Shell effects on the dielectric properties of core-shell quantum dots.

The dielectric properties in semiconductor quantum dots are crucial for exciton formation, migration, and recombination. Different from 3D bulk materials, the dielectric response is, however, ambiguous for the small-sized 0D dots in which the effect of outer atoms on the inner atoms is usually described qualitatively. Based on the first-principles calculated electron density, the polarizability of the core-shell CdSe@ZnS wurtzite quantum dots is decomposed into the distributional contributions among which the dipole polarizability of the core is proposed to measure the shell effect on the dielectric properties of core-shell quantum dots. The shell thickness dependence on the shell effect is then studied, which is significant for the outermost shell but decays rapidly in the additional shells. Moreover, this model gives explicit physical origins of the core dipole polarizability in the core-shell QDs, which is determined by the intra-shell polarization and inter-core-shell charge transfer. Our study proposes a new approach for studying the dielectric properties of core-shell quantum dots, which is effective and extendable for other low-dimensional structures.

Comparative Study of NO and CO Oxidation Reactions on Single-Atom Catalysts Anchored Graphene-like Monolayer.

Noble metal single-atom catalysts (NM-SACs) anchored at novel graphene-like supports has attracted enormous interests. Gas sensitivity, catalytic activity, and d-band centers of single NM (Pt and Pd) atoms at graphenylene (graphenylene-NM) are investigated using first-principle calculations. The adsorption geometries of gas reactants on graphenylene-NM sheets are analyzed. It is found that the adsorption energies of reactant species on graphenylene-Pt are larger than those on graphenylene-Pd, because the d-band center of the Pt atom is closeser to the Fermi level. The NO and CO oxidation reactions on graphenylene-NM are investigated via four catalytic mechanisms, including Langmuir-Hinshelwood (LH), Eley-Rideal (ER), New ER (NER), and termolecular ER (TER). The results show that the NO and CO oxidations via LH and TER mechanisms can occur owing to the relatively small energy barriers. Moreover, the interaction of 2NO+2CO via ER mechanism is the energetically more favorable reaction. Although the NO oxidation via the NER mechanism has rather low energy barriers, the reaction is unlikely to occur due to the low adsorption energy of O2 compared with CO and NO. This research may provide guidance for exploring the catalytic performance of SACs on graphene-like materials to remove toxic gas molecules.

Effect of Mn doping on the electron injection in CdSe/TiO2 quantum dot sensitized solar cells.

Promotion in power conversion efficiency is an appealing task for quantum dot-sensitized solar cells that have emerged as promising materials for the utilization of clean and sustainable energy. Doping of Mn atoms into quantum dots (QD) has been proven to be one of the effective approaches, although the origin of such a promotion remains controversial. While several procedures are involved in the power conversion process, electron injection from the QD to the semiconductor oxide substrate is focused on in this work using first-principles calculations. Based on the Marcus theory, the electron injection rates are evaluated for the quantum dot-sensitized solar cell models in which the pure and Mn-doped core-shell CdSe clusters are deposited on a semiconductor titanium dioxide substrate. Enhanced rates are obtained for the Mn-doped structure, which is in qualitative agreement with the experiments. A large number of dominant injection channels and strong QD-substrate coupling are responsible for the Mn-induced rate enhancement, which could be achieved by manipulating the band structure mapping between the QD and the semiconductor oxide. By addressing the role of an Mn dopant in the electron injection process, strategies for the promotion of electron injection rates are proposed for the design of quantum dot-sensitized solar cells.

Pressure-induced reconstructive phase transitions, polarization with metallicity, and enhanced hardness in antiperovskite MgCNi3.

In general, hydrostatic pressure can suppress electrical polarization, instead of creating and/or enhancing polarization like strain engineering. Here, a combination of first-principles calculations and CALYPSO crystal structures prediction is used to point out that hydrostatic pressure applied on antiperovskite MgCNi3 can stabilize polarization with metallicity, and thus a polar metal can exist under high pressure. Strikingly, the metallic polar phase of MgCNi3 exhibits an original linear-cubic coupling between polar and nonpolar modes, resulting in an asymmetrical double-well when the polarization is switched. Moreover, another novel phase of MgCNi3 under high pressure possesses an enhanced hardness stemming from a robust s-s electrons interaction of an unexpected C-C bond, rather than typical sp3 orbital hybridization. These discoveries open new routes to design superhard materials and polar metals.

Single-atom metal-modified graphenylene as a high-activity catalyst for CO and NO oxidation.

Herein, the adsorption behaviors and interactions of different gas species on single-metal atom-anchored graphenylene (M-graphenylene, M = Mn, Co, Ni, and Cu) sheets were investigated by first-principles calculations. At first, the single metal atom tends to adsorb on the hollow or bridge site of graphenylene, and the formed M-graphenylene systems exhibit varied magnetic properties. The reactants (NO, CO, O2, O, CO2, and NO2) adsorbed on the Mn-, Co-, and Ni-graphenylene sheets exhibit higher stability than those adsorbed on the Cu-graphenylene sheet. Moreover, the co-adsorption configurations of NO-O2, CO-O2, 2NO, and 2CO on the M-graphenylene sheets were comparably studied, which are considered as the initial states for NO and CO oxidation. It was found that the energy barriers for the formation of OONO and OOCO complexes on Mn-graphenylene by the Langmuir-Hinshelwood (LH) mechanism are larger than those in the case of Co-graphenylene (<0.4 eV). The possible reactions for the oxidation of 2CO by the 2NO molecules on the M-graphenylene sheets were also considered, because of the adsorbed NO molecules are more stable than the CO and O2 molecules. Furthermore, the energy barrier for the oxidation of CO on Mn-graphenylene via the Eley-Rideal (ER) mechanism (2NO + 2CO → 2CO2 + N2) is smaller (<0.3 eV) than those in the cases of other substrates. These results illustrate that the single-metal atom-modified graphenylene can be used as a potential novel carbon-based catalyst with high activity.

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.

Large polarization and dielectric response in epitaxial SrZrO3 films.

First-principles calculations are performed to investigate the ferroelectric and dielectric properties of (001) epitaxial SrZrO3 thin films under misfit strain. A rich phase diagram is predicted. By condensing the polar instability, the ferroelectric Pmc21 and Ima2 phases can coexist under tensile strain (about 3.7%-5.2%/5.7%). Combining in-plane ferroelectric (FExy) and out-of-plane in-phase antiferrodistortive (IAFDz) modes, another new Pmc21 state (P > 56 µC cm(-2)) occurs with increase in the tensile strain. The paraelectric I4/mcm and ferroelectric P4mm phases emerge around -3.2%/-3.7% and -6.4%/-7.4% compressive strain, respectively. The former exhibits an intense out-of-plane dielectric response, while the latter possesses a rather large polarization (∼ 110 µC cm(-2)). The large polarization and dielectric response are discussed in relationship to strain-driven structural distortion.

Core-shell interaction and its impact on the optical absorption of pure and doped core-shell CdSe/ZnSe nanoclusters.

The structural, electronic, and optical properties of core-shell nanoclusters, (CdSe)(x)@(CdSe)(y) and their Zn-substituted complexes of x = 2-4 and y = 16-28, were studied with density functional theory calculations. The substitution was applied in the cores, the shells, and/or the whole clusters. All these clusters are characterized by their core-shell structures in which the core-shell interaction was found different from those in core or in shell, as reflected by their bondlengths, volumes, and binding energies. Moreover, the core and shell combine together to compose a new cluster with electronic and optical properties different from those of separated individuals, as reflected by their HOMO-LUMO gaps and optical absorptions. With the substitution of Cd by Zn, the structural, electronic, and optical properties of clusters change regularly. The binding energy increases with Zn content, attributed to the strong Zn-Se bonding. For the same core/shell, the structure with a CdSe shell/core has a narrower gap than that with a ZnSe shell/core. The optical absorption spectra also change accordingly with Zn substitution. The peaks blueshift with increasing Zn concentration, accompanying with shape variations in case large number of Cd atoms are substituted. Our calculations reveal the core-shell interaction and its influence on the electronic and optical properties of the core-shell clusters, suggesting a composition-structure-property relationship for the design of core-shell CdSe and ZnSe nanoclusters.

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

Understanding the size-dependent structures and properties of ligand-capped nanoclusters in solvent is of particular interest for the design, synthesis and application of II-VI colloidal QDs. Using DFT and TDDFT calculations, we studied the structure and optical property evolution of the cysteine-capped (CdSe)N clusters of N = 1-10, 13, 16 and 19 in gas, toluene, water and alkaline aqueous solution, and made a comparison with their corresponding bare clusters. The cysteine binds with (CdSe)Nvia several patterns depending on the medium they exist in, affecting the cluster structures and in consequence their optical absorption. In general, the absorption bands of (CdSe)N blueshift when cysteine is added, and the shift varies with the interaction strength between the cluster and the ligand, and the dielectric constant of the solvent. However, bare clusters retain their size sensitivity, in particular the redshift trend with increasing cluster size, and some similarity was noted for the optical absorption of the bare and ligated clusters regardless of the gas or solvent media. Population analysis reveals that the excitations are mainly from orbitals distributing on the (CdSe)N part, while the ligand is negligibly involved in the excitations. This is an important feature for the II-VI QDs as biosensors with which the information of biomolecules is detected from the size dependent optical absorption or emission of the QDs other than the biomolecules.

Strain-Induced Structural Phase Transitions in Epitaxial (001) BiCoO3 Films: A First-Principles Study.

We have simulated BiCoO3 films epitaxially grown along (001) direction with density functional theory computations. Leading candidates for the lowest-energy phases have been identified. The tensile strains induce magnetic phase transition in the ground state (P4mm symmetry) from a C-type antiferromagnetic order to a G-type order for the in-plane lattice parameter above 3.922 Å. The G-type antiferromagnetic order will be maintained with larger tensile strains; however, a continuous structural phase transition will occur, combining the ferroelectric and antiferrodistortive modes. In particular, the larger tensile strain allows an isostructural transition, the so-called Cowley's ''Type Zero'' phase transitions, from Cc-(I) to Cc-(II), with a slight volume collapse. The orientation of ferroelectric polarization changes from the out-of-plane direction in the P4mm to the in-plane direction in the Pmc21 state under epitaxial tensile strain; meanwhile, the magnetic ordering temperature TN can be strikingly affected by the variation of misfit strain.

Laser-Driven Insulator-Metal Phase Transitions in CsPbI3 Quantum Dots and Influence of Doped Metal Nanowires.

All-inorganic CsPbI3 perovskite quantum dots (QDs) have received extensive attention in developing optoelectronic devices due to their outstanding properties. Here, using time-dependent density functional theory (TDDFT), the optical properties of the three distinct phases (α, γ, and δ) of the CsPbI3 QDs are investigated. Surprisingly, the δ phase structured QDs exhibit stronger optical absorption properties than the α and γ phase QDs when exposed to equivalent laser irradiation. Considering the quantum size effect, size regulation is also performed on the three structures, the results reveal a significant improvement in optical properties as the size increases in the direction of laser irradiation. More interestingly, Ag-hybrid QDs show better optical gain and maintain a laser-driven metallic state. Our results demonstrate the great potential of size adjustment and metal nanowire coupling in improving the optoelectronic properties of QDs and developing efficient photovoltaic devices.

Interlayer electron flow and field shielding in twisted trilayer graphene quantum dots.

While multilayer graphene (MLG) possesses excellent intralayer electron mobility, its interlayer electrical conductance exhibits great diversity that results in exotic phenomena and various applications in electronic devices. Driven by a vertical electric field, electron flow occurs across the layers, and its current is tunable by controlling the interlayer stacking and distance, disc size and field strength. The electron rearrangement induced by the external field is appropriately described by the polarizability that measures the electronic response against the applied field. Based on the field-induced electron density variations computed with a first-principles approach, a polarizability decomposition scheme is developed in this work to isolate the inter- and intra-layer contributions from the total polarizability of twisted trilayer graphene (TTG) quantum dots. The inter- and intra-layer counterparts reflect the charge transfer (CT) and field shielding effects among the layers, respectively. Shielded by the top and bottom layers, the middle layer is particularly effective in bridging, switching and promoting the interlayer electron flow. Large CT and shielding effects occur not only in the strongly coupled Bernal stacking, but also in the structures misorientating from the full-AAA stacking by a small twist angle. Moreover, both effects vary with the twist angle and disc size, indicating a controllable conductive/dielectric conversion in the vertical direction. In light of inter- and intralayer polarizability, our study addresses the precise modulation of interlayer conductance for TTG quantum dots, which is required in the microstructure design and performance manipulation of MLG-based electronic devices.

Interfacial interaction-driven rheological properties of quartz nanofluids from molecular dynamics simulations and density functional theory calculations.

Correlations of the shear viscosity of quartz nanofluids with particle concentration, particle size, and temperature were investigated with molecular dynamics simulations and density functional theory (DFT) calculations. A new understanding to the experimentally concluded correlations was addressed in terms of microscopic particle-water interfacial interaction in three aspects. First, the viscosity of quartz nanofluids at different particle concentrations, particle sizes, and temperatures were simulated using the equilibrium molecular dynamics simulations method to reproduce the experimental observations. At the same particle size, the nanofluid viscosity decreases significantly with temperature and increases with nanoparticle volume concentration, and at the same volume concentration, the nanofluid viscosity increases with the decrease of particle size. Second, DFT calculations confirm a stronger particle-water interaction than that among water molecules. The important role of particle-water interaction in the viscosity determination of nanofluids was revealed. Finally, a correlation was proposed to fit the simulated results and compared with earlier two-parameter correlations. One parameter in the correlation is indeed a constant, while the other is a function of SiO2-water interaction energy. Our study proposes a physical basis for the experimentally concluded correlations on the viscosity of nanofluids.

Quantum Chemistry Study on the Structures and Electronic Properties of Bimetallic Ca2-Doped Magnesium Ca2Mgn (n = 1-15) Clusters.

Here, by utilizing crystal structure analysis through the particle swarm optimization (CALYPSO) structural searching method with density functional theory (DFT), we investigate the systemic structures and electronic properties of Ca2Mgn (n = 1-15) clusters. Structural searches found that two Ca atoms prefer to occupy the external position of magnesium-doped systems at n = 2-14. Afterward, one Ca atom begins to move from the surface into the internal of the caged skeleton at n = 15. Calculations of the average binding energy, second-order difference of energies, and HOMO-LUMO gaps indicated that the pagoda construction Ca2Mg8 (as the magic cluster) has higher stability. In addition, the simulated IR and Raman spectra can provide theoretical guidance for future experimental and theoretical investigation. Last, further electronic properties were determined, including the charge transfer, density of states (DOS) and bonding characteristics. We hope that our work will provide theoretical and experimental guidance for developing magnesium-based nanomaterials in the future.

Wheat growth monitoring and yield estimation based on remote sensing data assimilation into the SAFY crop growth model.

Crop growth monitoring and yield estimate information can be obtained via appropriate metrics such as the leaf area index (LAI) and biomass. Such information is crucial for guiding agricultural production, ensuring food security, and maintaining sustainable agricultural development. Traditional methods of field measurement and monitoring typically have low efficiency and can only give limited untimely information. Alternatively, methods based on remote sensing technologies are fast, objective, and nondestructive. Indeed, remote sensing data assimilation and crop growth modeling represent an important trend in crop growth monitoring and yield estimation. In this study, we assimilate the leaf area index retrieved from Sentinel-2 remote sensing data for crop growth model of the simple algorithm for yield estimation (SAFY) in wheat. The SP-UCI optimization algorithm is used for fine-tuning for several SAFY parameters, namely the emergence date (D0), the effective light energy utilization rate (ELUE), and the senescence temperature threshold (STT) which is indicative of biological aging. These three sensitive parameters are set in order to attain the global minimum of an error function between the SAFY model predicted values and the LAI inversion values. This assimilation of remote sensing data into the crop growth model facilitates the LAI, biomass, and yield estimation. The estimation results were validated using data collected from 48 experimental plots during 2014 and 2015. For the 2014 data, the results showed coefficients of determination (R2) of the LAI, biomass and yield of 0.73, 0.83 and 0.49, respectively, with corresponding root-mean-squared error (RMSE) values of 0.72, 1.13 t/ha and 1.14 t/ha, respectively. For the 2015 data, the estimated R2 values of the LAI, biomass, and yield were 0.700, 0.85, and 0.61, respectively, with respective RMSE values of 0.83, 1.22 t/ha, and 1.39 t/ha, respectively. The estimated values were found to be in good agreement with the measured ones. This shows high applicability of the proposed data assimilation scheme in crop monitoring and yield estimation. As well, this scheme provides a reference for the assimilation of remote sensing data into crop growth models for regional crop monitoring and yield estimation.