Cage-Like Nanoclusters of ZnO Probed by Time-Resolved Photoelectron Spectroscopy and Theory

Tuning the photodynamics of sub-nanometer neutral chromium oxide clusters through sequential oxidation

Sub-nanometer neutral chromium oxide clusters were produced in the gas phase through laser ablation and their low-lying excited state lifetimes were measured using femtosecond pump-probe spectroscopy. Time-dependent density functional theory calculations relate the trends in experimental lifetimes to the cluster's electronic structure. The photoexcited (CrO₂)_n (n < 5) cluster transients with the absence of up to four O atoms (Cr_nO_2n-x, x < 5) exhibit a ∼30 fs and sub-ps lifetime, attributed to instantaneous metallic e-e scattering and vibrationally mediated charge carrier relaxation, respectively. A long-lived (>2 ps) response is found in both small and clusters with low O content, indicating that terminal CrO bonds facilitate efficient excited state relaxation. The ∼30 fs transient signal fraction grows nearly linearly with oxidation, matching the amount of O-2p to Cr-3d charge transfer character of the photoexcitation and suggesting a gradual transition between semiconducting and metallic behavior in chromium oxide clusters at the molecular level. The results presented herein suggest that the photocatalytic properties of chromium oxides can be tunable based on size and oxidation.

Increased Excited State Metallicity in Neutral Cr2On Clusters (n < 5) upon Sequential Oxidation

Excited state lifetimes of neutral Cr₂O_n (n < 5) clusters were measured using femtosecond pump-probe spectroscopy. Density functional theory calculations reveal that the excited state dynamics are correlated with changes in the cluster's electronic structure with increasing oxidation. Upon absorption of a UV (400 nm) photon, the clusters exhibit features attributed to three separate relaxation processes. All clusters exhibit similar subpicosecond lifetimes, attributed to vibrational relaxation. However, the ∼30 fs transient signal fraction grows linearly with oxidation, matching the amount of O to Cr charge transfer character of the photoexcitation and highlighting a gradual transition between semiconducting and metallic behavior at the molecular level. A long-lived (>2.5 ps) response is recorded only in clusters with significant d-electron character, suggesting that adiabatic relaxation back to the ground state is efficient in heavily oxidized clusters, due to the presence of terminal O atoms. The simple picture of sequential oxidation of Cr₂O_n reveals a linear variation in the contributions of each relaxation component to the total transient signals, therefore opening possibilities for the design of new molecular spintronic materials.

Effect of Passivation on Stability and Electronic Structure of Bulk-like ZnO Clusters

Electronic structure of nearly stoichiometric and nonstoichiometric clusters of ZnO having bulk-like wurtzite geometry passivated with fictitious hydrogen atoms are comparatively analyzed for structural evolution using density functional theory-based electronic structure calculations. A parameter, average binding energy per atomic number (ABE-number), is introduced for better insight of structural evolution. The stability of a cluster is determined by binding energy per atom and ABE-number, whereas structural evolution on the basis of spin-polarized energy spectrum is studied via site projected partial density of states (l-DOS). The overall structural evolution is mapped for bare and passivated ZnO clusters to l-DOS. The study has established a correlation between the stability of clusters and their l-DOS. O-excess and O-surfaced clusters are found to be more stable. The HOMO-LUMO gap varies from 0 to 6.3 eV by tuning the size, composition, and surface termination of the clusters. Present results reported for clusters of sizes up to ∼1 nm can pave a path for formulating strategies for experimental synthesis of ZnO nanoparticles for tuning the HOMO-LUMO gap.

Analyzing ZnO clusters through the density-functional theory

The potential energy surface of Zn _n O _n clusters (n = 2, 4, 6, 8) has been explored by using a simulated annealing method. For n = 2, 4, and 6, the CCSD(T)/TZP method was used as the reference, and from here it is shown that the M06-2X/TZP method gives the lowest deviations over PBE, PBE0, B3LYP, M06, and MP2 methods. Thus, with the M06-2X method we predict isomers of Zn _n O _n clusters, which coincide with some isomers reported previously. By using the atoms in molecules analysis, possible contacts between Zn and O atoms were found for all structures studied in this article. The bond paths involved in several clusters suggest that Zn _n O _n clusters can be obtained from the zincite (ZnO crystal), such an observation was confirmed for clusters with n = 2 - 9,18 and 20. The structure with n = 23 was obtained by the procedure presented here, from crystal information, which could be important to confirm experimental data delivered for n = 18 and 23.

The Zn12O12 cluster-assembled nanowires as a highly sensitive and selective gas sensor for NO and NO2

Motivated by the recent realization of cluster-assembled nanomaterials as gas sensors, first-principles calculations are carried out to explore the stability and electronic properties of Zn₁₂O₁₂ cluster-assembled nanowires and the adsorption behaviors of environmental gases on the Zn₁₂O₁₂-based nanowires, including CO, NO, NO₂, SO₂, NH₃, CH₄, CO₂, O₂ and H₂. Our results indicate that the ultrathin Zn₁₂O₁₂ cluster-assembled nanowires are particularly thermodynamic stable at room temperature. The CO, NO, NO₂, SO₂, and NH₃ molecules are all chemisorbed on the Zn₁₂O₁₂-based nanowires with reasonable adsorption energies, but CH₄, CO₂, O₂ and H₂ molecules are only physically adsorbed on the nanowire. The electronic properties of the Zn₁₂O₁₂-based nanowire present dramatic changes after the adsorption of the NO and NO₂ molecules, especially their electric conductivity and magnetic properties, however, the other molecules adsorption hardly change the electric conductivity of the nanowire. Meanwhile, the recovery time of the nanowire sensor at T = 300 K is estimated at 1.5 μs and 16.7 μs for NO and NO₂ molecules, respectively. Furthermore, the sensitivities of NO and NO₂ are much larger than that of the other molecules. Our results thus conclude that the Zn₁₂O₁₂-based nanowire is a potential candidate for gas sensors with highly sensitivity for NO and NO₂.

Cluster assembly route to a novel octagonal two-dimensional ZnO monolayer

To explore the possibility of cluster assembly resulting in a two-dimensional (2D) stable monolayer of ZnO, a systematic study is performed on the structural evolution of bare and passivated stoichiometric clusters of [Formula: see text] [Formula: see text], [Formula: see text], using density-functional-theory-based first principles electronic structure calculations. The changes in hybridization are investigated with the aid of the site-projected partial density of states and partial charge density, while the effect of passivation and size on the ionicity of the cluster is studied using Bader charge analysis. The structural and chemical properties are found to be influenced by the coordination number of atoms in the clusters irrespective of the coordinating species. The physical parameters and hybridization of the states for the clusters on passivation resemble those of the bulk. Passivation thus provides an environment that leads to the stability of the clusters. Cluster assembly using the stable cluster geometries of passivated clusters (without the passivating atoms) has been shown to lead to stable 2D structures. This stability has been studied on the basis of binding energy, vibrational frequency, phonon dispersion and thermal properties. A new octagonal 2D monolayer planar geometry of ZnO is predicted.

Size dependent structural and polymorphic transitions in ZnO: from nanocluster to bulk

We report on an extensive survey of (ZnO)_N nanostructures ranging from bottom-up generated nanoclusters to top-down nanoparticles cuts from bulk polymorphs. The obtained results enable us to follow the energetic preferences of structure and polymorphism in (ZnO)_N systems with N varying between 10-1026. This size range encompasses small nanoclusters with 10s of atoms and nanoparticles with 100s of atoms, which we also compare with appropriate bulk limits. In all cases the nanostructures and bulk systems are optimized using accurate all-electron, relativistic density functional theory based calculations with numeric atom centered orbital basis sets. Specifically, sets of five families of (ZnO)_N species are considered: single-layered and multi-layered nanocages, and bulk cut nanoparticles from the sodalite (SOD), body centered tetragonal (BCT), and wurtzite (WZ) ZnO polymorphs. Using suitable fits to interpolate and extrapolate these data allows us to assess the size-dependent energetic stabilities of each family. With increasing size our results indicate a progressive change in energetic stability from single-layered to multi-layered cage-like nanoclusters. For nanoparticles of around 2.6 nm diameter we identify a transitional region where multi-layered cages, SOD, and BCT nanostructures are very similar in energetic stability. This transition size also marks the size regime at which bottom-up nanoclusters give way to top-down bulk-cut nanoparticles. Eventually, a final crossover is found where the most stable WZ-ZnO polymorph begins to energetically dominate at N ∼ 2200. This size corresponds to an approximate nanoparticle diameter of 4.7 nm, in line with experiments reporting the observation of wurtzite crystallinity in isolated ligand-free ZnO nanoparticles of 4-5 nm size or larger.

From monomer to monolayer: a global optimisation study of (ZnO)n nanoclusters on the Ag surface

We employ global optimisation to investigate how oxide nanoclusters of increasing size can best adapt their structure to lower the system energy when interacting with a realistic extended metal support. Specifically, we focus on the (ZnO)@Ag(111) system where experiment has shown that the infinite Ag(111)-supported ZnO monolayer limit corresponds to an epitaxially 7 : 8 matched graphene-like (Zn(3)O(3))-based hexagonal sheet. Using a two-stage search method based on classical interatomic potentials and then on more accurate density functional theory, we report global minina candidate structures for Ag-supported (ZnO)n cluster with sizes ranging from n = 1-24. Comparison with the respective global minina structure of free space (ZnO)n clusters reveals that the surface interaction plays a decisive role in determining the lowest energy Ag-supported (ZnO)n cluster structures. Whereas free space (ZnO)n clusters tend to adopt cage-like bubble structures as they grow larger, Ag-supported (ZnO)n clusters of increasing size become progressively more like planar cuts from the infinite graphene-like ZnO single monolayer. This energetic favourability for planar hexagonal Ag-supported clusters over their 3D counterparts can be partly rationalised by the ZnO-Ag(111) epitaxial matching and the increased number of close interactions with the Ag surface. Detailed analysis shows that this tendency can also be attributed to the capacity of 2D clusters to distort to improve their interaction with the Ag surface relative to more rigid 3D bubble cluster isomers. For the larger sized clusters we find that the adsorption energies and most stable structural types appear to be rather converged confirming that our study makes a bridge between the Ag-supported ZnO monomer and the infinite Ag-supported ZnO monolayer.