Optical Properties of Neutral and Charged Low Band Gap Alternating Copolyfluorenes： TD-DFT Investigation

Electronic structure and optical properties of neutral and charged low band gap alternating copolyfluorenes （Green 1, which is based on alternating repeating units consisting of alkyl-substituted fluorene and a thiophene-[1,2,5]thiadiazolo-[3,4]quinoxaline-thiophene （T-TDQ-T） unit were investigated theoretically with time-dependent density functional theory （TD-DFT） method, and their excited state properties were further analyzed with 2D site and 3D cube representations. For neutral Green 1, the band gap, binding energy, exciton binding energy, and nuclear relaxation energy were obtained. The transition dipole moments of neutral and charged Green 1 are compared using 3D transition density, which reveals the orientation and strength of transition dipole moments. The charge redistribution of neutral and charged Green 1 upon excitation are displayed and compared with 3D charge difference density. The electron-hole coherences of neutral and charged Green 1 upon excitation are investigated with 2D site representation （transition density matrix）. The excited state properties of neutral Green 1 calculated with TD-DFT method are compared with that calculated with ZINDO method, which reveals the importance of electron-electron interaction （in TD-DFT） in the excited state properties.

Strong interfacial charge transfer between hausmannite manganese oxide and alumina for efficient photocatalysis

Well crystalline manganese oxide(Mn_(3)O_(4))nanoparticles anchored on gamma alumina(γ-Al_(2)O_(3))have been successfully tailored via a proficient and cost effective chemical process as an efficient material for photo catalysis.XRD indicated the composite formation ofγ-Al_(2)O_(3) and hausmannite structure of Mn_(3)O_(4).SEM and TEM revealed that hetero structure of Mn_(3)O_(4)/γ-Al_(2)O_(3) exhibits an amalgam of aggregated nanoparticles and nanorods.XPS demonstrated the chemical states of binary nanocomposite.The band gap tuning has been performed withγ-Al_(2)O_(3) nanoparticles by assimilating hausmannite Mn_(3)O_(4) particles into flower like microstructure of Al_(2)O_(3).The photoluminescence spectra affirmed the enhancement in charge separation in Mn_(3)O_(4)/γ-Al_(2)O_(3) binary hybrid photocatalyst.The band gap becomes narrow with the increase in concentrations of Mn_(3)O_(4).The narrowing of band gap is concorded with crystalline domains of primary aggregated particles.To elucidate the mechanism of the photocatalytic activity linear sweep voltammetry was performed.The results showed that Mn_(3)O_(4)/γ-Al_(2)O_(3) nanocomposite revealed the enhancement in current density as compared to pureγ-Al_(2)O_(3) which confirmed the electron transfer from Mn_(3)O_(4) toγ-Al_(2)O_(3) through the interfacial potential gradient in conduction bands.The optimum concentration of 6.0%Mn_(3)O_(4)/γ-Al_(2)O_(3) for hybrid structure showed an excellent photocatalytic activity under visible light due to narrow band gap energy.High degree distribution of Mn_(3)O_(4) nano architects overlying onγ-Al_(2)O_(3) induces a significant synergic effect betweenγ-Al_(2)O_(3) and hausmannite phase of manganese oxide(Mn_(3)O_(4)).This strong interfacial contact betweenγ-Al_(2)O_(3) and Mn_(3)O_(4) endures the quick transfer of photo generated charge carriers across interface.

Anomalous photoluminescence enhancement and resonance charge transfer in type-II 2D lateral heterostructures

Type-Ⅱband alignment can realize the efficient charge transfer and separation at the semiconductor heterointerface,which results in photoluminescence(PL)quenching.Recently,several researches demonstrated great enhancement of localized PL at the interface of type-Ⅱtwo-dimensional(2D)heterostructure.However,the dominant physical mechanism of this enhanced PL emission has not been well understood.In this work,we symmetrically study the exciton dynamics of type-Ⅱlateral heterostructures of monolayer MoS_(2) and WS_(2) at room temperatures.The strong PL enhancement along the one-dimensional(1D)heterointerface is associated with the trion emission of the WS_(2) shell,while a dramatic PL quenching of neutral exciton is observed on the MoS_(2) core.The enhanced quantum yield of WS2trion emission can be explained by charge-transfer-enhanced photoexcited carrier dynamics,which is facilitated by resonance hole transfer from MoS_(2) side to WS_(2) side.This work sheds light on the 1D exciton photophysics in lateral heterostructures,which has the potential to lead to new concepts and applications of optoelectronic device.

Band offset and electronic properties at semipolar plane Al N(1ī01)/diamond heterointerface

Tailoring the electronic states of the Al N/diamond interface is critical to the development of the next-generation semiconductor devices such as the deep-ultraviolet light-emitting diode, photodetector, and high-power high-frequency field-effect transistor. In this work, we investigate the electronic properties of the semipolar plane Al N（11^-01）/diamond heterointerfaces by using the first-principles method with regard to different terminated planes of Al N and surface structures of diamond（100） plane. A large number of gap states exist at semi-polar plane Al N（11^-01）/diamond heterointerface, which results from the N 2 p and C 2 s2 p orbital states. Besides, the charge transfer at the interface strongly depends on the surface termination of diamond, on which hydrogen suppresses the charge exchange at the interface. The band alignments of semi-polar plane Al N（11^-01）/diamond show a typical electronic character of the type-Ⅱ staggered band configuration. The hydrogen-termination of diamond markedly increases the band offset with a maximum valence band offset of 2.0 e V and a conduction band offset of 1.3 e V for the semi-polar plane N–Al N（11^-01）/hydrogenated diamond surface. The unique band alignment of this Type-Ⅱ staggered system with the higher CBO and VBO of the semi-polar Al N/HC（100） heterostructure provides an avenue to the development of robust high-power high-frequency power devices.

Selecting nitride host for Yb^(3+) toward near-infrared emission with low-energy charge transfer band

Yb^(3+)-doped phosphors have characteristic near-infrared(NIR)emissions,but their applications in phosphor-converted light-emitting-diodes(pc-LEDs)and Si solar cells are limited due to their mismatching excitation spectra.Here,we selected nitride La3 Si6 N11(LSN)as host material to achieve Yb^(3+)NIR emission upon low-energy charge transfer(CT)excitation.The obtained phosphor LSN:Yb^(3+)has a broad CT excitation band ranging from 250 to 500 nm and narrowband NIR emissions ranging from 950 to 1100 nm centered at 983 nm.On the basis of spectral data,the vacuum referred binding energies(VRBE)schemes are constructed to locate energy levels of all lanthanide ions in LSN.We also fabricated NIR pc-LED device using 395 nm LED chip to demonstrate the potential applications of LSN:Yb^(3+)phosphors.

Regulation of the photogenerated carrier transfer process during photoelectrochemical water splitting:A review

Photoelectrochemical(PEC)water splitting is considered as an ideal technology to produce hydrogen.Photogenerated carrier migration is one of the most important roles in the whole process of PEC water splitting.It includes bulk transfer inside of the photoelectrode and the exchange at the solid-liquid interface.The energy barriers during the migration process lead to the dramatic recombination of photogenerated hot carrier and the reducing of their redox capacity.Thus,an applied bias voltage should be provided to overcome these energy barriers,which brings the additional loss of energy.Plentiful researches indicate that some methods for the regulation of photogenerated hot carrier,such as p-n junction,unique transfer nanochannel,tandem nanostructure and Z-Scheme transfer structure et al.,show great potential to achieve high-efficient PEC water overall splitting without any applied bias voltage.Up to now,many reviews have summarized and analyzed the methods to enhance the PEC or photocatalysis water splitting from the perspectives of materials,nanostructures and surface modification etc.However,few of them focus on the topic of photogenerated carrier transfer regulation,which is an important and urgent developing technique.For this reason,this review focuses on the regulation of photogenerated carriers generated by the photoelectrodes and summarizes different advanced methods for photogenerated carrier regulation developed in recent years.Some comments and outlooks are also provided at the end of this review.

Constructing charge transfer channel between dopants and oxygen vacancies for enhanced visible-light-driven water oxidation

Photocatalytic water oxidation is a crucial step in water splitting,but is generally restricted by the slow kinetics.Therefore,it is necessary to develop high-performance water oxidation photocatalysts.Herein,the Fe-doped Bi2WO6 nanosheets with oxygen vacancies(OVs)were synthesized for enhanced photocatalytic water oxidation efficiency,showing a synergistic effect between Fe dopants and OVs.When a molar fraction of 2%Fe was doped into the Bi2WO6 nanosheets,the visible-light-driven photocatalytic oxygen evolution rate was increased up to 131.3μmol·h^(-1)·g_(cat)^(-1)under ambient conditions,which was more than 3 times that of pure Bi2WO6 nanosheets.The proper doping concentration of Fe could promote the formation of OVs and at the same time modulate the band structure of catalysts,especially the position of the valence band maximum(VBM),leading to effective visible-light absorption and enhanced oxidizing ability of photogenerated holes.With ameliorated localized electron distribution,fast charge transfer channel emerged between the OVs and adjacent metal atoms,which accelerated the charge carrier transfer and promoted the separation of photoexcited electrons and holes.This work provides feasible approaches for designing efficient two-dimensional semiconductor water oxidation photocatalysts that could utilize visible-light,which will make more use of solar energy.

通过氟掺杂调控TiO_(2)的d带中心以增强光催化产H_(2)O_(2)活性

光催化产双氧水(H_(2)O_(2))以太阳光、水和空气中的氧气作为原料,将光能转变为化学能,是一种绿色高效节能环保的新技术,具有较好的应用前景.光催化产H_(2)O_(2)主要包括三个关键步骤:(1)催化剂在高能入射光激发下产生光生电子和空穴;(2)光生电子-空穴对分离并迁移到催化剂表面;(3)光生电子与催化剂表面吸附的氧气发生反应生成超氧自由基,其继续与水和光生电子反应,产生H_(2)O_(2).因此,氧气在催化剂表面吸附性能的强弱对光催化产H_(2)O_(2)的性能有着重要影响.d带中心理论表明,金属的d带能级高低决定了催化剂表面活性位点对小分子物质的吸附强度,能级越高,催化剂对小分子物质的吸附能力越强.TiO_(2)具有制备简单、无毒、理化性质稳定、导价带位置跨越多个氧化还原电位等诸多优势,在光催化生产H_(2)O_(2)领域具有较好的应用前景.提升TiO_(2)的d带中心可以提高其对小分子物质如O_(2)的吸附性能,有效提升其光催化产H_(2)O_(2)的活性.本文从氟离子掺杂提升TiO_(2)的d带中心增强对O_(2)的吸附性能入手,通过第一性理论计算、电子顺磁共振实验、飞秒瞬态吸收光谱等方法研究光生载流子的传输机理,阐明F/TiO_(2)光催化产H_(2)O_(2)活性增强机制,并对TiO_(2)光催化产H_(2)O_(2)的前景提出了展望.首先,分别以钛酸四异丙酯和氟化铵作为钛源和氟源,通过溶胶-凝胶法结合高温煅烧制得了F/TiO_(2)光催化剂.第一性理论计算结果表明,F-体相掺杂导致TiO_(2)的电荷分布不均匀,使得d带中心上移,从而增强TiO_(2)与表面吸附O_(2)的相互作用,降低表面氧的吸附能,最终提高光催化生成H_(2)O_(2)的效率.电子顺磁共振实验结果表明,晶格中F-离子的存在诱导了还原性Ti^(3+)中心的形成,这些还原性Ti^(3+)中心可以提供电荷补偿所需的额外电子.O_(2)温度程序解吸实验结果表明,F/TiO_(2)对O_(2)的化学吸附能力高于纯TiO_(2),说明较低的反键轨道占用率可以增强Ti^(3+)对O_(2)的吸附.飞秒瞬态吸收光谱结果表明,光生电子从F/TiO_(2)的导带转移到Ti^(3+)表面态和表面F-离子上,加速了光生电子和空穴的分离;光生电子与吸附在F/TiO_(2)表面的O_(2)发生反应,加速了H_(2)O_(2)的生成.光催化产H_(2)O_(2)性能实验结果表明,F-掺杂TiO_(2)后,光催化生成H_(2)O_(2)的产率由277μmol·g^(-1)·h^(-1)提高到了467μmol·g-1·h^(-1).循环实验结果表明,F/TiO_(2)使用前后形貌和晶体结构几乎没有改变,且循环实验后氧空位和Ti3+中心依然存在,说明制得的F/TiO_(2)光催化剂具有良好的稳定性.综上所述,本文借助第一性理论计算并结合实验结果,从d带中心调控的角度揭示了F/TiO_(2)光催化产H_(2)O_(2)活性提高的机理,阐明了光催化产H_(2)O_(2)的反应机制.本研究为优化光催化剂与氧气之间的吸附强度,提高光催化产H_(2)O_(2)的性能提供了一种新策略,可为后续光催化产H_(2)O_(2)技术的改进和应用提供参考.

Interfacial Charge Transfer in Nanoscale Polymer Transistors

Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors.While the details remain elusive in many systems,this charge transfer has been inferred in a number of photoemission experiments.We present electronic transport measurements in very short channel(L<100 nm)transistors made from poly(3-hexylthiophene)(P3HT).As channel length is reduced,the evolution of the contact resistance and the zero gate voltage conductance are consistent with such charge transfer.Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts,consistent with charge transfer expectations.Alternating current scanning tunneling microscopy(ACSTM)provides further evidence that holes are transferred from Pt into P3HT,while much less charge transfer takes place at the Au/P3HT interface.

纳米晶体中稀土离子的发光性质及其变化机理研究

近年来,纳米晶体中稀土离子发光性质的研究越来越受到人们的广泛关注,这是因为纳米稀土发光材料在发光、高清显示、光电子纳米器件、生物荧光标记、激光和闪烁体等众多领域有着重要的应用前景。本项目采用软化学合成方法如水热法、溶胶-凝胶法等,通过合成工艺的调控,设计并合成出一系列不同颗粒尺寸、分散均匀、形貌可控的稀土离子掺杂氧化物(氟化物)微/纳米晶体,利用激发、发射、漫反射以及高分辨激光光谱等光谱分析手段对其发光性质进行研究,弄清影响发光行为的本质原因。同时,结合光谱实验数据,利用密度泛函理论和复杂晶体化学键介电理论方法进行理论计算,成功解释了光谱变化规律和不同稀土离子间能量传递机理,为相关稀土光谱研究奠定了理论和实验基础。

纳米Gd_2O_3中两种格位Eu^(3+)的电荷迁移态激发跃迁

观测了粒径分别为15,23,135 nm的立方相Gd_2O_3:Eu^(3+)的选择激发光谱、发射光谱和激发光谱。受强量子限域效应的影响,纳米Gd_2O_3:Eu^(3+)的激发光谱的强度表现出对颗粒尺寸的明显依赖性。用Jorgensen公式计算电荷迁移带的位置,与实验测得激发光谱中位置相一致。通过电荷迁移带不同位置的选择激发光谱可以分辨出立方相Gd_2O_3:Eu^(3+)中C_2和S_6格位Eu^(3+)的发光,从选择激发的发射光谱和激发光谱结果计算出C_2和S_6格位电荷迁移带的激发光谱,与实验结果相符合。

共掺杂对Y_2O_3:Eu^(3+)纳米晶结构和发光性质的影响

采用燃烧法制备不同离子(M:Li+,Na+,K+,Mg2+,Sr2+,Ba2+,B3+,Al3+)共掺杂的纳米Y2O3∶Eu3+粉末。系统地研究了各掺杂离子对纳米Y2O3∶Eu3+材料的结构、发光性质及其寿命的影响。比较发现,掺杂不仅可以调节纳米材料的尺寸,还可以影响材料的结晶性,尤其是后者对发光性质和荧光动力学过程,如荧光强度、电荷迁移带的位置和5D0的寿命等有重要的影响。

纳米Y_2O_3∶Eu^(3+)中S_6格位电荷迁移带的光学特性

在Y2 O3 ∶Eu3 + 体材料和纳米材料中 ,观察到紫外激发下处于S6格位的Eu3 + 的5D0 →7F1发射 ( 5 82nm)的强度 ,相对处于C2 格位的5D0 →7F0 发射 ( 5 80nm)的强度 ,随着激发波长在 2 0 0～ 3 0 0nm紫外区由长变短而增强。这一现象说明Y2 O3 ∶Eu3 + 中两种格位的电荷迁移带及基质激发的性质不同。光谱分解得出S6格位的电荷迁移带位于C2 格位电荷迁移带的高能侧 ,Y2 O3 基质倾向于向S6格位进行能量传递。与体材料相比 ,两种格位的电荷迁移带在纳米材料中都发生红移 ;相对于C2 格位的电荷迁移带 ,S6格位的电荷迁移带强度在纳米材料中比在体材料中明显降低 ,并对结果进行了讨论。

通过改变基质阳离子半径使O^(2-)→Eu^(3+)电荷迁移带红移

采用高温固相法在1 300℃合成了A_(0.98)Nb_2O_6∶Eu_(0.02)(A=Ca,Sr,Ba)荧光粉。X射线衍射(XRD)的结果表明烧结后得到的产物为纯相。利用稳态荧光光谱(PL)和漫反射光谱(DRS)对A_(0.98)Nb_2O_6∶Eu_(0.02)(A=Ca,Sr,Ba)的发光性质进行了研究。结果表明A_(0.98)Nb_2O_6∶Eu_(0.02)(A=Ca,Sr,Ba)荧光粉可以发射Eu^(3+)的特征红光,光强度按Ca Nb_2O_6>Sr Nb_2O_6>Ba Nb_2O_6从大到小排列。激发光谱中可以观察到Eu^(3+)离子的电荷转移跃迁(CT)和f-f跃迁吸收。其中CT吸收峰因基质阳离子半径的增大而发生了较大程度的红移,从270 nm红移到了330 nm。

Pb^(2+)与人血清白蛋白的相互作用

用平衡透析和差示吸收光谱法详细研究了Pb2+与人血清白蛋白(HSA)的相互作用.平衡透析研究结果表明,Pb2+与HSA的相互作用明显受到缓冲溶液pH影响,在pH 6.3和pH 5.4时,Pb2+在HSA上的强结合位点数分别是2.1个和1.1个,弱结合位点数分别为7.0个和2.4个,通过非线性最小二乘法拟合Bjerrum方程,首次报道了Pb2+-HSA体系的逐级稳定常数,Hill系数表明Pb2+与HSA的结合具有负协同效应.使用差示吸收光谱法研究物质的量比为1∶1的Pb2+-HSA体系的电荷转移谱带,发现组氨酸咪唑基是Pb2+在白蛋白中的优先配位基团.进一步根据Zn2+与Pb2+竞争结合HSA上的强结合位点,推断Pb2+在HSA中优先结合位点是位点A.

能带理论及其在半导体材料Te的电荷传输机制中应用

能带理论是一种描述晶体内电子在周期性势场中运动的近似理论,可用来分析晶体的导电性、PN异质结、导热性和掺杂问题等.通过分析能带理论的三种近似模型,并以半导体材料碲(Te)为例,阐述Te与多硫电解液间的电荷传输机制.

GdAlO_3∶Eu^(3＋)荧光粉的合成及发光特性研究

研究了不同退火机制下GdAlO3∶Eu3+荧光粉的光谱特性。采用高温固相反应法在空气气氛和还原气氛中分别合成了GdAlO3∶Eu3+荧光粉,讨论了在烧结过程中产生的色心的光谱性质及其对GdAlO3∶Eu3+发光强度、激发光谱和O2--Eu3+电荷迁移带位置的影响。研究了后退火对GdAlO3∶Eu3+光谱特性的影响,进一步解释了VUV激发下的能量传递机制。根据烧结气氛、烧结温度和后退火对色心以及GdAlO3∶Eu3+光谱特性的影响,找到了一条能有效消除色心获得高荧光强度的两步反应合成路线。

Au/Au^(3+)对硅酸盐玻璃中Eu^(3+)发光性质的影响

采用熔融法制备了新颖的单掺Eu3+,Au/Eu3+和Au3+/Eu3+共掺的硅酸盐玻璃,并对这些玻璃的发光特性进行了初步研究。测试了玻璃的吸收光谱、激发光谱和发射光谱。在经过还原热处理的样品中观察到了强的Au表面等离子共振吸收带,证明了体系中确实有Au的存在。结果表明由于Au3+的存在使得O2与Au3+发生电荷迁移,电荷迁移带的强度明显增强,发光强度在电荷迁移带(CT)240nm激发下有所增强,而在共振激发(394nm)下却减弱了。Eu3+荧光衰减寿命没有明显变化。

萘酞菁锌复合体系的光致电荷转移

合成了萘酞菁锌,利用傅立叶红外光谱、元素分析和MALDI-TOF质谱等手段表征了分子结构;循环伏安测试和吸收光谱确认了共轭体系的扩大使分子带隙下降.根据材料加工性能的不同,分别采用溶液法、层-层蒸镀(Layer-by-layere vaporation)法和单层分散旋涂法,将给体分子萘酞菁锌与三种受体分子1-(3-甲氧基羧基)丙基-1-苯基-[6,6]C61,C60和N,N'-二嘧啶基苝四羧基二酰亚胺进行了复合,通过研究复合前后荧光变化,确认了给体-受体两相界面处发生了由分子能级差引发的光致电荷转移,为制备更宽光伏响应范围的太阳能电池器件提供了潜在的新途径.

Al^(3+)/Yb^(3+)/P^(5+)掺杂对石英玻璃紫外透过和紫外激发荧光的影响

采用溶胶–凝胶法结合高温真空烧结工艺制备了不同浓度的Al^(3+)/Yb^(3+)/P^(5+)掺杂石英玻璃。研究了P^(5+)和Al^(3+)的引入对Yb^(3+)掺杂石英玻璃紫外透过和紫外激发荧光光谱,以及Yb4d电子结合能的影响,并初步探索了其机理。研究结果表明,Al^(3+)/Yb^(3+)/P^(5+)掺杂石英玻璃在190~300 nm波段的吸收主要来源于O2-→Yb^(3+)的电荷迁移吸收,其谱带位置和Yb4d电子结合能随Yb^(3+)的第二配位元素(Al、Si、P)电负性增大向高能方向移动。真空烧结条件下,引入Al^(3+)会引发石英玻璃中Yb^(3+)还原为Yb2+,其典型的吸收峰位于330 nm处;然而,在Al^(3+)/Yb^(3+)共掺的基础上再引入P^(5+),且P^(5+)/Al^(3+)摩尔比大于1时,可以有效抑制Yb2+的形成。紫外光激发引起的近红外发光(976 nm)是电子从电荷迁移态弛豫到Yb^(3+)激发态向基态跃迁的结果,可见发光(525 nm)归因于Yb2+的5d→4f跃迁。本文研究结果对通过优化工艺和调整组分制备出高性能的Yb^(3+)掺杂光纤具有一定的指导意义。