• First principles calculations of nanoscale phenomena (chemical reactions, molecular diffusion, vibrational dynamics and thermodynamics of nanomaterials).
• Multiscale-modeling of functional material.
• Structural stability and characteristics of nanoalloys.
• Atomistic studies of diffusion at solid surfaces.
• Thin film growth processes and surface morphological evolution of nanostructures.
Develop and implement strategies that foster student learning and integrate research and education; help recruit/retain students, particularly from underprivileged backgrounds.
Helped establish program at Kansas State University for enhancing retention and graduation rates of students from historically underrepresented groups; initiated projects which integrate research and education; co-organizer of the annual International Summer College, Nathiagali, Pakistan on .Physics and Contemporary Needs of Developing Countries., 1998-present; member, Editorial Board, Executive Committee, Journal of Physics Condensed Matter; Sigma Xi distinguished lecturer, 2004-06, frequent lecturer at public schools, member nominating committee of American Physical Society; hands-on workshops in computational modeling of materials at National Center for Physics, Pakistan.
Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets
We investigated the effect of intermolecular dipolar interactions on an ensemble of 100 three-dimensional systems of 5 nanomagnets, each with spin S=5, arranged in a cubic lattice. We employed the Landau-Lifshitz-Gilbert equation to solve for the magnetization curves for several values of the damping constant, the induction sweep rate, the lattice constant, the temperature, and the magnetic anisotropy. We find that the smaller the damping constant, the stronger the maximum induction required to produce hysteresis. The shape of the hysteresis loops also depends on the damping constant. We find further that the system magnetizes and demagnetizes at decreasing magnetic field strengths with decreasing sweep rates, resulting in smaller hysteresis loops. Variations of the lattice constant within realistic values (1.52.5 nm) show that the ...
We propose a combined density-functional-theorydynamical-mean-field-theory (DFT + DMFT) approach for reliable inclusion of electronelectron correlation effects in nanosystems. Compared with the widely used DFT + U approach, this method has several advantages, the most important of which is that it takes into account dynamical correlation effects. The formalism is illustrated through different calculations of the magnetic properties of a set of small iron clusters (number of atoms 2 ≤ N ≤ 5). It is shown that the inclusion of dynamical effects leads to a reduction in the cluster magnetization (as compared to results from DFT + U) and that, even for such small clusters, the magnetization values agree well with experimental estimations. These results justify confidence in the ability of the method to accurately describe the magnetic properties of clusters of interest to nanoscience.
A Surface Coordination Network Based on Substrate-Derived Metal Adatoms with Local Charge Excess
We present a coordination network, formed at a metal surface in a system facile to prepare because the required metal centers can be released in a measured fashion from the substrate by simple annealing. Analysis of the charge density in this system suggests that metal adatoms can have a pronounced donating character despite the electron deficiency of uncoordinated adatoms. The notion that metal adatoms present an anionic character in their interaction with molecular adsorbates could have significant and widespread implications (for example, in heterogeneous ...
Toward an Understanding of Ligand Selectivity in Nanocluster Synthesis
We performed scalar relativistic density functional theory (DFT) calculations using the projector augmented wave scheme (PAW) to examine the reactivity and selectivity of diphosphine ligands LM, with the formula PH2(CH2)MPH2 (spacer M = 3, 5), toward small-sized cationic Aun (n = 7-11) nanoclusters. By isolating the ligand-induced contribution to the stability condition, we show that such interaction selectively stabilizes the cationic Au11 cluster. Furthermore, we find that L5
Time-dependent density-matrix functional theory for biexcitonic phenomena
We formulate a time-dependent density-matrix functional theory (TDDMFT) approach for higher-order correlation effects like biexcitons in optical processes in solids based on a reduced two-particle density-matrix formalism within the normal orbital representation. A TDDMFT version of the Schr鰀inger equation for biexcitons in terms of one- and two-body reduced density matrices is derived, which leads to finite biexcitonic binding energies already with an adiabatic approximation. Biexcitonic binding energies for several bulk semiconductors are...
This work has been reported on the Physical Review B
[V. Turkowski, C. A. Ullrich, T. S. Rahman, and M. N. Leuenberger, Phys. Rev. B 82 205208 (2010)]
Selective oxidation of ammonia on RuO2(110)
We have used a combination of density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations to calculate the reaction rates for the selective oxidation of ammonia on RuO2(110). Our KMC simulations of 18 reactions among NHx (x=0-3) and OHx (x=0-2) species on RuO2(110) show 93% selectivity for NO, in close agreement with experiment (~95%). The chief factor in the high selectivity for NO on the RuO2(110) surface is the significantly reduced N diffusion (via N blocking) caused ...
This work has been reported on the Journal of Catalysis
[S. Hong, A. Karim, T. S. Rahman, K. Jacobi, G. Ertl, J. Catal. 276 371 (2010)]
Off-lattice pattern recognition scheme for KMC simulations
We report the development of a pattern-recognition scheme for the off-lattice self-learning kinetic Monte Carlo (KMC) method, one that is simple and flexible enough that it can be applied to all types of surfaces. In this scheme, to uniquely identify the local environment and associated processes involving three-dimensional (3D) motion of an atom or atoms, space around a central atom is divided into 3D rectangular boxes. The dimensions and the number of 3D boxes are determined by the accuracy with which a process needs to be identified and a process is described as the central atom moving to a neighboring vacant box accompanied by the motion of any other atom or atoms in its surrounding boxes.
Role of vdW in the tilted binding of amine molecules to Au(111)
We show that the inclusion of van der Waals interactions between the isolated molecule (3 amine molecules) and the Au(111) surface leads to good agreement with experimental data on the binding energies. Each molecule adsorbs with a small tilt angle. For the case of 1,4-diaminobenzene (BDA) our calculations reproduce the larger tilt angle measured by photoemission experiments, when intermolecular interactions are included. These results point to not only the importance of vdW interactions to molecule-surface binding, but also that of intermolecular interactions in determining the adsorption geometry and pattern formation.