Probing of an Adsorbate-Specific Excited State on an Organic Insulating Surface by Two-Photon Photoemission Spectroscopy

In this study, we investigate the photoexcited electronic states of ferrocene (Fc) molecules adsorbed on an organic insulating surface by two-photon photoemission spectroscopy. This insulating layer, composed of a decanethiolate self-assembled monolayer formed on an Au(111) substrate, enables us to probe the electronically excited states localized at the adsorbed Fc molecules. The adsorbate-specific state is resonantly excited by photons at 4.57 eV, which is 0.5 eV smaller than the energy of the first molecular Rydberg state of free Fc in the gas phase. This result indicates that the electrons are bound to both the excited hole formed in the adsorbate and the positive image charge induced in the substrate. The hybridized electronic characteristics of the adsorbate-specific state are responsible for the strong transition selectivity and short lifetime of the excited state

Energy Level Alignment of Organic Molecules with Chemically Modified Alkanethiolate Self-Assembled Monolayers

We have employed two-photon photoemission spectroscopy to nondestructively resolve the unoccupied energy levels of fullerene C₆₀ molecules deposited on alkanethiolate self-assembled monolayers (SAMs). By fluorine substitution of the hydrogen atoms in the alkyl chain, the work function (WF) increased from 4.3 eV for the alkanethiolate-SAM (H-SAM) to 5.7 eV for the fluorine-substituted SAM (F-SAM), owing to the formation of surface dipole layers. When C₆₀ is deposited on the H-SAM and F-SAM, the energy positions of the unoccupied/occupied levels of C₆₀ are pinned to the vacuum level (Fermi level (<i>E</i>_F) + WF). As a result of the energy level alignment, on the F-SAM, the relative energy from <i>E</i>_F of the highest occupied molecular orbital of C₆₀ almost equals that of the lowest unoccupied molecular orbital, implying that the C₆₀ film on the F-SAM exhibits both p- and n-type (ambipolar) charge transport properties, while C₆₀ is known as a typical n-type semiconductor. The energetics are preserved even with multilayered C₆₀ films at least up to ∼5 nm in thickness, showing that the dipole layers induced by SAMs are robust against molecular overlayers. Such a spectroscopic study on the energy levels for organic films will be of importance for further development of organic thin film devices

Photoexcited State Confinement in Two-Dimensional Crystalline Anthracene Monolayer at Room Temperature

Organic thin film electronics place a high demand on bottom-up technology to form a two-dimensionally (2D) functional unit consisting of a single molecular crystalline layer bound to a layered structure. As the strong interaction between a substrate and molecules makes it difficult to evaluate the electronic properties of organic films, the nature of electronic excited states has not been elucidated. Here, we study a 2D crystalline anthracene monolayer electronically decoupled by alkanethiolates on a gold substrate using scanning tunneling microscopy and time-resolved two-photon photoemission spectroscopy and unravel the geometric/electronic structures and excited electron dynamics. Our data reveal that dispersive 2D excited electrons on the surface can be highly coupled with an annihilation of nondispersive excitons that facilitate electron emission with vibronic interaction. Our results provide a fundamental framework for understanding photoexcited anthracene monolayer and show how the coupling between dispersive and nondispersive excited states may assist charge separation in crystalline molecular layers

Charge Transfer Complexation of Ta-Encapsulating Ta@Si₁₆ Superatom with C₆₀

The tantalum-encapsulating Si₁₆ cage nanocluster superatom (Ta@Si₁₆) has been a promising candidate for a building block of nanocluster-based functional materials. Its chemical states of Ta@Si₁₆ deposited on an electron acceptable C₆₀ fullerene film were evaluated by X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS, respectively). XPS results for Si, Ta, and C showed that Ta@Si₁₆ combines with a single C₆₀ molecule to form the superatomic charge transfer (CT) complex, (Ta@Si₁₆)⁺C₆₀^–. The high thermal and chemical robustness of the superatomic CT complex has been revealed by the XPS and UPS measurements conducted before and after heat treatment and oxygen exposure. Even when heated to 720 K or subjected to ambient oxygen, Ta@Si₁₆ retained its original framework, forming oxides of Ta@Si₁₆ superatom

Charge Separation at the Molecular Monolayer Surface: Observation and Control of the Dynamics

Charge separation dynamics relevant to an electron transfer have been revealed by time- and angle-resolved two-photon photoemission spectroscopy for an <i>n</i>-alkanethiolate self-assembled monolayer (SAM) on a Au(111) surface fabricated by a chemical-wet process. The electron was photoexcited into an image potential state located at 3.7 eV above the Fermi level (<i>E</i>_F), and it survived well for more than 100 ps on dodecanethiolate (C12)-SAM. The degree of electron separation is precisely controlled by selecting the length of the alkyl chain (C10–C18). We have also evaluated molecular conductivity at the specific electron energy of <i>E</i>_F + 3.7 eV. The tunneling decay parameter, β, was fitted by β_90 K = 0.097 Å^–1 and β_RT = 0.13 Å^–1. These values were one order smaller than that at around <i>E</i>_F by conventional contact probe methods