Origin of Differing Reactivities of Aliphatic Chains on H−Si(111) and Oxide Surfaces with Metal

Abstract

The interaction of deposited metals with monolayer films is critical to the understanding of, and ultimate utility of, the emerging arena of molecular electronics. We present the results of a thorough study of the interaction of vapor-deposited Au and Ag on alkane films attached to Si substrates. Two distinct categories of films are studied: C18 films formed from the hydrosilation reaction of octadecyl trichlorosilane with thin SiO2 layers and C18 films formed from the direct attachment of functionalized alkanes with hydrogen-terminated Si. Two direct attachment chemistries were studied: (Si)3−Si−O−C linkages of 1-octadecanol and octadecanal on H-terminated Si(111) and (Si)3−Si−C linkages of 1-octadecene on H-terminated Si(111). The reactivity of the films was studied with p-polarized backside reflection absorption spectroscopy (pb-RAIRS), sputter depth profiling X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), and device electrical tests. Independent of direct attachment chemistry, we report the remarkable observation that deposition of Au results in the displacement of the molecular film from the Si interface. In contrast, the directly attached molecular films are robust toward the deposition of Ag. For both metals, the C18 films formed by hydrosilation reactions on SiO2 remain at the interface. The results of monolayer stability with metal are linked to reactions between the metal and substrate. The displacement of the films by Au is attributed to Au insertion in the Si backbonds, in a reaction analogous to silicide formation. The results demonstrate that one must fully take into account the reactivity of the entire system, including substrates, molecular functional groups, and metal electrodes, when considering the robustness of molecules in metal junctions.

Molecules on Si: Electronics with Chemistry

“…[68] In other work powerful techniques for characterizing Si MOMS were used and, in part, adapted. [69][70][71][72] Significant contributions have been made to transport modeling for several Si-molecule systems. [60,73,74] Interfacial monolayers affect the junction electrostatics by the interfacial dipole they create/change (see Sections 3.2, 3.3, and 4.2).…”

“…Excellent reviews on the chemistry of wet [77][78][79] and gas-phase [10,31,80,81] adsorption have been published. The SiÀH bond needs to be activated for the reaction, which can be done by heating (150À200 8C), [44,71] UV illumination, [31,69] Lewis acid-mediated initiators, [42] or electrochemistry. [42,43] Alternatively, H is replaced by Cl, a better leaving group that can react with short Grignard reagents.…”

“…Many functionalized groups, apart from alkenes (C¼ À C), [31,69,80,86] can adsorb on H-terminated Si, such as alkynes (C¼C), [87] alcohols, [69,88] aldehydes, [69] thiols (SH) [72,89] or Grignard reagents. [42,83,90] Aromatic molecules are mostly electrochemically adsorbed as diazonium salts [43,62,91] or by photochemical binding of ethynylbenzene.…”

“…Moreover, metal films become opaque to experimental probes (both optical and charged particle ones) beyond a thickness of $10 nm. [70] Therefore, with few exceptions, [69,70,92,93] most characterizations are limited to surfaces and cannot detect molecules in buried interfaces. One immediate corollary of this situation is an uncertainty about possible changes, induced by contact deposition onto the molecules (cf.…”

“…also Section 7.2, below). [34,67,69,70,92,[94][95][96] Beyond structure and morphology, several surface analyses, mainly spectroscopic ones, can provide information on the electronic structure of the system. As such information cannot be extracted directly from the transport data, the use of complementary methods is crucial for understanding transport mechanisms.…”