单个半导体纳米晶多激子相关性质的研究

发布时间：2018-09-13 10:16

【摘要】：半导体纳米晶是一类由～100-10000个原子构成的纳米半导体材料,典型尺寸为1-10纳米。纳米晶的尺寸小于或近似于其激子玻尔半径,因此属于量子受限的材料体系。由于量子受限效应,纳米晶表现出光谱蓝移、能级分立等性质。光谱蓝移与纳米晶受限程度正相关,因此可以通过改变纳米晶尺寸、形状等实现对其能带结构的调控。分立的能级使得载流子间俄歇作用不再受动量守恒条件的限制,并且纳米晶纳米尺度的尺寸加大了载流子间的波函数交叠和库伦相互作用,这导致载流子间存在强烈的俄歇作用。快速的俄歇复合抑制了多激子的辐射复合,使得半导体纳米晶成为一种理想单光子源。同时,强烈的俄歇作用也促进了碰撞电离过程,增强了纳米晶载流子倍增效率。基于上述特性,半导体纳米晶在光电领域具有广泛的应用前景。例如,在LED领域,通过纳米晶的受限效应可以调控发光颜色,从而获得高质量白光光源;在量子信息领域,半导体纳米晶是很好的室温单光子源;在太阳能电池和光电探测器领域,纳米晶高效的载流子倍增效率能有效提高器件的光电转换效率。本文主要研究单个半导体纳米晶多激子相关性质,具体包括CsPbBr_3纳米晶单光子发射、荧光闪烁、光谱漂移的研究,CdSe纳米晶多激子复合、载流子倍增过程的研究。第二章我们主要研究单个CsPbBr_3纳米晶的光学性质。首先我们运用化学方法合成了平均尺寸为～9.4nm的CsPbBr_3纳米晶,这一尺寸接近激子玻尔半径,因此CsPbBr_3纳米晶属于量子受限体系。在室温下,CsPbBr_3纳米晶表现出荧光闪烁现象,这些随机出现的暗态来源于带电激子的非辐射俄歇复合所导致的荧光猝灭。俄歇作用同时也抑制了多激子的辐射复合,测量二阶相关函数确定了 CsPbBr_3纳米晶的单光子发射性质,并且钙钛矿纳米晶荧光波长可以通过量子受限效应和改变卤素元素(Cl,Br,I)组成进行人工调控,这使得钙钛矿纳米晶成为单光子源的有力竞争者。在低温下,单个CsPbBr_3纳米晶荧光光谱表现出光谱漂移的现象,光谱线宽也因此达到相对较宽的1meV。CsPbBr_3纳米晶在表现出与传统金属硫化物纳米晶类似的光学性质的同时,也具有自身的一些优点:CsPbBr_3纳米晶的吸收截面比传统纳米晶大两个量级,同时CsPbBr_3纳米晶快速的辐射复合也使其具有更高的单光子发射效率。第三章我们主要运用时间相关单光子计数的方法研究单个CdSe纳米晶的带电双激子。由于俄歇作用、缺陷态捕获等原因,纳米晶容易形成带电激子态。处于带电态的纳米晶如果同时吸收两个光子就形成了带电双激子态,带电双激子态通过俄歇过程形成带电单激子态,带电单激子态再次通过俄歇过程复合。我们测量单个伸长型CdSe纳米晶的荧光闪烁曲线,发现在高功率下,荧光闪烁曲线除了荧光闪烁的亮态和暗态,还额外出现了一个荧光强度介于两者之间的灰态。灰态来源于由于双激子俄歇作用带负电的纳米晶同时吸收两个光子而形成了带负电双激子态。统计灰态光子的二阶相关函数确认了其来源于带负电双激子的辐射复合,并且灰态的单指数荧光强度衰减曲线暗示了其发射的光子对具有相同的荧光寿命。由于带电激子态的干扰,纳米晶的载流子倍增效应一直饱受争议。为了排除干扰,第四章我们运用单分子光谱的方法研究单个CdSe纳米晶的载流子倍增效应。对于普通半导体纳米晶,带电激子态和中性激子的俄歇复合寿命一般在亚纳秒量级,这相对于中性单激子几十纳秒的辐射复合寿命大大缩短。考虑到现有的载流子倍增效率测量实验都是研究群体纳米晶样品,超快光谱测量技术难以准确地分辨出与双激子俄歇复合相关的真实载流子倍增信号和与带电激子俄歇复合相关的虚假载流子倍增信号。我们测量单个CdSe纳米晶荧光强度,通过荧光闪烁曲线的亮态和暗态可以方便地区分出中性激子和带电激子,进而得到可靠的载流子倍增效率。基于对大量单个纳米晶载流子倍增效率的测量,我们得到在266nm(光子能量为2.46倍带隙能量)激光激发下CdSe纳米晶的平均载流子倍增效率是20.2%。
[Abstract]:Semiconductor nanocrystals are a class of nano-semiconductor materials consisting of ~100-10 000 atoms with a typical size of 1-10 nm. The size of nanocrystals is smaller than or similar to the exciton Bohr radius, so they belong to the quantum confined material system. Because of the positive correlation between the confinement degree of nanocrystals, we can adjust the band structure of nanocrystals by changing the size and shape of nanocrystals. The discrete energy levels make the intermittence Auger interaction of carriers no longer restricted by the momentum conservation condition, and the nanocrystalline size enlarges the wave function overlap and Coulomb interaction between carriers, which leads to this. Rapid Auger recombination inhibits the radiative recombination of multiple excitons, making semiconductor nanocrystals an ideal single photon source. At the same time, strong Auger recombination promotes the collision ionization process and enhances the carrier multiplication efficiency of nanocrystals. For example, in the field of LED, high quality white light source can be obtained by controlling the luminous color of nanocrystals through the limited effect of nanocrystals; in the field of quantum information, semiconductor nanocrystals are good single photon source at room temperature; in the field of solar cells and photodetectors, nanocrystals have high efficiency of carrier doubling efficiency. In this paper, we mainly study the properties of single semiconductor nanocrystalline polyexciton, including single photon emission, fluorescence scintillation, spectral drift, CdSe nanocrystalline polyexciton recombination, carrier multiplication process. In the second chapter, we mainly study single CsPbBr_3 nanocrystalline. Optical properties. Firstly, CsPbBr_3 nanocrystals with an average size of 9.4 nm were synthesized by chemical method. The size of CsPbBr_3 nanocrystals is close to the exciton Bohr radius. Therefore, CsPbBr_3 nanocrystals belong to a quantum confined system. At room temperature, CsPbBr_3 nanocrystals exhibit fluorescence scintillation phenomena. These random dark states originate from the non-radiation of charged excitons. Fluorescence quenching induced by Auger recombination. Auger effect also inhibits the radiative recombination of multiple excitons. The single photon emission properties of CsPbBr_3 nanocrystals are determined by measuring the second-order correlation function. The fluorescence wavelength of perovskite nanocrystals can be controlled artificially by quantum confinement effect and changing the composition of halogen elements (Cl, Br, I), which makes it possible to control the emission of CsPbBr_3 nanocrystals artificially. Perovskite nanocrystals are powerful competitors for single photon sources. At low temperatures, the fluorescence spectra of single CsPbBr_3 nanocrystals exhibit spectral drift, which results in a relatively wide spectral linewidth of 1meV. CsPbBr_3 nanocrystals exhibiting optical properties similar to those of traditional metal sulfide nanocrystals. Advantages: The absorption cross section of CsPbBr_3 nanocrystals is two orders of magnitude larger than that of conventional nanocrystals, and the fast radiation recombination of CsPbBr_3 nanocrystals also leads to higher single photon emission efficiency. In the charged state, if two photons are absorbed at the same time, the charged double exciton state is formed. The charged double exciton state is formed by Auger process, and the charged single exciton state is recombined by Auger process. We measure the fluorescence of a single elongated CdSe nanocrystal. In addition to the bright and dark states of the fluorescence scintillation, a gray state with fluorescence intensity between the two states appears in the fluorescence scintillation curve at high power. The gray state originates from the double exciton Auger interaction of the negatively charged nanocrystals which absorb two photons at the same time. The second-order correlation function confirms that it originates from the radiative recombination of the negatively charged double excitons, and the decay curve of the single-exponential fluorescence intensity of the gray state implies that the photons emitted by the nanocrystals have the same fluorescence lifetime. The carrier multiplication effect in a single CdSe nanocrystal is studied by means of single molecule spectroscopy. For ordinary semiconductor nanocrystals, the Auger recombination lifetime of charged and neutral excitons is generally in the order of sub-nanosecond, which is much shorter than the radiation recombination lifetime of tens of nanoseconds of neutral excitons. Ultra-fast spectroscopy is difficult to accurately distinguish the true carrier multiplication signal related to double-exciton Auger recombination and the false carrier multiplication signal related to charged exciton Auger recombination. The average carrier multiplication efficiency of CdSe nanocrystals is 20.2% at 266 nm (photon energy 2.46 times band gap energy) based on the measurement of carrier multiplication efficiency of a large number of single nanocrystals.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TN304

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