Molecular-Scale Mechanistic Investigation of Oxygen Dissociation and Adsorption on Metal Surface-Supported Cobalt Phthalocyanine

Ultrahigh vacuum scanning tunneling microscopy and density functional theory are used to investigate adsorption of oxygen on cobalt phthalocyanine (CoPc), a promising nonprecious metal oxygen reduction catalyst, supported on Ag(111), Cu(111), and Au(111) surfaces at the molecular scale. Four distinct molecular and atomic oxygen adsorption configurations are observed for CoPc supported on Ag(111) surfaces, which are assigned as O₂/CoPc/Ag(111), O/CoPc/Ag(111), CoPc/(O)₂/Ag(111), and (O)₂/CoPc/Ag(111). In contrast, no oxygen adsorption is observed for CoPc supported on Cu(111) and Au(111) surfaces. The results show that for Ag(111), atomic O that is predominantly catalytically produced from the dissociation of molecular O₂ at metal surface step edges is responsible for the observed adsorption configurations. However, Cu(111) binds atomic O too strongly, and Au(111) does not produce atomic O. These results show the active role of the supporting metal surface in facilitating oxygen adsorption on CoPc.

The Future of Layer-by-Layer Assembly: A Tribute to ACS Nano Associate Editor Helmuth Möhwald

Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings. In this Perspective, dedicated to the memory of ACS Nano associate editor Prof. Dr. Helmuth Möhwald, we discuss the developments and applications that are to come in LbL assembly, focusing on coatings, bulk materials, membranes, nanocomposites, and delivery vehicles.

Borophene Synthesis on Au(111)

Borophene (the first two-dimensional (2D) allotrope of boron) is emerging as a groundbreaking system for boron-based chemistry and, more broadly, the field of low-dimensional materials. Exploration of the phase space for growth is critical because borophene is a synthetic 2D material that does not have a bulk layered counterpart and thus cannot be isolated via exfoliation methods. Herein, we report synthesis of borophene on Au(111) substrates. Unlike previously studied growth on Ag substrates, boron diffuses into Au at elevated temperatures and segregates to the surface to form borophene islands as the substrate cools. These observations are supported by ab initio modeling of interstitial boron diffusion into the Au lattice. Borophene synthesis also modifies the surface reconstruction of the Au(111) substrate, resulting in a trigonal network that templates growth at low coverage. This initial growth is composed of discrete borophene nanoclusters, whose shape and size are consistent with theoretical predictions. As the concentration of boron increases, nanotemplating breaks down and larger borophene islands are observed. Spectroscopic measurements reveal that borophene grown on Au(111) possesses a metallic electronic structure, suggesting potential applications in 2D plasmonics, superconductivity, interconnects, electrodes, and transparent conductors.

Electronic Coupling in Metallophthalocyanine-Transition Metal Dichalcogenide Mixed-Dimensional Heterojunctions

Mixed-dimensional heterojunctions, such as zero-dimensional (0D) organic molecules deposited on two-dimensional (2D) transition metal dichalcogenides (TMDCs), often exhibit interfacial effects that enhance the properties of the individual constituent layers. Here we report a systematic study of interfacial charge transfer in metallophthalocyanine (MPc) - MoS₂ heterojunctions using optical absorption and Raman spectroscopy to elucidate M core (M = first row transition metal), MoS₂ layer number, and excitation wavelength effects. Observed phenomena include the emergence of heterojunction-specific optical absorption transitions and strong Raman enhancement that depends on the M identity. In addition, the Raman enhancement is tunable by excitation laser wavelength and MoS₂ layer number, ultimately reaching a maximum enhancement factor of 30x relative to SiO₂ substrates. These experimental results, combined with density functional theory (DFT) calculations, indicate strong coupling between nonfrontier MPc orbitals and the MoS₂ band structure as well as charge transfer across the heterojunction interface that varies as a function of the MPc electronic structure.

Low-Frequency Carrier Kinetics in Perovskite Solar Cells

Hybrid organic-inorganic halide perovskite solar cells have emerged as leading candidates for third-generation photovoltaic technology. Despite the rapid improvement in power conversion efficiency (PCE) for perovskite solar cells in recent years, the low-frequency carrier kinetics that underlie practical roadblocks such as hysteresis and degradation remain relatively poorly understood. In an effort to bridge this knowledge gap, we perform here correlated low-frequency noise (LFN) and impedance spectroscopy (IS) characterization that elucidates carrier kinetics in operating perovskite solar cells. Specifically, we focus on planar cell geometries with a SnO₂ electron transport layer and two different hole transport layers-namely, poly(triarylamine) (PTAA) and spiro-OMeTAD. PTAA and spiro-OMeTAD cells with moderate PCEs of 5-12% possess a Lorentzian feature at ∼200 Hz in LFN measurements that corresponds to a crossover from electrode to dielectric polarization. In comparison, spiro-OMeTAD cells with high PCEs (>15%) show 4 orders of magnitude lower LFN amplitude and are accompanied by a cyclostationary process. Through a systematic study of more than a dozen solar cells, we establish a correlation with noise amplitude, PCE, and fill factor. Overall, this work establishes correlated LFN and IS as an effective methodology for quantifying low-frequency carrier kinetics in perovskite solar cells, thereby providing new physical insights that can rationally guide ongoing efforts to improve device performance, reproducibility, and stability.

Geometric imaging of borophene polymorphs with functionalized probes

A common characteristic of borophene polymorphs is the presence of hollow hexagons (HHs) in an otherwise triangular lattice. The vast number of possible HH arrangements underlies the polymorphic nature of borophene, and necessitates direct HH imaging to definitively identify its atomic structure. While borophene has been imaged with scanning tunneling microscopy using conventional metal probes, the convolution of topographic and electronic features hinders unambiguous identification of the atomic lattice. Here, we overcome these limitations by employing CO-functionalized atomic force microscopy to visualize structures corresponding to boron-boron covalent bonds. Additionally, we show that CO-functionalized scanning tunneling microscopy is an equivalent and more accessible technique for HH imaging, confirming the v_1/5 and v_1/6 borophene models as unifying structures for all observed phases. Using this methodology, a borophene phase diagram is assembled, including a transition from rotationally commensurate to incommensurate phases at high growth temperatures, thus corroborating the chemically discrete nature of borophene.

Borophene, or 2D boron, is highly polymorphic with many predicted lattice arrangements, complicating the identification of its atomic structure. Here, the authors use functionalized-tip scanning probe microscopy to directly resolve the atomic lattice structures of several borophene polymorphs.

Freestanding Ion Gels for Flexible, Printed, Multifunctional Microsupercapacitors

Freestanding ion gels (FIGs) provide unique opportunities for scalable, low-cost fabrication of flexible microsupercapacitors (MSCs). While conventional MSCs employ a distinct electrolyte and substrate, FIGs perform both functions, offering new possibilities for device integration and multifunctionality while maintaining high performance. Here, a capillarity-driven printing method is demonstrated to manufacture high-precision graphene electrodes on FIGs for MSCs. This method achieves excellent self-alignment and resolution (width: 50 μm, interdigitated electrode footprint: <1 mm²) and 100% fabrication yield (48/48 devices) and is readily generalized to alternative electrode materials including multiwalled carbon nanotubes (MWCNTs). The devices demonstrate good performance, including high specific capacitance (graphene: 0.600 mF cm^-2; MWCNT: 6.64 mF cm^-2) and excellent stability against bending, folding, and electrical cycling. Moreover, this strategy offers unique opportunities for device design and integration, including a bifacial electrode structure with enhanced capacitance (graphene: 0.673 mF cm^-2; MWCNT: 7.53 mF cm^-2) and improved rate performance, print-and-place versatility for integration on diverse substrates, and multifunctionality for light emission and transistor gating. These compelling results demonstrate the potential of FIGs for scalable, low-cost fabrication of flexible, printed, and multifunctional energy storage devices.

Fully Inkjet-Printed, Mechanically Flexible MoS2 Nanosheet Photodetectors

Solution-processed two-dimensional materials offer a scalable route toward next-generation printed devices. In this report, we demonstrate fully inkjet-printed photodetectors using molybdenum disulfide (MoS₂) nanosheets as the active material and graphene as the electrodes. Percolating films of semiconducting MoS₂ with high electrical conductivity are achieved with an ethyl cellulose-based ink formulation. Two classes of photodetectors are fabricated, including thermally annealed devices on glass with fast photoresponse of 150 μs and photonically annealed devices on flexible polyimide with high photoresponsivity exceeding 50 mA/W. The photonically annealed photodetector also reduces the curing time to milliseconds and maintains functionality over 500 bending cycles.

Hot Carrier and Surface Recombination Dynamics in Layered InSe Crystals

Layered indium selenide (InSe) is a van der Waals solid that has emerged as a promising material for high-performance ultrathin solar cells. The optoelectronic parameters that are critical to photoconversion efficiencies, such as hot carrier lifetime and surface recombination velocity, are however largely unexplored in InSe. Here, these key photophysical properties of layered InSe are measured with femtosecond transient reflection spectroscopy. The hot carrier cooling process is found to occur through phonon scattering. The surface recombination velocity and ambipolar diffusion coefficient are extracted from fits to the pump energy-dependent transient reflection kinetics using a free carrier diffusion model. The extracted surface recombination velocity is approximately an order of magnitude larger than that for methylammonium lead-iodide perovskites, suggesting that surface recombination is a principal source of photocarrier loss in InSe. The extracted ambipolar diffusion coefficient is consistent with previously reported values of InSe carrier mobility.

Polymer Doping Enables a Two-Dimensional Electron Gas for High-Performance Homojunction Oxide Thin-Film Transistors

High-performance solution-processed metal oxide (MO) thin-film transistors (TFTs) are realized by fabricating a homojunction of indium oxide (In₂ O₃ ) and polyethylenimine (PEI)-doped In₂ O₃ (In₂ O₃ :x% PEI, x = 0.5-4.0 wt%) as the channel layer. A two-dimensional electron gas (2DEG) is thereby achieved by creating a band offset between the In₂ O₃ and PEI-In₂ O₃ via work function tuning of the In₂ O₃ :x% PEI, from 4.00 to 3.62 eV as the PEI content is increased from 0.0 (pristine In₂ O₃ ) to 4.0 wt%, respectively. The resulting devices achieve electron mobilities greater than 10 cm² V^-1 s^-1 on a 300 nm SiO₂ gate dielectric. Importantly, these metrics exceed those of the devices composed of the pristine In₂ O₃ materials, which achieve a maximum mobility of ≈4 cm² V^-1 s^-1 . Furthermore, a mobility as high as 30 cm² V^-1 s^-1 is achieved on a high-k ZrO₂ dielectric in the homojunction devices. This is the first demonstration of 2DEG-based homojunction oxide TFTs via band offset achieved by simple polymer doping of the same MO material.

Suppressing Ambient Degradation of Exfoliated InSe Nanosheet Devices via Seeded Atomic Layer Deposition Encapsulation

With exceptional charge carrier mobilities and a direct bandgap at most thicknesses, indium selenide (InSe) is an emerging layered semiconductor that has generated significant interest for electronic and optoelectronic applications. However, exfoliated InSe nanosheets are susceptible to rapid degradation in ambient conditions, thus limiting their technological potential. In addition to morphological changes upon ambient exposure, the mobilities and current modulation on/off ratios of InSe transistors, as well as the responsivities of InSe photodetectors, decrease by over 3 orders of magnitude within 12 h of ambient exposure. In an effort to mitigate these deleterious effects, here we present an encapsulation scheme based on seeded atomic layer deposition that provides pinhole-free growth of alumina without compromising the intrinsic electronic properties of the underlying InSe. In particular, this encapsulation provides reproducible InSe field-effect transistor characteristics and InSe photodetector responsivities in excess of 10⁷ A/W following ambient exposure for time periods on the order of months. Because atomic layer deposition is a highly scalable and manufacturable process, this work will accelerate ongoing efforts to integrate InSe nanosheets into electronic and optoelectronic technologies.

Atomic-Scale Observation of Electrochemically Reversible Phase Transformations in SnSe2 Single Crystals

2D materials have shown great promise to advance next-generation lithium-ion battery technology. Specifically, tin-based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions-intercalation, conversion, and alloying-but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real-time and atomic-scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe₂ single crystals, using in situ high-resolution transmission electron microscopy complemented by first-principles calculations. Lithiation proceeds sequentially through intercalation, conversion, and alloying reactions (SnSe₂ → Li_x SnSe₂ → Li₂ Se + Sn → Li₂ Se + Li₁₇ Sn₄ ) in a manner that maintains structural and crystallographic integrity, whereas delithiation forms numerous well-aligned SnSe₂ nanodomains via a homogeneous deconversion process, but gradually loses the coherent orientation in subsequent cycling. Furthermore, alloying and dealloying reactions cause dramatic structural reorganization and thereby consequently reduce structural stability and electrochemical cyclability, which implies that deep discharge for Sn chalcogenide electrodes should be avoided. Overall, the findings elucidate atomistic lithiation and delithiation mechanisms in SnSe₂ with potential implications for the broader class of 2D metal chalcogenides.

Abrupt Thermal Shock of (NH4)2Mo3S13 Leads to Ultrafast Synthesis of Porous Ensembles of MoS2 Nanocrystals for High Gain Photodetectors

Ultrafast synthesis of high-quality transition-metal dichalcogenide nanocrystals, such as molybdenum disulfide (MoS₂), is technologically relevant for large-scale production of electronic and optoelectronic devices. Here, we report a rapid solid-state synthesis route for MoS₂ using the chemically homogeneous molecular precursor, (NH₄)₂Mo₃S₁₃·H₂O, resulting in nanoparticles with estimated size down to 25 nm only in 10 s at 1000 °C. Despite the extreme nonequilibrium conditions, the resulting porous MoS₂ nanoparticles remain aggregated to preserve the form of the original rod shape bulk morphology of the molecular precursor. This ultrafast synthesis proceeds through the rapid decomposition of the precursor and rearrangement of Mo and S atoms coupled with simultaneous efficient release of massive gaseous species, to create nanoscale porosity in the resulting isomorphic pseudocrystals, which are composed of the MoS₂ nanoparticles. Despite the very rapid escape of massive amounts of NH₃, H₂O, H₂S, and S gases from the (NH₄)₂Mo₃S₁₃·H₂O mm sized crystals, they retain their original shape as they convert to MoS₂ rather than undergo explosive destruction from the rapid escape process of the gases. The obtained pseudocrystals are made of aggregated MoS₂ nanocrystals exhibit a Brunauer-Emmett-Teller surface area of ∼35 m²/g with an adsorption average pore width of ∼160 Å. The nanoporous MoS₂ crystals are solution processable by dispersing in ethanol and water and can be cast into large-area uniform composite films. Photodetectors fabricated from these films show more than 2 orders of magnitude higher conductivity (∼6.25 × 10^-6 S/cm) and photoconductive gain (20 mA/W) than previous reports of MoS₂ composite films. The optoelectronic properties of this nanoporous MoS₂ imply that the shallow defects that originate from the ultrafast synthesis act as sensitizing centers that increase the photocurrent gain via two-level recombination kinetics.

Silicon-Phosphorene Nanocavity-Enhanced Optical Emission at Telecommunications Wavelengths

Generating and amplifying light in silicon (Si) continues to attract significant attention due to the possibility of integrating optical and electronic components in a single material platform. Unfortunately, silicon is an indirect band gap material and therefore an inefficient emitter of light. With the rise of integrated photonics, the search for silicon-based light sources has evolved from a scientific quest to a major technological bottleneck for scalable, CMOS-compatible, light sources. Recently, emerging two-dimensional materials have opened the prospect of tailoring material properties based on atomic layers. Few-layer phosphorene, which is isolated through exfoliation from black phosphorus (BP), is a great candidate to partner with silicon due to its layer-tunable direct band gap in the near-infrared where silicon is transparent. Here we demonstrate a hybrid silicon optical emitter composed of few-layer phosphorene nanomaterial flakes coupled to silicon photonic crystal resonators. We show single-mode emission in the telecommunications band of 1.55 μm ( E_g = 0.8 eV) under continuous wave optical excitation at room temperature. The solution-processed few-layer BP flakes enable tunable emission across a broad range of wavelengths and the simultaneous creation of multiple devices. Our work highlights the versatility of the Si-BP material platform for creating optically active devices in integrated silicon chips.

Inorganic "Conductive Glass" Approach to Rendering Mesoporous Metal-Organic Frameworks Electronically Conductive and Chemically Responsive

A representative mesoporous metal-organic-framework (MOF) material, NU-1000, has been rendered electronically conductive via a robust inorganic approach that permits retention of MOF crystallinity and porosity. The approach is based on condensed-phase grafting of molecular tin species onto the MOF nodes via irreversible reaction with hydroxyl and aqua ligands presented at the node surface, a self-limiting process termed solvothermal installation (of metal ions) in MOFs (SIM, a solution-phase analog of atomic layer deposition in MOFs). Treatment of the modified MOF with aerated steam at 120 °C converts the grafted tin molecules to tetratin(IV)oxy clusters, with the clusters being sited between insulating pairs of zirconia-like nodes (the zirconium component being key to endowing the parent material with requisite chemical and thermal stability). By introducing new O-H presenting ligands on the modified-MOF node, the high-temperature steam step additionally serves to reset the material to reactive form, thus enabling a second self-limiting tin-grafting step to be run (and after further steam treatment, enabling a third). Difference-envelop-density (DED) analyses of synchrotron-derived X-ray scattering data, with and without installed tin species, show that the clusters formed after one cycle are spatially isolated, but that repetitive SIM cycling adds metal and oxygen ions in a way that enshrouds nodes, links clusters, and yields continuous one-dimensional strands of oxy-tin(IV), oriented exclusively along the c axis of the MOF. Two-probe conductivity measurements show that the parent MOF and the version containing isolated oxy-tin(IV) clusters are electrically insulating, but that the versions featuring continuous strands show an electrical conductivity of 1.8 × 10^-7 S/cm after three Sn-SIM cycles. When combined with interdigitated microelectrodes, the solvent-free and conductive-glass-modified material (three Sn-SIM cycles) displays a substantial and persistent increase in electrical conductivity during exposure to 5% H₂, indicating a role for dissociated H₂ as an electronic dopant. The increase can be repetitively reversed by alternating H₂ with air, illustrating the ability of the conductive MOF to function as a resistive sensor for H₂ and suggesting further potential applications that may capitalize on the combination of high volumetric surface area, high mesoporosity, high chemical and thermal stability, and significant electrical conductivity.

Density-Gradient Control over Nanoparticle Supercrystal Formation

With the advent of DNA-directed methods to form "single crystal" nanoparticle superlattices, new opportunities for studying the properties of such structures across many length scales now exist. These structure-property relationships rely on the ability of one to deliberately use DNA to control crystal symmetry, lattice parameter, and microscale crystal habit. Although DNA-programmed colloidal crystals consistently form thermodynamically favored crystal habits with a well-defined symmetry and lattice parameter based upon well-established design rules, the sizes of such crystals often vary substantially. For many applications, especially those pertaining to optics, each crystal can represent a single device, and therefore size variability can significantly reduce their scope of use. Consequently, we developed a new method based upon the density difference between two layers of solvents to control nanoparticle superlattice formation and growth. In a top aqueous layer, the assembling particles form a less viscous and less dense state, but once the particles assemble into well-defined rhombic dodecahedral superlattices of a critical size, they sediment into a higher density and higher viscosity sublayer that does not contain particles (aqueous polysaccharide), thereby arresting growth. As a proof-of-concept, this method was used to prepare a uniform batch of Au nanoparticle (20.0 ± 1.6 nm in diameter) superlattices in the form of 0.95 ± 0.20 μm edge length rhombic dodecahedra with body-centered cubic crystal symmetries and a 49 nm lattice parameter (cf. 1.04 ± 0.38 μm without the sublayer). This approach to controlling and arresting superlattice growth yields structures with a 3-fold enhancement in the polydispersity index.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

Solution-Based Processing of Optoelectronically Active Indium Selenide

Layered indium selenide (InSe) presents unique properties for high-performance electronic and optoelectronic device applications. However, efforts to process InSe using traditional liquid phase exfoliation methods based on surfactant-assisted aqueous dispersions or organic solvents with high boiling points compromise electronic properties due to residual surface contamination and chemical degradation. Here, these limitations are overcome by utilizing a surfactant-free, low boiling point, deoxygenated cosolvent system. The resulting InSe flakes and thin films possess minimal processing residues and are structurally and chemically pristine. When employed in photodetectors, individual InSe nanosheets exhibit a maximum photoresponsivity of ≈5 × 10⁷ A W^-1 , which is the highest value of any solution-processed monolithic semiconductor to date. Furthermore, the surfactant-free cosolvent system not only stabilizes InSe dispersions but is also amenable to the assembly of electronically percolating InSe flake arrays without posttreatment, which enables the realization of ultrahigh performance thin-film photodetectors. This surfactant-free, deoxygenated cosolvent approach can be generalized to other layered materials, thereby presenting additional opportunities for solution-processed thin-film electronic and optoelectronic technologies.

Intermixing and periodic self-assembly of borophene line defects

Two-dimensional (2D) boron (that is, borophene) was recently synthesized following theoretical predictions^1-5. Its metallic nature and high in-plane anisotropy combine many of the desirable attributes of graphene⁶ and monolayer black phosphorus⁷. As a synthetic 2D material, its structural properties cannot be deduced from bulk boron, which implies that the intrinsic defects of borophene remain unexplored. Here we investigate borophene line defects at the atomic scale with ultrahigh vacuum (UHV) scanning tunnelling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Under suitable growth conditions, borophene phases that correspond to the v_1/6 and v_1/5 models are found to intermix and accommodate line defects in each other with structures that match the constituent units of the other phase. These line defects energetically favour spatially periodic self-assembly that gives rise to new borophene phases, which ultimately blurs the distinction between borophene crystals and defects. This phenomenon is unique to borophene as a result of its high in-plane anisotropy and energetically and structurally similar polymorphs. Low-temperature measurements further reveal subtle electronic features that are consistent with a charge density wave (CDW), which are modulated by line defects. This atomic-level understanding is likely to inform ongoing efforts to devise and realize applications based on borophene.

Revealing the Effects of Electrode Crystallographic Orientation on Battery Electrochemistry via the Anisotropic Lithiation and Sodiation of ReS2

The crystallographic orientation of battery electrode materials can significantly impact electrochemical performance, such as rate capability and cycling stability. Among the layered transition metal dichalcogenides, rhenium disulfide (ReS₂) has the largest anisotropic ratio between the two main axes in addition to exceptionally weak interlayer coupling, which serves as an ideal system to observe and analyze anisotropy of electrochemical phenomena. Here, we report anisotropic lithiation and sodiation of exfoliated ReS₂ at atomic resolution using in situ transmission electron microscopy. These results reveal the role of crystallographic orientation and anisotropy on battery electrode electrochemistry. Complemented with density functional theory calculations, the lithiation of ReS₂ is found to begin with intercalation of Li-ions, followed by a conversion reaction that results in Re nanoparticles and Li₂S nanocrystals. The reaction speed is highly anisotropic, occurring faster along the in-plane ReS₂ layer than along the out-of-plane direction. Sodiation of ReS₂ is found to proceed similarly to lithiation, although the intercalation step is relatively quicker. Furthermore, the microstructure and morphology of the reaction products after lithiation/sodiation show clear anisotropy along the in-plane and out-of-plane directions. These results suggest that crystallographic orientation in highly anisotropic electrode materials can be exploited as a design parameter to improve battery electrochemical performance.

Novel ALD Chemistry Enabled Low-Temperature Synthesis of Lithium Fluoride Coatings for Durable Lithium Anodes

Lithium metal anodes can largely enhance the energy density of rechargeable batteries because of the high theoretical capacity and the high negative potential. However, the problem of lithium dendrite formation and low Coulombic efficiency (CE) during electrochemical cycling must be solved before lithium anodes can be widely deployed. Herein, a new atomic layer deposition (ALD) chemistry to realize the low-temperature synthesis of homogeneous and stoichiometric lithium fluoride (LiF) is reported, which then for the first time, as far as we know, is deposited directly onto lithium metal. The LiF preparation is performed at 150 °C yielding 0.8 Å/cycle. The LiF films are found to be crystalline, highly conformal, and stoichiometric with purity levels >99%. Nanoindentation measurements demonstrate the LiF achieving a shear modulus of 58 GPa, 7 times higher than the sufficient value to resist lithium dendrites. When used as the protective coating on lithium, it enables a stable Coulombic efficiency as high as 99.5% for over 170 cycles, about 4 times longer than that of bare lithium anodes. The remarkable battery performance is attributed to the nanosized LiF that serves two critical functions simultaneously: (1) the high dielectric value creates a uniform current distribution for excellent lithium stripping/plating and ultrahigh mechanical strength to suppress lithium dendrites; (2) the great stability and electrolyte isolation by the pure LiF on lithium prevents parasitic reactions for a much improved CE. This new ALD chemistry for conformal LiF not only offers a promising avenue to implement lithium metal anodes for high-capacity batteries but also paves the way for future studies to investigate failure and evolution mechanisms of solid electrolyte interphase (SEI) using our LiF on anodes such as graphite, silicon, and lithium.

Assessing and Mitigating the Hazard Potential of Two-Dimensional Materials

The family of two-dimensional (2D) materials is comprised of a continually expanding palette of unique compositions and properties with potential applications in electronics, optoelectronics, energy capture and storage, catalysis, and nanomedicine. To accelerate the implementation of 2D materials in widely disseminated technologies, human health and environmental implications need to be addressed. While extensive research has focused on assessing the toxicity and environmental fate of graphene and related carbon nanomaterials, the potential hazards of other 2D materials have only recently begun to be explored. Herein, the toxicity and environmental fate of postcarbon 2D materials, such as transition metal dichalcogenides, hexagonal boron nitride, and black phosphorus, are reviewed as a function of their preparation methods and surface functionalization. Specifically, we delineate how the hazard potential of 2D materials is directly related to structural parameters and physicochemical properties and how experimental design is critical to the accurate elucidation of the underlying toxicological mechanisms. Finally, a multidisciplinary approach for streamlining the hazard assessment of emerging 2D materials is outlined, thereby providing a pathway for accelerating their safe use in a range of technologically relevant contexts.

Extrinsic polarization-controlled optical anisotropy in plasmon-black phosphorus coupled system

Two-dimensional black phosphorus (BP) has drawn extensive research interest due to its promising anisotropic photonic and electronic properties. Here, we study anisotropic optical absorption and photoresponse of exfoliated BP flakes at visible frequencies. We enhance this intrinsic optical anisotropy in BP flakes by coupling plasmonic rectangular nanopatch arrays that support localized surface plasmon resonances. In particular, by combining extrinsic anisotropic plasmonic nanostructures lithographically aligned with intrinsically anisotropic BP flakes, we demonstrate for the first time a combined anisotropic plasmonic-semiconductor coupling that provides significant control over the polarization-dependent optical properties of the plasmon-BP hybrid material system, enhancing polarization-sensitive responses to a larger degree. This hybrid material system not only unveils the plasmon-enhanced mechanisms in BP, but also provides novel controllable functionalities in optoelectronic device applications involving polarization-sensitive optical and electrical responses.

Transfer Printing of Sub-5 μm Graphene Electrodes for Flexible Microsupercapacitors

Printed graphene microsupercapacitors (MSCs) are attractive for scalable and low-cost on-chip energy storage for distributed electronic devices. Although electronic devices have experienced significant scaling to smaller formats, the corresponding miniaturization of energy storage components has been limited, with a typical resolution of ∼30 μm for printed graphene patterns to date. Transfer printing is demonstrated here for patterning graphene electrodes with fine line and spacing resolution less than 5 μm. The resulting devices exhibit an exceptionally small footprint (∼0.0067 mm²), which provides, to the best of our knowledge, the smallest printed graphene MSCs. Despite this, the devices retain excellent performance with a high areal capacitance of ∼6.63 mF/cm² along with excellent electrochemical stability and mechanical flexibility, resulting from an efficient nonplanar electrode structure and an optimized two-step photoannealing method. As a result, this miniaturization strategy facilitates the on-chip integration of printed graphene MSCs to power emerging electronic devices.

Self-Assembled Photochromic Molecular Dipoles for High-Performance Polymer Thin-Film Transistors

The development of high-performance multifunctional polymer-based electronic circuits is a major step toward future flexible electronics. Here, we demonstrate a tunable approach to fabricate such devices based on rationally designed dielectric super-lattice structures with photochromic azobenzene molecules. These nanodielectrics possessing ionic, molecular, and atomic polarization are utilized in polymer thin-film transistors (TFTs) to realize high-performance electronics with a p-type field-effect mobility (μ_FET) exceeding 2 cm² V^-1 s^-1. A crossover in the transport mechanism from electrostatic dipolar disorder to ionic-induced disorder is observed in the transistor characteristics over a range of temperatures. The facile supramolecular design allows the possibility to optically control the extent of molecular and ionic polarization in the ultrathin nanodielectric. Thus, we demonstrate a 3-fold increase in the capacitance from 0.1 to 0.34 μF/cm², which results in a 200% increase in TFT channel current.