| 1. |
- Jalali Roudsar, Amir A., 1965-
(författare)
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On ferromagnetic thin films and two-dimensional magneto-optic photonic crystals
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents results in two different neighboring areas of research: the magnetic properties of thin ferrite films and the application of the films in two-dimensional photonic crystals.In the first part, we investigate the accuracy of the customary method for determining the magnetic anisotropy constants of ferrite films by ferromagnetic resonance (FMR) experiment. We have improved the method and introduced an experimental procedure to obtain the anisotropy constants with higher precision. The magnetic anisotropy fields are obtained by using FMR on a (111)-oriented yttrium iron garnet (YIG) film made by pulsed laser deposition. Moreover, we found experimentally that the shapes of FMR spectra of laser deposited epitaxial YIG films strongly depend on the orientation of the magnetic bias field with respect to the crystalline axes of the film. Inhomogeneities of the constants of anisotropy throughout the film could be responsible for the complexity of the FMR spectra. We find the special directions of the applied magnetic field in which the contribution of the magnetocrystalline anisotropy has the smallest effect on the ferromagnetic resonance and therefore on the elements of the permeability tensor.In the second part, we study the electromagnetic wave propagation in two-dimensional (2D) dielectric and magneto-optic photonic crystals (PCs). We have proposed a 2D PC which is composed of magneto-optic material for the purpose of the enhancement of Faraday rotation in high transmission. It is assumed that the 2D PC contains a bismuth iron garnet (BIG) film either as the PC background medium or as a defect, embedded in the 2D PC. We have examined theoretically and computationally the increase in the Faraday rotation as well as the transmission of a plane-polarized plane wave incident onto these structures in the optical wavelength regime. Several important phenomena, with potential applications, are observed.
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| 2. |
- Liu, Zhengjun, et al.
(författare)
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Aluminum oxide mask fabrication by focused ion beam implantation combined with wet etching
- 2013
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 24:17, s. 175304-
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Tidskriftsartikel (refereegranskat)abstract
- A novel aluminum oxide (Al2O3) hard mask fabrication process with nanoscale resolution is introduced. The Al2O3 mask can be used for various purposes, but in this work it was utilized for silicon patterning using cryogenic deep reactive ion etching (DRIE). The patterning of Al2O3 is a two-step process utilizing focused ion beam (FIB) irradiation combined with wet chemical etching. Gallium (Ga+) FIB maskless patterning confers wet etch selectivity between the irradiated region and the non-irradiated one on the Al2O3 layer, and mask patterns can easily be revealed by wet etching. This method is a modification of Ga+ FIB mask patterning for the silicon etch stop, which eliminates the detrimental lattice damage and doping of the silicon substrate in critical devices. The shallow surface gallium FIB irradiated Al2O3 mask protects the underlying silicon from Ga+ ions. The performance of the masking capacity was tested by drawing pairs consisting of a line and an empty space with varying width. The best result was seven such pairs for 1 mu m. The smallest half pitch was 59 nm. This method is capable of arbitrary pattern generation. The fabrication of a freestanding single-ended tuning fork resonator utilizing the introduced masking method is demonstrated.
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| 3. |
- Singh, Aadesh P., 1982, et al.
(författare)
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Hydrogen induced interface engineering in Fe2O3-TiO2 heterostructures for efficient charge separation for solar-driven water oxidation in photoelectrochemical cells
- 2021
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 11:8, s. 4297-4307
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Tidskriftsartikel (refereegranskat)abstract
- Semiconductor heterostructure junctions are known to improve the water oxidation performance in photoelectrochemical (PEC) cells. Depending on the semiconductor materials involved, different kinds of junctions can appear, for instance, type II band alignment where the conduction and valence bands of the semiconductor materials are staggered with respect to each other. This band alignment allows for a charge separation of the photogenerated electron-hole pairs, where the holes will go from low-to-high valance band levels and vice versa for the electrons. For this reason, interface engineering has attracted intensive attention in recent years. In this work, a simplified model of the Fe2O3-TiO2 heterostructure was investigated via first-principles calculations. The results show that Fe2O3-TiO2 produces a type I band alignment in the heterojunction, which is detrimental to the water oxidation reaction. However, the results also show that interstitial hydrogens are energetically allowed in TiO2 and that they introduce states above the valance band, which can assist in the transfer of holes through the TiO2 layer. In response, well-defined planar Fe2O3-TiO2 heterostructures were manufactured, and measurements confirm the formation of a type I band alignment in the case of Fe2O3-TiO2, with very low photocurrent density as a result. However, once TiO2 was subjected to hydrogen treatment, there was a nine times higher photocurrent density at 1.50 V vs. the reversible hydrogen electrode under 1 sun illumination as compared to the original heterostructured photoanode. Via optical absorption, XPS analysis, and (photo)electrochemical measurements, it is clear that hydrogen treated TiO2 results in a type II band alignment in the Fe2O3-H:TiO2 heterostructure. This work is an example of how hydrogen doping in TiO2 can tailor the band alignment in TiO2-Fe2O3 heterostructures. As such, it provides valuable insights for the further development of similar material combinations. This journal is
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| 4. |
- Singh, Aadesh P., 1982, et al.
(författare)
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Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case
- 2020
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 10:55, s. 33307-33316
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Tidskriftsartikel (refereegranskat)abstract
- Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. The challenge is to develop corrosion resistant and chemically stable semiconductors that absorb sunlight in the visible region and, at the same time, have the band edges matching with the redox level of water. In this work, hematite (alpha-Fe2O3) thin films were prepared onto an indium-doped tin oxide (ITO; In:SnO2) substrate by e-beam evaporation of Fe, followed by air annealing at two different temperatures: 350 and 500 degrees C. The samples annealed at 500 degrees C show anin situdiffusion of indium from the ITO substrate to the surface of alpha-Fe2O3, where it acts as a dopant and enhances the photoelectrochemical properties of hematite. Structural, optical, chemical and photoelectrochemical analysis reveal that the diffusion of In at 500 degrees C enhances the optical absorption, increases the electrode-electrolyte contact area by changing the surface topology, improves the carrier concentration and shifts the flat band potential in the cathodic direction. Further enhancement in photocurrent density was observed byex situdiffusion of Ti, deposited in the form of nanodisks, from the top surface to the bulk. Thein situIn diffused alpha-Fe(2)O(3)photoanode exhibits an improved photoelectrochemical performance, with a photocurrent density of 145 mu A cm(-2)at 1.23 V-RHE, compared to 37 mu A cm(-2)for the photoanode prepared at 350 degrees C; it also decreases the photocurrent onset potential from 1.13 V to 1.09 V. However, the In/Ti co-doped sample exhibits an even higher photocurrent density of 290 mu A cm(-2)at 1.23 V(RHE)and the photocurrent onset potential decreases to 0.93 V-RHE, which is attributed to the additional doping and to the surface becoming more favorable to charge separation.
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