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Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution.
ACS Nano ( IF 15.8 ) Pub Date : 2020-04-23 , DOI: 10.1021/acsnano.0c01924 Huy A Nguyen , Indrajit Srivastava 1 , Dipanjan Pan 2 , Martin Gruebele 2
ACS Nano ( IF 15.8 ) Pub Date : 2020-04-23 , DOI: 10.1021/acsnano.0c01924 Huy A Nguyen , Indrajit Srivastava 1 , Dipanjan Pan 2 , Martin Gruebele 2
Affiliation
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Unlike quantum dots, photophysical properties of carbon dots (CDs) are not strongly correlated with particle size. The origin of CD photoluminescence has been related to sp2 domain size and the abundance of oxidized surface defects. However, direct imaging of surface-accessible spatially localized oxidized defects is still lacking. In this work, solvothermal-synthesized CDs are fractionated into different colors by polarity-based chromatography. We then study the mechanism of CD fluorescence by directly imaging individual CDs with subparticle resolution by scanning tunneling microscopy. Density of states imaging of CDs reveals that the graphitic core has a large bandgap that is inconsistent with observed fluorescence wavelength, whereas localized defects have smaller electronic gaps for both red-emitting dots (rCDs) and blue-emitting dots (bCDs). For individual bCDs within our laser tuning range, we directly image optically active surface defects (ca. 1-3 nm in size) and their bandgaps, which agree with the emission wavelength of the ensemble from which the bCDs were taken. We find that the emissive defects are not necessarily the ones with the smallest gap, consistent with quantum yields less than unity (0.1-0.26). X-ray photoelectron spectroscopy and pH-dependent fluorescence titration show that oxygen-containing surface-accessible protonatable functional groups (e.g., phenolic -OH, -COOH) define the chemical identity of the defects. This observation explains why we detect neither long-lived optical excitation of the core nor a correlation between size and emission wavelength. Instead, control over the number of oxygen-containing defects defines the emission wavelength, with more oxidized defects at the surface producing redder emission wavelengths.
中文翻译：
用亚单颗粒分辨率揭示碳点的荧光机理。
与量子点不同，碳点（CDs）的光物理性质与颗粒大小没有强烈关系。CD光致发光的起源与sp2域大小和氧化表面缺陷的数量有关。但是，仍然缺乏对表面可及的空间局部氧化缺陷的直接成像。在这项工作中，溶剂热合成的CD通过基于极性的色谱分离成不同的颜色。然后，我们通过扫描隧道显微镜通过亚粒子分辨率直接成像单个CD来研究CD荧光的机制。CD的状态成像密度表明，石墨核具有较大的带隙，与观察到的荧光波长不一致，而局部缺陷对于发红光点（rCD）和发蓝光点（bCD）具有较小的电子间隙。对于我们激光调谐范围内的单个bCD，我们直接成像光学活性表面缺陷（大小约为1-3 nm）及其带隙，这些缺陷与bCD所采集的整体的发射波长一致。我们发现，发射缺陷不一定是具有最小间隙的缺陷，这与小于1（0.1-0.26）的量子产率一致。X射线光电子能谱和pH依赖性荧光滴定表明，含氧的表面可接近质子化官能团（例如，酚-OH，-COOH）定义了缺陷的化学特性。该观察解释了为什么我们既未检测到核的长寿命光学激发，也未检测到尺寸与发射波长之间的相关性。相反，控制含氧缺陷的数量会定义发射波长，
更新日期：2020-04-23
中文翻译：
用亚单颗粒分辨率揭示碳点的荧光机理。
与量子点不同，碳点（CDs）的光物理性质与颗粒大小没有强烈关系。CD光致发光的起源与sp2域大小和氧化表面缺陷的数量有关。但是，仍然缺乏对表面可及的空间局部氧化缺陷的直接成像。在这项工作中，溶剂热合成的CD通过基于极性的色谱分离成不同的颜色。然后，我们通过扫描隧道显微镜通过亚粒子分辨率直接成像单个CD来研究CD荧光的机制。CD的状态成像密度表明，石墨核具有较大的带隙，与观察到的荧光波长不一致，而局部缺陷对于发红光点（rCD）和发蓝光点（bCD）具有较小的电子间隙。对于我们激光调谐范围内的单个bCD，我们直接成像光学活性表面缺陷（大小约为1-3 nm）及其带隙，这些缺陷与bCD所采集的整体的发射波长一致。我们发现，发射缺陷不一定是具有最小间隙的缺陷，这与小于1（0.1-0.26）的量子产率一致。X射线光电子能谱和pH依赖性荧光滴定表明，含氧的表面可接近质子化官能团（例如，酚-OH，-COOH）定义了缺陷的化学特性。该观察解释了为什么我们既未检测到核的长寿命光学激发，也未检测到尺寸与发射波长之间的相关性。相反，控制含氧缺陷的数量会定义发射波长，
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