RESUMEN
Measurement of electrical conductivity of conductive thin film deposited on a conductive substrate is important and challenging. An effective conductivity model was constructed for a bilayer structure to extract thin film conductivity from the measured Q-factor of a quasi-optical resonator. As a demonstration, aluminium films with thickness of 100 nm were evaporated on four silicon wafers whose conductivity ranges from ~101 to ~105 S/m (thus, the proposed method can be verified for a substrate with a wide range of conductivity). Measurement results at ~180 GHz show that average conductivities are 1.66 × 107 S/m (which agrees well with direct current measurements) with 6% standard deviation. The proposed method provides a contactless conductivity evaluation method for conductive thin film deposited on conductive substrate which cannot be achieved by the existing microwave resonant method.
RESUMEN
Conductive nanomaterials are widely studied and used. The four-point probe method has been widely used to measure nanomaterials' sheet resistance, denoted as . However, for materials sensitive to contamination or physical damage, contactless measurement is highly recommended if not required. Feasibility of evaluation using a one-port rectangular waveguide working on the microwave band in a contact-free mode is studied. Compared with existed waveguide methods, the proposed method has three advantages: first, by introducing an air gap between the waveguide flange and the sample surface, it is truly contactless; second, within the specified range of , the substrate's effect may be neglected; third, it does not require a matched load and/or metallization at the sample backside. Both theoretical derivation and simulation showed that the magnitude of the reflection coefficient decreased monotonously with increasing . Through calibration, a quantitative correlation of and was established. Experimental results with various conductive glasses showed that, for in the range of ~10 to 400 Ohm/sq, the estimation error of sheet resistance was below ~20%. The potential effects of air gap size, sample size/location and measurement uncertainty of are discussed. The proposed method is particularly suitable for characterization of conductive glass or related nanomaterials with in the range of tens or hundreds of Ohm/sq.
RESUMEN
The X-ray structure of methanol dehydrogenase (MEDH) from Paracoccus denitrificans (MEDH-PD) was determined at 2.5 A resolution using molecular replacement based on the structure of MEDH from Methylophilus methylotrophus W3A1 (MEDH-WA). The overall structures from the two bacteria are similar to each other except that the former has a longer C-terminal tail in each subunit and shows local differences in several insertion regions. The "X-ray sequence" of the segment alphaGly444-alphaLeu452 was established, including one insertion and seven replacements compared with the reported sequence. The primary electron acceptor of MEDH-PD is cytochrome c-551i (Cyt c551i). Based on the crystal structure of MEDH-PD and of the published structure of Cyt c551i, their interactions were investigated by molecular modeling. As a guide and starting point, the covalently attached cytochrome and PQQ domains of the alcohol dehydrogenase from Pseudomonas putida HK5 (ADH2B) were used. In the modeling, two molecules of Cyt c551i could be accommodated in their interaction with the MEDH heterotetramer in accordance with the two-fold molecular symmetry of the latter. Two models are proposed, in both of which electrostatic and hydrogen bonding interactions make major contributions to inter-protein binding. One of these models involves salt bridges from alphaArg99 of MEDH to the heme propionic acids of Cyt c551i and the other involves salt bridges from alphaArg426 of MEDH to Glu112 of Cyt c551i. Both involve salt bridges from alphaLys93 of MEDH to Asp75 of Cyt c551i. The size and nature of the cytochrome/quinoprotein heterodimer interfaces and calculations of electronic coupling and electron transfer rates favor one of these models over the other.