Electronic and optical properties of 3N-doped graphdiyne/MoS2 heterostructures tuned by biaxial strain and external electric field

The construction of van der Waals (vdW) heterostructures by stacking different two-dimensional layered materials have been recognised as an effective strategy to obtain the desired properties. The 3N-doped graphdiyne (N-GY) has been successfully synthesized in the laboratory. It could be assembled into a supercapacitor and can be used for tensile energy storage. However, the flat band and wide forbidden bands could hinder its application of N-GY layer in optoelectronic and nanoelectronic devices. In order to extend the application of N-GY layer in electronic devices, MoS₂ was selected to construct an N-GY/MoS₂ heterostructure due to its good electronic and optical properties. The N-GY/MoS₂ heterostructure has an optical absorption range from the visible to ultraviolet with a absorption coefficient of 10⁵ cm^-1. The N-GY/MoS₂ heterostructure exhibits a type-Ⅱ band alignment allows the electron-hole to be located on N-GY and MoS₂ respectively, which can further reduce the electron-hole complexation to increase exciton lifetime. The power conversion efficiency of N-GY/MoS₂ heterostructure is up to 17.77%, indicating it is a promising candidate material for solar cells. In addition, the external electric field and biaxial strain could effectively tune the electronic structure. Our results provide a theoretical support for the design and application of N-GY/MoS₂ vdW heterostructures in semiconductor sensors and photovoltaic devices.     

First principle study on interfacial interaction of black phosphorus and CsBr vdW heterostructure

Two dimensional crystalline materials have attracted much attentions due to the establishment of heterostructure that can adjust their electrical and optical properties, and have potential applications in lasers, light-emitting diodes, solar cells and high mobility transistors. And the interface engineering is an effective route to tune structural and electrical properties in semiconductor heterostructures. In this study, the electronic structure, charge transport and optical properties of monolayer caesium bromide and black phosphorus (CsBr/BP) heterostructure are calculated by the first principle based on density functional theory (DFT). It was found that the characteristics of electronic band structures of the monolayer CsBr and BP remain in the heterostructure, and the effective mass and carrier mobility are highly anisotropic. When the heterostructure is uniaxially stretched, the mobility of electron is greater than that of the hole, while the biaxial stretching is just the opposite, the mobility of hole is greater than that of the electron. In addition, compared with the CsBr monolayer, the light absorption of the heterostructure is significantly enhanced, especially in the infrared, indicating that the CsBr/BP heterostructure can be well applied to photovoltaic devices in the future.     

有机无机杂化固态太阳能电池的研究进展

近年来, 由于钙钛矿材料优良的光学吸收和电荷传导特性, 有机无机杂化固态太阳能电池取得了突破性的进展. 自2009年首次报道了光电转换效率为3.8%的钙钛矿太阳能电池以来, 该类电池的效率不断突破. 基于介孔薄膜的电池已取得了超过16.7%的认证光电转换效率, 基于平板异质结结构电池光电转换效率达到19.3%, 已接近传统硅基太阳能电池的光电转换效率. 本文将介绍有机无机杂化钙钛矿作为光电材料的光学物理结构特性, 以及在固态太阳能电池中的应用. 基于固态钙钛矿太阳能电池结构上的差异, 分别介绍其在多孔结构、平板异质结结构、柔性结构以及无空穴传导材料结构电池工作特性和各自优势, 以及影响电池特性的主要影响因素, 特别是钙钛矿成膜控制等. 并阐述对钙钛矿电池的理解和进一步提高固态钙钛矿电池光电转换效率需要关注的重点以及展望.     

Improved efficiency in GaAs solar cells by 1D and 2D nanopatterns fabricated by laser interference lithography

We demonstrate an improvement in efficiency of GaAs solar cells using front surface texturing with dielectric 1D and 2D nanopatterns obtained by a low cost laser interference lithography technique. The strong light scattering by the surface dielectric nanopatterns effectively increases the optical path of the incident light in the absorber layers resulting in an efficiency increase up to 23.5% compared to that of the reference solar cell. The observed efficiency improvement in the studied solar cells shows the potential use of low cost photoresist as an antireflection coating material and further application of other robust dielectric materials as texturing layer.     

Nonlinear properties of a graded-index photonic heterostructure

The optical properties of a one-dimensional (1D) photonic heterostructure with graded-index nonlinear materials are demonstrated theoretically. The influence of the gradation profile of the graded-index nonlinear layers on the linear and nonlinear responses of the structure are analysed. It is shown that the Q-factor of the defect mode and the threshold input intensity to achieve the optical bistability in the used photonic heterostructure depend on the gradation profile of the graded-index nonlinear layers.     

二维GaTe/Bi2Se3异质结：一类有潜力的Z型光解水催化剂

本文基于第一性原理方法，计算了二维GaTe/Bi₂Se₃异质结的电子结构、界面电荷转移、静电势分布、吸收光谱及光催化性质. 计算结果表明异质结是一个小能隙的准直接半导体，能有效捕获太阳光. 由于相对较强的界面內建极化电场和带边轻微弯曲，导致异质结中的光生电子和空穴分别有效分离在GaTe单层和Bi₂Se₃薄片上，可用于析氢和产氧. 这些理论计算结果意味着二维GaTe/Bi₂Se₃异质结是一类有潜力的Z型太阳能全解水催化剂.     

基于二维六方氮化硼材料的光子晶体非对称传输异质结构设计

二维六方氮化硼(hexagonal boron nitride,hBN)材料在产生光学稳定的超亮量子单光子光源领域有着潜在应用,有望用于量子计算和信息处理平台,已成为研究热点.而光学非对称传输设备是集成量子计算芯片中的关键器件之一.本文从理论上提出了一种基于hBN材料光子晶体异质结构的纳米光子学非对称光传输器件.运用平面波展开法研究了光子晶体的能带结构与等频特性,从理论上分析了hBN异质结构中可见光波非对称传输的可行性.同时,采用时域有限差分方法研究了可见光波段异质结构的晶格常数和半径对透射光谱的影响.研究结果显示,该结构实现了在610—684 nm波长范围内TE偏振光的非对称传输,在652 nm波长处正向透射率达到0.65,反向透射率为0.006,非对称传输透射对比度高达0.98.本文提出的结构模型为基于hBN的新型纳米光子器件设计提供了新的可能性,可用于不同功能光学器件的集成设计.     

A review of the interfacial properties of 2-D materials for energy storage and sensor applications

The discovery of two-dimensional (2D) materials like graphene inspired the researchers and scientists to develop new 2D materials. The 2D materials create extensive attention due to their novel electronic properties, large surface area, charging capacity, optical, biocompatible, unique physical and chemical properties. Many of these properties are an excellent requirement for an application of electrode for batteries and super-capacitors. The applications of 2D materials are not just confined to Opto and nano-electronics but a strong potential in gas, and biosensing technologies. The 2D materials are stackable through weak Van der Waals, therefore, used in alkali metal ion batteries as electrodes, this causes zero volume and area changes during the intercalation and deintercalation of alkali metal. Also, a large surface of 2D materials provides large storage capacity as compared to the bulk materials. The heterostructures based on 2D materials pay significant attention towards the optoelectronics, nanoelectronics and in alkali metal ion battery applications also. In this paper, we review the importance of heterostructure, stacking technique in interfacial synthesis, address their structural morphologies by the interface of 2D materials and its application for energy storage, gas, and biosensing applications. We will come up with an overview of interfacial characters and highlights about the advantages and individuality of 2D materials.     

Optical properties of two-dimensional perovskites

The optical properties of two-dimensional (2D) perovskites recently receive numerous research focus thanks to the strong quantum and dielectric confinement effects. In addition to the strong excitonic effect at room temperature, 2D perovskites also have appealing features that their optical properties can be flexibly tuned by alternating organic or inorganic layers. Particularly, 2D chiral perovskites and 2D perovskites based heterostructures are emerging as new platforms to extend their functionalities. To optimize performance of 2D perovskites-based optoelectronic devices, it is critical to understand the fundamentals and explore the strategies to engineer their optical properties. This review begins with an introduction to the excitons and self-trapped excitons of 2D perovskites. Subsequently, inorganic/organic layer effects on optical properties and 2D perovskites based heterostructures are discussed. We also discussed the nonlinear optical properties of 2D perovskite. We are looking forward to that this review can stimulate more efforts to understand and optimize the optical properties of 2D perovskites.     

Electronic and optical properties of an intrinsic type-I band alignment ZrS_2/SnS_2 van der Waals heterostructure for optoelectronic devices

The electronic and optical properties of the ZrS_2/SnS_2 van der Waals heterostructure have been investigated.We find out that the formed heterostructure has an intrinsic type-I band alignment. Moreover, the characteristics of optical absorption in the heterostructure can be enhanced to the amount of 106 in the ultraviolet light region. In addition, the tuning electronic properties of ZrS_2/SnS_2 heterostructure are very interesting, due to the transitions from type-I to type-II band alignment that can occur by applying an external electric field.These results suggest that the atomically thin materials ZrS_2/SnS_2 heterostructure will be utilized for flexible optoelectronic applications.     

Excitons in boron nitride nanotubes: dimensionality effects

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2, 2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed "optical gap" constancy for different tubes and bulk hexagonal BN.     

石墨烯-六方氮化硼面内异质结构的扫描隧道显微学研究

石墨烯-六方氮化硼面内异质结构因可调控石墨烯的能带结构而受到广泛关注. 本文介绍了在超高真空体系内, 利用两步生长法在两类对石墨烯分别有强和弱电子掺杂的基底, 即Rh(111)和Ir(111)上制备石墨烯-六方氮化硼单原子层异质结构. 通过扫描隧道显微镜及扫描隧道谱对这两种材料的形貌和电子结构进行研究发现: 石墨烯和六方氮化硼倾向于拼接生长形成单层的异质结构, 而非形成各自分立的畴区; 在拼接边界处, 石墨烯和六方氮化硼原子结构连续无缺陷; 拼接边界多为锯齿形型, 该实验结果与密度泛函理论计算结果相符合; 拼接界面处的石墨烯和六方氮化硼分别具有各自本征的电子结构, 六方氮化硼对石墨烯未产生电子掺杂效应.     

Thermally induced band hybridization in bilayer-bilayer MoS_2/WS_2 heterostructure

Transition metal dichalcogenides(TMDs), being valley selectively, are an ideal system hosting excitons. Stacking TMDs together to form heterostructure offers an exciting platform to engineer new optical and electronic properties in solid-state systems. However, due to the limited accuracy and repetitiveness of sample preparation, the effects of interlayer coupling on the electronic and excitonic properties have not been systematically investigated. In this report, we study the photoluminescence spectra of bilayer-bilayer MoS_2/WS_2 heterostructure with a type Ⅱ band alignment. We demonstrate that thermal annealing can increase interlayer coupling in the van der Waals heterostructures, and after thermally induced band hybridization such heterostructure behaves more like an artificial new solid, rather than just the combination of two individual TMD components. We also carry out experimental and theoretical studies of the electric controllable direct and indirect infrared interlayer excitons in such system. Our study reveals the impact of interlayer coupling on interlayer excitons and will shed light on the understanding and engineering of layer-controlled spin-valley configuration in twisted van der Waals heterostructures.     

Optical properties of highly excited direct gap semiconductors

The electronic excitations in direct gap semiconductors interact strongly with the photon field. We discuss both the experimental and the theoretical aspects of the optical properties of these materials under strong optical excitation. We distinguish between intermediate excitation levels at which the electronic excitations form a dense system of excitons and excitonic molecules and very high excitation levels at which a degenerate electron-hole plasma occurs. The optical spectra of dense excitonic systems, which are mainly observed in copper halides and II–VI compounds, are shown to be determined mainly by the interaction processes between excitonic molecules, polaritons and free carriers. The optical properties of the electron-hole plasma, which has been observed in II–VI and especially in III–V compounds, can be understood only by taking into account many-body effects, such as dynamical screening of the Coulomb interactions, plasmon-assisted transitions and excitonic enhancement.     

Fluorinated Boron Nitride Quantum Dots: A New 0D Material for Energy Conversion and Detection of Cellular Metabolism

Quantum dots encompass a broad spectrum of optical, catalytic, and electrochemical properties bringing in novel applications in catalysis, imaging, displays, and optoelectronics. Herein, the unanticipated broad‐spectrum light absorption and high fluorescence quantum yield in fluorinated boron nitride (FBN) quantum dots are discussed. A heterostructure of FBN quantum dots with a wide‐bandgap semiconductor, titania nanotube arrays, exhibits high photocatalytic activity as evidenced by high external quantum efficiency extending from ultraviolet to green region of the solar spectrum (≈24% at 400 nm). The high activity is confirmed using photoelectrochemical hydrogen evolution experiments. Further, it is demonstrated that high fluorescence quantum yield could be tapped for the detection of glycolytic activity in cancer cells compared to normal cells. This finding could shift the paradigm of molecular detection using quantum dots. The 0D structure and the gap states introduced through fluorination are believed to be responsible for these unprecedented characteristics of boron nitride.     

Field-enhanced charge flow in nanorod heterostructure solar cells

Recent studies on organic heterostructure solar cells have indicated that interface morphology plays an important role in determining the quantum efficiency. Hybrid heterostructure mixing donor and acceptor semiconductors appear to offer the best opportunity in achieving superior performance and there are indications that a network of percolated heterojunctions can be quite effective in promoting light absorption and exciton quenching. Charge transport and collection efficiency, however, appear to be more complex in the bulk heterostructure and the nature of charge flow depends largely on the type of current paths in existence. We report in this work the possible existence of field-assisted charge flow in the nanorod heterostructure solar cells when carriers of different polarity move in close proximity. The field-effect associated with the charge density gradients can exert a force on the nearby carriers resulting in an increase in the short-circuit current. The model is used to explain data reported in the literature on solar cells composed of TiO₂ nanorods embedded in a conjugated polymer.     

二维平面和范德瓦耳斯异质结的可控制备与光电应用

自石墨烯被发现以来,二维材料因其优异的特性获得了持续且深入的探索与发展,以石墨烯、六方氮化硼、过渡金属硫化物、黑磷等为代表的二维材料相关研究层出不穷.随着二维新材料制备与应用探索的不断发展,单一材料性能的不足逐渐凸显,研究者们开始考虑采用平面拼接和层间堆垛所产生的协同效应来弥补单一材料的不足,甚至获得一些新的性能.利用二维材料晶格结构的匹配构建异质结,实现特定的功能化,或利用范德瓦耳斯力进行堆垛,将不同二维材料排列组合,从而在体系里引入新的自由度,为二维材料的性质研究和实际应用打开了新的窗口.本文从原子制造角度,介绍了二维平面和范德瓦耳斯异质结材料的可控制备和光电应用.首先简要介绍了应用于异质结制备的常见二维材料的分类及异质结的相关概念,然后从原理上分类列举了常用的表征方法,随后介绍了平面和垂直异质结的制备方法,并对其光电性质及器件应用做了简要介绍.最后,对领域内存在的问题进行了讨论,对未来发展方向做出了展望.     

In_2O_3:Sn中间层改善B掺杂ZnO薄膜的性能及其应用研究

金属有机化学气相沉积(MOCVD)法生长的掺硼氧化锌(BZO)薄膜,具有天然的"类金字塔"绒面结构,作为硅基薄膜太阳电池的前电极具有良好的陷光效果.但直接获得的BZO薄膜表面形貌过于尖锐,影响后续硅基薄膜材料生长质量及太阳电池的光电转换效率.本文设计了以一层超薄In2O3:Sn(ITO)薄膜(～4 nm厚度)作为中间层的多层膜,并通过对顶层BZO薄膜的厚度调制,改善BZO薄膜的表面特性,薄膜结构为:glass/底层BZO/ITO/顶层BZO.合适厚度的顶层BZO薄膜有助于获得类似"菜花状"形貌特征,尖锐的表面趋于"柔和",而较厚的顶层BZO薄膜仍然保持"类金字塔状"结构."柔和"的BZO薄膜表面结构有助于提高后续生长薄膜电池的结晶质量.将获得的新型"三明治"结构多层膜应用于p-i-n型氢化微晶硅(μc-Si:H)薄膜太阳电池,相比传统的BZO薄膜,电池的量子效率QE在500—800 nm波长范围提高了～10%,并且电池的Jsc和Voc均有所提高.     

Sr_3Al_2O_6∶Tb~(3+),Yb~(3+)荧光粉的近红外量子剪裁效应

硅材料带隙与太阳光子光谱的失配导致了比较严重的光子损失,大大降低了硅太阳能电池的效率。为了减少入射光子的损失,可以利用具有近红外量子剪裁效应的光谱转换材料来提高硅太阳能电池的效率。本研究采用溶胶凝胶法制备了Sr_3Al_2O_6∶Tb~(3+),Yb~(3+)荧光粉,并研究了其近红外量子剪裁效应。实验结果表明:在320 nm的紫外光激发下,Sr_3Al_2O_6∶Tb~(3+),Yb~(3+)荧光粉发射出Tb~(3+):5D4→~7F_j的可见光;另外,由于Tb~(3+)、Yb~(3+)离子之间的合作能量传递,得到了Yb~(3+):~7F_(5/2)→7F7/2的近红外发光。荧光寿命衰减证明Tb~(3+)到Yb~(3+)之间的确存在合作能量传递,而且存在量子剪裁效应,其中,能量传递效率为35.9%,量子剪裁效率为135.9%。由于Yb~(3+)的发射光谱与硅太阳能电池的吸收匹配,Sr_3Al_2O_6∶Tb~(3+),Yb~(3+)荧光粉有可能作为潜在的光谱转换材料应用于硅太阳能电池以提高其光电转换效率。     

Diffractionless optical networking in an inverse opal photonic band gap micro-chip

We present a design for a photonic crystal (PC) all-optical micro-chip based on a three-dimensional (3D) inverse opal heterostructure intercalated with a two-dimensional (2D) triangular lattice photonic crystal slab. Within the 2D micro-chip layer, we demonstrate single-mode (diffractionless) waveguiding of light in air, throughout a bandwidth of more than 70 nm near 1.55 μm. This suggests that inverse opal photonic band gap (PBG) materials can facilitate on-chip optical networking functions over the telecommunication frequency band used in current-day optical fibers.