Jing Zhang

A piezotronic graphene artificial synapse based on a piezopotential powered/modulated electrical double layer graphene field effect transistor is developed to correlate the spatiotemporal information of the external strains with the excitatory postsynaptic current. Typical properties of a neuron synapse, such as potentiation/inhibition, plasticity, paired‐pulse facilitation, and dynamic modulation functions, are demonstrated.

Abstract

The human somatosensory system, consisting of receptors, transmitters, and synapses, functions as the medium for external mechanical stimuli perception and sensing signal delivery/processing. Developing sophisticated artificial sensory synapses with a high performance, uncomplicated fabrication process, and low power consumption is still a great challenge. Here, a piezotronic graphene artificial sensory synapse developed by integrating piezoelectric nanogenerator (PENG) with an ion gel–gated transistor is demonstrated. The piezopotential originating from PENG can efficiently power the synaptic device due to the formation of electrical double layers at the interface of the ion gel/electrode and ion gel/graphene. Meanwhile, the piezopotential coupling is capable of linking the spatiotemporal strain information (strain amplitude and duration) with the postsynaptic current. The synaptic weights can be readily modulated by the strain pulses. Typical properties of a synapse including excitation/inhibition, synaptic plasticity, and paired pulse facilitation are successfully demonstrated. The dynamic modulation of a sensory synapse is also achieved based on dual perceptual presynaptic PENGs coupling to a single postsynaptic transistor. This work provides a new insight into developing piezotronic synaptic devices in neuromorphic computing, which is of great significance in future self‐powered electronic skin with artificial intelligence, a neuromorphic interface for neurorobotics, human–robot interaction, an intelligent piezotronic transistor, etc.

A piezoelectric‐effect‐enhanced full‐spectrum TiO₂/BTO/Ag₂O photoanode is demonstrated with a high photoelectrochemical performance. The BaTiO₃ nanolayer between the n‐TiO₂ and p‐Ag₂O heterojunction provides a polar charge‐created piezoelectric field, which reduces the recombination rate of the photocarriers generated by p‐Ag₂O and n‐TiO₂ in the UV–vis–NIR full spectrum.

Abstract

Photoelectrochemical (PEC) water splitting offers a promising strategy for converting solar energy to chemical fuels. Herein, a piezoelectric‐effect–enhanced full‐spectrum photoelectrocatalysis with multilayered coaxial titanium dioxide/barium titanate/silver oxide (TiO₂/BTO/Ag₂O) nanorod array as the photoanode is reported. The vertically grown nanorods ensure good electron conductivity, which enables fast transport of the photogenerated electrons. Significantly, the insertion of a piezoelectric BaTiO₃ (BTO) nanolayer at the p‐type Ag₂O and n‐type TiO₂ interface created a polar charge‐stabilized electrical field. It maintains a sustainable driving force that attract the holes of TiO₂ and the electrons of Ag₂O, resulting in greatly increased separation and inhibited recombination of the photogenerated carriers. Furthermore, Ag₂O as a narrow bandgap semiconductor has a high ultraviolet–visible–near infrared (UV–vis–NIR) photoelectrocatalytic activity. The TiO₂/BTO/Ag₂O, after poling, successfully achieves a prominent photocurrent density, as high as 1.8 mA cm⁻² at 0.8 V versus Ag/Cl, which is about 2.6 times the TiO₂ nanorod photoanode. It is the first time that piezoelectric BaTiO₃ is used for tuning the interface of p‐type and n‐type photoelectrocatalyst. With the enhanced light harvesting, efficient photogenerated electron–hole pairs' separation, and rapid charge transfer at the photoanode, an excellent photoelectrocatalytic activity is realized.

The combination of the exciting properties of graphene with those of monolayer tungsten disulfide (WS 2 ) makes this heterostack of great interest for electronic, optoelectronic and spintronic applications. The scalable synthesis of graphene/WS 2 heterostructures on technologically attractive substrates like SiO 2 would greatly facilitate the implementation of novel two-dimensional (2D) devices. In this work, we report the direct growth of monolayer WS 2 via chemical vapor deposition (CVD) on single-crystal graphene arrays on SiO 2 . Remarkably, spectroscopic and microscopic characterization reveals that WS 2 grows only on top of the graphene crystals so that the vertical heterostack is selectively obtained in a bottom-up fashion. Spectroscopic characterization indicates that, after WS 2 synthesis, graphene undergoes compressive strain and hole doping. Tailored experiments show that such hole doping is caused by...

The mechanism underlying the ideality evolution of p‐type organic field‐effect transistors in air is revealed. Water and oxygen in air can act as a double‐edged sword in determining the device ideality. In light of this mechanism, a near‐ideal behavior is achieved by selectively doping the contact areas and controlling the measurement environment.

Abstract

Organic field‐effect transistors (OFETs) often deviate from ideal behaviors in air, which masks their intrinsic properties and thus significantly impedes their practical applications. A key issue of how the presence of air affects the ideality of OFETs has not yet been fully understood. It is revealed that air atmosphere may exert a double‐edged sword effect on the active semiconductor layer when determining the ideality of OFETs fabricated from p‐type crystalline organic semiconductors. Upon exposing the as‐fabricated device to air, water and oxygen mainly function as efficient p‐type dopants for the active layer in the contact regions, enhancing charge carrier injection and consequently improving device ideality. Nevertheless, as the exposure time increases, the trapping centers for the injected minority charge carriers appear in the channel region, leading to degradation of device ideality. Inspired by the double‐edged sword behavior of air, a near‐ideal OFET is achieved by ingeniously utilizing the doping/positive effect and eliminating the trapping/negative effect. The effect of air on the ideality of p‐type OFETs is clarified, which not only illuminates some common observations of OFETs in air but also offers useful guidance for the construction of high‐performance ideal OFETs.

Charged excitons (trions), with a large binding energy (≈60 meV) in anisotropic, atomically thin rhenium dichalcogenides (ReS₂), are discovered by tuning carrier density. Strongly polarized anisotropic emission from these trions are also experimentally observed.

Abstract

Experimentally observed, stable trions with large binding energy (≈25 meV) in atomically thin monolayer 2D transition metal dichalcogenides MX₂ (M = Mo, W, X = S, Se, and Te) with an isotropic crystal structure have been extensively studied. In contrast, the characteristics of trions in atomically thin 2D materials with an anisotropic crystal structure are not completely understood. Low‐temperature photoluminescence (PL) spectroscopy in few‐layer ReS₂ with an anisotropic crystal structure by applying a gate voltage is described. A new PL peak that emerges below the lower‐energy side of neutral excitons obtained by tuning the gate voltages is attributed to emission from negative trions. Furthermore, the trion binding energy that is strongly dependent on the layer thickness reaches a large value of ≈60 meV in 1L–ReS₂, which is ≈2 times larger than that in other isotropic 2D materials (MX₂). The enhancement of the binding energy reflects the quasi‐1D nature of the trions in anisotropic atomically thin ReS₂. These experimental observations will promote a better understanding of the optical response and applications in new categories of the anisotropic atomically thin 2D materials with a quasi‐1D nature.

A multibeam interference model is developed to analyze irregular scattering‐type scanning near‐field optical microscopy images of polaritons induced by small sample size or complex edges. This model extracts the polariton wave vectors and ratio of scattering rate to reflectivity at edge, which is important for studying van der Waals nanomaterials smaller than 10 µm and designing integrated nanophotonic devices.

Abstract

Van der Waals (vdW) materials are among the most promising candidates for photonic integrated circuits because they support a full set of polaritons that can manipulate light at deep subdiffraction nanoscale. It is possible to directly probe the propagating polaritons in vdW materials in real space via scattering‐type scanning near‐field optical microscopy, such that the wave vector and lifetime of the polaritons can be extracted from as‐measured interference fringes by Fourier analysis. However, this method is unsuitable for clutter interference patterns in samples exhibiting inadequate fringes due to small size (less than 10 µm) or complex edges that are often encountered in nanophotonic devices and new material characterization. Here, a multibeam interference model is developed to analyze complex images by disentangling them into periodic patterns and residue. By employing phase stationary approximation, polariton wave vector can be derived from offset ratio of the center point, and the ratio of polariton reflection and scattering rates at the edge is obtained from the ratio of the periodic and aperiodic patterns. This method can be widely used in the optical characterization of new vdW materials that are difficult to synthesize into large crystals, as well as nanophotonic integrated devices with unique boundaries.

In article number https://doi.org/10.1002/adfm.2019050561905056, Xuelin Yang, Kaihui Liu, Bo Shen, and co‐workers achieve the epitaxy of single‐crystalline gallium nitride (GaN) film on a complementary metal‐oxide‐semiconductor‐compatible Si(100) substrate by applying a one‐atom‐thick single‐crystalline graphene buffer layer. The monolayer graphene provides an in‐plane driving force for the uniform alignment of nitrides domains. This apppoach can also enable the growth of wafer‐scale hexagonal single‐crystalline films on amorphous or flexible substrates.

The development of E‐skin with dual functionality, integrating strain detection and alarm into a single device, is crucial for health monitoring. A one‐step laser‐induced graphene (LIG)‐based E‐skin that can detect faint biosignals (respiration, pulse, etc.) and issue thermoacoustic sound to warn when detecting some abnormal conditions (sleep apnea, sudden cardiac arrest).

Abstract

Diseases such as cardiovascular problems and sleep apnea cause mass deaths annually due to a lack of timely and portable monitoring and alarm measures. Various wearable devices for health monitoring have been intensely researched to reduce mortality. However, these devices themselves can only detect physiological signals; they cannot sound an alarm. Therefore, they must rely on mobile phones or other peripheral devices such as speakers or vibration motors to sound an alarm, which may result in a patient missing the optimal treatment. It is valuable to develop a self‐alarm health monitoring device with the dual functions of physiological signal detection and sound alarm simultaneously. A one‐step laser‐induced graphene (LIG)‐based electronic skin (E‐skin) is fabricated to perform health monitoring and alarm at the same time, which benefit from its both excellent mechanical and acoustical performance. These customized shutter‐patterned E‐skins have an ultrahigh sensitivity of 316.3 and can detect various biosignals such as wrist pulse, respiratory, etc. They also have a self‐alarm function and can sound an alarm when detecting abnormal situations. This study addresses the multifunctional integration required for multisensors, which will open further applications in wearable sensors and health‐care devices.

Light‐emitting field‐effect transistors are optoelectronic devices that combine switching and amplification with light emission. They can be created with a wide range of semiconductors from organic to inorganic and even nanoscale materials. Their unique structure and properties enable applications that include plasmonic or photonic interactions as well as optical memory.

Abstract

Light‐emitting field‐effect transistors (LEFETs) combine switching and amplification with light emission and thus represent an interesting optoelectronic device. They are not limited anymore to a few examples and specific materials but are nearly universal for a wide range of semiconductors, from organic to inorganic and nanoscale. This review introduces the basic working principles of lateral unipolar and ambipolar LEFETs and discusses recent examples based on various solution‐processed semiconducting materials. Applications beyond simple light emission are presented and possible future directions for light‐emitting transistors with added functionalities are outlined.

Any conventional garment or textile can be transformed into a waterproof, breathable, antibacterial, and self‐powered e‐textile by embroidering over their surface omniphobic triboelectric nanogenerators (R^F‐TENGs) with highly networked silver nanoflake electrodes. The high output power density and touch sensitivity of R^F‐TENGs and their compatibility with large‐scale production processes enable the cost‐effective fabrication of washable e‐textiles for emerging human–machine interface applications.

Abstract

Multifunctional electronic textiles (e‐textiles) incorporating miniaturized electronic devices will pave the way toward a new generation of wearable devices and human–machine interfaces. Unfortunately, the development of e‐textiles is subject to critical challenges, such as battery dependence, breathability, satisfactory washability, and compatibility with mass production techniques. This work describes a simple and cost‐effective method to transform conventional garments and textiles into waterproof, breathable, and antibacterial e‐textiles for self‐powered human–machine interfacing. Combining embroidery with the spray‐based deposition of fluoroalkylated organosilanes and highly networked nanoflakes, omniphobic triboelectric nanogenerators (R^F‐TENGs) can be incorporated into any fiber‐based textile to power wearable devices using energy harvested from human motion. R^F‐TENGs are thin, flexible, breathable (air permeability 90.5 mm s⁻¹), inexpensive to fabricate (<0.04$ cm⁻²), and capable of producing a high power density (600 µW cm⁻²). E‐textiles based on R^F‐TENGs repel water, stains, and bacterial growth, and show excellent stability under mechanical deformations and remarkable washing durability under standard machine‐washing tests. Moreover, e‐textiles based on R^F‐TENGs are compatible with large‐scale production processes and exhibit high sensitivity to touch, enabling the cost‐effective manufacturing of wearable human–machine interfaces.

We propose and analyze a mechanism for inducing spin Hall currents in ordinary (1H phase) monolayer transition metal dichalcogenides (TMDs) due to the nonlinear process of optical rectification. The photo-induced spin current is proportional to the light intensity, and originates from the intrinsic spin–orbit coupling in TMDs. The spin current spectrum is strongly influenced by electron–hole interactions, i.e. excitonic effects, analogous to the optical absorption. Remarkably, excitons change the temperature dependence of the induced spin current, to the point that the current direction can even be reversed by varying the temperature. This peculiar excitonic behavior is shown to emerge from the relative strength of two distinct mechanisms contributing to the optical response, i.e. a purely interband part and a mixed inter/intraband contribution. Our findings pave the path to the generation of dc spin currents in ordinary TMDs without external static electric or magnetic fields.

We studied the nucleation and growth of hexagonal BN (h-BN) on AlN template on c -plane sapphire by metalorganic vapor phase epitaxy as functions of growth temperature, deposition time, and triethylboron (TEB) partial pressure. A lateral growth rate of about 25 nm min −1 for h-BN nuclei was obtained by atomic force microscopy and a nucleation activation energy of 2.1 eV was extracted from the temperature dependence of the nucleation density. A large TEB flow rate strongly enhances the formation of h-BN nuclei. At a reduced TEB flow rate, we observed a significantly decreased nuclei density and a delay in nucleation due to TEB desorption. By fine tuning the growth parameters, single-crystalline multilayer h-BN was successfully formed on AlN surface, as confirmed by x-ray diffraction and transmission electron microscopy (TEM). The epitaxial relationship between h-BN and AlN was [0 0 0 1] h-BN || [0 0 0 1] AlN and [1 0   −1 0] h-BN || [...

Large‐area Bi₂O₂Se nanosheets are synthesized using a modified chemical vapor deposition method and a face‐down approach. A device fabricated with this material has a detection wavelength covering the ultraviolet–visible–near‐infrared range a photoresponsivity of 108 696 A W⁻¹ at 360 nm, an external quantum efficiency of 1.5 × 10⁷% at 405 nm, and a rise time reaches 32 µs. This is due to a combination of photogating, photovoltaic, and photothermal effects.

Abstract

Bi₂O₂Se, a high‐mobility and air‐stable 2D material, has attracted substantial attention for application in integrated logic electronics and optoelectronics. However, achieving an overall high performance over a wide spectral range for Bi₂O₂Se‐based devices remains a challenge. A broadband phototransistor with high photoresponsivity (R) is reported that comprises high‐quality large‐area (≈180 µm) Bi₂O₂Se nanosheets synthesized via a modified chemical vapor deposition method with a face‐down configuration. The device covers the ultraviolet (UV), visible (Vis), and near‐infrared (NIR) wavelength ranges (360–1800 nm) at room temperature, exhibiting a maximum R of 108 696 A W⁻¹ at 360 nm. Upon illumination at 405 nm, the external quantum efficiency, R, and detectivity (D*) of the device reach up to 1.5 × 10⁷%, 50055 A W⁻¹, and 8.2 × 10¹² Jones, respectively, which is attributable to a combination of the photogating, photovoltaic, and photothermal effects. The devices reach a −3 dB bandwidth of 5.4 kHz, accounting for a fast rise time (τ_rise) of 32 µs. The high sensitivity, fast response time, and environmental stability achieved simultaneously in these 2D Bi₂O₂Se phototransistors are promising for high‐quality UV and IR imaging applications.

An ultrasensitive and highly stretchable multifunctional strain sensor with timbre‐recognition ability based on high‐crack‐density vertical graphene is fabricated using an ultrasonic peeling method. The strain sensor has a gauge factor of 22 000 at a strain of 100%, and can distinguish frequencies of sounds higher than 2500 Hz.

Abstract

Stretchable/wearable strain sensors are attracting growing interest due to their broad applications in physical and physiological measurements. However, the development of a multifunctional highly stretchable sensor satisfying the requirements of ultrahigh sensitivity (able to distinguish sound frequency) remains a challenge. An ultrasensitive and highly stretchable multifunctional strain sensor with timbre‐recognition ability based on high‐crack‐density vertical graphene (VGr) is fabricated using an ultrasonic peeling (UP) method. It can distinguish frequencies of sounds higher than 2500 Hz. Detailed microscopic examinations reveal that their ultrahigh sensitivity stems from the formation of high‐density nanocracks in the graphitic base layer, which is bridged by the top branched VGr nanowalls. These nanocracks cut the VGr film into a large number of nanopieces, which increase the natural frequency of the sensors, enabling the sensors to distinguish the sound frequency. Demonstrations are presented to highlight the sensors' potential as wearable devices for human physiological signal and timbre detections. This is the first multifunctional highly stretchable strain sensor with timbre‐recognition ability.

A one‐step simplified and scalable fabrication method is demonstrated for the facile construction of highly integrated all‐solid‐state planar graphene‐based micro‐supercapacitors, free of metal current collectors, interconnects, and extra substrates. The resulting micro‐supercapacitors exhibit shape diversity, remarkable mechanical flexibility, customized output voltage and current, exceptional performance uniformity, and outstanding high‐temperature stability.

Abstract

The rapid development of miniature electronics has accelerated the demand for simplified and scalable production of micro‐supercapacitors (MSCs); however, the preparation of active materials, patterning microelectrodes, and subsequent modular integration of the reported MSCs are normally separated and are involved in multiple complex steps. Herein, a one‐step, cost‐effective strategy for fast and scalable fabrication of patterned laser‐induced graphene (LIG) for all‐solid‐state planar integrated MSCs (LIG‐MSCs) with various form factors of designable shape, exceptional flexibility, performance uniformity, superior modularization, and high‐temperature stability is demonstrated. Notably, using the conductive and porous LIG patterns composed of randomly stacked graphene nanosheets simultaneously acting as both microelectrodes and interconnects, the resulting LIG‐MSCs represent typical electrical double capacitive behavior, having an impressive areal capacitance of 0.62 mF cm⁻² and long‐term stability without capacitance degeneration after 10 000 cycles. Furthermore, LIG‐MSCs display exceptional mechanical flexibility and adjustable voltage and capacitance output through arbitrary arrangement of cells connected in series and in parallel, indicative of exceptional performance customization. Moreover, all‐solid‐state LIG‐MSCs working at ionogel electrolyte exhibit highly stable performance even at high temperature of 100 °C, with 90% capacitance retention over 3000 cycles, suggestive of outstanding reliability. Therefore, the LIG‐MSCs offer tremendous opportunities for miniature power source‐integrated microelectronics.

The interfacial properties of both n‐ and p‐MoS₂ field‐effect transistors with a wide thickness range and various gate stack structures are investigated. The full energy spectra of the interface state densities are extracted. The external strain dominates the interface at the conduction band side, while sulfur‐vacancy‐induced defect‐states dominate the valance band side.

Abstract

2D materials are promising to overcome the scaling limit of Si field‐effect transistors (FETs). However, the insulator/2D channel interface severely degrades the performance of 2D FETs, and the origin of the degradation remains largely unexplored. Here, the full energy spectra of the interface state densities (D _it) are presented for both n‐ and p‐ MoS₂ FETs, based on the comprehensive and systematic studies, i.e., full rage of channel thickness and various gate stack structures with h‐BN as well as high‐k oxides. For n‐MoS₂, D _it around the mid‐gap is drastically reduced to 5 × 10¹¹ cm⁻² eV⁻¹ for the heterostructure FET with h‐BN from 5 × 10¹² cm⁻² eV⁻¹ for the high‐k top‐gate. On the other hand, D _it remains high, ≈10¹³ cm⁻² eV⁻¹, even for the heterostructure FET for p‐MoS₂. The systematic study elucidates that the strain induced externally through the substrate surface roughness and high‐k deposition process is the origin for the interface degradation on conduction band side, while sulfur‐vacancy‐induced defect states dominate the interface degradation on valance band side. The present understanding of the interface properties provides the key to further improving the performance of 2D FETs.

For the first time, 2D ultrathin Te flakes (5 nm) are successfully realized by hydrogen‐assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm that two volatile intermediates increase the vapor pressure of the source and promote the reaction. Impressively, the Te‐flake‐based phototransistor shows giant gate‐dependent photoresponse.

Abstract

Tellurium (Te), as an elementary material, has attracted intense attention due to its potentially novel properties. However, it is still a great challenge to realize high‐quality 2D Te due to its helical chain structure. Here, ultrathin Te flakes (5 nm) are synthesized via hydrogen‐assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm the growth mechanism, which can be ascribed to the formation of volatile intermediates increasing vapor pressure of the source and promoting the reaction. Impressively, the Te flake‐based transistor shows high on/off ratio ≈10⁴, ultralow off‐state current ≈8 × 10⁻¹³ A, as well as a negligible hysteresis due to reducing thermally activated defects at 80 K. Moreover, Te‐flake‐based phototransistor demonstrates giant gate‐dependent photoresponse: when gate voltage varies from −70 to 70 V, I _on/I _off is increased by ≈40‐fold. The hydrogen‐assisted strategy may provide a new approach for synthesizing other high quality 2D elementary materials.

Nanohybrids with antagonistic properties (high capacitance and good conductivity) like pMoS₂‐nSNrGO are demonstrated among excellent electrode materials for ionic actuators. With a 670% bending improvement at a low voltage of 0.5 V and the ability to perform fast bending up to 15 Hz, the pMoS₂‐nSNrGO‐based actuators successfully act as soft fingers to touch fragile surfaces of smartphones to switch the flashlight.

Abstract

Future smart mobile electronics and wearable robotics that can perform delicate activities controlled by artificial intelligence can require rapid motion actuators working at low voltages with acceptable safety and improved energy efficiency. Accordingly, ionic soft actuators can have great potential over other counterparts because they exhibit gentle movements at low voltages, less than 2 V. However, these actuators currently show deficient performances at sub‐1 V voltages in the high‐frequency range because of the lack of electrode materials with the vital antagonistic properties of high capacitance and good conductivity. Herein, a mutually exclusive nanohybrid electrode (pMoS₂‐nSNrGO) is reported consisting of oxide‐doped p‐type molybdenum‐disulfide and sulfur‐nitrogen‐codoped n‐type reduced‐graphene‐oxide. The pMoS₂‐nSNrGO electrode derives high capacitance from MoS₂ and good charge transfer between the two components from p‐n nano‐junctions, resulting in excellent actuation performances (670% improvement compared with rGO electrode at 0.5 V and 1 Hz, together with fast responses up to 15 Hz). With such excellent performances, these actuators can be successfully applied to realize an artificial soft robotic finger system for delicately touching the fragile surfaces of smartphones and tablets. The mutually exclusive pMoS₂‐nSNrGO electrode can open a new way to develop high‐performance soft actuators for soft robotic applications in the future.

A comprehensive review of thermal transport of various emerging 2D semiconductors is provided here. The phonon‐related phenomenon is discussed alongside issues encountered in various applications based on them. Furthermore, a thorough understanding of phonon transport physics in 2D semiconductors to inform the thermal management of next‐generation nanoelectronic devices is provided, and strategies for controlling heat energy transport and conversion are also considered.

Abstract

The discovery of graphene has stimulated the search for and investigations into other 2D materials because of the rich physics and unusual properties exhibited by many of these layered materials. Transition metal dichalcogenides (TMDs), black phosphorus, and SnSe among many others, have emerged to show highly tunable physical and chemical properties that can be exploited in a whole host of promising applications. Alongside the novel electronic and optical properties of such 2D semiconductors, their thermal transport properties have also attracted substantial attention. Here, a comprehensive review of the unique thermal transport properties of various emerging 2D semiconductors is provided, including TMDs, black‐ and blue‐phosphorene among others, and the different mechanisms underlying their thermal conductivity characteristics. The focus is placed on the phonon‐related phenomena and issues encountered in various applications based on 2D semiconductor materials and their heterostructures, including thermoelectric power generation and electron–phonon coupling effect in photoelectric and thermal transistor devices. A thorough understanding of phonon transport physics in 2D semiconductor materials to inform thermal management of next‐generation nanoelectronic devices is comprehensively presented along with strategies for controlling heat energy transport and conversion.

Via systematic linear polarization‐ and helicity‐resolved Raman measurements, the fingerprint and chirality of phonons in 2D itinerant ferromagnet Fe₃GeTe₂ are elucidated for the first time. Importantly, the spin–phonon coupling is determined through analysis of temperature‐dependent phonon energies and lifetimes. Such spin–phonon coupling significantly enhances the Raman susceptibility. Finally, the Raman fingerprints associated with the degradation of Fe₃GeTe₂ are uncovered.

Abstract

Fe₃GeTe₂ has emerged as one of the most fascinating van der Waals crystals due to its 2D itinerant ferromagnetism, topological nodal lines, and Kondo lattice behavior. However, lattice dynamics, chirality of phonons, and spin–phonon coupling in this material, which set the foundation for these exotic phenomena, have remained unexplored. Here, the first experimental investigation of the phonons and mutual interactions between spin and lattice degrees of freedom in few‐layer Fe₃GeTe₂ is reported. The results elucidate three prominent Raman modes at room temperature: two A_1g(Γ) and one E_2g(Γ) phonons. The doubly degenerate E_2g(Γ) mode reverses the helicity of incident photons, indicating the pseudoangular momentum and chirality. Through analysis of temperature‐dependent phonon energies and lifetimes, which strongly diverge from the anharmonic model below Curie temperature, the spin–phonon coupling in Fe₃GeTe₂ is determined. Such interaction between lattice oscillations and spin significantly enhances the Raman susceptibility, allowing to observe two additional Raman modes at the cryogenic temperature range. In addition, laser radiation‐induced degradation of Fe₃GeTe₂ in ambient conditions and the corresponding Raman fingerprint is revealed. The results provide the first experimental analysis of phonons in this novel 2D itinerant ferromagnet and their applicability for further fundamental studies and application development.

For the first time, 2D ultrathin Te flakes (5 nm) are successfully realized by hydrogen‐assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm that two volatile intermediates increase the vapor pressure of the source and promote the reaction. Impressively, the Te‐flake‐based phototransistor shows a giant gate‐dependent photoresponse.

Abstract

Tellurium (Te), as an elementary material, has attracted intense attention due to its potentially novel properties. However, it is still a great challenge to realize high‐quality 2D Te due to its helical chain structure. Here, ultrathin Te flakes (5 nm) are synthesized via hydrogen‐assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm the growth mechanism, which can be ascribed to the formation of volatile intermediates increasing vapor pressure of the source and promoting the reaction. Impressively, the Te flake‐based transistor shows high on/off ratio ≈10⁴, ultralow off‐state current ≈8 × 10⁻¹³ A, as well as a negligible hysteresis due to reducing thermally activated defects at 80 K. Moreover, Te‐flake‐based phototransistor demonstrates giant gate‐dependent photoresponse: when gate voltage varies from −70 to 70 V, I _on/I _off is increased by ≈40‐fold. The hydrogen‐assisted strategy may provide a new approach for synthesizing other high quality 2D elementary materials.

A wafer‐scale high‐yield manufacturing process is proposed for degradable electronics with a demonstration of degradable system for real‐time environmental monitoring. High device yields, high‐uniformity, and great performance are achieved simultaneously, which provide new opportunities for degradable electronics to next‐generation ecofriendly sensing platforms for the coming Internet of Things (IoT) era.

Abstract

Degradable electronics that dissolve or disintegrate in the environment after completing target functions are highly desirable due to great capabilities to eliminate the disposal, retrieval, and recycling of electronic waste worldwide. Constructing electronic systems on water‐soluble substrates via transfer printing technology has emerged as a promising approach toward this goal. However, the current approach suffers from low yields and thus hinders the complexity and scale of the obtained system in practical applications. Here, a wafer‐scale manufacturing process is proposed for degradable systems with high yields. As a demonstration, chips based on carbon nanotube thin films are 100% successfully transferred to water‐soluble substrates with an average device yield of 96.6%. Great uniformity is also obtained in the transferred thin‐film transistors (TFTs) and integrated circuits with a minimum standard deviation of 55 and 60 mV in the threshold voltage of TFTs and switching threshold voltage of inverters, respectively. System‐level demonstration of real‐time environmental monitoring is implemented in a simulated ecosystem together with a degradation demonstration under artificial rain. With its combined great performance, processing robustness, and high yields, this technology provides new opportunities for batch manufacturing of degradable electronics and next‐generation ecofriendly sensing platforms for the coming Internet of Things era.

Nanoassembly growth model of low‐symmetry 2D materials is revealed to understand the formation mechanism of grain boundary and subdomain in CVD‐grown 1T′ ReS₂. The controlled construct of diverse grain boundary structures combined with their novel properties will open up new prospects for the grain boundary‐mediated engineering of material properties and applications.

Abstract

Grain boundaries (GBs) significantly affect the electrical, optical, magnetic, and mechanical properties of 2D materials. An anisotropic 2D material like ReS₂ provides unprecedented opportunities to explore novel GB properties, since the reduced lattice symmetry offers greater degrees of freedom to build new GB structures. Here the atomic structure and formation mechanism of unusual multidomain and diverse GB structures in the vapor phase synthesized ReS₂ atomic layers are reported. Using high‐resolution electron microscopy, two major categories of GBs are observed in each ReS₂ domain, namely, the joint GB including three structures, and the GBs formed from a reconstruction of Re4‐chains including seven different structures. Based on the experimental observations, a novel “nanoassembly growth model” is proposed to elucidate the growth process of ReS₂, where three types of Re4‐chain reconstruction give rise to a multidomain structure. Moreover, it is shown that by controlling the thermodynamics of the growth process, the structure and density of GB in the ReS₂ domain can be tailored. First‐principles calculations point to interesting new properties resulting from such GBs, such as a new electron state or ferromagnetism, which are highly sought after in the construction of novel 2D devices.

The interfacial properties of both n‐ and p‐MoS₂ field‐effect transistors with a wide thickness range and various gate stack structures are investigated. The full energy spectra of the interface state densities are extracted. The external strain dominates the interface at the conduction band side, while sulfur‐vacancy‐induced defect‐states dominate the valance band side.

Abstract

2D materials are promising to overcome the scaling limit of Si field‐effect transistors (FETs). However, the insulator/2D channel interface severely degrades the performance of 2D FETs, and the origin of the degradation remains largely unexplored. Here, the full energy spectra of the interface state densities (D _it) are presented for both n‐ and p‐ MoS₂ FETs, based on the comprehensive and systematic studies, i.e., full rage of channel thickness and various gate stack structures with h‐BN as well as high‐k oxides. For n‐MoS₂, D _it around the mid‐gap is drastically reduced to 5 × 10¹¹ cm⁻² eV⁻¹ for the heterostructure FET with h‐BN from 5 × 10¹² cm⁻² eV⁻¹ for the high‐k top‐gate. On the other hand, D _it remains high, ≈10¹³ cm⁻² eV⁻¹, even for the heterostructure FET for p‐MoS₂. The systematic study elucidates that the strain induced externally through the substrate surface roughness and high‐k deposition process is the origin for the interface degradation on conduction band side, while sulfur‐vacancy‐induced defect states dominate the interface degradation on valance band side. The present understanding of the interface properties provides the key to further improving the performance of 2D FETs.

Through both calculation and experimental demonstration, vertical growth of Bi₂O₂Se nanoplates on mica substrates is obtained with Bi₂O₃ serving as a seed layer, by changing the crystallographic disregistry and the corresponding adhesion energy between two‐dimensional material and substrate. These vertically grown Bi₂O₂Se nanoplates can be transferred conveniently to suspended substrates without contamination.

Abstract

As two‐dimensional (2D) layered materials attract more attention owing to their unique optical, electrical, and thermal properties, there are persistent efforts to grow high‐quality 2D layered materials for fundamental research and device applications. While large‐area 2D layered materials with high crystal quality can be obtained through chemical vapor transport, the strong binding between 2D layered materials and substrates poses a significant challenge for attempts to reveal their intrinsic properties and to use these 2D building blocks for constructing advanced heterostructured devices. Therefore, it would be ideal to grow high‐quality 2D materials with minimized contact and binding with substrate. Through both calculation and experiment, it is demonstrated that by introducing a seed layer at the nucleation stage, the crystallographic disregistry and the corresponding adhesion energy between 2D materials and substrate can be altered, resulting in a change of crystal surface in contact with the substrate, and therefore vertical growth of 2D materials on substrates. As an example, it is demonstrated that with Bi₂O₃ serving as a seed layer, vertical growth of 2D plates of Bi₂O₂Se on mica substrates can be realized. These vertically grown 2D nanoplates of Bi₂O₂Se can be conveniently transferred with their thermal properties investigated for the first time.

Large‐area Bi₂O₂Se nanosheets are synthesized using a modified chemical vapor deposition method and a face‐down approach. A device fabricated with this material has a detection wavelength covering the ultraviolet–visible–near‐infrared range a photoresponsivity of 108 696 A W⁻¹ at 360 nm, an external quantum efficiency of 1.5 × 10⁷% at 405 nm, and a rise time reaches 32 µs. This is due to a combination of photogating, photovoltaic, and photothermal effects.

Abstract

Bi₂O₂Se, a high‐mobility and air‐stable 2D material, has attracted substantial attention for application in integrated logic electronics and optoelectronics. However, achieving an overall high performance over a wide spectral range for Bi₂O₂Se‐based devices remains a challenge. A broadband phototransistor with high photoresponsivity (R) is reported that comprises high‐quality large‐area (≈180 µm) Bi₂O₂Se nanosheets synthesized via a modified chemical vapor deposition method with a face‐down configuration. The device covers the ultraviolet (UV), visible (Vis), and near‐infrared (NIR) wavelength ranges (360–1800 nm) at room temperature, exhibiting a maximum R of 108 696 A W⁻¹ at 360 nm. Upon illumination at 405 nm, the external quantum efficiency, R, and detectivity (D*) of the device reach up to 1.5 × 10⁷%, 50055 A W⁻¹, and 8.2 × 10¹² Jones, respectively, which is attributable to a combination of the photogating, photovoltaic, and photothermal effects. The devices reach a −3 dB bandwidth of 5.4 kHz, accounting for a fast rise time (τ_rise) of 32 µs. The high sensitivity, fast response time, and environmental stability achieved simultaneously in these 2D Bi₂O₂Se phototransistors are promising for high‐quality UV and IR imaging applications.

Noble‐transition‐metal dichalcogenide has emerged as a unique 2D material for fundamental studies and various applications. A comprehensive review is provided regarding the unique structure and novel physical properties for a wide range of applications. In addition, current progress and latest advances in the device application are summarized.

Abstract

Emerging classes of 2D noble‐transition‐metal dichalcogenides (NTMDs) stand out for their unique structure and novel physical properties in recent years. With the nearly full occupation of the d orbitals, 2D NTMDs are expected to be more attractive due to the unique interlayer vibrational behaviors and largely tunable electronic structures compared to most transition metal dichalcogenide semiconductors. The novel properties of 2D NTMDs have stimulated various applications in electronics, optoelectronics, catalysis, and sensors. Here, the latest development of 2D NTMDs are reviewed from the perspective of structure characterization, preparation, and application. Based on the recent research, the conclusions and outlook for these rising 2D NTMDs are presented.

Nature Nanotechnology, Published online: 03 October 2019; doi:10.1038/s41565-019-0554-3

An EU-funded open call aims at building a consortium to bridge basic and applied research on graphene and related atomically thin crystals for the development of integrated circuit technologies.

Nature Nanotechnology, Published online: 03 October 2019; doi:10.1038/s41565-019-0556-1

Graphene is being used commercially in large quantities in ways that are grounded in market realities, far from the much hyped ‘killer applications’.

Nature Nanotechnology, Published online: 03 October 2019; doi:10.1038/s41565-019-0555-2

This Review reflects on the 15 years of advances in the field of 2D materials towards commercialization of graphene and its future perspectives.