Atomic-Scale Three-Dimensional Local Solvation Structures of Ionic Liquids

Room-temperature ionic liquids are promising media for next-generation energy devices because of their various superior characteristics. Because device performance is often dictated by the solvation structures at the solid–liquid interfaces, particularly at the local reactive sites, their atomistic pictures are in great demand. However, there has been no experimental technique for their three-dimensional solvation structures. Here, we first demonstrate the measurement of the atomic-scale ionic liquids using a recently established ultralow-noise three-dimensional frequency-modulation atomic force microscopy technique supported by molecular dynamics simulations. We conducted the experiments in protic and aprotic aqueous solutions and reveal that the aprotic solvation structure exhibits the higher site specificity, which resolves atomic-scale surface charge distribution on mica because of the absence of the H-bonding network. Our methodology is also applicable to pure liquids and would be a breakthrough for expanding their future applications

Investigation of Molecular Chain Orientation Change of Polymer Crystals in Phase Transitions by Friction Anisotropy Measurement

Direct observation of the molecular orientation change in polymer crystals provides us visible information for understanding their structural phase-transition mechanisms. In this letter, we successfully identified the main-chain orientation of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) crystals over all directions using friction anisotropy measured by lateral-modulation friction force microscopy (LM-FFM). This technique made possible our investigation of molecular orientation changes caused by a ferroelectric phase transition and also a fabrication process for artificial nanometer-scale structures. These results give us visual information that is directly connected to the transition mechanisms

Atomic-Scale 3D Local Hydration Structures Influenced by Water-Restricting Dimensions

Hydration structures at solid–liquid interfaces mediate between the atomic-level surface structures and macroscopic functionalities in various physical, chemical, and biological processes. Atomic-scale local hydration measurements have been enabled by ultralow noise three-dimensional (3D) frequency-modulation atomic force microscopy. However, for their application to complicated surface structures, e.g., biomolecular devices, understanding the relationship between the hydration and surface structures is necessary. Herein, we present a systematic study based on the concept of the structural dimensionality, which is crucial in various scientific fields. We performed 3D measurements and molecular dynamics simulations with silicate surfaces that allow for 0, 1, and 2 degrees of freedom to water molecules. Consequently, we found that the 3D hydration structures reflect the structural dimensions and the hydration contrasts decrease with increasing dimension due to the enlarged water self-diffusion coefficient and increased embedded hydration layers. Our results provide guidelines for the analysis of complicated hydration structures, which will be exploited in extensive fields

Interlayer Resistance and Edge-Specific Charging in Layered Molecular Crystals Revealed by Kelvin-Probe Force Microscopy

Organic field-effect transistors (OFETs) having an active channel of solution-processed 2,7-dioctyl[1]­benzothieno­[3,2-<i>b</i>]­[1]­benzothiophene (C₈–BTBT) were investigated by Kelvin-probe force microscopy (KFM). We found step-like potential distributions in a channel region, suggesting that the interlayer resistance between the conjugated BTBT core layers is quite high and each conjugated layer is electrically isolated from one another by insulating alkyl chain layers. We also found a noticeable positive charging in the channel region especially at the step edges after the device operation. The observed charging was explained by long-lived positive charges on the trap sites, and the trap density at the step edge was estimated to be on the order of 10¹¹ cm^–2. The KFM measurements suggest that the device performance of the staggered C₈–BTBT OFETs could deteriorate due to the considerably high access resistance, which stems from the high interlayer resistance and/or by the site-specific charge trapping at the contact/semiconductor interface which originates from step edge structures

Atomic-Scale Three-Dimensional Local Solvation Structures of Ionic Liquids

Room-temperature ionic liquids are promising media for next-generation energy devices because of their various superior characteristics. Because device performance is often dictated by the solvation structures at the solid–liquid interfaces, particularly at the local reactive sites, their atomistic pictures are in great demand. However, there has been no experimental technique for their three-dimensional solvation structures. Here, we first demonstrate the measurement of the atomic-scale ionic liquids using a recently established ultralow-noise three-dimensional frequency-modulation atomic force microscopy technique supported by molecular dynamics simulations. We conducted the experiments in protic and aprotic aqueous solutions and reveal that the aprotic solvation structure exhibits the higher site specificity, which resolves atomic-scale surface charge distribution on mica because of the absence of the H-bonding network. Our methodology is also applicable to pure liquids and would be a breakthrough for expanding their future applications

Electrospray Deposition, Model, and Experiment: Toward General Control of Film Morphology

Poly(vinylidene fluoride) film formation with electrospray deposition has been studied with support of a droplet evaporation model. The input parameters of the model consist basically of the solvent, the solute concentration, the flow rate, and the solution conductivity. The model provides the droplet size, the solute concentration, the droplet velocity, and the shear stress of the droplet at impact as a function of the distance between the nozzle and the substrate. With some additional experimental information such as the size change of the film with spray distance and the viscosity of the solution, the growth rate of the film and the shear rate of the droplet at impact can be determined. Growth rate is shown to define distinct regimes of film formation. In those regimes, only a single factor or a limited number of factors controls the film morphology. The most important factors include the shear rate and the surface energy. It is found that at a specific range of growth rates only the shear rate determines the morphology of the polymer film. Growth rate, as the defining quantity of film morphology, is not limited to polymer solutions. Therefore, the growth rate, in combination with the control factors mentioned above, functions as a general framework through which understanding and control of film formation with electrospray deposition can be improved

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