This thesis discusses DC and AC transport properties of molecular devices from first principles. For dc bias, based on the non-equilibrium Green’s function (NEGF) technique coupled with the density functional theory (DFT), the dc current density distribution of a molecular device Al-C60-Al is numerically investigated from first principles. Due to the presence of non-local pseudo-potential, the conventional definition of current density is not suitable to describe the correct current density profile inside the molecular device. By using the new definition of current density which includes the contribution due to the nonlocal potential, our numerical results show that the new definition of current density J(r) conserves the current. In addition, the current obtained from the current density calculated inside the molecular device equals to that calculated from the Landauer-Büttiker formula. When the external bias is time dependent, a theoretical formalism to study the time dependent transport behavior of molecular device from first principles is proposed based on the non-equilibrium Green’s function (NEGF) and time dependent density functional theory (TDDFT). For the purpose of numerical implementation on molecular devices, a computational tractable numerical scheme is discussed in detail. The transient current of two molecular devices Al-1,4-dimethylbenzene-Al and Al-Benenze-Al are numerically studied from first principles. To overcome the computational complexity due to the memory term, a fast algorithm has been employed to speed up the calculation and CPU time has been reduced from the scaling N^3to N^2 log(_2^2)(N) for the step like pulse, where N is the number of time step in the time evolution of Green’s function. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xvii, 187 p.; also includes graphics(some col). Includes bibliographical references (p. 171-187). Available online via OhioLINK's ETD Center

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A series of zwitterionic materials of general formula R-D*-CH=C(CN)- CÖH4-C(CN)2', where D* is a pyridirium or quinolinium acceptor and R is a hydrophobic alkyl chain or an aryl group, were deposited using the Langmuir- Blodgett (LB) technique and their photochromic and nonlinear optical properties characterised. The materials are highly solvatochromic, exhibiting a broad photochromic charge transfer band in the visible region which bleaches when irradiated. LB films of the zwitterions, Z-ß-(1-hexadecyl-4-pyridinium)-a-cyano-4- styryldicyanomethanide (CMH33-P3CNQ) and the quinolinium analogue, Z-ß-(1- hexadecyl-4-quinolinium)-a-cyano-4-styryldicyanomethanide (CIÖH33-Q3CNQ), are non-centrosymmetric (Z-type). They exhibit sharp charge transfer bands at 495 n ana ses n respecivey with haf widths ai half maximum of 27 and zz mi. Unusually, mixed LB films of CIGH33-P3CNQ and C161-133-Q3CNQ exhibit a single sharp charge transfer band whose position is dependent upon the mole fraction and is finely tunable in the range 495 to 565 nm. These films are photobleached when irradiated at wavelengths which overlap the absorption bands, and may find potential applications a components of a multifrequency optical memory. Also, the unique peak wavelength tuning of the heteromolecular films enabled the effect of the position of the absorption band on second harmonic generation to be investigated for the first time. The zwitterionic materials have exciting norlinear optical properties and the strongest second harmonic intensity from any LB film has been obtained. LB films of the quinolinium zwitterion (CMH33-Q3CNQ) are non-centrosymmetric and the second harmonic intensity increases quadratically with the number of layers deposited to thicknesses of ca. 1 m. It is one of only five known materials to show such behaviour and the second-order susceptibility (xa) = 180 pm V* at 1064 nm) is the highest value obtained for a multilayer structure.