Blackstock Jason J, Donley Carrie L, Stickle William F, Ohlberg Douglas A A, Yang J Joshua, Stewart Duncan R, Williams R Stanley
Information and Quantum Systems Laboratory, Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, California 94304, USA.
J Am Chem Soc. 2008 Mar 26;130(12):4041-7. doi: 10.1021/ja710448e. Epub 2008 Mar 5.
X-ray photoelectron spectra (XPS) are reported from a series of buried titanium/organic monolayer interfaces accessed through sample delamination in ultrahigh vacuum (UHV). Conventional characterization of such buried interfaces requires ion-mill depth profiling, an energetic process that frequently destroys bonding information by chemically reducing the milled material. In contrast, we show that delaminating the samples at the metal/organic interface in vacuum yields sharp, nonreduced spectra that allow quantitative analysis of the buried interface chemistry. Using this UHV delamination XPS, we examine titanium vapor deposited onto a C18 cadmium stearate Langmuir-Blodgett monolayer supported on Au, SiO2, or PtO2 substrates. Titanium is widely used as an adhesion layer in organic thick film metallization as well as a top metal contact for molecular monolayer junctions, where it has been assumed to form a few-atoms-thick Ti carbide overlayer. We establish here that under many conditions the titanium instead forms a few-nanometers-thick Ti oxide overlayer. Both TiO2 and reduced TiOx species exist, with the relative proportion depending on oxygen availability. Oxygen is gettered during deposition from the ambient, from the organic film, and remarkably, from the substrate itself, producing substrate-dependent amounts of Ti oxide and Ti carbide "damage". On Au substrates, up to 20% of the molecular-monolayer carbon formed titanium carbide, SiO2 substrates approximately 15%, and PtO2 substrates <5%. Titanium oxide formation is also strongly dependent on the deposition rate and chamber pressure.
通过超高真空（UHV）中的样品分层获得了一系列掩埋钛/有机单层界面的X射线光电子能谱（XPS）。对于此类掩埋界面的传统表征需要离子束深度剖析，这是一个高能过程，经常会通过化学还原研磨材料而破坏键合信息。相比之下，我们表明在真空中在金属/有机界面处对样品进行分层可产生清晰、未还原的光谱，从而能够对掩埋界面化学进行定量分析。使用这种超高真空分层XPS，我们研究了沉积在支撑在Au、SiO2或PtO2衬底上的C18硬脂酸镉Langmuir-Blodgett单层上的钛蒸气。钛广泛用作有机厚膜金属化中的粘附层以及分子单层结的顶部金属接触，在这些应用中，人们认为它会形成几原子厚的碳化钛覆盖层。我们在此确定，在许多情况下，钛反而会形成几纳米厚的氧化钛覆盖层。TiO2和还原的TiOx物种都存在，相对比例取决于氧气的可用性。在沉积过程中，氧气从环境、有机膜以及显著地从衬底本身中被吸收，产生了取决于衬底的氧化钛和碳化钛“损伤”量。在Au衬底上，高达20%的分子单层碳形成了碳化钛，SiO2衬底约为15%，PtO2衬底<5%。氧化钛的形成也强烈依赖于沉积速率和腔室压力。
