Computational Prediction of Structures and Optical Excitations for Nanoscale Ultrasmall ZnS and CdSe Clusters

Influence of Capping Ligands, Solvent, and Thermal Effects on CdSe Quantum Dot Optical Properties by DFT Calculations

Cadmium selenide nanomaterials are very important materials in photonics, catalysis, and biomedical applications due to their optical properties that can be tuned through size, shape, and surface passivation. In this report, static and ab initio molecular dynamics density functional theory (DFT) simulations are used to characterize the effect of ligand adsorption on the electronic properties of the (110) surface of zinc blende and wurtzite CdSe and a (CdSe)₃₃ nanoparticle. Adsorption energies depend on ligand surface coverage and result from a balance between chemical affinity and ligand-surface and ligand-ligand dispersive interactions. In addition, while little structural reorganization occurs upon slab formation, Cd···Cd distances become shorter and the Se-Cd-Se angles become smaller in the bare nanoparticle model. This originates mid-gap states that strongly influence the absorption optical spectra of nonpassivated (CdSe)₃₃. Ligand passivation on both zinc blende and wurtzite surfaces does not induce a surface reorganization, and thus, the band gap remains nonaffected with respect to bare surfaces. In contrast, structural reconstruction is more apparent for the nanoparticle, which significantly increases its highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap upon passivation. Solvent effects decrease the band gap difference between the passivated and nonpassivated nanoparticles, the maximum of the absorption spectra being blue-shifted around 20 nm by the effect of the ligands. Overall, calculations show that flexible surface cadmium sites are responsible for the appearance of mid-gap states that are partially localized on the most reconstructed regions of the nanoparticle that can be controlled through appropriate ligand adsorption.

Density Functional Study to Investigate the Ability of (ZnS)n (n = 1-12) Clusters Removing Hg0, HgCl, and HgCl2 via Electron Localization Function and Non-Covalent Interactions Analyses

In this study, the density functional theory is used to study the ability of (ZnS)n clusters to remove Hg0, HgCl, and HgCl2 and reveals that they can be absorbed on (ZnS)n clusters. According to electron localization function (ELF) and non-covalent interactions (NCI) analyses, the adsorption of Hg0 on (ZnS)n is physical adsorption and the adsorption ability of (ZnS)n for removing Hg0 is weak. When (ZnS)n adsorbs HgCl and HgCl2, two new Hg-S and Zn-Cl bonds form in the resultant clusters. An ELF analysis identifies the formation of Hg-S and Zn-Cl bonds in (ZnS)nHgCl and (ZnS)nHgCl2. A partial density of states and charge analysis confirm that as Hg0, HgCl, and HgCl2 approach (ZnS)n clusters, atomic orbitals in Hg and Zn, Hg and S, as well as Zn and Cl overlap and hybridize. Adsorption energies of HgCl and HgCl2 on (ZnS)n clusters are obviously bigger than those of Hg0, indicating that HgCl and HgCl2 adsorption on (ZnS)n clusters is much stronger than that of Hg0. By combining ELF analysis, NCI analysis, and adsorption energies, the adsorption of HgCl, and HgCl2 on (ZnS)n clusters can be classified as chemical adsorption. The adsorption ability of (ZnS)n clusters for removing HgCl and HgCl2 is higher than that of Hg0.

Defective Nature of CdSe Quantum Dots Embedded in Inorganic Matrices

Quantum dots (QDs) embedded in inorganic matrices have been extensively studied for their potential applications in lighting, displays, and solar cells. While a significant amount of research studies focused on their experimental fabrication, the origin of their relatively low photoluminescence quantum yield has not been investigated yet, although it severely hinders practical applications. In this study, we use time-dependent density functional theory (TDDFT) to pinpoint the nature of excited states of CdSe QDs embedded in various inorganic matrices. The formation of undercoordinated Se atoms and nonbridging oxygen atoms at the QD/glass interface is responsible for the localization of a hole wave function, leading to the formation of low-energy excited states with weak oscillator strength. These states provide pathways for nonradiative processes and compete with radiative emission. The photoluminescence performance is predicted for CdSe QDs in different matrices and validated by experiments. The results of this study have significant implications for understanding the underlying photophysics of CdSe QDs embedded in inorganic matrices that would facilitate the fabrication of highly luminescent glasses.

Theoretical studies on the two-photon absorption of II-VI semiconductor nano clusters

Semiconductor clusters, Zn_nO_n, Zn_nS_n, and Cd_nS_n (n = 2–8), were optimized and the corresponding stable structures were acquired. The symmetry, bond length, bond angle, and energy gap between HOMO and LUMO were analyzed. According to reasonable calculation and comparative analysis for small clusters Zn₂O₂, Zn₂S₂, and Cd₂S₂, an effective method based on density function theory (DFT) and basis set which lay the foundation for the calculation of the large clusters have been obtained. The two-photon absorption (TPA) results show that for the nano clusters with planar configuration, sizes play important role on the TPA cross section, while symmetries determine the TPA cross section under circumstance of 3D stable structures. All our conclusions provide theoretical support for the development of related experiments.

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.

Interaction of H2O with (CuS)n, (Cu2S)n, and (ZnS)n small clusters (n = 1-4, 6): relation to the aggregation characteristics of metal sulfides at aqueous solutions

The interaction of H₂O onto small CuS, Cu₂S, and ZnS clusters was theoretically studied by Density Functional Theory computations to get insights into the aggregation characteristics of metal sulfides at aqueous solutions. The results show the charge-controlled interactions with polarized solvent molecules are favored on the ZnS clusters compared with CuS and Cu₂S clusters. Moreover, the chemical adsorption of H₂O molecules is energetically favored onto ZnS clusters with higher interaction energies of up to 35.4 kcal/mol compared with CuS and Cu₂S clusters (up to 31.3 kcal/mol), where the stability of H₂O adsorption decreases as the size of the clusters increases. However, thermochemical analysis shows that the adsorption of H₂O on copper sulfides is not a spontaneous process at room temperature. Additionally, the electrostatic energy of H₂O onto the Cu₂S and CuS clusters is lower than that associated with the H₂O-H₂O interactions, suggesting that copper precipitates prefer to bind between them at early stages of the precipitation process due to an unfavorable solvent-solute interaction. Dispersion forces play a relative key role in the interaction of water on copper sulfides, while for zinc sulfide clusters, the adsorption energy is slightly influenced by dispersion contributions. Accordingly, the aggregation of zinc sulfides in a water environment is expected to be lower compared with copper sulfides, and where the aggregation characteristics are not determined by the binding energy of the sulfides, but of the ability to interact with the solvent molecules. These statements were confirmed by experimental optical microscopy analysis and settling tests during precipitation processes in water. Therefore, this work allows proposing a simple strategy to study the aggregation characteristics of metal sulfides, which turns useful for use in hydrometallurgical applications.

Surface Chemistry of Semiconducting Quantum Dots: Theoretical Perspectives

Colloidal quantum dots (QDs) are near-ideal nanomaterials for energy conversion and lighting technologies. However, their photophysics exhibits supreme sensitivity to surface passivation and defects, of which control is problematic. The role of passivating ligands in photodynamics remains questionable and is a focus of ongoing research. The optically forbidden nature of surface-associated states makes direct measurements on them challenging. Therefore, computational modeling is imperative for insights into surface passivation and its impact on light-driven processes in QDs. This Account discusses challenges and recent progress in understanding surface effects on the photophysics of QDs addressed via quantum-chemical calculations. We overview different methods, including the effective mass approximation (EMA), time-dependent density functional theory (TDDFT), and multiconfiguration approaches, considering their strengths and weaknesses relevant to modeling of QDs with a complicated surface. We focus on CdSe, PbSe, and Si QDs, where calculations successfully explain experimental trends sensitive to surface defects, doping, and ligands. We show that the EMA accurately describes both linear and nonlinear optical properties of large-sized CdSe QDs (>2.5 nm), while TDDFT is required for smaller QDs where surface effects dominate. Both approaches confirm efficient two-photon absorption enabling applications of QDs as nonlinear optical materials. TDDFT also describes the effects of morphology on the optical response of QDs: the photophysics of stoichiometric, magic-sized X_nY_n (X = Cd, Pb; Y = S, Se) QDs is less sensitive to their passivation compared with nonstoichiometric X_n≠mY_m QDs. In the latter, surface-driven optically inactive midgap states can be eliminated by anionic ligands, explaining the better emission of metal-enriched QDs compared with nonmetal-enriched QDs. Ideal passivation of magic-sized QDs by amines and phosphine oxides leaves lower-energy transitions intact, while thiol derivatives add ligand-localized trap states to the band gap. Depending on its position, any loss of ligand from the QD's surface also introduces electron or hole traps, decreasing the QD's luminescence. TDDFT investigations of QD-ligand and QD-QD interactions provide an explanation of experimentally detected enhancement of blinking on-times in closely packed Si QDs and establish favorable conditions for hole transfer from the photoexcited CdSe QD to metal-organic dyes. While TDDFT well describes qualitative trends in optical response to stoichiometry and ligand modifications of QDs, it is unable to calculate highly correlated electronic states like biexcitons and magnetic-dopant-derived states. In these cases, multiconfiguration methods are applied to small nanoclusters and the results are extrapolated to larger-sized QDs, providing reasonable explanations of experimental observables. For light-driven dynamics, the electron-phonon couplings are important, and nonadiabatic dynamics (NAD) is applied. NAD based on first-principles calculations is a current grand challenge for the theory. However, it can be accomplished through sets of semiclassical approximations such as surface hopping (SH). We discuss validations of approximations used in photodynamics of ligated and doped QDs. Time-domain DFT-based SH-NAD reveals the ligand's role in ultrafast energy relaxation and the connection between the phonon bottleneck and the Zeno effect in CdSe QDs. The calculated results are helpful in controlling both dissipation and radiative processes in QDs via surface engineering and in explanations of experimental data.

Theoretical analysis of structures and electronic spectra in molecular cadmium chalcogenide clusters

We present calculated structural and optical properties of molecular cadmium chalcogenide nonstoichiometric clusters with a size range of less than 1 nm to more than 2 nm with well-defined chemical compositions and structures in comparison to experimental characterization and previous theoretical work. A unified treatment of these clusters to obtain a fundamental understanding of the size, ligand, and solvation effects on their optical properties has not been heretofore presented. The clusters belong to three topological classes, specifically supertetrahedral (Tn), penta-supertetrahedral (Pn), and capped supertetrahedral (Cn), where n is the number of metal layers in each cluster. The tetrahedrally shaped Tn clusters examined in this work are Cd(ER)4(2-) (T1), Cd4(ER)10(2-) (T2), and Cd10E4 (')(ER)16(4-) (T3), where R is an organic group, E and E' are chalcogen atoms (sulfur or selenium). The first member of the Pn series considered is M8E'(ER)16(2-). For the Cn series, we consider the first three members, M17E4 (')(ER)28(2-), M32E14 (')(ER)36L4, and M54E32 (')(ER)48L4(4-) (L = neutral ligand). Mixed ligand clusters with capping ER groups replaced by halogen or neutral ligands were also considered. Ligands and solvent were found to have a large influence on the color and intensity of the electronic absorption spectra of small clusters. Their effects are generally reduced with increasing cluster sizes. Blueshifts were observed for the first electronic transition with reduced size for both cadmium sulfide and cadmium selenide series. Due to weakly absorbing and forbidden transitions underlying the one-photon spectra, more care is needed in interpreting the quantum confinement from the clusters' lowest-energy absorption bands.

Origin of photoactivity in graphitic carbon nitride and strategies for enhancement of photocatalytic efficiency: insights from first-principles computations

Our computations suggest that CdSe-g-C₃N₄ and WTe₂-g-C₃N₄ heterostructured nanosheets are efficient for improving photocatalytic activities of g-C₃N₄.

A study of optical absorption of cysteine-capped CdSe nanoclusters using first-principles calculations

Optical absorption of cysteine-capped CdSe nanoclusters varies with cluster size, ligands and solvents.

Two-photon absorption in CdSe colloidal quantum dots compared to organic molecules

We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models.

Atomic structures and gram scale synthesis of three tetrahedral quantum dots

Luminescent semiconducting quantum dots (QDs) are central to emerging technologies that range from tissue imaging to solid-state lighting. However, existing samples are heterogeneous, which has prevented atomic-resolution determination of their structures and obscured the relationship between their atomic and electronic structures. Here we report the synthesis, isolation, and structural characterization of three cadmium selenide QDs with uniform compositions (Cd35Se20(X)30(L)30, Cd56Se35(X)42(L)42, Cd84Se56(X)56(L)56; X = O2CPh, L = H2N-C4H9). Their UV-absorption spectra show a lowest energy electronic transition that decreases in energy (3.54 eV, 3.26 eV, 3.04 eV) and sharpens as the size of the QD increases (fwhm = 207 meV, 145 meV, 115 meV). The photoluminescence spectra of all three QDs are broad with large Stokes shifts characteristic of trap-luminescence. Using a combination of single-crystal X-ray diffraction and atomic pair distribution function analysis, we determine the structures of their inorganic cores, revealing a series of pyramidal nanostuctures with cadmium terminated {111} facets. Theoretical and experimental studies on these materials will open the door to a deeper fundamental understanding of structure-property relationships in quantum-confined semiconductors.

Theoretical prediction of low-density nanoporous frameworks of zinc sulfide based on ZnnSn (n = 12, 16) nanocaged clusters

Low-density ZnS framework materials with novel structures and properties based on highly stable Zn_nS_n (n = 12, 16) clusters are designed and investigated.

Ligand induced circular dichroism and circularly polarized luminescence in CdSe quantum dots

Chiral thiol capping ligands L- and D-cysteines induced modular chiroptical properties in achiral cadmium selenide quantum dots (CdSe QDs). Cys-CdSe prepared from achiral oleic acid capped CdSe by postsynthetic ligand exchange displayed size-dependent electronic circular dichroism (CD) and circularly polarized luminescence (CPL). Opposite CPL signals were measured for the CdSe QDs capped with D- and L-cysteine. The CD profile and CD anisotropy varied with size of CdSe nanocrystals with largest anisotropy observed for CdSe nanoparticles of 4.4 nm. Magic angle spinning solid state NMR (MAS ssNMR) experiments suggested bidentate interaction between cysteine and the surface of CdSe. Time Dependent Density Functional Theory (TDDFT) calculations verified that attachment of L- and D-cysteine to the surface of model (CdSe)13 nanoclusters induces measurable opposite CD signals for the exitonic band of the nanocluster. The origin of the induced chirality is consistent with the hybridization of highest occupied CdSe molecular orbitals with those of the chiral ligand.