隧穿電子誘導的單分子單光子發(fā)射研究
發(fā)布時間：2018-08-22 14:49
【摘要】：單光子光源一直是量子信息技術相關領域研究的核心問題之一。現有的研究已經在多種單量子體系(量子點、原子、離子、分子、色心等等)實現了單光子發(fā)射。在這些體系中,分子單光子源具有出豐富的頻率選擇和穩(wěn)定、全同的光譜特征,表現出成為實用化單光子源的潛質。在已經實現的單光子發(fā)射體系中,受到光的衍射極限和電激勵器件尺寸的限制,往往需要借助分散手段構建低密度光源避免同時激發(fā)多個發(fā)光體而產生多光子事件。另一方面,電泵納米光源和單光子源對于納米光電集成十分重要,但嵌在兩個金屬納米電極之間的單個孤立的分子是否能發(fā)光、其發(fā)光特性是否為單光子發(fā)射卻一直沒有被證實,更不用說在納米尺度上表征和調控單光子光源。在本論文的工作中,我們利用掃描隧道顯微鏡的高度局域的隧穿電子激發(fā)及其亞納米分辨能力,可以選擇性地激發(fā)樣品表面孤立的單個分子,并借助隧道結內納米尺度的等離激元局域場增強效應,實現了單分子電致熒光。我們進一步對單分子電致熒光的出射光子強度的二階相關性進行檢測,證實了單分子電致發(fā)光過程表現出光子反聚束效應,是一種單光子發(fā)射現象。此外,這種單分子單光子輻射特性還可以通過改變納腔尺寸結構和在納米尺度操縱分子光源的構造進行調控和優(yōu)化。本論文由下面四章構成,各個章節(jié)的主要內容如下:第一章主要介紹全文相關的背景。我們首先從表面等離激元的概念引入,強調了納腔等離激元的增強效應;然后簡單介紹了掃描隧道顯微鏡(STM)和掃描隧道顯微鏡誘導發(fā)光技術(STML),并對實現掃描隧道顯微鏡誘導分子發(fā)光所需要的兩個必要條件——納腔等離激元共振增強和脫耦合層隔絕熒光淬滅——進行了闡述;隨后我們介紹了單光子源的物理背景、技術測量手段和獲得方式;最后,綜合以上內容論證了利用STML技術實現單分子單光子發(fā)射的創(chuàng)新性和可行性,并介紹了實驗中實際所使用的STM和光學檢測手段。在第二章中,我們首先研究了氧化銅作為脫耦合層實現單分子電致熒光的可能性,實驗中選取的熒光分子是并五苯和H2TBPP。我們發(fā)現,單個并五苯分子在氧化銅表面的吸附表現出很高的選擇性,但分子本征熒光仍然被淬滅,不過分子對襯底的等離激元發(fā)光具有偏壓依賴的強度調制關系:正偏壓下分子對等離激元發(fā)光有增強作用,而負偏壓下則是抑制作用。這種偏壓依賴的關系可以用表面和分子在正負偏壓區(qū)間不同的電子態(tài)強度分布來解釋。氧化銅表面隨后蒸鍍的部分H2TBPP分子團簇能夠表現出分子發(fā)光的特征,說明氧化銅作為脫耦合層具有一定程度的絕緣效果,能夠實現稍高位置的分子熒光。在第三章中,我們選用介電常數更高、絕緣效果更好的NaCl薄膜作為脫耦合層,選擇ZnTPP分子作為熒光分子研究了單分子電致發(fā)光。利用原位分子熱蒸發(fā)的技術,我們可以可控地使得ZnTPP分子在NaCl薄膜表面以單個形式存在,高分辨的STM圖像證明了這種樣品結構。隨后,我們成功地在ZnTPP分子上實現了單分子的電致熒光,值得注意的是,ZnTPP的分子熒光僅在負偏壓下出現。我們對ZnTPP分子的熒光進行了輻射特性的測量,結果表明出射光子被納腔垂直方向極化而形成沿探針方向的線偏振。由于該體系下ZnTPP分子的熒光強度仍然不夠強,因此我們未能得到信噪比足夠的二階相關函數測量數據。本章的實驗工作成功實現了單分子電致熒光,并證明了 NaCl薄膜作為脫耦合層的優(yōu)越性質,為后續(xù)單分子單光子發(fā)射實驗奠定了基礎。在第四章中,我們對第三章的實驗體系進行改進,一方面構造更多層的NaCl薄膜實現更好的脫耦合效果,另一方面將熒光分子換成在NaCl體系下發(fā)光強度更強的酞菁類分子(H2Pc、ZnPc),從而實現了電致單分子單光子發(fā)射。在這個體系中我們發(fā)現,四層NaCl上酞菁分子的STM電致熒光表現出強度足夠高的單分子電致發(fā)光。我們對發(fā)射光子的二階相關函數進行了檢測,實驗結果表明,四層NaCl上的單個H2Pc或ZnPc分子的光子發(fā)射均表現出明顯的光子反聚束效應,具有顯著的單光子發(fā)射特性,而且最好的單光子發(fā)射純凈度可以達到g2(0)=0.09。g2(0)的值沒有理想地降到零可能是因為HBT測量系統時間分辨率不夠和極少量等離激元輻射光子的影響。表面上同種分子在光譜和二階相關函數上表現出幾乎全同的特性。后續(xù)研究表明,納腔的結構尺寸會顯著影響單光子輻射的參數,這是由于分子與電極的距離越近則有更大的概率激發(fā)等離激元光子,從而影響單光子純凈度。利用STM的操縱能力,我們構建了分子二聚體形式的耦合體系,證明其發(fā)射光子統計同樣表現出光子反聚束效應,獲得了強度更高、線寬更窄的單光子發(fā)射。這一方面為調控單光子源提供了新的方法,而且也從實驗上證實了偶極相干耦合的分子聚合體構成了一種多分子糾纏的整體系統。
[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.
【學位授予單位】：中國科學技術大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O485
本文編號：2197427
[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.
【學位授予單位】：中國科學技術大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O485
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