Quantitative scanning near-field microwave microscopy for thin film dielectric constant measurement.

We combine a scanning near-field microwave microscope with an atomic force microscope for use in localized thin film dielectric constant measurement, and demonstrate the capabilities of our system through simultaneous surface topography and microwave reflection measurements on a variety of thin films grown on low resistivity silicon substrates. Reflection measurements clearly discriminate the interface between approximately 38 nm silicon nitride and dioxide thin films at 1.788 GHz. Finite element simulation was used to extract the dielectric constants showing the dielectric sensitivity to be Deltaepsilon(r)=0.1 at epsilon(r)=6.2, for the case of silicon nitride. These results illustrate the capability of our instrument for quantitative dielectric constant measurement at microwave frequencies.

Cloning the soil metagenome: a strategy for accessing the genetic and functional diversity of uncultured microorganisms.

Recent progress in molecular microbial ecology has revealed that traditional culturing methods fail to represent the scope of microbial diversity in nature, since only a small proportion of viable microorganisms in a sample are recovered by culturing techniques. To develop methods to investigate the full extent of microbial diversity, we used a bacterial artificial chromosome (BAC) vector to construct libraries of genomic DNA isolated directly from soil (termed metagenomic libraries). To date, we have constructed two such libraries, which contain more than 1 Gbp of DNA. Phylogenetic analysis of 16S rRNA gene sequences recovered from one of the libraries indicates that the BAC libraries contain DNA from a wide diversity of microbial phyla, including sequences from diverse taxa such as the low-G+C, gram-positive Acidobacterium, Cytophagales, and Proteobacteria. Initial screening of the libraries in Escherichia coli identified several clones that express heterologous genes from the inserts, confirming that the BAC vector can be used to maintain, express, and analyze environmental DNA. The phenotypes expressed by these clones include antibacterial, lipase, amylase, nuclease, and hemolytic activities. Metagenomic libraries are a powerful tool for exploring soil microbial diversity, providing access to the genetic information of uncultured soil microorganisms. Such libraries will be the basis of new initiatives to conduct genomic studies that link phylogenetic and functional information about the microbiota of environments dominated by microorganisms that are refractory to cultivation.