Aqueous Synthesis of Thiol-Capped CdTe Nanocrystals:  State-of-the-Art

Zheng Y., Gao S., Ying J. .

Wang, Zhang H., Zhang, Li, Sun, Yang B.

1-Thioglycerol-capped CdTe nanocrystals were used as core template to generate CdTe/CdS core−shell nanocrystals in aqueous solution by means of ultrasonic irradiation. The photoluminescence quantum...

Bertoni C., Gallardo D., Dunn S., Gaponik N., Eychmüller A.

Thiol-capped CdTe nanocrystals were used to fabricate light-emitting diodes, consisting of an emissive nanocrystal multilayer deposited layer by layer, sandwiched between indium tin oxide and aluminum electrodes. The emissive and electrical properties of devices with different numbers of nanocrystal layers were studied. The improved structural homogeneity of the nanocrystal multilayer allowed for stable and repeatable current- and electroluminescence-voltage characteristics. These indicate that both current and electroluminescence are electric-field dependent. Devices were operated under ambient conditions and a clear red light was detected. The best performing device shows a peak external efficiency of 0.51% and was measured at 0.35mA∕cm2 and 3.3V.

He Y., Sai L., Lu H., Hu M., Lai W., Fan Q., Wang L., Huang W.

Water-dispersed nanocrystals with high photoluminescence quantum yield (PLQY) and narrow size distribution are desirable for a variety of bioapplications. In this report, we present a facile method assisted by microwave irradiation for rapidly synthesizing high-quality CdTe nanocrystals in aqueous phase. The PLQY of CdTe nanocrystals prepared in very short time (15 min) reaches as high as 82%, and the fwhm value is merely 27 nm meanwhile. Moreover, the PLQY is increased to a remarkable 98% after further amelioration through the illumination method. The key synthesis parameters (molar ratio of reagents, pH values, reaction temperature, and time) are fully discussed, and the optimum conditions are illuminated in this paper. In addition, the mechanism about microwave irradiation improving the quality of nanocrystals is discussed as well.

Tekin E., Smith P. ., Hoeppener S., van den Berg A. ., Susha A. ., Rogach A. ., Feldmann J., Schubert U. .

Inkjet printing is used to produce well‐defined patterns of dots (with diameters of ca. 120 μm) that are composed of luminescent CdTe nanocrystals (NCs) embedded within a poly(vinylalcohol) (PVA) matrix. Addition of ethylene glycol (1–2 vol %) to the aqueous solution of CdTe NCs suppresses the well‐known ring‐formation effect in inkjet printing leading to exceptionally uniform dots. Atomic force microscopy characterization reveals that in the CdTe NC films the particle–particle interaction could be prevented using inert PVA as a matrix. Combinatorial libraries of CdTe NC–PVA composites with variable NC sizes and polymer/NC ratios are prepared using inkjet printing. These libraries are subsequently characterized using a UV/fluorescence plate reader to determine their luminescent properties. Energy transfer from green‐light‐emitting to red‐light‐emitting CdTe NCs in the composite containing green‐ (2.6 nm diameter) and red‐emitting (3.5 nm diameter) NCs are demonstrated.

Komarala V.K., Rakovich Y.P., Bradley A.L., Byrne S.J., Gun’ko Y.K., Gaponik N., Eychmüller A.

Surface plasmon (SP) enhanced photoluminescence (PL) from CdTe quantum dots (QDs) on monolayers of Au nanoparticles is investigated under both resonant and nonresonant conditions. Enhancement of the QD PL intensity is observed when the emission spectrum is redshifted with respect to the SP absorption resonance. Coupling to the SPs results in a redshift and broadening of the PL spectrum, and an increase in the PL decay rate. The largest coupling is observed for QD monolayers with peak emission at 667nm, producing a ten fold increase in PL intensity. No change in PL intensity and decay rate is observed at the SP resonance.

Tang Z., Zhang Z., Wang Y., Glotzer S.C., Kotov N.A.

In their physical dimensions, surface chemistry, and degree of anisotropic interactions in solution, CdTe nanoparticles are similar to proteins. We experimentally observed their spontaneous, template-free organization into free-floating particulate sheets, which resemble the assembly of surface layer (S-layer) proteins. Computer simulation and concurrent experiments demonstrated that the dipole moment, small positive charge, and directional hydrophobic attraction are the driving forces for the self-organization process. The data presented here highlight the analogy of the solution behavior of the two vastly different classes of chemical structures.

Niu H., Gao M.

Self-sacrifice: Poly(acrylic acid) was used to tune the diameter of 1D nanowires prepared from linear Cd2+/thioglycolic acid coordination polymer chains. The nanowires were then employed as sacrificial templates for preparing long CdTe nanotubes of different diameters (see the TEM images).

Shavel A., Gaponik N., Eychmüller A.

The aqueous synthesis of thiol-stabilized semiconductor CdTe colloidal nanocrystals has been revisited. We found optimal conditions for the synthesis of high-quality CdTe NCs through a study of the influence of the initial conditions (structure and concentration of Cd−thiol complexes) on the quality of the CdTe nanocrystals. A numerical calculation shows a clear correlation between the concentration of CdL (where L is (SCH2COO)2-) in the initial solution and the photoluminescence quantum efficiency of the CdTe nanocrystals.

Roither J., Pichler S., Kovalenko M.V., Heiss W., Feychuk P., Panchuk O., Allam J., Murdin B.N.

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230cm−1 from the amplified spontaneous emission. This high value makes the authors’ waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.

Liu Y., Chen W., Joly A.G., Wang Y., Pope C., Zhang Y., Bovin J., Sherwood P.

It is commonly believed that high-quality CdTe nanoparticles with strong luminescence can only be prepared under the protection of an inert gas such as nitrogen or argon. Here, we report the preparation of highly luminescent CdTe nanoparticles in air and compare their luminescence properties with CdTe nanoparticles made in nitrogen. We find that both water-soluble CdTe nanoparticles made in air and in nitrogen exhibit strong photoluminescence as well as upconversion luminescence at room temperature. However, differences do exist between the particles made in air and those made in nitrogen. In particular, the particles prepared in air display a faster growth rate, grow to larger sizes, and display stronger electron coupling relative to the particles prepared in nitrogen. X-ray photoelectron spectroscopy analysis indicates that the oxygen content in the nanoparticles synthesized in air is higher that that in particles synthesized in N(2), likely resulting in a higher availability of excess free cadmium. Cytotoxicity measurements reveal that the particles made in air appear slightly more toxic, possibly due to the excess of free cadmium.

Zhang H., Wang D., Yang B., Möhwald H.

The present article is devoted to systematically exploring the influence of various experimental variables, including the precursor concentration, the ligand nature, the counterion type, the Cd-to-Te molar ratio, pH, and temperature, on the aqueous growth of CdTe nanocrystals. The growth may be divided into two stages: the early fast growth stage and the later slow growth stage. The later stage is found to be dominated by Ostwald ripening (OR), being strongly dependent on all experimental conditions. In contrast, the early stage is dominated by adding monomers to nanocrystals, which may be dramatically accelerated by lowering precursor concentrations and using ligands with a molecular structure similar to that of thioglycolic acid (TGA). This fast growth stage is similar to that observed during organometallic growth of nanocrystals in hot organic media. On the basis of this finding, one-dimensional wurtzite CdTe nanostructures can be directly prepared in aqueous media by storing rather dilute precursor solution (2.4 mM with reference to ligand) in the presence of TGA at lower temperature (from room temperature to 80 degrees C). A low growth temperature is used to suppress OR during crystal growth. In addition, the simultaneous presence of both TGA-like ligand and 1-thioglycerol or 2-mercaptoethylamine leads to formation of colloidally stable 1D CdTe nanostructures with controlled aspect ratios.

Ray K., Badugu R., Lakowicz J.R.

We described, for the first time, the metal-enhanced fluorescence from the CdTe nanocrystals spin coated on silver island films (SIFs). CdTe nanocrystals show approximately 5-fold increase in fluorescence intensity, 3-fold decrease in lifetimes, and reduction in blinking on SIF surfaces that can be observed by ensemble and single-molecule fluorescence studies. The single-molecule study also provides further insight on the heterogeneity in the fluorescence enhancement and lifetimes of the CdTe nanocrystals on both glass and SIF surfaces, which is otherwise not possible to observe using ensemble measurements.

Byrne S.J., Corr S.A., Rakovich T.Y., Gun'ko Y.K., Rakovich Y.P., Donegan J.F., Mitchell S., Volkov Y.

We report the preparation and luminescence enhancement of thioglycolic acid (TGA) stabilised CdTe quantum dots (QDs) for use as live cell imaging tools in THP-1 macrophage cells. Short irradiating times utilising a high powered Hg lamp resulted in increases in luminescence efficiencies of up to ∼40% and permit significantly enhanced live imaging of the THP-1 cellular components. It was found that the TGA-stabilised QDs traverse the cell membrane, illuminating the cytoplasm and decorating the nuclear membrane. These studies highlight the potential use of photoetched CdTe QDs as probes for specific in vitro labelling.

Biebersdorf A., Dietmüller R., Susha A.S., Rogach A.L., Poznyak S.K., Talapin D.V., Weller H., Klar T.A., Feldmann J.

Semiconductor nanocrystals (SCNCs) made of CdSe, CdTe, and InP are used to photosensitize needlelike C(60) crystals. The photocurrent is increased by up to 3 orders of magnitude as compared with C(60) crystals without SCNCs. The photocurrent spectrum can be tuned precisely by the SCNC size and material, rendering the SCNC-functionalized C(60) crystals an excellent material for spectrally tuneable photodetectors. We explain the increased photocurrent as a result of photoexcited electrons transferring from the SCNCs to the C(60) crystals and causing photoconductivity, while the complementary holes remain trapped in the SCNCs.

Vecchio D.A., Hammig M.D., Kotov N.A.

Li N., Teng Z., Kim K., Qian Q., Zhang L., Yang Y., Leng M., Wang Y., Huang X.

Sevim Ünlütürk S., Taşcıoğlu D., Ozcelik S.

Abstract This review focuses on the recent progress of wet-chemistry-based synthesis methods for infrared (IR) colloidal quantum dots (CQD), semiconductor nanocrystals with a narrow energy bandgap that absorbs and/or emits infrared photos covering from 0.7 to 25 micrometers. The sections of the review are colloidal synthesis, precursor reactivity, cation exchange, doping and de-doping, surface passivation and ligand exchange, intraband transitions, quenching and purification, and future directions. The colloidal synthesis section is organized based on precursors employed: toxic substances such as mercury- and lead-based metals and non-toxic substances such as indium- and silver-based metal precursors. CQDs are prepared by wet-chemical methods that offer advantages such as precise spectral tunability by adjusting particle size or particle composition, easy fabrication and integration of solution-based CQDs (as inks) with complementary metal-oxide-semiconductors, reduced cost of material manufacturing, and good performances of IR CQD-made optoelectronic devices for non-military applications. These advantages may allow facile and materials’ cost-reduced device fabrications that make CQD-based infrared technologies accessible compared to optoelectronic devices utilizing epitaxially grown semiconductors. However, precursor libraries should be advanced to improve colloidal infrared quantum dot synthesis, enabling CQD-based IR technologies to be available to consumer electronics. As the attention of academia and industry to CQDs continues to proliferate, the progress of precursor chemistry for IR CQDs could be rapid.

Wegner K.D., Hildebrandt N.

Kang J.W., Jo I., Kim Y., Kim H., Kim K.

Nanostructures formed by self-assembly of semiconductor nanoparticles have unique properties that make them useful in various fields. In particular, cadmium telluride is being actively studied in the field of solar cells on the strengths of its long-term stability and low cost. In this study cadmium telluride nanoparticles were synthesized into nanowires in the dark and their evolutionary process was investigated. Thioglycolic acid was selected from among several thiol-based stabilizers. A ratio of 1.4:1 (thioglycolic acid to cadmium ion) was used rather than the traditional 2.4:1 ratio. In this study, nanowires were prepared by self-assembly of nanoparticles and the process of forming straight nanowires was observed. Synthesized nanowires were observed by using a scanning electron microscope and a transmission electron microscope, and the synthesized nanostructures were characterized. The wavelength of photoluminescence was converted from 549.9 nm to 553.3 nm as nanoparticles transformed into nanowires in an aqueous solution. The nanowire solution did not precipitate even after 60 days. The suggested synthetic procedure thus provides a viable pathway for the fabrication of nanomaterials.

Cuevas R.F., Prado S.J., Solano Reynoso V.C., Pradela Filho L.A., Menezes P.H., Balanta M.A.

Herein, CdTe/MnS core/shell nanoparticles dispersed in an aqueous solution have been synthesized. The formation of MnS semiconductor shell occurs by spontaneous self‐assembly. This process is activated by thermal hydrolysis that removes the excess of thiol and releases S2− ions. In this process, Mn2+ ions on the surface of the CdTe nanoparticles bind to S2− ions to produce a fine semiconducting layer of MnS. Measurements of Raman spectroscopy, optical absorption, and electrochemical measurements are performed. The Raman spectrum shows CdTe characteristic bands at 141 and 163 cm−1. Bands at 221 and 444 cm−1 are associated with the MnS structure. Cyclic voltammetry and differential pulse voltammetry are used to estimate the electrochemical gap at ≈2.47 eV. Absorption optical measurements show tree absorption bands. A broad band between 460 and 520 nm is associated with the first transition in CdTe nanoparticle. The absorption spectrum reveals an optical gap in the range of 2.41–2.33 eV for all the refluxed samples. These values are consistent with those obtained with the electrochemical measurements. The results evidence the formation of a core–shell semiconducting nanostructure made of CdTe nanoparticles coated with a spontaneously self‐assembled thin layer of MnS nanoparticles.

Rempel Andrey A., Ovchinnikov Oleg V., Weinstein Ilya A., Rempel Svetlana V., Kuznetsova Yulia V., Naumov Andrei V., Smirnov Mikhail S., Eremchev Ivan Yu., Vokhmintsev Alexander S., Savchenko Sergey S.

Quantum dots are the most exciting representatives of nanomaterials. They are synthesized using advanced methods of nanotechnology pertaining to both inorganic and organic chemistry. Quantum dots possess unique physical and chemical properties; therefore, they are used in very different fields of physics, chemistry, biology, engineering and medicine. It is not surprising that the Nobel Prize in chemistry in 2023 was given for discovery and synthesis of quantum dots. This review addresses modern methods for the synthesis of quantum dots and their optical properties and practical applications. In the beginning, a short insight into the history of quantum dots is given. Many gifted scientists, including chemists and physicists, were engaged in these studies. The synthesis of quantum dots in solid and liquid matrices is described in detail. Quantum dots are well-known owing to their unique optical properties; that is why the attention in the review is focused on the quantum-size effect. The causes for fascinating blinking of quantum dots and techniques for observation of a single quantum dot are considered. The last part of the review describes mportant applications of quantum dots in biology, medicine and quantum technologies.The bibliography includes 772 references.

Zhu Z., Sun S., Chai X., Gao J., Lu M., Wu Z., Gao Y., Feng T., Bai X., Zhang Y., Yan F., Yu W.W., Ke C.

AbstractGain medium‐doped active optical fibers for optical amplification and long‐range transmission have triggered a lot of researches. Traditional rare‐earth ion‐doped active fibers have non‐adjustable emissions, narrow band luminescence, limited gain enhancement, and other challenges. The unique optical properties of quantum dots (QDs) can further improve the doped optical fibers in the light amplification capacity, temperature sensitivity and to gain a broad‐spectrum coverage and lower laser threshold and other superiorities. However, there are few systematic discussions on the development of the technologies and applications of those QD‐doped optical fibers. In this review, the optical properties, temperature characteristics and optical amplification mechanisms of QD‐doped optical fibers are outlined. Then, the preparation technologies and the applications of QD‐doped optical fibers are highlighted. Additionally, the existing challenges and future development prospects of QD‐doped optical fibers are discussed.

Ruiz-Robles M.A., Solís-Pomar F.J., Travieso Aguilar G., Márquez Mijares M., Garrido Arteaga R., Martínez Armenteros O., Gutiérrez-Lazos C.D., Pérez-Tijerina E.G., Fundora Cruz A.

In this report, we present the results on the physicochemical characterization of cadmium telluride quantum dots (QDs) stabilized with glutathione and prepared by optimizing the synthesis conditions. An excellent control of emissions and the composition of the nanocrystal surface for its potential application in monoclonal antibody and biomarker testing was achieved. Two samples (QDYellow, QDOrange, corresponding to their emission colors) were analyzed by dynamic light scattering (DLS), and their hydrodynamic sizes were 6.7 nm and 19.4 nm, respectively. Optical characterization by UV-vis absorbance spectroscopy showed excitonic peaks at 517 nm and 554 nm. Photoluminescence spectroscopy indicated that the samples have a maximum intensity emission at 570 and 606 nm, respectively, within the visible range from yellow to orange. Infrared spectroscopy showed vibrational modes corresponding to the functional groups OH-C-H, C-N, C=C, C-O, C-OH, and COOH, which allows for the formation of functionalized QDs for the manufacture of biomarkers. In addition, the hydrodynamic radius, zeta potential, and approximate molecular weight were determined by dynamic light scattering (DLS), electrophoretic light scattering (ELS), and static light scattering (SLS) techniques. Size dispersion and the structure of nanoparticles was obtained by Transmission Electron Microscopy (TEM) and by X-ray diffraction. In the same way, we calculated the concentration of Cd2+ ions expressed in mg/L by using the Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-OES). In addition to the characterization of the nanoparticles, the labeling of murine myeloid cells was carried out with both samples of quantum dots, where it was demonstrated that quantum dots can diffuse into these cells and connect mostly with the cell nucleus.

Palomino R.A., Zon M.A., Fernández H., Porcal G.V., Arévalo F.J.

In order to obtain an emitter nanoprobe for applications in aqueous solutions, we synthesised quantum dots of CdSe (CdSe QDs) in aqueous solutions, which were stabilised with 3-morpholinoethanesulfonic (MES) and 3-mercaptopropionic (MPA) acids. Comprehensive photophysical and electrochemical studies were carried out in order to determine the optimum MPA:MES ratio that produces the highest electrogenerated chemiluminescence (ECL) intensity. We found that the variation between the relationship of MPA and MES directly affected the surface, the size and properties of the CdSe QDs. Characteristic parameters of CdSe QDs were determined by conventional photophysical and electrochemical techniques, where an excellent correlation between the two methods was found. In addition, diameter, quantum yield (ϕNC), bandgap (Eg) and lifetime of CdSe QDs were determined. It was also observed that as the proportion of MPA in the capping increased, the ϕNC increased, and that an increase in the proportion of MES in the capping increased the intensity of ECL. The surface state of quantum dots is responsible for the difference in the behaviour between fluorescence and electrochemiluminescence properties. These results allowed us to infer that the choice of dual-stabiliser will depend on the applications of the resulting quantum dots as nanoprobe emitters.

Zhang Y., Oberg C.P., Hu Y., Xu H., Yan M., Scholes G.D., Wang M.

Cai X., Wang B., Nian L., Zhao S., Xiao J.

We have developed a robust and versatile host–guest peptide toolbox to construct highly stable and specific HGP-QDs probes for imaging extracellular matrices and cells, providing an efficient tool for multiplex imaging and precision medicine.

Fonseca A.F., Giarola C.E., Carvalho T.A., Hojo de Souza F.S., Schiavon M.A.

Quantum dots (QDs) have attracted much attention and exhibit many attractive properties, including high absorption coefficient, adjustable bandgap, high brightness, long-term stability, and size-dependent emission. It is known that to obtain high-quality luminescent properties (i.e. emission color, color purity, quantum yield, and stability), the synthesis parameters must be precisely controlled. In this work, we have constructed a database with CdTe aqueous synthesis parameters and spectroscopic results and applied machine learning algorithms to better understand the influence of the main synthesis parameters of CdTe QDs on their final emission properties. A strong dependence of the final emission wavelength with the reaction time and surface ligands and precursors concentrations was demonstrated. These parameters adjusted synchronously were shown to be very useful for provide ideal synthesis conditions for the preparation of CdTe QDs with desirable emission wavelengths. Moreover, applying the algorithms correctly allows for obtaining information and insights into the growth kinetics of QDs under different synthetic conditions.

Rosebrock M., Graf R.T., Kranz D., Christmann H., Bronner H., Hannebauer A., Zámbó D., Dorfs D., Bigall N.C.

For a long time, researchers in nanochemistry have been exploring ways to create 3D structures using cross‐linked nanoparticles, such as lyogels and aerogels. In the present work, how simple modifications to the nanoparticle surface can be used to influence the resulting structure in a targeted manner is demonstrated. Specifically, positively charged surface ligands containing amine groups are compared to negatively charged ligands typically used, containing carboxylic acid groups, to generate network structures using different gelation agents. By utilizing bridging through S2− ions, a network structure of anisotropic CdSe/CdS nanorods is generated, packing them side by side at the nanoscopic level. The resulting structures exhibit improved fluorescence properties comparable to those of tip‐to‐tip connected networks but without harsh conditions for the nanoparticle surfaces. This innovative new method of gelation using S2− ions can achieve adequate photoluminescence quantum yields as well as prolonged fluorescence lifetimes compared to other network structures.

Soosaimanickam A., Manidurai P., Krishna Sundaram S., Sridharan M.B.

Synthesis of semiconductor nanocrystals through solution-based approaches has turned out huge interest among researchers. Out of other methods, preparation of colloidal semiconductor nanocrystals through hot-injection method is intensively investigated owing to its versatility. Importantly, the structure, shape and size of the nanocrystals can be precisely tuned with respect to the precursors and temperature. This influence a lot on the composition, physical, and chemical properties of the synthesized nanocrystals. Using hot-injection synthesis approach, the nanocrystals are prepared either in aqueous medium or in organic solvents. The acidic or basic nature of surface metal atoms integrate the ligand interaction and hence stability of the nanocrystals in solution is governed. Furthermore, with respect to the reaction time intervals and type of precursors, the absorption and emission spectra of the nanocrystals can be tuned. Diverse range of inorganic nanocrystals such as metallic nanostructures (ex: Ag, Au, Pt, Pd) metal-chalcogenide semiconductor nanocrystals (ex: CdS, ZnS, CdTe, CdSe), metal-oxide nanocrystals (ex: TiO2, CeO2), metal phosphides, metal nitrides and metal halide perovskite nanocrystals (RNH3PbX3 and CsPbX3, where R = alkyl group and X = Cl, Br and I) are prepared using this method and their structural, optical and morphological properties are evaluated. Interesting features such as polytyphism, different phase formation, surface charge and ligand metal interaction are dealt with respect to the reaction conditions. In this view, this chapter discuss about the advancements of hot-injection approach in synthesizing different group semiconductor nanocrystals, challenges and it’s future perspectives.