Selected Modify On Self-Assembled Monolayer By Electric Field Of Tip And Characterization

Atomic force microscope (AFM) has been widely used in biology, material science, nanostructure fabrication, and et al. Tip-induced local electrooxidation of surface groups on self-assembly monolayers is of great current interest because three dimensional self-assembly structures on nanometers scale can be generated on the oxidized monolayers. Such oxidized area is usually in the range of nanometers, and there are no effective methods to detect the transformation of top groups on such a small area. Herein we use SAM monolayers at the top of AFM tip as a model to study the oxidization of monolayer on nanometers scale. The aim of the study is to develop methods to characterize the electrooxidization of the monolayer at the tip. The main contents are as follows:1. Electrical characterization of Si tips using conducting atomic force microscopy. The electrical properties of n-dopant Si tip have been characterized in conducting atomic force microscopy under various conditions. Si tip with SiO2 layer on it presented complex electric properties: larger positive threshold bias, which is different from that of its doped semiconductor material. Silicon tip after removing SiO2 layer had smaller positive threshold bias, such bias varied with the loading force: smaller loading force corresponding to larger positive threshold bias, and it remained a constant at lower level for larger loading force. Humility of experiments influenced the threshold bias: lower RH (<25%) and larger loading force was in favor of getting stable threshold bias. The conductance increased remarkably in high relative humility although it was kept in a narrow range when RH lower than 40%. Loading force didn't affect the conductance in examined RH condition. One advantage of bare silicon tips over commercial conductive ones is that they smaller radius than gold-coated tips, this is in more favor of reaching single molecular electronics.2. Force titration to characterize the electrooxidization of self-assembly monolayer at the apex of AFM tips induced by electric field. Force titration methods have been developed to qualitatively determine the chemical groups left on the apex of OTS tip after eletrooxidized under varied bias. Pull-off force of the biased OTS tip on mica reference samples increases with the increase of bias. This indicates the alkyl groups have been modified to hydrophilic ones. The results of chemical force titration as a function of solution pH on hydroxyl-terminated SAMs showed that the chemical groups at the apex of OTS tip are carboxyl mixing with alkyl when with lower voltage (1.5 V), carboxyl groups with higher bias (5.0 V), and hydroxyl when the voltage was 10.0 V. Self-assembly monolayers via C-Si bonds on AFM tips after removing the silicon dioxide layer are of high quality, and shows excellent behavior in the electrooxidization experiments. A model was established to calculate the electrooxidized area at the top of tip, and the oxidized area is obtained under some bias.3. Patterns of electrooxidization induced by tip electric field on self-assembly monolayer. There is minimum bias about 7.5 V for silicon tip to form oxidization dots on OTS SAMs. The oxidized dots are very sensitive to relative humility (RH) and the biasing duration: higher RH and longer duration both lead to larger oxidized dots. Higher bias induces larger oxidized dots, while the loading force has little influence on the size of oxidized dots. It is easy to control the size of oxidized dots in a very small range on the SAMs made from alkene binding on H-Si surface. Typical oxidized dots of less 20 nm were obtained by 8 V bias. These oxidized dots can adsorb aqueous particles and form solid nanopatterns.