Photon upconversion in systems incorporating inorganic quantum dots (QDs) is of great interest for applications in solar energy conversion, bioimaging, and photodynamic therapy. Achieving high up-conversion efficiency requires not only high-quality inorganic nanoparticles, but also precise control of their surface functional groups. Gas-phase surface functionalization provides a new pathway towards controlling the surface of small inorganic nanoparticles. In this contribution, we utilize a one-step low-temperature plasma technique for the synthesis and in-flight partial functionalization of silicon QDs with alkyl chains. The partially functionalized surface is then modified further with 9-vinylanthracene via thermal hydrosilylation resulting in the grafting of 9-ethylanthracene (9EA) groups. We have found that the minimum alkyl ligand density necessary for quantum dot solubility also gives the maximum upconversion quantum yield, reaching 17% for silicon QDs with Si-dodecyl chains and an average of 3 9EA molecules per particle.
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Low temperature radical initiated hydrosilylation of silicon quantum dots
The photophysics of silicon quantum dots (QDs) is not well understood despite their potential for many optoelectronic applications. One of the barriers to the study and widespread adoption of Si QDs is the difficulty in functionalizing their surface, to make, for example, a solution-processable electronically-active colloid. While thermal hydrosilylation of Si QDs is widely used, the high temperature typically needed may trigger undesirable side-effects, like uncontrolled polymerization of the terminal alkene. In this contribution, we show that this high-temperature method for installing aromatic and aliphatic ligands on non-thermal plasma-synthesized Si QDs can be replaced with a low-temperature, radical-initiated hydrosilylation method. Materials prepared via this low-temperature route perform similarly to those created via high-temperature thermal hydrosilylation when used in triplet fusion photon upconversion systems, suggesting the utility of low-temperature, radical-initiated methods for creating Si QDs with a range of functional behavior.
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- Award ID(s):
- 1610412
- NSF-PAR ID:
- 10214040
- Date Published:
- Journal Name:
- Faraday Discussions
- Volume:
- 222
- ISSN:
- 1359-6640
- Page Range / eLocation ID:
- 190 to 200
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Photon upconversion may have the highest impact in biological applications because incoming photons transparent to tissue can be combined to make visible light useful for photodynamic therapy and imaging. The ability to use semiconductor nanocrystals as light absorbers for photon upconversion is important because their strong absorption profiles are synthetically tunable. In particular, the use of earth‐abundant, environmentally benign silicon quantum dots (QDs) as light absorbers for photon upconversion is very attractive. In this work, the authors demonstrate a general strategy employing both physical and chemical barriers to achieve air‐stable fusion of triplet excitons photosensitized by silicon QDs, crucial to practical applications of photon upconversion. Gel permeation chromatography (GPC) and dynamic light scattering (DLS) show that thermal hydrosilylation critical for colloidal stability and efficient triplet energy transfer creates a polymeric barrier to oxygen. This kinetic barrier to oxygen arises from the presence of cross‐linked surfactants and is complemented by the sacrificial oxidation of silicon QDs itself. Photon upconversion lasted longer than 4 days with quantum yields (QYs) as high as 7.5% (out of a maximum of 50%) using Si QD light absorbers with diphenylanthracene in methyl oleate. Oil‐in‐water micelles are air‐stable for 2 days with absolute upconversion QYs of 5.5%.
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The development of high quality, non-toxic ( i.e. , heavy-metal-free), and functional quantum dots (QDs) via ‘green’ and scalable synthesis routes is critical for realizing truly sustainable QD-based solutions to diverse technological challenges. Herein, we demonstrate the low-temperature all-aqueous-phase synthesis of silver indium sulfide/zinc (AIS/Zn) QDs with a process initiated by the biomineralization of highly crystalline indium sulfide nanocrystals, and followed by the sequential staging of Ag + cation exchange and Zn 2+ addition directly within the biomineralization media without any intermediate product purification. Therein, we exploit solution phase cation concentration, the duration of incubation in the presence of In 2 S 3 precursor nanocrystals, and the subsequent addition of Zn 2+ as facile handles under biomineralization conditions for controlling QD composition, tuning optical properties, and improving the photoluminescence quantum yield of the AIS/Zn product. We demonstrate how engineering biomineralization for the synthesis of intrinsically hydrophilic and thus readily functionalizable AIS/Zn QDs with a quantum yield of 18% offers a ‘green’ and non-toxic materials platform for targeted bioimaging in sensitive cellular systems. Ultimately, the decoupling of synthetic steps helps unravel the complexities of ion exchange-based synthesis within the biomineralization platform, enabling its adaptation for the sustainable synthesis of ‘green’, compositionally diverse QDs.more » « less
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null (Ed.)The photoluminescence (PL) response of porous Si has potential applications in a number of sensor and bioimaging techniques. However, many questions still remain regarding how to stabilize and enhance the PL signal, as well as how PL responds to environmental factors. Regenerative electroless etching (ReEtching) was used to produce photoluminescent porous Si directly from Si powder. As etched, the material was H-terminated. The intensity and peak wavelength were greatly affected by the rinsing protocol employed. The highest intensity and bluest PL were obtained when dilute HCl(aq) rinsing was followed by pentane wetting and vacuum oven drying. Roughly half of the hydrogen coverage was replaced with –RCOOH groups by thermal hydrosilylation. Hydrosilylated porous Si exhibited greater stability in aqueous solutions than H-terminated porous Si. Pickling of hydrosilylated porous Si in phosphate buffer was used to increase the PL intensity without significantly shifting the PL wavelength. PL intensity, wavelength and peak shape responded linearly with temperature change in a manner that was specific to the surface termination, which could facilitate the use of these parameters in a differential sensor scheme that exploits the inherent inhomogeneities of porous Si PL response.more » « less
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ABSTRACT In this work, by means of quantum chemistry (Density Functional Theory (DFT), PW6B95/def2-TZVPP; DLPNO-CCSD(T)/CBS), HCN polymerization [(HCN)1 − 4] initiated and catalysed by a siloxyl radical (Si-O•) on a model silica surface is analysed. Linear HCN polymers (pHCN) are obtained by a radical initiated mechanism at a SiO• site and are characterized by a -(HC-N)- skeleton due to radical localization on the terminal N atom and radical attack on the C centre. NC heterocycles are formed by cyclization of the linear SiO-(HCN)3 − 4 and are always thermodynamically preferred over their linear counterparts, acting as thermodynamic sinks. Of particular interest to the astrochemistry community is the formation of the N-heterocycle 1,3,5-triazine that can be released into the gas phase at relatively low T (ΔG† = 23.3 kcal/mol). Full hydrogenation of SiO-(HCN•) follows two reaction channels with products: (a) SiO-CH3 + •NH2 or (b) amino-methanol + Si•, though characterized by slow kinetics. Nucleophilic addition of H2O to the electron-rich SiO-(HCN•) shows an unfavourable thermodynamics as well as a high-activation energy. The cleavage of the linear (HCN)1−4 from the SiO• site also shows a high thermodynamic energy penalty (ΔG≥82.0 kcal/mol). As a consequence, the silicate surface will be passivated by a chemically active ‘pHCN brush’ modifying the surface physico-chemical properties. The prospect of surface-catalysed HCN polymers exhibiting a high degree of chemical reactivity and proposed avenues for the formation of 1,3,5-triazine and amino-methanol opens exciting new chemical pathways to Complex Organic Matter formation in astrochemistry.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9fd00144a
