Cadmium Chalcogenide Research Articles

Oxygen Management to Avoid Photo-Inactive Cd(S,Se) for Efficient Cd(Se,Te) Solar Cells

Our previous work has demonstrated that the formation of a penternary cadmium chalcogenide Cd(O,S,Se,Te) region can significantly reduce the front interface recombination in Cd(Se,Te)-based thin-film solar cells. In this work, we have shown that oxygen management during the device fabrication is crucial to form this region. While both the CdS and CdSe layer depositions and the postdeposition CdCl2 treatment should be conducted in the presence of oxygen, the CdTe deposition should be conducted in an oxygen-free atmosphere. Improper oxygen management leads to low device performance due to the formation of a photoinactive Cd(S,Se) region and reduced absorber quality. Additionally, we investigated the carrier transport and collection properties in devices with photoinactive Cd(S,Se) and photoactive Cd(O,S,Se,Te) at the front interface to gain comprehensive understanding of the mechanisms that resulted in improved efficiencies approaching 20%.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration.

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

Crystal host engineering for transition metal lasers

Much progress has been made in the power and energy scaling of mid-infrared lasers based on transition metal ions, such as Cr2 + , Co2 + , and Fe2 + , in zinc and cadmium chalcogenides. Still, the exploration of the physics of these devices is incomplete. In this work, we analyze absorption spectra collected from Fe2 + ions in several binary, ternary, and quaternary host crystals at temperatures from a few degrees Kelvin to room temperature. We use the zero-phonon lines of the low-temperature spectra to calculate the value of the crystal field energy of Fe2 + ions in these hosts. We plot these crystal field energies with respect to the anion–cation distance of the host crystal and show that their crystal field strengths deviate somewhat from the trend predicted by crystal field theory. We use a model which we previously developed to describe the upper state lifetime of Fe2 + ions to predict the steady-state radiative efficiency of Fe:II–VI materials with respect to temperature. The impact of relative crystalline disorder on the output characteristics of lasers based on Cr:ZnS, Cr:ZnSe, Fe:ZnSe, and Fe:CdMnTe is explored. The effect of decreased long-range order of the host crystal is observed in the broadening of the absorption spectra of Fe2 + -doped ternary and quaternary alloys, the broadening of the spectral linewidth of continuous-wave Fe:II–VI lasers, and a reduction in the portion of the Cr:ZnSe emission spectrum accessible for modelocked lasing. This survey provides a richer picture of the tradespaces that can be leveraged when producing laser devices based on transition metal chalcogenides.

Shape tunability of copper nanocrystals deposited on nanorods.

The significant role of metal particle geometry in dictating catalytic activity, selectivity, and stability is well established in heterocatalysis. However, this topic is rarely explored in semiconductor-metal hybrid photocatalytic systems, primarily due to the lack of synthetic control over this feature. Herein, we present a new synthetic route for the deposition of metallic Cu nanoparticles with spherical, elliptic, or cubic geometrical shapes, which are selectively grown on one side of the well-established CdSe@CdS nanorod photocatalytic system. An additional multipod morphology in which several nanorod branches are combined on a single Cu domain is presented as well. Cu is an earth-abundant low-cost catalyst known to promote a diverse gallery of organic transformations and is an excellent thermal and electrical conductor with interesting plasmonic properties. Its deposition on cadmium chalcogenide nanostructures is enabled here via mitigation of the reaction kinetics such that the cation exchange reaction is prevented. The structural diversity of these sophisticated nanoscale hybrid systems lays the foundations for shape-activity correlation studies and employment in various applications.

CdSexS1–x Alloyed Nanoplatelets with Continuously Tunable Blue-Green Emission

Cadmium chalcogenide nanoplatelets (NPLs) are established as promising materials for a wide variety of optoelectronic applications due to their properties surpassing in many aspects their counterpart nanocrystals (NCs) with other shapes. Most of these features arise from strong quantum confinement in the direction of thickness which can be tuned with precision down to one monolayer. However, atomic smoothness of their basal planes and hence the ability to change the NPL thickness only in discrete steps prevent precise tuning of absorption and photoluminescence spectra unlike in the case of quantum dots. Preparation of alloyed NCs provides a potential solution to this problem, but it is complicated by the different reactivities of chalcogenide sources, which becomes even more restrictive in the case of NPLs because they are more sensitive to alterations of reaction conditions. In this work, we overcome this obstacle by employing highly reactive stearoyl sulfide and selenide as chalcogen sources, which enable straightforward variation of the NPL composition and thickness by changing the ratio of chalcogen precursors and reaction temperature, respectively. Alloyed CdSexS1–x NPLs obtained exhibit tunable absorption and photoluminescence bands covering the blue-green region from 380 to 520 nm with bright band-edge emission and quantum yields of ∼30–50% due to their relatively small lateral size enabled by a much finer control of the lateral growth.

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.

Theoretical Investigation of the Electronic Spectra of Cadmium Chalcogenide 2D Nanoplatelets.

Although colloidal cadmium chalcogenide 2D nanoplatelets (NPLs) have recently demonstrated strongly enhanced one- and two-photon absorption (OPA, TPA) spectra, an understanding of the effects of quantum confinement in the lateral and vertical (thickness) dimensions is mostly lacking. In this work, we investigate theoretically CdS and CdSe NPLs passivated with formate and acetate ligands with thicknesses of two and three monolayers (MLs) and different lateral dimensions. Initial structures for CdS nanoplatelets were obtained using our recently developed deep neural network potential, and the low-energy geometries were subsequently optimized using density functional theory (DFT). Linear- and nonlinear-response calculations using time-dependent DFT (TDDFT) and the simplified Tamm-Dancoff approximation (sTDA) demonstrated good agreement between measured spectra and calculated TDDFT and sTDA spectra for 2 and 3 ML NPLs. The OPA red-shifts from 2 to 3 ML NPLs can be attributed to the electron delocalization in the lateral and vertical directions. TPA responses for CdS and CdSe NPLs were found to be dominated by weakly absorbing and forbidden OPA states.

Bandgap engineering of CdTe/CdSe rod-shaped core/shell and CdTeSe ellipsoidal alloy quantum dots with tunable and intense emission

In this work, core CdTe, core-shell CdTe/CdSe and alloy CdTeSe QDs are synthesized by aqueous solvent. TEM visually shows the rod-shaped core-shell structure with a transverse to longitudinal ratio of 6.5 and the ellipsoidal alloy structure with different crystal planes competing for growth. The XRD and Raman spectra of the three QDs illustrate the characteristics of the internal lattice structure. With the increase of reaction time, CdTe emission maximum (EM) shows a red shift of 84 nm. Stokes shift changes little, and full width half maximum (FWHM) increases slightly. Compared with the CdTe, the red shift of CdTe/CdSe EM is slowly first and then faster, with a total shift of 125 nm. Stokes shift increases obviously. The initial FWHM is larger than CdTe and the widening speed is gradually accelerated. The initial EM of CdTeSe is red shifted compared with CdTe, and the total red shift of EM is 72 nm. Stokes shift increases obviously. The initial FWHM is significantly wider than CdTe. FWHM first decreases and then increases with reaction time. CdTe shows obvious band edge exciton absorbance of photoluminescence excitation (PLE) spectrum between 356 nm and 400 nm. CdTe/CdSe shows double band edge exciton absorption at 354–373 nm and 401–436 nm due to wave function separated of core-shell structure. The PLE spectrum of CdTeSe shows an obvious upward trend at 306–373 nm due to high energy level nonradiation. The average lifetime of CdTe/CdSe increases by 53 ns compared with CdTe due to the spatially separated electrons and holes. The defect state of CdTeSe increases the proportion of fast delay, which is 2 ns shorter than the average lifetime of CdTe. The transition mechanism of core-shell indirect band gap and alloy direct band gap is explained according to the valence bands of each element shown by XPS and the simulation of energy band density of states. The thickness shell and the doping elements can be adjusted to provide rich variation in the optical properties. This provides a more definite direction for the applications of cadmium chalcogenide in the fields of photoelectric devices and biology. • Type I and type II transformation through Rod-shaped core/shell and ellipsoidal alloy QDs. • Tunable emission achieved by adjusting morphology, lattice structure and valence bond connection. • Precise and rich regulation of band gap engineering. • Providing a more definite direction for the applications of cadmium chalcogenide QDs.

Transformations of Magic-Size Clusters via Precursor Compound Cation Exchange at Room Temperature.

The transformation of colloidal semiconductor magic-size clusters (MSCs) from zinc to cadmium chalcogenide (ZnE to CdE) at low temperatures has received scant attention. Here, we report the first room-temperature evolution of CdE MSCs from ZnE samples and our interpretation of the transformation pathway. We show that when prenucleation stage samples of ZnE are mixed with cadmium oleate (Cd(OA)2), CdE MSCs evolve; without this mixing, ZnE MSCs develop. When ZnE MSCs and Cd(OA)2 are mixed, CdE MSCs also form. We propose that Cd(OA)2 reacts with the precursor compounds (PCs) of the ZnE MSCs but not directly with the ZnE MSCs. The cation exchange reaction transforms the ZnE PCs into CdE PCs, from which CdE MSCs develop. Our findings suggest that in reactions that lead to the production of binary ME quantum dots, the E precursor dominates the formation of binary ME PCs (M = Zn or Cd) to have similar stoichiometry. The present study provides a much more profound view of the formation and transformation mechanisms of the ME PCs.

The Sulfur Rich Fluorothiophosphate Dianions [S5 P2 F2 ]2- and [S3 PF]2- : Cluster and Chelation Control of P-S Heterolysis.

The sulfur rich difluoropentathiodiphosphate dianion [S5 P2 F2 ]2- , from fluoride addition to P4 S10 , has a somewhat checkered history and proves to be the main product of the reaction in acetonitrile. Its optimized synthesis, and structural characterization, as either a tetraphenylphosphonium or a tetrapropylammonium salt, [Nn Pr4 ]2 [S5 P2 F2 ] allows for the first coordination chemistry for this dianion. Reactions of [S5 P2 F2 ]2- with d10 metal ions of zinc(II), and cadmium(II), and d9 copper(II) resulted in a surprising diverse array of binding modes and structural motifs. In addition to the simple bis-chelate coordination of [S5 P2 F2 ]2- with zinc, cleavage of the P-S bond resulted in complexes with the unusual [S3 PF]2- fluorotrithiophosphate dianion. This was observed in two cluster complexes: a trinuclear cadmium complex with mixed [S5 P2 F2 ]2- /[S3 PF]2- ligands, [Cd3 (S5 P2 F2 )3 (S3 PF)2 ]4- as well as an octanuclear copper cluster, [Cu8 (S3 PF)6 ]4- which form rapidly at room temperature. These new metal/sulfur/ligand clusters are of relevance to understanding multimetal binding to metallothionines, and to potential capping strategies for the condensed nanoparticulate cadmium chalcogenide semiconductors CdS and CdSe.

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.

A comparative study of chalcogenide thin films for micro sensor applications

Cadmium chalcogenide thin films were deposited by different processes. Pulsed laser deposition, thermal evaporation, and sputtering were used to elaborate micro devices on glass substrate from CdS-AgI-Ag2S-As2S3, CdS-Ag2S-Sb2S3 and Cd-Ag-As2S3 starting materials respectively. The micro films were characterised using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemistry. Comparative structural characterisation leads us to understand sensors out-put signal. The best miniaturised sensor was selected. The very first electrochemical results are presented. The potentiometric response of Cd-Ag-As2S3 sensing matrix showed a quasi-Nernstian behaviour with a great detection limit. Moreover Cd-ISE is sensitive to silver (+I) in solution.

Photoemission of the Upconverted Hot Electrons in Mn-Doped CsPbBr3 Nanocrystals.

Hot electrons play a crucial role in enhancing the efficiency of photon-to-current conversion or photocatalytic reactions. In semiconductor nanocrystals, energetic hot electrons capable of photoemission can be generated via the upconversion process involving the dopant-originated intermediate state, currently known only in Mn-doped cadmium chalcogenide quantum dots. Here, we report that Mn-doped CsPbBr3 nanocrystals are an excellent platform for generating hot electrons via upconversion that can benefit from various desirable exciton properties and the structural diversity of metal halide perovskites (MHPs). Two-dimensional Mn-doped CsPbBr3 nanoplatelets are particularly advantageous for hot electron upconversion due to the strong exciton-dopant interaction mediating the upconversion process. Furthermore, nanoplatelets reveal evidence for the hot electron upconversion via long-lived dark excitons in addition to bright excitons that may enhance the upconversion efficiency. This study establishes the feasibility of hot electron upconversion in MHP hosts and demonstrates the potential merits of two-dimensional MHP nanocrystals in the upconversion process.

Light emitters based on II-VI chalcogenides with nanostructured surface

A modified surface was obtained on wide-gap II-VI compounds. The methods of thermal annealing for cadmium chalcogenides and chemical etching for the first time obtained heterolayers of atypical hexagonal modification of α-ZnSe have been improved. AFM topograms of surface nanostructures have been investigated and comprehensive studies of optical and luminescent properties have been carried out. The emission spectra were determined in the photon energy range 1.4–3.6 eV and the important role of recombination processes in the formation of the A band in the edge region was established. The dominant role of quantum-dimensional processes in the formation of the obtained B band of intense radiation in a wide spectral region of the visible and ultraviolet ranges has been established.

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Determination of stoichiometry deviation in wide-band II-VI semiconductors on the basis of equilibrium vapor phase composition

A method of determining stoichiometry deviation in cadmium and zinc chalcogenides that is based on the temperature dependence of the ratio of components partial pressures during evaporation of solid compounds in a limited volume has been suggested. The new method differs from methods implying the collection of excessive component during evaporation in large volumes. The method includes measuring vapor phase components partial pressures during material heating to above 800 K, solving a set of material balance equations and the electric neutrality equation, and calculating the stoichiometry deviation in the initial compound at room temperature. Intrinsic point defect concentrations are calculated using the method of quasichemical reactions. The independent variables in the set of equations are the sought stoichiometry deviation, the partial pressure of the metal and the concentration of free electrons. We show that the parameter of the material balance equation which determines the method’s sensitivity to stoichiometry deviation, i.e., the volume ratio of vapor and solid phases, can be considered constant during heating and evaporation unless this parameter exceeds 50. If the partial pressure is measured based on the optical density of the vapors, then the sensitivity of the method can be increased to not worse than 10-6 at.%.

Cadmium Chalcogenide (CdS, CdSe, CdTe) Quantum Dots for Solar‐to‐Fuel Conversion

Photocatalysis is considered a promising technology to alleviate the pressure of environmental pollution and energy shortage. Among various photocatalysts, cadmium chalcogenide quantum dots (CdX QDs) stand out due to their excellent visible light absorption ability and efficient charge separation properties. This review presents the atomic and electronic structures of CdX QDs and their synthetic strategies. In addition, the reaction mechanism of CdX QDs in photocatalytic solar‐to‐fuel conversion is discussed. Another focus of this review is the optimization strategies for CdX QDs to improve their photocatalytic performance. Finally, the challenges and the perspectives on CdX QDs are briefly proposed.

Determination of stoichiometry deviation in wide-band II–VI semiconductors on the basis of equilibrium vapor phase composition

A method has been suggested for determining stoichiometry deviation in cadmium and zinc chalcogenides based on the temperature dependence of the ratio of components partial pressures during evaporation of solid compounds in a limited volume. The new method differs from methods implying the collection of excessive component during evaporation in large volumes. The method includes measuring the partial pressures of vapor phase components during material heating to above 800 K, solving a set of material balance equations and the electric neutrality equation, and calculating the stoichiometry deviation in the initial compound at room temperature. Intrinsic point defect concentrations are calculated using the method of quasichemical reactions. The independent variables in the set of material balance equations are the sought stoichiometry deviation, the partial pressure of the metal and the concentration of free electrons. We show that the parameter of the material balance equation which determines the method’s sensitivity to stoichiometry deviation, i.e., the volume ratio of vapor and solid phases, can be considered constant during heating and evaporation if this parameter does not exceed 50. If the partial pressure is measured based on the optical density of the vapors, then the sensitivity of the method can be increased to not worse than 10–6 at.%.

Interparticle Distance Variation in Semiconductor Nanoplatelet Stacks (Adv. Funct. Mater. 24/2022)

Semiconductor Nanoplatelet Stacks In article number 2112621, Nadja C. Bigall and co-workers demonstrate a route to decrease the inter-nanoplatelet distance in polymer encapsulated cadmium chalcogenide nanoplatelet stacks. Thus, certain ligand exchanges are applied to minimize the distance by more than half in the case of CdSe nanoplatelets. Furthermore, the influence of the smaller distance on the charge carrier transfer is analysed by photo-electrochemical measurements.

Photoinstability aversion in perovskite solar cell by downconversion cadmium chalcogenide filters

Susceptibility to environmental factors, such as moisture, humidity, oxygen, heat, and ultraviolet (UV) light (photoinstability), has affected perovskite solar cell (PSC) stability in practical applications. To overcome the instability and performance degradation due to oxygen, humidity, and moisture, different strategies and encapsulation schemes have been proposed, and promising results have been obtained. However, photostability remains a major hurdle because UV light is an inherent part of the standard incident spectrum of PSCs. To prevent photoinstability and increase quantum energy harvesting levels, cadmium chalcogenide (CC) photoluminescent (PL) filters for downconverting the UV part of the incident spectra obtained for PSCs are proposed in this work. The concept was illustrated by matching 500-nm-thick CC-PL filters to the front glass of a PSC to form a CC-PL/glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)/perovskite/Ag structure. Cadmium sulfide (CdS), cadmium selenide, and cadmium telluride were taken as the CC materials. Practical measurements confirmed that the PSC with the CdS-PL filter can maintain 92% of its initial value under continuous light soaking for more than 100 h. Furthermore, this PSC exhibited the best improvement in power conversion efficiency.