Chun-Wei Pao

Multiscale molecular simulations of the nanoscale morphologies of P3HT:PCBM blends for bulk heterojunction organic photovoltaic cells

In this study, we developed a multiscale molecular simulation framework including coarse-grained (CG) molecular simulation, reverse-mapping, and morphology evaluation schemes to investigate the nanoscale morphologies of bulk heterojunction (BHJ) blend films comprising poly(3-hexylthiophene) (P3HT) and the methanofullerene derivative PCBM. A stable and phase-separated blend film with the fibrillar P3HT structure was observed after CG simulation of the thermal annealing process, and by the reverse-mapping technique the atomistic details—showing strong π–π interaction between thiophene rings—were retrieved. To evaluate the morphologies of P3HT:PCBM blends, a spatial-discretization scheme was developed. With such a scheme, we estimated the average domain sizes, interface-to-volume ratios, and percolation ratios of the blends at different P3HT:PCBM weight ratios. The average domain sizes determined through these simulations were in excellent agreement with those reported experimentally. Moreover, our simulations indicated that blend films having weight ratios close to 1 : 1 would have the highest interface-to-volume ratio and the most balanced charge carrier transport in both the P3HT and PCBM phases, consistent with the experimental observation that a 1 : 1 weight ratio is optimal for P3HT:PCBM blends. The multiscale molecular simulation framework proposed herein can be extended to investigating the morphologies of other photoactive layers of organic photovoltaic cells.

Ab initio calculations of the reaction pathways for methane decomposition over the Cu (111) surface

Growth of large-area, few-layer graphene has been reported recently through the catalytic decomposition of methane (CH(4)) over a Cu surface at high temperature. In this study, we used ab initio calculations to investigate the minimum energy pathways of successive dehydrogenation reactions of CH(4) over the Cu (111) surface. The geometries and energies of all the reaction intermediates and transition states were identified using the climbing image nudged elastic band method. The activation barriers for CH(4) decomposition over this Cu surface are much lower than those in the gas phase; furthermore, analysis of electron density differences revealed significant degrees of charge transfer between the adsorbates and the Cu atoms along the reaction path; these features reveal the role of Cu as the catalytic material for graphene growth. All the dehydrogenation reactions are endothermic, except for carbon dimer (C(2)) formation, which is, therefore, the most critical step for subsequent graphene growth, in particular, on Cu (111) surface.

Decoupling of CVD graphene by controlled oxidation of recrystallized Cu

Large-area graphene grown by chemical vapour deposition (CVD) is promising for applications; however, the interaction between graphene and the substrate is still not well understood. In this report, we use a combination of two non-destructive characterization techniques, i.e., electron backscatter diffraction (EBSD) and Raman mapping to locally probe the interface between graphene and copper lattices without removing graphene. We conclude that the crystal structure of the Cu grains under graphene layers is governed by two competing processes: (1) graphene induced Cu surface reconstruction favoring the formation of Cu(100) orientation, and (2) recrystallization from bulk Cu favoring Cu(111) formation. The underlying Cu grains, regardless of reconstruction or recrystallization, induce a large hydrostatic compression to the graphene lattice. Interestingly, the strong interaction could be decoupled by allowing the intercalation of a thin cuprous oxide interfacial-layer. The Cu2O layer is mechanically and chemically weak; hence, graphene films can be detached and transferred to arbitrary substrates and the Cu substrates could be re-used for graphene growth.

Anisotropic thermal conductivity of MoS2 nanoribbons: Chirality and edge effects

Previous studies of the thermal transport in MoS2 are limited to the 0° (zigzag) and 30° (armchair) chiralities. We investigate the anisotropic thermal transport properties of MoS2 nanoribbons with various crystal chiralities by employing the full-band phonon dispersion relations obtained from first-principle calculations. The ribbons with chiralities other than 0° and 30° always have lower thermal conductivity, yet a local maximum at 19.1°. In addition, the thermal conductivity can be further decreased by increasing the edge roughness due to the largely degraded longitudinal phonons. These findings suggest possibilities of obtaining a higher thermoelectric efficiency in MoS2 nanoribbons.

Low-temperature grown graphene films by using molecular beam epitaxy

Complete graphene film is prepared by depositing carbon atoms directly on Cu foils in a molecular beam epitaxy chamber at 300 °C. The Raman spectrum of the film has indicated that high-quality few-layer graphene is obtained. With back-gated transistor architecture, the characteristic current modulation of graphene transistors is observed. Following the similar growth procedure, graphitization is observed at room temperature, which is consistent with the molecular dynamics simulations of graphene growth.

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

The nanomorphologies of the bulk heterojunction (BHJ) layer of polymer solar cells are extremely sensitive to the electrode materials and thermal annealing conditions. In this work, the correlations of electrode materials, thermal annealing sequences, and resultant BHJ nanomorphological details of P3HT:PCBM BHJ polymer solar cell are studied by a series of large-scale, coarse-grained (CG) molecular simulations of system comprised of PEDOT:PSS/P3HT:PCBM/Al layers. Simulations are performed for various configurations of electrode materials as well as processing temperature. The complex CG molecular data are characterized using a novel extension of our graph-based framework to quantify morphology and establish a link between morphology and processing conditions. Our analysis indicates that vertical phase segregation of P3HT:PCBM blend strongly depends on the electrode material and thermal annealing schedule. A thin P3HT-rich film is formed on the top, regardless of bottom electrode material, when the BHJ layer is exposed to the free surface during thermal annealing. In addition, preferential segregation of P3HT chains and PCBM molecules toward PEDOT:PSS and Al electrodes, respectively, is observed. Detailed morphology analysis indicated that, surprisingly, vertical phase segregation does not affect the connectivity of donor/acceptor domains with respective electrodes. However, the formation of P3HT/PCBM depletion zones next to the P3HT/PCBM-rich zones can be a potential bottleneck for electron/hole transport due to increase in transport pathway length. Analysis in terms of fraction of intra- and interchain charge transports revealed that processing schedule affects the average vertical orientation of polymer chains, which may be crucial for enhanced charge transport, nongeminate recombination, and charge collection. The present study establishes a more detailed link between processing and morphology by combining multiscale molecular simulation framework with an extensive morphology feature analysis, providing a quantitative means for process optimization.

Correlation of nanoscale organizations of polymer and nanocrystals in polymer/inorganic nanocrystal bulk heterojunction hybrid solar cells: insights from multiscale molecular simulations

A comprehensive insight into the correlations of the nanoscale organizations of polymer and nanocrystals in polymer/inorganic nanocrystal bulk heterojunction (BHJ) hybrid solar cells is the key toward nanomorphology control for improving device performance. In this study, we investigated the organizations of both the polymer and nanocrystals in polymer/inorganic nanocrystal hybrid solar cells by performing multiscale molecular simulations of P3HT:TiO2 nanocrystal BHJs incorporating nanocrystals with two different dimensionalities, namely, zero-dimensional nanoparticles (NPs), and one-dimensional nanorods (NRs). We reveal that nanocrystal dimensionality has significant impacts on the polymer/nanocrystal organizations for polymer/inorganic nanocrystal hybrid blends. One-dimensional nanocrystals, such as TiO2 NRs, can effectively enhance the polymer degree of crystallinity as a result of preferential polymer chain alignment along the axial dimension of the NRs, thereby promoting hole transport; in addition, the elongated, anisotropic NRs significantly reduce the probability of electron hopping, and maintain a high specific interfacial area for efficient exciton dissociation. Therefore, the present study demonstrates the possibility of the nanoscale morphology control of polymer/inorganic nanocrystal BHJ hybrid blends via tuning the nanocrystal shapes, which is potentially helpful for developing next-generation polymer/inorganic nanocrystal hybrid electronic devices such as solar cells or thin film transistors.

PSII–LHCII Supercomplex Organizations in Photosynthetic Membrane by Coarse-Grained Simulation

Green plant photosystem II (PSII) and light-harvesting complex II (LHCII) in the stacked grana regions of thylakoid membranes can self-organize into various PSII-LHCII supercomplexes with crystalline or fluid-like supramolecular structures to adjust themselves with external stimuli such as high/low light and temperatures, rendering tunable solar light absorption spectrum and photosynthesis efficiencies. However, the mechanisms controlling the PSII-LHCII supercomplex organizations remain elusive. In this work, we constructed a coarse-grained (CG) model of the thylakoid membrane including lipid molecules and a PSII-LHCII supercomplex considering association/dissociation of moderately bound-LHCIIs. The CG interaction between CG beads were constructed based on electron microscope (EM) experimental results, and we were able to simulate the PSII-LHCII supramolecular organization of a 500 × 500 nm(2) thylakoid membrane, which is compatible with experiments. Our CGMD simulations can successfully reproduce order structures of PSII-LHCII supercomplexes under various protein packing fractions, free-LHCII:PSII ratios, and temperatures, thereby providing insights into mechanisms leading to PSII-LHCII supercomplex organizations in photosynthetic membranes.

Folding Sheets with Ion Beams

Focused ion beams (FIBs) are versatile tools with cross-disciplinary applications from the physical and life sciences to archeology. Nevertheless, the nanoscale patterning precision of FIBs is often accompanied by defect formation and sample deformation. In this study, the fundamental mechanisms governing the large-scale plastic deformation of nanostructures undergoing FIB processes are revealed by a series of molecular dynamic simulations. A surprisingly simple linear correlation between atomic volume removed from the film bulk and film deflection angle, regardless of incident ion energy and current, is revealed, demonstrating that the mass transport to the surface of material caused by energetic ion bombardment is the primary cause leading to nanostructure deformation. Hence, by controlling mass transport by manipulation of the incident ion energy and flux, it is possible to control the plastic deformation of nanostructures, thereby fabricating nanostructures with complex three-dimensional geometries.

The formation mechanisms and optical characteristics of GaSb quantum rings

The growth mechanisms and optical characteristics of GaSb quantum rings (QRs) are investigated. Although As-for-Sb exchange is the mechanism responsible for the dot-to-ring transition, significant height difference between GaSb quantum dots (QDs) and QRs in a dot/ring mixture sample suggests that the dot-to-ring transition is not a spontaneous procedure. Instead, it is a rapid transition procedure as long as it initiates. A model is established to explain this phenomenon. Larger ring inner diameters and heights of the sample with longer post Sb soaking time suggest that As-for-Sb exchange takes places in both vertical and lateral directions. The decreasing ring densities, enlarged ring inner/outer diameters and eventually flat GaSb surfaces observed with increasing growth temperatures are resulted from enhanced adatom migration and As-for-Sb exchange with increasing growth temperatures.