Colloidal semiconductor quantum dots(QDs)constitute a perfect material for ink-jet printable large area displays,photovoltaics,light-emitting diode,bio-imaging luminescent markers and many other applications.For this ...Colloidal semiconductor quantum dots(QDs)constitute a perfect material for ink-jet printable large area displays,photovoltaics,light-emitting diode,bio-imaging luminescent markers and many other applications.For this purpose,efficient light emission/absorption and spectral tunability are necessary conditions.These are currently fulfilled by the direct bandgap materials.Si-QDs could offer the solution to major hurdles posed by these materials,namely,toxicity(e.g.,Cd-,Pb-or As-based QDs),scarcity(e.g.,QD with In,Se,Te)and/or instability.Here we show that by combining quantum confinement with dedicated surface engineering,the biggest drawback of Si—the indirect bandgap nature—can be overcome,and a‘direct bandgap’variety of Si-QDs is created.We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling.The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain‘direct bandgap-like’(phonon-less)character.This results in efficient fast emission,tunable within the visible spectral range by QD size.These findings are fully justified within a tight-binding theoretical model.When the C surface termination is replaced by oxygen,the emission is converted into the well-known red luminescence,with microsecond decay and limited spectral tunability.In that way,the‘direct bandgap’Si-QDs convert into the‘traditional’indirect bandgap form,thoroughly investigated in the past.展开更多
This work studies Te doping effects on the direct bandgap photoluminescence(PL) of indirect Ga_(x)In_(1-x)P alloys(0.72 ≤ x ≤ 0.74). The temperature-dependent PL shows that the energy difference between direct Γ va...This work studies Te doping effects on the direct bandgap photoluminescence(PL) of indirect Ga_(x)In_(1-x)P alloys(0.72 ≤ x ≤ 0.74). The temperature-dependent PL shows that the energy difference between direct Γ valley and indirect X valleys is reduced due to the bandgap narrowing(BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. Carrier thermalization has been observed for Te-doped Ga_(x)In_(1-x)P samples, as integrated PL intensity increases with increasing temperature from 175 to 300 K. The activation energy for carrier thermalization is reduced as doping concentration increases. Both BGN effect and carrier thermalization contribute to the carrier injection into the Γ valley. As a result, the direct band transition is enhanced in the Te-doped indirect Ga_(x)In_(1-x)P alloys. Therefore, the PL intensity of the Ga_(0.74)In_(0.26) P sample with active doping concentration of 9 × 10^(17)cm^(-3)is increased by five times compared with that of a nominally undoped sample. It is also found that the PL intensity is degraded significantly when the doping concentration is increased to 5 × 10^(18)cm^(-3). From cross-section transmission electron microscopy,no large dopant clusters or other extended defects were found contributing to this degradation.展开更多
The III–V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades.In the present article,the bandgap tuning and its influen...The III–V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades.In the present article,the bandgap tuning and its influence on optical properties of In1-xGaxN/P,where(x=0.0,0.25,0.50,0.75,and 1.0)alloys are comprehensively analyzed by density functional theory based on full-potential linearized augmented plane wave method(FP-LAPW)and modified Becke and Johnson potentials(TB-mBJ).The direct bandgaps turn from 0.7 eV to 3.44 eV,and 1.41 eV to 2.32 eV for In1-xGaxN/P alloys,which increases their potentials for optoelectronic devices.The optical properties are discussed such as dielectric constants,refraction,absorption,optical conductivity,and reflection.The light is polarized in the low energy region with minimum reflection.The absorption and optical conduction are maxima in the visible region,and they are shifted into the ultraviolet region by Ga doping.Moreover,static dielectric constant e1(0)is in line with the bandgap from Penn’s model.展开更多
Strain-engineered silicon nanocrystals(SiNCs)have recently been shown to possess direct bandgap.Here,we report the observation of a rich structure in the single-nanocrystal photoluminescence spectra of strain-engineer...Strain-engineered silicon nanocrystals(SiNCs)have recently been shown to possess direct bandgap.Here,we report the observation of a rich structure in the single-nanocrystal photoluminescence spectra of strain-engineered direct-bandgap SiNCs in the temperature range of 9–300 K.The relationship between individual types of spectra is discussed,and the numerical modeling of spectral diffusion of the experimentally acquired spectra reveals a common origin for most types.The intrinsic spectral shape is shown to be a structure that contains three peaks,approximately 150 meV apart,each of which possesses a Si phonon substructure.Narrow spectral lines,reaching ≤ meV at 20 K,are detected.The observed temperature dependence of the spectral structure can be assigned to the radiative recombination of positively charged trions,in contrast to several previous reports linking a very similar shape to phonons in the surface capping layers.Our result serves as strong additional support for the direct-bandgap nature of the investigated SiNCs.展开更多
A new bismuth-based halide double perovskite Cs_(2)KBiCl_(6) was isolated successfully through solid-state reactions and investigated using X-ray and neutron diffraction.Rather than an ordered structure,the crystal st...A new bismuth-based halide double perovskite Cs_(2)KBiCl_(6) was isolated successfully through solid-state reactions and investigated using X-ray and neutron diffraction.Rather than an ordered structure,the crystal structure consists of shifted Cs,K,Bi,and Cl sites from the ideal positions with fractional occupancy in compensation,leading to variable local coordination of Cs^(+)ions,as revealed by^(133)Cs solid-state nuclear magnetic resonance spectroscopy.Cs_(2)KBiCl_(6) displays volume hysteresis at 5-298 K range upon heating and cooling.The Cs_(2)KBiCl_(6) has a direct bandgap of 3.35(2)eV and red-shift luminescence of around 600 nm upon Mn doping compared with the Na analogue.The stabilization of disordered structure in Cs_(2)KBiCl_(6) is related to two factors including the large-sized K^(+)cation which prefers to coordinate with more than six Cl^(-),and the Bi^(3+)with 6s^(2) lone pair which has a preference for a local asymmetric environment.These findings could have general application and help to understand the structure and property of halide perovskites.展开更多
通过合金化改性技术,Ge可由间接带隙半导体转变为直接带隙半导体.改性后的Ge半导体可同时应用于光子器件和电子器件,极具发展潜力.基于直接带隙Ge1-x Sn x半导体合金8带Kronig-Penny模型,重点研究了其导带有效状态密度、价带有效状态密...通过合金化改性技术,Ge可由间接带隙半导体转变为直接带隙半导体.改性后的Ge半导体可同时应用于光子器件和电子器件,极具发展潜力.基于直接带隙Ge1-x Sn x半导体合金8带Kronig-Penny模型,重点研究了其导带有效状态密度、价带有效状态密度及本征载流子浓度,旨在为直接带隙改性Ge半导体物理的理解及相关器件的研究设计提供有价值的参考.研究结果表明:直接带隙Ge1-x Sn x合金导带有效状态密度随着Sn组分x的增加而明显减小,价带有效状态密度几乎不随Sn组分变化.与体Ge半导体相比,直接带隙Ge1-x Sn x合金导带有效状态密度、价带有效状态密度分别低两个和一个数量级;直接带隙Ge1-x Sn x合金本征载流子浓度随着Sn组分的增加而增加,比体Ge半导体高一个数量级以上.展开更多
In_(2/3)PSe_(3)因其对称性破缺的晶体结构、良好的发光性质和直接带隙特性,在光电子器件领域具有巨大的应用潜力.本研究采用化学气相输运法合成了In_(2/3)PSe_(3)单晶并通过机械剥离方法获得了二维In_(2/3)PSe_(3)纳米片.借助二次谐波(...In_(2/3)PSe_(3)因其对称性破缺的晶体结构、良好的发光性质和直接带隙特性,在光电子器件领域具有巨大的应用潜力.本研究采用化学气相输运法合成了In_(2/3)PSe_(3)单晶并通过机械剥离方法获得了二维In_(2/3)PSe_(3)纳米片.借助二次谐波(second-harmonic generation,SHG)和光致发光(photoluminescence,PL)光谱对其非线性光学和发光性质进行了系统的研究,结果表明In_(2/3)PSe_(3)纳米片具有本征对称性破缺的晶体结构和优异的发光性质.通过微纳加工技术构筑了基于In_(2/3)PSe_(3)纳米片的光探测器并研究了其光探测性能.基于In_(2/3)PSe_(3)纳米片的光探测器在365 nm波长的光照下,具有极低的暗电流(25 f A)、优异的探测度(6.28×10^(11)Jones)、高的开关比(4×10^(4))以及超快的响应速度(14μs/24μs).这些优异的光电性能预示着In_(2/3)PSe_(3)有潜力成为新一代高性能光探测器的核心材料.展开更多
Two-dimensional magnets have received increasing attention since Cr_2Ge_2Te_6 and CrI_3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature,...Two-dimensional magnets have received increasing attention since Cr_2Ge_2Te_6 and CrI_3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature, a layered ferromagnetic semiconductor with high Curie temperature(Tc) is yet to be unveiled. Here, we theoretically predicted a family of high Tcferromagnetic monolayers, namely MnNX and CrCX(X = Cl, Br and I; C = S, Se and Te). Their Tcvalues were predicted from over 100 K to near 500 K with Monte Carlo simulations using an anisotropic Heisenberg model. Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy, i.e. ～10~1 for effective masses and ～10~2 for carrier mobilities, along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials.Together with the mobility anisotropy, it offers a spin-, dichroism-and mobility-anisotropy locking.These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices.展开更多
基金This work was financially supported by Stichting der Fundamenteel Onderzoek der Materie and Stichting voor de Technische Wetenschappen.Part of this work(CPU,JMJP andHZ)was financed by the Dutch Polymer Institute for funding of UCin Functional Polymer Systemsproject no.681,and(ANP andAAP)Russian Foundation for Basic Research and‘Dynasty’-Foundation of International Center for Fundamental Physics in Moscow.
文摘Colloidal semiconductor quantum dots(QDs)constitute a perfect material for ink-jet printable large area displays,photovoltaics,light-emitting diode,bio-imaging luminescent markers and many other applications.For this purpose,efficient light emission/absorption and spectral tunability are necessary conditions.These are currently fulfilled by the direct bandgap materials.Si-QDs could offer the solution to major hurdles posed by these materials,namely,toxicity(e.g.,Cd-,Pb-or As-based QDs),scarcity(e.g.,QD with In,Se,Te)and/or instability.Here we show that by combining quantum confinement with dedicated surface engineering,the biggest drawback of Si—the indirect bandgap nature—can be overcome,and a‘direct bandgap’variety of Si-QDs is created.We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling.The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain‘direct bandgap-like’(phonon-less)character.This results in efficient fast emission,tunable within the visible spectral range by QD size.These findings are fully justified within a tight-binding theoretical model.When the C surface termination is replaced by oxygen,the emission is converted into the well-known red luminescence,with microsecond decay and limited spectral tunability.In that way,the‘direct bandgap’Si-QDs convert into the‘traditional’indirect bandgap form,thoroughly investigated in the past.
文摘This work studies Te doping effects on the direct bandgap photoluminescence(PL) of indirect Ga_(x)In_(1-x)P alloys(0.72 ≤ x ≤ 0.74). The temperature-dependent PL shows that the energy difference between direct Γ valley and indirect X valleys is reduced due to the bandgap narrowing(BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. Carrier thermalization has been observed for Te-doped Ga_(x)In_(1-x)P samples, as integrated PL intensity increases with increasing temperature from 175 to 300 K. The activation energy for carrier thermalization is reduced as doping concentration increases. Both BGN effect and carrier thermalization contribute to the carrier injection into the Γ valley. As a result, the direct band transition is enhanced in the Te-doped indirect Ga_(x)In_(1-x)P alloys. Therefore, the PL intensity of the Ga_(0.74)In_(0.26) P sample with active doping concentration of 9 × 10^(17)cm^(-3)is increased by five times compared with that of a nominally undoped sample. It is also found that the PL intensity is degraded significantly when the doping concentration is increased to 5 × 10^(18)cm^(-3). From cross-section transmission electron microscopy,no large dopant clusters or other extended defects were found contributing to this degradation.
文摘The III–V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades.In the present article,the bandgap tuning and its influence on optical properties of In1-xGaxN/P,where(x=0.0,0.25,0.50,0.75,and 1.0)alloys are comprehensively analyzed by density functional theory based on full-potential linearized augmented plane wave method(FP-LAPW)and modified Becke and Johnson potentials(TB-mBJ).The direct bandgaps turn from 0.7 eV to 3.44 eV,and 1.41 eV to 2.32 eV for In1-xGaxN/P alloys,which increases their potentials for optoelectronic devices.The optical properties are discussed such as dielectric constants,refraction,absorption,optical conductivity,and reflection.The light is polarized in the low energy region with minimum reflection.The absorption and optical conduction are maxima in the visible region,and they are shifted into the ultraviolet region by Ga doping.Moreover,static dielectric constant e1(0)is in line with the bandgap from Penn’s model.
基金Czech Science Foundation Funding(Grant Nos.GPP204/12/P235(Katerina Kusova)and P108/12/G108(Ivan Pelant))and a L’Oreal-UNESCO for Women in Science scholarship(Katerina Kusova)are gratefully acknowledgedPart of this work was supported by the Czech-Japan collaborative project LG14246(Jan Valenta).
文摘Strain-engineered silicon nanocrystals(SiNCs)have recently been shown to possess direct bandgap.Here,we report the observation of a rich structure in the single-nanocrystal photoluminescence spectra of strain-engineered direct-bandgap SiNCs in the temperature range of 9–300 K.The relationship between individual types of spectra is discussed,and the numerical modeling of spectral diffusion of the experimentally acquired spectra reveals a common origin for most types.The intrinsic spectral shape is shown to be a structure that contains three peaks,approximately 150 meV apart,each of which possesses a Si phonon substructure.Narrow spectral lines,reaching ≤ meV at 20 K,are detected.The observed temperature dependence of the spectral structure can be assigned to the radiative recombination of positively charged trions,in contrast to several previous reports linking a very similar shape to phonons in the surface capping layers.Our result serves as strong additional support for the direct-bandgap nature of the investigated SiNCs.
基金the National Science Foundation of China(Nos.22090043 and 22161014)Guangxi Natural Science Foundation(Nos.2019GXNSFGA245006 and 2020GXNSFAA297220)the Foundation of Guilin University of Technology(No.GUTQDJJ2018115)for the financial support。
文摘A new bismuth-based halide double perovskite Cs_(2)KBiCl_(6) was isolated successfully through solid-state reactions and investigated using X-ray and neutron diffraction.Rather than an ordered structure,the crystal structure consists of shifted Cs,K,Bi,and Cl sites from the ideal positions with fractional occupancy in compensation,leading to variable local coordination of Cs^(+)ions,as revealed by^(133)Cs solid-state nuclear magnetic resonance spectroscopy.Cs_(2)KBiCl_(6) displays volume hysteresis at 5-298 K range upon heating and cooling.The Cs_(2)KBiCl_(6) has a direct bandgap of 3.35(2)eV and red-shift luminescence of around 600 nm upon Mn doping compared with the Na analogue.The stabilization of disordered structure in Cs_(2)KBiCl_(6) is related to two factors including the large-sized K^(+)cation which prefers to coordinate with more than six Cl^(-),and the Bi^(3+)with 6s^(2) lone pair which has a preference for a local asymmetric environment.These findings could have general application and help to understand the structure and property of halide perovskites.
文摘通过合金化改性技术,Ge可由间接带隙半导体转变为直接带隙半导体.改性后的Ge半导体可同时应用于光子器件和电子器件,极具发展潜力.基于直接带隙Ge1-x Sn x半导体合金8带Kronig-Penny模型,重点研究了其导带有效状态密度、价带有效状态密度及本征载流子浓度,旨在为直接带隙改性Ge半导体物理的理解及相关器件的研究设计提供有价值的参考.研究结果表明:直接带隙Ge1-x Sn x合金导带有效状态密度随着Sn组分x的增加而明显减小,价带有效状态密度几乎不随Sn组分变化.与体Ge半导体相比,直接带隙Ge1-x Sn x合金导带有效状态密度、价带有效状态密度分别低两个和一个数量级;直接带隙Ge1-x Sn x合金本征载流子浓度随着Sn组分的增加而增加,比体Ge半导体高一个数量级以上.
文摘In_(2/3)PSe_(3)因其对称性破缺的晶体结构、良好的发光性质和直接带隙特性,在光电子器件领域具有巨大的应用潜力.本研究采用化学气相输运法合成了In_(2/3)PSe_(3)单晶并通过机械剥离方法获得了二维In_(2/3)PSe_(3)纳米片.借助二次谐波(second-harmonic generation,SHG)和光致发光(photoluminescence,PL)光谱对其非线性光学和发光性质进行了系统的研究,结果表明In_(2/3)PSe_(3)纳米片具有本征对称性破缺的晶体结构和优异的发光性质.通过微纳加工技术构筑了基于In_(2/3)PSe_(3)纳米片的光探测器并研究了其光探测性能.基于In_(2/3)PSe_(3)纳米片的光探测器在365 nm波长的光照下,具有极低的暗电流(25 f A)、优异的探测度(6.28×10^(11)Jones)、高的开关比(4×10^(4))以及超快的响应速度(14μs/24μs).这些优异的光电性能预示着In_(2/3)PSe_(3)有潜力成为新一代高性能光探测器的核心材料.
基金supported by the National Natural Science Foundation of China(11274380,91433103,11622437 and 61674171)the Fundamental Research Funds for the Central Universities of China+2 种基金the Research Funds of Renmin University of China(16XNLQ01)the Strategic Priority Research Program of Chinese Academy of Sciences(XDB30000000)supported by the Outstanding Innovative Talents Cultivation Funded Programs 2017 of Renmin University of China
文摘Two-dimensional magnets have received increasing attention since Cr_2Ge_2Te_6 and CrI_3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature, a layered ferromagnetic semiconductor with high Curie temperature(Tc) is yet to be unveiled. Here, we theoretically predicted a family of high Tcferromagnetic monolayers, namely MnNX and CrCX(X = Cl, Br and I; C = S, Se and Te). Their Tcvalues were predicted from over 100 K to near 500 K with Monte Carlo simulations using an anisotropic Heisenberg model. Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy, i.e. ～10~1 for effective masses and ～10~2 for carrier mobilities, along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials.Together with the mobility anisotropy, it offers a spin-, dichroism-and mobility-anisotropy locking.These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices.
