Piezophototronic Effect in Single-Atomic-Layer MoS2 for Strain-Gated Flexible Optoelectronics

Theoretical study of piezoelectric and light absorption properties, and carrier mobilities of Janus TiPX (X = F, Cl, and Br) monolayers

Janus TiPX (X = F, Cl, and Br) monolayers were systematically investigated through first-principles calculations. The Janus TiPX monolayers exhibit mechanical and dynamic stability. Two monolayers are indirect bandgap semiconductors, except the TiPBr monolayer, which has the features of a quasi-direct bandgap semiconductor. Biaxial strain can modify the band gap of single layers. The Janus TiPX monolayers have remarkable flexibility and piezoelectric properties. In particular, the TiPF monolayer shows high horizontal (44.18 pm V-1) and vertical piezoelectric coefficients (-3.59 pm V-1). These values exceed those of conventional bulk materials, like GaN (3.1 pm V-1) and α-quartz (2.3 pm V-1). All of the monolayers have absorption coefficients of 105 cm-1 for visible and ultraviolet (UV) light, which are one order of magnitude greater than that of MoSSe. Furthermore, TiPX monolayers have high carrier mobility. Janus TiPX monolayers represent a class of two-dimensional (2D) materials with exceptional properties and multifunctionality, holding significant promise for various applications in piezoelectric sensors, solar cells, and nano-electronic devices.

Manipulating 2D Materials through Strain Engineering

This review explores the growing interest in 2D layered materials, such as graphene, h-BN, transition metal dichalcogenides (TMDs), and black phosphorus (BP), with a specific focus on recent advances in strain engineering. Both experimental and theoretical results are delved into, highlighting the potential of strain to modulate physical properties, thereby enhancing device performance. Various strain engineering methods are summarized, and the impact of strain on the electrical, optical, magnetic, thermal, and valleytronic properties of 2D materials is thoroughly examined. Finally, the review concludes by addressing potential applications and challenges in utilizing strain engineering for functional devices, offering valuable insights for further research and applications in optoelectronics, thermionics, and spintronics.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Directionally-Resolved Phononic Properties of Monolayer 2D Molybdenum Ditelluride (MoTe2) under Uniaxial Elastic Strain

Understanding the phonon characteristics of two-dimensional (2D) molybdenum ditelluride (MoTe2) under strain is critical to manipulating its multiphysical properties. Although there have been numerous computational efforts to elucidate the strain-coupled phonon properties of monolayer MoTe2, empirical validation is still lacking. In this work, monolayer 1H-MoTe2 under uniaxial strain is studied via in situ micro-Raman spectroscopy. Directionally dependent monotonic softening of the doubly degenerate in-plane E2g1 phonon mode is observed with increasing uniaxial strain, where the E2g1 peak red-shifts -1.66 ± 0.04 cm-1/% along the armchair direction and -0.80 ± 0.07 cm-1/% along the zigzag direction. The corresponding Grüneisen parameters are calculated to be 1.09 and 0.52 along the armchair and zigzag directions, respectively. This work provides the first empirical quantification and validation of the orientation-dependent strain-coupled phonon response in monolayer 1H-MoTe2 and serves as a benchmark for other prototypical 2D transition-metal tellurides.

Photoluminescence of Two-Dimensional MoS2 Nanosheets Produced by Liquid Exfoliation

Extraordinary properties of two-dimensional materials make them attractive for applications in different fields. One of the prospective niches is optical applications, where such types of materials demonstrate extremely sensitive performance and can be used for labeling. However, the optical properties of liquid-exfoliated 2D materials need to be analyzed. The purpose of this work is to study the absorption and luminescent properties of MoS₂ exfoliated in the presence of sodium cholate, which is the most often used surfactant. Ultrasound bath and mixer-assisted exfoliation in water and dimethyl sulfoxide were used. The best quality of MoS₂ nanosheets was achieved using shear-assisted liquid-phase exfoliation as a production method and sodium cholate (SC) as a surfactant. The photoluminescent properties of MoS₂ nanosheets varied slightly when changing the surfactant concentrations in the range C(SC) = 0.5-2.5 mg/mL. This work is of high practical importance for further enhancement of MoS₂ photoluminescent properties via chemical functionalization.

Emerging Trends in 2D TMDs Photodetectors and Piezo-Phototronic Devices

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.

Synthesis of Large-Area Single- to Few-Layered MoS2 on an Ionic Liquid Surface

Large-area fabrication of transition metal dichalcogenides via environmentally friendly and efficient processes has been a long-standing issue in the field of two-dimensional (2D) materials. Here, we report that single- to few-layered MoS₂ sheets with an average size of the order of micrometers have been successfully synthesized on an ionic liquid surface by a modified low-pressure chemical vapor deposition (LP-CVD) method without the assistance of catalysts. It is found that the MoS₂ sheets grown on the liquid substrate exhibit a complete molecular crystal structure, which is confirmed by transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy measurements. The interlayer spacing does not change significantly with the increase of the MoS₂ layers, corresponding to a layer-by-layer growth pattern. The growth mechanism of the MoS₂ sheets is presented according to the experimental results. The work provides a new and simple method of preparing more molecular crystals on liquid substrates and will contribute to further research in this field.

Piezoelectricity-modulated optical recombination dynamics of monolayer-MoS2/GaN-film heterostructures

Dynamic manipulation of optoelectronic responses by mechanical stimuli is promising for developing wearable electronics and human-machine interfacing. Although 2D-3D hybrid heterostructures can bring advancements in optoelectronics, their dynamic optical responses to external strains remain rarely studied. Here, we demonstrate the strain-tuned recombination dynamics of monolayer-MoS₂ and thin-film-GaN heterostructures. We find that optical excitons in the heterostructures, apart from trions, can be markedly modulated by strains. We argue that MoS₂ piezoelectric dipoles across the interfaces lead to curved band diagrams, in which optical excitons dissociate into spatially separated quasi-particles and concurrently relocate to the maxima of valence bands and the minima of conduction bands. With the increase in tensile strains, the photoluminescence (PL) intensity of the heterostructures shows quenched responses. Noticeably, the change in PL spectra strongly depends on the directions of the applied strains because of the lateral piezoelectric periodicity of MoS₂ flakes. This work not only helps in understanding the underlying physics of the decreased PL intensities upon applying strains but also demonstrates a feasible way (i.e., strains) to manipulate the PL efficiency of 2D-material-based optoelectronics.

Effect of MoSe2 nanoribbons with NW30 edge reconstructions on the electronic and catalytic properties by strain engineering

Monolayer transition metal dichalcogenides (TMDs), typical two-dimensional semiconductors, have been extensively studied for their extraordinary physical properties and utilized for nanoelectronics and optoelectronics. However, the finite samples and discontinuity in the synthesis process of TMD materials definitely induce defect edges in nanoribbons and greatly influence the device performance. Here, we systematically studied the atomic structures, energetic and mechanical stability, and electronic and catalytic properties of MoSe₂ nanoribbons on the basis of experiments. Clear benefits of ZZSe-Mo-NW30 edged nanoribbons were found to evidently increase the dynamic stability according to our first-principles calculations. Meanwhile, unsaturated Mo atoms at the edge sites induced local magnetic moments up to 0.54 μ_B and changed the chemical environments of adjacent Se atoms, which acted as active sites for the hydrogen evolution reaction (HER) with a lower onset potential of -0.04 eV. The external tensile strain on these nanoribbons can have negligible effects on the electronic and catalytic properties. The onset potential of the ZZSe-Mo-NW30 edged nanoribbons only changed 0.03 eV under critical tensile strain. The atomic-scale research of edge reconstructions in TMD materials provides new opportunities to modulate the synthesis mechanism for experiments and defect-engineering applications in electrochemical catalysts.

2D-materials-integrated optoelectromechanics: recent progress and future perspectives

The discovery of two-dimensional (2D) materials has gained worldwide attention owing to their extraordinary optical, electrical, and mechanical properties. Due to their atomic layer thicknesses, the emerging 2D materials have great advantages of enhanced interaction strength, broad operating bandwidth, and ultralow power consumption for optoelectromechanical coupling. The van der Waals (vdW) epitaxy or multidimensional integration of 2D material family provides a promising platform for on-chip advanced nano-optoelectromechanical systems (NOEMS). Here, we provide a comprehensive review on the nanomechanical properties of 2D materials and the recent advances of 2D-materials-integrated nano-electromechanical systems and nano-optomechanical systems. By utilizing active nanophotonics and optoelectronics as the interface, 2D active NOEMS and their coupling effects are particularly highlighted at the 2D atomic scale. Finally, we share our viewpoints on the future perspectives and key challenges of scalable 2D-materials-integrated active NOEMS for on-chip miniaturized, lightweight, and multifunctional integration applications.

Effect of the Substrate on MoS2 Monolayer Morphology: An Integrated Computational and Experimental Study

Synthesis of two-dimensional materials, specifically transition metal dichalcogenides (TMDs), with controlled lattice orientations is a major barrier to their industrial applications. Controlling the orientation of as-grown TMDs is critical for preventing the formation of grain boundaries, thus reaching their maximum mechanical and optoelectronic performance. Here, we investigated the role of the substrate's crystallinity in the growth orientation of 2D materials using reactive molecular dynamics (MD) simulations and verified with experimental growth using the chemical vapor deposition (CVD) technique. We considered MoS₂ as our model material and investigated its growth on crystalline and amorphous silica and sapphire substrates. We revealed the role of the substrate's energy landscape on the orientation of as-grown TMDs, where the presence of monolayer-substrate energy barriers perpendicular to the streamlines hinder the detachment of precursor nuclei from the substrate. We show that MoS₂ monolayers with controlled orientations could not be grown on the SiO₂ substrate and revealed that amorphization of the substrate changes the intensity and equilibrium distance of monolayer-substrate interactions. Our simulations indicate that 0° rotated MoS₂ is the most favorable configuration on a sapphire substrate, consistent with our experimental results. The experimentally validated computational results and insight presented in this study pave the way for the high-quality synthesis of TMDs for high-performance electronic and optoelectronic devices.

Local Structure of Sulfur Vacancies on the Basal Plane of Monolayer MoS2

The nature of the S-vacancy is central to controlling the electronic properties of monolayer MoS₂. Understanding the geometric and electronic structures of the S-vacancy on the basal plane of monolayer MoS₂ remains elusive. Here, operando S K-edge X-ray absorption spectroscopy shows the formation of clustered S-vacancies on the basal plane of monolayer MoS₂ under reaction conditions (H₂ atmosphere, 100-600 °C). First-principles calculations predict spectral fingerprints consistent with the experimental results. The Mo K-edge extended X-ray absorption fine structure shows the local structure as coordinatively unsaturated Mo with 4.1 ± 0.4 S atoms as nearest neighbors (above 400 °C in an H₂ atmosphere). Conversely, the 6-fold Mo-Mo coordination in the crystal remains unchanged. Electrochemistry confirms similar active sites for hydrogen evolution. The identity of the S-vacancy defect on the basal plane of monolayer MoS₂ is herein elucidated for applications in optoelectronics and catalysis.

Piezo-phototronic effect on photocatalysis, solar cells, photodetectors and light-emitting diodes

The piezo-phototronic effect (a coupling effect of piezoelectric, photoexcitation and semiconducting properties, coined in 2010) has been demonstrated to be an ingenious and robust strategy to manipulate optoelectronic processes by tuning the energy band structure and photoinduced carrier behavior. The piezo-phototronic effect exhibits great potential in improving the quantum yield efficiencies of optoelectronic materials and devices and thus could help increase the energy conversion efficiency, thus alleviating the energy shortage crisis. In this review, the fundamental principles and challenges of representative optoelectronic materials and devices are presented, including photocatalysts (converting solar energy into chemical energy), solar cells (generating electricity directly under light illumination), photodetectors (converting light into electrical signals) and light-emitting diodes (LEDs, converting electric current into emitted light signals). Importantly, the mechanisms of how the piezo-phototronic effect controls the optoelectronic processes and the recent progress and applications in the above-mentioned materials and devices are highlighted and summarized. Only photocatalysts, solar cells, photodetectors, and LEDs that display piezo-phototronic behavior are reviewed. Material and structural design, property characterization, theoretical simulation calculations, and mechanism analysis are then examined as strategies to further enhance the quantum yield efficiency of optoelectronic devices via the piezo-phototronic effect. This comprehensive overview will guide future fundamental and applied studies that capitalize on the piezo-phototronic effect for energy conversion and storage.

Recent Development of Multifunctional Sensors Based on Low-Dimensional Materials

With the demand for accurately recognizing human actions and environmental situations, multifunctional sensors are essential elements for smart applications in various emerging technologies, such as smart robots, human-machine interface, and wearable electronics. Low-dimensional materials provide fertile soil for multifunction-integrated devices. This review focuses on the multifunctional sensors for mechanical stimulus and environmental information, such as strain, pressure, light, temperature, and gas, which are fabricated from low-dimensional materials. The material characteristics, device architecture, transmission mechanisms, and sensing functions are comprehensively and systematically introduced. Besides multiple sensing functions, the integrated potential ability of supplying energy and expressing and storing information are also demonstrated. Some new process technologies and emerging research areas are highlighted. It is presented that optimization of device structures, appropriate material selection for synergy effect, and application of piezotronics and piezo-phototronics are effective approaches for constructing and improving the performance of multifunctional sensors. Finally, the current challenges and direction of future development are proposed.

Mechanomaterials: A Rational Deployment of Forces and Geometries in Programming Functional Materials

The knowledge of mechanics of materials has been extensively implemented in developing functional materials, giving rise to recent advances in soft actuators, flexible electronics, mechanical metamaterials, tunable mechanochromics, regenerative mechanomedicine, etc. While conventional mechanics of materials offers passive access to mechanical properties of materials in existing forms, a paradigm shift is emerging toward proactive programming of materials' functionality by leveraging the force-geometry-property relationships. Here, such a rising field is coined as "mechanomaterials". To profile the concept, the design principles in this field at four scales is first outlined, namely the atomic scale, the molecular scale, the manipulation of nanoscale materials, and the microscale design of structural materials. A variety of techniques have been recruited to deliver the multiscale programming of functional mechanomaterials, such as strain engineering, capillary assembly, topological interlocking, kirigami, origami, to name a few. Engineering optical and biological functionalities have also been achieved by implementing the fundamentals of mechanochemistry and mechanobiology. Nonetheless, the field of mechanomaterials is still in its infancy, with many open challenges and opportunities that need to be addressed. The authors hope this review can serve as a modest spur to attract more researchers to further advance this field.

Generating large out-of-plane piezoelectric properties of atomically thin MoS2via defect engineering

We calculated the piezoelectric properties of asymmetrically defected MoS₂ using density functional theory. By creating uneven numbers of defects on either side of two-dimensional MoS₂, the out-of-plane centrosymmetry of the charge distribution is clearly broken, and the out-of-plane piezoelectric response is induced. The largest out-of-plane piezoelectric response is associated with the highest defect ratio for MoS₂ to be semiconducting. We calculated the critical defect density of the metal-insulator transition of the asymmetrically defected MoS₂ to be 9.90 × 10¹⁴ cm^-2 and chemical formula MoS_1.22. The d₃₃ of the multilayer of optimally defected MoS₂ is found to be greater than those of AlN and ZnO, and in the same order of magnitude as lead zirconate titanate. All two-dimensional transition metal dichalcogenides can in principle be fabricated as piezoelectric with this approach. The required defect engineering is readily available with various types of ion irradiation or plasma treatment. By controlling the dose of the ion, the defect ratio and hence the piezoelectricity can be tuned. Such asymmetrically defected transition metal dichalcogenides can easily be integrated into two-dimensional transition metal dichalcogenide based devices, which is hard for conventional piezoelectric thin films to rival.

Spatial Mapping of Electrostatic Fields in 2D Heterostructures

In situ electron microscopy is an effective tool for understanding the mechanisms driving novel phenomena in 2D structures. However, due to practical challenges, it is difficult to address these technologically relevant 2D heterostructures with electron microscopy. Here, we use the differential phase contrast (DPC) imaging technique to build a methodology for probing local electrostatic fields during electrical operation with nanoscale spatial resolution in such materials. We find that, by combining a traditional DPC setup with a high-pass filter, we can largely eliminate electric fluctuations emanating from short-range atomic potentials. Using a method based on this filtering algorithm, a priori electric field expectations can be directly compared with experimentally derived values to readily identify inhomogeneities and potentially problematic regions. We use this platform to analyze the electric field and charge density distribution across layers of hBN and MoS₂.

Flexo-photoelectronic effect in n-type/p-type two-dimensional semiconductors and a deriving light-stimulated artificial synapse

Flexoelectricity and photoelectricity with their coupled effect (the so-called flexo-photoelectronic effect), are of increasing interest in the study of electronics and optoelectronics in van der Waals layered semiconductors. However, the related device design is severely restricted owing to the ambiguous underlying physical nature of flexo-photoelectronic effects originating from the co-manipulation of light and strain-gradients. Here, flexoelectric polarization and the flexo-photoelectronic effect of few-layered semiconductors have been multi-dimensionally investigated from high-resolution microscopic characterization on the nanoscale, physics analysis, and deriving a device design. We found that two back-to-back built-in electric fields form in bent InSe and WSe₂, and greatly modulate the transport behaviors of photogenerated carriers, further facilitating the separation of photogenerated electron-hole pairs and trapping the holes/electrons in InSe or WSe₂ channels, recorded in realtime by a home-made technique of lighting Kelvin probe force microscopy (KPFM). The slow release of trapped carriers contributes to the photoconductance relaxation after illumination. Utilizing the photoconductance relaxation, a light-stimulated artificial synapse based on the flexo-photoelectronic effect of bent InSe has been achieved. Significantly, all the pair-pulse facilitation (PPF) behavior, spike frequency-dependent excitatory post-synaptic current (EPSC) and the transition from short-term memory (STM) to long-term memory (LTM) have been successfully realized in this artificial synapse. This work adds to the investigation of flexo-photoelectronic effects on 2D optoelectronics, and moves towards the development of 2D neuromorphic electronics.

MoS2-Decorated/Integrated Carbon Fiber: Phase Engineering Well-Regulated Microwave Absorber

Phase engineering is an important strategy to modulate the electronic structure of molybdenum disulfide (MoS2). MoS2-based composites are usually used for the electromagnetic wave (EMW) absorber, but the effect of different phases on the EMW absorbing performance, such as 1T and 2H phase, is still not studied. In this work, micro-1T/2H MoS2 is achieved via a facile one-step hydrothermal route, in which the 1T phase is induced by the intercalation of guest molecules and ions. The EMW absorption mechanism of single MoS2 is revealed by presenting a comparative study between 1T/2H MoS2 and 2H MoS2. As a result, 1T/2H MoS2 with the matrix loading of 15% exhibits excellent microwave absorption property than 2H MoS2. Furthermore, taking the advantage of 1T/2H MoS2, a flexible EMW absorbers that ultrathin 1T/2H MoS2 grown on the carbon fiber also performs outstanding performance only with the matrix loading of 5%. This work offers necessary reference to improve microwave absorption performance by phase engineering and design a new type of flexible electromagnetic wave absorption material to apply for the portable microwave absorption electronic devices.

Strain-Modulated Photoelectric Responses from a Flexible α-In2Se3/3R MoS2 Heterojunction

Semiconducting piezoelectric α-In₂Se₃ and 3R MoS₂ have attracted tremendous attention due to their unique electronic properties. Artificial van der Waals (vdWs) heterostructures constructed with α-In₂Se₃ and 3R MoS₂ flakes have shown promising applications in optoelectronics and photocatalysis. Here, we present the first flexible α-In₂Se₃/3R MoS₂ vdWs p-n heterojunction devices for photodetection from the visible to near infrared region. These heterojunction devices exhibit an ultrahigh photoresponsivity of 2.9 × 10³ A W^-1 and a substantial specific detectivity of 6.2 × 10¹⁰ Jones under a compressive strain of - 0.26%. The photocurrent can be increased by 64% under a tensile strain of + 0.35%, due to the heterojunction energy band modulation by piezoelectric polarization charges at the heterojunction interface. This work demonstrates a feasible approach to enhancement of α-In₂Se₃/3R MoS₂ photoelectric response through an appropriate mechanical stimulus.

Revealing Electrical-Poling-Induced Polarization Potential in Hybrid Perovskite Photodetectors

Despite recent rapid advances in metal halide perovskites for use in optoelectronics, the fundamental understanding of the electrical-poling-induced ion migration, accounting for many unusual attributes and thus performance in perovskite-based devices, remain comparatively elusive. Herein, the electrical-poling-promoted polarization potential is reported for rendering hybrid organic-inorganic perovskite photodetectors with high photocurrent and fast response time, displaying a tenfold enhancement in the photocurrent and a twofold decrease in the response time after an external electric field poling. First, a robust meniscus-assisted solution-printing strategy is employed to facilitate the oriented perovskite crystals over a large area. Subsequently, the electrical poling invokes the ion migration within perovskite crystals, thus inducing a polarization potential, as substantiated by the surface potential change assessed by Kelvin probe force microscopy. Such electrical-poling-induced polarization potential is responsible for the markedly enhanced photocurrent and largely shortened response time. This work presents new insights into the electrical-poling-triggered ion migration and, in turn, polarization potential as well as into the implication of the latter for optoelectronic devices with greater performance. As such, the utilization of ion-migration-produced polarization potential may represent an important endeavor toward a wide range of high-performance perovskite-based photodetectors, solar cells, transistors, scintillators, etc.

Design and Integration of a Layered MoS2/GaN van der Waals Heterostructure for Wide Spectral Detection and Enhanced Photoresponse

Molybdenum disulfide (MoS₂) as a typical two-dimensional (2D) transition-metal dichalcogenide exhibits great potential applications for the next-generation nanoelectronics such as photodetectors. However, most MoS₂-based photodetectors hold obvious disadvantages including a narrow spectral response in the visible region, poor photoresponsivity, and slow response speed. Here, for the first time, we report the design of a two-dimensional MoS₂/GaN van der Waals (vdWs) heterostructure photodetector consisting of few-layer p-type MoS₂ and very thin n-type GaN flakes. Thanks to the good crystal quality of the 2D-GaN flake and the built-in electric field in the interface depletion region of the MoS₂/GaN p-n junction, photogenerated carriers can be rapidly separated and more excitons are collected by electrodes toward the high photoresponsivity of 328 A/W and a fast response time of 400 ms under the illumination of 532 nm light, which is seven times faster than pristine MoS₂ flake. Additionally, the response spectrum of the photodetector is also broadened to the UV region with a high photoresponsivity of 27.1 A/W and a fast response time of 300 ms after integrating with the 2D-GaN flake, exhibiting an advantageous synergetic effect. These excellent performances render MoS₂/GaN vdWs heterostructure photodetectors as promising and competitive candidates for next-generation optoelectronic devices.

Giant Piezoresistive Effect and Strong Bandgap Tunability in Ultrathin InSe upon Biaxial Strain

The ultrathin nature and dangling bonds free surface of 2D semiconductors allow for significant modifications of their bandgap through strain engineering. Here, thin InSe photodetector devices are biaxially stretched, finding, a strong bandgap tunability upon strain. The applied biaxial strain is controlled through the substrate expansion upon temperature increase and the effective strain transfer from the substrate to the thin InSe is confirmed by Raman spectroscopy. The bandgap change upon biaxial strain is determined through photoluminescence measurements, finding a gauge factor of up to ≈200 meV %-1. The effect of biaxial strain on the electrical properties of the InSe devices is further characterized. In the dark state, a large increase of the current is observed upon applied strain which gives a piezoresistive gauge factor value of ≈450-1000, ≈5-12 times larger than that of other 2D materials and of state-of-the-art silicon strain gauges. Moreover, the biaxial strain tuning of the InSe bandgap also translates in a strain-induced redshift of the spectral response of the InSe photodetectors with ΔE cut-off ≈173 meV at a rate of ≈360 meV %-1 of strain, indicating a strong strain tunability of the spectral bandwidth of the photodetectors.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Ultrafast nucleation and growth of high-quality monolayer MoSe2 crystals via vapor-liquid-solid mechanism

The controlled production of two-dimensional atomically thin transition metal dichalcogenides (TMDs) is fundamentally important for their device applications. However, the synthesis of large-area and high-quality TMD monolayers remains a challenge due to the lack of sufficient understanding of growth mechanisms, especially for the chemical vapor deposition (CVD). Here we report molten-salt assisted CVD growth of highly crystalline MoSe₂ monolayers via a novel vapor-liquid-solid (VLS) mechanism. Our results show that the growth rate of the VLS-grown monolayer MoSe₂ is about 40 times faster than that of MoSe₂ grown via the vapor-solid (VS) mechanism, which makes the fabrication of 100 μm domains for ∼2 min and a uniform monolayer film within 5 min. The ultrafast growth of monolayer MoSe₂ crystals benefits from the synergic effect of one-dimensional VLS growth and two-dimensional VS edge expansion. Moreover, these MoSe₂ monolayers exhibit high crystal quality and enhanced photoluminescence due to efficient Se-vacancy repairing by the doping of halogen atoms. These findings provide a new understanding of MoSe₂ growth and open up an opportunity for the rapid synthesis of high-quality TMD monolayers and heterostructures.

Recent Advances in Strain-Induced Piezoelectric and Piezoresistive Effect-Engineered 2D Semiconductors for Adaptive Electronics and Optoelectronics

The development of two-dimensional (2D) semiconductors has attracted widespread attentions in the scientific community and industry due to their ultra-thin thickness, unique structure, excellent optoelectronic properties and novel physics. The excellent flexibility and outstanding mechanical strength of 2D semiconductors provide opportunities for fabricated strain-sensitive devices and utilized strain tuning their electronic and optic-electric performance. The strain-engineered one-dimensional materials have been well investigated, while there is a long way to go for 2D semiconductors. In this review, starting with the fundamental theories of piezoelectric and piezoresistive effect resulted by strain, following we reviewed the recent simulation works of strain engineering in novel 2D semiconductors, such as Janus 2D and 2D-Xene structures. Moreover, recent advances in experimental observation of strain tuning PL spectra and transport behavior of 2D semiconductors are summarized. Furthermore, the applications of strain-engineered 2D semiconductors in sensors, photodetectors and nanogenerators are also highlighted. At last, we in-depth discussed future research directions of strain-engineered 2D semiconductor and related electronics and optoelectronics device applications.

Peculiar piezoelectricity of atomically thin planar structures

The emergence of piezoelectricity in two-dimensional (2D) materials has represented a milestone towards employing low-dimensional structures for future technologies. 2D piezoelectric materials possess unique and unprecedented characteristics that cannot be found in other morphologies; therefore, the applications of piezoelectricity can be substantially extended. By reducing the thickness into the 2D realm, piezoelectricity might be induced in otherwise non-piezoelectric materials. The origin of the enhanced piezoelectricity in such thin planes is attributed to the loss of centrosymmetry, altered carrier concentration, and change in local polarization and can be efficiently tailored via surface modifications. Access to such materials is important from a fundamental research point of view, to observe the extraordinary interactions between free charge carriers, phonons and photons, and also with respect to device development, for which planar structures provide the required compatibility with the large-scale fabrication technologies of integrated circuits. The existence of piezoelectricity in 2D materials presents great opportunities for applications in various fields of electronics, optoelectronics, energy harvesting, sensors, actuators and biotechnology. Additionally, 2D flexible nanostructures with superior piezoelectric properties are distinctive candidates for integration into nano-scale electromechanical systems. Here we fundamentally review the state of the art of 2D piezoelectric materials from both experimental and theoretical aspects and report the recent achievements in the synthesis, characterization and applications of these materials.

P-type Doping in Large-Area Monolayer MoS2 by Chemical Vapor Deposition

Molybdenum disulfide (MoS₂) with excellent properties has been widely reported in recent years. However, it is a great challenge to achieve p-type conductivity in MoS₂ because of its native stubborn n-type conductivity. Substitutional transition metal doping has been proved to be an effective approach to tune their intrinsic properties and enhance device performance. Herein, we report the growth of Nb-doping large-area monolayer MoS₂ by a one-step salt-assisted chemical vapor deposition method. Electrical measurements indicate that Nb doping suppresses n-type conductivity in MoS₂ and shows an ambipolar transport behavior after annealing under the sulfur atmosphere, which highlights the p-type doping effect via Nb, corresponding to the density functional theory calculations with Fermi-level shifting to valence band maximum. This work provides a promising approach of two-dimensional materials in electronic and optoelectronic applications.

Photoresponsive 2D polymeric Langmuir–Blodgett films of 2,3,6,7,10,11-hexaiminotriphenylene

A photoelectric polymeric film of 2,3,6,7,10,11-hexaiminotriphenylene (HATP) was prepared via aerobic oxidative polymerization using Langmuir–Blodgett (LB) technique.

Preparation of MoS2 nanoflowers with rich active sites as an efficient adsorbent for aqueous organic dyes

In this study, molybdenum disulfide (MoS₂) was used as an adsorbent to quickly and efficiently remove Rhodamine B (RhB) from wastewater.

Accurate Method To Determine the Mobility of Transition-Metal Dichalcogenides with Incomplete Gate Screening

van der Waals layered transition-metal dichalcogenides usually exhibit high contact resistance because of the induced Schottky barriers, which occur at nonideal metal-semiconductor contacts. These barriers usually contribute to an underestimation in the determination of mobility, when extracted by standard, two-terminal methods. Furthermore, in devices based on atomically thin materials, channels with thicknesses of up to a few layers cannot completely screen the applied gate bias, resulting in an incomplete potential drop over the channel; the resulting decreased field effect causes further underestimation of the mobility. We demonstrate a method based on Kelvin probe force microscopy, which allows us to extract the accurate semiconductor mobility and eliminates the effects of contact quality and/or screening ability. Our results reveal up to a sevenfold increase in mobility in a monolayer device.

Scaled-up Direct-Current Generation in MoS2 Multilayer-Based Moving Heterojunctions

Techniques for scaling-up the direct-current (dc) triboelectricity generation in MoS₂ multilayer-based Schottky nanocontacts are vital for exploiting the nanoscale phenomenon for real-world applications of energy harvesting and sensing. Here, we show that scaling-up the dc output can be realized by using various MoS₂ multilayer-based heterojunctions including metal/semiconductor (MS), metal/insulator (tens of nanometers)/semiconductor (MIS), and semiconductor/insulator (a few nanometers)/semiconductor (SIS) moving structures. It is shown that the tribo-excited energetic charge carriers can overcome the interfacial potential barrier by different mechanisms, such as thermionic emission, defect conduction, and quantum tunneling in the case of MS, MIS, and SIS moving structures. By tailoring the interface structure, it is possible to trigger electrical conduction resulting in optimized power output. We also show that the band bending in the surface-charged region of MoS₂ determines the direction of the dc power output. Our experimental results show that engineering the interface structure opens up new avenues for developing next-generation semiconductor-based mechanical energy conversion with high performance.

Growth and Strain Engineering of Trigonal Te for Topological Quantum Phases in Non-Symmorphic Chiral Crystals

Strained trigonal Te has been predicted to host Weyl nodes supported by a non-symmorphic chiral symmetry. Using low-pressure physical vapor deposition, we systematically explored the growth of trigonal Te nanowires with naturally occurring strain caused by curvature of the wires. Raman spectra and high mobility electronic transport attest to the highly crystalline nature of the wires. Comparison of Raman spectra for both straight and curved nanowires indicates a breathing mode that is significantly broader and shifted in frequency for the curved wires. Strain induced by curvature during growth therefore may provide a simple pathway to investigate topological phases in trigonal Te.

Piezotronics and Piezo-phototronics of Third Generation Semiconductor Nanowires

With the fast development of nanoscience and nanotechnology in the last 30 years, semiconductor nanowires have been widely investigated in the areas of both electronics and optoelectronics. Among them, representatives of third generation semiconductors, such as ZnO and GaN, have relatively large spontaneous polarization along their longitudinal direction of the nanowires due to the asymmetric structure in their c-axis direction. Two-way or multiway couplings of piezoelectric, photoexcitation, and semiconductor properties have generated new research areas, such as piezotronics and piezo-phototronics. In this review, an in-depth discussion of the mechanisms and applications of nanowire-based piezotronics and piezo-phototronics is presented. Research on piezotronics and piezo-phototronics has drawn much attention since the effective manipulation of carrier transport, photoelectric properties, etc. through the application of simple mechanical stimuli and, conversely, since the design of new strain sensors based on the strain-induced change in semiconductor properties.

Two-dimensional layered materials: from mechanical and coupling properties towards applications in electronics

With the increasing interest in nanodevices based on two-dimensional layered materials (2DLMs) after the birth of graphene, the mechanical and coupling properties of these materials, which play an important role in determining the performance and life of nanodevices, have drawn increasingly more attention. In this review, both experimental and simulation methods investigating the mechanical properties and behaviour of 2DLMs have been summarized, which is followed by the discussion of their elastic properties and failure mechanisms. For further understanding and tuning of their mechanical properties and behaviour, the influence factors on the mechanical properties and behaviour have been taken into consideration. In addition, the coupling properties between mechanical properties and other physical properties are summarized to help set up the theoretical blocks for their novel applications. Thus, the understanding of the mechanical and coupling properties paves the way to their applications in flexible electronics and novel electronics, which is demonstrated in the last part. This review is expected to provide in-depth and comprehensive understanding of mechanical and coupling properties of 2DLMs as well as direct guidance for obtaining satisfactory nanodevices from the aspects of material selection, fabrication processes and device design.

Hysteresis and its impact on characterization of mechanical properties of suspended monolayer molybdenum-disulfide sheets

The hysteresis phenomenon frequently arises in two-dimensional (2D) material nanoindentation, which is generally expected to be excluded from characterizing the elastic properties due to the imperfect elastic behaviour. However, the underlying mechanism of hysteresis and its effect on the characterization of the mechanical properties of 2D materials remain unclear. Cyclic loadings are exerted on the suspended monolayer molybdenum-disulfide (MoS2) films in atomic force microscopy (AFM) nanoindentation experiments. The elastic hysteresis loops are observed for most of the force-displacement curves. The friction/wear between the AFM silicon tip and the MoS2 monolayer is deemed to be dominant compared to the friction between the monolayer and the silicon dioxide substrate after the analysis, as determined using the finite element method (FEM) simulation. The loading force-displacement curves instead of the unloading curves have been used to deduce the elastic mechanical properties using a modified regression equation. The mean value of the obtained Young's modulus of monolayer MoS2, E, is equal to 209 ± 18 GPa, which is close to the inherent stiffness value, predicted by first principles calculation. Our results have confirmed that it is not obligatory to exclude the sample data with hysteresis behaviour for characterizing the elastic properties of 2D materials. In addition, all sample sheets have finally been penetrated and the mean breaking stress value, σmax, is 36.6 ± 0.9 GPa, determined using the radius value of the worn tip. Furthermore, the effect of the loading force and the shape/size of the suspended monolayer MoS2 sheets on the hysteresis behaviour in the 2D nanoindentation have also been analyzed and discussed, exhibiting interesting trends. Our findings provide guidance for the characterization of the mechanical properties of 2D materials using the AFM nanoindentation and the experimental samples with elastic hysteresis behaviour.

Regulation of Two-Dimensional Lattice Deformation Recovery

The lattice directly determines the electronic structure, and it enables controllably tailoring the properties by deforming the lattices of two-dimensional (2D) materials. Owing to the unbalanced electrostatic equilibrium among the dislocated atoms, the deformed lattice is thermodynamically unstable and would recover to the initial state. Here, we demonstrate that the recovery of deformed 2D lattices could be directly regulated via doping metal donors to reconstruct electrostatic equilibrium. Compared with the methods that employed external force fields with intrinsic instability and nonuniformity, the stretched 2D molybdenum diselenide (MoSe2) could be uniformly retained and permanently preserved via doping metal atoms with more outermost electrons and smaller electronegativity than Mo. We believe that the proposed strategy could open up a new avenue in directly regulating the atomic-thickness lattice and promote its practical applications based on 2D crystals.

A piezo-phototronic enhanced serrate-structured ZnO-based heterojunction photodetector for optical communication

ZnO-based heterojunction photodetectors have been widely used in various fields such as optical imaging and health monitoring. As for the traditional planar heterojunction interface, their limited optical absorption will place restrictions on the full photoelectric potential of ZnO nanorods, which severely restrains the commercial applications of ZnO-based photodetectors. Herein, using an intrinsically octahedral structure of p-type Cu2O and one-dimensional ZnO arrays, the newly designed serrate-structured heterojunction was constructed, whose unique serrate-structured interface of ZnO/Cu2O is highly conducive to the aggrandizing of optical absorption. The as-fabricated photodetector could achieve a high on/off ratio up to 1000 and an optimum photocurrent of 24.90 μA under 1.41 mW mm-2 (405 nm) illumination without bias voltage, which was 2.5 times higher than that of the planar-structured photodetector, and the response time was as quick as 1.6 ms. When the additional external strain was 0.39%, the performance was dramatically enhanced more than 5 times due to the synergism of the piezo-phototronic effect and the serrate-structured design. Based on this, we successfully developed designed photodetector arrays with an excellent optical communication performance of transmitting information. Prospectively, this kind of unique serrate-structured heterojunction design will open up a possible opportunity for high performance photodetectors based on structural engineering.

Enhanced NO2 gas sensing of a single-layer MoS2 by photogating and piezo-phototronic effects

NO₂ sensors with ultrahigh sensitivity are demanded for future electronic sensing systems. However, traditional sensors are considerably limited by the relative low sensitivity, high cost and complicated process. Here, we report a simply and reliable flexible NO₂ sensor based on single-layer MoS₂. The flexible sensor exhibits high sensitivity to NO₂ gas due to ultra-large specific surface area and the nature of two-dimensional (2D) semiconductor. When the NO₂ is 400 ppb (parts per billion), compared with the dark and strain-free conditions, the sensitivity of the single-layer sensor is enhanced to 671% with a 625 nm red light-emitting diode (LED) illumination of 4 mW/cm² power under 0.67% tensile strain. More important, the response time is dramatically reduced to ∼16 s and it only needs ∼65 s to complete 90% recovery. A theoretical model is proposed to discuss the microscopic mechanisms. We find that the remarkable sensing characteristics are the result of coupling among piezoelectricity, photoelectricity and adsorption-desorption induced charges transfer in the single-layer MoS₂ Schottky junction based device. Our work opens up the way to further enhancements in the sensitivity of gas sensor based on single-layer MoS₂ by introducing photogating and piezo-phototronic effects in mesoscopic systems.

A ternary SnS1.26Se0.76 alloy for flexible broadband photodetectors

Layered two-dimensional (2D) materials often display unique functionalities for flexible 2D optoelectronic device applications involving natural flexibility and tunable bandgap by bandgap engineering. Composition manipulation by alloying of these 2D materials represents an effective way in fulfilling bandgap engineering, which is particularly true for SnS_2xSe_2(1−x) alloys showing a continuous bandgap modulation from 2.1 eV for SnS₂ to 1.0 eV for SnSe₂. Here, we report that a ternary SnS_1.26Se_0.76 alloy nanosheet can serve as an efficient flexible photodetector, possessing excellent mechanical durability, reproducibility, and high photosensitivity. The photodetectors show a broad spectrum detection ranging from visible to near infrared (NIR) light. These findings demonstrate that the ternary SnS_1.26Se_0.76 alloy can act as a promising 2D material for flexible and wearable optoelectronic devices.

Bandgap engineering of a ternary SnS_1.26Se_0.76 alloy for flexible broadband photodetectors.

Near-Field Coupled Integrable Two-Dimensional InSe Photosensor on Optical Fiber

Two-dimensional (2D) van der Waals layered materials possess innate advantages as integrable sensors, due to their thinness, flexibility, and sensitivity. They can be seamlessly integrated onto surfaces with different geometries where detection for near-field signal is desired. In this study, we develop a device transfer technique to integrate device assemblies based on 2D materials onto an arbitrary smooth surface. Such technique utilizes a sacrificial polymer underlayer and achieves clean and nondestructive full device transfer. For demonstration, we transferred a complete 2D multilayer InSe photodetector device onto a stripped optical fiber. Due to the extreme vicinity of the 2D photodetector with the fiber core, the device can effectively couple with the evanescent field and accurately detect information transmitted inside the optical fiber. In addition, these super thin flexible device assemblies can be integrated onto the fibers themselves to non-invasively monitor the optical fiber performance. The demonstration of optically coupled, conformal 2D devices on substrates of different form factors can enable a variety of near-field optical and sensing applications.

Ultra-high performance flexible piezopotential gated In1-xSnxSe phototransistor

Flexible optoelectronic devices facilitated by the piezotronic effect have important applications in the near future in many different fields ranging from solid-state lighting to biomedicine. Two-dimensional materials possessing extraordinary mechanical strength and semiconducting properties are essential for realizing nanopiezotronics and piezo-phototronics. Here, we report the first demonstration of piezo-phototronic properties in In_1-xSn_xSe flexible devices by applying systematic mechanical strain under photoexcitation. Interestingly, we discover that the dark current and photocurrent are increased by five times under a bending strain of 2.7% with a maximum photoresponsivity of 1037 AW^-1. In addition, the device can act as a strain sensor with a strain sensitivity up to 206. Based on these values, the device outperforms the same class of devices in two-dimensional materials. The underlying mechanism responsible for the discovered behavior can be interpreted in terms of piezoelectric potential gating, allowing the device to perform like a phototransistor. The strain-induced gate voltage assists in the efficient separation of photogenerated charge carriers and enhances the mobility of In_1-xSn_xSe, resulting in good performance on a freeform surface. Thus, our multifunctional device is useful for the development of a variety of advanced applications and will help meet the demand of emerging technologies.

Tunable WSe2-CdS mixed-dimensional van der Waals heterojunction with a piezo-phototronic effect for an enhanced flexible photodetector

Due to the absence of bond fracture and atomic reconstruction under strain, vdWs structures hold great promise in flexible electronic/optoelectronic applications. Besides all-2D heterojunctions, the dangling-bond-free surfaces of 2D materials also enable vdWs interaction with other materials of different dimensionalities, forming mixed-dimensional vdWs heterostructures. Such structures allow a much broader selection of materials and may provide a promising approach to compensate for the intrinsic weakness of 2D crystals before realizing their full potential. In this study, we present the fabrication of a WSe2-CdS mixed-dimensional vdWs p-n heterojunction for flexible photodetection. A strain-tunable vdWs interface was demonstrated and the photoresponse was dramatically enhanced with the piezo-phototronic effect. The photocurrent can be increased by ∼110% under a compressive strain of -0.73% and the corresponding photoresponsivity reaches up to 33.4 A W-1. The enhancement originates from realigned local energy-band tilting at the WSe2-CdS interface by strain-induced piezopolarization, which promotes the transport process of photoexcited carriers. Our work provides a new route to a tunable vdWs interface other than with electrostatic gating, which may inspire the development of novel flexible vdWs optoelectronics.

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, 2D materials, wurtzite-type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.

Ultrathin Piezotronic Transistors with 2 nm Channel Lengths

Because silicon transistors are rapidly approaching their scaling limit due to short-channel effects, alternative technologies are urgently needed for next-generation electronics. Here, we demonstrate ultrathin ZnO piezotronic transistors with a ∼2 nm channel length using inner-crystal self-generated out-of-plane piezopotential as the gate voltage to control the carrier transport. This design removes the need for external gate electrodes that are challenging at nanometer scale. These ultrathin devices exhibit a strong piezotronic effect and excellent pressure-switching characteristics. By directly converting mechanical drives into electrical control signals, ultrathin piezotronic devices could be used as active nanodevices to construct the next generation of electromechanical devices for human-machine interfacing, energy harvesting, and self-powered nanosystems.

Piezotronic Effect Enhanced Flexible Humidity Sensing of Monolayer MoS2

We report the piezotronic effect on the performance of humidity detection based on a back-to-back Schottky contacted monolayer MoS₂ device. By introducing an upswept mechanical strain, the in-plane electrical polarization can be induced at the MoS₂/metal junction region. The polarization charges can modify the Schottky barrier height at the interface of MoS₂/metal junction, subsequently improving the sensitivity of the humidity sensing. An energy band diagram is proposed to explain the experiment phenomenon of the humidity sensor. This work provides a simple way to enhance the sensitivity of ultrathin two-dimensional-materials-based sensors by the piezotronic effect, which has great potential applications in electronic skin, human-computer interfacing, gas sensing, and environment monitoring.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.