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Bulk heterojunction is one key concept leading to breakthrough in organic photovoltaics. The active layer is expectantly formed of distinct morphologies that carry out their respective roles in photovoltaic performance. The morphology-performance relationship however remains stymied, because unequivocal morphology at the nanoscale is not available. We used scattering-type scanning near-field optical microscopy operating with a visible light source (visible s-SNOM) to disclose the nanomorphology of P3HT:PCBM and pBCN:PCBM blends. Donor and acceptor domain as well as intermixed phase were identified and their intertwined distributions were mapped. We proposed energy landscapes of the BHJ active layer to shed light on the roles played by these morphologies in charge separation, transport and recombination. This study shows that visible s-SNOM is capable of profiling the morphological backdrop pertaining to the operation of high performance organic solar cells.

In this work, the optical properties of Au nanoparticles processed by centrifugation techniques are studied. Most of the literature related to the control of nanoparticle size has focused on different preparation parameters; however, the wide size distribution is commonly an issue for follow-up investigations and further applications. Therefore, we developed a method in which specific-diameter particles can be effectively separated using different centrifugal procedures. The initial nanoparticle solution with a primary absorption peak at 534 nm is separated into discernible resonance wavelengths from 526 to 537 nm, with corresponding particle sizes from 30 to 55 nm. For the atomic force microscopy analysis of nanoparticle size, a dry cetyltrimethylammonium bromide (CTAB) film often covers the particles and interferes with the measurement; thus, CTAB has to be removed. However, if too much CTAB is removed, the surface of the Au nanoparticle becomes unstable, and the particles aggregate. Accordingly, we used UV spectroscopy to monitor the CTAB content; properly adjust the rotational speed and the number of centrifugation stages; and design a method that can effectively remove impurities, avoid clustering, and enable particle size measurement. The usually complicated procedures and high cost of preparation of specific-size Au nanoparticles are greatly simplified and reduced by the convenient extraction process proposed in this work, which would benefit related research and applications.

The strategy of using the thermal expansion of copper single crystal can provide an approach of homogenous strain along the basal plane to study the strain characteristic of graphene with temperature. Using an in-situ Raman measurement under a ultra-high vacuum (UHV) environment, the ability to remove contaminations allowed the direct observation of the strain property in as-grown chemical vapor deposition (CVD)-graphene with temperature on a Cu(100) substrate. In this study, the strain coefficients of G and G’ band with temperature were investigated from the in-situ temperature-dependent Raman spectra of the as-grown CVD-graphene on the single crystal of Cu(100) under UHV. By eliminating the minor contributions of the lattice expansion and anharmonic phonon scattering effects, we were able to estimate the strain coefficient of the G and G’ bands of graphene over a wide temperature from 100 K to 800 K. Based on the strain coefficients and the correlation map of the G- and G’-band frequencies, we made a re...

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)R30° surface, as an archetype of SAMs on compound surfaces, with in situ 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 spec...

Deposition of organic molecules on inorganic substrates is an important step in the fabrication of organic electronic devices with designated optoelectronic properties. Their interaction with the surface underneath and the interaction among themselves influence the molecular orientation of the first adsorbed layer and thus energy and charge transport properties of the grown organic films. We show that these interactions of trans-stilbene, trans,trans-distyrylbenzene, and trans-azobenzene adsorbed on Ag/Ge(111) -(√3 × √3)R30° surface are reflected on their Raman signatures. Detailed spectral analysis reveals the characteristic vibrational features corresponding to the molecules residing in the first adlayer and in the layers above, separately. The results further show that for all the three different molecular systems, the integrated intensity of the multilayer Raman signature increases linearly with the molecular coverage. This study explores potential utilization of Raman spectroscopy as a sensitive, non...

The vibrational features of self-organized submonolayer trans-stilbene on the Ag/Ge(111)−(√3 × √3)R30° surface have been investigated by Raman spectroscopy in ultrahigh vacuum at 100 K. A red shift in the C═C stretching mode of the olefinic group at 1625 cm−1 and enhanced Raman activity of the peak at 1568 cm−1 were observed, as compared with those corresponding to a multilayer trans-stilbene. Electronic interaction between trans-stilbene and the underneath surface and Raman spectra were calculated based on density functional theory. The calculated Raman spectra are in good agreement with the experimental results, supporting the calculated molecular geometry of the adsorbed trans-stilbene, which exhibits a predominant bond elongation in the olefinic group and a torsional angle between the phenyl ring and the olefinic plane. Analysis of the partial density of states shows that the lowest unoccupied molecular orbital is broadened and lowered to cross the surface Fermi level by the interaction with the surfa...

The adsorption and desorption of stilbene on Ag/Ge(111)-(√3 × √3)R30° (Ag/Ge(111)-√3) were investigated using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory (DFT). Both trans- and cis-stilbenes form a (2 × 1) overlayer structure on Ag/Ge(111)-√3 at a coverage of ∼1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. At a coverage of less than 1 ML, the TPD of cis-stilbene shows only one peak, attributed to submonolayer desorption. The TPD peaks are indistinguishable for desorption of trans-stilbene from the surface submonolayer and multilayer. This is due to the simultaneous desorption and/or thinning of adsorbed multilayers during the TPD process, as determined from the STM analysis of adsorbed trans-stilbene structures before and after annealing. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(...

The adsorption and self-organized monolayers of trans,trans-distyrylbenzene (tt-DSB) and cis,cis-distyrylbenzene (cc-DSB) on Ag/Ge(111)-( × )R30° (Ag/Ge(111)- ) were studied by low-temperature scan...

The photodissociation of CH≡CCH2X (X=Br and Cl) at 248 and 193 nm was studied using an angle-resolved beam apparatus. Both the translational energy distribution P(Et) and the anisotropy parameters β of the photoproducts were derived from the time-of-flight spectra. Excitation at 248 nm, CH≡CCH2Br underwent exclusively a direct C–Br bond fission with anisotropic recoiling fragments. By increasing the photon energy to 193 nm, three competing primary dissociation channels were observed: (1) Br+C3H3 (fast, 26%), (2) Br+C3H3 (slow, 44%), and (3) HBr+C3H2 (30%), with average translational energies of 34, 12, and 9 kcal/mol, respectively. From the measured β values, all reactions were shown to proceed extremely fast within a fraction of rotational period. On the other hand, excitation at 193 nm, CH≡CCH2Cl underwent a predominant C–Cl bond fission with a minor channel producing the HCl+C3H2 fragments. The results are discussed in terms of either a direct or an indirect dissociation process arising from the transi...

The photolysis of CHCl=CCl2 at 193 nm was investigated by translational spectroscopy. Two distinct product translational distributions were derived for the C–Cl bond fissions. The internally excited C2HCl2 fragment from the main dissociation channel is shown to decompose totally to produce the Cl+C2HCl products.