My primary interest is in applying quantum mechanical and chemical methods within the field of electron transport calculations in mesoscopic systems. My work aims at understanding the electronic structure of nano materials and establishing a link between structures and properties. With this in mind, I employ tools and methods of theoretical and computational chemistry and physics, such as Density Functional Theory (DFT), Non-Equlibrium Green's Function (NEGF) formalism and numerous ab initio techniques. I am also interested in performing calculations to explore possible new materials for solar cells, batteries and non-CMOS devices. I have a broad interest in drug design and modeling, transition-metal and organometallic chemistry, molecule sensors and electronics. A guiding principle in my work is that theory and calculations should be used in synergy with experiments, addressing topical problems and providing insight that informs experimental practice.
Electron transport in mesoscopic systems
Transition-metal and organometallic chemistry
'Excellent computer simulations are done for a purpose. The most valid purposes are to explore uncharted territory, to resolve a well-posed scientific or technical question, or to make a good design choice.'
Recent grants worked on
NEGF simulator development
The project is based on research in NEGF quantum transport simulation technology and its incorporation in efficient 3D simulation tools. The work includes improvement of efficiency of 3D NEGF simulation using couple mod simulation techniques and incorporation of the quantum transport simulation module in the 3D Glasgow 'atomistic' simulator. The emphasis is on the improved speed and convergence of trapped charges and interface roughness. Nanowire and FinFET transistors are considered.
Molecular-Metal-Oxide-nanoelectronicS (M-MOS): Achieving the Molecular Limit
The main aim of the project is to demonstrate functional circuits using molecular metal-oxides (MMOs), connecting self-assembled MMOs into top-down, lithographically defined CMOS architectures with the ultimate aim of achieving the molecular limit in data storage and processing: i.e., realising inorganic, single molecule transistors. This project is highly creative and adventurous, proposing that inorganic molecules could be reliably used in the fabrication of nano-electronic devices that take advantage of the intrinsic electronic properties of molecules as switchable molecular semiconductors.
2007 - 2011
Electronic structure/function relationships inmetal nanowires: components for molecularelectronics
The project revolves around understanding the link between electronic structure and electron transport properties of extended metal atom chain (EMAC). These complexes are built from helical array of oligo-α-pyridyl ligands which is used to support a frame of metal centres. These structures have been the subject of a protracted debate in the inorganic chemistry community due to their polymorphism – they exist is symmetric and unsymmetic forms. My current objective is to relate the fundamental electronic structure of these EMAC complexes to their conductance as measured, for example, by STM. Ultimately, an understanding of these phenomena will be essential to the development of new computer architectures based on molecular-scale components.
Vihar P. Georgiev, Ewan A. Towie, Asen Asenov, IEEE Transaction on Electron Devices, 2013, 60 (3), 965-971
In this paper, we report the first systematic study of quantum transport simulation of the impact of precisely positioned dopants on the performance of ultimately scaled gate-all-around silicon nanowire transistors (NWTs) designed for digital circuit applications. Due to strong inhomogeneity of the self-consistent electrostatic potential, a full 3-D real-space nonequilibrium Green function formalism is used. The simulations are carried out for an n-channel NWT with 2.2 x 2.2 nm2 cross section and 6-nm channel length, where the locations of the precisely arranged dopants in the source–drain extensions and in the channel region have been varied. The individual dopants act as localized scatters, and hence, impact of the electron transport is directly correlated to the position of the single dopants. As a result, a large variation in the on-current and a modest variation of the subthreshold slope are observed in the ID–VG characteristics when comparing devices with microscopically different discrete dopant configurations. The variations of the current–voltage characteristics are analyzed with reference to the behavior of the transmission coefficients.
Vihar P. Georgiev, P. J. Mohan, Daniel DeBrincat, John E. McGrady, Coordination Chemistry Reviews, 2013, 1 (257), 290-298
The extended metal atoms chains (EMACs) consitute a diverse group of molecules differing in both length and identity of the constituent metal ions. They therefore present important challenges to our understanding of metal-metal bonding and its relationship to structural, magnetic and electron transport properties. In this review we summarise our current understanding of the origins of low-symmetry distortions in these chains and their impact on current flow through the molecule. We conclude that structure, magnetochemistry and resistance are all intimately related through a shared dependence on the energies and orientations of the frontier orbitals.
Vihar P. Georgiev, W. M. C. Sameera, John E. McGrady, Phys. Chem. C , 2012,116 (38), pp 20163–20172
Density functional theory, in conjunction with non-equilibrium Green’s functions, is used to explore the attenuation of the resistance of Cox wires along the series Co3(dpa)4(NCS)2, Co5(tpda)4(NCS)2, Co7(teptra)4(NCS)2. At very low bias (0 < V < 25 mV) the conductance, G, decreases in the order G(Co3) > G(Co5) > G(Co7), consistent with experiment and with an anticipated inverse relationship between conductance and chain length. At higher voltages, however, the current-voltage responses of all three are striking non-linear, and above 50 mV G(Co5) > G(Co3) > G(Co7). The very different behavior of the members of this homologous series can be traced to the different symmetries and multiplicities of their respective ground states, which in turn control the properties of the dominant transport channels.
P. J. Mohan, Vihar P. Georgiev, John E. McGrady, Chem. Sci., 2012, 3, 1319-1329.
Density functional theory, in conjunction with non-equilibrium Green’s functions, is used to reconcile the structural, magnetic and electron transport properties of a triruthenium extended metal atom chain, Ru3(dpa)4(NCS)2. The distinct bending of the Ru-Ru-Ru core in this species is traced to strong second order mixing between levels of σ and π symmetry that are near degenerate in the linear geometry. The dominant electron transport channel is formed by the LUMO, an orbital of π* symmetry that lies just above the Fermi level of the gold electrode. The bending has a substantial impact on electron transport in that it induces a spin crossover from a quintet to a singlet which in turn brings the LUMO much closer to the Fermi level. The presence of significant net π bonding in the metal chains also broadens the π/πnb/π* manifold, such that the channel is not strongly perturbed by the electric field, even at a bias of 1.0 V. The presence of a robust π symmetry conduction channel marks the triruthenium systems out as quite distinct from its first row counterparts, Cr3(dpa)4(NCS)2 and Co3(dpa)4(NCS)2, where current flows primarily through the σ framework.
Vitesh Mistry, Vihar P. Georgiev, John E. McGrady, Comptes Rendus Chimie, 2012, 15(2-3), 176 - 183.
Density functional theory in conjunction with non-equilibrium Green's functions is used to explore the electron transport properties of a series of molecules based on the face-shared bioctahedral (M2Cl9) motif. The metal-metal bond orders in the chosen molecules, [Rh2Cl9]3–, [Ru2Cl9]3– and [Mo2Cl9]3– vary from 0 (Rh) to 1 (Ru) and 3 (Mo), and the calculations indicate that there is a direct correlation between conductance and bond order. The [Mo2Cl9]3– case is particularly interesting as it is well known from crystallographic studies to be very flexible, the Mo–Mo bond length varying over a range of ∼0.35 Å depending on cation. The upper limit of this range marks the point where homolytic cleavage of the δπ components of the triple bond is complete, and this has a marked impact on electron transport. The localization of the metal-based orbitals means that those on the left (source) and right (drain) sides respond very differently to applied bias, giving rise to resonance effects at particular bias voltages, and hence to negative differential resistance effects.
Vihar P. Georgiev, John E. McGrady, J. Am. Chem. Soc., 2011, 133(32), 12590–12599.
In the field of molecular electronics, an intimate link between the delocalization of molecular orbitals and their ability to support current flow is often assumed. Delocalization, in turn, is generally regarded as being synonymous with structural symmetry, for example, in the lengths of the bonds along a molecular wire. In this work, we use density functional theory in combination with nonequilibrium Green’s functions to show that precisely the opposite is true in the extended metal atom chain Cr3(dpa)4(NCS)2 where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr3 core do indeed reduce the effectiveness of these π channels, but this is largely irrelevant to electron transport at low bias simply because they lie far below the Fermi level. Instead, the dominant pathway is through higher-lying orbitals of σ symmetry, which remain essentially unperturbed by even quite substantial distortions. In fact, the conductance is actually increased marginally because the σnb channel is displaced upward toward the Fermi level. These calculations indicate a subtle and counterintuitive relationship between structure and function in these metal chains that has important implications for the interpretation of data emerging from scanning tunnelling and atomic force microscopy experiments.
Vihar P. Georgiev and John. E. McGrady, Inorg. Chem.,2010, 49(12), 5591- 5597.
Density functional theory in conjunction with nonequilibrium Green’s functions has been used to explore charge transport through the cobalt-based extended metal atom chain, Co3(dpa)4(NCS)2. The isolated molecule has a doublet ground state, and the singly occupied σ nonbonding orbital proves to be the dominant transport channel, providing spin filtering efficiencies in excess of 90%. The metal chain differs from typical organic conductors in that the π orbitals that form the contact with the gold electrode are orthogonal to the transport channel. As a result, the rehybridization of these π levels by the applied electric field has only a minor impact on the current, allowing spin filtering to persist even at biases in excess of 1 V.
M. Velinova, V. Georgiev, T. Todorova, G. Madjarova, A. Ivanova, A. Tadjer, Journal of Molecular Structure: THEOCHEM, 2010, 955, 97–108.
The aim of this theoretical study is the investigation of hydrocarbon stability as a function of BN- and B-substitution degree. The targets are anthracene and phenanthrene because they resemble the zig-zag and armchair peripheries of graphene sheets. The computations are made at the RHF and MP2 level with various basis sets. The conclusions drawn are based on the results from calculations for more than 600 different structures created in a systematic way where pairs of carbon atoms are replaced by the respective number of B and N, or solely B-atoms. A common feature of all stable substituted molecules is the tendency for least fragmentation of the carbonskeleton, particularly, for preservation of maximum number of intact carbon aromatic cycles. The major dissimilarity between the BN- and B-substituted molecules are the preferred doping patterns and positions: the favorable structures feature alternation of B and N in the BN-derivatives and disjoint borons separated by C2 fragments in the B-ones; the nitrogen atoms substitute predominantly secondary carbons, while borons prefer tertiary sites in the BN-substituted hydrocarbons and secondary positions in the B-modified structures. Overall, the anthracene isomers are more stable. The energy and AO contributions to the LUMOs of the studied species are rationalized in terms of electron-accepting properties of the latter, estimating their potential performance as oxidants. Energy depression of LUMOs is achieved mainly in B-substituted hydrocarbons due to large coefficients on the borons causing pronounced delocalization of the orbital. The findings can be used for directed synthesis of novel modified carbon-based materials for practical purposes.
F. Dietz, K. Mullen, M. Baumgarten, V. Georgiev, N. Tyutyulkov, Chem.Phys.Letters, 2004, 389, 135-139.
The magnetic properties of a class of 1-D organic polymers with stable 2-azaphenalenyl radicals within the elementary units have been investigated theoretically. The nature and magnitude of the magnetic coupling between the electrons within the half-filled band (HFB) of the polymers were determined using the band theory. In all cases the π-electrons within the HFB are ferromagnetically coupled with the main contribution of the Coulomb- and indirect exchange to the effective Heisenberg integral.