Chemical Reactions Involving the Surface of Metal Chalcogenide Quantum Dots

Abstract

This invited feature article focuses on the chemical reactions involving the surface ions of colloidal quantum dots (Qdots). Emphasis is placed on ion-exchange, redox, and complexation reactions. The pursuit of reactions involving primarily the cations on the surface results in changes in the optical properties of the Qdots and also may confer new properties owing to the newly formed surface species. For example, the cation-exchange reaction, leading to systematic removal of the cations present on the as-synthesized Qdots, enhances the photoluminescence quantum yield. On the other hand, redox reactions, involving the dopant cations in the Qdots, could not only modulate the photoluminescence quantum yield but also give rise to new emission not present in the as-synthesized Qdots. Importantly, the cations present on the surface could be made to react with external organic ligands to form inorganic complexes, thus providing a new species defined as the quantum dot complex (QDC). In the QDC, the properties of Qdots and the inorganic complex are not only present but also enhanced. Furthermore, by varying reaction conditions such as the concentrations of the species and using a mixture of ligands, the properties could be further tuned and multifunctionalization of the Qdot could be achieved. Thus, chemical, magnetic, and optical properties could be simultaneously conferred on the same Qdot. This has helped in externally controlled bioimaging, white light generation involving individual quantum dots, and highly sensitive molecular sensing. Understanding the species (i.e., the newly formed inorganic complex) on the surface of the Qdot and its chemical reactivity provide unique options for futuristic technological applications involving a combination of an inorganic complex and a Qdot.

Charge Transport Characteristics of Surface‐Complexed Quantum Dot in a Thin Film Transistor

“…This observation is also supported from the previously reported results that the emission lifetime of Mn dopant remains the same following attachment of thiol ligand to the surface of Mn‐doped CdS/ZnS nanocrystal . On the other hand, the shorter emission lifetime (λ em = 480 nm) of Zn(QS) 2 surface complex matches with our earlier reported value, which clearly indicated the formation of complex on the surface of Qdots . However, the emission of Qdot at 595 nm was quenched significantly after formation of Zn(QS) 2 complex with no considerable changes in the emission lifetime of Qdot at λ em = 595 nm.…”

“…On the other hand, the inter‐QDC spacing would depend on the geometry of the complex on the surface. Our earlier observations indicated the formation of an octahedral complex, which would make the inter particle separation shorter in comparison to other arrangements of ligand attachment on the surface. Hence, we anticipate that the reduction in hole mobility and relatively better electron transport in QDC‐TFT is because of attachment of HQS ligand to the surface of Qdot in a geometry similar to HQ, i.e., octahedral .…”

“…Surface complexation, i.e., the formation of luminescent inorganic complex(es) on the surface of a metal chalcogenide Qdot is a much simpler and cost‐effective surface modification approach. It has been used for phase transfer of hydrophobic Qdots, targeted cellular imaging, pH sensing, and fabrication of single component nontoxic white light emitting nanocomposite . Recently, white light emitting (WLE) Qdots have gained intense interest in the context of replacing the existing WLE solid state devices.…”

The quantum dot-FRET-based detection of vitamin B12 at a picomolar level

“…Environmentally sustainable metal-based QDs, especially Mn 2+ -doped ZnS QDs, have demonstrated their usefulness in bioimaging, white light generation, light emitting devices and optical sensing due to their long-wavelength atomic orange-red emission, photostable nature and lower toxicity. [17][18][19][20][21][22][23] For example, (i) white light generation could be possible from Mn 2+doped ZnS QDs followed by complexing their surface with two luminescent inorganic complexes, (ii) a reversible pH nanoprobe (in the physiological range 6.5-10.6) can be fabricated by complexing the surface of Mn 2+ -doped ZnS QDs with external chelating ligands, (iii) a white light emitting hydrogel, with the capability of enzyme packaging, could also be fabricated followed by engineering of the surface of Mn 2+ -doped ZnS QDs with ionic liquids. [19][20][21] Thus, the biocompatibility, ease of aqueous-based fabrication and lower toxicity of Mn 2+ -doped ZnS QDs make them a prime choice for the luminescence-based sensitive and selective detection of VB12.…”

Quantum Dots: A Review from Concept to Clinic

“…[30] This has been demonstrated that the in vivo toxicity of Si QDs is ten times safer than Cd-containing QDs. [31] Among various types of QDs, [32,33] zinc sulfide (ZnS) QDs are particularly suitable to apply as a green and efficient nanophotocatalyst for many kinds of pollutants removal, due to their long-term stability, large retention capability, and more efficacy for the pollutant remediation using only a small amount of QD nanophotocatalysts. [34] Also, ZnS has been the most popular choice in the production of QDs in photocatalytic applications because of its higher surface area to volume ratio compared to the bulk, which allows more enhanced photon absorption at the nano-interfaces.…”