One-Dimensional Zinc Oxide Nanomaterials for Application in High-Performance Advanced Optoelectronic Devices

Abstract

Unlike conventional bulk or film materials, one-dimensional (1D) semiconducting zinc oxide (ZnO) nanostructures exhibit excellent photoelectric properties including ultrahigh intrinsic photoelectric gain, multiple light confinement, and subwavelength size effects. Compared with polycrystalline thin films, nanowires usually have high phase purity, no grain boundaries, and long-distance order, making them attractive for carrier transport in advanced optoelectronic devices. The properties of one-dimensional nanowires—such as strong optical absorption, light emission, and photoconductive gain—could improve the performance of light-emitting diodes (LEDs), photodetectors, solar cells, nanogenerators, field-effect transistors, and sensors. For example, ZnO nanowires behave as carrier transport channels in photoelectric devices, decreasing the loss of the light-generated carrier. The performance of LEDs and photoelectric detectors based on nanowires can be improved compared with that of devices based on polycrystalline thin films. This article reviews the fabrication methods of 1D ZnO nanostructures—including chemical vapor deposition, hydrothermal reaction, and electrochemical deposition—and the influence of the growth parameters on the growth rate and morphology. Important applications of 1D ZnO nanostructures in optoelectronic devices are described. Several approaches to improve the performance of 1D ZnO-based devices, including surface passivation, localized surface plasmons, and the piezo-phototronic effect, are summarized.

1. Introduction

The small size of one-dimensional (1D) nanomaterials leads to unique electrical, mechanical, chemical, and optical properties that are attractive for application in nanoscience and nanotechnology. In particular, 1D zinc oxide (ZnO) is a representative nanomaterial with excellent properties, such as ultrahigh intrinsic photoelectric gain and multiple array light confinement. Thus, ZnO nanomaterials have been well studied in recent years with the goal of constructing advanced optoelectronic devices with improved performance [1]. It is anticipated that 1D ZnO nanomaterials could be used as an important basic unit of nanostructured systems. Several types of 1D ZnO nanostructures have been reported, including nanowires [2,3,4,5,6], nanorods [7,8,9], nanobelts [10,11,12,13], and nanotubes [14,15,16]. Considering actual application requirements, 1D ZnO nanomaterials are usually considered an ideal medium to construct optoelectronic devices. The performance of optoelectronic devices could be improved using 1D ZnO nanomaterials because of their excellent properties, such as carrier transport with high mobility, efficient light confinement, and light emission. Thus, 1D ZnO nanowires are usually used in devices for applications such as light emission, photon detection, and biochemical sensing. In addition, nanowire-like structures could be considered as an ideal system for physical studies investigating the carrier transport, light confinement, and energy loss behaviors of 1D confined objects. Therefore, 1D ZnO nanowires are favorable not only for examining the basic phenomena of low-dimensional systems, but also for constructing new types of nanodevices that exhibit high performance.

As a member of the II-VI family, ZnO possesses properties such as a wide direct band gap of 3.37 eV, large exciton binding energy of 60 meV at room temperature, and suitable features for feasible production of nanomaterials. Potential applications of ZnO materials include optical waveguides [17], varistors [18], solid-state lighting [19], gas sensors [20], and transparent conductive films. Because of its wide band gap, ZnO is a promising candidate material for use in solid-state optoelectronics that emit in the blue or ultraviolet (UV) spectral range, including lasers. In addition, the large exciton binding energy of ZnO means that it can display efficient excitonic emission even at room temperature [21].

Various fabrication methods of 1D ZnO nanomaterials have been developed, such as vapor transport [5], hydrothermal reaction [8,11,16,22], electrodeposition [12,23], chemical vapor deposition (CVD) [24,25], molecular beam epitaxy [26], and pulsed laser deposition [27]. These methods can be used to obtain samples on substrates such as Si, quartz, and sapphire. Each method has its specific merits and inevitable weaknesses. The gas vapor approaches can fabricate high-quality single-crystalline ZnO nanostructures. However, these processes require high temperatures (about 450–1000 °C) and often have other limitations, including poor sample uniformity, narrow substrate choice, and low product yield. Solution processing is one of the most commonly used methods to prepare 1D ZnO nanomaterials because of its low cost, controllability, and compatibility with large-area manufacturing.

Compared with other 1D nanomaterials, ZnO has three prominent advantages: first, it is a semiconductor with a piezoelectric effect, which is the basis for electric mechanical coupling sensors and inverters; second, the biosecurity and biocompatibility of ZnO is relatively high, so it can be used in medicine; third, ZnO has the largest range of known nanostructures, including nanowires [2,3,4,5,6], nanorods [7,8,9], nanobelts [10,11,12,13], nanotubes [14,15,16], nanoplates [28,29,30], nanospheres [31], nanotrees [32], nanoleaves [33], and nanosails [34]. In addition, ZnO tetrapod [35], microrod/microwire [36,37], porous array [38], matrix [39], and 3D network nanostructure [20,40] have been obtained. At present, many researchers are focusing on constructing devices based on ZnO nanomaterials. New devices based on 1D ZnO nanomaterials are constantly being developed, such as room-temperature lasers, light-emitting diodes (LEDs), photodetectors, sensors, and transistors. These devices containing 1D ZnO nanomaterials exhibit better performance than that of devices with thin films and bulk materials. The excellent performance of new devices based on 1D ZnO nanomaterials reveal their great potential for applications in many fields.

Previously, there were some review articles about ZnO nanostructure, for example, Wang et al. reviewed various nanostructures of ZnO using solid-vapor phase technique, growth mechanisms, the application of nanobelts in nanosensors, field effect transistors, and nanoresonators [1]. Yi et al., reviewing the fabrication method and doping of ZnO nanorods, also discussed the properties related to ZnO nanorods, including luminescence, field emission, electron transport, as well as various applications [41]. Wang comprehensively reviewed various ZnO nanostructures fabricated by wet chemical methods, their doping and alloying, patterned growth, application in catalysis, surface modification, sensing, optoelectronic, and so on [42]. The review mainly focuses on the effect of growth conditions on 1D ZnO nanostructures and achievements of 1D ZnO-based photodetectors and LEDs, and, in particular, the promising approaches to improve the performance of LEDs and photodetectors.

In this review, we briefly introduce the properties of ZnO, review the growth methods of 1D ZnO nanomaterials, and describe the progress and current status of photoelectric devices (LEDs and photodetectors) based on 1D ZnO nanomaterials. Section 2 provides a brief introduction to the basic properties of 1D ZnO nanostructures. Section 3 covers the fabrication of 1D ZnO nanostructures, including CVD, hydrothermal reaction, and electrodeposition, and describes the influence of growth parameters on the growth rate and morphology of 1D ZnO nanostructures. Finally, we focus on LEDs and photodetectors based on 1D ZnO nanomaterials. The recent achievements of ZnO-based photodetectors and LEDs with different device structures and promising approaches to improve the performance of LEDs and photodetectors—including surface passivation, localized surface plasmons resonance (LSPR), and the piezo-phototronic effect—are discussed.

2. Basic Properties of ZnO

ZnO is II-VI compound semiconductor with direct wide band gap of 3.37 eV at room temperature. ZnO has three typical crystal structures: wurtzite, zinc blende, and rock salt. The thermodynamically stable phase of ZnO at ambient temperature and pressure is wurtzite. The zinc blende structure can be stabilized by growth on cubic substrates. The rock-salt structure can be fabricated at high pressure [21].

Wurtzite ZnO has a hexagonal structure with lattice parameters a and c and belongs to the P6₃mc space group. The lattice parameter a mostly ranges from 0.32475 to 0.32501 nm, and c typically ranges from 0.520642 to 0.52075 nm [43]. The deviation of ZnO lattice constants from the ideal values is probably caused by the presence of point defects, such as zinc antisites, oxygen vacancies, and dislocations [21]. As illustrated in the schematic of the wurtzite ZnO structure shown in Figure 1, both Zn and O atoms arrange in a hexagonal close-packed (hcp) pattern and are stacked alternately along the c-axis. Furthermore, each kind of atom is surrounded by four atoms of the other kind, and the O²⁻ or Zn²⁺ of two adjacent layers form a tetrahedral structure. The resulting non-centrosymmetric structure of wurtzite ZnO can result in piezoelectricity and the thermoelectric effect. A ZnO single crystal exhibits very strict crystallographic polarity. The polar plane of such a crystal is the basal plane, which is composed of the Zn (0001) plane and O $(000\overline{1}$) plane, inducing a normal dipole moment, spontaneous polarization along the c-axis, and a distinct surface energy [1,21].

3. Growth of ZnO Nanostructures

ZnO probably possesses the richest variety of reported nanostructures, which are typically categorized into three groups: zero-dimensional, 1D, and two-dimensional. The four most common types of 1D ZnO nanostructures are nanowires [2,3,4,5,6], nanorods [7,8,9], nanobelts [10,11,12,13], and nanotubes [14,15,16]. Many approaches have been used to fabricate 1D ZnO nanostructures, such as CVD [4,5,6], hydrothermal processing [8,11,16,22], electrochemical deposition [12,23], and magnetron sputtering [44], with and without a catalyst.

3.1. Chemical Vapor Deposition

CVD is the one most commonly used methods to synthesize 1D ZnO nanostructures. In the CVD synthesis process, ZnO powder, Zn powder, or another Zn compound is used as the raw material after evaporation, reduction/oxidation, decomposition/combination, and other physical and chemical changes. The growth process is commonly carried out in a horizontal tube furnace, which consists of a horizontal tube heater, an alumina or quartz tube, a gas supply, and a control system. The fabrication of 1D ZnO nanostructures by CVD is generally affected by the synthesis parameters, such as the growth temperatures of the source and substrate, the distance between the source and substrate, heating rate, carrier gas (including gas species and flow rate), tube diameter, pressure, and atmosphere. Usually, the vapor–liquid–solid (VLS) process is used for 1D ZnO nanostructure growth via CVD, which requires a catalyst such as gold (Au) [5], copper (Cu) [45], stannum (Sn) [46], or cobalt (Co) [47]. The liquid droplet serves as a preferential site for absorption of the gas-phase reactant until the droplet is saturated and then becomes the nucleation site for crystallization. During growth, the catalyst droplet regulates the growth direction and controls the diameter of the nanowire [1]. Control of the nanowire diameter has also been achieved by varying the Au layer thickness. As shown in Figure 2, when the Au film coated on a substrate was 5 nm thick, the ZnO nanowires had diameters of 80–120 nm and lengths of 10–20 µm. When the Au film was 3 nm thick, the ZnO nanowires had diameters of 40–70 nm and lengths of 5–10 µm [5]. In addition to using external catalysts, Zn itself can act as a catalyst under certain conditions to prepare 1D ZnO nanostructures [48].

The influence of different catalyst thin films on the morphology and even nucleation process of ZnO nanomaterials varies. Osgoods et al. systematically studied ZnO nanowire growth using diverse metal catalysts (Au, Ag, Ni, and Fe) and substrates (Si, sapphire) with different structures and crystal orientation. SEM images of the nanowires are shown in Figure 3 [41,42,43,44,45,46,47,48,49]. The ZnO nanowires grew through the vapor–solid mechanism in the case of an iron (Fe) catalyst and via the VLS process using Au, Ag, or Ni as the catalyst. The ZnO nanostructures grown using different catalysts possessed differences in not only size and the ratio of length to diameter, but also the atomic composition ratio of Zn to O. Thus, the relative intensity of the oxygen vacancy-related emission in photoluminescence spectra of nanostructures grown on different thin films varied.

A ZnO thin film or other transition layer can be prepared on the substrate to facilitate the nucleation growth of 1D ZnO nanostructures. Fang et al. [6] demonstrated that ZnO thin films fabricated by electron-beam evaporation can provide nucleation sites to control the growth direction of ZnO nanowires. In addition, the density of ZnO nanowires could be adjusted by varying the thickness of the ZnO thin film, as shown in Figure 4. Yang and coworkers fabricated radial ZnO nanowire arrays on Si substrates with an amorphous carbon thin layer without a metal catalyst [50].

3.2. Hydrothermal Method

The hydrothermal method is a very powerful technique for the growth of 1D ZnO nanomaterials, which involves chemical reaction of substances in solution at a certain temperature and pressure to fabricate the nanomaterials. This technique enables the synthesis of most materials with the required physical and chemical properties. Compared with other conventional preparation methods, the hydrothermal method has many advantages, such as low cost, simplicity, limited pollution, good nucleation and shape control, and low-temperature (<200 °C) growth at higher pressure in the presence of an appropriate solvent. In addition, the size distribution of materials obtained by the hydrothermal technique is uniform, with hardly any aggregation. The shape and size of the prepared materials are influenced by several processing parameters, such as the molar ratio and concentration of the reactant, temperature, and reaction time.

Vayssieres et al. [2,51] first used the hydrothermal method to fabricate ZnO nanorods in 2001. Equimolar mixed solutions of zinc nitrate (Zn(NO₃)₂) and hexamethylenetetramine (C₆H₁₂N₄, HMT) with different concentrations (1–10 mM) were heated at a constant temperature of 95 °C for several hours. The products were cleaned and then dried to give ZnO nanorod arrays. Various materials were used as substrates in their hydrothermal growth of ZnO nanostructures; for example, polycrystalline fluorine-doped tin oxide (FTO)-coated glass, single-crystalline sapphire, and Si/SiO₂ wafers with ZnO thin films. The experiments showed that the diameter of ZnO nanorods was controlled by the concentration of the precursors. As the concentration of the solution was lowered from 10 to 1 mM, the diameter of the nanorods decreased from 100–200 nm to 10–20 nm. SEM images of the ZnO nanorods fabricated by the hydrothermal method are shown in Figure 4. In this approach, only two kinds of chemical solutions with good solubility are needed. The two solutions can readily mix uniformly and promote ZnO formation, greatly simplifying and lowering the cost of the experimental process. The simple setup for hydrothermal growth of ZnO nanostructures is also beneficial to improve the purity of products and study the influence of synthetic parameters on nanostructure formation.

The method developed by Vayssieres and colleagues has since been greatly improved and remains in common use because of its simplicity. To improve the quality of the product and the controllability of the deposition process, a crystalline seed layer can be prepared on the substrate prior to hydrothermal growth. In 2003, Yang’s group grew uniform ZnO nanowires on various substrates (e.g., Si (100) wafers and flexible polydimethylsiloxane films) using a two-step hydrothermal method [52]. The first step involved the preparation of a seed layer of ZnO nanocrystals with a diameter of 5–10 nm, which were spin-cast several times on the substrate to give a seed-layer thickness of 50–200 nm. The second step was ZnO nanowire growth by the hydrothermal method. The substrate with a seed crystal layer was placed in an open crystallizing dish filled with an equimolar solution of zinc nitrate hydrate and methenamine or diethylenetriamine (0.025 M) at 90 °C. Other research groups have prepared seed crystals using techniques such as pulsed laser deposition [14], magnetron sputtering [53], the sol–gel method [22,54], and atomic layer deposition [55]. Using small, homogeneous ZnO nanocrystals as a seed layer can provide nanowires that are small in diameter, well aligned, and in intimate contact with the substrate. In addition, the existing seeds could bypass the nucleation step, promoting growth on the seeds rather than nucleation in the homogeneous bulk solution. ZnO nanowires tend to grow wherever there are ZnO seeds; therefore, the density of nanowires is usually quite high [42]. As a result, the density of ZnO nanowires can be controlled by the thickness of the seed layer [22].

Although it is practical and effective to prepare ZnO nanostructures by the hydrothermal method using Zn(NO₃)₂ and HMT as raw materials, this approach does have shortcomings. For example, it is difficult to obtain ZnO nanomaterials with a large ratio of length to diameter. To increase the aspect ratio of the ZnO nanostructures, poly(ethylenimine) can be added to the reaction solution [56]. The addition of a low concentration of sodium citrate can alter the morphology of the ZnO nanorods. Sun et al. [57] prepared ZnO nanorods via cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal oxidation of Zn metal at 180 °C. CTAB played important roles in the growth of ZnO nanorods, affecting the erosion process and growth orientation.

Kim et al. [8] prepared single-crystalline vertically aligned ZnO nanorod arrays on various substrates (Si, SiO₂, and FTO glass) using a polymer-templated hydrothermal method at a low temperature of 93 °C. To ensure that the ZnO nanorods grew along the c-axis [0001] direction and to minimize the crystal mismatch with the substrate, a c-oriented ZnO seed layer was first fabricated by the sol–gel method. The morphologies of the resulting nanostructures could be transformed from rodlike to pencil-like, needle-like, tubelike, treelike, and spherelike by exploiting the structural polarity of the ZnO nanorods and adjusting the growth parameters, including the seed layer, pH, and temperature, as summarized in Figure 5. Thus, Kim and coworkers developed a simple process to obtain ZnO nanorods with controllable size, density, and lattice.

3.3. Electrochemical Deposition

Electrochemical deposition has been widely used to obtain uniform and large-area growth of ZnO nanostructures [58]. Electrochemical deposition has many advantages, such as scalability, low cost, and easy handling [59]. Compared with ordinary chemical solution methods (e.g., hydrothermal methods), electrochemical deposition includes an external driving electric field in the reaction process, thus promoting reactions that are otherwise nonspontaneous. A standard three-electrode system with a saturated Ag/AgCl electrode as the reference electrode, Pt as the counter electrode (anode), and growth substrate as the working electrode is the typical setup for electrochemical deposition. In this case, the substrate serves as the cathode, so it must be conductive regardless of whether it is flat or curved. The substrate cathode is placed parallel to the anode in an aqueous solution of zinc salt (electrolyte). The chemical reaction is carried out in an electrolytic cell placed in a water bath to maintain a constant temperature.

Various groups have fabricated 1D ZnO nanostructures on FTO [60,61], indium-doped tin oxide (ITO) [62], polycrystalline Zn foil [63], and Si [64] substrates. Mari et al. [65] studied the influence of the bath temperature, current density, and deposition time on the growth rate of ZnO nanorods. Their results showed that the length of the nanorods increased with the current density and deposition time. Temperature had the opposite effect: the nanorods were longer when the deposition temperature was low. Xu and coworkers investigated the morphological control of ZnO nanostructures induced by addition of different capping agents, such as potassium chloride (KCl), ammonium fluoride (NH₄F), ammonium acetate (CH₃COONH₄), and ethylenediamine (EDA) [62]. The morphology of the ZnO structures evolved from hexagonal rods to rhombohedral rods and woven nanoneedles upon varying the compositions of the mixed capping agents, as depicted in Figure 6.

Generally, zinc chloride (ZnCl₂) is used as the Zn²⁺ precursor when ZnO is deposited from the reduction of O₂ or H₂O₂. Zaera et al. [66] examined the effect of chloride (Cl⁻) concentration, which was mainly supplied by the KCl supporting electrolyte, on the mechanism of ZnO nanowire growth driven by O₂ reduction. The rate of oxygen reduction decreased with rising Cl⁻ concentration, and the deposition rate and dimensions of ZnO nanowire arrays could be tuned by varying the Cl⁻ concentration of the solution, as shown in Figure 7. The effect of different anions (Cl⁻, SO₄^2–, and CH₃COO⁻) on the reduction of dissolved O₂ in the deposition solution has also been studied [67]. The observed differences in the O₂ reduction rate were explained by the different adsorption behaviors of the anions. Therefore, the different adsorption behaviors of the anions on ZnO surfaces could affect the generation rate of hydroxide ions and influence the morphology and growth rate of the ZnO nanowires. It was also demonstrated that the diameter and length of ZnO nanowires could be adjusted by varying the type of anions in the deposition solution. In particular, the nanowires grown in the presence of Cl⁻ and CH₃COO⁻ possessed low and high aspect ratios, respectively.

ZnO nanowires can also be fabricated by electrochemical deposition assisted by templates, including anodic aluminum oxide (AAO) [68,69,70], polycarbonate membranes [71,72], and porous films. AAO templates are widely used in the preparation of ZnO nanowires because of their simplicity and suitability for large-area, well-distributed nanowire fabrication. AAO templates can be prepared through a two-step anodization process [68,70]. The AAO template can be removed after nanowire growth, leaving the free-standing nanowires. Yi et al. [68] synthesized Zn nanowires consisting of the wurtzite hcp-Zn phase by electrodeposition on an AAO template with nanometer-sized pores. ZnO nanowires were then obtained by annealing the Zn nanowires at different temperatures for 10 h. Research has shown that this technique can provide not only ZnO nanowires, but also other semiconductor oxide nanostructures. However, ZnO and Al₂O₃ are amphoteric oxides, which makes the selective removal of the AAO membrane from the ZnO nanowires difficult. Polycarbonate membrane templates can be easily dissolved in chloroform, which makes them attractive to prepare free-standing 1D ZnO nanostructures. Wang and colleagues fabricated randomly distributed ZnO nanowires using a polycarbonate template [71]. Zeng et al. fabricated ZnO nanowire lines and bundle arrays on polystyrene (PS) template, as sown in Figure 8. The density of ZnO nanowires on Si substrate with 200 nm PS sphere template has decrease tendency with the heating time [73].

4. Applications of 1D ZnO Nanostructures

4.1. Light-Emitting Diodes

An important feature of 1D ZnO nanomaterials is their excellent light transmission characteristics, which makes them attractive for use in high-efficiency UV LEDs and laser devices. ZnO nanomaterials display a direct wide band gap and large exciton binding energy at room temperature. UV LEDs have practical applications in solid-state lighting, UV photolithography, high-density data storage, and biomedical analysis [74]. The basis of LED construction is formation of a p–n junction. Unintentionally doped ZnO usually presents n-type conductivity. It is difficult to obtain p-type ZnO of high quality because of its low dopant solubility, deep acceptor level, and self-compensation effect. Therefore, the initial ZnO LEDs were constructed with other p-type semiconductors to form p–n heterojunctions that exhibited electroluminescence (EL). In the early stages, ZnO light-emitting devices were mainly based on thin films. After the thin-film LEDs were realized, many researchers shifted their attention to the study of LEDs containing 1D ZnO nanomaterials.

In 2006, Chang and coworkers fabricated a heterojunction device consisting of a ZnO nanowire array on an n-GaN film on a sapphire substrate and the p-type polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). This hybrid LED exhibited multiple EL peaks at forward bias, including ZnO band edge emission at about 383 nm and other peaks centered at 430, 640, and 748 nm [75]. Zhang et al. [76] synthesized an n-type ZnO nanowire array on a p-type GaN film by CVD to produce a hybrid LED with high brightness. UV–blue EL emission was observed from the heterojunction diode, which displayed a blue shift as the forward bias increased. Tang et al. [77] fabricated an LED based on single n-ZnO/p-AlGaN heterojunction nanowires. A schematic illustration and SEM image of their LED are displayed in Figure 9a,b, respectively. When the injection current was 4 µA, UV EL emission with a peak at 394 nm was observed, as shown in Figure 9c.

Heterojunction diodes have been fabricated using n-type ZnO nanomaterials and p-type organic/inorganic materials such as GaN [78,79,80], AlGaN, Si [81,82], MgZnO [83], NiO [84], Cu₂O, SiC, N,N′-di(naphth-2-yl)-N,N′-diphenylbenzidine (NPB) [85], PEDOT/PSS [75], and [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) [86]. Representative results for heterojunction LEDs containing 1D ZnO nanostructures, along with a brief description of the corresponding device structure, applied bias, and EL performance, are summarized in

Table 1. 1D ZnO nanostructure-based heterojunction LEDs.

Device Structure	Turn-On Voltage	Threshold Voltage of Emission	Bias/Current	Wavelength of EL Peak	Ref.
ZnO nanowire arrays/P-GaN	3 V	4.4 V	5–7 V/0.5–1.5 mA	397 nm	[78]
ZnO nanowire arrays/P-GaN	3 V	4.4 V	6.5 V/2.6 mA	397 nm	[79]
n-GaN/ZnO nanorod/P-GaN	5 V		6 V	382 nm, 430 nm	[80]
Sb-doped p-ZnO nanowire arrays/n-GaN	3.74 V	4.0 V	16 V	391 nm	[87]
n-ZnO/p-AlGaN	2 V		4 µA	394 nm	[77]
ITO/n-ZnO nanorod array/p+-Si	5 V (5 mA)	6 V	9.5 V/29.5 mA	450 nm, 700 nm	[81]
n-ZnO nanorod/p-Si			9.5 V	387 nm, 535 nm	[88]
p-Zn0.68Mg0.32O:N/n-ZnO nanowire	4.2 V		5 mA/125 mA cm2	390 nm	[83]
p-NiO/n-ZnO nanowire	2.5 V	7 V	15 V	385 nm, 570 nm	[84]
ZnO nanowire/polymer (PEDOT/PSS)		<3 V	3 V	383, 430, 640, 748 nm	[75]
ZnO nanorod/NPB			22 mA/cm2	342 nm	[85]
ZnO nanorods/MEH-PPV			28 V	380, 580, 615, 640, 747 nm,	[86]

.

Many research groups are committed to obtaining p-type ZnO materials and have observed EL emission from ZnO p–n homojunctions. To date, p-type ZnO nanostructures have been realized by doping ZnO with group-V elements, including N [89], P [74,90], As [91,92], and Sb [93]. However, there have been relatively few reports of p–n homojunction LEDs based on 1D ZnO nanostructures.

Sun and coworkers obtained p-ZnO by As ion implantation into as-grown ZnO nanorods and then constructed ZnO nanorod homojunction LEDs [92]. A schematic illustration of the homojunction LED is presented in Figure 10a. At forward bias, UV emission originating from free-exciton recombination centered at about 380 nm was dominant, and a weak broad peak centered at about 630 nm was observed in EL spectra collected at room temperature. The second harmonic of UV emission centered at about 760 nm was also detected, as displayed in Figure 10b. The same group also used P as an acceptor dopant to fabricate ZnO nanorod homojunction LEDs by P ion implantation [74]. Strong UV emission attributed to near-band-edge emission and weak visible emission related to deep-level defects were observed from both devices at forward bias.

However, there are some problems associated with heterojunction and homojunction LEDs based on 1D ZnO nanostructures, which can prevent the development and application of ZnO-based LEDs. For example, the heterostructured devices suffer from p-ZnO doping and large lattice mismatch. A Schottky-type LED based on Au/ZnO nanowires was constructed to overcome these issues [94]. A schematic illustration of the LED is shown in Figure 11a. The EL spectra in Figure 11b reveal that the LED displayed excitonic EL emission at about 380 nm at high forward bias.

Although there have been some encouraging achievements in LEDs based on 1D ZnO nanostructures, there are still some problems like the degradation of efficiency and stability caused by surface defects, surface adsorption, the low quality of p-type ZnO, and/or a high content of heterointerface defects. Surface passivation has been recognized as a promising approach to improve the EL performance of ZnO-based LEDs. In this approach, a dielectric/semiconductor material is coated on the surface of the 1D ZnO nanomaterial to form a core/shell nanostructure. Liu et al. [95] fabricated two types of LEDs on p-GaN films using MgZnO-coated and bare ZnO nanorod arrays as active layers, as shown in Figure 12a. The EL performance of two types of LEDs is compared in Figure 12b. A strong emission peak at 387 nm was present in the EL spectra of both devices. The EL intensity of the MgZnO-coated LED was much higher than that of the uncoated one, which was attributed to the enhancement of the radiative recombination of the ZnO nanorods caused by the effective passivation of dangling bonds on the nanorod surface and the carrier confinement effect offered by the MgZnO coating. The stability of both devices exposed to ambient air for over a year at the same injection current of 5 mA was investigated at regular intervals; the results are displayed in Figure 12c. The ZnO near-band-edge UV emissions of both devices gradually decayed over time, but the decay rate of the uncoated device was faster than that of the MgZnO-coated LED. Therefore, the MgZnO-coated device exhibited higher stability than that of the uncoated one because surface adsorption on the nanorods was suppressed by the MgZnO coating. SiO₂ [96] and MgO [97] can also be used as passivation materials to improve the performance of 1D ZnO nanostructure LEDs.

Another effective approach to improve the luminous efficiency of LEDs is to introduce LSPs into the LED structure. Semiconductor excitons/photons can be resonantly coupled with the metal LSPs and then scattered/re-emitted into free space as radiation. This approach provides additional recombination/extraction pathways, which can increase the spontaneous radiation rate, internal quantum efficiency, and light extraction efficiency of the LEDs. Liu and coworkers fabricated ZnO nanorod array/p-GaN film heterostructure LEDs embedded in an Ag nanoparticle/polymethyl methacrylate (PMMA) composite [98]. A typical cross-sectional SEM image and schematic illustration of such an LED are shown in Figure 13a,b, respectively. The UV emission from ZnO excitons was enhanced more than 13-fold in EL spectra (Figure 13c). Lu et al. [99] constructed an Al nanoparticle-decorated n-ZnO nanorod/p-GaN film LED. They obtained 30-fold EL enhancement compared with that of the device without Al nanoparticles because of the effect of Al LSPs.

To further improve the performance of LEDs, the two methods mentioned above can be combined. Zhang et al. [100] fabricated LSP-enhanced waveguide-type UV LEDs via sputtering Ag nanoparticles onto ZnO/MgZnO core/shell nanorod array/p-GaN film heterostructures. After decoration with Ag nanoparticles, the UV emission intensity of the ZnO/MgZnO core/shell nanorod array showed a ~9-fold enhancement compared with that of the device without Ag nanoparticles because of coupling with LSPs. The next year, the same group constructed LSP-enhanced UV LEDs by spin-coating Ag nanoparticles on ZnO/SiO₂ core/shell nanorod array/p-GaN heterostructures; a schematic diagram of this LED structure is illustrated in Figure 14a [96]. The EL intensity was enhanced when both Ag nanoparticles and a SiO₂ spacer layer were coated on the surface of the ZnO nanorods, as shown in Figure 14b. The effect of the SiO₂ spacer-layer thickness on the UV emission intensity was investigated. A maximum EL enhancement of ~7-fold was obtained when the thickness of the SiO₂ spacer layer was 12 nm. In contrast, a maximum PL enhancement of ~3.5-fold was achieved when the SiO₂ spacer layer was 16 nm thick. The EL enhancement was attributed to the combined effects of internal quantum efficiency improvement caused by exciton–LSP coupling and extraction efficiency enhancement induced by photon–LSP coupling.

A near-UV LED based on a ZnO nanorod/MEH-PPV heterostructure using a ZnS layer as a buffer layer was reported by Wang and colleagues [101]. The intensity of near-UV EL was enhanced 10-fold compared with that of that of the device without a ZnS layer. In particular, the turn-on voltage was obviously decreased. In addition, the ZnS layer could induce recombination center of carriers, which regulated the EL wavelength of the heterojunction. Wei et al. [102] fabricated ZnO nanowire/p-GaN heterojunction LEDs containing ZnS particles to sensitize the ZnO nanowires. Localized states formed at the ZnO/ZnS interface, generating a built-in electric field, which captured electrons and holes at the ZnO/ZnS interface, where they recombined to result in EL emission from ZnO. Moreover, the piezo-phototronic effect can improve the efficiency of flexible ZnO nanowire/p-polymer hybridized LEDs [103,104]. Wang et al. [104] fabricated a hybrid LED based on a ZnO nanowire/p-polymer heterostructure. The external quantum efficiency of the hybrid LED at least doubled upon applying strain, reaching 5.92%.

4.2. Photodetectors

UV photodetectors are important photoelectric devices that occupy an indispensable position in today’s information society. UV photodetectors are widely used in the military (e.g., UV control and guidance, UV alarms, and UV resistance), as well as in civil fields such as fire alarms, UV communication, ozone hole detection, and water purification. Therefore, the further development of UV detection technology has important implications in modern national defense and daily life. The importance and universality of UV photodetectors in military and civilian fields also make them a focus of current research.

ZnO has a wide direct band gap at room temperature of about 3.37 eV, along with high chemical and thermal stability, strong ability to resist radiation damage, and low content of electron-induced defects. Because of these favorable features, ZnO is widely used in UV detectors. Early ZnO UV detectors were mainly based on membrane structures [105]. However, with the development of low-dimensional materials, photodetectors based on 1D nanomaterials have attracted interest. Compared with UV detectors based on thin films and bulk materials, those based on 1D ZnO nanostructures display higher responsivity and internal photoconductive gain. This is because 1D nanomaterials have a large surface-to-volume ratio and the presence of deep-level surface trap states can prolong the lifetime of photogenerated electrons and holes. In addition, the low dimensionality of the active area can shorten the transit time of carriers [4,106]. There are several types of photodetector structures, such as photoconductive, metal–semiconductor–metal (MSM), Schottky [107], and p–n junction [108,109,110] photodetectors.

In 2002, Yang and coworkers first detected the UV response of an individual ZnO nanowire [111]. The conductance of a single ZnO nanowire was very sensitive to UV light. The resistivity of a ZnO nanowire reversibly changed by 4–6 orders of magnitude under UV illumination. In 2007, Soci et al. [4] reported a UV photodetector based on a single ZnO nanowire with an internal photoconductive gain as high as ~10⁸. While the photoconductive detector showed slow response times because of possible persistent photoconductivity effects, it usually worked with an external power source.

To realize ZnO-based UV photodetectors with high sensitivity and fast response time, a Schottky contact instead of an ohmic contact was adopted [107,112,113,114,115]. For example, Zhou and coworkers fabricated a Schottky junction-type photodetector that used ZnO nanowire and Pt metal electrode to form Schottky contact [98]. The UV sensitivity of the photodetector was enhanced by ~10⁴, and its recovery time was markedly shortened from ~417 to ~0.8 s, as shown in Figure 15.

Nie and coworkers [115] reported a Schottky junction UV photodetector based on transparent monolayer graphene (MLG)/ZnO nanorods. The photodetector showed a ratio of photocurrent to dark current of approximately 3 orders of magnitude under 365 nm UV illumination at a bias of −1 V. The responsivity and photoconductive gain of the detector were 113 A W⁻¹ and 385, respectively (Figure 16). The response speed of the UV photodetector was rather fast, with rise and fall times of 0.7 and 3.6 ms, respectively, which was attributed to the formation of a Schottky barrier, high crystal quality of the ZnO nanorod array, and strong light-trapping effect of the structure.

Several kinds of 1D ZnO nanostructure-based p–n junction photodetectors have been reported [108,110,116,117], which possess the advantages of low applied fields and no oxygen dependency over Schottky junction detectors. In particular, p–n junction photodetectors work at rather high bias voltage to inhibit the recombination of electron–hole pairs, which increases the detection capability of UV signals [108,118]. Tahani et al. [108] developed a photodetector based on p-Si/n-ZnO nanotube heterojunctions, which is shown in Figure 17. Under 365 nm UV illumination with an incident power intensity of 1 mW cm⁻² at a bias of −2 V, the responsivity and detectivity of the detector were 21.51 A W⁻¹ and 1.26 × 10¹² cm Hz^1/2 W⁻¹, along with an external quantum efficiency of 73.1 × 10²% (effective area = ~0.79 mm²). The rise and fall times of the heterojunction device were 0.44 and 0.599 s, respectively.

Recently, self-powered ZnO photodetectors have attracted research interest because they can work in some poor conditions without an external power source. According to their interface features, self-powered photodetectors have three types of structures: Schottky junction, p–n junction, and photoelectrochemical. Compared with the other two types, ZnO-based p–n junction UV detectors are more suitable for development as self-powered photodetectors because of their much lower applied fields, faster response times, and high stability.

Table 2. Recently reported self-powered 1D ZnO nanostructure-based UV photodetectors.

Device Structure	Light of Detection, Power	UV–Visible Rejection Ratio	On/Off Ratio	Responsivity	Rise/Decay Time	Ref.
ZnO/Sb-doped ZnO nanowire	365 nm, 0.3 mW cm−2		26.7	26.5	30 ms/30 ms	[119]
n-ZnO nanowire/P-GaN film					20 µs/219 µs	[120]
n-ZnO nanorods/i-MgO/p-GaN	350 nm	R350 nm/R500 nm) = 34.5	8000	0.32 A/W		[121]
ZnO nanorods/Si				UV/Visible: 0.3 A W−1/0.5 A W−1		[122]
ZnO Nanowire/CuSCN	370 nm, 100 mW/cm2	R370 nm/R500 nm) = 100		0.02 A/W		[123]
ZnO Nanorod/CuSCN	380 nm	100		0.0075 A W−1	5 ns/6.7 us
PEDOT:PSS/ZnO micro-/nanowire			1000			[124]
n-ZnO/p-NiO core–shell nanowire				0.493 mA W−1	1.38 ms/10.0 ms and 30.3 ms	[125]
p-NiO/n-ZnO nanorod array	355 nm			0.44 mA W−1	0.23 s/0.21 s	[126]
p-NiO/n-ZnO nanowire	380 nm, 0.36 mW/cm2			1.4 mA/W		[127]
CH3NH3PbI3 perovskite/ZnO nanorod				3.9 A W−1 300 nm	0.3 s/0.7 s	[128]
ZnO/Cu2O nanowire						[129]

summarizes representative results for reported 1D ZnO nanostructure-based self-powered UV photodetectors, accompanied with a brief description of the corresponding device characteristics, detection wavelengths, power intensity, and photodetector performance.

Various approaches have been adopted to improve the photoresponse performance of 1D ZnO nanostructure photodetectors for use in practical applications. Generally, surface and interface carrier transport modulation are the main routes used to raise photoresponsivity [130]. For instance, Lao et al. [131] manipulated and functionalized the polymer polystyrene sulfate (PSS) on the surface of ZnO nanobelts using a layer-by-layer self-assembly method. The sensitivity of the PSS-coated ZnO nanobelt-based detector was improved by close to 5 orders of magnitude compared with that of the device lacking PSS, which was ascribed to the energy levels introduced by PSS serving as a “hopping” state or bridge for electron transfer, effectively increasing the excitation probability of an electron to the conduction band.

In addition, functional devices consisting of a noble metal (e.g., Au [132,133] and Ag [134,135,136,137]) coated on semiconductor nanostructures have inspired extensive research effort. Such devices can realize superior optoelectronic properties to those without a metal coating because their LSPR effect leads to strong scattering and absorption of incident light, efficient separation of photogenerated electron–hole pairs, and rapid transport of charge carriers at the metal/semiconductor interface. Liu and coworkers fabricated ZnO nanowire photodetectors coated with Au nanoparticles [132]. The ratio of photocurrent to dark current of the ZnO photodetector with Au nanoparticles reached up to 5 × 10⁶, which was much larger than that of the device without Au nanoparticles (around 10³). Moreover, the decay time of the device decreased from ~300 s to ~10 s after coating with Au nanoparticles.

Al is an abundant and inexpensive metal that is a better plasmonic material than Au and Ag in the UV range because of the negative real part and relatively low imaginary part of its dielectric function [138]. Lu et al. [139] used LSP resonance mediated by Al nanoparticles to optimize the UV response of a ZnO nanorod array photodetector. The responsivity of the photodetector was improved from 0.12 to 1.59 A W⁻¹ after modification with Al nanoparticles and the ratio of photocurrent to dark current was enhanced 6-fold compared with that of the bare one, as displayed in Figure 18. Furthermore, the rise and decay times of the photodetector decreased from 0.8 and 0.85 s, respectively, to less than 0.03 and 0.035 s, respectively, after Al nanoparticle decoration.

The transition metal Co is an effective candidate material for the surface modification of photodetectors. Buddha and coworkers fabricated a UV photodetector based on a Co-coated ZnO (Co–ZnO) nanorod array [140]; a schematic of the device is displayed in Figure 19a. The presence of defect states in the Co–ZnO nanorod detector raised the photocurrent compared with that of the device with bare ZnO nanorods. The Co–ZnO nanorod photodetector was sensitive to an external magnetic field; its response current increased by about 186% under an applied magnetic field of 2400 G compared with that without an applied magnetic field at the same bias voltage (Figure 19b). Furthermore, the response and recovery times of the photodetector decreased from 38 and 195 s, respectively, to less than 1.2 and 7.4 s, respectively, upon Co decoration, as shown in Figure 19c,d.

Recently, many researchers have attempted to improve the performance of photodetectors based on ZnO 1D nanostructures through the piezo-phototronic effect [141,142,143,144,145,146] and obtained some noticeable achievements. ZnO with a non-centrosymmetric crystal structure possesses high polarization. Therefore, the generation, transport, separation, and recombination process of carriers can be controlled by the piezopotential caused by the strain in this piezoelectric semiconductor material. It is worth noting that the coupled piezoelectric polarization, optoelectronic, and semiconducting properties of 1D ZnO nanomaterials offer an opportunity to construct functional devices with improved optoelectronic performance. Wang et al. [146] demonstrated that the responsivity of an MSM ZnO micro-/nanowire photodetector was enhanced through the piezoelectric effect. Zhang et al. [143] fabricated a self-powered photodetector based on a metal–insulator–semiconductor (MIS) Pt/Al₂O₃/ZnO structure; a schematic diagram of the device is shown in Figure 20a. The Schottky barrier height (SBH) was markedly enhanced to 0.739 eV by introducing the ultrathin dielectric layer (Al₂O₃) at the interface. The SBH could be actively adjusted by the modulation of the piezopolarization-induced built-in electric field under compressive strain, as shown in Figure 20b. The responsivity and detectivity of the photodetector increased from 0.644 μA W⁻¹ and 2.92 × 10⁶ cm Hz^0.5 W⁻¹, respectively, without compressive strain to 0.644 μA W⁻¹ and 2.92 × 10⁶ cm Hz^0.5 W⁻¹, respectively, under compressive strain of −1.0%.

5. Conclusions

In conclusion, low-dimensional ZnO nanomaterials could be suitable for use in advanced optoelectronic devices, such as LEDs and photodetectors, because of their favorable properties. This review focused on the fabrication methods and growth parameters that influence the properties of the obtained 1D ZnO nanostructures. The fabricated nanomaterials could be used to construct novel devices with improved performance for certain applications. The properties, preparation, and device applications of 1D ZnO nanostructures were reviewed in detail. Importantly, several promising approaches to improve device performance, including surface passivation, LSPs, and piezo-phototronic effects, were discussed.

Considering the wide application potential of 1D ZnO nanomaterials and their excellent performance in devices, they can satisfy the requirements of “smaller, faster, colder” for electronics, which is expected to be extended to practical applications in the near future. The applications of 1D ZnO nanomaterials have expanded greatly in recent years, and the performance of various devices has been continuously improved. Novel high-performance devices based on 1D ZnO nanomaterials will certainly be an important research focus of the nanotechnology industry in the 21st century. With the improvement of preparation technology, deepening of research, constant improvement of device performance, and progress of nanotechnology industrialization, 1D ZnO nanomaterials will play an increasingly important role in the fields of energy, environmental protection, information science technology, biomedicine, security, and national defense.

The fundamental properties and applications of 1D ZnO nanomaterials have been explored extensively to optimize their advantages, providing a route to meet practical requirements in nanoscience and nanotechnology. Because 1D ZnO nanomaterials possess a large binding exciton energy, wide band gap, high carrier mobility, and non-centrosymmetric structure, they can be used to couple different properties such as semiconductivity, piezoelectricity, and photoexcitation together to develop next-generation devices with attractive properties such as intelligent features and high efficiency. At present, much attention has been paid to LEDs and photodetectors based on 1D ZnO nanomaterials, and more and more interdisciplinary research has been conducted, such as biochemical applications of 1D ZnO nanomaterials. It is anticipated that the carrier transport in 1D ZnO nanomaterials can be well tuned to maximize conversion efficiency. Through the combination of semiconducting properties and piezo-phototronic and size effects, the energy band structure of devices could be tuned to modulate charge carrier transport behavior, and device performance could be improved by controlling the SBH. Therefore, coupled effects can be readily applied to 1D ZnO nanomaterial-based devices to improve performance. We anticipate that research on 1D ZnO nanomaterial-based optoelectronic devices, such as LEDs and photodetectors, will yield further progress.

6. Perspective

There has been extensive and fundamental research performed on 1D ZnO nanomaterials and devices, resulting in methods to construct intelligent, smart, functional units with advanced performances for potential applications. One-dimensional ZnO nanomaterials generally have non-central symmetry crystal structure, high crystal quality, large aspect ratio, high carrier mobility, wide band gap, and other characteristics that can be applied for coupling with properties such as photoexcitation, piezoelectricity, and semiconductivity for designing next-generation, newly emerging devices. At present, many types of 1D ZnO nanodevices have emerged with promising applications, for example, energy harvesting, piezo-phototronic light emitting, photodetecting, nano-biosensing, and so on, based on 1D ZnO nanomaterials. When considering the design factors for building several kinds of nanodevices based on 1D ZnO nanomaterial, charge carrier generation, transport, and recombination dynamics are generally considered by studying band gap structure, non-central symmetry induced piezotronic effects, surface or interface contact properties, and so on. Thus, conversion efficiency could be key in the overall consideration for building advanced optoelectric devices with improved performance. Until now, it was anticipated that the charge carrier transport behaviors of the 1D ZnO nanomaterials could be well modulated for building devices based on piezotronic and piezo-phototronic effects. Self-powered 1D ZnO nanodevices could be constructed with outperforming properties based on the above effects, thus, next-generation, intelligent products with smart volume could be built by applying the mentioned ideas. By optimally combining their properties based on 1D ZnO nanomaterials, the performance of these devices, such as nanogenerators, LEDs, photodetectors, nanosensors, and memory devices, can be improved by modulating charge carrier transport behavior, interface barrier structures, piezotronic effect, and so on. It is believed that the coupled effects can be well applied for modulating carrier transport behaviors in 1D ZnO nanodevices for improving their performance. Further, newly emerging 1D ZnO nanodevices could be constructed for promising applications by studying its known effects and other properties through doping and surface modification. Finally, we anticipate that research work on 1D ZnO nanodevices—such as LEDs, photodetectors, solar cells, nanosensors, nanogenerators, and others—will yield continuous progress, and novel optoelectric, light emitting, electric nanodevices could be created through further efforts.