Impact of D₂O/H₂O solvent exchange on the emission of HgTe and CdTe quantum dots: Polaron and energy transfer effects

We have studied light emission kinetics and analyzed carrier recombination channels in HgTe quantum dots that were initially grown in H2O. When the solvent is replaced by D2O, the nonradiative recombination rate changes highlight the role of the vibrational degrees of freedom in the medium surrounding the dots, including both solvent and ligands. The contributing energy loss mechanisms have been evaluated by developing quantitative models for the nonradiative recombination via (i) polaron states formed by strong coupling of ligand vibration modes to a surface trap state (nonresonant channel) and (ii) resonant energy transfer to vibration modes in the solvent. We conclude that channel (i) is more important than (ii) for HgTe dots in either solution. When some of these modes are removed from the relevant spectral range by the H2O to D2O replacement, the polaron effect becomes weaker and the nonradiative lifetime increases. Comparisons with CdTe quantum dots (QDs) served as a reference where the resonant energy loss (ii) a priori was not a factor, also confirmed by our experiments. The solvent exchange (H2O to D2O), however, is found to slightly increase the overall quantum yield of CdTe samples, probably by increasing the fraction of bright dots in the ensemble. The fundamental study reported here can serve as the foundation for the design and optimization principles of narrow bandgap quantum dots aimed at applications in long wavelength colloidal materials forinfrared light emitting diodes and photodetectors.We acknowledge financial support by the grant from the Research Grants Council of the Hong Kong S.A.R., China (project CityU 11302114). MIV acknowledges financial support from the FCT (Portugal)

Enhancing Light Absorption and Charge Transfer Efficiency in Carbon Dots Through Graphitization and Core Nitrogen Doping

Single-source precursor syntheses have been devised for the preparation of structurally similar graphitic carbon dots (CDs), with (g-N-CD) and without (g-CD) core nitrogen doping for artificial photosynthesis. An order of magnitude improvement has been realized in the rate of solar (AM1.5G) H$_{2}$ evolution using g-N-CD (7950 μmolH2 (gCD)$^{−1}$ h$^{−1}$ ) compared to undoped CDs. All graphitized CDs show significantly enhanced light absorption compared to amorphous CDs (a-CD) yet undoped g-CD display limited photosensitizer ability due to low extraction of photogenerated charges. Transient absorption spectroscopy showed that nitrogen doping in g-N-CD increases the efficiency of hole scavenging by the electron donor and thereby significantly extends the lifetime of the photogenerated electrons. Thus, nitrogen doping allows the high absorption coefficient of graphitic CDs to be translated into high charge extraction for efficient photocatalysis.Oppenheimer PhD scholarship, Poynton PhD scholarship, Marie Curie postdoctoral fellowship, FRQNT postdoctoral fellowship, ERC Intersolar project, Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development), OM

Aggregated Molecular Fluorophores in the Ammonothermal Synthesis of Carbon Dots

Recently, molecular fluorophores were shown to be formed in the bottom-up chemical synthesis, contributing to the emission of carbon dots (CDs), derived from a citric acid precursor. We applied an ammonothermal synthesis toward CDs, employing two reactants citric acid and supercritical ammonia functioning as both solvent and precursor. The resulting nanoparticles are identified as amorphous aggregates of molecular fluorophores based on citrazinic acid derivatives, which resemble many of the emission features typically reported to be characteristic for CDs. The aggregates absorb and emit at short and long wavelengths of the spectrum, a feature prior ascribed to intrinsic CD core and surface states, respectively. We identify three emission states: a high energy and a low energy aggregate state as well as an energy transfer state between both. Energy transfer is triggered only upon excitation within a narrow high energy spectral range, resulting in a characteristic blue-green double emission. The high energy aggregate state exhibits a trapping mechanism elongating emission lifetime. To further analyze aggregated molecular fluorophores, we studied aqueous solutions and films of citrazinic acid and polyvinylpyrrolidone and demonstrated their concentration dependent optical behavior. Since fluorophore aggregates reproduce the emissive features of CDs, the contribution of sp2/sp3 carbonized products and graphitic domains to the emission features of CDs must be carefully evaluated in future studies. 2017 American Chemical Society.The authors thank Dr. Klaus Speck (Ludwig-Maximilians-Universität, Munich) for valuable discussions regarding the chemistry of citric acid. This work was funded by NPRP grant no. 8-878-1-172 from the Qatar National Research Fund (a member of the Qatar Foundation).Scopu

Carbonization conditions influence the emission characteristics and the stability against photobleaching of nitrogen doped carbon dots

Carbon Dots (CDs), fabricated by hydrothermal bottom-up synthesis, are complex materials, whose optical properties are influenced by multiple factors. The presence of domains of conjugated sp2 carbon, which are formed upon carbonization of precursors at high temperature; nitrogen doping; and as recently shown, the presence of molecular fluorophores, are contributing to the emission of such CDs. We conducted a series of syntheses, each designed with specific precursors and reaction conditions that reveal the contribution of the above-mentioned factors. Specifically, we enforced carbonization by using graphene oxide as a precursor material; favored the formation of molecular fluorophores by conducting the synthesis at lower temperature and ambient pressure; and employed two different nitrogen precursors, namely ethylenediamine and triethanolamine. We compared and analyzed the distinct optical properties of the resulting products; furthermore, to address the relationship of CDs and molecular fluorophores, we examined photobleaching characteristics of these materials, under exposure to UV irradiation. From the analysis of the emission lifetimes, we revealed the quenching of the molecular fluorophores, whereas species with a higher degree of carbonization offer protection through incorporation into a carbon matrix. Based on a detailed comparison of different carbon dot species, our study provides novel physical insights into the origin of the luminescence properties of carbon dot derived nanomaterials. 1 The Royal Society of Chemistry.This work was supported by NPRP grant No. 8-878-1-172 from the Qatar National Research Fund (A Member of the Qatar Foundation).Scopu

Influence of Doping and Temperature on Solvatochromic Shifts in Optical Spectra of Carbon Dots

Solvatochromic shifts in nitrogen-doped and nitrogen–sulfur-co-doped carbon dots are studied by analyzing absorption, photoluminescence excitation and photoluminescence emission spectra, and their emission lifetimes in two different solvents, protic water (H₂O) and aprotic dimethyl sulfoxide (DMSO). We identify three emission bands belonging to the sp²-hybridized core, the edge, and the functional surface groups of carbon dots, as well as surface-attached fluorophores that emit within the edge band energy range. Edge and surface bands show opposite solvatochromic shifts solely depending on the doping heteroatoms. We are able to reproduce emission shifts observed in DMSO by heating CDs in H₂O from 7 to 87 °C, when the polarity and hydrogen-bonding strength of the solvent are reduced. Intrinsic edge band transitions are found to be strongly influenced by the solvent polarity, as charge transfer processes dominate. Surface band transitions are found to be influenced especially by hydrogen bonding between the carbon dots and the solvent. Together, these processes lead to characteristic, solvatochromic blue and red shifts of the emission bands. Furthermore, we observe strong emission quenching in the edge band but emission enhancement in the surface band of carbon dots in DMSO. This is attributed to quenched organic fluorophores that are formed during the carbon dot synthesis, leaving only intrinsic edge band emission while the radiative decay in the surface band is enhanced. As a result, the edge band of nitrogen–sulfur-co-doped carbon dots switches from an excitation-independent, fluorophore-like emission to an excitation-dependent emission associated with intrinsic edge states

Impact of D₂O/H₂O Solvent Exchange on the Emission of HgTe and CdTe Quantum Dots: Polaron and Energy Transfer Effects

We have studied light emission kinetics and analyzed carrier recombination channels in HgTe quantum dots that were initially grown in H₂O. When the solvent is replaced by D₂O, the nonradiative recombination rate changes highlight the role of the vibrational degrees of freedom in the medium surrounding the dots, including both solvent and ligands. The contributing energy loss mechanisms have been evaluated by developing quantitative models for the nonradiative recombination <i>via</i> (i) polaron states formed by strong coupling of ligand vibration modes to a surface trap state (nonresonant channel) and (ii) resonant energy transfer to vibration modes in the solvent. We conclude that channel (i) is more important than (ii) for HgTe dots in either solution. When some of these modes are removed from the relevant spectral range by the H₂O to D₂O replacement, the polaron effect becomes weaker and the nonradiative lifetime increases. Comparisons with CdTe quantum dots (QDs) served as a reference where the resonant energy loss (ii) a priori was not a factor, also confirmed by our experiments. The solvent exchange (H₂O to D₂O), however, is found to slightly increase the overall quantum yield of CdTe samples, probably by increasing the fraction of bright dots in the ensemble. The fundamental study reported here can serve as the foundation for the design and optimization principles of narrow bandgap quantum dots aimed at applications in long wavelength colloidal materials for infrared light emitting diodes and photodetectors