Ultrasonic controllable synthesis of sulfur-functionalized metal-organic frameworks (S-MOFs) and their application in piezo-photocatalytic rapid reduction of hexavalent chromium (Cr).

The United Nations' Sustainable Development Goals (SDGs) are significant in guiding modern scientific research. In recent years, scholars have paid much attention to MOFs materials as green materials. However, piezo catalysis of MOFs materials has not been widely studied. Piezoelectric materials can convert mechanical energy into electrical energy, while MOFs are effective photocatalysts for removing pollutants. Therefore, it is crucial to design MOFs with piezoelectric properties and photosensitivity. In this study, sulfur-functionalized metal-organic frameworks (S-MOFs) were prepared using organic sulfur-functionalized ligand (H2TDC) ultrasonic synthesis to enhance their piezoelectric properties and visible light absorption. The study demonstrated that the S-MOFs significantly enhanced the reduction of a 10 mg/L solution of hexavalent chromium to 99.4 % within 10 min, using only 15 mg of catalyst. The orbital energy level differences of the elements were analyzed using piezo response force microscopy (PFM) and X-ray photoelectron spectroscopy (XPS). The results showed that MOFs functionalized with sulfur atom ligands have a built-in electric field that facilitates charge separation and migration. This study presents a new approach to enhance the piezoelectric properties of MOFs, which broadens their potential applications in piezo catalysis and piezo-photocatalysis. Additionally, it provides a sustainable method for reducing hexavalent chromium, contributing to the achievement of sustainable development goals, specifically SDG-6, SDG-7, SDG-9, and SDG-12.

Introducing hafnium to atomically small- and medium-sized tin clusters (HfSnn0/-/2- (n = 4-17)): A computational investigation of geometrical and growth behavior, spectral properties, electronic configuration and thermochemistry.

The global and local minimum configurations of single Hf atom doped Sn clusters are conducted via density function theory (DFT) combined with artificial bee colony algorithm (ABCluster). Furthermore, DFT method is also used to systematically investigate on their structural growth evolution, spectral and electronic information, thermochemical properties following the size of tin clusters doped Hf atom. Structurally, the ground-state geometries of neutral, anion and di-anion are discovered that, from n = 4, the number of Sn atoms in cluster, HfSnn0/-/2- adsorb additional Sn atom on the prior architecture one by one until forming n = 17 for HfSnn-10/-, as well as forming n = 16 for HfSnn-12-. And for the HfSn110/- and HfSn102- as beginning the species veritably develop sealed architectures. The strongest vibrational modes of sealed nanoclusters are stretching modes of Hf atom with infrared actives and breathing modes of the Sn cage framework with Raman actives, respectively. The natural population analysis (NPA) elucidates the stronger relationship between the Hf atoms and the tin frameworks in sealed clusters than that in unsealed clusters. The results of thermochemical properties, molecular orbital shell (MOs), adaptive natural density partitioning (AdNDP) and ultraviolet visible absorption spectrum (UV-Vis) indicate that, the HfSn16 with high symmetry of Td exhibits thermochemical stability and optoelectronic properties, which is utilized potentially as zero-dimensional unit of self-assembling fluorescent nanomaterials.

Structural Growth Pattern, Electronic Configurations, and Spectral and Thermochemistry Properties of ZrSnn0/-/2- (n = 4-17) Nanoscale Compounds: A Systematic Study Using Density Functional Theory.

By performing density functional theory (DFT) calculations for geometric optimization in conjunction with the artificial bee colony algorithm for cluster (ABCluster) global search approach, the ground-state structures of the neutral, anionic, and dianionic ZrSnn0/-/2- (n = 4-17) nanoscale compounds are obtained. Their structural growth evolution, spectral information, and electronic and thermochemical properties are investigated. Regarding the architectural evolution of the neutral, anion, and dianionic species, ZrSnn0/-/2- (n = 4-17) compounds possess two different stages of adsorption patterns in which, when n = 4-7 and n = 8-17, ZrSn40/-/2- and ZrSn80/-/2- compounds as the basic motif adsorb Sn atoms to become the larger clusters, respectively. The simulated photoelectron spectra (PES) of anionic compounds are in good agreement with the available experimental PES. The infrared and Raman spectra can be summarized as follows: under infrared vibrational modes, the sealed cages of ZrSnn0/-/2- compounds belong to the deformation mode, and under Raman vibrational modes, they belong to the breathing mode of the Sn cage framework. The density of states (DOS) spectra and natural population analysis (NPA) indicate that the interaction between the Zr atom and Snn frameworks of capsulated compounds has been developing stronger than for unsealed compounds. The results of thermochemical properties, molecular orbital shell (MOs) analysis, and ultraviolet-visible (UV-vis) absorption spectrum indicate that the neutral ZrSn16 nanoscale compound possesses not only both thermodynamic and chemical stability but also far-infrared sensing and optoelectronic properties and hence, is the best building block motif for new multipurpose nanoscale materials.

Theoretical Insights into the Geometrical Evolution, Photoelectron Spectra, and Vibrational Properties of YGe n - (n = 6-20) Anions: From Y-Linked to Y-Encapsulated Structures.

The structural evolution behavior of germanium anionic clusters doped with the rare-earth metal yttrium, YGe n - (n = 6-20), has been investigated using a mPW2PLYP density functional scheme and an ABCluster structure searching technique. The results reveal that with increasing cluster size n, the structure evolution pattern is from the Y-linked framework (n = 10-14), where Y serves as a linker (the Y atom bridges two germanium subclusters), to the Y-encapsulated framework (n = 15-20), where the Y atom is located in the center of the Ge cage. The simulated PES spectra show satisfying agreement with the experimental PES spectra for n = 12-20, which reveals that the global minimum structures reported here are reliable. In particular, the anionic YGe16 - nanocluster is found to be the most stable structure in the size range of n = 6-20 through analyzes of the relative stability, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, spherical jellium model, and isochemical shielding surface. Moreover, spectral properties such as infrared and Raman spectra were reported. In addition, the UV-vis spectra of the YGe16 - nanocluster are in good agreement with solar energy distribution, showing that such substances serve as multifunctional building blocks to be potentially used in optoelectronic devices or solar energy converters.

Structural evolution, photoelectron spectra and vibrational properties of anionic GdGe n - (n = 5-18) nanoalloy clusters: a DFT insight.

The structural growth of Gd-doped germanium anionic nanoclusters, GdGe n - (n = 5-18), has been explored via quantum chemistry calculations using the mPW2PLYP method and an unprejudiced structural searching technique known as ABCluster. The optimized geometries exhibited that when n = 10-14, the structural evolution favors the Gd-linked configuration where the Gd atom as a connector bridges two Ge subgroups, while the Gd atom is encapsulated in a closed cage-like Ge frame when n = 15-18. The properties like magnetic moment, charge transfer, relative stability, HOMO-LUMO gap, photoelectron spectra, and infrared and Raman spectra have been predicted. The information of these spectra could provide extra approaches to experimentally determine the electronic structures and equilibrium configuration of these compounds. The largest spin magnetic moment of 7 µ B is attained via half-filled 4f states. The GdGe16 - nanocluster is determined to be a superatom because its total valence of 75 electrons can be distributed to the orbital sequence of 1S21P6(4f7)1D101F142S22P21G182P42D10, which complies with not only Hund's rule, but also the spherical jellium model. Particularly, its UV-Vis spectra match well with solar energy distribution. Such materials act as nano multifunctional building units potentially used in solar energy converters or ultra-highly sensitive near-infrared photodetectors.

Structural Evolution, Electronic Structures, and Vibrational Properties of Anionic LuGen (n = 5-17) Clusters: From Lu-Linked to Lu-Encapsulated Configurations.

The structural evolution pattern and electronic properties of Lu-doped germanium anion clusters, LuGen- (n = 5-17), have been investigated using a global search method combined with a double hybrid density functional theory and by comparing the theoretical PES spectra with the experimental ones. It is found that, for the structural growth patterns, a Lu-linked configuration is preferred for n = 10-14 in which the Lu atom as a linker connects two Ge subclusters and a Lu-encapsulated Ge cage-like motif is preferred for n = 15-17. The simulated PES spectra agree with experimental ones, revealing that the current global minimum structures are the true minima. The properties such as relative stability, charge transfer, highest-energy occupied molecular orbital-lowest-energy unoccupied molecular orbital (HOMO-LUMO) gap, IR, Raman, and ultraviolet-visible (UV-vis) spectra have been evaluated. The results of IR and Raman spectra could provide additional ways to experimentally identify the structure of these clusters. The results of stability, HOMO-LUMO gap, and UV-vis spectra could make the LuGe16- cluster the most suitable building block for further development as a potential optoelectronic material.

Unraveling the Mechanism of Aerobic Alcohol Oxidation by a Cu/pytl-ß-Cyclodextrin/TEMPO Catalytic System under Air in Neat Water.

The mechanism for the oxidation of p-tolylmethanol to p-tolualdehyde catalyzed by a Cu/pytl-ß-cyclodextrin/TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidinyl-1-oxy) catalytic system under air in neat water is fully investigated by density functional theory (DFT). Four possible pathways (paths A → D) are presented. The calculated TOF = 0.67 h-1 for path A is consistent with the experimental TOF = 1.9 h-1 but much lower than that for path D (TOF = 1.1 × 105 h-1). The results demonstrate that path A is the dominant pathway under the optimal experimental conditions, even though path D is more kinetically favorable. This is because the concentration of precatalyst 11 [(pytl-ß-CD)CuII(OH)] in path D is too low to start path D, so p-tolylmethanol oxidation can only proceed via path A. This finding implies that the relative concentration of precatalysts in a one-pot synthesis experiment plays a vital role in the aerobic alcohol oxidation reaction. Based on this finding, we speculate that the direct use of the presynthesized precatalyst 11 or addition of an appropriate amount of NaOH to the reaction solution, but with the total amount of the base added unchanged, is a good way to improve its catalytic activity. Meanwhile, the solvent water was not found to directly participate in the catalytic active sites for the oxidation of alcohols but rather inhibited it by forming the hydrogen-bonded network.

Thermochemical Properties and Growth Mechanism of the Ag-Doped Germanium Clusters, AgGe n λ with n = 1-13 and λ = -1, 0, and +1.

A systematic investigation of the silver-doped germanium clusters AgGe n with n = 1-13 in the neutral, anionic, and cationic states is performed using the unbiased global search technique combined with a double-density functional scheme. The lowest-energy minima of the clusters are identified based on calculated energies and measured photoelectron spectra (PES). Total atomization energies and thermochemical properties such as electron affinity (EA), ionization potential (IP), binding energy, hardness, and highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap are obtained and compared with those of pure germanium clusters. For neutral and anionic clusters, although the most stable structures are inconsistent when n = 7-10, their structure patterns have an exohedral structure except for n = 12, which is a highly symmetrical endohedral configuration. For the cationic state, the most stable structures are attaching structures (in which an Ag atom is adsorbed on the Ge n cluster or a Ge atom is adsorbed on the AgGe n-1 cluster) at n = 1-12, and when n = 13, the cage configuration is formed. The analyses of binding energy indicate that doping of an Ag atom into the neutral and charged Ge n clusters decreases their stability. The theoretical EAs of AgGe n clusters agree with the experimental values. The IP of neutral Ge n clusters is decreasing when doped with an Ag atom. The chemical activity of AgGe n is analyzed through HOMO-LUMO gaps and hardness, and the variant trend of both versus cluster size is slightly different. The accuracy of the theoretical analyses in this paper is demonstrated successfully by the agreement between simulated and experimental results such as PES, IP, EA, and binding energy.

Study on the growth behavior and photoelectron spectroscopy of neodymium-doped silicon nanoclusters NdSin0/- (n = 8-20) with a double-hybrid density functional theory.

Structural evolution, magnetic moment, and thermochemical and spectral properties of NdSin0/- (n = 8-20) nanoclusters were studied. Optimized structures for NdSin demonstrated that the configuration with quintet ground state prefers Nd-substituted for a Si of the most stable Sin + 1 (n = 8-11) structure to Nd-linked configuration with Si9 tricapped trigonal prism subcluster (n = 12-19). Finally, the configuration prefers to Nd-encapsulated into Si cage framework (n = 20). For anion, the evolution at the quartet state prefers Nd-linked structure for n = 8-19 (excluded 9), and prefers Nd-encapsulated structure of n = 20. The spectral information including electron affinity, vertical detachment energy, and simulated photoelectron spectroscopy were also observed. The 4f electrons of Nd atom in NdSin with n = 8-10 hardly participate in bonding, but take part in remaining neutral clusters and all anionic NdSin- clusters. The calculations of average bond energy, HOMO-LUMO gap, and chemical bonding analyses reveal that NdSi20- possesses perfect thermodynamic and ideal chemical stability, making it as the most appropriate constitutional units for novel multi-functional semiconductors.

Probing the electronic structures and properties of neutral and charged FeSin(-1,0,+1) (n = 1-6) clusters using ccCA theory.

The equilibrium structures and electronic properties such as relative stabilities, electron affinities, and charge transfer of small neutral and charged FeSin(-1,0,+1) (n = 1-6) clusters have been systematically studied using the high level of the correlation consistent Composite Approach (ccCA) method. The lowest-energy geometries of these clusters can be regarded as "substitutional structure." It is derived from Sin + 1(and/or Si-n + 1) clusters by replacing a silicon atom with an iron atom. The adiabatic electron affinities (AEAs) and the adiabatic ionization potentials (AIPs) have also been predicted by ccCA schemes for FeSin(n = 1-6) and their ions. The dissociation energies of an iron or a silicon atom from the ground-state structure of FeSin clusters have been evaluated to check relative stabilities of FeSin(-1,0,+1) (n = 1-6) clusters. Compared with other clusters, neutral and charged FeSi2 possess higher stability. As for the neutral clusters and the negatively charged ions, the theoretical charges of the iron atom in FeSin (n = 1-6) species (except for FeSi and FeSi2-) show that silicon clusters act as an electron donor. For the cationic species, however, the charge transfers from iron atom to silicon clusters (except for FeSi3+) show that the iron atom acts as electron donor.

In-situ formation of carboxylate species on TiO2 nanosheets for enhanced visible-light photocatalytic performance.

It still remains challenge for expanding the photo-response range of TiO2 with dominant {0 0 1} facets due to the hardly achieving modification of the electronic structure without destroying the formation of TiO2 high energy facets. Herein, we report the construction of carboxylate species modified TiO2 nanosheets with dominant {0 0 1} facets by employing ethanol as a carbon source through a low-temperature (300 °C) carbonization method. The as-obtained samples were investigated in detail by using various characterization techniques. The results indicate that the carboxylate species derived from the oxidation and carbonization of ethanol are coordinated to the {0 0 1} facets in a bidentate bridging mode. The electron-withdrawing carboxylate species induce TiO2 to form a lower valence band edge and a narrower bandgap, which enhances the oxidation ability of photogenerated holes and expands the photo-response range. The partially carbonized carboxylate species can also act as a photosensitizer to induce visible-light photocatalytic activity of TiO2 nanosheets. In addition, the carboxylate species can further promote the separation of photogenerated charge carriers. The findings of this work may provide a new perspective for tuning the band structure of TiO2 with dominant {0 0 1} facets and improving its photocatalytic performance.

Structural and electronic properties of nanosize semiconductor HfSin0/-/2- (n = 6-16) material: a double-hybrid density functional theory investigation.

Equilibrium geometries, thermodynamic stabilities, chemical reactivities, and electronic properties of neutral, mono-, and di-anionic Hf-doped silicon nanoclusters HfSin0/-2- (n = 6-16) are calculated by employing an ABCluster global search technique combined with mPW2PLYP scheme. Based on the concordance between simulated and experimental PES, the true global minima are confirmed for n = 6, 9, and 12-16. Optimized geometries for neutral HfSin nanoclusters can be divided into three stages: first, Hf atom prefers locating on the surface site of the cluster for n = 6-9, which can be obtained by adding one, two, three, and four Si atoms to HfSi5 tetragonal bipyramid, respectively (denoted as additive type); then, Hf atom is surrounded by Si atoms with half-cage configuration for n = 10-13; finally, Hf atom is encapsulated into Si cage pattern for n = 14-16. For mono-anions, it is from additive type (n = 6-11) to the cagelike configuration with Hf atom resided in silicon clusters (n = 12-16). For di-anions, it is additive type (n = 6-9) to the Hf-linked configuration (n = 10-11), and in the end to the Hf-encapsulated cagelike motif (n = 12-16). The thorough analysis of stability and chemical bonding revealed that the neutral HfSi16 and di-anionic HfSi152- are magic nanoclusters with good thermodynamic and chemical stability, which may make them as the most suitable building block for new functional materials. We suggest that the experimental PES of HfSin- with n = 7, 8, 10, and 11 should be further examined due to the lack of comparably low electron binding energy peaks.

Facile synthesis of multi-type carbon doped and modified nano-TiO2 for enhanced visible-light photocatalysis.

Nano-TiO2 is a type of environment-friendly and inexpensive substance that could be used for photocatalytic degradation processes. In this study, the multi-type carbon species doped and modified anatase nano-TiO2 was innovatively synthesized and developed to overcome the deficiency of common nano-TiO2 photocatalysts. The multi-type carbon species were derived from tetrabutyl titanate and ethanol as the internal and external carbon sources, respectively. Meanwhile, diverse characterization methods were applied to investigate the morphology and surface properties of the photocatalyst. Finally, the visible-light photocatalytic degradation activity of the collected samples was evaluated by using methyl orange as a model pollutant. The promotion mechanism of multi-type carbon species in the photocatalytic process was also discussed and reported. The results in this work show that the doping and modification of multi-type carbon species successfully narrows the bandgap of nano-TiO2 to expand the light absorption range, reduces the valence band position to improve the oxidation ability of photogenerated holes, and promotes the separation of photogenerated charge carriers to improve quantum efficiency. In addition, the further modification of the external carbon source can promote the surface adsorption of MO and stabilize the multi-type carbon species on the surface of nano-TiO2.

Study on Structural Evolution, Thermochemistry and Electron Affinity of Neutral, Mono- and Di-Anionic Zirconium-Doped Silicon Clusters ZrSin0/-/2- (n = 6-16).

We have carried out a global search of systematic isomers for the lowest energy of neutral and Zintl anionic Zr-doped Si clusters ZrSin0/-/2- (n = 6-16) by employing the ABCluster global search method combined with the mPW2PLYP double-hybrid density functional. In terms of the evaluated energies, adiabatic electron affinities, vertical detachment energies, and agreement between simulated and experimental photoelectron spectroscopy, the true global minimal structures are confirmed. The results reveal that structural evolution patterns for neutral ZrSin clusters prefer the attaching type (n = 6-9) to the half-cage motif (n = 10-13), and finally to a Zr-encapsulated configuration with a Zr atom centered in a Si cage (n = 14-16). For Zintl mono- and di-anionic ZrSin-/2-, their growth patterns adopt the attaching configuration (n = 6-11) to encapsulated shape (n = 12-16). The further analyses of stability and chemical bonding make it known that two extra electrons not only perfect the structure of ZrSi15 but also improve its chemical and thermodynamic stability, making it the most suitable building block for novel multi-functional nanomaterials.

Structural growth pattern of neutral and negatively charged yttrium-doped silicon clusters YSi n 0/- (n=6-20): from linked to encapsulated structures.

A global search for the low energy of neutral and anionic doped Si clusters YSi n 0/- (n = 6-20) was performed using the ABCluster global search technique coupled with a hybrid density functional method (mPW2PLYP). In light of the calculated energies and the measured photoelectron spectroscopy values, the true minima of the most stable structures were confirmed. It is shown that the structural growth pattern of YSi n - (n = 6-20) is from Y-linked two subcluster structure to a Y-encapsulated structure in Si cages, while that of YSi n (n = 6-20) is from substitutional to linked structures, and as the number of Si atoms increases, it evolves toward the encapsulated structure. Superatom YSi20 - with a high-symmetry endohedral I h structure has an ideal thermodynamic stability and chemical reactivity, making it the most suitable building block for novel optical, optoelectronic photosensitive or catalytic nanomaterials.

Structural Stability and Evolution of Scandium-Doped Silicon Clusters: Evolution of Linked to Encapsulated Structures and Its Influence on the Prediction of Electron Affinities for ScSi n ( n = 4-16) Clusters.

Sc-doped semiconductor clusters are the simplest transition metal- and rare-earth metal-doped semiconductor clusters. In this work, the structural evolution behavior and electronic properties of Sc-doped neutral and anionic Si n ( n = 4-16) clusters were studied using the ABCluster global search technique coupled with a hybrid density functional method. The results revealed that although neutral and anionic configurations are different for ScSi n ( n = 6-14) clusters, the evolution pattern of the ground-state structures is consistent (evolution of linked to encapsulated structures starting from n = 14). The good agreement between the theoretical and experimental photoelectron spectra demonstrated that the obtained anionic global minimum structures are reasonable. The excellent agreement between the adiabatic electron affinities corrected by considering the structural correction factor and the experimental data indicated that the structural correction factor is important for reproducing the experimental data and that the obtained ground-state structures for the neutral ScSi n clusters reported herein are reliable. The relative stability and chemical bonding analysis showed that the fully encapsulated ScSi16- cluster is a magic cluster with good thermodynamic and chemical stability.

Probing structure, thermochemistry, electron affinity, and magnetic moment of thulium-doped silicon clusters TmSi n (n = 3-10) and their anions with density functional theory.

The most stable structures and electronic properties of TmSi n (n = 3-10) clusters and their anions have been probed by using the ABCluster global search technique combined with the PBE, TPSSh, and B3LYP density functional methods. The results revealed that the most stable structures of neutral TmSi n and their anions can be regarded as substituting a Si atom of the ground state structure of Si n + 1 with a Tm atom. The reliable AEAs, VDEs and simulated PES of TmSi n (n = 3-10) are presented. Calculations of HOMO-LUMO gap revealed that introducing Tm atom to Si cluster can improve photochemical reactivity of the cluster. The NPA analyses indicated that the 4f electron of Tm atom in TmSi n (n = 3-10) and their anions do not participate in bonding. The total magnetic moments of TmSi n are mainly provided by the 4f electrons of Tm atom. The dissociation energy of Tm atom from the most stable structure of TmSi n and their anions has been calculated to examine relative stability.

Stability and electronic properties of praseodymium-doped silicon clusters PrSin (n = 12-21).

The neutral PrSi n (n = 12-21) species considering various spin configurations were systematically studied using PBE0 and B3LYP schemes in combination with relativistic small-core potentials (ECP28MWB) for Pr atoms and cc-pVTZ basis set for Si atoms. The total energy, growth-pattern, equilibrium geometry, relative stability, hardness, charge transfer, and magnetic moments are calculated and discussed. The results reveal that when n < 20, the ground-state structure of PrSi n evaluated to be prolate clusters. Starting from n = 20, the ground-state structures of PrSi n are evaluated to be endohedral cagelike clusters. Although the relative stabilities based on various binding energies and different functional is different from each other, the consensus is that the PrSi13, PrSi16, PrSi18, and PrSi20 are more stable than the others, especially the PrSi20. Analyses of hardness show that introducing Pr into Si n (n = 12-21) elevates the photochemical sensitivity, especially for PrSi20. Calculated result of magnetic moment and charge transfer shows that the 4f electrons of Pr in the clusters are changed, especially in endohedral structures such as PrSi20, in which one electron transfers from 4f to 5d orbital. That is, the 4f electron of Pr in the clusters participates in bonding. The way to participate in bonding is that a 4f electron transfers to 5d orbital. Although the 4f electron of Pr atom participates in bonding, the total magnetic moment of PrSi n is equal to that of isolated Pr atom. The charge always transfers from Pr atom to Si n cluster for the ground state structures of PrSin (n = 12-19), but charge transfer is reverse for n ≥ 20. The largest charge transfer for endohedral structure reveals that the bonding between Pr and Si n is ionic in nature and very strong. The fullerenelike structure of PrSi20 is the most stable among all of these clusters and can act as the building blocks for novel functional nanotubes.

Density-functional study of the structures and properties of holmium-doped silicon clusters HoSi n (n = 3-9) and their anions.

The structures and properties of Ho-doped Si clusters, including their adiabatic electron affinities (AEAs), simulated photoelectron spectra (PESs), stabilities, magnetic moments, and charge-transfer characteristics, were systematically investigated using four density-functional methods. The results show that the double-hybrid functional (which includes an MP2 correlation component) can accurately predict the ground-state structure and properties of Ho-doped Si clusters. The ground-state structures of HoSi n (n = 3-9) are sextuplet electronic states. The structures of these Ho-doped Si clusters (aside from HoSi7) are substitutional. The ground-state structures of HoSi n- are quintuplet electronic states. Their predicted AEAs are in excellent agreement with the experimental ones. The mean absolute error in the theoretical AEAs of HoSi n (n = 4-9) is only 0.04 eV. The simulated PESs for HoSi n- (n = 5-9) are in good agreement with the experimental PESs. Based on its simulated PES and theoretical AEA, we reassigned the experimental PES of HoSi4- and obtained an experimental AEA of 2.2 ± 0.1 eV. The dissociation energies of Ho from HoSi n and HoSi n- (n = 3-9) were evaluated to test the relative stabilities of the clusters. HOMO-LUMO gap analysis indicated that doping the Si clusters with the rare-earth metal atom significantly increases their photochemical reactivity. Natural population analysis showed that the magnetic moments of HoSi n (n = 3-9) and their anions derive mainly from the Ho atom. It was also found that the magnetic moments of Ho in the HoSi n clusters are larger than the magnetic moment of an isolated Ho atom.

Reexamination of structures, stabilities, and electronic properties of holmium-doped silicon clusters HoSi n (n = 12-20).

The total energies, growth patterns, equilibrium geometries, relative stabilities, hardnesses, intramolecular charge transfer, and magnetic moments of HoSi n (n = 12-20) clusters have been reexamined theoretically using two different density functional schemes in combination with relativistic small-core Stuttgart effective core potentials (ECP28MWB) for the Ho atoms. The results show that when n = 12-15, the most stable structures are predicted to be exohedral frameworks with a quartet ground state, but when n = 16-20, they are predicted to be endohedral frameworks with a sextuplet ground state. These trend in stability across the clusters (gauged from their dissociation energies) was found to be approximately the same regardless of the DFT scheme used in the calculations, with HoSi13, HoSi16, HoSi18, and HoSi20 calculated to be more stable than the other clusters. The results obtained for cluster hardness indicated that doping the Ho atom into Si13 and Si16 leads to the most stable HoSi n clusters, while doping Ho into the other Si n clusters increases the photochemical sensitivity of the cluster. Analyses of intracluster charge transfer and magnetic moments revealed that charge always shifts from the Ho atom to the Si n cluster during the creation of exohedral HoSi n (n = 12-15) structures. However, the direction of charge transfer is reversed during the creation of endohedral HoSi n (n = 16-20) structures, which implies that Ho acts as an electron acceptor when it is encapsulated in the Si n cage. Furthermore, when the most stable exohedral HoSi n (n = 12-15) structures are generated, the 4f electrons of Ho are virtually unchanged and barely participate in intracluster bonding. However, in the most stable endohedral HoSi n (n = 16-20) frameworks, a 4f electron does participate in bonding. It does this by transferring to the 5d orbital, which hybridizes with the 6s and 6p orbitals and then interacts with Si valence sp orbitals. Meanwhile, the total magnetic moments of the HoSi n (n = 16-20) clusters are considerably higher than those of HoSi n (n = 12-15). Interestingly, the endohedral HoSi16 and HoSi20 clusters can be viewed as the most suitable building blocks for novel high-density magnetic storage nanomaterials and for novel optical and optoelectronic photosensitive nanomaterials, respectively.