Plasmonic excitations and coupling in atomic wires

Metal-induced atomic wires on the semiconductor surfaces are the ultimate limit of long-range ordered quasi-one-dimensional electronic systems. In this thesis, the plasmonic excitations and their coupling with the structural parameters, as well as the role of the embedding medium on the Au atomic wires, self-organized on flat and high-indexed Si surfaces were investigated. The atomic wire systems in the focus of this thesis, Si(111)-(5×2)-Au and Si(hhk)-(1×2)-Au, were generated by adsorption of submonolayers of gold onto the reconstructed surfaces. These systems feature a broad range of characteristics that can exemplarily be classified based on the number of atomic chains per terrace, inter-wire spacing, local structure of step edges, number of metallic bands and band filling. High-resolution EELS-LEED and SPA-LEED were used as the main experimental methods to address the mentioned properties and furthers. These experimental results were compared with the atomistic DFT calculations. Moreover, for the Si(553)-Au system, IR transmittance experiments were performed to investigate the optically active standing wave formation after oxidation. As a prominent property of conductive electrons, collective excitation is strongly coupled to the crystal lattice, electronic band structure, electronic and spatial confinement as well as properties of the surrounding physical media. This makes plasmonic excitation an adequate tool to probe a variety of interactions and coupling between those parameters associated with metallic structures. Taking this advantage, the present thesis investigates plasmonic excitations and their coupling to structural and environmental parameters. As a result of this investigation, a strong cross-coupling of the electronic and structural properties was revealed. For example, electronic doping to the Si(553)-Au system enhances order along the wires, which also results in band gap opening at the same time. Moreover, modification of individual structural motifs on the Si(557)-Au surface leads to a unique rearrangement of the band structure while preserving the metallicity. Also, combining the plasmon dispersion with calculated band structures, the kinetic sequence of the oxidation of the different atomic groups could be studied. For most investigated systems, an almost quantitative agreement between atomistic calculations and plasmon spectroscopy results was achieved, validating the calculated band structure and model used. In particular, the unoccupied part of the band structure was investigated for atomic wire systems. As an example, for the very first time unoccupied electronic band structure of the Si(111)-(5×2)-Au surface was investigated by comparing plasmon dispersion with the available DFT calculated bands. Moreover, doping of the latter surface with surplus Au and atomic H resulted in a metal-insulator transition. However, due to their highly robust anisotropic structure and electronic properties, the doping and oxidation of Si(hhk)-Au systems underline more specific mechanisms in the fine-tuning of metallicity. Some of these particular mechanisms include switching of bands, electronic interaction of adjacent terraces, cross-talking of dimerization and band gap opening, self-healing of defects, robust metallicity due to the site-specific oxidation, and indirect charge transfer to the metallic states, etc. have been extensively studied in this thesis.Metallische atomare Drähte auf Halbleiteroberflächen stellen die physikalische die Grenze für langreichweitige geordnete quasi-eindimensionale elektronische Systeme dar. In dieser Arbeit wurden sowohl plasmonische Anregungen und deren Kopplung mit strukturellen Parametern, als auch die Rolle des Trägermaterials auf atomare Golddrähte untersucht, die durch Selbstorganisation auf flachen und gestuften Silizium-Flächen hergestellt wurden. Schwerpunkt der vorliegenden Arbeit sind die Systeme Si(111)-(5×2)-Au und Si(hhk)-(1×2)-Au, die durch die Adsorption von Submonolagen Gold auf rekonstruierte Siliziumoberflächen hergestellt wurden. Diese Systeme zeigen eine Reihe von Charakteristika, die anhand der Anzahl an atomaren Ketten pro Terrasse, dem Abstand der Ketten, der lokalen Struktur der Stufenkanten und der Anzahl metallischer Bänder klassifiziert werden können. Hochauflösendes EELS-LEED und SPA-LEED wurden als experimentelle Hauptmethoden zur Analyse der genannten Eigenschaften verwendet. Die experimentellen Ergebnisse wurden mit atomistischen DFT-Rechnungen verglichen und diskutiert. Weiterhin, wurden für das Si(553)-Au-system IR-Transmissionsexperimente durchgeführt, um die optisch stehenden Welle nach der Oxidation zu untersuchen. Als markante Eigenschaft metallischer Ladungsträger ist deren kollektive Anregung stark verknüpft mit dem Kristallgitter, der elektronischen Bandstruktur, der räumlichen Einengung sowie mit den Eigenschaften des umgebenden Materials. Dies macht plasmonische Anregung zu einem adäquaten Werkzeug um die vielen Wechselwirkungen und Kopplungen, die mit den metallischen Strukturen verbunden sind, zu untersuchen. Basierend auf diesem Ansatz, analysiert die vorliegende Arbeit die plasmonischen Anregungen und ihre Kopplung zu strukturellen und Umgebungsparametern. Als ein Ergebnis dieser Untersuchungen, konnte eine starke Kopplung zwischen den elektronischen und strukturellen Parametern aufgezeigt werden. Beispielweise verbessert elektronische Dotierung des Si(553)-Au-Systems die Ordnung in den Ketten, bei gleichzeitiger Öffnung einer Bandlücke. Weitere Modifikationen von strukturellen Gegebenheiten in dem System führen zu einer einzigartigen Neuordnung der Bandstruktur bei Aufrechterhaltung der metallischen Eigenschaften. Des Weiteren konnte durch eine Kombination der Plasmonendispersion und der Bandstruktur die kinetischen Sequenz des Oxidationsprozesses verschiedener atomarer Gruppen untersucht werden. Für einen Großteil der untersuchten Systeme, konnte eine nahezu quantitative Übereinstimmung zwischen den atomaren Berechnungen und den Ergebnissen der Plasmonenspektroskopie erzielt werden, wodurch sowohl die berechnete Bandstruktur als auch das verwendete Modell verifiziert wurden. Im Speziellen konnte erstmalig der unbesetzte Teil der Bandstuktur atomarer Kettensysteme untersucht werden, beispielweise die unbesetzte Bandstruktur von Si(111)-(5×2)-Au, die durch den Vergleich der Plasmonendispersion mit den durch DFT Rechnungen ermittelten Bändern untersucht wurde. Eine Dotierung der genannten Oberfläche mit überschüssigen Gold- oder Wasserstoffatomen führt zu einem Metall-Isolator-Übergang. Durch die Robustheit der anisotropen Struktur und der elektronischen Eigenschaften, stellt die Dotierung und Oxidation von Si(hhk)-Gold Systemen spezifische Mechanismen für die Feinabstimmung der metallischen Eigenschaften heraus. Einige dieser Effekte, wie das Abschalten von Bändern, elektronische Wechselwirkung zwischen benachbarten Terrassen, die gegenseitige Beeinflussung von Dimerisation und der Öffnung einer Bandlücke, Selbstheilung von Defekten, robuste metallische Charakteristika durch die positionsspezifische Oxidation, der indirekte Ladungsübertrag auf die metallischen Zustände, etc. wurden in dieser Arbeit detailliert untersucht

Preparation and optical studies of PbS nanoparticles

In the present report formation of nano-sized PbS in MA/octene-1 copolymer matrix at 80 C temperature is<br>being reported. A size and distribution of particles were observed in AFM results and the images are correlated<br>with the results on X-ray diffraction measurements. The structure and phase of the PbS nanoparticles<br>were characterized by X-ray diffraction (XRD). XRD studies reveal that as-synthesized PbS nanoparticles are<br>in single phase cubic structure and the grain size have been calculated 10–15 nmfrom XRD results. The size<br>distribution was further supported by UV/Vis absorption and photoluminescence (PL) spectroscopy of the<br>colloid nanoparticles. The obtained nanocomposites show an emission peak at 418 nm

Optical and structural studies of ZnS nanoparticles synthesized via chemical in situ

tZnS nanoparticles (NPs) have been synthesized by the facile chemical route with a narrow size dis-tribution in the MA/octene-1 copolymer matrix and effect of reaction time has been discussed. X-raydiffraction pattern confirms the pure cubic phase of ZnS with 5–7 nm average crystal sizes which arein good agreement with the AFM and UV–vis measurements. Absorption spectra exhibit a strong blueshift from the bulk with the 3.98 eV optical band gap which clearly indicates the strong size confinementeffect. Thermogravimetric analyses show increased thermal stability of the nanocomposite comparedto the copolymer. The possible growth mechanism of the particles formation and stabilization has beendiscussed

CuS nanoparticles synthesized by a facile chemical route under different pH conditions

The CuS/polymer nanocomposites were synthesized by a<br>facile chemical route under different pH conditions, and the<br>influence of pH on the optical and structural properties was<br>studied by FT-IR, UV-VIS and photoluminescence spectroscopy,<br>thermogravimetric analysis, atomic force microscopy<br>and powder X-ray diffraction techniques

Synthesis and studies of CdS and ZnS-PE NBR modi fi ed thermoplastic elastomeric copolymer nanocomposite film

The CdSandZnSnanoparticleshavebeensynthesizedsuccessfullybythesimplechemicalmethodof<br>“layer-by-layer” in thefunctionalizedthermoplasticelastomericNBR/PEcopolymer films. Structuraland<br>optical propertieshavebeencharacterizedbyFT-IR,XRD,AFM,UV–vis andPLspectroscopymeasure-<br>ments. TheXRDresultsindicatethatCdSandZnSnanoparticleswereformedwithhexagonalandcubic<br>phase withtheirpreferredorientationalongthe[002]and[111]inthepolymericmatrix,respectively.<br>The opticalmeasurementsoftheCdSandZnSnanoparticlesareapparentlyblue-shiftedcomparedtoa<br>bulk whichindicatesthedecreasesoftheparticlessize.Thedirectallowedbandgapwasestimated<br>2.67 eVforCdSand3.72eVforZnSnanoparticles.Accordingtothecharacterizationmethods,itwas<br>revealedthattheaveragegrainsizeforthebothmaterialstakesthevaluesin5–10nm

Metal Sulfide Photocatalysts for Hydrogen Generation: A Review of Recent Advances

Metal-sulfide nanostructures have piqued the interest of researchers for decades due to their intriguing optoelectronic properties. Indeed, significant advances and improvements have been made in various fundamental aspects for cutting-edge applications, such as water splitting and hydrogen production. Furthermore, rising demand for low-dimensional materials due to lower material consumption and improved performance due to quantum size effects has spurred research on semiconducting metal sulfides. Consequently, size-controllable nanostructures with diverse morphologies have been fabricated and studied for potential applications. However, the photocatalytic hydrogen evolution rate is still limited mainly by fast recombination rate, poor solar energy utilization and lack of surface-active sites for H2 reduction. This review will highlight particularly recent findings in metal-sulfide-based photocatalysts for hydrogen evolution reactions, considering the swift development and excellent research in this field. Following a brief overview of fundamental properties, we will explore state-of-the-art strategies for enhancing H2 generation efficiencies over the pristine, heterostructured and co-catalayzed metal-sulfide photocatalysts

Synthesis and characterization of Ag2SPVA-fullerene (C60) nanocomposites

Wepresentthechemicalsynthesisofsilversulfide nanocrystals(NCs)inPVAmatrixandtheextended<br>characteristicsoftheobtainednanocompositeswithfullerene.Thesampleswerepreparedwiththe<br>different concentrationoffullereneandPVAforcompressionoftheirfractionrelationwiththeoptical<br>and phaseproperties.UV–vis spectraofAg2S showsharpexcitonicfeaturesandalargeblueshiftfrom<br>the bulkmaterial.TheNCsgrainsizewasdeterminedabout10–15nmwhichisconfirmed bydifferent<br>techniquessuchasScanningelectronmicroscopy(SEM),UV–visible absorptionandX-rayDiffractometer<br>(XRD). TheSEMimagesdemonstratethatthenanoparticlesoverlappedandstabilizedwithapolymer<br>matrix. TheEDAXresultindicatesthatthepreparednanocompositesarecomposedofpurephaseAg,S,C<br>and anyotherirrelevantmixtureshavenotbeendetecte

Synthesis and characterization of CdS nanocrystals and maleic anhydride octene-1 copolymer nanocomposite.pdf

In the present work, in-situ chemical co-precipitation method was employed for the preparation of CdS<br>nanocrystals in the copolymer matrix. The process has been carried out successfully and the nanocomposites<br>have been obtained with excellent optical and structural properties. During the process it was<br>determined that the grain size of nanoparticles depend on regulation of the reaction conditions and on<br>the proportions of the precursors in this method. Also the results have shown that the sizes of the nanoparticles<br>increase with the increase of temperature and this shifts revealed in measurements. Surface<br>morphology and crystallinity have been studied by AFM and XRD techniques, respectively. The average<br>size of nanocrystals was calculated 2–6 nm by AFM and XRD measurements. Consequently, it have been<br>detected that elaborated CdS nanocrystals demonstrate new interesting structural and optical properties.<br>A detailed optical property of the obtained CdS/MA-octene-1 nanocomposite material is done by characterizing<br>UV–Vis, FT-IR and PL spectrophotometric methods

How adsorbates alter the metallic behavior of quasi-1D electron systems of the Si(5 5 3)-Au surface

The plasmonic signals of quasi-1D electron systems are a clear and direct measure of their metallic behavior. Due to the finite size of such systems in reality, plasmonic signals from a gold-induced superstructure on Si(5 5 3) can be studied with infrared spectroscopy. The infrared spectroscopic features have turned out to be extremely sensitive to adsorbates. Even without geometrical changes of the surface superstructure, the effects of doping, of the adsorbate induced electronic surface scattering, and of the electronic polarizability changes on top of the substrate surface give rise to measurable changes of the plasmonic signal. Especially strong changes of the plasmonic signal have been observed for gold, oxygen, and hydrogen exposure. The plasmonic resonance gradually disappears under these exposures, indicating the transion to an insulating behavior, which is in accordance with published results obtained from other experimental methods. For C70 and, as shown here for the first time, TAPP-Br, the plasmonic signal almost retains its original intensity even up to coverages of many monolayers. For C70, the changes of the spectral shape, e.g. of electronic damping and of the resonance position, were also found to be marginal. On the other hand, TAPP-Br adsorption shifts the plasmonic resonance to higher frequencies and strongly increases the electronic damping. Given the dispersion relation for plasmonic resonances of 1D electron systems, the findings for TAPP-Br indicate a push-back effect and therefore stronger confinement of the free charge carriers in the quasi-one-dimensonal channel due to the coverage by the flat TAPP-Br molecules. On the gold-doped Si(5 5 3)-Au surface TAPP-Br acts as counter dopant and increases the plasmonic signal