Ruhua Tao

Electrical, optical and structural properties of CuCrO2 films prepared by pulsed laser deposition

Transparent conducting CuCrO2 thin films were prepared by pulsed laser deposition (PLD) from a nanocrystalline CuCrO2 target. The derived CuCrO2 films were highly c-axis oriented deposited at 800 K. The microstructural, electrical as well as optical properties were studied. It was found that the films were relatively smooth and behaved as semiconductors. The energy band of the CuCrO2 films is constructed based on the Mott–Davis model in order to investigate the conduction mechanism. The transmittances of the films in the visible region are about 60–80% with direct band gaps of about 3.2 eV. The results suggested that CuCrO2 films could be successfully prepared by the PLD method, which can broaden the applications of the transparent conducting oxide films.

Magnetic and electrical properties of p-type Mn-doped CuCrO2 semiconductors

Mn-doped CuCrO2 polycrystalline semiconductors were synthesized by the sol–gel method. The valence state of the Mn ions in the CuCr1−xMnxO2 samples was identified as 3+ by analysing their lattice parameters, their electrical properties and their x-ray photoelectron spectra. The temperature dependence of the susceptibility of all samples exhibits paramagnetic behaviour at high temperature. For x = 0.20 the samples exhibit a clear ferromagnetic (FM) transition at 120 K. Clear hysteresis loops indicate that FM order exists in the Mn-doped samples at 50 K. All samples behave like semiconductors. The electrical and magnetic properties indicate that the ferromagnetism can be attributed to the double exchange interaction between the Mn3+ and Cr3+ in CuCr1−xMnxO2 semiconductors.

Room temperature deposition of amorphous p-type CuFeO2 and fabrication of CuFeO2/n-Si heterojunction by RF sputtering method

Transparent conducting amorphous p-type CuFeO2 (CFO) thin film was prepared by radio-frequency (RF) magnetron sputtering method at room temperature using polycrystalline CuFeO2 target. Amorphous structure of as-deposited film was confirmed by XRD. XPS analysis convinced that the chemical state of Cu+ and Fe3+ in the film, and the chemical composition of the thin films is close to the stoichiometry of CuFeO2. Surface morphology of the film was analysed by SEM studies. p-type nature and concentration of carriers was investigated by Hall effect measurement. The p–n heterojunction in the structure of Al/n-Si/p-CuFeO2/Al showed good rectifying behaviour with a forward and reverse currents ratio of 555 at 2 V. The turn-on voltage and reverse leakage current values were found to be 0.9 V and 4 μA at −2 V. Further, the conduction mechanism of forward bias voltage was controlled by thermionic emission (TE) and trap-space charge limited current (TCLC) mechanisms.

Fabrication and characterization of ZnO nanowires grown on Ti substrate

Zinc oxide (ZnO) with a wide band gap of 3.37 eV, and a large exciton binding energy of 60 mV at room temperature, is one of the most important n-type semiconductor, that has potential applications in the area of short-wavelength optoelectronic devices, gas sensors, solar cells, and field emitters. Some advanced nanodevices based on one-dimensional (1-D) ZnO nanomaterials have been successfully demonstrated in the past few years. The types of substrate have a great influence on the properties of ZnO nanostrctural devices. Semiconductor substrates such as Si and Al2O3 were widely used for the collection or epitaxial growth of ZnO nanostructures, for metal substrate, Fe and Cu foil has also been used as substrate, there are few reports on ZnO nanowires grown on Ti foil, Ti is an important electrode metal that ohmic contact can be appropriately achieved, which is critical for semiconductor device application. Besides, both Ti and ZnO show good biocompatibility, it is expected that ZnO nanowires/ Ti show good performance on bio-sensors. In this paper, 1-D ZnO nanostructures have been successfully fabricated on the conductive Ti substrate via a vapor phase transport (VPT) method by carbothermal reduction of ZnO and graphite powder mixture in a tube furnace at 850°C. The final products were characterized by means of field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), high-solution transmission electron microscope (HRTEM) (equipped with selected area electron diffraction, SAED), and photoluminescence (PL) spectroscopy. FE SEM results show that dense, ultra-long (>10μm), and locally aligned ZnO nanowire arrays were grown on the Ti foil. The diameter of nanowires exhibits a wide range from 150 nm to about 500nm. Structural characterizations (XRD, SAED, HRTEM) indicate the as synthesized nanostructures have a ZnO wurtzite structure and are perfect single crystalline without any defects or impurities. The growth direction is [0001]. Optical property (PL spectrum) shows strong UV emission is detected in our sample.

Structural and electrical properties of delafossite CuMO2 (M=Al, Cr, Y) semiconductors and their exploitation for ozone detection

Single phase polycrystalline pellets of CuMO2 (M = Al, Cr, Y) semiconductors with delafossite structure were prepared by sol-gel method and solid state reaction, respectively. The XRD results shows that structure can be indexed as 3R-CuAlO2 (JCPDF No. 35-1401), 3R-CuCrO2 (JCPDF No. 89-6744) and 2H-CuYO2 (JCPDF No. 76-1422), respectively. The conductivities of CuMO2 are thermally activated in the measured temperature range with the activation energy EA about 0.24eV, 0.34eV and 0.25eV, respectively. The conductivities of CuMO2 decrease monotonously with the increase of radius of M cation. This phenomenon coincides with the previous theoretical studies that the hole conduction path of CuMO2 was predominantly in the Cu-ions layers, and the Cu-Cu spacing (or α-axis length) was governed by the M cation size, which modifies the wave function overlap between Cu-ions and results in decrease of the conductivities. The room temperature ozone sensing properties of CuMO2 (M = Al, Cr, Y) polycrystalline pellets were studied. Ozone-purified air-ozone circles were used to measure the ozone gas sensing properties of all the specimens, which are similar to the practical measurement environments. The relative humidity of the environment was controlled around 60 RH% ± 5 RH%. The temperature was controlled around 300K ± 0.5K. Except for CuYO2 pellets, CuAlO2 and CuCrO2 pellets show reversible responds to ozone gas at room temperature. The room temperature ozone sensing properties of CuCrO2 film prepared by pulsed laser deposition was also studied. The response time of CuCrO2 film is about 3.5 min to 90% of the final value and the recovery time is about 2 min to 10% of the steady state signal under ozone concentration of 600 ppm. Though the performance is not yet sufficiently high for practical use, the delafossites CuMO2 (M = Al and Cr), as parent compounds of room temperature ozone sensing materials, are recommendable for further studies on the improvement of ozone sensing properties.

Oxygen Vacancy Defects Boosted High Performance p-Type Delafossite CuCrO2 Gas Sensors

p-type ternary oxides can be extensively explored as alternative sensing channels to binary oxides with diverse structural and compositional versatilities. Seeking a novel approach to magnify their sensitivities toward gas molecules, e.g., volatile organic compounds (VOCs), will definitely expand their applications in the frontier area of healthcare and air-quality monitoring. In this work, delafossite CuCrO2 (CCO) nanoparticles with different grain sizes have been utilized as p-type ternary oxide sensors. It was found that singly ionized oxygen vacancies (Vo•) defects, compared with the grain size of CCO nanoparticles, play an important role in enhancing the charge exchange at the VOCs molecules/CCO interface. In addition to suppressing the hole concentration of the sensor channel, the unpaired electron trapped in Vo• provides an active site for chemisorptions of environmental oxygen and VOCs molecules. The synergetic effect is responsible for the observed increase of sensitivity. Furthermore, the sensitive (Vo• defect-rich) CCO sensor exhibits good reproducibility and stability under a moderate operation temperature (<325 °C). Our work highlights that Vo• defects, created via either in situ synthesis or postannealing treatment, could be explored to rationally boost the performance of p-type ternary oxide sensors.

Study of aluminum coating prepared by PVD used as anti-corrosion in liquid lead-bismuth

Liquid metals-such as lead (Pb) or lead-bismuth (PbBi) are used as reactor core coolants for accelerator driven systems (ADS) proposed for high-level radioactive waste transmutation. Compatibility of steels with liquid PbBi is a key problem because steels are attacked by dissolution of the components in PbBi, so it has to form a stable coating on steel surface. There are many methods to prepare anti-corrosion coatings on steel, such as hot dipping, pack cementation, plasma spaying, and physical vapor deposition (PVD). Compared with other methods, the PVD method is easy to control the thickness of the coating and the obtained coatings are dense which is crucial to the anti-corrosion ability of the coatings. In this letter, PVD aluminum coatings are developed on the surface of T91 steel and different heat-treatment atmosphere is used to adjust the microstructure, aluminum content, and the phase of the coatings. It is found that the coatings have good adherence ability with steel. The aluminum content and the phase of the coating can be adjusted by the heat-treatment atmosphere. Corrosion tests are performed in oxygen-saturated liquid PbBi at 550 ◦ C for 1000 h, the phase and composition of the coating do not change drastically. All the results indicate that the PVD is a useful method to prepare coatings on the surface of steel used in liquid PbBi.

Transparent conducting CuCr1-xMgxO2 films prepared by pulsed laser deposition

Transparent conducting CuCr1-xMgxO2 thin films were prepared by pulsed laser deposition (PLD) from polycrystalline CuCr1-xMgxO2 targets. The derived CuCr1-xMgxO2 films were highly c-axis oriented deposited at higher substrate temperature. The micro structural, electrical as well as optical properties were studied. It was found that the films were relatively smooth, and behaved as semiconductors. The transmittances of the films in the visible region are about 80% with direct band gaps about 3.15eV. The results suggested that CuCr1-xMgxO2 films could be successfully prepared by PLD, which can broaden the applications of the transparent conducting oxides films.

Transparent conducting CuCr 1-x Mg x O 2 films prepared by pulsed laser deposition

Transparent conducting CuCr1-xMgxO2 thin films were prepared by pulsed laser deposition (PLD) from polycrystalline CuCr1-xMgxO2 targets. The derived CuCr1-xMgxO2 films were highly c-axis oriented deposited at higher substrate temperature. The micro structural, electrical as well as optical properties were studied. It was found that the films were relatively smooth, and behaved as semiconductors. The transmittances of the films in the visible region are about 80% with direct band gaps about 3.15eV. The results suggested that CuCr1-xMgxO2 films could be successfully prepared by PLD, which can broaden the applications of the transparent conducting oxides films.

Room temperature ultraviolet emission from microcrystallite ZnO thin films

Strong room temperature ultraviolet emission is observed in highly oriented ZnO microcrystallite films prepared on (001) sapphire substrates by pulsed laser deposition method (PLD). The influences of substrate temperature, distance between the target and the substrate, oxygen pressure, and laser energy density on structural and optical properties were systemically studied by x-ray diffraction (XRD), scanning electron microscopy (SEM) and room temperature photoluminescence (PL) measurements. The XRD as well as the SEM results showed that all the four experiment parameters had obvious effects on the structure and micrograph of the derived ZnO films. From the room temperature photoluminescence spectra, all the derived samples have a strong ultraviolet (UV) emission about 376nm, and the visible emission is depressed greatly. Additionally, the results showed that the processing parameters could obviously affect the PL properties and the reasons were also discussed. From our results, the UV PL intensity is strongly depending both on the crystallinity and the stoichiometry of the ZnO films.