The Next Generation Technology for Light Emitting Displays by Louzhen Fan from Oasis Publishers

Clearwater, FL, October 23, 2018 --( Louzhen Fan from Oasis Publishers has invented a next-generation technology for Light Emitting Diodes. Quantum dots have emerged as the next-generation technology for Light Emitting Diodes (LEDs) used in lighting and displays. These materials emit pure, saturated color that can be tuned simply by changing the size of the dots. Some of the unique advantages of quantum dots include outstanding color purity (full-width-at-half-maximum (FWHM) ~30 nm), high brightness (up to ~200,000 cd m−2), and low operating voltage (Vturn-on < 2 V).

Recent intensive researches (from 2006) have shown that light emitting Carbon Quantum Dots or CQDs offer an alternative to traditional semiconductor quantum dots such as the Cd2+/Pb2+-based quantum dots in light-emitting displays. CQDs are made up of quantum sized carbon of size less than 10 nm, and provide unique advantages in terms of carbon’s high stability, low cost, high abundance, and environment-friendliness.

However, there is a known limitation that CQDs give broad emission and inferior color-purity with FWHM commonly exceeding 80 nm, and this limits CQDs’ applications in display technology, where high color-purity is a prerequisite. This research from Louzhen Fan proves that Triangular CQDs (or T-CQDs) can be designed to deliver light emission (from blue to red) with an unprecedented narrow bandwidth of 30 nm, and this finding invalidates the above discussed general limitation with respect to CQDs.

In this breakthrough research, Louzhen Fan demonstrates that Triangular CQDs can produce multi-colored, narrow bandwidth emission, with high color-purity (FWHM of 30 nm), giving quantum yield up to 54–72%.

Synthesis of Triangular CQDs was accomplished by a rational synthesis of triangular CQDs by choosing a three-fold symmetric phloroglucinol (PG) as the reagent (a triangulogen), together with a tri-molecular reaction route designed into the neighboring active -OH and -H groups for six-membered ring cyclization, propagating to the target high color-purity Narrow Bandwidth Emission T-CQDs (NBE-T-CQDs).

The results also showed bright multi-colored emissions of blue (B), green (G), yellow (Y), and red (R) from the NBET-CQDs solutions, with gradually increasing sizes from 1.9 nm, to 2.4, 3.0, and 3.9 nm, respectively, as expected from the quantum confinement effect.

The ultraviolet photoelectron spectroscopy (UPS) revealed up-shifted highest occupied molecular orbital (HOMO) levels from -5.18 to -4.92 eV and down-shifted lowest unoccupied molecular orbital (LUMO) levels from -2.55 to -2.85 eV for the NBE-T-CQDs, from blue to red. From this observation, Louzhen Fan. conclude that the quantum confinement effect dominates the electronic and optical properties of the NBE-T-CQDs.

As part of the study, the important physical parameters of exciton binding energy of carbon materials were obtained for the first time, and Louzhen Fan. consider this as one of the most significant achievements of this study. The parameters were obtained by integrating PL emission intensity as a function of temperature (175–295 K).

To gain insights on the high color purity and its intrinsic relation with the structure of the NBE-T-CQDs, Louzhen Fan. conducted detailed studies on structural characterizations. Further analysis was conducted using HAADF-STEM images, wide-area TEM images, six-fold symmetric Fast Fourier.

It has been found that there is a correlation between the triangular structure and the narrow bandwidth emission of NBE-T-CQDs. The studies also demonstrated reduced electron-phonon coupling in the NBE-T-CQDs. The higher degree of delocalization lead to higher structural stability of the unique triangular structure of the NBE-T-CQDs, which in turn resulted in dramatically reduced electron–phonon coupling. This contributed to the high color-purity excitonic emission and narrow FWHM of PL spectra of NBE-T-CQDs as demonstrated by the temperature-dependent PL spectra.

Louzhen Fan conducted and elaborated theoretical investigations which proved that high color-purity excitonic emission of the NBE-T-CQDs was due to the unique highly crystalline triangular structure, functionalized with pure electron-donating hydroxyl groups at the edge sites, showing highly delocalized charges and outstanding structural stability.

On the basis of the research and analysis, Louzhen Fan and team, extend the study by fabricating an experimental LED from blue to red, with the NBE-T-CQDs blended with poly(N-vinyl carbazole) (PVK) as the active emission layer. This multi-colored LED based on the NBE-T-CQDs demonstrated high color-purity (FWHM of 30–39 nm), high maximum luminance (1882–4762 cd m−2), and current efficiency of 1.22–5.11 cd A-1, rivalling the well-developed inorganic QDs-based LEDs.

Carbon-based light emitting displays can be the future prospect in next-generation technological frontiers of carbon photonics and optoelectronics. In this elaborate research, Louzhen Fan demonstrated that High Color-Purity Triangular Carbon Quantum Dots can revolutionize the next generation LED display technology with its superior characteristics, compared to traditional semiconductor LEDs.
Oasis Publishers
Louzhen Fan
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