Resonance structures have long been employed by RF and microwave devices ranging from antennas, to wave guides. These resonance structures have exhibited an enormous amount of control over the wavelength selectivity, polarization, and directivity of the electromagnetic radiation which couples to the structure. Traditional geometrical optics has alternatively used discrete optical components such as lenses, gratings, and polarizers to accomplish equivalent control over optical radiation. This dissertation contributes to the larger body of literature that applies lessons learned in RF and microwave resonance structures, to nanoscale resonance structures. Optoelectronic nanoscale resonance structures were fabricated and characterized using both experimental and numerical methods. Two nanoscale resonance structures were investigated: an antenna inspired Yagi-Uda array, and a metasurface inspired interdigitated structure. Experimental devices containing the nanoscale resonance structures were fabricated on semiconducting substrates forming metal-semiconductor-metal photodiodes. The spectral response of the nanoscale resonance photodiode was determined by measuring the photocurrent or photovoltage resulting from incident monochromatic light which was swept through wavelengths from 400 nm to 2000 nm. The previously mentioned Yagi-Uda based device exhibited two maxima in photoresponse at 1110 nm and 1690 nm. Effective wavelength scaling was applied to the Yagi-Uda nanoantenas, and consistency was demonstrated between the theoretical effective wavelength and experimental photoresponse maxima. The spectral response of the interdigitated structure demonstrated good qualitative agreement with the finite element modeled absorbance in an equivalent structure. Analysis of the modeled absorbance suggests that hot electron injection contributes to the photoresponse, and the spectral response of the detector device may be tuned by varying the geometrical parameters of the device. An optimized device was proposed that could improve photodetection efficiency using nanoscale resonance devices. Antenna inspired nanoscale resonance structures may be used to probe fundamental physical phenomena such as hot carrier generation, hot carrier transport, and surface plasmon resonances. Combined optical and electrical-optoelectronic devices exploiting these phenomena may be realized for a variety of applications, eliminating some or all of the discrete optical components required for optoelectronic systems and hence significantly reducing the SWaP cost of optoelectronic systems. / Doctor of Philosophy / Resonance structures have long been employed by RF and microwave devices ranging from antennas, to wave guides. These resonance structures have exhibited an enormous amount of control over radio waves. Traditional optics has alternatively used discrete components such as lenses, gratings, and polarizers to accomplish equivalent control over light waves. This dissertation contributes to the larger body of literature that applies lessons learned in RF and microwave resonance structures, to nanoscale resonance structures. Optoelectronic nanoscale resonance structures were fabricated and characterized using both experimental and computational methods. Two nanoscale resonance structures were investigated: an antenna inspired Yagi-Uda array, and a metasurface inspired interdigitated structure. The ability of both devices to detect light of a particular wavelength was then tested. The photoresponse of the device containing a Yagi-Uda array is consistent with RF Yagi-Uda antennas when considered in accordance with the concept of effective wavelength. The experimental response of the interdigitated structure demonstrated good qualitative agreement with the computational modeled absorbance in an equivalent structure. Analysis of the modeled absorbance suggests that the spectral response of the detector device may be tuned by varying the geometrical parameters of the device. An optimized device was proposed that could improve photodetection efficiency using nanoscale resonance devices. Antenna inspired nanoscale resonance structures may be used to probe fundamental physical phenomena such as hot carrier generation, hot carrier transport, and surface plasmon resonances. Combined optical and electrical-optoelectronic devices exploiting these phenomena may be realized for a variety of applications, eliminating some or all of the discrete optical components required for optoelectronic systems and hence significantly reducing the SWaP cost of optoelectronic systems.

No description available.

Graphene, a one-atom-thick sheet of hexagonally packed carbon atoms, is a novel material with high electron mobility due to its unique linear and gapless electronic band structure. Its broadband absorption and unusual doping properties, along with superb mechanical flexibility make graphene of promising application in optoeletronic devices such as solar cell, ultrafast photodetectors, and terahertz modulators. How- ever, the current performance of graphene-based devices is quite unacceptable owning to serious limitations by its inherently small absorption cross section and low quan- tum efficiency. Fortunately, nanoscale optical antennas, consisting of closely spaced, coupled metallic nanoparticles, have fascinating optical response since the collective oscillation of electrons in them, namely surface plasmons, can concentrate light into a subwavelength regime close to the antennas and enhance the corresponding field considerably. Given that optical antenna have been applied in various areas such as subwavelength optics, surface enhanced spectroscopies, and sensing, they are also able to assist graphene to harvest visible and near-infrared light with high efficiency. Moreover, the efficient production of hot electrons due to the decay of the surface plasmons can be further implemented to modulate the properties of graphene. Here we choose plasmonic oligomers to serve as optical antenna since they pos- sess tunable Fano resonances, consisting of a transparency window where scattering is strongly suppressed but absorption is greatly enhanced. By placing them in di- rect contact with graphene sheet, we find the internal quantum efficiency of hybrid antenna-graphene devices achieves up to 20%. Meanwhile, doping effect due to hot electron is also observed in this device, which can be used to optically tune the elec- tronic properties of graphene.

No description available.

Doctor of Philosophy / Department of Chemistry / Jun Li / Solar energy as a sustainable resource is a promising alternative to fossil fuels to solve the tremendous global energy crisis. Development of three generation of solar cells has promoted the best sunlight to electricity conversion efficiency above 40%. However, the most efficient solar cells rely on expensive nonsustainable raw materials in device fabrication. There is a trend to develop cost-effective biophotovoltaics that combines natural photosynthetic systems into artificial energy conversion devices such as dye sensitized solar cells (DSSCs). In this research, a model system employs natural extract light-harvesting complex II (LHCII) as a light-absorbing sensitizer to interface with semiconductive TiO₂ and plasmonic nanoparticles in DSSCs. The goal of this research is to understand the fundamental photon capture, energy transfer and charge separation processes of photosynthetic pigment-protein complexes along with improving biophotovoltaic performance based on this model system through tailoring engineering of TiO₂ nanostructures, attaching of the complexes, and incorporating plasmonic enhancement. The first study reports a novel approach to linking the spectroscopic properties of nanostructured LHCII with the photovoltaic performance of LHCII-sensitized solar cells (LSSCs). The aggregation allowed reorganization between individual trimers which dramatically increased the photocurrent, correlating well with the formation of charge-transfer (CT) states observed by absorption and fluorescence spectroscopy. The assembled solar cells demonstrated remarkable stability in both aqueous buffer and acetonitrile electrolytes over 30 days after LHCII being electrostatically immobilized on amine-functionalized TiO₂ surface. The motivation of the second study is to get insights into the plasmonic effects on the nature of energy/charge transfer processes at the interface of photosynthetic protein complexes and artificial photovoltaic materials. Three types of core-shell (metal@TiO₂) plasmonic nanoparticles (PNPs) were conjugated with LHCII trimers to form hybrid systems and incorporated into a DSSC platform built on a unique open three-dimensional (3D) photoanode consisting of TiO₂ nanotrees. Enhanced photon harvesting capability, more efficient energy transfer and charge separation at the LHCII/TiO₂ interface were confirmed in the LHCII-PNP hybrids, as revealed by spectroscopic and photovoltaic measurements, demonstrating that interfacing photosynthesis systems with specific artificial materials is a promising approach for high-performance biosolar cells. Furthermore, the final study reveals the mechanism of hot electron injection by employing a mesoporous core-shell (Au@TiO₂) network as a bridge material on a micro-gap electrode to conduct electricity under illumination and comparing the photoconductance to the photovolatic properties of the same material as photoanodes in DSSCs. Based on the correlation of the enhancements in photoconductance and photovoltaics, the contribution of hot electrons was deconvoluted from the plasmonic near-field effects.

Obtaining abundant, clean, sustainable energy has become an increasingly large need globally. To date, solar cells have had a limited impact in meeting energy demands. This is primarily due to their relatively high cost and low power conversion efficiencies. Sensitized solar cells, or Grätzel cells, have the potential for being made with low cost materials, and achieving power conversion efficiency high enough to economically compete with fossil fuels. Understanding the dynamics of charge carriers as they separate at the interface of the light absorbing donor and their semiconducting acceptor becomes an important first step in the realization of an inexpensive and efficient sensitized solar cell. Presented is the theory of treating electrons at donor-acceptor interfaces, and why time-resolved surface second harmonic generation (TR-SHG) is used to probe the dynamics of charge carriers at these interfaces. A series of experiments are described where various preparations of thin films of sensitizers on single crystal titanium dioxide, a common acceptor in Grätzel cells, are prepared and studied. TR-SHG studies of thin films of colloidal PbSe and CdSe QDs showed remarkably different electron cooling and transfer dynamics. The electron cooling in PbSe is thermally activated in PbSe QDs. By cooling samples, electron transfer from higher excited “hot” states was observed. Contrary, for CdSe QDs electron transfer rates were dependent on the energy of the excited state. When higher states were excited, charge transfer rates decreased, indicating that only low energy, electrically “cold”, states participate in charge transfer. When carbon based grapheme QDs are used, the electron dynamics mimic PbSe QDs. In this system, increasing the pump energy leads to slower recombination rates, indicating that electrons have to drift further back to the interface. / text

Thesis (MScEng (Electrical and Electronic Engineering))--Univerity of Stellenbosch, 2011. / ENGLISH ABSTRACT: The design and simulation of hot electron bolometer sensors for the detection of millimeter and terahertz wave radiation is presented. These devices can be used for the formation of images or for spectroscopy in these frequency ranges. Many molecules resonate and have absorption spectra over these ranges, allowing for the identi cation, non-destructively and at a distance, of the constitution of many di erent materials. The penetrating ability of the radiation makes mm-wave and THz wave detectors ideal for security imaging. The design and simulation of these devices predicts performance as mixers and as interferometers. Manufacturing processes used while fabricating these thin lm devices are outlined and experimental results are reported. / AFRIKAANSE OPSOMMING: Hierdie tesis bespreek die ontwerp en simulasie van warm-elektron bolometers vir die waarneming van millimetergolf- en terahertz-straling. Sodanige toestelle kan in bogenoemde frekwensiebereike gebruik word vir beeldvorming of spektroskopie. Aangesien heelwat molekules binne hierdie bande resoneer, kan die samestelling van 'n verskeidenheid materiale oor 'n afstand en op 'n nie-vernietigende wyse gedenti seer word. Die deurdringingsvermo van millimetergolf- en terahertz-straling maak sulke detektors ideaal vir beeldvorming in veiligheidstoepassings. Met die ontwerp en simulasie van hierdie toestelle word gewys wat hul werkverrigting as mengers en interferometers kan wees. Die prosesse wat gebruik is om hierdie dun lmtoestelle mee te vervaardig word ook bespreek en eksperimentele resultate word aangebied.

Thanks to high Tc of 40 K, high Jc of > 10^7 A.cm^-2, and no weak link behavior across the grain boundary in MgB2 material. This highest Tc among all conventional BCS superconductors, and better material properties of MgB2 compared to high Tc cuprate superconductors makes this material attractive for many applications including, but not limited to, power cables, Josephson junction based electronic devices, SRF cavities, THz sensors and single photon counters. Ultrathin superconducting films are a key element in various detectors utilized in remote sensing over a large part of the entire electromagnetic spectrum. The superconducting hot electron bolometer (HEB) mixer is a crucial detector for high-resolution spectroscopy at THz frequencies. The state-of-the-art NbN phonon-cooled HEB mixers have a relatively narrow (IF) bandwidth ~ 3- 4 GHz as a direct result of the poor acoustic transparency of the film-substrate interface and low sound velocity in NbN reducing the phonon escape time in the film. Alternatively, MgB2 displays a very short τe-ph ~ ps. The phonon escape time is also short due to the high sound velocity in the material (~ 7 Km.s^-2) thus giving rise to a broader IF bandwidth. Also, smaller magnetic penetration depth (λ ≈ 40 nm) of MgB2 makes material of choices for single photon detector application. The response time of an SNSPD is proportional to the square of its magnetic penetration depth λ. Therefore, MgB2 may potentially operate 10-fold faster than the NbN (λ =200 nm) based SNSPD. In this work, I present my effort to fabricate high quality ultrathin superconducting MgB2 films on 6H-SiC (0001) substrates, and study their superconducting and electronic properties. C- epitaxial 10 nm showed Tc of above 36 K, while residual resistivity of up to 26 μΩ.cm was achieved. Critical currents of more than 6 × 10^6 A · cm^−2 at 20 K have been measured for the films with thicknesses iv ranging from 10 to 100 nm. Fishtail structures have been observed in the magnetic field dependence of the critical current density for the thinnest of these films, indicating the presence of defects, which act as vortex pinning centers. From the magnetic field dependence, an average distance between adjacent pinning centers of 35 nm has been obtained for the thinnest films. Ultrathin film as thin as 1.8 nm (6 unit cells) can be achieved by Hybrid Physical-Chemical Vapor Deposition (HPCVD) followed by low angle Ar ion milling. These post processed films exhibit better superconducting properties compared to directly grown films. The 1.8 nm, showed Tc > 28 K and Jc > 10^6 A/cm^2 4 K. The surface roughness of the films was significantly improved and the suppression of Tc from the bulk value is much slower in milled films than in as-grown films. These results show the great potential of these ultrathin films for superconducting devices and present a possibility to explore superconductivity in MgB2 at the 2D limit. Finally, I measured the upper critical field of MgB2 films of various thickness and extracted their thickness dependent in-plane intraband diffusivities by using Gurevich model developed for two-band MgB2 superconductor in dirty limit. Results showed that π band diffusivity (Dπ) decreases rapidly from 71.12 cm^2/s for 100 nm film to 4.6 cm^2/s for 5 nm film where as �� band diffusivity (����) decreases much slower from 2.8 cm^2/s for 100 nm film to 0.8 cm^2/s for 5 nm film. This larger Dπ than ���� indicates the cleaner π band. / Physics

The advancement of next generation technologies is reliant on our ability to engineer matter at the nanoscale. Since the morphological features of nanomaterials dictate their chemical and physical properties, a significant effort has been put forth to develop syntheses aimed at fine tuning their size, shape and composition. This massive effort has resulted in a maturing colloidal chemistry containing an extensive collection of morphologies with compositions nearly spanning the entire transition of the periodic table. While colloidal nanoparticles have opened the door to promising applications in fields such as cancer theranostics, drug delivery, catalysis and sensing; the synthetic protocols for the placement of nanomaterials on surfaces, a requisite for chip-based devices, are ill-developed. This dissertation serves to address this limitation by highlighting a series of syntheses related to the design of substrate-based nanoparticles whose size, shape and composition are controllably engineered to a desired endpoint. The experimental methods are based on a template-mediated approach which sees chemical modifications made to prepositioned thermally assembled metal nanostructures which are well bonded to a sapphire substrate. The first series of investigations will highlight synthetic routes utilizing galvanic replacement reactions, where the prepositioned templates are chemically transformed into hollow nanoshells. Detailed studies are provided highlighting discoveries related to (i) hollowing, (ii) defect transfer, (iii) strain induction, (iv) interdiffusion, (v) crystal structure and (vi) the localized surface plasmon resonance (LSPR). The second series of investigations, based on heterogeneous nucleation, have Au templates serve as nucleation sites for metal atoms arriving in either the solution- or vapor phase. The solution-phase heterogeneous nucleation of Ag on Au reveals that chemical kinetics (injection rate & precursor concentration) can be used to control the nature of how Ag atoms grow on the Au template. It was discovered that (i) slow kinetics leads to an anisotropic growth mode (heterodimeric structures), (ii) fast kinetics causes a very uniform deposition (Au-Ag coreshell morphology, or Au@Ag) and (iii) medium kinetics produces structures with an intermediate morphology (truncated octahedron). In the second case, where the nucleation event is carried out at high temperatures, the Ag vapor is sourced from a sublimating foil onto adjacent Au templates. This process drives the composition and morphology from a Au Wulff-shape to a homogeneous Au-Ag nanoprism. By tracking over time the (i) morphological features, (ii) LSPR and (iii) composition; insights into the fundamental atomic scale growth mechanisms are elucidated. Overall, substrate-based template-mediated syntheses have proven to be an effective route for directing growth pathways toward a desired endpoint giving rise to an impressive new group of complex substrate-based nanostructures with asymmetric, core-shell and hollowed morphologies. While this dissertation is focused heavily on the development of synthetic procedures aimed at generating substrate-based plasmonic nanomaterials, the last chapter will serve to highlight a series of on-going studies aimed at defining these nanomaterials as highly effective heterogeneous catalysts. Several examples are shown including (i) nanoparticle films synthesize via sputter deposition, (ii) mechanically induced nanotexturing of bulk copper foils, (iii) ultra-small AuPd nanoparticles synthesized via pulse laser, (iv) substrate-based AuCu nanoprisms and (v) the Wulff in a Cage Morphology. / Mechanical Engineering

Transistor Quantique InAs à Electrons Chauds: Fabrication submicronique et étude à haute fréquenceL'objectif de cette thèse est le développement de la technologie d'un transistor à électrons chauds constitué d'une hétérostructure quantique InAs/AlSb et exploitant un transport électronique résonant ultrarapide, le QHET (Quantum Hot Electron Transistor). Ce travail a permis l'étude approfondie de ses propriétés et performances à haute fréquence. L'étude aborde tous les aspects, de la conception, la croissance épitaxiale, la technologie de fabrication à la caractérisation statique et dynamique. Ce travail de thèse s'est effectué principalement à l'Institut d'Electronique du Sud (IES), sous la direction de Roland Teissier, et pour partie à l'Institut d'Electronique de Microélectronique et Nanotechnologie (IEMN) sous la direction de Mohamed Zaknoune. Nous avons, dans premier temps, mis en œuvre à l'IES une technologie double mésa afin de fabriquer les transistors avec l'émetteur de 10x10µm². La technologie en grande dimension est aisément réalisable et surtout reproductible. Elle nous a permis de travailler sur un grand nombre de structures transistor fabriquées par epitaxie par jets moléculaires (EJM) sur substrats InAs, afin d'en étudier le transport électronique et d'optimiser leur dessin. Le premier résultat marquant a été d'augmenter le gain statique jusqu'à une valeur de 15 grâce à une modification de la structure de l'émetteur qui une injection plus efficace puis l'utilisation d'une base fine de 85Å, qui améliore le temps de transit. Dans un deuxième temps, nous avons travaillé au sein de l'IES sur l'évolution de la technologie vers des dimensions intermédiaires dont la dimension la plus petite est de 1 µm de largeur. Cette technologie nous a donné une amélioration de performance des QHET grâce à la réduction des résistances et des capacités parasites des composants. Nous avons aussi travaillé à l'IEMN pour développer une technologie submicronique qui permet d'atteindre une largeur d'émetteur de 0.3 µm grâce à l'utilisation de la lithographie électronique. Cette technologie de fabrication plus performante nous a permis de mieux comprendre le fonctionnement du QHET. Et d'atteindre une régime de fonctionnement à forte densité de courant jusqu'à près de 1MA/ cm². Enfin, nous avons développé la structure et la technologie qui vont nous permettre d'évaluer la réponse à haute fréquence des QHET. Un point important a été de à disposer de la structure active du transistor sur un substrat isolant qui permette de réduire les éléments parasites durant la mesure en fréquence. Nous avons développé deux solutions : le transfert de substrat et la croissance métamorphique directement sur un substrat GaAs isolant.Les composants fabriqués par transfert de susbtrat présentent des valeurs de fréquence de transition FT de 77GHz et de fréquence d'oscillation FMAX de 88GHz. Les échantillons métamorphiques ont démontré de meilleures performances avec un FT de 170GHz et un FMAX supérieur à 200GHz. Ces résultats constituent les meilleurs dynamiques de transistors à électrons chauds à température ambiante. Ces études ont également fait progresser la compréhension du transport à haute fréquence dans ces composants. Ils permettent de comprendre les limitations actuelles et de proposer des pistes d'amélioration. / This work aims to develop a new high speed transistor in a vertical transport configuration that exploits the favourable transport properties of III-V semiconductor heterostructures based on InAs. This transistor is similar to a heterojunction bipolar transistor (HBT), but has theoretical assets to overcome the fundamental high speed limits of electron transport in HBT. Our approach uses the concept of hot electron transistor in an original InAs/AlSb quantum heterostructure, that we called a quantum hot electron transistor (QHET) or quantum cascade transistor (QCT). This research was almost done in Southern Electronics Institute (IES) under supervision of Dr. Roland Teissier and other work was realized in Micro-Nanotechnology Electronics Institute (IEMN) under supervision of Dr. Mohamed Zaknoune. The QHET is a unipolar vertical transport device made of a InAs/AlSb quantum heterostructure. Its first advantage over npn HBTs is the low base sheet resistance of 250 Ω/□ , accessible with moderate n-type doping levels (typically 1018 cm-3), which is a key parameter for high speed operation. Secondly, electron transport in the short (typically 100nm) bulk InAs collector is mostly ballistic with calculated transit times much shorter than in InP-based devices. We already developed the design and technology of QHET and demonstrated its resonant transports at cryogenic temperature and its improved static operation in smaller device. From these results, we come to develop our QHET structures to achieve high current gain. Using quantum design of thin base, the current gain is about 15. We fabricated QHET with emitter width scaled down to 0.3µm, using a state of the art electron beam lithography process. The junctions are defined using selective chemical etching. The base contact is self-aligned on the emitter contact. We achieved base resistance lower than 50Ω, comparable to state of the art HBTs. The small dimension allowed reaching the high current density regime of up to 1 MA/cm² required for high frequency operation. The static current gain is about 10, but could be increased up to 14 using a new quantum design. The collector breakdown voltage is greater than 1.2 V.Towards high frequency measurement, the substrate must be non-conducting material but InAs substrate is not available. Two technologies were proposed: transferred substrate and metamorphic substrate. For transferred substrate technology, we obtained a response of cutoff frequency of 77 GHz for FT and 88 for FMAX. For metamorphic substrate technology, we performed the growth of the transistor structures on a semi-insulating GaAs substrate. We used a thin GaSb buffer layer for metamorphic growth of the active part of the transistor, with an adequate growth procedure that allows forming mainly 90° misfit dislocations at the interface between the GaAs and GaSb. This technique permits more convenient and reliable processing of the devices, as compared to use of the more standard AlSb thick buffer layer. The frequency response was determined from S-parameters measured with a network analyser up to a frequency of 70 GHz. The measured gains, after de-embedding of the connection parasitic for a device with 0.5x4µm² emitter for JC=350kA/cm² (Ic= 6.0mA, Ib= 0.7mA, Vce=1.3V). The frequency dependence is not conventional on this device, with a resonance in the current gain close to 10 GHz and a slope different from -20 dB/decade for Mason's unilateral gains. Nevertheless, we could extract the cut-off frequencies FT=172 GHz from H21 and FMAX =230 GHz using -20dB/decade extrapolation of maximum stable gain (MSG). The present results confirmed the validity of this novel device concept. In addition, this is the first demonstration of the ability of a hot electron transistor to operate at high frequency at room temperature.