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1
Luo, Ming. "Transition-metal ions in II-VI semiconductors ZnSe and ZnTe /." Morgantown, W. Va. : [West Virginia University Libraries], 2006. https://eidr.wvu.edu/etd/documentdata.eTD?documentid=4630.
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Thesis (Ph. D.)--West Virginia University, 2006.Title from document title page. Document formatted into pages; contains xiv, 141 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 135-141).
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Kashefi-Naini, A. "A study of some transition metal-silicon Schottky barrier diodes." Thesis, University of Kent, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.375200.
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Kajikawa, Hiroaki. "Slow dynamics in the semiconductor-metal transition region of liquid chalcogens." 京都大学 (Kyoto University), 2007. http://hdl.handle.net/2433/136830.
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Peleckis, Germanas. "Studies on diluted oxide magnetic semiconductors for spin electronic applications." Access electronically, 2006. http://www.library.uow.edu.au/adt-NWU/public/adt-NWU20070821.145447/index.html.
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Proskuryakov, Yuri. "Interactions, localisation and the metal to insulator transition in two-dimensional semiconductor systems." Thesis, University of Exeter, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.288367.
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Hall, Ralph Stephen. "Photocapacitance studies of transition metal related deep levels in III-V and II-VI semiconducters." Thesis, University of St Andrews, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329476.
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Stollenwerk, Tobias [Verfasser]. "Ferromagnetic Semiconductor-Metal Transition in Heterostructures of Electron Doped Europium Monoxide / Tobias Stollenwerk." Bonn : Universitäts- und Landesbibliothek Bonn, 2013. http://d-nb.info/1045276324/34.
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Ramanathan, Sivakumar. "Optical and electrical properties of compound and transition metal doped compound semiconductor nanowires." VCU Scholars Compass, 2009. http://scholarscompass.vcu.edu/etd/1667.
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Nanotechnology is the science and engineering of creating functional materials by precise control of matter at nanometer (nm) length scale and exploring novel properties at that scale. It is vital to understand the quantum mechanical phenomena manifested at nanometer scale dimensions since that will enable us to precisely engineer quantum mechanical properties to realize novel device functionalities. This dissertation investigates optical and electronic properties of compound and transition metal doped compound semiconductor nanowires with a view to exploiting them for a wide range of applications in semiconductor electronic and optical devices. In this dissertation work, basic concepts of optical and electronic properties at low dimensional structures will be discussed in chapter 1. Chapter 2 discusses the nanofabrication technique employed to fabricate highly ordered nanowires. Using this method, which is based on electrochemical self-assembly techniques, we can fabricate highly ordered and size controlled nanowires and quantum dots of different materials. In Chapter 3, we report size dependent fluorescence spectroscopy of ZnSe and Mn doped ZnSe nanowires fabricated by the above method. The nanowires exhibit blue shift in the emission spectrum due to quantum confinement effect, which increases the effective bandgap of the semiconductor. We found that the fluorescence spectrum of Mn doped ZnSe nanowires shows high luminescence efficiency, which seems to increase with increasing Mn concentration. These results are highly encouraging for applications in multi spectral displays. Chapter 4 investigates field emission results of highly ordered 50 nm tapered ZnO nanowires that were also fabricated by electrochemical self-assembly. Subsequent to fabrication, the nanowires tips are exposed by chemical etching which renders the tips conical in shape. This tapered shape concentrates the electric field lines at the tip of the wires, and that, in turn, increases the emission current density while lowering the threshold field for the onset of field emission. Measurement of the Fowler-Nordheim tunneling current carried out in partial vacuum indicates that the threshold electric field for field emission in 50-nm diameter ZnO nanowires is 15 V/µm. In this study we identified the key constraint that can increase the threshold field and reduce emission current density. In Chapter 5 we report optical and magnetic measurement of Mn-doped ZnO nanowires. Hysterisis measurements carried out at various temperatures show a ferromagnetic behavior with a Curie temperature of ~ 200 K. We also studied Mn-doping of the ZnO nanowires. The room temperature fluorescence spectroscopy of Mn-doped ZnO nanowires shows a red-shift in the spectra compared to the undoped ZnO nanowires possibly due to strain introduced by the dopants in the nanowires. Finally, in Chapter 6, we report our study of the ensemble averaged transverse spin relaxation time (T2*) in InSb thin films and nanowires using electron spin resonance (ESR) measurement. Unfortunately, the nanowires contained too few spins to produce a detectable signal in our apparatus, but the thin films contained enough spins (> 109/cm2) to produce a measurable ESR signal. We found that the T2* decreases rapidly with increasing temperature between 3.5 K and 20 K, which indicates that spin-dephasing is primarily caused by spin-phonon interactions.
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Wright, Trevor. "A comparison of the metal-insulator transitions amporphous metal-semiconductor alloys." Thesis, University of Cambridge, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.264356.
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Hart, Lewis. "Novel transition metal dichalcogenide semiconductors and heterostructures." Thesis, University of Bath, 2018. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.760986.
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Rhenium diselenide and rhenium disulphide are layered semiconductors that belong to the transition metal dichalcogenide (TMD) family. Like graphene and other TMDs, these materials can be exfoliated down to a few atomic layers. However, unlike other TMDs, the rhenium dichalcogenides are only stable in a triclinic structure that exhibits in-plane anisotropy. This anisotropy manifests itself in the vibrational, optical and electronic transport properties ofthese crystals. Ab initio calculations and experimental results are presented to describe the Raman spectra of the rhenium dichalcogenides. From Raman spectroscopy the anisotropy of these crystals can be observed. Flipping a flake (a C2 rotation about an axis in the layer plane) is not a symmetry of the system. Therefore, there are two non-equivalent vertical orientations. Raman spectroscopy can be used to identify whether a flake is facing "up" or "down". The latticedynamics of these crystals are described using a simple ball and spring model. It is shown that low mass impurities, such as sulphur, in ReSe2 can occupy four non-equivalent positions of the unit cell; there are four local vibrational modes corresponding to these four positions and Raman spectroscopy can be used to find them. An unusual experimental geometry (edge-on excitation) helps enhance these signals. The electronic band structures of bulk ReSe2 and ReS2 are explored using angle-resolved photoemission spectroscopy (ARPES). From the measurements and complementary DFT calculations it is shown that: (i) there is anisotropy in the electronic dispersions; (ii) the valence band maxima are not located along any of the high symmetry directions; and (iii) both of these crystals have indirect band gaps. The rhenium dichalcogenides were thought to act as electronically decoupled monolayers; it is demonstrated that this is not the case and that thereis signicant electronic coupling between the layers. Finally, ARPES results of a monolayer of ReSe2 are presented; again, anisotropy in the electronic band structure is observed.
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Silber, Georg Thomas. "Molecular semiconductors based on transition metal complexes." Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/18018.
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The field of organic, or molecular, electronics is currently dominated by both polymeric and molecular organic materials, while considerably less research efforts are devoted to transition metal based complexes. Despite this, such compounds can offer advantages including additional tuneability of the spatial distribution and energy levels of the frontier orbitals or stable paramagnetic species by manipulating the metal-ligand interactions which may be accomplished selectively modifying the ligand framework or changing the central metal. A series of Ni(II) and Cu(II) acenaphthenequinone bis(thiosemicarbazonato) complexes were prepared and characterised using X-ray diffraction, cyclic voltammetry, UV/Vis and EPR spectroscopy, as well as magnetic susceptibility and field effect transistor measurements and computational calculations. The observed charge transport properties are discussed in terms of the structural and electronic trends both within the series and in the context of the two more established analogue series, namely the bis(3-thiosemicarbazonato) and the diacetyl bis(3-thiosemicarbazonato) metal complexes. The Ni(II) analogues of the acenaphthenequinone bis(thiosemicarbazonato) family were found to exhibit p-type charge transport with mobilities between 10¯9 and 10¯5 cm2V¯1s¯1 depending on the exocyclic substitutent and resulting packing pattern. The observed results were rationalised in terms of the reorganisation energy and the charge transfer integrals. A series of 4,4`-phenyl-substituted nickel dithiolene complexes was synthesised and characterised. Initially with the aim of investigating the effect of varying the para-substituent of the phenyl ring on the charge transport properties, these efforts were undermined by the poor processability of these molecules by both vapour and solution phase methods. As a result, n-type charge transport could be observed under ambient conditions only for the phenyl and 4-bromo-phenyl substituted analogues, but the device performance was extremely poor. Nonetheless, the calculated reorganisation energies, charge transfer integrals and predicted mobilities were encouraging and may prompt further work on these materials. An all-organic analogue series of 4,4`-(4-halogen-phenyl)-substituted tetrathiafulvalenes was also investigated. The hole transport materials displayed mobilities of between 10¯3 and 10¯7 cm2V¯1s¯1 for both solution and vapour processed devices, depending on the nature of the halogen. These results are discussed in terms of their molecular properties and the calculated charge transport parameters and put in context of the performance of the 4,4`-bis(phenyl)-substituted benchmark analogue. Interestingly, the obtained crystal structure of the bromo-substituted analogue showed the molecule to be in the cis conformation, an observation that is unprecedented for simple, 4-phenyl,5-hydrogen substituted tetrathiafulvalenes, and indicates that both conformers are initially formed. Finally, a series of 4,4`-(2-alkyl)thienyl substituted nickel dithiolene salts and tetrathiafulvalenes was synthesised and characterised. While the charge transport properties of the former were not further investigated due to the low solubility of the neutral species, the tetrathiafulvalenes were incorporated into FET devices via solution processing. All exhibited comparatively high conductivity at room temperature (1.6x10¯3S m¯1), exceeding that of their quarterthiophene analogues. This masked the observed gate effects but indicates potential applications as conducting or charge transfer materials. While the two resolved analogues displayed trans geometry in the single crystal structures, powder diffraction and preliminary DSC measurements indicate that the materials displayed at least one additional phase, which once again likely corresponded to the cis conformer.
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Neuenschwander, Jürg. "A high pressure low temperature study on rare earth compounds : semiconductor to metal transition /." [S.l.] : [s.n.], 1988. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=8668.
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Collins-McIntyre, Liam James. "Transition-metal doped Bi2Se3 and Bi2Te3 topological insulator thin films." Thesis, University of Oxford, 2015. http://ora.ox.ac.uk/objects/uuid:480ea55a-5cac-4bab-a992-a3201f10f4c5.
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Topological insulators (TIs) are recently predicted, and much studied, new quantum materials. These materials are characterised by their unique surface electronic properties; namely, behaving as band insulators within their bulk, but with spin-momentum locked surface or edge states at their interface. These surface/edge crossing states are protected by the underlying time-reversal symmetry (TRS) of the bulk band structure, leading to a robust topological surface state (TSS) that is resistant to scattering from impurities which do not break TRS. Their surface band dispersion has a characteristic crossing at time reversal invariant momenta (TRIM) called a Dirac cone. It has been predicted that the introduction of a TRS breaking effect, through ferromagnetic order for instance, will open a band-gap in this Dirac cone. It can be seen that magnetic fields are not time reversal invariant by considering a solenoid. If time is reversed, the current will also reverse in the solenoid and so the magnetic field will also be reversed. So it can be seen that magnetic fields transform as odd under time reversal, the same will be true of internal magnetisation. By manipulating this gapped surface state a wide range of new physical phenomena are predicted, or in some cases, already experimentally observed. Of particular interest is the recently observed quantum anomalous Hall effect (QAHE) as well as, e.g., topological magneto-electric effect, surface Majorana Fermions and image magnetic monopoles. Building on these novel physical effects, it is hoped to open new pathways and device applications within the emerging fields of spintronics and quantum computation. This thesis presents an investigation of the nature of magnetic doping of the chalcogenide TIs Bi2Se3 and Bi2Te3 using 3d transition-metal dopants (Mn and Cr). Samples were grown by molecular beam epitaxy (MBE), an ideal growth method for the creation of high-quality thin film TI samples with very low defect densities. The grown films were investigated using a range of complementary lab-based and synchrotron-based techniques to fully resolve their physical structure, as well as their magnetic and electronic properties. The ultimate aim being to form a ferromagnetic ground state in the insulating material, which may be expanded into device applications. Samples of bulk Mn-doped Bi2Te3 are presented and it is shown that a ferromagnetic ground state is formed below a measured TC of 9-13 K as determined by a range of experimental methodologies. These samples are found to have significant inhomogeneities within the crystal, a problem that is reduced in MBE-grown crystals. Mn-doped Bi2Se3 thin films were grown by MBE and their magnetic properties investigated by superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD). These reveal a saturation magnetisation of 5.1 μB/Mn and show the formation of short-range magnetic order at 2.5 K (from XMCD) with indication of a ferromagnetic ground state forming below 1.5 K. Thin films of Cr-doped Bi2Se3 were grown by MBE, driven by the recent observation of the QAHE in Cr-doped (Bi1−xSbx)2Te3. Investigation by SQUID shows a ferromagnetic ground state below 8.5 K with a saturation magnetisation of 2.1 μB/Cr. Polarised neutron reflectometry shows a uniform magnetisation profile with no indication of surface enhancement or of a magnetic dead layer. Further studies by extended x-ray absorption fine structure (EXAFS) and XMCD elucidate the electronic nature of the magnetic ground state of these materials. It is found that hybridisation between the Cr d and Se p orbitals leads to the Cr being divalent when doping on the Bi3+ site. This covalent character to the electronic structure runs counter to the previously held belief that divalent Cr would originate from Cr clusters within the van der Waals gap of this material. The work overall demonstrates the formation of a ferromagnetic ground state for both Cr and Mn doped material. The transition temperature, below which ferromagnetic order is achieved, is currently too low for usable device applications. However, these materials provide a promising test bed for new physics and prototype devices.
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Kim, Changsu. "Optical, laser spectroscopic, and electrical characterization of transition metal doped ZnSe and ZnS nano- and microcrystals." Birmingham, Ala. : University of Alabama at Birmingham, 2009. https://www.mhsl.uab.edu/dt/2009r/kim.pdf.
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Thesis (Ph. D.)--University of Alabama at Birmingham, 2009.Title from PDF title page (viewed Feb. 3, 2010). Additional advisors: Renato Camata, Derrick Dean, Chris M. Lawson, Andrei Stanishevsky, Sergey Vyazovkin. Includes bibliographical references (p. 133-140).
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Watson, Deborah Lee. "Quantum interference effects in the magnetoresistance of semiconductor structures near the metal to insulator transition." Thesis, University of Exeter, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.286547.
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Racke, David. "Measuring and Controlling Energy Level Alignment at Hybrid Organic/Inorganic Semiconductor Interfaces." Diss., The University of Arizona, 2015. http://hdl.handle.net/10150/556212.
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In this dissertation, I present the results of my research regarding hybrid semiconductor interfaces between organic and inorganic semiconductors. Using photoemission spectroscopy, I elucidate the important role of defect-induced electronic states within the inorganic semiconductor phase. These states significantly affect both the energy level alignment and the charge carrier dynamics at the hybrid interface. I demonstrate that the behavior of these hybrid semiconductor interfaces is complex and not well characterized by current models for organic semiconductor interfaces. Specifically, I show that hybrid interfaces host unique electronic phenomena that depend sensitively on the surface structure of the inorganic semiconductor. I also demonstrate new applications of photoemission spectroscopies that enable the direct analysis of important properties of inorganic semiconductors, including charge carrier behavior near hybrid interfaces and the electronic character of defect-induced energy levels. The research presented here focuses on two different n-type inorganic semiconductors, tin disulfide (SnS₂) and zinc oxide (ZnO). SnS₂ is a layered transition metal dichalcogenide that presents an atomically flat and inert surface, ideal for sensitively probing electronic interactions at the hybrid interface. To probe the electronic structure of the SnS₂ surface, I used a variety of organic molecules, including copper phthalocyanine, vanadyl naphthalocyanine, chloro-boron subphthalocyanine, and C₆₀. ZnO has a complex surface structure that can be modified by simple experimental procedures; it was therefore used as a tunable semiconductor substrate where the effects of altered electronic structure can be observed. By carefully studying the origin of hybrid interfacial interactions, these research projects provide a first step in explicitly elucidating the fundamental mechanisms that determine the electronic properties of hybrid interfaces.
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Shen, Pin-Chun. "Large-area CVD growth of two-dimensional transition metal dichalcogenides and monolayer MoS₂ and WS₂ metal-oxide-semiconductor field-effect transistors." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/112003.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 54-55).
Two-dimensional semiconducting materials such as MoS₂ and WS₂ have been attractive for use in ultra-scaled electronic and optoelectronic devices because of their atomically-thin thickness, direct band gap, and lack of dangling bonds. Methods for large-area growth of 2D semiconducting materials are needed to bring them to practical applications. This thesis aims to develop reliable methods for growing high-quality monolayer MoS₂ and WS₂ by CVD and explore their intrinsic electrical transport properties for electronic and optoelectronic device applications. The as-grown monolayer MoS₂ and WS₂ exhibit n-type semiconducting behavior with excellent optical properties. Various techniques are employed to characterize the CVD-grown materials, including photoluminescence, UV-visible absorption, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. Moreover, the electronic transport characteristics of single-layer CVD-grown MoS₂ and WS₂ field-effect transistors with a back-gated configuration are demonstrated.
by Pin-Chun Shen.
S.M.
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Muckel, Franziska [Verfasser], and Gerd [Akademischer Betreuer] Bacher. "Transition metal doped colloidal semiconductor nanocrystals : from functionality to device development / Franziska Muckel ; Betreuer: Gerd Bacher." Duisburg, 2018. http://d-nb.info/1155722787/34.
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Muckel, Franziska Elisabeth [Verfasser], and Gerd [Akademischer Betreuer] Bacher. "Transition metal doped colloidal semiconductor nanocrystals : from functionality to device development / Franziska Muckel ; Betreuer: Gerd Bacher." Duisburg, 2018. http://d-nb.info/1155722787/34.
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Simo, Aline. "Physical properties of vanadium dioxide nanoparticles: application as 1-d nanobelts room temperature for hydrogen gas sensing." Thesis, University of the Western Cape, 2013. http://hdl.handle.net/11394/4581.
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Philosophiae Doctor - PhDTransition metal oxides magneli phases present crystallographic shear structure which is of great interest in multiple applications because of their wide range of valence, which is exhibited by the transition metals. The latter affect chemical and physical properties of the oxides. Amongst them we have nanostructures VO2 system of V and O components which are studied including chemical and physical reactions based on non-equilibrium thermodynamics. Due to their structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure, these oxides are generally used as catalytic materials which are prepared by common methods under mild conditions presenting distortion or defects in the case of VO2. Existence of an intermediate phase is proved using an x-ray thermodiffraction experiment providing structural information as the nanoparticles are heated. Potential application as gas sensing device has been the first time obtained due to the high surface to volume ratio, and good crystallinity, purity of the material and presence of suitable nucleating defects sites due to its n-type semiconductor behavior. In addition, annealing effect on nanostructures VO2 nanobelts shows a preferential gas reductant of Ar comparing to the N2 gas. Also, the hysteresis loop shows that there is strong size dependence to annealing treatment on our samples. This is of great interest in the need of obtaining high stable and durable material for Mott insulator transistor and Gas sensor device at room temperature.
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Fan, Junpeng. "Synthesis and advanced structural and magnetic characterization of mesoporous transition metal–doped sno2 powders and films." Doctoral thesis, Universitat Autònoma de Barcelona, 2017. http://hdl.handle.net/10803/457982.
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Aquesta Tesi doctoral comprèn la síntesi mitjançant nanoemmotllament (de l’anglès, nanocasting) i autoassemblatge per evaporació induïda (de l’anglès, evaporation–induced self–assembly) i la caracterització exhaustiva de pols i capes de SnO2 mesoporós dopat amb Ni i Cu. L’origen de les propietats magnètiques d’aquests materials es discuteix en detall.
En primer lloc, es van sintetitzar per nanoemmotllament a partir de motlles de sílice KIT–6, pols mesoporosa ordenada de SnO2 dopada amb diferents quantitats de Ni. Es va verificar la replicació correcta del motlle de sílice mitjançant microscòpia electrònica de rastreig. No es van detectar fases extres atribuïbles a Ni o NiO en els corresponents difractogrames excepte per a la mostra amb el dopatge més alt (9 at.% Ni), per a la qual es va observar la presència de NiO com a fase secundària. Es va estudiar l’estat d’oxidació i la distribució espaial de Ni en la pols mitjançant espectroscòpia fotoelectrònica de raigs X i espectroscòpia de pèrdua d’energia d’electrons, respectivament. Les mostres dopades amb Ni presenten resposta ferromagnètica tant a temperatura ambient com a baixa temperatura, com a conseqüència de la presència d’espins no compensats a la superfície de nanopartícules de NiO i vacants d’oxigen.
En segon lloc, es van sintetitzar capes primes continues i mesoporoses de SnO2 dopades amb Ni a partir de diferents relacions molars [Ni(II)]/[Sn(IV)] mitjançant un procés d’autoassemblatge sol–gel, utilitzant el copolímer tribloc P–123 com a agent director d’estructura. Una caracterització estructural profunda va evidenciar l’obtenció d’una estructura nanoporosa 3–D, de gruix comprès entre els 100 i 150 nm, i mida de porus de 10 nm. Els experiments de difracció de raigs X d’incidència rasant van posar de manifest que el Ni ocupava posicions substitucionals en la xarxa tipus rutil del SnO2, tot i que les anàlisis per dispersió d’energies de raigs X també van revelar la presència de petits clústers de NiO en les capes produïdes a partir de les relacions molars [Ni(II)]/[Sn(IV)] més elevades. Convé remarcar que les propietats magnètiques de les capes mesoporoses varien significativament en funció del percentatge de dopant. Les capes de SnO2 no dopades presenten un comportament diamagnètic, mentre que les dopades amb Ni mostren un clar senyal paramagnètic amb una petita contribució ferromagnètica.
En tercer lloc, també es van estudiar les propietats magnètiques de pols mesoporosa ordenada de SnO2 dopada amb Cu, obtinguda mitjançant nanoemmotllament a partir de sílice KIT–6. Per bé que una eventual contaminació amb impureses de Fe or la presència de vacants d’oxigen podrien explicar el comportament ferromagnètic observat a temperatura ambient, el ferromagnetisme a baixa temperatura es va atribuir únicament a la naturalesa nanoestructurada de les nanopartícules antiferromagnètiques de CuO formades (espins no compensats i shape–mediated spin canting). La menor temperatura de bloqueig, situada entre 30 i 50 K, i l’existència de petits desplaçaments verticals en els cicles d’histèresi van confirmar efectes de mida en les nanopartícules de CuOThis Thesis dissertation covers the synthesis by means of nanocasting and evaporation–induced self–assembly (EISA) methods as well as the advanced characterization of Ni, Cu–doped mesoporous SnO2 powders and films. The origin of the magnetic properties in these materials is also discussed in detail. Firstly, ordered mesoporous SnO2 powders doped with different Ni amounts were synthesized by nanocasting from mesoporous KIT–6 silica. Successful replication of the silica template was verified by scanning electron microscopy. No extra phases attributed to Ni or NiO were detected in the corresponding X–ray diffractograms except for the sample with the highest doping amount (e.g., 9 at.% Ni), for which NiO as secondary phase was observed. The oxidation state and spatial distribution of Ni in the powders was investigated by X–ray photoelectron spectroscopy and electron energy loss spectroscopy, respectively. Ni–containing powders exhibit ferromagnetic response at low and room temperatures, due to uncompensated spins at the surface of NiO nanoparticles and the occurrence of oxygen vacancies. Secondly, continuous mesoporous Ni–doped SnO2 thin films were synthesized from variable [Ni(II)]/[Sn(IV)] molar ratios through a sol–gel based self–assembly process, using P–123 triblock copolymer as a structure directing agent. A deep structural characterization revealed a truly 3–D nanoporous structure with thickness in the range of 100–150 nm, and average pore size about 10 nm. Grazing incidence X–ray diffraction experiments indicated that Ni had successfully substituted Sn in the rutile–type lattice, although energy–dispersive X–ray analyses also revealed the occurrence of small NiO clusters in the films produced from high [Ni(II)]/[Sn(IV)] molar ratios. Interestingly, the magnetic properties of these mesoporous films significantly vary as a function of the doping percentage. The undoped SnO2 films exhibit a diamagnetic behaviour, whereas a clear paramagnetic signal with small ferromagnetic contribution dominates the magnetic response of the Ni–doped mesoporous films. Thirdly, the magnetic properties of ordered mesoporous Cu–doped SnO2 powders, prepared by hard–templating from KIT–6 silica, were also studied. While Fe contamination or the presence of oxygen vacancies might be a plausible explanation of the room temperature ferromagnetism, the low–temperature ferromagnetism was mainly and uniquely assigned to the nanoscale nature of the formed antiferromagnetic CuO nanoparticles (uncompensated spins and shape–mediated spin canting). The reduced blocking temperature, which resided between 30 and 5 K, and small vertical shifts in the hysteresis loops confirmed size effects in the CuO nanoparticles.
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Wilkinson, Aidan. "Transport phenomena in two-phase systems." Thesis, Loughborough University, 2017. https://dspace.lboro.ac.uk/2134/25574.
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The physics of two-phase systems is explored here, particularly magneto-transport and low temperature d.c. conductivity in thin films. The extraordinary magnetoresistance (EMR) effect was analysed in the context of previous experimental and theoretical considerations. The magnetoresistance (MR) may be enhanced by up to two orders of magnitude by changing the geometry. This was investigated using finite element analysis. Thin film samples consisting of a layered structure of Germanium-Tin-Germanium (Ge-Sn-Ge) were created in collaboration with Shandong University in China. Ge layers were kept at a constant thickness across all samples, with variable Sn thickness. Regions of Sn form island-like shapes ten times larger than the average film thickness, as is seen in scanning electron microscope (SEM) images. Raman spectroscopy was conducted on these samples, from which it is concluded that the Ge layers are amorphous in nature. It was seen that there is a relationship between the electrical resistance and the film thickness which is indicative of a metal-insulator transition (MIT). The temperature dependence of resistivity was subsequently investigated. The temperature coefficient of resistivity (TCR) of the samples is seen to become negative as the thickness of the Sn layer is reduced below a certain critical thickness. Depending on their thickness, samples were designated as metallic or insulator, and various models associated with metals and insulators fitted to the data. While it is impossible to be absolutely certain of the validity of each of the models, some are a better fit than others. The same temperature dependence of resistivity was measured with an applied magnetic field. This is compared with the previous EMR investigation, however the MR of the samples is only of the order of a few percent which corresponds to ordinary MR, seen in most metals. The magnetic field measurements suppress a resistivity down-turn at very low temperatures (T < 10K) which suggests the presence of superconductivity. Analysis of dr=dT shows that the onset of superconductivity is lower for samples with a lower Sn thickness. Additionally, the deposition rate of the Sn layer affects the resistivity significantly; a higher deposition rate causes a decrease in resistivity. It is supposed that this is due to a change in the microstructure of the film. Finally, piezo-resistivity was considered by applying mechanical compression to the samples. The added pressure causes a drop in resistivity.
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Zhou, Shengqiang. "Transition metal implanted ZnO: a correlation between structure and magnetism." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2008. http://nbn-resolving.de/urn:nbn:de:bsz:14-ds-1209998012687-36583.
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Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors question the origin of this ferromagnetism, i.e. if the observed ferromagnetism stems from ferromagnetic precipitates rather than from carriermediated magnetic coupling of ionic impurities, as required for a diluted magnetic semiconductor. In this thesis, this question will be answered for transition-metal implanted ZnO single crystals. Magnetic secondary phases, namely metallic Fe, Co and Ni nanocrystals, are formed inside ZnO. They are - although difficult to detect by common approaches of structural analysis - responsible for the observed ferromagnetism. Particularly Co and Ni nanocrystals are crystallographically oriented with respect to the ZnO matrix. Their structure phase transformation and corresponding evolution of magnetic properties upon annealing have been established. Finally, an approach, pre-annealing ZnO crystals at high temperature before implantation, has been demonstrated to sufficiently suppress the formation of metallic secondary phases.
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Gorini, Lorenzo. "Electrical contact properties of ultrathin transition metal dichalcogenide sheets." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2018. http://amslaurea.unibo.it/16884/.
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The graphene discovery led to advances in exfoliation and synthetic techniques, and the lack of a bandgap in graphene has stimulated the research for new 2D semiconducting materials. Transition metal dichalcogenides (TMDCs), semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), have recently been isolated. TMDCs exhibit a unique combination of atomic-scale thickness, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, valleytronics and optoelectronics.
According to optical measurements, single-layer WS2 sheets exhibit a direct band gap of at least 2.0 eV. Because of its strong spin-orbit coupling induced valence band splitting, WS2 shows spin-valley coupling, even in few-layer sheets , which may allow easier observation of the valley Hall effect than in the other TMDCs.
The thesis reviews the theoretical background of TMDCs and their optoelectronic properties. It also reports on the fabrication of field-effect transistors based on few-layer sheets of WS2 and the investigation of their electronic transport properties. Particularly the project focuses on improving the interface between the metal contact and WS2 sheet, where annealing improves the contact transparency. Together with van der Pauw geometry, annealing allows four-terminal measurements to be performed and the pristine properties of the material to be recovered at room temperature, where the devices show n-type behaviour and a linear I-V curve.
The promising improvements and the electronic properties shown in this thesis make WS2 interesting for future applications in valleytronic devices.
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Liu, William K. "Electron spin dynamics in quantum dots, and the roles of charge transfer excited states in diluted magnetic semiconductors /." Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/8588.
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Cho, Suyeon. "Synthesis and characterization of refractory oxides doped with transition metal ions." Phd thesis, Université Paris Sud - Paris XI, 2011. http://tel.archives-ouvertes.fr/tel-00856580.
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In this study, the oxygen-deficient TiO2, SrTiO3 systems and transition metal ion (Cr or V) doped TiO2, SrTiO3 and SrZrO3 systems have been investigated. We prepared samples as polycrystals, single crystals and thin films for various desires. Their structural, physical and electronic properties were measured by bulk-sensitive techniques (X-Ray Diffraction, SQUID and Electro Paramagnetic Resonance) or surface-sensitive techniques (Photoemission spectroscopy and X-ray absorption spectroscopy). The measurement of SQUID and EPR showed not only their magnetic properties but also the valence state of Cr dopant. We verified the valence state of Cr ions in oxides and found the key parameters of sample synthesis which control the valence state of Cr ions. Segregated phases such as SrCrO4 were formed when the samples were synthesized under O2 rich environment. The surface properties of Cr doped SrZrO3 films are also discussed. We found the synthesis conditions which influence on not only the behavior of Cr ions but also the resistive-switching behaviors. Various resistive-switching behaviors seem to depend on the surface chemistry of films. We found that the accumulation of Cr3+ on film surface provides a clean interface without any non-stoichiometric oxides and that this sharp interface termination results in a good performance of resistive-switching.
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Carnio, Edoardo. "The metal-insulator transition in doped semiconductors : an ab initio approach." Thesis, University of Warwick, 2018. http://wrap.warwick.ac.uk/106449/.
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In this thesis we study the Anderson metal-insulator transition starting from an atomistically correct ab initio description of a doped semiconductor. In particular, we use density functional theory to simulate model systems of sulphur-doped silicon (Si:S) with few impurities in a large cell. From the resulting Kohn-Sham Hamiltonian, we build an effective tight-binding Hamiltonian for larger systems with an arbitrary number of dopants. Our effective model assumes the same potential around single and paired impurities, for up to ten nearest neighbours and disregarding configurations of three and more close impurities. We generate up to a thousand disorder realisations for systems of 16 3 to 22 3 atoms and a large range of impurity concentrations. From the diagonalisation of these realisations we study the formation of an impurity band in the band gap of the host semiconductor. With increasing impurity concentration, this band undergoes an Anderson metal-insulator transition, namely (i) it approaches and merges with the conduction band and (ii) its states delocalise starting from the band centre. From the multifractal fluctuations of the wave functions near criticality, we characterise the Anderson transition in terms of its critical concentration nc and exponent. We identify two regimes: for energies in a “hybridization region”, where the conduction band seems to influence the impurity band, we observe an increase from v ≈ 0:5 to v ≈ 1, compatibly with the experimental values; deeper in the band, instead, the estimates of v fluctuate between 1 and 1:5, compatibly with v ≈ 1:59 (v ≈1:3) found in the Anderson model without (with) electron-electron interactions. Our results suggest a possible resolution of the long-standing exponent puzzle due to the interplay between conduction and impurity states.
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Lancaster, Adam. "Mid-IR waveguide lasers in transition metal doped II-VI semiconductors." Thesis, Heriot-Watt University, 2017. http://hdl.handle.net/10399/3342.
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This thesis focuses on the development of compact Mid-IR waveguide laser systems in transition metal doped II-VI semiconductors. These waveguide lasers have been developed to produce high brightness and compact sources in the Mid-IR. The wavelength tuneable operation of Cr:ZnSe waveguides was investigated. The waveguides were fabricated using ultrafast laser inscription. The waveguide laser demonstrated a tuning range of 2080 - 2883 nm, with more than 15 mW of output power across the full range. This resulted in a tuneable bandwidth of 39 THz, which is the widest range demonstrated by a Mid-IR waveguide laser. Modelocking of a Cr:ZnSe waveguide laser was investigated using a SESAM. This resulted in the demonstration of a CW modelocked laser with a PRF of 308 MHz, pulse energy of 0.3 nJ and a pulse width of 638 fs. In addition, a CW modelocked laser was demonstrated with a PRF of 1.03 GHz, this is the fastest PRF of any Cr:ZnSe laser demonstrated. Other transition metal doped II-VI semiconductors were investigated. Waveguides were developed in Cr:ZnS because of its improved thermal properties compared with Cr:ZnSe for future power scaling applications. The Cr:ZnS waveguide laser emitted 97 mW at a central wavelength of 2333 nm. Waveguide laser operation was investigated in Fe:ZnSe for laser operation in the 3.7 to 5 μm spectral band. Firstly, low-loss laser waveguide parameters were investigated in Fe:ZnSe. The passive characterisation found the waveguide propagation losses to be as low as 0.4 dB/cm, which is much lower than those demonstrated in other ULI cladding waveguides in Cr:ZnSe and Cr:ZnS. The low-loss waveguides were inserted inside a cryostat for CW laser operation. A free running CW laser operation with an Fe:ZnSe waveguide laser was demonstrated. An output power of 76 mW was produced at 4122 nm with a pump power of 908 mW. This work highlights the operational wavelength versatility of ULI fabricated depressed cladding waveguides.
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Durandurdu, Murat. "Polyamorphism in Semiconductors." Ohio University / OhioLINK, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1040060243.
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Zhou, Shengqiang. "Transition metal implanted ZnO: a correlation between structure and magnetism." Doctoral thesis, Technische Universität Dresden, 2007. https://tud.qucosa.de/id/qucosa%3A23718.
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Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors question the origin of this ferromagnetism, i.e. if the observed ferromagnetism stems from ferromagnetic precipitates rather than from carriermediated magnetic coupling of ionic impurities, as required for a diluted magnetic semiconductor. In this thesis, this question will be answered for transition-metal implanted ZnO single crystals. Magnetic secondary phases, namely metallic Fe, Co and Ni nanocrystals, are formed inside ZnO. They are - although difficult to detect by common approaches of structural analysis - responsible for the observed ferromagnetism. Particularly Co and Ni nanocrystals are crystallographically oriented with respect to the ZnO matrix. Their structure phase transformation and corresponding evolution of magnetic properties upon annealing have been established. Finally, an approach, pre-annealing ZnO crystals at high temperature before implantation, has been demonstrated to sufficiently suppress the formation of metallic secondary phases.
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Herwadkar, Aditi Dr. "Electronic structure and magnetism in some transition metal nitrides: MN-doped ScN, dilute magnetic semiconductor and CrN, Mott insulator." Case Western Reserve University School of Graduate Studies / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=case1164816868.
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Herwadkar, Aditi A. "Electronic structure and magnetism in some transition metal nitrides Mn-doped ScN, dilute magnetic semiconductor and CrN, Mott insulator /." online version, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=case1164816868.
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Gatuna, Ngigi wa. "Intrinsic vacancy chalcogenides as dilute magnetic semiconductors : theoretical investigation of transition-metal doped gallium selenide /." Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/10595.
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Crowley, Kyle McKinley. "Electrical Characterization, Transport, and Doping Effects in Two-Dimensional Transition Metal Oxides." Case Western Reserve University School of Graduate Studies / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=case1597327584506971.
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Makino, Yukio. "Chemical Interpretation of Superconductivity by Valence Electron Parameters." 京都大学 (Kyoto University), 2014. http://hdl.handle.net/2433/188509.
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Leighton, Christopher. "Persistent photoconductivity and the metal-insulator transition in Cd(_1-x)Mn(_x)Te:In." Thesis, Durham University, 1997. http://etheses.dur.ac.uk/5008/.
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The persistent photoconductivity (PPC) effect in the diluted magnetic semiconductor Cd(_1-x)Mn(_x)Te:In has been studied in detail. Electrical transport measurements have been made on a large number of samples to build up an understanding of the phototransport properties of this material. In particular, the compositional dependence of the phototransport parameters has been measured up to x ≈ 0.2. Several samples exhibit an elevated temperature PPC effect which has been interpreted in terms of the formation of multiple DX centres. These samples can have a quenching temperature of up to 190 K, suggestmg that Cd(_1-x)Mn(_x)Te:In could be an interesting material in terms of applications of room temperature persistent photoconductors. The low field magnetoresistance has been measured and analysed quantitatively in order to attempt to identify the origin of the magnetoresistive effects in insulating and metallic samples. The positive magnetoresistance has been found to be linked with the magnetization of the sample. An anomalous negative magnetoresistance has been observed tinder certain experimental conditions. This negative magnetoresistance has been interpreted in terms of the formation of bound magnetic polarons and their contribution to spin-disorder scattering. The main body of this thesis is concerned with the study of the Metal-Insulator Transition (MIT). The PPC effect allows us to study the MIT in a continuous fashion by fine timing the carrier density by illumuiation. In this way we have made the first zero magnetic field study of the MIT in a magnetic semiconductor. The critical behaviour has been found to be consistent with the scaling theory of electron localization, which predicts a critical form σ = σ(_0)(n/n(_c) – 1)(^v). The critical conductivity exponent, v was determined to be close to one, while the critical carrier density, n(_e), was found to be ~ 2 x 10(^17) cm(^-3), for x = 0.08. The temperature dependence of the conductivity has been quantitatively analysed m both the metallic and insulating phases. On the insulating side of the transition, variable range hopping (VRH) conduction has been observed at low temperatures (down to 300 mK). The temperature dependence is consistent with VRH conduction with electron-electron interaction effects taken into account. In the metallic phase the temperature dependence of the conductivity (up to ~ 1 K) is consistent with a model where the zero temperature value of the conductivity is corrected by electron-electroninteraction effects, and the effects of weak localization. The magnitudes of these corrections are found to be in reasonable agreement with theoretical predictions. The electrical transport has also been studied in the weakly localized regime in Cd(_1-x)Mn(_x)Te:In and Cd(_1-x)Mn(_x)Te:In, Al. A rapid decrease in the conductivity occurs at low temperatures ( < 1.5 K). This is interpreted in terms of the effect of the s-d exchange interaction, which leads to the formation of bound magnetic polarons. It is suggested that this drop in conductivity can only be observed in the paramagnetic phase, and that spinglassordering has a significant effect on the temperature dependence of the conductivity at low temperatures.
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Eiting, Christopher James. "Growth of III-V nitride materials by MOCVD for device applications /." Digital version accessible at:, 1999. http://wwwlib.umi.com/cr/utexas/main.
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Docherty, Callum James. "Terahertz spectroscopy of graphene and other two-dimensional materials." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:98c03952-dc3f-442b-bbc0-d8397645cc1b.
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In this thesis, two-dimensional materials such as graphene are tested for their suitability for opto-electronic applications using terahertz time domain spectroscopy (THz-TDS). This ultrafast all-optical technique can probe the response of novel materials to photoexcitation, and yield information about the dynamics of the material systems. Graphene grown by chemical vapour deposition (CVD) is studied using optical-pump THz-probe time domain spectroscopy in a variety of gaseous environments in Chapter 4. The photoconductivity response of graphene grown by CVD is found to vary dramatically depending on which atmospheric gases are present. Adsorption of these gases can open a local bandgap in the material, allowing stimulated emission of THz radiation across the gap. Semiconducting equivalents to graphene, molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), grown by CVD, are investigated in Chapter 5. These members of the transition metal dichalcogenide family show sub-picosecond responses to photoexcitation, suggesting promise for use in high-speed THz devices. In Chapter 6, an alternative production route to CVD is studied. Liquid-phase exfoliation offers fast, easy production of few-layer materials. THz spectroscopy reveals that the dynamics of these materials after photoexcitation are remarkably similar to those in CVD-grown materials, offering the potential of cheaper materials for future devices. Finally in Chapter 7, it is shown that carbon nanotubes can be used to make ultrafast THz devices. Unaligned, semiconducting single walled carbon nanotubes can be photoexcited to produce an ultrafast, dynamic THz polariser. The work in this thesis demonstrates the potential for these novel materials in future opto-electronic applications. THz spectroscopy is shown to be an important tool for the characterisation of new materials, providing information that can be used to understand the dynamics of materials, and improve production methods.
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Kortan, Victoria Ramaker. "Transition-metal dopants in tetrahedrally bonded semiconductors: symmetry and exchange interactions from tight-binding models." Diss., University of Iowa, 2015. https://ir.uiowa.edu/etd/1865.
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It has become increasingly apparent that the future of electronic devices can and will rely on the functionality provided by single or few dopant atoms. The most scalable physical system for quantum technologies, i.e. sensing, communication and computation, are spins in crystal lattices. Diamond is an excellent host crystal offering long room temperature spin coherence times and there has been exceptional experimental work done with the nitrogen vacancy center in diamond demonstrating many forms of spin control. Transition metal dopants have additional advantages, large spin-orbit interaction and internal core levels, that are not present in the nitrogen vacancy center. This work explores the implications of the internal degrees of freedom associated with the core d levels using a tight-binding model and the Koster-Slater technique. The core d levels split into two separate symmetry states in tetrahedral bonding environments and result in two levels with different wavefunction spatial extents. For 4d semiconductors, e.g. GaAs, this is reproduced in the tight-binding model by adding a set of d orbitals on the location of the transition metal impurity and modifying the hopping parameters from impurity to its nearest neighbors. This model does not work in the case of 3d semiconductors, e.g. diamond, where there is no physical reason to drastically alter the hopping from 3d dopant to host and the difference in wavefunction extent is not as pronounced. In the case of iron dopants in gallium arsenide the split symmetry levels in the band gap are responsible for a decrease in tunneling current when measured with a scanning tunneling microscope due to interference between two elastic tunneling paths and comparison between wavefunction measurements and tight-binding calculations provides information regarding material parameters. In the case of transition metal dopants in diamond there is less distinction between the symmetry split d levels. When considering pairs of transition metal dopants an important quantity is the exchange interaction between the two, which is a measure of how fast a gate can be operated between the pair and how well entanglement can be created. The exchange interaction between pairs of transition metal dopants has been calculated in diamond for several directions in the (110) plane, and for select transition metal dopants in gallium arsenide. In tetrahedral semiconductors transition metal dopants provide an internal degree of freedom due to the symmetry split d levels and this included functionality makes them special candidates for single spin based quantum technologies as well as physical systems to learn about fundamental physics.
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Nur, Baizura Binti Mohamed. "Study on photoluminescence quantum yields of atomically thin-layered two-dimensional semiconductors transition metal dichalcogenides." Kyoto University, 2018. http://hdl.handle.net/2433/233854.
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Le, Thi Ly. "Preparation of transition metal oxide thin films used as solar absorbers." Thesis, Toulouse 3, 2016. http://www.theses.fr/2016TOU30120/document.
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Ce travail de thèse a porté sur l'élaboration de nanoparticules et de couches minces d'oxydes spinelles mixtes de MxCo2-xMnO4 (M = Ni, Cu, Zn ; x = 0, 0,15, 0,30, 0,60) semi-conducteurs, absorbants de lumière avec d'intéressantes applications potentielles pour la photo-catalyse et le photovoltaïque. Le premier chapitre présente tout d'abord une vue globale du contexte énergétique à l'échelle mondiale et des ressources d'énergie renouvelables, alternatives aux énergies fossiles les plus répandues. Une revue détaillée est ensuite faite des différents matériaux et systèmes employés dans la fabrication de cellules solaires, en portant une attention plus particulière à un nouveau type de cellules photovoltaïques en couches minces, dites " Tout-oxyde ", basées notamment sur l'utilisation d'oxydes de type spinelle utilisés comme absorbeurs solaires. Le deuxième chapitre présente les techniques expérimentales de synthèse et de caractérisation utilisées lors de ce travail de thèse. Le procédé de polycondensation inorganique, optimisé au laboratoire, utilisé pour synthétiser les poudres d'oxydes à basse température (T < 120 °C) sans agent organique complexe est décrit. Ensuite, les méthodes de préparation de dispersions colloïdales à l'ambiante dans l'éthanol et de films minces homogènes d'oxydes par trempage-retrait sont explicitées. Le troisième chapitre présente les résultats détaillés des structures atomiques et électroniques des matériaux de base à l'étude, issus de calculs par la méthode Density Functional Theory (DFT), réalisés en collaboration avec le laboratoire CEMES de Toulouse. Les résultats des calculs de densités électroniques et détermination de structures de bandes, réalisés pour la première fois à notre connaissance, sur l'ensemble de la solution solide MnxCo3-xO4 (0 = x = 3), sont comparés à nos données expérimentales, obtenues notamment sur les largeurs de bande interdite (gap) à partir de mesures optiques faites sur couches minces. Un gap de 0,8 eV est calculé, qui serait dû à des transitions inter-métalliques en sites B. Deux gaps à 1,5 et 2,2 eV, obtenus expérimentalement dans l'UV-VIS, qui augmentent avec la quantité de manganèse, correspondraient à des transitions respectives B-A et O-B, respectivement. Les propriétés magnétiques de ces matériaux sont également discutées. Le quatrième chapitre présente l'élaboration et la caractérisation (micro-)structurale des poudres et des couches minces d'oxydes de type spinelle. Toutes les compositions (Co2MnO4 dopé au Ni, Cu ou Zn) cristallisent dans une phase cubique. Les nanoparticules sont sphériques avec la taille variant entre 20 et 50 nm. Les couches minces homogènes ont été déposées sur quartz, alumine, nitrure de titane et platine afin de mesurer leurs propriétés électriques et optiques. Une température de frittage environ de 1000 °C sous air a été déterminée par dilatométrie et les couches sont stables jusqu'à 900 °C quel que soit le substrat. En revanche, seules les couches déposées sur platine permettent d'atteindre la température de frittage sous air (et d'accroitre la compacité donc la conductivité des couches) sans réaction avec le substrat. Le chapitre cinq présente les variations des propriétés optiques et électriques des couches minces avec le frittage. Les propriétés d'absorbance de lumière des couches minces ainsi préparées, mesurées sur une gamme de longueurs d'ondes du domaine spectral UV-visible, montrent deux bandes d'absorbance, correspondantes à deux valeurs de gap pour chaque composition. La propriété d'absorbance des couches minces augmente dans la gamme du visible après frittage et les gaps diminuent. Les couches minces sont plus compactées. La résistivité des couches minces diminue de 105 à 102 Omega.cm avec l'augmentation de la température de 20 à 300 ºC. Une étude parallèle, basée sur la préparation de films minces absorbants de lumière de Co2MnO4 et Cu2O par la technique de Pulsed Laser Deposition (PLD) est également présentéeThe present thesis deals with the synthesis and structural characterization of transition metals doped cobalt and manganese based spinel oxides MxCo2-xMnO4 (with M = Ni, Cu, Zn and x = 0, 0.15, 0.30, 0.60), in relationships with their conduction and optical properties. These materials are good p-type semiconductors and light absorbers in the UV and visible regions, therefore interesting for photo-catalysis and photovoltaics. The first chapter is a brief overview of the energy context and nature of global warming, renewable energy resources and a literature review of materials used for solar cells including the newly studied system type based on all-oxide photovoltaics. Chapter two presents all the experimental methods and characterization techniques used for this research work. The inorganic polycondensation method optimized in our laboratory and used for synthesizing spinel oxide powders at low temperature (T < 120 °C) without complex organic agents is described. Then, the preparation of colloidal dispersions stabilized at room temperature using an azeotrope solution based on absolute ethanol and water only is described, in order to obtain homogenous oxide thin films by the dip-coating technique. The third chapter presents detailed results on the atomic and electronic structures of the materials under study performed by using a full density functional theory investigation thanks to a collaboration with the CEMES. First principles electronic structure calculations were performed for the first time to our knowledge over the whole spinel oxide solid solution range MnxCo3-xO4 (0 = x = 3), and compared with our experimental data. A small band gap of ~ 0.8 eV is calculated, due to metal-metal transitions in B sites. The experimental band gaps observed at 1.5 and 2.2 eV, which increase with the amount of manganese, would correspond to B-A and O-B transitions, respectively. The magnetic properties of these materials are also discussed. Chapter four shows the experimental details of the preparation and characterization of the spinel oxide powders, colloidal dispersions and thin films. All samples (Ni, Cu or Zn-doped Co2MnO4) are well crystallized with a single cubic spinel oxide phase. Nanoparticles are spherical and their diameters vary from 20 to 50 nm, doping with Zn, Ni to Cu, mainly due to steric effects. Homogenous oxide thin films were deposited on quartz, alumina, titanium nitride and platinum in order to measure their optical and electrical properties, and to increase the film compactness (thus electrical conductivity and light absorbance) after thermal treatment. Thin films are well preserved up to 900 °C in air and can handle higher temperatures (up to 1000 ºC) on platinum without reaction with the substrate. Chapter five deals with the optical and electrical properties of thin films before and after sintering. The optical properties were measured over a wide range of wavelengths (UV-VIS). The optical properties of spinel oxide thin films show two strong absorption band gaps for each composition at the UV front and close to 700 nm in wavelength. These band gaps are direct and mostly lower than 2 eV for the first band. Both band gaps increase with further doping and decrease after annealing. Thin film resistivity is about 105 .cm at room temperature and decreases with increasing temperature (a few tens of 20cm at 300 ºC). In parallel to the soft chemistry method and dip-coating technique used to prepare our spinel oxide thin layers, Pulsed Laser Deposition technique was used to prepare pure Co2MnO4 and Cu2O dense thin films. Their structural and optical main features are discussed
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Liu, Mingde. "Magnetization-steps spectroscopy in dilute magnetic semiconductors and in molecular magnetism /." Thesis, Connect to Dissertations & Theses @ Tufts University, 1998.
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Thesis (Ph.D.)--Tufts University, 1998 .Adviser: Yaacov Shapira. Submitted to the Dept. of Physics. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;
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Channam, Venkat Sunil Kumar. "Synthesis of strongly correlated oxides and investigation of their electrical and optical properties." Thesis, Toulouse, INPT, 2017. http://www.theses.fr/2017INPT0075/document.
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Les oxydes fortement corrélés sont largement étudiés pour l'hôte d'applications uniques, telles que la supraconductivité à haute température, la magnéto-résistance colossale, les commandes exotiques magnétiques, chargées et orbitales et les transitions isolant-métal. Les oxydes métalliques de transition qui forment la majorité des systèmes d'oxydes corrélés et des oxydes de vanadium, en particulier VO2 et V2O5, sont les deux systèmes les plus préférés parmi les chercheurs pour plusieurs applications. Dans cette thèse, la croissance et la caractérisation de VO2 et V2O5 sont discutées avec un accent particulier sur la propriété optique, en particulier les propriétés thermochromiques. Traditionnellement, le comportement SMT et l'infrarouge reflètent la zone de focalisation pour la recherche VO2 et c'est seulement jusqu'à récemment que la VO2 est traitée comme un système beaucoup plus complexe et a été étudiée comme un métamatériel naturellement désordonné très réactif près de la température de transition de phase où le matériau présente des matériaux semi-conducteurs et métalliques Coexistence de phase. Étant donné que chaque phase de VO2 a des propriétés optiques et électriques distinctes, elle contrôle l'étendue des transitions de phase par une modulation de température précise, permet d'exploiter le matériau pour de nouvelles propriétés, comme la modulation d'émissivité dans la région NIR et pour la création de motifs IR réversibles et réinscriptibles. Le V2O5 est traditionnellement considéré comme un matériau TCR élevé et considéré comme un matériau de choix pour une application allant de la catalyse, des capteurs de gaz aux batteries au lithium. Dans cette étude, nous nous concentrons sur les propriétés optiques du matériau, en particulier la nature thermochromique de la gamme visible des revêtements V2O5 synthétisés par recuit oxydatif des revêtements VOx développés par MOCVD. L'impact du dopage et de la production sélective de vacance d'oxygène sur la propriété thermochromique est discutéStrongly correlated oxides are studied widely for the host of unique applications, such as hightemperature superconductivity, colossal magneto resistance, exotic magnetic, charge and orbital ordering, and insulator-to-metal transitions. Transitional metal oxides which form the majority of the correlated oxide systems and oxides of Vanadium, especially VO2 and V2O5 are the two most favourite systems among researchers for several applications. In this thesis, the growth and characterization of VO2 and V2O5 are discussed along with a special focus on the optical property, especially thermochromic properties. Traditionally SMT behaviour and Infrared reflectively was the focus area for VO2 research, and its only until recently that VO2 is being treated as a much more complex system and investigated as highly responsive naturally disordered metamaterial near the phase transition temperature where the material exhibits semiconducting and metallic phase co-existence. Since each phase of VO2 has a distinct optical and electrical properties, controlling the extent of phase transitions by accurate temperature modulation, enables exploitation of the material for new properties like emissivity modulation in the NIR region and for creating IR visible reversible and rewritable patterns. V2O5 is traditionally seen as a high TCR material and regarded as material of choice for application ranging from catalysis, gas sensors to lithium batteries. In this study, however we focus on the optical properties of the material, especially the visible range thermochromic nature of V2O5 coatings synthesised by oxidative annealing of MOCVD grown VOx coatings. The impact of doping and selective oxygen vacancy generation on the thermochromic property are discussed
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Venkatachalam, Anusha. "Investigation of self-heating and macroscopic built-in polarization effects on the performance of III-V nitride devices." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/29669.
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Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010.Committee Chair: Yoder, Douglas; Committee Member: Graham, Samuel; Committee Member: Allen, Janet; Committee Member: Klein, Benjamin; Committee Member: Voss, Paul. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Silva, Luciane Janice Venturini da. "Transição semicondutor-metal em nanocristais de VO2 termoeletricamente ativada." Universidade Federal de Santa Maria, 2015. http://repositorio.ufsm.br/handle/1/3933.
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Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorIn this thesis, structural and electrical characteristics are investigated around the thermally triggered semiconductor to metal transition in VO2 thin films. The films, the metallics electrodes, as well as SiO2 buffer layers have been deposited by reactive magnetron sputtering onto Si substrates. The crystallographic and morphological characteristics have been observed through measurements of X-Ray diffraction as a function of the temperature, and atomic force microscopy (AFM). The nanoscale electrical characterization have been performed using a measurement system via nano-tips. The results of X-ray diffraction at room temperature revealed that the samples are polycrystalline and are strongly textured in the < 011 > direction, which is almost perpendicular to the substrate plane. The X-Ray diffraction spectra have been extracted at different temperatures to follow the crystallographic transition experienced by VO2 near the transition temperature. For films deposited on SiO2 (without electrodes) and the Ta electrode at temperatures below the critical temperature for the transition, the material presented in the monoclinic phase M1. Within the range of temperatures that comprises the transition occurs progressive appearance of the peak corresponding to the (110) plane of R rutile phase. Within a range at relatively higher temperatures, there is a coexistence of phases R and M1 and M2 may be the M2 monoclinic. As would be expected, the peak of rutile structure grows to the point of being virtually the only present when the temperature reaches about 80°C. The transition from one crystallographic film VO2 with Pd electrode was accompanied by diffraction measured at room temperature. The peak (011) of phase M1 is much smaller compared to the samples deposited on Ta electrode. However, contrary to the Ta electrode film which is likely to have grown in the shape of very small nano-grain or even amorphous form, the Pd electrode film is polycrystalline and highly textured. The transport properties during the electrical phase transition were investigated using injection of electrical current perpendicular to the sample plane. Films grown on Ta electrodes showed abrupt semiconductor-metal phase transitions in different nano-crystallites VO2. The IV characteristics of the film on the Pd electrode had an S-NDR region, specifically attributed to the formation of a filamentary current flow between the Pd probe and the electrode. The details of this phenomenon could not be established definitively, but if in fact the electrical transition is present in nano-crystallites measured, it was suggested that the origin of this conducting channel may be related to reminiscent earlier phase transitions.
Nesta tese, realizou-se uma investigação estrutural e elétrica em torno da transição semicondutor-metal desencadeada termicamente em filmes finos de VO2. Os filmes foram depositados por magnetron sputtering reativo, os eletrodos metálicos, bem como camadas buffers de SiO2 sobre os substratos de Si foram depositados por magnetron sputtering. As características cristalográficas e morfológicas foram evidenciadas através de medidas de difração de raios-X em função da temperatura e microscopia de força atômica (AFM), respectivamente. A caracterização elétrica, em nanoescala foi realizada utilizando-se um sistema de medidas via nano-ponteiras. Os resultados de difração de raios-X à temperatura ambiente revelaram que as amostras são policristalinas e estão fortemente texturizados com a direção < 011 > praticamente perpendicular ao plano do substrato. Os difratogramas em função da temperatura foram realizados para acompanhar a transição cristalográfica que o VO2 apresenta próximo a temperatura de 68°C. Para os filmes depositados sobre SiO2 (sem eletrodo) e sobre o eletrodo de Ta, em temperaturas abaixo da temperatura crítica para a transição, o material apresentou-se na fase monoclínica M1. Na faixa de temperaturas que compreende a transição, ocorre o surgimento progressivo do pico correspondente ao plano (110) da fase rutila R. Para uma faixa relativamente grande de temperaturas, há uma coexistência das fases M1 e R e, eventualmente da monoclínica M2. Como seria de se esperar, o pico da estrutura rutila cresce até o ponto de ser praticamente o único presente, quando a temperatura atingiu cerca de 80°C. A transição cristalográfica de um filme de VO2 com eletrodo de Pd foi acompanhada por medidas de difração à temperatura ambiente. O pico (011) da fase M1 é muito menor comparado ao das amostras depositadas sobre eletrodo de Ta. Porém, contrariamente ao eletrodo de Ta, que provavelmente tenha crescido na forma de nano-grãos muito pequenos ou mesmo na forma amorfa, o filme de Pd depositado é policristalino e bastante texturizado. As propriedades de transporte durante a transição de fase elétrica forma investigadas utilizando-se injeção de corrente elétrica perpendicular ao plano da amostra. Esta investigação, para os filmes crescidos sobre eletrodo de Ta, mostraram abruptas transições de fase semicondutor-metal em diferentes nano-cristalitos de VO2. As características I-V do filme com eletrodo de Pd apresentaram uma região com S-NDR, especificamente atribuída à formação de um regime filamentar de corrente entre a ponteira e o eletrodo de Pd. Os detalhes deste fenômeno não puderam ser estabelecidos de forma definitiva, mas se de fato a transição elétrica está presente nos nano-cristalitos medidos, sugeriu-se que a origem deste canal condutor pode estar relacionada com transições de fase anteriores e remanescentes.
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Mahani, Mohammad Reza. "Magnetic solotronics near the surface of a semiconductor and a topological insulator." Doctoral thesis, Linnéuniversitetet, Institutionen för fysik och elektroteknik (IFE), 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-40398.
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Technology where a solitary dopant acts as the active component of an opto-electronic device is an emerging field known as solotronics, and bears the promise to revolutionize the way in which information is stored, processed and transmitted. Magnetic doped semiconductors and in particular (Ga, Mn)As, the archetype of dilute magnetic semiconductors, and topological insulators (TIs), a new phase of quantum matter with unconventional characteristics, are two classes of quantum materials that have the potential to advance spin-electronics technology. The quest to understand and control, at the atomic level, how a few magnetic atoms precisely positioned in a complex environment respond to external stimuli, is the red thread that connects these two quantum materials in the research presented here. The goal of the thesis is in part to elucidate the properties of transition metal (TM) impurities near the surface of GaAs semiconductors with focus on their response to local magnetic and electric fields, as well as to investigate the real-time dynamics of their localized spins. Our theoretical analysis, based on density functional theory (DFT) and using tight-binding (TB) models, addresses the mid-gap electronic structure, the local density of states (LDOS) and the magnetic anisotropy energy of individual Mn and Fe impurities near the (110) surface of GaAs. We investigate the effect of a magnetic field on the Mn acceptor LDOS measured in cross-sectional scanning tunneling microscopy, and provide an explanation of why the experimental LDOS images depend weakly on the field direction despite the strongly anisotropic nature of the Mn acceptor wavefunction. We also investigate the effects of a local electrostatic field generated by nearby charged As vacancies, on individual and pairs of ferromagnetically coupled magnetic dopants near the surface of GaAs, providing a means to control electrically the exchange interaction of Mn pairs. Finally, using the mixed quantum-classical scheme for spin dynamics, we calculate explicitly the time evolution of the Mn spin and its bound acceptor, and analyze the dynamic interaction between pairs of ferromagnetically coupled magnetic impurities in a nanoscaled semiconductor. The second part of the thesis deals with the theoretical investigation of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi2Se3 TI, using an approach that combines DFT and TB calculations. Our analysis clarifies the crucial role played by the spatial overlap and the quasi-resonant coupling between the Mn-acceptor and the topological surface states inside the Bi2Se3 band gap, in the opening of a gap at the Dirac point. Strong electronic correlations are also found to contribute significantly to the mechanism leading to the gap, since they control the hybridization between the p orbitals of nearest-neighbor Se atoms and the acceptor spin-polarization. Our results explain the effects of inversion-symmetry and time-reversal symmetry breaking on the electronic states in the vicinity of the Dirac point, and contribute to clarifying the origin of surface-ferromagnetism in TIs. The promising potential of magnetic-doped TIs accentuates the importance of our contribution to the understanding of the interplay between magnetic order and topological protected surface states.
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Fichou, Denis. "L'interface oxyde de zinc/électrolyte : étude des processus primaires." Paris 6, 1986. http://www.theses.fr/1986PA066259.
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Matsui, Masafuyu. "Role of Interchain Interaction in Determining the Band Gap of Trigonal Selenium: A Density Functional Theory Study with a Linear Combination of Bloch Orbitals." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/195940.
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Lima, Francisco Anderson de Sousa. "Application of transition-metal-oxide-based nanostructured thin films on third generation solar cells." reponame:Repositório Institucional da UFC, 2015. http://www.repositorio.ufc.br/handle/riufc/14584.
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LIMA, F. A. S. Application of transition-metal-oxide-based nanostructured thin films on third generation solar cells. 2015. 225 f. Tese (Doutorado em Ciência de Materiais) – Centro de Tecnologia, Universidade Federal do Ceará, Fortaleza, 2015.Submitted by Marlene Sousa (mmarlene@ufc.br) on 2015-12-17T12:45:41Z No. of bitstreams: 1 2015_tese_faslima.pdf: 24015209 bytes, checksum: a66470eb7a55b6b3c2a5e8544c6d4d32 (MD5)
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One of the greatest challenges of our time is to devise means to provide energy in a sustainable way to attend an exponentially growing demand. The energy demand is expected to grow 56% by 2040. In this context, the use of clean and sustainable sources of energy is imperative. Among these sources, solar energy is the only one which can meet the total world energy requirement even considering such large growth in demand. The solar power incident on the Earth's surface every second is equivalent to 4 trillion 100-watt light bulbs. Photovoltaic solar cells are one of several ways to harness solar energy. These cells convert solar energy directly into electricity. Commercial photovoltaic devices are already a reality, but their share of the world energy matrix is still quite small, mainly due to the high costs. Next generation photovoltaics open a number of new possibilities for photovoltaic energy applications that can potentially decrease the overall cost of energy production. Transition metal semiconductor oxides are promising materials that can be produced by low cost methods and o er interesting new features. The use of these materials in next generation photovoltaics is therefore a very promising and interesting application. In this thesis work zinc, titanium and vanadium oxides were used in next generation solar cells. Thin lms of zinc oxide were synthesized by the low cost and environmentally friendly techniques of electrodeposition and hydrothermal synthesis and applied as working electrodes in highly e cient dye sensitized solar cells (DSSCs). The lms were characterized by structural and optical techniques while the cells were tested by current vs: voltage and quantum e ciency measurements. The e ciencies of these cells were as high as 2.27% using ZnO thin lms without any post deposition treatment. Moreover, natural dyes extracted from plants of northeastern Brazil were applied as sensitizers in DSSCs assembled with commercial available TiO 2 as working electrode. The natural dyes were extracted employing very simple methods and were characterized by XPS and UPS techniques. Their band alignments were shown to be compatible with the TiO 2 as well as with the mediator electrolyte. The e ciency of DSSCs sensitized with natural dyes were as high as 1.33%. Finally, water based V 2 O 5 was used as hole transport medium (HTM) in conventional organic solar cells (OSCs) and ITO-free, plastic OSCs. The results obtained with V 2 O 5 were compared with the results obtained from cells assembled with PEDOT:PSS, which is the most used HTM. This comparison showed that the use of V 2 O 5 as HTM can lead to more e cient OSCs. The stability of these devices were evaluated by tests applying the ISOS standards ISOS-D-1, ISOS-L-1 and ISOS-O-1. A UV- lter and a protective graphene oxide (GO) layer were employed seeking to improve the stability of OSCs. The combination of both UV- lter and GO protective layer was shown to be the most e ective way to improve the stability of these devices
Um dos maiores desa os do nosso tempo e desenvolver formas para fornecer energia de forma sustent avel para atender uma demanda que cresce exponencialmente e que dever a crescer 56% at e 2040. Neste contexto, o uso de fontes limpas e sustent aveis de energia e um imperativo. Entre essas fontes, a energia solar e a unico que pode satisfazer a ne- cessidade total de energia do mundo, mesmo considerando o crescimento na demanda. A pot^encia solar incidente na superf cie da Terra a cada segundo e equivalente a 4 trilh~oes de l^ampadas de 100 watts. C elulas solares fotovoltaicas s~ao uma das v arias maneiras de aproveitar a energia solar, convertendo-a diretamente em eletricidade. Dispositivos com- erciais fotovoltaicos j a s~ao uma realidade, mas a sua participa c~ao na matriz energ etica mundial ainda e muito pequena, principalmente devido aos seus custos elevados. C elulas fotovoltaicas de nova gera c~ao abrem uma s erie de novas possibilidades para aplica c~oes de energia fotovoltaica que pode diminuir o custo total de produ c~ao de energia. Oxidos semicondutores de metais de transi c~ao s~ao materiais promissores que podem ser produzi- dos atrav es de m etodos de baixo custo e que possuem caracter sticas interessantes. Por conseguinte, o uso destes materiais em energia fotovoltaica de pr oxima gera c~ao se apre- senta com uma aplica c~ao promissora. Nesta tese de doutorado oxidos de zinco, tit^anio e van adio foram utilizados em c elulas solares de pr oxima gera c~ao. Filmes nos de oxido de zinco foram sintetizados por eletrodeposi c~ao e s ntese hidrot ermica. Os lmes foram apli- cados como eletrodos de trabalho em c elulas solares sensibilizadas por corante (DSSCS) altamente e cientes. Os lmes foram caracterizados por t ecnicas estruturais e oticas en- quanto que as c elulas foram testadas por medidas de corrente vs: voltagem e de e ci^encia qu^antica. A e ci^encia dessas c elulas atingiu 2,27% utilizando lmes nos de ZnO sem qualquer tratamento p os-deposi c~ao. Al em disso, corantes naturais extra dos de plan- tas do nordeste do Brasil foram aplicados como sensibilizadores em DSSCs montadas com TiO 2 comercial utilizado como eletrodo de trabalho. Os corantes naturais foram extra das empregando m etodos simples e foram caracterizados por espectroscopia de fotoel etrons excitados por raios X e por radia c~ao ultravioleta, XPS e UPS respectivamente. Seus alin- hamentos de banda se mostraram compat veis com o TiO 2 e com o eletrodo de regenera c~ao. A e ci^encia das DSSCs sensibilizadas com corantes naturais chegou a 1,33%. Finalmente, V 2 O 5 a base de agua foi usado como material transportador de buracos (HTM) em c elulas solares org^anicas (OSCs) convencionais e OSCs de pl astico constru das sem ITO. Os re- sultados obtidos com V 2 O 5 foram comparados com os resultados de c elulas constru das com PEDOT:PSS, que e o HTM mais utilizado. Esta compara c~ao revelou que o uso de V 2 O 5 como HTM pode levar a OSCs mais e cientes. A estabilidade destes dispositivos foi avaliada por testes aplicando os padr~oes ISOS-D-1, ISOS-L-1 e ISOS-O-1. O uso de ltros ultravioleta e de uma camada protetora de oxido de grafeno reduzido foi testado com o intuito de melhorar a estabilidade desses dispositivos. O uso de uma combina c~ao de ambos se mostrou a forma mais efetiva de melhorar a estabilidade das OSCs
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Koperski, Maciej. "Propriétés optiques des couches minces de dichalcogénures de métaux de transition." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY019/document.
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L’étude intitulée « optical properties of thin layers of transition metal dichalcogenides » vise les phénomènes physiques émergeant à la limite de la bidimensionnalité, lorsque l’épaisseur du composé ciblé est à l’échelle atomique. Les effets de la dimensionnalité réduite sur les propriétés physiques furent initialement explorés dans le graphène. Les études concernant ce composé se concentrent surtout sur ses propriétés de transport puisque le graphène lui-même n’a pas bande interdite. Les sc-TMD, en plus de présenter une structure atomique et électronique similaire au graphène (vallée aux points K dans la zone de Brillouin), peuvent également être produit sous forme de couche monoatomique. Ainsi, plusieurs études révélèrent que ces composés en couche mince combinent des propriétés découlant de leur caractère 2D en plus des caractéristiques typiques des semi-conducteurs. De plus, la dimensionnalité de ces composés joue un rôle important dans la structure électronique. Plus précisément, les sc-TMD présentent dans le régime tridimensionnel un gap indirect qui devient direct lorsque les composés sont sous la forme d’une monocouche. Cette thèse est une étude complète des propriétés optiques des composés sc-TMD. Le manuscrit en question est divisé en cinq parties : trois sections principales précédées par une introduction. L’ensemble est complété par une annexe présentant des études complémentaires sur un autre composé 2D : le nitrure de bore hexagonal (h-BN).Introduction :Les propriétés fondamentales des composés étudiés sont présentées en mettant l’accent sur celles qui jouent un rôle important dans la réponse optique de sc-TMD. Plus précisément, on y retrouve des informations sur la structure cristalline et la structure de bandes électroniques. Cette section détaille également le processus de préparation des échantillons ainsi que les divers montages expérimentaux utilisés.Chapitre 1 : caractérisation optique de base des excitons en résonance dans les couches et les multicouches de sc-TMD.La réponse optique des composés (surtout le MoSe2 et le WSe2) obtenue par spectroscopie de réflexion et par spectroscopie d’émission est interprétée. En particulier, l’impact du nombre de monocouches et de la température sur celle-ci est discuté. De plus, une étude complémentaire de ces propriétés optiques résolue temporellement y est insérée.Chapitre 2 : Spectroscopie Zeeman des excitons résonants sous champ magnétique.L’évolution des résonances optiques en fonction d’un champ magnétique appliqué perpendiculairement aux couches est l’objet de cette section. Un modèle phénoménologique décrivant la dépendance en champs magnétique de l’énergie des états électroniques est dérivé directement des résultats expérimentaux présentés dans cette section. L’effet de pompage optique est également étudié dans la monocouche de WSe2, effet qui est très sensible aux champs magnétiques.Chapitre 3 : Émetteurs de photons uniques dans les couches minces de sc-TMD.La découverte de raies d’émission fines et localisées sur de minces cristallites de sc-TMD est présentée, suivie d’une étude approfondie sur leur nature et leurs propriétés. Entre autres, leur évolution en fonction de la température, leur sensibilité aux champs magnétiques appliqués et leur polarisation sont discutées. Finalement, la spectroscopie de corrélation de photon est utilisée pour vérifier le caractère « source de photon unique » de ces émetteurs.Annexe A : Émetteurs de photons uniques dans le nitrure de bore hexagonal.Le h-BN partage de nombreuses caractéristiques avec les sc-TMD tout en se distinguant de ceux-ci par la présence d’un gap électronique significativement plus grand. Certaines régions cristallines se comportent comme des défauts ponctuels, dans les matériaux caractérisés par leur large gap, en présentant des raies d’émission fines. Ces régions partagent une similarité frappante avec les émetteurs de photons uniques observés dans le WSe2The research reported in the thesis entitled ‘Optical properties of thin layers of transition metal dichalcogenides’ focuses on physical phenomena which emerge in the limit of two-dimensional (2D) miniaturisation when the thickness of fabricated films reaches an atomic scale. The importance of such man-made structures has been revealed by the dynamic research on graphene: a single atomic plane of carbon atoms arranged in honeycomb lattice. Graphene is intrinsically gapless and therefore mainly explored with respect to its electric properties. The investigation of semiconducting materials which can also display the hexagonal crustal structure and which can be thinned down to individual layers, bridges the concepts characteristic of graphene-like systems (K-valley physics) with more conventional properties of semiconductors. This has been indeed demonstrated in a number of recent studies of ultra-thin films of semiconducting transition metal dichalcogenides (sc-TMD). Particularly appealing, from the point of view of optical studies, is a transformation of the bandgap alignment of sc-TMD films, from the indirect bandgap bulk crystals to the direct bandgap system in single layers. The presented thesis work provides a comprehensive optical characterisation of thin structures of sc-TMD crystals. The manuscript is divided into five parts: three main chapters with a preceding introduction and the appendix reporting the supplementary studies of another layered material: hexagonal boron nitride.Introduction. The fundamental properties of the investigated crystals are presented, especially those which are important from the point of view of optical studies. The discussion includes information on the crystal structure, Brillouin zone and electronic band structure. Also, the general description of the samples’ preparation process and experimental set-up is provided.Chapter 1. Basic optical characterisation of excitonic resonances in mono- and multi-layers of sc-TMDs. The optical response, as seen in the reflectance and luminescence spectra of thin sc-TMDs is analysed (mostly for MoSe2 and WSe2 materials). The impact of the number of layers and temperature on the optical resonances is studied and interpreted in details. The complementary time-resolved study is also presented.Chapter 2. Zeeman spectroscopy of excitonic resonances in magnetic fields. The evolution of the optical resonances in an external magnetic field, applied perpendicularly to the layers of sc-TMD materials is investigated. Based on these results, a phenomenological model is developed aiming to describe the linear with magnetic field contributions to the energy of individual electronic states in fundamental sub-bands of sc-TMD monolayers. Furthermore, the effects of optical pumping are investigated in WSe2 monolayers, which can be tuned by tiny magnetic fields.Chapter 3. Single photon sources in thin sc-TMD flakes. The discovery of localised narrow lines emitting centres has been in thin sc-TMD flakes is presented. An investigation of their fundamental properties is discussed. This includes the measurements of temperature and magnetic field evolution of the photoluminescence lines, and the analysis of the polarisation properties and the excitation spectra as well as photon correlation measurements.Appendix A. Single photon emitters in boron nitride crystals. Hexagonal boron nitride also belongs to the family of layered materials, but it exhibits much larger band gap than semiconducting transition metal dichalcogenides. A narrow lines emitting centres has been observed in boron nitride structures, which reveal multiple similarities to defect centres in wide gap materials. They are characterised in a similar manner as the emitting centres in WSe2