白宝石衬底上生长的MgxZn1-xO晶体薄膜的结构和光学性能

在白宝石 (sapphire)衬底上低温外延生长出了MgxZn1 -xO晶体薄膜 .x射线衍射 (XRD)及能量色散x射线 (EDX)分析表明 ,MgxZn1 -xO薄膜的晶体结构依赖于薄膜中Mg的组分x,随着Mg组分的增大 ,MgxZn1 -xO薄膜的结构从与ZnO晶体一致的六方结构转变为与MgO晶体一致的立方结构 .对MgxZn1 -xO薄膜的紫外透射光谱及紫外光致荧光谱 (UVPL)的分析表明 ,随着Mg组分的增大 ,光学吸收边产生明显的蓝移 ,表明MgxZn1 -xO晶体薄膜的带隙增大 ,且带隙连续可调 .吸收光谱和XRD测量显示 ,带隙高达 5 6 5eV的MgxZn1 -xO晶体薄膜与MgO之间的晶格失配仅为0 16 % .     

硅基硒纳米颗粒的发光特性研究

由于尺寸缩小引起的量子效应, 硒(Se) 材料的低维纳米结构具有更高的光响应和低的阈值激射等特性, 因此成为纳米电子与纳米光电子器件领域一个重要的研究方向. 本文通过对非晶硒薄膜的快速热退火来制备硒纳米颗粒, 退火温度在100–180℃之间时, 结晶后的硒纳米颗粒均为三角晶体结构, 其颗粒尺寸随退火温度的增加而线性增大. 光致发光谱测试发现三个发光峰, 分别位于1.4eV, 1.7eV和1.83eV. 研究发现位于1.4eV处的发光峰来源于非晶硒缺陷发光, 位于1.83eV处的发光峰来源于晶体硒的带带跃迁发光; 而位于1.7eV处的发光峰强度随激发功率增强而指数增大, 且向短波长移动, 该发光峰应该来源于非晶硒与硒纳米颗粒界面处的施主-受主对复合发光. 关键词： 硅基 硒纳米颗粒 光致发光 施主-受主对     

氢化物气相外延生长高质量GaN膜生长参数优化研究

系统研究了低温成核层生长时间、高温生长时的V/Ⅲ 比以及生长温度对氢化物气相外延生长GaN膜晶体质量的影响. 研究发现合适的低温成核层为后续高温生长提供成核中心, 并能有效降低外延膜与衬底间的界面自由能, 促进成核岛的横向生长; 优化的V/Ⅲ比和最佳生长温度有利于降低晶体缺陷密度, 促进横向生长, 增强外延膜的二维生长. 利用扫描电子显微镜、原子力显微镜、高分辨X射线衍射、 低温光致发光谱和室温拉曼光谱对优化条件下生长的GaN外延膜进行了结构和光电特性表征. 测试结果表明, 膜表面平整光滑, 呈现二维生长模式表面形貌; (002)和(102)面摇摆曲线半高宽分别为317和343 arcsec; 低温光致发光谱中近带边发射峰为3.478 eV附近的中性施主束缚激子发射峰, 存在11 meV的蓝移, 半高宽为10 meV, 并且黄带发光强度很弱;常温拉曼光谱中E₂ (high) 峰发生1.1 cm^-1蓝移.结果表明, 优化条件下生长的GaN外延膜具有良好的晶体质量和光电特性, 但GaN 膜中存在压应力. 关键词： 氮化镓 氢化物气相外延 低温成核层     

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.     

温和氧气等离子体对薄层二硒化钼的荧光调控

近年来,二硒化钼(MoSe_2)作为二维过渡金属硫属化合物(TMDs)中的一员,引起学术界和产业界的高度关注和广泛研究.薄层MoSe_2由于其比单层MoSe_2厚度大所以光密度较高,在光电器件领域中具有潜在的应用.采用一种处于电容放电模式(E-mode)的温和氧气电感耦合等离子体对薄层MoSe_2进行处理,实现其荧光增强到15倍.结合拉曼、荧光、原子力显微镜和X射线光电子能谱仪的表征,发现氧等离子中的氧原子通过物理和化学吸附修复了薄层MoSe_2的原始缺陷,从而使其荧光大大增强,同时氧气的化学吸附诱导在MoSe_2层中引入了p型掺杂,由负激子转变为中性激子,此研究为调控薄层MoSe_2的光学特性提供了新途径.     

ZnO六方纳米管晶体的制备与光致发光性能

常压下在空气中以氧化锌和碳粉为原料,利用高温碳热还原反应法制备出了自组装ZnO纳米晶体,采用SEM、XRD、PL等手段对产物进行了表征。结果表明产物为自组装多层六方管状结构的晶体,且是单一的六角晶系纤锌矿ZnO相。分析表明其生长机理为首先形成ZnO薄片,然后薄片再卷曲为管状物,其中碳热能和静电能为卷曲驱动力。室温光致发光谱显示,ZnO六方管晶体在以545nm为中心的范围内,形成较强较宽的绿光发射峰,而红光发射的强度增幅尤其突出。     

2H-PbI_2晶体的电子结构和力学性质

采用基于密度泛函平面波赝势方法(PWP)方法,计算了六角晶系2H-PbI2晶体的电子结构、力学性质和硬度.采用局域密度近似(LDA)方法计算的晶格常数、带隙、弹性常数与实验值和理论值符合较好.计算表明,2H-PbI2是一种直接带隙的半导体,带隙大约为2.38eV.运用复杂晶体硬度计算公式计算了六角晶系2H-PbI2晶体的硬度,硬度值大约为2.54GPa.还发现2H-PbI2晶体的各向异性非常明显.     

带隙可调的CdS纳米晶薄膜的化学浴制备和光学性质

CdS是一种直接带隙半导体,室温下其禁带宽度约为2.4eV,是一种良好的太阳能电池窗口层材料和过渡层材料。分别以CdCl2和(NH2)2CS作为镉源和硫源,用化学淀积法在玻璃上生长CdS纳米薄膜,考察了Cd2 浓度、淀积温度、淀积时间以及溶液pH值对CdS成膜的影响。紫外可见光吸收谱和荧光光谱的结果表明,在样品的制备过程中,通过改变反应条件如化学试剂的浓度、加热温度、加热时间等来控制薄膜中颗粒的尺寸大小,随着反应温度的逐渐降低或反应时间的减少等可以使得到的CdS纳米晶薄膜中晶粒尺寸逐渐减小,带隙增加;镉离子浓度越小或氨水浓度越大,所得CdS纳米晶薄膜带隙越大。     

2H-PbI2晶体的电子结构和力学性质

采用基于密度泛函平面波赝势方法(PWP)方法,计算了六角晶系2H-PbI2晶体的电子结构、力学性质和硬度。采用局域密度近似(LDA)方法计算的晶格常数、带隙、弹性常数与实验值和理论值符合较好。计算表明,2H-PbI2是一种直接带隙的半导体,带隙大约为2。38eV。运用复杂晶体硬度计算公式计算了六角晶系2H-PbI2晶体的硬度,硬度值大约为2. 54 GPa。还发现2H-PbI2晶体的各向异性非常明显。     

水热合成ZnO:Cd纳米棒的微结构及光致发光特性

采用水热法制备了ZnO和不同掺杂浓度的ZnO:Cd纳米棒,通过SEM,XRD、拉曼光谱等的分析,研究了ZnO和ZnO:Cd的微结构并测试分析了其光致发光特性.结果表明,ZnO和ZnO:Cd纳米棒呈六角纤锌矿结构,Cd掺杂使得纳米棒体积更小.由于内部张应力的影响,Cd掺杂使得材料光学带隙减少.当掺杂浓度为2%时,合成的材料光致发光谱中出现了位于2.67 eV处,由导带底和Zn空位(VZn)缺陷能级跃迁造成的蓝光发射峰,并且Cd的掺入使得位于2.90 eV附近的紫光发射峰强度增强,对于研究ZnO蓝紫发光器件具有重要的意义.     

Strain-modulated excitonic gaps in mono-and bi-layer MoSe_2

Photoluminescence(PL) and Raman spectra under uniaxial strain were measured in mono- and bi-layer MoSe_2 to comparatively investigate the evolution of excitonic gaps and Raman phonons with strain. We observed that the strain dependence of excitonic gaps shows a nearly linear behavior in both flakes. One percent of strain increase gives a reduction of ~42 meV(~35 me V) in A-exciton gap in monolayer(bilayer) MoSe_2. The PL width remains little changed in monolayer MoSe_2 while it increases rapidly with strain in the bilayer case. We have made detailed discussions on the observed strain-modulated results and compared the difference between monolayer and bilayer cases. The hybridization between 4d orbits of Mo and 4p orbits of Se, which is controlled by the Se–Mo–Se bond angle under strain, can be employed to consistently explain the observations. The study may shed light into exciton physics in few-layer MoSe_2 and provides a basis for their 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.     

Structural,electronic,and magnetic properties of vanadium atom-adsorbed MoSe_2 monolayer

Using the first-principles calculations, we study the structural, electronic, and magnetic properties of vanadium adsorbed MoSe_2 monolayer, and the magnetic couplings between the V adatoms at different adsorption concentrations. The calculations show that the V atom is chemically adsorbed on the MoSe_2 monolayer and prefers the location on the top of an Mo atom surrounded by three nearest-neighbor Se atoms. The interatomic electron transfer from the V to the nearestneighbor Se results in the polarized covalent bond with weak covalency, associated with the hybridizations of V with Se and Mo. The V adatom induces local impurity states in the middle of the band gap of pristine MoSe_2, and the peak of density of states right below the Fermi energy is associated with the V- dz~2 orbital. A single V adatom induces a magnetic moment of 5 μBthat mainly distributes on the V-3d and Mo-4d orbitals. The V adatom is in high-spin state, and its local magnetic moment is associated with the mid-gap impurity states that are mainly from the V-3d orbitals. In addition,the crystal field squashes a part of the V-4s electrons into the V-3d orbitals, which enhances the local magnetic moment.The magnetic ground states at different adsorption concentrations are calculated by generalized gradient approximations(GGA) and GGA+U with enhanced electron localization. In addition, the exchange integrals between the nearest-neighbor V adatoms at different adsorption concentrations are calculated by fitting the first-principle total energies of ferromagnetic(FM) and antiferromagnetic(AFM) states to the Heisenberg model. The calculations with GGA show that there is a transition from ferromagnetic to antiferromagnetic ground state with increasing the distance between the V adatoms. We propose an exchange mechanism based on the on-site exchange on Mo and the hybridization between Mo and V, to explain the strong ferromagnetic coupling at a short distance between the V adatoms. However, the ferromagnetic exchange mechanism is sensitive to both the increased inter-adatom distance at low concentration and the enhanced electron localization by GGA+U, which leads to antiferromagnetic ground state, where the antiferromagnetic superexchange is dominant.     

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.     

氢化二维过渡金属硫化物的稳定性和电子特性:第一性原理研究

用氢对单层二维过渡金属硫化物(TMDs)进行功能化是调节单层TMDs电子性质的既有效又经济的方法.采用密度泛函理论,对单层TMDs (MX_2 (M=Mo, W; X=S, Se, Te))的稳定性和电子性质进行理论研究,发现在单层MX_2 的层间有一个比其表面更稳定的氢吸附位点.当同阳离子时,随着阴离子原子序数的增加, H原子与MX_2 层的结合越强,氢化单层MX_2 结构越稳定;相反,同阴离子时,随着阳离子原子序数的增加, H原子与MX_2 层的结合越弱.氢原子从MoS_2的表面经层间穿越到另一表面的扩散势垒约为0.9 eV.氢化对单层MX_2 的电子特性也会产生极大的影响,主要表现在氢化实现了MX_2 体系从无磁性到磁性体系的过渡.表面氢化会使MX_2 层的带隙急剧减小,而层间氢化使MX_2 的电子结构从半导体转变为金属能带.     

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.     

Monolayer MoS2 of high mobility grown on SiO2 substrate by two-step chemical vapor deposition

We report a novel two-step ambient pressure chemical vapor deposition (CVD) pathway to grow high-quality MoS₂ monolayer on the SiO₂ substrate with large crystal size up to 110 μm. The large specific surface area of the pre-synthesized MoO₃ flakes on the mica substrate compared to MoO₃ powder could dramatically reduce the consumption of the Mo source. The electronic information inferred from the four-probe scanning tunneling microscope (4P-STM) image explains the threshold voltage variations and the n-type behavior observed in the two-terminal transport measurements. Furthermore, the direct van der Pauw transport also confirms its relatively high carrier mobility. Our study provides a reliable method to synthesize high-quality MoS₂ monolayer, which is confirmed by the direct 4P-STM measurement results. Such methodology is a key step toward the large-scale growth of transition metal dichalcogenides (TMDs) on the SiO₂ substrate and is essential to further development of the TMDs-related integrated devices.     

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.     

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.