Single ZnO Nanorods Research Articles

Unraveling Anisotropic and Pulsating Etching of ZnO Nanorods in Hydrochloric Acid via Correlative Electron Microscopy.

Despite much technical progress achieved so far, the exact surface and shape evolution during wet chemical etching is less unraveled, especially in ionically bonded ceramics. Herein, by using in situ liquid cell transmission electron microscopy, a repeated two-stage anisotropic and pulsating periodic etching dynamic is discovered during the pencil shape evolution of a single crystal ZnO nanorod in aqueous hydrochloric acid. Specifically, the nanopencil tip shrinks at a slower rate along [0001̅] than that along the ⟨101̅0⟩ directions, resulting in a sharper ZnO pencil tip. Afterward, rapid tip dissolution happens due to accelerated etching rates along various crystal directions. Concurrently, the vicinal base region of the original nanopencil tip emerges as a new tip followed by the repeated sequence of tip shrinking and removal. The high-index surfaces, such as {101̅m} (m = 0, 1, 2, or 3) and {21̅ 1̅n} (n = 0, 1, 2, or 3), are found to preferentially expose in different ratios. Our 3D electron tomography, convergent beam electron diffraction, middle-angle bright-field STEM, and XPS results indicate the dissociative Cl- species were bound to the Zn-terminated tip surfaces. Furthermore, DFT calculation suggests the preferential Cl- passivation over the {101̅1} and (0001) surfaces of lower energy than others, leading to preferential surface exposures and the oscillatory variation of different facet etching rates. The boosted reactivity due to high-index nanoscale surface exposures is confirmed by comparatively enhanced chemical sensing and CO2 hydrogenation activity. These findings provide an in-depth understanding of anisotropic wet chemical etching of ionic nanocrystals and offer a design strategy for advanced functional materials.

Nanopatterned rGO/ZnO:Al seed layer for vertical growth of single ZnO nanorods

In this work, we demonstrate a novel low-cost template-assisted route to synthesize vertical ZnO nanorod arrays on Si (100). The nanorods were grown on a patterned double seed layer comprised of reduced graphene oxide (rGO) and Al-doped ZnO nanoparticles. The seed layer was fabricated by spray-coating the substrate with graphene and then dip-coating it into a Al-doped ZnO sol–gel solution. The growth template was fabricated from a double-layer resist, spin-coated on top of the rGO/ZnO:Al seed layer, and patterned by colloidal lithography. The results show a successful chemical bath deposition of vertically aligned ZnO nanorods with controllable diameter and density in the nanoholes in the patterned resist mask. Our novel method can presumably be used to fabricate electronic devices on virtually any smooth substrate with a thermal budget of 1 min at 300 °C with the seed layer acting as a conductive strain-relieving back contact. The top contact can simply be made by depositing a suitable transparent conductive oxide or metal, depending on the specific application.

Modulation of bandgap and electrical conductivity in europium doped single ZnO nanorod device

We investigate the effect of europium doping on the optical properties and electrical conductivity of hydrothermally grown ZnO nanorods. The widening of optical bandgap with europium doping occurs because of the Burstein-Moss effect upto 0.8% and bandgap renormalization is observed at higher concentration. The temperature-dependent resistivity measurements of the individual nanorods reveal that the intrinsic electrical conductivity is a combination of thermally activated and nearest-neighbor hopping conduction processes at high and low-temperature regimes, respectively. We can successfully correlate that the modulation of carrier concentration in the defect D and D− bands can tune both bandgap and electron transport in Eu doped ZnO nanorods.

Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide.

Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS2 and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS2. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS2 showed enhanced luminescence of MoS2 and multimodal emission.

Modulation of Bandgap and Electrical Conductivity in Europium Doped Single Zno Nanorod Device

Modulation of Bandgap and Electrical Conductivity in Europium Doped Single Zno Nanorod Device

Fabrication of ZnO Nanobrushes by H2-C2H2 Plasma Etching for H2 Sensing Applications.

Zinc oxide has widespread use in diverse applications due to its distinct properties. Many of these applications benefit from controlling the morphology on the nanoscale, where for example gas sensing is strongly enhanced for high surface-to-volume ratios. In this work the formation of novel ZnO nanobrushes by plasma etching treatment as a new approach is presented. The morphology and structure of the ZnO nanobrushes are studied in detail by transmission and scanning electron microscopy. It is revealed that ZnO nanobrush structures are fabricated by self-patterned preferential etching of ZnO microtetrapods in a hydrogen-acetylene plasma. The etching process was found to be most effective at 1% C2H2 admixture. Nanowire arrays are formed enabled by sidewall passivation due to a-C:H deposition. The nanobrush structures are further stabilized by simultaneous deposition of a SiOx layer from the opposite direction. Highly sensitive (gas response S = 148), selective, and fast (response time 15s, recovery time 6 s) hydrogen sensors are fabricated from single nanobrushes. Single nanobrush sensors show enhanced sensing performance in increased gas response S of at least 10 times and improved response as well as recovery times when compared to nonporous single ZnO nanorod sensors due to the small diameters (≈50 nm) of the formed nanowires as well as the strongly enhanced surface-to-volume ratio of the nanobrushes by a factor of more than 10.

Interface design for electrically pumped ultraviolet nanolaser from single ZnO-nanorod

Nanolaser, especially electrically pumped nanolaser, has attractive applications in optoelectronic or photonic interconnection, optical communication, biosensing, medical imaging and 3D display. It is in challenge how to inject charge carrier effectively for sufficient gain and how to overcome the heavy optical loss in such small sized laser device. Facing to the issues, an individual hexagonal ZnO nanorod was bonded on p-GaN to construct a heterojunction through PEDOT/HfO2 buffer in our case. Optically, the inserted buffer with low reflective index reduces the optical loss, hence decreases the lasing threshold from 1.55 μW to 1.32 μW of the nanocavity. Electrically, a reliable electrical contact is guaranteed by inserting the organic polymer PEDOT facilely. More importantly, PEDOT and HfO2 form a step energy band configuration in the p-GaN/ZnO heterojunction for efficient charge carrier injection, thus contribute to improve radiative recombination. As the injection current reached 6 mA, the FWHM of the emission spectrum quickly narrowed from ~ 30 to 1.4 nm, and the intensity increase dramatically. This demonstrated the UV lasing behaviors in a nano-scaled heterojunction diode.

One-step synthesis CdS/single crystal ZnO nanorod heterostructures with high photocatalytic H2 production ability

CdS/single crystal ZnO nanorod heterostructure (CdS/ZnO) was fabricated via microwave synthesis method, and the structural, morphological and photocatalytic hydrogen generation was tested. Studied results demonstrated that CdS particles were decorated on the surface of single crystal ZnO nanorod with crystallite size about 43 nm, and CdS nanoparticle significantly enhanced the photocatalytic hydrogen generation ability of the heterostructure by increasing the specific surface area, promoting the separation, migration of photoinduced carriers and increasing the light absorption. Pure ZnO exhibits bandgap energy of 3.09 eV, while the CdS/ZnO sample possessed a more narrow band gap (2.62 eV) than ZnO nanorod. Under the irradiation of visible light, the highest photocatalytic hydrogen production rate of 4076 µmolg−1h−1 is observed over the CdS/ZnO with the content of CdS is 38 wt%, which is about 13.59 and 76.91 times more active than pure ZnO nanorod and CdS nanoparticle. In addition, the specific surface area of the composite sample reaches 43.375 m2/g, and the instantaneous photocurrent reaches 130 μA /cm2. The present microwave radiation method is simple, easy to control, and timesaving, thereby providing a efficient route for the preparation of metal oxide-based photocatalytic material with high hydrogen evolution activity.

Low-Cost Electrodeposition of Size-Tunable Single-Crystal ZnO Nanorods

In this paper we report a low cost, simple, electrochemical method for large-area growth of single crystal ZnO nanorods. The method utilizes a metallic zinc foil as the source of the necessary zinc ions for ZnO growth on indium-doped tin oxide (ITO) glass slides. The method is thoroughly discussed and investigated varying all the parameters involved. The resulting ZnO nanorods are highly oriented along c-axis and densely packed, while their length and diameter can be tuned by varying the growth parameters. Two different types of seed layers on the ITO glass slides are tested. A seed layer made by spin coating of ZnO nanoparticles results in a twofold increase of the ZnO nanorod surface density as compared with a ZnO thin film seed layer by physical vapor deposition. Additionally, the effect of oxygen supply during electrodeposition was investigated as a crucial regulatory parameter not only for the geometrical and topological characteristics of the ZnO nano-arrays but for their physical properties as well.

NO2 Gas Sensing Characters of Polycrystaline ZnO Nanorods

ZnO has attracted much attention as an important semiconductor material, since it has a wide band gap (3.4eV), and excellent stability. It has been extensively researched to detect many gases, such as H2, C2H5OH, NH3 and VOC(volatile organic compounds). Herein, we developed a MOF derived ZIF-8 coated ZnO nanorods(ZnO@ZIF) as a precursor by solvethermal route, then calcined at 500 ºC in air, during the process, the exposure facets of ZnO nanorods have changed. The polycrystaline ZnO nanorods shows higher response to 1-5 ppm NO2 at 140ºC compared with single crystal ZnO nanorods. This provides a new etching way to change the surface facets by using ZIF coated and calcined in air.

Effect of oxygen nonstoichiometry on the photoelectrochemical performance of oxide-nanorod based TiO2/Sb2S3 and ZnO/Sb2S3 heterostructured photoanodes

Single crystalline ZnO and TiO2 nanorods are grown on fluorine-doped tin oxide (FTO) substrates by hydrothermal method. The nanorods are annealed under air and reducing conditions to alter the oxygen nonstoichiometry and hence the mid-bandgap defect states. Such an annealing process is shown to impart significant change on the photoelectrochemical (PEC) performance of the photoelectrodes. Large photocurrent densities (J) of 0.78 mA cm−2 are obtained for air annealed (AA) TiO2 nanorods (TNR) compared to hydrogen annealed (HA) TNR (J = 0.36 mA cm−2). ZnO nanorods (ZNR), on the contrary, shows photocurrent density of 0.76 mA cm−2 and 0.36 mA cm−2 for ZNR-HA and ZNR-AA photoanodes, respectively. The contrasting difference in the PEC performance is attributed to the synergetic effect of interfacial impedance with electrolytes and the oxygen nonstoichiometry. Further, to overcome the limitation of light absorption by these materials owing to their wide bandgap, TiO2 and ZnO nanorods are coated with Sb2S3 by chemical bath deposition to form heterostructured TNR-AA/Sb2S3(CBD) and ZNR-HA/Sb2S3(CBD) thin films. Such heterostructures exhibit enhanced photocurrent values of ∼1.39 mA cm−2 and 3.36 mA cm−2 (at 1.6 V versus Ag/AgCl), respectively. The PEC performances of the nanorods are analyzed in terms of the annealing conditions and subsequent introduction of defect states in the bandgap. The present study shows the importance of oxygen defect control at the interface between the oxide and chalcogenide, and its role in the betterment of PEC performance in TiO2/Sb2S3 and ZnO/Sb2S3 heterostructure photoanodes.

Flower-Like ZnO Nanorods Synthesized by Microwave-Assisted One-Pot Method for Detecting Reducing Gases: Structural Properties and Sensing Reversibility.

In this work, flower-like ZnO nanorods (NRs) were successfully prepared using microwave-assisted techniques at a low temperature. The synthesized NRs exhibited a smooth surface and good crystal structure phase of ZnO. The sharp peak of the XRD and Raman spectrum confirmed the high crystallinity of these ZnO NRs with a pure wurtzite structure. The nanorods were ~2 μm in length and ~150 nm in diameter, respectively. The electron diffraction pattern confirmed that the single crystal ZnO nanorods aligned along the [001] plane. The NRs were applied to fabricate a gas sensor for reducing gases such as CH4, CO, and H2. The sensor showed a good performance and sensitivity toward the target gases. However, its response toward CH4 and CO was higher compared to H2 gas. Although the operating temperature was varied from room temperature (RT) up to 350°C, the sensor did not show a response toward any of the target gases in the range of RT-150°C, but dramatic enhancement of the sensor response was observed at 200°C, and up to higher temperatures. This behavior was ascribed to the activity of the smooth surface and the reactivity of surface oxygen species with the targeted gases. The sensor response was measured at various gas concentrations, where the calibration curve was shown. The gas sensing mechanism was described in terms of the reaction of the gases with the transformed oxygen species on the surface of the oxides.

Mediated ferromagnetism in ZnO nanorods with heavily codoped MnFe

The ferromagnetic properties are investigated in single crystal ZnO nanorods with heavy codoping of 28 at. % Mn and 1 at. % Fe fabricated by vapor phase transport. The electron energy loss spectroscopy and x-ray photoelectron spectroscopy confirmed that Mn and Fe dopants are, respectively, +2 and +3 valence states, where Raman scattering exhibited additional peaks at 308 cm−1, 458 cm−1, 480 cm−1, 504 cm−1, 516 cm−1, 530 cm−1, and 569 cm−1, associated with MnFe dopants. Compared with undoped and Mn doped ZnO nanostructures, the MnFe-codoped ZnO nanorods are strongly ferromagnetic below Tc of 39 K and around room temperature. The ferromagnetic behavior can be mainly attributed to the carrier-mediated Ruderman–Kittel–Kasuya–Yosida mechanism below Tc and the defect-induced bound magnetic polaron mechanism around room temperature.

Photoluminescence of ZnO Nanowires: A Review.

One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy−cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands.

Highly Rectifying Heterojunctions Formed by Annealed ZnO Nanorods on GaN Substrates.

We study the effect of thermal annealing on the electrical properties of the nanoscale p-n heterojunctions based on single n-type ZnO nanorods on p-type GaN substrates. The ZnO nanorods are prepared by chemical bath deposition on both plain GaN substrates and on the substrates locally patterned by focused ion beam lithography. Electrical properties of single nanorod heterojunctions are measured with a nanoprobe in the vacuum chamber of a scanning electron microscope. The focused ion beam lithography provides a uniform nucleation of ZnO, which results in a uniform growth of ZnO nanorods. The specific configuration of the interface between the ZnO nanorods and GaN substrate created by the focused ion beam suppresses the surface leakage current and improves the current-voltage characteristics. Further improvement of the electrical characteristics is achieved by annealing of the structures in nitrogen, which limits the defect-mediated leakage current and increases the carrier injection efficiency.

Formation of nanoripples on ZnO flat substrates and nanorods by gas cluster ion bombardment.

In the present study Ar+ cluster ions accelerated by voltages in the range of 5–10 kV are used to irradiate single crystal ZnO substrates and nanorods to fabricate self-assembled surface nanoripple arrays. The ripple formation is observed when the incidence angle of the cluster beam is in the range of 30–70°. The influence of incidence angle, accelerating voltage, and fluence on the ripple formation is studied. Wavelength and height of the nanoripples increase with increasing accelerating voltage and fluence for both targets. The nanoripples formed on the flat substrates remind of aeolian sand ripples. The ripples formed at high ion fluences on the nanorod facets resemble well-ordered parallel steps or ribs. The more ordered ripple formation on nanorods can be associated with the confinement of the nanorod facets in comparison with the quasi-infinite surface of the flat substrates.

Photocatalytic performance of single crystal ZnO nanorods and ZnO nanorods films under natural sunlight

To solve the low catalytic activity and recycling difficulty of ZnO nanomaterials, a simple and large-scale synthesized method was used to prepare single crystalline ZnO nanorods with visible light response, and then ZnO membrane was fabricated using carbon fiber (CF) cloth as support. The resultant ZnO nanorods were carriedout with X-ray diffraction (XRD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), Raman spectra, X-ray photoelectron spectroscopy (XPS), Ultraviolet spectrophotometer, and fluorescence spectrum. The experimental results display that the prepared ZnO nanorods own single crystalline structure and outstanding photocatalytic performance, which the removal time of all RhB is 15 min at UV light, 60 min at visible light and 100 min at natural sunlight, respectively. Moreover, ZnO nanorod membrane can completely remove RhB within 6 h under natural sunlight, meaning that the synthesized ZnO nanorods and ZnO nanorod membrane have potential industrial application prospect.

Electrical properties of nanoscale p-n heterojunctions formed between a single ZnO nanorod and GaN substrate

We study electronic transport in single vertically oriented n-type ZnO nanorods on p-type GaN substrates using a nanoprobe in a scanning electron microscope. The ZnO nanorods are prepared by chemical bath deposition on both plain GaN substrates and the substrates locally modified by focused ion beam. The heterojunctions on focused ion beam-modified substrates show rectifying current-voltage characteristics, while the characteristics of the plain structures are symmetrical. Adsorption/desorption processes on the surface of ZnO nanorods strongly affect their electrical properties. We demonstrate that the electronic transport in the nanorods grown on focused ion beam-modified substrates is less sensitive to adsorption/desorption processes, which is related to their uniform nucleation and higher crystalline quality.

Tuning ZnO nanorods photoluminescence through atmospheric plasma treatments

Room temperature atmospheric plasma treatments are widely used to activate and control chemical functionalities at surfaces. Here, we investigated the effect of atmospheric pressure plasma jet (APPJ) treatments in reducing atmosphere (Ar/1‰ H2 mixture) on the photoluminescence (PL) properties of single crystal ZnO nanorods (NRs) grown through hydrothermal synthesis on fluorine-doped tin oxide glass substrates. The results were compared with a standard annealing process in air at 300 °C. Steady-state photoluminescence showed strong suppression of the defect emission in ZnO NRs for both plasma and thermal treatments. On the other side, the APPJ process induced an increase in PL quantum efficiency (QE), while the annealing does not show any improvement. The QE in the plasma treated samples was mainly determined by the near band-edge emission, which increased 5–6 fold compared to the as-prepared samples. This behavior suggests that the quenching of the defect emission is related to the substitution of hydrogen probably in zinc vacancies (VZn), while the enhancement of UV emission is due to doping originated by interstitial hydrogen (Hi), which diffuses out during annealing. Our results demonstrate that atmospheric pressure plasma can induce a similar hydrogen doping as ordinarily used vacuum processes and highlight that the APPJ treatments are not limited to the surfaces but can lead to subsurface modifications. APPJ processes at room temperature and under ambient air conditions are stable, convenient, and efficient methods, compared to thermal treatments to improve the optical and surface properties of ZnO NRs, and remarkably increase the efficiency of UV emission.

Position- and Polarization-Specific Waveguiding of Multi-Emissions in Single ZnO Nanorods.

We examine multiphoton-produced optical signals waveguided through single ZnO nanorods (NRs) using a newly developed, scanning offset-emission hyperspectral microscopy (SOHM) technique. Specifically, we concurrently analyze waveguiding behaviors of sum-frequency generation (SFG), deep-trap emissions (DTE), and coherent anti-Stokes Raman scattering (CARS) occurring in individual ZnO NRs. SOHM acquires spectrally-indexed and spatially-resolved intensity maps/spectra of waveguided light intensity while excitation/emission collection positions and light polarization are scanned. Hence, the powerful measurement capabilities of SOHM enable quantitative analyses of the different ZnO NR waveguiding behaviors specific to the multiphoton-generated emissions as a function of measurement position, light-matter interaction geometry, and the optical origin of the guided signal. We subsequently reveal the distinct waveguiding behaviors of single ZnO NRs pertaining to the SFG-, DTE-, and CARS-originated signals and discuss particularly attractive ZnO NR properties in CARS waveguiding.