Differentiation of molecules in a mixed self-assembled monolayer of H- and Cl-terminated bicyclo [2.2.2]octane derivatives
Differentiation of molecules in a mixed self-assembled monolayer of H- and Cl-terminated bicyclo [2.2.2]octane derivatives
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DOI：
10.1088/0957-4484/17/7/s03
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发表时间：
2006-04-14
期刊：
影响因子：
3.5
通讯作者：
Fujihira, M
中科院分区：
文献类型：
--
作者：
Fujii, S;Fujihira, M
利用带有CaF 2纳米团簇的改性Si针尖对Au（111）表面混合自组装单分子层（SAM）中的双环[2.2.2]辛烷（BCO）衍生物的H-和Cl-端基进行了成像，以区分具有不同电负性的两个末端。为了实现这一点，我们制作了一个新的样品保持器，在其上可以并排安装CaF 2单晶和Au（111）上的混合SAM。我们将带有两个样品的保持器转移到真空（UHV）原子力显微镜（AFM）室中。在UHV下切割CaF 2单晶时，在分离的2 mm内出现与SAM表面平行的新鲜且干净的CaF 2（111）表面。通过反复使Si尖端与CaF 2（111）表面接触来制备改性Si尖端。所得的改性针尖可根据针尖终止离子（即分别为F-和Ca 2+离子）的符号对CaF 2（111）表面上的Ca 2+和F-亚晶格的原子周期性成像，如先前所报道的（Foster等，2002 Phys. Rein B 66 235417）。使用已知的尖端终止离子的修改后的Si针尖，我们观察到的Cl-终端周围的H-终端在混合SAM的非接触（NC）原子力显微镜。在此，Cl-末端由于其电负性而带负电荷，因此具有Cl-末端的BCO部分被C δ（+）-Cl δ-永久偶极终止，而H-末端几乎是中性的。Cl-末端显得更亮（更有吸引力）和黑暗在NC-AFM图像中，根据尖端终止离子的符号，即分别在修饰的尖端上的Ca 2+和F-，尽管图像对比度和尖端终止离子的符号之间的关系并不总是完美的，因为尖端终止离子在纳米团簇上的不稳定性。本方法可用于区分不同电负性的端基。
H- and Cl-terminal groups of bicyclo[2.2.2] octane (BCO) derivatives in a mixed self-assembled monolayer (SAM) on Au(111) were imaged using a modified Si tip with a CaF2 nanocluster to differentiate the two terminals, which have different electronegativities. In order to achieve this we fabricated a new sample holder, on which a CaF2 single crystal and the mixed SAM on Au(111) could be mounted side by side. We transferred the holder with the two samples into a ultrahigh vacuum (UHV) atomic force microscopy (AFM) chamber. Upon cleaving the CaF2 single crystal under UHV, a fresh and clean CaF2(111) surface parallel with the SAM surface appeared within 2 mm of the separation. The modified Si tip was prepared by repeatedly making contact between a Si tip and the CaF2(111) surface. The resulting modified tip could image the atomic periodicity of a Ca2+ and an F- sublattice on the CaF2(111) surface depending on the sign of the tip-terminating ion, i.e. an F- and a Ca2+ ion, respectively, as reported previously (Foster et al 2002 Phys. Rein B 66 235417). Using the modified Si tip with the known tip-terminating ion, we observed the Cl-terminal in the surrounding H-terminals in the mixed SAM by noncontact (NC) AFM. Here, the Cl-terminal is negatively charged due to its electronegativity and thus the BCO moiety with the Cl-terminal is terminated by a C delta(+)-Cldelta- permanent dipole, while the H-terminal is almost neutral. The Cl-terminal appeared brighter (more attractive) and darker (more repulsive) than the surrounding H-terminals in NC-AFM images depending on the sign of the tip-terminating ion, i.e. a Ca2+ and an F- on the modified tip, respectively, although the relationship between the image contrast and the sign of the tip-terminating ion was not always perfect because of the instability of the tip-terminating ion on the nanocluster. The present method can be used to distinguish terminal groups with different electronegativities.