Improved Strain Transfer Efficiency in Large-Area Two-Dimensional MoS2 Obtained by Gold-Assisted Exfoliation.

Strain engineering represents a pivotal approach to tailoring the optoelectronic properties of two-dimensional (2D) materials. However, typical bending experiments often encounter challenges, such as layer slippage and inefficient transfer of strain from the substrate to the 2D material, hindering the realization of their full potential. In our study, using molybdenum disulfide (MoS2) as a model 2D material, we have demonstrated that layers obtained through gold-assisted exfoliation on flexible polycarbonate substrates can achieve high-efficient strain transfer while also mitigating slippage effects, owing to the strong interfacial interaction established between MoS2 and gold. We employ differential reflectance and Raman spectroscopy for monitoring strain changes. We successfully applied uniaxial strains of up to 3% to trilayer MoS2, resulting in a notable energy shift of 168 meV. These values are comparable only to those obtained in encapsulated samples with organic polymers.

Electrical properties of disordered films of van der Waals semiconductor WS2 on paper.

One of the primary objectives in contemporary electronics is to develop sensors that are not only scalable and cost-effective but also environmentally sustainable. To achieve this goal, numerous experiments have focused on incorporating nanomaterial-based films, which utilize nanoparticles or van der Waals materials, on paper substrates. In this article, we present a novel fabrication technique for producing dry-abraded van der Waals films on paper, demonstrating outstanding electrical characteristics. We assess the quality and uniformity of these films by conducting a spatial resistivity characterization on a 5 × 5 cm2 dry-abraded WS2 film with an average thickness of 25 µm. Employing transfer length measurements with varying channel length-to-width ratios, we extract critical parameters, including sheet resistance and contact resistance. Notably, our findings reveal a resistivity approximately one order of magnitude lower than previous reports. The film's inherent disorder manifests as an asymmetric distribution of resistance values for specific geometries. We explore how this behavior can be effectively modeled through a random resistance network (RRN), which can reproduce the experimentally observed resistance distribution. Finally, we investigate the response of these devices under applied uniaxial strain and apply the RRN model to gain a deeper understanding of this process.

Strain-Enhanced Large-Area Monolayer MoS2 Photodetectors.

In this study, we show a direct correlation between the applied mechanical strain and an increase in monolayer MoS2 photoresponsivity. This shows that tensile strain can improve the efficiency of monolayer MoS2 photodetectors. The observed high photocurrent and extended response time in our devices are indicative that devices are predominantly governed by photogating mechanisms, which become more prominent with applied tensile strain. Furthermore, we have demonstrated that a nonencapsulated MoS2 monolayer can be used in strain-based devices for many cycles and extensive periods of time, showing endurance under ambient conditions without loss of functionality. Such robustness emphasizes the potential of MoS2 for further functionalization and utilization of different flexible sensors.

Large Biaxial Compressive Strain Tuning of Neutral and Charged Excitons in Single-Layer Transition Metal Dichalcogenides.

The absorption and emission of light in single-layer transition metal dichalcogenides are governed by the formation of excitonic quasiparticles. Strain provides a powerful technique to tune the optoelectronic properties of two-dimensional materials and thus to adjust their exciton energies. The effects of large compressive strain in the optical spectrum of two-dimensional (2D) semiconductors remain rather unexplored compared to those of tensile strain, mainly due to experimental constraints. Here, we induced large, uniform, biaxial compressive strain (∼1.2%) by cooling, down to 10 K, single-layer WS2, MoS2, WSe2, and MoSe2 deposited on polycarbonate substrates. We observed a significant strain-induced modulation of neutral exciton energies, with blue shifts up to 160 meV, larger than in any previous experiments. Our results indicate a remarkably efficient transfer of compressive strain, demonstrated by gauge factor values exceeding previous results and approaching theoretical expectations. At low temperatures, we investigated the effect of compressive strain on the resonances associated with the formation of charged excitons. In WS2, a notable reduction of gauge factors for charged compared to neutral excitons suggests an increase in their binding energy, which likely results from the effects of strain added to the influence of the polymeric substrate.

High-Throughput Mechanical Exfoliation for Low-Cost Production of van der Waals Nanosheets.

A method is presented for scaling up the production of flakes of van der Waals materials via mechanical exfoliation. Using a roll-to-roll setup and an automatized, massive parallel exfoliation process, adhesive tapes with a high density of nanosheets of van der Waals materials are produced. The technique allows for obtaining a good trade-off between large lateral size and excellent area scalability, while also maintaining low cost. The potential of the method is demonstrated through the successful fabrication of field effect transistors and flexible photodetectors in large batches. This low-cost method to produce large area films out of mechanically exfoliated flakes is very general, and it can be applied to a variety of substrates and van der Waals materials and, moreover, it can be used to combine different van der Waals materials on top of each other. Therefore, it is believed that this production method opens an interesting avenue for fabrication of low-cost devices while maintaining a good scalability and performance.

Approaching the Intrinsic Properties of Moiré Structures Using Atomic Force Microscopy Ironing.

Stacking monolayers of transition metal dichalcogenides (TMDs) has led to the discovery of a plethora of new exotic phenomena, resulting from moiré pattern formation. Due to the atomic thickness and high surface-to-volume ratio of heterostructures, the interfaces play a crucial role. Fluctuations in the interlayer distance affect interlayer coupling and moiré effects. Therefore, to access the intrinsic properties of the TMD stack, it is essential to obtain a clean and uniform interface between the layers. Here, we show that this is achieved by ironing with the tip of an atomic force microscope. This post-stacking procedure dramatically improves the homogeneity of the interfaces, which is reflected in the optical response of the interlayer exciton. We demonstrate that ironing improves the layer coupling, enhancing moiré effects and reducing disorder. This is crucial for the investigation of TMD heterostructure physics, which currently suffers from low reproducibility.

Direct Magnetic Evidence, Functionalization, and Low-Temperature Magneto-Electron Transport in Liquid-Phase Exfoliated FePS3.

Magnetism and the existence of magnetic order in a material is determined by its dimensionality. In this regard, the recent emergence of magnetic layered van der Waals (vdW) materials provides a wide playground to explore the exotic magnetism arising in the two-dimensional (2D) limit. The magnetism of 2D flakes, especially antiferromagnetic ones, however, cannot be easily probed by conventional magnetometry techniques, being often replaced by indirect methods like Raman spectroscopy. Here, we make use of an alternative approach to provide direct magnetic evidence of few-layer vdW materials, including antiferromagnets. We take advantage of a surfactant-free, liquid-phase exfoliation (LPE) method to obtain thousands of few-layer FePS3 flakes that can be quenched in a solvent and measured in a conventional SQUID magnetometer. We show a direct magnetic evidence of the antiferromagnetic transition in FePS3 few-layer flakes, concomitant with a clear reduction of the Néel temperature with the flake thickness, in contrast with previous Raman reports. The quality of the LPE FePS3 flakes allows the study of electron transport down to cryogenic temperatures. The significant through-flake conductance is sensitive to the antiferromagnetic order transition. Besides, an additional rich spectra of electron transport excitations, including secondary magnetic transitions and potentially magnon-phonon hybrid states, appear at low temperatures. Finally, we show that the LPE is additionally a good starting point for the mass covalent functionalization of 2D magnetic materials with functional molecules. This technique is extensible to any vdW magnetic family.

Biodegradable albumen dielectrics for high-mobility MoS2 phototransistors.

This work demonstrates the fabrication and characterization of single-layer MoS2 field-effect transistors using biodegradable albumen (chicken eggwhite) as gate dielectric. By introducing albumen as an insulator for MoS2 transistors high carrier mobilities (up to ~90 cm2 V-1 s-1) are observed, which is remarkably superior to that obtained with commonly used SiO2 dielectric which we attribute to ionic gating due to the formation of an electric double layer in the albumen MoS2 interface. In addition, the investigated devices are characterized upon illumination, observing responsivities of 4.5 AW-1 (operated in photogating regime) and rise times as low as 52 ms (operated in photoconductivity regime). The presented study reveals the combination of albumen with van der Waals materials for prospective biodegradable and biocompatible optoelectronic device applications. Furthermore, the demonstrated universal fabrication process can be easily adopted to fabricate albumen-based devices with any other van der Waals material.

Enhanced Field-Effect Control of Single-Layer WS2 Optical Features by hBN Full Encapsulation.

The field-effect control of the electrical and optical properties of two-dimensional (2D) van der Waals semiconductors (vdW) is one important aspect of this novel class of materials. Thanks to their reduced thickness and decreased screening, electric fields can easily penetrate in a 2D semiconductor and thus modulate their charge density and their properties. In literature, the field effect is routinely used to fabricate atomically thin field-effect transistors based on 2D semiconductors. Apart from the tuning of the electrical transport, it has been demonstrated that the field effect can also be used to modulate the excitonic optical emission of 2D transition metal dichalcogenides such as MoS2 or WSe2. In this paper, we present some recent experiments on the field-effect control of the optical and excitonic properties of the monolayer WS2. Using the deterministic transfer of van der Waals materials, we fabricate planar single-layer WS2 devices contacted by a gold electrode and partially sandwiched between two insulating hexagonal boron nitride (hBN) flakes. Thanks to the planar nature of the device, we can optically access both the hBN encapsulated and the unencapsulated WS2 regions and compare the field-effect control of the exciton population in the two cases. We find that the encapsulation strongly increases the range of tunability of the optical emission of WS2, allowing us to tune the photoluminescence emission from excitons-dominated to trions-dominated. We also discuss how the full encapsulation of WS2 with hBN helps reduce spurious hysteretic effects in the field-effect control of the optical properties, similar to what has been reported for 2D vdW field-effect transistors.

Improving the conductivity of graphite-based films by rapid laser annealing.

We present a method to anneal devices based on graphite films on paper and polycarbonate substrates. The devices are created using four different methods: spray-on films, graphite pencil-drawn films, liquid-phase exfoliated graphite films, and graphite powder abrasion-applied films. We characterize the optical properties of the films before and after laser annealing and report the two-terminal resistance of the devices for increased laser power density. We find the greatest improvement (16× reduction) in the resistance of spray-on film devices starting from 25.0 kΩ and reaching 1.6 kΩ at the highest annealing power densities. These improvements are attributed to local laser ablation of binders, stabilizers, and solvent residues left in the film after fabrication. This work highlights the utility of focused laser annealing for spray-on, drawn, printed, and abrasion fabricated films on substrates sensitive to heat/thermal treatments.

Waveguide-Integrated MoTe2 p-i-n Homojunction Photodetector.

Two-dimensional (2D) materials, featuring distinctive electronic and optical properties and dangling-bond-free surfaces, are promising for developing high-performance on-chip photodetectors in photonic integrated circuits. However, most of the previously reported devices operating in the photoconductive mode suffer from a high dark current or a low responsivity. Here, we demonstrate a MoTe2 p-i-n homojunction fabricated directly on a silicon photonic crystal (PC) waveguide, which enables on-chip photodetection with ultralow dark current, high responsivity, and fast response speed. The adopted silicon PC waveguide is electrically split into two individual back gates to selectively dope the top regions of the MoTe2 channel in p- or n-types. High-quality reconfigurable MoTe2 (p-i-n, n-i-p, n-i-n, p-i-p) homojunctions are realized successfully, presenting rectification behaviors with ideality factors approaching 1.0 and ultralow dark currents less than 90 pA. Waveguide-assisted MoTe2 absorption promises a sensitive photodetection in the telecommunication O-band from 1260 to 1340 nm, though it is close to MoTe2's absorption band-edge. A competitive photoresponsivity of 0.4 A/W is realized with a light on/off current ratio exceeding 104 and a record-high normalized photocurrent-to-dark-current ratio of 106 mW-1. The ultrasmall capacitance of p-i-n homojunction and high carrier mobility of MoTe2 promise a high dynamic response bandwidth close to 34.0 GHz. The proposed device geometry has the advantages of employing a silicon PC waveguide as the back gates to build a 2D material p-i-n homojunction directly and simultaneously to enhance light-2D material interaction. It provides a potential pathway to develop 2D material-based photodetectors, laser diodes, and electro-optic modulators on silicon photonic chips.

An ultraminiaturized spectrometer.

Scaling down spectrometers could allow their application in consumer devices.

Combining Freestanding Ferroelectric Perovskite Oxides with Two-Dimensional Semiconductors for High Performance Transistors.

We demonstrate the fabrication of field-effect transistors based on single-layer MoS2 and a thin layer of BaTiO3 (BTO) dielectric, isolated from its parent epitaxial template substrate. Thin BTO provides an ultrahigh-κ gate dielectric effectively screening Coulomb scattering centers. These devices show mobilities substantially larger than those obtained with standard SiO2 dielectrics and comparable with values obtained with hexagonal boron nitride, a dielectric employed for fabrication of high-performance two-dimensional (2D) based devices. Moreover, the ferroelectric character of BTO induces a robust hysteresis of the current vs gate voltage characteristics, attributed to its polarization switching. This hysteresis is strongly suppressed when the device is warmed up above the tetragonal-to-cubic transition temperature of BTO that leads to a ferroelectric-to-paraelectric transition. This hysteretic behavior is attractive for applications in memory storage devices. Our results open the door to the integration of a large family of complex oxides exhibiting strongly correlated physics in 2D-based devices.

Broadband-tunable spectral response of perovskite-on-paper photodetectors using halide mixing.

Paper offers a low-cost and widely available substrate for electronics. It possesses alternative characteristics to silicon, as it shows low density and high flexibility, together with biodegradability. Solution processable materials, such as hybrid perovskites, also present light and flexible features, together with a huge tunability of the material composition with varying optical properties. In this study, we combine paper substrates with halide-mixed perovskites for the creation of low-cost and easy-to-prepare perovskite-on-paper photodetectors with a broadband-tunable spectral response. From the bandgap tunability of halide-mixed perovskites we create photodetectors with a cut-off spectral onset that ranges from the NIR to the green region, by increasing the bromide content on MAPb(I1-xBrx)3 perovskite alloys. The devices show a fast and efficient response. The best performances are observed for pure I and Br perovskite compositions, with a maximum responsivity of ∼400 mA W-1 on the MAPbBr3 device. This study provides an example of the wide range of possibilities that the combination of solution processable materials with paper substrates offers for the development of low-cost, biodegradable and easy-to-prepare devices.

Paper-based broadband flexible photodetectors with van der Waals materials.

Layered metal chalcogenide materials are exceptionally appealing in optoelectronic devices thanks to their extraordinary optical properties. Recently, their application as flexible and wearable photodetectors have received a lot of attention. Herein, broadband and high-performance paper-based PDs were established in a very facile and inexpensive method by rubbing molybdenum disulfide and titanium trisulfide crystals on papers. Transferred layers were characterized by SEM, EDX mapping, and Raman analyses, and their optoelectronic properties were evaluated in a wavelength range of 405-810 nm. Although the highest and lowest photoresponsivities were respectively measured for TiS3 (1.50 mA/W) and MoS2 (1.13 µA/W) PDs, the TiS3-MoS2 heterostructure not only had a significant photoresponsivity but also showed the highest on/off ratio (1.82) and fast response time (0.96 s) compared with two other PDs. This advantage is due to the band offset formation at the heterojunction, which efficiently separates the photogenerated electron-hole pairs within the heterostructure. Numerical simulation of the introduced PDs also confirmed the superiority of TiS3-MoS2 heterostructure over the other two PDs and exhibited a good agreement with the experimental results. Finally, MoS2 PD demonstrated very high flexibility under applied strain, but TiS3 based PDs suffered from its fragility and experience a remarkable drain current reduction at strain larger than ± 0.33%. However, at lower strains, all PDs displayed acceptable performances.

Chip-integrated van der Waals PN heterojunction photodetector with low dark current and high responsivity.

Two-dimensional materials are attractive for constructing high-performance photonic chip-integrated photodetectors because of their remarkable electronic and optical properties and dangling-bond-free surfaces. However, the reported chip-integrated two-dimensional material photodetectors were mainly implemented with the configuration of metal-semiconductor-metal, suffering from high dark currents and low responsivities at high operation speed. Here, we report a van der Waals PN heterojunction photodetector, composed of p-type black phosphorous and n-type molybdenum telluride, integrated on a silicon nitride waveguide. The built-in electric field of the PN heterojunction significantly suppresses the dark current and improves the responsivity. Under a bias of 1 V pointing from n-type molybdenum telluride to p-type black phosphorous, the dark current is lower than 7 nA, which is more than two orders of magnitude lower than those reported in other waveguide-integrated black phosphorus photodetectors. An intrinsic responsivity up to 577 mA W-1 is obtained. Remarkably, the van der Waals PN heterojunction is tunable by the electrostatic doping to further engineer its rectification and improve the photodetection, enabling an increased responsivity of 709 mA W-1. Besides, the heterojunction photodetector exhibits a response bandwidth of ~1.0 GHz and a uniform photodetection over a wide spectral range, as experimentally measured from 1500 to 1630 nm. The demonstrated chip-integrated van der Waals PN heterojunction photodetector with low dark current, high responsivity and fast response has great potentials to develop high-performance on-chip photodetectors for various photonic integrated circuits based on silicon, lithium niobate, polymer, etc.

Eco-Friendly Disposable WS2 Paper Sensor for Sub-ppm NO2 Detection at Room Temperature.

We developed inexpensive and disposable gas sensors with a low environmental footprint. This approach is based on a biodegradable substrate, paper, and features safe and nontoxic electronic materials. We show that abrasion-induced deposited WS2 nanoplatelets on paper can be employed as a successful sensing layer to develop high-sensitivity and selective sensors, which operate even at room temperature. Its performance is investigated, at room temperature, against NO2 exposure, finding that the electrical resistance of the device drops dramatically upon NO2 adsorption, decreasing by ~42% (~31% half a year later) for 0.8 ppm concentration, and establishing a detection limit around~2 ppb (~3 ppb half a year later). The sensor is highly selective towards NO2 gas with respect to the interferents NH3 and CO, whose responses were only 1.8% (obtained for 30 ppm) and 1.5% (obtained for 8 ppm), respectively. Interestingly, an improved response of the developed sensor under humid conditions was observed (tested for 25% relative humidity at 23 °C). The high-performance, in conjunction with its small dimensions, low cost, operation at room temperature, and the possibility of using it as a portable system, makes this sensor a promising candidate for continuous monitoring of NO2 on-site.

Strongly Anisotropic Strain-Tunability of Excitons in Exfoliated ZrSe3.

The effect of uniaxial strain on the band structure of ZrSe3 , a semiconducting material with a marked in-plane structural anisotropy, is studied. By using a modified three-point bending test apparatus, thin ZrSe3 flakes are subjected to uniaxial strain along different crystalline orientations monitoring the effect of strain on their optical properties through micro-reflectance spectroscopy. The obtained spectra show excitonic features that blueshift upon uniaxial tension. This shift is strongly dependent on the direction along which the strain is being applied. When the flakes are strained along the b-axis, the exciton peak shifts at ≈60-95 meV %-1 , while along the a-axis, the shift only reaches ≈0-15 meV %-1 . Ab initio calculations are conducted to study the influence of uniaxial strain, applied along different crystal directions, on the band structure and reflectance spectra of ZrSe3 , exhibiting a remarkable agreement with the experimental results.

Integrating superconducting van der Waals materials on paper substrates.

Paper has the potential to dramatically reduce the cost of electronic components. In fact, paper is 10 000 times cheaper than crystalline silicon, motivating the research to integrate electronic materials on paper substrates. Among the different electronic materials, van der Waals materials are attracting the interest of the scientific community working on paper-based electronics because of the combination of high electrical performance and mechanical flexibility. Up to now, different methods have been developed to pattern conducting, semiconducting and insulating van der Waals materials on paper but the integration of superconductors remains elusive. Here, the deposition of NbSe2, an illustrative van der Waals superconductor, on standard copy paper is demonstrated. The deposited NbSe2 films on paper display superconducting properties (e.g. observation of Meissner effect and resistance drop to zero-resistance state when cooled down below its critical temperature) similar to those of bulk NbSe2.