ZnO Single-crystal Microtubes Research Articles

Optimized optical vapor supersaturated precipitation for time-saving growth of ultrathin-walled ZnO single-crystal microtubes

Here we report growth of 〈0 0 0 1〉-oriented ultrathin-walled ZnO single-crystal microtubes with diameter of 75–250 μm and facet wall of <500 nm in thickness by optimized optical vapor supersaturated precipitation (OVSP). The mechanism of ultrathin-walled microtube formation during OVSP is revealed. The presintering temperature of precursor rod in the range of 600–800 °C is found to be critical to achieve the complete hexagonal cross-sectional geometry of microtube. The facet wall is then thinned down to ∼450 nm with the temperature holding time increasing during OVSP under the lamp power of 60%@6000 W. The ultraviolet photoluminescence indicates the exciton-exciton collisions (i.e. p-band) boosted in the ultrathin-walled ZnO microtube. Considering the time-consuming process in presintering of precursor rod in the Molysili furnace, the in-situ optical vapor supersaturated precipitation (IOVSP) is developed, in which the optical presintering is first performed in the image furnace using the lamp power of 30%@6000 W for 6 h and then the lamp power is directly increased to 60%@6000 W for microtube growth by OVSP. The cooling process and transfer of precursor rod from the Molysili furnace to the image furnace are eliminated and the growth time can therefore be saved 56.7%. The finished microtube demonstrates a perfect hexagonal cross section with smooth surface and thin facet wall of ∼500 nm enhancing exciton-exciton collisions. The present work provides a time-saving in-situ method to grow high-quality ultrathin-walled ZnO single-crystal microtubes served as optical microcavities in the future for the applications in micro/nanophotonics.

Synthesis of highly conductive thin-walled Al-doped ZnO single-crystal microtubes by a solid state method

ZnO has attracted considerable attention in fundamental studies and practical applications for the past decade due to its outstanding performance in gas sensing, photocatalytic degradation, light harvesting, UV-light emitting/lasing, etc. The large-sized thin-walled ZnO (TW-ZnO) microtube with stable and rich VZn-related acceptors grown by optical vapor supersaturated precipitation (OVSP) is a novel multifunctional optoelectronic material. Unfortunately, the OVSP cannot achieve doping due to the vapor growth process. To obtain doped TW-ZnO microtubes, a solid state method is introduced in this work to achieve thin-walled Al-doping ZnO (TW-ZnO:Al) microtubes with high electrical conductivity. The morphology and microstructures of ZnO:Al microtubes are similar to undoped ones. The Al3+ ions are confirmed to substitute Zn2+ sites and Zn(0/−1) vacancies in the lattice of ZnO by EDS, XRD, Raman and temperature-dependent photoluminescence analyses. The Al dopant acting as a donor level offers massive free electrons to increase the carrier concentrations. The resistivity of the ZnO:Al microtube is reduced down to ∼10−3 Ω·cm, which is one order of magnitude lower than that of the undoped microtube. The present work provides a simple way to achieve doped ZnO tubular components for potential device applications in optoelectronics.

Experimental and numerical study on growth of high-quality ZnO single-crystal microtubes by optical vapor supersaturated precipitation method

In this work, high-quality free-standing ZnO single-crystal microtubes with hexagonal cross-section were fabricated by an optical image furnace. Optical vapor supersaturated precipitation (OVSP) and axial photo-thermal-decomposition were proposed to interpret the microrods growth and microtubes formation, respectively. The maximum dimensions of the grown microtube were 5mm in length, 100µm in diameter and 1µm in facet wall thickness. In our previous work, a new room-temperature photoluminescence (PL) peak (~392nm) of ZnO microtubes was attributed to VZn-related donor-acceptor-pairs (DAP) transition. This work further confirmed the VZn-related acceptors widely existing during ZnO microrods/ microtubes growth by OVSP. The effects of major growth parameters (e.g. lamp power, filament geometry and growth platform shape) on temperature field at the growth platform of precursor rod were studied by a finite element model as well. The lamp power of 65% (1500W), thick single-filament and appropriate conical growth platform were optimized for a uniform temperature field to achieve consistent finish quality of microtubes and prevent twin-microtubes formation. This work would be beneficial for batch growth of the novel ZnO microtubes/microrods with high quality for a variety of applications.

Investigation and characterization of ZnO single crystal microtubes

Morphological, structural, and optical characterization of microwave synthesized ZnO single crystal microtubes were investigated in this work. The structure and morphology of the ZnO microtubes are characterized using X-ray diffraction (XRD), single crystal diffraction (SCD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). The results reveal that the as-synthesized ZnO microtube has a highly regular hexagonal cross section and smooth surfaces with an average length of 650–700μm, an average outer diameter of 50μm and wall thickness of 1–3μm, possessing a single crystal wurtzite hexagonal structure. Optical properties of ZnO single crystal microtubes were investigated by photoluminescence (PL) and ultraviolet-visible (UV-vis) absorption techniques. Room-temperature PL spectrum of the microtube reveal a strong UV emission peak at around 375.89nm and broad and a weak visible emission with a main peak identified at 577nm, which was assigned to the nearest band-edge emission and the deep-level emission, respectively. The band gap energy of ZnO microtube was found to be 3.27eV.

ZnO single crystal microtubes: Synthesis, growth mechanism, and geometric structure using direct microwave irradiation

ZnO single crystal microtubes have been successfully synthesized using direct microwave irradiation via the catalyst-free vapor–solid (V–S) process. Field emission scanning electron microscopy (FE-SEM) showed that ZnO single crystal microtubes exhibit a symmetrical 6-facet structure with an exact hexagonal hollow. ZnO single crystal microtubes have an average outer diameter of 100µm and a length of over 300µm, and the wall thickness ranges from 1 to 2µm. X-ray diffraction (XRD) reveals a single high-intensity diffraction peak along the (101) direction, suggesting good crystallization of the synthesized ZnO single crystal microtubes. Single crystal diffraction (SCD) reveals the obvious polar nature of the ZnO single crystal microtube structure. The room-temperature photoluminescence (PL) spectrum shows an intense ultraviolet (UV) emission at 375.68nm, with weak and broad visible light emissions in the range between 435nm and 700nm at a maximum peak of 588.66nm.

UV Photoresponse and Field Emission Properties of ZnO Single Crystal Microtubes

We present the UV photodetection properties and the field emission properties of ZnO single-crystal microtubes synthesized using a microwave-heating growth method. The ZnO microtubes exhibited relatively fast UV photoresponse with a cut-off wavelength ∼ 370 nm, indicating their potential application as UV detectors with high efficiency and low cost. The turn-on voltage for the as-grown and post-annealed ZnO microtube samples were 5.6 V/μ m and 6.4 V/μ m, respectively. The emission current density reached about 11 mA/cm 2 for the as-grown sample at an applied field of 20 V/μ m, and 12.5 mA/cm 2 for the post-annealed sample at an applied field of 13 V/μ m. Due to its relative large tip area (in comparison with ZnO nanomaterials) of the ZnO microtube, the intrinsic field emission enhancement factor β of the ZnO microtube was relatively low; however, the experimental β values were quite high, indicating strong field emission characteristics of the ZnO microtubes.

Field Emission Properties of ZnO Single Crystal Microtubes

Field emission properties of ZnO single-crystal microtubes were investigated in this work. The turn-on voltages for the as-grown and postannealed ZnO microtube samples were 5.6 and 6.4 V/μm, respectively. The emission current density was 11 mA/cm2 at an applied field of 20 V/μm for the as grown ZnO microtube and 12.5 mA/cm2 at an applied field of 13 V/μm for the postannealed ZnO microtube. Due to the relatively large tip area (in comparison with ZnO nanomaterials) of the ZnO microtubes, the intrinsic field emission enhancement factor β of the ZnO microtube was comparatively low (estimated to be around 6.5). However, the experimental β values were quite high (418 for the as-grown ZnO microtube and 1466 for the postannealed ZnO microtube), indicating strong field emission characteristics of the ZnO microtubes.

ZnO microtube ultraviolet detectors

ZnO stands a good chance of being a candidate material for solar-blind UV detection because of its direct band-gap of 3.37 eV and high photoresponse. In this work, we present the UV photodetection properties of ZnO single crystal microtubes synthesized using a microwave-heating growth method. The ZnO microtubes exhibited relatively fast UV photoresponse with a cut-off wavelength ∼370 nm, indicating their potential application as UV detectors with high efficiency and low cost.

Investigation of hexagonal microtube ZnO on silicon by capacitance-voltage measurements

The model and the intrinsic carrier concentration of hexagonal ZnO single-crystal microtubes are investigated by capacitance-voltage (C-V) measurements. The film fabricated by hydrothermal method on p-type silicon (111) is composed of microtubes with hexagonal tubular structure, which have diameters of 3–4μm and lengths in the range of 10–20μm. In this article, the structure of ZnO∕Si was analyzed and modeled, and the total capacitance model of the sample was deduced by analyzing the C-V characteristics of the ZnO. The small-signal equivalent scheme of the sample capacitance is also obtained. The C-V profiling calculated by the model agreed with the measured C-V curve. As a simple application of the model, the intrinsic carrier-concentration distribution of ZnO was extracted.

Zinc oxide microtubes prepared by optical thermal evaporation

ZnO single-crystal microtubes were fabricated by an optical thermal evaporation method. The microtubes showed hexagonal hollow morphology with well faceted side surfaces, with cross-sectional dimensions of 20–100 µm and lengths of several millimetres. Air flow is found to be the critical experimental parameter for the formation of ZnO microtubes with different cross-sections. Under the excitation of 325 nm, photoluminescence of the microtubes was obtained with a peak wavelength of 492 nm.

Zinc oxide single-crystal microtubes

ZnO single-crystal microtubes were fabricated using an encapsulated microwave growth method. The ZnO crystals are grown in hexagonal hollow tube form with a well faceted end and side surfaces, which have cross-sectional dimensions of 100to250μm, lengths of 3–5mm, and wall thickness of 1–2μm. Under optical excitation, a strong near-band-edge emission was obtained at a peak wavelength of 377.8nm with a full width at half maximum of 11nm. The ZnO microtubes exhibited a highly selective UV light response with a cut-off wavelength at ∼370nm, and excellent electron field emission properties with an emission current density of 11mA∕cm2 at an applied field of ∼20V∕μm.