Unraveling Molecular Adsorption with Surface Raman Spectroscopy: trans-Stilbene, trans,trans-Distyrylbenzene, and trans-Azobenzene on Ag/Ge(111)

Abstract

Deposition of organic molecules on inorganic substrates is an important step in the fabrication of organic electronic devices with designated optoelectronic properties. Their interaction with the surface underneath and the interaction among themselves influence the molecular orientation of the first adsorbed layer and thus energy and charge transport properties of the grown organic films. We show that these interactions of transstilbene, trans,trans-distyrylbenzene, and trans-azobenzene adsorbed on Ag/Ge(111) -( 3 × 3)R30°surface are reflected on their Raman signatures. Detailed spectral analysis reveals the characteristic vibrational features corresponding to the molecules residing in the first adlayer and in the layers above, separately. The results further show that for all the three different molecular systems, the integrated intensity of the multilayer Raman signature increases linearly with the molecular coverage. This study explores potential utilization of Raman spectroscopy as a sensitive, noninvasive tool to characterize the intricate molecule-surface interaction and to quantify physisorbed molecular systems.

Exploring Azobenzenethiol Adsorption on the Ag/Ge(111) Surface with Surface Raman Spectroscopy

“…The intensity of the 1437 cm –1 peak for the 1 ML adsorption is larger than that for the 0.3 ML adsorption, while the intensities of 1423 cm –1 peak at the two adsorption coverages are almost equal. This result can be compared with our previous works on stilbene and azobenzene adsorbed on Ag/Ge(111)-√3. , As an instance of azobenzene, the NN stretch undergoes a significant red-shift of 14 cm –1 for submonolayer trans -azobenzene as compared to multilayer case. It is also known that the molecular conformation and adsorption orientation are expected to vary at different coverages.…”

“…We have taken these advantages to study various adsorbate systems: hydrogenated Si and Ge surfaces; 29−31 and silbene, distyrylbenzene, and azobenzene on Ag/Ge(111)-√3. 17 In particular, we have identified the first adsorbed layer and the adsorbates above it with their distinct Raman signatures and have utilized their respective signatures to determine the molecular coverage, confirming the potential of using NSRS as a sensitive, noninvasive tool to characterize surface adsorbates. These unique competencies would render the NSRS study of SAMs a brand new analytic way to reveal the intricate interactions involved in a molecular scale.…”

“…Is it possible that the 1423 and 1437 cm –1 peaks, corresponding to adsorbed molecules with different tilt angles, represent these two different adsorption sites? In light of the strong interaction between the azobenzene moiety and the silver atoms, as revealed from our previous study, , it is expected that the adsorption site for the lying-down adsorbates is the silver trimer on the Ag/Ge(111)-√3 surface . The answer to this question would become more illuminated when these adsorption kinetics are compared with that on the bare Ge(111) surface.…”

“…Although many techniques have been utilized in the adsorption studies of SAMsincluding scanning probe microscopy (STM) for molecule-resolved distribution of adsorbates, X-ray photoemission spectroscopy (XPS) for surface chemical composition, sum-frequency generation spectroscopy (SFG) for surface molecular orientation and vibrational identification, surface-enhanced Raman scattering (SERS) for detecting diminutive molecular trace on nanostructured metal surfaces, − etc.the challenge nonetheless has always been how to acquire physically meaningful information from a minute amount of surface adsorbates (from submonolayer to one monolayer). In this study, we performed in situ polarized Raman spectroscopic measurements of the adsorption of azoSH on the Ag/Ge(111)-√3 surface in vacuum, providing several unique characteristics delineated below that are impossible or hard to come by with the aforementioned onesthe advantages of using normal surface Raman spectroscopy (NSRS) to interrogate molecular adsorption on surfaces have been explained and demonstrated in our previous publications. , In brief, it is capable of noninvasively acquiring the surface composition and structure from the obtained vibrational signature; its high energy resolution enables the resolution of minutely disparate interactions happening upon adsorption; the adsorbate orientation can be deduced with polarized Raman spectroscopy; the amount of adsorbates can be quantified with the procured Raman signal strength. We have taken these advantages to study various adsorbate systems: hydrogenated Si and Ge surfaces; − and silbene, distyrylbenzene, and azobenzene on Ag/Ge(111)-√3 .…”

“…In this study, we performed in situ polarized Raman spectroscopic measurements of the adsorption of azoSH on the Ag/Ge(111)-√3 surface in vacuum, providing several unique characteristics delineated below that are impossible or hard to come by with the aforementioned onesthe advantages of using normal surface Raman spectroscopy (NSRS) to interrogate molecular adsorption on surfaces have been explained and demonstrated in our previous publications. , In brief, it is capable of noninvasively acquiring the surface composition and structure from the obtained vibrational signature; its high energy resolution enables the resolution of minutely disparate interactions happening upon adsorption; the adsorbate orientation can be deduced with polarized Raman spectroscopy; the amount of adsorbates can be quantified with the procured Raman signal strength. We have taken these advantages to study various adsorbate systems: hydrogenated Si and Ge surfaces; − and silbene, distyrylbenzene, and azobenzene on Ag/Ge(111)-√3 . In particular, we have identified the first adsorbed layer and the adsorbates above it with their distinct Raman signatures and have utilized their respective signatures to determine the molecular coverage, confirming the potential of using NSRS as a sensitive, noninvasive tool to characterize surface adsorbates.…”