| Field evaporation of carbon as positive and negative ions from the clean graphite(001) plane surface under STM pulsed-field condition has been investigated. We use the charge-exchange model, taking the potential from a simple empirical atomic potential model. The removed carbon atom considered here initially resides in the perfect graphite surface, rather than being an adsorbed atom. Assuming a clean tungsten tip as counterelectrode, threshold electric fields for $rm Csp-, Csp+, Csp{2+}$, and C$sp{3+}$ have been obtained. We further consider the possibility of field evaporation of C clusters. Our calculations indicate that C atoms can field-evaporate as members of ionic clusters at much lower threshold fields than a single carbon atom can.;Nanometer scale modifications on graphite samples at various low temperature by using a home-made cryogenic STM have been investigated. Both gold and tungsten tips are used in our modification experiments. Hillocks appear more often than holes after voltage pulses, a feature which is different from the results at room temperature. Among those nanometer structures obtained at low temperature, multiple hillock and hole-hill structures after one voltage pulse are often obtained in our experiments.;Nanometer scale structures on gold samples are also obtained by using our STM at room temperature. Besides hole and hillock structures, multiple hillock and hill-hole structures after one voltage pulse appear with high probability. Multiple tips or protrusions on the tip apex are suggested to cause the multiple hillock structures.;Attempts to make nanometer scale modifications on $rm Bisb2Srsb2CaCusb2Osb8$ are also made without success even with the maximum pulse amplitude, although the STM-produced etching is observed on $rm Bisb2Srsb2CaCusb2Osb8$ samples. |
