低温下二硫化钼电子迁移率研究

二硫化钼(MoS₂)是已知的二维半导体材料中光电性能最优秀的材料之一. 单原子层厚的MoS₂是禁带宽度为1.8 eV 的二维直接带隙半导体材料, 可以用来发展新型的纳米电子器件和光电功能器件. 本论文利用玻尔兹曼平衡方程输运理论研究低温时MoS₂系统的电输运性质, 计算得到了MoS₂电子迁移率的解析表达式. 研究发现, 低温时MoS₂ 的迁移率与衬底材料的介电常数的平方成正比; 与系统的电子浓度对带电杂质的浓度的比率n_e/n_i 成线性关系. 因此, 选用介电常数高的衬底材料, 适当提高MoS₂系统的载流子浓度, 同时降低杂质的浓度, 可以有效提高MoS₂系统的迁移率. 研究结果为探索以MoS₂为基础的新型纳米光电器件的研究和实际应用提供了理论依据. 关键词： 二硫化钼 迁移率 电输运 平衡方程     

Tunable terahertz transmission behaviors and coupling mechanism in hybrid MoS2 metamaterials

A hybrid metamaterial with the integration of molybdenum disulfide(MoS₂)overlayer is proposed to manipulate the terahertz(THz)wave.The simulated results indicate that the introduction of MoS₂ layer could significantly modify the resonant responses with large resonance red-shift and bandwidth broadening due to the depolarization field effect,especially for the structure on the small permitivity substrate.Additionally,the wide-band modulator in off-resonant region and a switch effect at resonance can be achieved by varying the conductivity of MoS₂ layer.Further theoretical calculations based on the Lorentz coupling model are consistent with the simulated results,explicating the response behaviors originate from the coupling between MoS₂ overlayer and the metastructure.Our results could provide a possibility for active control THz modulator and switchable device based on the MoS₂ overlayer and advance the understanding of the coupling mechanism in hybrid structures.     

Reduction of low‐frequency noise in multilayer MoS2 FETs using a Fermi‐level depinning layer

Two‐dimensional transition metal dichalcogenides (TMDCs) are potential candidate materials for future thin‐film field effect transistors (FETs). However, many aspects of this device must be optimized for practical applications. In addition, low‐frequency noise that limits the design window of electronic devices, in general, must be minimized for TMD‐based FETs. In this study, the low‐frequency noise characteristics of multilayer molybdenum disulphide (MoS₂) FETs were investigated in detail, with two different contact structures: titanium (Ti) metal–MoS₂ channel and Ti metal–TiO₂ interlayer–MoS₂ channel. The results showed that the noise level of the device with a TiO₂ interlayer reduced by one order of magnitude compared with the device without the TiO₂ interlayer. This substantial improvement in the noise characteristics could be explained using the carrier number of fluctuation model. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of temperature–bias annealing on the hysteresis and subthreshold behavior of multilayer MoS2 transistors

The transfer characteristics (I_D–V_G) of multilayers MoS₂ transistors with a SiO₂/Si backgate and Ni source/drain contacts have been measured on as‐prepared devices and after annealing at different temperatures (T_ann from 150 °C to 200 °C) under a positive bias ramp (V_G from 0 V to +20 V). Larger T_ann resulted in a reduced hysteresis of the I_D–V_G curves (from ～11 V in the as‐prepared sample to ～2.5 V after T_ann at 200 °C). The field effect mobility (～30 cm² V^–1 s^–1) remained almost unchanged after the annealing. On the contrary, the subthreshold characteristics changed from the common n‐type behaviour in the as‐prepared device to the appearance of a low current hole inversion branch after annealing. This latter effect indicates a modification of the Ni/MoS₂ contact that can be explained by the formation of a low density of regions with reduced Schottky barrier height (SBH) for holes embedded in a background with low SBH for electrons. Furthermore, a temperature dependent analysis of the subthreshold characteristics revealed a reduction of the interface traps density from ～9 × 10¹¹ eV^–1cm^–2in the as‐prepared device to ～2 × 10¹¹ eV^–1cm^–2after the 200 °C temperature–bias annealing, which is consistent with the observed hysteresis reduction.

Schematic representation of a back‐gated multilayer MoS₂ field effect transistor (left) and transfer characteristics (right) measured at 25 °C on an as‐prepared device and after the temperature–bias annealing at 200 °C under a positive gate bias ramp from 0 V to +20 V.     

Band renormalization and spin polarization of MoS2 in graphene/MoS2 heterostructures

Transition metal dichalcogenides exhibit spin–orbit split bands at the K‐point that become spin polarized for broken crystal inversion symmetry. This enables simultaneous manipulation of valley and spin degrees of freedom. While the inversion symmetry is broken for monolayers, we show here that spin polarization of the MoS₂ surface may also be obtained by interfacing it with graphene, which induces a space charge region in the surface of MoS₂. Polarization induced symmetry breaking in the potential gradient of the space charge is considered to be responsible for the observed spin polarization. In addition to spin polarization we also observe a renormalization of the valence band maximum (VBM) upon interfacing of MoS₂ with graphene. The energy difference between the VBM at the Γ‐point and K‐point shifts by ～150 meV between the clean and graphene covered surface. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

二维辉钼材料及器件研究进展

经过几十年的发展, 集成电路的特征尺寸将在10–15年内达到其物理极限, 替代材料的研究迫在眉睫. 石墨烯曾被寄予厚望, 但由于其缺乏带隙限制了在数字电路领域的应用. 近年来, 单层及多层辉钼材料由于具有优异的半导体性能, 有可能超过石墨烯成为硅的替代者而引起了微纳电子领域的广泛关注. 本文对近二年国际上辉钼半导体器件研制、辉钼半导体材料的性能 表征及制备方法研究等方面的进展进行了综述, 并对大面积单层材料的研制提出了值得关注的方向. 关键词： 2')" href="#">MoS₂ 辉钼材料 纳米材料 集成电路     

Low‐temperature photoluminescence of oxide‐covered single‐layer MoS2

We present a photoluminescence study of single‐layer MoS₂ flakes on SiO₂ surfaces. We demonstrate that the luminescence peak position of flakes prepared from natural MoS₂, which varies by up to 25 meV between individual flakes, can be homogenized by annealing in vacuum. We use HfO₂ and Al₂O₃ layers prepared by atomic layer deposition to cover some of our flakes. In these flakes, we observe a suppression of the low‐energy luminescence peak which appears in asprepared flakes at low temperatures. We infer that this peak originates from excitons bound to surface adsorbates. We also observe different temperature‐induced shifts of the luminescence peaks for the oxide‐covered flakes. This effect stems from the different thermal expansion coefficients of the oxide layers and the MoS₂ flakes. It indicates that the single‐layer MoS₂ flakes strongly adhere to the oxide layers and are therefore strained. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Giant two‐photon absorption in monolayer MoS2

Strong two‐photon absorption (TPA) in monolayer MoS₂ is demonstrated in contrast to saturable absorption (SA) in multilayer MoS₂ under the excitation of femtosecond laser pulses in the near‐infrared region. MoS₂ in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO₂/Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS₂ possesses a nonsaturation TPA coefficient as high as ∼(7.62 ±0.15) ×10³ cm/GW, larger than that of conventional semiconductors by a factor of 10³. As a result of TPA, two‐photon pumped frequency upconverted luminescence is observed directly in the monolayer MoS₂. For the multilayer MoS₂, the SA response is demonstrated with the ratio of the excited‐state absorption cross section to ground‐state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS₂ is ∼97 GW/cm², larger than that of the multilayer MoS₂ of ∼78 GW/cm².

     

水热法合成纳米花状二硫化钼及其微观结构表征

本文以钼酸钠、硫代乙酰胺为前驱体, 硅钨酸为添加剂, 成功用水热法合成高纯度纳米花状二硫化钼. 产物特性用X射线衍射(XRD)、能量色散谱(EDS)、扫描电子显微镜(SEM)进行表征. XRD和EDS图显示实验产物为二硫化钼, 且其结晶度和层状堆垛良好. SEM图谱则表明二硫化钼为纳米花状结构, 颗粒直径300 nm左右, 由几十上百片花瓣组成, 每片花瓣厚度十个纳米左右. 通过以硅钨酸为变量的梯度实验, 研究发现, 硅钨酸对于纳米花状MoS₂的形成具有重要作用, 不添加硅钨酸, 无法形成纳米花状MoS₂, 此外, 硅钨酸的剂量会影响合成MoS₂的大小和形貌. 本文还对纳米花状二硫化钼的形成机理做了初步的讨论.     

无机类富勒烯MoS2纳米材料的制备与表征

采用简单的沉淀法 ,利用聚乙二醇作为分散剂 ,盐酸羟铵为还原剂 ,以硫化铵为硫源合成了具有无机类富勒烯结构的纳米二硫化钼 ,通过粉末X射线衍射 (XRD)、扫描电镜 (SEM )和高分辨透射电镜 (HRTEM)等方法对其形貌和结构进行了表征 .结果表明 ,用聚乙二醇做分散剂 ,使其吸附在前驱物表面使颗粒环境呈现出一个相对隔绝的状态 ,在煅烧过程中 ,其空间位阻作用有利于MoS2 纳米颗粒形成无机类富勒烯结构 .     

低维MoS_2混合纳米片的发光特性

采用液相剥离法剥离MoS2块体材料,通过选择合适的剥离剂、超声时间、超声功率得到含有不同尺寸且分散均匀的MoS2混合纳米薄片悬浮溶液。在360 nm光激发下,这种悬浮液表现出单层MoS2及小尺寸MoS2纳米颗粒的复合发光特征。与微机械剥离得到的单层MoS2的发光特性相比,这种液相法得到的混合纳米薄片在512 nm处的最强发光峰位发生明显蓝移。混合纳米薄片在横向尺度上所产生的量子限制效应可能是导致该峰位蓝移的主要原因。     

Significant Enhancement of Hydrogen Production in MoS2/Cu2ZnSnS4 Nanoparticles

Hydrogen produced from water splitting is a renewable and clean energy source. Great efforts have been paid in searching for inexpensive and highly efficient photocatalysts. Here, significant enhancement of hydrogen production has been achieved by introducing ≈1 mol% of MoS₂ to Cu₂ZnSnS₄ nanoparticles. The MoS₂/Cu₂ZnSnS₄ nanoparticles showed a hydrogen evolution rate of ≈0.47 mmol g⁻¹ h⁻¹ in the presence of sacrificial agents, which is 7.8 times that of Cu₂ZnSnS₄ nanoparticles (0.06 mmol g⁻¹ h⁻¹). In addition, the MoS₂/Cu₂ZnSnS₄ nanoparticles exhibited high stability, and only ≈3% of catalytic activity was lost after a long time irradiation (72 h). Microstructure investigation on the MoS₂/Cu₂ZnSnS₄ nanoparticles reveals that the intimate contact between the nanostructured MoS₂ and Cu₂ZnSnS₄ nanoparticles provides an effective one‐way expressway for photogenerated electrons transferring from the conduction band of Cu₂ZnSnS₄ to MoS₂, thus boosting the lifetime of charge carriers, as well as reducing the recombination rate of electrons and holes.     

Na、 Be、 Mg掺杂单层MoS_2的第一性原理研究

本文基于第一性原理研究了Na、 Be、 Mg掺杂单层MoS_2的稳定性、能带结构、态密度以及电荷分布.得到Be掺杂单层MoS_2体系在实验上较容易实现,在三者掺杂体系中稳定性最强.与此同时,掺杂体系的带隙值都降低,有利于电子的跃迁,增强了导电性能;掺杂原子打破了原体系的平衡关系,导致周边S原子p轨道上的多余的电子会与近邻Mo原子d轨道上的电子产生相互作用;平衡的打破,也导致了杂质原子周围存在着电荷聚集和损失的现象.     

缺陷对单层MoS2电子结构的影响

为了研究缺陷对单层MoS2的电子结构, 本文基于密度泛函理论框架下的第一性原理, 采用数值基组的方法计算了MoS2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现：Mo位缺陷、S位缺陷的MoS2的能带结构中的价带顶与导带底都在Q点, 为直接带隙材料; 其中Mo位缺陷体的禁带区域都出现5条新能级, S位缺陷体的禁带区域出现了3条新能级; 缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言, Mo位缺陷体的费米能级处出现了峰值, 表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现, Mo缺陷周围存在着负电荷聚集的现象, S缺陷周围存在正电荷聚集的现象.     

缺陷对单层MoS_2电子结构的影响

为了研究缺陷对单层MoS_2的电子结构,本文基于密度泛函理论框架下的第一性原理,采用数值基组的方法计算了MoS_2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现:Mo位缺陷、S位缺陷的MoS_2的能带结构中的价带顶与导带底都在Q点,为直接带隙材料;其中Mo位缺陷体的禁带区域都出现5条新能级,S位缺陷体的禁带区域出现了3条新能级;缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言,Mo位缺陷体的费米能级处出现了峰值,表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现,Mo缺陷周围存在着负电荷聚集的现象,S缺陷周围存在正电荷聚集的现象.     

MoS2摩擦表面氧化与电子转移

利用电子自旋共振谱（ＥＳＲ）和Ｘ射线光电子谱（ＸＰＳ）研究了ＭｏＳ２摩擦表面的氧化行为和摩擦表面氧化的电子转移,发现Ｍｏ２在摩擦失效过程中Ｍｏ＾４＋与氧作用生成稳定的Ｍｏ＾６＋终态氧化物,其间经过Ｍｏ＾５＋过渡态。深入揭示了ＭｏＳ２摩擦表面氧化过程的复杂性,指出Ｍｏ在摩擦表面氧化过程中以多种化学状态存在,Ｍｏ原子的氧化是Ｍｏ４ｄ轨道上的单电子转移过程。     

Co对单层MoS_2电子结构与磁性的影响

为了研究Co对单层MoS_2电子结构和磁性的影响,本文基于第一性原理,采用数值基组的方法计算了Co吸附式掺杂、Co替代式掺杂单层MoS_2的能带结构、态密度以及分析了其结构的稳定性.结果发现:Co替换式掺杂体系的形成能较低,实验上容易实现;Co在Mo位吸附的稳定性强于在S位吸附;Mo位吸附体系的总磁矩为0.999μB,其磁矩的主要来源于Co原子的吸附所贡献的0.984μB,Co原子的掺杂体系总磁矩为1.029μB,其磁矩的主要由Co原子替代掉一个Mo原子所贡献的磁矩为0.9444μB,相比于吸附体系,Co原子对磁矩的贡献率有所降低;无论是Co吸附在单层MoS_2表面还是Co直接替代掉Mo原子的掺杂体系,Co原子3d轨道的引入是引起单层MoS_2体系磁性的主要原因.     

V,Cr,Mn掺杂MoS2磁性的第一性原理研究

基于第一性原理的自旋极化密度泛函理论分别研究了过渡金属V, Cr, Mn掺杂单层MoS₂的电子结构、 磁性和稳定性. 结果表明: V和Mn单掺杂均能产生一定的磁矩, 而磁矩主要集中在掺杂的过渡金属原子上, Cr单掺杂时体系不显示磁性. 进一步讨论双原子掺杂MoS₂ 体系中掺杂原子之间的磁耦合作用发现, Mn掺杂的体系在室温下显示出稳定的铁磁性, 而V掺杂则表现出非自旋极化基态. 形成能的计算表明Mn掺杂的MoS₂体系相对V和Cr 掺杂结构更稳定. 由于Mn掺杂的MoS₂ 不仅在室温下可以获得比较好的铁磁性而且其稳定性很高, 有望在自旋电子器件方面发挥重要的作用. 关键词： 2')" href="#">单层MoS₂ 掺杂 铁磁态 第一性原理