Search

The clean production of hydrogen from water using sunlight has emerged as a sustainable alternative toward large-scale energy generation and storage. However, designing photoactive semiconductors that are suitable for both light harvesting and water splitting is a pivotal challenge. Atomically thin transition metal dichalcogenides (TMD) are considered as promising photocatalysts because of their wide range of available electronic properties and compositional variability. However, trade-offs between carrier transport efficiency, light absorption, and electrochemical reactivity have limited their prospects. We here combine two approaches that synergistically enhance the efficiency of photocarrier generation and electrocatalytic efficiency of two-dimensional (2D) TMDs. The arrangement of monolayer WS

Control of the precursor transport through oxygen dosing yields increased MoS2 coverage and increased connectivity between crystalline MoS2 domains.

Scheme and band diagram for Au NPs combined with a CuxO thin film are synthesized by a self-assembly process and electrochemical modification. Au/CuxO for developing carbon capture and storage performance.

We here present a planarized solid-state chemical reaction that can produce transition metal monochalcogenide (TMMC) 2D crystals with large lateral extent and finely controllable thickness down to individual layers. The enhanced lateral diffusion of a gaseous reactant at the interface between a solid precursor and graphene was found to provide a universal route towards layered TMMCs of different compositions. A unique layer-by-layer growth mechanism yields atomically abrupt crystal interfaces and kinetically controllable thickness down to a single TMMC layer. Our approach stabilizes 2D crystals with commonly unattainable thermodynamic phases, such as β-Cu2S and γ-CuSe, and spectroscopic characterization reveals ultra-large phase transition depression and interesting electronic properties. The presented ability to produce large-scale 2D crystals with high environmental stability was applied to highly sensitive and fast optoelectronic sensors. Our approach extends the morphological, compositional, and thermodynamic complexity of 2D materials.

Distinguishing a multitude of optical labels is crucial to improving the spatial and temporal resolution of bioimaging. However, current multicolor imaging approaches are limited by the spectral overlap of employed fluorophores. We here discern different instances of a single optical label type through their emission intensity. Such multilevel optical labels are enabled by an optical writing process that permanently modifies their spectral response in a predictable manner and by a separate spectral feature that serves as normalization in the presence of sample variability. The proposed approach was realized by independently controlling the emission properties of highly functionalized fluorescent nanodiamond. Upon laser irradiation, the contribution of the spectral region associated with the N3 color center decreases in a predictable and permanent fashion, while the nitrogen vacancy (NV) emission remains stable. This selective photobleaching of N3 centers was found to originate from a two-photon-assisted dissociation process that results in a 105 higher mobility of photoexcited carriers in N3 centers compared to NV. The resulting write once read many (WORM) memory exhibits multiple distinct memory levels that can be stored and read out with high robustness and reproducibility. The potential of our approach was demonstrated by characterizing markers in HeLa cells with high fidelity, despite the complex emission background. Finally, direct manipulation of label information inside of cells was demonstrated, opening up new routes in advanced bioimaging.

The carrier conduction in 2D materials is more sensitive to surface-bound disorder than bulk materials and is thought to limit their achievable performance in electronic devices. To date, charged impurity scattering is considered the main source of interaction between ionic adsorbates and carriers in 2D materials. We here observe a previously unknown source of carrier scattering in graphene upon interaction with ionic impurities. Different from charged impurity scattering, these “neutral scatterers” do not depend on carrier concentration and yield a sixfold mobility variation at similar doping. Comparison of different ionic residue from various metal etchants reveals a universal mechanism that controls the carrier mean free path. Raman spectroscopy suggests that inhomogeneous charge distribution is the source of neutral scatterers and we extract an optical fingerprint for their presence. The charge heterogeneity thus acts as an additional degree of freedom in graphene’s carrier transport and its consideration can explain the transition from ambipolar to unipolar charge transport in graphene. Our results not only provide new insight into the carrier transport of 2D materials in the presence of disorder and provide guidelines for enhancing the performance of graphene devices but also enable novel device concepts in graphene.

Despite significant progress in the fabrication and application of semiconductor materials for optical emitters and sensors, few materials can cover the cyan-gap between 450 and 500 nm. We here introduce a robust and facile method to deposit amorphous Sb2S3 films with a bandgap of 2.8 eV. By exploiting the tunable functionality of graphene, a two-dimensional material, efficient deposition from a chemical was achieved. Ozone-generated defects in the graphene were shown to be required to enhance the morphology and quality of the material and comprehensive characterization of the seed layer and the Sb2S3 film were applied to design an optimal deposition process. The resulting material exhibits efficient carrier transport and high photodetector performance as evidenced by unprecedented responsivity and detectivity in semiconductor/graphene/glass vertical heterostructures. (112 A/W, 2.01 × 1012 Jones, respectively).

In this work, we present efficient, robust, and transparent electromagnetic interference (EMI) shielding by a hybrid material comprised of a nickel (Ni) mesh and a conformal graphene coating. We de...

The assembly of graphene and other two-dimensional (2D) materials into artificial crystals termed van-der-Waals stacks has great potential to produce new materials without precedence in nature and develop novel electronic devices. To reliably assemble 2D materials into such structures, however, a better understanding of the transfer process is required. Here we report a quantitative approach to examining the adhesion behavior during viscoelastic stamping of 2D materials. By measuring the adhesion of graphene to different carrier substrates and varying the peeling speed we have identified the range of adhesion of samples. The result shows that the adhesion occurs between graphene-graphene and graphene-SiO2 substrate have a higher value than the ability of polydimethylsiloxane (PDMS) stamp to pick up. The impact of surface modification and alternative substrates is investigated and our results provide guidelines to realize an effective fabrication method for two-dimensional heterostructure devices.

Two-dimensional (2D) thermoelectrics have shown enhanced performance compared to bulk thermoelectrics but cannot easily be improved through nanostructuring, since phonon-boundary-scattering becomes...

Two-dimensional (2D) material nanocomposites have emerged as a material system for discovering new physical phenomena and developing novel devices. However, because of the low density of states of most two-dimensional materials such as graphene, the heterostructure of nanocomposites suffers from an enhanced depletion region, which can greatly reduce the efficiency of the charge carrier transfer and deteriorate the device performance. To circumvent this difficulty, here we propose an alternative approach by inserting a second 2D mediator with a heavy effective mass having a large density of states in-between the heterojunction of 2D nanocomposites. The mediator can effectively reduce the depletion region and form a type-II band alignment, which can speed up the dissociation of electron-hole pairs and enhance charge carrier transfer. To illustrate the principle, we demonstrate a novel stretchable photodetector based on the combination of graphene/ReS

In this study, we synthesized gradient MoS

Optical rectennas extend the electromagnetic wave rectification process into the visible regime and provide a route toward high-performance photodetection and energy harvesting. Here, the promise of 2D materials toward on-chip optical rectennas is demonstrated. A self-aligned patterning process yields lateral MIM structures where a nanometer-sized air gap separates a 2D material contact from a metal electrode. This device can be scalably produced in large arrays using established microfabrication techniques. Different from previous approaches, the performance of the 2D rectenna can be adjusted through electrostatic gating. Optimization of the band alignment leads to strong rectification at wavelengths around 500 nm and clear polarization control. Comparison of wavelength-dependent rectenna performance with a photon-assisted tunneling model reveals a tenfold increase in photon-electron coupling over nanotube-based rectennas. The results highlight the potential of 2D material-based rectennas for future quantum computing applications.

Barrier-guided CVD growth could provide a new route to printed electronics by combining high quality 2D materials synthesis with scalable and cost-effective deposition methods. Unfortunately, we observe the limited stability of the barrier at growth conditions which results in its removal within minutes due to hydrogen etching. This work describes a route towards enhancing the stability of an ink-jet deposited barrier for high resolution patterning of high quality graphene. By modifying the etching kinetics under confinement, the barrier film could be stabilized and high resolution barriers could be retained even after 6 hours of graphene growth. Thus produced microscopic graphene devices exhibited an increase in conductivity by 6 orders of magnitude and a decrease in defectiveness by 48 times yielding performances that are superior to devices produced by traditional lithographical patterning which indicates the potential of our approach for future electronic applications.

Graphene has shown great potential as electrochemical electrodes in energy storage and sensor applications due to its unique combination of semiconducting and metallic properties. We here demonstrate that graphene’s semimetallic nature imparts it with a continuously tunable electrochemical reactivity. Extrinsic doping was shown to modify the reaction rate of graphene microelectrode arrays and a direct correlation between graphene’s linearly varying density of states and its electron transfer rate was established. Dynamic control of the charge transfer process enabled the variation of graphene’s reaction rate over 1 order of magnitude and was confirmed by a simple Gerischer–Marcus charge transfer kinetics model. The observed fine control over graphene’s electrochemical properties enabled a 2-fold increase in the resolution of an electrochemical impedance sensor. These results not only explain previous observations of graphene’s spatially varying electrochemical reactivity and highlight the importance of do...

The quality of CVD-grown graphene is intimately linked to the morphology of the employed catalyst. One commonly employed route to enhanced catalyst quality is recrystallization prior to graphene growth. The dimensions of a catalyst's single-crystalline domains, however, are limited by the stability of dislocations at the grain boundaries. We here employ a solid material in contact with the catalyst as a sink for dislocations. It was found that the interaction between Cu-foil and a solid cap significantly alters the recrystallization kinetics and achievable grain size. This development originates from an improved strain-relaxation of up to 1% during recrystallization which can support the formation of commonly unstable surface orientations. Correlation of diffraction measurements and atomic force microscopy reveals a direct impact of the strain relaxation on the decrease in copper surface roughness. Finally, we demonstrate the improvement of the quality of graphene grown on thus prepared foils through spectroscopic and carrier transport measurements.

Nanoscrolls are a class of nanostructures where atomic layers of 2D materials are stacked consecutively in a coaxial manner to form a 1D spiral topography. Self-assembly of chemical vapor deposition grown 2D WS2 monolayer into quasi-1D van der Waals scroll structure instigates a plethora of unique physiochemical properties significantly different from its 2D counterparts. The physical properties of such nanoscrolls can be greatly manipulated upon hybridizing them with high-quantum-yield colloidal quantum dots, forming 0D/2D structures. The efficient dissociation of excitons at the heterojunctions of QD/2D hybridized nanoscrolls exhibits a 3000-fold increased photosensitivity compared to the pristine 2D-material-based nanoscroll. The synergistic effects of confined geometry and efficient QD scatterers produce a nanocavity with multiple feedback loops, resulting in coherent lasing action with an unprecedentedly low lasing threshold. Predominant localization of the excitons along the circumference of this helical scroll results in a 12-fold brighter emission for the parallel-polarized transition compared to the perpendicular one, as confirmed by finite-difference time-domain simulation. The versatility of hybridized nanoscrolls and their unique properties opens up a powerful route for not-yet-realized devices toward practical applications.

Lateral heterojunctions in two-dimensional (2D) materials have demonstrated potential for high-performance sensors because of the unique electrostatic conditions at the interface. The increased complexity of producing such structures, however, has prevented their widespread use. We here demonstrate the simple and scalable fabrication of heterojunctions by a one-step synthesis process that yields photodetectors with superior device performance. Catalytic conversion of a solid precursor at optimized conditions was found to produce lateral nanostructured junctions between graphene domains and 3 nm thin amorphous carbon films. Carrier transport in these heterojunctions was found to proceed by minimizing the path through the amorphous carbon barriers, which results in a self-selective Schottky emission process with high uniformity and low emission barriers. We demonstrate the potential of thus produced heterojunctions by realizing a photodetector that combines an ultrahigh detectivity of 10

Two-dimensional materials have shown great promise to enable novel wearable electronic devices ranging from sensors to energy generators. These developments are due to their high mechanical robustness, which allow them to retain high performance even at large deformations. Under these conditions, however, good electrical contacts become an important issue that cannot be addressed with conventional materials. Liquid metals could overcome this limitation by providing soft and compliant electrodes but to date no realistic heterointegration of nanomaterials and complex liquid metal contacts has been attempted. We here demonstrate the application of micrometer-sized electrical contacts to flexible, fragile and rough 2D materials using patterned liquid metal contacts. A novel deposition method enables the scalable and facile production of large arrays of contacts in arbitrary geometries. This ability permitted the single-step, fabrication-free and contamination-free production of concentric liquid metal-contacted graphene field effect transistors of comparable performance to traditional devices. We demonstrate that the contacts can be removed without damaging the 2D materials allowing the contacts to be reused. Finally, good contact could be made to complex morphologies and three-dimensional substrates, which highlights the potential of our approach to the characterization and application of nanomaterials in electronics.

We study convex optimization problems that feature low-rank matrix solutions. In such scenarios, non-convex methods offer significant advantages over convex methods due to their lower space complexity, as well as practical faster convergence. Under mild assumptions, these methods feature global convergence guarantees. In this paper, we extend the results on this matter by following a different path. We derive a non-Euclidean optimization framework in the non-convex setting that takes nonlinear gradient steps on the factors. Our framework enables the possibility to further exploit the underlying problem structures, such as sparsity or low-rankness on the factorized domain, or better dimensional dependence of the smoothness parameters of the objectives. We prove that the non-Euclidean methods enjoy the same rigorous guarantees as their Euclidean counterparts under appropriate assumptions. Numerical evidence with Fourier ptychography and FastText applications, using real data, shows that our approach can enhance solution quality, as well as convergence speed over the standard non-convex approaches.

Graphene, a monoatomic sheet of graphite, could find application as a transparent conducting film (TCF) in solar cells and electronic devices. Currently available graphene, however, does not meet industry requirements in conductivity for these uses. We here demonstrate that ad-layers, discontinuous graphene islands on top of a complete graphene layer, can significantly enhance graphene's TCF-performance. Two routes were employed to produce controllable densities of ad-layers. Electropolishing pretreatment of the catalyst was found to affect the ad-layer nucleation. Alternatively, variation of the growth parameters was employed to influence the density of ad-layer dimensions. By analyzing the coverage-dependent carrier transport, we identified a continuous enhancement even by small ad-layers. This surprising behavior was quantitatively explained by a high conductivity parallel current path through the ad-layers. The resulting ad-layer-covered graphene exhibits a high and tunable performance as a transparent conductor.

In this study, we investigated the effect of UV irradiation of graphene on the hydrothermal growth of ZnO nanowires (NWs). We found that the density of ZnO NWs varied with respect to the UV irradiation time, with the highest density of ZnO NWs being achieved on a graphene surface that had been irradiated for 10 min. Transmission electron microscopy revealed that the ZnO NWs grew vertically on the graphene. The hydrophobicity of the graphene decreased after UV irradiation, enhancing the seeding and hydrothermal growth of ZnO NWs. Raman spectroscopic analysis revealed that UV irradiation of the graphene produced defects, which served as trapping centers during the seeding process. When UV irradiation was applied for more than 10 min, the number of carbon atoms with dangling bonds decreased, thereby decreasing the resulting density of ZnO NWs on the graphene. This facile strategy for growing uniform ZnO NWs on chemically inert graphene after UV irradiation should be suitable for adoption in other growth systems, potentially allowing the development of novel hybrid systems for use in electronic and optoelectronic devices.

Correction for ‘Patterned liquid metal contacts for high density, stick-and-peel 2D material device arrays’ by Yen-Lin Chen et al., Nanoscale, 2018, 10, 14510–14515.

The purpose of the current study was to assess the relationship between drinking and severe physical and sexual victimization in a sample of 989 college women over five years. Participants completed a web-based survey each fall semester, beginning as first time incoming freshman, and continuing each year for five years. The survey was comprehensive in assessing drinking, victimization, and relevant covariates. Women were followed whether they remained at university or not. Prior year same type of severe victimization predicted current year victimization, both severe physical and sexual. However, prior year drinking did not predict current year severe victimization. Prior year severe sexual victimization predicted current year drinking. Our findings of a longitudinal relationship between severe sexual victimization and subsequent increases in drinking suggests that college women may be drinking to cope with negative sequelae that they experience as a result of the victimization. We did not find the same longitudinal relationship between drinking and severe physical or sexual victimization, suggesting that a reciprocal relationship does not exist between drinking and victimization among college women. We did find that severe sexual victimization decreased across college, suggesting that the year prior to and the first year of college may be a critical period for intervening to reduce risk for severe victimization.