Deciphering Anomalous Raman Features of Regioregular Poly(3-hexylthiophene) in Ordered Aggregation Form

Poly­(3-hexylthiophene) (P3HT), being a prototypic conjugated polymer, bears a high charge mobility that is sensitive to its packing configuration in the condensed phase. Despite its extensive experimental study with X-ray diffraction, its specified packing structure still remains stymied. This study searched for possible structures of crystalline P3HT and identified the one that holds a simulated Raman spectrum most approximate to the experimental one of ordered P3HT aggregates in the frozen solvent. The spectral correspondence shows that the Raman-active C–C stretch peak exhibits a red shift in frequency, while the CC stretch peak displays a blue shift as the layer planarity of P3HT is relaxed. Moreover, the CC peak splits into two when adjacent thiophene rings in the P3HT chain hold a dihedral angle of 22° with respect to each other. This study demonstrates that Raman spectroscopy plus first-principles simulations can serve as a powerful tool to resolve fine structures of molecular crystals

Large-Scale Uniform Two-Dimensional Hexagonal Arrays of Gold Nanoparticles Templated from Mesoporous Silica Film for Surface-Enhanced Raman Spectroscopy

A good surface-enhanced Raman spectroscopy (SERS) substrate requires precise control of the enhancement factor in large area which may be achieved with large-scale hot spot engineering. Here, we present a facile method for synthesizing 2-D hexagonally patterned gold nanoparticle arrays on centimeter-sized substrates of mesoporous silica thin films with vertical nanochannels by chemical reduction. Scanning electron microscopy images showed densely packed gold nanoparticles directly anchored on the openings of vertical mesopores (∼5 nm) leading to 2 nm nanogaps between the gold nanoparticles. The gold nanoparticle arrays showed red-shifted localized surface plasmon resonance spectra due to strong couplings between close-packed gold. The dense on-substrate 2 nm plasmonic nanogaps lead to highly enhanced local electric field and excellent macroscopic uniformity in SERS

Selective SERS Detecting of Hydrophobic Microorganisms by Tricomponent Nanohybrids of Silver–Silicate-Platelet–Surfactant

Nanohybrids consisting of silver nanoparticles (Ag), clay platelets, and a nonionic surfactant were prepared and used as the substrate for surface-enhanced Raman scattering (SERS). The nanoscale silicate platelets (SP) (with dimensions of 100 × 100 nm² and a thickness of ∼1 nm) were previously prepared from exfoliation of the natural layered silicates. The tricomponent nanohybrids, Ag-SP-surfactant (Ag-SP-S), were prepared by in situ reduction of AgNO₃ in the presence of clay and the surfactant. The clay platelets with a large surface area and ionic charge (ca. 18 000 sodium ions per platelet) allowed for the stabilization of Ag nanoparticles in the range of 10–30 nm in diameter. With the addition of a nonionic surfactant such as poly­(oxyethylene) alkyl ether, the tricomponent Ag-SP-S nanohybrids possessed an altered affinity for contacting microorganisms. The particle size and interparticle gaps between neighboring Ag on SP were characterized by TEM. The surface tension of Ag-SP and Ag-SP-S in water implied different interactions between Ag and hydrophobic bacteria (Escherichia coli and Mycobacterium smegmatis). By increasing the surfactant content in Ag-SP-S, the SERS peak intensity was dramatically enhanced compared to the Ag-SP counterpart. The nanohybrids, Ag-SP and Ag-SP-S, with the advantages of varying hydrophobic affinity, floating in medium, and 3D hot-junction enhancement could be tailored for use as SERS substrates. The selective detection of hydrophobic microorganisms and larger biological cells makes SERS a possible rapid, label-free, and culture-free method of biodetection

Dependence of Adenine Raman Spectrum on Excitation Laser Wavelength: Comparison between Experiment and Theoretical Simulations

We acquired the Raman spectra of adenine in powder and aqueous phase using excitation lasers with 532, 633, and 785 nm wavelengths for the region between 300 and 1500 cm^–1. In comparison to the most distinct peak at 722 cm^–1, the peaks between 1200 and 1500 cm^–1 exhibited a characteristic increase in cross-section with decreasing excitation wavelength in both phases. This trend can be reproduced by different density functional theory (DFT) calculations for the adenine molecule in the gas phase as well as in the aqueous phase. Furthermore, from the calculation on the π-stacked dimer, hydrogen-bonded dimer, and trimer, we find that this trend toward excitation laser wavelength is not sensitive to the packing. When comparing the Raman spectra given by different excitation wavelength, one should take care in analyzing the cross-section, and present day DFT calculations are able to capture general trends in the excitation laser wavelength dependence of the Raman activity

Exploring Azobenzenethiol Adsorption on the Ag/Ge(111) Surface with Surface Raman Spectroscopy

Self-assembled monolayers (SAMs) formed with thiols on surfaces represent the most representative system of such kind. Their detailed adsorption orientation and kinetics are however rarely elucidated completely, making the development of the SAM systems mostly based on try-and-error approach. We have studied the adsorption of azobenzenethiol (azoSH) on the Ag/Ge(111)-(√3 × √3)<i>R</i>30° surface, as an archetype of SAMs on compound surfaces, with <i>in situ</i> surface Raman spectroscopy. Two different adsorbates have been identified with their vibrational signatures and orientations. They respectively correspond to the two adsorption sites of this compound surface system, owing to distinct molecule–surface interactions, and both exhibit Langmuir adsorption behavior. These traits are compared with that on the Ge(111) surface, bearing homogeneous adsorption propensity, where one precursor of adsorption has been identified. The revelation of the detailed adsorption traits of azoSH has demonstrated that surface Raman spectroscopy is expedient in revealing complex adsorption behaviors of the SAM systems

Revealing Ordered Polymer Packing during Freeze-Drying Fabrication of a Bulk Heterojunction Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61-butyric Acid Methyl Ester Layer: In Situ Optical Spectroscopy, Molecular Dynamics Simulation, and X‑ray Diffraction

Formation of ordered poly­(3-hexylthiophene-2,5-diyl) (P3HT) molecular stacking during the freeze-drying process is tracked with in situ spectroscopy of Raman scattering, absorption, and photoluminescence. Raman spectra of pristine P3HT dissolved in 1,2-dichlorobenzene show that P3HT polymers undergo drastic ordered aggregation upon being lower than 0 °C, at which the solubility of P3HT is reached, as evidenced by the emergence of pronounced red-shifted, narrow Raman peaks (1422 and 1435 cm^–1) caused by intermolecular coupling. The absorption and photoluminescence spectra bear similar temperature dependence as the results of Raman. Aggregation of P3HT is further confirmed by coarse-grained molecular dynamics simulation showing the enhanced order parameters of distance and orientation between P3HT chains upon cooling. The incorporation of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) does not significantly alter the P3HT packing configuration, as verified by nearly identical Raman features observed in P3HT:PCBM mixing solution upon cooling. While optical spectroscopy and MD simulation portrayed the short-range order of P3HT aggregates, grazing-incident X-ray diffraction exposed the long-range order by the pronounced diffraction spots corresponding to the lamellar stacking of P3HT. This study demonstrates the ability of Raman spectroscopy to reveal the short-range order of polymer packing, while the in situ monitoring illustrates that the ability of freeze-drying to separate molecular aggregation from solvent removal thus is advantageous for photovoltaic device fabrication without resorting to trial and error

Additional file 1: of Core-shell of FePt@SiO2-Au magnetic nanoparticles for rapid SERS detection

TEM images of core-shell nanoparticles. Figure S1. TEM images of (A) FePt@SiO2-N and (B) gold nanoparticles (scale bar: 50 nm). Figure S2. TEM images of Au-FePt@SiO2-N with various EDS concentration: (A) 0 mM, (B) 0.1 M, (C) 0.2 M, (D) 0.3 M, (E) 0.4 M and (F) 0.5 M (scale bar: 50 nm). Figure S3. TEM images of Au-FePt@SiO2-N (0.3 M) with various gold concentration: (A) 0 μM, (B) 47.6 μM, (C) 95.2 μM, (D)142.8 μM, (E)190.4 μM, and (F) 238 μM (scale bar, 100 nm