A detailed analysis has been carried out of the advantages and disadvantages of using carbon fibers and flat graphite structures with activated surfaces as materials for field electron emission cathodes. In this article we present the results of original experimental studies on the elementary composition of carbon materials and on the effect of thermal annealing on their structure and emission properties and on the reduction of the work function when cesium is implanted into these materials. We also assess the potential of irradiation techniques for producing flat field emission cathodes with activated surfaces, and discuss a method for predicting how the shape of the emitting surface changes as a result of bombardment by low-energy residual-gas ions. Suggestions are made about the best design of field electron emission cathodes for various practical purposes.