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Use this search facility to find out more about the profile of our HPC-Europa2 visitors, the type of work they have been doing, and their project achievements.
Combining organic species with gold, or transition metals in general, usually has a substantial impact on the optical, electronic, and magnetic properties of the compound. Trinuclear complexes of coinage metals have been found to posses interesting photochemical properties. Cyclic trinuclear Au(I) complexes are especially intriguing, as they exhibit bright phosphorescence at room temperature, tuneable across the whole visible spectrum, charge-transfer properties, and are sensitive to multiple stimuli. They are the main focus of this project. The specific systems of this project will be the stacked, cyclic trinucelar Au(I) carbeniate complexes, recently synthesised and experimentally characterised at the University of North Texas. The compounds have been found to potentially provide the broad emission bands necessary for the production of white light. For this highly coveted feature, a careful tuning of the organic ligands is necessary. Further, the emission and excitation spectra of the complexes have been found to crucially depend on the packing arrangements of the species. The photophysical properties of these species depend heavily on intermolecular interactions, the most important of which is the aurophilic attraction. In order to first conclusively establish the correct structure of these complexes, a combination of experiment and high-level modelling is necessary. Establishing a reliable method for modelling this class of compounds, would enable in silico tuning of the properties. Thus, a main objective of this project is to uncover the theoretical methods capable of delivering the required accuracy. Studying gold and gold-containing complexes computationally is very challenging. Gold has many electrons, increasing the computational effort of simulating their correlated behaviour. The problem is further complicated by the significant effects arising from relativity. Density functional theory has thus been extensively used in the study of gold-containing species, due to the usually high quality:computational cost ratio. Standard DFT methods do not provide a sufficiently accurate description of the aurophilic attraction bonding gold atoms together, however. As shown by Johansson, Furche, and co-workers, already structural properties require functionals that include true non-local effects, that is, meta-generalised gradient approximations. Electronic excitations computed at DFT level also suffer from problems. These difficulties are reasonably well-known in the case of normal organic compounds, but very little is known of the performance of DFT when it comes to accurately describing excitations for complexes incorporating several heavy atoms. From a practical point-of-view, the most problematic short-coming of contemporary DFT is, arguably, the inability to describe charge-transfer excitations correctly. These types of excitations can be expected to be important in the trinuclear Au-complexes of the project. Thus, wave-function based methods need to be employed. The complexes to be studied are large, and even with supercomputing power, the choice of reliable quantum chemical methods that can be employed is limited. The approximate second-order coupled cluster method CC2 has recently been very cost-efficiently implemented, and will be one of the methods used in the study. The main work-horse of the study will, however, be the second-order algebraic diagram construction ADC(2) approximation. This method was recently thoroughly benchmarked and applied in the host group, and found to be a cost-effective alternative..
However efficiently implemented, computing excitations at correlated wave-function level is still restricted to a scalar relativistic approximation. Spin-orbit (SO) effects can however be large when the systems contain heavy metals. Therefore, excitation energies including spin-orbit coupling will be performed at DFT level.
The project got well underway, although due to exceptionally long queueing times, only a minor part of the planned calculations could be finished during the visit. The initial benchmark-type studies on smaller systems were succesfully completed, and a good understanding of the required level of theory for studying the gold complexes was obtained, however. Importantly, the collaboration between host and guest was crucial for obtaining the insight required for completing the project. The collaboration continues, as access to the supercomputing facilities is available also beyond the limits of the physical visit.
As a side-product of the visit, other projects where the interests of the host and guest coincide were uncovered, which lead to further collaborative projects, where the strengths of both groups combined are expected to lead to highly interesting science.
To develop models useful in many-body physics. While the model for solid HD is still under construction (a model including phonons, rotations, and their couplings is under way), here the question of polarized insulation in strongly correlated models was addressed via variational methods.
This project achieved the simple description of a metal-insulator transition (and metal-metal transitions away from half-filling) in which bound excitons play a key role. The metal-insulator transition in strongly correlated systems is in many ways still an open question. Variational wavefunctions (Baeriswyl and Baeriswyl-Gutzwiller projected wavefunctions) were studied via the well-known Gutzwiller ansatz applied in reciprocal space. The phase diagram was calculated resulting in a metal-insulator transition in which exciton binding plays the key role both in the metal-insulator transition at half-filling and the metal-metal transition away from half-filling. For the Baeriswyl-Gutzwiller projected wavefunction the transition occurs between a correlated metallic state of the Gutzwiller type and an insulator with bound excitons.
Hematite (α- Fe2O3) being, one of the most abundant low band gap semi conductor (~2.1eV) material on earth is well known to be an energetically appropriate material for the sustainable and carbon free solar energy conversion by means of water splitting . Though its low cost, non toxicity and stability over a wide range of pH made them promising, it’s poor charge transport properties still remain as a drawback. Recently, there have been reported a series of advances made by many research groups that can combat these challenges to a small extend.
Our aim is to improve its catalytic activity by using solid solution with some other catalytically active corundum type oxides like Cr2O3, Al2O3, etc. The Cr2O3 is not studied much from a perspective of photocatalysis. Recent experiments on Cr dopped Fe2O3 nanoparticles showed improved photocatalytic activity. The extremely high life time of excitons in the Ruby laser lead us to think about testing the variation lifetime of the Fe2O3-Cr2O3 solid solution by including a small percentage of Al2O3. In our laboratory, recently we synthesised a few compositions of Cr(1-x)Al(x)O3 and Fe(1-x)Cr(x)O3 mixed oxides with solution combustion method and the results are promising. As the preparation of the solid solutions within the entire composition spectrum is quite complex and time consuming, the results from DFT based computation is a better alternative and would lead the experiments towards the fruit full path of the photocatalytic hydrogen production with the expenditure of less time. The use many transition metals and all electron Gaussian basis sets would need much computational infrastructure, HPC- Europa program is known to be very successful in providing computational facilities needed for such large scale calculations.
The main objectives of the proposed project were
The second aim of the proposed visit was to initiate an intensive collaboration with the host research group lead by Dr. Vicente Timon Salinero (Departamento de Geoquímica Ambiental, Department of Environmental Geochemistry, Granada, Spain). It is well known that the phoyllosilicates are getting a key attention among the researchers due to their potential applications in wide range of science such as geosciences, chemistry, physics, materials science, and in agriculture. Recently, it is noted that the substitution of transition metals on the phyllosilicate system dramatically changes their electronic properties. Since these minerals are of low symmetry, the application of density functional theory, in order to explore these effects using Gaussian basis sets is quite challenging due to high computational expenses. We wrote a small project on the effect of increase in Fe- and Ti content on the electronic properties of Biotite mineral, and will utilize the infrastructure provided by the HPC- Europa project for successful completion of the proposed research work.
Large scale Density Functional Theory Based simulations have been performed with in the HPC background using the computational code CRYASTAL. An intensive collaboration with the host researcher helped me to achieve the aim of the study successfully. The main intention of the study was to explore the effect of impurity cations like Cr and Al on the electronic and charge transport properties of corundum structured hematite (α-Fe2O3 crystal).
We found out that doping of these cations have substantial effect on the electronic properties of the hematite system. In order to explore the full effects we have created a three unit supercell of α-Fe2O3 (with 12-Iron cations and 18 oxygen anions) within a homogeneity range and we created binary and ternary solid solutions of Fe2O3, Cr2O3, and Al2O3. We obtained the ground state electronic properties of the analyzed systems. We have synthesized a few compositions in the laboratory and compared their electronic properties with the data that we have obtained with the HPC- aided calculations.
Binary and ternary solid solutions of α- Fe2O3, Cr2O3, Al2O3 has been created by successive substitution of the Fe cations by Al and Cr cations in the stable antiferromagnetic composition. A full geometry relaxation has been performed for all the compounds. We found that the c/a ratio remains almost constant throughout the entire composition range. The volume of the single formula unit decreases slightly with increase in the aluminum content. This is due to the strain induced by aluminum and might result in improved charge transport properties. Regarding the electronic properties, in the case of ternary solid solution the band gap slightly decreases with increase in the Cr and Al content in the solid solution. This is in fair agreement with our experimental results. The conduction band minimum is nearly dispersion less due to the higher contribution from Fe-3d orbital.
The valence band maximum is a mixture of metal (Fe and Cr) 3d and oxygen 2p contributions. The contribution of Cr 3d orbital dominates the Fe -3d orbital. Due to the hybrid nature of the valence band, the band structure shows little bit more dispersion.
The binary solid solution of Fe2O3- Cr2O3 shows a decrease in band gap with the increase in Cr content and in the case of Fe2O3-Al2O3 solid solution the case is reverse and shows an increase in the band gap with the Al content as expected.
The second aim of my visit was to initiate a long term intensive collaboration with the host researcher. We have analyzed the effect of transition metal cations on the phyllosilicate mineral and the main part of the results shows that the with the increase in Fe- content (Biotite) there is an insulator to conductor transition in phyllosilicates.
Finally, the HPC- Europa2 programme was extremely useful in the successful completion of the project. We are thankful to the HPC group for providing us the facilities and computational time, which helped us to advance our research far from expected. The CRYSTAL code used in our calculation was perfectly parallelized within the HPC environment. A couple of scientific papers and proceedings on the results obtained from our calculations are on process.
We hope to get more computing time for simulating advanced and challenging multiferroics in the field of photocatalysis, in the next calls.