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类型 基础研究 预答辩日期 2017-11-30
开始(开题)日期 2016-02-25 论文结束日期 2017-09-14
地点 东南大学九龙湖校区物理南楼205 论文选题来源 973、863项目     论文字数 5.4 (万字)
题目 新型二维及量子点材料电子与光学性质的理论研究
主题词 二维材料,量子点,吸收发射谱,异质结,含时密度泛函理论
摘要 石墨烯、磷烯等超薄二维材料及相应的量子点结构具有丰富的物理化学性质,在电子与光电子领域展现了广泛的应用前景,引起了人们的格外关注。在这些低维纳米材料的制备和应用过程中,我们需要对其性质进行调节控制以满足具体的需求,如:通过改变尺寸的大小、掺杂、表面修饰以及材料复合等等。同时,在合成过程中产生的各种缺陷对其性质也会产生影响。为了清晰地理解这些因素对材料电子结构和光学性质的影响,基于密度泛函或含时密度泛函理论,结合多体理论,我们对石墨烯和黑磷量子点、氧化锌/蓝磷异质结及砷烯基复合材料的电子结构和光学性质进行了系统研究,主要研究内容如下: 1) 氮掺杂石墨烯量子点光吸收与发射机制研究。氮掺杂能够调节石墨烯量子点荧光波长,增加光稳定性以及提高量子产率,具有良好的应用前景。但是,氮掺杂对石墨烯量子点光吸收和发射的影响机制一直存在争议。我们的研究表明,不同种类和位置的氮掺杂导致石墨烯量子点具有不同的吸收和发射特性:中心氮掺杂在石墨烯量子点带隙中引入中间态,导致石墨烯量子点荧光湮灭;边界氮掺杂可以调节石墨烯量子点的激发态能级位置,引起光谱红移或蓝移,同时增强荧光发射强度,提高量子产率。光谱的蓝移红移与石墨烯量子点和掺杂氮前线轨道之间的杂化密切相关:杂化均匀往往导致光谱蓝移;反之,光谱红移。我们的理论首次清晰地阐明了氮掺杂实验中的各种争议。我们还揭示了溶剂效应对量子点光学性质的影响,极性强的溶液导致光谱红移,同时增强量子点的吸收发射强度。 2) 黑磷量子点光学性质的反常尺寸效应。理解黑磷纳米结构的电子跃迁性质对其在电子和光电子器件的应用是至关重要的。采用含时密度泛函理论方法,我们系统地研究了黑磷量子点在电子结构、光吸收和光发射方面的尺寸效应。黑磷量子点电子带隙和吸收带隙是正常的量子尺寸效应,即:尺寸越大,带隙越小,尺寸与带隙成负相关关系。但对于光学发射带隙,我们观察到了反常的尺寸效应:当尺寸从0.8 nm 到1.8 nm时,光发射带隙逐渐增大,当尺寸大于1.8 nm时,光发射带隙显示出减小的趋势。这种反常发射现象源于激发态结构迟豫导致结构畸变,进而减小了带隙;且尺寸越小,畸变就越显著。结构畸变和量子尺寸效应的竞争作用最终导致了反常光发射现象。 3) 黑磷量子点对点缺陷和氧缺陷的光吸收容忍性。黑磷量子点具有极好的光学性质,在生物医学及光电子器件方面具有潜在应用价值。黑磷量子点在制备过程中,不可避免地会引入各种各样的点缺陷,同时,黑磷在空气环境中也很容易被氧化进而引入氧缺陷。目前,这些缺陷对黑磷量子点电子和光学性质的影响还不清楚。针对这一问题,我们采用含时密度泛函理论方法,系统地研究了点缺陷和氧缺陷对黑磷量子点电子和光学性质的影响。结果表明,无论是点缺陷还是氧缺陷,都存在两种情况:(1) 对于缺陷位磷原子的配位数为3的类型,黑磷量子点具有极好的光吸收容忍性;(2) 对于缺陷位磷原子的配位数不为3的,无论是是大于还是小于3,黑磷量子点带隙中都出现了中间态,导致黑磷量子点光吸收通道的堵塞,吸光能力锐减。有趣的是,对于这种情况,我们采用氢气充分钝化,可使黑磷量子点的光吸收性质得到恢复。因此,黑磷量子点具有很好的光吸收容忍性。 4) 氧化锌/蓝磷异质结:优异的光生电子空穴对分离效率。载流子分离对于光电器件的应用至关重要。在单一半导体材料中,因为光生电子空穴的波函数交叠,电子空穴对很容易复合。通过设计第二类带边结构的复合材料,能有效地解决这一问题。基于第一性原理计算,我们设计了氧化锌/蓝磷异质结,该结构不但具有第二类异质结带边结构,而且两种材料之间存在促进电子空穴分离的内建电场。这些因素极大地促进了光生电荷从氧化锌层转移到蓝磷层,显著增加了电子空穴对的分离效率。相比于它们单一的材料,异质结的光吸收能力也得到了增强。而且,通过施加垂直电场,异质结能够从间接带隙转化成直接带隙,这也将进一步促进光生电荷的转移。 5) 高效的光伏和光催化双功能材料:砷烯基异质结。砷烯,具有合适的带隙,高的载流子迁移率和优异的光学性质,在光伏与光催化领域具有潜在的应用。然而,砷烯在大气环境中的稳定性,精确的带边位置及光生电子和空穴的分离效率仍没有得到系统研究。采用多体微扰GW方法结合带边外推的方法,我们获得了准确的带边信息,并发现砷烯与二硫化钼、TCNQ、TCNNQ等能构成第二类带边结构异质结,这些异质结构具有合适的带隙和带边能级,满足光催化水裂解的要求。并且,把这些异质结应用到光伏领域,砷烯作为电子供体材料,其他的对材料作为电子受体材料,能量转换效率达到20%左右。同时,砷烯良好的稳定性也增加了砷烯基异质结在光伏与光催化领域应用的竞争力。
英文题目 THEORETICAL STUDY OF THE ELECTRONIC AND OPTICAL PROPERTIES OF NOVEL TWO DIMENSIONAL AND QUANTUM DOTS MATERIAL
英文主题词 two-dimensional materials, quantum dots, absorption and emission spectra, heterostructures, time-dependent density functional theory
英文摘要 Ultrathin two-dimensional materials and corresponding quantum dots, such as graphene and phosphorene, exhibiting outstanding physical and chemical properties and holding potential applications in electronics and optoelectronics, have attracted world-wide attention. In the process of fabrication or application of materials, we need to tune and control their properties to meet various requirements by changing the size of materials, doping, surface modification, or heterojuncting. Moreover, the preparation of material can also introduce defects which may influence the properties of materials. To make clear how these factors influence the electronic and optical properties, in this thesis, we systematically study graphene or black phosphorene quantum dots, graphitic zinc oxide/blue phosphorene heterostructure and arsenene-based heterostructure with molybdenum disulfide or organic molecules within the framework of density functional theory (DFT) or time-dependent (TD) DFT in combination with many-body Green’s function (GW). The underlying mechanism such as absorption and emission in nitrogen doping graphene quantum dots, anomalous size dependence of optical properties and photo-absorption tolerance of defects in black phosphorus quantum dots are revealed. 1) Revealing the underlying absorption and emission mechanism of nitrogen doping graphene quantum dots. Nitrogen-doped graphene quantum dots (N-GQDs) hold promising application in electronics and optoelectronics because of excellent photo-stability, tunable photoluminescence and high quantum yield. However, the absorption and emission mechanisms have been debated for years. Here, by employing TDDFT, we demonstrate that different N-doping types and positions give rise to different absorption and emission behaviors, which successfully addresses the inconsistence observed in different experiments. Specifically, the center doping creates mid-states, rendering as non-fluorescent, while the edge N-doping modulates the energy levels of excited states and increases the radiation transition probability, thus enhances the fluorescence strength. More importantly, the even hybridization of frontier orbitals between edge N-doping and GQD leads to the blue-shift for both absorption and emission spectra, while the uneven hybridization of frontier orbitals induces the red-shift. Solvent effects on N-GQDs are further explored by the conductor-like screening model and it is found that strong polarity of solvent can cause the red-shift and enhance the intensity of both absorption and emission spectra. 2) Anomalous size dependence of optical properties in black phosphorus quantum dots. Understanding electron transitions in black phosphorus nanostructures plays a crucial role for applications in electronics and optoelectronics. By employing TDDFT calculations, we systematically study the size-dependent electronic, optical absorption and emission properties of black phosphorus quantum dots (BPQDs). Both the electronic gap and the absorption gap follow an inversely proportional law to the diameter of BPQDs in conformity to the quantum confinement effect. In contrast, the emission gap exhibits anomalous size dependence in the range of 0.8-1.8 nm which is blue-shift with the increase of size. The anomaly, in fact, arises from the structure distortion induced by the excited state relaxation and it leads to huge Stokes shift in small BPQDs. 3) Photo-absorption tolerance of intrinsic point defects and oxidation in black phosphorus quantum dots. BPQDs exhibit excellent optical and photothermal properties and promising applications in optoelectronics and biomedicine. However, various intrinsic structural defects and oxidation are nearly unavoidable in preparation of BPQDs and how they affect the electronic and optical properties remains unclear. By employing TDDFT, we reveal that there are two types of photo-absorption in BPQDs for both point defects and oxidation. A close structure-absorption relation is unraveled: BPQDs are defect-tolerant and show excellent photo-absorption as long as the coordination number (CN) of defective P atoms is 3; In contrast, the unsaturated or oversaturated P atoms with CN≠3 create in-gap-states (IGSs) and completely quench the optical absorption. An effective way to eliminate the IGSs and repair the photo-absorption of defective BPQDs via sufficient hydrogen passivation is further proposed. 4) Efficient carrier separation in graphitic zinc oxide and blue phosphorus van der waals heterostructure. Efficient carrier separation is the key to the application of photoelectric device. However, photo-generated electron-hole pairs in simplex semiconductors generally occupy the same regions spatially and are easy to recombine. We design a graphitic zinc oxide (g-ZnO) based intrinsic type-II heterostructure, g-ZnO/blue phosphorus (Blue-P), based on first-principles calculations. The type-II band offsets and large built-in electric field ensure the photogenerated electrons easily migrating from g-ZnO to Blue-P, which significantly enhances the separation of electron-hole pairs. Improved optical absorption is also observed in the heterostructure. Furthermore, the perpendicular external electric field can greatly modulate band edges and achieve a direct band gap at Г point, which provides further promotion in the separation of carriers. 5) Arsenene-based heterostructures: high-efficient bifunctional candidates for photovoltaic and photocatalytic applications based on accurate band alignments. Utilizing many-body perturbation GW method with an extrapolation technique, we obtain accurate band edges and find that arsenene-based heterostructures constructed with molybdenum disulfide or ornanic molecules tetracyano-quinodimethane and tetracyanonaphtho-quinodimethane possess suitable band gap, energy level of band edges which can satisfy the requirements of photocatalytic water splitting. Moreover, the power conversion efficiency of these arsenene-based heterostructures is predicted to be ~ 20% in photovoltaic solar-cell application. The arsenene serves as the donor, and other materials as acceptor. The bifunctional performance of arsenene-based heterostructures possess promising potential in optoelectronics applications.
学术讨论
主办单位时间地点报告人报告主题
物理学院 2015-4-15 田家炳南楼203 牛相宏 Optical properties on BPQDs
物理学院 2015-11-04 田家炳南楼203 牛相宏 Unusual size dependence of the optical emission gap in BPQDs
物理学院 2016-4-20 田家炳南楼203 牛相宏 The Heterojunction of semiconduction monolayer materials with QDs for excitonic Solar cells Application
物理学院 2016-12-14 田家炳南楼203 牛相宏 Some designs on how to effectively spearate electron and hole for solar cell applications
物理学院 2017-02-23 田家炳南楼203 牛相宏 The heterojunction of semiconducting monolayer materials with organic molecule for excitonic solar cell and photocatalytic water-splitting application
物理学院 2017-6-30 田家炳南楼205 陈中方 How to survive in machine learning era: some recent lessons
物理学院 2014-10-09 田家炳南楼203 牛相宏 ADF学习及算例
物理学院 2014-12-01 田家炳南楼203 牛相宏 N掺杂对石墨烯量子点光学性质的影响
     
学术会议
会议名称时间地点本人报告本人报告题目
第九届计算纳米科学与能源材料国际研讨会 2016-06-22--2016-06-26 上海 Absorption and emission mechanism of graphene and BPQDs
2017 美国物理年会 2017.03.11-2017.03.17 New Orleans LA USA Anomalous size dependence of optical properties in BPQDs
     
代表作
论文名称
Anomalous Size Dependence of Optical Properties in Black Phosphorus Quantum Dots
Ab initio study of the low-lying electronic states of [Be-Kr]+
Ground and excited states of [Be-Xe]+: A multireference configuration interaction study
Extensive theoretical studies on the low-lying electronic states of BBr+
 
答辩委员会组成信息
姓名职称导师类别工作单位是否主席备注
黎书华 正高 教授 博导 南京大学
李兴鳌 正高 教授 博导 南京邮电大学
孙岳明 正高 教授 博导 东南大学
孙建 正高 教授 博导 南京大学
汪军 正高 教授 博导 东南大学
      
答辩秘书信息
姓名职称工作单位备注
周跫桦 其他 讲师 东南大学