Isolating the Nonlinear Optical Response of a MoS2_2 Monolayer under Extreme Screening of a Metal Substrate

Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D) direct bandgap semiconductors, hold promise for advanced optoelectronic and photocatalytic devices. Interaction with three-dimensional (3D) metals, like Au, profoundly affects their optical properties, posing challenges in characterizing the monolayer's optical responses within the semiconductor-metal junction. In this study, using precise polarization-controlled final-state sum frequency generation (FS-SFG), we successfully isolated the optical responses of a MoS$_2$ monolayer from a MoS$_2$/Au junction. The resulting SFG spectra exhibit a linear lineshape, devoid of A or B exciton features, attributed to the strong dielectric screening and substrate induced doping. The linear lineshape illustrates the expected constant density of states (DOS) at the band edge of the 2D semiconductor, a feature often obscured by excitonic interactions in week-screening conditions such as in a free-standing monolayer. Extrapolation yields the onset of a direct quasiparticle bandgap of about $1.65\pm0.20$ eV, indicating a strong bandgap renormalization. This study not only enriches our understanding of the optical responses of a 2D semiconductor in extreme screening conditions but also provides a critical reference for advancing 2D semiconductor-based photocatalytic applications.Comment: 14 pages, 4 figures + supplemental materia

Electron dynamics in a heterogeneous system: thin Ag films on Si(100)

Kennerknecht C, Dantscher S, Pfeiffer W, Autzen O, Wesenberg C, Hasselbrink E. Electron dynamics in a heterogeneous system: thin Ag films on Si(100). Surface Science. 2006;600(18):4269-4274.Photodesorption dynamics is strongly influenced by the electronic structure and electron dynamics of the substrate. Here we use time-resolved two-photon photoemission spectroscopy to investigate electron relaxation and transport in thin ultra-flat epitaxial Ag films on Si(I 00). Two-photon photoemission spectra obtained for excitation with 3.1 eV photons reveal that electron emission occurs preferentially from the Ag film. In contrast, the photoelectrons generated by two-photon excitation with 4.7 eV photons originate from the Si substrate. This conclusion is supported by intensity dependent spectral changes that can only be reconciled if both excitation steps occur in the Si substrate. In addition, this is corroborated by time-resolved experiments that reveal slow electron relaxation dynamics as it is expected for Si. Our results indicate that the excitation in the Si substrate and the transmission of electrons through the thin Ag film play an important role in electron dynamics in the heterogeneous system and can well be responsible for the film thickness and excitation wavelength dependence of photodesorption of NO2. (c) 2006 Elsevier B.V. All rights reserved

The interactions of Na, NO, and H2O on the graphite (0001) surface

The reactions of Na and NO on the (0001) surface of graphite and the influence of coadsorbed water on these reactions have been studied by thermal desorption spectroscopy. The products of the NO+Na reactions are dependent on the partial concentrations in the initial coadsorbed layer. For NO:Na dose ratios less than 1:1, N2 formation is dominant. For higher NO doses, this is superseded by N2O formation. In all cases sodium is oxidized by the NO. This leads to carbonate formation, which subsequently decomposes to release CO2 and CO. The addition of H2O at low coverages to the Na+NO system complicates the reactions. It results in ammonia formation by two independent mechanisms. The formation of NH3 is strongly dependent on the water dose. In addition to generating NH3, coadsorbed water alters the sodium oxidation pathway resulting in an enhanced formation of CO2 at certain coverages. Large H2O coverages block the NO reaction pathways by forming an inert "hypermetalated" hydroxide overlayer. The surface composition of this hydroxide is of the type Na2OH or Na3OH. Decomposition of this overlayer results in the desorption of a significant fraction of stable Na2OH molecules

2-Propanol Interacting with Co3O4(001):A combined AIMD and vSFS study

The interaction of 2-propanol with Co3O4(001) was studied by vibrational sum fre-quency spectroscopy (vSFS) and by ab initio molecular dynamics (AIMD) simulations of 2-propanol dissolved in a water film to gain insight at the molecular level into the pathways of catalytic oxidation. The experimental study has been performed under near ambient condition, where the presence of water vapor is unavoidable, resulting in a water film on the sample and thereby allowing us to mimic the solution-water interface. Both experiment and theory conclude that 2-propanol adsorbs molecularly. The lack of dissociation is attributed to the adsorption geometry of 2-propanol in which the O-H bond does not point towards the surface. Furthermore, the copresent water not only competitively adsorbs on the surface but also inhibits 2-propanol deprotonation. The calculations reveal that the presence of water deactivates the lattice oxygen, thereby reducing the surface activity. This finding sheds light on the multifaceted role of water at the interface for the electrochemical oxidation of 2-propanol in aqueous solution as recently reported. At higher temperatures 2-propanol remains molecularly adsorbedon Co3O4(001) until it desorbs with increasing surface temperature