.12–15 However, most of these studies are based on vacuum processes that require very high temperatures (>500 °C). High temperature facilitates the incorporation of the dopant into the crystal lattice; this may cause an unclear interface of the homojunction owing to the interdiffusion of dopants.16 Moreover, high-temperature processes preclude the use of flexible substrates because they are unable to endure high temperatures. This sets limitations on their applications in flexible devices. Unlike the vacuum processes, solution processes have the special advantage of low-temperature processability that significantly reduces the cost. Thus, progressive demands for low-cost and flexible LEDs encourage researchers to use cheap and abundant materials that can be synthesized by using low-temperature solution methods. Three different studies have been carried out on solution processed ZnO homojunction LED. Nguyen et al. reported ZnO coaxial nanorods (NRs)-based homojunction ultraviolet (UV) LEDs by using potassium (K)-doped p-ZnO.17 Here, n-type ZnO NRs were grown on n-GaN substrates, which was followed by the growth of coaxial K-doped p-ZnO NRs. This approach employed the expensive GaN template that was grown at 1020 °C on sapphire substrates. In another report, Fang et al. fabricated phosphorous (P)-doped p-ZnO NRs/n-ZnO NRs homojunction LEDs with the help of an additional n-type ZnO film.18 Here, the researchers applied a high annealing temperature of 800 °C and moreover, they did not observe any light emission from the homojunction LEDs. Hsu et al. reported on undoped p-type ZnO by controlling native defects


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with the help of a seed layer and applied it to fabricate a ZnO homojunction LED by coupling it with electrodeposited n-type ZnO.19 However, An additional seed layer may seriously affect light emission by absorbing and scattering the emitted photons. In a typical hydrothermal method, zinc ions are hydrolyzed and nucleated under a supersaturated state of the aqueous solution resulting in various nanostructured ZnO crystals. Although low-temperature solution processes are very simple, there is still an inherent limitation regarding the initiation of crystal growth. Thus, in order to grow single-crystal ZnO using lowtemperature solution methods, it is helpful to use a seed material such as ZnO or GaN.20,21 However, using a seed material is cumbersome because the growth parameters of ZnO absolutely follow the conditions of a pre-deposited seed material such as solution concentration, layer thickness, and post-annealing temperature.22–25 Moreover, inserting an additional seed layer in an LED may deteriorate the performance of the device by incorporating additional interfacial trap states. Therefore, to achieve a high-performance ZnO homojunction LED, seed-layer-free epitaxial growth of n-type and p-type ZnO nanocrystals (NCs) must be accomplished at a considerably low process temperature. In this paper, we demonstrate a facile and effective way to fabricate a ZnO p-n homojunction LED. By using a low temperature hydrothermal method antimony (Sb)-doped p-type ZnO nanodisks (NDs) were synthesized. These ZnO NDs were used as a template for the seed-layerfree epitaxial growth of n-type ZnO NRs. The fabricated ZnO homojunction LED exhibited a clear p-n junction rectifying behavior with a bright orange-red light emission.


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EXPERIMENTAL SECTION Synthesis of Sb-doped p-type ZnO NDs. The Sb-doped ZnO NDs were synthesized hydrothermally using a teflon-lined stainless steel autoclave with a capacity of 50 mL. Here, zinc acetate dihydrate (2.5 mmol) and trisodium citrate dihydrate (0.8 mmol) were used as the zinc precursor and the structure-directing agent, respectively. Antimony (III) acetate [Sb(CH3CO2)3] was selected as the precursor of Sb (p-type dopant), and its atomic concentration was controlled to be 0.5, 1, 1.5, and 2 at.% to that of Zn. All the above reagents were separately dissolved in 10 mL of deionized (DI) water (18.25 MΩ, Millipore), and then they were mixed together. The final volume of the aqueous solution was made up to 40 mL. Then 2 M of NaOH was slowly added until the pH of the prepared solution became 9.0 based on measurements using a pH meter (SP2300, Suntex). All chemicals were supplied by Sigma Aldrich. The 40 mL of final solution was transferred into the autoclave reactor and kept in an oven at 120 °C for 8 h. After the reaction, the autoclave was cooled down to room temperature naturally. A white precipitate composed of Sb-doped p-type ZnO NDs was collected by a centrifuge and rinsed with DI water and absolute ethanol for three consecutive times. Synthesis of n-type ZnO NRs. To synthesize the undoped n-type ZnO NRs on a monolayer of Sb-doped ZnO NDs, a conventional hydrothermal method was adopted with an optimized temperature of 95 °C and a reaction time of 4 h.26,27 The Sb-doped ZnO NDs arrays were immersed in a chemical bath containing an aqueous solution of 20 mm of zinc acetate dihydrate with an equal concentration of hexamethylenetetramine (HMTA). The reaction was carried out under optimized conditions. After the reaction was complete, the samples were rinsed in DI water and dried on a hot plate at 110 °C for 10 min.


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Device fabrication. The glass substrates were cleaned by sonication in acetone, methanol, and DI water sequentially for 10 min each. Then a 100 nm-thick Au layer for the bottom contact was deposited on the cleaned substrates by using an electron-beam evaporator. In order to obtain a Sb-doped p-type ZnO NDs array, an alcohol-assisted monolayer assembly technique developed by our group was used.28,29 Here, a monolayer of p-type ZnO NDs was formed at the air-water interface and subsequently transferred onto the Au-coated glass substrates. Next, the samples were annealed at 250 °C for 1 h on a hot plate in air ambiance. Then, an AZ 5214E photoresist (PR) was spin-coated to fill the gaps between the p-type ZnO NDs. This PR layer on the p-type ZnO NDs was dry-etched by using oxygen plasma of 150 W in a reactive ion etching (RIE) chamber. Next, n-type ZnO NRs were grown on the arrayed p-type ZnO NDs homoepitaxially according to the procedure mentioned above. The PR molding and etching processes were repeated to deposit the top-contact electrode. Finally, an indium zinc oxide (IZO)-Ag-IZO electrode composed of an oxide-metal-oxide (OMO) multilayer with a corresponding thickness of 40-12-40 nm was deposited by using a dot-patterned shadow mask with a diameter of 550 µm inside the in-situ sputtering at room temperature. Characterization and measurement. The surface morphologies of both p-type ZnO NDs and n-type ZnO NRs were characterized by a scanning electron microscope (SEM) (S-5000, Hitachi). The structural properties of the materials were investigated by using x-ray diffraction (XRD) (SmartLab, Rigaku) and a high-resolution transmission electron microscope (HRTEM) (JEMARM 200F, JEOL) with a focused ion beam system (JIB-4601F, JEOL). The origin of the p-type conductivity of Sb-doped ZnO NDs was studied by using x-ray photoelectron spectroscopy (XPS) (K-alpha, Thermo U.K.). The Sb-doping profile in the p-type ZnO was examined by using timeof-flight secondary ion mass spectra (TOF-SIMS) (TOF SIMS 5, ION TOF). The atomic


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percentage of Sb was verified by using energy-dispersive spectra (EDS) (JEM-ARM 200F, JEOL). The electrical properties of the n-ZnO NRs/p-ZnO NDs homojunction were analyzed by using

a

semiconductor

parameter

analyzer

(B-1500A,

Agilent

Technologies).

The

electroluminescence (EL) spectroscopy from this p-n homojunction was measured by using Andor SOLIS simulation software combined with a charge-coupled device (CCD) camera (DV401A-BV). To investigate the optical properties of both p-ZnO NDs and n-ZnO NRs, roomtemperature photoluminescence (PL) spectroscopy was performed in a μ-PL measurement system (Dongwoo Optron) using an IK3252R-E He-Cd laser (λ = 325 nm) coupled with a MonoRa 320i monochromator and an Andor SOLIS simulation package.

RESULTS AND DISCUSSION In order to achieve p-type conductivity in ZnO, Sb was doped by using a low temperature facile chemical route with little change in the existing methods.30,31 Here, citrates were used as a structure-directing agent, which are strongly adsorbed on a (0002) facet of ZnO and suppress the crystal growth along the [0002] direction, resulting in lateral or in-plane growth.32,33 Thus, a disk shape can be obtained by using citrates. In order to apply Sb-doped ZnO as a freestanding template for seed-layer-free growth of n-type ZnO NRs, the shape of ZnO was optimized by varying concentrations of [Sb(CH3CO2)3]. The atomic percentage of Sb was varied from 0.5 to 2 at.% to that of Zn. In solution methods, doping is carried out by adsorption of dopants onto the surface of the growing host material and being buried by it. The doping of Sb in ZnO NDs is proposed as: 𝑆𝑆𝑆𝑆 3+ + 2𝐻𝐻𝐻𝐻 − + 2𝑂𝑂𝑂𝑂 − ↔ 𝑆𝑆𝑆𝑆𝐶𝐶2 − + 2𝐻𝐻2 𝑂𝑂

(1)


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where, HC- denotes citrate ion and SbC2- denotes Sb-citrates complex. Upon introducing citrates into a reaction system containing Sb ions, HC- produces SbC2-. This complex is adsorbed on the surfaces of growing crystals of ZnO, which is subsequently followed by desorption of HCleaving behind Sb ions on the ZnO-surface. These Sb ions are eventually buried in the host lattice of the growing ZnO. A SEM image of the 0.5 at.% Sb-doped ZnO NDs revealed a clear ND shape with an average diameter of 650 nm, as shown in Figure 1a, whereas the rest were found to be of a bumpy rod shape with rough surfaces (Figure S1a–c). As the concentration of Sb increased, the shape changed gradually from NDs to NRs. It is assumed that increasing the concentration of Sb precursor adds more CH3COO‒ into the solution and changes its pH, which ultimately modifies the shape of ZnO. To confirm the crystallinity of Sb-doped ZnO NCs with different concentrations of Sb precursor, XRD analyses were carried out as shown in Figure S1d. The 0.5 at.% Sb-doped ZnO NDs exhibited a predominant (0002) peak. As the concentration of Sb precursor increased, this peak became weak, indicating degradation of the crystallinity. In addition, the Sb concentration and distribution of Sb-doped ZnO were measured by EDS and TOF-SIMS, as shown in Figures S2 and S3, respectively. The EDS results revealed that the Sb at.% increased linearly as the concentration of Sb precursor increased, implying the successful incorporation of Sb atoms in the ZnO NCs. TOF-SIMS results also showed a uniform depth profile of Sb atoms in the ZnO NCs. Even though the ZnO NCs doped with higher concentrations of Sb precursor had a higher Sb at.% with a uniform distribution of Sb atoms, they exhibited poor crystallinity along the c-axis. However, in order to use ZnO NCs as a template for the growth of a homoepitaxial p-n junction, the crystallinity along the c-axis is the most important factor because undoped n-type ZnO NRs are predominantly grown along the c-


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axis. Therefore, in this study, 0.5 at.% Sb-doped ZnO NDs were chosen as a template material for the epitaxial growth of undoped n-type ZnO NRs. The structural and optical properties of the optimized 0.5 at.% Sb-doped ZnO NDs were investigated by using a HRTEM, XRD, and room temperature PL. Figures 1b and c show the TEM images of the NDs, confirming a predominant crystallinity along the [0002] direction. Figure 1c shows the HRTEM image revealing a clear lattice fringe with a spacing of 0.26 nm, which corresponds to the distance between two adjacent (0002) planes. The inset of Figure 1c shows the selected-area electron diffraction (SAED) pattern where a single crystallinity of the NDs prevails along the c-axis, which affirms its potential as a template for the growth of ZnO NRs. The XRD patterns of the 0.5 at.% Sb-doped NDs also confirm a predominant growth of the NDs along the c-axis, as shown in Figure 1d. In addition, room-temperature PL spectra exhibit a strong near-bandedge emission (NBE) at 372 nm with a weak deep-level emission (DLE), as shown in Figure 1e. Thus, it is believed that the facilely synthesized Sb-doped ZnO NDs have an insignificant number of deep-level defects. In order to identify the binding states of the Sb atoms in the ZnO lattice, XPS analyses were performed. The full-scan XPS spectra of the 0.5 at.% Sb-doped ZnO NDs exhibited peaks exclusively related to Zn, O, and Sb, confirming the chemical purity of the NDs, as shown in Figure 2a. Figure 2b shows the narrow-scan XPS spectra of Zn 2p with a spin-orbital splitting (SOS) value of 23 eV between Zn 2p3/2 and Zn 2p1/2. To determine the presence of Zn-related defect states, Zn 2p3/2 was deconvoluted according to the asymmetry of the peak, as shown in the inset of the figure. The lofty peak at 1023.3 eV can be attributed to the Zn bonded with O atoms in the ZnO lattice, whereas the peak at 1022.3 eV is a result of the presence of Zn vacancies (VZn).34 The core-level XPS spectra related to O 1s and Sb 3d5/2 overlapped, as shown in Figure


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2c. The eminent peak was deconvoluted into two peaks. The major peak at 530.1 eV is assigned to the O atoms bonded with Zn in the ZnO lattice.35 The adjacent shoulder peak at 531.6 eV is attributed to the presence of Sb atoms that substituted for Zn and bonded to O atoms.36,37 However, as the Sb 3d5/2 peak overlapped with O 1s, further clarification was required. Thus, the narrow-scan XPS spectra of Sb 4d were also provided, as shown in Figure 2d. The figure shows a prominent peak at 34.7 eV, which is attributed to Sb (III) atoms bonded with O atoms in the ZnO lattice.38,39 The absence of metallic Sb in the XPS spectra reveals the proper incorporation of the dopants into the ZnO lattice by the low-temperature solution method. Therefore, the XPS analyses evidently indicate the formation of an (SbZn–2VZn) acceptor complex resulting in p-type conductivity in the Sb-doped ZnO NDs.40 The existing reports also strongly support the p-type conductivity owing to (SbZn–2VZn) acceptors in the Sb-doped ZnO.40–42 The large sizemismatched Sb atom replaces one Zn atom and subsequently creates two Zn vacancies to become stable in the ZnO lattice system. The formation of the complex can be expressed by the following steps:

𝑆𝑆𝑆𝑆𝑍𝑍𝑍𝑍 3+ + 𝑉𝑉𝑍𝑍𝑍𝑍 2− → (𝑆𝑆𝑆𝑆𝑍𝑍𝑍𝑍 − 𝑉𝑉𝑍𝑍𝑍𝑍 )+

(𝑆𝑆𝑆𝑆𝑍𝑍𝑍𝑍 − 𝑉𝑉𝑍𝑍𝑍𝑍 )+ + 𝑉𝑉𝑍𝑍𝑍𝑍 2− → (𝑆𝑆𝑆𝑆𝑍𝑍𝑍𝑍 − 2𝑉𝑉𝑍𝑍𝑍𝑍 )−

(2) (3)

By using a monolayer of Sb-doped p-type ZnO NDs as a freestanding template for the growth of undoped n-type ZnO NRs, a p-n homojunction LED was fabricated. The schemes in Figure 3 show the fabrication steps with corresponding SEM images. First, a monolayer of Sb-doped ZnO NDs was prepared at the air-water interface and subsequently transferred onto the 100 nm-thick


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Au-coated glass substrates, as depicted in Figure 3a.28,29 The solution of p-type ZnO NDs dispersed in 1-butanol was slowly injected dropwise into a petri dish containing DI water. As the solution was spread onto the water surface, a two-dimensional monolayer of p-type ZnO NDs was spontaneously formed at the air-water interface. Then, this array was carefully transferred onto the Au-coated glass substrates, followed by post-annealing at 250 °C for 1 h on a hot plate in air ambiance to form an intimate contact between the Au electrode and the p-type ZnO NDs. Figure 3b shows the self-assembled array of NDs on the substrates after transferring, which is represented by a top-view SEM image. Next, the gaps between the NDs were filled with non-conducting PR as an encapsulating agent to prevent any electrical shorts between the Au and n-type ZnO. The PR on the top of the NDs was dry-etched by reactive oxygen ions in order to grow ZnO NRs. Figure 3c and the corresponding tilted-view SEM image represent the PR-molded array of Sb-doped ZnO NDs after dry etching. Then, undoped n-type ZnO NRs were grown on the exposed surface of the ZnO NDs array at 95 °C for 4 h in order to achieve a p-n homojunction. The scheme and the corresponding tilted-view SEM image after the growth of n-type ZnO NRs are shown in Figure 3d. In the next step, the PR molding and etching processes were repeated to deposit the top electrode. Figure 3e and its corresponding SEM image show the samples with the exposed surface of the ZnO NRs before deposition of the top-electrode material. In order to fabricate a high-performance surface-emission LED, a transparent IZO-Ag-IZO OMO electrode was used as the top-electrode material. An excellent transmittance of 87.7 % in the visible region with a low sheet resistance of 5.65 Ω/□ was achieved after optimizing the thickness of the IZO-Ag-IZO layers at 40-12-40 nm. The details of the optimization and characterization were described in our


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previous study.43 Figure 3f represents the complete n-ZnO NRs/p-ZnO ND homojunction structure with the transparent top OMO electrode. The structural and optical properties of the as-grown ZnO NRs are shown in Figure 4. The cross-sectional SEM image reveals that n-type ZnO NRs with a length of 2.7 μm were grown vertically on the monolayer of Sb-doped p-type ZnO NDs, as shown in Figure 4a. The inset of the figure shows the top-view SEM image, indicating a uniform and dense growth of the ZnO NRs on the Sb-doped p-type ZnO NDs. The structural properties of the interface between the nZnO NRs and the p-ZnO NDs were investigated by using TEM. Figure 4b shows the lowmagnification TEM image of the interface. Figure 4c exhibits the magnified HRTEM image of the highlighted part from Figure 4b. The continuous lattice fringe at the interface confirmed the epitaxial growth of n-type ZnO NRs along the [0002] direction on the Sb-doped p-type ZnO NDs. The measured interplanar lattice spacing of 0.51 nm in the ZnO NRs indicates that they are grown along the [0001] direction, which is in accordance with the orientation of Sb-doped ZnO NDs. The inset of Figure 4c is the SAED pattern of the interface exhibiting the epitaxial nature of the homojunction. It is noteworthy that homoepitaxy has a certain edge over heteroepitaxy or a physical junction, as the latter induces numerous trap centers in terms of defects, which deteriorate the device performance by hindering the flow of carriers. In particular, for an LED, continuous lattice periodicity at the interface is crucial because it functions in the depletion region. In order to take advantage of the waveguide nature of the NRs, vertical growth with single crystallinity is highly anticipated.44 The XRD pattern also confirms that ZnO NRs were predominantly grown along the [0002] direction, as shown in Figure 4d. The superior structural properties of ZnO NRs demonstrate the potential of Sb-doped ZnO NDs as a template for the


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growth of the NRs. Room-temperature PL spectra of the ZnO NRs reveal a weak peak at 382 nm, which is related to NBE with a broad eminent peak at 620 nm related to DLE, covering most of the visible region as shown in Figure 4e. The high-intensity and broad-visible-range DLE indicate the presence of native defects in the ZnO NRs.47 Thus, it is believed that, being located deep into the bandgap, the native point defects facilitate charge transition to the defect levels instead of direct band-to-band transition. The current density-voltage (J-V) characteristics in Figures 5a and b demonstrate the feasibility of OMO and Au electrodes for ohmic contacts with as-grown n-type ZnO NRs and Sb-doped p-type ZnO NDs, respectively. The insets of the figures represent schemes of the corresponding electrode-semiconductor-electrode (ESE) structures. Both J-V curves indicate the ohmic nature of the electrode-semiconductor contacts. It is noteworthy that the ohmic contact between the Au and Sb-doped ZnO NDs evidently confirms the p-type conductivity of the NDs. In addition, the high current level of the ESE structures (Figure 5b and c) reflects the low contact resistance of the electrode-semiconductor interface. Figure 5c shows the rectifying J-V characteristics of the 0.5 at.% Sb-doped p-ZnO NDs/n-ZnO NRs homojunction diode with a turn-on voltage of approximately 2.5 V. The inset of the figure shows a schematic of the device under dc forward bias. The measured current level of the device was observed to be significantly high with a current density of 18.66 Acm-2 at 4 V. The EL spectra of the 0.5 at.% p-ZnO NDs/n-ZnO NRs homojunction LED under different forward bias is shown in Figure 6a. As the forward bias was increased from 2 V to 5 V, the EL intensity was also increased linearly, indicating an increased flow of charge carriers that facilitates more radiative recombinations.44 The deconvoluted EL spectra at a forward bias of 5 V are shown in Figure 6b, which contains four peaks: (i) a peak with a high-energy emission at


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378 nm, (ii) a peak at 400 nm, (iii) a peak at 612 nm corresponding to orange emission, and (iv) a peak at 742 nm related to red emission. Light-emitting images from the LEDs under different forward bias (2, 3, 4, and 5 V) is exhibited in Figure 6c. The inset of the figure shows an optical microscopic (OM) image of the transparent top OMO electrode with a diameter of 550 μm on the LED. The emitted orange-red light was visible to the naked eye and was also very stable. In accordance with the EL spectra, the brightness of the light was also increased with increasing dc forward bias. This was a result of the larger number of photon emission. The luminance of the LED was measured to be 1.73 cd/m2 under a dc forward bias of 5 V and the corresponding current efficiency was calculated to be 2.75×10-6 cd/A. Notably, no similar study in the framework of low-temperature solution-processed ZnO nanostructures-based homojunction LED is reported that provides brightness and efficiency.17–19 However, the performance of the current LED in comparison to that of the quantum dots-based45 and organic materials-based46 LEDs was found to be inferior, which was attributed to the dot-like emission. The discontinuous, dot-like emission was attributed to the spatial distribution of the ZnO NDs and NRs along with their height difference. After the formation of p-type ZnO ND monolayer on Au-coated glass substrates, inter-NDs gaps were appeared which were further transferred in n-type ZnO NRs (Figure S4). In addition, during PR-molding the shorter NDs and NRs were covered and those were not in direct contact with NRs and OMO electrode, respectively. Therefore, the height difference and inter-gaps between the adjacent nanostructures resulted in discontinuities in the pn junction producing dot-like emission. Figure 6d reveals the results of a stability test of the LED under various forward bias from 2 V to 5 V for 100 s. The currents were very stable throughout the entire period of operation regardless of the amount of injected current. In comparison, to verify the effect of the shape and crystallinity of the Sb-doped p-type ZnO on the light emission,


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p-n homojunction LEDs with 1, 1.5, and 2 at.% Sb-doped p-type ZnO NRs were also fabricated. The emission images of these LEDs were found to be very localized and weak under a forward bias of 5 V, as shown in Figure S5. The Sb-doped ZnO NRs with a higher Sb at.% showed high surface roughness and worse crystallinity, as shown in Figure S1, that severely deteriorated the interface of the p-n homojunction. To investigate the origin of the emission, band diagrams of the LED (before and after forming a junction) with probable radiative transitions are shown schematically in Figure 7. The small peak at 378 nm is assigned to the band-to-band transition as supported by the NBE in the PL spectra of the ZnO NRs (Figure 4e). The dominant peak at 400 nm is considered to be a result of the transition from a zinc interstitial (Zni) to a valence-band maximum (VBM).47 The energy levels corresponding to the Zni native defects are located at 0.2 eV below the conduction-band minimum (CBM), which instigates electrons for non-radiative relaxation from CBM rather than a band-to-band transition. The other peaks at 612 and 742 nm in the visible region are attributed to the transitions from Zni to an oxygen interstitial (Oi) and from oxygen vacancy (VO) to VBM, respectively.48

CONCLUSIONS An inorganic n-ZnO NRs/Sb-doped p-ZnO NDs homojunction LED was successfully fabricated using a low-temperature solution process. It was observed that the 0.5 at.% Sb-doped ZnO NCs were predominantly oriented along the c-axis with a ND shape, which was further used as a freestanding template for epitaxial growth of ZnO NRs. The XPS spectra revealed (SbZn– 2VZn) acceptor complex in the ZnO NDs. The undoped n-type ZnO NRs, grown directly on ptype ZnO NDs were found to be single crystalline [0002] with an epitaxial interface. The current-


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voltage properties of the ZnO homojunction revealed a typical p-n diode nature with a turn-on voltage of 2.5 V. Moreover, this p-n homojunction was found to be stable under different voltage stresses. The EL spectra of the p-n ZnO homojunction LED exhibited two major peaks at 612 nm and 742 nm in the visible region corresponding to orange and red colors, respectively, along with an asymmetric high-energy peak at approximately 400 nm. The visible-range emission was found to be defect-induced which was in accordance with the PL spectra of ZnO NRs. The peak at 612 nm was assigned to the transition from Zni to Oi, whereas the peak at 742 nm was attributed to a transition from VO to VBM. The high-energy peak around 400 nm was deconvoluted into two sub-peaks, viz. the peak at 378 nm was related to the band-to-band transition, and another peak at 400 nm was assigned to the transition from Zni to VBM. The complete p-n ZnO homojunction LED emitted significantly strong orange-red light under a forward bias of 5 V, which was visible by naked eye. Therefore, it is believed that this study can pave the way for applications of p-n homojunction LEDs based on ZnO NCs by using lowtemperature and low-cost solution processes.


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FIGURES

Figure 1. (a) Top-view SEM image of the facilely synthesized 0.5 at.% Sb-doped ZnO NDs after transferring on the glass substrate, revealing an average diameter of 650 nm. (b) Lowmagnification TEM image of the same NDs showing a growth direction of [0002]. (c) HRTEM image indicating a clear lattice fringe with an interplanar spacing of 0.26 nm. Inset: Corresponding SAED pattern revealing a single crystallinity of the NDs. (d) XRD spectra of the Sb-doped ZnO NDs confirming a predominant growth along the c-axis. (e) Room-temperature PL spectra of the the NDs indicating a strong NBE at 372 nm with an insignificant number of deep-level defects.


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Figure 2. (a) Full-scan XPS spectra of the 0.5 at.% Sb-doped ZnO NDs confirming the chemical purity of the NDs by containing peaks exclusively related to Zn, O, and Sb. (b) Core-level Zn 2p spectra with an SOS value of 23 eV. Inset: Deconvoluted Zn 2p3/2 spectra revealing the existence of Zn vacancies. (c) Sb 3d-O1s overlapped spectra showing peaks related to O bonded with Zn and Sb bonded with O. (d) Core-level Sb 4d spectra revealing the presence of Sb (III) atoms replacing Zn atoms and bonded with O atoms in the ZnO lattice.


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Figure 3. Schematic fabrication process of the ZnO homojunction LED along with corresponding SEM images; (a) Alcohol-assisted formation of monolayer assembly of Sb-doped ZnO NDs. (b) Transferred monolayer of ZnO NDs assembly onto Au-coated glass substrate. (c) After PR molding and etching of ZnO NDs assembly. (d) Homoepitaxial growth of ZnO NRs on the molded NDs assembly. (e) After PR molding and etching of ZnO NRs. (f) The device with OMO top-electrode.


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Figure 4. (a) Cross-sectional SEM image of the vertically grown ZnO NRs with a length of 2.7 µm on the monolayer of the 0.5 at.% Sb-doped ZnO NDs. Inset: Top-view SEM image of the same revealing the density and uniformity of the ZnO NRs. (b) Low-magnification TEM image of the interface between Sb-doped p-type ZnO NDs and n-type ZnO NRs. (c) HRTEM image of the interface indicating the epitaxial growth of the ZnO NRs on the Sb-doped ZnO NDs with a prominent lattice fringe. Inset: Corresponding SAED pattern revealing a single crystallinity of the interface. (d) XRD pattern of the as-grown vertical ZnO NRs confirming a predominant growth along the c-axis. (e) Room-temperature PL spectra of the ZnO NRs indicating the presence of large number of defect states.


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Figure 5. (a) J-V characteristics of Au/0.5 at.% Sb-doped ZnO NDs/Au structure indicating the p-type conductivity of the NDs by exhibiting an ohmic nature. Inset: Scheme of the ESE structure under dc bias. (b) J-V characteristics of OMO/as-grown ZnO NRs/OMO structure revealing an ohmic contact between the transparent OMO electrode and the ZnO NRs. Inset: Scheme of the ESE structure under dc bias. (c) J-V characteristics of the ZnO homojunction LED showing a typical p-n diode nature with a turn-on voltage of 2.5 V. Inset: Scheme of the LED under dc forward bias.


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Figure 6. (a) EL spectra of the ZnO homojunction LED under various dc forward bias indicating a broad-visible-range photon emission. EL intensity increases linearly with increasing dc forward voltage. (b) Deconvoluted EL spectra of the LED at a forward bias of 5 V revealing four prominent peaks. (c) The illuminated LED images under various forward bias indicating a strong orange-red emission at 5 V. Inset: OM image of the transparent top OMO electrode with a diameter of 550 µm. (d) Stability test results of the ZnO homojunction LED with a voltage stress for 100 s under different dc forward bias.


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Figure 7. Schematic energy band diagrams of n-ZnO NRs and Sb-doped p-ZnO NDs (a) before forming a junction with their bandgap (Eg), electron affinity (χ), and relative work functions (φ1 and φ2) and (b) after forming a junction. The highlighted circle exhibits the emission sources of the n-ZnO NRs/Sb-doped p-ZnO NDs homojunction LED.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: SEM images of Sb-doped ZnO NCs (1, 1.5, and 2 at.%). XRD patterns of Sb-doped ZnO NCs (0.5, 1, 1.5, and 2 at.%). Sb concentrations of Sb-doped ZnO NCs measured by EDS. Depth profiles of Sb-doped ZnO NCs measured by TOF-SIMS. LED images at 5 V for Sb-doped ZnO NCs (1, 1.5, 2 at.%). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The author declares no competing financial interest. ACKNOWLEDGMENTS

This work was supported by the Technology Innovation Program (10051207, Development of flexible inorganic light-emitting device fabrication technology based on metal oxide nanosemiconductor by solution process) funded by the Ministry of Trade, industry & Energy (MI, Korea), and the LG Display academic industrial cooperation program.


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Figure 1. (a) Top-view SEM image of the facilely synthesized 0.5 at.% Sb-doped ZnO NDs after transferring on the glass substrate, revealing an average diameter of 650 nm. (b) Low-magnification TEM image of the same NDs showing a growth direction of [0002]. (c) HRTEM image indicating a clear lattice fringe with an interplanar spacing of 0.26 nm. Inset: Corresponding SAED pattern revealing a single crystallinity of the NDs. (d) XRD spectra of the Sb-doped ZnO NDs confirming a predominant growth along the c-axis. (e) Room-temperature PL spectra of the the NDs indicating a strong NBE at 372 nm with an insignificant number of deep-level defects. 118x79mm (300 x 300 DPI)


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Figure 2. (a) Full-scan XPS spectra of the 0.5 at.% Sb-doped ZnO NDs confirming the chemical purity of the NDs by containing peaks exclusively related to Zn, O, and Sb. (b) Core-level Zn 2p spectra with an SOS value of 23 eV. Inset: Deconvoluted Zn 2p3/2 spectra revealing the existence of Zn vacancies. (c) Sb 3d-O1s overlapped spectra showing peaks related to O bonded with Zn and Sb bonded with O. (d) Core-level Sb 4d spectra revealing the presence of Sb (III) atoms replacing Zn atoms and bonded with O atoms in the ZnO lattice. 143x115mm (300 x 300 DPI)



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Figure 3. Schematic fabrication process of the ZnO homojunction LED along with corresponding SEM images; (a) Alcohol-assisted formation of monolayer assembly of Sb-doped ZnO NDs. (b) Transferred monolayer of ZnO NDs assembly onto Au-coated glass substrate. (c) After PR molding and etching of ZnO NDs assembly. (d) Homoepitaxial growth of ZnO NRs on the molded NDs assembly. (e) After PR molding and etching of ZnO NRs. (f) The device with OMO top-electrode. 87x44mm (300 x 300 DPI)


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Figure 4. (a) Cross-sectional SEM image of the vertically grown ZnO NRs with a length of 2.7 µm on the monolayer of the 0.5 at.% Sb-doped ZnO NDs. Inset: Top-view SEM image of the same revealing the density and uniformity of the ZnO NRs. (b) Low-magnification TEM image of the interface between Sb-doped p-type ZnO NDs and n-type ZnO NRs. (c) HRTEM image of the interface indicating the epitaxial growth of the ZnO NRs on the Sb-doped ZnO NDs with a prominent lattice fringe. Inset: Corresponding SAED pattern revealing a single crystallinity of the interface. (d) XRD pattern of the as-grown vertical ZnO NRs confirming a predominant growth along the c-axis. (e) Room-temperature PL spectra of the ZnO NRs indicating the presence of large number of defect states. 129x94mm (300 x 300 DPI)



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Figure 5. (a) J-V characteristics of Au/0.5 at.% Sb-doped ZnO NDs/Au structure indicating the p-type conductivity of the NDs by exhibiting an ohmic nature. Inset: Scheme of the ESE structure under dc bias. (b) J-V characteristics of OMO/as-grown ZnO NRs/OMO structure revealing an ohmic contact between the transparent OMO electrode and the ZnO NRs. Inset: Scheme of the ESE structure under dc bias. (c) J-V characteristics of the ZnO homojunction LED showing a typical p-n diode nature with a turn-on voltage of 2.5 V. Inset: Scheme of the LED under dc forward bias. 48x13mm (300 x 300 DPI)


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Figure 6. (a) EL spectra of the ZnO homojunction LED under various dc forward bias indicating a broadvisible-range photon emission. EL intensity increases linearly with increasing dc forward voltage. (b) Deconvoluted EL spectra of the LED at a forward bias of 5 V revealing four prominent peaks. (c) The illuminated LED images under various forward bias indicating a strong orange-red emission at 5 V. Inset: OM image of the transparent top OMO electrode with a diameter of 550 µm. (d) Stability test results of the ZnO homojunction LED with a voltage stress for 100 s under different dc forward bias. 126x90mm (300 x 300 DPI)



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Figure 7. Schematic energy band diagrams of n-ZnO NRs and Sb-doped p-ZnO NDs (a) before forming a junction with their bandgap (Eg), electron affinity (χ), and relative work functions (φ1 and φ2) and (b) after forming a junction. The highlighted circle exhibits the emission sources of the n-ZnO NRs/Sb-doped p-ZnO NDs homojunction LED. 111x84mm (300 x 300 DPI)


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Low-Temperature Facile Synthesis of Sb-Doped p-Type ZnO

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