Self-Assembly And Reaction Of Alkanes On Metal And Metal Oxide Surfaces

Molecular self-assembly on surfaces is defined as a process by which ordered aggregates form spontaneously on the surface upon the adsorption of molecules.The self-assembled structures are determined by the subtle balance between molecule-molecule and molecule-substrate interactions.Molecular self-assembly is also affected by external factors.A large number of studies have also been conducted to reveal the effect of substrates on molecular self-assembly,including the effects of different surfaces,different crystal planes of the same metal,and reconstructed structures of the metals.Nevertheless,the methods that can be applied to control the self-assembly on surfaces are still limited.Normal alkanes are one of the earliest organic molecules used for the investigation of self-assembly on surfaces,and can also be used as a template for the growth of metals or organics.We chose alkanes as target molecules to study how the arrangement of molecules on the surface is regulated by the substrate surface.In addition,the rise of on-surface chemistry has recently become an important research method in the field of surface science.On the basis of studying the self-assembly of alkanes,selecting a suitable substrate for the selective activation of alkanes on the surface for further research is of potential practical significance for the conversion of alkanes to high-value chemicals and efficient use of alkanes.This thesis includes following contents by using ultra-high-vacuum low temperature scanning tunneling microscope as the main characterization tool:(1)Regulating the direction of alkane self-assembly by polymer-modified step edgesControlling the orientations of the constitutes in self-assembled monolayers on surfaces is an effective means to construct high quality organic devices.Herein,we utilize linear polymer-modified step edges as a simple and peculiar template for regulating the subsequent self-assembly of alkane molecules(n-C32H66).With this strategy,all alkane molecules orientate along the step edge([011]directions).Combining with DFT calculations,we figure out the mechanism.The relative strong van der Waals interactions between the parallel aligned carbon chains decrease the total energy.As a consequence,the homogeneous orientation of the pre-decorated polymers leads to the well alignments of the self-assembly structures.(2)Self-assembly of alkanes on the surface of Cu2O(111)The self-assembly of alkane molecules on the surface of various ordered Cu2O(111)surfaces represents a new strategy to explore the self-assembly of alkane on metal oxides.By controlling the temperature of substrate,the partial pressure and the dosing time of O2,various ordered structures such as "29" Cu2O(111)and "44" Cu2O(111)were formed on Cu(111)surfaces.Depositing n-C 32H66 molecules on "29" and "44" Cu2O(111)surfaces,different adsorption configurations were observed by low temperature scanning tunneling microscope.The difference in the force balance between the alkane molecule-molecule and molecule-substrate on the surfaces of Cu2O(111)structures results in different molecular assembled structures.(3)Structural changes of Cu2O(111)induced by the adsorption of alkanesUnder milder conditions,the Cu2O(111)surface can catalyze the oxidative dehydrogenation of alkanes,resulting in the structural changes of the Cu2O(111)surfaces.By thermally activating the sample,the oxidative dehydrogenation of short-chain alkanes(i-C4H10)and long-chain alkanes(n-C32H66)was studied on Cu2O(111)surface.Structural changes of Cu2O(111),specifically the formation of a new triangular structure was observed by STM.Our research is expected to provide new ideas for the reaction of alkanes on the Cu2O(111)surface.