Method to Probe Glass Transition Temperatures of Polymer Thin Films

A new methodology was developed to probe glass transition temperatures (<i>T</i>_gs) of polymer thin films supported on gold (Au) substrates and confined between two solid (silica and silver) surfaces based on the surface plasmon polariton (SFPP) signals generated by sum frequency generation (SFG) spectroscopy. The measured <i>T</i>_gs for polymer (poly­(methyl methacrylate), poly­(benzyl methacrylate) and poly­(ethyl methacrylate)) thin films supported on Au substrates showed similar thickness-dependent trend, that is, the <i>T</i>_g decreased as the thin film thickness decreased due to the free surface effect. However, the measured <i>T</i>_g of the (poly­(methyl methacrylate)) thin films confined between two solid surfaces increased significantly with respect to the bulk value, indicating the strong interfacial effect when the free surface was replaced by a buried interface. This method to measure the <i>T</i>_g can be applied to study different polymer thin films supported on metal surfaces or confined between two solid surfaces with different surface chemistries. More importantly, SFG has the unique selectivity and sensitivity to study surfaces and interfaces, providing the feasibility to develop SFG into a powerful tool to detect surface, interfacial, and bulk <i>T</i>_gs of a polymer thin film simultaneously in the future

Terahertz Phase Shift and Its Modulation in NdGaO₃ Single Crystals

Finding appropriate materials to shape the terahertz (THz) wave is highly desired due to the increasing development of practical devices and application systems. In this work, the THz phase shifts of four typical oxide crystals, i.e., NdGaO3 (NGO), quartz, (LaAlO3)0.3(Sr2AlTaO6)0.35, and sapphire, have been investigated by using THz time-domain spectroscopy. It is interesting to find that the NGO crystal is the only one that shows a distinct phase shift. The phase shift (Δφ) of a ∼500 μm-thick NGO crystal is almost linearly dependent on the THz frequency and it reaches as large as ∼94° at 1.5 THz with a temperature variation from 100 to 400 K. Moreover, the THz phase shift of NGO possesses crystal anisotropy and Δφ(100) > Δφ(001) > Δφ(110). In addition, the effects of both electric field and laser illumination on the THz phase shift in the NGO crystal have also been explored. It is found that the electric field (∼260 V/cm) has negligible effect, while the laser illumination can efficiently cause a noticeable THz shift and Δφ ∼ 78° with good manipulation stability can be achieved with 20 J/cm2 light fluence. These findings suggest that NGO crystals are an appropriate candidate for THz phase modulator and their sensitivity and stability are expected to have a great technological impact and offer prospects for their applications in THz optics

Detecting Surface Hydration of Poly(2-hydroxyethyl methacrylate) in Solution <i>in situ</i>

Understanding the interfacial molecular structures of antifouling polymers in solutions is extremely important in research and applications related to chemistry, biology, and medicine. However, it is generally challenging to probe such buried solid/liquid interfaces <i>in situ</i>. We herein report a molecular-level study on detecting the interfacial molecular structures of an antifouling hydrogel material, poly­(2-hydroxyethyl methacrylate) (PHEMA), in contact with water and bovine serum albumin (BSA) solution <i>in situ</i> using sum frequency generation (SFG) vibrational spectroscopy. To compare to and validate our <i>in situ</i> experiments, molecular-level structures of the substrate/PHEMA interface before and after water exposure were also detected. The detected strong O–H vibrational signals from water and hydroxyethyl and carbonyl vibrational signals from PHEMA prove that the PHEMA surface hydration was attributed to the interaction between water and PHEMA side hydrophilic groups. SFG experimental results verify that the adsorbed BSA molecules at the PHEMA/solution interface were disorderly arranged, supported by data from the laser scanning confocal microscopic (LSCM) experiment. This indicates the weak interaction between the BSA molecules and PHEMA surface. This direct detection of the surface hydrated structures of PHEMA sheds light on understanding the interfacial properties of antifouling materials in aqueous environments. The capability reported here to probe the PHEMA/solution interface and the hidden substrate/PHEMA interface after water exposure can be applied to investigate a broad range of interfaces of antifouling materials

Sum Frequency Generation of Interfacial Lipid Monolayers Shows Polarization Dependence on Experimental Geometries

Sum frequency generation (SFG) vibrational spectroscopy has been widely employed to investigate molecular structures of biological surfaces and interfaces including model cell membranes. A variety of lipid monolayers or bilayers serving as model cell membranes and their interactions with many different molecules have been extensively studied using SFG. Here, we conducted an in-depth investigation on polarization-dependent SFG signals collected from interfacial lipid monolayers using different experimental geometries, i.e., the prism geometry (total internal reflection) and the window geometry (external reflection). The different SFG spectral features of interfacial lipid monolayers detected using different experimental geometries are due to the interplay between the varied Fresnel coefficients and second-order nonlinear susceptibility tensor terms of different vibrational modes (i.e., ss and as modes of methyl groups), which were analyzed in detail in this study. Therefore, understanding the interplay between the interfacial Fresnel coefficients and χ⁽²⁾ tensors is a prerequisite for correctly understanding the SFG spectral features with respect to different experimental geometries. More importantly, the derived information in this paper should not be limited to the methyl groups with a <i>C</i>_3<i>v</i> symmetry; valid extension to interfacial functional groups with different molecular symmetries and even chiral interfaces could be expected

Qualitative and Quantitative Analyses of the Molecular-Level Interaction between Memantine and Model Cell Membranes

Sum frequency generation (SFG) vibrational spectroscopy was employed to study the interaction between memantine (a water-soluble drug for treating Alzheimer’s disease) and lipid bilayers (including zwitterionic PC and negatively charged PG lipid bilayers) at the molecular level in real time and <i>in situ</i>. SFG results revealed how the memantine affected these lipid bilayers in terms of the lipid dynamics, average tilt angle (θ), as well as angle distribution width (σ). It was found that memantine could adsorb onto the zwitterionic PC surface but did not affect the flip-flop rate of the PC bilayer even in the presence of 5.0 mM memantine, indicating the negligible interaction between memantine and the PC bilayer. However, for the negatively charged PG bilayer, it was found that the outer PG leaflet could be significantly destroyed by memantine at a relatively low memantine concentration (1.0 mM), while the inner PG leaflet remained intact. Besides, the θ and σ of CD₃ groups in the outer PG lipid leaflet were calculated to be ∼82.0° and ∼19.5° after adding 5 mM memantine, respectively, indicating that these CD₃ groups were prone to lie down at the membrane surface (versus the surface normal) with the addition of 5 mM memantine while nearly standing up without the addition of drug molecules. These monolayer- and molecular-level results could hardly be obtained by other techniques. To the best of our knowledge, this is the first experimental attempt to quantify the drug-induced orientational changes of lipid molecules within a lipid bilayer. The present work provided an in-depth understanding on the interaction between memantine and model cell membranes, which will potentially benefit the development of new drugs for neurodegenerative diseases involving drug–membrane interaction

Green-Solvent-Processable Low-Cost Fluorinated Hole Contacts with Optimized Buried Interface for Highly Efficient Perovskite Solar Cells

Solution-processed hole contact materials, as an indispensable component in perovskite solar cells (PSCs), have been widely studied with consistent progress achieved. One bottleneck for the commercialization of PSCs is the lack of hole contact materials with high performance, cost-effective preparation, and green-solvent processability. Therefore, the development of versatile hole contact materials is of great significance. Herein, we report two novel donor–acceptor (D–A)-type hole contact molecules (FMPA–BT-CA and 2FMPA–BT-CA) with low cost and alcohol-based processability by utilizing a fluorination strategy. We showed that the fluorine atoms lead to the lowered highest occupied molecular orbital (HOMO) energy levels and larger dipole moments for FMPA–BT-CA and 2FMPA–BT-CA. Moreover, fluorination also improves the buried interfacial interaction between hole contacts and perovskite. As a result, a remarkable power conversion efficiency (PCE) of 22.37% along with good light stability could be achieved for green-solvent-processed FMPA–BT-CA-based inverted PSC devices, demonstrating the great potential of environmentally compatible hole contacts for highly efficient PSCs