High-Sensitivity Raman Scattering Substrate Based on Au/La_0.7Sr_0.3MnO₃ Periodic Arrays

We have developed Au/La0.7Sr0.3MnO3 (Au/LSMO) periodic arrays with tunable surface plasmon properties that can be used as novel surface-enhanced Raman scattering (SERS) substrates. The periodic arrays are created by electron beam lithography of LSMO resist and metal film deposition. The LSMO electron beam resist is unique in that it exhibits either positive or negative resist behaviors depending on the electron beam dosage. Interestingly, surface plasmon behavior of the arrays can be controlled by just changing the electron beam dosage when presented with a fixed design pattern. Scanning confocal microscopy and spectral microreflectometry have been adapted to directly demonstrate this unique behavior. Furthermore, we show that our novel Au/LSMO array can be used as a high-sensitivity Raman scattering substrate. To illustrate this working principle, the Au/LSMO periodic array is applied to enhance the Raman scattering of a thin film containing 0.1 wt % poly-3-hexylthiophene (P3HT) in poly(methyl methacrylate) (PMMA). By controlling the geometry of the patterned substrate that exhibits gold surface plasmon near the excitation wavelength, we can enhance the intensity of Raman scattering of P3HT at 1350 cm−1 up to 4 orders of magnitude as compared with previously generated planar Au substrates

Quantitative Nanoorganized Structural Evolution for a High Efficiency Bulk Heterojunction Polymer Solar Cell

We have developed an improved small-angle X-ray scattering (SAXS) model and analysis methodology to quantitatively evaluate the nanostructures of a blend system. This method has been applied to resolve the various structures of self-organized poly(3-hexylthiophene) /C61-butyric acid methyl ester (P3HT/PCBM) thin active layer in a solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by a different annealing process can provide a flexible approach and better understanding to enhance the power conversion of the P3HT/PCBM solar cell. The quantitative structural characterization by this method includes (1) the mean size, volume fraction, and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration, and (4) the correlation length of the PCBM molecular network within the P3HT phase. The above terms are correlated well with the device performance. The various structural evolutions and transformations (growth and dissolution) between PCBM and P3HT with the variation of annealing history are demonstrated here. This work established a useful SAXS approach to present insight into the modeling of the morphology of P3HT/PCBM film. In situ GISAXS measurements were also conducted to provide informative details of thermal behavior and temporal evolution of PCBM-related structures during phase separation. The results of this investigation significantly extend the current knowledge of the relationship of bulk heterojunction morphology to device performance

Reaction Kinetics and Formation Mechanism of TiO₂ Nanorods in Solution: An Insight into Oriented Attachment

The reaction kinetics and formation mechanism of oriented attachment for shaped nanoparticles in solution are not well-understood. We present the reaction kinetics and formation mechanism of organic-capped anatase TiO₂ nanorods in solution as a case study for the oriented attachment process using small-angle X-ray scattering (SAXS) and transmission electronic microscopy. The SAXS analysis qualitatively and quantitatively provides in-depth understanding of the mechanism, including the structural evolution, interparticle interaction, and spatial orientation of nanoparticles developed from nanodots to nanorods during the nucleation, isotropic, and anisotropic growth steps. The present study demonstrates the growth details of oriented attachment of nanoparticles in solution. An ordered lamellar structure in the solution is constructed by the balance of interaction forces among surface ligands, functional groups, and solvent molecules serving as a natural template. The template allows the alignment of spherical nanoparticles into ordered chain arrays and facilitates simultaneous transformation from spherical to rod shape via proximity attachment. The proposed model reveals an insight into the oriented attachment mechanism. This multistep formation mechanism of TiO₂ nanorods in solution can provide the fundamental understanding of how to tune the shape of nanoparticles and further control the aggregation of spatial nanorod networks in solution

Manipulation of Nanoscale Phase Separation and Optical Properties of P3HT/PMMA Polymer Blends for Photoluminescent Electron Beam Resist

A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The λmax of the film absorption is blue-shifted from 520 to 470 nm, and its λmax of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration

Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives

The aim of this study is to quantitatively investigate the effect of different fullerene type (PC₆₀BM and PC₇₀BM) on various morphological structures and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) P3HT/PC_<i>x</i>BM solar cells. The solar cells are fabricated by spin coating without thermal annealing. The quantitative investigations of three-dimensional self-organized nanostructures are performed by using combined grazing-incidence small- and wide-angle X-ray scattering technique (GISAXS/GIWAXS). Two types of nanostructures are observed due to the phase separation in the BHJ films during the processing. They include (1) intercalated PC_<i>x</i>BM molecules around boundary of P3HT crystalline domain and within amorphous domain and (2) aggregated PC_<i>x</i>BM clusters in PC_<i>x</i>BM domains. The lamellar spacing of P3HT crystalline domains in P3HT/PC₇₀BM is larger than that in P3HT/PC₆₀BM. This result indicates more interfacial areas are generated between PC₇₀BM and P3HT at the molecular scale for more efficient charge separation. On the other hand, the size, volume fraction, partial attachment, and spatial distribution of PC₆₀BM clusters are larger than that of PC₇₀BM clusters, which reveals more efficient electron transport in P3HT/PC₆₀BM. We deduce the correlation between nanostructures and PCE (3.25% and 2.64%, respectively, for P3HT/PC₇₀BM and P3HT/PC₆₀BM). The structure of fullerene intercalated with P3HT rather than the size of fullerene cluster plays a major role in the PCE performance of BHJ solar cell without thermal annealing

Nanoparticle-Tuned Self-Organization of a Bulk Heterojunction Hybrid Solar Cell with Enhanced Performance

We demonstrate here that the nanostructure of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction (BHJ) can be tuned by inorganic nanoparticles (INPs) for enhanced solar cell performance. The self-organized nanostructural evolution of P3HT/PCBM/INPs thin films was investigated by using simultaneous grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS) technique. Including INPs into P3HT/PCBM leads to (1) diffusion of PCBM molecules into aggregated PCBM clusters and (2) formation of interpenetrating networks that contain INPs which interact with amorphous P3HT polymer chains that are intercalated with PCBM molecules. Both of the nanostructures provide efficient pathways for free electron transport. The distinctive INP-tuned nanostructures are thermally stable and exhibit significantly enhanced electron mobility, external quantum efficiency, and photovoltaic device performance. These gains over conventional P3HT/PCBM directly result from newly demonstrated nanostructure. This work provides an attractive strategy for manipulating the phase-separated BHJ layers and also increases insight into nanostructural evolution when INPs are incorporated into BHJs