Session Y10: Surface Science of Organic Molecular Solids, Films, and Nanostructures - Materials Synthesis, Deposition, and Device preparation

Magnetism of ultra-short one dimensional atomic chains

Driven by almost a century of theoretical work, the physical realization of one dimensional (1D) magnets has become essential to address pressing problems such as quantum criticality, many-body, spin transport, the emergence of new (topological) magnetic phases, and the extent and persistence of short- and long magnetic interactions as a function of length.

We report structural and magnetic properties of one-dimensional Fe chains as a function of length in the 10 to 200 atoms range. These Fe chains are grown using iron phthalocyanine (FePc) thin films and FePc/ metal-free-phthalocyanine (H₂Pc) superlattices (SLs). The length of 1D Fe chains is precisely controlled by the deposited thickness of the FePc layers.

Although structurally identical, 1D Fe chains formed in films and superlattices have different magnetic behavior. In films, the coercive field remains almost constant whereas in SLs increases with the length of the chain. This difference can be explained using a semi-classical model, which combines short range direct Exchange and Dzyaloshinskii-Moriya interactions. The increase of the coercive field in SLs is attributed to a magnetization reversal process which is governed by chiral symmetry breaking produced by a weak magnetic anisotropy. This anisotropy originates at the extreme of the Fe chains by proximity with the H₂Pc layers and is observable by element selective X-ray absorption spectroscopy (XAS).
  

Coordination dependence of cooperative effects in spin crossover [Fe{H2B(pz)2}2(bipy)] thin film

The electronic state of Fe(II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine) thin film on Al₂O₃ has been investigated by magnetometry (SQUID) and X-ray absorption spectroscopy (XAS) in both the total electronic yield mode and the photo-luminescence yield mode. The transition temperature of the spin crossover transition has a 20 K difference between the cooling and heating sequence in magnetometry and X-ray absorption spectroscopy taken in both the total electronic yield and photo-luminescence yield modes, indicating cooperative effects in the Fe(II) spin crossover complex. The differences in the hysteresis loop for the spin crossover transition for this Fe(II) spin crossover complex at the surface of the molecular film, extracted from X-ray absorption taken in the total electronic yield mode, differs from the bulk, as obtained from magnetometry and photo-luminescence yield mode X-ray absorption. This indicates the intermolecular interactions between the spin crossover molecules, at the surface of the Fe(II) spin crossover complex thin film, differs from bulk.   

Local charge accumulation at a trinuclear metal-organic nanostructure on a surface

Coordination chemistry relies on harnessing active metal sites within organic matrices. Polynuclear complexes — consisting of organic ligands bound to several metal atoms — are relevant due to their electronic and magnetic properties, and to their potential for functional reactivity pathways. However, their synthesis remains challenging, with few geometries and configurations that have been achieved. Here, we synthesize — via supramolecular chemistry on a noble metal surface — one-dimensional metal-organic nanostructures composed of terpyridine-based molecules coordinated with well-defined polynuclear iron clusters. Combining low-temperature scanning probe microscopy techniques, density functional theory and x-ray absorption spectroscopy, we demonstrate that the coordination motif consists of coplanar terpyridine groups linked via a quasi-linear tri-iron node with mixed positive valence and a metal–metal bond configuration. This unusual linkage is stabilized by local accumulation of electrons between cations, ligands and metal surface. This morphology, enabled by the bottom-up on-surface synthesis, yields an electronic structure that hints at a chemically active polynuclear metal center, paving the way for nanomaterials with novel catalytic and magnetic functionalities.   

Electron Induced Disordering and Decomposition of Alkanethiol Self-assembled Monolayers on Au(111)

Self-assembled monolayers (SAMs) are used for applications such as molecular electronics, selective deposition, and other forms of surface modification. Lithography within the semiconductor industry is adopting shorter wavelengths of light such that the interaction of secondary electrons with the organic resist is becoming the primary mechanism for photo-initiated electro-chemical reactions. To study the interaction of low energy electrons with thin organic films, measurements have been performed on electron induced disordering and decomposition of 1-decanethiol molecules grown via vapor phase deposition on Au(111). These monolayers arrange into two phases commonly referred to as lying down and standing up. The lying down phase is a physisorbed layer that is only weakly interacting with the substrate via Van der Waals forces. Conversely, the standing up phase is a chemisorbed species that is more strongly bound to the substrate. Surface analysis techniques were used to characterize the monolayers before and after electron exposure. LEED was used to determine the structure of the SAM and the rate of disordering and decomposition. TPD was used to evaluate the thermal stability of the attached SAMs and the resulting desorption products after electron exposure.   

Assessing Monomer and Aggregate Populations in Squaraine-Based Organic Solar Cells

Higher efficiencies of organic photovoltaic (OPV) devices have often been correlated with structural order within the solid state active layer of the device. Structural order is thought to improve charge mobility and energy transfer since both rely on effective orbital overlap between molecules. Nevertheless, questions remain about the role of electronic aggregates since they may act as energy traps for excited state species after photoexcitation, thereby leading to reduced overall efficiency. We aim to demonstrate how OPV efficiency is indeed related to aggregate (vs. monomer) populations through quantitative measurement. Population can be measured through absorption measurements and with a knowledge of the extinction coefficient of the species in question, along with the path length for the light through the sample. Experimentally, we will focus on the thickness measurements for a set of squaraine films. Squaraines are interesting because their efficiencies are significantly impacted by the exact geometry of aggregation, which can be controlled through chemical tuning. We will therefore use AFM thickness measurements coupled with spectroscopic techniques to confirm the importance of aggregation for molecular design strategy and fully optimized OPV devices.   

Tuning the tunneling decay coefficient in self-assembled monolayer junctions

Having insight into charge transport across organic-inorganic interfaces is important for the development and improvement of molecular electronic devices. Within the regime of quantum tunneling transport, the charge transfer rate strongly depends on the tunneling decay coefficient β, which determines the decline of the current across the junction as a function of the length and the barrier height. For self-assembled monolayer (SAM) junctions, it is well-known that the value of β depends on the specific electronic structure of the molecular backbone. In our combined experimental and computational study, we demonstrate that β of a non-conjugated SAM junction can be lowered significantly by using different halogens as termination on one side of the molecular backbone. With our experiments we can also show that this lowering of the tunneling coefficient is correlated with a change in the dielectric constant. Furthermore, our calculations allow for understanding how the frontier orbitals and transmission channels are effected by the change of the end-group.   

Coherent X-ray measurement of local step-flow propagation during growth on polycrystalline C60 thin film surfaces

Vacuum deposition of C₆₀ on a graphene-coated surface is investigated with X-ray Photon Correlation Spectroscopy in surface-sensitive conditions. Local step-flow is observed through the observation of oscillatory correlations in the later stages of growth after crystalline mounds have formed. An important aspect of the work is that coherent X-rays do not average over complex structures, and this allows us to monitor the growth on polycrystalline surfaces without loss of information. The experimental results show that the step-flow velocity must be nonuniform, and we model the velocity of each step-edge as being a simple function of the lengths of the terraces above and below it. This model predicts that the steps become almost stationary near the edges of the mounds where the local terrace length is very small, and the average slope of the surface is large. It was not previously known that such nonuniform and disordered step arrays as we have observed would follow such a simple growth law. This work shows that the use of coherent X-ray scattering provides an approach to better understand surface dynamics and fluctuations during crystal growth.   

Differences in Self-Assembly of Spherical C60 and Planar PTCDA on Rippled Graphene Surfaces

It was recently recognized that two-dimensional (2D) graphene exhibits nonplanar aberrations such as a rippled surface. Understanding the self-assembly of organic semiconductor molecules on monolayer 2D curved graphene surfaces is a paramount issue for ultimate application. Herein, we report on the preparation of fullerene, C₆₀ and perylenetetracarboxylic dianhydride (PTCDA) molecules adsorbed on a rippled graphene surface. We find that the C₆₀ form a quasi-hexagonal close packed (hcp) structure, while the PTCDA form a disordered herringbone structure. These 2D layer systems have been characterized by STM imaging and DFT approaches. The DFT results exhibit interaction energies for adsorbed molecule/rippled graphene complexes located in the 2D graphene valley sites that are significantly larger in comparison with adsorbed planar/molecule graphene 2D complexes. In addition, we report that the adsorbed PTCDA prefer different orientations when the rippled graphene peak regions are compared to the valley regions. This difference in orientations causes the PTCDA molecules to form a disordered herringbone structure on the rippled graphene surface.   

Effect of Admixture on Domain Morphology Transitions in Phase Separation of PCBM:tn-ZnPc from CCl3 Solution

We present results of investigations aimed at understanding how small mutually immiscible organic molecules self-assemble into domains during phase separation from liquid solutions. As a prototypical system we investigated molecular mixtures consisting of tetranitro zinc-phthalocyanine (tn-ZnPc), an electron donor, and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), an electron acceptor, in chloroform and deposited on native oxide-covered Si(111) substrates. Previously we showed that for a 1:1 mixture PCBM:tn-ZnPc which has much higher solubility, precipitates onto the silicon substrate with domain morphologies which vary widely over the range of supersaturation rates investigated. For the highest rates the domains are isotropic, and apparently amorphous – with a mesoscale near-periodicity. Slowing the supersaturation rate causes an abrupt change in the domain size and shape, to micron-scale faceted domains; we speculated that either (1) a transition from spontaneous decomposition kinetics to nucleation and growth of crystalline domains or (2) a “cascade” series of transitions through increasingly stable structures was responsible for this. Here we present results in which the admixture of PCBM:tn-ZnPc is varied systematically to distinguish between these two mechanisms.   

Competing on-surface reaction pathways of bifunctional anthracene precursors to obtain organometallic networks

On surface synthesis is a versatile bottom up fabrication approach to create covalent nanostructures with atomic precision. Several organic covalent structures have been created by this approach, such as graphene based structures or organometallic networks. Aiming at the synthesis of a porous network, we study the hierarchy of chemical reactions of 10-bromoanthracene-9-yl-boronic-acid (BABA) precursors that are vapour sublimated onto crystalline surfaces under ultra-high-vacuum conditions. Our scanning tunnelling microscopy and spectroscopy characterization unveils that BABA dehydration to produce boroxine rings and metal substitution of the halogen group take place simultaneously leading to the formation of an organometallic honeycomb network, similarly to the case of using a simple phenyl core instead of anthracene [1]. In our case, however, the competition between these two reactions is extremely dependent on the catalytic role of the substrate, its surface termination and its temperature during the deposition of the precursor. As a consequence, the reaction pathway can be readily controlled by the choice of the catalytic surface (Ag(001), Ag(111) or Ag(111)).

1. J. Phys. Chem. C 2012, 116, 7, 4819