多壁纳米碳管的频率上转换效应研究

实验上测量了多壁纳米碳管的吸收光谱和光致发光谱,观察到了多壁纳米碳管的光频率上转换效应,激发波长为1064 nm,发射光谱为带状光谱,峰值波长为780 nm.由吸收光谱上观察到了纳米碳管的态密度分布的范霍夫奇点,这些奇点对应的吸收峰位置为685nm,719nm和894nm.上转换过程是纳米碳管的电子经双光子吸收,再经无辐射跃迁布居在范霍夫奇点,最后经辐射跃迁而产生荧光.     

电场调控双层石墨烯纳米带的电子结构和光学性质

利用基于密度泛函理论的第一性原理方法,研究了外加电场作用下双层AA堆垛的Armchair边缘石墨烯纳米带(BAGNRs)的电子结构和光学性质. BAGNRs具有半导体特性,其带隙随带宽(宽度为4～12个碳原子)的增加而振荡性减小.当施加电场后,BAGNRs的带隙随着电场强度的增加而逐渐减小,带隙越大对电场值的变化越敏感.当电场值为0.5 V/?时,所有BAGNRs的带隙都为零. BAGNRs具有各向异性的光学性质,其介电函数在垂直极化方向为半导体特性,而在平行极化方向为金属特性.在外加电场的作用下,BAGNRs的介电函数、吸收系数、折射系数、反射系数、电子能量损失系数和光电导率,其峰值向低能量区域移动,即产生红移现象.电场增强了能带间的跃迁几率.纳米带宽度对这些光学性质参数具有不同程度的影响.研究结果解释了电场调控BAGNRs光学性质的规律和微观机理.     

纤锌矿结构ZnO/Mg_xZn_(1-x)O量子阱中带间光吸收的尺寸效应和三元混晶效应

对于纤锌矿结构ZnO/MgxZn1-xO有限深单量子阱结构,考虑内建电场、导带弯曲及材料掺杂对实际异质结势的影响,利用有限差分法和自洽法数值求解Schr?dinger方程和Poisson方程,获得电子(空穴)的本征能级和本征波函数.进而,采用费米黄金法则讨论带间光吸收的尺寸效应和三元混晶效应.结果表明:三元混晶材料MgxZn1-xO中Mg组分的增加会增强垒层和阱层的内建电场强度,使得电子(空穴)平均位置靠近左(右)垒,导致带间跃迁吸收峰呈指数减小且发生蓝移;ZnO/MgxZn1-xO量子阱带间跃迁吸收峰随阱宽增大而减小,吸收峰发生红移.所得结果可为改善异质结构材料和器件的光电性能提供理论指导,以期获得实际应用所需的光学吸收频谱和波长.     

扭转形变对石墨烯吸附O原子电学和光学性质影响的电子理论研究

基于密度泛函理论的第一性原理方法研究了扭转形变对石墨烯吸附O体系结构稳定性、电子结构和光学性质,包括吸附能、带隙、吸收系数及反射率的影响.研究发现,吸附O原子后,距O原子最近的C原子被拔起,导致石墨烯平面发生扭曲.吸附能计算表明,扭转形变使石墨烯吸附O原子体系结构稳定性下降,而扭转程度对结构稳定性影响微弱.能带结构分析发现,O原子的吸附使石墨烯由金属变成半导体,扭转形变发生时,可实现其从半导体到金属、再到半导体特性的转变.扭转角为12°的吸附O原子体系为间接带隙,而其他出现带隙的体系均为直接带隙.与本征石墨烯受扭体系相比,吸附O原子体系的电子结构对扭转形变的敏感度降低,其中扭转角在10°—16°范围内变化时,带隙始终稳定在0.11 eV附近,即在此扭转角范围内始终对应窄带隙半导体.在光学性能中,受扭转形变的吸附体系吸收系数和反射率峰值较未受扭转形变石墨烯吸附O原子体系均减弱,且随着扭转程度的加剧,均出现红移到蓝移的转变.     

层数调控磷烯能带与光学性质的研究

近年来,黑磷作为兼具石墨烯和过渡金属硫化物之长的新型二维材料而倍受关注.本文基于密度泛函理论,研究了不同厚度黑磷的电子结构与光学性质.结果表明,黑磷的性质与其厚度密切相关,可通过厚度调整实现能带与光学性质的可调控性.层间相互作用导致费米能级附近价带和导带的劈裂,是造成黑磷带隙随层数减小的根本原因.黒磷的静态折射率和静态反射率的大小均随层数的增大有增大的趋势,并且各层黑磷的反射峰均位于紫外光波段.黑磷对光的吸收涵盖了可见光到紫外光区域,对光的损失范围小于4eV.本文基于能带图和分波态密度图,从电子跃迁的角度分析了黑磷各项光学性质的变化情况,旨在为黑磷的带隙及光学性质层数可调控性提供理论依据.     

单层与双层石墨烯的光学吸收性质研究

采用紧束缚模型分别描述单层、双层石墨烯的能带结构, 利用光子-电子相互作用的二阶微扰理论分别计算单光子和双光子吸收系数.计算结果表明: 单层石墨烯单光子吸收系数为常数, 约为6.8×10⁷ m^-1, 即单层石墨烯对入射光的吸收率约为2.3%; 双层石墨烯的单光子吸收比单层石墨烯的单光子吸收强, 且随入射光波长呈分段性变化.单层石墨烯的双光子吸收系数与波长λ⁴成正比; 双层石墨烯双光子吸收系数在红外波段(～ 3100 nm处)有一个很强的共振吸收峰. 研究结果可为石墨烯材料在光电子器件的研究和制作方面提供指导. 关键词： 石墨烯 光学吸收 紧束缚模型     

锗硅量子阱结构带间吸收边研究

利用光电流谱的方法对锗硅量子阱结构的带间吸收边进行了研究.实验观察到了在不同的偏压和温度下,锗硅量子阱结构的带间吸收边谱线发生了有规律的变化.通过对锗硅量子阱材料的光电流谱的带间跃迁吸收边的拟合,得到了硅导带到锗价带的能带宽度分别为1.043 eV和1.050 eV.随着外加电场的增强,带边的吸收曲线向低能方向移动.通过理论计算得到了带间跃迁吸收边的漂移量与外加电场的关系,并与实验吻合较好.随着温度的降低,带间吸收边向高能方向偏移,对于这一现象给出了定性的解释,并通过拟合得到了禁带宽度随温度的变化率.     

THz谐振腔型石墨烯光电探测器的设计

由于石墨烯在太赫兹波范围内只发生带内跃迁,相比在可见光范围内,其光学吸收特性有显著优势,通过集成石墨烯与谐振腔,将太赫兹波限制在腔内,可进一步增强石墨烯对太赫兹波的吸收.采用麦克斯韦方程组并结合电磁场边界条件,研究了单层石墨烯在太赫兹波段范围内的光吸收机理;推导出石墨烯的传输矩阵和吸收系数方程,发现在太赫兹波段石墨烯的吸收是在可见光波段吸收的9—22倍;通过建立谐振腔型石墨烯光电探测器在太赫兹波段的光吸收模型及求解探测器吸收率方程,发现在0.12 THz处,吸收率可达0.965,相比无腔状态下石墨烯在太赫兹波段的最大吸收率0.5,提高了93%;优化设计器件结构参数并表征,最终器件响应度最高达到236.7 A/W,半高全宽为0.035 THz.理论分析表明,采用谐振腔型石墨烯光电探测器对太赫兹波进行探测,具有高吸收率、高响应度.研究结果对于太赫兹谐振腔型石墨烯光电探测器的设计和应用提供了理论参考.     

硫化锡电子结构和光学性质的量子尺寸效应

基于密度泛函理论的第一性原理计算,系统研究了硫化锡(SnS)晶体、纳米单层及多层的结构稳定性、电子结构和光学性质.结果表明:由于相对弱的层间范德瓦尔斯力作用,SnS单层纳米片可以像石墨烯等二维材料一样从块体中剥离出来;受制于量子尺寸效应和层间相互作用的影响,SnS的结构稳定性随层数减少而逐渐减弱,其带隙随层数减少而逐渐增大;由于材料的本征激发和吸收取决于电子结构,因此改变SnS材料的层数可以到达调控其光学性质的目的;SnS块体和纳米结构的主要光学吸收峰起源于Sn-5s,5p和S-2p轨道之间的电子跃迁;并且从块体到单层纳米结构,SnS的光学吸收峰出现明显的蓝移.本文的研究将有助于SnS材料在太阳能电池领域的应用.     

金属有机化学气相淀积技术制备GaMnN薄膜材料光学性质研究

研究由MOCVD 技术制备的 GaMnN 外延薄膜光吸收谱.实验发现Mn掺杂后较未掺杂GaN吸收系数在近紫外区增加,在吸收谱低能区144 eV附近观察到吸收峰,吸收系数随Mn浓度的增加而增大.实验结果与基于密度泛函理论的第一性原理计算结果一致,结合理论计算分析认为144 eV附近的吸收峰源于Mn³⁺离子e态与t₂态间的带内跃迁⁵T₂→⁵E. 关键词： GaMnN MOCVD 密度泛函理论 光学性质     

Theoretical study of broadband near-field optical spectrum of twisted bilayer graphene

We theoretically study the broadband near-field optical spectrum of twisted bilayer graphene (TBG) at various twist angles near the magic angle using two different models. The spectrum at low Fermi energy is characterized by a series of peaks that are almost at the same energies as the peaks in the far-field optical conductivity of TBG. When the Fermi energy is near a van Hove singularity, an additional strong peak appears at finite energy in the near-field spectrum, which has no counterpart in the optical conductivity. Based on a detailed calculation of the plasmon dispersion, we show that these spectroscopic features are associated with interband and intraband plasmons, which can provide critical information about the local band structure and plasmonic excitations in TBG. The near-field peaks evolve systematically with the twist angle, so they can serve as fingerprints for identifying the spatial dependent twist angle in TBG samples. Our findings pave the way for future experimental studies of the novel optical properties of TBG in the nanoscale.     

Optical spectroscopy of graphene: From the far infrared to the ultraviolet

The unique electronic structure of graphene leads to several distinctive optical properties. In this brief review, we outline the current understanding of two general aspects of optical response of graphene: optical absorption and light emission. We show that optical absorption in graphene is dominated by intraband transitions at low photon energies (in the far-infrared spectral range) and by interband transitions at higher energies (from mid-infrared to ultraviolet). We discuss how the intraband and interband transitions in graphene can be modified through electrostatic gating. We describe plasmonic resonances arising from the free-carrier (intraband) response and excitonic effects that are manifested in the interband absorption. Light emission, the reverse process of absorption, is weak in graphene due to the absence of a band gap. We show that photoluminescence from hot electrons can, however, become observable either through femtosecond laser excitation or strong electrostatic gating.     

Terahertz laser based on optically pumped graphene: Model and feasibility of realization

We consider a terahertz laser based on optically pumped graphene layer and bilayer as the active media and suggested waveguide structure. Using the developed model, we calculate the spectral dependences the dynamic conductivity of the optically pumped graphene layer and bilayer associated with the interband and intraband transitions, estimate the pumping optical power required for lasing, and demonstrate the feasibility of realization of such a laser. The article is published in the original.     

Optical conductivity of twisted bilayer graphene near the magic angle

We theoretically study the band structure and optical conductivity of twisted bilayer graphene(TBG) near the magic angle considering the effects of lattice relaxation. We show that the optical conductivity spectrum is characterized by a series of peaks associated with the van Hove singularities in the band structure, and the peak energies evolve systematically with the twist angle. Lattice relaxation effects in TBG modify its band structure, especially the flat bands, which leads to significant shifts of the peaks in the optical conductivity. These results demonstrate that spectroscopic features in the optical conductivity can serve as fingerprints for exploring the band structure, band gap, and lattice relaxation in magic-angle TBG as well as identifying its rotation angle.     

Pure spin photocurrents in low-dimensional structures

As is well known, the absorption of circularly polarized light in semiconductors results in optical orientation of electron spins and helicity-dependent electric photocurrent, and the absorption of linearly polarized light is accompanied by optical alignment of electron momenta. Here, we show that the absorption of unpolarized light leads to the generation of a pure spin current, although both the average electron spin and electric current vanish. We demonstrate this for direct interband and intersubband as well as indirect intraband (Drude-like) optical transitions in semiconductor quantum wells.     

A first principle calculation of electronic and dielectric properties of electrically gated low-buckled mono and bilayer silicene

The structural, electronic and dielectric properties of mono and bilayer buckled silicene sheets are investigated using density functional theory. A comparison of stabilities, electronic structure and effect of external electric field are investigated for AA and AB-stacked bilayer silicene. It has been found that there are no excitations of electrons i.e. plasmons at low energies for out-of-plane polarization. While for AB-stacked bilayer silicene 1.48 eV plasmons for in-plane polarization is found, a lower value compared to 2.16 eV plasmons for monolayer silicene. Inter-band transitions and plasmons in both bilayer and monolayer silicene are found relatively at lower energies than graphene. The calculations suggest that the band gap can be opened up and varied over a wide range by applying external electric field for bilayer silicene. In infra-red region imaginary part of dielectric function for AB-stacked buckled bilayer silicene shows a broad structure peak in the range of 75–270 meV compared to a short structure peak at 70 meV for monolayer silicene and no structure peaks for AA-stacked bilayer silicene. On application of external electric field the peaks are found to be blue-shifted in infra-red region. With the help of imaginary part of dielectric function and electron energy loss function effort has been made to understand possible interband transitions in both buckled bilayer silicene and monolayer silicene.     

The optical conductivity of bilayer zigzag-edge graphene nanoribbons with external transverse electric fields

The optical absorption properties of bilayer zigzag-edge graphene nanoribbons (BL-ZGNRs) with external transverse electric fields are investigated by taking into account the Coulomb interaction effect in the Hartree-Fock approximation. We study the phase transitions of BL-ZGNRs induced by external electric fields and also the optical selection rules for the incident light polarized along the longitudinal and transverse directions. We find that the excitations from the edge states are crucial for the optical properties of BL-ZGNRs in the antiferromagnetic phase. We show that the low energy part of the optical absorption can be modulated by the external transverse electric field, and there is a broad band low frequency absorption enhancement for the transverse-polarized incident light in the charge-polarized state of BL-ZGNRs.     

扭转双层石墨烯物理性质、制备方法及其应用的研究进展

石墨烯是一种准二维蜂窝网状结构新型纳米材料,石墨烯的层数和构型对其性能产生重要影响.固体中准粒子的量子状态由其本身的对称性质所决定,扭转双层石墨烯打破了对称性,引起了强烈的层间耦合作用,改变了扭转双层石墨烯的电子能带、声子色散、形成能垒等物性,产生了独特的性能,如可以连续调控带隙0-250 meV,光电效应的响应度相比于单层石墨烯提高了80倍,因此对扭转双层石墨烯功能化研究有重大意义.本文同时还论述了扭转双层石墨烯向类金刚石转变的理论与实验研究进展,发现扭转双层石墨烯呈现出具有类金刚石结构与性能特征.进一步阐述调控扭转双层石墨烯的扭转角度对其内在性能的影响,揭示这种新型纳米结构在原子层次的行为特征.最后介绍了如何调控制备扭转双层石墨,分析其调控机理,讨论了各种制备工艺的不足与发展趋势.因此本文从扭转双层石墨烯的输运性质、晶体结构转变、制备三个方面展开阐述,并对其在先进电子器件领域的潜在应用进行了展望.     

Probing interlayer coupling in twisted single‐crystal bilayer graphene by Raman spectroscopy

Twisted bilayer graphene, in which interlayer interaction plays a critical role in this coupled system, is characterized for its angle‐dependent electronic and optical properties. Here, we present a systematic Raman study of single‐crystal twisted bilayer graphene grains, with the spectra of each bilayer graphene precisely correlated to its twist angle using combined transmission electron microscopic technique. Van Hove singularities develop as a result of band rehybridization at the crossing Dirac cones of the two layers, giving rise to a critical twist angle that determines the energy separation between the saddle points in the band structure and the resonance Raman spectra accordingly. The 2D mode becomes sensitive to the twist angle, showing the angle‐dependent position, peak width, and intensity. Our results interpreted in the framework of angle‐dependent double resonance scattering provide an important experimental perspective in understanding the coupled bilayer graphene system. Copyright © 2014 John Wiley & Sons, Ltd.