Synthesis of uniform two-dimensional MoS_(2) films via thermal evaporation

Two-dimensional(2D)molybdenum disulfide(MoS_(2))holds great potential for various applications such as electronic devices,catalysis,lubrication,anti-corrosion and so on.Thermal evaporation is a versatile thin film deposition technique,however,the conventional thermal evaporation techniques face challenges in producing uniform thin films of MoS_(2) due to its high melting temperature of 1375℃.As a result,only thick and rough MoS_(2) films can be obtained using these methods.To address this issue,we have designed a vacuum thermal evaporation system specifically for large-scale preparation of MoS_(2) thin films.By using K2MoS4 as the precursor,we achieved reliable deposition of uniform polycrystalline MoS_(2) thin films with a size of 50 mm×50 mm and controllable thickness ranging from 0.8 to 2.4 nm.This approach also allows for patterned deposition of MoS_(2) using shadow masks and sequential deposition of MoS_(2) and tungsten disulfide(WS_(2)),similar to conventional thermal evaporation techniques.Moreover,we have demonstrated the potential applications of the obtained MoS_(2) thin films in field effect transistors(FETs),memristors and electrocatalysts for hydrogen evolution reaction(HER).

Aligned Graphene Nanoribbons and Crossbars from Unzipped Carbon Nanotubes

Aligned graphene nanoribbon (GNR) arrays have been made by unzipping of aligned single-walled and few-walled carbon nanotube (CNT) arrays. Nanotube unzipping was achieved by a polymer-protected Ar plasma etching method, and the resulting nanoribbon array can be transferred onto any chosen substrate. Atomic force microscope (AFM) imaging and Raman mapping on the same CNTs before and after unzipping confirmed that ~80% of CNTs were opened up to form single layer sub-10 nm GNRs. Electrical devices made from the GNRs (after annealing in H2 at high temperature) showed on/off current (Ion/Ioff) ratios up to 103 at room temperature, suggesting the semiconducting nature of the narrow GNRs. Novel GNR-GNR and GNR-CNT crossbars were fabricated by transferring GNR arrays across GNR and CNT arrays, respectively. The production of such ordered graphene nanoribbon architectures may allow for large scale integration of GNRs into nanoelectronics or optoelectronics.

Phase Engineering of Two-Dimensional Transition Metal Dichalcogenides

Transition metal dichalcogenides(TMDs)possess a large number of two-dime nsional(2D)materials with novel physical and chemical properties and hold great potential applications in electronic devices,optical devices as well as catalysts.TMDs usually have poly-phases,such as 2H,3R and 1T.Chemical and physical properties,including electrical conductivity,superconductivity,magnetism and catalytic activity,are different for different phases of TMDs.Therefore,great efforts have been made to obtain a specific pure phase of 2D TMD materials.Here,we review the recent phase engineering research for 2D TMDs,including ion insertion,alying,temperature,defects,strain and electric field.

Growth of large-area aligned pentagonal graphene domains on high-index copper surfaces

Single-crystal graphene domains grown by chemical vapor deposition （CVD） intrinsically tend to have a six-fold symmetry; however, several factors can influence the growth kinetics, which can in turn lead to the formation of graphene with different shapes. Here we report the growth of oriented large-area pentagonal single-crystal graphene domains on Cu foils by CVD. We found that high-index Cu planes contributed selectively to the formation of pentagonal graphene. Our results indicated that lattice steps present on the crystalline surface of the underlying Cu promoted graphene growth in the direction perpendicular to the steps and finally led to the disappearance of one of the edges forming a pentagon. In addition, hydrogen promoted the formation of pentagonal domains. This work provides new insights into the mechanism of graphene growth.

Metallic and ferromagnetic MoS2 nanobelts with vertically aligned edges

Edge effects are predicted to significantly impact the properties of low dimensional materials with layered structures. The synthesis of low dimensional materials with copious edges is desired for exploring the effects of edges on the band structure and properties of these materials. Here we developed an approach for synthesizing MoS2 nanobelts terminated with vertically aligned edges by sulfurizing hydrothermally synthesized MoO3 nanobelts in the gas phase through a kinetically driven process; we then investigated the electrical and magnetic properties of these metastable materials. These edge-terminated MoS2 nanobelts were found to be metallic and ferromagnetic, and thus dramatically different from the semiconducting and nonmagnetic two-dimensional （2D） and three-dimensional （3D） 2H-MoS2 materials. The transitions in electrical and magnetic properties elucidate the fact that edges can tune the properties of low dimensional materials. The unique structure and properties of this one-dimensional （1D） MoS2 material will enable its applications in electronics, spintronics, and catalysis.

Fast growth of large single-crystalline WS_(2) monolayers via chemical vapor deposition

Two-dimensional(2D)tungsten disulfide(WS2)has emerged as a promising ultrathin semiconductor for building high-performance nanoelectronic devices.The controllable synthesis of WS2 monolayers(1L)with both large size and high quality remains as a challenge.Here,we developed a new approach for the chemical vapor deposition(CVD)growth of WS2 monolayers by using K2WS4 as the growth precursor.The simple chemistry involved in our approach allowed for improved controllability and a fast growth rate of~30μm·min−1.We achieved the reliable growth of 1L WS2 flakes with side lengths of up to~500μm and the obtained WS2 flakes were 2D single crystals with low density of defects over a large area as evidenced by various spectroscopic and microscopic characterizations.In addition,the large 1L WS2 single crystals we obtained showed higher electrical performance than their counterparts grown with previous approaches,demonstrating the potential of our approach in producing high quality and large 2D semiconductors for future nanoelectronics.

行为者社会阶层特征对道德判断的影响

社会阶层对道德判断的影响一直是社会学和心理学研究者关注的话题.本研究从行为者的角度出发,通过研究一(n=295)和研究二(n=299),从道德行为和不道德行为两个方面,考察行为者社会阶层特征是否影响道德判断的严厉程度.研究结果发现,人们对高低社会阶层行为者的道德判断标准存在差异,相比于低社会阶层行为者,在道德行为上,被试对高社会阶层行为者的道德判断更宽松,主要表现在道德基础的关爱领域和公平领域;而在不道德行为上,相比于低社会阶层行为者,被试对高社会阶层行为者的道德判断更严厉,主要表现在道德基础的伤害领域和颠覆领域.在此研究基础上,本研究进一步提出道德判断的社会阶层效应模型,为探索构建和谐、稳定的道德社会提供理论支持.

Highly crystalline ReSe2 atomic layers synthesized by chemical vapor transport

Two-dimensional (2D) anisotropic rhenium diselenide (ReSe2) has attracted lots ofattention due to its promising applications in electronics and optoelectronics. However,controlled synthesis of high quality ultrathin ReSe2 remains as a challenge.Here we developed an approach for synthesizing high quality 2D ReSe2 flakes witha thickness down to monolayer by chemical vapor transport (CVT) through carefullytuning the growth kinetics. The atomic structures and anisotropy of theobtained ReSe2 flakes were intensively characterized with scanning transmissionelectron microscope and angle-resolved polarized Raman spectroscopy. Fieldeffecttransistors fabricated on the CVT-grown ReSe2 flakes showed n-typesemiconducting behavior with an on/off current ratio of 105 and a mobility up to
5 cm2 V−1 s−1, which is comparable to mechanically exfoliated flakes and isobvious higher than the samples synthesized with other approaches. This study notonly make high quality 2D ReSe2 easily accessible for both fundamental and applicationexplorations but also sheds new lights on the chemical synthesis of otheranisotropic 2D materials.

Growth of single-walled carbon nanotubes from Ag15 cluster catalysts

Chirality-specific growth of single-walled carbon nanotubes(SWNTs) remains a challenge for their practical applications in electronics. Here, we explored the surface growth of SWNTs by utilizing the atomic-precise silver cluster complex [Ag_(15){1,3,5–(C:C)_3–C_6H_3}_2(Py[8])_3–(CF_3SO_3)_3](CF_3SO_3)_6(Py[8] is abbreviation for octamethylazacalix[8]pyridine) as a catalyst precursor. The diameters of most acquired SWNTs distributed in the range of 1.2–1.4 nm, which is suitable for making high performance field-effect transistors. The high quality of the obtained SWNTs was evidenced by Raman spectroscopy and electrical measurements. Successful growth of high quality SWNTs in this study foresees that rational design of metal-organic complexes as growth catalysts can open up a new avenue for the controllable synthesis of SWNTs.

In situ identification of active sites during electrocatalytic hydrogen evolution

The rational design of efficient,low cost,and durable catalysts is critical for the industrial applications of electrocatalytic hydrogen production.A key step towards the structure design of high-performance catalysts for hydrogen evolution reaction(HER)relies on the in situ identification of the catalytic active sites in the process of HER,which is of great challenge.In this review,we summarize the recent advances on the in situ investigation of the active sites on low dimensional catalysts for HER.We highlight the characterization techniques used for this purpose,including scanning electrochemical microscopy(SECM),scanning electrochemical cell microscopy(SECCM),electrochemical scanning tunneling microscopy(EC-STM),in situ liquid phase transmission electron microscopy(LP-TEM),and in situ spectroscopic tools.We conclude with an overview of the main technical limitations for the current approaches and give an outlook to future opportunities in this emerging field.

Atmospheric Degradation and Performance Recovery of Two-Dimensional MoS2 Field Effect Transistor

Two-dimensional(2D)transition metal dichalcogenides(TMDCs)showed great potentials in 2D nanoelectronic devices due to their abundant and unique properties.The performance stability of the 2D TMDCs devices turns into one of the keys for their practical applications but has been rarely explored.Here,we investigated stability of MoS_(2)devices in ambient condition and contributed the device performance degradation to the surface oxidation of the contact metals with low work function,which increased the contact barrier and hindered the electron injection.We developed a new approach to recover the performance of the aged devices through the selective doping of contacts with organolithium,which prolonged the lifetime of MoS_(2)devices.Our work not only provides important insights into the stability of 2D TMDCs devices,but also opens up a new avenue for optimizing the performance of 2D MoS_(2)devices.

Fast Scan mode of scanning electrochemical microscopy:In-situ characterization of phase transition and mapping the hydrogen evolution activity for MoS_(2)

Scanning electrochemical microscopy(SECM)is an attractive technology to in-situ characterize the structural evolution and catalytic performance for various electrocatalysts.However,spatial and temporal resolution coupling are still the obstacles that limit its wide applications.Herein,a new operation mode,Fast Scan mode,was developed by improving the dual-pass scan mode,designing novel hardware structure,and employing thermal drift calibration software to achieve a high spatial and temporal resolution simultaneously.The temporal speed can achieve 4 Hz for a high spatial resolution(less than 30 nm)image.This operation mode was employed to dynamically track the phase transition process of molybdenum disulfide(MoS_(2))over time and characterize the hydrogen evolution reaction(HER)catalytic activity on the edge of semiconducting MoS_(2)quantitatively while minimizing the diffusional broadening effect and total amount of catalytic products generated above the surface.This new approach should be useful for in-situ tracking dynamic electrochemical processes,establishing the structure-activity relationship for structural complex electrocatalysts,and offering a strategy for high-speed scanning with other electrochemical imaging techniques.

Carrier mobility tuning of MoS_(2) by strain engineering in CVD growth process

Strain engineering is proposed to be an effective technology to tune the properties of two-dimensional(2D)transition metal dichalcogenides(TMDCs).Conventional strain engineering techniques(e.g.,mechanical bending,heating)cannot conserve strain due to their dependence on external action,which thereby limits the application in electronics.In addition,the theoretically predicted strain-induced tuning of electrical performance of TMDCs has not been experimentally proved yet.Here,a facile but effective approach is proposed to retain and tune the biaxial tensile strain in monolayer MoS_(2) by adjusting the process of the chemical vapor deposition(CVD).To prove the feasibility of this method,the strain formation model of CVD grown MoS_(2) is proposed which is supported by the calculated strain dependence of band gap via the density functional theory(DFT).Next,the electrical properties tuning of strained monolayer MoS_(2) is demonstrated in experiment,where the carrier mobility of MoS_(2) was increased by two orders(~0.15 to~23 cm^(2)·V^(−1)·s^(−1)).The proposed pathway of strain preservation and regulation will open up the optics application of strain engineering and the fabrication of high performance electronic devices in 2D materials.