Grazing Incidence Wide-angle X-ray Research Articles

The continuous fiber networks with a balanced bimodal orientation of P(NDI2OD-T2) by controlling solution nucleation and face-on and edge-on crystallization rates

The conjugated semiconducting polymer film morphology with long fiber and optimizing molecular packing is strongly correlated with charge transport properties. Herein, we report a method to obtain continuous fiber networks with a balanced bimodal orientation of poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)] thin films by controlling solution nucleation and edge-on and face-on crystallization rates. This is enabled by adding ethylene glycol (EG) (the optimal content is 3 vol%) to P(NDI2OD-T2) in chloroform solution which improves the polarization of the NDI2OD units along the backbone direction and solution pre-aggregation. Therefore, the crystal nuclei are formed in solution state during the three-phase line receding as characterized by in situ polarized optical microscopy. Moreover, the peak intensity of the aggregated state increases with solvent evaporation, illustrating that the nucleation and crystal growth proceed simultaneously in the drying process as revealed by in situ ultraviolet–visible absorption spectra. The film morphology of continuous fibrous networks with an average fiber length of 531 nm is formed. In addition, the slopes of (100) peak area vs time in the out-of-plane and in-plane directions are similar, indicating that the edge-on and face-on crystallization rates are almost equal as obtained from in situ grazing incidence wide-angle X-ray scattering. Thus, the bimodal orientation with an edge-on and face-on mixing ratio is approximately 1. The average electron mobility of the field effect transistors of P(NDI2OD-T2) films with this continuous fiber network reaches to 0.2 cm2 V−1 s−1, an almost 3-fold improvement compared with devices without EG addition films.

Strain Relaxation on Perovskite Surface via Light-Enhanced Ionic Homogeneity.

The efficiency and stability of perovskite solar cells (PSCs) can be either deteriorated or enhanced by strain at interfaces, which is sensitive to various external conditions, particularly light illumination. Here we investigated the vertical strain distribution in perovskite films synthesized under light illumination with various wavelengths. The films were formed by reacting formamidinium iodide (FAI)/methylammonium chloride (MACl) vapor with vapor-deposited PbI2 (CsBr) films. Strain in the films was evaluated with incident-angle-dependent grazing-incidence wide-angle X-ray scattering, which showed out-of-plane compressive and in-plane tensile strains, particularly on the surface. Short-wavelength light relaxed the strain on the perovskite surface via promotion of ionic diffusion, including FA, MA, Cs, and I, to reach vertical ionic homogeneity. With the charge trap concentration being reduced, both the efficiency and stability were greatly improved. This finding provides deep insight into the effect of light on strain in PSCs.

Crystallization and phase separation in PEDOT:PSS/PEO blend thin films: Influence on mechanical and electrical properties at the nanoscale

Thin films of polymeric blends composed of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and polyethylene oxide (PEO) were investigated by varying the concentration of the two components over the entire range of compositions and the molecular weight of PEO. Phase separation and crystallization have been studied at different length scales by combining Atomic Force Microscopy (AFM) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) using synchrotron light. Several different arrangements in the thin films of the two polymers constituting the blend were observed and discussed in detail, providing information about their reciprocal influence at the micro- and nano-scale. In addition to that, we were able to estimate quantitative nanomechanical and nanoelectrical properties via AFM and finally revealing the dependence of the thin films' physical properties on their composition and structure. By varying the blend composition, we achieved different coating capability, mechanical properties, and electrical conductivity. Furthermore, depending on the PEO molecular weight, the electrical response of the resulting blends’ thin films shows some differences. In the low range concentration, the blend thin films with high molecular weight PEO present coarser conducting paths than in those with the low molecular weight counterparts. For intermediate concentrations, a more effective phase segregation of PEDOT:PSS and PEO is achieved for high molecular weight PEO. These differences are also translated to different electrical conductivity.

A wide-angle X-ray scattering laboratory setup for tracking phase changes of thin films in a chemical vapor deposition chamber.

The few-layer transition metal dichalcogenides (TMD) are an attractive class of materials due to their unique and tunable electronic, optical, and chemical properties, controlled by the layer number, crystal orientation, grain size, and morphology. One of the most commonly used methods for synthesizing the few-layer TMD materials is the chemical vapor deposition (CVD) technique. Therefore, it is crucial to develop in situ inspection techniques to observe the growth of the few-layer TMD materials directly in the CVD chamber environment. We demonstrate such an in situ observation on the growth of the vertically aligned few-layer MoS2 in a one-zone CVD chamber using a laboratory table-top grazing-incidence wide-angle X-ray scattering (GIWAXS) setup. The advantages of using a microfocus X-ray source with focusing Montel optics and a single-photon counting 2D X-ray detector are discussed. Due to the position-sensitive 2D X-ray detector, the orientation of MoS2 layers can be easily distinguished. The performance of the GIWAXS setup is further improved by suppressing the background scattering using a guarding slit, an appropriately placed beamstop, and He gas in the CVD reactor. The layer growth can be monitored by tracking the width of the MoS2 diffraction peak in real time. The temporal evolution of the crystallization kinetics can be satisfactorily described by the Avrami model, employing the normalized diffraction peak area. In this way, the activation energy of the particular chemical reaction occurring in the CVD chamber can be determined.

Direct In Situ Conversion of Lead Iodide to a Highly Oriented and Crystallized Perovskite Thin Film via Sequential Deposition for 23.48% Efficient and Stable Photovoltaic Devices.

In the sequential deposition method of perovskite films, the crystallinity and microstructure of PbI2 are often sacrificed to solve the problem of an incomplete reaction between organic halide and lead halide. As a result, the crystal orientation of the perovskite film prepared by the sequential deposition method is generally worse than that of the perovskite film prepared by a one-step antisolvent method. Here, we preplaced formamidine formate (FAFa) on the buried interface to regulate the formation mechanism from PbI2 to perovskite. As shown by the XPS measurement of the perovskite buried interface, the HCOO- anion of FAFa first partially replaces I- to coordinate with Pb2+. With the subsequent annealing process, some HCOO- anions were released and migrated upward, which promoted the recrystallization of PbI2, obtaining a PbI2 film with enhanced crystallinity and orientation. Additionally, the lift-off process proves that the HCOO- anions suppress the anion vacancy defects enriched at the buried interface and promote charge transport because the HCOO- anions are small enough to adapt to the iodide vacancy. Grazing incidence wide-angle X-ray scattering and X-ray diffraction measurements show that the in situ conversion mechanism is responsible for the PbI2-to-perovskite process, resulting in the highly oriented perovskite film without increasing the residual PbI2 content in the perovskite film. As a result, our strategies enabled a champion power conversion efficiency of 23.48% with improved storage stability and photostability. This work provides a new strategy to improve the crystallinity of sequential deposition perovskites without destabilizing the device due to more PbI2 residues.

Improved order and transport in C60 thin films grown on SiO2 via use of transient templates

The performance of C60 semiconducting films is linked to the degree of crystallinity and ordering, properties that strongly depend on the substrate, and growth conditions. Substrate–molecule interactions can be specifically tailored by employing growth templates to achieve a desired thin film structure. However, the presence of a growth template after the film deposition is usually not desirable as it may change the properties of the layer of interest. The ability to remove a growth template without any disruption to the active layer would be highly beneficial. A simple method of template removal by annealing is presented here. A variety of small organic molecules (perfluoropentacene, [6]phenacene, and α-sexithiophene) were used as a growth template to obtain a high-quality well-ordered C60 thin film. In situ grazing-incidence wide-angle x-ray scattering was employed to study the structural changes of C60 thin films during template removal. While a slight disturbance of the thin film structure was observed during template removal caused by evaporated molecules from the growth template escaping through the C60 layer, the disruption is only temporary. When the annealing process is concluded, only the well-ordered C60 thin film directly on top of SiO2 is left, which is not achievable without the use of a growth template. Improved crystallinity and grain size of such a thin film, when compared to preparation without a growth template, lead to a significant improvement of the charge carrier mobility. Importantly, template removal prevents the formation of undesired ambipolar transistor characteristics.

Nanomechanical and structural study of Au38 nanocluster Langmuir-Blodgett films using bimodal atomic force microscopy and X-ray reflectivity

HypothesisLangmuir-Blodgett (LB) technique allows the deposition of gold nanoparticles and nanoclusters (atomically precise nanoparticles below 2 nm in diameter) onto solid substrates with an unprecedented degree of control and high transfer ratios. Nanoclusters are expected to follow the crinkle folding mechanism, which promotes the formation of trilayers of nanoparticles but kinetically disfavors the formation of the fourth layer. ExperimentsLB films of Au38(SC2H4Ph)24 nanocluster were prepared at a range of surface pressures in the bilayer/trilayer regime and their internal structure was analyzed with X-ray Reflectivity (XRR) and Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). Bimodal atomic force microscopy (AFM) imaging was used to quantify the elastic modulus, which can be correlated with the topography at the same point on the surface. FindingsNanocluster bilayers and trilayers exhibited the elastic moduli of ca. 1.2 GPa and 0.9 GPa respectively. Films transferred in the 20–25 mN/m surface pressure regime displayed a particular propensity to form highly vertically organized trilayers. Further compression resulted in disorganization of the layers. Crucially, the use of two cantilevers of contrasting stiffness for bimodal AFM measurements has demonstrated a new approach to quantify the mechanical properties of ultrathin films without the use of deconvolution algorithms to remove the substrate contribution.

Quasi-epitaxial growth of BaTiS3 films

Perovskite chalcogenides have emerged as a new class of semiconductors with tunable band gap in the visible-infrared region. High quality thin films are critical to understand the fundamental properties and realize the potential applications based on these materials. We report growth of quasi-epitaxial thin films of quasi one-dimensional (quasi-1D) hexagonal chalcogenide BaTiS$_3$ by pulsed laser deposition. We identified the optimal growth conditions by varying the growth parameters such as the substrate temperature and H2S partial pressure and examined their effects on the thin film structure. High resolution thin film X-Ray diffraction shows strong texture in the out-of-plane direction, whereas no evidence of in-plane relationship between the film and the substrate is observed. Grazing incidence wide-angle X-ray scattering and scanning transmission electron microscopy studies reveal the presence of weak epitaxial relationships of the film and the substrate, despite a defective interface. Our study opens up a pathway to realize quasi-1D hexagonal chalcogenide thin films and their heterostructures with perovskite chalcogenides.

Utilization of Catechol End-Functionalized PMMA as a Macromolecular Coupling Agent for Ceramic/Fluoropolymer Piezoelectric Composites

An approach based on the use of a macromolecular coupling agent and the aim to improve the interfacial adhesion between piezoelectric ceramics and piezoelectric polymer matrix in piezoelectric composites is presented. Poly(methyl methacrylate) (PMMA) bearing a catechol moiety was used as a macromolecular coupling agent, as it is known to be miscible to piezoelectric fluoropolymers and catechol groups can strongly bind to a large variety of surfaces. Thus, entanglement between the PMMA chains and the amorphous segments of the fluoropolymer would ensure the desired interfacial adhesion. Well-defined PMMA was synthesized via RAFT polymerization using 2-cyano-2-propyl dodecyl trithiocarbonate as a chain-transfer agent. The PMMA ω-chain end was then functionalized with a catechol group via a one-pot aminolysis/thia-Michael addition procedure using a dopamine acrylamide (DA) derivative as a Michael acceptor. The presence of the catechol moiety at the chain end of PMMA was controlled by 1H NMR and cyclic voltammetry measurements. The resulting PMMA-DA was then grafted onto the surface of a lead-free piezoelectric ceramic film (i.e., a thin film of H2O2-activated (Bi0.5Na0.5)TiO3 (BNT) with a large contact area). The increase of the water contact angle confirmed the efficiency of the grafting. A commercial piezoelectric copolymer P(VDF-co-TrFE) was then spin-coated onto the modified BNT surface to form a bilayer composite. The composite cross section prepared by cryofracture was examined by scanning electron microscopy and revealed that the ceramic/polymer interface of the BNT-PMMA/P(VDF-co-TrFE) bilayer composite exhibits a much better cohesion than its counterpart composite prepared from nonmodified BNT. Moreover, the grazing incidence wide-angle X-ray scattering confirmed that the copolymer crystal structure was not impacted by the presence of the PMMA-DA coupling agent. A strong piezoelectric response was locally detected by piezoresponse force microscopy. This study highlights the potential of PMMA-DA as a macromolecular coupling agent to improve the ceramic/polymer interface in piezoelectric composite materials.

A Highly Conductive n-Type Coordination Complex with Thieno[3,2-b]thiophene Units: Facile Synthesis, Orientation, and Thermoelectric Properties.

An organometallic nickel complex containing thieno[3,2-b]thiophene units was designed and synthesized. Composite films of the resulting nickel complex and polyvinylidene difluoride, which can be fabricated via a simple solution process under atmospheric conditions, exhibit remarkably high n-type conductivity (>200 S cm-1). Moreover, the thermoelectric power factor of the n-type composite film was proven to be air stable. A grazing-incidence wide-angle X-ray diffraction analysis indicated a significant impact of introducing the thieno[3,2-b]thiophene core into the backbone of the nickel complex on the orientation within the composite films.

Establishing Self‐Dopant Design Principles from Structure–Function Relationships in Self‐n‐Doped Perylene Diimide Organic Semiconductors

Self-doping is a particular doping method that has been applied to a wide range of organic semiconductors. However, there is a lack of understanding regarding the relationship between dopant structure and function. A structurally diverse series of self-n-doped perylene diimides (PDIs) is investigated to study the impact of steric encumbrance, counterion selection, and dopant/PDI tether distance on functional parameters such as doping, stability, morphology, and charge-carrier mobility. The studies show that self-n-doping is best enabled by the use of sterically encumbered ammoniums with short tethers and Lewis basic counterions. Additionally, water is found to inhibit doping, which concludes that thermal degradation is merely a phenomenological feature of certain dopants, and that residual solvent evaporation is the primary driver of thermally activated doping. In situ grazing-incidence wide-angle X-ray scattering studies show that sample annealing increases the π-π stacking distance and shrinks grain boundaries for improved long-range ordering. These features are then correlated to contactless carrier-mobility measurements with time-resolved microwave conductivity before and after thermal annealing. The collective relationships between structural features and functionality are finally used to establish explicit self-n-dopant design principles for the future design of materials with improved functionality.

Nanoscale Networks of Metal Oxides by Polymer Imprinting and Templating for Future Adaptable Electronics

While network formation is prevalent in nature, networks are generally not expected in inorganic structures. Exceptions are those cases in which surface states become important, such as nanoparticles. However, even in these cases, the morphology of these networks is difficult to control and they show a large degree of disorder. In this work, we show that highly ordered and interconnected nanoscale networks of functional metal oxides can be fabricated by a combination of polymer imprinting and polymer templating through solution processable methods. We report the fabrication of a number of functional oxide networks (i.e., BiFeO3, SrTiO3, La0.7Ca0.3MnO3, and HfO2) from solution, showing that all the oxide materials tried so far are able to follow the self-assembled network morphology dictated by the polymer structure. These networks were characterized for the overall structure by scanning electron microscopy and atomic force microscopy (AFM). Grazing incidence small angle X-ray scattering showed a good imprint quality on the mm2 scale for the combined networks, which is challenging given that multiple processing steps were involved during the fabrication. The material stoichiometries were investigated by X-ray photoemission spectroscopy and the crystal phases by grazing incidence wide angle X-ray scattering. When electronic functionality is anticipated, the networks behave as expected: conducting AFM on the La0.7Ca0.3MnO3 networks confirmed the conductive character of this composition; and piezoresponse force microscopy of the BiFeO3 network is consistent with the presence of ferroelectric behavior. These nanoscale networks show promise for future applications in adaptable electronics, such as neuromorphic computing or brain-inspired information processing.

Stepwise sulfurization of MoO3 to MoS2 thin films studied by real-time X-ray scattering

Chemical vapor deposition (CVD) is commonly used for the large-scale synthesis of the films of two-dimensional (2D) transition metal dichalcogenide (TMD) materials, including MoS2 thin films, which have potential applications in optoelectronics, catalysis, and nanoelectronics. As far as thin film of MoS2 is concerned, the influence of deposition parameters on the chemical reactions in the growth process via CVD synthesis has not yet been exhaustively studied. Here we present a comprehensive, time-resolved, in-situ study of the growth of MoS2 thin film from MoO3 by sulfurization using the grazing-incidence wide-angle X-ray scattering (GIWAXS) in laboratory conditions. We tracked the basic chemical reactions during the sulfurization process in real time by monitoring the phase transformations of MoO3 to MoO2 and MoO2 to MoS2 via a MoOS2 transient phase, although it was not possible to observe it directly. Preferential crystallographic orientation of MoO3, and MoO2 phases during the growth was confirmed. In addition, their activation energies were determined from the crystallization kinetics parameters. The present work highlights the importance of understanding the basic chemical reactions inside the CVD reactor as a prerequisite for optimizing the properties of the final 2D TMD films.

Biomimetic Self-Assembling Metal-Organic Architectures with Non-Iridescent Structural Coloration for Synergetic Antibacterial and Osteogenic Activity of Implants.

Materials in nature feature versatile and programmable interactions to render macroscopic architectures with multiscale structural arrangements. By rationally combining metal-carboxylate and metal-organophosphate coordination interactions, Au25(MHA)18 (MHA, 6-mercaptohexanoic acid) nanocluster self-assembled structural color coating films and phytic acid (PA)-metal coordination complexes are sequentially constructed on the surface of titanium implants. The Lewis acid-base coordination principle applies for these metal-organic coordination networks. The isotropic arrangement of nanoclusters with a short-range order is investigated via grazing incidence wide-angle X-ray scattering. The integration of robust M-O (M = Ti, Zr, Hf) and labile Cu-O coordination bonds with high connectivity of Au25(MHA)18 nanoclusters enables these artificial photonic structures to achieve a combination of mechanical stability and bacteriostatic activity. Moreover, the colorless and transparent PA-metal complex layer allows the viewing of the structural color and surface wettability switching to hydrophilic and makes feasible the interfacial biomineralization of hydroxyapatite. Collectively, these modular metal-organic coordination-driven assemblies are predictive and rational material design strategies with tunable hierarchy and diversity. The complete metal-organic architectures will not only help improve the physicochemical properties of the bone-implant interface with synergistic antibacterial and osseointegration activities but also can boost surface engineering of medical metal implants.

Surface crystalline structure of thin poly(l-lactide) films determined by the long-range substrate effect

Due to the stepwise crystallization behavior of polymorphous poly(l-lactide) (PLLA) films, the surface crystalline structures of thin PLLA films supported on Si/SiOx substrates with various thicknesses of both the film and adsorbed layer were investigated separately by grazing incidence wide-angle X-ray diffraction (GIWAXD). An unusual highly packed α-form crystal was observed at the surface at a crystallization temperature of 346 K, which transformed to a loosely packed δ-form (or called α′-form) crystal with decreasing film thickness or increasing adsorbed layer thickness. The mobility of the surface chains of PLLA films was scaled by the surface crystallization rate to elucidate the substrate effect on the corresponding surface crystalline structure. It was found that if the surface mobility is suppressed by the long-range substrate effect from much faster to slower than that in the bulk, then the surface crystalline structure changes from α-crystal to δ-crystal. A phase diagram was ultimately constructed to predict the surface crystalline structure at 346 K in thin PLLA films with various thicknesses of both the film and adsorbed layer.

Halide Remixing under Device Operation Imparts Stability on Mixed‐Cation Mixed‐Halide Perovskite Solar Cells

Mixed-halide mixed-cation hybrid perovskites are among the most promising perovskite compositions for application in a variety of optoelectronic devices due to their high performance, low cost, and bandgap-tuning capabilities. Instability pathways such as those driven by ionic migration, however, continue to hinder their further progress. Here, an operando variable-pitch synchrotron grazing-incidence wide-angle X-ray scattering technique is used to track the surface and bulk structural changes in mixed-halide mixed-cation perovskite solar cells under continuous load and illumination. By monitoring the evolution of the material structure, it is demonstrated that halide remixing along the electric field and illumination direction during operation hinders phase segregation and limits device instability. Correlating the evolution with directionality- and depth-dependent analyses, it is proposed that this halide remixing is induced by an electrostrictive effect acting along the substrate out-of-plane direction. However, this stabilizing effect is overwhelmed by competing halide demixing processes in devices exposed to humid air or with poorer starting performance. The findings shed new light on understanding halide de- and re-mixing competitions and their impact on device longevity. These operando techniques allow real-time tracking of the structural evolution in full optoelectronic devices and unveil otherwise inaccessible insights into rapid structural evolution under external stressconditions.

Confined crystallization and polymorphism in iPP thin films

The morphological and structural evolution of isotactic polypropylene (iPP) in thin and ultrathin films casted by hot-solution spin coating is investigated by means of atomic force microscopy (AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). When reducing the film thickness from 300 nm to 15 nm, changes from a 3D to a quasi-2D morphology are observed. The quasi-2D confined morphology is found in the 220-40 nm thickness range and is characterized by a predominant edge-on orientation (EO) of the lamellae. A transition to the flat-on (FO) orientation paired to a drastic decrease of the sample roughness is observed for thicknesses below 30 nm. Surprisingly, a mixed morphology where orthorhombic γ-phase branches develop from FO α-phase lamellae is observed at the lowest prepared thickness of 15 nm. The unexpected appearance of the γ-phase for the iPP grade used here is due to the favourable flat-on lamellar orientation at the lowest film thickness. Thanks to a quantitative analysis of the GIWAXS patterns, the relative amount between edge-on and flat-on oriented lamellae (χ EO /χ FO ) is calculated, providing useful insights on the real morphological composition of the iPP thin films, a task that is very challenging to achieve with microscopy techniques. • Isotactic polypropylene thin film prepared by hot-solution spin coating present confined crystallization behaviour. • Morphology evolution is investigated via AFM, nanostructural composition and lamellar orientation is studied by means of GIWAXS. • Lamellae maintain a mixed morphological structure, preferential orientation changes from edge-on to flat-on on the substrate. • Formation of the orthorhombic γ-phase is induced by increasing the degree of confinement ( i.e. : decreasing film thickness).

High molecular weight polymeric acceptors based on semi-perfluoroalkylated perylene diimides for pseudo-planar heterojunction all-polymer organic solar cells

In this study bulk heterojunction (BHJ) and pseudo-planar heterojunction (PPHJ) organic solar cells (OSCs) were prepared and studied with perylene diimides (PDIs)-based polymeric acceptors P(4CF8CH-PDI-TT) and P(20CH-PDI-TT). The PDI polymers were used in PPHJ all-polymer solar cells for the first time. Its high molecular weight (Mn of 94.8 kDa), high mobility (μe of 8.40 × 10−3 cm2 V−1 s−1), and excellent solubility make semi-perfluoroalkylated polymer P (4CF8CH-PDI-TT) well polymeric acceptor for PPHJ OSCs. A series of measurements including photoluminescence quenching, dependence of light intensity, carrier mobility, atomic force microscopy, transmission electron microscopy, grazing-incidence wide-angle X-ray scattering, and depth analysis X-ray photoelectron spectroscopy were carried out. The results show that PPHJ devices prepared by the layer-by-layer spin-coating method have better charge transport, higher and more balanced carrier mobility, less exciton recombination loss, and more favorable film morphology than BHJ devices. Consequently, the PPHJ OSCs based on P(4CF8CH-PDI-TT) and P(20CH-PDI-TT) gave power conversion efficiencies (PCE) of 3.43% and 3.15%, respectively, which are higher than 1.71% and 1.10% of BHJ OSCs. It demonstrates that semi-perfluoroalkylated strategy is effective for the construction of high-performance polymeric semiconductors with the PDI block.

All-Printed Ultrahigh-Responsivity MoS2 Nanosheet Photodetectors Enabled by Megasonic Exfoliation.

Printed 2Dmaterials, derived from solution-processed inks, offer scalable and cost-effective routes to mechanically flexible optoelectronics. With micrometer-scale control and broad processing latitude, aerosol-jet printing (AJP) is of particular interest for all-printed circuits and systems. Here, AJP is utilized to achieve ultrahigh-responsivity photodetectors consisting of well-aligned, percolating networks of semiconducting MoS2 nanosheets and graphene electrodes on flexible polyimide substrates. Ultrathin (≈1.2nm thick) and high-aspect-ratio (≈1μm lateral size) MoS2 nanosheets are obtained by electrochemical intercalation followed by megasonic atomization during AJP, which not only aerosolizes the inks but also further exfoliates the nanosheets. The incorporation of the high-boiling-point solvent terpineol into the MoS2 ink is critical for achieving a highly aligned and flat thin-film morphology following AJP as confirmed by grazing-incidence wide-angle X-ray scattering and atomic force microscopy. Following AJP, curing is achieved with photonic annealing, which yields quasi-ohmic contacts and photoactive channels with responsivities exceeding 103 AW-1 that outperform previously reported all-printed visible-light photodetectors by over three orders of magnitude. Megasonic exfoliation coupled with properly designed AJP ink formulations enables the superlative optoelectronic properties of ultrathin MoS2 nanosheets to be preserved and exploited for the scalable additive manufacturing of mechanically flexible optoelectronics.

Solid Phase Epitaxy of Single Phase Two-Dimensional Layered InSe Grown by MBE.

Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In2Se3, β-In2Se3, γ-In2Se3, etc.). In this work, the 2D α-In2Se3 film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In2Se3 surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In2Se3 phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In2Se3 to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).