Wonsuk Cha

Nanoscopic Management of Molecular Packing and Orientation of Small Molecules by a Combination of Linear and Branched Alkyl Side Chains

We synthesized a series of acceptor-donor-acceptor-type small molecules (SIDPP-EE, SIDPP-EO, SIDPP-OE, and SIDPP-OO) consisting of a dithienosilole (SI) electron-donating moiety and two diketopyrrolopyrrole (DPP) electron-withdrawing moieties each bearing linear n-octyl (O) and/or branched 2-ethylhexyl (E) alkyl side chains. X-ray diffraction patterns revealed that SIDPP-EE and SIDPP-EO films were highly crystalline with pronounced edge-on orientation, whereas SIDPP-OE and SIDPP-OO films were less crystalline with a radial distribution of molecular orientations. Near-edge X-ray absorption fine structure spectroscopy disclosed an edge-on orientation with a molecular backbone tilt angle of ∼22° for both SIDPP-EE and SIDPP-EO. Our analysis of the molecular packing and orientation indicated that the shorter 2-ethylhexyl groups on the SI core promote tight π-π stacking of the molecular backbone, whereas n-octyl groups on the SI core hinder close π-π stacking to some degree. Conversely, the longer linear n-octyl groups on the DPP arms facilitate close intermolecular packing via octyl-octyl interdigitation. Quantum mechanics/molecular mechanics molecular dynamics simulations determined the optimal three-dimensional positions of the flexible alkyl side chains of the SI and DPP units, which elucidates the structural cause of the molecular packing and orientation explicitly. The alkyl-chain-dependent molecular stacking significantly affected the electrical properties of the molecular films. The edge-on oriented molecules showed high hole mobilities in organic field-effect transistors, while the radially oriented molecules exhibited high photovoltaic properties in organic photovoltaic cells. These results demonstrate that appropriate positioning of alkyl side chains can modulate crystallinity and molecular orientation in SIDPP films, which ultimately have a profound impact on carrier transport and photovoltaic performance.

Core–shell strain structure of zeolite microcrystals

Zeolites are crystalline aluminosilicate minerals featuring a network of 0.3-1.5-nm-wide pores, used in industry as catalysts for hydrocarbon interconversion, ion exchangers, molecular sieves and adsorbents. For improved applications, it is highly useful to study the distribution of internal local strains because they sensitively affect the rates of adsorption and diffusion of guest molecules within zeolites. Here, we report the observation of an unusual triangular deformation field distribution in ZSM-5 zeolites by coherent X-ray diffraction imaging, showing the presence of a strain within the crystal arising from the heterogeneous core-shell structure, which is supported by finite element model calculation and confirmed by fluorescence measurement. The shell is composed of H-ZSM-5 with intrinsic negative thermal expansion whereas the core exhibits a different thermal expansion behaviour due to the presence of organic template residues, which usually remain when the starting materials are insufficiently calcined. Engineering such strain effects could have a major impact on the design of future catalysts.

Demonstration of Feasibility of X-Ray Free Electron Laser Studies of Dynamics of Nanoparticles in Entangled Polymer Melts

The recent advent of hard x-ray free electron lasers (XFELs) opens new areas of science due to their exceptional brightness, coherence, and time structure. In principle, such sources enable studies of dynamics of condensed matter systems over times ranging from femtoseconds to seconds. However, the studies of “slow” dynamics in polymeric materials still remain in question due to the characteristics of the XFEL beam and concerns about sample damage. Here we demonstrate the feasibility of measuring the relaxation dynamics of gold nanoparticles suspended in polymer melts using X-ray photon correlation spectroscopy (XPCS), while also monitoring eventual X-ray induced damage. In spite of inherently large pulse-to-pulse intensity and position variations of the XFEL beam, measurements can be realized at slow time scales. The X-ray induced damage and heating are less than initially expected for soft matter materials.

High Crystalline Dithienosilole-Cored Small Molecule Semiconductor for Ambipolar Transistor and Nonvolatile Memory

We characterized the electrical properties of a field-effect transistor (FET) and a nonvolatile memory device based on a solution-processable low bandgap small molecule, Si1TDPP-EE-C6. The small molecule consisted of electron-rich thiophene-dithienosilole-thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) units. The as-spun Si1TDPP-EE-C6 FET device exhibited ambipolar transport properties with a hole mobility of 7.3×10(-5) cm2/(Vs) and an electron mobility of 1.6×10(-5) cm2/(Vs). Thermal annealing at 110 °C led to a significant increase in carrier mobility, with hole and electron mobilities of 3.7×10(-3) and 5.1×10(-4) cm2/(Vs), respectively. This improvement is strongly correlated with the increased film crystallinity and reduced π-π intermolecular stacking distance upon thermal annealing, revealed by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) measurements. In addition, nonvolatile memory devices based on Si1TDPP-EE-C6 were successfully fabricated by incorporating Au nanoparticles (AuNPs) as charge trapping sites at the interface between the silicon oxide (SiO2) and cross-linked poly(4-vinylphenol) (cPVP) dielectrics. The device exhibited reliable nonvolatile memory characteristics, including a wide memory window of 98 V, a high on/off-current ratio of 1×10(3), and good electrical reliability. Overall, we demonstrate that donor-acceptor-type small molecules are a potentially important class of materials for ambipolar FETs and nonvolatile memory applications.

High Performance of Low Band Gap Polymer-Based Ambipolar Transistor Using Single-Layer Graphene Electrodes

Bottom-contact bottom-gate organic field-effect transistors (OFETs) are fabricated using a low band gap pDTTDPP-DT polymer as a channel material and single-layer graphene (SLG) or Au source/drain electrodes. The SLG-based ambipolar OFETs significantly outperform the Au-based ambipolar OFETs, and thermal annealing effectively improves the carrier mobilities of the pDTTDPP-DT films. The difference is attributed to the following facts: (i) the thermally annealed pDTTDPP-DT chains on the SLG assume more crystalline features with an edge-on orientation as compared to the polymer chains on the Au, (ii) the morphological features of the thermally annealed pDTTDPP-DT films on the SLG electrodes are closer to the features of those on the gate dielectric layer, and (iii) the SLG electrode provides a flatter, more hydrophobic surface that is favorable for the polymer crystallization than the Au. In addition, the preferred carrier transport in each electrode-based OFET is associated with the HOMO/LUMO alignment relative to the Fermi level of the employed electrode. All of these experimental results consistently explain why the carrier mobilities of the SLG-based OFET are more than 10 times higher than those of the Au-based OTFT. This work demonstrates the strong dependence of ambipolar carrier transport on the source/drain electrode and annealing temperature.

Exploration of crystal strains using coherent x-ray diffraction

We measured coherent x-ray diffraction (CXD) on zeolite micro- crystals in order to gain information on internal density distribution and to learn more about the strain developed during the synthesis and attachment process on the substrate. From the distortion and asymmetry of the diffraction pattern on the (020) Bragg peak, the strain field distribution is estimated. We inverted the diffraction patterns from a less strained crystal to obtain the three-dimensional image of the shape and internal strain fields using the error reduction and hybrid input-output phase retrieval algorithms. We also show a few examples of characteristic distortion modes relevant to CXD of zeolites.

Polarized Raman spectroscopy of Cu-poor and Zn-rich single-crystal Cu2ZnSnSe4

Cu 2ZnSnSe4 (CZTSe) is a p-type semiconductor which has been developed as an absorber layer of polycrystalline thin film solar cells. Generally, Cu-poor and Zn-rich compositions tend to give the highest solar conversion efficiencies. Raman spectroscopy has been used to detect secondary phases such as ZnSe and Cu 2SnSe3 in CZTSe thin films. However, the fundamental phonon modes in single-crystal CZTSe with a composition matching that of high-efficiency thin film solar cells have not yet been fully understood. We performed polarized Raman measurements on Cu-poor and Zn-rich single-crystal CZTSe and identified 12 peaks, including two low-frequency peaks. By comparing the polarization dependence of the Raman peaks with a group theoretical analysis, we concluded that the crystal structure of CZTSe single-crystal is kesterite and made appropriate peak assignments.

Coherent X-ray scattering beamline at port 9C of Pohang Light Source II

The coherent X-ray scattering beamline at the 9C port of the upgraded Pohang Light Source (PLS-II) at Pohang Accelerator Laboratory in Korea is introduced. This beamline provides X-rays of 5-20 keV, and targets coherent X-ray experiments such as coherent diffraction imaging and X-ray photon correlation spectroscopy. The main parameters of the beamline are summarized, and some preliminary experimental results are described.

Ultrafast Three-Dimensional Integrated Imaging of Strain in Core/Shell Semiconductor/Metal Nanostructures

Visualizing the dynamical response of material heterointerfaces is increasingly important for the design of hybrid materials and structures with tailored properties for use in functional devices. In situ characterization of nanoscale heterointerfaces such as metal-semiconductor interfaces, which exhibit a complex interplay between lattice strain, electric potential, and heat transport at subnanosecond time scales, is particularly challenging. In this work, we use a laser pump/X-ray probe form of Bragg coherent diffraction imaging (BCDI) to visualize in three-dimension the deformation of the core of a model core/shell semiconductor-metal (ZnO/Ni) nanorod following laser heating of the shell. We observe a rich interplay of radial, axial, and shear deformation modes acting at different time scales that are induced by the strain from the Ni shell. We construct experimentally informed models by directly importing the reconstructed crystal from the ultrafast experiment into a thermo-electromechanical continuum model. The model elucidates the origin of the deformation modes observed experimentally. Our integrated imaging approach represents an invaluable tool to probe strain dynamics across mixed interfaces under operando conditions.

Coherent X‐ray Diffraction Imaging of Zeolite Microcrystals (abstract)

We measured coherent x‐ray diffraction (CXD), an emerging technique to obtain three‐dimensional internal and external images of crystals, on ZSM‐5 zeolite microcrystals to get internal density distribution and to map deformation field of strain. The experiments were performed at the beamline 34‐ID‐C in Advanced Photon Source at Argonne National Laboratory in the US. The CXD patterns of ZSM‐5 zeolite microcrystals with sizes of 2 μm by monochromatic coherent x‐rays with energy of 9 keV were obtained under continuously surrounding and Bragg conditions as a function of temperature. The oversampled diffraction patterns are inverted to obtain three‐dimensional images of the shapes and internal strain fields of zeolite microcrystals using phase retrieval algorithms of error reduction and a hybrid input‐output method. The internal density and strain distribution as a function of temperature are discussed.