在云母基底上十八烷基膦酸单分子层的飞行时间二次离子质谱研究。

Francis J T, Nie H-Y, McIntyre N S, Briggs D

Surface Science Western, Room G-1, Western Science Centre, The University of Western Ontario, London, Ontario N6A 5B7, Canada.

Langmuir. 2006 Oct 24;22(22):9244-50. doi: 10.1021/la061456i.

In this paper, time-of-flight secondary ion mass spectrometry (ToF-SIMS) under static conditions was used to investigate self-assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) formed on freshly cleaved muscovite mica substrates. The coverage of OPA on mica ranged from 20 to 100%, with a film thickness of 1.7+/-0.2 nm, which was determined by atomic force microscopy (AFM) imaging. The relative intensity of the specific secondary ion species associated with the OPA and with the exposed mica substrate exhibited good correlation with surface coverage. An excellent correlation was also observed (R2=0.98) between the relative SIMS [OPA-H]- intensity and the surface carbon concentration (OPA C 1s, in atomic %) from XPS at the prescribed surface coverage. The observation of positive and negative OPA molecular attachment of secondary ions involving the substrate species is discussed in terms of the chemical affinity of the OPA phosphonate headgroup for the cleaved mica surface as well as the sampling depth. In addition, the OPA molecular attachment species formed with the potassium ions on the cleaved mica substrate dominated the positive secondary ion mass spectrum in the high-mass range. A temperature-dependent, ToF-SIMS study employing in situ heating of a 100% coverage OPA monolayer revealed that the molecules begin to diffuse above approximately 80 degrees C, resulting in a decrease in the relative secondary ion yield of the OPA-specific secondary ions. This observation is hypothesized to be due to a decrease in the effective coverage of the substrate by the OPA molecules, which in turn could be due to the formation of multilayers upon heating in an effort to minimize the energy of the system. The interesting behavior of the novel OPA dimer species as a function of temperature is also reported. It was observed that the relative intensity of OPA and the mica-specific secondary ion peak intensities to that of Si (mica substrate) provides an effective means to estimate the change in coverage at elevated temperatures.

在本文中，采用静态条件下的飞行时间二次离子质谱（ToF-SIMS）来研究在刚解理的白云母云母基底上形成的十八烷基膦酸（OPA）自组装单分子层（SAMs）。通过原子力显微镜（AFM）成像确定，OPA在云母上的覆盖范围为20%至100%，膜厚为1.7±0.2 nm。与OPA以及暴露的云母基底相关的特定二次离子物种的相对强度与表面覆盖率呈现出良好的相关性。在规定的表面覆盖率下，相对SIMS [OPA-H]-强度与XPS测定的表面碳浓度（OPA C 1s，原子百分比）之间也观察到了极好的相关性（R2 = 0.98）。从OPA膦酸酯头基对解理云母表面的化学亲和力以及采样深度的角度讨论了涉及基底物种的二次离子的OPA分子正负附着的观察结果。此外，在解理云母基底上与钾离子形成的OPA分子附着物种在高质量范围内主导了正二次离子质谱。一项采用原位加热100%覆盖的OPA单分子层的温度依赖性ToF-SIMS研究表明，分子在大约80℃以上开始扩散，导致OPA特定二次离子的相对二次离子产率降低。据推测，这一观察结果是由于OPA分子对基底的有效覆盖率降低，而这反过来可能是由于加热时形成多层以尽量减少系统能量所致。还报道了新型OPA二聚体物种随温度变化的有趣行为。观察到OPA和云母特定二次离子峰强度与Si（云母基底）峰强度的相对强度提供了一种估计高温下覆盖率变化的有效方法。