Earth Sciences & Env.
Engineering & Tech.
Information & Comm. Tech.
Life Sciences & Biotech
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.
We aimed to carry out ab-initio calculations (in the framework of the Density Functional Theory) on silicon-germanium nanowires (SiGe NWs) doped with boron (B) and phosphorous (P) impurities and oriented along the  direction. The main motivation of the project was related to the crucial and decisive role that doping has in all the technological and device applications of SiGe NWs [1,2] and to the paucity of theoretical efforts on doped SiGe NWs’ s structural, electronic and transport properties. The main objectives of the project were the following: i) A complete comprehension of the role of B and P impurities on the structural and geometrical stability of the SiGe NWs, with particular emphasis on the different structural changes that B and P atoms can induce in different region of the wire (i.e. Si or Ge region). ii) To obtain a complete description about the thermodynamical stability of the single-doped and co-doped structures in order to give useful informations for the synthesis of these type of wires. This objective can be reached by calculating the Formation Enthalpy of the NWs and evaluating how this quantity is affected by the size’s wire, by the position of the impurities in the wire, by the relative distance impurity-impurity in the unit cell. iii) To obtain a complete description of role of doping and co-doping on the electronic properties of the wires; this can be reached by calculating band structure and density of states of the single-doped and co-doped NWs and comparing them with the undoped ones. iv) To analyze the nature of spatial wave function localization in order to show the possible separation between the impurity and the donor carrier. v) To calculate, through the Landauer approach, the transport properties of the NWs, pointing out how much crucial is the role of the impurity on the electronic mobility. We intended to perform all the calculations with the SIESTA code for structural and electronic calculations and with the TRANSIESTA code for the transport calculations, in particular the optimized version especially reengineered for the massively parallel machines of the Barcelona Supercomputing Centre (BSC), which could have permitted us to satisfy our computational requirements. To perform simulations which can be more close to the reality of the experiment our purpose was to study NWs with diameters ranging from 1.6 nm to 3 nm, that means systems with a unit cell containing nearly two hundreds atoms; we knew that this aspect would have required a very huge computational demand, that only with a high performance computing facility could have been satisfied. Moreover in order to analyze the effect of doping for these type of systems, we have intended to perform calculations with a unit cell that is four, five or six times the unit cell of the undoped case; this choice was made in order to minimize all the possible interactions between the impurities and to try to make the concentration of the impurity more close to the experiment; moreover, for each SiGe NWs, our aim was to study the effect of B-doping, P-doping and co-doping in the Si region and in the Ge region of the wire. This aspect should have increased the number of configurations on which performing calculations, making this part of the work the most demanding from a computational point of view; infact the considered unit cells were made of more than six hundreds atoms, which means that we should have used a very large number of processors in order to reduce as much as possible the CPU time. We applied for a six weeks HPC-Europa2 visit, which has been split in two shorter ones, in order to satisfy the demands of the two groups involved.
References:  J. Xiang et al., Nature 441, 489 (2005);  L. Lauhon et al., Nature 420, 57 (2002)
We can state that the main objectives of the project have been successfully reached. We have performed first-principles calculations (based on Density Functional Theory and using the SIESTA code) concerning SiGe nanowires (SiGe NWs), doped with B and P impurities, oriented along the  direction and with diameters of 1.6 and 2.4 nm. We have focused on the core-shell geometry which recently has been the most interesting for all the technological and device applications of SiGe NWs. In particular we have analyzed how the effect of an impurity can modify the structural and electronic properties of a core-shell SiGe NW. The analysis of the structural properties of a B and P doped SiGe NW has been carried out analyzing the energetics of an impurity substitution in the core and in the shell of Sicore/Geshell (Si/Ge) and Gecore/Sishell (Ge/Si) NWs and evaluating the formation energy for each case. Since the the definition of formation energy (FE) of an impurity for a coaxial nanowire is not straightforward and there is a paucity of theoretical efforts on this argument, we have deeply studied the phenomena developing a new theoretical formulation for FE of doped core-shell SiGe NWs, starting from the generalization of Zhang and Northrup formula for reduced dimensionality system. Our results show that given a certain coaxial structure (Ge/Si or Si/Ge) the impurities prefer to occupy lattice sites located into the wire shell, pointing out the central role of geometry for this type of quantity. The only exception to this behavior is the P-doping of Si/Ge, where the Si-core doping is favored, therefore favoring a particular type of chemical bonding (Si-P bonds) with respect to the geometry of the system. Maybe the origin of this result can be ascribed to the fact that for the FE of a P impurity into a pure Si NW is smaller than the one in a pure Ge NW. As regards as the electronic properties we have analyzed B and P doping into the core and into the shell for both the types of core-shell NWs. In particular we have analyzed how the impurity type (B or P) and position (core or shell) can modify band structure and wave function localization given a certain coaxial structure. The results of our calculations show how, in contrast with B and P doping of pure Si and pure Ge NWs (for which several studies have demonstrated that the doping mechanism is inefficient at very small diameters), an efficient n-type and p-type doping can be reached in the case of core-shell SiGe NWs; this very interesting result is a consequence of the type II band offset between Si and Ge, which implies that valence states are on the germanium part of the wire while conduction states are on the silicon one. In particular in the case of Ge/Si NW with a P impurity into the core the impurity level falls inside the conduction band, yielding an electron at its bottom. This result is related to the type II band-offset that comes out at Si/Ge interface. In a pure Ge NW of the same size of the core, the impurity level would have been deep into the band gap and very difficult to activate at typical device temperatures. In this case instead, analyzing the wave function localization, we have found that the bottom of conduction band is on silicon-shell, which are below the impurity and all the germanium-core states; it means that the impurity does not need to be thermally activated. This indicates the formation of a one-dimensional electron gas that can have relevant importance for device applications. Other interesting case is the configuration with a B atom into the silicon shell of a Ge/Si NW. Again the impurity level is deep into the valence states and we have the formation of one-dimensional hole gas. These types of results can be easily extended to the case of Si/Ge NWs. The demonstration of an efficient doping for these types of wires can open substantial opportunities for the understanding doping mechanism at nanoscale and for improving its technological application.
Two-dimensional (2D) turbulence is characterized by the conservation of two quadratic invariants, energy and enstrophy. Unlike classical three-dimensional turbulence where only energy is conserved, the extra invariant implies that there exists two inertial ranges with energy flowing to large scales and enstrophy to small scales. When considered in a finite box, energy is forced to accumulate at the largest scale, i.e. at the scale of the box-size.
If the system is continuously forced, then a large scale coherent structure or condensate can form which contains the majority of the energy in the system. In a doubly periodic box, this condensate consists of either a vortex dipole or zonal flow. Little is known how the appearance of a condensate effects the turbulence in the system. Some preliminary results on the flow profile of the condensate in the form of a vortex dipole have already been analysed, however there is still much more than needs to be considered.
Our objectives are to investigate how the appearance of a condensate effects the turbulent flow. We do this by performing high resolution numerical simulations of the 2D Naiver-Stokes equations and to examine the fluxes of energy and momentum in physical space using single-point, second and third order, velocity moments to gain insight into how turbulence leads to the formation and growth of a condensate. Then we will consider the two-point velocity moments to study the behavior of the energy flux in Fourier space and consequently the turbulent spectra. Subsequently, we will be able to study the degree of anisotropy in the turbulence with respect to spatial coordinate and scale associated with the presence of a condensate.
So far we have been able to perform several high performance numerical simulations of the 2D Naiver-Stokes equations in various parameter regimes. We have been able to compare the second and third order, single-point, velocity moments between the numerical simulations and theoretical predictions. This has enabled us to verify the behavior of these statistics with parameter variability and, more importantly, uncover new physics related to the interaction between coherent structures and the turbulent background. For instance, we have discovered that the power-law profile of the vortex dipole is robust with regards to different dissipation (viscous and hyper-viscous) mechanisms at small scales, although the vortex core radius is not. Moreover, we find that the vorticity concentrated in the coherent structure scales with the inverse square root of the friction dissipation coefficient.
After the discovery that the conjugated polymers in doped form can be very good organic conductors, they became one of the most studied targets in the recent decades, with interesting and useful possible applications. A challenging representative of the conjugated polymers is polyaniline (PA). A number of reasons make PA topical: cheap monomer, facile and high-yield polymerization, prominent stability of the polymer. PA has potential to be utilized in electrochromic windows, gas sensors, light-emitting diodes, antistatic coatings , solar cells . Doping with a protonic acid can convert polyaniline from a semi-conductor - emeraldine base, to a conductor - emeraldine salt (ES). Such doping includes counterions (the ions, which neutralize the charge resulting from the chain protonation) in the polymer matrix. The process induces substantial changes in the structural, electric, magnetic and optical properties of the polymer. ESR experiments  demonstrate that emeraldine salt can exist in two forms differing in their magnetic behavior - paramagnetic (polaronic) and magnetically inactive (bipolaronic). Elucidation of their nature as well as the structural, charge and spin (for the polarons) density changes invoked by their formation, and their relative stability can be achieved at the molecular level using the methods of quantum chemistry [4, 5]. Thus, the object of the current study is the emeraldine salt. The bipolaronic form is modeled as singlet and the polaronic one – in the highest spin state (triplet for a single tetramer, quintet for a stack of two tetramer chains). The two forms also differ in the position of the counterions with respect to the chain. A recent study  indicates the important role the water molecules play for the chains ordering and for PA conductivity. This effect can be explored by means of molecular simulations, too.
The main goal of the current study is the investigation of the influence of environmental effects (water molecules and neighboring chains) on the structural, electronic and spin characteristics of the HCl-doped emeraldine salt at the ab initio level. Such calculations will bring the model systems closer to the real one.
. D. Bowman, B. R. Mattes, Synth. Metals 2005, 154, 29.
. Z. Liu, J. Zhou, H. Xue, L. Shen, H. Zang, W. Chen, Synth. Metals 2006, 156, 721.
. P. K. Kahol, A. Raghunathan, B. J. McCormick, Synth. Metals 2004, 140, 261.
. J. Petrova, J. Romanova, G. Madjarova, A. Ivanova, A. Tadjer Prog. Theor. Chem. Phys. 2009, 20, 219.
. J. Petrova, J. Romanova, G. Madjarova, A. Ivanova, A. Tadjer J. Phys. Chem. B 2011, 115, 3765.
. N. Gospodinova, D. Ivanov, D. Anokhin, I. Mihai, L. Vidal, S. Brun, J. Romanova, A. Tadjer, Macromol. Rapid Commun. 2008, 30, 29.
In order to accomplish the aim, first it is necessary to outline a computational protocol comprising an appropriate alternative to the accurate but time- and resource-consuming CCSD(T) and MP2 methods. At this stage the smallest representative oligomer of PA – the tetramer – in the polaronic and bipolaronic form was used. In addition, a number of explicit water molecules forming the first solvation shell of the counterions (chloride ions) were included. Thanks to the access to high throughput facilities, in particular to CINES, provided by the HPC-Europa2 program, a broad range of functionals – M05, M06, M06HF, M062x, M06L, PBE0, PBEh, LC-wPBE, wB97XD, B3LYP, B3LYP-D, BLYP, LC-BLYP, BLYP-D, was tested. The influence of the basis set on the results was estimated, too. The data for structure – bond length alternation, torsion angles along the chain, number and type of hydrogen bonds, structure of the water molecules around the counterions and the chain, and charge density distribution, were compared.
The interaction of one/two water molecules with every type of nitrogen along the tetramer was assessed. Different positions of water with respect to the nitrogen atom were considered. The binding energy was estimated. According to the results for the energy, the oligomer – water configurations for the next calculations were constructed. These consisted of three water molecules – one for the nitrogen in the middle of the chain and two for the terminal nitrogen atom.
Next, the polaron and bipolaron tetramer with three water molecules were optimized. Clusters of two tetramers in all possible mutual alignments (p-stacked and/or lateral) in singlet and quintet forms were built from the latter structures. In summary, the effect of intermolecular interactions on the structure and charge distribution was evaluated and the most energetically favorable mutual orientation of the two chains was determined.