Session B17: Organic Film Structure, Properties, and Dynamics

Photochemical Stability in Vapor-deposited Organic Thin Films Can be Modulated by Glass Packing

Photochemically robust materials are desired for organic electronics. While chemical structure is typically altered to improve photostability, we show that photostability of organic amorphous thin films can be significantly improved by packing them more tightly via physical vapor deposition. Disperse Orange 37, a push-pull azobenzene with fast thermal isomerization, can be made 50 times more stable against photoisomerization if deposition temperatures are chosen correctly. Photostability was determined by measuring density and molecular orientation changes during irradiation. We further show that the enhanced photostability in vapor-deposited glasses is a general phenomenon by using a non-push-pull azobenzene, 4,4’-diphenyl azobenzene (DPA). By mixing DPA into the glass host of celecoxib, we directly measure populations of trans and cis via UV-Vis and show that the rate of photoisomerization varies as a function of the substrate temperature. Photostability correlates with density of packing, where the optimum glass is about 30 times more photostable than the liquid-cooled glass. These results provide a molecular explanation for enhanced photostability in glasses, and they provide insight in designing organic electronics by making denser materials to achieve longer lifetimes.   

Spontaneous van der Waals epitaxy of high-quality 2D chalcogenide thin film

Explosive interests have been focused on layer structured two-dimensional (2D) chalcogenides due to the discoveries of exotic physical properties, such as high-performance thermoelectricity, superior carrier mobility, ultrafast charge transfer, topological insulator, and superconductivity. In parallel, the demands on high-quality 2D chalcogenide films for device applications are increasing since such properties originate from their unique structural ordering and depend on crystal quality. However, the difficulties in controlling epitaxy with defect density, the lack of suitable substrate, and the limited understanding of growth mechanism for 2D chalcogenide film have been major obstacles for the further advances of these materials. We demonstrate new approaches enabling the vdWE of 2D chalcogenide epitaxial films on conventional substrates. As a proof of concept, highly-oriented bismuth antimony telluride thermoelectric films were epitaxially grown on 2D (graphene) and 3D (α-Al₂O₃) substrates by pulsed laser deposition via vdWE utilizing the natural surface reaction of the substrate with chalcogen. It was confirmed that this unusual vdWE renders the high-quality 2D chalcogenide film with high carrier mobility, low defect density, and exceptionally low thermal conductivity.   

Chain Length Dependence of the Dielectric Constant and Polarizability in Conjugated Organic Thin Films

Dielectric materials are ubiquitous in optics, electronics and materials science. Recently, there has been new effort to characterize the dielectric performance of thin-films made of polarizable molecule assemblies, at both theoretical and experimental levels. In this context, we present the relation between the polarizability of the constituting molecule and the film dielectric constant, using periodic Density Functional Theory (DFT) calculations, for conjugated and saturated chains.
We particularly explore the implication of the peculiar chain length dependence of the polarizability for conjugated molecules. We show and explain from DFT calculations and a simple Clausius-Mossotti-like depolarization model how this chain-length dependence is affected by the collective polarization. A high sensitivity of the dielectric constant to the thin-film thickness is expected for conjugated molecules. This latter can increase by a factor of 3-4 at a reasonable coverage, just by extending the molecular length. This significantly limits the decline of the thin-film capacitance with the inverse of the film thickness, generally observable for non-conjugated materials.

[1] Van Dyck, Marks, Ratner, ACS Nano, 2017, 10.1021/acsnano.7b01807   

Atomic Force Microscopy (AFM) study of elastic modulus of artificial phospholipid membranes

The study of artificial phospholipid membranes (PMs) on plane substrates has become relevant to gain insights into the physical behaviour of cell membranes. Here, we analyse the temperature dependence of Young’s modulus and adhesion of several PMs (DPPC, DMPC and DSPC) by Atomic Force Microscopy (AFM) and Surface Force Spectroscopy (SFS) measurements. Phospholipids were deposited onto silicon substrates by physical vapour deposition (PVD) in high vacuum. Using Raman spectroscopy, we find that the chemical structure of our phospholipids remains unchanged after PVD. AFM measurements in liquid confirm the self-assembly of the phospholipid bilayer. Young’s modulus and adhesion measurements obtained by SFS show two phase transitions, consistent with the ripple-gel transition and the gel-liquid crystalline phases. With this we have shown that several types of PMs can be vapour deposited in high vacuum, regaining their structure and mechanical properties after proper hydration. This study opens new pathways to assemble phospholipid mixtures by vapour deposition.   

Ultrafast Transformation of Provitamin D Analogs in Lipid Bilayers

Ultrafast photochemical transformations of 7-Dehydrocholesterol (DHC, Provitamin D₃), DHC-acetate and Ergosterol (Ergo, Provitamin D₂) to previtamin (D₃, D₃-acetate and D₂) occurs upon a ring-opening reaction in the excited state where a cyclohexadiene (CHD) chromophore embedded within the molecules opens to form a hexatriene previtamin D species. The ring opening of isolated CHD happens on a sub-picosecond time scale, therefore it is necessary to use ultrafast techniques such as transient absorption to capture the dynamics of the molecules in the excited state. Here we are studying the excited state dynamics of DHC and its analogs in liposomes as a model for biologically relevant skin membranes. DHC has been studied extensively in isotropic solvents where the molecule is free to isomerize to previtamin D₃, yielding a 0.5ps fast component and 1-2ps long component while in the anisotropic liposomes a third, longer lived, 11ps component is also observed. To understand the contribution in biological membranes we extend our studies to DHC analogs, DHC-Acetate and Ergo in isotropic solvents and in lipid bilayers.   

Impedance Spectroscopy to Discern Grain Boundaries in Copper Phthalocyanine Thin Films

The thin film growth of phthalocyanine molecules, which play a key role in photovoltaic and gas sensing devices, is important to electronic and magnetic properties. Impedance spectroscopy is used to study copper phthalocyanine thin films in order to disentangle the contributions of the crystal and the unavoidable grain boundaries created under different growth conditions and provide complimentary data to atomic force microscopy. The spectroscopy data is fit with an equivalent circuit model to determine resistance, capacitance, and activation energy for different grain morphologies. The Cole-Cole plots show either one or two peaks, which are attributed to the crystalline bulk and the grain boundary regions. The grain boundary resistance component changes by three orders of magnitude when varying the grain morphology, and the capacitance changes by one order of magnitude. The resistance of the grain boundary shows a minimum near the growth temperature of 450 K. The capacitance and the activation energy peak at the same temperature. The impedance spectroscopy results help determining optimal deposition conditions for electronic applications of small molecular thin films.   

Tuning the Electronic Structure of Two-Dimensional Covalent Organic Frameworks (2D-COFs) by an Asymmetrical Bonding Scheme

Recently, there has been a growing interest in two-dimensional covalent organic frameworks (2D COFs) and its bottom-up synthesis from molecular precursors provides an important new methodology for designing and fabricating such 2D materials. The electronic structure of 2D-COFs is highly dependent on the chemical bonds within these networks. Here we report the synthesis and characterization of a porphyrin-based single-layer 2D COF with a square lattice. The rational design of molecular precursors leads to an asymmetrical bonding scheme in which each porphyrin core is in a different chemical bonding environment than its four nearest neighboring porphyrin cores. This is achieved via an in situ UHV condensation coupling reaction between two different molecular precursors on a Au(111) surface. The chemical structure and local electronic properties of the resulting COF were explored using scanning tunneling microscopy (STM) and spectroscopy (STS). The different bonding environment of the porphyrin cores within this asymmetrically bonded COF results in the formation of a type II heterojunction within a COF network.   

Tailoring the growth and electronic structures of organic molecular thin films

Understanding and control of heterointerfaces between organic and inorganic materials are critical for the development of organic electronics, molecular electronics, molecular/biological sensors, and energy harvesting devices. On the one hand, achieving large-scale molecular ordering on inorganic substrates remains a significant challenge that requires a thorough understanding of the growth mechanism. On the other hand, elucidating the charge behavior, electronic structures and energy-level alignment at the heterointerfaces are crucially important for enhancing the transport properties in hybrid devices. In this talk, I will discuss our recent discovery of the anisotropic crystalline step-flow growth of the prototypical metal phthalocyanine molecules on the deactivated Si(111)-B surface. I will then address the growth mechanism and show that the molecular ordering and molecular orientation can be effectively controlled through selective orbital coupling between the molecule and substrate. Finally, I will illustrate the effects of substrate electrostatic screening, mediated by interfacial charge transfer, on molecular electronic structures and energy-level alignment at the heterointerfaces.   

Thickness Dependence and Substrate Interaction of Thin Film C60 Band Structure

Influence of novel substrates on buckminsterfullerene (C₆₀) thin films is examined through high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements. Thickness dependence of the substrate interaction and band hybridization is investigated as well as alkali metal doping effects. Experimental observations are compared with theoretical models and calculations for a deeper understanding of the physics driving these unique low-dimensional C₆₀ thin films.   

A new electrode design for ambipolar carrier injection in organic semiconductors and its applications to light emitting diodes

Organic semiconductors (OSCs) have attracted much attention for low-cost, flexible and human-friendly optoelectronics. However, achieving high electron injection efficiency is difficult from air-stable electrodes and cannot be equivalent to that of holes. Here, we present a novel concept of electrode composed of a bilayer of tetratetracontane (TTC) and polycrystalline organic semiconductors (pc-OSC) covered by a metal (M) thin film layer. Field-effect transistors of single crystal organic semiconductors (sc-OSCs) with the new electrodes of M/pc-OSC/TTC (M: Ca or Au) show both highly-efficient electron and hole injection. Contact resistances of electron and hole injection from Ca/pc-OSC/TTC and Au/pc-OSC/TTC are lower than those from pure Ca for electrons and Au for holes, respectively. Equivalent high field-effect mobilities of holes (22 cm²V^–1s^–1) and electrons (5.0 cm²V^–1s^–1) were observed in a rubrene single crystal, which are the highest among field-effect transistors with electrodes so far proposed. The new electronde are useful in high performance and air stable lihgt emitting field-effect transistors.   

Transverse Thermal Diffusivity of Polymer and Small Molecule Organic Semiconductors Measured with an ac-Photothermal Technique

We have developed a simplified ac-photothermal apparatus [1] for measurement of the transverse (i.e. through-plane for partly aligned polymers and interlayer for layered crystals) thermal diffusivity of small samples. Our technique is essentially the Fourier transform of the laser flash method. The sample, with a typical area of 5 mm^2 and heated with chopped light, is placed in the dewar of an infrared detector close to the detector, which measures the thermal radiation from the back of the sample. For optically opaque samples, an analysis of the complex frequency dependence of the detector signal gives the transverse diffusivity; results will be presented for free-standing PEDOT:PSS films and samples of cellulose nanofibrils coated with PEDOT:PSS (samples provided by X. Crispin, Linkoping U.) For samples which are not opaque, the same analysis, overlooking the finite optical absorption length, can lead to a very large overestimate of the diffusivity. Here we show how the technique can be adapted and present a more complete analysis for less absorbing samples, and present results for TIPS-pentacene (provided by J. Anthony, U. Kentucky), correcting the value we previously presented [1].
[1] J. W. Brill, et al, J. Appl. Phys. 118, 235501 (2015)   

Two-dimensional Phosphorus Carbide: Competition between sp2 and sp3 Bonding

We propose previously unknown allotropes of phosphorus carbide (PC) in the stable shape of an atomically thin layer. Different stable geometries, which result from the competition between sp2 bonding found in graphitic C and sp3 bonding found in black P, may be mapped onto 2D tiling patterns that simplify categorizing of the structures. Depending on the category, we identify 2D-PC structures that can be metallic, semi-metallic with an anisotropic Dirac cone, or direct-gap semiconductors with their gap tunable by in-layer strain.