Complex Surface Diffusion Mechanisms of Cobalt Phthalocyanine Molecules on Ag(100)

Single-molecular diffusivity and long jumps of large organic molecules: CoPc on Ag(100)

Energy dissipation and the transfer rate of adsorbed molecules do not only determine the rates of chemical reactions but are also a key factor that often dictates the growth of organic thin films. Here, we present a study of the surface dynamical motion of cobalt phthalocyanine (CoPc) on Ag(100) in reciprocal space based on the helium spin-echo technique in comparison with previous scanning tunnelling microscopy studies. It is found that the activation energy for lateral diffusion changes from 150 meV at 45-50 K to ≈100 meV at 250-350 K, and that the process goes from exclusively single jumps at low temperatures to predominantly long jumps at high temperatures. We thus illustrate that while the general diffusion mechanism remains similar, upon comparing the diffusion process over widely divergent time scales, indeed different jump distributions and a decrease of the effective diffusion barrier are found. Hence a precise molecular-level understanding of dynamical processes and thin film formation requires following the dynamics over the entire temperature scale relevant to the process. Furthermore, we determine the diffusion coefficient and the atomic-scale friction of CoPc and establish that the molecular motion on Ag(100) corresponds to a low friction scenario as a consequence of the additional molecular degrees of freedom.

Superflux of an organic adlayer towards its local reactive immobilization

On-surface mass transport is the key process determining the kinetics and dynamics of on-surface reactions, including the formation of nanostructures, catalysis, or surface cleaning. Volatile organic compounds (VOC) localized on a majority of surfaces dramatically change their properties and act as reactants in many surface reactions. However, the fundamental question "How far and how fast can the molecules travel on the surface to react?" remains open. Here we show that isoprene, the natural VOC, can travel ~1 μm s-1, i.e., centimeters per day, quickly filling low-concentration areas if they become locally depleted. We show that VOC have high surface adhesion on ceramic surfaces and simultaneously high mobility providing a steady flow of resource material for focused electron beam synthesis, which is applicable also on rough or porous surfaces. Our work established the mass transport of reactants on solid surfaces and explored a route for nanofabrication using the natural VOC layer.

Surface Diffusion Aided by a Chirality Change of Self-Assembled Oligomers under 2D Confinement

Chirality switching of self-assembled molecular structures is of potential interest for designing functional materials but is restricted by the strong interaction between the embedded molecules. Here, we report on an unusual approach based on reversible chirality changes of self-assembled oligomers using variable-temperature scanning tunneling microscopy supported by quantum mechanical calculations. Six functionalized diazomethanes each self-assemble into chiral wheel-shaped oligomers on Ag(111). At 130 K, a temperature far lower than expected, the oligomers change their chirality even though the molecules reside in an embedded self-assembled structure. Each chirality change is accompanied by a slight center-of-mass shift. We show how the identical activation energies of the two processes result from the interplay of the chirality change with surface diffusion, findings that open the possibility of implementing various functional materials from self-assembled supramolecular structures.

Standard deviation of microscopy images used as indicator for growth stages

Photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM) can easily distinguish between organic molecules adsorbed in crystallites or in the wetting layers as well as the bare metal substrate due to their different electronic properties. Already before (and during) the condensation of such solid phases (2D islands or 3D crystallites), there is a dilute 2D gas phase. Such a 2D gas phase consists of molecules, which are highly mobile and diffuse across the surface. The individual molecules are too small to be resolved in PEEM/LEEM images. Here, we discuss, how image features below and above the resolution limit of a PEEM/LEEM affect the mean electron yield and its (normalized) standard deviation. We support our findings with two experimental examples: the deposition of cobalt phthalocyanine (CoPc) on Ag(100) and of perfluoro-pentacene on Ag(110). Our results demonstrate, how a spatial and temporal analysis of image series can be used to obtain information about molecular phases, which cannot be directly resolved in microscopy images.

Quantifying entropic barriers in single-molecule surface diffusion

The quantitative role of entropy in the surface diffusion of molecules with many degrees of freedom is still not well understood. Here, we quantify entropic diffusion barriers as well as attempt frequencies by performing a systematic decomposition of the Arrhenius equation for single oligophenyl molecules of various lengths (two to six phenyl rings and benzene as the reference) on an amorphous silica surface using extensive molecular dynamics simulations. Attempt frequencies evaluated from velocity auto-correlation functions are found close to k_BT/h, the frequency factor of transition state theory. Importantly, we find large positive entropy contributions to the free energy barrier of diffusion up to 55%, increasing with molecular length with 4.1 kJ/mol/phenyl ring. The entropic barrier is about 40%-60% of the entropy of the molecule surface adsorption free energy, revealing that at the transition states, the molecules can liberate a major part of their conformational states, increasing with length. The substantial role of the internal degrees of freedom for the diffusive dynamics is explicitly demonstrated by studying internally constrained, "rigid" version of the molecules. Finally, we discuss also rotational diffusion and the role of surface vibrations. Our results affirm that it is essential for quantitative studies and interpretation of surface diffusion of complex molecules to consider internal entropic effects.

Zinc(II) tetraphenylporphyrin on Au(111) investigated by scanning tunnelling microscopy and photoemission spectroscopy measurements

Porphyrins are a versatile class of molecules, which have attracted attention over the years due to their electronic, optical and biological properties. Self-assembled monolayers of porphyrins were widely studied on metal surfaces in order to understand the supramolecular organization of these molecules, which is a crucial step towards the development of devices starting from the bottom-up approach. This perspective could lead to tailor the interfacial properties of the surface, depending on the specific interaction between the molecular assembly and the metal surface. In this study, we revisit the investigation of the assembly of zinc-tetraphenylporphyrins on Au(111) in order to explore the adsorption of the molecular network on the noble metal substrate. The combined analysis of scanning tunneling microscopy (STM) imaging and core levels photoemission spectroscopy measurements support a peculiar arrangement of the ZnTPP molecular network, with Zn atoms occupying the bridge sites of the Au surface atoms. Furthermore, we prove that, at few-layers coverage, the interaction between the deposited layers allows a relevant molecular mobility of the adlayer, as observed by STM and supported by core levels photoemission analysis.

Understanding the Enhancement of Surface Diffusivity by Dimerization

Beyond dilute coverage, the collective diffusion of molecules might enhance material transport. We reveal an enhanced mobility of molecular dimers by separating two motions, diffusion and rotation, of CO dimers on elemental Ag(100) as well as on a dilute Cu alloy of Ag(100). From time-lapsed scanning tunneling microscopy movies recorded between 15 and 25 K, we determine the activation energy of dimer diffusion on elemental Ag(100) to be, at (40±2)  meV, considerably smaller than the one for monomer diffusion, at (72±1)  meV. The alloyed Cu atoms reduce the dimer mobility facilitating to determine their rotational barrier separately to be (39±3)  meV. Disentangling different degrees of freedom suggests that a rotational motion is at the origin of enhanced dimer diffusivity.

Translation of metal-phthalocyanines adsorbed on Au(111): from van der Waals interaction to strong electronic correlation

Using first-principles calculations, we investigate the binding energy for six transition metal - phthalocyanine molecules adsorbed on Au(111). We focus on the effect of translation on molecule - surface physical properties; van der Waals interactions as well as the strong correlation in d orbitals of transition metals are taken into account in all calculations. We found that dispersion interaction and charge transfer have the dominant role in the molecule-surface interaction, while the interaction between the transition metal and gold has a rather indirect influence over the physics of the molecule-surface system. A detailed analysis of the physical properties of the adsorbates at different geometric configurations allows us to propose qualitative models to account for all values of interface dipole charge transfer and magnetic moment of metal-phthalocyanines adsorbed on Au(111).

Ordering of Zn-centered porphyrin and phthalocyanine on TiO2(011): STM studies

Zn(II)phthalocyanine molecules (ZnPc) were thermally deposited on a rutile TiO₂(011) surface and on Zn(II)meso-tetraphenylporphyrin (ZnTPP) wetting layers at room temperature and after elevated temperature thermal processing. The molecular homo- and heterostructures were characterized by high-resolution scanning tunneling microscopy (STM) at room temperature and their geometrical arrangement and degree of ordering are compared with the previously studied copper phthalocyanine (CuPc) and ZnTPP heterostructures. It was found that the central metal atom may play some role in ordering and growth of phthalocyanine/ZnTPP heterostructures, causing differences in stability of upright standing ZnPc versus CuPc molecular chains at given thermal annealing conditions.

Unveiling orbital coupling at the CoPc/Bi(111) surface by ab initio calculations and photoemission spectroscopy

Orbital coupling is revealed at the CoPc/Bi(111) interface with the local magnetic moment retained in CoPc.

Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?

Mass transport at a surface is a key factor in heterogeneous catalysis. The rate is determined by excitation across a translational barrier and depends on the energy landscape and the coupling to the thermal bath of the surface. Here we use helium spin-echo spectroscopy to track the microscopic motion of benzene adsorbed on Cu(001) at low coverage (θ ∼ 0.07 ML). Specifically, our combined experimental and computational data determine both the absolute rate and mechanism of the molecular motion. The observed rate is significantly higher by a factor of 3.0 ± 0.1 than is possible in a conventional, point-particle model and can be understood only by including additional molecular (rotational) coordinates. We argue that the effect can be described as an entropic contribution that enhances the population of molecules in the transition state. The process is generally relevant to molecular systems and illustrates the importance of the pre-exponential factor alongside the activation barrier in studies of surface kinetics.

Local modification of intermolecular interactions at a sub-molecule level

The local modification of intermolecular interactions in nickel-phthalocyanine molecules (NiPCs) is investigated on an Au(111) substrate using scanning tunneling microscopy. When the molecules are physisorbed on the substrate, they repel each other due to induced charge dipole moments. However, when the NiPC is chemisorbed on the substrate through the dehydrogenation of one of its ligands by a bias pulse, we find that a nearby physisorbed NiPC is attracted to the dehydrogenated ligand and trapped. Using our experimental results in combination with density functional theory calculations, we show that the observed attraction can be ascribed to the local charge redistribution around the dehydrogenated ligand of the chemisorbed NiPC. Furthermore, we demonstrate that desorption of the attracted NiPC from the trapped site can be readily controlled by changing the density of NiPCs around the dehydrogenated ligand.

Temperature calibration for diffusion experiments to sub-Kelvin precision

Arrhenius plots are often used to determine energy barriers and prefactors of thermally activated processes. The precision of thus determined values depends crucially on the precision of the temperature measurement at the sample surface. We line out a procedure to determine the absolute temperature of a metal sample in a cryogenic scanning tunneling microscope between 5 K and 50 K with sub-Kelvin precision. We demonstrate the dependence of prefactor and diffusion energy on this calibration for diffusion of CO on Cu(111) and on Ag(100) measured in the temperature range from 30 K to 38 K and 19 K to 23 K, respectively.