Scanning electron microscope technique for measuring electrical conductivity: Application to tetrathiafulvalene-tetracyanoquinodimethane

A new technique for measuring the electrical conductivity of small samples and its application to the organic conductor tetrathiafulvalenetetracyanoquinodimethane (TTF-TCNQ) is reported. A movable current sour(:e provided by the electron beam of a scanning electron microscope is used to map out the potential distribution on crystal faces containing the a-b crystallographic axes. Silver paint contacts are used to return the beam current to ground and measure voltage changes as the beam position is moved. The results of the new technique are confirmed and complemented by the conventional movable contact method and the extension of both methods to low temperature is discussed. The potential distributions for our samples reveal frequently occurring irregularities in current flow which are attributable to sample imperfections and inhomogeneities in the silver paint contacts. Methods are presented whereby the commonly reported conductivities 0a and 0b can be determined despite the presence of certain current flow irregularities; room temperature values are found to be: 0b = 490 ±80 (ncm)-l and sigma-a = 1.21 ±.15 (Ohmcm)-l. The relationship of sigma-a and sigma-b to the elements of the correctly expressed conductivity tensor for TTF-TCNQ is clarified. The influence of contact inhomogeneities on four-probe measurements of the temperature dependence of the £-axis conductivity as determined with an electrolytic tank model are also presented. It is found that there is a large probability of slightly underestimating conductivity, but that it is possible in a small number of cases to greatly overestimate conductivity.U of I OnlyThesi