| The computational design of nanomaterials is one of the most exciting activities within the field of nanotechnology. Four types of molecular devices are explored here: a molecular cation sensor, a metallacarborane rotor, a PAH wave mixer and bioconjugated CdSe molecular clusters.;First, the electronic conduction of a crown ether-embedded molecular wire is studied with the non-equilibrium Green's function method. The differential conductance decreases upon cation adsorption. The conductance drop at low bias is larger for proton than the alkali cations. The conductance change may therefore be exploited to indicate molecular recognition. Second, density functional theory (DFT) is used to examine the conformations and frontier orbitals of a nickel metallacarborane. The interpretation of spectroscopy using computational modeling demonstrates that the metallacarborane may be driven by photo or redox control. Third, the frequency mixing property of a polycyclic aromatic hydrocarbon is simulated by propagating the one-electron density matrix for the Mickel molecular orbitals. Two polyacetylene wires, driven at different electric field frequencies, yield an output electronic oscillation containing frequency components indicative of nonlinear mixing. Local potential shifts simulating isolated heteroatoms or impurities are shown to change the output spectrum.;Finally, several conjugated cadmium selenide molecular clusters are studied. The binding of the peptide 3xCCCha to (CdSe)13 is studied with a universal force field and parameter method 3. The thermochemical energies show that the hydrocarbon side chains stabilize the peptide coating. Peptide adsorption also red-shifts the HOMO-LUMO gap of (CdSe)133xCCCha compared to (CdSe)13. The magnitude of red-shift varies with the number of thiol bonds. To examine the peptide-induced red-shift, the binding of methanethiolate and L-cysteine to (CdSe)6 is examined with DFT calculations. The adsorbate inserts sulfur 3p orbitals which behave as hole traps. The HOMO-LUMO gaps of [(CdSe)6SCH3]- and (CdSe)6Cys therefore become smaller compared to (CdSe) 6.;Accordingly, time-dependent DFT results show that the lowest energy excitations of the conjugated molecular clusters are red-shifted compared to (CdSe) 6. Similar effects are observed when surfactant passivation is simulated by coating the molecular clusters with ammonia. For (CdSe)6 Cys, the magnitude of red-shift depends on the cysteine isomer; this dependence may be exploited to signal pH changes. |
