| 1. |
- Pappas, S. D., et al.
(författare)
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A Cost-Effective Growth of SiOx Thin Films by Reactive Sputtering : Photoluminescence Tuning
- 2011
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Ingår i: Journal of Nanoscience and Nanotechnology. - : American Scientific Publishers. - 1533-4880 .- 1533-4899. ; 11:4, s. 3684-3687
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Tidskriftsartikel (refereegranskat)abstract
- We present a new cost-effective method to produce substoichiometric SiO2 thin films by means of a simple sputter-coater operated at a base pressure of 1 x 10(-3) mbar. During sputtering air is introduced through a fine valve so that the sputtering gas is a mixture of air/Ar. High-resolution electron microscopy shows the formation of amorphous SiOx thin films for the as-deposited samples. The index x approaches 1 when the ratio of the partial pressure of air/Ar tends to 0.1. On the other hand, pure silica is formed when the ratio of the partial pressure of air/Ar approaches 0.5. The films in the as-deposited state show intense green yellow photoluminescence. This fades away with short annealing under air at 950 degrees C. If on the other hand, prolonged annealing is performed under Argon atmosphere at 1000 degrees C, red-infrared photoluminescence is recorded due to the formation of Si nanocrystals embedded in SiO2. This simple method could be suitable for the production of thin SiOx films with embedded nanocrystals for optoelectronic or photovoltaic applications.
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| 2. |
- Pappas, S. D., et al.
(författare)
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Growth and Experimental Evidence of Quantum Confinement Effects in Cu(2)O and CuO Thin Films
- 2011
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Ingår i: Journal of Nano Research. - 1662-5250. ; 15, s. 69-74
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Tidskriftsartikel (refereegranskat)abstract
- Thin Cu films of thickness 0.4 - 150 nm were deposited via radio frequency magnetron sputtering on Si(100) wafers, corning glass and quartz. Subsequently the Cu films were oxidized in ambient air at 230 degrees C and 425 degrees C in order to produce single-phase Cu(2)O and CuO, respectively. Selected samples were measured in the transmission geometry with the help of an ultraviolet - visible spectrophotometer. From the absorption spectra of the films, it was found that the gap E(B) for the dipole allowed transitions showed blue shifts of about 1.2 eV for the Cu(2)O thinnest film (0.75 nm), whereas the E(direct) for the direct gap transitions showed blue shifts of about 0.16 eV for the CuO thinnest film (0.7 mm). The blue shift of the energy gap in the copper-oxide semiconductors is an indication of the presence of strong quantum confinement effects.
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| 3. |
- Pappas, S. D., et al.
(författare)
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Layering and temperature-dependent magnetization and anisotropy of naturally produced Ni/NiO multilayers
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 112:5, s. 053918-
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Tidskriftsartikel (refereegranskat)abstract
- Ni/NiO multilayers were grown by magnetron sputtering at room temperature, with the aid of the natural oxidation procedure. That is, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (similar to 1.2 nm) is formed. Simulated x-ray reflectivity patterns reveal that layering is excellent for individual Ni-layer thickness larger than 2.5 nm, which is attributed to the intercalation of amorphous NiO between the polycrystalline Ni layers. The magnetization of the films, measured at temperatures 5-300 K, has almost bulk-like value, whereas the films exhibit a trend to perpendicular magnetic anisotropy (PMA) with an unusual significant positive interface anisotropy contribution, which presents a weak temperature dependence. The power-law behavior of the multilayers indicates a non-negligible contribution of higher order anisotropies in the uniaxial anisotropy. Bloch-law fittings for the temperature dependence of the magnetization in the spin-wave regime show that the magnetization in the multilayers decreases faster as a function of temperature than the one of bulk Ni. Finally, when the individual Ni-layer thickness decreases below 2 nm, the multilayer stacking vanishes, resulting in a dramatic decrease of the interface magnetic anisotropy and consequently in a decrease of the perpendicular magnetic anisotropy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4750026]
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| 4. |
- Pappas, S. D., et al.
(författare)
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Natural Nanomorphous Ni/NiO Magnetic Multilayers : Structure and Magnetism of the High-Ar Pressure Series
- 2014
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Ingår i: Journal of Nanoscience and Nanotechnology. - : American Scientific Publishers. - 1533-4880 .- 1533-4899. ; 14:8, s. 6103-6107
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Tidskriftsartikel (refereegranskat)abstract
- Natural nanomorphous Ni/NiO multilayers have exhibited interesting magnetic properties, such as an unusual positive surface anisotropy and perpendicular magnetic anisotropy. Most attention has been paid to multilayers prepared by radio frequency magnetron sputtering under relatively low (3x10(-3) mbar) Ar pressure. Here we report on the correlation between structural and magnetic properties for a new series of multilayers, prepared under relatively high (3x10(-2) mbar) Ar pressure. The crystalline Ni individual layer thickness ranges between 5-8 nm. The amorphous NiO layer thickness is constant, about 1.1 nm thick. X-ray reflectivity showed that in some of the multilayers the high-order Bragg peaks become broader and diminish quickly. Cross-section transmission electron microscopy reveals that this occurs because the first bilayers are formed in accordance to the growth conditions, while the ones near the top are vanished. Despite the deterioration of the interface quality, all samples show tendency for perpendicular magnetic anisotropy even for large bilayer thickness of about 9 nm. Similar tendency is observed even by a 330 nm thick non-multilayered Ni film grown under the same conditions. This observation reveals the important role of strain and magnetoelastic anisotropy as a source of perpendicular magnetic anisotropy in the Ni/NiO multilayers.
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| 5. |
- Poulopoulos, P., et al.
(författare)
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Growth and Magnetism of Natural Multilayers
- 2011
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Ingår i: Journal of Nano Research. - 1662-5250. ; 15, s. 95-103
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we present a simple method to fabricate high quality Ni/NiO multilayers with the use of a single magnetron sputtering head. Namely, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (similar to 1nm) is formed by natural oxidation. The process is reproducible and the result is the formation of a multilayer with excellent layering. Magnetization hysteresis loops recorded at 5 K and room temperature reveal a tendency for perpendicular magnetic anisotropy as the thickness of the individual Ni layers decreases. It is shown that the Ni/NiO interface has sizeable positive surface/interface anisotropy, i.e. it favors the development of perpendicular magnetic anisotropy. This is rather unusual for a Ni-based multilayered system and may render Ni/NiO multilayers useful for magneto-optical recording applications.
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| 6. |
- Poulopoulos, P., et al.
(författare)
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Positive surface and perpendicular magnetic anisotropy in natural nanomorphous Ni/NiO multilayers
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 96:20, s. 202503-
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Tidskriftsartikel (refereegranskat)abstract
- Ni/NiO multilayers with excellent sequencing are grown via radiofrequency magnetron sputtering with the use of one Ni target and natural oxidation. Ni layers consist of very small Ni nanocrystals interrupted by amorphous NiO layers. When Ni is deposited at 0.3 Pa Ar-pressure, the hard-magnetization axis is the film normal and saturation field decreases by decreasing Ni layer thickness. Considerable positive surface anisotropy is found, which is remarkable for Ni-based multilayers. If Ni is deposited at 3 Pa Ar-pressure, perpendicular magnetic anisotropy is observed at low temperatures even for 5.4 nm thick Ni layers. This anisotropy results in the formation of stripe magnetic domains.
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| 7. |
- Delimitis, Andreas, et al.
(författare)
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Microstructural Investigation of SiOx Thin Films Grown by Reactive Sputtering on (001) Si Substrates
- 2012
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Ingår i: Journal of Nano Research. - 1661-9897. ; 17, s. 147-156
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Tidskriftsartikel (refereegranskat)abstract
- In the Current Study, the Structural Characteristics of Siox Thin Films Grown by Magnetron Sputtering on Si Substrates Are Reported. High Resolution Transmission Electron Microscopy Revealed the Formation of Amorphous Siox Films for the as-Deposited Samples, as Well as the Ones Annealed in Ambient Air for 30 Min at 950oC and of Si Nanocrystals, Embedded in Amorphous Siox, after Ar Annealing for 1-4 Hours at 1000oC. the Nanocrystals, with Sizes up to 6 Nm, Predominately Exhibit {111} Lattice Planes. Energy-Dispersive X-Ray Analysis Showed that the Si/O Ratio Is between 0.5-1, I.e. the Amorphous Films Comprise of a Mixture of Sio2 and Sio. Phase Images and Corresponding Strain Maps Created Using Fourier Filtering Revealed a Uniform Contrast in the Nanocrystals, which Shows that the Si Lattice Constant Does Not Vary Significantly. the Residual Strain Variations, around 4%, May Account for the Possible Existence of a Small Percentage of Highly Disordered Si or Siox Residual Clusters inside the Regular Si Matrix, in Full Agreement with Photoluminescence Measurements Performed on the same Materials.
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| 8. |
- Kapaklis, Vassilios, 1978-, et al.
(författare)
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Structure and Magnetic Properties of hcp and fcc Nanocrystalline Thin Ni Films and Nanoparticles Produced by Radio Frequency Magnetron Sputtering
- 2010
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Ingår i: Journal of Nanoscience and Nanotechnology. - : American Scientific Publishers. - 1533-4880 .- 1533-4899. ; 10:9, s. 6024-6028
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Tidskriftsartikel (refereegranskat)abstract
- We report on the growth of thin Ni films by radio frequency magnetron sputtering in Ar-plasma. The growth temperature was about 350 K and the films were deposited on various substrates such as glass, silicon, sapphire and alumina. The thickness of the thinnest films was estimated by the appearance of Kiessig fringes up to about 2 theta = 8 degrees in the small-angle X-ray diffraction pattern, as expected for high-quality atomically-flat thin films. With the help of this, a quartz balance system was calibrated and used for measuring the thickness of thicker samples with an accuracy of better than 5%. Structural characterization via X-ray diffraction and high resolution transmission electron microscopy revealed an Ar-gas pressure window, where single phase hcp Ni films may be grown. The magnetic response of the Ni films was checked at room temperature via a newly established and fully automatic polar magneto-optic Kerr effect magnetometer. The hcp films show no magnetic response. Interestingly, the magnetic saturation field of fcc films deposited at low Ar pressure is comparable to the one of bulk Ni, while the one of fcc films deposited at high Ar pressures is decreased, revealing the presence of residual strain in the films. Finally, it is shown that it is possible to form films which contain magnetic Ni fcc nanoparticles in a non-magnetic hcp matrix, i.e., a system interesting for technological applications demanding a single Ni target for its production.
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