隧穿电子诱导的单分子单光子发射研究

发布时间：2018-08-22 14:49

【摘要】：单光子光源一直是量子信息技术相关领域研究的核心问题之一。现有的研究已经在多种单量子体系(量子点、原子、离子、分子、色心等等)实现了单光子发射。在这些体系中,分子单光子源具有出丰富的频率选择和稳定、全同的光谱特征,表现出成为实用化单光子源的潜质。在已经实现的单光子发射体系中,受到光的衍射极限和电激励器件尺寸的限制,往往需要借助分散手段构建低密度光源避免同时激发多个发光体而产生多光子事件。另一方面,电泵纳米光源和单光子源对于纳米光电集成十分重要,但嵌在两个金属纳米电极之间的单个孤立的分子是否能发光、其发光特性是否为单光子发射却一直没有被证实,更不用说在纳米尺度上表征和调控单光子光源。在本论文的工作中,我们利用扫描隧道显微镜的高度局域的隧穿电子激发及其亚纳米分辨能力,可以选择性地激发样品表面孤立的单个分子,并借助隧道结内纳米尺度的等离激元局域场增强效应,实现了单分子电致荧光。我们进一步对单分子电致荧光的出射光子强度的二阶相关性进行检测,证实了单分子电致发光过程表现出光子反聚束效应,是一种单光子发射现象。此外,这种单分子单光子辐射特性还可以通过改变纳腔尺寸结构和在纳米尺度操纵分子光源的构造进行调控和优化。本论文由下面四章构成,各个章节的主要内容如下:第一章主要介绍全文相关的背景。我们首先从表面等离激元的概念引入,强调了纳腔等离激元的增强效应;然后简单介绍了扫描隧道显微镜(STM)和扫描隧道显微镜诱导发光技术(STML),并对实现扫描隧道显微镜诱导分子发光所需要的两个必要条件——纳腔等离激元共振增强和脱耦合层隔绝荧光淬灭——进行了阐述;随后我们介绍了单光子源的物理背景、技术测量手段和获得方式;最后,综合以上内容论证了利用STML技术实现单分子单光子发射的创新性和可行性,并介绍了实验中实际所使用的STM和光学检测手段。在第二章中,我们首先研究了氧化铜作为脱耦合层实现单分子电致荧光的可能性,实验中选取的荧光分子是并五苯和H2TBPP。我们发现,单个并五苯分子在氧化铜表面的吸附表现出很高的选择性,但分子本征荧光仍然被淬灭,不过分子对衬底的等离激元发光具有偏压依赖的强度调制关系:正偏压下分子对等离激元发光有增强作用,而负偏压下则是抑制作用。这种偏压依赖的关系可以用表面和分子在正负偏压区间不同的电子态强度分布来解释。氧化铜表面随后蒸镀的部分H2TBPP分子团簇能够表现出分子发光的特征,说明氧化铜作为脱耦合层具有一定程度的绝缘效果,能够实现稍高位置的分子荧光。在第三章中,我们选用介电常数更高、绝缘效果更好的NaCl薄膜作为脱耦合层,选择ZnTPP分子作为荧光分子研究了单分子电致发光。利用原位分子热蒸发的技术,我们可以可控地使得ZnTPP分子在NaCl薄膜表面以单个形式存在,高分辨的STM图像证明了这种样品结构。随后,我们成功地在ZnTPP分子上实现了单分子的电致荧光,值得注意的是,ZnTPP的分子荧光仅在负偏压下出现。我们对ZnTPP分子的荧光进行了辐射特性的测量,结果表明出射光子被纳腔垂直方向极化而形成沿探针方向的线偏振。由于该体系下ZnTPP分子的荧光强度仍然不够强,因此我们未能得到信噪比足够的二阶相关函数测量数据。本章的实验工作成功实现了单分子电致荧光,并证明了 NaCl薄膜作为脱耦合层的优越性质,为后续单分子单光子发射实验奠定了基础。在第四章中,我们对第三章的实验体系进行改进,一方面构造更多层的NaCl薄膜实现更好的脱耦合效果,另一方面将荧光分子换成在NaCl体系下发光强度更强的酞菁类分子(H2Pc、ZnPc),从而实现了电致单分子单光子发射。在这个体系中我们发现,四层NaCl上酞菁分子的STM电致荧光表现出强度足够高的单分子电致发光。我们对发射光子的二阶相关函数进行了检测,实验结果表明,四层NaCl上的单个H2Pc或ZnPc分子的光子发射均表现出明显的光子反聚束效应,具有显著的单光子发射特性,而且最好的单光子发射纯净度可以达到g2(0)=0.09。g2(0)的值没有理想地降到零可能是因为HBT测量系统时间分辨率不够和极少量等离激元辐射光子的影响。表面上同种分子在光谱和二阶相关函数上表现出几乎全同的特性。后续研究表明,纳腔的结构尺寸会显著影响单光子辐射的参数,这是由于分子与电极的距离越近则有更大的概率激发等离激元光子,从而影响单光子纯净度。利用STM的操纵能力,我们构建了分子二聚体形式的耦合体系,证明其发射光子统计同样表现出光子反聚束效应,获得了强度更高、线宽更窄的单光子发射。这一方面为调控单光子源提供了新的方法,而且也从实验上证实了偶极相干耦合的分子聚合体构成了一种多分子纠缠的整体系统。
[Abstract]:Single photon source has been one of the key issues in the field of quantum information technology. The existing research has realized single photon emission in a variety of single quantum systems (quantum dots, atoms, ions, molecules, color centers, etc.). In these systems, molecular single photon sources have abundant frequency selectivity and stability, identical spectral characteristics. Limited by the diffraction limit of light and the size of electric exciting devices, it is often necessary to construct low-density light sources by means of dispersion to avoid multi-photon events by simultaneously exciting multiple light emitters. On the other hand, electrically pumped nano-light sources and single-photon sources are used. It is very important for nano-optoelectronic integration, but whether a single isolated molecule embedded between two metal nano-electrodes can emit light or not and whether its luminescent properties are single photon emission have not been confirmed, let alone characterizing and regulating single photon light sources at nano-scale. Highly localized tunneling electron excitation and its sub-nano resolution enable the selective excitation of individual molecules isolated on the surface of the sample, and the single molecule electrofluorescence is realized by means of the local field enhancement effect of nano-scale plasmon in the tunnel junction. We further investigate the second-order dependence of the photon intensity of the single molecule electrofluorescence emission. In addition, the single-molecule single-photon emission characteristics can be controlled and optimized by changing the size of the nanocavity and manipulating the structure of the molecular light source at the nanoscale. The main contents of the chapters are as follows: In the first chapter, we introduce the concept of surface plasmon, and emphasize the enhancement effect of nanocavity plasmon. Then we briefly introduce the scanning tunneling microscopy (STM) and the scanning tunneling microscopy induced luminescence (STML) technology, and the realization of the temptation of scanning tunneling microscopy (STM). Two necessary conditions for conducting molecule luminescence-resonance enhancement of nanocavity plasmon and fluorescence quenching of decoupled layer isolation-are described; then we introduce the physical background of single-photon source, technical measurement methods and acquisition methods; finally, we demonstrate the realization of single-photon emission using STML technology. In the second chapter, we first studied the possibility of using copper oxide as a decoupling layer to achieve single molecule electrofluorescence. The fluorescent molecules selected in the experiment were pentacene and H2TBPP. We found that a single pentacene molecule was on the surface of copper oxide. The adsorption exhibits high selectivity, but the intrinsic fluorescence of the molecule is still quenched. However, the molecule has a bias-dependent intensity modulation relation to the PL emission on the substrate: the molecule enhances PL emission under positive bias, while the molecule inhibits PL emission under negative bias. Some H2TBPP clusters evaporated on the surface of copper oxide exhibit molecular luminescence characteristics, indicating that copper oxide as a decoupling layer has a certain degree of insulation effect and can achieve a slightly higher position of molecular fluorescence. Single molecule electroluminescence was studied by using ZnTPP molecule as fluorescent molecule. Using in situ molecular thermal evaporation technique, we can controllably make ZnTPP molecule exist in a single form on the surface of NaCl film. High resolution STM images confirmed the structure of the sample. It is noteworthy that the molecular fluorescence of ZnTPP occurs only under negative bias. We have measured the radiation characteristics of the fluorescence of ZnTPP molecule. The results show that the emitted photons are polarized perpendicular to the nanocavity and form a linear polarization along the probe direction. The fluorescence intensity of the electron is still not strong enough, so we can not get enough second-order correlation function measurement data of signal-to-noise ratio. The experimental work in this chapter successfully realized the single molecule electrofluorescence, and proved the superior properties of NaCl film as a decoupling layer, which laid the foundation for the follow-up single molecule single photon emission experiment. In the third chapter, we improved the experimental system. On the one hand, more layers of NaCl thin films were constructed to achieve better decoupling effect. On the other hand, the fluorescent molecules were replaced by phthalocyanines (H2Pc, ZnPc) with stronger luminescence intensity in the NaCl system. Thus, single photon emission was realized. The STM electroluminescence of cyanine molecules exhibits high enough intensity single-molecule electroluminescence. The second-order correlation function of emitted photons is measured. The experimental results show that the photon emission of a single H2Pc or ZnPc molecule on four-layer NaCl exhibits obvious photon anti-bunching effect, and has remarkable single-photon emission characteristics, and is the best. The single photon emission purity can reach g2(0)=0.09.g2(0) without ideal reduction to zero may be due to the lack of time resolution of the HBT measurement system and the influence of very few photons emitted by plasmons. The structure size can significantly influence the parameters of the single photon radiation, because the closer the molecule is to the electrode, the more probable the plasmon photons are excited, thus affecting the single photon purity. The single photon emission with higher intensity and narrower linewidth is obtained by the beam effect, which provides a new method to control the single photon source and also proves experimentally that the dipole coherently coupled molecular aggregates constitute a whole system of multi-molecular entanglement.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O485

【相似文献】