In our theory group, we design model nanoscale systems to test certain hypotheses
and ideas, and investigate real nanosystems in collaboration with experimental groups.
In a variety of systems, we model molecular and electronic transport, self-assembly,
catalysis, and other phenomena. The modeling is realized with ab-initio
electronic structure and quantum transport methods, atomistic and coarse-grained
molecular dynamics (MD) simulations, and other semi-classical techniques.
We have also developed new methodologies in quantum transport (Phys. Rev. B 56, 7293 (1997)), and coherent control (Rev. Mod. Phys. 79, 53 (2007)), and discovered unique phenomena (Phys. Rev. Lett. 88, 056803 (2002)), and quantum states (Phys. Rev. Lett. 89, 135506 (2002)) (journal covers right). In order to perform precise modeling and realize efficient visualization of large and complex molecular systems, we build our own computer clusters based on multicore processors and graphical cards (GPU).
Molecular Transport and Nanofluidics
Ion and molecular drag on nanotubes
We have extensively studied molecular and electronic drag phenomena in nanotubes. We have predicted that electric current passing through carbon nanotubes can induce drag of atoms and molecules inside the nanotubes and on their surfaces (Phys. Rev. Lett. 82, 5373 (1999)). We have also discovered the opposite effect, where polar liquids floating around nanotubes generate electric currents in them (Phys. Rev. Lett. 86, 131 (2001)). Both phenomena were later experimentally observed.
Furthermore, we have shown that polar liquids (water) floating inside nanotubes can drag polar molecules and ions on their surfaces (JACS (Communication) 128, 15984 (2006)). Similarly, ions driven by electric fields inside the nanotubes can drag polar droplets on their surfaces (Phys. Rev. Let. 101, 046103 (2008)). We have even realized dragging of DNA solvated in physiological solutions and adsorbed on nanotubes by ionic solutions passing through the tubes.
Ion and molecular transport through synthetic nanopores
We also design systems with high selectivity for ion and molecular passage (JACS 130, 16448 (2008)), controlled by mechanical, optical or electronic means. We are developing multiscale methodologies to describe molecular transport around polarizable nanoscale material surfaces. Our hybrid methodologies should address in a consistent manner classical molecular transport accompanied by quantum electronic transport in such systems.
We also perform atomistic MD simulations to investigate selective passage of solvated molecules in a variety of functionalized nanochannels (carbon nanotubes, carbon nanocones, and other synthetic channels (J. Phys. Chem. B 114, 1174 (2010)). The selectivity and passage rates through these systems can be controlled by mechanical means (mutual twist of the system components).
Control of rotary molecular motion at the nanoscale
We study molecular nanomachines, in particular, explore motility of model nanoscale systems in liquid media and on their surfaces. Since these systems operate at low Reynolds numbers, irreversibility in their motion is essential for their locomotion, propulsion and pumping. For example, we have designed molecular propellers (Phys. Rev. Let. 98, 266102 (2007)), formed by carbon nanotube rotors with attached aromatic blades. We have shown that the blade chemistry determines the pumping efficiency of these systems (hydrophylic blades become clogged). In applications, these propellers might be driven by biological or synthetic motors. To this goal, we have designed electron-tunneling motors (Phys. Rev. Let. 101, 186808 (2008)).
Recently, we have shown that locomotion of larger objects on liquid surfaces (water), such as nanorods, can be achieved by external fields. We demonstrated that nanorods with photoactive coating ligands can roll and carry cargo on air/water interfaces, when driven by light (Phys. Rev. Lett. 103, 246103 (2009)).
Self-assembly of nanoscale systems
Spontaneous and guided assembly of nanoparticle superlattices and molecular nanostructures
In the second major direction of our research, we study realistic superlattices formed of colloidal semiconductor nanoparticles (NP) and nanorods (NR) with the goal to understand the conditions under which they can be self-assembled. The results of our semi-classical simulations, based on minimization of the interparticle coupling forces, agree with the experimentally obtained results. Moreover, we show that non-local dipoles formed in NPs and NRs, and their screening by conducting substrates are both crucial for the lattice formation on material surfaces (Nano Letters 7, 1213 (2007) and Nano Letters 8, 3605 (2008)). We also model by atomistic and coarse-grained MD simulations the self-assembly of gold NPs covered with surfactants into self-supporting close-packed monolayers prepared experimentally, and investigate their stability and other properties.
Further, we use atomistic and coarse-grained MD simulations to study induced folding of planar graphene structures. We have discovered that water nanodroplets deposited on graphene flakes can act as catalytic elements that initiate and guide their self-assembly (Nano Letters 9, 3766 (2009)). Nanodroplets can induce rapid bending, folding, sliding, rolling and zipping of the planar nanostructures, which can lead to the assembly of nanoscale sandwiches, capsules, knots and rings. We are testing if interactions of graphene flakes and ribbons with other nanoscale objects can induce graphene folding into stable nanostructures and materials.
We have shown that graphene monolayers optimally doped with boron and nitrogen atoms, or small ligands, can serve as highly specific docking nests for molecules (Small 3, 580 (2007)). Once docked on the doped graphene, the molecules can be detected by vibrational means. In this way, one can distinguish between different enantiomers docked in these nests with planar chirality (Nano Res. ASAP (2010)). We also study chiral molecules adsorbed on twisted and helical carbon nanotubes (Nano Letters 6, 1918 (2006)).
Bio-inspired hybrid materials: Assembly of nanostructures inside phospholipid membranes
Recently, we started to study by coarse-grained MD simulations the self-assembly of graphene sheets and surfactant-coated NP clusters inside phospholipid membranes. These hybrid superstructures may be prepared by forming hydrated micelles with the hydrophobic cargo (graphene, nanoparticle), which can then be fused with the membranes (ACS Nano 4, 229 (2010)). The self-assembled systems might be potentially used as biosensors and bioelectronic materials.
In collaboration with Prof. Hayat Onyuksel (Dept. of Biopharmaceutical Sciences, UIC), we investigate lipid-based micelles used in efficient and targeted delivery of anticancer drugs. We model by atomistic MD simulations sterically stabilized phospholipid micelles (SSM) and evaluate the Gibbs energies of drugs with varying degrees of hydrophobicity / amphiphilicity solubilized inside the SSM. Recently, we have developed GPU-based lab supercomputers to perform large-scale simulations of these nanomedicines. The computations are also carried at several supercomputer networks.
Design of functional carbonaceous materials
Many of the systems designed and studied in our group need certain degree of optimization. In collaboration with Dr. Peter Zapol and Dr. Larry Curtiss, from the Argonne National Laboratory, we study novel catalysts based on carbonaceous materials. Durable and inexpensive catalysts need to be developed to replace traditional metal materials. We use density functional theory to explore the structures and the energetics of hybrid materials made of metal and nitrogen atoms incorporated into carbon nanostructures (J. Phys. Chem. C. 113, 21629 (2009)). We have also designed a large number of functional carbonaceous materials (Coor. Chem. Rev. 253, 2852 (2009)).