Enhancement of Photon Upconversion in Rare Earth-Doped Carbon Quantum Dots
Authors: P. Vijeesh , J. Ananthu E. , P. Liyana V. 
Published: January 2025

Background:
Among the various types of quantum dots, Carbon Quantum Dots
(CQDs) have emerged as a particularly promising class of nanomaterials. CQDs are characterized
by their tunable photoluminescence, high chemical stability, low toxicity, and excellent
biocompatibility, making them suitable for a wide range of applications, including bioimaging,
sensing, drug delivery, and energy-related devices. Recent research has focused on enhancing
the optical properties of CQDs through doping with rare earth elements, which introduces
unique photoluminescence properties due to their distinct electronic configurations and
energy transitions. Photon upconversion, a process where lower-energy photons are absorbed
and re-emitted as higher-energy photons, is a key area of interest in CQD research. This phenomenon
is particularly useful in applications that require high-energy ultraviolet light, such
as bioimaging and photocatalysis. The ability of CQDs to exhibit photon upconversion,
alongside their traditional downconversion photoluminescence, adds to their versatility and
potential for innovative applications.

Objective:
The objective of this study is to synthesize and characterize pure and rare earthdoped
Carbon Quantum Dots (CQDs) using a hydrothermal method with gelatin as the precursor.
The research aims to investigate the photoluminescent properties of these CQDs, with a
particular focus on their photon upconversion capabilities and emission stability. By exploring
the effects of doping and synthesis conditions on the optical characteristics of CQDs, the
study seeks to enhance their potential for applications in fields such as bioimaging, fluorescent
marking, solar cell efficiency enhancement, and other technologies requiring stable and reliable
luminescence. The ultimate goal is to demonstrate the suitability of these synthesized
CQDs for various scientific and practical applications, contributing to advancements in nanomaterial
research and technology.

Method:
The hydrothermal bottom-up method for synthesizing Carbon Quantum Dots
(CQDs) involves dissolving 0.5 grams of gelatin in 25 mL of doubly deionized water with
continuous stirring to create a uniform solution. This solution is then transferred into a 50 mL
Teflon-lined autoclave, which is placed in a muffle furnace set to 160°C. The mixture undergoes
a hydrothermal reaction under controlled heat and pressure for 4 hours, converting the
gelatin into CQDs. After heating, the autoclave is allowed to cool gradually, stabilizing the
synthesized CQDs with distinctive optoelectronic properties.

Results:
Gelatin-based pure and doped Carbon Quantum Dots (CQDs) were synthesized using
the hydrothermal method, and their photoluminescent and up-conversion properties were
studied. Photoluminescence was observed at different excitation frequencies. At an excitation
wavelength of 314 nm, the emission wavelengths for P-CQD, C-CQD, and L-CQD were 395
nm, 402 nm, and 398 nm, respectively. For an excitation wavelength of 341 nm, the emissions
were 402 nm, 422 nm, and 417 nm. Photon up-conversion was examined using a 420 nm excitation,
showing emission frequency variations with doping.

Conclusion:
The synthesized gelatin-based pure and doped CQDs exhibited distinct photoluminescent
and up-conversion properties, with emission wavelengths varying according to the excitation
frequencies and types of doping. The observed shifts in emission wavelengths, especially
under up-conversion at 420 nm excitation, demonstrate that doping influences the optical behavior
of CQDs. This tunability of emission frequencies is particularly promising for frequency
conversion applications, such as enhancing the spectral absorbance range of solar cells, potentially
improving their efficiency by enabling better utilization of the solar spectrum.