Institute of Ion Beam Physics and Materials Research: Annual Report 2001

Summary of the scientific activities of the institute in 2001 including selected highlight reports, short research contributions and an extended statistics overview

Understanding the Electronic Structure and Electron Transfer Kinetics of Titanium Dioxide Photoanodes and Analyzing Parameters Affecting Flatband Potentials in Metal Oxides

Rutile TiO2 shows promise for being used as a photoanode semiconductor in dye-sensitized photoelectrosynthesis cells (DSPECs), devices that use sunlight to drive the production of solar fuels. The addition of a TiO2 coating or shell onto a mesoporous nanocrystalline TiO2 photoanode substrate has been shown to improve device efficiency for water oxidation, yet little is known about why this change improves the performance of DSPECs. In this research, TiOx shells were deposited using atomic layer deposition onto rutile TiO2 nanorods and the electrochemical effects of the deposition were probed to help elucidate changes to the electronic structure induced by the shell. Rutile TiO2 was found to have a monoenergetic collection of deep trap states that is positive in potential to an exponential trap distribution in the band gap below the conduction band minimum. When increasing the TiOx shell thickness, the deep trap states of rutile TiO2 shifted to more positive potentials without changing the density of these states. In addition, the band gap of the material was found to decrease as shell thickness increased as quantified using diffuse reflectance spectroscopy. From photoelectrochemical impedance spectroscopy, the calculated rates of back-electron transfer were lower for samples with a TiOx shell compared to samples of rutile TiO2 without a shell. Metal oxides like TiO2 are gaining a lot of attention for their applications in energy technologies. A useful parameter used in the application of metal oxides is the flatband potential, yet values for the flatband potential are widely variable in the literature. A meta-analysis of flatband potential values for TiO2, SnO2, and ZnO was performed to study what variables impact the flatband potential for n-type metal oxides. Flatband potential values shifted –59 mV/pH with the exception of ZnO thin film flatband potentials, which showed an apparent lack of dependence on solution pH likely. The flatband potentials for anatase TiO2 nanotubes were shifted ~ 0.4V positive of other anatase TiO2 morphologies. Without the nanotube data points, anatase TiO2 and rutile TiO2 did not have a significant difference in mean flatband potential values, in contrast to what is often assumed for these two crystalline phases. Flatband potentials for ZnO appeared to shift negatively with increasing cation concentration, though previous literature precedence with other metal oxides suggests that the flatband potential should not be affected by non-proton cations in aqueous solutions. The findings of these analyses demonstrate the need to recognize the sensitivity of flatband potentials to multiple factors and the spread of flatband potential values that exist even between similar nanomaterials.Bachelor of Scienc

Mid-wavelength infrared type-II InAs/GaSb superlattice for photodetectors

Type-II superlattices (T2SLs) have emerged as a promising technology for infrared detectors compared to the state-of-art mercury cadmium telluride (MCT). T2SLs have shown great potential for mid-wavelength infrared (MWIR) detectors but have yet to attain their theoretically predicted performance. Simulation, fabrication, and characterisation were utilised in this research project to improve the performance levels of MWIR T2SL detectors. Using the molecular beam epitaxy (MBE) reactor, different interfacial growth schemes were used to accommodate internal strain caused by the lattice mismatches in T2SL. The project involves band heterostructure simulations, growth schemes, structural and optical characterisations, and electrical characterisation of fabricated T2SL p-i-n diode. Reference T2SL samples were evaluated by X-ray diffraction (X-RD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). An 8-band k·p approach for band heterostructure simulation explains PL experimental findings. Photoluminescence (PL) measurements probe the band structure and bandgap energy information. T2SL p-i-n diodes fabrication process was optimised using standard photolithography. The diode electrical performance of the T2SLs was examined by current-voltage (I-V) measurements using a cryogenic probe station. Investigation of structural and optical properties of the grown T2SL samples with interfacial growth schemes, namely incorporating an InSb compensation interface (IF) layer, was carried out to improve the material quality. AFM and TEM measurements revealed structural degradation due to the additional strain and lattice mismatch introduced by the InSb IF layers. However, including the InSb IF layer has improved the optical property of the T2SL. Band heterostructure simulation was performed to understand the possibility of atomic intermixing and segregation at the T2SL interfaces. Fabrication processes of T2SL single-pixel diodes were performed by wet etch, dry etch, and a combination of both approaches. The I-V characteristics revealed that the current density of the wet-etched devices is improved by approximately four orders of magnitude at low temperatures in comparison with the dry-etched devices, but they are comparable at high operating temperatures (HOT). Lastly, developments of wet etching processes were investigated using inorganic solutions, such as hydrochloric and phosphoric acids and organic solutions, including citric acid

Fundamentals and Recent Advances in Epitaxial Graphene on SiC

This book is a compilation of recent studies by recognized experts in the field of epitaxial graphene working towards a deep comprehension of growth mechanisms, property engineering, and device processing. The results of investigations published within this book develop cumulative knowledge on matters related to device-quality epaxial graphene on SiC, bringing this material closer to realistic applications

Multifunctional Organic-Inorganic Hybrid Nanophotonic Devices

The emergence of optical applications, such as lasers, fiber optics, and semiconductor based sources and detectors, has created a drive for smaller and more specialized devices. Nanophotonics is an emerging field of study that encompasses the disciplines of physics, engineering, chemistry, biology, applied sciences and biomedical technology. In particular, nanophotonics explores optical processes on a nanoscale. This dissertation presents nanophotonic applications that incorporate various forms of the organic polymer N-isopropylacrylamide (NIPA) with inorganic semiconductors. This includes the material characterization of NIPA, with such techniques as ellipsometry and dynamic light scattering. Two devices were constructed incorporating the NIPA hydrogel with semiconductors. The first device comprises a PNIPAM-CdTe hybrid material. The PNIPAM is a means for the control of distances between CdTe quantum dots encapsulated within the hydrogel. Controlling the distance between the quantum dots allows for the control of resonant energy transfer between neighboring quantum dots. Whereby, providing a means for controlling the temperature dependent red-shifts in photoluminescent peaks and FWHM. Further, enhancement of photoluminescent due to increased scattering in the medium is shown as a function of temperature. The second device incorporates NIPA into a 2D photonic crystal patterned on GaAs. The refractive index change of the NIPA hydrogel as it undergoes its phase change creates a controllable mechanism for adjusting the transmittance of light frequencies through a linear defect in a photonic crystal. The NIPA infiltrated photonic crystal shows greater shifts in the bandwidth per ºC than any liquid crystal methods. This dissertation demonstrates the versatile uses of hydrogel, as a means of control in nanophotonic devices, and will likely lead to development of other hybrid applications. The development of smaller light based applications will facilitate the need to augment the devices with control mechanism and will play an increasing important role in the future

Solution Synthesized Nanostructured Thermoelectric Materials

Thermoelectric heat engines are currently used in several niche applications for electricity generation and cooling. Many additional applications would be practical if thermoelectric materials with improved figures of merit could be made. Over the past twenty years, many nanostructured materials have been shown to possess improved figures of merit compared to their bulk counterparts mostly due to the reduction in thermal conductivity associated with nanostructured materials. Several classes of solution synthesized nanostructured materials have achieved high figures of merit, yet significant room for improvement exists for solution synthesized nanostructured PbTe

Towards Oxide Electronics:a Roadmap

At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore's law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community. Oxide science and technology has been the target of a wide four-year project, named Towards Oxide-Based Electronics (TO-BE), that has been recently running in Europe and has involved as participants several hundred scientists from 29 EU countries. In this review and perspective paper, published as a final deliverable of the TO-BE Action, the opportunities of oxides as future electronic materials for Information and Communication Technologies ICT and Energy are discussed. The paper is organized as a set of contributions, all selected and ordered as individual building blocks of a wider general scheme. After a brief preface by the editors and an introductory contribution, two sections follow. The first is mainly devoted to providing a perspective on the latest theoretical and experimental methods that are employed to investigate oxides and to produce oxide-based films, heterostructures and devices. In the second, all contributions are dedicated to different specific fields of applications of oxide thin films and heterostructures, in sectors as data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics

Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II

Wide bandgap (WBG) semiconductors are becoming a key enabling technology for several strategic fields, including power electronics, illumination, and sensors. This reprint collects the 23 papers covering the full spectrum of the above applications and providing contributions from the on-going research at different levels, from materials to devices and from circuits to systems

Glassy Materials Based Microdevices

Microtechnology has changed our world since the last century, when silicon microelectronics revolutionized sensor, control and communication areas, with applications extending from domotics to automotive, and from security to biomedicine. The present century, however, is also seeing an accelerating pace of innovation in glassy materials; as an example, glass-ceramics, which successfully combine the properties of an amorphous matrix with those of micro- or nano-crystals, offer a very high flexibility of design to chemists, physicists and engineers, who can conceive and implement advanced microdevices. In a very similar way, the synthesis of glassy polymers in a very wide range of chemical structures offers unprecedented potential of applications. The contemporary availability of microfabrication technologies, such as direct laser writing or 3D printing, which add to the most common processes (deposition, lithography and etching), facilitates the development of novel or advanced microdevices based on glassy materials. Biochemical and biomedical sensors, especially with the lab-on-a-chip target, are one of the most evident proofs of the success of this material platform. Other applications have also emerged in environment, food, and chemical industries. The present Special Issue of Micromachines aims at reviewing the current state-of-the-art and presenting perspectives of further development. Contributions related to the technologies, glassy materials, design and fabrication processes, characterization, and, eventually, applications are welcome