Defect engineering in two-dimensional materials

Since the discovery of graphene in 2004, two-dimensional (2D) materials have attracted worldwide interest. They are proved to be the most promising materials for next generation electronic and optoelectronic devices, including transistor, photodetector, sensor, modulator and light-emitting diode. Defects, e.g. vacancies, adatoms, edges, grain boundaries, and substitutional impurities, are inevitable in 2D materials[1]. They will influence the performance of the materials in many aspects such as mechanical, electrical, optical and optoelectronic properties. For example, the presence of sulfur vacancies (SVs) leads to electron donor states within the electronic bandgap. This increases electron concentration and results in n-type characteristic in as-prepared MoS2. They could also give rise to hopping transport behavior in low carrier density and act as scattering centers to reduce the carrier mobility in MoS2. Thus, defect engineering, namely, eliminating the unfavorable defects and introducing beneficial defects is very meaningful, and would be a promising strategy to realize high performance electronic and optoelectronic devices based on 2D materials.

Achieving high-performance multilayer MoSe2 photodetectors by defect engineering

Optoelectronic properties of MoSe2 are modulated by controlled annealing in air.Characterizations by Raman spectroscopy and XPS demonstrate the introduction of oxygen defects.Considerable increase in electron and hole mobilities reveals the highly improved electron and hole transport.Furthermore,the photocurrent is enhanced by nearly four orders of magnitudes under 7 nW laser exposure after annealing.The remarkable enhancement in the photoresponse is attributed to an increase in hole trapping centers and a reduction in resistance.Furthermore,the annealed photodetector shows a fast time response on the order of 10 ms and responsivity of 3×10^(4) A/W.

Accurate Assessment and Tracking the Process of Liver-Specific Injury by the Residual Tissue Activity of Carboxylesterase 1 and Dipeptidyl Peptidase 4

Accurately assessing and tracking the progression of liver-specific injury remains a major challenge in the field of biomarker research.Here,we took a retrospective validation approach built on the mutuality between serum and tissue biomarkers to characterize the liver-specific damage of bile duct cells caused by a-naphthyl isothiocyanate(ANIT).We found that carboxylesterase 1(CES1),as an intrahepatic marker,and dipeptidyl peptidase 4(DPP-IV),as an extrahepatic marker,can reflect the different pathophysiologies of liver injury.Levels of CES1 and DPP-IV can be used to identify liver damage itself and the inflammatory state,respectively.While the levels of the conventional serological biomarkers alkaline phosphatase(ALP),alanine aminotransferase(ALT),and aspartate aminotransferase(AST)were all concomitantly elevated in serum and tissues after ANIT-induced injury,the levels of bile acids decreased in bile,increased in serum,and ascended in intrahepatic tissue.Although the level of γ-glutamyl transpeptidase(γ-GT)changed in an opposite direction,the duration was much shorter than that of CES1 and was quickly restored to normal levels.Therefore,among the abovementioned biomarkers,only CES1 made it possible to specifically determine whether the liver cells were destroyed or damaged without interference from inflammation.CES1 also enabled accurate assessment of the anti-cholestasis effects of ursodeoxycholic acid(UDCA;single component)and Qing Fei Pai Du Decoction(QFPDD;multicomponent).We found that both QFPDD and UDCA attenuated ANIT-induced liver damage.UDCA was more potent in promoting bile excretion but showed relatively weaker anti-injury and antiinflammatory effects than QFPDD,whereas QFPDD was more effective in blocking liver inflammation and repairing liver damage.Our data highlights the potential of the combined use of CES1(as an intrahepatic marker of liver damage)and DPP-IV(as an extrahepatic marker of inflammation)for the accurate evaluation and tracking of liver-specific injury—an application that allows for the differentiation of liver damage and inflammatory liver injury.

Direct writing-in and visualizing reading-out data storage with high capacity in low-cost plastics

The explosive growth of the global data volume demands new and advanced data storage methods.Here,we report that data storage with ultrahigh capacity(~1 TB per disc)can be realized in low-cost plastics,including polycarbonate(PC),precipitated calcium carbonate(PCC),polystyrene(PS),and polymethyl methacrylate(PMMA),via direct fs laser writing.The focused fs laser can modify the fluorescence of written regions on the surface and in the interior of PMMA,enabling threedimensional(3D)information storage.Through the 3D laser processing platform,a 50-layer data record with low bit error(0.96%)is archived.Visual reading of data is empowered by the fluorescence contrast.The broad variation of fluorescence intensity assigns 8 gray levels,corresponding to 3 bits on each spot.The gray levels of each layer present high stability after longterm aging cycles,confirming the robustness of data storage.Upon single pulse control via a high-frequency electro-optic modulator(EOM),a fast writing speed(~1 kB/s)is achieved,which is limited by the repetition frequency of the fs laser.

High-performance silicon−graphene hybrid plasmonic waveguide photodetectors beyond 1.55μm

Graphene has attracted much attention for the realization of high-speed photodetection for silicon photonics over a wide wavelength range.However,the reported fast graphene photodetectors mainly operate in the 1.55μm wavelength band.In this work,we propose and realize high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon−graphene hybrid plasmonic waveguide,which enables efficient light absorption in graphene at 1.55μm and beyond.When operating at 2μm,the present photodetector has a responsivity of ~70 mA/W and a setup-limited 3 dB bandwidth of >20 GHz.When operating at 1.55μm,the present photodetector also works very well with a broad 3 dB bandwidth of >40 GHz(setup-limited)and a high responsivity of ~0.4 A/W even with a low bias voltage of−0.3 V.This work paves the way for achieving highresponsivity and high-speed silicon-graphene waveguide photodetection in the near/mid-infrared ranges,which has applications in optical communications,nonlinear photonics,and on-chip sensing.

Realization of vertical and lateral van der Waals heterojunctions using two-dimensional layered organic semiconductors

Van der Waals （vdW） heterojunctions based on two-dimensional （2D） atomic crystals have been extensively studied in recent years. Herein, we show that both vertical and lateral vdW heterojunctions can be realized with layered molecular crystals using a two-step physical vapor transport （PVT） process. Both types of heterojunctions show clean and sharp interfaces without phase mixing under atomic force microscopy （AFM）. They also exhibit a strong interfacial built-in electric field similar to that of their inorganic counterparts. These heterojunctions have greater potential for device applications than individual materials. The lateral heterojunction （LHJ） devices show rectifying characteristics due to the asymmetric energy barrier for holes at the interface, while the vertical heterojunction （VHJ） devices behave like metal-insulator-semiconductor tunnel junctions, with pronounced negative differential conductance （NDC）. Our work extends the concept of vdW heterojunctions to molecular materials, which can be generalized to other layered organic semiconductors （OSCs） to obtain new device functionalities.

Defect Engineering in 2D Materials: Precise Manipulation and Improved Functionalities

Two-dimensional(2D)materials have attracted increasing interests in the last decade.The ultrathin feature of 2D materials makes them promising building blocks for next-generation electronic and optoelectronic devices.With reducing dimensionality from 3D to 2D,the inevitable defects will play more important roles in determining the properties of materials.In order to maximize the functionality of 2D materials,deep understanding and precise manipulation of the defects are indispensable.In the recent years,increasing research efforts have been made on the observation,understanding,manipulation,and control of defects in 2D materials.Here,we summarize the recent research progress of defect engineering on 2D materials.The defect engineering triggered by electron beam(e-beam),plasma,chemical treatment,and so forth is comprehensively reviewed.Firstly,e-beam irradiation-induced defect evolution,structural transformation,and novel structure fabrication are introduced.With the assistance of a high-resolution electron microscope,the dynamics of defect engineering can be visualized in situ.Subsequently,defect engineering employed to improve the performance of 2D devices by means of other methods of plasma,chemical,and ozone treatments is reviewed.At last,the challenges and opportunities of defect engineering on promoting the development of 2D materials are discussed.Through this review,we aim to build a correlation between defects and properties of 2D materials to support the design and optimization of high-performance electronic and optoelectronic devices.

Thickness-dependent enhanced optoelectronic performance of surface charge transfer-doped ReS2 photodetectors

Surface charge transfer doping has been widely utilized to tune the electronic and optical properties of semiconductor photodetectors based on low-dimensional materials.Although many studies have been conducted on the performance(response time,responsivity,etc.)of doped photodetectors and their mechanisms,they merely examined a specific thickness and did not systematically explore the dependence of doping effects on the number of layers.This work performs a series of investigations on ReS_(2)photodetectors with different numbers of layers and demonstrates that the p-dopant tetrafluorotetracyanoquinodimethane(F_(4)-TCNQ)converts the deep trap states into recombination centers for few-layer ReS_(2)and induces a vertical p-n junction for thicker ReS_(2).A response time of 200 ms is observed in the decorated 2-layer ReS_(2)photodetector,more than two orders of magnitude faster than the response of the pristine photodetector,due to the disappearance of deep trap states.A current rectification ratio of 30 in the F_(4)-TCNQ-decorated sandwiched ReS_(2)device demonstrates the formation of a vertical p-n junction in a thicker ReS_(2)device.The responsivity is as high as 2,000 A/W owing to the strong carrier separation of the p-n junction.Different thicknesses of ReS_(2)enable switching of the prominent operating mechanism between transforming deep trap states into recombination centers and forming a vertical p-n junction.The thicknessdependent doping effect of a two-dimensional material serves as a new mechanism and provides a scheme toward improving the performance of other semiconductor devices,especially optical and electronic devices based on low-dimensional materials.

Controllable n-type doping in WSe_(2)monolayer via construction of anion vacancies

The successful applications of two-dimensional(2 D)transition metal dichalcogenides highly rely on rational regulation of their electronic properties.The nondestructive and controllable doping strategy is of great importance to implement 2 D materials in electronic devices.Herein,we propose a straightforward and effective method to realize controllable n-type doping in WSe_(2)monolayer by electron beam irradiation.Electrical measurements and photoluminescence(PL)spectra verify the strong n-doping in electron beam-treated WSe_(2)monolayers.The n-type doping arises from the generation of Se vacancies and the doping degree is precisely controlled by irradiation fluences.Due to the n-dopinginduced narrowing of the Schottky barrier,the current of back-gated monolayer WSe_(2)is enhanced by an order of magnitude and a$8?increase in the electron filed-effect mobility is observed.Remarkably,it is a moderate method without significant reduction in electrical performance and severe damage to lattice structures even under ultra-high doses of irradiation.

Temperature-sensitive and solvent-resistance hydrogel sensor for ambulatory signal acquisition in“moist/hot environment”

To realize continuously and stably work in a“moist/hot environment”,flexible electronics with excellent humid resistance,antiswelling,and detection sensitivity are demanding.Herein,a solvent-resistant and temperature-ultrasensitive hydrogel sensor was prepared by combining MXene and quaternized chitosan(QCS)with the binary polymer chain.The strong electrostatic interaction between the QCS chain and the poly(acrylic acid)(PAA)network endows the hydrogel stability against solvent erosion,high temperature,and high humidity.The strong dynamic interaction between MXene and polymer matrix significantly improves the mechanical properties and sensing(strain and temperature)sensitivity of the hydrogel.The hydrogel strain sensor exhibits a high gauge factor(5.53),temperature/humidity tolerance(equilibrium swelling ratio of 2.5%at 80℃),and excellent cycle stability,which could achieve a remote and accurate perception of complex human motion and environment fluctuation under aquatic conditions.Moreover,the hydrogel sensor exhibits impressive thermal response sensitivity(-3.183%/℃),ultrashort response time(<2.53 s),and a low detection limit(<0.5℃)in a wide temperature range,which is applied as an indicator of the body surface and ambient temperature.In short,this study broadens the application scenarios of hydrogels in persistent extreme thermal and wet environments.

Monolayer Graphene as a Saturable Absorber in a Mode-Locked Laser

我们证明单层 graphene 的内在的性质允许它当时，为锁模式的纤维激光充当一个更有效的可饱和的吸收器与多层的 graphene 相比。单层 graphene 的吸收能与多层的 graphene，有像皱纹的缺点的 graphene，或 functionalized graphene 相比在更低的刺激紧张被浸透。单层 graphene 有 65.9% 的显著地大的调整深度，而多层的 graphene 的调整深度极大地由于 nonsaturable 吸收被减少并且散布损失。微微秒 ultrafast 激光脉搏(1.23 ps ) 能作为一个可饱和的吸收器用单层 graphene 被产生。由于 ultrafast 松驰时间，更大的调整深度和单层 graphene 的更低的散布损失，它以塑造能力，脉搏稳定性，和产量精力的脉搏比多层的 graphene 更好表现。

Raman Spectroscopy and Imaging of Graphene

Graphene has many unique properties that make it an ideal material for fundamental studies as well as for potential applications.Here we review recent results on the Raman spectroscopy and imaging of graphene.We show that Raman spectroscopy and imaging can be used as a quick and unambiguous method to determine the number of graphene layers.The strong Raman signal of single layer graphene compared to graphite is explained by an interference enhancement model.We have also studied the effect of substrates,the top layer deposition,the annealing process,as well as folding(stacking order)on the physical and electronic properties of graphene.Finally,Raman spectroscopy of epitaxial graphene grown on a SiC substrate is presented and strong compressive strain on epitaxial graphene is observed.The results presented here are highly relevant to the application of graphene in nano-electronic devices and help in developing a better understanding of the physical and electronic properties of graphene.

Defects as a factor limiting carrier mobility in WSe2： A spectroscopic investigation

The electrical performance of two-dimensional transition metal dichalcogenides （TMDs） is strongly affected by the number of structural defects. In this work, we provide an optical spectroscopic characterization approach to correlate the number of structural defects and the electrical performance of WSe2 devices. Low-temperature photoluminescence （PL） spectra of electron-beam-lithography- processed WSe2 exhibit a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an electron-beam-free transfer-electrode technique, we successfully prepared a backgated WSe2 device containing a limited amount of defects. A maximum hole mobility of approximately 200 cm2.V -1.s-1 was achieved because of the reduced scattering sources, which is the highest reported value for this type of device. This work provides not only a versatile and nondestructive method to monitor the defects in TMDs but also a new route to approach the room-temperature phonon-limited mobility in high-performance TMD devices.

How defects influence the photoluminescence of TMDCs

Two-dimensional(2D)transition metal dichalcogenide(TMDC)monolayers,a class of ultrathin materials with a direct bandgap and high exciton binding energies,provide an ideal platform to study the photoluminescence(PL)of light-emitting devices.Atomically thin TMDCs usually contain various defects,which enrich the lattice structure and give rise to many intriguing properties.As the influences of defects can be either detrimental or beneficial,a comprehensive understanding of the internal mechanisms underlying defect behaviour is required for PL tailoring.Herein,recent advances in the defect influences on PL emission are summarized and discussed.Fundamental mechanisms are the focus of this review,such as radiative/nonradiative recombination kinetics and band structure modification.Both challenges and opportunities are present in the field of defect manipulation,and the exploration of mechanisms is expected tofacilitate the applications of 2D TMDCs in the future.

Ultrasensitive graphene-Si position-sensitive detector for motion tracking

Motion tracking has attracted great attention in the fields of real-time tracking,nanorobotics,and targeted therapy.For achieving more accurate motion tracking,the highly sensitive position-sensitive detector(PSD)is desirable.Here,we demonstrate a meliorated PSD based on graphene-Si heterojunction for motion tracking.The position sensitivity of PSD was improved by employing surface engineering of graphene.Through modulating the transport property of graphene,nearly 20-fold increase of sensitivity was achieved under weak light,and at the same time,the detection limit power was reduced to^2 nW.A motion tracking system was developed based on the improved PSD,and human arm swing was tracked,which demonstrated high sensitivity and real-time tracking capabilities of the PSD.In addition,the PSD can support up to^10 kHz high-frequency tracking.This work provides a new strategy for improving the performance of PSD,and promotes the development of two-dimensional materials in novel optoelectronic devices.

Synergistic graphene/aluminum surface plasmon coupling for zinc oxide lasing improvement

Collective oscillations of free electrons generate plasmons on the surface of a material. A whispering-gallery microcavity effectively confines the light field on its surface based on the total reflection from its internal wall. When these two kinds of electromagnetic waves meet each other, the stimulated emissions from an individual ZnO microrod were enhanced more than 50-fold and the threshold was reduced after the whispering-gallery microcavity was coated with a monolayer of graphene and A1 nanoparticles. The improvement of the lasing performance was attributed to the synergistic energy coupling of the graphene/A1 surface plasmons with ZnO excitons. The lasing characteristics and the coupling mechanism were investigated systematically.

Position-sensitive detectors based on two-dimensional materials

Two-dimensional(2D)materials have attracted great attention in optoelectronics because of their unique structure,optical and electrical properties.Designing high-performance photodetectors and implementing their applications are eager to promote the development of 2D materials.Position-sensitive detector(PSD)is an optical inspection device for the precise measurements of position,distance,angle,and other relevant physical variables.It is a widely used component in the fields of tracking,aerospace,nanorobotics,and so forth.Essentially,PSD is also a photodetector based on the lateral photovoltaic effect(LPE).This article reviews recent progress in high-performance PSD based on 2D materials.The high-sensitive photodetectors and LPE involved in 2D photodetectors are firstly discussed.Then,we introduce the research progress of PSD based on 2D materials and analyze the carrier dynamics in different device structures.Finally,we summarize the functionalities and applications of PSD based on 2D materials,and highlight the challenges and opportunities in this research area.

Controlling phase transition in WSe_(2) towards ideal n-type transistor

Two-dimensional(2D)transition metal dichalcogenides(TMDs)have been rapidly established as promising building blocks for versatile atomic scale circuits and multifunctional devices.However,the high contact resistance in TMDs based transistors seriously hinders their applications in complementary electronics.In this work,we show that an Ohmic homojunction n-type tungsten diselenide(WSe_(2))transistor is realized through spatially controlling cesium(Cs)doping region near the contacts.We find that the remarkable electron doping effect of Cs stimulates a semiconductor to metal(2H to 1T')phase transition in WSe_(2),and hence the formation of 2H-1T’hetero-phase contact.Our method significantly optimizes the WSe_(2) transport behavior with a perfect low subthreshold swing of-61 mV/dec and ultrahigh current on/off ratio exceeding-10^(9).Meanwhile,the electron mobility is enhanced by nearly 50 times.We elucidate that the ideal n-type behavior originates from the negligible Schottky barrier height of~19 meV and low contact resistance of-0.9Ωk·μm in the 2H-1T’homojunction device.Moreover,based on the Ohmic hetero-phase configuration,a WSe_(2) inverter is achieved with a high gain of~270 and low power consumption of-28 pW.Our findings envision Cs functionalization as an effective method to realize ideal Ohmic contact in 2D WSe_(2) transistors towards high performance complementary electronic devices.