II-VI Semiconductor Nano-Structures for On-Chip integrated Photonics

Nanowires (NWs) and nanobelts (NBs) have been widely studied and fabricated into on-chip photodetectors, biosensors, LEDs/lasers, solar cells and computational components. Their highly tunable physical, electronic and optical properties have generated interest in this field over the past two decades. While there is tremendous potential for nano-structured devices, the wide spread application of NWs/NBs has been hindered by the difficulty in integrating multiple NW or NB structures together into more complex devices. This problem requires a completely novel approach to what has been previously attempted in order to effectively couple on-chip light sources, waveguides and detectors. Multiple factors must be considered including optical power of nanoscale light sources, propagation losses in waveguides and responsivity of nano-scale detectors. Only in combination is it possible to have fully on-chip integrated devices. In this thesis we report the design, optimization and fabrication of coupled self-aligned NB LED emitters and photodetectors. An etched cut is made into a single Cadmium Sulfide NB providing the ability to fabricate each section of a single NB into a separate device. This opens possibilities for on-chip devices such biological sensors. This self-aligned structure can also be coupled to an external light source. Additionally, we present a method for waveguideing and modulating second harmonic generation (SHG) in Cadmium Sulfide NBs as a light source for on-chip measurements. SHG is a coherent and tunable frequency doubled light source so the input laser does not interfere with measurements on-chip. The ability to reliably fabricate more complex devices with nano-structures will continue the trend of portability and point-of-care technology by integrating bulk components such as lasers and photodetectors onto on-chip devices

Piezoelectric Semiconducting Nanowires

© 2018 Elsevier Inc. Piezoelectric semiconducting nanowires have generated much interest due to the interplay of their mechanical, electrical, and optical properties, which paves the way for potential applications in mechanical energy harvesting as well as sensing. The nature of piezoelectricity in these nanowires is governed by the crystalline phases present, which in turn can be controlled during the nanowire growth process. This chapter provides insight into the manifestation of piezoelectricity in semiconducting nanowires, the effect of growth on their piezoelectric properties, and importantly, how piezoelectricity is characterized at the nanoscale in these materials. Energy-related applications of semiconducting piezoelectric nanowires are described in detail, including their incorporation into nanogenerators for energy harvesting, as well as in piezotronic and photo-piezotronics devices based on the electromechanical and opto-electromechanical interactions taking place in piezoelectric semiconductor-nanowire junction-based devices. Advances in nanofabrication, nanoscale characterization, and device engineering, coupled with a greater understanding and control of piezoelectricity in semiconducting nanowires, will ultimately help unlock the full potential of these fascinating nanomaterials.European Research Council (Grant no. ERC–2014–STG–639526, NANOGEN

Large-Scale Patterned Oxide Nanostructures: Fabrication, Characterization and Applications

Nanotechnology is experiencing a flourishing development in a variety of fields covering all of the areas from science to engineering and to biology. As an active field in nanotechnology, the work presented in this dissertation is mostly focused on the fundamental study about the fabrication and assembly of functional oxide nanostructures. In particular, Zinc Oxide, one of the most important functional semiconducting materials, is the core objective of this research, from the controlled growth of nanoscale building blocks to understanding their properties and to how to organize these building blocks. Thermal evaporation process based on a single-zone tube furnace has been employed for synthesizing a range of 1D nanostructures. By controlling the experimental conditions, different morphologies, such as ultra-small ZnO nanobelts, mesoporous ZnO nanowires and core-shell nanowire were achieved. In order to pattern the nanostructures, a large-scale highly-ordered nanobowl structure based on the self-assembly of submicron spheres was created and utilized as patterning template. The growth and patterning techniques were thereafter integrated for aligning and patterning of ZnO nanowires. The aligning mechanisms and growth conditions were thoroughly studied so as to achieve a systematic control over the morphology, distribution and density. The related electronic and electromechanical properties of the aligned ZnO nanowires were investigated. The feasibility of some potential applications, such as photonic crystals, solar cells and sensor arrays, has also been studied. This research may set a foundation for many industrial applications from controlled synthesis to nanomanufacturing.Ph.D.Committee Chair: Wang, Zhong Lin; Committee Co-Chair: Summers, Christopher J.; Committee Member: Dupuis, Russell D.; Committee Member: Wagner, Brent; Committee Member: Wong, C. P

Towards metal chalcogenide nanowire-based colour-sensitive photodetectors

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2016/6 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. Authors are grateful to Reinis Ignatans for XRD measurements.In recent years, nanowires have been shown to exhibit high photosensitivities, and, therefore are of interest in a variety of optoelectronic applications, for example, colour-sensitive photodetectors. In this study, we fabricated two-terminal PbS, In2S3, CdS and ZnSe single-nanowire photoresistor devices and tested applicability of these materials under the same conditions for colour-sensitive (405 nm, 532 nm and 660 nm) light detection. Nanowires were grown via atmospheric pressure chemical vapour transport method, their structure and morphology were characterized by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), and optical properties were investigated with photoluminescence (PL) measurements. Single-nanowire photoresistors were fabricated via in situ nanomanipulations inside SEM, using focused ion beam (FIB) cutting and electron-beam-assisted platinum welding; their current-voltage characteristics and photoresponse values were measured. Applicability of the tested nanowire materials for colour-sensitive light detection is discussed.ISSP UL Nr. SJZ/2016/6; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Nanomechanical and Electromechanical Characterization on Zinc Oxide Nanobelt Using Atomic Force Microscopy

ZnO nanobelts are a group of quasi-one-dimensional nanostructures that have a unique rectangle-like cross section, with typical widths of several hundred nanometers, width-to-thickness ratios of 1 to 10, and lengths of tens to hundreds of micron meters. They are the promising candidates for nanoscale ultrahigh frequency resonator, nanosensors and nanoactuators due to their well-defined geometry, prefect single crystallinity and excellent piezoelectric properties. In this study, we revealed the following unique nanomechanical and electromechanical properties of a single ZnO nanobelt using Atomic Force Microscopy. (i) Elastic modulus, hardness and fracture toughness of the ZnO nanobelt are much smaller than its bulk counterpart. (ii) Strong photoinduced elastic effect is observed in ZnO nanobelt, while there is no effect in its bulk counterpart. (iii) The effective piezoelectric coefficient, d33, of ZnO nanobelt is frequency dependent and much higher than that of its bulk counterpart. These unique properties cannot be accounted for using macroscale theory such as continuum mechanics, while they may be related to surface effects thanks to the high surface to volume ratio of ZnO nanobelt. One of the consequences of our findings is that these properties are fundamental to the real applications and may be universal phenomena in quasi-one-dimensional semiconducting nanostructures, which are becoming building blocks of MEMS (Micro Electromechanical Systems) and NEMS (Nano Electromechanical Systems)

Synthesis, characterization and application of ZnO nanomaterials

In this thesis, high temperature vapor deposition method has been extensively used to synthesize nanomaterials. One of the as-synthesized nanostructures is superlattice-structured nanohelix, which is made of two types of alternating and periodically distributed long crystal strips. The manipulation of the nanohelix showed super-elasticity and special fracture mechanism. The other widely studied nanomaterial is vertically aligned ZnO nanowire array, which is epitaxially grown on GaN and SiC substrates. Several manipulation methods such as e-beam lithography (EBL), dielectrophoresis, and in situ direct manipulation, have been developed, so that the mechanical and electrical properties of a single nanowire can be characterized, which provide essential references for fabricating bridged nanowire based devices. Specifically, an improved atomic force microscope (AFM) based method has been developed to accurately measure the elastic modulus of bridged ZnO nanowires. Bridged nanostructure is an extremely important configuration in planar MEMS/NEMS devices and this new approach provides insights to the importance of boundary conditions. Novel physical and statistical models have been firstly developed to obtain better estimate of elastic modulus. For electrical properties of bridged nanowires, it is found that the direct contact of ZnO nanowire and Au electrodes displays a back-to-back Schottky behavior. Self-assembled monolayer (SAM) can improve the mechanical contact and increase the conductance. These devices with Schottky contacts show much better UV sensing performance than the ones with Ohmic contacts. Barrier height change is believed to play an important role in a lot of sensors. A thermionic emission-diffusion model is deduced to successfully explain the current change in a strain sensor. This thesis clearly exhibits the unique properties of ZnO nanomaterials and provides deeper understanding to methodologies as well as the phenomena. With further exploration, ZnO nanomaterials should be able to better understood and utilized, and come close to the next step of commercialization.Ph.D.Committee Chair: Wang, Zhong Lin; Committee Member: Gall, Kenneth A.; Committee Member: Snyder, Robert L.; Committee Member: Wong, Ching-Ping; Committee Member: Wu, C.F. Jef

HIGH-PRESSURE STUDY OF NANOSTRUCTURED SnO2 BY RAMAN SPECTROSCOPY AND X-RAY DIFFRACTION WITH SYNCHROTRON RADIATION

Tin dioxide (SnO2) nanowires and nanobelts were compressed in a diamond anvil cell to around 40 gigapascal (GPa) at room temperature followed by decompression. Raman spectroscopy was employed to monitor the pressure effect on the SnO2 nanobelts. Synchrotron X-ray powder diffraction measurements were also carried out on both nanomaterials during both compression and decompression. The diffraction patterns were analyzed quantitatively by the Rietveld refinement. Pressure-induced phase transformations were observed in both nanomaterials and compared with those observed previously in the bulk materials. However, transition pressures, phase abundances, reversibilities, as well as compressiblities were found to be different from those of the bulk materials in both nanomorphologies. The mechanisms attributed to these differences were discussed. Finally, scanning electron microscopy (SEM) images of nanobelts and nanowires were collected before compression and after decompression. Morphologies were found to be significantly altered by high pressures, providing insight into these pressure-induced transformation

NIR-emissive Alkynylplatinum(II) Terpyridyl Complex as a turn-on selective probe for heparin quantification by induced helical self-assembly behaviour

The extent of self-assembly viametal–metal and π-π stacking interactions, induced by the polyanionic biopolymers, enables the class of alkynylplatinum(II) terpyridyl complexes to be applicable for the sensing of important biomacromolecules through the monitoring of spectral changes. Strong demand arises for the design of selective and practical detection techniques for the quantification of heparin, a highly negative-charged polysaccharidethat can function as anticoagulant, due to the prevention of hemorrhagic complications upon overdose usage.Aconvenient sensing protocol for the detection of UFH and LMWH, two common forms of heparins in clinical use, in buffer and biological medium has been demonstrated with the spectral changes associated with the induced self-assembly of a NIR-emissive platinum(II) complex. The detection range has been demonstrated to cover clinical dosage levels and the structurally similar analogues can be effectively differentiated based on their anionic charge density and the formation of supramolecular helical assembly of the platinum(II) complex with them ...postprin

II-VI Core-Shell Nanowires: Synthesis, Characterizations and Photovoltaic Applications

The emergence of semiconducting nanowires as the new building blocks for photovoltaic (PV) devices has drawn considerable attention because of the great potential of achieving high efficiency and low cost. In special, nanowires with a coaxial structure, namely, core-shell structures have demonstrated significant advantages over other device configurations in terms of radial charge collection and cost reduction. In this dissertation, several core-shell nanowire structures, including ZnO/ZnSe, ZnO/ZnS, and CdSe/ZnTe, have been synthesized and the photovoltaic devices processed from a ZnO/ZnS core-shell nanowire array and a single CdSe/ZnTe core-shell nanowire have been demonstrated. By combining the chemical vapor deposition and pulsed laser deposition (PLD) techniques, type-II heterojunction ZnO/ZnSe and ZnO/ZnS core-shell nanowire array were synthesized on indium-tin-oxide substrates. Their structures and optical properties have been investigated in detail, which revealed that, despite highly mismatched interfaces between the core and shell, both systems exhibited an epitaxial growth relationship. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the core with the shell provides the evidence that the charge separation and collection in the type II core-shell nanowire is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selection and device operation mechanisms for achieving their maximum energy conversion efficiencies at a low cost and in an environmentally friendly manner. In order to achieve a high quality interface in the core-shell nanowire, CdSe and ZnTe, which have close lattice parameters and thermal expansion coefficients, were chosen to fabricate nanowire solar cells. ZnTe and CdSe nanowires were first synthesized by thermal evaporation and the shells were subsequently deposited by PLD. ZnTe/CdSe nanowires represented an inhomogeneous coating while the CdSe/ZnTe core-shell exhibited a conformal coating with obvious ZnTe eptilayer. The final PV device based on an individual CdSe/ZnTe nanowire demonstrated an efficiency of ~1.7%. In addition, a controllable synthesis of CdSe nanowire array on muscovite mica substrate was presented, providing the possibility to harvest hybrid energies in an all-inorganic nanowire array

Ternary II-VI 1D nanomaterials: synthesis, properties and applications

Ph.DDOCTOR OF PHILOSOPH