Zinc manganese oxide (ZnMnO) grown by metal-organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) exhibit absorption band edge tunability with Mn incorporation. ZnMnO with good crystal quality oriented in the (002) direction was grown using MOCVD. ALD-grown ZnMnO was amorphous but exhibited high crystallinity after annealing at 800 °C for 1 hour. ZnMnO using both growth techniques showed an overall reduction in band edge with Mn incorporation, but the trend was scattered. The band edge reduction is influenced by energy states and oxidation states of Mn incorporated in ZnMnO rather than the cumulative Mn incorporation. Incorporation of Mn with oxidation states favorable to achieve bandgap tunability could be controlled with growth techniques and growth conditions. Control over energy states introduced by Mn in ZnMnO could enable application of ZnMnO for various areas of interest including spintronics, photovoltaics, photodiodes, and sensing.
The role of metal-organic precursors specifically gadolinium precursors on the resulting magnetic properties of gadolinium-doped gallium nitride (GaGdN) is investigated. Gadolinium-doping is expected to render spin-related magnetic properties in GaN for spintronic applications. To achieve and understand this, GaGdN was grown using metalorganic chemical vapor deposition using two types of gadolinium precursors - tris (2,2,6,6-tetramethyl-3,5- heptanedionate) gadolinium ((TMHD)3Gd) and tris(cyclopentadienyl) gadolinium (Cp3Gd). GaGdN grown using (TMHD)3Gd showed Anomalous Hall Effect and ferromagnetism at room temperature (RT). GaGdN grown using Cp3Gd showed ordinary Hall Effect with no signs of ferromagnetism or any spin polarization. Oxygen from (TMHD)3Gd incorporated in GaGdN during the MOCVD growth could be responsible for the differences in magnetic properties. GaGdN shows properties at RT that are conducive for spintronic applications. However, metal-organic precursors and corresponding presence of oxygen significantly influence the spin-related capabilities of GaGdN. This work contributes towards understanding the mechanisms for spin-related properties of GaGdN that can enable its RT spintronic applications.
Transition metal-doped zinc oxide (ZnTMO) grown using metal-organic chemical vapor deposition exhibited spinrelated properties at room temperature that are driven by secondary phases, defects, transition metal clusters, and extrinsic carrier scattering. ZnTMO has been an interesting material for spintronics and has shown ferromagnetism at room temperature in the literature. However, the mechanism for spin-characteristics is not clear. In this work, Anomalous Hall Effect measurements were performed to investigate these mechanisms. A non-linear trend of transverse Hall resistvity with externally applied field indicated spin polarization in the material. However, variations in transverse resistivity with applied magnetic field follow same trend as four-point Van der Pauw resistivity. Hence, the Anomalous Hall Effect is likely not due to intrinsic free carriers. Spin-related properties of ZnTMO are dominantly influenced by secondary phases, native defects, and other extrinsic carrier-scattering centers such as transition metal clusters. This work contributes towards the understanding of ZnTMO for spintronic applications and provides better clarity regarding the underlying phenomena for its spin-related properties.
This study aimed at fabricating the PVDF-based piezoelectric energy harvester with high flexibility and efficiency. PVDF Nano-fiber was synthesized by using the electrospinning process. This study utilized the ink-jet printer to make the silver contact for the PVDF energy harvester. Later, the energy harvester was coated with Polydimethylsiloxane (PDMS) with different thickness. The result shown that the PVDF energy harvester tend to harvest the vibration at the frequency lower than 100 Hz. Moreover, the coating layer can decrease the energy harvesting efficiency due to its vibration absorption. This result was further validated by the Comsol Multiphysics simulation.
Nickel-doped zinc oxide (ZnNiO) was grown on sapphire by metal organic chemical vapor deposition (MOCVD) with varying Ni content under two growth conditions of 400°C/100 Torr and 450°C/30 Torr. Elemental composition indicated that Ni could occupy Zn and O/interstitial sites in ZnNiO. Ni-doping in ZnO resulted in shifts in X-ray diffraction (002) peak, and introduced a (111) phase. Absorption spectrum showed a reduction in near band edge with Ni content in both the samples’ sets. Samples grown at 400°C/100 Torr had a band gap reduction from 3.276 eV to 3.269 eV, while those synthesized at 450°C/30 Torr showed reduction from 3.287 eV to 3.260 eV. The bandgap reduction rate was influenced by growth conditions, and sites activated for Ni incorporation during the growth. Nickel could introduce shallow energy states near the valence band in ZnNiO, and result in a reduction in the bandgap. A potential for bandgap tunability, and controllable introduction of energy states in zinc oxide with transition metal doping by MOCVD, could widen the application range of zinc oxide-based materials for energy harvesting and electronics.
Electromechanical impedance (EMI) technique coupled with the piezoelectric sensor has been explored as a promising non-destructive testing (NDT) method to determine the quality of cementitious materials. Among the piezoelectric sensors, Lead Zirconate Titanate (PZT) is the most commonly used piezo-ceramic materials for conducting the EMI because of the high sensitivity and high piezoelectric constant. However, the inherent brittleness of PZT limited their potential application as embedded sensors for in-situ monitoring of materials/structure properties. To improve the durability and flexibility of the PZT sensor, we have systematically investigated the effect of different polymer coating methods of PZT on their sensitivity and effectiveness of EMI sensors. Specifically, two polymer coating agents were used to encapsulate the PZT sensor including the flexible organic polymer compound - Polydimethylsiloxane (PDMS) and the rigid polymer compound- Polyepoxides (Epoxy) We have also investigated the feasibility of EMI technique with the surface bonded sensor and embedded sensor to test the compressive strength gain of mortars. The compressive strength test of samples was also conducted using conventional mechanical methods per ASTM C109 as a baseline at the first 5th to 8th hour. The EMI signatures were collected for all the samples at the same time. Our preliminary results have shown a high correlation of EMI methods’ results with the compressive testing results for two different polymers coated PZT sensors, which indicated the polymer-coated PZT sensors can be used as an effective NDT method for in-situ monitoring of concrete compressive strength gain at the very early age.
Nowadays, strength monitoring of concrete structures by the nondestructive method has gained more attention. Strength monitoring is not only important to determine the readiness of structures for service, but also to ensure the safety of the structure itself during the construction. The main goal of in-situ monitoring the strength gain in concrete is to obtain reliable information about the quality of concrete which can be further used to assess construction schedule and process of concrete structures, such as determining the optimal traffic opening time. The current methods are unreliable, inefficient, costly and partially destructive. To address these challenges, this paper aims to investigate the feasibility of employing polymer based piezoelectric sensors to characterize the in-situ compressive strength gain of concrete at the early ages. The pitch-catch approach was considered for the active sensing approach. In this approach, a piezoelectric transducer acts an actuator to propagates the Lamb waves while another piezoelectric device works as a sensor to detect the signal. A PVDF sensor was fabricated using electrospinning method, and a commercial PZT transducer was used to generate the Lamb waves. The developed piezoelectric-based set-up has shown a promising technique for strength development of cementitious materials at early ages.
Zinc oxide (ZnO) is an earth abundant wide bandgap semiconductor of great interest in the recent years. ZnO has many unique properties, such as non-toxic, large direct bandgap, high exciton binding energy, high transparency in visible and infrared spectrum, large Seebeck coefficient, high thermal stability, high electron diffusivity, high electron mobility, and availability of various nanostructures, making it a promising material for many applications. The growth techniques of ZnO is reviewed in this work, including sputtering, PLD, MOCVD and MBE techniques, focusing on the crystalline quality, electrical and optical properties. The problem with p-type doping ZnO is also discussed, and the method to improve p-type doping efficiency is reviewed. This paper also summarizes the current state of art of ZnO in thermoelectric and photovoltaic applications, including the key parameters, different device structures, and future development.
ZnO-based materials show promise in energy harvesting applications, such as piezoelectric, photovoltaic and thermoelectric. In this work, ZnO-based vertical Schottky barrier solar cells were fabricated by MOCVD de- position of ZnO thin films on ITO back ohmic contact, while Ag served as the top Schottky contact. Various rapid thermal annealing conditions were studied to modify the carrier density and crystal quality. Greater than 200 nm thick ZnO films formed polycrystalline crystal structure, and were used to demonstrate Schottky solar cells. I-V characterizations of the devices showed photovoltaic performance, but but need further development. This is the first demonstration of vertical Schottky barrier solar cell based on wide bandgap ZnO film. Thin film and bulk ZnO grown by MOCVD or melt growth were also investigated in regards to their room- temperature thermoelectric properties. The Seebeck coefficient of bulk ZnO was found to be much larger than that of thin film ZnO at room temperature due to the higher crystal quality in bulk materials. The Seebeck coefficients decrease while the carrier concentration increases due to the crystal defects caused by the charge carriers. The co-doped bulk Zn0:96Ga0:02Al0:02O showed enhanced power factors, lower thermal conductivities and promising ZT values in the whole temperature range (300-1300 K).
The wide-bandgap semiconductor ZnO has gained major interest in research community for its unique properties and wide range of applications. In this review article, we present synthesis techniques and a few emerging applications for ZnO. Common techniques for growing ZnO films are discussed briefly, and a detailed discussion of MOCVD growth of ZnO is provided citing previous experimental reports on this technique by our group and others. A few important and distinctive uses of ZnO are discussed for various applications focusing on the current limitations of ZnO to realize its feasibility in these applications.
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