### Recent Submissions

• #### Skyrmion battery effect via inhomogeneous magnetic anisotropy

(Applied Physics Reviews, AIP Publishing, 2021-04-14) [Article]
Magnetic skyrmions are considered a promising candidate for the next-generation information processing technology. Being topologically robust, magnetic skyrmions are swirling spin textures that can be used in a broad range of applications from memory devices and logic circuits to neuromorphic computing. In a magnetic medium lacking inversion symmetry, magnetic skyrmion arises as a result of the interplay among magnetic exchange interaction, Dzyaloshinskii-Moriya interaction, and magnetic anisotropy. Instrumental to the integrated skyrmion-based applications are the creation and manipulation of magnetic skyrmions at a designated location, absent any need of a magnetic field. In this paper, we propose a generic design strategy to achieve that goal and a model system to demonstrate its feasibility. By implementing a disk-shaped thin film heterostructure with an inhomogeneous perpendicular magnetic anisotropy, stable sub-100-nm size skyrmions can be generated without magnetic field. This structure can be etched out via, for example, focused ion beam microscope. Using micromagnetic simulation, we show that such heterostructure not only stabilizes the edge spins of the skyrmion but also protects its rotation symmetry. Furthermore, we may switch the spin texture between skyrmionic and vortex-like ones by tuning the slope of perpendicular anisotropy using a bias voltage. When embedded into a magnetic conductor and under a spin polarized current, such heterostructure emits skyrmions continuously and may function as a skyrmion source. This unique phenomenon is dubbed a skyrmion battery effect. Our proposal may open a novel venue for the realization of all-electric skyrmion-based devices.
• #### Can a recipe for the growth of single-layer graphene on copper be used in different chemical vapor deposition reactors?

(Chemistry, an Asian journal, Wiley, 2021-04-13) [Article]
In the last decade, catalytic chemical vapor deposition (CVD) has been intensively explored for the growth of single-layer graphene (SLG). Despite the scattering of guidelines and procedures, variables such as the surface texture/chemistry of catalyst metal foils, carbon feedstock, and growth process parameters have been well-scrutinized. Still, questions remain on how best to standardize the growth procedure. The possible correlation of recipes between different CVD setups is an example. Here, two thermal CVD reactors were explored to grow graphene on Cu foil. The design of these setups was entirely distinct, one being a "showerhead" cold-wall type, whereas the other represented the popular "tubular" hot-wall type. Upon standardizing the Cu foil surface, it was possible to develop a recipe for cm 2 -scale SLG growth that differed only on the carrier gas flow rate used in the two reactors.
• #### Lithium-Ion Desolvation Induced by Nitrate Additives Reveals New Insights into High Performance Lithium Batteries

(Advanced Functional Materials, Wiley, 2021-04-02) [Article]
Electrolyte additives have been widely used to address critical issues in current metal (ion) battery technologies. While their functions as solid electrolyte interface forming agents are reasonably well-understood, their interactions in the liquid electrolyte environment remain rather elusive. This lack of knowledge represents a significant bottleneck that hinders the development of improved electrolyte systems. Here, the key role of additives in promoting cation (e.g., Li+) desolvation is unraveled. In particular, nitrate anions (NO3−) are found to incorporate into the solvation shells, change the local environment of cations (e.g., Li+) as well as their coordination in the electrolytes. The combination of these effects leads to effective Li+ desolvation and enhanced battery performance. Remarkably, the inexpensive NaNO3 can successfully substitute the widely used LiNO3 offering superior long-term stability of Li+ (de-)intercalation at the graphite anode and suppressed polysulfide shuttle effect at the sulfur cathode, while enhancing the performance of lithium–sulfur full batteries (initial capacity of 1153 mAh g−1 at 0.25C) with Coulombic efficiency of ≈100% over 300 cycles. This work provides important new insights into the unexplored effects of additives and paves the way to developing improved electrolytes for electrochemical energy storage applications.
• #### Theory-Guided Synthesis of Highly Luminescent Colloidal Cesium Tin Halide Perovskite Nanocrystals

(Journal of the American Chemical Society, American Chemical Society (ACS), 2021-04-01) [Article]
The synthesis of highly luminescent colloidal CsSnX<sub>3</sub> (X = halogen) perovskite nanocrystals (NCs) remains a long-standing challenge due to the lack of a fundamental understanding of how to rationally suppress the formation of structural defects that significantly influence the radiative carrier recombination processes. Here, we develop a theory-guided, general synthetic concept for highly luminescent CsSnX<sub>3</sub> NCs. Guided by density functional theory calculations and molecular dynamics simulations, we predict that, although there is an opposing trend in the chemical potential-dependent formation energies of various defects, highly luminescent CsSnI<sub>3</sub> NCs with narrow emission could be obtained through decreasing the density of tin vacancies. We then develop a colloidal synthesis strategy that allows for rational fine-tuning of the reactant ratio in a wide range but still leads to the formation of CsSnI<sub>3</sub> NCs. By judiciously adopting a tin-rich reaction condition, we obtain narrow-band-emissive CsSnI<sub>3</sub> NCs with a record emission quantum yield of 18.4%, which is over 50 times larger than those previously reported. Systematic surface-state characterizations reveal that these NCs possess a Cs/I-lean surface and are capped with a low density of organic ligands, making them an excellent candidate for optoelectronic devices without any postsynthesis ligand management. We showcase the generalizability of our concept by further demonstrating the synthesis of highly luminescent CsSnI<sub>2.5</sub>Br<sub>0.5</sub> and CsSnI<sub>2.25</sub>Br<sub>0.75</sub> NCs. Our findings not only highlight the value of computation in guiding the synthesis of high-quality colloidal perovskite NCs but also could stimulate intense efforts on tin-based perovskite NCs and accelerate their potential applications in a range of high-performance optoelectronic devices.
• #### Optically and Electrocatalytically Decoupled Si Photocathodes with a Porous Carbon Nitride Catalyst for Nitrogen Reduction with Over 61.8% Faradaic Efficiency

The photoelectrochemical (PEC) approach is attractive as a promising route for the nitrogen reduction reaction (NRR) toward ammonia (NH<sub>3</sub> ) synthesis. However, the challenges in synergistic management of optical, electrical, and catalytic properties have limited the efficiency of PEC NRR devices. Herein, to enhance light-harvesting, carrier separation/transport, and the catalytic reactions, a concept of decoupling light-harvesting and electrocatalysis by employing a cascade n<sup>+</sup> np<sup>+</sup> -Si photocathode is implemented. Such a decoupling design not only abolishes the parasitic light blocking but also concurrently improves the optical and electrical properties of the n<sup>+</sup> np<sup>+</sup> -Si photocathode without compromising the efficiency. Experimental and density functional theory studies reveal that the porous architecture and N-vacancies promote N<sub>2</sub> adsorption of the Au/porous carbon nitride (PCN) catalyst. Impressively, an n<sup>+</sup> np<sup>+</sup> -Si photocathode integrating the Au/PCN catalyst exhibits an outstanding PEC NRR performance with maximum Faradaic efficiency (FE) of 61.8% and NH<sub>3</sub> production yield of 13.8 µg h<sup>-1</sup> cm<sup>-2</sup> at -0.10 V versus reversible hydrogen electrode (RHE), which is the highest FE at low applied potential ever reported for the PEC NRR.
• #### Effective Doping of Square/Octagon-Phase Arsenene by Adsorption of Organic Molecules

(Advanced Theory and Simulations, Wiley, 2021-03-27) [Article]
Adsorption of organic molecules can be a better choice than traditional chemical doping to achieve effective doping of 2D materials. The adsorption of organic molecules on square/octagon-phase (S/O-phase) arsenene is investigated and promoted the possibility of developing p–n junctions. In particular, it is found that adsorption of F4-TCNQ or TCNE molecules leads to effective p-doping. Adsorption of DMPD or TTF molecules leads to less effective n-doping. Interestingly, in the case of TTF adsorption strain engineering can be used to greatly improve the material properties. Therefore, both effective n- and p-doping of S/O-phase arsenene can be realized.
• #### Strain-Directed Layer-By-Layer Epitaxy Toward van der Waals Homo- and Heterostructures

(ACS Materials Letters, American Chemical Society (ACS), 2021-03-25) [Article]
Transition-metal dichalcogenide (TMDC) homo- and heterostacks hold tantalizing prospects for being integrated as active components in future van der Waals (vdW) electronics and optoelectronics. However, most TMDC homo- and heterostacks are created by onerous mechanical exfoliation, followed by a mixing-and-matching process. While versatile enough for pilot demonstrations, these strategies are clearly not scalable for practical technologies and widespread implementations. Here, we report a two-step epitaxy strategy that promotes the growth of second-layer TMDCs on the basal plane of the first TMDCs epilayer. The first-layer TMDCs are grown on substrates where the tensile strength can be tuned by the control of chemical environments. The succeeding epilayers then prefer to grow layer-by-layer on the highly tensile-strained first layers. The result is the growth of high-density TMDC homo (WSe2) bilayers and hetero (WSe2–MoS2) bilayers with an exceedingly high yield (>99% bilayers) and uniformity. A density functional theory simulation further sheds light on how strain engineering shifts the subsequent layer growth preference. Second-harmonic generation and high-angle annular dark-field scanning transmission electron microscopy collectively attest to the AB and AA′ stacking between the TMDC epi- and overlayers. The proposed strategy could be a versatile platform for synthesizing diverse arrays of vdW homo- and heterostacks, thus providing prospects for realizing large-scale and layer-controllable two-dimensional electronics.
• #### Engineering Band-Type Alignment in CsPbBr 3 Perovskite-Based Artificial Multiple Quantum Wells

Semiconductor heterostructures of multiple quantum wells (MQWs) have major applications in optoelectronics. However, for halide perovskites—the leading class of emerging semiconductors—building a variety of bandgap alignments (i.e., band-types) in MQWs is not yet realized owing to the limitations of the current set of used barrier materials. Here, artificial perovskite-based MQWs using 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), tris-(8-hydroxyquinoline)aluminum, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as quantum barrier materials are introduced. The structures of three different five-stacked perovskite-based MQWs each exhibiting a different band offset with CsPbBr3 in the conduction and valence bands, resulting in a variety of MQW band alignments, i.e., type-I or type-II structures, are shown. Transient absorption spectroscopy reveals the disparity in charge carrier dynamics between type-I and type-II MQWs. Photodiodes of each type of perovskite artificial MQWs show entirely different carrier behaviors and photoresponse characteristics. Compared with bulk perovskite devices, type-II MQW photodiodes demonstrate a more than tenfold increase in the rectification ratio. The findings open new opportunities for producing halide-perovskite-based quantum devices by bandgap engineering using simple quantum barrier considerations.
• #### Achieving room-temperature M2-phase VO2 nanowires for superior thermal actuation

(Nano Research, Springer Nature, 2021-03-24) [Article]
Vanadium dioxide (VO2) has emerged as a promising micro-actuator material for its large amplitude and high work density across the transition between the insulating (M1 and M2) and metallic (R) phase. Even though M2–R transition offers about 70% higher transformation stress than M1–R structural phase transition, the application of the M2 phase in the micro-actuators is hindered by the fact that previously, M2 phase can only stay stable under tensile stress. In this work, we propose and verify that by synthesizing the VO2 nanowires under optimized oxygen-rich conditions, stoichiometry change can be introduced into the nanowires (NWs) which in turn yield a large number free-standing single-crystalline M2-phase NWs stable at room temperature. In addition, we demonstrate that the output stress of the M2-phase NWs is about 65% higher than that of the M1-phase NWs during their transition to R phase, quite close to the theoretical prediction. Our findings open new avenues towards enhancing the performance of VO2-based actuators by using M2–R transition. [Figure not available: see fulltext.].
• #### [Cu 15 (PPh 3 ) 6 (PET) 13 ] 2+ : a Copper Nanocluster with Crystallization Enhanced Photoluminescence

(Small, Wiley, 2021-03-19) [Article]
Due to their atomically precise structure, photoluminescent copper nanoclusters (Cu NCs) have emerged as promising materials in both fundamental studies and technological applications, such as bio-imaging, cell labeling, phototherapy, and photo-activated catalysis. In this work, a facile strategy is reported for the synthesis of a novel Cu NCs coprotected by thiolate and phosphine ligands, formulated as [Cu<sub>15</sub> (PPh<sub>3</sub> )<sub>6</sub> (PET)<sub>13</sub> ]<sup>2+</sup> , which exhibits bright emission in the near-infrared (NIR) region (≈720 nm) and crystallization-induced emission enhancement (CIEE) phenomenon. Single crystal X-ray crystallography shows that the NC possesses an extraordinary distorted trigonal antiprismatic Cu<sub>6</sub> core and a, unique among metal clusters, "tri-blade fan"-like structure. An in-depth structural investigation of the ligand shell combined with density functional theory calculations reveal that the extended CH···π and π-π intermolecular ligand interactions significantly restrict the intramolecular rotations and vibrations and, thus, are a major reason for the CIEE phenomena. This study provides a strategy for the controllable synthesis of structurally defined Cu NCs with NIR luminescence, which enables essential insights into the origins of their optical properties.
• #### Two-Dimensional Tetrahex-GeC2: A Material with Tunable Electronic and Optical Properties Combined with Ultrahigh Carrier Mobility

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2021-03-19) [Article]
Based on first-principles calculations, we propose a novel two-dimensional (2D) germanium carbide, tetrahex-GeC<sub>2</sub>, and determine its electronic and optical properties. Each Ge atom binds to four C atoms, in contrast to the known 2D hexagonal germanium carbides. Monolayer tetrahex-GeC<sub>2</sub> possesses a narrow direct band gap of 0.89 eV, which can be effectively tuned by applying strain and increasing the thickness. Its electron mobility is extraordinarily high (9.5 × 10<sup>4</sup> cm<sup>2</sup>/(V s)), about 80 times that of monolayer black phosphorus. The optical absorption coefficient is ∼10<sup>6</sup> cm<sup>-1</sup> in a wide spectral range from near-infrared to near-ultraviolet, comparable to perovskite solar cell materials. We obtain high cohesive energy (5.50 eV/atom), excellent stability, and small electron/hole effective mass (0.19/0.10 <i>m</i><sub>0</sub>). Tetrahex-GeC<sub>2</sub> turns out to be a very promising semiconductor for nanoelectronic, optoelectronic, and photovoltaic applications.
• #### Adjusting the energy of interfacial states in organic photovoltaics for maximum efficiency

(Nature Communications, Springer Nature, 2021-03-19) [Article]
AbstractA critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states. This requires small energetic offsets often detrimental for high external quantum efficiency (EQE). Here, we obtain OSC with both non-radiative voltage losses (0.24 V) and photocurrent losses (EQE > 80%) simultaneously minimised. The interfacial CT states separate into free carriers with ≈40-ps time constant. We combine device and spectroscopic data to model the thermodynamics of charge separation and extraction, revealing that the relatively high performance of the devices arises from an optimal adjustment of the CT state energy, which determines how the available overall driving force is efficiently used to maximize both exciton splitting and charge separation. The model proposed is universal for donor:acceptor (D:A) with low driving forces and predicts which D:A will benefit from a morphology optimization for highly efficient OSC.
• #### Impact of Photoluminescence Reabsorption in Metal-Halide Perovskite Solar Cells

(Solar RRL, Wiley, 2021-03-17) [Article]
The precise quantification of the impact of photoluminescence reabsorption (PLr) in metal-halide perovskite solar cells (PSCs) has remained challenging. Here, the PLr effect is examined by combined time-resolved photoluminescence (TRPL) spectroscopy and time-resolved terahertz spectroscopy (TRTS) and a model is proposed which relates both the PLr and non-radiative recombination rate (k$_{nr}$) to the quasi-Fermi-level-splitting (QFLS). PLr is shown to be beneficial for the QFLS when k$_{nr}$ is below a critical value of ∽7×10$^5$ s$^{−1}$; at high k$_{nr}$ PLr is detrimental to the QFLS. By incorporating PLr into a two-diode model that allows extraction of the effective k$_{nr}$, the series resistance (r$_s$), and the shunt resistance (r$_{sh}$) in PSCs, we find that neglecting PLr overestimates the effective k$_{nr}$, while it does not affect the value of r$_s$ and r$_{sh}$. Our findings provide insight into the impact of the PLr effect on metal-halide PSCs.
• #### Successes and Challenges of Core/Shell Lead Halide Perovskite Nanocrystals

(ACS Energy Letters, American Chemical Society (ACS), 2021-03-17) [Article]
Newly emerging perovskite nanocrystals (NCs) have shown a huge potential to be utilized in a gamut of optoelectronic devices due to their superior photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile synthesis protocols at low cost. Despite the enormous progress made in synthetic protocol development, their poor stability against environmental stressors remains a major shortcoming that significantly restricts their practical applications and future commercialization. Of particular interest, core/shell NC engineering has fueled significant progress not only to improve the luminescent properties, reduce exciton recombination, suppress non-radiative recombination, and enhance the charge carrier transport but also, perhaps more importantly, to improve the semiconductor materials’ stability under harsh environmental conditions. Accordingly, this architecture represents a promising avenue to alleviate the stability issue and, therefore, could push the devices’ operational stability and performance forward. In this Focus Review, we explore the successes and challenges of recently reported perovskite core/shell heterostructures and summarize the synthesis methods, the photophysics after shelling, the theoretical approaches, and the applications. Finally, we conclude with a discussion of new opportunities and suggestions to push this research area a step forward.
• #### [Ag9(1,2-BDT)6]3–: How Square-Pyramidal Building Blocks Self-Assemble into the Smallest Silver Nanocluster

(Inorganic Chemistry, American Chemical Society (ACS), 2021-03-17) [Article]
The emerging promise of few-atom metal catalysts has driven the need for developing metal nanoclusters (NCs) with ultrasmall core size. However, the preparation of metal NCs with single-digit metallic atoms and atomic precision is a major challenge for materials chemists, particularly for Ag, where the structure of such NCs remains unknown. In this study, we developed a shape-controlled synthesis strategy based on an isomeric dithiol ligand to yield the smallest crystallized Ag NC to date: [Ag<sub>9</sub>(1,2-BDT)<sub>6</sub>]<sup>3-</sup> (1,2-BDT = 1,2-benzenedithiolate). The NC's crystal structure reveals the self-assembly of two Ag square pyramids through preferential pyramidal vertex sharing of a single metallic Ag atom, while all other Ag atoms are incorporated in a motif with thiolate ligands, resulting in an elongated body-centered Ag<sub>9</sub> skeleton. Steric hindrance and arrangement of the dithiolated ligands on the surface favor the formation of an anisotropic shape. Time-dependent density functional theory based calculations reproduce the experimental optical absorption features and identify the molecular orbitals responsible for the electronic transitions. Our findings will open new avenues for the design of novel single-digit metal NCs with directional self-assembled building blocks.
• #### Crossover from diffusive to superfluid transport in frustrated magnets

(Physical Review B, American Physical Society (APS), 2021-03-16) [Article]
We investigate the spin transport across the magnetic phase diagram of a frustrated antiferromagnetic insulator and uncover a drastic modification of the transport regime from spin diffusion to spin superfluidity. Adopting a triangular lattice accounting for both nearest-neighbor and next-nearest-neighbor exchange interactions with easy-plane anisotropy, we perform atomistic spin simulations on a two-terminal configuration across the full magnetic phase diagram. We found that as long as the ground state magnetic moments remain in-plane, irrespective of whether the magnetic configuration is ferromagnetic, collinear, or noncollinear antiferromagnetic, the system exhibits spin superfluid behavior with a device output that is independent of the value of the exchange interactions. When the magnetic frustration is large enough to compete with the easy-plane anisotropy and cant the magnetic moments out of the plane, the spin transport progressively evolves towards the diffusive regime. The robustness of spin superfluidity close to magnetic phase boundaries is investigated and we uncover the possibility for proximate spin superfluidity close to the ferromagnetic transition.
• #### Temperature-dependent electronic ground state charge transfer in van der Waals heterostructures

(arXiv, 2021-03-14) [Preprint]
Electronic charge rearrangement between components of a heterostructure is the fundamental principle to reach the electronic ground state. It is acknowledged that the density of states distribution of the components governs the amount of charge transfer, but a notable dependence on temperature has not yet been considered, particularly for weakly interacting systems. Here, we experimentally observe that the amount of ground state charge transfer in a van der Waals heterostructure formed by monolayer MoS2 sandwiched between graphite and a molecular electron acceptor layer increases by a factor of three when going from 7 K to room temperature. State-of-the-art electronic structure calculations of the full heterostructure that account for nuclear thermal fluctuations reveal intra-component electron-phonon coupling and inter-component electronic coupling as the key factors determining the amount of charge transfer. This conclusion is rationalized by a model applicable to multi-component van der Waals heterostructures.
• #### Toward Large-Scale Ga2O3 Membranes via Quasi-Van Der Waals Epitaxy on Epitaxial Graphene Layers

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2021-03-12) [Article]
Epitaxial growth using graphene (GR), weakly bonded by van der Waals force, is a subject of interest for fabricating technologically important semiconductor membranes. Such membranes can potentially offer effective cooling and dimensional scale-down for high voltage power devices and deep ultraviolet optoelectronics at a fraction of the bulk-device cost. Here, we report on a large-area β-Ga<sub>2</sub>O<sub>3</sub> nanomembrane spontaneous-exfoliation (1 cm × 1 cm) from layers of compressive-strained epitaxial graphene (EG) grown on SiC, and demonstrated high-responsivity flexible solar-blind photodetectors. The EG was favorably influenced by lattice arrangement of SiC, and thus enabled β-Ga<sub>2</sub>O<sub>3</sub> direct-epitaxy on the EG. The β-Ga<sub>2</sub>O<sub>3</sub> layer was spontaneously exfoliated at the interface of GR owing to its low interfacial toughness by controlling the energy release rate through electroplated Ni layers. The use of GR templates contributes to the seamless exfoliation of the nanomembranes, and the technique is relevant to eventual nanomembrane-based integrated device technology.
• #### Implication of polymeric template agent on the formation process of hybrid halide perovskite film

(Nanotechnology, IOP Publishing, 2021-03-10) [Article]
The use of polymeric additives supporting the growth of hybrid halide perovskites has proven to be a successful approach aiming at high quality active layers targeting optoelectronic exploitation. A detailed description of the complex process involving the self-assembly of the precursors into the perovskite crystallites in presence of the polymer is, however, still missing. Here we take starch:CH3NH3PbI3 (MAPbI3) as example of highly performing composite, both in solar cells and light emitting diodes, and study the film formation process through differential scanning calorimetry and in situ time-resolved grazing incidence wide-angle X-ray scattering, performed during spin coating. These measurements reveal that starch beneficially influences the nucleation and growth of the perovskite precursor phase, leading to improved structural properties of the resulting film which turns into higher stability towards environmental conditions.
• #### M2X Monolayers as Anode Materials for Li Ion Batteries

(Physical Review Applied, American Physical Society (APS), 2021-03-09) [Article]
Electrochemically efficient electrode materials are required for clean energy storage in Li ion batteries. We predict two-dimensional hexagonal metal nitrides, borides, and phosphides (Sc2B, Sc2N, Y2B, Y2N, and Y2P) and evaluate the feasibility of experimental realization. The materials combine excellent metallicity, as required for electrodes, with Li binding energies providing high storage capacity and a low average open-circuit voltage. In contrast to two-dimensional silicene, borophene, and SnS2, we observe negligible structural distortions during Li adsorption and extraction, which results in high reversibility and a long cycle life. Superionic Li diffusion enables fast charge or discharge of next-generation Li ion batteries.