Thermal properties of two-dimensional materials

Two-dimensional(2D) materials, such as graphene, phosphorene, and transition metal dichalcogenides(e.g., Mo S2 and WS2), have attracted a great deal of attention recently due to their extraordinary structural, mechanical, and physical properties. In particular, 2D materials have shown great potential for thermal management and thermoelectric energy generation. In this article, we review the recent advances in the study of thermal properties of 2D materials. We first review some important aspects in thermal conductivity of graphene and discuss the possibility to enhance the ultra-high thermal conductivity of graphene. Next, we discuss thermal conductivity of Mo S2 and the new strategy for thermal management of Mo S2 device. Subsequently, we discuss the anisotropic thermal properties of phosphorene. Finally, we review the application of 2D materials in thermal devices, including thermal rectifier and thermal modulator.     

Ballistic thermal transport in black and blue phosphorene nanoribbons and in-plane heterostructures

The thermal transport properties of black and blue phosphorene nanoribbons and in-plane heterostructures are systematically investigated by non-equilibrium Green's-function method. Both edge shape and width have a sensitive influence on the thermal conductance of pristine black and blue phosphorene nanoribbons and they all exhibit a clear anisotropic thermal performance. Interestingly, the in-plane heterostructures possess a tunable thermal conductance which depends on the percentage of black phosphorene nanoribbons and the way how they are linked. These findings will provide new applications in nanoelectronic and thermoelectric devices based on phosphorene.     

Anisotropic Transport on Monolayer and Multilayer Phosphorene in the Presence of an Electric Field

We demonstrate theoretically the anisotropic quantum transport of electrons through an electric field on monolayer and multilayer phosphorene. Using the long-wavelength Hamiltonian with continuum approximation, we find that the transmission probability for transport through an electric field is an oscillating function of incident angle, electric field intensity, as well as the incident energy of electrons. By tuning the electric field intensity and incident angle, the channels can be transited from opaque to transparent. The conductance through the quantum waveguides depends sensitively on the transport direction because of the anisotropic effective mass, and the anisotropy of the conductance can be tuned by the electric field intensity and the number of layers. These behaviors provide us an efficient way to control the transport of phosphorene-based microstructures.     

离子辐照对磷烯热导率的影响及其机制分析

通过离子辐照产生缺陷,可以非常有效地调控磷烯诸多物理性质.本文应用分子动力学方法模拟离子辐照磷烯的过程,给出了缺陷的形成概率与入射离子能量、离子种类以及离子入射角度之间的关系,并且应用非平衡态分子动力学计算辐照后磷烯热导率的变化.以缺陷形成概率为切入点,系统地研究了辐照离子的能量、辐照剂量、离子的种类以及离子的入射角度对磷烯热导率的影响.应用晶格动力学方法研究了空位缺陷对磷烯声子参与率的影响,并计算了声子局域模式的空间分布.基于量子微扰和键弛豫理论,指出空位缺陷明显降低磷烯热导率的最重要物理机制是空位缺陷附近的低配位原子对声子强烈散射.本文研究可为缺陷工程调控磷烯的热输运性质提供理论参考.     

磷烯基二维范德瓦尔斯异质结的理论表征

本文基于单层黑磷和蓝磷,理论设计出二维范德瓦尔斯异质结、能带结构、态密度、Bader电荷布局、电荷密度差分图及光吸收谱等,计算结果表明它是典型的第二型异质结,有利于光生载流子分离,且可见光捕获能力显著增强.内禀的界面极化电场能有效阻止光生电子-空穴的复合.表明磷烯基二维范德瓦尔斯异质结是一类性能优异的光解水催化剂.     

Thermal transport in lithium-ion battery: A micro perspective for thermal management

In recent years, lithium ion (Li-ion) batteries have served as significant power sources in portable electronic devices and electric vehicles because of their high energy density and rate capability. There are growing concerns towards the safety of Li-ion batteries, in which thermal conductivities of anodes, cathodes, electrolytes and separator play key roles for determining the thermal energy transport in Li-ion battery. In this review, we summarize the state-of-the-art studies on the thermal conductivities of commonly used anodes, cathodes, electrolytes and separator in Li-ion batteries, including both theoretical and experimental reports. First, the thermal conductivities of anodes and cathodes are discussed, and the effects of delithiation degree and temperature of materials are also discussed. Then, we review the thermal conductivities of commonly used electrolytes, especially on solid electrolytes. Finally, the basic concept of interfacial thermal conductance and simulation methods are presented, as well as the interfacial thermal conductance between separator and cathodes. This perspective review would provide atomic perspective knowledge to understand thermal transport in Li-ion battery, which will be beneficial to the thermal management and temperature control in electrochemical energy storage devices.     

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.     

Recent Advances in the Study of Phosphorene and its Nanostructures

Phosphorene, a single layer of black phosphorus, has attracted considerable attention recently due to its intriguing structures and fascinating electronic properties. In particular, its remarkable properties, such as high charge carrier mobility, direct band-gap semiconducting characteristics, and strong anisotropies in electro-optical and thermo-mechanical properties, etc., are opening up brand-new opportunities for its applications in nanoelectronics, optoelectronics, sensors, energy conversion, and advanced engineering materials, etc. In this article, we present recent advances in the study of phosphorene and its derivatives (nanoribbons, nanotubes, fullerenes, and heterostructures) with special emphasis on structures, morphologies, properties (electronic, optical, magnetic, thermal, mechanical), and applications (transistors, phonon detectors, digital circuits, sensors, thermoelectric materials, Li-ion batteries). In addition, routes for modifying these properties by physical and chemical functionalization, defect engineering, strain engineering, and electric fields are discussed. Our intent is to present a state-of-the-art view in this fast-evolving field, with a balanced theoretical and experimental perspective.     

Ballistic Transport through a Strained Region on Monolayer Phosphorene

We investigate quantum transport of carriers through a strained region on monolayer phosphorene theoretically.The electron tunneling is forbidden when the incident angle exceeds a critical value. The critical angles for electrons tunneling through a strain region for different strengths and directions of the strains are different.Owing to the anisotropic effective masses, the conductance shows a strong anisotropic behavior. By tuning the Fermi energy and strain, the channels can be transited from opaque to transparent, which provides us with an efficient way to control the transport of monolayer phosphorene-based microstructures.     

Strong interlayer coupling in phosphorene/graphene van der Waals heterostructure: A first-principles investigation

Based on first-principles calculations within the framework of density functional theory, we study the electronic properties of phosphorene/graphene heterostructures. Band gaps with different sizes are observed in the heterostructure, and charges transfer from graphene to phosphorene, causing the Fermi level of the heterostructure to shift downward with respect to the Dirac point of graphene. Significantly, strong coupling between two layers is discovered in the band spectrum even though it has a van der Waals heterostructure. A tight-binding Hamiltonian model is used to reveal that the resonance of the Bloch states between the phosphorene and graphene layers in certain K points combines with the symmetry matching between band states, which explains the reason for the strong coupling in such heterostructures. This work may enhance the understanding of interlayer interaction and composition mechanisms in van der Waals heterostructures consisting of two-dimensional layered nanomaterials, and may indicate potential reference information for nanoelectronic and optoelectronic applications.     

First-principles analysis of phonon thermal transport properties of two-dimensional WS_2/WSe_2 heterostructures

The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.     

磷烯的研究进展与展望

黑磷晶体的单原子层结构被定义为磷烯，它具有独特的褶皱形态和一些区别于其它二维晶体材料的特性，如可调控的直接带隙，高开关比，高载流子迁移率以及优异的光学饱和吸收特性等，使其在纳米电子和纳米光学领域具有潜在应用价值.此外，蓝磷烯被理论计算所预测，它是黑磷烯的一种同素异形体，具有许多类似黑磷烯的优异特性.本文主要介绍了当前两种构型磷烯的研究进展，包括黑/蓝磷烯各自的晶体结构、制备方法、物理特性和稳定性；最后对目前磷烯研究中存在的问题与挑战提出了一些见解和展望.     

Large positive and negative magnetoresistance in armchair phosphorene nanoribbons

We investigate the tunnel magnetoresistance (TMR) in an armchair phosphorene nanoribbon modulated by two ferromagnetic stripes. It is shown that large TMR can be achieved by applying a perpendicularly electric field to the phosphorene plane. We find that the TMR can be adjusted by an external gate voltage, and the TMR oscillates periodically from positive to negative by a slight change of the gate voltage. This characteristic can be observed in a wide region of exchange splitting values. Our findings open the way to design phosphorene-based spintronics nanodevices, and it may contribute to the future low power spintronic applications.     

Electronic Transport and Thermopower in 2D and 3D Heterostructures—A Theory Perspective

The impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials is discussed herein. Recent progress in understanding electronic transport in heterostructures of 2D materials ranging from graphene to transition metal dichalcogenides, their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS₂ lateral interfaces), and vertical van der Waals heterostructures is reviewed. Work on thermopower in 2D heterojunctions, as well as their applications in creating devices such as resonant tunneling diodes (RTDs), is also discussed. Last, the focus turns to work in 3D heterostructures. While transport in 3D heterostructures has been researched for several decades, here recent progress in theory and simulation of quantum effects on transport via the Wigner and non‐equilibrium Green's functions approaches is reviewed. These simulation techniques have been successfully applied toward understanding the impact of heterojunctions on transport properties and thermopower, which finds applications in energy harvesting, and electron resonant tunneling, with applications in RTDs. In conclusion, tremendous progress has been made in both simulation and experiments toward the goal of understanding transport in heterostructures and this progress will soon be parlayed into improved energy converters and quantum nanoelectronic devices.     

二维范德瓦尔斯异质结构的制备与物性研究

二维范德瓦尔斯材料(可简称二维材料)已发展成为备受瞩目的材料大家族,而由其衍生的二维范德瓦尔斯异质结构的集成、性能及应用是现今凝聚态物理和材料科学领域的研究热点之一.二维范德瓦尔斯异质结构为探索丰富多彩的物理效应和新奇的物理现象,以及构建新型的自旋电子学器件提供了灵活而广阔的平台.本文从二维材料的转移技术着手,介绍二维范德瓦尔斯异质结构的构筑、性能及应用.首先,依据湿法转移和干法转移的分类,详细介绍二维范德瓦尔斯异质结构的制备技术,内容包括转移技术的通用设备、常用转移方法的具体操作步骤、三维操纵二维材料的方法、异质界面清洁.随后介绍二维范德瓦尔斯异质结构的性能和应用,重点介绍二维磁性范德瓦尔斯异质结构,并列举在二维范德瓦尔斯磁隧道结和摩尔超晶格领域的应用.因此,二维材料转移技术的发展和优化将进一步助力二维范德瓦尔斯异质结构在基础科学研究和实际应用上取得突破性的成果.     

二维范德瓦尔斯异质结构的制备与物性研究

二维范德瓦尔斯材料(可简称二维材料)已发展成为备受瞩目的材料大家族,而由其衍生的二维范德瓦尔斯异质结构的集成、性能及应用是现今凝聚态物理和材料科学领域的研究热点之一.二维范德瓦尔斯异质结构为探索丰富多彩的物理效应和新奇的物理现象,以及构建新型的自旋电子学器件提供了灵活而广阔的平台.本文从二维材料的转移技术着手,介绍二维范德瓦尔斯异质结构的构筑、性能及应用.首先,依据湿法转移和干法转移的分类,详细介绍二维范德瓦尔斯异质结构的制备技术,内容包括转移技术的通用设备、常用转移方法的具体操作步骤、三维操纵二维材料的方法、异质界面清洁.随后介绍二维范德瓦尔斯异质结构的性能和应用,重点介绍二维磁性范德瓦尔斯异质结构,并列举在二维范德瓦尔斯磁隧道结和摩尔超晶格领域的应用.因此,二维材料转移技术的发展和优化将进一步助力二维范德瓦尔斯异质结构在基础科学研究和实际应用上取得突破性的成果.     

Transport in quasi one-dimensional spin-1/2 systems

We present numerical results for the spin and thermal conductivity of one-dimensional (1D) quantum spin systems. We contrast the properties of integrable models such as the spin-1/2 XXZ chain against nonintegrable ones such as frustrated and dimerized chains. The thermal conductivity of the XXZ chain is ballistic at finite temperatures, while in the nonintegrable models, this quantity is argued to vanish. For the case of frustrated and dimerized chains, we discuss the frequency dependence of the transport coefficients. Finally, we give an overview over related theoretical work on intrinsic and extrinsic scattering mechanisms of quasi-1D spin systems.     

聚噻吩单链量子热输运的第一性原理研究

聚噻吩块体通常被视为绝热材料,其热导率小于1W·m~(-1)·K~(-1).但近年发现对于室温下沿聚噻吩分子链方向排列的无定形聚噻吩纳米纤维,其热导率高于聚噻吩块体,可达4.4W·m~(-1)·K~(-1).为了相对准确地揭示纳米尺度聚噻吩单链热输运的微观特征,从量子力学出发,在密度泛函理论计算的基础上,应用中间插入延展方法结合非平衡格林函数方法,对长度为25.107nm、包含448个原子的聚噻吩单链的量子热输运及其同位素效应进行了研究,并与分子动力学方法模拟的结果进行了详细比较.结果表明:室温下32 nm长的纯聚噻吩单链热导率上限高达30.2 W·m~(-1)·K~(-1),与铅的热导率35 W·m~(-1).K~(-1)相近;相同掺杂比例(原子百分数)下C元素热导的同位素效应比S元素显著;室温下聚噻吩单链中~(12)C,~(13)C等比例随机掺杂时的同位素效应最为显著,此时聚噻吩单链的平均热导至少降低了30%;室温下纯聚噻吩单链的热导随C的相对原子质量增加近似呈反比例减小,随S的相对原子质量增加呈非线性单调增加.该研究对认识和调控聚噻吩这种新型功能材料的热输运特性具有积极的价值.     

锂离子电池相关材料的Raman光谱学研究

锂离子电池是目前综合性能最好的可充电池。本文总结我们实验室用Raman光谱学研究锂离子电池相关材料的一些结果 ,包括聚合物电解质的微结构和离子输运机制 ,低温热解碳负极材料的结构表征和锂离子在其中的嵌入 /脱出机理 ,元素替代引起正极材料LiMn2 O4的结构变化以及在充放电过程中电极 /电解质界面形成的钝化层的性质及其对电池性能的影响