Inorganic surface passivation of PbS nanocrystals resulting in strong photoluminescent emission

Strong photoluminescent emission has been obtained from 3 nm PbS nanocrystals in aqueous colloidal solution, following treatment with CdS precursors. The observed emission can extend across the entire visible spectrum and usually includes a peak near 1.95 eV. We show that much of the visible emission results from absorption by higher-lying excited states above 3.0 eV with subsequent relaxation to and emission from states lying above the observed band-edge of the PbS nanocrystals. The fluorescent lifetimes for this emission are in the nanosecond regime, characteristic of exciton recombination.

Investigation of the role of cadmium sulfide in the surface passivation of lead sulfide quantum dots

Surface passivation of PbS nanocrystals (NC), resulting in strong photoluminescence, can be achieved by the introduction of CdS precursors. The role of CdS in the surface passivation of PbS NCs is uncertain, as the crystalline structure of CdS and PbS are different, which should impede effective epitaxial overgrowth. Absorption spectroscopy is used to show that the CdS precursors strongly interact with the PbS NC surface. Electron microscopy reveals that the introduction of CdS precursors results in an increased particle size, consistent with overcoating. However, we also find the process to be highly non-uniform. Nevertheless, evidence for epitaxial growth is found, suggesting that effective surface passivation may be possible.

Influence of average size and interface passivation on the spectral emission of Si nanocrystals embedded in SiO2

The correlation between the structural (average size and density) and optoelectronic properties [band gap and photoluminescence (PL)] of Si nanocrystals embedded in SiO2 is among the essential factors in understanding their emission mechanism. This correlation has been difficult to establish in the past due to the lack of reliable methods for measuring the size distribution of nanocrystals from electron microscopy, mainly because of the insufficient contrast between Si and SiO2. With this aim, we have recently developed a successful method for imaging Si nanocrystals in SiO2 matrices. This is done by using high-resolution electron microscopy in conjunction with conventional electron microscopy in dark field conditions. Then, by varying the time of annealing in a large time scale we have been able to track the nucleation, pure growth, and ripening stages of the nanocrystal population. The nucleation and pure growth stages are almost completed after a few minutes of annealing time at 1100°C in N2 and afterward the ensemble undergoes an asymptotic ripening process. In contrast, the PL intensity steadily increases and reaches saturation after 3-4 h of annealing at 1100°C. Forming gas postannealing considerably enhances the PL intensity but only for samples annealed previously in less time than that needed for PL saturation. The effects of forming gas are reversible and do not modify the spectral shape of the PL emission. The PL intensity shows at all times an inverse correlation with the amount of Pb paramagnetic centers at the Si-SiO2 nanocrystal-matrix interfaces, which have been measured by electron spin resonance. Consequently, the Pb centers or other centers associated with them are interfacial nonradiative channels for recombination and the emission yield largely depends on the interface passivation. We have correlated as well the average size of the nanocrystals with their optical band gap and PL emission energy. The band gap and emission energy shift to the blue as the nanocrystal size shrinks, in agreement with models based on quantum confinement. As a main result, we have found that the Stokes shift is independent of the average size of nanocrystals and has a constant value of 0.26±0.03 eV, which is almost twice the energy of the Si¿O vibration. This finding suggests that among the possible channels for radiative recombination, the dominant one for Si nanocrystals embedded in SiO2 is a fundamental transition spatially located at the Si¿SiO2 interface with the assistance of a local Si-O vibration.

Surface passivation of crystalline silicon by Cat-CVD amorphous and nanocrystalline thin silicon films

In this work, we study the electronic surface passivation of crystalline silicon with intrinsic thin silicon films deposited by Catalytic CVD. The contactless method used to determine the effective surface recombination velocity was the quasi-steady-state photoconductance technique. Hydrogenated amorphous and nanocrystalline silicon films were evaluated as passivating layers on n- and p-type float zone silicon wafers. The best results were obtained with amorphous silicon films, which allowed effective surface recombination velocities as low as 60 and 130 cms -1 on p- and n-type silicon, respectively. To our knowledge, these are the best results ever reported with intrinsic amorphous silicon films deposited by Catalytic CVD. The passivating properties of nanocrystalline silicon films strongly depended on the deposition conditions, especially on the filament temperature. Samples grown at lower filament temperatures (1600 °C) allowed effective surface recombination velocities of 450 and 600 cms -1 on n- and p-type silicon.

Surface passivation and optical characterization of Al2O3/a-SiCx stacks on c-Si substrates

Postprint (published version)

Surface Passivation of Nanoporous TiO2 via Atomic Layer Deposition of ZrO2 for Solid-State Dye-Sensitized Solar Cell Applications

We report here the utilization of atomid layer deposition to passivate surface map states in mosoporous TiO2 nanoparticles for solid state dye sensitized solar cells based on 9,9'-spirobifluorene (spiro-OMeTAD). By depositing ZrO2 films with angstrom-level precision, coating the mesoporous TiO2 produces over a two-fold enhancement in short-circuit current density, as compared to a control device. Impedance spectroscopy measurements provide evidence that the ZrO2 coating reduces recombination lossed at the TiO2/spiro-OMeTAD interface and passivates localized surface states. Low-frequency negative capacitances, frequently observed in nanocomposite solar cells, have been associated with the surface-state mediated charge transfer from TiO2 to the spiro-OMeTAD.

Manufacturing and characterization of amorphous silicon alloys passivation layers for silicon hetero-junction solar cells

Nel presente lavoro di tesi magistrale sono stati depositati e caratterizzati film sottili (circa 10 nm) di silicio amorfo idrogenato (a-Si:H), studiando in particolare leghe a basso contenuto di ossigeno e carbonio. Tali layer andranno ad essere implementati come strati di passivazione per wafer di Si monocristallino in celle solari ad eterogiunzione HIT (heterojunctions with intrinsic thin layer), con le quali recentemente è stato raggiunto il record di efficienza pari a 24.7% . La deposizione è avvenuta mediante PECVD (plasma enhanced chemical vapour deposition). Tecniche di spettroscopia ottica, come FT-IR (Fourier transform infrared spectroscopy) e SE (spettroscopic ellipsometry) sono state utilizzate per analizzare le configurazioni di legami eteronucleari (Si-H, Si-O, Si-C) e le proprietà strutturali dei film sottili: un nuovo metodo è stato implementato per calcolare i contenuti atomici di H, O e C da misure ottiche. In tal modo è stato possibile osservare come una bassa incorporazione (< 10%) di ossigeno e carbonio sia sufficiente ad aumentare la porosità ed il grado di disordine a lungo raggio del materiale: relativamente a quest’ultimo aspetto, è stata sviluppata una nuova tecnica per determinare dagli spettri ellisometrici l’energia di Urbach, che esprime la coda esponenziale interna al gap in semiconduttori amorfi e fornisce una stima degli stati elettronici in presenza di disordine reticolare. Nella seconda parte della tesi sono stati sviluppati esperimenti di annealing isocrono, in modo da studiare i processi di cristallizzazione e di effusione dell’idrogeno, correlandoli con la degradazione delle proprietà optoelettroniche. L’analisi dei differenti risultati ottenuti studiando queste particolari leghe (a-SiOx e a-SiCy) ha permesso di concludere che solo con una bassa percentuale di ossigeno o carbonio, i.e. < 3.5 %, è possibile migliorare la risposta termica dello specifico layer, ritardando i fenomeni di degradazione di circa 50°C.

New laser-based approaches to improve the passivation and rear contact quality in high effiency crystalline silicon solar cells

Laser processing has been the tool of choice last years to develop improved concepts in contact formation for high efficiency crystalline silicon (c-Si) solar cells. New concepts based on standard laser fired contacts (LFC) or advanced laser doping (LD) techniques are optimal solutions for both the front and back contacts of a number of structures with growing interest in the c-Si PV industry. Nowadays, substantial efforts are underway to optimize these processes in order to be applied industrially in high efficiency concepts. However a critical issue in these devices is that, most of them, demand a very low thermal input during the fabrication sequence and a minimal damage of the structure during the laser irradiation process. Keeping these two objectives in mind, in this work we discuss the possibility of using laser-based processes to contact the rear side of silicon heterojunction (SHJ) solar cells in an approach fully compatible with the low temperature processing associated to these devices. First we discuss the possibility of using standard LFC techniques in the fabrication of SHJ cells on p-type substrates, studying in detail the effect of the laser wavelength on the contact quality. Secondly, we present an alternative strategy bearing in mind that a real challenge in the rear contact formation is to reduce the damage induced by the laser irradiation. This new approach is based on local laser doping techniques previously developed by our groups, to contact the rear side of p-type c-Si solar cells by means of laser processing before rear metallization of dielectric stacks containing Al2O3. In this work we demonstrate the possibility of using this new approach in SHJ cells with a distinct advantage over other standard LFC techniques.

Semiconductor measurement technology : techniques for measuring the integrity of passivation overcoats on integrated circuits /

Mode of access: Internet.

Corrosion and passivation of steel in concrete

A study of several chemical and electrochemical factors which affect the behaviour of embedded steel in cement pastes and concrete has been made. The effects of internal and external sources of chloride ions on the pore solution chemistry of Portland cement pastes, with and without additions of anodic corrosion inhibitors, have been studied using a pore solution expression device which has enabled samples of pore solution to be expressed from hardened cement pastes and analysed for various ionic species. Samples of pure alite and tricalcium aluminate have been prepared and characterised with respect to morphology, free lime content and fineness. Kinetics of diffusion of chloride ions in hardened pastes of alite and alite blended with tricalcium aluminate have been investigated and an activation energy obtained for the diffusion process in alite. The pore structures of the hardened pastes and the chloride ion binding capacity of alite have also been determined. Concrete cylinders containing embedded steel with four different surface conditions were exposed to various environments. The electrochemical behaviour of the steel was monitored during the period of exposure by means of rest potential measurements and the steel corrosion products analysed before and after being embedded. An examination was made of the nature of the interfacial zones produced between the embedded steel and cement. Rest potential measurements were monitored for steel embedded in alite paste in the presence of chloride ions and cement paste containing various levels of inhibitors in combination with chloride ions. In the latter case the results were supported by polarisation resistance determinations.

Étude et passivation des défauts introduits par la gravure de vias sur cellule photovoltaïque triple jonction

Malgré l'augmentation constante de l'efficacité des cellules photovoltaïques multi-jonctions destinées au photovoltaïque concentré, des pertes de performances subsistent à haute concentration solaire. Elles sont principalement causées par un ombrage excessif dû aux métallisations ou par effet Joule à cause de la résistance série. Une des solutions à ce problème est de reporter le contact métallique en face avant sur la face arrière grâce à des vias métallisés et isolés électriquement. Avec cette architecture, les pertes dues à l'effet Joule et à l'ombrage seront limitées et des gains en efficacité sont attendus. Toutefois, l'intégration de vias sur des cellules photovoltaïques triple jonction favorise la recombinaison électron-trou en surface et peut provoquer une perte de performances de ces dispositifs. Ce mémoire présente les travaux de recherche effectués visant à étudier précisément cette problématique ainsi qu'à proposer des solutions pour limiter ces pertes. L'objectif est d'évaluer les pertes de performances de cellules photovoltaïques triple jonction suite à l'intégration de vias. Dans un second temps, l'objectif secondaire vise à limiter les pertes grâce à des traitements de passivation. Les résultats et solutions qu'apporte ce projet représentent une étape clé dans la réalisation de cette nouvelle architecture de contact électrique pour cellules photovoltaïques. En effet, les conclusions de ce projet de recherche permettent de valider la possibilité d'obtenir des gains en efficacité grâce à cette architecture. De plus, les procédés de microfabrication présentés dans ce projet de recherche proposent des solutions afin d'intégrer des vias sur ces hétérostructures tout en limitant les pertes en performances.

Estudo eletroquímico comparativo do aço ISO 5832-9 em diferentes meios de interesse biológico

Foi estudado o comportamento eletroquímico a 37°C do aço inoxidável ISO 5832-9, em meios de NaCl 0,9 %, de Ringer Lactato e meio mínimo de Eagle (MEM), por voltametria linear e análises da superfície por microscopia eletrônica de varredura (MEV) e por espectroscopia por dispersão de energia (EDS). Foram feitos ensaios mecânicos e testes de toxicidade. O aço ISO 5832-9 se encontra passivado no potencial de corrosão e não apresenta corrosão por pite nos três meios estudados em toda faixa de potencial investigada, desde o potencial de corrosão até 50 mV acima do potencial de transpassivação. Em meio de MEM, no entanto, as análises por MEV e EDS mostraram que o referido aço, nesse valor mais elevado de potencial, apresentou um comportamento diferente, com perda das inclusões de óxido de manganês. Os potenciais de corrosão, Ecorr (potencial de circuito aberto estacionário) bem como os valores de densidade de corrente de passivação, variaram na seguinte ordem: Ecorr, RL < Ecorr, NaCl < Ecorr, MEM. e jMEM << jRL ≅ jNaCl. No ensaio de citotoxicidade, o aço foi caracterizado como não-tóxico.

Structural and optical analysis of GaAsP/GaP core-shell nanowires

The structural and optical properties of GaAsP/GaP core-shell nanowires grown by gas source molecular beam epitaxy were investigated by transmission electron microscopy, Raman spectroscopy, photoluminescence (PL), and magneto-PL. The effects of surface depletion and compositional variations in the ternary alloy manifested as a redshift in GaAsP PL upon surface passivation, and a decrease in redshift in PL in the presence of a magnetic field due to spatial confinement of carriers.

EIS study of Ti-23Ta alloy in artificial saliva

Although titanium and Ti-6Al-4V alloy have been widely used as dental materials, possible undesirable effects such as cytotoxic reactions and neurological disorder due to metal release led to the development of more corrosion resistant and V and Al free titanium alloys, containing Nb, Zr, Mo and Ta atoxic elements. Fluoride containing products used in the prevention of plaque formation and dental caries can affect the stability of the passive oxide films formed on the Ti alloys. In this work, the corrosion behaviour of the new Ti-23Ta alloy has been evaluated in artificial saliva of different pH and fluoride concentration using electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy study showed that the oxide film formed on the alloy in artificial saliva consists of an inner compact film and an outer porous layer. The corrosion resistance of Ti-23Ta alloy which is reduced by increasing F concentration or decreasing pH is related to the resistance of the inner compact layer. The presence of fluoride and low pH of the saliva enhance the porosity of the oxide film and its dissolution.

Investigation on the intergranular corrosion resistance of the AISI 316L(N) stainless steel after long time creep testing at 600 degrees C

The effect of precipitation on the corrosion resistance of AISI 316L(N) stainless steel previously exposed to creep tests at 600 degrees C for periods of up to 10 years, has been studied. The corrosion resistance was investigated in 2 M H(2)SO(4)+0.5 M NaCl+0.01 M KSCN solution at 30 degrees C by electrochemical methods. The results showed that the susceptibility to intergranular corrosion was highly affected by aging at 600 degrees C and creep testing time. The intergranular corrosion resistance decreased by more than twenty times when the creep testing time increased from 7500 h to 85,000 h. The tendency to passivation decreased and less protective films were formed on the creep tested samples. All tested samples also showed susceptibility to pitting. Grain boundary M(23)C(6) carbides were not found after long-term exposure at 600 degrees C and the corrosion behavior of the creep tested samples was attributed to intermetallic phases (mainly sigma phase) precipitation. (C) 2007 Elsevier Inc. All rights reserved.