Up-conversion photoluminescence (UCPL) is an optical process in which the electrons from ground state absorb lower energy photon (NIR and/or IR) to excite to the excited state and the electrons will return to the ground state while emitting higher energy photon (UV and/or Visible light). However, the occurrence of UCPL in carbon-based nanomaterials was rare to be found. Herein, we provide better understanding and confirmation to the phenomenon of UCPL in graphene quantum dots (GQDs). From excitation-dependent PL measurement, it shows that GQDs have excitation-independent PL emission. Furthermore, the PL emission can still be observed even when it was excited using low energy photons, confirming the phenomenon of UCPL. We performed temperature-dependent PL measurement to further confirm the credibility of UCPL phenomenon. It showed that as increasing temperature, the UCPL intensity grows higher, showing the contribution of phonon in UCPL process. Therefore, we confirm the UCPL phenomenon and due the phonon contribution in the process, we conclude that anti-Stokes photoluminescence (ASPL) is the UCPL process in our materials.
Luminescent solar concentrator (LSC) is an optoelectronic device which converts direct and diffused sunlight into electricity when coupled with photovoltaic (PV) cells around its edge. Large Stokes shift is beneficial to minimize the reabsorption loss and improve the overall efficiency of LSCs. In this study, the fabricated Gold-doped silver nanoclusters embedded in Polyvinylpyrrolidone (Au-AgNCs@PVP) matrix exhibited significantly large Stokes shift of about ~200nm with spectral overlap integral of 0.03 and Urbach tail observed on the absorption spectra along ~480 to 600nm. This is a typical behavior of emissions from self-trapped excitons (STEs) due to the lattice distortion caused by a strong exciton-phonon coupling. The strong coupling is evident by having small steepness constant around ~0.32 extracted from the temperature dependent Urbach energy, thus, proving the photoluminescence originating from STEs.
Graphene quantum dots (GQDs) are one kind of carbon-based nanomaterials which can be used for numerous applications, such as energy conservation, luminescent solar concentrators, bioimaging, and biosensing. It has low toxicity, high conductivity and it shows exceptional optical properties, including photoluminescence (PL) emission which could be adjusted from blue to red emission depending on the solvent. Another interesting properties found in GQDs is anti-Stokes photoluminescence (ASPL). However, the mechanism of ASPL in GQDs was still unclear. In this study, GQDs were prepared with 1,3,6-trinitropyrene as the precursor, then dissolved in toluene (GQDs@TL). The results show that GQDs@TL has PL emission peak at ~595 nm when excited at ~530 nm and ~700 nm. It showed that GQDs@TL has large energy gain (~310 meV). To further understand the mechanism of ASPL, additional temperature-dependent measurements were done. We found that the large energy gain could be gained owing to the contribution of phonon energy and hot-band absorption energy (EHBA) coming from molecular and lattice vibration. Therefore, this study will conclude the mechanism of ASPL.
KEYWORDS: Fluorescence, Silver, Matrices, Time correlated single photon counting, Temperature metrology, Solids, Metals, Solid state physics, Photoluminescence, Thin films
Ligand-protected metal nanoclusters (NCs) have emerged as prospective less-toxic luminescent materials which can be beneficial for numerous applications, such as bioimaging, biosensing, photo-induced catalytic process, and optoelectronics. The most intriguing properties of NCs are long photoluminescence (PL) lifetime with broadband PL emission, making them more attractive to be developed. However, low PL Quantum Yield (PLQY) and unclear excitonic dynamics mechanism strongly hinder their practical utilization in daily basis. Therefore, to address these issues, gold-doped silver nanoclusters were synthesized using one-pot synthesis method, and were covalently bonded in a solid matrix, polyvinylpyrrolidone (PVP). The fabricated gold-doped silver nanoclusters embedded in PVP matrix (AuAgNCs@PVP) have bright red emission centered at approximately 650 nm with microseconds PL lifetime. By probing the nanosecond time resolution, it was revealed that two distinct decay profile (nano- and micro-seconds) exist in AuAgNCs@PVP, showing the occurrence of Thermally Activated Delayed Fluorescence (TADF). We also found that after the absorption of photons, thermal equilibrium between singlet and triplet states were quickly reached due to fast intersystem crossing and reverse intersystem crossing (ISC/RISC) process owing to the small singlet triplet energy splitting. Hence, we proposed the mechanism behind broadband PL emission with long PL lifetime was due to Thermally Equilibrated Delayed Fluorescence (TEDF).
Heavy-metal-containing quantum dots (QDs), such as CdSe-based quantum dots (QDs) have been applied to lightconversion nano-phosphors due to tunable emission and pure colors. Unfortunately, those QDs involve toxic elements and synthesize in a hazardous halogenated solvent. Therefore, Eco-friendly gold nano-clusters (AuNCs@GSH) in solution phase have gained much attention for promising applications in biophotonics. For the first time, we explore the feasibility of aqueous-solution-processed AuNCs@GSH as luminescent species for promising applications in "green" luminescent solar concentrators (LSCs) by investigating their photophysical properties. Due to ligand-to-metal chargetransfer (LMCT) state, we found that such "green" LSCs formed by Zn-AuNCs@GSH dispersed in a polymer matrix exhibit large Stokes shift and small scattering losses. Compared to AuNCs@GSH, the Zn-AuNCs@GSH dispersed in a polymer matrix could suppress non-radiative recombination rates, inducing the enhancement of luminescence and the increase of PL-QY from 2% to 40%.
The CdSe-based quantum dots (QDs) have been applied to light-conversion nano-phosphors due to tunable emission and pure colors. However, these cadmium-containing QDs was strongly toxic and synthesized in a hazardous solvent. In addition, conventional QD nano-phosphors with a small Stokes shift suffered from reabsorption losses and aggregation-induced photoluminescence (PL) quenching in the solid state. Therefore, there is a need to develop nanophosphors with a large Stokes shift. Here, we demonstrate one-pot synthesis of gold nanoclusters (AuNCs) using 3- aminopropyltrimethoxysilane (APS) and glutathione as protection ligand with a large Stokes shift. The gold nanoclusters with a large Stokes shift can mitigate the aggregation-induced PL quenching and reabsorption losses, which would be potential candidates for "green" nano-phosphors.
We describe the use of UV light under different radiation time induces a variety of fluorescence wavelength of gold
quantum clusters. First, we synthesize blue-emitted gold quantum clusters by dissolving the gold trichloride in pure
toluene. To simplify the expression, we assume that the several featured PL peak (425, 450, 470 nm) is the signal for
blue-emitted gold quantum clusters. Undergo UV irradiation can brighten and broaden the PL spectra of gold quantum
clusters, which are observed by the evolutional spectra versus exposure time. After UV light exposure, the major
population of gold quantum clusters @425nm decreased and turned to gold quantum clusters@450nm, followed by the
growing population of gold quantum clusters@470nm clusters. Until 2 hour exposure, the spectra become broad with
major peak shifted to 525 nm. The tunable spectra from blue to green attributes to the induced growth of gold quantum
clusters by UV irradiation. The UV energy indeed tunes and broadens the emission covering the whole visible-spectra
range. Finally, we also utilize via proper selection of organic surfactant (such as: trioctyl phosphine, TOP) can coordinate
the quantum yield enhancement of blue-emitted gold quantum clusters under UV irradiation. The experiment method is
easily for gold quantum clusters synthesis. Thus we expect this materials can be developed for fluorescence labeling
application in the future.
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