Chemical reactions on solid surfaces: Atomistic observations by scanning tunneling microscopy

A scanning tunneling microscope has been developed which allows manipulation and control of the surface as well as scanning tunneling microscopy without transfer of the crystal off the manipulator to the microscope. The microscope is of the ‘Johnnie Walker’ type and is lowered onto the crystal, which is itself housed in a versatile transfer carriage. In this fashion surfaces can be ion bombarded, chemically treated, etc. and examined by the microscope at temperatures ranging from 120–400 K with atomic resolution without change in surface temperature. With this microscope we have studied the reaction of carbon monoxide with oxygen on Cu(110). At an ambient pressure of 10−4 Torr carbon monoxide reacts with the p(2 × 1) islands of oxygen at a measurable rate only at 400 K or above. Reaction is approximately 1000 times faster along the (001) direction of the oxide islands than along the (110) direction. Reaction does not occur internal to the island unless there are defects to initiate the reaction. At 400 K the oxide islands do not maintain an equilibrium shape or distribution when reacting with ambient carbon monoxide, though in the presence of a low pressure of oxygen they appear to do so. At 150 K the p(2 × 1) oxide islands are inert to reaction with carbon monoxide and, additionally, short CuO chains are also unreactive. Reaction between pre-adsorbed carbon monoxide and ambient oxygen is observed, implying that mobile oxygen atoms which have not yet formed surface-bound pseudomolecules, can react with adsorbed carbon monoxide.