13 results on '"W Calvet"'
Search Results
2. Efficient plasmonic scattering of colloidal silver particles through annealing-induced changes
- Author
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Guanchao Yin, Sven Ring, Bernd Stannowski, Yan Lu, Matthias Ballauff, Rutger Schlatmann, W Calvet, and A Ott
- Subjects
Materials science ,business.industry ,Annealing (metallurgy) ,Scanning electron microscope ,Mechanical Engineering ,technology, industry, and agriculture ,Nanoparticle ,Bioengineering ,General Chemistry ,Light scattering ,Crystallinity ,Optics ,X-ray photoelectron spectroscopy ,Chemical engineering ,Mechanics of Materials ,General Materials Science ,sense organs ,Particle size ,Plasmonic solar cell ,Electrical and Electronic Engineering ,business - Abstract
We present an experimental study on the influence of annealing temperature on morphological changes of colloidal silver nanoparticles (Ag NPs) and their optical response. Monodisperse colloidal Ag NPs with diameter of 164 nm ± 15 nm have been fabricated by a facile two-step synthesis approach. The annealing effects on the Ag NPs have been investigated by means of optical measurements, scanning electron microscopy, x-ray photoelectron spectroscopy and x-ray diffraction. By annealing up to 440 °C morphology and chemical compositions of the Ag NPs changed. These changes affect the particle size and distribution, surface morphology, crystallinity and, most importantly, the oxidation state of the surface layer. The removal of an oxide layer leads to stronger light scattering from the nanoparticles and decreases parasitic light absorption at wavelengths above 400 nm. Strong light trapping has been observed for a silicon thin film with embedded annealed Ag NPs. This study demonstrates that colloidal Ag NPs for plasmonic solar cells need to be carefully processed and that they can potentially achieve high scatter efficiencies.
- Published
- 2014
3. Raman scattering structural evaluation of CuInS2 thin films
- Author
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Alejandro Pérez-Rodríguez, J. Álvarez-García, W Calvet, Roland Scheer, Joan Ramon Morante, Albert Romano-Rodriguez, and T. Jawhari
- Subjects
Chemistry ,business.industry ,Metals and Alloys ,Surfaces and Interfaces ,Molecular physics ,Spectral line ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,symbols.namesake ,Optics ,X-ray Raman scattering ,Molecular vibration ,Materials Chemistry ,symbols ,Coherent anti-Stokes Raman spectroscopy ,Crystallite ,Thin film ,business ,Raman spectroscopy ,Raman scattering - Abstract
This work reports the spectral characterisation of CuInS 2 (CIS) layers for solar cell devices by Raman scattering. The Raman spectrum from CIS is characterised by a dominant A 1 mode at approximately 290 cm −1 , being the position and width of the mode sensitive to the CIS structural quality. Besides, layers with poor structural quality show the appearance in the spectra of an additional line, not allowed for chalcopyrite crystals, at approximately 305 cm −1 . The relative intensity of this line has been proposed as a further marker for film quality. To deepen in the knowledge of the origin of this mode, Raman scattering measurements are performed in both polycrystalline and epitaxial layers under different conditions, including resonant Raman and different polarisation configurations. The origin of the mode is discussed in terms of (i) the presence of phases with different symmetry as Cu-Au ordering, and (ii) the possible existence of defect-related local vibrational modes.
- Published
- 2001
4. Thin CuInS2 Films Prepared by MOMBE: Interface and Surface Properties
- Author
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W. Calvet, Christian Pettenkofer, and Carsten Lehmann
- Subjects
Diffraction ,Materials science ,Chalcopyrite ,Analytical chemistry ,chemistry.chemical_element ,Epitaxy ,Adsorption ,chemistry ,X-ray photoelectron spectroscopy ,visual_art ,visual_art.visual_art_medium ,Deposition (phase transition) ,Layer (electronics) ,Carbon - Abstract
CuInS2 films were prepared on Si(111) by MOMBE using standard MBE Cu and In sources and Ditertbutyldisulfide (TBDS) as an organic sulfur precursor. The films were analyzed in situ by XPS, LEED and UPS. Deposition at 250°C yields chalcopyrite films [1] with admixtures of CuSi2 and CuIn alloys. Deposition at higher temperatures up to 550°C was used to clarify the feasibility of the process. High quality LEED diffraction patterns show epitaxial growth but CuSi2 precipitations are still observed for deposition on Si(111). A buffer layer of indiumsulfide showed no considerable effect on the interface morphology. The obtained LEED pattern are in accordance with the epitaxial relation Si{111}||CuInS2{112}. Even for temperatures as high as 550°C no incorporation of Carbon or residual hydrocarbons in the film or adsorbed at the surface were detected.
- Published
- 2005
5. Electronic, structural and chemical properties of GaAs/ZnSe heterovalent interfaces as dependent on MBE growth conditions and ex situ annealing.
- Author
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T A Komissarova, M V Lebedev, S V Sorokin, G V Klimko, I V Sedova, S V Gronin, K A Komissarov, W Calvet, M N Drozdov, and S V Ivanov
- Subjects
ELECTRIC properties of zinc selenide ,MOLECULAR beam epitaxy ,SURFACE reconstruction ,VALENCE bands ,CHEMICAL bonds - Abstract
A study of electronic, structural and chemical properties of GaAs/ZnSe heterovalent interfaces (HI) in dependence on molecular beam epitaxy (MBE) growth conditions and post-growth annealing was performed. Initial GaAs surface reconstructions ((2 × 4)As or c(4 × 4)As) and ZnSe growth mode (MBE or migration-enhanced epitaxy (MEE)) were varied for different undoped and n-doped heterovalent structures. Although all the structures have low extended defect density (less than 10
6 cm−2 ) and rather small (less than 5 nm) atomic interdiffusion at the HI, the structural, chemical and electronic properties of the near-interface area (short-distance interdiffusion effects, dominant chemical bonds, and valence band offset values) as well as electrical properties of the n-GaAs/n-ZnSe heterovalent structures were found to be influenced strongly by the MBE growth conditions and post-growth annealing. [ABSTRACT FROM AUTHOR]- Published
- 2017
- Full Text
- View/download PDF
6. Efficient plasmonic scattering of colloidal silver particles through annealing-induced changes.
- Author
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A Ott, S Ring, G Yin, W Calvet, B Stannowski, Y Lu, R Schlatmann, and M Ballauff
- Subjects
SILVER nanoparticles ,PLASMONICS ,COLLOIDAL networks ,CRYSTALLINITY ,SCANNING electron microscopy - Abstract
We present an experimental study on the influence of annealing temperature on morphological changes of colloidal silver nanoparticles (Ag NPs) and their optical response. Monodisperse colloidal Ag NPs with diameter of 164 nm ± 15 nm have been fabricated by a facile two-step synthesis approach. The annealing effects on the Ag NPs have been investigated by means of optical measurements, scanning electron microscopy, x-ray photoelectron spectroscopy and x-ray diffraction. By annealing up to 440 °C morphology and chemical compositions of the Ag NPs changed. These changes affect the particle size and distribution, surface morphology, crystallinity and, most importantly, the oxidation state of the surface layer. The removal of an oxide layer leads to stronger light scattering from the nanoparticles and decreases parasitic light absorption at wavelengths above 400 nm. Strong light trapping has been observed for a silicon thin film with embedded annealed Ag NPs. This study demonstrates that colloidal Ag NPs for plasmonic solar cells need to be carefully processed and that they can potentially achieve high scatter efficiencies. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
7. Structural and Chemical Properties of NiO x Thin Films: Oxygen Vacancy Formation in O 2 Atmosphere.
- Author
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Blume R, Calvet W, Ghafari A, Mayer T, Knop-Gericke A, and Schlögl R
- Abstract
NiO
x films on Si(111) were put in contact with oxygen at elevated temperatures. During heating and cooling in oxygen atmosphere Near Ambient Pressure (NAP)-XPS and -XAS and work function (WF) measurements reveal the creation and replenishing of oxygen vacancies in dependence of temperature. Oxygen vacancies manifest themselves as a distinct O1s feature at 528.9 eV on the low binding energy side of the main NiO peak as well as by a distinct deviation of the Ni2p3/2 spectral features from the typical NiO spectra. DFT calculations reveal that the presence of oxygen vacancies leads to a charge redistribution and altered bond lengths of the atoms surrounding the vacancies causing the observed spectral changes. Furthermore, we observed that a broadening of the lowest energy peak in the O K-edge spectra can be attributed to oxygen vacancies. In the presence of oxygen vacancies, the WF is lowered by 0.1 eV., (© 2023 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)- Published
- 2023
- Full Text
- View/download PDF
8. Structural and chemical properties of NiO x thin films: the role of oxygen vacancies in NiOOH formation in a H 2 O atmosphere.
- Author
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Blume AR, Calvet W, Ghafari A, Mayer T, Knop-Gericke A, and Schlögl R
- Abstract
NiO
x films grown from 50 nm thick Ni on Si(111) were put in contact with oxygen and subsequently water vapor at elevated temperatures. Near ambient pressure (NAP)-XPS and -XAS reveal the formation of oxygen vacancies at elevated temperatures, followed by H2 O dissociation and saturation of the oxygen vacancies with chemisorbing OH. Through repeated heating and cooling, OH-saturated oxygen vacancies act as precursors for the formation of thermally stable NiOOH on the sample surface. This is accompanied by a significant restructuring of the surface which increases the probability of NiOOH formation. Exposure of a thin NiOx film to H2 O can lead to a partial reduction of NiOx to metallic Ni accompanied by a distinct shift of the NiOx spectra with respect to the Fermi edge. DFT calculations show that the formation of oxygen vacancies and subsequently Ni0 leads to a state within the band gap of NiO which pins the Fermi edge.- Published
- 2023
- Full Text
- View/download PDF
9. Visualizing anisotropy in the surface oxidation of germanium by wet etching of patterned nanowedges: proof of concept.
- Author
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Abrenica GHA, Lebedev MV, Pacco A, Calvet W, Mayer T, and van Dorp DH
- Abstract
We study the anisotropy in surface oxidation for Ge(100) and (110) in HCl/H2O2 solution complemented by synchrotron X-ray photoemission spectroscopy (SXPS) measurements integrated with an in situ etching chamber. Visual anisotropic demonstration is confirmed by lithographic Ge nanowedges.
- Published
- 2019
- Full Text
- View/download PDF
10. Investigation of the potassium fluoride post deposition treatment on the CIGSe/CdS interface using hard X-ray photoemission spectroscopy - a comparative study.
- Author
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Ümsür B, Calvet W, Steigert A, Lauermann I, Gorgoi M, Prietzel K, Greiner D, Kaufmann CA, Unold T, and Lux-Steiner MCh
- Abstract
The impact of the potassium fluoride post deposition treatment on CIGSe chalcopyrite absorbers is investigated by means of depth resolved hard X-ray photoemission spectroscopy of the near surface region. Two similar, slightly Cu-poor CIGSe absorbers were used with one being treated by potassium fluoride prior to the chemical bath deposition of an ultrathin CdS layer. The thickness of the CdS layer was chosen to be in the range of about 10 nm in order to allow the investigation of the CIGSe/CdS interface by the application of hard X-rays, increasing the information depth up to 30 nm. Besides strong intermixing on both samples, an increased Cu depletion of the KF treated absorber was observed in combination with an increased accumulation of Cd and S. In addition, a general shift of about 0.15 eV to higher binding energies of the CIGSe valence band at the absorber surface as well as the CIGSe and CdS related core levels was measured on the KF treated sample. This phenomenon is attributed to the impact of additional cadmium which acts as donor and releases further electrons into the conduction band of the absorber. Finally, the electrons accumulate at the CdS surface after having passed the interface region. This additional surface charge leads to a pronounced shift in the photoemission spectra as observed on the KF treated CIGSe absorber compared to the non-treated absorber.
- Published
- 2016
- Full Text
- View/download PDF
11. Efficient plasmonic scattering of colloidal silver particles through annealing-induced changes.
- Author
-
Ott A, Ring S, Yin G, Calvet W, Stannowski B, Schlatmann R, and Ballauff M
- Abstract
We present an experimental study on the influence of annealing temperature on morphological changes of colloidal silver nanoparticles (Ag NPs) and their optical response. Monodisperse colloidal Ag NPs with diameter of 164 nm ± 15 nm have been fabricated by a facile two-step synthesis approach. The annealing effects on the Ag NPs have been investigated by means of optical measurements, scanning electron microscopy, x-ray photoelectron spectroscopy and x-ray diffraction. By annealing up to 440 °C morphology and chemical compositions of the Ag NPs changed. These changes affect the particle size and distribution, surface morphology, crystallinity and, most importantly, the oxidation state of the surface layer. The removal of an oxide layer leads to stronger light scattering from the nanoparticles and decreases parasitic light absorption at wavelengths above 400 nm. Strong light trapping has been observed for a silicon thin film with embedded annealed Ag NPs. This study demonstrates that colloidal Ag NPs for plasmonic solar cells need to be carefully processed and that they can potentially achieve high scatter efficiencies.
- Published
- 2014
- Full Text
- View/download PDF
12. Silicon based tandem cells: novel photocathodes for hydrogen production.
- Author
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Calvet W, Murugasen E, Klett J, Kaiser B, Jaegermann W, Finger F, Hoch S, Blug M, and Busse J
- Abstract
A photovoltaic tandem cell made of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) was investigated as a photocathode for hydrogen evolution in a photoelectrochemical device. The electronic and electrochemical properties of the samples were characterized using X-ray photoemission spectroscopy (XPS) and cyclic voltammetry (CV), whereas the morphology of the surface in contact with the electrolyte was investigated by scanning electron microscopy (SEM). The electric efficiency of the tandem cell was determined to be 5.2% in a photoelectrochemical (PEC) setup in acidic solution which is only about half of the photovoltaic efficiency of the tandem cell. A significant improvement in efficiency was achieved with platinum as a catalyst which was deposited by physical vapour deposition (PVD) under ultra-high vacuum (UHV) conditions.
- Published
- 2014
- Full Text
- View/download PDF
13. Formation of an electron hole doped film in the α-Fe2O3 photoanode upon electrochemical oxidation.
- Author
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Gajda-Schrantz K, Tymen S, Boudoire F, Toth R, Bora DK, Calvet W, Grätzel M, Constable EC, and Braun A
- Abstract
Solar hydrogen generation by water splitting in photoelectrochemical cells (PEC) is an appealing technology for a future hydrogen economy. Hematite is a prospective photoanode material in this respect because of its visible light conjugated band gap, its corrosion stability, its environmentally benign nature and its low cost. Its bulk and surface electronic structure has been under scrutiny for many decades and is considered critical for improvement of efficiency. In the present study, hematite films of nominally 500 nm thickness were obtained by dip-coating on fluorine doped tin oxide (FTO) glass slides and then anodised in 1 molar KOH at 500, 600, and 700 mV for 1, 10, 120 and 1440 minutes under dark conditions. X-ray photoelectron spectra recorded at the Fe 3p resonant absorption threshold show that the e(g) transition before the Fermi energy, which is well developed in the pristine hematite film, becomes depleted upon anodisation. The spectral weight of the e(g) peak decreases with the square-root of the anodisation time, pointing to a diffusion controlled process. The speed of this process increases with the anodisation potential, pointing to Arrhenius behaviour. Concomitantly, the weakly developed t(2g) peak intensity becomes enhanced in the same manner. This suggests that the surface of the photoanode contains Fe(2+) species which become oxidized toward Fe(3+) during anodisation. The kinetic behaviour derived from the experimental data suggests that the anodisation forms an electron hole doped film on and below the hematite surface.
- Published
- 2013
- Full Text
- View/download PDF
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