Jiuxiang Dai

A novel metal-mirror-enhanced n–p–n van der Waals heterostructure is designed. The device exhibits excellent performance including high blackbody photoresponsivity up to 0.77 A W⁻¹, high specific detectivity of 8.61 × 10¹⁰ cm Hz^1/2 W⁻¹ under blackbody radiation, and fast response speed of ≈4 µs.

Abstract

Room-temperature-operating highly sensitive mid-wavelength infrared (MWIR) photodetectors are utilized in a large number of important applications, including night vision, communications, and optical radar. Many previous studies have demonstrated uncooled MWIR photodetectors using 2D narrow-bandgap semiconductors. To date, most of these works have utilized atomically thin flakes, simple van der Waals (vdW) heterostructures, or atomically thin p–n junctions as absorbers, which have difficulty in meeting the requirements for state-of-the-art MWIR photodetectors with a blackbody response. Here, a fully depleted self-aligned MoS₂-BP-MoS₂ vdW heterostructure sandwiched between two electrodes is reported. This new type of photodetector exhibits competitive performance, including a high blackbody peak photoresponsivity up to 0.77 A W⁻¹ and low noise-equivalent power of 2.0 × 10⁻¹⁴ W Hz^−1/2, in the MWIR region. A peak specific detectivity of 8.61 × 10¹⁰ cm Hz^1/2 W⁻¹ under blackbody radiation is achieved at room temperature in the MWIR region. Importantly, the effective detection range of the device is twice that of state-of-the-art MWIR photodetectors. Furthermore, the device presents an ultrafast response of ≈4 µs both in the visible and short-wavelength infrared bands. These results provide an ideal platform for realizing broadband and highly sensitive room-temperature MWIR photodetectors.

Nature Communications, Published online: 22 August 2022; doi:10.1038/s41467-022-32582-9

Nature Photonics, Published online: 22 August 2022; doi:10.1038/s41566-022-01053-4

The second-order phase transition behavior of the grains in the polycrystalline system is highly dependent on the surrounding grain boundaries environment. Atomic-scale observations show that the dominating effect of different grain boundaries leads to coexistence of high- and low-temperature phases of Cu₂Se in one grain.

Abstract

Phase transition is a physical phenomenon that attracts great interest of researchers. Although the theory of second-order phase transitions is well-established, their atomic-scale dynamics in polycrystalline materials remains elusive. In this work, second-order phase transitions in polycrystalline Cu₂Se at the transition temperature are directly observed by in situ aberration-corrected transmission electron microscopy. Phase transitions in microcrystalline Cu₂Se start at the grain boundaries and extend inside the grains. This phenomenon is more pronounced in nanosized grains. Analysis of phase transitions in nanocrystalline Cu₂Se with different grain boundaries demonstrates that grain boundary energy dominates unsynchronized phase transition behavior. This suggests that the energy of grain boundaries is the key factor influencing the energetic barrier for initiation of phase transition. The findings advance atomic-scale understanding of second-order phase transitions, which is crucial for the control of this process in polycrystalline materials.

Nature Electronics, Published online: 23 August 2022; doi:10.1038/s41928-022-00819-6

Nature Electronics, Published online: 23 August 2022; doi:10.1038/s41928-022-00831-w

Nature Electronics, Published online: 23 August 2022; doi:10.1038/s41928-022-00822-x

The use of Ti₃C₂T _x as an electrode to build Ti₃C₂T _x -MoS₂ vdWs contact can improve the performance of the 2D field effect transistors, including alleviating the Fermi level pinning and decreasing the Schottky barrier height to 121 eV. Moreover, a simple and time-saving technique has been proposed to tune the work function of Ti₃C₂T _X electrode in the range of 4.33 to 5.32 eV.

Abstract

The quality of the contact between source/drain electrodes and 2D transition metal dichalcogenides plays a decisive role in improving transistor performance. Understanding the mechanisms of Fermi level pinning (FLP) and finding out the strategies to solve FLP problems can further promote the development of 2D electronics. In this study, the suppressing effect of MXene on FLP in MoS₂ transistors by using Ti₃C₂T _x as an electrode to build a Ti₃C₂T _x -MoS₂ heterostructure is systematically studied. A simple and time-saving ultraviolet ozone technique to tune the work function of the Ti₃C₂T _x electrode in the range of 4.33–5.32 eV is proposed, and a low Schottky barrier height of 121 meV is achieved. The van der Waals contact between Ti₃C₂T _x and MoS₂ can alleviate the FLP effectively, and the pinning factor can be greatly optimized from 0.28 (metal electrode) to 0.87 (MXene electrode). This study can pave the way for extensive use of MXene and provide a new strategy to eliminate the negative effects of FLP in 2D materials-based electronic devices.

The focused electron beam is capable of triggering the phase transition from the semiconducting T’’ phase to metallic T’ and T phases in 2D rhenium disulfide (ReS₂) and rhenium diselenide (ReSe₂) monolayers, rendering ultra-precise phase patterning technique even in sub-nanometer scale.

Abstract

Phase patterning in polymorphic two-dimensional (2D) materials offers diverse properties that extend beyond what their pristine structures can achieve. If precisely controllable, phase transitions can bring exciting new applications for nanometer-scale devices and ultra-large-scale integrations. Here, the focused electron beam is capable of triggering the phase transition from the semiconducting T’’ phase to metallic T’ and T phases in 2D rhenium disulfide (ReS₂) and rhenium diselenide (ReSe₂) monolayers, rendering ultra-precise phase patterning technique even in sub-nanometer scale is found. Based on knock-on effects and strain analysis, the phase transition mechanism on the created atomic vacancies and the introduced substantial in-plane compressive strain in 2D layers are clarified. This in situ high-resolution scanning transmission electron microscopy (STEM) and in situ electrical characterizations agree well with the density functional theory (DFT) calculation results for the atomic structures, electronic properties, and phase transition mechanisms. Grain boundary engineering and electrical contact engineering in 2D are thus developed based on this patterning technique. The patterning method exhibits great potential in ultra-precise electron beam lithography as a scalable top-down manufacturing method for future atomic-scale devices.

A methylammonium lead bromide single crystal thin film/molybdenum disulfide (MoS₂) vertical p-n heterostructure based photodetector has been built, which shows excellent photovoltaic characteristics with power conversion efficiency of 8% and prominent photoelectric properties with responsivity of 368 mA W⁻¹ under 532 nm laser, resulting from the built-in electric field and the shortened transmit distance for the photogenerated carriers in this heterostructure.

Abstract

Methylammonium lead bromide (MAPbBr₃) single crystal thin film shows great opportunities in high-performance optoelectronic devices due to its high absorption coefficient, high photoelectric conversion efficiency, and low trap-state density properties. In order to fabricate a highly sensitive self-powered photodetectors, herein, a MAPbBr₃ single crystal thin film/molybdenum disulfide (MoS₂) vertical p-n heterostructure based photodetector has been built by typical polymer film assisted dry transfer method and micromachining technique. Attributing to the built-in electric field and the shortened transmit distance for the photogenerated carriers in this heterostructure, the device shows excellent photovoltaic characteristics with a maximum output electrical power of 7 nW and power conversion efficiency of 8% at 0.34 V under 532 nm laser. Moreover, prominent photoelectric properties with responsivity of 368 mA W⁻¹ and detectivity of 3.74 × 10¹² Jones for 532 nm laser without any bias have been achieved, which are ranking high among the organic–inorganic hybrid perovskites based self-power photodetectors. These results demonstrate that the MAPbBr₃ single crystal thin film/MoS₂ vertical heterostructure can pave a new way to develop high-performance photovoltaic devices.

npj 2D Materials and Applications, Published online: 16 August 2022; doi:10.1038/s41699-022-00329-1

Nature Nanotechnology, Published online: 16 August 2022; doi:10.1038/s41565-022-01192-3

The Ti-doped MoS₂ monolayers with ultra-high photoluminescence (PL) intensity are successfully synthesized through chemical vapor transport. The PL intensity from the Ti-doped MoS₂ is 85-fold stronger than that from the pristine MoS₂. The giant PL enhancement is attributed to dopant-induced O-Ti-S units and improved interaction between the monolayer and the mica substrate, increasing the PLQY and suppressing the nonradiative recombination. These results provide a new route of structural modification for 2D materials and pave the way for applying TMDCs to develop high-performance optoelectrical devices and electronic components.

Abstract

Substitutional doping of 2D transition metal dichalcogenides (TMDCs) has been recognized as a promising strategy to tune their optoelectronic properties for a wide array of applications. However, controllable doping of TMDCs remains a challenging issue due to the natural doping of these materials. Here, the controllable growth of Ti-doped MoS₂ monolayers is demonstrated via the chemical vapor transport method, and the atomic embedded structure is confirmed by scanning transmission electron microscope with a probe corrector measurements. Furthermore, the grown Ti-doped MoS₂ monolayer exhibits giant photoluminescence (PL), 85-fold stronger than a pristine MoS₂ monolayer prepared by the same method. The giant PL enhancement is attributed to dopant-induced O-Ti-S units and improved interaction between the monolayer and the mica substrate, increasing the photoluminescence quantum yield and facilitating radiation recombination. The successful growth of Ti-doped MoS₂ monolayer and the improvement of its optical and electrical properties by Ti doping may provide a promising method to engineer the optoelectronic properties of 2D TMDCs materials.

Inkjet-printed all-oxide CMOS inverters are fabricated with In₂O₃-based NMOS and Cu _x O-based PMOS thin film transistors (TFTs). The mesoporous In₂O₃ edge-FET architecture demonstrates colossal On-current (1.02 mA µm⁻¹), whereas PMOS devices show field-effect mobility of 0.5 cm² V⁻¹ s⁻¹ and On/Off ratio >7 × 10³. The single-step annealed CMOS inverters show signal gain of 31 and power dissipation of 4 nW at low supply voltage.

Abstract

Oxide semiconductors are becoming the materials of choice for modern-day display industries. The performance of solution-processed oxide thin film transistors (TFTs) has also improved dramatically over the last few years. However, while oxygen deficient n-type semiconductors can demonstrate excellent electronic transport, the performance of p-type materials has remained unsatisfactory. Consequently, only the n-type semiconductor-based pseudo-complementary metal oxide semiconductor (CMOS) technology has attracted tremendous interests recently; yet, the high power dissipation remains a problem. Here, this work demonstrates all-oxide CMOS invertors with high-performance narrow-channel n-type TFTs, which can compensate for the limited carrier mobility of the p-type transistors. These n-type TFTs are fabricated with polymer-templated mesoporous In₂O₃ and with a device geometry that allows near-vertical current transport, thereby rendering the TFT channel lengths to be equal to the semiconductor film thickness (≈50 nm). Unprecedented On-current (1.02 mA µm⁻¹) and transconductance (950 µS µm⁻¹) are achieved. The CuO-based p-type TFTs also show a device mobility of no less than 0.5 cm² V⁻¹ s⁻¹. The printed all-oxide CMOS inverters are found to operate at very low supply voltages and demonstrate sharp transfer curves with maximum signal gain of 31 and low power dissipation of only 4 nW, at a supply voltage of 1 V.

A hybrid structure strategy is developed utilizing a MoS₂/SnS bulk-heterojunction-like film as photosensitizer, which improves its infrared photoresponsivity by 3 orders of magnitude. It maintains excellent photodetection performance on flexible substrates, exhibiting its potential applications in wearable and portable electronics. The work highlights the promising strategy of designing and constructing the hybrid films for achieving low-cost and high-performance photodetectors.

Abstract

Flexible electronics is one of the hotspots of interdisciplinary research and can promote disruptive technology for post-Moore applications in the field of biomedical, electronic skin, wearable devices, etc. 2D materials have triggered great interest in flexible electronic devices since they have tunable bandgaps, excellent mechanical flexibility, good chemical stability, and outstanding optical properties. Their reducible atomic thickness greatly facilitates the design and construct for bending, crimping, and folding, whereas it comes at the expense of superior optoelectronic properties. Herein, a highly-bendable flexible photodetector based on MoS₂/SnS amorphous blended film is fabricated with a broadband response from 473 to 1550 nm. The responsivity at 808 nm is 1128 times larger than that of intrinsic MoS₂ photodetector accompanied by a four-fold acceleration in response time. It can bear a small bending radius as low as 3 mm for over 2000 bending–flatting cycles without a drastic performance decay. Moreover, it shows an ultrastrong stability in photoresponse exposed to air for 200 days. This work provides a promising candidate for high performance flexible photodetectors and opens up the prospect of 2D bulk blended materials in a facile all-in-one strategy for multifunctional flexible optoelectronics devices and systems.

Large single crystals of calcite are generated from amorphous calcium carbonate (ACC) using a dynamic control strategy in which local heating is used to define when and where nucleation occurs. Morphologies ranging from discs to serpentine strips are generated by independently controlling nucleation and growth, and the mechanism and energetics of the transformation are studied using in situ transmission electron microscopy.

Abstract

The use of amorphous calcium carbonate (ACC) as a precursor phase affords organisms with outstanding control over the formation of calcite and aragonite biominerals. Essential to this strategy is that the ACC is maintained within confined volumes in the absence of bulk water. This ensures that the ACC undergoes a pseudomorphic transformation and that the organism can independently control nucleation and growth. However, comparable control has proven hard to achieve in synthetic systems. Here, a straightforward method is demonstrated for controlling the crystallization of ACC thin films in which nucleation is first triggered using a heated probe, and then growth is sustained by incubating the film at a lower temperature. By independently controlling nucleation and growth, sub-millimeter calcite single crystals can be generated when and where it is desired, morphologies ranging from discs to squares to serpentine strips can be created, and arrays of crystals formed. The mechanism and energetics of crystallization of the ACC are studied using in situ transmission electron microscopy and continuity between the ACC and calcite at the growth front is demonstrated. It is envisaged that this method can be applied to the formation of large single crystals of alternative functional materials that form via amorphous precursor phases.

In this study, a large-area transparent transistor array using low temperature grown MoS₂ on an inexpensive glass substrate is fabricated. The MoS₂ transistors show uniform photoresponse under visible light over a large area. These findings provide a new platform toward the development of transparent smart glass technology.

Abstract

MoS₂-based transparent electronics can revolutionize the state-of-the-art display technology. The low-temperature synthesis of MoS₂ below the softening temperature of inexpensive glasses is an essential requirement, although it has remained a long persisting challenge. In this study, plasma-enhanced chemical vapor deposition is utilized to grow large-area MoS₂ on a regular microscopic glass (area ≈27 cm²). To benefit from uniform MoS₂, 7 × 7 arrays of top-gated transparent (≈93% transparent at 550 nm) thin film transistors (TFTs) with Al₂O₃ dielectric that can operate between −15 and 15 V are fabricated. Additionally, the performance of TFTs is assessed under irradiation of visible light and estimated static performance parameters, such as photoresponsivity is found to be 27 A W⁻¹ (at λ = 405 nm and an incident power density of 0.42 mW cm⁻²). The stable and uniform photoresponse of transparent MoS₂ TFTs can facilitate the fabrication of transparent image sensors in the field of optoelectronics.

A simple, inexpensive, and reproducible soft lithographic method is demonstrated for the area selective chemical vapor deposition growth of 2D transition metal dichalcogenide (TMD) monolayers and multilayers. The application possibilities of the patterned TMDs are demonstrated in field effect transistors, high responsivity photodetectors, and memtransistor devices.

Abstract

A simple, large area, and cost-effective soft lithographic method is presented for the patterned growth of high-quality 2D transition metal dichalcogenides (TMDs). Initially, a liquid precursor (Na₂MoO₄ in an aqueous solution) is patterned on the growth substrate using the micromolding in capillaries technique. Subsequently, a chemical vapor deposition step is employed to convert the precursor patterns to monolayer, few layers, or bulk TMDs, depending on the precursor concentration. The grown patterns are characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and photoluminescence spectroscopy to reveal their morphological, chemical, and optical characteristics. Additionally, electronic and optoelectronic devices are realized using the patterned TMDs and tested for their applicability in field effect transistors and photodetectors. The photodetectors made of MoS₂ line patterns show a very high responsivity of 7674 A W⁻¹ and external quantum efficiency of 1.49 × 10⁶%. Furthermore, the multiple grain boundaries present in patterned TMDs enable the fabrication of memtransistor devices. The patterning technique presented here may be applied to many other TMDs and related heterostructures, potentially advancing the fabrication of TMDs-based device arrays.

A new type of an atomistic synaptic network on an atomically thin van der Waals heterostructure, where the ultrasmall cells built with trilayer tungsten disulfide (WS₂) semiconductor serve as a gate-tunable photoactive synapse, is demonstrated. UV pulses onto the memristor generate dopants at atomic-level precision by direct light–lattice interactions, leading to the accurate modulation of the synaptic cells.

Abstract

A new type of atomically thin synaptic network on van der Waals (vdW) heterostructures is reported, where each ultrasmall cell (≈2 nm thick) built with trilayer WS₂ semiconductor acts as a gate-tunable photoactive synapse, i.e., a photo-memtransistor. A train of UV pulses onto the WS₂ memristor generates dopants in atomic-level precision by direct light–lattice interactions, which, along with the gate tunability, leads to the accurate modulation of the channel conductance for potentiation and depression of the synaptic cells. Such synaptic dynamics can be explained by a parallel atomistic resistor network model. In addition, it is shown that such a device scheme can generally be realized in other 2D vdW semiconductors, such as MoS₂, MoSe₂, MoTe₂, and WSe₂. Demonstration of these atomically thin photo-memtransistor arrays, where the synaptic weights can be tuned for the atomistic defect density, provides implications for a new type of artificial neural networks for parallel matrix computations with an ultrahigh integration density.

Nature Communications, Published online: 13 August 2022; doi:10.1038/s41467-022-32259-3