石墨烯封装单层二硫化钼的热稳定性研究

将单层二硫化钼用石墨烯进行封装,构造了石墨烯和二硫化钼的范德瓦耳斯异质结构,并且分别在氩气（Ar）和氢气（H₂）氛围下,详细研究了被封装的二硫化钼的热稳定性.结果表明：在氩气氛围中,石墨烯封装的二硫化钼在400–1000℃下一直保持稳定,而石墨烯和氧化硅上裸露的二硫化钼在1000℃时几乎全部分解;在氢气氛围中,石墨烯封装的二硫化钼在400–1000℃下一直稳定存在,而石墨烯和氧化硅上裸露的二硫化钼在800℃下已经完全分解.综上可得,在氩气和氢气的氛围下,被石墨烯封装的二硫化钼的热稳定性得到了显著的提高.该研究通过用石墨烯将单层的二硫化钼进行封装以提高其热稳定性,在未来以单层二硫化钼作为基础材料的电子器件中,可以保证其在高温下能够正常工作.该研究也为提高其他二维材料的热稳定性提供了一种可行的方法和思路.     

Enhanced photoluminescence of monolayer MoS2 on stepped gold structure

Different MoS₂/Au heterostructures can play an important role in tuning the photoluminescence (PL) and optoelectrical properties of monolayer MoS₂. Previous studies of PL of MoS₂/Au heterostructures were mainly limited to the PL enhancement by using different Au nanostructures and PL quenching of monolayer MoS₂ on flat Au surfaces. Here, we demonstrate the enhanced excitonic PL emissions of monolayer MoS₂/Au heterostructures on Si/SiO₂ substrates. By transferring the continuous monolayer MoS₂ onto a stepped Au structure consisting of 60-nm and 100-nm Au films, the MoS₂/Au-60 and MoS₂/Au-100 heterostructures exhibit enhanced PL emissions, each with a blue-shifted PL peak in comparison with the MoS₂/SiO₂. Furthermore, the PL intensity of MoS₂/Au-60 is about twice larger than that of MoS₂/Au-100. The different enhanced excitonic PL emissions in MoS₂/Au heterostructures can be attributed to the different charge transfer effects modified by the stepped Au structure. This work may provide an insight into the excitonic PL and charge transfer effect of MoS₂ on Au film and yield novel phenomena in MoS₂/Au heterostructures for further study of PL tuning and optoelectrical properties.     

二硫化钼/石墨烯异质结的界面结合作用及其对带边电位影响的理论研究

采用基于色散修正的平面波超软赝势方法研究了二硫化钼/石墨烯异质结的界面结合作用及其对电荷分布和带边电位的影响.研究表明二硫化钼与石墨烯之间可以形成范德瓦耳斯力结合的稳定堆叠结构.通过能带结构计算,发现二硫化钼与石墨烯的耦合导致二硫化钼成为n型半导体,石墨烯转变成小带隙的p型体系.并通过电子密度差分图证实了界面内二硫化钼附近聚集负电荷,石墨烯附近聚集正电荷,界面内形成的内建电场可以抑制光生电子-空穴对的复合.石墨烯的引入可以调制二硫化钼的能带,使其导带底上移至-0.31 eV,提高了光生电子还原能力,有利于光催化还原反应.     

Electronic and optical properties of 3N-doped graphdiyne/MoS2 heterostructures tuned by biaxial strain and external electric field

The construction of van der Waals (vdW) heterostructures by stacking different two-dimensional layered materials have been recognised as an effective strategy to obtain the desired properties. The 3N-doped graphdiyne (N-GY) has been successfully synthesized in the laboratory. It could be assembled into a supercapacitor and can be used for tensile energy storage. However, the flat band and wide forbidden bands could hinder its application of N-GY layer in optoelectronic and nanoelectronic devices. In order to extend the application of N-GY layer in electronic devices, MoS₂ was selected to construct an N-GY/MoS₂ heterostructure due to its good electronic and optical properties. The N-GY/MoS₂ heterostructure has an optical absorption range from the visible to ultraviolet with a absorption coefficient of 10⁵ cm^-1. The N-GY/MoS₂ heterostructure exhibits a type-Ⅱ band alignment allows the electron-hole to be located on N-GY and MoS₂ respectively, which can further reduce the electron-hole complexation to increase exciton lifetime. The power conversion efficiency of N-GY/MoS₂ heterostructure is up to 17.77%, indicating it is a promising candidate material for solar cells. In addition, the external electric field and biaxial strain could effectively tune the electronic structure. Our results provide a theoretical support for the design and application of N-GY/MoS₂ vdW heterostructures in semiconductor sensors and photovoltaic devices.     

Interface engineering of transition metal dichalcogenide/GaN heterostructures: Modified broadband for photoelectronic performance

The GaN-based heterostructures are widely used in optoelectronic devices, but the complex surface reconstructions and lattice mismatch greatly limit the applications. The stacking of two-dimensional transition metal dichalcogenide (TMD = MoS₂, MoSSe and MoSe₂) monolayers on reconstructed GaN surface not only effectively overcomes the larger mismatch, but also brings about novel electronic and optical properties. By adopting the reconstructed GaN (0001) surface with adatoms (N-ter GaN and Ga-ter GaN), the influences of complicated surface conditions on the electronic properties of heterostructures have been investigated. The passivated N-ter and Ga-ter GaN surfaces push the mid-gap states to the valence bands, giving rise to small bandgaps in heterostructures. The charge transfer between Ga-ter GaN surface and TMD monolayers occurs much easier than that across the TMD/N-ter GaN interfaces, which induces stronger interfacial interaction and larger valence band offset (VBO). The band alignment can be switched between type-I and type-II by assembling different TMD monolayers, that is, MoS₂/N-ter GaN and MoS₂/Ga-ter GaN are type-II, and the others are type-I. The absorption of visible light is enhanced in all considered TMD/reconstructed GaN heterostructures. Additionally, MoSe₂/Ga-ter GaN and MoSSe/N-ter GaN have larger conductor band offset (CBO) of 1.32 eV and 1.29 eV, respectively, extending the range from deep ultraviolet to infrared regime. Our results revel that the TMD/reconstructed GaN heterostructures may be used for high-performance broadband photoelectronic devices.     

基于滑动势能面的二维材料原子尺度摩擦行为的量化计算

近年来,二维材料优异的摩擦特性成为人们关注的焦点,然而目前缺乏理论上对其摩擦力进行快速、有效、精确的计算预测方法.本文提出采用密度泛函理论计算真实体系的滑动势能面,利用得到的"数值型势能面"替代传统的解析势函数,并结合Prandtl-Tomlinson模型,量化求解具有复杂形状势能面的真实二维材料体系的摩擦行为.基于该方法,揭示了原子力显微镜实验中观察到的石墨烯Moire纹超晶格结构的双周期"黏-滑"摩擦现象;理论预测了二维材料异质结构的层间超低摩擦现象,相对于同质材料,其静摩擦力和滑动摩擦力均成数量级降低,发现势能面起伏和驱动弹簧刚度均会影响层间相对滑动路径,进而对层间的摩擦行为产生影响.该方法同样可拓展到其他van der Waals作用主导的界面摩擦体系.     

Li掺杂少层MoS2的电荷分布及与石墨和氮化硼片的比较

基于第一性原理计算, 研究了Li掺杂的少层(1-3层) MoS₂的电荷分布, 并与石墨片和BN片的电荷分布特征进行了比较. 与石墨片和BN片相同的是: 电荷转移的大部分只发生在Li与最靠近Li的第一层MoS₂之间. 然而, 第二层和第三层MoS₂也能获得10%的转移电荷, 而石墨片和BN片的第二层和第三层得不到2%的电荷. 结合静电能和功函数的分析可知, MoS₂、石墨片和BN片的电荷分布主要由层间的静电相互作用和功函数来决定. 这些研究结果对于揭示具有多层结构的电荷分布特征及其电子器件的设计提供了理论支持.     

(Cu,N)共掺杂TiO2/MoS2异质结的电子和光学性能:杂化泛函HSE06

在密度泛函理论的基础上,系统地研究了Cu/N（共）掺杂的TiO₂/MoS₂异质结体系的几何结构、电子结构和光学性质.计算发现,TiO₂/MoS₂异质结的带隙相比于纯的TiO₂（101）表面明显变小,Cu/N（共）掺杂TiO₂/MoS₂异质结体系的禁带宽度也明显地减小,这导致光子激发能量的降低和光吸收能力的提高.通过计算Cu/N（共）掺杂TiO₂/MoS₂的差分电荷密度,发现光生电子与空穴积累在掺杂后的TiO₂（101）表面和单层MoS₂之间,这表明掺杂杂质体系可以有效地抑制光生电子-空穴对的复合.此外,我们计算了在不同压力下TiO₂/MoS₂异质结的几何、电子和光学性质,发现适当增加压力可以有效提高异质结的光吸收性能.本文结果表明,Cu/N（共）掺杂TiO₂/MoS₂异质结和对TiO₂/MoS₂异质结加压都能有效地提高材料的光学性能.     

Energy band alignment of 2D/3D MoS2/4H-SiC heterostructure modulated by multiple interfacial interactions

The interfacial properties of MoS₂/4H-SiC heterostructures were studied by combining first-principles calculations and X-ray photoelectron spectroscopy. Experimental (theoretical) valence band offsets (VBOs) increase from 1.49 (1.46) to 2.19 (2.36) eV with increasing MoS₂ monolayer (1L) up to 4 layers (4L). A strong interlayer interaction was revealed at 1L MoS₂/SiC interface. Fermi level pinning and totally surface passivation were realized for 4H-SiC (0001) surface. About 0.96e per unit cell transferring forms an electric field from SiC to MoS₂. Then, 1L MoS₂/SiC interface exhibits type I band alignment with the asymmetric conduction band offset (CBO) and VBO. For 2L and 4L MoS₂/SiC, Fermi level was just pinning at the lower MoS₂ 1L. The interaction keeps weak vdW interaction between upper and lower MoS₂ layers. They exhibit the type II band alignments and the enlarged CBOs and VBOs, which is attributed to weak vdW interaction and strong interlayer orbital coupling in the multilayer MoS₂. High efficiency of charge separation will emerge due to the asymmetric band alignment and built-in electric field for all the MoS₂/SiC interfaces. The multiple interfacial interactions provide a new modulated perspective for the next-generation electronics and optoelectronics based on the 2D/3D semiconductors heterojunctions.     

Toplayer-dependent crystallographic orientation imaging in the bilayer two-dimensional materials with transverse shear microscopy

Nanocontact properties of two-dimensional (2D) materials are closely dependent on their unique nanomechanical systems, such as the number of atomic layers and the supporting substrate. Here, we report a direct observation of toplayer-dependent crystallographic orientation imaging of 2D materials with the transverse shear microscopy (TSM). Three typical nanomechanical systems, MoS₂ on the amorphous SiO₂/Si, graphene on the amorphous SiO₂/Si, and MoS₂ on the crystallized Al₂O₃, have been investigated in detail. This experimental observation reveals that puckering behaviour mainly occurs on the top layer of 2D materials, which is attributed to its direct contact adhesion with the AFM tip. Furthermore, the result of crystallographic orientation imaging of MoS₂/SiO₂/Si and MoS₂/Al₂O₃ indicated that the underlying crystalline substrates almost do not contribute to the puckering effect of 2D materials. Our work directly revealed the top layer dependent puckering properties of 2D material, and demonstrate the general applications of TSM in the bilayer 2D systems.     

Growth behaviors and emission properties of Co-deposited MAPbI3 ultrathin films on MoS2

Hybrid organic-inorganic perovskite thin films have attracted much attention in optoelectronic and information fields because of their intriguing properties. Due to quantum confinement effects, ultrathin films in nm scale usually show special properties. Here, we report on the growth of methylammonium lead iodide (MAPbI₃) ultrathin films via co-deposition of PbI₂ and CH₃NH₃I (MAI) on chemical-vapor-deposition-grown monolayer MoS₂ as well as the corresponding photoluminescence (PL) properties at different growing stages. Atomic force microscopy and scanning electron microscopy measurements reveal the MoS₂ tuned growth of MAPbI₃ in a Stranski-Krastanov mode. PL and Kelvin probe force microscopy results confirm that MAPbI₃/MoS₂ heterostructures have a type-II energy level alignment at the interface. Temperaturedependent PL measurements on layered MAPbI₃ (at the initial stage) and on MAPbI₃ crystals in averaged size of 500 nm (at the later stage) show rather different temperature dependence as well as the phase transitions from tetragonal to orthorhombic at 120 and 150 K, respectively. Our findings are useful in fabricating MAPbI₃/transition-metal dichalcogenide based innovative devices for wider optoelectronic applications.     

二维材料及其异质结的声子物理研究

声子是固体最重要的元激发之一,是理解材料摩尔热容、德拜温度以及热膨胀系数等热力学性质的基础,同时电声子相互作用也决定了固体的电导和超导等特性。拉曼光谱是表征固体声子物理的重要实验手段,不仅能表征材料的结构和质量,还能提供材料声子性质、电子能带结构、电声耦合等信息。文章将拉曼光谱应用于二维材料及其范德瓦尔斯异质结的声子物理研究。先简单介绍二维材料的层间振动声子模式和层内振动声子模式,其中层间振动声子模式的频率可用线性链模型来计算,而强度则可用层间键极化率模型来解释;同类层内振动声子模式的Davydov劈裂峰之间的频率差异可用范德瓦尔斯模型拟合。随后,将这些模型推广到二维范德瓦尔斯异质结中,以转角多层石墨烯、MoS₂/石墨烯和hBN/WS₂为例介绍了范德瓦尔斯异质结的声子谱,阐述如何应用线性链模型和经典键极化率模型计算层间振动模的频率和强度,并由此给出二维范德瓦尔斯异质结的界面耦合强度和各层间呼吸模的电声耦合强度等重要参数。     

Tuning band alignment and optical properties of 2D van der Waals heterostructure via ferroelectric polarization switching

Favourable band alignment and excellent visible light response are vital for photochemical water splitting. In this work, we have theoretically investigated how ferroelectric polarization and its reversibility in direction can be utilized to modulate the band alignment and optical absorption properties. For this objective, 2D van der Waals heterostructures (HTSs) are constructed by interfacing monolayer MoS₂ with ferroelectric In2Se3. We find the switch of polarization direction has dramatically changed the band alignment, thus facilitating different type of reactions. In In₂Se₃/MoS₂/In₂Se₃ heterostructures, one polarization direction supports hydrogen evolution reaction and another polarization direction can favour oxygen evolution reaction. These can be used to create tuneable photocatalyst materials where water reduction reactions can be selectively controlled by polarization switching. The modulation of band alignment is attributed to the shift of reaction potential caused by spontaneous polarization. Additionally, the formed type-II van der Waals HTSs also significantly improve charge separation and enhance the optical absorption in the visible and infrared regions. Our results pave a way in the design of van der Waals HTSs for water splitting using ferroelectric materials.     

非共振圆偏振光作用下单层二硫化钼电子结构及其自旋/谷输运性质

基于紧束缚近似下的低能有效哈密顿模型和久保线性响应理论,研究了外部非共振圆偏振光作用下单层二硫化钼(MoS_2)电子结构及其自旋/谷输运性质.研究结果表明:单层MoS_2布里渊区K谷和K′谷附近自旋依赖子带间的能隙随着非共振右旋圆偏振光引起的有效耦合能分别线性增大和先减小后增大,随着非共振左旋圆偏振光引起的有效耦合能分别先减小后增大和线性增大,实现了系统能带结构有趣的半导体-半金属-半导体转变.此外,单层MoS_2在外部非共振圆偏振光作用下,呈现有趣的量子化横向霍尔电导和自旋/谷霍尔电导,自旋极化率在非共振右/左旋圆偏振光有效耦合能±0.79 eV附近达到最大并发生由正到负或由负到正的急剧转变,谷极化率随着非共振圆偏振光有效耦合能先增大后减小并在其绝对值0.79-0.87 eV范围内达到100%.因而,可以利用外部非共振圆偏振光将单层MoS_2调制成自旋/谷以及光电特性优异的新带隙材料.     

层状二硫化钼研究进展

近年来,层状二硫化钼由于其特殊的类石墨烯结构和独特的物理化学性质已成为国内外研究的热点.本文综述了层状二硫化钼的物理结构、价带结构和光学性质;介绍了制备方法,包括生长制备和剥离制备.生长制备的原料包括四硫代钼酸铵((NH4)_2MoS_4)、钼(Mo)和三氧化钼(MoO_3)等.剥离制备包括微机械剥离、液相超声法、锂离子插层法和电化学锂离子插层法等.归纳了层状二硫化钼在场效应晶体管、传感器和存储方面的应用,展望了层状二硫化钼的研究前景.     

金衬底调控单层二硫化钼电子性能的第一性原理研究

基于密度泛函的第一性原理研究了金衬底对单层二硫化钼电子性能的调控作用. 从结合能、能带结构、电子态密度和差分电荷密度四个方面进行了深入研究. 结合能计算确定了硫原子层在界面的排布方式, 并指出这种吸附结构并不稳定. 能带结构分析证实了金衬底与单层二硫化钼形成肖特基接触, 并出现钉扎效应. 电子态密度分析表明金衬底并没有影响硫原子和钼原子之间的共价键, 而是通过调控单层二硫化钼的电子态密度增加其导电率. 差分电荷密度分析表明单层二硫化钼的导电通道可能在界面处产生. 研究结果可对单层二硫化钼晶体管的建模和实验制备提供指导.     

Interfacial defect engineering and photocatalysis properties of hBN/MX2 (M = Mo,W, and X = S,Se heterostructures

Van der Waals (VDW) heterostructures have attracted significant research interest due to their tunable interfacial properties and potential applications in many areas such as electronics, optoelectronic, and heterocatalysis. In this work, the influences of interfacial defects on the electronic structures and photocatalytic properties of hBN/MX₂ (M = Mo, W, and X=S, Se) are studied using density functional theory calculations. The results reveal that the band alignment of hBN/MX₂ can be adjusted by introducing vacancies and atomic doping. The type-I band alignment of the host structure is maintained in the heterostructure with n-type doping in the hBN sublayer. Interestingly, the band alignment changed into the type-II heterostructrue due to V_B defect and p-type doping is introduced into the hBN sublayer. This can conduce to the separation of photo-generated electron-hole pairs at the interfaces, which is highly desired for heterostructure photocatalysis. In addition, two Z-type heterostructures including hBN(Be_B)/MoS₂, hBN(Be_B)/MoSe₂, and hBN(V_N)/MoSe₂ are achieved, showing the decreasing of band gap and ideal redox potential for water splitting. Our results reveal the possibility of engineering the interfacial and photocatalysis properties of hBN/MX₂ heterostructures via interfacial defects.     

Mechanical properties of lateral transition metal dichalcogenide heterostructures

Transition metal dichalcogenide(TMD)monolayers attract great attention due to their specific structural,electronic and mechanical properties.The formation of their lateral heterostructures allows a new degree of flexibility in engineering electronic and optoelectronic dervices.However,the mechanical properties of the lateral heterostructures are rarely investigated.In this study,a comparative investigation on the mechanical characteristics of 1H,IT'and 1H/1T'heterostructure phases of different TMD monolayers including molybdenum disulfide(M0S₂)molybdenum diselenide(MoSe₂),Tungsten disulfide(WS₂),and Tungsten diselenide(WSe₂)was conducted by means of density functional theory(DFT)calculations.Our results indicate that the impact of the lateral heterostructures has a relatively weak mechanical strength for all the TMD monolayers.The significant correlation bet ween the mechanical properties of the TMD monolayers and their structural phases can be used to tune their stiffness of the materials.Our findings,therefore,suggest a novel strategy to manipulate the mechanical characteristics of TMDs by engineering their structural phases for their practical applications.     

Role of graphene in improving catalytic behaviors of AuNPs/MoS2/Gr/Ni-F structure in hydrogen evolution reaction

The efficient production of hydrogen through electrocatalytic decomposition of water has broad prospects in modern energy equipment. However, the catalytic efficiency and durability of hydrogen evolution catalyst are still very deficient, which need to be further explored. Here in this work, we prove that introducing a graphene layer (Gr) between the molybdenum disulfide and nickel foam (Ni-F) substrate can greatly improve the catalytic performance of the hybrid. Owing to the excitation of local surface plasmon resonance (LSPR) of gold nanoparticles (NPs), the electrocatalytic hydrogen releasing activity of the MoS₂/Gr/Ni-F heterostructure is greatly improved. This results in a significant increase in the current density of AuNPs/MoS₂/Gr/Ni-F composite material under light irradiation and in the dark at 0.2 V (versus reversible hydrogen electrode (RHE)), which is much better than in MoS₂/Gr/Ni-F composite materials. The enhancement of hydrogen release can be attributed to the injection of hot electrons into MoS₂/Gr/Ni-F by AuNPs, which will improve the electron density of MoS₂/Gr/Ni-F, promote the reduction of H₂O, and further reduce the activation energy of the electrocatalyst hydrogen evolution reaction (HER). We also prove that the introduction of graphene can improve its stability in acidic catalytic environments. This work provides a new way of designing efficient water splitting system.     

掺杂对金属-MoS_2界面性质调制的第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法,计算了卤族元素掺杂对金属-MoS_2界面性质的影响,包括缺陷形成能、电子能带结构、差分电荷密度以及电荷布居分布.计算结果表明:卤族元素原子倾向于占据单层MoS_2表面的S原子位置;对于单层MoS_2而言,卤族元素的掺杂将在禁带中引入杂质能级以及导致费米能级位置的移动.对于金属-MoS_2界面体系,结合Schottky-Mott模型,证明了卤族元素的掺杂可以有效地调制金属-MoS_2界面间的肖特基势垒高度.发现F和Cl原子的掺杂将会降低体系的肖特基势垒高度.相比之下,Br和I原子的掺杂却增大了体系的肖特基势垒高度.通过差分电荷密度和布居分布的分析,阐明了肖特基势垒高度的被调制是因为电荷转移形成的界面偶极矩的作用导致.研究结果解释了相关实验现象,并给二维材料的器件化应用提供了调节手段.