Charge transfer between the Au(111) surface and adsorbed C60: Resonant photoemission and new core-hole decay channels

Surface-Enhanced Raman Scattering Stimulated by Strong Metal-Molecule Interactions in a C60 Single-Molecule Junction

Specifying the geometric and electronic structures of a metal-molecule interface at the single-molecule level is crucial for the improvement of organic electronics. A single-molecule junction (SMJ) can be used to investigate interfaces because it can be regarded as an elementary unit of the interface structure. Although considerable efforts have been made to this end, the detection of structural changes in SMJs associated with metal-molecule interactions remains challenging. In this study, we detected the surface-enhanced Raman scattering (SERS) signal originating from the metal-molecule interaction change induced by a local structural change in a C₆₀ SMJ. This junction has attracted wide attention owing to its unique electronic and vibronic properties. We fabricated a C₆₀ SMJ using a lithographically fabricated Au electrode and measured the SERS spectra along with the current-voltage (I-V) response. By continuous measurement of SERS for the C₆₀ SMJ, we obtained SERS spectra dependent on the local structural change. The analysis of the I-V response revealed that the vibration energy shift originates from the change in the local structure for different Au-C₆₀ interactions. Based on the discrimination of the states in accordance with the Au-C₆₀ interaction, we found that the probability of SERS for geometry with a large Au-C₆₀ interaction was enhanced.

From high quality packing to disordered nucleation or phase separation in donor/acceptor interfaces: ClAlPc-C60 on Au(111)

The dramatic consequences that the orientation adopted by the molecular dipoles, in diverse arrays of chloroaluminum phthalocyanine (ClAlPc) on Au(111), have on the ulterior adsorption and growth of C60 are explored by means of an all scanning probe microscopy approach. The unidirectional downwards organization of the molecular dipoles at the first layer reduces charge transfer from the metal to C60. Imbalance between attractive and repulsive interactions of the fullerenes are crucial for their ordered supramolecular aggregation. The effect at the basis of such self-assembling seems to be released by the all upwards dipole orientation adopted on the ClAlPc second layer. The low electronic corrugation of the bilayer results in a higher mobility of the fullerenes which for similar coverages diffuse large distances to reach uncovered first layer regions. Density functional theory calculations corroborate the experimental observations indicating the relevance of charge transfer, potential energy surface corrugation, C60 on-surface diffusion barriers and screening. The structure of the co-adsorbed C60 and ClAlPc layers strongly depends on the deposition sequence. Phase-separation, where each molecule adopts the single-component assembly, occurs if C60 is deposited first. The present results contribute to understanding the influence of the dipolar nature of molecular layers on the electronic and structure of donor/acceptor heterojunctions, which is crucial for device design via engineering the energy level alignment at organic-organic and organic-metal interfaces.

Adsorption and charge transfer interactions of bi-isonicotinic acid on Ag(111)

The adsorption and charge transfer dynamics of the organic molecule bi-isonicotinic acid (4,4-dicarboxy-2,2-bipyridine) on single crystal Ag(111) has been studied using synchrotron radiation-based photoemission, x-ray absorption, and resonant core spectroscopies. Measurements for multilayer and monolayer coverage are used to determine the nature of the molecule-surface interactions and the molecular orientation. An experimental density of states for the monolayer with respect to the underlying metal surface is obtained by combining x-ray absorption spectroscopy at the N 1s edge and valence photoemission to measure the unoccupied and occupied valence states, respectively. This shows that the lowest unoccupied molecular orbital in the core-excited state lies energetically below the Fermi level of the surface allowing charge transfer from the metal into this orbital. Resonant photoelectron spectroscopy was used to probe this charge transfer in the context of super-spectator and super-Auger electron transitions. The results presented provide a novel interpretation of resonant core-level spectroscopy to explore ultra-fast charge transfer between an adsorbed organic molecule and a metal surface through the observation of electrons from the metal surface playing a direct role in the core-hole decay of the core-excited molecule.

Hybridization-Induced Carrier Localization at the C60 /ZnO Interface

Electronic coupling and ground-state charge transfer at the C60 /ZnO hybrid interface is shown to localize carriers in the C60 phase. This effect, revealed by resonant X-ray photoemission, arises from interfacial hybridization between C60 and ZnO. Such localization at carrier-selective electrodes and interlayers may lead to severely reduced carrier harvesting efficiencies and increased recombination rates in organic electronic devices.

Ultrafast electron injection into photo-excited organic molecules

State-of-the-art X-ray spectroscopy allows femtosecond gating of energy levels of photo-excited molecules on a metal substrate enabling ultrafast and bi-directional charge transfer across the interface with controllable dependence on the molecular adsorption geometry.

Influence of steric hindrance on the molecular packing and the anchoring of quinonoid zwitterions on gold surfaces

The N-substituent on quinonoid zwitterions influences the molecules packing and impacts their anchoring on gold surfaces.

Ultrafast Bidirectional Charge Transport and Electron Decoherence at Molecule/Surface Interfaces: A Comparison of Gold, Graphene, and Graphene Nanoribbon Surfaces

We investigate bidirectional femtosecond charge transfer dynamics using the core-hole clock implementation of resonant photoemission spectroscopy from 4,4'-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core-hole creation in all systems, opening an additional decay channel for the core-hole, involving electron donation from substrate to the molecule. Furthermore, using the core-hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface.

Vacancy formation on C60/Pt (111): unraveling the complex atomistic mechanism

The interaction of fullerenes with transition metal surfaces leads to the development of an atomic network of ordered vacancies on the metal. However, the structure and formation mechanism of this intricate surface reconstruction is not yet understood at an atomic level. We combine scanning tunneling microscopy, high resolution and temperature programmed-x-ray photoelectrons spectroscopy, and density functional theory calculations to show that the vacancy formation in C60/Pt(111) is a complex process in which fullerenes undergo two significant structural rearrangements upon thermal annealing. At first, the molecules are physisorbed on the surface; next, they chemisorb inducing the formation of an adatom-vacancy pair on the side of the fullerene. Finally, this metastable state relaxes when the adatom migrates away and the vacancy moves under the molecule. The evolution from a weakly-bound fullerene to a chemisorbed state with a vacancy underneath could be triggered by residual H atoms on the surface which prevent a strong surface-adsorbate bonding right after deposition. Upon annealing at about 440 K, when all H has desorbed, the C60 interacts with the Pt surface atoms forming the vacancy-adatom pair. This metastable state induces a small charge transfer and precedes the final adsorption structure.

Quantifying through-space charge transfer dynamics in π-coupled molecular systems

Understanding the role of intermolecular interaction on through-space charge transfer characteristics in π-stacked molecular systems is central to the rational design of electronic materials. However, a quantitative study of charge transfer in such systems is often difficult because of poor control over molecular morphology. Here we use the core-hole clock implementation of resonant photoemission spectroscopy to study the femtosecond charge-transfer dynamics in cyclophanes, which consist of two precisely stacked π-systems held together by aliphatic chains. We study two systems, [2,2]paracyclophane (22PCP) and [4,4]paracyclophane (44PCP), with inter-ring separations of 3.0 and 4.0 Å, respectively. We find that charge transfer across the π-coupled system of 44PCP is 20 times slower than in 22PCP. We attribute this difference to the decreased inter-ring electronic coupling in 44PCP. These measurements illustrate the use of core-hole clock spectroscopy as a general tool for quantifying through-space coupling in π-stacked systems.

Altering the static dipole on surfaces through chemistry: molecular films of zwitterionic quinonoids

The adsorption of molecular films made of small molecules with a large intrinsic electrical dipole has been explored. The data indicate that such dipolar molecules may be used for altering the interface dipole screening at the metal electrode interface in organic electronics. More specifically, we have investigated the surface electronic spectroscopic properties of zwitterionic molecules containing 12π electrons of the p-benzoquinonemonoimine type, C(6)H(2)(···NHR)(2)(···O)(2)(R = H (1), n-C(4)H(9) (2), C(3)H(6)-S-CH(3) (3), C(3)H(6)-O-CH(3) (4), CH(2)-C(6)H(5) (5)), adsorbed on Au. These molecules are stable zwitterions by virtue of the meta positions occupied by the nitrogen and oxygen substituents on the central ring, respectively. The structures of 2-4 have been determined by single crystal X-ray diffraction and indicate that in these molecules, two chemically connected but electronically not conjugated 6π electron subunits are present, which explains their strong dipolar character. We systematically observed that homogeneous molecular films with thickness as small as 1 nm were formed on Au, which fully cover the surface, even for a variety of R substituents. Preferential adsorption toward the patterned gold areas on SiO(2) substrates was found with 4. Optimum self-assembling of 2 and 5 results in ordered close packed films, which exhibit n-type character, based on the position of the Fermi level close to the conduction band minimum, suggesting high conductivity properties. This new type of self-assembled molecular films offers interesting possibilities for engineering metal-organic interfaces, of critical importance for organic electronics.