STM Study of the Self-Assembly of Biphenyl-3,3',5,5'-Tetracarboxylic Acid and Its Mixing Behavior with Coronene at the Liquid-Solid Interface.

We report a scanning tunneling microscopy (STM) study of the molecular self-assembly of biphenyl-3,3',5,5'-tetracarboxylic acid (BPTC) at the octanoic acid/graphite interface. STM revealed that the BPTC molecules generated stable bilayers and monolayers under high and low sample concentrations, respectively. Besides hydrogen bonds, the bilayers were stabilized by molecular π-stacking, whereas the monolayers were maintained by solvent co-adsorption. A thermodynamically stable Kagomé structure was obtained upon mixing BPTC with coronene (COR), while kinetic trapping of COR in the co-crystal structure was found by the subsequent deposition of COR onto a preformed BPTC bilayer on the surface. Force field calculation was conducted to compare the binding energies of different phases, which helped to provide plausible explanations for the structural stability formed via kinetic and thermodynamic pathways.

A Paradigm Shift from 2D to 3D: Surface Supramolecular Assemblies and Their Electronic Properties Explored by Scanning Tunneling Microscopy and Spectroscopy.

Exploring supramolecular architectures at surfaces plays an increasingly important role in contemporary science, especially for molecular electronics. A paradigm of research interest in this context is shifting from 2D to 3D that is expanding from monolayer, bilayers, to multilayers. Taking advantage of its high-resolution insight into monolayers and a few layers, scanning tunneling microscopy/spectroscopy (STM/STS) turns out a powerful tool for analyzing such thin films on a solid surface. This review summarizes the representative efforts of STM/STS studies of layered supramolecular assemblies and their unique electronic properties, especially at the liquid-solid interface. The superiority of the 3D molecular networks at surfaces is elucidated and an outlook on the challenges that still lie ahead is provided. This review not only highlights the profound progress in 3D supramolecular assemblies but also provides researchers with unusual concepts to design surface supramolecular structures with increasing complexity and desired functionality.

Flow-induced-crystallization: tailoring host-guest supramolecular co-assemblies at the liquid-solid interface.

Here, we report that using the method of simply contacting a sample solution droplet with a piece of tissue paper can create a solvent flow (capillary force). During this process, the dynamics and solvent removal can promote the formation and stabilization of a meta-stable linear quasi-crystal composed of p-terphenyl-3,5,3',5'-tetracarboxylic acid (TPTC) molecules, which would otherwise pack into thermodynamically favored random tiling. The tailored quasi-crystal (linear) template allows atop it higher-efficiency accommodation of fullerene molecules (C60) from 40.1% to 97.5%, compared with that obtained in the random-tiling (porous) case. Overall, the result of this study presents an unusual yet remarkably simple strategy for tailoring complex host-guest supramolecular systems at the liquid-solid interface.

Monolayer and Bilayer Formation of Molecular 2D Networks Assembled at the Liquid/Solid Interfaces by Solution-Based Drop-Cast Method.

In recent years, extending self-assembled structures from two-dimensions (2D) to three-dimensions (3D) has been a paradigm in surface supramolecular chemistry and contemporary nanotechnology. Using organic molecules of p-terphenyl-3,5,3',5'-tetracarboxylic acid (TPTC), and scanning tunneling microscopy (STM), we present a simple route, that is the control of the solute solubility in a sample solution, to achieve the vertical growth of supramolecular self-assemblies, which would otherwise form monolayers at the organic solvent/graphite interface. Presumably, the bilayer formations were based on π-conjugated overlapped molecular dimers that worked as nuclei to induce the yielding of the second layer. We also tested other molecules, including trimesic acid (TMA) and 1,3,5-tris(4-carboxyphenyl)-benzene (BTB), as well as the further application of our methodology, demonstrating the facile preparation of layered assemblies.

Enhancing the photovoltaic performance of hybrid heterojunction solar cells by passivation of silicon surface via a simple 1-min annealing process.

Solution-based heterojunction technology is emerging for facile fabrication of silicon (Si)-based solar cells. Surface passivation of Si substrate has been well established to improve the photovoltaic (PV) performance for the conventional bulk Si cells. However, the impact is still not seen for the heterojunction cells. Here, we developed a facile and repeatable method to passivate the Si surface by a simple 1-min annealing process in vacuum, and integrated it into the heterojunction cells with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) or carbon nanotube (CNT). A thin and dense oxide layer was introduced on the Si surface to provide a high-quality hole transport layer and passivation layer. The layer enhanced the power conversion efficiency from 9.34% to 12.87% (1.38-times enhancement) for the PEDOT:PSS/n-Si cells and from 6.61% to 8.52% (1.29-times enhancement) for the CNT/n-Si cells. The simple passivation is a promising way to enhance the PV performance of the Si cells with various solution-based heterojunctions.

Single small molecule-assembled nanoparticles mediate efficient oral drug delivery.

Oral drug delivery, which requires surviving the harsh environment in the gastrointestinal (GI) tract and penetrating the intestinal epithelium, has not been achieved using simple formulation nanoparticles (NPs). Medicinal natural products (MNPs) have been widely used in traditional medicine for disease management through oral consumption. However, most pharmacologically active compounds within MNPs do not have the properties suitable for oral applications. We hypothesize that some MNPs contain natural nanomaterials that can convert those compounds into oral formulations by forming NPs. After screening 66 MNPs, we identified five classes of small molecules that form NPs, many of which are capable of efficient drug encapsulation and GI penetration. We show that one of them, dehydrotrametenolic acid (DTA), is capable of mediating oral delivery for effective disease treatment. We determine that DTA NPs assemble through hydrogen bonding and penetrate the GI tract via apical sodium-dependent bile acid transporter. Our study reveals a novel class of single component, small molecule- assembled NPs for oral drug delivery, and suggests a novel approach to modernizing MNPs through nanomaterial discovery.