Chalcogenide Nanocrystals Research Articles

Synthesis of Wurtzite Cu2ZnSnS4 Nanosheets with Exposed High-Energy (002) Facets for Fabrication of Efficient Pt-Free Solar Cell Counter Electrodes

Two-dimensional (2D) semiconducting nanomaterials have generated much interest both because of fundamental scientific interest and technological applications arising from the unique properties in two dimensions. However, the colloidal synthesis of 2D quaternary chalcogenide nanomaterials remains a great challenge owing to the lack of intrinsic driving force for its anisotropic growth. 2D wurtzite Cu2ZnSnS4 nanosheets (CZTS-NS) with high-energy (002) facets have been obtained for the first time via a simple one-pot thermal decomposition method. The CZTS-NS exhibits superior photoelectrochemical activity as compared to zero-dimensional CZTS nanospheres and comparable performance to Pt counter electrode for dye sensitized solar cells. The improved catalytic activity can be attributed to additional reactive catalytic sites and higher catalytic reactivity in high-energy (002) facets of 2D CZTS-NS. This is in accordance with the density functional theory (DFT) calculations, which indicates that the (002) facets of wurtzite CZTS-NS possess higher surface energy and exhibits remarkable reducibility for I3− ions. The developed synthetic method and findings will be helpful for the design and synthesis of 2D semiconducting nanomaterials, especially eco-friendly copper chalcogenide nanocrystals for energy harvesting and photoelectric applications.

Using lead chalcogenide nanocrystals as spin mixers: a perspective on near-infrared-to-visible upconversion.

The process of upconversion leads to emission of photons higher in energy than the incident photons. Near-infrared-to-visible upconversion, in particular, shows promise in sub-bandgap sensitization of silicon and other optoelectronic materials, resulting in potential applications ranging from photovoltaics that exceed the Shockley-Queisser limit to infrared imaging. A feasible mechanism for near-infrared-to-visible upconversion is triplet-triplet annihilation (TTA) sensitized by colloidal nanocrystals (NCs). Here, the long lifetime of spin-triplet excitons in the organic materials that undergo TTA makes upconversion possible under incoherent excitation at relatively low photon fluxes. Since this process relies on optically inactive triplet states, semiconductor NCs are utilized as efficient spin mixers, absorbing the incident light and sensitizing the triplet states of the TTA material. The state-of-the-art system uses rubrene with a triplet energy of 1.14 eV as the TTA medium, and thus allows upconversion of light with photon energies above ∼1.1 eV. In this perspective, we review the field of lead sulfide (PbS) NC-sensitized near-infrared-to-visible upconversion, discuss solution-based upconversion, and highlight progress made on solid-state upconversion devices.

Environmentally friendly and earth-abundant colloidal chalcogenide nanocrystals for photovoltaic applications

A review of recent advances in the synthesis of environmentally friendly and earth-abundant colloidal chalcogenide nanocrystals for solar cells.

Solar light harvesting with multinary metal chalcogenide nanocrystals.

The paper reviews the state of the art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into the electric current or the accumulation of solar energy in the form of products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to produce the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

Colloidal Synthesis and Thermoelectric Properties of CuFeSe₂ Nanocrystals.

Copper-based chalcogenides that contain abundant, low-cost and environmentally-friendly elements, are excellent materials for numerous energy conversion applications, such as photocatalysis, photovoltaics, photoelectricity and thermoelectrics (TE). Here, we present a high-yield and upscalable colloidal synthesis route for the production of monodisperse ternary I-III-VI2 chalcogenides nanocrystals (NCs), particularly stannite CuFeSe2, with uniform shape and narrow size distributions by using selenium powder as the anion precursor and CuCl2·2H2O and FeCl3 as the cationic precursors. The composition, the state of valence, size and morphology of the CuFeSe2 materials were examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM), respectively. Furthermore, the TE properties characterization of these dense nanomaterials compacted from monodisperse CuFeSe2 NCs by hot press at 623 K were preliminarily studied after ligand removal by means of hydrazine and hexane solution. The TE performances of the sintered CuFeSe2 pellets were characterized in the temperature range from room temperature to 653 K. Finally, the dimensionless TE figure of merit (ZT) of this Earth-abundant and intrinsic p-type CuFeSe2 NCs is significantly increased to 0.22 at 653 K in this work, which is demonstrated to show a promising TE materialand makes it a possible p-type candidate for medium-temperature TE applications.

Growth Mechanism and Surface State of CuInS2 Nanocrystals Synthesized with Dodecanethiol.

Ternary metal chalcogenide nanocrystals (NCs) offer exciting opportunities as novel materials to be explored on the nanoscale showing optoelectronic properties tunable with size and composition. CuInS2 (CIS) NCs are the most widely studied representatives of this family as they can be easily prepared with good size control and in high yield by reacting the metal precursors (copper iodide and indium acetate) in dodecanethiol (DDT). Despite the widespread use of this synthesis method, both the reaction mechanism and the surface state of the obtained NCs remain elusive. Here, we perform in situ X-ray diffraction using synchrotron radiation to monitor the pre- and postnucleation stages of the formation of CIS NCs. SAXS measurements show that the reaction intermediate formed at 100 °C presents a periodic lamellar structure with a characteristic spacing of 34.9 Å. WAXS measurements performed after nucleation of the CIS NCs at 230 °C demonstrate that their growth kinetics depend on the degree of precursor conversion achieved in the initial stage at 100 °C. NC formation requires the cleavage of S-C bonds. We reveal by means of combined 1D and 2D proton and carbon NMR analyses that the generated dodecyl radicals lead to the formation of a new thioether species R-S-R. The latter is part of a ligand double layer, which consists of dynamically bound dodecanethiolate ligands as well as of head-to-tail bound R-S-R molecules. This ligand double layer and a high ligand density (3.6 DDT molecules per nm2) are at the origin of the apparent difficulty to functionalize the surface of CIS NCs obtained with the DDT method.

Chemical insights into the formation of Cu2ZnSnS4 films from all-aqueous dispersions for low-cost solar cells

Cu2ZnSnS4 (CZTS) shows great potential for photovoltaic application because of its non-toxic earth-abundant components and good optoelectronic properties. Combining low-cost and environmentally friendly routes would be the most favorable approach for the development of CZTS solar cells. In this context, development of Cu2ZnSnS4 (CZTS) films from all-aqueous CZTS nanocrystals inks represents an interesting challenge. Here, we have highlighted a condensation regulation by the alkali ion size observed in the alkali series Li+ < Na+ < K+ < Rb+ < Cs+, and demonstrated the chemical stability of Cu2ZnSnS4 surfaces in basic aqueous dispersions. Data such as optimal nanocrystal size, critical cracking thickness and average thickness to fabricate micron crack-free films from all-aqueous chalcogenide nanocrystals dispersions were determined. From these results, a proof of concept for the formation of a crack-free film of 2.2 μm formed from an all-aqueous CZTS nanocrystals ink is given. When employing low-cost materials, removal of carbon impurities represents another important challenge. With the objective to fabricate residue-free films, a specific annealing strategy is proposed involving a high temperature purification step under Se partial pressure. Carbon removal is thus achieved via the CSe2 gas formation, simultaneously to the amorphous domains crystallization as demonstrated by Raman spectroscopy. These source data favoring the formation of residue-free, crack-free, annealed films should assist the large scale development of CZTS solar cells from low-cost and environmentally friendly, all –aqueous inks.

Chalcogenide glasses with embedded ZnS nanocrystals: Potential mid‐infrared laser host for divalent transition metal ions

AbstractChalcogenide glasses (ChGs) containing II‐VI chalcogenide (ChG) nanocrystals such as ZnS/Se have recently been intensively studied as promising mid‐infrared nonlinear optics and laser materials, yet preparation of pure‐phase II‐VI nanocrystals embedded in ChGs via controlled crystallization is still very challenging. In this study, a new system of ChGs and glass ceramics (GCs), viz., (100−x)As2S3–xZnSe (x = 0 ~ 30 mol%), is synthesized, and its physical and optical properties including density, molar volume, microhardness, glass transition temperature, glass network structure, transmission, and refractive index are comprehensively characterized. Significantly, it is initially demonstrated that pure ZnS nanocrystals can be precipitated in GCs simply by a thermal treatment process. The composition and thermal treatment temperature dependencies of crystallization are studied using X‐ray diffraction spectroscopy, and the morphology of the nanocrystals by high‐resolution transmission electron microscope. The ChG GCs with embedded ZnS nanocrystals retaining good transparency can be a potential host laser material for divalent transition metals (e.g., Cr2+/Fe2+, etc.), and thus used for ultrabroadband tunable continuous or ultra‐short‐pulsed mid‐infrared fiber lasers.

A Selective Cation Exchange Strategy for the Synthesis of Colloidal Yb3+-Doped Chalcogenide Nanocrystals with Strong Broadband Visible Absorption and Long-Lived Near-Infrared Emission.

Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4f-4f emission with the strong broadband absorption and low-phonon-energy crystalline environment of semiconductors to make new solution-processable spectral-conversion nanophosphors, but synthesis of this class of materials has proven extraordinarily challenging because of fundamental chemical incompatibilities between lanthanides and most intermediate-gap semiconductors. Here, we present a new strategy for accessing lanthanide-doped visible-light-absorbing semiconductor nanocrystals by demonstrating selective cation exchange to convert precursor Yb3+-doped NaInS2 nanocrystals into Yb3+-doped PbIn2S4 nanocrystals. Excitation spectra and time-resolved photoluminescence measurements confirm that Yb3+ is both incorporated within the PbIn2S4 nanocrystals and sensitized by visible-light photoexcitation of these nanocrystals. This combination of strong broadband visible absorption, sharp near-infrared emission, and long (>400 μs) emission lifetimes in a colloidal nanocrystal system opens promising new opportunities for both fundamental-science and next-generation spectral-conversion applications such as luminescent solar concentrators.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

High photosensitivity and wide operation voltage in two-dimensional CdS nano-crystal layer embedded a-InGaZnO hybrid phototransistors

We suggest a design for the channel stacking sequence in hybrid phototransistors consisting of an oxide layers and two-dimensional (2D) chalcogenide nano-crystals to develop good electrical performance and a good photoresponse. The hybrid channels of the phototransistors were composed of a photo-insensitive oxide [amorphous InGaZnO (a-InGaZnO)] for effective charge transport and a visible photosensitive chalcogenide (CdS) nano-crystal. Interestingly, the electrical performance, photosensitivity, and recovery properties of the hybrid phototransistor depended strongly on the order of the channel stacking sequence. When the CdS nano-crystal layers were located close to the gate interface in the hybrid channel, significant degradation of the field-effect mobility and recovery behavior was observed, which was due to the increase in gate-interface trap density caused by many surfaces of nano-crystals. In contrast to the negative Vth shift for the a-InGaZnO/2D CdS nano-crystal stacking sequence, the phototransistor using an embedded-absorber channel structure (a-InGaZnO/2D CdS nano-crystals/a-InGaZnO) exhibited a considerably higher off-current under green irradiation, which is attributed to simultaneous realization of effective generation of photocarriers from the CdS nano-crystals and charge transport via the high-mobility upper a-InGaZnO layer. In particular, this hybrid phototransistor exhibited a wide operation voltage window as well as high photosensitivity and photoresponsivity, without any degradation of the oxide thin film transistor's performance.

Multifunctional Cu1.94S-Bi2S3@polymer nanocomposites for computed tomography imaging guided photothermal ablation

The doping of radiocontrast agent such as bismuth (Bi) in copper chalcogenide nanocrystals for computed tomography (CT) imaging guided photothermal therapy (PTT) has drawn increasing attention. However, the doping of Bi often suffers from the weak CT signal due to the low Bi doping concentration and deteriorates the PTT efficacy of copper chalcogenides. Here we report a multifunctional nanoprobe by encapsulating both Cu1.94S and Bi2S3 nanocrystals into a biocompatible poly(amino acid) matrix with size of ~85 nm for CT imaging guided PTT. The amount of nanocrystals and the ratio of Cu1.94S-to-Bi2S3 in the multifunctional nanocomposites (NCs) are tunable toward both high photothermal conversion efficiency (~31%) and excellent CT imaging capability (27.8 HU g L−1). These NCs demonstrate excellent effects for photothermal ablation of tumors after intratumoral injection on 4T1 tumor-bearing mice. Our study may provide a facile strategy for the fabrication of multifunctional theranostics towards simultaneous strong CT signal and excellent PTT.

Hydrophilic, Hole-Delocalizing Ligand Shell to Promote Charge Transfer from Colloidal CdSe Quantum Dots in Water

Colloidal cadmium chalcogenide nanocrystals are usually stabilized in polar solvents by functionalizing the surface with a layer of hydrophilic ligands. While these ligands protect against aggregation, they also present a steric barrier that hinders surface access. In applications that require charge transfer to and from nanocrystals, colloidal stability and surface access for redox species are therefore difficult to reconcile. This work assesses the possibility of a more dynamic ligand shell that not only provides stability to nanocrystals but also promotes charge transfer without the need for ligand removal. We use transient absorption spectroscopy to study CdSe quantum dots functionalized with hydrophilic, hole-delocalizing dithiocarbamate ligands in water for the first time, and find that a conjugated ligand facilitates charge transfer to redox species in solution.

Role of Crystal Structure and Chalcogenide Redox Properties on the Oxidative Assembly of Cadmium Chalcogenide Nanocrystals.

Oxidative assembly of metal chalcogenide nanocrystals (NCs) enables the formation of 2-D (dense) and 3-D porous structures without the presence of intervening ligands between particles that can moderate transport properties. This route has been demonstrated to be successful for a range of single-component structures including CdQ, PbQ, and ZnQ (Q = S, Se, Te). En route to the controllable assembly of multicomponent nanostructures, the roles of Q redox properties (2Q2- → Q22- + 2e) responsible for particle cross-linking and the native structure (cubic zinc blende vs hexagonal wurtzite) in the kinetics of assembly in single-component CdQ NCs are evaluated using time-resolved dynamic light scattering (TR-DLS). For wurtzite CdQ, the rates follow the ease of oxidation, with telluride as the fastest, followed by selenide and sulfide. However, when comparing CdS wurtzite (w) and zinc blende (zb), the cubic NCs exhibit surprisingly slow kinetics. NMR studies reveal the zb structure to have lower ligand coverage (by a factor of 4) relative to that of w, and the formation of free disulfide (the product of ligand oxidation) is slow. This is attributed to differences in the surface energies of w and zb facets, with w having polar (0001) facets of high energy compared to the neutral facets of the zb structure. The zb-CdS NCs prepared by low-temperature synthesis methods are likely to suffer from surface defects that may moderate reactivity. EPR studies suggest that zb-CdS has paramagnetic sulfur vacancies not present in w-CdS. These data suggest that structure plays an unexpectedly large role in the kinetics of CdQ NC oxidative assembly, providing a useful lever to moderate activities in multicomponent assemblies.

Q-switching Yb3+: YAG lasers based on plasmon resonance nonlinearities of Cu2-xSe@Cu2-xS nanorods.

Copper(II) chalcogenide nanocrystals, including Cu2-xY (Y=S, Se, and Te), have an intense localized surface plasmon resonance (LSPR) band in the near-infrared (NIR) region. In this research, colloidal Cu2-xSe@Cu2-xS nanorods were synthesized using the organometallic colloidal and cation-exchange methods. The dynamics of LSPR were investigated using ultrafast laser pulses via pump-probe experiments in the NIR region. Investigation of the transient absorption spectra revealed an LSPR spectral band from approximately 850 to 1350nm, with a center wavelength of 1030nm. The kinetics of the recovering plasmon maximum, probed at the peak wavelength of 1030nm, exhibited a strong nonlinear response for plasmonic absorption, with a modulation depth exceeding 25% in the transmitted signal under a pump fluence of 3.97 mJ/cm2. The ultrafast nonlinear optical properties of these plasmonic nanoparticles could be used as excellent saturable absorbers (SAs) in ultrafast lasers. A compact passively Q-switched Yb3+:YAG microchip laser with a Cu2-xSe@Cu2-xS SA was investigated. Furthermore, a maximum average output power of 187mW was obtained, with a pulse energy of 4.11μJ, pulse duration of 8.5μs, and repetition rate of 45.45KHz at a pump power of 8.7W.

Tuning Blinking Behavior of Highly Luminescent Cesium Lead Halide Nanocrystals through Varying Halide Composition

Colloidal cesium lead halide nanocrystals (CsPbX3, where X = Cl, Br, and I) are newcomer optoelectronic materials and have been successfully utilized as bright light sources and high-efficiency photovoltaics due to their facial solution processability. However, detailed understanding of their fundamental chemistry and photophysics behavior are urgently required for utilizing the full potential of the perovskite nanocrystals. In this work, we described a simple and efficient approach for regulating the blinking behavior of the CsPbX3 nanocrystals through varying halide composition. We systematically investigated the ensemble and single-particle optical properties of the mixed-halide CsPbBrxI3–x and CsPbBr3–yIy nanocrystals via two different anion-exchange routes (Br– ions → I– ions and I– ions → Br– ions). The power-law fitting results demonstrated that both the “on” and “off” events of their fluorescence exhibited a similar power-law rule to those of the cadmium chalcogenide nanocrystals (e.g., CdSe nanoc...

"Click" on Alkynylated Carbon Quantum Dots: An Efficient Surface Functionalization for Specific Biosensing and Bioimaging.

Surface functionalization is an essential pre requisite for wide and specific applications of nanoparticles such as photoluminescent (PL) carbon quantum dots (CQDs), but it remains a major challenge. In this report, alkynylated CQDs, prepared from carboxyl-rich CQDs through amidation with propargylamine in the presence of 1,1'-carbonyldiimidazole, were modified efficiently with azido molecular beacon DNA through a copper(I)-catalyzed alkyne-azide cycloaddition reaction (CuAAC). As a proof-of-concept, the DNA-modified CQDs are then bonded with gold nanoparticles (AuNPs, 5 nm) through a gold-sulfur bond. Owing to the emission enhancement, this complex can then be applied to the recognition of a single-base- mismatched target. The same functionalizing strategy applied to click the alkynylated CQDs with a nuclear localization sequence (NLS) peptide showed that the NLS-modified CQDs could target the nuclei specifically. These results indicate that surface functionalization of CQDs through a nonstoichiometric copper chalcogenide nanocrystal- (nsCuCNC-) catalyzed click reaction is efficient, and has significant potential in the fields of biosensing and bioimaging.

Chalcogenide and pnictide nanocrystals from the silylative deoxygenation of metal oxides

Transition metal chalcogenides and pnictides are interesting in optoelectronics and catalysis. We present a generalized method to transform preformed metal oxide nanocrystals into sulfides, selenides, or phosphides via silylative deoxygenation with trimethylsilyl reagents. In all reactions studied, simultaneous decomposition of excess silyl reagent produces a protective, amorphous silica coating around the newly formed nanocrystals.

Design of TPGS-functionalized Cu3BiS3 nanocrystals with strong absorption in the second near-infrared window for radiation therapy enhancement.

Integrating radiation therapy with high-depth photothermal therapy in the second near-infrared (NIR) window is highly required for efficient treatment of deep-seated tumor cells. Here, we constructed a multifunctional nano-theranostic with bimetallic chalcogenide nanocrystals (NCs) functionalized with amphiphilic d-α-tocopherol polyethylene glycol 1000 succinate (TPGS-Cu3BiS3). Benefiting from the strong absorbance of both X-ray and NIR light in the second NIR window, TPGS-Cu3BiS3 CNs can not only deposit more radiation dose to trigger enhanced radiation damage in vivo, but also conduct photo-induced hyperthermia for thermal ablation in the second NIR window and effective improvement of tumor oxygenation to overcome the hypoxia-associated radio-resistance of tumors. Moreover, copper ions on the surface of TPGS-Cu3BiS3 NCs are capable of catalyzing the Fenton-like and Haber-Weiss reactions to produce highly reactive hydroxyl radicals, leading to the increase in the level of oxygen radicals and further enhance cancer cell destruction. Apart from their therapeutic application, by means of X-ray computer tomography imaging as well as multispectral optoacoustic tomography imaging, TPGS-Cu3BiS3 NCs also have the potential as a nano-theranostic to offer remarkable therapeutic outcome for deep-seated tumor cells in imaging-guided synergistically enhanced radiation therapy.