Session P52: Focus Topic: Surface, Interface, and Thin Film Science of Organic Molecular Solids II

Detecting Stable Surface Adsorbates with Bayesian Optimization

Determining stable structures of organic molecules at inorganic surfaces requires both quantum mechanics and thorough exploration of the potential energy surface (PES). This is prohibitively expensive with density-functional theory (DFT). We combine DFT with artificial intelligence (AI) for global atomistic structure search of stable adsorbates. Bayesian Optimization Structure Search (BOSS) [1] is a new AI tool, which accelerates the structure search via a strategic sampling of the PES. BOSS minimizes the number of expensive DFT simulations to compute the complete PES. This allows a clear identification of the most stable minimum energy structures and the barriers between them.

We apply BOSS to study the adsorption of a camphor molecule on the Cu(111) surface as a function of molecular orientation and translations. We identify the optimal structure and 8 unique types of stable adsorbates, in which camphor chemisorbs via oxygen (global minimum) or physisorbs via hydrocarbons to the Cu(111) surface. This study demonstrates that the new cross-disciplinary tools, like BOSS, allow us to identify complex interface structures and properties, and tune the functionality of advanced materials.

[1] M. Todorović, M. Gutmann, J. Corander and P. Rinke, npj Comput. Mater. 2019, 5, 35.   

Unusual self-assembly of chloroaluminium phthalocyanine on graphite

We report an unusual self-assembled layer structure of chloroaluminium phthalocyanine (ClAlPc) molecules on highly ordered pyrolytic graphite (HOPG), in which a close-packed well-ordered monolayer is separated from the substrate by a relatively disordered buffer layer, as revealed using scanning tunneling microscopy (STM). Our close-packed monolayer has a nearly rectangular lattice, instead of the distinctly different square lattice for the more commonly observed well-ordered bilayer structure. This may be due to the dominance of intermolecular interactions within the monolayer when the influence from the substrate is shielded by the buffer layer. Density Functional Theory (DFT) calculations and Reduced Density Gradient (RDG) analysis indicate that the dominant intermolecular interaction within the unusual layer is likely the London dispersion force.   

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

Vapor deposition of C₆₀ on a graphene coated surface is investigated in real-time by utilizing coherent hard X-rays. X-ray Photon Correlation Spectroscopy is performed in grazing incidence to achieve surface-sensitivity. Local step-flow is monitored through the observation of oscillatory correlations in the later stages of growth after crystalline mounds have formed. Coherent X-rays do not average over complex structures, and this allows us to monitor growth on polycrystalline surfaces without loss of information. The results show that the step-flow velocity is 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. Sensitivity to local step-flow is enhanced by coherent mixing of X-rays scattered from the average mound structure with those scattered from the step array. This effect is a version of heterodyne scattering, where the scattering from the average step array can be considered to be a quasi-static reference signal. This work shows that the use of coherent X-ray scattering provides an approach to better understand surface dynamics and fluctuations during crystal growth.   

Realizing nearly-free-electron like conduction band in a molecular film through mediating intermolecular van der Waals interactions

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 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 that optimizes the intermolecular π-π couplings, resulting in a nearly-free-electron (NFE) like 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. The fact that the π-π vdW interactions does 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.

Reference：
1. X. X. Cui, D. Han, H. L. Guo, L. W. Zhou, J. S. Qiao, Q. Liu, Zh. H. Cui, Y. F. Li, C. W. Lin, L. M. Cao, W.Ji, H. Petek, M. Feng. Nat. Comm. 10:3374 (2019).
2. H. L. Guo, X. X. Cui, W. Q. Zhou, D. Han, C. W. Lin, L. M. Cao, M. Feng. J. Appl. Phys. 124, 045301 (2018)
  

Predicting χ of polymer blends using atomistic "morphing" simulations

The Flory Huggins interaction parameter χ governs phase behavior in polymer blends and block copolymers. We have previously used molecular dynamic (MD) simulations and thermodynamic integration during the morphing of one species to another, to compute the excess free energy of mixing in coarse-grained, bead-spring model of polymer blends. Using this method, we have studied the effect on χ of factors like stiffness mismatch, chain architecture and Lennard Jones interactions. In this work, we use united-atom MD simulations and the morphing method to calculate χ for real polymer blends: (1) poly(ethylene) - poly(ethylene oxide), (2) poly(styrene) - poly(2-vinyl pyridine), (3) poly(isoprene) - saturated (polyisoprene) and (4) poly(styrene) - poly(α-methyl styrene). These examples require different kinds of morphing- LJ interactions and partial charges of atoms (cases 1 and 2), transform double bond to single bond (case 3) and disappearance of atoms (case 4). All simulations used force field parameters adopted from TRAPPE. Our χ values from simulations are in reasonable agreement with experiment, but are sensitive to force field parameters used in the simulation.   

Visualizing Molecular Orientational Ordering and Electronic Structure in CsnC60 thin films

Alkali-doped fullerides exhibit a wealth of unusual phases that remain controversial by nature. Here a cryogenic scanning tunneling microscopy and first-principles calculation are combined to investigate the sub-molecular structural and electronic properties of expanded fullerene C₆₀^n- films on inert graphene with various cesium (Cs) doping. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of C₆₀^n- anions, yet the tunneling conductance spectra are robustly characteristic of energy gaps, hallmarks of the Jahn-Teller distortions and electron correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of n = 1, 2, 3 and 4, apart from which it moves toward the band edges with concomitant electronic states in the gap. Our findings establish the universality of Jahn-Teller instability, and clarify the relationship among the doping, structural and electronic structures in Cs_nC₆₀ fullerides.   

Facet Specific Adsorption as a Route to Remediation of Chlorinated Organic Contaminants

The deployment of Palladium (Pd) based technologies in contaminant removal would require multiples of improvement 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, structural organization. We separate the adsorption organic molecules to atop, bridge and hollow types. Effects of temperature are additionally provided to correlate to adsorption isotherms. The Pd {110} surface templates an interlocked structure for both TCE and TCB, while having the least favorable adsorption energies. The Pd {111} surface provides stability to TCB. All the energy calculation has been done for each adsorption type and is consistent with the result of Potential of Mean Force (PMF).   

Evidence for an accumulation layer at the donor-acceptor interface in organic solar cells

Expected band bending at typical donor-acceptor interfaces in solar cells suggests an accumulation layer could develop for sufficiently high bias, though this is rarely discussed in the photovoltaic literature. There is evidence from bilayer organic field effect transistors as well as organic light emitting transistors that high mobility interfacial accumulation layers exist [1], as well as a report on unexpectedly long diffusion lengths in a solar cell [2]. Our model of dark current in solar cells, which carefully considers multiple physical contributions to the total current, suggests the role of an accumulation layer in supporting observed high current densities. Here we present bulk electrical measurements on bilayer transistors, along with scanning Kelvin probe force microscopy capable of measuring band bending and density of states. Materials studied include CuPC/NTCDA bilayer transistors.

[1] H. Wang and D. Yan (2010), “Organic heterostructures in organic field-effect transistors,” NPG Asia Materials, 2(2), 69-78.
[2] Q. Burlingame et al. (2018), “Centimetre-scale electron diffusion in photoactive organic heterostructures,” Nature, 554, 77-80.   

Controlling energy level alignment at a chromophore/TiO2 interface using a co-adsorbed helical peptide

The sensitization of wide band gap transition metal oxide semiconductors by chromophores has become ubiquitous in the field of photovoltaic devices. Since performance hinges on the charge transfer across the interface, which in turn depends upon the alignment of the chromophore frontier orbitals with respect to the substrate band edges, it is crucial to find ways of controlling this energy alignment.

Here, we present a novel and versatile approach of controlling this energy alignment via a “mixed-layer” solution sensitization process, where a monolayer of a dipole containing helical peptide is prepared on a single crystal TiO₂(110) surface and successively exposed to a solution of a Zinc Tetraphenylporphyrin (ZnTPP) derivative. Using UV and X-ray photoemission spectroscopies, we find that (1) repeated exposure to the ZnTPP derivative progressively displaces the peptides on the surface, and (2) the highest occupied molecular orbital and N 1s core level of the ZnTPP derivative within the mixed layer are located 300 meV lower in energy than their respective counterparts on a pure ZnTPP-derivative monolayer. These results are consistent with a simple parallel plate capacitor model of the intermixed peptide dipole layer.   

Investigating the monolayer structure of caffeine molecules on Au(111)

Polymorphism is an important property of pharmaceutical ingredients in terms of their bioavailability. As a bottom-up approach we investigated the formation of monolayers of caffeine molecules on a Au(111) surface to understand caffeine's polymorph behavior.
In this work we analyzed the structure formation of caffeine molecules on Au(111) by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. All investigations were conducted at monolayer coverage at room temperature.
By combining these powerful surface science techniques, we demonstrate successful formation of caffeine monolayers including molecular alignment in two mirrored directions of the Au(111) surface. We propose a structure model for the two found surface domains in a three-molecular unit cell. This contributes to an understanding of the driving forces for surface structure formation which is essential for potential applications.   

Spiropyran/Merocyanine adsorption on a Gold Surface

Spiropyran is a photochromic, three-dimensional molecular switch which can be converted to Merocyanine, a planar isomer of Spiropyran where the central C-O bond has been broken. The adsorption behavior of a specific molecule, namely naphtho-merocyanine on an Au(111) substrate has been investigated using Scanning Tunneling Microscopy (STM) and computational chemistry methods. The experiments show a strong preference of dimer formation of the merocyanine molecule. Topographical and spectroscopic measurements are used to identify two distinguished configurations, an elongated dimer and a compact dimer. With the help of DFT and Molecular Mechanics calculations these two configurations as well as the specific merocyanine isomer can be identified and energy calculations including adsorption geometries could be performed. This is a first step in investigating possible pathways toward the switching behavior of merocyanine and spiropyran on this substrate.   

Investigating Electronic structure at Nano-Bio Interface to optimize Excitonic Solar cell architecture

A study of the Bacteriorhodopsin/TiO₂ and Bacteriorhodopsin/Perovskite interfaces associated with the photoactive chromophore in bacteriorhodopsin (bR) and its various mutant (D96N, triple glutamic mutations E9Q/E194Q/E204Q) forms allowed the explanation of the effective coupling existing between the π electrons of the chromophore and the local environment of the Schiff base when adsorbed to a substrate (Das et al., ACS applied materials & interfaces 11.34 (2019)). X-ray Photoemission Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) experiments have been used to explain some of the optical properties for various substrates such as Mesoporous TiO₂ and Perovskite. Density functional calculations (DFT) for both the ground state and the excited state has been carried in parallel with the experimental measurements. We discussed how to compensate the charge offset, to restore the Valence Band and XPS of C1s while mapping the occupied and unoccupied states around both sides of the Fermi level. In specific, we calculated the electronic band structure of the bR Chromophore using DFT+U methodology. The goal of this investigation is to optimize future Solar cell architecture (Das et al., The Journal of Physical Chemistry C (2019)).   

Light-induced molecular dipole reordering in halide perovskites

Light-induced molecular dipole’s orientation has been suggested to be a critical degree of freedom to affect the performance of hybrid perovskite-based optoelectronic devices. In this work, we utilized light-illuminated cross-sectional scanning tunneling spectroscopy to map simultaneously the organic cation’s dipole orientation and the corresponding electrostatic potential with atomic resolution. Our discovery provides real space experimental evidence of the dipole reordering under illumination to create a deep one-dimensional potential energy well. The result may be directly responsible for the efficient carrier transport and suppressed carrier recombination of hybrid perovskite as an excellent photovoltaic and optoelectronic material.