Oxide and carbide formation at titanium/organic monolayer interfaces

Area of Science:

• Materials Science
• Surface Science
• Analytical Chemistry

Background:

• Characterizing buried interfaces is crucial for electronic devices.
• Ion-milling depth profiling often damages delicate interface chemistry.
• Understanding metal-organic interfaces informs adhesion and junction performance.

Purpose of the Study:

• To develop and apply a novel UHV delamination XPS technique for analyzing buried Ti/organic interfaces.
• To investigate the chemical state of titanium deposited on organic monolayers.
• To determine the influence of substrate type and deposition conditions on interface formation.

Main Methods:

• Sample delamination in ultrahigh vacuum (UHV) to access buried interfaces.
• X-ray photoelectron spectroscopy (XPS) for chemical state analysis.
• Titanium deposition on Langmuir-Blodgett monolayers (C18 cadmium stearate) on Au, SiO2, and PtO2 substrates.

Main Results:

• UHV delamination XPS provides sharp, non-reduced spectra, enabling quantitative analysis.
• Titanium primarily forms a Ti oxide overlayer (TiO2 and reduced TiOx), not a thin Ti carbide layer as previously assumed.
• Significant substrate-dependent Ti carbide formation (up to 20% on Au) occurs due to oxygen gettering from the organic film and substrate.
• Titanium oxide formation is influenced by oxygen availability, deposition rate, and chamber pressure.

Conclusions:

• UHV delamination XPS is a superior method for characterizing buried metal-organic interfaces without artifacts.
• Titanium's interaction with organic monolayers is complex, involving substantial oxide formation and substrate-dependent carbide generation.
• The findings challenge previous assumptions about titanium adhesion layers and metal contacts in molecular electronics.