基于硫蒸气处理转移的单层二硫化钼的荧光增强

采用化学气相沉积法在SiO2/Si衬底上制备了单层MoS2,再通过300℃硫蒸气处理用聚甲基丙烯酸甲酯(Polymethyl Methacrylate,PMMA)转移下的单层MoS2.使用原子力显微镜、真空荧光检测和拉曼光谱等手段表征了样品的形貌和光致发光性能.结果表明:经过硫蒸气处理转移后的单层MoS2的光致发光强度比由化学气相沉积法制备的未处理的单层MoS2的光致发光强度增强了约5倍.光致发光强度增强是由于在硫蒸气处理过程中,单层MoS2的部分硫空位被硫原子纳米团簇所填补,从而提高了光致发光效率.此外,分别将单层MoS2转移到SiO2/Si衬底、石英、三氧化铝及氟化镁衬底再经过硫处理后,也观察到了类似的荧光增强现象.     

真空中固液界面的离子束分析研究

原位实时地高精度测量固液界面的元素或离子(电荷)组成和动态变化对于界面反应和相互作用研究非常重要,但是传统的高分辨离子束分析实验在真空环境中不能直接测量液体样品。本文研制了一种固体-液体界面探针,该探针使用氮化硅-铝纳米复合膜作为真空密封窗和电化学电极,利用复旦大学核微探针成功开展了真空中固体-液体界面探针0.01 mol/L氯化钡和1 mol/L氯化镧溶液样品固体-液体界面的卢瑟福背散射(RBS)分析和粒子激发X射线(PIXE)分析。实验结果表明,真空环境下,固液界面探针纳米薄窗可承受2 MeV He+离子注量为1.0×1018 ions/cm2的辐照。微区PIXE分析成功获得了固液界面探针结构的元素分布。通过对卢瑟福背散射能谱进行分析,获取了20 nm分辨的电极界面微米深溶液中的La, Cl元素浓度。在1 mol/L的LaCl3固液界面电极表面,负电压(–2.3 V)时电解质离子在电极表面高浓度聚集,正电压(+2.3 V)时电解质在电极表面呈低浓度分布,在约1 250 nm深处电解质溶液趋向于体浓度。     

Adsorption of 5-halouracils on Au(111)

The immobilization of 5-halouracils on Au(111) has been studied by soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Multilayer and monolayer films of 5-chlorouracil were deposited on the Au(111) surface by evaporation in vacuum while films of 5-bromouracil were absorbed in two modes: from the gas phase under ultrahigh vacuum conditions and from solution. The photoemission spectra of C, N and O 1s, Br 3d and Cl 2p as well as the absorption spectra at the N and O K-edges were measured for monolayers of 5-halouracil films and the nature of the bonding with the Au(111) surface has been determined. From the angular dependence of the NEXAFS spectra at the O and K-edges, we conclude that these 5-halouracils are lying nearly parallel to the Au(111) surface. Distinct chemical states and surface adsorption geometry of the 5-BrU molecules for monolayer coverage prepared in two different ways have been found.     

Surface tension and density measurements for indium and uranium using a sessile-drop apparatus with glow discharge cleaning

The sessile-drop method is used to measure the surface tension and density of liquid indium and uranium under high vacuum. Measurements are made over the temperature range 156–500°C for In and at the melting point for U. Surface oxides are efficiently removed with a glow discharge system. Drop profiles are captured by photograph and processed using nonlinear regression to yield the surface tension and density. In this regression procedure, normal distances from calculated profiles to data points are minimized. For indium, the density and surface tension measurements yield _mp = 7.05 × 10³kg/m³, d/dT = −0.776 kg/m³·°C, and γ_mp = 0.568 N/m, dγ/dT = −9.45 × 10⁻⁵ N/m·°C. The results for uranium at the melting point are _mp = 17.47 × 10³ kg/m³ and γ_mp = 1.653 N/m.     

超细Ni80Cr20合金丝的冷拉拔制备及表征

开展特定直径超细Ni80Cr20合金丝的冷拉拔制备研究,并通过扫描电子显微镜(SEM)、X射线衍射仪(XRD)和金属丝强力仪等对超细合金丝进行了表征。研究结果表明:烧头过程最佳电解液浓度为0.05mol/L,时间为3s;制备的镍铬合金丝直径为24.54μm,表面光滑,尺寸精准;拉拔后晶粒尺寸减小,退火后晶粒尺寸长大;拉拔态镍铬丝由于加工硬化,其伸长率由16%降低至1.88%;退火可改善镍铬丝性能,其断裂强度降低,伸长率增大,合金塑性增强,氮气退火丝的断裂强度大于真空退火丝的断裂强度,伸长率则相反。     

Surface potential measurement on organic ultrathin film by Kelvin probe force microscopy using a piezoelectric cantilever

We measured surface potential (SP) on a ?-conju-gated thiophene oligomer monolayer film deposited on a metallic substrate by Kelvin probe force microscopy using a piezoelectric cantilever. Since the cantilever has a relatively large spring constant (calculated as about 150 N/m), the frequency modulation detection method was used for tip-sample distance regulation in order to achieve high-sensitivity SP measurement. A contact potential difference between monolayer regions and the metallic substrate was clearly observed in an SP image. Furthermore, an apparent change in a contrast of the SP image was observed while the sample was irradiated with ultraviolet light.     

HfSe2 monolayer stability tuning by strain and charge doping

Strain and charge doping are the effective ways to modulate the electronic and phonon properties of materials. The effects of biaxial tensile strains and charge dopings on the stabilities of HfSe₂ monolayer have been systematically investigated using first-principles methods. Its two-dimensional Young's modulus is only 65.4 N/m, and it is easy to be stretched. When the tensile strain is applied on HfSe₂ monolayer, two of its phonon modes soften with one frequency decreasing to zero at critical strain. Our results show that electron and hole dopings could suppress the softening of phonon modes, and significantly enhance the ideal strength by 28% and 36%, respectively. The calculations for electronic structures and phonon dispersions provide the theoretical references for future nano-device designing.     

基于第一性原理的锂空气电池钌掺杂石墨烯正极 氧还原反应机理研究

双电解液锂空气电池因其高理论能量密度受到广泛研究,但电池正极侧氧还原反应(ORR)速率低,其反应速率是限制锂空气电池发展的主要因素之一.本文提出了以钌(Ru)掺杂单层石墨烯作为正极ORR催化剂,采用第一性原理计算nRu (n=1～3)掺杂石墨烯的电子结构和氧气在Ru掺杂石墨烯表面的吸附性能,并以过渡态搜索方法获得ORR反应路径,研究碱性溶液中Ru掺杂单层石墨烯作用下的ORR机理.研究结果表明,经Ru原子掺杂后,石墨烯能够获得稳定的掺杂结构,且电导率显著提升.同原始单层石墨烯相比,Ru掺杂石墨烯增强了对O₂的吸附能力.在三Ru(n=3)掺杂石墨烯表面进行的ORR无需克服任何能垒.此外,三Ru掺杂石墨烯表面对OH基团的吸附能最低,有利于ORR的连续进行.研究表明三Ru掺杂石墨烯有望成为一种新型的ORR催化剂以提高双电解液锂空气电池的性能.     

Deposition of monolayer and bulk lead on Ag(111) studied in vacuum and in an electrochemical cell

The early stages of Pb condensation on Ag(111) have been studied in ultrahigh vacuum and in an electrochemical cell. The vacuum deposited films were investigated by AES, LEED and work function measurements, and the electrolytically deposited films in situ by cyclic voltammetry, potential step experiments and reflectance spectroscopy. Despite the very different environments a similar growth behaviour is observed. In both cases deposition starts with the formation of a uniform monolayer on Ag(111), which occurs in three steps: random adsorption at very low coverages followed by a √3 × √3 superstructure and finally, a hexagonal close packed layer. From an analysis of the AES data for the vacuum deposited film a particular case of Stranski-Krastanov type is found for the subsequent growth. The formation of ordered submonolayer structures which is clearly detectable by LEED, is inferred for the electrolytically formed layer since a similar coverage dependence in the reflectance is observed. From the underpotential shift of the electrolytically formed monolayer an excess binding energy of about 0.3 eV is deduced for the Pb adatoms on Ag(111).     

Intercalation of silver atoms under a graphite monolayer on Ni(111)

The process of silver intercalation under a graphite monolayer (GM) grown on the (111) nickel single-crystal face, GM/Ni(111), is studied. The experiments were conducted in ultrahigh vacuum. The systems were formed in situ in a vacuum chamber under direct monitoring of each stage in the formation of the systems by angle-resolved UV photoelectron spectroscopy and LEED. The possibility of silver intercalation in the GM/Ni(111) system was studied in the course of deposition of various amounts of the metal on the given subject with subsequent heat treatment. It was established that the process occurs optimally under cyclic alternation of the operations of adsorbate (Ag) deposition on the GM/Ni(111) surface and subsequent annealing of the system. In the intermediate stages of GM/Ag/Ni(111) formation, the GM on Ni(111) was found to exist in two phases. Ag intercalation under a graphite monolayer on Ni(111) at room temperature was verified.     

Protoluminescence study of ultra-high vacuum cleaved germanium

We have studied the photoluminescence of Ge samples cleaved in ultra-high vacuum to better understand the role of intrinsic surface states in radiative and non-radiative recombination. We find that the surface states associated with the as-cleaved surface (2 × 1 reconstructed 〈111〉) are very efficient in quenching the band-to-band recombination at T ～ 90 K. The transitions through surface states are predominantly non-radiative. The adsorption of oxygen, up to a half monolayer, markedly reduces this quenching effect by removing the surface state bands. Some preliminary experimental results on the question of radiative transitions involving the surface states is presented. No luminescence unambiguously attributable to surface state transitions is observed for wavelengths shorter than 5 μm.     

Comparison of single-layer and bilayer InAs/GaAs quantum dots with a higher InAs coverage

Epitaxially grown self-assembled InAs quantum dots (QDs) have found applications in optoelectronics. Efforts are being made to obtain efficient quantum-dot lasers operating at longer telecommunication wavelengths, specifically 1.3 μm and 1.55 μm. This requires narrow emission linewidth from the quantum dots at these wavelengths. In InAs/GaAs single layer quantum dot (SQD) structure, higher InAs monolayer coverage for the QDs gives rise to larger dots emitting at longer wavelengths but results in inhomogeneous dot-size distribution. The bilayer quantum dot (BQD) can be used as an alternative to SQDs, which can emit at longer wavelengths (1.229 μm at 8 K) with significantly narrow linewidth (∼16.7 meV). Here, we compare the properties of single layer and bilayer quantum dots grown with higher InAs monolayer coverage. In the BQD structure, only the top QD layer is covered with increased (3.2 ML) InAs monolayer coverage. The emission line width of our BQD sample is found to be insensitive towards post growth treatments.     

固体电解质Rb4Cu16I7Cl13制备及离子导电性研究

液态固体电解质材料的离子电导率低,安全性问题在一定程度上限制了其发展与应用,而固体电解质材料在室温下具有很好的稳定性和高的离子电导率值,具有较好的应用前景.本文采用机械化学球磨法制备固体电解质Rb₄Cu₁₆I₇Cl₁₃粉末,探索制备工艺和球磨参数,对其晶体结构进行解析、观察粉体微观结构、通过交流阻抗谱及等效电路分析得到了离子电导率与活化能、并详细探讨其离子传导性能与晶体结构的关系以及化学成分稳定性进行研究.实验结果表明,在480 rpm转速下球磨6 h时可得到纯的固体电解质Rb₄Cu₁₆I₇Cl₁₃物相.粉体晶粒尺寸分布均匀,均在20 nm-400 nm之间,室温下固体电解质Rb₄Cu₁₆I₇Cl₁₃离子电导率可达到0.213 S/cm且活化能为0.087(9)eV.在真空干燥条件下存放5天和12天后观察了微观形貌和化学稳定性...     

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.     

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.     

GeI2 monolayer: a model thermoelectric material from 300 to 600 K

ABSTRACT

In this work, the electronic structure, optical properties and thermoelectric properties of the GeI₂ monolayer are calculated by the first principles with the Boltzmann transport equation. The monolayer is calculated as an indirect band gap semiconductor with an indirect band gap of a value 2.19?eV. This GeI₂ monolayer is good for absorbing low-energy photons, and it is insensitive to high-energy photons. The material is stable at temperatures up to 600?K, so we calculated the thermal conductivity (K_L), Seebeck coefficient (S), power factor (PF) and thermoelectric figure of merit (ZT) of the GeI₂ monolayer at various carrier concentrations from 300 to 600?K. Due to the lower group velocity, the GeI₂ monolayer has a lower thermal conductivity of 0.48?W/m?K at 300K. In P-type doping, the power factor can up to 0.11?mW/m?K², and its ZT value is 4.04 at 600?K of the GeI₂ monolayer, indicating that the GeI₂ monolayer is a potential thermoelectric material.     

TiS2-graphene heterostructures enabling polysulfide anchoring and fast electrocatalyst for lithium-sulfur batteries: A first-principles calculation

Lithium-sulfur batteries have attracted attention because of their high energy density. However, the "shuttle effect" caused by the dissolving of polysulfide in the electrolyte has greatly hindered the widespread commercial use of lithium-sulfur batteries. In this paper, a novel two-dimensional TiS₂/graphene heterostructure is theoretically designed as the anchoring material for lithium-sulfur batteries to suppress the shuttle effect. This heterostructure formed by the stacking of graphene and TiS₂ monolayer is the van der Waals type, which retains the intrinsic metallic electronic structure of graphene and TiS₂ monolayer. Graphene improves the electronic conductivity of the sulfur cathode, and the transferred electrons from graphene enhance the polarity of the TiS₂ monolayer. Simulations of the polysulfide adsorption show that the TiS₂/graphene heterostructure can maintain good metallic properties and the appropriate adsorption energies of 0.98-3.72 eV, which can effectively anchor polysulfides. Charge transfer analysis suggests that further enhancement of polarity is beneficial to reduce the high proportion of van der Waals (vdW) force in the adsorption energy, thereby further enhancing the anchoring ability. Low Li₂S decomposition barrier and Li-ion migration barrier imply that the heterostructure has the ability to catalyze fast electrochemical kinetic processes. Therefore, TiS₂/graphene heterostructure could be an important candidate for ideal anchoring materials of lithium-sulfur batteries.     

Flow Characteristics in the Exhaust of a Pulsed Megawatt Gas Fed Arc

The transient flow generated by a pulsed, megawatt-level, gas-fed arc with an applied magnetic nozzle has been examined with a new design piezoelectric pressure transducer. Sensor thermal conduction and accelerations have been examined and eliminated in the 500?sec period of plasma flow. Existence of a large magnitude cold gas pressure front of 20?sec duration has been reconfirmed and its relationship to the following plasma flow of about 200?sec duration has been examined for the first time. At a point 30 cm from the arc source, initially near vacuum conditions (typically with an arc current of 11.2 kA and 1 tesla applied magnetic field), a pressure pulse of unionized gas with a magnitude of 104 N/m2 is followed by plasma flows with nearly constant impact pressure of 103 N/m2. Pressure and number density in this plasma region are seen to decrease with applied magnetic field strength. With electron density derived from Thomson scattering measurements (1020 m-3) plasma flow velocities on the order of 5 × 104 m/sec are calculated.     

The electrodeposition of copper onto UHV-prepared GaAs(0 0 1) surfaces

Synchrotron surface X-ray diffraction has been used to investigate in situ the morphology and epitaxy of monolayer amounts of copper electrodeposited from aqueous electrolyte onto ultra-high vacuum prepared, smooth, Ga- or As-terminated GaAs(0 0 1) surfaces. The fcc lattice of the epitaxial Cu islands is rotated by 5° and tilted by about 9° with respect to the GaAs substrate lattice, leading to eight symmetry equivalent domains of Cu islands terminated by {1 1 1} facets.     

Magnetic properties of AlN monolayer doped with group 1A or 2A nonmagnetic element: First-principles study

The electronic structure, magnetic properties, and mechanism of magnetization in two-dimensional(2D) aluminum nitride(AlN) monolayer doped with nonmagnetic elements of group 1A(Li, Na, K) or group 2A(Be, Mg, Ca) were systematically investigated using first-principles studies. Numerical results reveal that the total magnetic moments produced by group 1A and group 2A nonmagnetic doping are 2.0μB and 1.0μB per supercell, respectively. The local magnetic moments of the three N atoms around the doping atom are the primary moment contributors for all these doped AlN monolayers. The p orbital of the dopant atom contributes little to the total magnetic moment, but it influences adjacent atoms significantly, changing their density of states distribution, which results in hybridization among the p orbitals of the three closest N atoms, giving rise to magnetism. Moreover, the doped AlN monolayer, having half-metal characteristics,is a likely candidate for spintronic applications. When two group 1A or group 2A atoms are inserted, their moments are long-range ferromagnetically coupled. Remarkably, the energy of formation shows that, if the monolayer has been grown under N-rich conditions, substitution of a group 2A atom at an Al site is easier than substitution of a group 1A atom.