六角星形MoSe_2双层纳米片的制备及其光致发光性能

采用化学气相沉积法,以三氧化钼作为钼源,硒粉作为硒源,在H_2/Ar气氛下生长出硒化钼纳米片.扫描电镜、X射线衍射表征结果表明,MoSe_2产物呈六角星状,横向尺寸约10μm,具有很好的晶体质量和结构.拉曼光谱表征其结构,确定其为双层纳米片.研究表明,高温反应时间对双层纳米片的生长具有重要的影响.通过对双层纳米片的生长机理的探究,推测其经历了3个生长过程:在高温下,Mo源和Se源被气化成气态分子并发生硒化反应形成晶核;晶核呈三角形外延生长;当反应时间持续增加,在空间位阻效应的影响下,晶体以中心原子岛为核,外延耦合生长出第二层三角形,最终形成六角星状双层纳米片.光致发光光谱结果表明,六角星状MoSe_2双层纳米片在1.53 eV处具有直接带隙和1.78 eV处具有间接带隙,其较宽范围的激发光谱响应预测其在光电探测器件领域具有潜在的应用前景.     

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.     

Ultrafast interlayer photocarrier transfer in graphene–MoSe_2 van der Waals heterostructure

We report the fabrication and photocarrier dynamics in graphene–MoSe_2 heterostructures. The samples were fabricated by mechanical exfoliation and manual stacking techniques. Ultrafast laser measurements were performed on the heterostructure and MoSe_2 monolayer samples. By comparing the results, we conclude that photocarriers injected in MoSe_2 of the heterostructure transfer to graphene on an ultrafast time scale. The carriers in graphene alter the optical absorption coefficient of MoSe_2. These results illustrate the potential applications of this material in optoelectronic devices.     

Se和MoSe_2纳米片的结构和发光性能

以Se粉和MoO_3粉为源材料,利用热丝化学气相沉积在N_2中制备了Se和MoSe_2纳米片.利用场发射扫描电子显微镜、透射电子显微镜、X射线能谱仪、显微Raman光谱仪和X射线光电子谱仪对Se和MoSe_2纳米片的结构和组成进行了系统研究.结果表明:Se粉和M0O_3粉的混合与否直接影响了Se和MoSe_2纳米片的形成和结构;当Se粉和MoO_3粉充分混合时形成Se纳米片,而Se和MoO_3粉分开放置时则形成MoSe_2纳米片.研究发现这是由于Se和MoO_3粉的混合与否使Se和MoO_3在气相中的不同反应所致.对Se和MoSe_2纳米片的发光性能研究表明,它们分别产生了774,783和784 nm的发光峰,不同于单层MoSe_2纳米片的发光性能.这些结果丰富了对二维Se基纳米材料的合成和光学性能的知识,有助于对Se基二维纳米材料的光电器件的研制.     

单层WSe2、MoSe2激子发光的压力诱导K-Λ交互转变

采用机械剥离法在金刚石对顶砧中制备了单层WSe₂和MoSe₂样品,利用高压微区荧光光谱测量技术,在氩传压介质环境下对其激子发光行为进行了高压调控研究。其中单层WSe₂的中性和负电激子演化趋势在2.43 GPa处出现拐点,单层MoSe₂中性激子发光在3.7 GPa处发生了劈裂。结合第一性原理计算分析,确认该不连续现象的产生机制为压力诱导的导带底K-Λ交互转变。该结果可以扩大至整个二维层状材料体系,为发展激子器件垫定基础。     

基于磁圆二向色谱的单层MoS_2激子能量和线宽温度依赖特性

以二硫化钼(MoS_2)为代表的过渡金属硫属化物属于二维层状材料,样品可以薄至单层.单层MoS_2是一种直接带隙半导体,在纳米逻辑器件、高速光电探测、纳米激光等领域具有广阔的应用前景.在实际应用中,温度是影响半导体材料能带结构和性质的主要因素之一.因此研究单层二维材料能带的温度依赖特性对理解其物理机理以及开展器件应用具有重要的意义.目前,在广泛采用的测量单层MoS_2反射谱的研究中,激子峰往往叠加在一个很强的光谱背底上,难以准确分辨激子的峰位和线宽.基于自行搭建的显微磁圆二向色谱系统,研究了单层MoS_2在65—300 K温度范围内的反射谱和磁圆二向色谱,结果表明磁圆二向色谱在研究单层材料激子能量和线宽方面具有明显的优势.通过分析变温的磁圆二向色谱,得到了不同温度下的A,B激子的跃迁能量和线宽.通过对激子能量和线宽的温度依赖关系进行拟合,进一步讨论了声子散射对激子线宽的影响.     

单层与双层WSe2纳米片层的光致发光

采用气相沉积法制备了WSe₂二维纳米材料,对其低温光致发光谱进行了研究。结果表明：随着WSe₂层数的增加,其光致发光强度单调下降;当WSe₂层数从单层增加为双层时,其发光强度急剧下降,表明其能带结构已从直接带隙转变为间接带隙。进一步研究了双层WSe₂的变温光致发光谱,发现随着温度的升高,双层WSe₂发光峰中A峰峰位的变化基本符合半导体带隙的温度变化规律,而I峰峰位红移与温度基本成线性关系,表明双层WSe₂同时存在间接和直接跃迁,且直接跃迁和间接跃迁特性不同。     

Au的金属颗粒对二硫化钼发光增强

二硫化钼(MoS₂)是一种层状的二维过渡金属硫族化合物材料,从块体到单层,禁带由间接带隙变为直接带隙,由于通常机械剥落的单层MoS₂是n型掺杂的,使得其发光效率仍然很低. 在本文中,采用匀胶机旋涂的方法将共振吸收峰在514 nm附近的纳米金颗粒尽可能均匀的铺在单层、双层以及多层的MoS₂样品表面,发现单层和双层样品的光致发光谱(PL谱)分别增强了约30倍和2倍同时伴随着峰位的蓝移,而多层样品的发光强度也略有增强. 拉曼特性揭示了纳米金颗粒对单层和双层MoS₂样品产生了明显的p型掺杂,从而增强了发光;同时纳米金颗粒的表面等离子激元效应对激发光的天线作用也是增强MoS₂的光致发光的一个因素. 关键词： 二硫化钼 光致发光 p型掺杂 Au纳米颗粒     

一种简单的化学气相沉积法制备大尺寸单层二硫化钼

最近单层二硫化钼以其直接带隙的性质及在电子器件、催化、光电等领域中的潜在应用而备受关注.化学气相沉积法能够制备出高质量、大尺寸且性能优良的单层二硫化钼,但其制备工艺比较复杂.本文采用简化的化学气相沉积法在蓝宝石衬底上制备出了大尺寸的单晶二硫化钼.清洗衬底时,只需要简单的清洁,不需要用丙酮、食人鱼溶液(H_2SO_4/H_2O_2=3:1)等处理,这样既减少了操作步骤,又避免了潜在的危险.升温时直接从室温加热到生长的温度,不必分段升温,并且采用常压化学气相沉积法,不需要抽真空等过程,使得实验可以快捷方便地进行.光学显微镜、拉曼光谱和光致发光谱的结果表明,生长的二硫化钼为规则的三角形单层,边长为50μm左右,远大于机械剥离的样品.     

Photoemission oscillation in epitaxially grown van der Waals β-In2Se3/WS2 heterobilayer bubbles

Thin films of millimeter-scale continuous monolayer WS₂ have been grown on SiO₂/Si substrate, followed by the deposition of β-In₂Se₃ crystals on monolayer WS₂ to prepare In₂Se₃/WS₂ van de Waals heterostructures by a two-step chemical vapor deposition (CVD) method. After the growth of In₂Se₃ at elevated temperatures, high densities of In₂Se₃/WS₂ heterostructure bubbles with monolayer to multilayer β-In₂Se₃ crystals atop are observed. Fluorescence of the resultant β-In₂Se₃/WS₂ heterostructure is greatly enhanced in intensity upon the formation of bubbles, which are evidenced by the Newton's rings in optical image owing to constructive and destructive interference. In photoluminescence (PL) mapping images of monolayer β-In₂Se₃/monolayer WS₂ heterobilayer bubble, significant oscillatory behavior of emission intensity is demonstrated due to constructive and destructive interference. However, oscillatory behaviors of peak position are also observed and come from a local heating effect induced by an excitation laser beam. The oscillatory mechanism of PL is further verified by changing the exterior pressure of bubbles placed in a home-made vacuum chamber. In addition, redshifted in peak position and broadening in peak width are observed due to strain effect during decreasing the exterior pressure of bubbles.     

碳纳米管/二硒化钼有机玻璃的非线性吸收、非线性散射和光限幅特性

MoSe₂的禁带宽度较窄（1.1–1.5 eV）,且具有可调谐的激子光电效应,这样使其在光致发光、光电晶体管、太阳能电池和光学非线性等方面具有潜在的应用价值.然而,纯的MoSe₂的光生电子空穴复合率较高,限制了其在某些光学领域中的应用.通过设计MoSe₂的复合材料,可以降低材料的光生电子空穴复合率,从而扩展其应用领域.首先,通过热溶剂法合成CNT/MoSe₂复合材料;然后,通过浇铸法将其分散在甲基丙烯酸甲酯（MMA）中制备成有机玻璃,其中MMA会聚合成聚甲基丙烯酸甲酯（PMMA）,并利用改进的Z-扫描技术首次对CNT/MoSe₂/PMMA有机玻璃的非线性吸收、非线性散射和光限幅特性进行了研究.研究表明,随着输入能量的变化,通过调节输入能量,CNT/MoSe₂/PMMA有机玻璃表现出饱和吸收（SA）和从SA到反饱和吸收的转变.结合材料特性及应用条件要求,可以得到CNT/MoSe₂/PMMA有机玻璃在光学设备,如光学限制器和锁模/调Q激光器等方向具有较好的应用前景.     

Dispersion of neutron spin resonance mode in Ba0.67K0.33Fe2As2

We report an inelastic neutron scattering investigation on the spin resonance mode in the optimally hole-doped iron-based superconductor Ba_0.67K_0.33Fe₂As₂ with T_c=38.2 K. Although the resonance is nearly two-dimensional with peak energy E_R≈14 meV, it splits into two incommensurate peaks along the longitudinal direction ([H, 0, 0]) and shows an upward dispersion persisting to 26 meV. Such dispersion breaks through the limit of total superconducting gaps ∆_tot=|∆_k|+|∆_k+Q|(about 11-17 meV) on nested Fermi surfaces measured by high resolution angle resolved photoemission spectroscopy (ARPES). These results cannot be fully understood by the magnetic exciton scenario under s^±-pairing symmetry of superconductivity, and suggest that the spin resonance may not be restricted by the superconducting gaps in the multi-band systems.     

单轴压力下Ge2X2Te5(X=Sb,Bi)薄膜拓扑相变的第一性原理研究

随着拓扑绝缘体的发现, 材料拓扑物性的研究成为凝聚态物理研究的热点领域. 本文基于第一性原理计算, 研究了化合物Ge₂X₂Te₅ (X=Sb, Bi) 的块体结构和二维单层和双层薄膜结构的拓扑物性, 以及单双层薄膜在垂直方向单轴压力下的拓扑量子相变. 研究发现, A型原子序列排列的这两种化合物都是拓扑绝缘体, 其单层薄膜都是普通金属, 而双层薄膜都是拓扑金属, 单层和双层薄膜在单轴加压过程中都没有发生拓扑量子相变; 这两种化合物的B型原子序列的晶体是普通绝缘体, 其所对应的薄膜, Ge₂Sb₂Te₅单层是普通金属, 双层薄膜和Ge₂Bi₂Te₅的单层和双层薄膜均为普通绝缘体, 但是在单轴加压过程中B 型原子序列所对应的单层和双层薄膜都转变为拓扑金属.     

Strain-induced magnetism in ReS_2 monolayer with defects

We investigate the effects of strain on the electronic and magnetic properties of ReS_2 monolayer with sulfur vacancies using density functional theory.Unstrained ReS_2 monolayer with monosulfur vacancy(V_s) and disulfur vacancy(V_(2S))both are nonmagnetic.However,as strain increases to 8%,V_S-doped ReS_2 monolayer appears a magnetic half-metal behavior with zero total magnetic moment.In particular,for V_(2S)-doped ReS_2 monolayer,the system becomes a magnetic semiconductor under 6%strain,in which Re atoms at vicinity of vacancy couple anti-ferromagnetically with each other,and continues to show a ferromagnetic metal characteristic with total magnetic moment of 1.60μb under 7%strain.Our results imply that the strain-manipulated ReS_2 monolayer with V_S and V_(2S) can be a possible candidate for new spintronic applications.     

氢化物气相外延生长的GaN膜中的应力分析

对在c面蓝宝石上用氢化物气相外延法(HVPE)生长的六方相纤锌矿结构的GaN膜中的应力进行了分析。高分辨X射线衍射(002)面和(102)面摇摆曲线扫描(半高宽数值分别为317和358角秒)表明生长的GaN膜具有较好的晶体质量。利用高分辨X射线衍射技术准确测量了制备的GaN膜的晶格常数,并计算得到GaN膜中面内双轴应变和面外双轴应变分别为3.37×10-4和-8.52×10-4,等静压应变为-7.61×10-5。拉曼光谱和激光光致发光谱测试表明HVPE-GaN外延膜具有较好的光学特性,利用拉曼光谱的E₂模式特征峰和激光光致发光谱中近带边发射峰的频移定量计算了外延膜中的面内双轴压应力和等静压应力。两种方法得到的面内双轴压应力较为相符。     

Nonlinear optical characteristics of monolayer MoSe2

In this study, we utilized picosecond pulses from an Nd:YAG laser to investigate the nonlinear optical characteristics of monolayer MoSe₂. Two‐step growth involving the selenization of pulsed‐laser‐deposited MoO₃ film was employed to yield the MoSe₂ monolayer on a SiO₂/Si substrate. Raman scattering, photoluminescence (PL) spectroscopy, and atomic force microscopy verified the high optical quality of the monolayer. The second‐order susceptibility χ⁽²⁾ was calculated to be ～50 pm V^?1 at the second harmonic wavelength ～810 nm, which is near the optical gap of the monolayer. Interestingly, our wavelength‐dependent second harmonic scan can identify the bound excitonic states including negatively charged excitons much more efficiently, compared with the PL method at room temperature. Additionally, the MoSe₂ monolayer exhibits a strong laser‐induced damage threshold ～16 GW cm^?2 under picosecond‐pulse excitation_. Our findings suggest that monolayer MoSe₂ can be considered as a promising candidate for high‐power, thin‐film‐based nonlinear optical devices and applications.     

电化学析氢反应中单层MoSe2氢吸附机理第一性原理研究

基于密度泛函理论的第一性原理方法,本文计算了单层2H相MoSe₂纳米材料表面及两种边缘（Mo原子边缘、Se原子边缘）不同活性位点、不同氢原子吸附率下的氢吸附吉布斯自由能（Gibbs free energy,用△G_H⁰表示）,并且将对应的微观结构进行了系统分析比较,得出△G_H⁰最接近于0 eV的吸附位点及相应的吸附率.同时,结合差分电荷密度和电负性理论,分析了单层MoSe₂两种边缘氢吸附的电荷转移及成键特性,进一步解释了不同吸附位点呈现的结构与能量趋势.最后,通过基于密度泛函理论的第一性原理分子动力学模拟,研究了高温热运动对两种边缘氢吸附的影响,获得了氢原子发生脱附的临界温度及对应的微观动态过程.该理论研究从原子尺度揭示了单层2H相MoSe₂纳米材料边缘不同位点在不同温度下对氢原子吸附和脱附的微观机理,证实了Mo原子边缘的畸变和重构行为,加深了对实验中单层2H相MoSe₂边缘在不同温度下氢吸附机理的理解,为实验中通过控制MoSe₂边缘设计廉价高效的析氢催化剂提供理论参考.     

Excited state biexcitons in monolayer WSe2 driven by vertically grown graphene nanosheets with high-density electron trapping edges

Interface engineering in atomically thin transition metal dichalcogenides (TMDs) is becoming an important and powerful technique to alter their properties, enabling new optoelectronic applications and quantum devices. Interface engineering in a monolayer WSe₂ sample via introduction of high-density edges of standing structured graphene nanosheets (GNs) is realized. A strong photoluminescence (PL) emission peak from intravalley and intervalley trions at about 750 nm is observed at the room temperature, which indicated the heavily p-type doping of the monolayer WSe₂/thin graphene nanosheet-embedded carbon (TGNEC) film heterostructure. We also successfully triggered the emission of biexcitons (excited state biexciton) in a monolayer WSe₂, via the electron trapping centers of edge quantum wells of a TGNEC film. The PL emission of a monolayer WSe₂/GNEC film is quenched by capturing the photoexcited electrons to reduce the electron-hole recombination rate. This study can be an important benchmark for the extensive understanding of light–matter interaction in TMDs, and their dynamics.     

钙钛矿(C6H5CH2NH3)2PbBr4拉曼光谱研究

运用激光拉曼光谱实验和密度泛函理论计算研究了450～1 700 cm-1光谱范围内有机-无机杂化钙钛矿材料(C₆H₅CH₂NH₃)₂PbBr₄的振动模式特性。对比实验所得拉曼光谱和理论计算所得拉曼光谱,发现密度泛函理论计算可以很好的模拟(C₆H₅CH₂NH₃)₂PbBr₄有机部分的分子振动模式。同时通过比较分析密度泛函理论计算和参考文献,对450～1 700 cm-1光谱范围内的拉曼峰的分子振动模式进行了初步的归属,并发现该光谱范围内的拉曼峰主要是由(C₆H₅CH₂NH₃)₂PbBr₄分子中有机部分振动所产生的。     

Tuning the energy gap of bilayer α-graphyne by applying strain and electric field

Our density functional theory calculations show that the energy gap of bilayer α-graphyne can be modulated by a vertically applied electric field and interlayer strain. Like bilayer graphene, the bilayer α-graphyne has electronic properties that are hardly changed under purely mechanical strain, while an external electric field can open the gap up to 120 meV. It is of special interest that compressive strain can further enlarge the field induced gap up to 160 meV, while tensile strain reduces the gap. We attribute the gap variation to the novel interlayer charge redistribution between bilayer α-graphynes.These findings shed light on the modulation of Dirac cone structures and potential applications of graphyne in mechanicalelectric devices.