Session S56: Focus Topic: Surface, Interface, and Thin Film Science of Organic Molecular Solids II

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Incorporation of single molecule magnets (SMMs) as quantum materials into thin-film heterojunctions is a necessary first step for implementation of useful quantum devices. We present results of our efforts to use independent thermal control of both source and substrate temperatures to obtain fully covered, intact, and well crystallized SMM films on a variety of substrates. The SMM studied is TbPc₂ with its quasi-planar double-decker geometry. Characterization by XRD, RAMAN, MOKE, SQUID, and vertical transport allows an assessment of how the SMMs change their electronic and magnetic properties as film thickness approaches monolayer dimensions and interface effects become increasingly important¹.

1. Eufemio Moreno Pineda et al., Dalton Trans., 2016, 45, 18417–18433   

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Regardless of many hazardous effects, phthalates are used as plasticizers in many plastic product-manufacturing processes. They can be leached out to its surroundings with time. Thus, a facile method of detection of phthalates in different environments is crucial. A film of highly conducting Multi-Walled Carbon Nanotubes (buckypaper) was developed and the Faradaic electrochemical impedance of the phthalate-adsorbed buckypaper was used as the detection parameter to identify the presence of phthalates. The detection was done by observing the change of Faradaic impedance in the Nyquist plot and with the shift in unique peak system displayed in the Bode Phase plot compared to the pristine buckypaper. With this technique, Di(2-ethylhexyl) phthalate, Dioctyl phthalate, and Di(2-propylheptyl) phthalate in methanol solutions was successfully detected. Sensitivity of this novel technique towards some other aromatic molecules such as xylene, bisphenol A, toluene, and naphthalene is negligible as they show low Faradaic impedance compared to buckypaper alone. The adsorption of phthalate esters onto the buckypaper increases interfacial charge transfer resistivity resulting low electric conductance. Therefore, with this technique a sensor could be developed to detect phthalates in solutions.   

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Copper oxide based catalysts have the promising capability to selectively reduce CO₂ into useful products, however the details of the mechanism are not well understood. Scanning tunneling microscopy (STM) studies performed on oxidized Cu(111) single crystal surfaces allow for the investigation of the fundamentals of this process on an atomic scale. Oxidized copper surfaces were generated via annealing under ambient conditions, and then subsequently annealed in ultra-high vacuum under a range of temperatures from 100 °C to 500 °C. After a low temperature vacuum anneal to remove adsorbates, STM revealed an oxidized structure characterized by grains tens of nanometers in size. Additional high temperature vacuum annealing led to STM imaging of flat atomically resolved oxide surfaces, which do not correspond to the “29” or “44” reconstructions in the literature. Differential conductance spectroscopy indicates a thin oxide layer with a gap-like feature of roughly 500 mV. After STM characterization of the surfaces, CO₂ was dosed in situ under cryogenic (5K) conditions. Individual CO₂ molecules were not observed, however scratchiness in the imaging following deposition indicates physisorbed CO₂ being perturbed by the apex of the tip.   

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Surface-confined self-assembly and metal-ligand coordination is a versatile method for creating and tuning the properties of low-dimensional nanostructures. Here we study the interaction between gold (Au) atoms and dicyanoanthracene (DCA) molecules adsorbed on Ag(111), characterized at 5 K by means of scanning tunneling microscopy (STM) and spectroscopy (STS), and atomic force microscopies (ncAFM). We observed a crystalline two-dimensional self-assembly consisting of close-packed DCA-Au-DCA units, in which a single Au atom binds covalently to a carbon atom at the anthracene ends. This conclusion is based on submolecular resolution ncAFM imaging achieved with a carbon monoxide (CO) functionalized tip probe, combined with atomic-scale STM manipulation of the DCA-Au-DCA units. The formation of such covalently bonded organometallic units indicates a selective on-surface reaction with a relatively low activation barrier which could be exploited to synthesize future functional nanomaterials.   

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Bottom-up synthesized covalent organic frameworks (COFs) provide a means to customize the properties of a 2D polymer from its molecular precursors. The 2D-crystalline DTPA (dimethylmethylene-bridged triphenylamine) COF is synthesized on metal surfaces through Ullman-type coupling¹. Theory predicts that the as-grown COF electronic structure is semiconducting. Heating in vacuum selectively cleaves methyl groups from the monomer bridge sites, creating a new COF. Enticing electronic properties are predicted, depending on the termination of the bridge sites including a half-metal (fully spin-polarized density of states at the Fermi energy) for the H-terminated case². Using cryogenic STM, we present new information on the bridge configuration of the demethylated structure, on the electronic structure of the DTPA COF in its methylated and demethylated forms, and on the COF/substrate interaction. Furthermore, by studying a range of DTPA self-assemblies, we have gained a deeper understanding of the electronic bands of the fully grown COF.

[1] Bieri et al., Chem. Commun. 47 (37) 10239 (2011).
[2] Kan et al., J. Am. Chem. Soc.134 (13) 5718 (2012).   

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Excitons in 2D transition metal dichalcogenide (TMD) semiconductors have large oscillator strength, while molecular excitons exhibit highly tunable emission. A molecular-2D material heterojunction offers a platform to combine the properties of the excitons in the disparate materials. In our study, we investigate the coupling of the WSe₂ B exciton with the 0-0 exciton of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). We observe evidence of a hybrid exciton with the strong absorption character of the WSe₂ B exciton leading to enhanced (20-fold) Raman scattering of the PTCDA modes. The interaction is modeled using a coupled oscillator model, which is supported by excitation energy dependent Raman measurements. We also find that the Raman enhancement decreases with increasing WSe₂ thickness due to the increased reflectivity of the heterostructure. Our findings elucidate the effects of nearly degenerate exciton coupling in 2D TMD-molecule systems and how they may lead to novel optical processes.   

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The properties of a material depend on its structure, and no theoretical prediction of the most stable thin film structures through traditional, exhaustive first-principle studies is feasible due to the combinatorial explosion in the number of possible polymorphs.

For monolayers, the machine-learning based SAMPLE approach [1] can already circumvent this problem, by using a few hundred DFT calculations to evaluate the energy of millions of possible polymorphs through Bayesian Linear Regression. It is our intention to extend the applicability of SAMPLE from monolayers to (meta)stable thin films.

Assuming a Frank-van der Merwe growth mechanism is at play, we can investigate thin-film structures by considering consecutive monolayers forming on top of one another.
As a first step, we study the formation of a second layer on top of the most stable monolayer predicted by SAMPLE for a well known system, Benzoquinone on Ag(111).
We evaluate the ways in which the electronic properties of the substrate can promote the formation of different second layers, and we give an assessment of the impact of such a change on the layer-to-layer charge transport rate in the thin film, as one can predict within the hopping regime.

[1] Hörmann et al., Computer Physics Communications 244, 143–155, 2019   

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The degree of electron delocalization in organic semiconductors is critical for their adoption in electronic and optoelectronic applications.Electron delocalization, however, is usually facile through chemical bonds in conjugated organic materials, but is rarely optimal when the noncovalent intermolecular van der Waals (vdW) forces define the self-assembly and the intermolecular electronic coupling. As a typical vdW material, C₆₀ molecules form solids through the attractive vdW force, thus are characterized with flat electronic bands with inconsequential dispersions and hopping transport, which is unfavourable for their practical applications.
To enhance intermolecular electronic hybridization, we introduce a method to mediate the vdW interactions between C₆₀ molecules through a substrate control of intermolecular interactions. We show by STM and theory that the black phosphorus (BP) substrate organizes C₆₀ molecules into a compressed monolayer and imposes a favorable orientation that optimizes the intermolecular π-π couplings, resulting in a nearly-free-electron (NFE) like lowest unoccupied molecular orbital (LUMO) band in C₆₀ monolayers, acquiring an effective mass of 0.53-0.70 and has carrier mobility of ~200 to 440 cm²V^-1s^-1. Our results show that the desirable NFE band does happen for a molecule like C₆₀if the π-π interactions are favored by an appropriate substrate template. The fact that the π-π vdW interactions could trigger the NFE like electronic band formation implies that templating of intermolecular interactions by vdW forces on otherwise weakly interacting substrates provides a promising strategy for tailoring remarkable electronic properties in organic materials for electronics and optoelectronics.   

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The deployment of Palladium (Pd) based technologies in contaminant removal would require multiples of improvements in reactivity specific to a given feed. Control in Pd nanostructure has shown as an approach that would allow scalable realization of gains in it. We report the behavior of two common chlorinated organic contaminants, trichloroethylene (TCE) and 1,3,5-trichlorobenzene (TCB), on three facets of Pd in terms of their adsorption, packing, dynamics and structural organization. Effect of temperature is additionally provided to correlate to adsorption isotherms. We found the Pd {110} surface template to be an interlocked structure for both TCE and TCB which molecules exhibit a standing geometry with lower binding energy and form interdigitated layer. This layer decreases the dynamics and residence time of the molecules. Both TCE and TCB display a mixed geometry on Pd {100} and {111}. The Pd {111} surface provides stability to both organic molecules due to more population of flat molecule and leading to slower dynamics than {100} facet. We separate the adsorption organic molecules to atop, bridge and hollow types. We found out that the atop type exhibits a unique packing structure that brings about the different adsorption behavior.   

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The decomposition of organic matter results in production of biogas, a renewable source that is composed primarily of CO2 and CH4. The CH4 concentration, acceptable for low efficiency combustion, is insufficient for high-performance applications. It is important to develop efficient and low-cost methods to separate CO2 and CH4. Adsorption/desorption cycles is one of the possible alternatives. Here we analyze computationally the effect of surface functionalization by polar groups on the adsorption of CO2 and CH4 in activated carbon (AC). The AC is modeled as slit-shaped pores formed by graphene fragments separated by distances between 8 and 20 Å [1-4] for different concentrations of hydroxyl and epoxy groups added randomly. The addition of polar groups significantly improves the selectivity of CO2 and CH4, especially at low (< 1 atm) and intermediate (1-10 atm) pressures and ca. room temperature or above. The mechanism of such added selectivity is the strong electrostatic interaction between the surface groups and the electric quadrupole of CO2.
[1] Z. Valleroy, et al., Langmuir 36, 3690−3702 (2020).
[2] M. Golebiowska, et al., Carbon 50, 225−234 (2012).
[3] L. Ortiz, et al., Mater. Res. Express 3, 055011 (2016).
[4] A. Albesa, et al., Langmuir 24, 3836−3840 (2008).   

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Metal clusters stabilized by surface passivation with organic molecules have potential applications in diverse fields such as optics, microelectronics, catalysis, and biochemical analysis. The diffusion of the organic-metal hybrid clusters on solid substrates seriously affect the performance of the clusters in many practical applications. Here, we report the observation of a mass migration of C₆₀-Au clusters at room temperature using scanning tunneling microscopy and molecular dynamic simulations. We found that the mass migration induced by thermal energy fluctuations, which is important for small systems. A close-packed C₆₀−Au cluster with a regular geometric shape can transfer into a liquid cluster owing to the sudden injection of thermal energy. The liquid cluster then migrates to a different site where it releases the extra energy and reassembles back into its initial structure. The break−reassembled behavior seems to be the dominant diffusion process for the C₆₀−Au cluster and perhaps also plays an important role in organic-metal hybrid clusters.
[J. Phys. Chem. C 2020, 124, 9990−9995]