Biomedical subjects
Publications and source records attributed to Liuming Yan.
We discuss the relationship between the inelastic electron tunneling spectroscopy (IETS) and vibronic coupling constant within the Green's function formalism at a level of perturbation theory approximation. We also compare our results with experimental measurements. Our results can provide insights into the mechanism of active vibronic modes for IETS.
The side chain of NAFION, a proton conductive membrane used as electrolyte in low-temperature fuel cells, is modeled with perfluorobutane sulfonic acid. Density functional theory is used to characterize structures and energetics of hydration of the model system interacting with a proton solvated with up to 24 water molecules and analyze interactions of some of these hydrated complexes with O(2) adsorbed on Pt(3). It is found that at least three water molecules are needed to ionize the sulfonic acid, and higher degrees of hydration induce the formation of cages where the water molecules are held together via complex hydrogen-bond networks. The interaction between the complex formed by the ionized acid and the hydrated proton, in contact with a bridge-adsorbed O(2)-Pt(3), promotes the protonation of the adsorbed O(2). Upon protonation, the O(2)-Pt(3) system evolves from hydrophobic to hydrophilic behavior, which may facilitate further interfacial contact.
Signals carrying information are encoded in molecular vibrational waves (vibronics) rather than in electric currents as widely done in microelectronics. We demonstrate theoretically that signals can be transmitted along a long polypeptide molecule; the signal is modulated in a terahertz carrier corresponding to a frequency of an intrinsic vibrational mode of the backbone of the polypeptide, via amplitude and frequency modulations. The modulated carrier is coupled as a vibrational wave to the polypeptide at one end of the molecule and propagates for more than 168 angstroms towards the other end. Digital signal processing techniques are used to recover the modulated signals.
It is proposed and demonstrated using molecular dynamics and digital signal processing techniques that molecular vibrations can be used to transport signals in molecular circuits, revealing that signals transmitted along polypeptide molecules by a frequency-modulated carrier in the terahertz domain consume only 0.2 eV to successfully transfer one information bit; this energy is several orders of magnitude smaller than the several thousands of electronvolts needed using electrons in present electronic devices.
The molecular and electronic structures and electron transport characteristics of a Co complex are investigated using first principles calculations. The Co complex belongs to the D(2d) point group, and its two ligands are perpendicular to each other. The central atom Co forms a distorted octahedron with six donor N atoms. In a low oxidation state, the bond length between Co and pyrrole nitrogen, 1.849 A, is much shorter than the distance between Co and pyridine nitrogen, 2.168 A, while, in a high oxidation state, the bond length differences between Co and pyrrole nitrogen, 1.814 A, and between Co and pyridine nitrogen, 1.990 A, are not as large as those in the Co2+ complex. The HOMO energy of the low oxidation state is very close to the Fermi level of bulk Au, allowing hole creation in the molecule. On the other hand, the LUMO energy of the high oxidation state is close to the Au Fermi level, allowing a low barrier for electron injection from the Au cathode to the molecule. These structural characteristics make the Co complex a good hole-conduction molecule. The density of states, transmission probability, and I-V characteristics are evaluated using the Green function approach.
Reproducible negative differential resistance (NDR)-like switching behavior is observed in NanoCells. This behavior is attributed to the formation of filaments and clusters between the discontinuous gold films. Control experiments are performed by self-assembly of insulating molecules between the gold islands and conducting molecules on these islands. Additional control experiments are performed by removing the filaments and clusters between islands using a piranha bath. The results are consistent with theoretical predictions and extend the domain of molecular electronics based in organic molecules to include nanosized clusters as active units. This facilitates a scenario where synthetically accessible organic molecules, with defined characteristics, can be adjusted by metallic nanoclusters as an in situ fine-tuning element, able to compensate for the lack of addressing in the nanosize regime.