Thermal reaction of trimethylphosphine and triethylphosphine on Cu(110)

Adsorption and thermal decomposition of trimethylphosphine (PMe₃) and triethylphosphine (PEt₃) on the Cu(110) surface are investigated using temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation. Both PMe₃ and PEt₃ chemisorb molecularly on three adsorption sites on a Cu(110) surface at 100 K and are mostly desorbed intact upon annealing to 520 K. However, a fraction of chemisorbed PMe₃ and PEt₃ undergoes decomposition via dealkylation. Chemisorbed PEt₃ undergoes thermal decomposition to a greater extent than PMe₃. PMe₃ molecules decompose thermally to form surface CH₃ and phosphorus through PMe₂ as an intermediate. Surface CH₃ eventually disproportionates to form CH₄ and surface carbon at a temperature above 420 K. Chemisorbed PEt₃ molecules undergo stepwise deethylation (PEt₃ → PEt₂ → PEt → P), resulting in surface C₂H₅ and phosphorus; the surface C₂H₅ moiety further decomposes to evolve C₂H₄ and H₂ through β-hydride elimination. Hence a Cu film grown from a CVD precursor containing PEt₃ ligands is almost free of carbon, but however, may be deposited with a higher concentration of phosphorus, relative to PMe₃.