Chalcogenide Nanocrystals Research Articles

Modification of Copper-based chalcogenide nanocrystals' interconnections for efficient hole transportation in perovskite solar cell

Cu-based chalcogenide p-type semiconductors in the form of nanocrystals can be used to deposit low-temperature and stable hole-transport layers (HTL) for n-i-p perovskite solar cells (PSCs). In this study, the synthesis parameters are modified to improve the hole-extraction of CuIn0.75Ga0.25S2 (CIGS) layer. By optimizing the amount of oleylamine as the capping agent, the internal series resistance of PSCs decreases significantly while preventing aggregation of CIGS nanocrystals. Increasing the growth temperature from 220°C to 275°C leads to an increase in the size of CIGS nanocrystals. This results in an improvement in the open-circuit voltage of PSCs with FTO/c-TiO2/m-TiO2/Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3/CIGS/Au structure from 0.941 to1.092 V, and Fill Factor from 60 to 73 on average. Finally, a nanocomposite of CIGS and a small amount of doped Spiro-OMeTAD is applied in PSCs with ITO/SnO2/(FA,MA)Pb(I,Cl,Br)3/HTL/Au structure. The achieved efficiency of 18.72% is comparable to that of PSCs with highly doped Spiro OMeTAD (18.5%).

Ternary Ag3AuS2 Nanocrystals for Thin-Film Solar Cells.

The concept of clean and pollution-free energy development has promoted the rise of environmentally friendly silver-based chalcogenide nanocrystal (NC) solar cells, but currently reported silver-based NCs need complex synthesis processes at high temperatures that may bring zerovalent noble metal impurities for their high redox potentials. In this study, we report a facile synthesis of novel Ag3AuS2 NCs by injecting highly active oleylamine sulfur complexes as sulfur sources into metal precursor solutions at low temperatures of 60 °C. The obtained Ag3AuS2 NCs exhibit broad absorption spectra and high molar extinction coefficients (106 M-1 cm-1). Then, the Ag3AuS2 NCs are applied as the light-absorbing active layer in environmentally friendly thin-film solar cells. By introducing a hybrid mixture of charge acceptors and donors (NCs/P3HT hybrid film) at the interface, the device gains an absorption increment and enhanced charge extraction, achieving a final power conversion efficiency of 3.38%. This work demonstrates the enormous potential of Ag3AuS2 NCs from low-temperature preparation for photovoltaic applications.

(Invited) Photon Upconversion: Getting Molecules and Nanocrystals to Talk Triplets

Photon upconversion offers the prospect of combining low energy near-infrared radiation to make high energy photons in the visible wavelengths to improve photovoltaics and introduce new bioimaging techniques. In order to harness the intrinsic ability of colloidal semiconductor nanocrystals to couple strongly with light, it is important to efficiently outcouple energy from photoexcited quantum dots (QDs), much like how nature uses molecular antennas to direct light during photosynthesis. This talk focuses on the organic-inorganic interface for triplet-fusion based photon upconversion, where orbital overlap between the QD donor and molecular acceptor is critical for efficient energy transduction. In the past 8 years, we have learnt a great deal about how the electronic interactions between chalcogenide nanocrystals and acene conjugated molecules affect the photon upconversion quantum yield. We apply the lessons learnt to extending photon upconversion to silicon nanocrystal light absorbers, addressing synthetic challenges with invaluable insight from time-resolved spectroscopy.

Magnesium Chalcogenide Nanocrystals and Their Core/Shell Nanostructures

AbstractMagnesium chalcogenide with wide bandgap semiconductor is an important candidate for the next generation semiconductor for optoelectronic devices. Colloidal magnesium chalcogenide (MgS, MgSe, and MgTe) nanocrystals are produced with the colloidal method, showing the optoelectronic properties for wide bandgap semiconductors in the violet region. The shape and size of magnesium chalcogenide nanocrystals are precisely controlled via tuning the precursors and ligands. Magnesium chalcogenide are further deposited on ZnSe and ZnS quantum dots for shell materials to enhance fluorescence emission due to the wide bandgap and suitable lattice in magnesium chalcogenide. Magnesium chalcogenide in nanoscale exhibits the potential for wide‐band semiconductors and the shell materials to light‐emitting quantum dots.

Removing Cadmium Impurities from Cation‐Exchange‐Derived CuInSe2/CuInS2 Nanorods for Enhanced Infrared Emission and Photodetection

AbstractMetal chalcogenide nanocrystals with variable composition and shape can conveniently be produced using cation exchange synthesis; however, the presence of cation‐containing ligands inherited from the starting material often results in contamination of the final product. To address this issue, a two‐step ligand replacement strategy is developed to fabricate CuInSe2/CuInS2 nanorods from CdSe/CdS nanorods via removal of Cd‐phosphonates from an intermediate Cu2‐xSe/Cu2‐xS phase used in the cation exchange conversion. This synthetic approach furnishes CuInSe2/CuInS2 nanorods with cadmium content below 1 at.%, and high photoluminescence quantum yields reaching 40% in the near‐infrared spectral range. Transient absorption studies reveal that the band alignment in the CuInSe2/CuInS2 heterostructure features a quasi‐type II character, with an electron localized in the core and a hole wavefunction spread over the entire nanorod. The efficient passivation of the core and the reduced Cd content leads to excitonic emission with full width at half maximum down to 110 meV, superimposed with a broad emission band from copper‐induced defects. Field‐effect transistors based on cadmium‐free CuInSe2/CuInS2 nanorods show two orders of magnitude lower noise current density compared with the cadmium‐rich devices. The responsivity and specific detectivity of these devices reach 230 mA W−1 and 108 Jones, respectively, under near‐infrared excitation at room temperature.

Redox Chemistries for Vacancy Modulation in Plasmonic Copper Phosphide Nanocrystals.

Copper phosphide (Cu3-xP) nanocrystals are promising materials for nanoplasmonics due to their substoichiometric composition, enabling the generation and stabilization of excess delocalized holes and leading to localized surface plasmon resonance (LSPR) absorption in the near-IR. We present three Cu-coupled redox chemistries that allow postsynthetic modulation of the delocalized hole concentrations and corresponding LSPR absorption in colloidal Cu3-xP nanocrystals. Changes in the structural, optical, and compositional properties are evaluated by powder X-ray diffraction, electronic absorption spectroscopy, 31P magic-angle spinning solid-state nuclear magnetic resonance spectroscopy, and elemental analysis. The redox chemistries presented herein can be used to access nanocrystals with LSPR energies of 660-890 meV, a larger range than has been possible through synthetic tuning alone. In addition to utilizing previously reported redox chemistries used for copper chalcogenide nanocrystals, we show that the largest structural and LSPR modulation is achieved using a divalent metal halide and trioctylphosphine. Specifically, nanocrystals treated with zinc iodide and trioctylphosphine have the smallest unit-cell volume (295.2 Å3) reported for P63cm Cu3-xP, indicating more Cu vacancies than have been previously observed. Overall, these redox chemistries present valuable insight into controlling the optical and structural properties of Cu3-xP.

Dual-drive acoustic micromixer for rapid nucleation and ultrafast growth of perovskite nanoparticles.

All-inorganic cesium lead halide perovskites have garnered significant attention owing to their favorable optical properties. Microfluidics-based acoustic mixers are capable of achieving rapid nucleation and ultrafast growth kinetics. Nevertheless, conventional acoustic mixers rely on the response of microstructures to the acoustic field for mixing fluids, the majority of these disturbances occur in the central region of the channel, with minimal impact on the fluid within the side walls. This paper proposes a novel acoustic mixer that combines the effects of sharp corners and bubbles in response to the acoustic field, thereby producing effective disturbance of the fluid throughout the channel. The combined effect enables the micromixer to achieve complete mixing at different inlet flow ratios with mixing times as low as 5 ms. The superiority of acoustic mixers in controlling the nanocrystal formation stage was further validated through the synthesis of chalcogenide nanocrystals using the LARP method. The millisecond mixing time facilitated the rapid formation of nanocrystals and their subsequent rapid growth. The results demonstrate that the green luminescence intensity at 520 nm of the samples synthesized using the acoustic micromixer is 118% higher than that of the samples synthesized using an intermittent reactor. The novel micromixer broadens the range of applications and offers a promising avenue for the large-scale continuous synthesis of high-quality lead-halide perovskite nanocrystals (NCs).

Polytypic metal chalcogenide nanocrystals.

By engineering chemically identical but structurally distinct materials into intricate and sophisticated polytypic nanostructures, which often surpass their pure phase objects and even produce novel physical and chemical properties, exciting applications in the fields of photovoltaics, electronics and photocatalysis can be achieved. In recent decades, various methods have been developed for synthesizing a library of polytypic nanocrystals encompassing IV, III-V and II-VI polytypic semiconductors. The exceptional performances of polytypic metal chalcogenide nanocrystals have been observed, making them highly promising candidates for applications in photonics and electronics. However, achieving high-precision control over the morphology, composition, crystal structure, size, homojunctions, and periodicity of polytypic metal chalcogenide nanostructures remains a significant synthetic challenge. This review article offers a comprehensive overview of recent progress in the synthesis and control of polytypic metal chalcogenide nanocrystals using colloidal synthetic strategies. Starting from a concise introduction on the crystal structures of metal chalcogenides, the subsequent discussion delves into the colloidal synthesis of polytypic metal chalcogenide nanocrystals, followed by an in-depth exploration of the key factors governing polytypic structure construction. Subsequently, we provide comprehensive insights into the physical properties of polytypic metal chalcogenide nanocrystals, which exhibit strong correlations with their applications. Thereafter, we emphasize the significance of polytypic nanostructures in various applications, such as photovoltaics, photocatalysis, transistors, thermoelectrics, stress sensors, and the electrocatalytic hydrogen evolution. Finally, we present a summary of the recent advancements in this research field and provide insightful perspectives on the forthcoming challenges, opportunities, and future research directions.

Toward Surface Chemistry of Semiconductor Nanocrystals at an Atomic-Molecular Level.

ConspectusProperties of colloidal semiconductor nanocrystals with a single-crystalline structure are largely dominated by their surface structure at an atomic-molecular level, which is not well understood and controlled, due to a lack of experimental tools. However, if viewing the nanocrystal surface as three relatively independent spatial zones (i.e., crystal facets, inorganic-ligands interface, and ligands monolayer), we may approach an atomic-molecular level by coupling advanced experimental techniques and theoretical calculations.Semiconductor nanocrystals of interest are mainly based on compound semiconductors and mostly in two (or related) crystal structures, namely zinc-blende and wurtzite, which results in a small group of common low-index crystal facets. These low-index facets, from a surface-chemistry perspective, can be further classified into polar and nonpolar ones. Albeit far from being successful, the controlled formation of either polar or nonpolar facets is available for cadmium chalcogenide nanocrystals. Such facet-controlled systems offer a reliable basis for studying the inorganic-ligands interface. For convenience, here facet-controlled nanocrystals refer to a special class of shape-controlled ones, in which shape control is at an atomic level, instead of those with poorly defined facets (e.g., typical spheroids, nanorods, etc).Experimental and theoretical results reveal that type and bonding mode of surface ligands on nanocrystals is facet-specific and often beyond Green's classification (X-type, Z-type, and L-type). For instance, alkylamines bond strongly to the anion-terminated (0001) wurtzite facet in the form of ammonium ions, with three hydrogens of an ammonium ion bonding to three adjacent surface anion sites. With theoretically assessable experimental data, facet-ligands pairing can be identified using density functional theory (DFT) calculations. To make the pairing meaningful, possible forms of all potential ligands in the system need to be examined systematically, revealing the advantage of simple solution systems.Unlike the other two spatial zones, the ligands monolayer is disordered and dynamic at an atomic level. Thus, an understanding of the ligands monolayer on a molecular scale is sufficient for many cases. For colloidal nanocrystals stably coordinated with surface ligands, their solution properties are dictated by the ligands monolayer. Experimental and theoretical results reveal that solubility of a nanocrystal-ligands complex is an interplay between the intramolecular entropy of the ligands monolayer and intermolecular interactions of the ligands/nanocrystals. By introducing entropic ligands, solubility of nanocrystal-ligands complexes can be universally boosted by several orders of magnitude, i.e., up to >1 g/mL in typical organic solvents. Molecular environment in the pseudophase surrounding each nanocrystal plays a critical role in its chemical, photochemical, and photophysical properties.For some cases, such as the synthesis of high-quality nanocrystals, all three spatial zones of the nanocrystal surface must be taken into account. By optimizing nanocrystal surface at an atomic-molecular level, semiconductor nanocrystals with monodisperse size and facet structure become available recently through either direct synthesis or afterward facet reconstruction, implying full realization of their size-dependent properties.

Colloidal Synthesis of Multinary Alkali-Metal Chalcogenides Containing Bi and Sb: An Emerging Class of I-V-VI2 Nanocrystals with Tunable Composition and Interesting Properties.

The growth mechanism and synthetic controls for colloidal multinary metal chalcogenide nanocrystals (NCs) involving alkali metals and the pnictogen metals Sb and Bi are unknown. Sb and Bi are prone to form metallic nanocrystals that stay as impurities in the final product. Herein, we synthesize colloidal NaBi1-xSbxSe2-ySy NCs using amine-thiol-Se chemistry. We find that ternary NaBiSe2 NCs initiate with Bi0 nuclei and an amorphous intermediate nanoparticle formation that gradually transforms into NaBiSe2 upon Se addition. Furthermore, we extend our methods to substitute Sb in place of Bi and S in place of Se. Our findings show the initial quasi-cubic morphology transforms into a spherical shape upon increased Sb substitution, and the S incorporation promotes elongation along the <111> direction. We further investigate the thermoelectric transport properties of the Sb-substituted material displaying very low thermal conductivity and n-type transport behavior. Notably, the NaBi0.75Sb0.25Se2 material exhibits an ultralow thermal conductivity of 0.25 W·m-1·K-1 at 596 K with an average thermal conductivity of 0.35 W·m-1·K-1 between 358 and 596 K and a ZTmax of 0.24.

Colloidal 2D Lead Chalcogenide Nanocrystals: Synthetic Strategies, Optical Properties, and Applications.

Lead chalcogenide nanocrystals (NCs) are an emerging class of photoactive materials that have become a versatile tool for fabricating new generation photonics devices operating in the near-IR spectral range. NCs are presented in a wide variety of forms and sizes, each of which has its own unique features. Here, we discuss colloidal lead chalcogenide NCs in which one dimension is much smaller than the others, i.e., two-dimensional (2D) NCs. The purpose of this review is to present a complete picture of today's progress on such materials. The topic is quite complicated, as a variety of synthetic approaches result in NCs with different thicknesses and lateral sizes, which dramatically change the NCs photophysical properties. The recent advances highlighted in this review demonstrate lead chalcogenide 2D NCs as promising materials for breakthrough developments. We summarized and organized the known data, including theoretical works, to highlight the most important 2D NC features and give the basis for their interpretation.

A Universal Synthesis Strategy for Lanthanide Sulfide Nanocrystals with Efficient Photocatalytic Hydrogen Production.

As an important lanthanide (Ln)-based functional materials, the Ln chalcogenides possess unique properties and various applications. However, the controllable synthesis of Ln chalcogenide nanocrystals still faces great challenges because of the rather poor affinity between Ln and chalcogenide ions (S, Se, Te) as well as strong preference of combination with existed oxygen. Herein, a facile but general heterogeneous nucleation synthetic strategy is established toward a series of colloidal ternary Cu Ln sulfides nanocrystals using the Ln dithiocarbamates and CuIas precursors. To extend this synthetic protocol, similar strategy is used to prepare six kinds of high quality CuLnS2 nanocrystals, while the bulk ones are only obtained by the traditional solid-state reaction at rigorous condition. Importantly, high-entropy nanocrystals CuLnS2 and CuEux Ln2-x S3 which contain six Lnelements (Nd, Sm, Gd, Tb, Dy) are readily obtained by the co-decomposed process attributed to their similar diffusion speed. As a proof-of-concept application, CuEu2 S3 nanocrystals showed efficient photocatalytic hydrogen production properties.

Effective Landé factors of electrons and holes in lead chalcogenide nanocrystals

The Land\'e or g-factors of charge carriers in solid state systems provide invaluable information about response of quantum states to external magnetic fields and are key ingredients in description of spin-dependent phenomena in nanostructures. We report on the comprehensive theoretical analysis of electron and hole g-factors in lead chalcogenide nanocrystals. By combining symmetry analysis, atomistic calculations, and extended k.p theory, we relate calculated linear-in-magnetic field energy splittings of confined electron states in nanocrystals to the intravalley g-factors of the multi-valley bulk materials, renormalized due to the quantum confinement. We demonstrate that this renormalization is correctly reproduced by analytical expressions derived in the framework of the extended k.p model.

Solution-phase synthesis of group 3-5 transition metal chalcogenide inorganic nanomaterials.

The versatility of early transition metal chalcogenide nanomaterials, including chalcogenide perovskites, has attracted enormous attention for a variety of applications, such as photovoltaics, photocatalysis, and optoelectronic devices. These nanomaterials exhibit unique electronic and optical properties, allowing for a broad range of applications, depending on their chemical composition and crystal structure. However, solution-phase synthesis of early transition metal chalcogenide nanocrystals is challenging due, in part, to their high crystallization energy and oxophilicity. In this feature article, we explore various synthetic routes reported for inorganic ternary and binary sulfide and selenide nanomaterials that include transition metals from groups 3, 4, and 5. By systematically comparing different synthetic approaches, we identify trends and insights into the chemistry of these chalcogenide nanomaterials.

Recent advances in heavily doped plasmonic copper chalcogenides: from synthesis to biological application

Copper-based chalcogenide compounds have emerged as alternative materials to Cd- or Pb-based traditional semiconductors and have drawn significant attention. Compared with widely reported semiconductors, copper chalcogenide nanocrystals (NCs) with abundant copper defects and vacancies present p-type features. Additionally, the migration of free hole carriers in copper-based chalcogenide NCs produced a metal-like local surface plasmon resonance (LSPR) effect. In this review, we focused on the plasmonic copper chalcogenide NCs achieved through a heavily doped strategy. The copper sulfur compounds with versatile atomic ratios and complex crystal structures exhibit rich electrical, optical, and magnetic properties, making them highly promising for a broad range of applications, from energy conversion to biomedical fields. Therefore, our main focus is on the classification of copper chalcogenide synthesis strategies, theoretical studies of doping, doping strategies, and biological applications. We aim to analyze the trends of copper-based chalcogenide nanomaterials for clinical applications by summarizing previous studies and presenting designs and concepts in a brief manner.

The Synergy of Lead Chalcogenide Nanocrystals in Polymeric Bulk Heterojunction Solar Cells.

Photoactive polymer and quantum dots (QDs)/nanocrystals (NCs)-based bulk heterojunction (BHJ) solar cells have the combined positivity of organic semiconductors and inorganic components, which can enable a high carrier mobility and absorption coefficient. Additionally, the NCs also provide the opportunity to tune the band gap to obtain enhanced absorption in a broad solar spectrum. Among the semiconductors, lead chalcogenide NCs are of particular interest due to their good photosensitivity in the near-infrared (NIR) region of the solar spectrum. These NCs have large exciton Bohr radii (18, 46, and 150 nm for PbS, PbSe, and PbTe, respectively) and tunable sizes depending on the optical bandgaps between 0.3 and 1.5 eV. Independently, lead chalcogenide NCs have been studied extensively for different applications; however, uses in polymer-NC-based bulk heterojunction solar cells are limited. This Review has been structured on the lead chalcogenide NCs incorporated in polymer composite-based bulk heterojunction solar cells covering the material, properties, and solar cell performance to find the issues and explore future opportunities.

Ligand and local surface strain-assisted doping for cadmium chalcogenide II-VI quantum dots

Different specific preparation methods of Ag-doped cadmium chalcogenide nanocrystals (NCs) in the post-synthetic stage were proposed by many groups. These methods usually emphasize the Ag precursors but ignore other factors that can influence the Ag-doping process. We used Ag film to dope the CdSe and CdS quantum dots to achieve the Ag-dopant photoluminescence (PL). That Ag-doping process can be modified effectively via tuning the surface stoichiometry, ligands, and morphology of cadmium chalcogenide quantum dots. Ag source is not the key factor influencing the doping process. We attributed those phenomena to that the local surface strain caused by those factors can lower the barrier of the Ag–Cd cation exchange. We also found that Ag+ can passivate the chalcogenide-rich QDs to enhance the band edge emission. This conclusion can explain the diversity of Ag-doping in various cadmium chalcogenide nanocrystals. The pure Ag-dopant PL was obtained by artificially constructing a strain-rich surface on CdSe QDs.

(Invited) Ligand Exchange at Chalcogenide and Perovskite Nanocrystal Surfaces Examined Via Isothermal Titration Calorimetry

I will describe our group’s efforts to quantify ligand association and exchange reactions on semiconductor nanocrystal (NC) surfaces via isothermal titration calorimetry (ITC). ITC provides a direct thermal measurement of reaction progress. As shown in earlier work by our group and others examining ligand exchange on chalcogenide and pnictide quantum dots, ITC is able to access the enthalpy change of reaction, equivalency, and effective equilibrium constants, including in cases that are challenging to resolve by NMR alone. Here, we will describe the extension of this approach to ligand exchange reactions on halide perovskite NCs, and to ligand exchange on chalcogenide NCs in aqueous solution. Such advances are needed to contribute a quantitative description of NC surface chemistry toward applications such as advanced optoelectronic devices, bioimaging probes, and photocatalysis.

Size-dependent radiative recombination characteristics of isolated CuInS2 nanocrystals

Ternary chalcopyrite semiconductor nanocrystals of the I–III–VI2 type are potential nontoxic alternatives to Cd- and Pb-based chalcogenide nanocrystals, which can be adopted for light emission and photovoltaic applications. Most I–III–VI2 nanocrystals exhibit luminescence originated from defect-related recombination, which is characterized by a broad spectrum with a large Stokes shift regardless of the size-dependent spectral shift due to the quantum size effect. However, the origin of the luminescence in I–III–VI2 nanocrystals is still under debate, and deeper investigations of the carrier recombination processes are necessary to gain further understanding of the luminescence mechanisms. For this purpose, we investigated the temperature dependence of the luminescence properties of CuInS2 nanocrystals down to cryogenic temperatures to suppress the nonradiative recombination. The intrinsic luminescence properties of the isolated CuInS2 nanocrystals were successfully evaluated by eliminating the influence of the interactions among the nanocrystals, which is different from previous studies on nanocrystal aggregates. Both the thermal quenching behavior and the temperature dependence of the carrier recombination rate could be satisfactorily described using a simple model that assumes a thermally activated nonradiative recombination. The radiative recombination rates of the CuInS2 nanocrystals were found to be 106–107 s−1, and a characteristic cubic dependence of the radiative recombination rate on the nanocrystal diameter was found.

Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties.

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, single-nanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron-hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.