CdSe Clusters Research Articles

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

CdSe cluster-modified biogenic α-FeOOH based on macroporous biochar for Fenton-like reaction of As(III)

In this study, CdSe cluster-modified biogenic α − FeOOH based on macroporous biochar was constructed from rhizosphere iron plaque via pyrolysis. An ultrahigh H2O2 production rate of 655.75 μmol/L was achieved by the photocatalysis-self-Fenton system within 30 min under visible light irradiation. α-FeOOH, with abundant iron ions, can directly activate H2O2 to produce abundant ⋅OH radicals. It was found that As(III) (10 mg/L) could be completely degraded within 20 min. Electron spin resonance spectroscopy (ESR) and density functional theory (DFT) revealed that a suitable energy band structure was achieved by the introduction of CdSe clusters; thus, O2 was mainly reduced to H2O2 through a two-electron reduction route in the reaction system. The photogenerated electrons promoted the conversion of Fe(III)/Fe(II) by reacting with Fe(III) in α-FeOOH. The presence of H2O2 and Fe2+ in this system accelerates the oxidation of As(III) by a Fenton-like reaction.

Peptoid-directed assembly of CdSe nanoparticles.

The high information content of proteins drives their hierarchical assembly and complex function, including the organization of inorganic nanomaterials. Peptoids offer an organic scaffold very similar to proteins, but with a wider solubility range and easily tunable side chains and functional groups to create a variety of self-assembling architectures with atomic precision. If we could harness this paradigm and understand the factors that govern how they direct nucleation and assembly of inorganic materials to design order within such materials, new dimensions of function and fundamental science would emerge. In this work, peptoid tubes and sheets were explored as platforms to assemble colloidal quantum dots (QDs) and clusters. We have successfully synthesized CdSe QDs with difunctionalized capping ligands containing both carboxylic acid and thiol groups and mixed them with maleimide containing peptoids, to create an assembly of the QDs on the peptoid surface via a covalent linkage. This conjugation was seen to be successful with peptoid tubes, sheets and CdSe QDs and clusters. The particles were seen to have a high preference for the peptoid surface but non-specific interactions with carboxylic acid groups on the peptoids limited control over QD density via maleimide conjugation. Replacing the carboxylic acid groups with methoxy ethers, however, allowed for control over QD density as a function of maleimide concentration. 1H NMR analysis demonstrated that binding of QDs to peptoids involved a subset of surface ligands bound through the carboxylate functional group, allowing the distal thiol to engage in a covalent linkage to the maleimide. Overall, we have shown the compatibility and control of CdSe-peptoid interactions via a covalent linkage with varying peptoid structures and CdSe particles to create complex hybrid structures.

Exploring the Correlation between Stability, Fluxionality, and Absorption Spectra of Ultrasmall CdSe Clusters: A Computational Study

Various properties of (CdSe)3n clusters (n = 1–9) were obtained using computational techniques. Using a simple force field, a simulated annealing technique was employed to explore the potential energy landscape and identify the isomers available at low temperatures. The lowest energy isomer was then optimized using density functional theory, and the quantities associated with their stability and optoelectronics were calculated. The possibility of fluxionality for a given range of temperatures and solvents was characterized by computing thermodynamic properties and inherent structure energies. All studied systems give rise to energetically stable hollow structures composed of six- and four-membered rings that changed from spherical to tubular as the number of atoms increased. For a solvent of dielectric constant equal to three, fluxionality was observed at 300 K for almost all clusters considered. The corresponding band gap was found to be ∼3 eV and relatively independent of the size of the clusters in this size range. The absorption coefficients (as calculated from the dielectric functions) obtained from fluxionality effects are in agreement with the experimental absorption profiles.

Periodic FTO IOs/CdS NRs/CdSe Clusters with Superior Light Scattering Ability for Improved Photoelectrochemical Performance.

Periodic fluorine-doped tin oxide inverse opals (FTO IOs) grafted with CdS nanorods (NRs) and CdSe clusters are reported for improved photoelectrochemical (PEC) performance. This hierarchical photoanode is fabricated by a combination of dip-coating, hydrothermal reaction, and chemical bath deposition. The growth of 1D CdS NRs on the periodic walls of 3D FTO IOs forms a unique 3D/1D hierarchical structure, providing a sizeable specific surface area for the loading of CdSe clusters. Significantly, the periodic FTO IOs enable uniform light scattering while the abundant surrounded CdS NRs induce additional random light scattering, combining to give multiple light scattering within the complete hierarchical structure, significantly improving light-harvesting of CdS NRs and CdSe clusters. The high electron collection ability of FTO IOs and the CdS/CdSe heterojunction formation also contribute to the enhanced charge transport and separation. Due to the incorporation of these enhancement strategies in one hierarchical structure, FTO IOs/CdS NRs/CdSe clusters present an improved PEC performance. The photocurrent density of FTO IOs/CdS NRs/CdSe clusters at 1.23 V versus reversible hydrogen electrode reaches 9.2 mA cm-2 , which is 1.43 times greater than that of CdS NRs/CdSe clusters and 3.83 times of CdS NRs.

Spectral shift, electronic coupling and exciton delocalization in nanocrystal dimers: insights from all-atom electronic structure computations.

Delocalization of excitons promoted by electronic coupling between clusters or quantum dots (QD) changes the dynamical processes in nanostructured aggregates enhancing energy transport. A spectroscopic shift of the absorption spectrum upon QD aggregation is commonly observed and ascribed to quantum mechanical coupling between neighbouring dots but also to exciton delocalization over the sulphur-based ligand shell or to other mechanisms as a change in the dielectric constant of the surrounding medium. We address the question of electronic coupling and exciton delocalization in nanocrystal aggregates by performing all-atom electronic structure calculations in models of colloidal QD dimers. The relation between spectral shift, interdot coupling and exciton delocalization is investigated in atomistic detail in models of dimers formed by CdSe clusters kept together by bridging organic ligands. Our results support the possibility of obtaining exciton delocalization over the dimer and point out the crucial role of the bridging ligand in enhancing interdot electronic coupling.

Size- and Charge-Dependent Optoelectronic Properties of CdSe Clusters: Evolution of the Optical Gap from Molecular to Bulk Behavior.

In the present work, the optical response of isolated (CdSe)n+ clusters with n = 3-6 is probed by measuring the photodissociation cross section in the photon energy range ℏω = 1.9-4.9 eV. In this joint experimental and theoretical study, the experimental observations are analyzed with time-dependent density functional theory and equation-of-motion coupled cluster theory. Structural candidates for the time-dependent excited-state calculations are obtained via global optimization by employing a genetic algorithm. The combined experimental and theoretical approach allows the discrimination of cluster geometries in the molecular beam experiments. From n ≥ 5, three-dimensional structures are found. Already for n = 6, light absorption in the red spectral range is observed. This observation is discussed with respect to the size dependence of the optical behavior of finite systems taking experimental and theoretical work on bare and ligated CdSe clusters and nanoparticles into account. Particularly, the influence of the net charge and ligands is considered. This allows a detailed discussion of the size-dependent evolution of the optical properties starting from molecular species over to nanoclusters and nanoparticles and finally to bulk CdSe.

Computational insights into structural and optical properties of P-containing and N-containing ligands capped CdSe clusters

ABSTRACTThe ligand effects on the structures and properties (energetics, binding energies, charge distribution and optical properties) of the (CdSe)n clusters (n = 3, 6, and 10) with P-containing (PH3, PH2Me, PHMe2 and PMe3) and N-containing (NH3, NH2Me, NHMe2 and NMe3) have been studied using density functional theory. The P atom and N atom in the ligands interact with Cd and form Cd–P and Cd–N bonds. The influence of P-containing ligands can be enhanced with increasing CH3 of ligands, while the N-containing ligands influence slightly change. A blueshift in absorption band was predicted for the clusters with increasing CH3 of P-containing ligands. We also found that the calculated binding energies for various ligands are found to decrease in the order PMe3 > NH2Me > NHMe2 > NH3 > NMe3 > PHMe2 > PH2Me > PH3. The use of hydrogen atom for modelling of the CdSe cluster passivating ligands is found to yield unphysical results as well.

Capping Structure of Ligand-Cysteine on CdSe Magic-Sized Clusters.

Ligand molecules capping on clusters largely affect the formation and stabilization mechanism and the property of clusters. In semiconductor CdSe clusters, cysteine is used as one of the ligands and allows the formation of ultrastable (CdSe)34 magic-sized clusters. Cysteine has sulfhydryl, amine, and carboxylate groups, all of which have coordination ability to the CdSe surface, and the bonding states of the three functional groups of ligand–cysteine on the CdSe core have not been determined. In this work, the capping structure of ligand–cysteine is examined by performing Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. FT-IR, XPS, and 1H, 13C, and 23Na magic-angle spinning NMR show that the sulfhydryl group of ligand–cysteine forms a sulfur–cadmium bond with a cadmium atom at the CdSe surface, while the carboxylate group does not contribute to the protection of the CdSe core and binds to a sodium ion contained as a counterion. 15N–{77Se} through-bond J-single quantum filtered NMR experiment reveals that the amine group of ligand–cysteine has no coordination to selenium atoms. By considering the N–Cd bond forming ratio (∼43%) revealed in our previous work, which is confirmed in this work by analyzing 13Cα signal intensity (∼42%), we concluded that cysteine capping on (CdSe)34 occurs in two ways: one involves both the sulfur–cadmium and nitrogen–cadmium bonds, and the other bears only the sulfur–cadmium bond.

Formation energies of CdSe wurtzoid and diamondoid clusters formed from Cd and Se atomic clusters

Cadmium Selenide thermodynamic formation energies at the molecular and nanoscale range are investigated using density functional theory. The investigation is performed using wurtzoid and diamondoid clusters that represent the wurtzite and zincblende structures at the molecular and nanoscale size range for a cluster number of atoms n ≤ 26. Cd and Se atomic clusters are optimized and used to provide component atomic cluster energies. Although both Cd and Se clusters at the nanoscale have different phases than bulk, the results show that Gibbs free energy, enthalpy, and entropy of formation of CdSe are close to their experimental bulk energies of formation within errors of experimental measurements. CdSe wurtzoids generally have higher absolute (more negative) Gibbs free energy of formation than CdSe diamondoids indicating more stable wurtzoid molecules which is also the case at bulk. The absolute Gibbs free energy of wurtzoids is also higher than experimental value (more negative) because of surface effects at the nanoscale. Enthalpy of formation indicates an exothermic reaction of Cd and Se clusters as is the case at bulk. The entropy of formation of all clusters is size-sensitive and converges towards bulk experimental measurements. Both wurtzoids and diamondoids members contain Cd13Se13 cluster which is the most investigated magic CdSe cluster.

Co2+-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions

Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (>21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)13 clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co-Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small-sized clusters.

Systematic Study of Structure, Stability, and Electronic Absorption of Tetrahedral CdSe Clusters with Carboxylate and Amine Ligands.

In this work, we carried out a systematic investigation to assess the effects of ligands on the structure, stability, and absorption spectra of ultrasmall CdSe tetrahedral quantum dots, where the cores of small tetrahedral quantum dots have been postulated to be stabilized by amine and carboxylate ligands. We found that amine and carboxylate ligands form extensive hydrogen bonding networks, which provide thermodynamic stability to the clusters. On the basis of the optimized structures, good agreement between observed and computed spectra was obtained. The ligands were also found to have a large influence on the color and intensity of the electronic absorption spectra, particularly for the small clusters, which were previously monitored with in situ UV-visible absorbance spectroscopy. Our work provides an understanding of the effect of ligands that influence thermodynamic stability and electronic absorption of ultrasmall quantum dots, thus potentially motivating further experimental exploration.

Development of effective stochastic potential method using random matrix theory for efficient conformational sampling of semiconductor nanoparticles at non-zero temperatures

The relationship between structure and property is central to chemistry and enables the understanding of chemical phenomena and processes. Need for an efficient conformational sampling of chemical systems arises from the presence of solvents and the existence of non-zero temperatures. However, conformational sampling of structures to compute molecular quantum mechanical properties is computationally expensive because a large number of electronic structure calculations are required. In this work, the development and implementation of the effective stochastic potential (ESP) method is presented to perform efficient conformational sampling of molecules. The overarching goal of this work is to alleviate the computational bottleneck associated with performing a large number of electronic structure calculations required for conformational sampling. We introduce the concept of a deformation potential and demonstrate its existence by the proof-by-construction approach. A statistical description of the fluctuations in the deformation potential due to non-zero temperature was obtained using infinite-order moment expansion of the distribution. The formal mathematical definition of the ESP was derived using the functional minimization approach to match the infinite-order moment expansion for the deformation potential. Practical implementation of the ESP was obtained using the random-matrix theory method. The developed method was applied to two proof-of-concept calculations of the distribution of HOMO-LUMO gaps in water molecules and solvated CdSe clusters at 300 K. The need for large sample size to obtain statistically meaningful results was demonstrated by performing 105 ESP calculations. The results from these prototype calculations demonstrated the efficacy of the ESP method for performing efficient conformational sampling. We envision that the fundamental nature of this work will not only extend our knowledge of chemical systems at non-zero temperatures but also generate new insights for innovative technological applications.

Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy.

Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long-standing goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.

Theoretical characterization on the size-dependent electron and hole trapping activity of chloride-passivated CdSe nanoclusters

Ligand passivation is often used to suppress the surface trap states of semiconductor quantum dots (QDs) for their continuous photoluminescence output. The suppression process is related to the electrophilic/nucleophilic activity of surface atoms that varies with the structure and size of QD and the electron donating/accepting nature of ligand. Based on first-principles-based descriptors and cluster models, the electrophilic/nucleophilic activities of bare and chloride-coated CdSe clusters were studied to reveal the suppression mechanism of Cl-passivated QDs and compared to experimental observations. The surface atoms of bare clusters have higher activity than inner atoms and their activity decreases with cluster size. In the ligand-coated clusters, the Cd atom remains as the electrophilic site, while the nucleophilic site of Se atoms is replaced by Cl atoms. The activities of Cd and Cl atoms in the coated clusters are, however, remarkably weaker than those in bare clusters. Cluster size, dangling atoms, ligand coverage, electronegativity of ligand atoms, and solvent (water) were found to have considerable influence on the activity of surface atoms. The suppression of surface trap states in Cl-passivated QDs was attributed to the reduction of electrophilic/nucleophilic activity of Cd/Se/Cl atoms. Both saturation to under-coordinated surface atoms and proper selection for the electron donating/accepting strength of ligands are crucial for eliminating the charge carrier traps. Our calculations predicted a similar suppressing effect of chloride ligands with experiments and provided a simple but effective approach to assess the charge carrier trapping behaviors of semiconductor QDs.

Annealing temperature dependence of structural, optical, and thermal properties of CdSe thin films grown on porous anodic alumina

In this study, cadmium selenide (CdSe) were synthesizd by a chemical route process using l-cysteine (l-cyst) as a ligand, cadmium acetate as a Cd source and Na2SeO3 as a Se precurseur. CdSe nanocrystals were embedded in porous anodic alumina (PAA) layer by the chemical bath deposition (CBD) method. The influence of annealing temperature in air-atmosphere from 150 to 350 °C on the microstructural, optical properties, and thermal conductivity of CdSe in PAA template has been studied by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, UV–Vis and photothermal deflection (PTD) techniques. The annealing temperature at 300 and 350 °C resulted in a considerable change in the surface morphology of CdSe film on the PAA layer, from a nonhomgeneous CdSe clusters into microrods array with a pyramidal ends. Raman spectroscopy measurements show an enhancement in the crystallinity of CdSe film on PAA layer and an increase of the average radius size of the nanocrystallites from 2.41 to 2.78 nm with the increase of air-annealing temperature. The optical reflectivity result of CdSe on PAA template showed that the crystal quality was significantly improved after the annealing step, bringing about an enhancement in optical absorption and a decrease in bandgap energy. The thermal conductivity Kth was studied via PTD technique, and the Kth increase was depicted with thermal treatement from 4.40 to 5.22 W M−1 K−1.

A heat capacity model of T3/2 dependence for quantum dots.

This article addresses the heat capacity of quantum dots (QDs) using density functional theory (DFT). By analyzing the evolution of phonon density of states and heat capacity as CdSe clusters grow from a molecular cluster into larger quantum confined solids, we have shown that their heat capacity does not fit very well with the Debye T3 model. We observed that the number of phonon modes, which is discrete, increases as the particles grow, and the dispersion relation shows a quadratic behavior in contrast to the bulk solids whose dispersion relation is linear and equal to the sound velocity. The phonon density of states showed a square root variation with respect to frequency whereas that of the bulk is a quadratic variation of frequency. From the observed variation in the phonon density of states and holding the fact that the atomic vibrations in solids are elastically coupled, we have re-derived the expression for total energy of the QDs and arrive at a T3/2 model of heat capacity, which fits very well to the observed heat capacity data. These results give promising directions in the understanding of the evolution of the thermophysical properties of solids.

Ion Exchange Transformation of Magic-Sized Clusters

Ultrasmall semiconductor clusters are exciting materials because of their molecularly precise structures and their unique optical spectra. “Magic-sized” CdSe clusters are transformed into their Cu2Se counterparts by means of ion exchange. We leverage the molecularly precise structure and high sensitivity of these clusters to investigate the mechanism of cation exchange. We optically identify a metastable intermediate in the solid-state transformation. Isolation and characterization of this intermediate provide insight into the dynamic structural rearrangement of the cationic sublattice in the course of cation exchange and the role of ligand passivation. Such understanding of the dynamics of ion exchange at the solid–liquid interface could help engineer improved materials for solid-state electrolytes and energy storage devices.

Computational Study of the Resonance Enhancement of Raman Signals of Ligands Adsorbed to CdSe Clusters through Photoexcitation of the Cluster

This paper describes density-functional-theory-based computations of resonance Raman (RR) spectra of ligand molecules adsorbed to the surface of a Cd16Se13 cluster. Signals from asymmetric vibrational modes of ligand binding groups, such as the asymmetric O–C–O stretching modes of carboxylates, are enhanced relative to the symmetric vibrational modes when the excitation energy is on-resonance with the excitonic energy of the cluster. Certain ligand molecules have frontier orbitals with the correct energies and symmetries to mix with the orbitals of the CdSe cluster, and as a result, the wave functions of the electron and the hole delocalize from the cluster onto the ligand molecules; experimentally, this delocalization results in a bathochromic shift of the band edge excitonic absorption. Increased excitonic delocalization results in greater vibronic coupling between the exciton and the ligand vibrations and, on average, preferential enhancements in the RR signals of those vibrations. This work suggests t...

Computational Study of Magic-Size CdSe Clusters with Complementary Passivation by Carboxylic and Amine Ligands

The electronic and optical properties of tetrahedral CdSe magic clusters (average diameter ∼1.5 nm) protected by carboxyl and amine ligands, which correspond to previously reported experimental structures, are studied using density functional theory. We find extreme ligand packing densities, capping every single dangling bond of the inorganic core, strong dependence of the Z-type metal carboxylate binding on the amount of excess amine, and potential for improved photoluminescence upon replacing phenyl ligands with alkanes. The computed absorption spectra of the Cd35Se20 cluster agree well with experiments, resolving the 0.2 eV splitting of the first exciton peak due to spin–orbit coupling. We discuss the origin of the significant broadening of the optical spectra as due to phonons and structural variations in the ligand configurations and inorganic core apexes.