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- Analysis of Particle Size Distributions of Quantum Dots: From Theory to Application
- Microbial synthesis of chalcogenide semiconductor nanoparticles: a review
Biophys J — Anal Bioanal Chem — Hum Gene Ther — Langmuir — Cell Stem Cell — Histochem Cell Biol — J Nanobiotechnology Stem Cells — Dev Dyn — Sykova E, Jendelova P In vivo tracking of stem cells in brain and spinal cord injury [review]. Prog Brain Res — J Phys Chem B — Biophys J 97 2 — Exp Cell Res — J Biophotonics — Chem Rec — J Neurosci — Dahan M From analog to digital: exploring cell dynamics with single quantum dots.
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Curr Pharm Biotechnol — Guo P, Wei C Quantum dots for robust and simple assays using single particles in nanodevices [review]. Nanomedicine — Nucleic Acids Res Nanotechnology Neurosurgery — Angiogenesis — Texier I, Josser V In vivo imaging of quantum dots. Br J Cancer — Cai W, Chen X Preparation of peptide-conjugated quantum dots for tumor vasculature-targeted imaging.
Nat Protoc — Cancer Biomark — Adv Exp Med Biol — Chapter Unit Small — J Biomed Opt Lancet Oncol 7 8 — Nat Med — J Immunol Meth — Smith RA, Giorgio TD Quantitative measurement of multifunctional quantum dot binding to cellular targets using flow cytometry.
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Cytometry A — Abrams B, Dubrovsky T Quantum dots in flow cytometry. Nat Medicine — Lab Hematol — J Am Chem Soc — Ornberg RL, Harper TF, Liu H Western blot analysis with quantum dot fluorescence technology: a sensitive and quantitative method for multiplexed proteomics. Int J Mol Sci — Eur Biophys J — So, it is plausible to assume that different magic-sized clusters have local minima in chemical potentials. Previous investigations have noticed that magic-sized nanoclusters are frequently observed in the nucleation stage during the synthesis of elongated nanocrystals.
The formation of magic-sized nanoclusters originates from their local minimum chemical potential because of the closed-shell configurations. The formation of magic-sized nanoclusters happens under a relatively high chemical potential and they are only stable at relatively high monomer concentrations due to their extremely small sizes.
It is suggested that the magic-sized nanoclusters can undergo two pathways after the formation.
Analysis of Particle Size Distributions of Quantum Dots: From Theory to Application
This process is highly favored with a lower monomer concentration present in the solution. It has also been observed that magic-sized clusters are formed only at the high monomer chemical potentials needed to form the rod-shaped nanocrystals, thus their chemical potential corresponds to local minima in the progression from precursors to final nanorods.
Jiang and Kelly tried to propose a mechanism to explain the formation of magic-sized clusters MSCs and to fit the current available experimental data [ 20 ]. A number of species are discussed in the system including the nuclei, the magic-sized clusters, nanorods representing nanocrystals with highly anisotropic shapes , and nanospheres nanocrystals with low aspect ratio.
The central feature of the mechanism is the fast equilibrium between monomers and the magic-sized clusters. The monomer concentration has a saturation value of M 0. The magic-sized clusters can only form when the monomer concentration exceeds M 0 and will dissolve back when it is lower than the saturation value. The condensation and dissolution processes happen at a very fast rate so that magic-sized clusters serve as a reservoir for monomers.
In this proposed mechanism, the possibility of magic-sized clusters serving as intermediate between monomer and regular nanorods is excluded. Based on the chemical potential relation indicated above, several monomer concentration regimes can be identified. When the monomer concentration exceeds the saturation value M 0 , the additional monomer will form magic-sized clusters to keep monomer concentration within the saturation value M 0.
When the monomer concentration decreases below M 0 , no magic-sized clusters can be formed and the existing clusters will dissolve back to form monomers.
The biggest difference between the two proposed mechanisms lies in the possibilities of magic-sized nanoclusters directly towards regular nanocrystals with larger size. Further well-designed experiments are expected to help elucidating the puzzle. Theoretical calculations have been carried out to help understanding the MSQD forming mechanism.
First-principle calculations using ultrasoft pseudopotentials and the generalized gradient approximation show that CdSe n to be cage-like polyhedral [ 21 ]. The Cd and Se ions will connect alternatively to form zigzag networks composed of four- and six-membered rings.
Furthermore, the cage can be stabilized by filling with a core inside with connections to the cage. Thus, a selection of a highly symmetric cage with the right size of core will impose stringent restrictions for forming a stable nanostructure.
By calculation, this novel 3D core-cage structure favors to take specific atom numbers to maximize the binding energy. The theoretical prediction fits well with the experiment results including time-of-flight mass spectra, extended X-ray absorption fine structure EXAFS , analysis and AFM estimate. One interesting phenomenon they observed is also the appearance of regular QD by simply heating MSQD to a higher temperature, the absorption peak of MSQD remains without shift although the intensity decreases. The presence of an intermediate state is still unknown.
Solvothermal method in aqueous medium called hydrothermal has been extensively utilized for nanoparticle synthesis. However, the earlier efforts to synthesize semiconductor quantum dots are largely restricted by the inferior product properties. Also, large amount of works generated nanoparticles without quantum confinement, without tight control of particle size and morphology.
Recently, there are a number of efforts reporting the solvothermal synthesis of quantum dots in organic solvents. Solvothermal could provide evaluated temperature and pressure thus generates unique synthesis condition for nanocrystal growth. The solvothermal method employs similar synthesis method as the normal batch synthesis, usually starting from the mixing of two individual precursors at lower temperature, then increases to the desired growth temperature in a sealed autoclave for the nanocrystal production.
Different sizes of PbSe nanocrystals could be obtained by varying the initial Pb-to-Se ratio. Although the synthesized quantum dots show narrow size distribution and quantum confined absorption spectra, the reported photoluminescence efficiency is still unknown or lower than those reported from organometallic or its modified method.
This indicates a lower quality of quantum dots possibly originated from higher defects level. Quantum dots made from aqueous synthesis is especially attractive for biological application due to their compatibility with water.
Microbial synthesis of chalcogenide semiconductor nanoparticles: a review
Also compared to the organic-based synthesis, aqueous synthesis is cheaper, less toxic and more environmental friendly. Unfortunately, the quality of the as-prepared quantum dots has relatively low quantum yield and large size distribution compared to organic approach samples. The additional post-synthesis treatments such as size-selective precipitation, selective photochemical etching, and surface modifications could further improve the quantum dot quality.
However, those treatments add additional complexity and cost thus are not desirable for large-scale production. Thiols and thioalkyl acids including thioglycolic acid TGA and 3-mercaptopropionic acid MPA are popular stabilizers used for aqueous synthesized quantum dots. Most of the aqueous synthesis utilized the mixing of two precursors containing the respective anions and cations to react at high temperature.
The reaction mechanism follows the similar discussion as the organometallic approach featuring the nucleation and growth stages [ 37 ]. A different strategy developed by Li utilized a water—oil heterogeneous system to synthesize nanocrystal and quantum dots [ 38 ]. The reaction is based on a general phase transfer and separation mechanism [ 39 ]. Hydrothermal synthesis uses water instead of organic solvents as the reaction medium. Although numerous papers report the hydrothermal synthesis of nanoparticles, few of them are within the quantum dot category with desired properties needed for applications.
The hydrothermal synthesis also starts from two precursors containing the cation and anion species, with mixing and heating towards the growth temperature for quantum dot production. The pH value of Cd precursor needs to be adjusted to 9. Unlike the reaction in organic medium, pH value is essential for aqueous-based hydrothermal reaction and needs tight control to achieve desirable results.
Also, the reaction temperature and precursor ratios are considered important parameters to affect quantum dot formation as well. Compared to normal aqueous synthesis, hydrothermal method could provide a higher reaction temperature exceeding the boiling point of water and a higher pressure which could favor the formation of nanocrystal.
The quality of the synthesized quantum dots has improved significantly in recent years. Although the best value of the QD yield is comparable to the parameters via organic approach, the size distribution is still wider with further optimization needed. Hydrothermal and aqueous synthesis of nanocrystal quantum dots is relatively new compared to the well-developed organometallic synthesis and its derived method. Although attractive material properties such as narrow size distribution and high photoluminescence comparable to organic synthesized quantum dots have been reported, relatively rare device applications have been explored.
The real application in energy field requires a more complementary investigation with close interaction with device scientists needed. The formation of functional thin film and the corresponding device performance need further evaluations before concluding them as feasible approaches for energy applications. Due to the increased demand of quantum dots for various applications, scientists have begun to explore the large-scale synthesis of nanocrystal quantum dots by various means.
Although there are a number of companies capable of generating a large amount of quantum dots, they are not discussed here due to the limit of their experiment details. All the discussions below are based on published journal papers and patents available.
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Compared to general chemistry, chemical engineering has accumulated extensive theoretical and experiment experience in large-scale synthesis. It is wise to apply chemical engineering guideline to guide our behavior in large-scale synthesis. Most of the syntheses mentioned above are categorized as heterogeneous reactions. In these kinds of reactions, the temperature, pressure, and compositions are obviously important variables. Besides that, the momentum transport, energy heat transport, and mass transport are also key variables for large-scale industrial synthesis.