Recently, Rogach's group realized solidstate-luminescent CD/silica composite, which possessed high quantum yield (QY) and PL output with relatively high CD loading (19.2 wt%) by gelation. [18] However, this process was limited to hydroxyl-rich CD only and still suffered from aggregation in extremely high concentration.Fundamental solution is to develop a selfquenching-resistant CD even without dispersion matrices. Polymer CD (PCD), a polymer-like CD consisting of crosslinked matrix and sub-fluorophores, has been studied for solid-state luminescence because crosslinking may prevent π-π interaction. [20][21][22][23] Several groups synthesized quenching-resistant PCD, but those showed red-shifted emission with noticeable color change compared to solutionstate fluorescence. [24][25][26][27] Such spectral shift in solid-state usually occurs with nonradiative decay competitively, which leads to a decrease of QY. In addition, the emission color change in solidstate needs additional optimization process for the fabrication of optoelectronics, so that the energy transfer as an origin for ACQ and red-shift should be strictly avoided.Herein, we synthesized a blue-emitting PCD with the same emission center at 434 nm in both solution-and solid-state through a hydrothermal reaction. By structural and spectroscopic analysis, the origin of solid-state luminescence was investigated. We further identified a reason for the emission color consistency of PCD from dilute solution to powder form by comparing with CD that showed red-shifted emission in solid-state. We also fabricated LED using PCD as a color-converting layer, and it had a luminous efficacy of 12.03 lm W −1 without significant color change even with extremely high loading fraction of 50 wt%. With the increased loading fraction (20 wt%), we enhanced brightness of our LED up to four times compared to the LED fabricated with conventional low loading fraction (0.1-5 wt%) using PCD. Compared to red-shifted CD (RCD)-based LED with the conventional loading fraction, the luminous flux of PCD-based LED with 20 wt% increased by 3 to 15 times. This result would be a great interest to those who study CD-based LED. Results and DiscussionSolid-state luminescent polymer carbon dot (PCD) was synthesized using citric acid (CA) and diethylenetriamine (DETA) through a hydrothermal reaction at 70 °C (Figure 1a). Product Solid-state luminescent carbon dot without dispersion matrices is studied to overcome aggregation-caused quenching, but it usually shows spectral shift between solution-and solid-state accompanying a dramatic decrease of fluorescence intensity, which hinders a real application. Herein, polymer carbon dot (PCD) showing solid-state luminescence without red-shift is synthesized by low-temperature reaction. The PCD solution and solid have the same emission peaks at 434 nm and similar absolute quantum yields of 68.2% and 62.7%, respectively, which is uncommon feature compared to other carbon dots. The mechanism for solid-state luminescence and emission color consistency is invest...

Realization of luminescent carbon dots (CDs) in the solid state has been a critical issue for their applications in various fields, such as optoelectronic devices and inkjet printing. However, luminescence self-quenching of CDs in an aggregated state limits the development of their applications necessarily demanding solid-state phosphors. Here, we report the origin of luminescence quenching of CDs in terms of the sp2 domain content, and we realized the solid-state luminescent CDs by controlling the degree of crystallinity. We compared self-quenched CDs and self-quenching-resistant CDs with structural analysis and figured out that a high mass ratio of urea regulated the amount of the sp2 domain in CDs, exhibiting yellow emission at 530 nm in a solid state. We also confirmed that through the carbonization time control, reducing the degree of crystallinity in CDs enabled initially nonluminescent CDs to show a quantum yield of 7.8% at 518 nm in the solid state, providing antiself-quenching property. This research would open the way to the development of solid-state CDs and their applications.

The dominant emission color of multi-color emissive carbon dots under a single excitation source is governed by the interparticle distance.