مجله دانش آزمایشگاهی ایران
سال دوم شماره 1 (پیاپی 5، بهار 1393)

One of the valuable capabilities of scanning tunneling microscopy (STM) is measuring single molecule conductance. For this purpose, a molecular junction should be created between an electrode (gold STM tip) and a molecule with a suitable functional group for creating junction (such as thiol groups). The conductance is measured in a controlled environment under the certain potential. One of the substantial challenges of this technique is making junctions repeatedly. Developing this method will play an important role in improvement of molecular electronics field. In this paper, theoretical and practical principles of measuring single molecule conductance are studied.

The method for carbon-hydrogen-nitrogen determination based on dynamic rapid combustion of the sample, gas chromatographic sep-aration and thermalconductivity measurement of the gases N2,CO2 and H2O is critically considered and improved .In order to achieve instantan-cous combustion of all types of organic compounds , oxidizable metallic containers are used for the sample and dropped in the hot combustion zone while the carrier gas is momentarily enriched with pure oxygen .Flash combustion of the sample is followed by catalytic oxidation on a newly developed active catalyst .This reagent, adsorption-free and thermally resistant ,consist of pure chromic oxide, mechanically compressed and granulated .Copper treated with silver is used as reducing reagent .Unwanted gases are eliminated with Co3O4 containing silver ,and by utilizing the chemiadsorption properties of the column packing .Measurements are made by integrating and printing the electrical signal .The analytical process is controlled by means of a potentiometric recorder .The increased combustion capacity ,the practical absence of blanks and the very extended linearity of response of the detector make possible CHN determination in sample sizes of 0.1-3 mg .Except for sample weighing and calculation of results from the printed integration values ,all the operations are automatically controlled by a preset program.

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The structural characterization of these disparate materials often requires techniques beyond routine application of basic transmission electron microscopy (TEM) technique. With care in sample preparation and operating technique, the full range of TEM techniques from high-resolution imaging to energy-filtered imaging and high-angle annular dark field imaging can be used to obtain more detailed information about the aerogels. This paper serves as a guide to introduce TEM modes for investigating aerogel architectures.

Today, the scanning electron microscope (hereinafter abbreviated to SEM) is utilized not only in medical science and biology, but also in diverse fields such as materials development, metallic materials, ceramics, and semiconductors. This instrument is getting easier to use with the progress of electronics and introduction of new techniques. Anybody can now take micrographs after short-time training in its operational procedure. However, when one has begun to use the instrument, he cannot always take satisfactory photos. When the photo is not sharp enough, or when necessary information cannot be obtained, it is necessary to think what causes it. This paper is based on a set of common problems that arise when working with scanning electron microscope and fixing these problems will be discussed as well as how to achieve the maximum desired results from the sample. It is hoped that this article will be of help to those who are currently using Scanning Electron Microscope or will be used in the future.