Electrochemical Scanning Tunneling Microscopy Study Of Copper And Palladium On MPS-Modified Au(111)

In the past decades self-assembly monolayers (SAMs) of sulfur-containing organic molecules have been successfully applied to modify the properties of the metal surface (e.g., corrosion resistance, wear resistance, wettability, adhesion, lubricity) and to control the transport of ionic species across the solid/liquid interface. The most important application of organosulfur compounds in metal electrodeposition is to manufacture submicrometer scaled interconnects in microelectronic devices. In the metal deposition process, mercaptosulfonate compounds are usually used as growth accelerating additives to control the metal growth behavior and the deposit morphology. In this thesis, we study the adsorption behaviors of MPS on Au(111) and deposition of Cu and Pd onto self-assembly molecules by employing cyclic voltammetry and ECSTM. The research contents and achievements are listed as follows:1、 Result of cyclic voltammetry reveals the stable potential region for the absorption of mercaptopropylsulfonic acid(MPS) molecules and the electrochemical behavior of molecules. When the potential was negative than-200 mV vs. SCE, a reduction peak corresponding to the reductive desorption of adsorbed MPS from Au (111) electrode surface was observed. As the potential moved above-200 mV, MPS molecules oxidatively reabsorbed on the Au (111) electrode surface. The mercaptopropylsulfonic acid (MPS) on Au (111) is stable in the so-called double layer region between-200 mV and 550 mV. A steep increase of the oxidative current potential at 800 mV, and it arose from the surface polarization which caused the oxidation of water and electrode itself and leaded to desorption of MPS molecules finally. ECSTM experiments have confirmed that the MPS molecules are stable in the potential region between 100 mV and 700 mV.2、The influences on Cu electrodeposition with and without adding MPS were compared. Firstly, we study the electrodeposition of Cu onto Au (111) electrode in the solution of 1mM CUSO4+0.1 M H2SO4, CV revealed two well-defined peaks of at 230 mV and 30 mV (vs. SCE), which were the typical Cu-UPD peaks, the Cu-OPD began at the potential negative than-20 mV. When Au(111) was covered with MPS in the solution of 1 mM CuSO4+1 mM MPS+0.1 M H2SO4, the amount of Cu deposited in the UPD region were less than the solution without MPS, the potential of the Cu-UPD and bulk Cu deposition shifted to 240 mV and-100 mV respectively. Therefore, adding MPS in the electrolyte inhibits the electrodeposition of Cu. ECSTM images exhibited growth of Cu electrodeposition, the electrodeposition of Cu onto Au (111) surface was a two-dimensional growth in the UPD region. When Au(111) electrode was covered with MPS molecular, a Cu film with distributed grains of different size was formed over MPS-modified Au(111) electrode surface and the grain-refining effect of MPS was demonstrated.3、The influences on Pd electrodeposition with and without adding MPS were compared. In the solution of 0.1 mM H2PdCl4+0.1 M H2SO4, CV of Au (111) electrode revealed that Pd UPD starts at 570 mV (vs. SCE), the cathodic current kept almost constant in the potential region more negative than 450 mV. When Au(111) was covered with MPS in the solution of 0.1 mM H2PdCl4+0.1 M H2SO4, the Pd deposited after the desorption of MPS. Therefore, adding MPS in the electrolyte inhibits the electrodeposition of Pd on Au(111). ECSTM experiments exhibited the process of Pd electrodeposition, when the potential was negative than 590 mV, the first Pd layer onto Au(111) showed a two-dimensional epitaxial growth and Pd covered the entire surface at 360 mV, the second layer deposited on the first layer of Pd at 390 mV. In comparison, when Au(111) electrode was covered with MPS molecules, Pd deposition was almost fully hindered. Because the PdCl42- complex was too big to adsorb on the MPS modified Au(111) electrode. In the potential region that MPS adsorbed, very few of Pd could be deposited. After the MPS reductively desorbed, Pd bulk deposition was observed. The grain-refining effect of MPS did not happen in the Pd electrodepostion on Au(111).