Yuhui Lu

Molecular quantum-dot cellular automata: From molecular structure to circuit dynamics

We establish a method for exploring the dynamics of molecular quantum-dot cellular automata (QCA) devices by hierarchically combining the techniques of quantum chemistry with the nonequilibrium time-dependent coherence vector formalism. Single QCA molecules are characterized using ab initio quantum chemistry methods. We show how to construct a simple model Hamiltonian for each QCA cell based on parameters extracted from the ab initio calculation. The model Hamiltonian captures well the relevant switching behavior and can then be used to calculate the time-dependent coherence vector, including thermal and nonequilibrium behavior. This enables us to explore dynamic behavior and power dissipation for various QCA devices and circuits.

Through-Bond versus Through-Space Coupling in Mixed-Valence Molecules: Observation of Electron Localization at the Single-Molecule Scale

Scanning tunneling microscopy (STM) is used to study two dinuclear organometallic molecules, meta-Fe2 and para-Fe2, which have identical molecular formulas but differ in the geometry in which the metal centers are linked through a central phenyl ring. Both molecules show symmetric electron density when imaged with STM under ultrahigh-vacuum conditions at 77 K. Chemical oxidation of these molecules results in mixed-valence species, and STM images of mixed-valence meta-Fe2 show pronounced asymmetry in electronic state density, despite the structural symmetry of the molecule. In contrast, images of mixed-valence para-Fe2 show that the electronic state density remains symmetric. Images are compared to constrained density functional (CDFT) calculations and are consistent with full localization of charge for meta-Fe2 on to a single metal center, as compared with charge delocalization over both metal centers for para-Fe2. The conclusion is that electronic coupling between the two metal centers occurs through the bonds of the organic linker, and through-space coupling is less important. In addition, the observation that mixed-valence para-Fe2 is delocalized shows that electron localization in meta-Fe2 is not determined by interactions with the Au(111) substrate or the position of neighboring solvent molecules or counterion species.

Molecular electronics–from structure to circuit dynamics

In this work, we study the dynamics of molecular quantum-dot cellular automata (QCA) devices by combining the technique of quantum chemistry and coherence vector formalism. First, we apply the quantum chemistry ab initio calculation on the single molecule, and construct a simple model Hamiltonian for each QCA cell based on the calculation. For a three-dot molecule discussed in this work, we show a 3×3 Hamiltonian is sufficient to describe its switching behavior. The second step is to use the model Hamiltonian to calculate the coherence vector evolved as time, from which we will investigate the dynamic behavior of various QCA devices and circuits. Finally, we discuss the structure-functionality relation between the molecular structure and circuit dynamics.

Synthesis of a Neutral Mixed‐Valence Diferrocenyl Carborane for Molecular Quantum‐Dot Cellular Automata Applications

The preparation of 7-Fc(+) -8-Fc-7,8-nido-[C2 B9 H10 ](-) (Fc(+) FcC2 B9 (-) ) demonstrates the successful incorporation of a carborane cage as an internal counteranion bridging between ferrocene and ferrocenium units. This neutral mixed-valence Fe(II) /Fe(III) complex overcomes the proximal electronic bias imposed by external counterions, a practical limitation in the use of molecular switches. A combination of UV/Vis-NIR spectroscopic and TD-DFT computational studies indicate that electron transfer within Fc(+) FcC2 B9 (-) is achieved through a bridge-mediated mechanism. This electronic framework therefore provides the possibility of an all-neutral null state, a key requirement for the implementation of quantum-dot cellular automata (QCA) molecular computing. The adhesion, ordering, and characterization of Fc(+) FcC2 B9 (-) on Au(111) has been observed by scanning tunneling microscopy.

Molecular cellular networks: A non von Neumann architecture for molecular electronics

The two fundamental limitations of the present computing paradigm are power dissipation from transistor switching and the architectural von Neumann bottleneck that segregates processing from memory. We examine a cellular architecture which radically intermixes memory and processing, and which is based on a transistor-less approach to representing binary information using the arrangement of charge within the molecule. Representing bits by molecular configuration, rather than a current switch, yields the limits of functional density and low power dissipation. Matching a new computational element to a new architectural framework could enable general purpose computing to evolve along a new roadmap.