R. Mundle

Transparent conductive oxides: Plasmonic materials for telecom wavelengths

We show that despite of low loss, silver and gold are not suitable for a variety of nanoplasmonic applications in the infrared range, which require compact modes in single-interface plasmonic waveguides. At the same time, degenerate wide-band-gap semiconductors can serve as high-quality plasmonic materials at telecom wavelengths, combining fairly high compactness and relatively low loss. Their plasmonic properties in the near-infrared can be compared to those of gold in the visible range. The same materials can be used in a variety of non-plasmonic metamaterials applications, including transformation optics and invisibility cloaking.

Metal-like conductivity in transparent Al: ZnO films

This letter reports on the metal-like conductivity down to 125K in highly crystalline transparent (⩾85% in the visible region) Al:ZnO films grown on sapphire and glass substrates by pulsed-laser deposition technique. Significantly different surface morphologies were found on both types of films. Temperature dependent resistivity measurements of the films grown at 450°C on sapphire and glass show metal-like conductivity with electrical resistivity, ∼1.77×10−4 and ∼3.92×10−4Ωcm, respectively, at room temperature followed by either a residual conductivity or a metal-semiconductor transition at low temperature due to the localization effect caused by the defects.

Surface plasmon excitation via Au nanoparticles in n-CdSe∕p-Si heterojunction diodes

We report on the significant enhancement of photocurrent in pn heterojunction diode, consisting of n-CdSe∕p-Si substrates, in situ deposited with Au nanoparticles on the surface by the pulsed-laser deposition technique. This is attributed due to the large enhancement in electromagnetic field that occurs in the vicinity of the metal surface, causing surface plasmons. The large enhancement in Raman and photoluminescence intensity was observed due to surface plasmon resonance. Our results suggest that the photodetectors, optoelectronic, such as high-performance thin-film solar cells, optical communication, and sensing devices, including bio- and molecular sensors, can be fabricated with improved functionality.

Pulsed-laser deposited Er:ZnO films for 1.54μm emission

High-quality Er:ZnO films were grown by the pulsed-laser deposition technique at high temperature followed by in situ annealing. The films demonstrate remarkable crystalline quality and array of self-assembled grains. Although the films show very low electrical resistivity (∼6.41×10−4Ωcm) at room temperature, a semiconductor-metal transition was observed at 190K for low doping in contrast to semiconductor behavior for high doping. The films show pronounced room temperature emission at 1.54μm, illustrating the activation of Er3+ ions in ZnO matrix. Furthermore, no quenching effects in 1.54μm emission characteristics were observed up to 2wt% of Er doping in ZnO at room temperature.

Extreme tunability in aluminum doped Zinc Oxide plasmonic materials for near-infrared applications

Plasmonic materials (PMs), featuring large static or dynamic tunability, have significant impact on the optical properties due to their potential for applications in transformation optics, telecommunications, energy, and biomedical areas. Among PMs, the carrier concentration and mobility are two tunable parameters, which control the plasma frequency of a metal. Here, we report on large static and dynamic tunability in wavelengths up to 640 nm in Al-doped ZnO based transparent conducting degenerate semiconductors by controlling both thickness and applied voltages. This extreme tunability is ascribed to an increase in carrier concentration with increasing thickness as well as voltage-induced thermal effects that eventually diminish the carrier concentration and mobility due to complex chemical transformations in the multilayer growth process. These observations could pave the way for optical manipulation of this class of materials for potential transformative applications.

Energy harvesting in semiconductor-insulator-semiconductor junctions through excitation of surface plasmon polaritons

We have demonstrated a simple approach for developing a photovoltaic device consisting of semiconductor-insulator-semiconductor (SIS) heterojunction using surface plasmon polaritons (SPPs) generated in one of the semiconductors (Al:ZnO) and propagated through the dielectric barrier (SiO2) to other (Si). This robust architecture based on surface plasmon excitation within an SIS device that produces power based on spatial confinement of electron excitation through plasmon absorption in Al:ZnO in a broad spectrum of visible to infrared wavelengths enhancing the photovoltaic activities. This finding suggests a range of applications for photovoltaics, sensing, waveguides, and others using SPPs enhancement on semiconductors without using noble metals.

Synthesis and magnetic characterizations of manganite-based composite nanoparticles for biomedical applications

We report chemically synthesized highly crystalline lanthanum strontium manganite (LaSrMnO3) and Eu-doped Y2O3 and their composites. The synthesis yields nanoparticles of size 30–40nm. Magnetic measurements performed on nanoparticles and composites show magnetic transition at about 370K with a superparamagnetic behavior at room temperature. The ferromagnetic resonance studies of the nanoparticles show large linewidth due to surface strains. The composite nanoparticles also display luminescent behavior when irradiated with ultraviolet light. The manganites as well their composite with the luminescent nanoparticles may be very useful for biomedical applications.

Surface plasmon resonance in CdSe semiconductor coated with gold nanoparticles

We have grown CdSe semiconductor films on glass substrates and the films were coated with Au nanoparticles of 10 nm in size by the pulsed-laser deposition technique. The films demonstrate a large enhancement of Raman intensity and photoluminescence of CdSe semiconductor via excitation of surface plasmon resonances in proximate gold metal nanoparticles deposited on the surface of CdSe film. These observations suggest a variety of approaches for improving the performance of devices such as photodetectors, photovoltaics, and related devices, including biosensors.

Electrical conductivity and photoresistance of atomic layer deposited Al-doped ZnO films

Al-doped ZnO films were deposited by the atomic layer deposition (ALD) on both glass and sapphire (0001) substrates. The Al composition of the films was varied by controlling the Zn:Al pulse cycle ratios. The films were characterized by the atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and optical measurements. The Film resistivity was measured as a function of Zn:Al cycle ratios as well as temperature for films grown at various substrate temperature used for ALD deposition. The resistivity of the ALD grown films decreases significantly, and so as the increase in the carrier concentration as the cycle ratio increases. The systematic measurements of temperature dependence of resistivity of films at various cycle ratios clearly demonstrate the crossover of the metal–semiconductor–insulator phase with the function of temperature as well as the cycle ratios. The average transmission of all films is greater than 85% and the optical absorption increases significantly in the visib...

ZnO/Al:ZnO Transparent Resistive Switching Devices Grown by Atomic Layer Deposition for Memristor Applications

ZnO has intrinsic semiconductor conductivity because of an unintentional doping mechanism resulting from the growth process that is mainly attributable to oxygen vacancies (VO) positioned in the bandgap. ZnO has multiple electronic states that depend on the number of vacancies and the charge state of each vacancy. In addition to the individual electron states, the vacancies have different vibrational states. We developed a high-temperature precursor vapor mask technique using Al2O3 to pattern the atomic layer deposition of ZnO and Al:ZnO layers on ZnO-based substrates. This technique was used to create a memristor device based on Al:ZnO thin films having metallic and semiconducting and insulating transport properties ZnO. We demonstrated that adding combination of Al2O3 and TiO2 barrier layers improved the resistive switching behavior. The change in the resistance between the high- and low-resistivity states of the memristor with a combination of Al2O3 and TiO2 was approximately 157%. The devices were exposed to laser light from three different laser diodes. The 450 nm laser diode noticeably affected the combined Al2O3 and TiO2 barrier, creating a high-resistivity state with a 2.9% shift under illumination. The high-resistivity state shift under laser illumination indicates defect shifts and the thermodynamic transition of ZnO defects.