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1.
Phys Chem Chem Phys ; 22(25): 14099-14108, 2020 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-32542259

RESUMO

The formation of small polarons in CeO2-x compounds has been investigated mainly on solids, compact surfaces, and large nanoparticles. However, those findings cannot be easily transferred to small ceria clusters, where size effects might play a crucial role. In this work, we report a hybrid density functional theory investigation within the Heyd-Scuseria-Ernzerhof functional to elucidate the response of the Ce oxidation state upon the adsorption of F, Na, Ni, Pd, and Pt on the (CeO2)6 cluster. Among the selected species, only the Na and Ni adatoms contribute to the formation of a single small-polaron neighboring the CeIII+ cation (i.e., change from CeIV+ to CeIII+) accompanied by a local distortion in the cluster structure, which can be explained by the large magnitude of the charge transfer from the adatoms to the cluster and change in the nature of the Ce f-states (delocalized to localized). The same effect is also obtained by adding a single electron to the (CeO2)6 cluster. The Pd and Pt adatoms yield only small charge transfer to the (CeO2)6 cluster, which is not enough to affect the Ce oxidation state. As expected, F binds to the cationic Ce sites and leads to the same effects as obtained by removing a single electron from the cluster, which implies the formation of a localized hole with O p-character above the highest occupied molecular orbital accompanied also by a local structural distortion; however, it does not affect the Ce oxidation state.

2.
Inorg Chem ; 2020 Jun 24.
Artigo em Inglês | MEDLINE | ID: mdl-32578989

RESUMO

Reaction of the Cu(I) sources, [Cu5](Mes)5 and [(iDipp)CuOtBu] (Mes = mesityl; iDipp = 1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-2-ylidene) with the Zn(I) complex [Zn2](Cp*)2 leads to a mixture of intermetallic Cu/Zn clusters with a distribution of species that is dependent on the stoichiometric ratio of the reactants, the reaction time, as well as the temperature. Systematic and careful investigation of the product mixtures rendered the isolation of two new clusters possible, i.e., the Zn-rich, red cluster 1, [CuZn10](Cp*)7 = [Cu(ZnZnCp*)3(ZnCp*)4], as well as the Cu-rich, dark-green cluster 2 [Cu10Zn2](Mes)6(Cp*)2. Structure and bonding of these two species was rationalized with the help of density functional theory calculations. Whereas 1 can be viewed as an 18-electron Cu center coordinated to four ZnCp* and three ZnZnCp* one-electron ligands (with some interligand bonding interaction), compound 2 is better to be described as a six-electron superatom cluster. This unusual electron count is associated with a prolate distortion from a spherical superatom structure. This unexpected situation is likely to be associated with the ZnCp* capping units that offer the possibility to strongly bind to the top and the bottom of the cluster in addition to the bridging mesityl ligands stabilizing the Cu core of the cluster.

3.
Phys Chem Chem Phys ; 22(15): 8067-8076, 2020 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-32239075

RESUMO

Understanding the formation of nano-interfaces between metallic clusters and nanoscale metal-oxides is an important step towards using such systems for catalytic applications. Thus, in this work, we employ density functional theory calculations to study the TMn-(ZrO2)13 interactions, for TM = Fe, Co, Ni, or Cu, and n = 1, 4, and 8. We found a general trend for adsorption and interaction energies (ad/int) for all cluster sizes, with . In terms of size effects, both adsorption and interaction (frozen adsorbed structures) energies become stronger with increasing cluster sizes due to the increase in the number of TM atoms in direct contact with the (ZrO2)13 nanocluster. The structural and electronic properties change for each TMn/(ZrO2)13 system, indicating that these properties could be tuned through variables like the TM species, cluster size and morphology (isomers with different structures). The results also indicate that, from the studied TMs, Ni (Cu) should form the smallest (largest) clusters when supported on the (ZrO2)13 nanoclusters. These and other results discussed here help understand the formation of the nano-interface in the TM-ZrO2 systems, which can be useful in the development of new catalysts.

4.
Phys Chem Chem Phys ; 2020 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-32293626

RESUMO

Adsorption is a crucial preliminary step for the conversion of CO2 into higher-value chemicals, nonetheless, the atomistic understanding of how substrate particle size affects this step is still incomplete. In this study, we employed density functional theory to investigate the effects of particle size on the adsorption of model molecules involved in the CO2 transformations (CO2, CO, H2O and H2) on Con, Nin and Cun particles with different sizes (n = 13, 55, 147) and on the respective close-packed surfaces. We found significant size-dependence of the adsorption properties for physisorbed (linear) and chemisorbed (bent) CO2 on the substrates and distinct (symmetric or asymmetric) stretching of the C-O bonds, which can play a crucial role to understand the CO2 dissociation pathways. For CO and H2, some properties showed small oscillations, due to size effects that induced alternation of the adsorption site preference for different particle sizes; for H2O, the adsorption properties were almost independent of particle size. The presence of low-coordinated adsorption sites resulted in a trend for stronger adsorption and greater charge transfer for smaller clusters. Fixing the size-independent factors (e.g., type of metal), our results show that CO2 adsorption on transition-metal clusters is significantly affected by particle size, suggesting that substrate particle size could be a key factor to understand and control the catalytic transformations of CO2.

5.
J Chem Inf Model ; 60(2): 537-545, 2020 Feb 24.
Artigo em Inglês | MEDLINE | ID: mdl-31917570

RESUMO

In this work, we report an ab initio investigation based on density functional theory calculations within van der Waals D3 corrections to investigate the adsorption properties and activation of CO2 on transition-metal (TM) 13-atom clusters (TM = Ru, Rh, Pd, Ag), which is a key step for the development of subnano catalysts for the conversion of CO2 to high-value products. From our analyses, which include calculations of several properties and the Spearman correlation analysis, we found that CO2 adopts two distinct structures on the selected TM13 clusters, namely, a bent CO2 configuration in which the OCO angle is about 125 to 150° (chemisorption), which is the lowest energy CO2/TM13 configuration for TM = Ru, Rh, Pd. As in the gas phase, the linear CO2 structure yields the lowest energy for CO2/Ag13 and several higher energy configurations for TM = Ru, Rh, Pd. The bent CO2 (activated) is driven by a chemisorption CO2-TM13 interaction due to the charge transfer from the TM13 clusters toward CO2, while a weak physisorption interaction is obtained for the linear CO2 on the TM13 clusters. Thus, the CO2 activation occurs only in the first case and it is driven by charge transfer from the TM13 clusters to the CO2 molecule (i.e., CO2-δ), which is confirmed by our Bader charge analysis and vibrational frequencies.

6.
J Chem Phys ; 151(21): 214301, 2019 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-31822101

RESUMO

The identification of the most important descriptors that drive the activation CO2 on transition-metal (TM) catalysts is a crucial step toward the conversion of CO2 into value-added chemicals; however, our atomistic understanding is far from satisfactory. Thus, aiming at the potential use of TM clusters in the conversion of CO2, we report density functional theory calculations of CO2, CO, H2O, and H2 adsorption on TM13 clusters (TM = Fe, Co, Ni, and Cu). Among the descriptors to evaluate the activation of the studied molecules, we found that the bond lengths increase, angles decrease, and their energetic variations upon the adsorption are the most important ones. From the structural response in anionic gas-phase molecules, the charge transfer toward CO2 and CO is pointed as relevant in their activation, and our results and analyses suggest that the adsorption on 3d TM13 clusters promote this charge donation process, decreasing in the order Fe13 > Co13 > Ni13 > Cu13. For CO2 and CO on Cu13, the activation was observed for highest energy configurations, indicating that is necessarily an additional driving force to occur the molecular activation on this material. Also, energetic parameters, adsorption energy, and interaction energy indicated that the strength of the adsorption is not necessarily proportional to the activation; it is difficult to point out these parameters as descriptors. Our results also provide interesting insights about steps of the CO2 reduction mechanism within the context of the modified Fischer-Tropsch synthesis.

7.
Phys Chem Chem Phys ; 21(48): 26637-26646, 2019 Dec 11.
Artigo em Inglês | MEDLINE | ID: mdl-31774074

RESUMO

Mixed CeO2-ZrO2 nanoclusters have the potential to play a crucial role in nanocatalysis, however, the atomistic understanding of those nanoclusters is far from satisfactory. In this work, we report a density functional theory investigation combined with Spearman rank correlation analysis of the energetic, structural and electronic properties of mixed CenZr15-nO30 nanoclusters as a function of the composition (n = 0, 1,…,14, 15). For instance, we found a negative excess energy for all putative global minimum CenZr15-nO30 configurations with a minimum at about n = 6 (i.e., nearly 40% Ce), in which both the oxygen anion surroundings and cation radii play a crucial role in the stability and distribution of the chemical species. We found a strong energetic preference of Zr4+ cations to occupy larger coordination number sites, i.e., the nanocluster core region, while the Ce4+ cations are located near vacuum exposed O-rich regions. As expected, we obtained an almost linear decrease of the average bond lengths by replacing Ce4+ by Zr4+ cations in the (ZrO2)15 nanoclusters towards the formation of mixed CenZr15-nO30 nanoclusters, which resulted in a shift towards higher vibrational frequencies. Besides, we also observed that the relative stability of the mixed oxides is directly correlated with the increase (decrease) of the Zr d-state (Ce f-state) contribution to the highest occupied molecular orbital with the increase of the Zr content, hence driving the gap energy towards higher values.

8.
J Chem Phys ; 151(20): 204301, 2019 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-31779323

RESUMO

Atomistic understanding of thermodynamic processes such as phase transitions in nanoalloys is crucial to improve real-life applications of Pt-based nanocatalysts. In this work, we investigate the thermodynamic properties of 55-atom PtCo and PtNi nanoalloys and compare them to reference unary systems, Pt55, Co55, and Ni55. Our results are based on the combination of the parallel tempering Monte Carlo and the revised basin-hopping Monte Carlo algorithms with many-body Gupta potentials, and furthermore, density functional theory calculations were employed to validate the adopted Gupta parameters and to analyze electronic effects induced by structural changes derived from temperature effects. We identified first-order phase transitions for Pt55, Co55, Pt30Co25, Ni55, and Pt40Ni15 at 727, 1027, 1003, 914, and 1051 K, respectively. Thus, alloying unary Pt nanoclusters with Ni and Co leads to an increase in the melting temperature, indicating that the nanoalloys are able to sustain higher temperatures while maintaining their structure. A low-temperature solid-solid transition was also identified for Pt55, which is characterized by a change from a face-centered cubic like structure (putative global minimum configuration) to the icosahedron structure. The structural transformations led by the temperature increase induce small changes on the total density of states, namely, a slight shift of the d-band center toward the highest occupied molecular orbital with increasing temperature, which was found for all considered nanoclusters.

9.
Phys Chem Chem Phys ; 21(41): 23076-23084, 2019 Oct 24.
Artigo em Inglês | MEDLINE | ID: mdl-31595273

RESUMO

An improved atomistic understanding of the W-based two-dimensional transition-metal dichalcogenides (2D TMDs) is crucial for technological applications of 2D materials, since the presence of tungsten endows these materials with distinctive properties. However, our atomistic knowledge on the evolution of the structural, electronic, and energetic properties and on the nanoflake stability of such materials is not properly addressed hitherto. Thus, we present a density functional theory (DFT) study of stoichiometric (WQ2)n nanoflakes, with Q = S, Se, Te, and n = 1,…,16, 36, 66, and 105. We obtained the configurations with n = 1,…,16 through the tree growth algorithm whereas the nanoflakes with n = 36, 66, and 105 were generated from fragments of 2D TMDs with an abundant diversity of shapes and edge configurations. We found that all the most stable nanoflakes present the same Q-terminated edge configuration. Furthermore, in isomers with n = 1,…,16 sizes, nanoflakes with triangular shapes and their derivatives, such as the rhombus geometry, define magic numbers, whereas for n > 16, triangular shapes were also found for the most stable structures, because they preserve the edge configuration. A strong modulation of the Hirshfeld charges, depending on chalcogen species and core or edge position, is also observed. The modulation of the Hirshfeld charge due to the nature of the W metal atoms makes the energetic 1D → 1T' transition of (WQ2)n differ in nanoflake size in relation to (MoQ2)n nanoflakes. Our analysis shows the interplay between edge configuration, coordination environment, and shape that determines the stability of nanoflakes, and allows us to describe design principles for stable 1T' stoichiometric nanoflakes of various sizes.

10.
Phys Chem Chem Phys ; 21(16): 8434-8444, 2019 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-30949640

RESUMO

Experimentally, steric and inductive effects have been suggested as key parameters in the adsorption and reactivity of alcohols on transition-metal (TM) surfaces, however, our atomistic understanding of the behavior of alcohols in catalysis is far from satisfactory, in particular, due to the role of hydroxy groups in the adsorption properties of C3 alcohols on TM surfaces. In this study, we investigated those effects through ab initio calculations based on density functional theory employing a semilocal exchange-correlation functional within van der Waals corrections (the D3 framework) for the adsorption of C3 alcohols with different numbers and positions of OH groups, namely, propane, 1-propanol, 2-propanol, 1,2-propanediol, 1,3-propanediol and glycerol, on the compact Ni(111), Pd(111) and Pt(111) surfaces. As expected, we found that the adsorption energy is affected by the number of hydroxy groups with similar values for each pair of regioisomers, which clearly indicates the effect of the number of OH groups. Based on Bader charge analysis, we found an effective charge transfer from the C3 molecules to the substrates, which can explain the reduction in the work function due to adsorption. Upon adsorption, the alpha carbon to the OH group closest to the surface and the central carbon are the most positively charged atoms, which increases the lability of their bonded H atoms. In addition, the depletion of electron density between the C-H and O-H bonds closer to the surfaces corroborated their stretching, suggesting that the proximity of the adsorbates to the surfaces affects the acidity of these H atoms, as well as inductive effects within the molecules.

11.
J Phys Chem Lett ; 10(3): 685-692, 2019 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-30681851

RESUMO

Gold nanoclusters have been the focus of numerous computational studies, but an atomistic understanding of their structural and dynamical properties at finite temperature is far from satisfactory. To address this deficiency, we investigate gold nanoclusters via ab initio molecular dynamics, in a range of sizes where a core-shell morphology is observed. We analyze their structure and dynamics using state-of-the-art techniques, including unsupervised machine-learning nonlinear dimensionality reduction (sketch-map) for describing the similarities and differences among the range of sampled configurations. In the examined temperature range between 300 and 600 K, we find that whereas the gold nanoclusters exhibit continuous structural rearrangement, they are not amorphous. Instead, they clearly show persistent motifs: a cationic core of 1-5 atoms is loosely bound to a shell which typically displays a substructure resulting from the competition between locally spherical versus planar fragments. Besides illuminating the properties of core-shell gold nanoclusters, the present study proposes a set of useful tools for understanding their nature in operando.

12.
ACS Appl Mater Interfaces ; 11(1): 1529-1537, 2019 Jan 09.
Artigo em Inglês | MEDLINE | ID: mdl-30525366

RESUMO

Tuning the magnetic properties of materials is a demand of several technologies; however, our microscopic understanding of the process that drives the enhancement of those properties is still unsatisfactory. In this work, we combined experimental and theoretical techniques to investigate the handling of magnetic properties of FeCo thin films via the thickness-tuning of a gold film used as an underlayer. We grow the samples by the deposition of polycrystalline FeCo thin films on the Au underlayer at room temperature by a magnetron sputtering technique, demonstrating that the lattice parameter of the sub-20 nm thickness gold underlayer is dependent on its thickness, inducing a stress up to 3% in sub-5 nm FeCo thin films deposited over it. Thus, elastic-driven variations for the in-plane magnetic anisotropy energy, Ku, up to 110% are found from our experiments. Our experimental findings are in excellent agreement with ab initio quantum chemistry calculations based on density functional theory, which helps to build up an atomistic understanding of the effects that take place in the tuning of the magnetic properties addressed in this work. The handling mechanism reported here should be applied to other magnetic films deposited on different metallic underlayers, opening possibilities for large-scale fabrication of magnetic components to be used in future devices.

13.
Phys Chem Chem Phys ; 20(37): 24210-24221, 2018 Sep 26.
Artigo em Inglês | MEDLINE | ID: mdl-30209491

RESUMO

Experimental and theoretical studies have suggested the use of strain effects to design efficient catalysts for direct alcohol fuel cells. However, our atomistic understanding of the adsorption of alcohols on strained transition-metal (TM) catalysts is still far from satisfactory. Here, we report an ab initio investigation based on density functional theory within the van der Waals D3 correction to explore the adsorption properties of methanol, ethanol and glycerol on Pt3Ni(111) alloys under different conditions. For that, we selected five TM substrates, namely, (i) Ni(111), (ii) Pt12Ni4/Pt12Ni4/Ni(111) (compressive strain), (iii) Pt16/Pt8Ni8/Pt3Ni(111) (without strain), (iv) Pt16/Pt8Ni8/Pt(111) (tensile strain) and (v) Pt(111). As expected, the physical and chemical properties of the Pt3Ni thin-layers are affected by the strain induced by the underlayer TM substrate, and hence, we can tune the adsorption properties of alcohols. In general, the magnitude of the alcohol adsorption energy increases in the following order Pt16/Pt8Ni8/Pt3Ni(111) < Pt16/Pt8Ni8/Pt(111) < Pt12Ni4/Pt12Ni4/Ni(111), which correlates with the d-band center and the effective charge on the adsorption sites, i.e., the coulombic contribution plays an important role in the adsorption. Structural and electronic density analyses indicate that, upon adsorption, the O-H and C-H bonds weaken and their breaking should be the first steps in the decomposition of alcohols. From the Bader charge analysis, we found that the TM atom directly below the bonding O loses charge to neighboring atoms, which polarizes the surface and changes the substrate work function. Although a significant enhancement of energetic and structural properties was found, the addition of the D3 correction does not change our qualitative results except for improving the dependence of the adsorption energy with the alcohol size.

14.
Angew Chem Int Ed Engl ; 57(44): 14630-14634, 2018 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-29981271

RESUMO

The paramagnetic cluster [Cu43 Al12 ](Cp*)12 was obtained from the reaction of [CuMes]5 and [AlCp*]4 (Cp*=η5 -C5 Me5 ; Mes=mesityl). This all-hydrocarbon ligand-stabilized M55 magic atom-number cluster features a Mackay-type nested icosahedral structure. Its open-shell 67-electron superatom configuration is unique. Three unpaired electrons occupy weakly antibonding jellium states. The situation prefigures the formation of a conduction band, which is in line with the measured temperature-independent magnetism. Steric protection by twelve Cp* ligands suppresses the intrinsic polyradicalar reactivity of the Cu43 Al12 core.

15.
J Chem Phys ; 149(24): 244702, 2018 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-30599733

RESUMO

The adsorption of Zr on the CeO2 surfaces can lead to the formation of ZrO2-like structures, which can play a crucial role in the catalytic properties of Ce x Zr1-x O2 as support for transition-metal catalysts; however, our atomistic understanding is far from satisfactory, and hence, it affects our capacity to engineer the combination of ZrO2-CeO2 for catalysis applications. Here, we investigate the adsorption of Zr n (n = 1 - 4) atoms on CeO2(111) surfaces through density functional theory with the Hubbard model and bring new insights into the Zr-CeO2 interaction and the formation of ZrO2-like structures on ceria. We found that the Zr atoms oxidize to Zr4+ and strongly interact with the O2- anions, reducing the surface Ce4+ cations to Ce3+ (4 Ce atoms per Zr adatom), which stabilizes the system by more than 10 eV per Zr. As more Zr is adsorbed, the O2- species migrate from the sub-surface to interact with the on-surface Zr adatoms in hcp sites, producing a full ZrO2-like monolayer, which contributes to reduce the strain induced by the increased size of the Ce3+ cations compared with Ce4+. The simulated partial and full ZrO2-like structure thicknesses agree with the experimental measurements. In addition, we found an unprecedented trend for the on-surface Zr atoms: our calculations show that they are less stable than Zr replacing Ce3+ atoms from the first cation layer. Therefore, under sufficiently high temperatures, one expects the formation of a Ce2O3-like/c-ZrO2/CeO2 structure, which may completely change the reactivity of the surface.

16.
Phys Chem Chem Phys ; 19(23): 15484-15502, 2017 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-28580970

RESUMO

Subnanometric transition-metal (TM) clusters have attracted great attention due to their unexpected physical and chemical properties, leastwise compared to their bulk counterparts. An in-depth understanding of the evolution of the properties as a function of the number of atoms for such systems is a basic prerequisite to leverage countless applications, from catalysis to magnetic storage, as well as to answer fundamental questions related to their intrinsic stability. Here, we reported a systematic density functional study to investigate the structural, electronic properties and stability of all TMn (30 elements) unary clusters as a function of the number of atoms (n = 2-15). We provided the complete structural patterns for all TM periodic table groups, considering the growth evolution as well as the main trends of the structural and electronic properties. The combination of the occupation of the bonding/anti-bonding d-states and the s-d hybridization is found to be the main stabilization mechanism, helping in the understanding of the structural patterns. Most TMn clusters have a magic number of atoms, for which there are peaks in s-d hybridization and null electric dipole moments. Thus, our extensive and comparative study addresses size effects along with the evolution of d-orbital occupation for the TMn gas-phase cluster properties.

17.
J Chem Phys ; 146(16): 164304, 2017 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-28456198

RESUMO

Although several studies have been reported for Pt55 and Au55 nanoclusters, our atomistic understanding of the interplay between the adsorbate-surface interactions and the mechanisms that lead to the formation of the distorted reduced core (DRC) structures, instead of the icosahedron (ICO) structure in gas phase, is still far from satisfactory. Here, we report a density functional theory (DFT) investigation of the role of the adsorption effects of PH3 (one lone pair of electrons) and SH2 (two lone pairs) on the relative stability of the Pt55 and Au55 nanoclusters. In gas phase, we found that the DRC structures with 7 and 9 atoms in the core region are about 5.34 eV (Pt55) and 2.20 eV (Au55) lower in energy than the ICO model with Ih symmetry and 13 atoms in the core region. However, the stability of the ICO structure increases by increasing the number of adsorbed molecules from 1 to 18, in which both DRC and ICO structures are nearly degenerate in energy at the limit of 18 ligands, which can be explained as follows. In gas phase, there is a strong compression of the cationic core region by the anionic surface atoms induced by the attractive Coulomb interactions (core+-surface-), and hence, the strain release is obtained by reducing the number of atoms in the cationic core region, which leads to the 55 atoms distorted reduced core structures. Thus, the Coulomb interactions between the core+ and surface- contribute to break the symmetry in the ICO55 structure. On the other hand, the addition of ligands on the anionic surface reduces the charge transfer between the core and surface, which contributes to decrease the Coulomb interactions and the strain on the core region of the ICO structure, and hence, it stabilizes a compact ICO structure. The same conclusion is obtained by adding van der Waals corrections to the plain DFT calculations. Similar results are obtained by the addition of steric effects, which are considered through the adsorption of triphenylphosphine (PPh3) molecules on Au55, in which the relative stability between ICO and DRC is the same as for PH3 and SH2. However, for Pt55, we found an inversion of stability due to the PPh3 ligand effects, where ICO has higher stability than DRC by 2.40 eV. Our insights are supported by several structural, electronic, and energetic analyses.

18.
J Chem Phys ; 146(6): 064114, 2017 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-28201917

RESUMO

A basic requirement for an atom-level understanding of nanoclusters is the knowledge of their atomic structure. This understanding is incomplete if it does not take into account temperature effects, which play a crucial role in phase transitions and changes in the overall stability of the particles. Finite size particles present intricate potential energy surfaces, and rigorous descriptions of temperature effects are best achieved by exploiting extended ensemble algorithms, such as the Parallel Tempering Monte Carlo (PTMC). In this study, we employed the PTMC algorithm, implemented from scratch, to sample configurations of LJn (n=38, 55, 98, 147) particles at a wide range of temperatures. The heat capacities and phase transitions obtained with our PTMC implementation are consistent with all the expected features for the LJ nanoclusters, e.g., solid to solid and solid to liquid. To identify the known phase transitions and assess the prevalence of various structural motifs available at different temperatures, we propose a combination of a Leader-like clustering algorithm based on a Euclidean metric with the PTMC sampling. This combined approach is further compared with the more computationally demanding bond order analysis, typically employed for this kind of problem. We show that the clustering technique yields the same results in most cases, with the advantage that it requires no previous knowledge of the parameters defining each geometry. Being simple to implement, we believe that this straightforward clustering approach is a valuable data analysis tool that can provide insights into the physics of finite size particles with few to thousand atoms at a relatively low cost.

19.
J Phys Condens Matter ; 29(8): 085501, 2017 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-28060770

RESUMO

The optical band gap, extracted from absorption measurements, defines the figure of merit for transparent conducting oxides (TCOs). In many oxides, such as [Formula: see text] or [Formula: see text], inversion symmetry introduces a selection rule that blocks transitions from the valence-band maximum to the conduction-band minimum. This raises the absorption threshold and enlarges the optical gap relative to the fundamental band gap. Here, we present density-functional computations identifying two optical gaps, either of which can be detected, depending on the optical light intensity. Under strong illumination, weak transitions from [Formula: see text]-points near the valence-band maximum contribute significantly to the absorption spectrum and define an optical gap matching the fundamental gap. Low optical intensities by contrast give prominence to the large optical gap determined by the selection rule. While experimental conditions have favored observation of the former optical gap in [Formula: see text], in contrast, absorption measurements in [Formula: see text] have focused on the latter. Our findings explain the disparity between the optical and fundamental gaps in bixbyite [Formula: see text] and predict that, measured under low illumination, the optical gap for rutile [Formula: see text] will increase, from 3.60 eV to 4.34 eV.

20.
J Phys Condens Matter ; 29(3): 035402, 2017 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-27849630

RESUMO

There is a great interest to design two-dimensional (2D) chalcogenide materials, however, our atomistic understanding of the major physical parameters that drive the formation of 2D or three-dimensional (3D) chalcogenides is far from satisfactory, in particular, for complex quaternary systems. To address this problem, we selected a set of quaternary 2D and 3D chalcogenide compounds, namely, [Formula: see text] (A = Li, K, Cs; Q = S, Se, Te), which were investigated by density functional theory calculations within van der Waals (vdW) corrections. Employing experimental crystal structures and well designed crystal modifications, we found that the average atomic radius of the alkali-metal, A, and chalcogen, Q, species play a crucial role in the stability of the 2D structures. For example, the 2D structures are energetically favored for smaller [Formula: see text] and larger [Formula: see text] average atomic radius, while 3D structures are favored at intermediate average atomic radius. Those results are explained in terms of strain minimization and Coulomb repulsion of the anionic species in the structure framework. Furthermore, the equilibrium lattice parameters are in excellent agreement with experimental results. Thus, the present insights can help in the design of stable quartenary 2D chalcogenide compounds.

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