Tyndall National Institute

Cork, Ireland

Tyndall National Institute

Cork, Ireland

Tyndall National Institute at UCC in Cork, Ireland named for John Tyndall, scientist, is one of Europe's leading research centres, specialising in ICT hardware research, commercialisation of technology and the education of next generation researchers. Tyndall has over 450 researchers, engineers, students and support staff focused on research and the commercialisation of technology through industry collaboration. Tyndall’s research spans a range of technologies from atoms to systems in the areas of photonics, microsystems and micro-nanoelectronics and addresses key challenges in the areas of Communications, Energy, Health and the Environment. Queen Elizabeth II visited the research centre as part of her state visit to Ireland on 20 May 2011. Wikipedia.

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In recent experiments Tada et al. have shown that TiO 2 surfaces modified with iron oxide display visible light photocatalytic activity. This paper presents first principles simulations of iron oxide clusters adsorbed at the rutile TiO 2 (110) surface to elucidate the origin of the visible light photocatalytic activity of iron oxide modified TiO 2. Small iron oxide clusters adsorb at rutile (110) surface and their presence shifts the valence band so that the band gap of the composite is narrowed towards the visible, thus confirming the origin of the visible light activity of this composite material. The presence of iron oxide at the TiO 2 surface leads to charge separation, which is the origin of enhanced photocatalytic efficiency, consistent with experimental photoluminesence and photocurrent data. Surface modification of a metal oxide is thus an interesting route in the development of visible light photocatalytic materials. © the Owner Societies 2011.

Usman M.,Tyndall National Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The demonstration of isotropic polarization response from semiconductor quantum dots (QDs) is a crucial step towards the design of several optoelectronic technologies. Among many parameters that impact the degree of polarization (DOP [n - ]) of a QD system, the shape asymmetry is a critical factor. We perform multi-million-atom simulations to study the impact of the elliptical shapes on the electronic and polarization properties of single and vertically stacked InAs QDs. The comparison between a low aspect ratio (AR) and a high AR QD reveals that the electronic and the polarization properties strongly depend on the AR of the QD; the elongation of a tall QD allows tuning of the DOP [n - ] over a much wider range. We then extend our analysis to an experimentally reported vertical stack of nine QDs (9-VSQDs) that has shown significant potential to achieve isotropic polarization properties. We analyze the contribution from the shape asymmetry in the large, experimentally measured, in-plane polarization anisotropy. Our analysis shows that the orientation of the base elongation controls the sign of the DOP [n - ]; however, the magnitude of the base elongation has only a very little impact on the magnitude of the DOP [n - ]. We further predict that the elliptical shape of the 9-VSQDs can only tune either DOP [110] or DOP [1̄10] for the isotropic response. Our model results, in agreement with the TEM findings, suggest that the experimentally grown 9-VSQDs has either a circular or a slightly [1̄10] elongated base. Overall, the detailed investigation of DOP [n - ] as a function of the QD shape asymmetry provides a theoretical guidance for the continuing experimental efforts to achieve tailored polarization properties from QD nanostructures for the design of optical devices. © 2012 American Physical Society.

Elliott S.D.,Tyndall National Institute
Langmuir | Year: 2010

Wepresent mechanisms for atomic layer deposition of Ru, Rh, Pd, Os, Ir, or Pt metal from homoleptic precursors and oxygen. The novel mechanistic feature is that combustion of ligands produces transient hydroxyl groups on the surface, which can undergo Brønsted-type elimination of a further ligand or water from the surface. Each ligand therefore releases one electron for reduction of the metal. The growth reaction may be described as oxide-catalyzed redox decomposition of the precursor. To validate the mechanism against experiment, we derive analytical expressions for product ratios and the growth rate in terms of saturating coverages. © 2010 American Chemical Society.

O'Brien S.,Tyndall National Institute
Optics Letters | Year: 2014

We present a design method and numerical results describing the construction of distributed feedback grating filters that support discrete combs of transmission resonances. These filter designs define open superstructure grating resonators with transmission channels that can be placed at predetermined frequencies, such as those defined by the wavelength division multiplexing grid or by a secondary frequency comb source. Focusing on a specific example with 40 GHz channel spacing, we optimize an active structure that defines three low-threshold lasing modes. How our design approach relates to filter synthesis techniques based on cascaded grating resonators is also discussed. © 2014 Optical Society of America.

Zhao J.,Tyndall National Institute
Optics Express | Year: 2014

We experimentally demonstrate and numerically investigate a discrete-Fourier-transform (DFT) based offset quadrature-amplitudemodulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) system. We investigate the scheme using a set of square-rootraised-cosine functions and a set of super-Gaussian functions as signal spectra. It is shown that offset-QAM OFDM exhibits negligible penalty for all investigated spectra, in contrast to rectangular-function based Nyquist FDM (N-FDM) and sinc-function based conventional OFDM (C-OFDM). The required guard interval (GI) length for dispersion compensation in offset-QAM OFDM is analyzed and shown to scale with twice the subcarrier spacing rather than the full OFDM bandwidth. Experimental results show that 38-Gb/s offset-16QAM OFDM supports 600-km fiber transmission with negligible penalty in the absence of GI while a GI length of eight is required in C-OFDM. Further numerical simulations show that by avoiding the GI, 112-Gb/s polarization multiplexed offset-4QAM OFDM can achieve 23% increase in net data rate over C-OFDM under the same transmission reach. We also discuss the design of the pulse-shaping filter in the DFT-based implementation and show that when compared to NFDM, the required memory length of the filter for pulse shaping can be reduced from 60 to 2 in offset-QAM OFDM regardless of the fiber length. © 2014 Optical Society of America.

Nolan M.,Tyndall National Institute
Chemical Physics Letters | Year: 2010

In ceria the formation of oxygen vacancies plays a key role. Density functional theory, corrected for on-site Coulomb interactions (DFT + U) provides a reasonable description of oxygen vacancies, but has issues with the U dependence. We present a hybrid HSE06 study of oxygen vacancies in the (1 1 0) and (1 0 0) ceria surfaces. We find (i) the oxygen vacancy formation energy is larger with hybrid DFT compared with DFT + U, (ii) localised Ce3+ ions form and (iii) the position of the Ce3+ gap state is in good agreement with experiment. Our results provide important information for assessing the reliability of the DFT + U approach. © 2010 Elsevier B.V. All rights reserved.

Nolan M.,Tyndall National Institute
Journal of Materials Chemistry | Year: 2011

With increasing interest in new catalytic materials based on doping of cerium dioxide with other metal cations, it is necessary to have an atomic level understanding of the factors that impact on the structural and electronic properties of doped ceria as well as its reactivity. We present in this paper simulations of the ceria (111) and (110) surfaces doped with divalent cations Pd and Ni using density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U) and hybrid DFT (using the screened exchange HSE06 functional). Structural distortions due to doping are strong in both surfaces and the most stable structure for both dopants arises through compensation of the dopant +2 valence through oxygen vacancy formation. Both dopants also lower the formation energy of the active oxygen vacancy in each surface, confirming the potential for these dopants to be used in ceria based materials for catalysis or solid oxide fuel cells, where the oxygen vacancy formation energy is important. When comparing DFT + U and hybrid DFT, although the qualitative descriptions provided by both DFT approaches are similar, we do find that the energetics of oxygen vacancy formation are quantitatively different and the importance of this point is discussed. © 2011 The Royal Society of Chemistry.

Nolan M.,Tyndall National Institute
Chemical Communications | Year: 2011

Density functional theory simulations show that modifying rutile TiO 2 with metal oxide nanoclusters produces composite materials with potential visible light photocatalytic activity. © The Royal Society of Chemistry 2011.

Titanium dioxide is an important and widely studied photocatalytic material, but to achieve photocatalytic activity under visible-light absorption, it needs to have a narrower band gap and reduced charge carrier recombination. First-principles simulations are presented in this paper to show that heterostructures of rutile TiO2 modified with nanoclusters of MgO and Ga2O3 will be new photocatalytically active materials in the UV (MgO-TiO2) and visible (Ga2O3-TiO 2) regions of the solar spectrum. In particular, our investigations of a model of the excited state of the heterostructures demonstrate that upon light excitation electrons and holes can be separated onto the TiO2 surface and the metal oxide nanocluster, which will reduce charge recombination and improve photocatalytic activity. For MgO-modified TiO2, no significant band gap change is predicted, but for Ga2O 3-modified TiO2, we predict a band gap change of up to 0.6 eV, which is sufficient to induce visible light absorption. Comparisons with unmodified TiO2 and other TiO2-based photocatalyst structures are presented. © 2012 American Chemical Society.

Nolan M.,Tyndall National Institute
Journal of Physical Chemistry C | Year: 2011

In this paper, we use density functional theory corrected for on-site Coulomb interactions (DFT + U) and hybrid DFT (HSE06 functional) to study the defects formed when the ceria (110) surface is doped with a series of trivalent dopants, namely, Al 3+, Sc 3+, Y 3+, and In 3+. Using the hybrid DFT HSE06 exchange-correlation functional as a benchmark, we show that doping the (110) surface with a single trivalent ion leads to formation of a localized M Ce / + O O • (M = the 3+ dopant), O - hole state, confirming the description found with DFT + U. We use DFT + U to investigate the energetics of dopant compensation through formation of the 2M Ce ′ +V O ̈ defect, that is, compensation of two dopants with an oxygen vacancy. In conjunction with earlier work on La-doped CeO 2, we find that the stability of the compensating anion vacancy depends on the dopant ionic radius. For Al 3+, which has the smallest ionic radius, and Sc 3+ and In 3+, with intermediate ionic radii, formation of a compensating oxygen vacancy is stable. On the other hand, the Y 3+ dopant, with an ionic radius close to that of Ce 4+, shows a positive anion vacancy formation energy, as does La 3+, which is larger than Ce 4+ (J. Phys.: Condens. Matter 2010, 20, 135004). When considering the resulting electronic structure, in Al 3+ doping, oxygen hole compensation is found. However, Sc 3+, In 3+, and Y 3+ show the formation of a reduced Ce 3+ cation and an uncompensated oxygen hole, similar to La 3+. These results suggest that the ionic radius of trivalent dopants strongly influences the final defect formed when doping ceria with 3+ cations. In light of these findings, experimental investigations of these systems will be welcome. © 2011 American Chemical Society.

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