Oxide and Carbide Formation at Titanium/Organic Monolayer Interfaces

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• 作者：Jason J. Blackstock ; Carrie L. Donley ; William F. Stickle ; Douglas A. A. Ohlberg ; J. Joshua Yang ; Duncan R. Stewart ; R. Stanley Williams
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：March 26, 2008
• 年：2008
• 卷：130
• 期：12
• 页码：4041 - 4047
• 全文大小：2290K
• 年卷期：v.130,no.12(March 26, 2008)
• ISSN：1520-5126

文摘

X-ray photoelectron spectra (XPS) are reported from a series of buried titanium/organic monolayerinterfaces accessed through sample delamination in ultrahigh vacuum (UHV). Conventional characterizationof such buried interfaces requires ion-mill depth profiling, an energetic process that frequently destroysbonding information by chemically reducing the milled material. In contrast, we show that delaminating thesamples at the metal/organic interface in vacuum yields sharp, nonreduced spectra that allow quantitativeanalysis of the buried interface chemistry. Using this UHV delamination XPS, we examine titanium vapordeposited onto a C₁₈ cadmium stearate Langmuir-Blodgett monolayer supported on Au, SiO₂, or PtO₂substrates. Titanium is widely used as an adhesion layer in organic thick film metallization as well as a topmetal contact for molecular monolayer junctions, where it has been assumed to form a few-atoms-thick Ticarbide overlayer. We establish here that under many conditions the titanium instead forms a few-nanometers-thick Ti oxide overlayer. Both TiO₂ and reduced TiO_x species exist, with the relative proportiondepending on oxygen availability. Oxygen is gettered during deposition from the ambient, from the organicfilm, and remarkably, from the substrate itself, producing substrate-dependent amounts of Ti oxide and Ticarbide "damage". On Au substrates, up to 20% of the molecular-monolayer carbon formed titanium carbide,SiO₂ substrates ~15%, and PtO₂ substrates <5%. Titanium oxide formation is also strongly dependent onthe deposition rate and chamber pressure.