Structural Characterization of 4-Bromostyrene Self-Assembled Monolayers on Si(111)

Abstract

Organic functionalization of silicon holds promise for a variety of applications ranging from molecular electronics to biosensing. Because the performance and reliability of organosilicon devices will be intimately tied to the detailed structure of the organic adlayers, it is imperative to develop systematic strategies for forming and characterizing self-assembled monolayers (SAMs) on silicon with submolecular spatial resolution. In this study, we use 4-bromostyrene for the photochemical growth of Br-terminated SAMs on Si(111). A variety of experimental and theoretical techniques including atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), and density functional theory (DFT) have been employed to determine the coverage and conformation of the 4-bromostryene molecules within the SAM. In particular, AFM verifies a continuous and atomically flat SAM, and the XRR data indicate a SAM thickness of 8.50 A and a molecular coverage of 46% of the surface silicon atoms. Because the DFT calculations indicate a molecular length of 8.89 A, the measured XRR thickness implies a molecular tilt angle of approximately 17 degrees. The XRR analysis also suggests that the Br atoms are preserved on top of the SAM in agreement with XPS measurements that show bromine bound solely to carbon and not to silicon. XRF reveals a Br atomic coverage of 50%, again in close agreement to that found by XRR. Single-crystal Bragg diffraction XSW is used to generate a three-dimensional map of the Br distribution within the SAM, which in conjunction with the XRR result suggests that the 4-bromostyrene molecules are tilted such that the Br atoms are located over the T4 sites at a height of 8.50 A above the top bulklike Si(111) layer. The direction of molecular tilt toward the T4 sites is consistent with that predicted by the DFT calculation. Overall, through this unique suite of complementary structural characterization techniques, it is concluded that the Br functional handle is preserved at the top of the SAM and is available for further substitutional chemistry.

Molecular Self‐Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin‐Film Transistor Applications

“…[139,140] One of the more extensively studied methods uses the functionalized reconstructed surface under ultrahigh vacuum (UHV), where molecular precursors with an unsaturated head group attach to the H-Si surface via a cycloaddition reaction. [141] Many studies have used the Si-H surface to investigate the electrical properties of SAMs, and a recent review describes these types of systems in more detail. [70] …”

Substituent Variation Drives Metal/Monolayer/Semiconductor Junctions from Strongly Rectifying to Ohmic Behavior

“…However, the saturated system does not allow for large dipole variation and the non‐conjugated molecules can add a significant transport barrier,19 drawbacks that are absent for aromatic ring‐based monolayers. Such monolayers, directly bound to Si, were prepared photochemically,22–24 electrochemically,25, 26 or by using the reactive (2 × 1) Si(100) surface under UHV conditions 4, 27. Nevertheless, the molecular effect on transport was rather weak 24, 26…”

“…Br‐terminated modification of Si22, 39 that keeps its excellent electrical quality is important, because it offers varied possibilities for chemical reactions for further substitution chemistry of the monolayer 40–42. Strikingly, the Br‐styrene junction (green curve of Figure 2a) is above that of the H‐Si junction (dark red curve of Figure 2a), i.e., more current passes through the junction with a close to 1 nm thick interfacial molecular layer!…”

Chemical manipulation of multifunctional hydrocarbons on silicon surfaces

“…Dense and well-characterized monolayers of styrene derivatives, such as 4-bromostyrene, can also be grown on Si(111) in a similar manner [411,412]. The features shown in Fig.…”