### Recent Submissions

• #### Defect Passivation in Perovskite Solar Cells by Cyano-Based π-Conjugated Molecules for Improved Performance and Stability

(Advanced Functional Materials, Wiley, 2020-07-09) [Article]
Defects at the surface and grain boundaries of metal–halide perovskite films lead to performance losses of perovskite solar cells (PSCs). Here, organic cyano-based π-conjugated molecules composed of indacenodithieno[3,2-b]thiophene (IDTT) are reported and it is found that their cyano group can effectively passivate such defects. To achieve a homogeneous distribution, these molecules are dissolved in the antisolvent, used to initiate the perovskite crystallization. It is found that these molecules are self-anchored at the grain boundaries due to their strong binding to undercoordinated Pb2+. On a device level, this passivation scheme enhances the charge separation and transport at the grain boundaries due to the well-matched energetic levels between the passivant and the perovskite. Consequently, these benefits contribute directly to the achievement of power conversion efficiencies as high as 21.2%, as well as the improved environmental and thermal stability of the PSCs. The surface treatment provides a new strategy to simultaneously passivate defects and enhance charge extraction/transport at the device interface by manipulating the anchoring groups of the molecules.
• #### Enhanced UV emission of GaN nanowires functionalized by wider bandgap solution-processed p-MnO quantum dots

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-07-04) [Article]
GaN-based UV light emitting devices suffer from low efficiency. To mitigate this issue, we hybridized GaN nanowires (NWs) grown on Si substrates by plasma-assisted molecular beam epitaxy with solution-processed p-type MnO quantum dots (QDs) characterized by a wider bandgap (~ 5 eV) than that of GaN. Further investigations reveal that the photoluminescence intensity of the GaN NWs increases up to ~ 3.9-fold (~ 290%) after functionalizing them with p-MnO QDs, while the internal quantum efficiency is improved by ~1.7-fold. Electron energy loss spectroscopy (EELS) incorporated into transmission electron microscopy (TEM) reveals an increase in the density of states in QD-decorated NWs compared to the bare ones. The advanced optical and EELS analyses indicate that the energy transfer from the wider-bandgap p-MnO QDs to n-GaN NW leads to substantial emission enhancement and a greater radiative recombination contribution, due to the good band alignment between MnO QDs and GaN NW. This work provides valuable insight into an environmentally-friendly strategy for improving UV device performance.
• #### A Highly Conductive Titanium Oxynitride Electron-Selective Contact for Efficient Photovoltaic Devices.

(Advanced materials (Deerfield Beach, Fla.), Wiley, 2020-07-03) [Article]
High-quality carrier-selective contacts with suitable electronic properties are a prerequisite for photovoltaic devices with high power conversion efficiency (PCE). In this work, an efficient electron-selective contact, titanium oxynitride (TiOx Ny ), is developed for crystalline silicon (c-Si) and organic photovoltaic devices. Atomic-layer-deposited TiOx Ny is demonstrated to be highly conductive with a proper work function (4.3 eV) and a wide bandgap (3.4 eV). Thin TiOx Ny films simultaneously provide a moderate surface passivation and enable a low contact resistivity on c-Si surfaces. By implementation of an optimal TiOx Ny -based contact, a state-of-the-art PCE of 22.3% is achieved for a c-Si solar cell featuring a full-area dopant-free electron-selective contact. Simultaneously, conductive TiOx Ny is proven to be an efficient electron-transport layer for organic photovoltaic (OPV) devices. A remarkably high PCE of 17.02% is achieved for an OPV device with an electron-transport TiOx Ny layer, which is superior to conventional ZnO-based devices with a PCE of 16.10%. Atomic-layer-deposited TiOx Ny ETL on a large area with a high uniformity may help accelerate the commercialization of emerging solar technologies.
• #### Bulk Spin Torque-Driven Perpendicular Magnetization Switching in L10 FePt Single Layer.

(Advanced materials (Deerfield Beach, Fla.), Wiley, 2020-06-30) [Article]
Due to its inherent superior perpendicular magnetocrystalline anisotropy, the FePt in L10 phase enables magnetic storage and memory devices with ultrahigh capacity. However, reversing the FePt magnetic state, and therefore encoding information, has proven to be extremely difficult. Here, it is demonstrated that an electric current can exert a large spin torque on an L10 FePt magnet, ultimately leading to reversible magnetization switching. The spin torque monotonically increases with increasing FePt thickness, exhibiting a bulk characteristic. Meanwhile, the spin torque effective fields and switching efficiency increase as the FePt approaches higher chemical ordering with stronger spin-orbit coupling. The symmetry breaking that generates spin torque within L10 FePt is shown to arise from an inherent structural gradient along the film normal direction. By artificially reversing the structural gradient, an opposite spin torque effect in L10 FePt is demonstrated. At last, the role of the disorder gradient in generating a substantial torque in a single ferromagnet is supported by theoretical calculations. These results will push forward the frontier of material systems for generating spin torques and will have a transformative impact on magnetic storage and spin memory devices with simple architecture, ultrahigh density, and readily application.
• #### Mechanism of wettability alteration of the calcite {101̄4} surface

(Physical Chemistry Chemical Physics, Royal Society of Chemistry (RSC), 2020-06-29) [Article]
<p>We propose that formation of Na$^{+}$ hydrates plays an important role in the wettability alteration of the calcite {101̄4} surface.</p>
• #### Induced spin textures at 3d transition metal–topological insulator interfaces

(Physical Review B, American Physical Society (APS), 2020-06-26) [Article]
While some of the most elegant applications of topological insulators, such as the quantum anomalous Hall effect, require the preservation of Dirac surface states in the presence of time-reversal symmetry breaking, other phenomena such as spin-charge conversion rather rely on the ability for these surface states to imprint their spin texture on adjacent magnetic layers. In this Rapid Communication, we investigate the spin-momentum locking of the surface states of a wide range of monolayer transition metals (3d-TM) deposited on top of Bi2Se3 topological insulators using first-principles calculations. We find an anticorrelation between the magnetic moment of the 3d-TM and the magnitude of the spin-momentum locking induced by the Dirac surface states. While the magnetic moment is large in the first half of the 3d series, following Hund’s rule, the spin-momentum locking is maximum in the second half of the series. We explain this trend as arising from a compromise between intra-atomic magnetic exchange and covalent bonding between the 3d-TM overlayer and the Dirac surface states. As a result, while Cr and Mn overlayers can be used successfully for the observation of the quantum anomalous Hall effect or the realization of axion insulators, Co and Ni are substantially more efficient for spin-charge conversion effects, e.g., spin-orbit torque and charge pumping.
• #### Flexible C6BN Monolayers As Promising Anode Materials for High-Performance K-Ion Batteries

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-06-25) [Article]
K-ion batteries attract extensive attention and research efforts because of the high energy density, low cost, and high abundance of K. Although they are considered suitable alternatives to Li-ion batteries, the absence of high-performance electrode materials is a major obstacle to implementation. On the basis of density functional theory, we systematically study the feasibility of a recently synthesized C6BN monolayer as anode material for K-ion batteries. The specific capacity is calculated to be 553 mAh/g (K2C6BN), i.e., about twice that of graphite. The C6BN monolayer is characterized by high strength (in-plane stiffness of 309 N/m), excellent flexibility (bending strength of 1.30 eV), low output voltage (average open circuit voltage of 0.16 V), and excellent rate performance (diffusion barrier of 0.09 eV). We also propose two new C6BN monolayers. One has a slightly higher total energy (0.10 eV) than the synthesized C6BN monolayer, exhibiting enhanced electronic properties and affinity to K. The other is even energetically favorable due to B-N bonding. All three C6BN monolayers show good dynamical, thermal, and mechanical stabilities. We demonstrate excellent cyclability and improved conductivity by K adsorption, suggesting great potential in flexible energy-storage devices.
• #### Structurally Tunable Two-Dimensional Layered Perovskites: From Confinement and Enhanced Charge Transport to Prolonged Hot Carrier Cooling Dynamics.

(The journal of physical chemistry letters, American Chemical Society (ACS), 2020-06-24) [Article]
Two-dimensional (2D) layered metal halide perovskites are potential alternatives to three-dimensional perovskites in optoelectronic applications owing to their improved photostabilities and chemical stabilities. Recent investigations of 2D metal halide perovskites have demonstrated interesting optical and electronic properties of various structures that are controlled by their elemental composition and organic spacers. However, photovoltaic devices that utilize 2D perovskites suffer from poor device efficiency due to inefficient charge carrier separation and extraction. In this Perspective, we shed light on confinement control and structural variation strategies that provide better parameters for the efficient collection of charges. The influence of these strategies on the exciton binding energies, charge-carrier mobilities, hot-carrier dynamics, and electron-phonon coupling in 2D perovskites is thoroughly discussed; these parameters highlight unique opportunities for further system optimization. Beyond the tunability of these fundamental parameters, we conclude this Perspective with the most notable strategies for attaining 2D perovskites with reduced bandgaps to better suit photovoltaic applications.
• #### Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C60 Organic Solar Cell Blends.

(The journal of physical chemistry letters, American Chemical Society (ACS), 2020-06-23) [Article]
Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):C60 blends to address charge generation and recombination by transient ultrafast spectroscopy. We gain relevant new insights to the functioning of dilute-donor (5% TAPC) fullerene-based BHJs compared to molecularly intermixed systems (50% TAPC). First, we show that intermolecular charge-transfer (CT) excitons in the C60 clusters of dilute BHJs rapidly localize to Frenkel excitons prior to dissociating at the donor:acceptor interface. Thus, both Frenkel and CT excitons generate photocurrent over the entire fullerene absorption range. Second, we selectively monitor interfacial and bulk C60 clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation. Third, we identify a fast (<1 ns) recombination channel, whereby free electrons recombine with trapped holes on isolated TAPC molecules. This can harm the performance of dilute solar cells, unless the electrons are rapidly extracted in efficient devices.
• #### Rapid Photonic Processing of High-Electron-Mobility PbS Colloidal Quantum Dot Transistors.

(ACS applied materials & interfaces, American Chemical Society (ACS), 2020-06-23) [Article]
Recent advances in solution-processable semiconducting colloidal quantum dots (CQDs) have enabled their use in a range of (opto)electronic devices. In most of these studies, device fabrication relied almost exclusively on thermal annealing to remove organic residues and enhance inter-CQD electronic coupling. Despite its widespread use, however, thermal annealing is a lengthy process, while its effectiveness to eliminate organic residues remains limited. Here, we exploit the use of xenon flash lamp sintering to post-treat solution-deposited layers of lead sulfide (PbS) CQDs and their application in n-channel thin-film transistors (TFTs). The process is simple, fast, and highly scalable and allows for efficient removal of organic residues while preserving both quantum confinement and high channel current modulation. Bottom-gate, top-contact PbS CQD TFTs incorporating SiO2 as the gate dielectric exhibit a maximum electron mobility of 0.2 cm2 V-1 s-1, a value higher than that of control transistors (≈10-2 cm2 V-1 s-1) processed via thermal annealing for 30 min at 120 °C. Replacing SiO2 with a polymeric dielectric improves the transistor's channel interface, leading to a significant increase in electron mobility to 3.7 cm2 V-1 s-1. The present work highlights the potential of flash lamp annealing as a promising method for the rapid manufacture of PbS CQD-based (opto)electronic devices and circuits.
• #### Efficient Hybrid Mixed Ion Perovskite Photovoltaics: In Situ Diagnostics of the Roles of Cesium and Potassium Alkali Cation Addition

(Solar RRL, Wiley, 2020-06-19) [Article]
Perovskite photovoltaics have made extraordinary progress in power conversion efficiency (PCE) and stability owing to process and formulation development. Perovskite cell performance benefits from the addition of alkali metal cations, such as cesium (Cs+) and potassium (K+) in mixed ion systems, but the underlying reasons are not fully understood. Here, we study the solidification of perovskite layers incorporating 5, 10, to 20% of Cs+ and K+ using in situ grazing incidence wide-angle X-ray scattering. We found that K+-doped solutions yield non-perovskite 4H phase rather than the 3C perovskite phase. For Cs+-doped formulations, both 4H and 3C phases are present at 5% Cs+, while the 3C perovskite phase forms in 10% Cs+-doped formulations, with undesirable halide segregation occurring at 20% Cs+. Post-deposition thermal annealing converts the intermediate 4H phase to the desirable 3C perovskite phase. Importantly, perovskite layers containing 5% of Cs+ or K+ exhibit reduced concentration of trap states, enhanced carrier mobility and lifetime. By carefully adjusting the Cs+ or K+ concentration to 5%, we demonstrate perovskite cells with a ≈5% higher average PCE than cells utilizing a higher cation concentrations. The study provides unique insights into the crystallization pathways towards perovskite phase engineering and improved cell performance.
• #### Micron Thick Colloidal Quantum Dot Solids

(Nano Letters, American Chemical Society (ACS), 2020-06-16) [Article]
Shortwave infrared colloidal quantum dots (SWIRCQDs) are semiconductors capable of harvesting across the AM1.5G solar spectrum. Today’s SWIR-CQD solar cells rely on spin-coating; however, these films exhibit cracking once thickness exceeds ∼500 nm. We posited that a blade-coating strategy could enable thick QD films. We developed a ligand exchange with an additional resolvation step that enabled the dispersion of SWIRCQDs. We then engineered a quaternary ink that combined highviscosity solvents with short QD stabilizing ligands. This ink, bladecoated over a mild heating bed, formed micron-thick SWIR-CQD films. These SWIR-CQD solar cells achieved short-circuit current densities (Jsc) that reach 39 mA cm−2 , corresponding to the harvest of 60% of total photons incident under AM1.5G illumination. External quantum efficiency measurements reveal both the first exciton peak and the closest Fabry−Perot resonance peak reaching approximately 80%this is the highest unbiased EQE reported beyond 1400 nm in a solution-processed semiconductor.
• #### Graphene Origami with Highly Tunable Coefficient of Thermal Expansion

(ACS Nano, American Chemical Society (ACS), 2020-06-15) [Article]
The coefficient of thermal expansion, which measures the change in length, area, or volume of a material upon heating, is a fundamental parameter with great relevance for many applications. Although there are various routes to design materials with targeted coefficient of thermal expansion at the macroscale, no approaches exist to achieve a wide range of values in graphene-based structures. Here, we use molecular dynamics simulations to show that graphene origami structures obtained through pattern-based surface functionalization provide tunable coefficients of thermal expansion from large negative to large positive. We show that the mechanisms giving rise to this property are exclusive to graphene origami structures, emerging from a combination of surface functionalization, large out-of-plane thermal fluctuations, and the three-dimensional geometry of origami structures.
• #### Effects of gas adsorption on monolayer Si2BN and implications for sensing applications

(Journal of Physics: Condensed Matter, IOP Publishing, 2020-06-15) [Article]
Using density functional theory, we investigate the adsorption behavior of CO, NH3, and NO molecules on monolayer Si2BN. The energetically favorable structural configurations along with their adsorption energies, charge transfers, and electronic properties are discussed. The CO and NH3 molecules show physisorption with moderate adsorption energies, whereas the NO molecule is subject to chemisorption. We further calculate the current–voltage characteristics using the non-equilibrium Green's function formalism. Significant anisotropy is observed for the armchair and zigzag directions, consistent with the anisotropy of the electronic band structure. Pronounced enhancement of the resistivity upon gas adsorption indicates that monolayer Si2BN is promising as gas sensing material.
• #### Selective Electrocatalytic Oxidation of Biomass-Derived 5-Hydroxymethylfurfural to 2,5-Diformylfuran: from Mechanistic Investigations to Catalyst Recovery

(ChemSusChem, Wiley, 2020-06-15) [Article]
The catalytic transformation of bio-derived compounds, specifically 5-hydroxymethylfurfural (HMF), into value-added chemicals may provide sustainable alternatives to crude oil and natural gas-based products. HMF can be obtained from fructose and successfully converted to 2,5-diformylfuran (DFF) by an environmentally friendly organic electrosynthesis performed in an ElectraSyn reactor, using cost-effective and sustainable graphite (anode) and stainless-steel (cathode) electrodes in an undivided cell, eliminating the need for conventional precious metal electrodes. In this work, the electrocatalysis of HMF is performed by using green solvents such as acetonitrile, γ-valerolactone, as well as PolarClean, which is used in electrocatalysis for the first time. The reaction parameters and the synergistic effects of the TEMPO catalyst and 2,6-lutidine base are explored both experimentally and through computation modeling. The molecular design and synthesis of a size-enlarged C 3-symmetric tris-TEMPO catalyst are also performed to facilitate a sustainable reaction work-up through nanofiltration. The obtained performance is then compared with those obtained by heterogeneous TEMPO alternatives recovered by using an external magnetic field and microfiltration. Results show that this new method of electrocatalytic oxidation of HMF to DFF can be achieved with excellent selectivity, good yield, and excellent catalyst recovery.
• #### Fully Inkjet-Printed, Ultrathin and Conformable Organic Photovoltaics as Power Source Based on Cross-Linked PEDOT:PSS Electrodes

(Advanced Materials Technologies, Wiley, 2020-06-15) [Article]
Ultra-lightweight solar cells have attracted enormous attention due to their ultra-conformability, flexibility, and compatibility with applications including electronic skin or miniaturized electronics for biological applications. With the latest advancements in printing technologies, printing ultrathin electronics is becoming now a reality. This work offers an easy path to fabricate indium tin oxide (ITO)-free ultra-lightweight organic solar cells through inkjet-printing while preserving high efficiencies. A method consisting of the modification of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) ink with a methoxysilane-based cross-linker (3-glycidyloxypropyl)trimethoxysilane (GOPS)) is presented to chemically modify the structure of the electrode layer. Combined with plasma and solvent post-treatments, this approach prevents shunts and ensures precise patterning of solar cells. By using poly(3-hexylthiophene) along rhodanine-benzothiadiazole-coupled indacenodithiophene (P3HT:O-IDTBR), the power conversion efficiency (PCE) of the fully printed solar cells is boosted up to 4.73% and fill factors approaching 65%. All inkjet-printed ultrathin solar cells on a 1.7 µm thick biocompatible parylene substrate are fabricated with PCE reaching up to 3.6% and high power-per-weight values of 6.3 W g−1. After encapsulation, the cells retain their performance after being exposed for 6 h to aqueous environments such as water, seawater, or phosphate buffered saline, paving the way for their integration in more complex circuits for biological systems.
• #### Photothermoelectric Response of Ti3C2Tx MXene Confined Ion Channels

(ACS Nano, American Chemical Society (ACS), 2020-06-15) [Article]
With recent growing interest in biomimetic smart nanochannels, a biological sensory transduction in response to external stimuli has been of particular interest in the development of biomimetic nanofluidic systems. Here we demonstrate the MXene-based subnanometer ion channels that convert external temperature changes to electric signals via preferential diffusion of cations under a thermal gradient. In particular, coupled with a photothermal conversion feature of MXenes, an array of the nanoconfined Ti3C2Tx ion channels can capture trans-nanochannel diffusion potentials under a light-driven axial temperature gradient. The nonisothermal open-circuit potential across channels is enhanced with increasing cationic permselectivity of confined channels, associated with the ionic concentration or pH of permeant fluids. The photothermoelectric ionic response (evaluated from the ionic Seebeck coefficient) reached up to 1 mV·K–1, which is comparable to biological thermosensory channels, and demonstrated stability and reproducibility in the absence and presence of an ionic concentration gradient. With advantages of physicochemical tunability and easy fabrication process, the lamellar ion conductors may be an important nanofluidic thermosensation platform possibly for biomimetic sensory systems.
• #### Mixed-state electron ptychography enables sub-angstrom resolution imaging with picometer precision at low dose.

(Nature communications, Springer Science and Business Media LLC, 2020-06-14) [Article]
Both high resolution and high precision are required to quantitatively determine the atomic structure of complex nanostructured materials. However, for conventional imaging methods in scanning transmission electron microscopy (STEM), atomic resolution with picometer precision cannot usually be achieved for weakly-scattering samples or radiation-sensitive materials, such as 2D materials. Here, we demonstrate low-dose, sub-angstrom resolution imaging with picometer precision using mixed-state electron ptychography. We show that correctly accounting for the partial coherence of the electron beam is a prerequisite for high-quality structural reconstructions due to the intrinsic partial coherence of the electron beam. The mixed-state reconstruction gains importance especially when simultaneously pursuing high resolution, high precision and large field-of-view imaging. Compared with conventional atomic-resolution STEM imaging techniques, the mixed-state ptychographic approach simultaneously provides a four-times-faster acquisition, with double the information limit at the same dose, or up to a fifty-fold reduction in dose at the same resolution.
• #### GIWAXS-SIIRkit: scattering intensity, indexing and refraction calculation toolkit for grazing-incidence wide-angle X-ray scattering of organic materials

(Journal of Applied Crystallography, International Union of Crystallography (IUCr), 2020-06-12) [Article]
<jats:p>Grazing-incidence wide-angle X-ray scattering (GIWAXS) has become an increasingly popular technique for quantitative structural characterization and comparison of thin films. For this purpose, accurate intensity normalization and peak position determination are crucial. At present, few tools exist to estimate the uncertainties of these measurements. Here, a simulation package is introduced called <jats:italic>GIWAXS-SIIRkit</jats:italic>, where SIIR stands for scattering intensity, indexing and refraction. The package contains several tools that are freely available for download and can be executed in MATLAB. The package includes three functionalities: estimation of the relative scattering intensity and the corresponding uncertainty based on experimental setup and sample dimensions; extraction and indexing of peak positions to approximate the crystal structure of organic materials starting from calibrated GIWAXS patterns; and analysis of the effects of refraction on peak positions. Each tool is based on a graphical user interface and designed to have a short learning curve. A user guide is provided with detailed usage instruction, tips for adding functionality and customization, and exemplary files.</jats:p>
• #### Highly Transparent and Conductive Electrodes Enabled by Scalable Printing-and-Sintering of Silver Nanowires

(Nanotechnology, IOP Publishing, 2020-06-12) [Article]
Silver nanowires (Ag NWs) have promised well for flexible and transparent electronics. However, It remains an open question on how to achieve large-scale printing of Ag NWs with high optical transparency, electrical conductivity, and mechanical durability for practical applications, though extensive research for more than one decade. In this work, we propose a possible solution that integrates screen printing of Ag NWs with flash-light sintering (FLS). We demonstrate that the use of low-concentration, screen-printable Ag NW ink enables large-area and high-resolution patterning of Ag NWs. A critical advantage comes from the FLS process that allows low-temperature processing, short operational time, and high output rate - characteristics that fit the scalable manufacturing. Importantly, we show that the resultant Ag NW patterns feature low sheet resistance (1.1-9.2 Ohm/sq), high transparency (75.2-92.6%), and thus a remarkable figure of merit comparable to state of the art. These outstanding properties of Ag NW patterns, together with their scalable fabrication method we proposed, would facilitate many Ag NW-based applications, such as transparent heater, stretchable displays, and wearable devices; here, we demonstrate the novel design of flexible and transparent radio frequency 5G antennas.